WO2019194686A1

WO2019194686A1 - Nanosilica from rice hull ash as a component of a nanofluids coolant and methods thereof

Info

Publication number: WO2019194686A1
Application number: PCT/PH2019/000002
Authority: WO
Inventors: Ma-Cristine Concepcion IGNACIO
Original assignee: University Of The Philippines Los Banos
Priority date: 2018-04-05
Filing date: 2019-03-22
Publication date: 2019-10-10

Abstract

The invention nanosilica as a component of nanofluid allows for the utilization of crop waste such as rice hull ash to obtain nanosilica particles. The utilization of rice hull ash provides for a cheaper alternative for obtaining nanosilica particles. Nanosilica particles obtained had particles starting at 46.5nm, and a spherical structure. Nanofluids with nanosilica in deionized water, and in ethylene glycol: deionized water were produced. 0.25-3% volume of nanostructured silica was added to deionized water, and or EG:dW mixtures of varying ratio (25:75, 50:50, 75:25). Nanofluids produced were stable for up to 10 days, wherein the most stable nanofluid using deionized water is 0.5% volume of nanosilica, whereas in EG:dW is 0.25% volume of nanosilica in 50:50 EG:dW. Nanofluids produced were subjected to varying temperatures and was found to have better thermophysical properties compared to deionized water: higher thermal conductivity, thermal diffusivity, lower specific heat, etc. These characteristics allows for the invention to be used as a heat transfer enhancement.

Description

NANOSILICA FROM RICE HULL ASH AS A COMPONENT OF NANOFLUIDS COOLANT AND METHODS THEREOF

TECHNICAL FIELD OF THE INVENTION

5 The invention relates to the use of nanosilica as a component of nanofluids specifically as coolants to vehicles, machinery, and other heat generating equipment.

BACKGROUND OF THE INVENTION lo Heat transfer using fluids (coolants) is widely used in modem technology such as vehicles, machinery and other heat generating equipment (Yu et al., 2009). The ability of these coolants to effectively transfer and disperse heat enhances the performance of the machine and, at the same time, decreases detrimental effects caused by overheating. However, the thermal conductivity of liquids is very low is compared to those of solids. This limits the liquid's ability to effectively transfer heat (Chopkar et al., 2006). Studies revealed that addition of solid particles in a liquid base fluid increased the thermal conductivity of the base fluid into which the particles are dispersed.

Further studies showed that nanoparticles, solid particles in the nano-size scale 20 ranging from 1 to 100 nm, are better heat conductors due to their higher surface- to-volume ratio. This results to uniform particle distribution in the base fluid. Increasing the mass concentration of dispersed nanoparticles further improves the thermal properties of the base fluid (Asadzadeh et al., 2012). Also, particles in the nanoscale are more stable due to a decreased tendency to settle; this 25 reduces the chances of clogging the machine.

The addition of micrometer- or millimeter-sized solid metal or metal oxide particles to the base fluid shows enhanced thermophysical properties especially in the thermal conductivity of resultant heat transfer fluids. However, the presence

SUBSTITUTE SHEET¹ (RULE 26) of milli- or micro-sized particles contributed many problems such as erosion and clogging of the heat transfer channels. To prevent these problems, a novel concept of "nanofluids" has been proposed. These fluids contain suspensions of nanoparticles, which effectively increase the thermal conductivity of the base fluid. Commonly used base fluids for heat-transferring applications are water, engine oil and ethylene glycol. These fluids must be non-reactive to the nanoparticles dispersed in it to form a stable solution. Apart from the application in the field of heat transfer, nanofluids can be also be synthesized for magnetic, electrical, chemical and biological applications. Nanosilica is used since it is a stable nanoparticle and is readily available for synthesis. Rice hull, which is considered to be one of the biggest agricultural wastes in the world, contains about 90% silica by mass upon combustion. Because of this, rice hull ash has lately become the leading raw material for silica production since it is cheap and readily accessible. Current research in the use of silicon particles as the solid component of nanofluids involve the synthesis of silicon from different compounds. These chemical reactions pave the way for the creation of silicon particles which can either be in different forms (such as sol), micro sized, and or even nanosized. Although this is the case, the research of these silicon particles revolve on the use of different chemicals to synthesize silicon, rather than the use of crop waste such as rice hull ash. One such research in the use of rice hull ash is one taught by Zhang, Zhiliang et al. in the 2016 study entitled. “Rice Husk Ash-Derived Silica Nanofluids: Synthesis and Stability Study.” Zhang, et al found that nanofluids have shown great potential in heat/mass transfer applications. However, their practical applications are limited by the high production cost and low stability. In their study, a low-cost agricultural waste, rice husk ash (RHA), was used as a silicon source to the synthesis of silica nanofluids. First, silica nanoparticles with an average size of 47 nm were synthesized. Next, by dispersing the silica nanoparticles in water with ultrasonic vibration, silica nanofluids were formed. The results indicated that the dispersibility and stability of nanofluids were highly dependent on sonication time and power, dispersant types and concentrations, as well as pH; an optimal experiment condition could result in the highest stability of silica nanofluid. After 7 days storage, the nanofluid showed no sedimentation, unchanged particle size, and zeta potential. The results of Zhang’s study demonstrated that there is a great potential for the use of RHA as a low-cost renewable resource for the production of stable silica nanofluids. There was, however, no mention in the Zhang study regarding the use of RHA in enhancing the thermal properties of heat transfer fluids.

SUMMARY OF THE INVENTION

The invention finds alternative ways of enhancing the thermal properties of heat transfer fluids and to utilize agricultural by-products like rice hull ash (RHA), which has been proven to be a viable and low-cost alternative source of nanosilica particles with 92.85% purity and 82% recovery. Fluids with nano-scaled particles form a stable suspension and provide impressive improvements in the thermal properties of base fluids. AFM analysis of the nanosilica powder gave a size range of 46.5 nm. The stability of the nanofluids based on sediment photography and UV-Vis spectrophotometry showed that 0.5% deionized water-based nanofluids was stable for 10 days and 0.25% of 50:50 EG:dW mixture-based nanofluids was stable and without significant sedimentation for 7 to 10 days compared to other EG:dW mixture-based nanofluids. Measured density of the nanofluids did not vary much relative to the base fluids. An increase in dynamic viscosity was observed in deionized water-based nanofluid by as much as 159.90%. It was observed that the 25:75 EG:dW ratio has the fastest flow rate while the 75:25 has the slowest. Thus, the concentration of ethylene glycol greatly affects the flow rate of the solution. Measured specific heat of the nanofluids formulated in different volume concentrations decreased compared to base fluids. The increase of thermal conductivity of water-based nanofluids compared to deionized water reached 45% after dispersing nanosilica powder at different volume concentrations measured at temperatures ranging 30-70°C while 55.12% for EG:dW mixture-based nanofluids. The performance of the most stable deionized water (0.5% volume concentration) and 75:25 EG:dW mixture-based nanofluids (0.5% volume concentration) in a heat exchanger was determined to be 0.57% to 6.02% and 13.49 to 35.40 %, respectively. The enhancement of thermophysical properties and heat transfer of nanosilica-in-fluid dispersion allows for the potential use of the nanofluids as a heat transfer fluid.

