1. Fluid thermal conductivity measurement apparatus, comprising:

a measuring tube for flowing a fluid to be measured;

a conveying device for flowing the fluid to be measured through the measuring tube;

and the electric heating device is used for heating the fluid to be measured flowing through the measuring pipe.

2. The fluid thermal conductivity measurement device of claim 1, wherein the delivery device comprises a delivery tube, a mechanical pump and a fluid reservoir, both of which are connected to the delivery tube; the measuring tube is connected to both ends of the delivery tube.

3. A fluid heat transfer coefficient measuring device according to claim 1, wherein a liquid filling port (2) is provided in the delivery pipe.

4. A fluid thermal conductivity measurement apparatus according to claim 2, wherein a cooling means is provided on the transport pipe for cooling the fluid to be measured.

5. The fluid thermal conductivity measuring apparatus according to claim 1, wherein a first sensor for measuring the temperature of the wall of the measuring tube is provided on the outer wall of the measuring tube, and a second sensor for measuring the temperature of the fluid to be measured in the measuring tube is provided in the measuring tube.

6. Fluid thermal conductivity measurement device according to claim 1, wherein the electrical heating means is an electrical heating wire (6) wound around the measurement tube.

7. The fluid thermal conductivity measurement device of claim 6, wherein an insulating layer is further provided on the measurement pipe.

8. A method for measuring thermal conductivity of a fluid based on the apparatus for measuring thermal conductivity of a fluid according to any one of claims 1 to 7, comprising:

s1, creating a flow condition of laminar flow of fluid in the measuring tube, and creating a boundary condition of uniform heat flow density heating;

s2, measuring the difference between the wall temperature and the fluid temperature of the fully developed section of the measuring tubeMeasuring the length of said measuring sectionLMeasuring the heating power of the heating wireUIAnd calculating the numerical value of the thermal conductivity coefficient of the measured fluid:

in the formula, areT _w,iIs the wall temperature of the ith measurement point;T _ithe fluid temperature at the ith measurement point; n is the number of measurement points.

9. The method of claim 8, wherein the boundary condition for creating uniform heat flux density heating is heating the measuring tube by an electric heating wire or a heating rod; the flow condition for creating laminar flow is to control the flow speed of the fluid to be detected, and the flow speed is preferably 0.05-0.1 m/s.

10. The method of claim 8, wherein the wall temperature is measured by a thermocouple provided in the wall, and the fluid temperature is measured by a sheathed thermocouple provided in the measuring section; the condition of the fully developed section of the measuring pipe is that the difference value of the wall surface temperature and the fluid temperature does not change along with time and position, and the preferable scheme is that the interpolation value of the wall surface temperature and the fluid temperature is controlled to be 1-2 ℃ per hour, and the temperature difference of different positions is controlled to be 1 ℃.

Background

The heat conductivity coefficient is a parameter for representing the quality of the heat conduction performance of the material, and has important application in engineering. The heat conductivity coefficients of different substances are different, and even if the same material is adopted, the heat conductivity coefficient value is also related to factors such as temperature, and the like, so that the accurate measurement of the numerical value of the heat conductivity coefficient of the substance has important significance for engineering application and academic research. The current common methods for measuring the heat conductivity coefficient comprise a steady state method and an unsteady state method, wherein the steady state method comprises a longitudinal heat flow method, a radial heat flow method, a direct electric heating method, a thermoelectric method and the like; the unsteady state method includes a periodic heat flow method, an instantaneous heat flow method, and the like. However, these methods are mainly aimed at measuring the thermal conductivity of solid, and for liquid, the thermal conductivity is small, so that the liquid is a poor thermal conductor, and convection is easily generated when the liquid is heated, so that the thermal conductivity of the liquid can be accurately measured. Meanwhile, many methods and devices for measuring the thermal conductivity of liquid are optimized and improved on the basis of the above method, but it is difficult to eliminate the influence of convection.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide a method and an apparatus for measuring the thermal conductivity of a liquid based on convective heat transfer, which starts from the boundary layer theory of fluid in convective heat transfer and utilizes the thermal conductivity of the fluid in the boundary layer to measure the thermal conductivity of the liquid. The problem that errors exist in the measured heat conductivity coefficient caused by convection in a steady state method and an unsteady state method is solved.

Technical scheme

In order to solve the problems of the existing method and device for measuring the heat conductivity coefficient of the liquid, the invention adopts the technical scheme that:

a fluid thermal conductivity measurement device, comprising:

a measuring tube for flowing a fluid to be measured;

a conveying device for flowing the fluid to be measured through the measuring tube;

and the electric heating device is used for heating the fluid to be measured flowing through the measuring pipe.

