CN112748145A - Double-flow-method specific heat capacity measuring device and method - Google Patents
Double-flow-method specific heat capacity measuring device and method Download PDFInfo
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Abstract
A double flow method specific heat capacity measuring device and method, including calorimeter part and heating control part; the calorimeter component comprises a vacuum cavity, the top of the vacuum cavity is provided with a lower flange and an upper flange end cover matched with the lower flange, and the upper flange end cover is provided with a vacuum pumping pipeline; a second U-shaped pipe and a first U-shaped pipe which are identical in structure are arranged inside the vacuum cavity, the upper ends of the second U-shaped pipe and the first U-shaped pipe are communicated with the lower ends of the two-way pipe, heating wires are wound on the outer sides of the second U-shaped pipe and the first U-shaped pipe, and through holes of a first temperature sensor and a second temperature sensor are respectively arranged at the two ends of the heating wires; the heating control part is positioned outside the calorimeter and is connected with the heating wire. The device can accurately measure the specific heat capacity of the liquid, has good stability, and wider measurable temperature and pressure range, can be used for measuring the specific heat of dilute electrolyte solution and organic matters in industry and scientific research, and can also be used for teaching of professional course experiments related to energy power.
Description
Technical Field
The invention belongs to the technical field of fluid thermophysical property measurement, and particularly relates to a specific heat capacity measuring device and method, which can be used for teaching of professional course experiments related to energy power and thermodynamics and can also be used for measuring the specific heat capacity of a fluid working medium in industry and scientific research.
Background
The specific heat capacity is one of the most important thermophysical properties of the fluid and represents the capability of an object to absorb or dissipate heat, the specific heat capacities of different substances are different, and the specific heat capacity of the same substance can change along with the change of temperature. The specific heat capacity is one of the parameters which are obtained primarily in the field of energy and chemical engineering, is widely applied in actual production and life, has important application value in the fields of energy, materials, aerospace, medicine, engineering thermodynamics and the like, and has important significance for thermodynamic cycle, chemical process calculation and the like. Important information such as material structure, phase change mechanism and the like can be obtained through specific heat measurement. Therefore, the accurate measurement of the specific heat capacity has very important significance. The current methods for measuring the specific heat of the fluid mainly comprise adiabatic calorimetry, mixing method, differential scanning calorimetry, flow type adiabatic calorimetry and the like.
The adiabatic calorimetry is characterized in that the heat loss is reduced as much as possible by arranging an adiabatic screen and a radiation screen, the heated heat is ensured to be fully absorbed by a sample, the adiabatic calorimeter is complex in structure, the time consumption for reaching the heat balance process in the experiment is too long, and the test precision depends on the adiabatic condition. The mixing method is a method for mixing an object with known heat capacity and temperature with a sample to be measured, and the specific heat capacity of the sample to be measured is obtained through calculation. At present, two types of copper-calorimeters and ice-calorimeters are used more, and the falling method mainly aims at measuring the specific heat capacity of the solid. The differential scanning calorimetry is an indirect measurement method, the specific heat capacity difference is reflected by measuring the relation between the power difference and the temperature of the measured substance and the reference substance input under the program control temperature, the specific heat of the substance measured by the differential scanning calorimetry is greatly influenced by the professional skill of an operator, the instrument condition and the experimental environment, and the precision and the accuracy of the method depend on the calibration of a standard substance. The flow type adiabatic calorimetry mainly aims at measuring the specific heat capacity of fluid, the main measuring method is that the fluid flowing through a calorimeter stably absorbs certain heat when flowing through a heater, certain temperature rise can be generated before and after heating, and the specific heat capacity of a substance to be measured can be obtained according to the specific heat measuring principle. The flow type adiabatic calorimetry method mainly aims at fluid with higher pressure, is relatively simple in measurement method, relatively wide in temperature measurement and pressure measurement range and relatively high in measurement accuracy, and is the most common measurement method at present. For dilute electrolyte solution, the difference between the specific heat capacity of the solution and the specific heat capacity of the solvent is extremely small, and an effective result cannot be obtained by direct measurement; in addition, the problem also exists in some organic solution mixtures, and the direct specific heat capacity measurement of the organic substance simple substance and the organic substance mixed solution cannot distinguish the difference.
Disclosure of Invention
The invention aims to provide a device and a method for measuring specific heat capacity of a substance by a double-flow method, which can accurately and efficiently measure the specific heat capacity of the substance to be measured and have higher measurement precision.