DETAILED DESCRIPTION OF THE INVENTION

1. Purification of Rice Hull Ash

Rice hull ash (RHA) was subjected to reflux in a 0.1 N HC1 solution for 2 hours. The mixture was then filtered and the residue was washed with deionized water, The filtrate was discarded. The residue was placed in an oven and dried at 100° C overnight. The dried RHA was then placed in a muffled furnace and was ignited at 650° C for 6 hours.

2. Extraction and Purification of Silica

The purified RHA was refluxed in a solution of 3.0 N NaOH for three hours. The mixture was filtered and the residue was discarded. The pH of the filtrate was adjusted to pH 2.5 using 5.0 N H₂SO₄, and then neutralized using concentrated ammonium hydrochloride. The resulting mixture was filtered and the residue (silica) was washed with hot deionized water. The residue was dried overnight in an oven at 100°C. The resulting white solid was ground into fine powder then was purified via reflux in a 6.0 N HCI solution for 4 hours. The resulting mixture was again filtered and the residue was washed with hot deionized water until neutral. The residue was then dried in an oven at 100° C overnight. 3. Production of Nanosilica

Nanosilica was produced by dissolving the purified silica in a 2.5 N NaOH solution. The mixture was stirred for 12 hours to dissolve the silica powder into the solution. The pH was adjusted to 2.5 using 5.0 N H2SO4 and then neutralized using concentrated ammonium hydroxide. Afterwards, the mixture was subjected to ultrasonication for 6 hours. It was then filtered and the residue (nanosilica) was collected and dried in a thermostatted oven at 120° C for two days.

4. Characterization of Nanosilica

The physical properties and purity of nanosilica were determined. The particle size of nanosilica was determined using scanning electron microscope (SEM) and atomic force microscope (AFM). The SEM analysis was done using a JEOL JSM-6010V Scanning Electron Microscope. The AFM analysis was done using a Park Systems XE-70 Atomic Force Microscope. The surface area and pore size of the nanosilica was done using a Nova 2200e Brunauer-Emmett-Teller (BET) instrument.

Samples were also examined with Energy Dispersive X-Ray Spectroscopy (EDX). The silicon content of the nanopowder was estimated from the EDX data based on the assumption that all the silicon was in the form of silica.

5. Preparation of Nanofluid

The nanosilica powder was placed in a 250 mL Erlenmeyer flask, followed by the addition of deionized water. The mixture was stirred continuously for 30 minutes using a magnetic stirrer and was then placed in an ultrasonic vibrator and sonicated continuously for 5 hours. Also, three mass-per-volume concentrations (0.5, 0.75, 1.0 % m/v) of nanofluids were prepared by dispersing the nanosilica into 50:50 ethylene glycol: deionized water (EG:dW) solutions. The nanofluid suspensions were subjected to stirring and sonication for 6 hours continuously to ensure uniform particle distribution.

Three mass-per-volume concentrations (0.25, 0.50, 0.75% m/v) of nanofluids were prepared by dispersing the nanosilica powder into 25:75, 75:25 and 50:50 ethylene glycol: deionized water solutions. The suspension was subjected to stirring for 12 hours followed by sonication for 6 hours to ensure uniform particle distribution. 6. Characterization of Nanofiuid

The particle size of nanosilica dispersed in deionized and ethylene:deionized water solutions was determined transmission electron microscope (TEM). The TEM analysis was done using a JEOL JEM 1010 Transmission Electron Microscope and TEM of RITM.

7. Stability Evaluation of Nanofluid

Stability of the nanofluids was determined by observing its UV-Vis spectrum and sedimentation photographs for ten days. For the sedimentation photography analysis, nanofluids were placed in transparent vials and were remained undisturbed for ten days. The sedimentation of the nanofluids at various days was compared using digital photographs.

For the UV-Vis spectroscopy analysis, the nanofluids to be evaluated were placed into cuvettes and absorbance at wavelength of 600 nm was read using a Shimadzu UV mini-1240 spectrophotometer.

8. Determination of Thermophysical Properties of Nanofluid Density and viscosity of the nanofluids were determined using pycnometer method and viscometer method at a temperature range from 30° to 70° C with 10-degree increments. The literature values of the density of water at each temperature are known.

A heat-flux type TA Instruments Q20 differential scanning calorimeter (DSC) was used to measure the specific heat of the samples. The classical three-step method for measuring specific heat capacity was used:

l . Equilibrate and remain isothermal at 30° C for one minute, 2. Ramp to 75^° C at 10° C/min,

3. Remain isothermal 75° C for one minute.

The formula below was then used to calculate the specific heat of the sample, cp, sample at 30^°C, 50°C and 70°C:

Where m_ref and rn_Sampie are the weights of the reference and sample, respectively; Qref and Qsampie are the heat flux of the reference and sample, respectively; and Cpref is the known specific heat of the reference. The reference used in this experiment is sapphire. The thermal conductivity of the nanofluids were determined using the transient hot-wire apparatus.

Other properties like kinematic viscosity, Prandtl number and thermal diffusivity were calculated based from the measured density, specific heat, thermal conductivity and dynamic viscosity of the nanofluids. 9. Fabrication of the Transient Hot-Wire Apparatus

The design was mainly based on the study based by Kostic and Simham (2009); however, the values of the resistances in the Wheatstone bridge cannot be exactly equal to the values of resistances. This was purely because of the commercially available materials in the country and the method of data acquisition. Before constructing the actual circuit, it was simulated using the Multisim software. The simulation helps in predicting the actual values of the output of the circuit. The platinum wire served as the temperature sensor of the fluid. The change of resistance of the wire will determine the change of temperature and, the change of resistance will be verified using a Wheatstone bridge. A Wheatstone bridge is a simple form of bridge circuit with the purpose of measuring small changes in resistances. Precision measurements of component values have been yielded for a long time using various forms of bridges, including the Wheatstone bridge.

