JPH07253404A - Liquid thermal conductivity measuring method using natural convection heat transfer - Google Patents

Abstract

PURPOSE:To calculate a heat transfer coefficient of liquid by measuring a heat transfer amount of the liquid by utilizing steady state convection heat transfer in the liquid along a heat generator. CONSTITUTION:A similarity rule exists for a convective heat transfer in liquid along a heat generator. That is, when geometric conditions of a heating surface are decided, convective heat transfer characteristics can be represented in terms of the relationship between a dimensionless heat transfer coefficient (Nusselt number Nu) and a dimensionless number (Rayleigh number Ra) indicating the strength of the convection irrespective of the hydrodynamic properties and the thermal physical properties of the liquid. When the heated surface has a simple shape, a convective heat transfer measuring system is introduced by the formula indicating the relationship Nu=a+bRa<c> (a, b, c are constants determined according to the system). Liquid having unknown heat transfer coefficient is filled in this measuring system, the temperature of the heating surface is raised, and a transfer heat quantity (consumed power) in a steady state is measured. This operation is executed a plurality of times by altering the raised temperature, the consumed power and the raised temperature on the heating surface in the steady state at this time are measured and the unknown heat transfer coefficient can be derived by calculation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring principle for measuring a thermophysical property (thermal conductivity) inherent in a liquid by utilizing heat transfer by natural convection generated in the liquid along a heating surface.

[0002]

2. Description of the Related Art Conventionally, a nonsteady method and a steady method are known as methods for measuring the thermal conductivity of liquid. The steady-state method is a method in which a horizontal thin slit-shaped space (gap) is filled with a liquid and heated from above to prevent the occurrence of natural convection and to measure the thermal conductivity of the liquid. This method generally tends to complicate the fabrication of the device and is generally not popular. On the other hand, the unsteady thin wire heating method is a typical unsteady method. In this method, the electric power consumed by the platinum wire and the temperature rise of the platinum wire are raised in the initial stage of measurement when the heat conduction before convection is dominant by electrically heating the thin platinum wire stretched vertically. The thermal conductivity of the liquid is determined from the change with time. The unsteady thin wire heating method is being established as a method for directly measuring the thermal conductivity with the highest accuracy.

[0003]

However, since the unsteady thin wire heating method applies an electric current directly to the platinum thin wire, it is necessary to electrically insulate the thin wire for the measurement of a liquid having electrical conductivity. Various insulation methods for thin wires have been devised for this purpose. These insulation methods are 200-
Although its effectiveness has been partially confirmed at 300 ° C or lower, it is generally extremely difficult to directly measure the thermal conductivity of a high temperature liquid. In addition, since the heating wire used in the thin wire heating method is very thin, such as several tens of microns, it must be handled with extreme care, and even a liquid at room temperature is difficult to use for current position measurement.

[0004]

SUMMARY OF THE INVENTION The present invention seeks to measure the thermal conductivity of a liquid by utilizing steady state convection heat transfer in the liquid along a heating element. Since this method is a stationary method, it is not necessary to reduce the heat capacity of the heating portion, and in general, electrical insulation can be easily provided. Therefore, the heating wire can be thickened by the linear heat source method, and the current position can be easily measured. It can also be applied to the thermal conductivity of electrically conductive liquids and high temperature solutions.

[0005]