In order to achieve the above purpose, the invention adopts the following scheme to realize:
a device for measuring specific heat capacity of a substance by a double-flow method comprises a calorimeter component and a heating control component; the calorimeter component comprises a vacuum cavity, a lower flange and an upper flange end cover matched with the lower flange are arranged at the top of the vacuum cavity, and a vacuumizing pipeline is arranged on the upper flange end cover; a second U-shaped pipe and a first U-shaped pipe which are identical in structure are arranged inside the vacuum cavity, the upper ends of the second U-shaped pipe and the first U-shaped pipe are communicated with the lower ends of the two-way pipe, heating wires are wound on the outer sides of the second U-shaped pipe and the first U-shaped pipe, and through holes of a first temperature sensor and a second temperature sensor are respectively arranged at the two ends of the heating wires;
an inlet pipeline, a buffer pipeline and an outlet pipeline are arranged outside the vacuum cavity, the outlet of the inlet pipeline is connected with the inlet of the first U-shaped pipe, the outlet of the first U-shaped pipe is connected with the inlet of the buffer pipeline, the outlet of the buffer pipeline is connected with the inlet of the second U-shaped pipe, and the outlet of the second U-shaped pipe is connected with the outlet pipeline;
the heating control part is positioned outside the calorimeter and is connected with the heating wire.
The invention has the further improvement that two ends of the heating wire are respectively provided with a red copper block; the red copper block is sleeved on the second U-shaped pipe and the first U-shaped pipe, and a through hole for placing the first temperature sensor and the second temperature sensor is formed in the red copper block;
the heating control component comprises a Wheatstone bridge, a PID controller and a program-controlled power supply, the Wheatstone bridge is connected with a phase-locked amplifier, the phase-locked amplifier is connected with the PID controller, the PID controller is connected with the program-controlled power supply, and the program-controlled power supply is connected with the heating wires;
the vacuum cavity, the inlet pipeline, the buffer pipeline and the outlet pipeline are all immersed in the constant-temperature water bath; the inlet of the inlet pipeline is connected with a high-pressure pump; and polishing the inner wall of the vacuum cavity.
The invention has the further improvement that heat-conducting silicone grease is filled between the red copper block and the gaps of the first temperature sensor and the second temperature sensor; the first temperature sensor and the second temperature sensor both adopt sheet type Pt-100 platinum resistance thermometer chips, two wiring terminals of the chips are respectively connected with silver-plated copper wire signal shielding wires, and the silver-plated signal shielding wires are led out through aviation plugs arranged on the flange end covers; and aluminum foil paper is wound on the heating wire, the first temperature sensor and the second temperature sensor.
The invention has the further improvement that the vacuum cavity is made of stainless steel, the vacuum cavity is arranged on a base in the constant-temperature water bath, and the outer side of the constant-temperature water bath is provided with heat-insulating cotton;
the upper flange end cover is made of stainless steel, the inlet of the vacuumizing pipeline is connected with a vacuum pump, the aviation plug is used for leading out a lead from the vacuum cavity, and the lower surface of the upper flange end cover is provided with a groove;
the lower flange is made of stainless steel, and a groove for placing an O-shaped ring is formed in the upper surface of the lower flange; the lower flange is welded with the upper end of the vacuum cavity;
the heating wire is an enameled nickel-chromium wire with the diameter of 0.15 mm; two wiring ends of the heating wire are respectively connected with copper wires with the diameter not more than 0.8mm, and the copper wires are led out through aviation plugs.
Firstly, vacuumizing a vacuum cavity to below 10Pa, and enabling a standard substance to sequentially flow through a first U-shaped pipe, a buffer pipeline, a second U-shaped pipe and an outlet pipeline through an inlet pipeline; applying heating voltage to the heating wire to make the second temperature sensor higher than the first temperature sensor by 2K, recording the output signal S of the phase-locked amplifier and the addition on the first U-shaped tubeHeating power P of hot wirer；
Keeping the flow rate constant, and replacing the standard substance with a sample to be detected to continue sample introduction; when a sample to be detected flows to the first U-shaped pipe, keeping the heating voltage of the heating wire on the second U-shaped pipe unchanged, adjusting the heating voltage of the heating wire on the first U-shaped pipe, enabling the output signal of the phase-locked amplifier to be recovered to S, and recording the current heating power P of the heating wire on the first U-shaped pipes；
According to heating power PrCurrent heating power PsCoefficient of heat leakage, density value rho of standard substance at experimental temperature and pressurerAnd density value rho of sample to be tested at experimental temperature and pressuresAnd specific heat capacity c of standard substancerObtaining the specific heat capacity c of the sample to be measureds。
The invention is further improved in that the specific heat capacity c of the sample to be measuredsObtained by the following formula:
wherein F is the volume flow rate and F is the heat leakage coefficient value.
The invention is further improved in that the heat leakage coefficient is obtained by the following process: measuring the resistance values of the first temperature sensor and the second temperature sensor on the first U-shaped pipe, and recording the constant flow rate F of the heating wire on the first U-shaped piperHeating power PrAnd measuring the resistance difference value delta R of the second temperature sensor and the first temperature sensor; changing the flow rate to the current flow rate FsMaking the resistance difference between the second temperature sensor and the first temperature sensor be a resistance difference value delta R, and recording the current heating power Ps(ii) a And changing the flow speed for multiple times, calculating to obtain the corresponding heat leakage coefficient value under each flow speed, and finally taking the average value of the heat leakage coefficients calibrated for multiple times in the adjacent area of the measured power as the final value heat leakage coefficient value.