Using Multisim, the initial stage of the simulation is to construct an existing Wheatstone bridge circuit used in a previous study in thermal conductivity. The values used were exactly equal to understand how the circuit was used and to able to know how to modify the circuit. Figure 1 below displays the simulation circuit.

Figure 1. Simulation of the Wheatstone bridge using Multisim The DC Electronic Load (R1) is represented by a potentiometer whose resistance can be varied to balance the bridge. The platinum wire in the thermal conductivity cell (R5) is also represented by a potentiometer because it changes resistance through time. The capacitor, connected in parallel with the voltage output of the bridge, was placed to serve as a low-pass filter to smoothen the output and avoid certain spikes and noise.

The simulation demonstrated the significance of balancing the Wheatstone bridge before performing the experiment. The circuit is balanced by injecting a small amount of input voltage such as 0.1 V and adjusting the resistance of the DC electronic load until the voltage output reads approximately 0 V. Increasing the input voltage will cause imbalance to the circuit resulting to the heating of the platinum, adjusting its resistance to balance the circuit until it reaches steady- state.

The fundamental need is the fluid sample, which implies that the casing of the apparatus is of utmost importance. The reservoir must minimize the sample volume needed for the experiment; at the same time, it must be sufficient to obtain the characteristics of the fluid. In addition, reducing the sample volume would lessen the heating power needed from the wire and shorten the duration of the data gathering.

Considering the different features of the hot-wire cell in the previous studies, the basis was their recommendations. For example, to be able to apply Fourier's Law, the heat flux of the line heat source must flow radially; hence, the cell should be cylindrical. Moreover, if the set-up is intended to be available for future use, it should be easy to maintain. The design is shown in Figure 2. The initial dimensions of the drawing is the same with the cell in the study by Kostic & Simham (2009). However, the values were found to be not commercially available. The measurements were adjusted according to local market availability.

The Metals Industry Research and Development Center fabricated the thermal hot-wire cell.

Figure 2. Isometric View of the resulting design rendered using AUTOCAD

The smallest diameter of a stainless pipe obtainable in the market is 1 inch, and the next available diameter is 1.25 inches. The half pipe which will hold the platinum wire should have a smaller diameter than the main cell; hence, the smallest diameter should be used. The purpose of the half pipe is to separate the sample fluid from the electrical wirings. Figure 3 is the drawing of the half pipe using AUTOCAD.

Holes were drilled to give way and to guide the wires soldered to the platinum wire, as well as for the placement of the thermocouples. The top part hole for the thermocouple is 75° to the right with the platinum wire, viewed in front, as the reference. The other hole for the thermocouple in the bottom part is located at 15° to the left with the platinum wire, viewed in front, as the reference.

Figure 3. Drawing of the Half-Pipe using AUTOCAD

The drawing was scaled using the diameter of the outer pipe as the base, adjusting the other dimensions up to four decimal places. The actual diameter of the half pipe is 1 inch instead of 1.0310 inches since it is the commercially available smallest diameter of a stainless steel pipe.

In Figure 4, the drawing for the two identical parts attached to the top and bottom of the half-pipe is shown. The purpose of this part is to fasten the off-centered half pipe and give space for the electrical wiring attached to the platinum wire.

Figure 4. Drawing of the Off-centered Alignment Ring using AUTOCAD The raw material of this component was a solid stainless steel shaft with 1.25- inch diameter. The opening at the side is where the wires go in and out; whereas, the fissure on the off-centered part is for visual checking whether the fluid covers the entire length of the platinum wire. The center hole is where a pin is fixed— it will hold the hot wire. However, due to constraints, the dimensions in the drawing were approximated up to two decimal places in the actual measurements of the fabricated cell.

The diameter of the platinum wire is 76 m which why is it is very fragile and difficult to install. Since fluids have conductive properties, it is possible that the current may disperse throughout the sample instead of flowing only through the wire resulting to ambiguous measurement of heat generated by the wire and polarization of the wire surface. Hence, electrical insulation of the wire is essential (Nagasaka and Nagashima, 1981).

According to the study by Yu and Choi, (2006), for most of the engineering applications, the numerical simulation and experimental test results prove that the measurement error due to the insulation coating is very small if the slope of the temperature rise over logarithmic time is computed for large time values. Moreover, it was also stated that there is no correction necessary even if the thickness of the insulation coating is comparable to the radius of the wire, and its thermal conductivity is lower than that of the sample fluid.

Most of the previous experiments suggest that the coating of the wire should be made of Teflon. It is an excellent insulation and it is highly resistant to chemical reaction, corrosion and crack due stresses and high temperature. Due to unavailability, high-heat-resistant Teflon spray paint was used as replacement for Teflon spray. In Figure 5 shows the Teflon spray paint coating the platinum wire.

Figure 5. Teflon Spray Paint Coated Platinum wire fixed in the Hot-wire Holder

The initial design for the stand is a solid stainless steel shaft with 4-inch diameter. Aside from being excessively heavy, this shaft is expensive and rare. Therefore, a 2-inch diameter solid stainless steel shaft welded to a stand with similar material was used instead. The final fabricated disassembled Thermal Conductivity hot-wire cell is shown in Figure 6 and the assembled view is show in Figure 7.

Figure 6. Disassembled fabricated Thermal Conductivity Hot-Wire Cell

Figure 7. Assembled view of the Thermal Conductivity Hot-wire Cell.

After constructing the hot-wire cell, the next phase was to build the circuit to start the data gathering. The schematic diagram for the data gathering is shown in Figure 8. The circuit was to be tested first in the actual set up to verify whether the components chosen would suffice.

Figure 8. Schematic Diagram of the Circuit used in the data gathering

At the first few days of the experiment, only voltmeters were used to measure the voltages across the resistances of the bridge, input and output. And since the output should be with respect to time, a timer was placed beside the voltmeter. The readings were recorded using a video camera. As shown in Figure 9, the first set up for the data gathering using voltmeters.

Figure 9. The initial set-up for data gathering using voltmeters

The possible error for this set-up is overwhelming, thus, an alternative has to be made. A device that can measure voltages with respect to time is needed to lessen the error in the data encoding. The only available devices with this kind of feature was a power quality analyzer and a digital power meter. The power quality analyzer can function as a data logger, but since it is designed to read large amounts of voltages, it cannot measure below 1 V. For this reason, the digital power meter is preferable for more accurate measurement. In addition, it can read instantaneous voltages and display the time; however, it cannot record values like a data logger. The photographic view the set-up is shown in Figure 10.