There is a similarity law for convective heat transfer in a liquid along a heating element. That is, if the geometrical conditions of the heating surface or heating body are determined, the convection heat transfer characteristics of the liquid are dimensionless heat transfer, regardless of the hydrodynamic properties (viscosity, etc.) and thermophysical properties (thermal conductivity, etc.) of each liquid. Coefficient (Nussert number Nu) and dimensionless number (Rayleigh number R
It can be uniquely expressed in relation to a). When the heating surface or the heating body has a simple shape, the relation is usually expressed by the following equation. Nu = a + bRa ^c (1) However, a, b and c are constants determined by the system (geometrical shape of the heating surface or heating body) and are independent of the physical properties of each fluid. There now exists a system whose convective heat transfer is described by equation (1). A liquid whose physical properties are unknown is put into this measurement system to raise the temperature of the heating surface, and the amount of heat transfer (power consumption) in a steady state is measured. This operation is repeated multiple times while changing the temperature rise temperature. From the relation of the equation (1) and the definitions of the Nusselt number and the Rayleigh number, the following relation is established between the power consumption q at this time and the temperature rise ΔT of the heating surface in the steady state. q ₁ d / kΔT ₁ = a + bRa ₁ ^c (2) q ₂ d / kΔT ₂ = a + bRa ₂ ^c = a + b (ΔT ₂ / ΔT ₁ ) ^c Ra ₁ ^c (3) where subscript The numbers indicate the order of measurement. Further, d and k are the representative length of the system and the thermal conductivity of the liquid. By taking the ratio of the equations (2) and (3), the unknown constant Ra ₁ can be solved. Further, by substituting the result into the equation (3), the following equation is obtained for the unknown thermal conductivity k. k = q ₁ d / (T ₁ (a + bRa ₁ ^c )) (4)

[0006]

[Example] In order to confirm the principle of thermal conductivity of a liquid proposed in this instruction, a cylindrical heating element (probe) having a length of 155 mm and a diameter of 2 mm is vertically installed in a liquid-filled cylindrical container. Carried out. The outline of the experimental apparatus is shown in FIG. Power was supplied to the probe using a constant power source. The temperature of the heating element was measured using a thermocouple installed inside the probe. It is known that the following equation holds for the amount of heat transfer from the probe at this time. Nu = 0.5635 + 0.2034Ra ^1/4 (5) Water and ethanol at 20 ° C. were used as the liquids to be measured. The power supply to the probe is changed, the probe temperature ΔT and the power supply q in the steady state are measured, and the equation (4)
Using (5) and (5), the thermal conductivity k was calculated for water and ethanol. The values of q and ΔT when water and ethanol were used as the liquids to be measured are shown in FIG. 2, and the thermal conductivities of water and ethanol obtained from the measurement results are shown in FIG. as a result,
The thermal conductivity of water is from 0.59 [w / mK] to 0.
Although there was a variation in the value during 62 [w / mK], the average was 0.612 [w / mK]. This value agrees with the literature value with an error of 3% or less. Also,
0.19 [w / mK] to 0.2 for ethanol
A variation in values was observed during 1 [w / mK], and an average value of 0.200 [w / mK] was obtained. This value was about 19% larger than the literature value of 0.168 [w / mK]. The cause of the increase in the error in the measurement of ethanol is considered to be that the thermal conductivity of the object to be measured decreased and the heat leakage from the portion where the probe was in contact with the atmosphere increased.

[0007]

As described above, according to the method for measuring the thermal conductivity of a liquid according to the present invention, the thermal conductivity of the liquid is calculated by measuring the amount of heat transfer in the presence of natural convection. You can do it.

[Brief description of drawings]

FIG. 1 shows an abbreviated view of an experimental apparatus that was performed using an elongated cylindrical heating element (probe).

FIG. 2 shows a heat transfer amount q and a probe temperature increase ΔT when water and ethanol are measured.

3 is a result of thermal conductivity k calculated based on the result of FIG. 2, where the horizontal axis shows the number of times calculated by taking a ratio, and the vertical axis shows the thermal conductivity at that time.

[Explanation of symbols]

a constant b constant d representative length [m] k thermal conductivity [w / mK] n number of calculations Nu Nusselt number q heat transfer amount per unit area [w / m ² ] Ra Rayleigh number superscript c power of Rayleigh number Subscript 1 Measurement example 1 2 Measurement example 2

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Sumi 1-10-13 Takahashi, Tagajo City, Miyagi Prefecture

Claims (1)

[Claims] 1. A heating element having a simple geometrical shape such as a cylinder or a flat plate is brought into contact with a liquid or a melt which is a sample to be measured, and a plurality of heat transfer amounts by natural convection are changed by changing the temperature of the heating element. Measure times. The non-dimensional number (Rayleigh number) showing the convection intensity and the non-dimensional heat transfer coefficient (Nussert number) of the measurement system are calculated from the steady heat transfer amount and the temperature difference between the heating element and the liquid, and the Rayleigh number and Nusselt number A principle that determines the thermal conductivity of liquids using relationships.