In a further development of the invention, the value of the heat leakage coefficient is calculated by the following formula:
in the formula, FrFor a constant flow rate, FsIs the current flow rate.
The invention is further improved in that the heating power PrWith the current heating power being PsRepresented by the formula:
Pr＝F·ρr·cr·ΔT+Lr (1)
Ps＝F·ρs·cs·ΔT+Ls (2)
in the formula, PrFor heating power, PsF is the volume flow rate for the current heating power; rhorAnd ρsRespectively representing the density values of the standard substance and the sample to be tested at the experimental temperature and pressure; c. CrAnd csRespectively is the specific heat capacity of the standard substance and the sample to be measured; delta T is the temperature difference between the second temperature sensor and the first temperature sensor under the action of the heating wire on the first U-shaped pipe, LrAnd LsRespectively the heat dissipation loss of the standard substance and the sample to be measured to the external environment.
The invention is further improved in that the output signal of the phase-locked amplifier is recovered to be S when the sample to be detected flows into the first U-shaped tube, and the second U-shaped tube is filled with standard substances.
Compared with the prior art, the invention has the following beneficial effects:
according to the double-flow-method specific heat capacity measuring device, the second U-shaped pipe is used as a reference purpose in a mode of arranging two identical U-shaped pipes, and fixed resistance values are only provided for two bridge arms of a Wheatstone bridge. The first U-shaped pipe is used for measurement, and when a standard substance and a sample to be measured flow through the first U-shaped pipe in sequence, different heating voltages are given so that the temperature rises in the two times are consistent; because the temperature of the first temperature sensor at the front end of the heating wire is constant at the constant temperature of the constant temperature bath, the temperature of the second temperature sensor at the rear end of the heating wire can be changed due to the change of a flowing medium in the first U-shaped pipe, the temperature change of the second temperature sensor is reflected by monitoring a voltage difference signal of the Wheatstone bridge, and the temperature rise of the first temperature sensor and the temperature rise of the second temperature sensor are consistent with each other twice through the input of the programmable power supply, so that the voltage difference signals of the Wheatstone bridge are consistent. In the process of calculating the specific heat capacity, two physical quantities, namely temperature difference and flow rate, do not appear any more, the influence caused by fluctuation of the temperature and the flow rate is eliminated, and the calculation precision can be improved to a great extent. The method has short time consumption and greatly simplifies the experimental process.
Furthermore, in order to reduce heat dissipation loss when the fluid is heated as much as possible, the first U-shaped pipe and the second U-shaped pipe are arranged in the vacuum cavity, polishing is avoided in the vacuum cavity, and heat conduction, convection heat exchange and radiation heat exchange losses are reduced.
Furthermore, in order to reduce the heat loss of the temperature sensor lead and the heating wire lead caused by heat conduction, the leads are fixed outside the U-shaped pipe and then led out.
Further, to reduce heat loss from the exit section of the vacuum chamber due to thermal conduction, increasing the length reduces the temperature gradient along the conduit.
Further, in order to reduce the radiation heat exchange in the vacuum cavity, the aluminum foil is wound on the heating wire and the first temperature sensor and the second temperature sensor.
Furthermore, the outer layer of the constant temperature bath is wrapped with thick heat preservation cotton and firmly fixed by an aluminum foil adhesive tape.
Furthermore, the temperature sensor is a high-precision sheet type PT100 platinum resistor, and the deviation of the heat capacity of the temperature sensor to the measurement result can be ignored; the response time of the temperature sensor is about 0.3s, and the sampling time of the PID controller is 3s, so that the influence of temperature hysteresis on the control action is avoided. In addition, the lead-out wire of the temperature sensor is a silver-plated shielding signal wire, so that external signal interference can be shielded, and signals acquired by the phase-locked amplifier are more stable; the diameter of the silver-plated signal shielding wire is not more than 0.2mm, and the influence of the heat capacity of the lead on the measurement result is further reduced.
Furthermore, the vacuum cavity of the calorimeter body is sealed by adopting the concave-convex groove flange, so that the vacuum tightness is good, the flange structure is convenient to disassemble, and the inspection or part replacement in the vacuum cavity is convenient.
Furthermore, after the aviation plug at the upper flange end cover leads out the conducting wire from the vacuum cavity, the conducting wire can be prevented from being soaked in water bath for a long time under the action of the protective sleeve. The protective sleeve and the upper flange end cover are in threaded connection, so that the disassembly is convenient, and the loosened wire is conveniently reinforced.