A fluid thermal conductivity measurement method based on the fluid thermal conductivity measurement device is characterized by comprising the following steps:

s1, creating a flow condition of laminar flow of fluid in the measuring tube, and creating a boundary condition of uniform heat flow density heating;

s2, measuring the difference between the wall temperature and the fluid temperature of the fully developed section of the measuring tubeMeasuring the length L of the measuring section, measuring the heating power UI of the heating wire, and calculating the numerical value of the heat conductivity coefficient of the measured fluid:

in the formula, is T_w,iIs the wall temperature of the ith measurement point; t is_iThe fluid temperature at the ith measurement point; n is the number of measurement points.

For laminar flow of a fluid within a circular tube, the continuity equation and momentum equation of a two-dimensional steady-state flow are used:

continuity equation:

the momentum equation:

wherein u and v are flow rates, and p, rho and v are pressure, density and kinematic viscosity respectively. In the fully developed section, there is only a velocity in the flow direction in the flow channel cross section, the velocity in the radial direction is negligible, and the velocity u is 0 at the wall surface. The pressure p is a function of the flow direction x only, and is uniform across the flow channel, so that

The velocity profile obtained was:

the laminar flow energy equation of the fluid in the circular tube is as follows:

in the fully developed segment, a certain infinitesimal segment dx is selected, and the energy balance relation on the infinitesimal segment is as follows:

for the condition of constant heat flow density, the corresponding boundary conditions are:

the solution temperature distribution is:

and the surface heat transfer coefficient of the convection heat transfer in the tube is as follows:

therefore, in the fully developed section of laminar fluid flow in the circular tube, the convective heat transfer coefficient in the circular tube can be obtained by measuring the applied heat flux density, the wall surface temperature and the temperature of the fluid, and the heat conductivity coefficient of the fluid can be obtained by measuring the pipe diameter of the circular tube and the convective heat transfer coefficient in the circular tube.

The invention relates to a device for measuring the heat conductivity coefficient of fluid, which comprises a mechanical pump, a liquid injection port, a liquid storage tank, a condenser, a heating wire and a heat insulating material. The measured liquid is added into the whole device through the liquid injection port, and the liquid injection is stopped when the volume of the measured liquid in the liquid storage tank reaches 2/3 of the volume of the liquid storage tank, so that the liquid can be ensured to fill the pipeline in the measuring process; after flowing out of the liquid storage tank, the measured liquid flows through the mechanical pump, the flow rate is increased after being pressurized by the mechanical pump, the flow rate of the measured liquid is controlled by controlling the power of the mechanical pump, and the Reynolds number of the measured liquid is controlled to be less than 2300, so that the measured liquid is in a laminar flow state at the measuring section; then the measured fluid flows through the measuring section, and the pipeline of the measuring section is uniformly wound and heated by the electric heating wire, so that the condition of approximate uniform heat flux density is caused; the pipeline and the electric heating wire of the measuring section are wrapped by the heat insulating material, so that heat loss is reduced; the measured fluid is heated by the heating wire to raise the temperature, and the heat boundary layer on the wall surface of the pipe has a process of growing from zero to know the convergence and the center line of the pipe. When the thermal boundary layer is converged on the central line of the pipe, the intensity of heat exchange, namely the Knudsen number, is kept unchanged, namely, the thermal boundary layer enters a full development section. The heat exchange intensity can be obtained by measuring the temperature difference and the wall temperature of the fluid at a certain distance in the fully developed section, and the heat conductivity coefficient of the fluid is calculated according to the constant of the Knoop number; the length of the measuring section is long enough to ensure that the fluid enters the fully developed section during measurement; the measured fluid flowing out of the measuring section flows through the condenser and is cooled by the cooling medium of the condenser, and the temperature of the measured fluid is reduced to the temperature at the inlet of the measuring section; the fluid to be measured then flows into the reservoir.

The whole length of the measuring section is L, the diameter of the circulating pipeline is d, when the measured fluid enters the fully-developed section, a plurality of measuring points are arranged along the pipeline, and the fluid temperature of the measuring points is T_nWall temperature of T_w,nThe distance between the starting measuring point and the ending measuring point is L_f. The heating quantity is provided by the electric heating wire, and the heating quantity is equal to the power of the electric heating wire, namely the product of the voltage U and the current I of the electric heating wire. Thus the measured fluidThe thermal conductivity coefficient is:

wherein h is the convective heat transfer coefficient, and the calculation formula is as follows:

wherein q is_wFor the heat flux density applied to the measured fluid, the formula is calculated as:

the delta T is the logarithmic mean temperature difference between the measured fluid and the pipe wall, and the calculation formula is as follows:

bringing formulas 1.14-1.16 into formula 1.13, wherein the obtained thermal conductivity is as follows:

therefore, in the present apparatus, the thermal conductivity of the fluid can be determined by measuring the length of the measurement section, the wall surface temperature and the fluid temperature at each measurement point, and the applied thermal load.