Figure 10. The final experiment set-up using a power meter

After recording the output values, the parameters for thermal conductivity were computed. The equation for the resistance of the hot-wire is the equation for variable resistance in a Wheatstone bridge given below. The following equations were used in the study by Kostic and Simham (2009).

where: is the resistance across the platinum wire (Ohms)

K_j is the value of the resistor in series with the DC Load (Ohms)

IR is the value set to the DC Electronic Load (Ohms)

R is the value of resistor in series with the hot-wire cell (Ohms)

1··'·_h is the input voltage (Volts)

1 „ i is the output voltage (Volts)

After computing for the resistance, the voltage across the hot-wire is computed using the equation below.

where:

R_{y l} is the resistance across the platinum wire (Ohms)

R-i is the resistor value in series with the hot-wire cell (Ohms)

t';„

input voltage (Volts)

l'._v; is the voltage across the hot-wire (Volts)

The heat flux generated at any instant was then calculated using the equation given below.

where:

q is the heat flux per length at any intant of time (W/m)

t,. is the resistance across the platinum wire (Ohms)

I·'_».; is the voltage across the hot-wire (Volts)

L_v is the length of the platinum wire (Meters)

After computing for the heat flux, the change of temperature can be now determined using the following equation.

where:

AT is the change of temperature (°C)

AR_IV is the change of resistance of the hot-wire (Ohms)

R_y,a is the initial resistance of the platinum wire (Ohms)

a is equal to 0.02656 (constant) over by i^._/U

The change of temperature will be computed for every voltage output across the bridge and will be plotted over time in logarithmic scale. The linear slope of the plot will to the calculation for the thermal conductivity using the equation below.

where:

Ay is the thermal conductivity (W/m °C)

q is the heat flux per length at any instant time (W/m)

Before applying voltage to the circuit, the bridge has to be balanced. Balancing the resistances of the bridge is done by adjusting the set value of the DC electronic load until the output voltage reads zero. When an input voltage is applied, the resistance of the platinum wire will change. This change will cause the bridge to be unbalanced, resulting to a voltage output across the bridge. Since the resistor in series with the wire will eventually heat up due to the current, the carbon resistor is replaced with a power resistor to ensure durability.

Before using the set-up for actual measurement of the thermal conductivity of a nanofluid, its reliability must first be confirmed by calibrating the set-up until the value of the thermal conductivity of a standard fluid obtained has tolerable percent error. After balancing the bridge, the output voltage was determined using the final experiment set-up. The values were then encoded and the change of temperature was then calculated. The computed values were then plotted over logarithmic time to acquire its slope, which is needed for the computation of the thermal conductivity. The obtained thermal conductivity is compared to the standard value by computing for the percent error. The resulting value must be within the tolerable range before the set-up can be considered for actual measurements for nanofluids.

The percent error must not be greater than 5%. If the computed error is not within tolerable range, the values of resistances in the arms of the bridge should be replaced. At this point, the adjustment of values can be determined by trial and error. The performance of the nanofluid as coolant in heat exchanger was determined using an assembled heat exchanger setup shown in Figure 1 1 . The temperature at the inlet and outlet of the fluid streams were read at 30° C, 50° C and 70° C. For the heat transfer calculation, the Reynolds number, Nusselt number and heat transfer coefficients were calculated for various temperatures from 30° C to 70° C.

Figure 11. Schematic diagram of the heat exchanger set-up used

8. Nanosilica Powder and Nanofluid Characterization

The rice hull ash (RHA) was first subjected to acid leaching using 0.001 N HC1 in order to effectively remove traces of inorganic metals such as magnesium, potassium, calcium and manganese. The acid-leached RHA was dried in a thermostatted oven and was then ashed in a muffled furnace at 650° C. This was done to decompose cellulose and lignin as well as to oxidize residue carbon. The purified RHA was subjected to alkali reflux using 3.0 N NaOH in order to form water soluble sodium silicate from the silicon dioxide present in the purified RHA, as shown by the reaction below:

Si0₂ + 2NaOH -> Na₂Si0₃ + H₂0

Ash sodium silicate Filtration of the sodium silicate solution was done. The pH of the filtrate was decreased to 2.5 using 5.0 N H2SO4. At pH 2.0, complete precipitation of silica from sodium silicate is observed, as shown by the reaction below:

Na₂Si0₃ + H₂S04 -> Si0₂ + Na₂S0₄ + H₂0

Sodium dioxide

The acid was then neutralized to pH 7.0 using ammonium hydroxide. The silica that precipitated out was filtered and the residue was dried in a thermostatted oven at 120° C overnight.

The silica powder synthesized was further purified by acid leaching with 6.0 N HC1 to further remove inorganic metals present in the silica powder. The mixture was filtered and the purified silica was dissolved in 2.5 N NaOH and stirred for ten hours to disperse the silica particles in the solution. The pH of the solution was then adjusted to 7.5 in order to precipitate out the nanosilica particles, which were then subjected to sonication for three hours in order to fully disperse the particles and prevent agglomeration. The mixture was filtered and the nanosilica residue was dried in a thermostatted oven at 50° C for two days. The small-scale synthesis of nanosilica was done. It was determined that the average percent yield of nanosilica from 10 grams RHA is 78.8534%, while for the large-scale synthesis, the average percent yield from 100 grams RHA is 82.3874% as shown in tables 1 and 2. Table 1. Data on weight of small-scale production of nanosilica after each step

Table 2. Data weight of large-scale production of nanosilica after each step

The synthesis of nanosilica from RHA using the method stated above yielded 78% for small-scale and 82% nanosilica powder for large-scale production. The nanosilica, upon subjecting to AFM determined to have a particle size diameter starting 46.5 nm. Comparing the size of the nanosilica powder obtained from Thuadaij and Nuntiya (2008), the method used is considered acceptable for producing nanostructured silica with high yield.

The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size samples are given in Table 3. The results show that BET surface area of nanosilica powder treated with ethylene glycol is 203.271 m² /gram. These results show that the surface area per unit volume of nanosilica particle is much larger (million times) than that of microparticles. Godson et al. (2010), observed that nanoparticles have larger relative surface areas to improve heat transfer and stability of the suspensions. Kapur (1985) reported the surface area of silica from rice hull husk is from 60 m² g-¹ to 80 m² g-¹ when the husk was incinerated at 350° C and 600° C, respectively. These results show that the surface area per unit volume of nanosilica particle is much larger (million times) than those of microparticles. Thus, the nanosilica powder that was produced has a higher surface area which will provide more heat transfer surface between particles.