In the measurement, a standard substance is taken as a reference, and a Wheatstone bridge signal of the standard substance after being heated and stabilized is firstly collected; after the front end of the inlet pipeline is switched to a sample to be detected, the voltage of the two ends of the heating wire is controlled, so that the voltage difference output signal of the Wheatstone bridge is consistent with that before; and after the specific heat capacity measuring process is finished, calibrating the heat leakage coefficient by using a standard substance based on a variable flow rate method, and finally obtaining the specific heat capacity of the sample to be measured through data processing. The device of the invention can complete the measurement and control process only in 10-15 minutes, thereby greatly improving the efficiency. The device can accurately measure the specific heat capacity of the liquid, has good stability, and wider measurable temperature and pressure range, can be used for measuring the specific heat of dilute electrolyte solution and organic matters in industry and scientific research, and can also be used for teaching of professional course experiments related to energy power.
Drawings
FIG. 1 is a view showing the structure of the apparatus of the present invention.
Fig. 2 is a plan view of the vacuum chamber.
FIG. 3 is a schematic diagram of a bit Wheatstone bridge.
Fig. 4 is a flowchart of the operation of the heating control unit of the present invention.
In the figure, 1 is a lead protective sleeve, 2 is a vacuumizing pipeline, 3 is an outlet pipeline, 4 is a constant temperature water bath, 5 is an upper flange end cover, 6 is a lower flange, 7 is a second U-shaped pipe, 8 is a red copper block, 9 is a vacuum cavity, 10 is a buffer pipeline, 11 is a heating wire, 12 is a second temperature sensor, 13 is a first temperature sensor, 14 is a first U-shaped pipe, 15 is a two-way pipe, 16 is an aviation plug, and 17 is an inlet pipeline.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
Referring to fig. 1, 2 and 3, the present invention includes a calorimeter block and a heating control block; the calorimeter component comprises a flange-sealed vacuum cavity 9, a lower flange 6 and an upper flange end cover 5 matched with the lower flange 6 are arranged at the top of the vacuum cavity 9, four two-way pipes 15, a vacuum-pumping pipeline 2 and an aviation plug 16 are arranged on the upper flange end cover 5, and a lead protective sleeve 1 is arranged at the upper part of the aviation plug 16; two identical U-shaped pipes, namely a second U-shaped pipe 7 and a first U-shaped pipe 14, are arranged inside the vacuum chamber 9, the upper ends of the second U-shaped pipe 7 and the first U-shaped pipe 14 are fixed at the lower side interface of the two channels 15 of the upper flange end cover 5 of the vacuum chamber 9, heating wires 11 are wound on the outer sides of the second U-shaped pipe 7 and the first U-shaped pipe 14, and two ends of each heating wire 11 are respectively provided with a red copper block 8; two through holes are formed in the red copper block 8, one through hole can enable the second U-shaped pipe 7 and the first U-shaped pipe 14 to penetrate through, and silver soldering is adopted among the red copper block 8, the second U-shaped pipe 7 and the first U-shaped pipe 14; the other through hole is used for inserting the first temperature sensor 13 and the second temperature sensor 12, and heat-conducting silicone grease is filled between the red copper block 8 and the gaps of the first temperature sensor 13 and the second temperature sensor 12.
An inlet pipeline 17, a buffer pipeline 10 and an outlet pipeline 3 are respectively arranged outside the vacuum cavity 9, and the inlet pipeline 17, the buffer pipeline 10 and the outlet pipeline 3 are all connected with the upper side interface of the two-way pipe 15 of the upper flange end cover 5; the vacuum chamber 9 and its connected external inlet line 17, buffer line 10 and outlet line 3 are completely immersed in the thermostatic water bath 4.
The heating control component is located outside the calorimeter and comprises a Wheatstone bridge, a PID controller and a programmable power supply, wherein the Wheatstone bridge is formed by connecting four temperature sensor leading-out wires, a voltage difference signal of the Wheatstone bridge is used as an input value of the PID controller, the output of the PID controller is used for determining an output voltage value of the programmable power supply, and the output voltage of the programmable power supply provides heating voltage for the heating wires 11 wound outside the second U-shaped tube 7 and the first U-shaped tube 14.
The vacuum cavity 9 is made of stainless steel (also comprises other materials capable of realizing vacuum sealing), and is sealed by a concave-convex groove flange; the size of the vacuum chamber 9 is
The height of the vacuum chamber 9 is 200mm, and the inner wall surface of the vacuum chamber 9 is polished; the vacuum cavity 9 is arranged on a base in the constant-temperature water bath 4, and the constant-temperature water bath 4 is wrapped by heat-preservation cotton for heat insulation.