Table 3. BET surface area, pore volume and pore size analysis of Nanosilica

Aside from the SEM analysis and BET method, nanosilica powder was also examined using energy dispersive x-ray spectroscopy. This is an analytical technique used for the elemental analysis or chemical characterization of the nanosilica powder. Table 4 summarizes the elements that were found in the nanosilica powder sample with 92.853% Si content and the graph of EDX analysis is shown in Figure 12. The method provided can synthesize nanosilica powder of high purity and good particle size and morphology.

Table 4. Summary of elements found in the nanosilica powder sample

Figure 12. Graph of EDX analysis of nanosilica

Results from the characterization proved that the silica synthesized from rice hull ash is in the nano-scale. The AFM image showed that the particles range between 50-100 nm while the TEM image confirmed that sizes of the nanosilica particles are less than 50 nm. The BET analysis data is summarized in Table 1. Results show that surface area of the nanosilica is relatively large (around 200 m²/g). Similar surface area results were reported by Thuadaij et al., (2008), and Yaghoubi et al. (201 1). EDX analysis data showed that the nanosilica produced was of 92% purity.

Three concentrations (1 %, 2%, 3% w/v) of the resulting deionized water- based nanofluids were prepared and subjected to transmission electron microscopy (TEM) as seen in figure 13. Also, three 75:25 ethylene glycokdeionized water ratio nanofluids in varying nanosilica volume concentrations (0.25, 0.50 and 0.75 %) were prepared and subjected to transmission electron microscopy (TEM) as seen in figure 14.

Figure 13. TEM image of 1 % (left), 2% (center), and 3% (right) w/v nanofluid concentration at 300000x magnification

Figure 14. TEM images of 0.75% (left), 0.50% (center), and 0.25% w/v nanosilica dispersed in 75:25 EG:deionized water based nanofluid concentration

It was observed from the photos that the size of the nanosilica powder dispersed in the deionized water and EG:dW mixtures is less than 50 nm and has a spherical morphology. As defined by Choi (1995), nanofluids are colloidal mixtures of nanoparticles (1 -100 nm particles of metal, oxides, carbides, nitrides or nanotubes) in host fluids. From the results, the size of the particles was maintained in the nanoscale after dispersion in the base fluid, which is a good indication of stability and non-agglomeration of nanosilica powder. 9. Stability of Deionized Water Based Nanofluids

Stability of nanofluids is usually dependent on the ability of the nanoparticles dispersed in the liquid medium to remain suspended for long periods of time. These nanoparticles must remain suspended in order to effectively disperse heat and prevent clogging in the passageways of machinery.

For the first year, the stability of the nanofluid was determined by observing sedimentation photographs and absorbance via UV-Vis spectrometer. Two concentrations (0.5%, 1.0% w/v) of nanofluid were prepared by dispersing nanosilica powder in deionized water. These solutions were placed in vials and were remain undisturbed for 10 days. The sedimentation was observed by taking pictures of each vial for 10 days. The appearance of the nanofluid in the vials in this period was observed. Small changes in the nanofluid will indicate the stability of the solution. The absorbance of the nanofluid concentrations was determined at 600 nm for a period of 10 days. The variation of supernatant particle concentration can be observed by the changes in absorbance of the solution. Table 5 and 6 summarize the absorbance of 0.5% and 1.0% solutions at the 10 day period, respectively.

Table 5. Absorbance values of 0.5% w/v nanosilica dispersed in deionized water solution

Table 6. Absorbance values of 1.0% w/v nanosilica dispersed in deionized water solution

It can be observed in Table 6 that there is an evident decrease in the absorbance of the 1.0% w/v nanofluid (NF) solution as the number of days increase as compared to the absorbance of the 0.5% w/v NF solution, as shown in Table 5. It is also observed that the absorbance of the 0.5% w/v NF solution has no apparent pattern in decrease or increase compared to the absorbance of the 1.0% w/v NF solution. With reference to the published work of Wen and Ding (2004), A1203 nanoparticle dispersed in water for 0.2-1.2% volume fractions with dispersant was stable for 1 wk after subjecting to 20 h of sonication. Sonication and mechanical stirring are done to break down the aggregates of nanoparticles (Wu et al. 2009). This shows that the suspended nanosilica particles in the 0.5% w/v NF solution tend to stay suspended longer as compared to the 1.0 w/v NF solution, hence, it can be said that the 0.5% w/v NF solution is more stable than the 1.0 w/v NF solution.

For AI₂O3 nanoparticle dispersed in water without dispersant in 2-10% volume fraction, the stability lasted for several days only (Li and Peterson, 2006). Lee et a/. (1999) reported CuO nanoparticles dispersed in water in 1 -5% volume fractions produced large agglomerations which is an indication of instability.

10. Stability of Ethylene Glycol: Water Based Nanofluids For the second year, the absorbance of the nanofluid concentrations was determined at 600 nm for a period of 10 days. The variation of supernatant particle concentration can be observed by the changes in absorbance of the solution. Tables 7, 8 and 9 summarize the absorbance of 0.25%, 0.50% and 0.75% solutions of varying ethylene glycol: deionized water ratios at the end of the 10 day period. Also, graphs of absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 25:75, 50:50 and 75:25 ethylene glycoLdeionized water solution are shown in Figures 15 to 17, respectively.

Table 7. Absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 25:75 ratio of ethylene glycol: deionised water solution

Table 8. Absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 50:50 ratio of ethylene glycol: deionized water solution

Table 9. Absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 75:25 ratio of ethylene glycol: deionized water solution

Figure 15. Graph of absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 25:75 ethylene glycokdeionized water solution

Figure 16. Graph of absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 50:50 ethylene glycohdeionized water solution

Figure 17. Graph of absorbance values of 0.25%, 0.50% and 0.75% w/v nanosilica dispersed in 75:25 ethylene glycokdeionized water solution

It can be observed in Tables 7, 8, and 9 that there is an evident decrease in the absorbance of all the nanofluid (NF) solution as the number of days increase. However, at day 6, there is less settling observed in the 0.25 and 0.5% w/v concentrations as compared to the 0.75% w/v concentration thus a higher nanoparticle concentration takes longer to stabilize compared to nanoparticles of lower concentrations.

11. Thermophysical Properties of the Deionized Water-based Nanofluids The density of four different concentrations (0.5, 1.0, 2.0, 3.0% w/v) of nanosilica dispersed in deionized water was determined. Table 10 summarizes the densities obtained at varying temperatures.

Table 10. Densities of nanosilica dispersed in nanofluid at different concentrations and temperatures

It can be observed in Table 10 that as the nanosilica concentration increases, the density of the solution increases, and as the temperature increases, the density of the solution decreases. Similar data were reported by Mahbubul et al., (201 1) and Rudyak (2013).

The heat flux of the nanosilica, 0.5% and 1.0% NF were measured using TA Instruments Q20 differential scanning calorimeter and the specific heat of the samples were determined.

Table 11. Specific heat of nanosilica (nS) and nanosilica-dispersed in deionised water

Specific heat is defined as the amount of heat required in changing a unit mass of a substance by one degree in temperature; therefore, for a coolant to be effective, it must have a high specific heat value. The values obtained exhibit a decreasing trend as well as negative values as nanoparticle concentration increases as shown in Table 11. According to O'Hanley et al. (2011 ), increase in nanoparticle concentration leads to decrease in specific heat capacity. Studies also done by Namburu et al. (2007), showed that specific heat of nanosilica nanofluid decreases with increasing particle volume concentration meaning, less heat input is required to increase the temperature of the nanofluid. Lu et al. (2013) also obtained similar results.

These may be due to Ostwald ripening which causes growth of nanoparticle size. Ostwald ripening is defined as a thermodynamically driven spontaneous process, which causes evolution of inhomogeneous structures over time. Larger particles in a suspension are more energetically stable than smaller particles due to the lower surface-to-volume ratio of large particles. Due to energy conservation, molecules on the surface of smaller particles have a tendency to detach and reattach itself to the stable surface of the larger particle thereby increasing its size while smaller particles continue to shrink. Therefore, larger nanoparticles would grow in size over time, decreasing its surface area and reducing its ability to enhance thermodynamic properties.

Before the measurement of the thermal conductivities of the nanofluid at different temperatures and volume concentrations, the transient hot wire apparatus was calibrated first using the base fluid. Given in Table 12, the measured thermal conductivities for deionized water obtained a percent error range from 0.15% to 0.92@ only compared to the theoretical values. This result shows that the transient hot wire apparatus developed will give reliable measurements for the thermal conductivities of the nanofluids. Table 12. Calibration Data of the Transient Hot Wire Apparatus using deionised water

The thermal conductivity of the nanofluid in three volume concentrations was measured at temperatures 30° C to 70° C with 10° C interval and summarized in Table 13. To exclude the effect of natural convection, data were collected from 5-10 seconds. The effect of nanosilica loading in the deionized water is evidently shown in Figure 18. The thermal conductivity of nanofluid increases as concentration of nanosilica powder increases. The measured thermal conductivities also show a nonlinearly increasing value in increasing concentration of nanosilica powder.

Table 13. Summary of measured thermal conductivities (W/m°C) of nanosilica-in-fluid dispersion (nanofluids)

Figure 18. Thermal conductivities of nanofluids at different volume concentrations measured at different temperatures from 30 to 70° C.

Given that nanosilica powder has a higher value for thermal conductivity than the base fluid, increasing the concentration of nanosilica powder dispersed into deionized water will also increase the thermal conductivity of the base fluid as shown in Figure 18. And since the surface to volume ratio is higher for nanosize diameters of particles, a more uniform distribution of particles gives a better thermal conductivity enhancement.

Another way to review the enhancement of thermal conductivity is to plot the thermal conductivity ratio (kni/kb) of each volume concentration against temperature. The enhancement is shown in Figure 19 reached up to approximately 50%. At 0.5% volume concentration the calculated thermal conductivity ratio means a 0 % to 9 % increased in thermal conductivity. While for a 3% volume concentration the enhancement ranges from 7 to 45 %; the minimum value taken at 30° C while the highest increased in thermal conductivity was obtained at 70° C.

Figure 19. The thermal conductivity ratio of different volume concentrations measured at 30° C to 70° C temperatures

The viscosity of the nanofluid was measured using the Ostwald viscometer. The dependence of dynamic viscosity on temperature is fairly significant for volume concentrations of nanofluid. The viscosities were measured at five temperatures from 30° to 70° C with 10° C interval. It is believed that viscosity is as critical as thermal conductivity in engineering systems because the nanofluid was expected to show an increase in thermal conductivity without an increase in pressure drop, which in turn is related to fluid viscosity.

Table 14. Summary of measured viscosity (g/m.s) of nanosilica-in-fluid dispersion (nanofluids)

The average viscosities of nanofluid at different volume concentrations at different temperatures shows a decreasing pattern with increasing temperature which is consistent with published work of Li et al. (2002) and Wang et al. (1999). The viscosities measured for nanofluids were higher in values compared to the viscosity of water at different temperatures with a percent increase ranges from 27.82% to 159.90%.

Table 15. Percent difference of measured nanofluid viscosity compared to the viscosity of water

The desirable heat transfer increase is offset by the undesirable increase in pressure drop, which needs more studies, and researches especially in the stability of nanofluids which is believed to be the reason for having high viscosities.

From table 16, the computed Prandtl numbers of nanosilica-in-fluid dispersion (nanofluids) at different temperatures and volume concentrations are shown. The Prandtl number of water as the control is also tabulated for reference. As shown in the results in Figure 20, Prandtl number of nanofluids at different volume concentrations decreases with increase in temperature and volume concentrations. Also, higher values of nanofluid Prandtl numbers were calculated compared to water Prandtl number.

Table 16. Computed Prandtl number of nanosilica-in-fluid dispersion (nanofluids) and Prandtl number of water used

Figure 20. Computed Prandtl number of nanosilica-in-fluid dispersion (nanofluids) at different volume concentrations

Table 16, the computed thermal diffusivity of nanofluids from the measured density, thermal conductivity and specific heats is shown and from the results, the calculated values are higher than the thermal diffusivity of water. From Figure 21 , thermal diffusivities of nanosilica-in-fluid dispersion show an increasing trend as temperature increases from 30° C to 70° C at different volume concentrations. Also shown in Figure 19, as the volume concentration of nanosilica dispersed in deionized water increases, the thermal diffusivity also increases at different temperatures.