The upper flange end cover 5 is made of stainless steel (also comprises other materials capable of realizing vacuum sealing), and the size of the upper flange end cover 5 is
The thickness is 8 mm; the two-way pipe 15, the vacuumizing pipeline 2 and the aviation plug 16 of the upper flange end cover 5 are welded, the two-way pipe 15 is used for connecting an inner pipeline and an outer pipeline of the vacuum cavity 9, the vacuumizing pipeline 2 is used for connecting a vacuum pump, the aviation plug 16 is used for leading out a lead from the vacuum cavity 9, a section of M36 multiplied by 1mm of thread is arranged above the welding position of the upper flange end cover 5 and the aviation plug 16, the thread is used for connecting the upper flange end cover 5 and the lead protection sleeve 1, and a groove is formed in the lower surface of the upper flange end cover 5.
The lower flange 6 is made of stainless steel (including other materials capable of realizing vacuum sealing), and the size of the lower flange 6 is
The thickness is 7 mm; the upper surface of the lower flange 6 is provided with a groove for placing an O-shaped ring; the lower flange 6 is matched with the upper end of the vacuum cavity 9 and then welded.
The second U-shaped pipe 7, the first U-shaped pipe 14, the inlet pipeline 17, the buffer pipeline 10 and the outlet pipeline 3 are all
The length of the inlet pipeline 17 and the length of the outlet pipeline 3 are both 3m, and the length of the buffer pipeline 10 is 15 m; the vacuum-pumping pipeline 2 is
The stainless steel material of (2).
The vacuum pumping pipeline 2 is connected with a vacuum pump. An inlet of an inlet pipeline 17 is connected with a high-pressure pump, an outlet of the inlet pipeline 17 is connected with the upper end of a first two-way pipe of an upper flange end cover 5, the lower end of the first two-way pipe is connected with an inlet of a first U-shaped pipe 14 in a vacuum cavity 9, an outlet of the first U-shaped pipe 14 is connected with the lower end of a second two-way pipe of the upper flange end cover 5, the upper end of the second two-way pipe is connected with an inlet of a buffer pipeline 10, an outlet of the buffer pipeline 10 is connected with the upper end of a third two-way pipe of the upper flange end cover 5, the lower end of the third two-way pipe is connected with an inlet of a second U-shaped pipe 7 in the vacuum cavity 9, an outlet of the second U-shaped pipe 7 is connected with the lower.
The heating wire 11 is an enameled nickel-chromium wire, and the diameter of the enameled nickel-chromium wire is 0.15 mm; two terminals of the heating wire 11 are respectively provided with a pure copper wire with the diameter not more than 0.8mm, and the pure copper wire is led out through an aviation plug 16 of the upper flange end cover 5 of the vacuum cavity 9.
The first temperature sensor 13 and the second temperature sensor 12 both adopt sheet type Pt-100 platinum resistance thermometer chips, the size of the chip is 1.6 multiplied by 3.2 multiplied by 1mm, two wiring terminals of the chip are respectively provided with a silver-plated copper wire signal shielding wire with the diameter not more than 0.2mm, and the silver-plated signal shielding wire is led out through an aviation plug 16 of the upper flange end cover 5 of the vacuum cavity 9. In order to reduce the radiation heat exchange in the vacuum cavity, the aluminum foil is wound on the heating wire and the first temperature sensor and the second temperature sensor.
Referring to fig. 3, four temperature sensors are connected to form a wheatstone bridge, the wheatstone bridge is connected to a lock-in amplifier, a voltage difference signal of the wheatstone bridge is collected by the lock-in amplifier, and the lock-in amplifier has an effect of removing noise and retaining a signal with a selected frequency; the built-in oscillating signal source of the phase-locked amplifier provides direct current voltage for the Wheatstone bridge, and the amplitude of the direct current voltage is 50 mV. The bridge voltage difference signal collected by the phase-locked amplifier is used as an input value of the PID controller, the output of the PID controller is used for determining the output voltage value of the programmable power supply, and the output voltage of the programmable power supply provides heating voltage for the heating wire 11 wound outside the U-shaped pipe.
Referring to fig. 4, the data acquisition of the heating control unit is divided into three parts, 1) phase-locked amplifier input parameter setting and output acquisition; 2) the initial parameters of the PID controller are obtained by a response curve method, the input of the PID controller is the output value of the phase-locked amplifier, and the output of the PID controller is the voltage value of the program control power supply; 3) and controlling the output voltage of the programmable power supply and collecting the real-time current value of the programmable power supply.
The pipelines through which the standard substance and the sample to be detected flow are capillary tubes with the diameter of phi 1.6 multiplied by 0.3mm, and the small pipe diameter can well meet the requirement of temperature uniformity. In order to fully preheat the fluid before the fluid enters the vacuum cavity to ensure that the fluid reaches the preset temperature, fluent simulation is carried out on the inlet pipeline, and the length is finally confirmed to be 3 m; the inlet line is wound in a spiral shape and is in direct contact with the thermostatic bath. A buffer section for storing enough standard substances is arranged between the outlet of the first U-shaped pipe and the inlet of the second U-shaped pipe of the vacuum cavity, the buffer pipeline is also placed in a constant-temperature oil bath, in order to meet the requirement that the standard substances are always in the second U-shaped pipe within the acting time of the PID controller, the length of the buffer section required is estimated to be 15m according to the flow speed in the experiment, and the control duration of 6-7 minutes can be provided. The purpose of the outlet section line was to reduce the errors caused by heat conduction, the outlet section line was also placed in a constant temperature oil bath, also 3m in length.
A method for measuring the specific heat capacity of liquid based on the device comprises the following steps: firstly, vacuumizing a vacuum cavity 9 to below 10Pa, controlling the temperature of the constant-temperature water bath 4 to be within +/-0.01 ℃ of the difference value of the temperature and the given temperature;
selecting a substance with known specific heat capacity as a standard substance, wherein the specific heat capacity of the standard substance is crThe used standard substance depends on the type of the sample to be detected, and the thermal physical properties of the standard substance are very close to those of the sample to be detected; opening the high-pressure pump to enable the standard substance to flow through the first U-shaped pipe 14, the buffer pipeline 10, the second U-shaped pipe 7 and the outlet pipeline 3 in sequence through the inlet pipeline 17;
in order to make the second temperature sensor 12 have a 2K temperature rise compared with the first temperature sensor 13, a certain heating voltage is provided for the heating wire 11, the Wheatstone bridge voltage difference signal E collected by the phase-locked amplifier is observed, and after the Wheatstone bridge voltage difference signal E is stabilized within +/-0.03 mu V, the output signal S of the phase-locked amplifier and the first temperature sensor are recordedHeating power P of heating wire 11 on U-shaped pipe 14r；
Keeping the flow rate constant, and replacing the standard substance in front of the high-pressure pump with a sample to be detected for continuous sample introduction; when a sample to be detected flows in the tube and reaches the first U-shaped tube 14, the heating voltage of the heating wire 11 on the second U-shaped tube 7 is kept unchanged, the PID controller is used for automatically adjusting the heating voltage of the heating wire 11 on the first U-shaped tube 14, the output signal of the phase-locked amplifier is still S, and the current heating power P of the heating wire 11 on the first U-shaped tube 14 at the moment is recordedsIn the process, the standard substance always flows through the second U-shaped pipe 7.
From the law of conservation of heat, the following calculation can be obtained:
Pr＝F·ρr·cr·ΔT+Lr (1)
Ps＝F·ρs·cs·ΔT+Ls (2)
in the formula, PrFor heating power, PsF is the volumetric flow rate for the current power, which remains constant during the experiment; rhorAnd ρsRespectively representing the density values of the standard substance and the sample to be tested at the experimental temperature and pressure; c. CrAnd csRespectively is the specific heat capacity of the standard substance and the sample to be measured; delta T is the temperature difference between the second temperature sensor 12 and the first temperature sensor 13 under the action of the heating wire 11 on the first U-shaped pipe 14, LrAnd LsRespectively the heat dissipation loss of the standard substance and the sample to be measured to the external environment.
Induced heat leakage coefficient f:
then, the above three calculation equations can be used together:
the heat leakage coefficient is usually obtained by calibrating a variable flow rate method of a standard substance, and when only the standard substance exists in the formula and the flow rates of the standard substance are different, the formula is deformed as follows:
in the formula, FrFor a constant flow rate, FsIs the current flow rate.
The change of the specific heat capacity of the standard substance is simulated by changing the flow rate of the standard substance, and the heating power variation is measured, so that the heat leakage coefficient value can be obtained.
Only the second temperature sensor 12 and the first temperature sensor 13 on the first U-shaped pipe 14 and the heating wire 11 are used for calibration.
The calibration process is as follows: measuring the resistance values of the first temperature sensor 13 and the second temperature sensor 12 on the first U-shaped pipe 14 after the specific heat measurement process is finished at a certain temperature; the constant flow rate F of the heating wire 11 on the first U-shaped tube 14 in the experiment is recordedrHeating power PrAnd measures the difference value Δ R between the resistances of the second temperature sensor 12 and the first temperature sensor 13; changing the flow rate to the current flow rate FsObserving the resistance difference between the second temperature sensor 12 and the first temperature sensor 13, manually adjusting the output voltage to return to the resistance difference Δ R, and recording the current power P at that times(ii) a The flow velocity can be changed for many times, the corresponding heat leakage coefficient value under each flow velocity is obtained through calculation, and finally the average value of the heat leakage coefficients calibrated for many times in the adjacent area of the measured power is taken as the final value.