Figure 21. Calculated thermal diffusivity of nanosilica-in-fluid dispersion (nanofluids) at different temperatures The results show that nanofluid produced has higher values for thermal diffusivity than the base fluid, meaning the nanofluid diffuses heat quickly. This enhancement is caused by the anomalous increase of thermal conductivity of the nanofluid. From Table 17 and Figures 22, the computed kinematic viscosities of nanofluid show an increasing pattern with volume concentration. But the change in kinematic viscosities with volume concentration is not significant; this is expected since the volume concentrations used are considered small. As temperature changes from 30° C to 70° C, nanofluid kinematic viscosity decreases same as with the control (deionized water).

The increased in kinematic viscosity of nanosilica-in-fluid dispersion was due to the nanosilica powder loading. Compared to the base fluid, the nanosilica- in-fluid dispersion kinematic viscosity has higher values.

Table 17. Computed kinematics viscosity (m²/s) of nanofluids at different temperatures

Figure 22. Computed kinematic viscosity of nanosilica-in-fluid dispersion (nanofluids) at different temepratures

11. Thermophysical Properties fo Ethylene Glycol: Deionized Water based Nanofluids

The density of three different concentrations (0.25, 0.5, 0.75% w/v) of nanosilica dispersed in various ethylene glycol: deionized water ratio was determined. Tables 18 to 20 summarizes the densities obtained at varying temperatures and shows that the density of nanofluids taken at temperatures ranging 30-70°C increases when the concentration of nanosilica increases. This is expected since adding solid particles to the base fluid will definitely result to a higher density (Lee et al. 1999).

Table 18. Density of a 25:75 ethylene glycol: deionized water ratio nanofluid with various concentrations at different temperatures

Table 19. Density of a 50:50 ethylene glycol: deionized water ratio nanofluid with various concentrations at different temperatures

Table 20. Density of a 25:75 ethylene glycol: deionized water ratio nanofluid with various concentrations at different temperatures

The thermal conductivity of the ethylene glycol: deionized water based nanofluids in three volume concentrations was measured at temperatures 30° C to 70°C with 20°C interval and summarized in Tables 21 -23. The thermal conductivity of nanofluid increases as concentration of nanosilica powder increases. The measured thermal conductivities also show a nonlinearly increasing value in increasing concentration of nanosilica powder. According to Wu et al. (2009), the thermal conductivity of nanofluid increases as concentration of nanosilica powder increases. In most researches, the relationship of thermal conductivity and volume concentration is linear. It is confirmed by many experimental data, such as the research on ALCb/water nanofluids containing AI2O3 particles by Lee et al. (1999), wherein thermal conductivity enhancement was observed to be 20% at 5% volume concentration and 10% at 4% volume concentration.

Table 21. Thermal conductivities of 25:75 ethylene glycobdeionized water ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70°C

Table 22. Thermal conductivities of 50:50 ethylene glycol: deionized water ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70° C

Table 23. Thermal conductivities of 75:25 ethylene glycol: deionized water ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70°C

10

The increased in thermal conductivities for the different EG:dW ratio nanofluids are shown in Figures 23 to 25. The effect of nanosilica loading in the thermal conductivity of ethylene glycohdeionized water nanofluids is evidently to have a maximum increased of almost 55% in the 75:25 EG:dW ratio with 0.75% volume concentration of nanosilica powder.

Figure 23. Percent increased in thermal conductivity of 25:75 EG:dW ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70°C

Figure 24. Percent increased in thermal conductivity of 50:50 EG:dW ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70°C

Figure 25. Percent increased in thermal conductivity of 75:25 EG:dW ratio nanofluid at different volume concentrations measured at different temperatures from 30 to 70 ° C The viscosities of three different concentrations (0.25, 0.50, 0.75% w/v) of nanosilica dispersed in a mixture of 25:75, 50:50 and 75:25 ethylene glycol: deionized water ratios was determined. The results are summarized in Table 24, 25 and 26.

Table 24. Viscosity of a 25:75 ethylene glycol: deionized water ratio of nanofluid with various concentrations at different temperatures

Table 25. Viscosity of a 50:50 ethylene glycol: deionized water ratio of nanofluid with various concentrations at different temperatures

Table 26. Viscosity of a 75:25 ethylene glycol: deionized water ratio of nanofluid with various concentrations at different temperatures

The results show that increasing the temperature decreases the viscosity of each solution while an increase in the concentration increases the viscosity of each solution. It can also be observed that the 25:75 EG:dW ratio has the fastest flow rate while the 75:25 has the slowest. Thus the concentration of ethylene glycol greatly affects the flow rate of the solution. The heat flux of the nanosilica, 0.25%, 0.50% and 0.75% nanofluid concentrations were measured using TA Instruments Q20 differential scanning calorimeter and the specific heat of the samples were determined using formula 1 -1. The calculated specific heats of the nanofluid suspensions are summarized in Tables 27, 28 and 29. Table 27. Specific heat of nanosilica-dispersed in 25:75 EG:dW mixture Table 28. Specific heat of nanosilica-dispersed in 50:50 EG:dW mixture

Table 29. Specific heat of nanosilica-dispersed in 75:25 EG:dW mixture

Specific heat is defined as the amount of heat required changing a unit mass a substance by one degree in temperature therefore for a coolant to be effective, it must have a high specific heat value. The data obtained shows that increase in concentration of nanoparticles decreases the specific heat of lo the nanofluid.

From Table 30, the computed Prandtl numbers of ethylene glycol deionized water based nanofluids at different temperatures and volume concentrations are shown. The Prandtl number of base fluid as the control is also tabulated for reference. The computed Prandtl numbers of nanofluids at different volume concentrations decreases with increase in temperature and while increases when the volume concentration increases. Also, lower values of nanofluid Prandtl numbers were calculated compared to base fluid Prandtl number.

Table 30. Computed Prandtl number of EG:dW based nanofluids

The results show that ethylene glycol: deionized water basefluid nanofluids produced has higher values for thermal diffusivity than the base fluid, meaning the nanofluid diffuses heat quickly as shown in Table 31 . This enhancement is caused by the anomalous increase of thermal conductivity of the nanofluid.

Table 31. Computed Thermal diffusivity of ethylene glycokdeionized water based nanofluids

From Table 32, the computed kinematic viscosities of the ethylene glycol: deionized based nanofluids show an increasing pattern with volume concentration. But the change in kinematic viscosities with volume concentration is not significant; this is expected since the volume concentrations used are considered small. As temperature changes from 30°C to 70°C, nanofluid kinematic viscosity decreases same as with the control (basefluid). Table 32. Computed kinematic diffusivity of ethylene glycol: deionized water based nanofluids

12. Performance Analysis of Nanofluid as Coolant in Heat Exchanger

The heat exchanger set-up will be used to determine the effect of nanofluid as coolant. One tank contains water to be heated while the other tank contains the nanofluid that will be used to cool the heated water. The measured increase in heat transfer of water-based nanofluids at different volume concentrations is summarized in Tables 33 and 34.

Table 33. Measured percent increased in heat transfer of water-based nanofluids in various volumes at different temperatures

As shown in Table 33, the deionized water-based nanofluids exhibited an increased in heat transfer compared to the base fluid. For 0.5% volume concentration the increased ranges from 0.57% to 6.02%. The highest increased of 29.26% in heat transfer was produced using the 3% volume concentration at 70°C. Considering the thermophysical properties of the three ratios of EG:dW nanofluids, the 75:25 ratio has the most promising properties given that the increased in thermal conductivity reached up to 55% and has the lowest viscosity among the nanofluids produced, thus it is the most ideal type of nanosilica-in-fluid dispersion (nanofluid). The 75:25 EG:dW ratio nanofluid was used in the heat exchanger set-up to determine its performance by computing the percent increased in heat transfer. Table 36 shows that the percent increased in heat transfer for the three different volume concentration of nanosilica powder in 75:25 EG:dW ratio. From the results, the increased reached up to 50% as compared to a base fluid (75:25 EG:dW).

Table 34. Percent Increased in Heat Transfer of 75:25 EG:dW ratio nanofluid in various volume concentration at different temperatures

Claims

CLAIMS:

1. A composition for a nanofluid comprising of nanostructured silica dissolved in a fluid.

2. The composition according to claim 1 wherein the nanostructured silica is derived from rice hull ash.

3. The composition according to claim 2 wherein the nanostructured silica recovered is equal to or more than 78%.

4. The composition according to claim 3 wherein the nanostructured silica recovered is spherical in shape, and has a dimension starting at 46.5 nm.

5. The composition according to claim 4 wherein the BET surface area of the nanostructured silica is around 203 m²/g, with an adsorption average pore radius of 62A.

6. The composition according to claim 1 wherein the fluid used is either deionized water, or a mixture of ethylene glycol: deionized water,

7. The composition according to claim 6 wherein the mixture of ethylene glycol: deionized water are 25:75, 50:50, and 75:25

8. The composition according to claim 1 wherein the volume of nanostructured silica in the nanofluid is between 0.25-3%.

9. The composition according to claim 6 wherein the nanofluid is stable for up to 10 days.

10. The composition according to claim 6 or 7 wherein the most stable nanofluid using deionized water is 0.5% volume concentration of nanosilica-in-fluid dispersion.

11. The composition according to claim 6 or 7 wherein the most stable nanofluid using ethylene glycol: deionized water is 0.25% volume concentration of nanosilica dispersed in 50:50 EG:dW.

12. The composition according to claim 1 wherein the nanofluid retains the particle size of the nanostructured silica to be less than 50nm.

13. A composition of nanofluid for use as heat transfer enhancement,

14. The composition according to claim 13 wherein thermal conductivity of the nanofluid changes as the temperature increases, and is higher than that of water.

15, The composition according to claims 13 or 14 wherein thermal conductivity of the nanofluid is within the range of 0.618-0.959 W/m°C at 30-70°C.

16. The composition according to claim 13 wherein viscosity of the nanofluid changes as temperature increases.

17, The composition according to claims 13 or 16 wherein the viscosity of the nanofluid is between the range of 0.99-1.1 g/ms of nanofluids at 30-70°C.

18. The composition according to claim 13 wherein the nanofluid has a higher thermal conductivity and thermal diffusivity, and a lower specific heat than water.

19. A method of producing nanosilica from rice hull ash (RHA) comprising the steps of:

a. subjecting the RHA to reflux in a 0.1 N HC1 solution for two hours; b. filtering the mixture and washing the residue with deionized water; c. discarding the filtrate;

d. drying the residue in an oven at 100° C for at least eight hours;

e. igniting the dried RHA in a muffled furnace at 650° C for 6 hours; f. refluxing the RHA in a solution of 3.0 N NaOH for three hours;

g, filtering the mixture and discarding the residue;

h. adjusting the pH of the filtrate to pH 2.5 using 5.0 N H₂SO_4;

i. neutralizing the filtrate using concentrated ammonium hydrochloride; j. filtering the mixture and washing the silica residue with hot deionized water;

k. drying the residue for at least eight hours in an oven at 100°C;

L. grinding the resulting white solid into fine powder;

m. purifying the powder via reflux in a 6.0 N HCI solution for four hours; n. filtering the resulting mixture and washing the residue with hot

deionized water until neutral; o. drying the residue in an oven at 100° C for at least eight hours;

p. dissolving the purified silica in a 2.5 N NaOH solution;

q. stirring the mixture was stirred for twelve hours to dissolve the silica powder into the solution;

r. adjusting the pH of the solution to 2.5 using 5.0 N H₂SO₄;

s. neutralizing the solution using concentrated ammonium hydroxide; t. subjecting the mixture to ultrasonication for six hours;

u. filtering the mixture and collecting the nanosilica residue; and v. drying the nanosilica residue in a thermostatted oven at 120° C for two days.

20. A method of preparing nanofluid from nanosilica produced in claim 19 comprising the steps of:

a. placing the nanosilica powder in a 250 mL Erlenmeyer flask;

b. adding deionized water;

c. stirring the mixture was stirred continuously for 30 minutes using a magnetic stirrer;

d. placing the mixture in an ultrasonic vibrator and sonicated continuously for five hours;

e. preparing three mass-per-volume concentrations (0.5, 0.75, 1.0 % m/v) of nanofluids by dispersing the nanosilica into 50:50 ethylene glycol: deionized water (EG:dW) solutions;

f. subjecting the nanofluid suspensions to stirring and sonication

continuously for six hours continuously to ensure uniform particle distribution;

g. preparing three mass-per-volume concentrations (0.25, 0.50, 0.75% m/v) of nanofluids by dispersing the nanosilica powder into 25:75, 75:25 and 50:50 ethylene glycol: deionized water solutions; and h. subjecting the suspension to stirring for twelve hours followed by

sonication for six hours to ensure uniform particle distribution.