P obtained finally from the measurementrAnd PsCalculated heat leakage coefficients f and rhor、ρsAnd crSubstituting into the calculation formula (4) to obtain the specific heat capacity c of the sample to be measureds。
The specific embodiment is as follows: specific heat capacity measurement of sodium chloride solution
5 sodium chloride solutions with concentrations of 0.2, 0.1, 0.05, 0.03 and 0.1mol/kg were prepared using water with abundant data thermophysical properties as a standard substance, and their specific heat capacities at 0.1MPa and 30 ℃ were measured, and 3 replicates were performed for each concentration in order to verify the reproducibility of the experiment, and the specific experimental results are shown in Table 1.
For the measurement of the specific heat capacity of the dilute solution, the specific heat capacity is generally used for obtaining the apparent molar heat capacity of the dilute solution, and the measurement result of the experiment is also calculated to obtain a corresponding apparent molar heat capacity value and compared with a literature value, so that the deviation between the calculated value and the literature value is small.
Table 1 example 1 specific heat capacity measurement results
In the table, m is the molar mass concentration,
to calculate the resulting apparent molar heat capacity,
the apparent molar heat capacity given in the literature, Δ is the calculated deviation from the literature value.
According to the invention, two sections of identical U-shaped pipeline structures are arranged, and in the experimental process, a Wheatstone bridge signal of a standard substance after the standard substance is heated and stabilized is firstly acquired by taking the standard substance as a reference; after the front end of the inlet pipeline is switched to a sample to be detected, the voltage of the two ends of the heating wire is controlled, so that the voltage difference output signal of the Wheatstone bridge is consistent with that before; and after the specific heat capacity measuring process is finished, calibrating the heat leakage coefficient by using a standard substance based on a variable flow rate method, and finally obtaining the specific heat capacity of the sample to be measured through data processing. The device of the invention can complete the measurement and control process only in 10-15 minutes, thereby greatly improving the efficiency. The device can accurately measure the specific heat capacity of the liquid, has good stability, and wider measurable temperature and pressure range, can be used for measuring the specific heat of dilute electrolyte solution and organic matters in industry and scientific research, and can also be used for teaching of professional course experiments related to energy power.
Claims (10)
1. A device for measuring specific heat capacity of a substance by a double-flow method is characterized by comprising a calorimeter component and a heating control component; the calorimeter component comprises a vacuum cavity (9), a lower flange (6) and an upper flange end cover (5) matched with the lower flange (6) are arranged at the top of the vacuum cavity (9), and a vacuumizing pipeline (2) is arranged on the upper flange end cover (5); a second U-shaped pipe (7) and a first U-shaped pipe (14) which are identical in structure are arranged inside the vacuum cavity (9), the upper ends of the second U-shaped pipe (7) and the first U-shaped pipe (14) are communicated with the lower ends of the two channels (15), heating wires (11) are wound on the outer sides of the second U-shaped pipe (7) and the first U-shaped pipe (14), and a first temperature sensor (13) and a second temperature sensor (12) are respectively arranged at two ends of each heating wire (11);
an inlet pipeline (17), a buffer pipeline (10) and an outlet pipeline (3) are arranged outside the vacuum cavity (9), the outlet of the inlet pipeline (17) is connected with the inlet of the first U-shaped pipe (14), the outlet of the first U-shaped pipe (14) is connected with the inlet of the buffer pipeline (10), the outlet of the buffer pipeline (10) is connected with the inlet of the second U-shaped pipe (7), and the outlet of the second U-shaped pipe (7) is connected with the outlet pipeline (3);
the heating control component is positioned outside the calorimeter and is connected with the heating wire (11).
2. The device for measuring the specific heat capacity of the substance by the dual-flow method according to claim 1, wherein two ends of the heating wire (11) are respectively provided with a red copper block (8); the second U-shaped pipe (7) and the first U-shaped pipe (14) are sleeved with the red copper block (8), and through holes for placing the first temperature sensor (13) and the second temperature sensor (12) are formed in the red copper block (8);
the heating control component comprises a Wheatstone bridge, a PID controller and a program-controlled power supply, the Wheatstone bridge is connected with a phase-locked amplifier, the phase-locked amplifier is connected with the PID controller, the PID controller is connected with the program-controlled power supply, and the program-controlled power supply is connected with the heating wire (11);
the vacuum cavity (9), the inlet pipeline (17), the buffer pipeline (10) and the outlet pipeline (3) are all immersed in the constant-temperature water bath (4); the inlet of the inlet pipeline (17) is connected with a high-pressure pump; the inner wall of the vacuum cavity (9) is polished.
3. The device for measuring the specific heat capacity of the substance by the dual-flow method according to claim 1, wherein a heat-conducting silicone grease is filled between the red copper block (8) and the gaps of the first temperature sensor (13) and the second temperature sensor (12); the first temperature sensor (13) and the second temperature sensor (12) both adopt sheet type Pt-100 platinum resistance thermometer chips, two wiring ends of the chips are respectively connected with silver-plated copper wire signal shielding wires, and the silver-plated signal shielding wires are led out through an aviation plug (16) arranged on the flange end cover (5); and aluminum foil paper is wound on the heating wire, the first temperature sensor and the second temperature sensor.
4. The device for measuring the specific heat capacity of the substance by the dual-flow method according to claim 3, wherein the vacuum cavity (9) is made of stainless steel, the vacuum cavity (9) is placed on a base in the constant-temperature water bath (4), and heat-insulating cotton is arranged outside the constant-temperature water bath (4);
the upper flange end cover (5) is made of stainless steel, the inlet of the vacuumizing pipeline (2) is connected with a vacuum pump, the aviation plug (16) is used for leading out a lead from the vacuum cavity (9), and the lower surface of the upper flange end cover (5) is provided with a groove;
the lower flange (6) is made of stainless steel, and a groove for placing an O-shaped ring is formed in the upper surface of the lower flange (6); the lower flange (6) is welded with the upper end of the vacuum cavity (9);
the heating wire (11) is an enameled nickel-chromium wire with the diameter of 0.15 mm; two wiring ends of the heating wire (11) are respectively connected with copper wires with the diameter not more than 0.8mm, and the copper wires are led out through an aviation plug (16).
5. A method for measuring the specific heat capacity of a liquid based on the device of claim 2,
firstly, a vacuum cavity (9) is vacuumized to be below 10Pa, and standard substances flow through a first U-shaped pipe (14), a buffer pipeline (10), a second U-shaped pipe (7) and an outlet pipeline (3) through an inlet pipeline (17) in sequence; the second temperature sensor (12) is higher than the first temperature sensor (13)2K by applying a heating voltage to the heating wire (11), and the output signal S of the phase-locked amplifier and the heating power P of the heating wire (11) on the first U-shaped pipe (14) are recordedr；
Keeping the flow rate constant, and replacing the standard substance with a sample to be detected to continue sample introduction; when a sample to be detected flows to the first U-shaped pipe (14), keeping the heating voltage of the heating wire (11) on the second U-shaped pipe (7) unchanged, adjusting the heating voltage of the heating wire (11) on the first U-shaped pipe (14), recovering the output signal of the phase-locked amplifier to S, and recording the current heating power P of the heating wire (11) on the first U-shaped pipe (14)s；
According to heating power PrCurrent heating power PsCoefficient of heat leakage, density value rho of standard substance at experimental temperature and pressurerAnd density value rho of sample to be tested at experimental temperature and pressuresAnd specific heat capacity c of standard substancerObtaining the specific heat capacity c of the sample to be measureds。
6. The method for measuring the specific heat capacity of a liquid according to claim 5, wherein the specific heat capacity c of the sample to be measuredsObtained by the following formula:
wherein F is the volume flow rate and F is the heat leakage coefficient value.
7. A method for measuring the specific heat capacity of a liquid according to claim 5, wherein the heat leakage coefficient is obtained by: measuring the resistance values of a first temperature sensor (13) and a second temperature sensor (12) on the first U-shaped pipe (14), and recording the constant flow rate F of a heating wire (11) on the first U-shaped pipe (14)rHeating power PrAnd measuring the difference value delta R of the resistance values of the second temperature sensor (12) and the first temperature sensor (13); changing the flow rate to the current flow rate FsThe resistance difference between the second temperature sensor (12) and the first temperature sensor (13) is made to be a resistance difference value delta R, and the current heating power P is recordeds(ii) a Changing the flow velocity for many times, calculating to obtain the corresponding heat leakage coefficient value under each flow velocity, and finally calibrating for many times in the adjacent area of the measured powerThe average value of the heat leakage coefficient is used as the final value of the heat leakage coefficient value.
8. A method for measuring the specific heat capacity of a liquid according to claim 5, wherein the value of the heat leakage coefficient is calculated by the following formula:
in the formula, FrFor a constant flow rate, FsIs the current flow rate.
9. A method for measuring the specific heat capacity of a liquid according to claim 5, wherein the heating power PrWith the current heating power being PsRepresented by the formula:
Pr＝F·ρr·cr·ΔT+Lr (1)
Ps＝F·ρs·cs·ΔT+Ls (2)
in the formula, PrFor heating power, PsF is the volume flow rate for the current heating power; rhorAnd ρsRespectively representing the density values of the standard substance and the sample to be tested at the experimental temperature and pressure; c. CrAnd csRespectively is the specific heat capacity of the standard substance and the sample to be measured; delta T is the temperature difference between the second temperature sensor (12) and the first temperature sensor (13) under the action of the heating wire (11) on the first U-shaped pipe (14), LrAnd LsRespectively the heat dissipation loss of the standard substance and the sample to be measured to the external environment.
10. A method for measuring the specific heat capacity of liquid according to claim 5, characterized in that the output signal of the phase-locked amplifier is recovered to be S from the time when the sample to be measured flows to the first U-shaped tube (14), and the second U-shaped tube (7) is filled with standard substance.
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