When the difference value between the wall surface temperature and the fluid temperature of each measuring point does not change along with time and position, the preferable scheme is that the change of the interpolation value between the wall surface temperature and the fluid temperature is controlled to be 1-2 ℃ per hour, the temperature difference between different positions is controlled to be 1 ℃, and the measured fluid in the measuring points can be considered to reach a steady state and a fully developed section.

Further, the measurement method is formulated using round tubes, but the method is also applicable to tubes of other shapes than round tubes.

Furthermore, the measuring section is wrapped by a heat insulating material, and the heat insulating material contains a material for shielding radiation heat.

Further, the fluid flowing state in the measuring section is laminar flow.

Further, the length of the measuring section is sufficiently long.

Further, the pipe wall of the pipeline is as thin as possible.

Further, the measuring section measures the fluid to reach a steady state and a fully developed section.

Further, the condenser may be in the form of, but not limited to, a plate heat exchanger.

Furthermore, the liquid storage tank can be additionally provided with a liquid level meter.

Further, the number of the measurement points is preferably 5 to 10.

Advantageous effects

(1) A heat conductivity coefficient measuring method based on convection heat transfer is particularly suitable for liquid with high viscosity, the heat conductivity coefficient of the liquid is measured by utilizing the heat exchange characteristic of a laminar flow fully-developed section in a circular tube, the measuring precision is high, and the problem that the error of a measuring result is large due to convection caused by a traditional steady-state method is solved.

(2) All parts of the device for measuring the heat conductivity of the fluid, which is designed based on the measuring method, can change parameters according to the measuring requirement, and the flow of the circulating device can be adjusted by adjusting the rotating speed of the mechanical pump; the heating power of the electric heating wire and the condensing power of the condenser can be adjusted; the measurement of the heat conductivity coefficients of fluids with different temperatures and different classes is realized.

(3) The device is simple, has low requirements on the measured fluid and the measuring equipment.

Drawings

FIG. 1 is a schematic view of a device for thermal conductivity measurement based on convective heat transfer;

FIG. 2 is a schematic view of a measurement section of the thermal conductivity measurement apparatus;

FIG. 3 is a schematic cross-sectional view of a measurement section of the thermal conductivity measurement apparatus;

in the figure: 1-a mechanical pump; 2-liquid filling port; 3, a liquid storage tank; 4-a condenser; 5-heat insulating material; 6-electric heating wire.

Detailed Description

The invention is further described with reference to the accompanying drawings in which:

the invention relates to a liquid thermal conductivity coefficient measuring device designed based on a liquid thermal conductivity coefficient measuring method, which comprises a mechanical pump 1, a liquid filling port 2, a liquid storage tank 3, a condenser 4 and a measuring section as shown in figure 1.

The measuring section is a section of pipe which is uniformly wrapped by an electric heating wire 6 and a heat insulating material 5.

The mechanical pump 1 is used for providing power for the circulating flow of the measured fluid, and simultaneously the flow rate is controlled by adjusting the power of the mechanical pump 1, so that the measured fluid entering the measuring section keeps a laminar flow state; the liquid filling port 2 is used for filling the measured fluid into the measuring device, and the measured fluid can be discharged from the device after the measurement is finished; the liquid storage tank 3 is used for storing part of the liquid to be measured, ensuring that the liquid to be measured can fill the pipeline and eliminating the interference of gas on measurement; the condenser 4 is used for cooling the fluid heated by the electric heating wire and ensuring that the inlet temperature of the measured fluid entering the measuring section is the same; the measuring section is used for measuring the heat conductivity coefficient of the measured fluid, and the electric heating wire 6 is used for generating the boundary condition of uniform heat flux density heating; the heat insulation material 5 has the function of reducing the heat loss of the electric heating wire, so that the heat generated by the electric heating wire 6 is completely used for heating the fluid in the pipeline; and secondly, the heat loss of the measured fluid caused by heat conduction and radiation is reduced, and the measurement precision is improved.

The specific measurement steps are as follows:

s1, filling the liquid to be measured into the measuring device through the liquid filling port 2, filling the pipeline in the measuring device with the liquid to be measured, observing the liquid level in the liquid storage tank 3, and enabling the volume of the liquid to be measured to reach 2/3 of the volume of the liquid storage tank 3.

S2, starting the mechanical pump 1 and the condenser 4, and controlling the power of the mechanical pump 1 to enable the flow velocity of the fluid to be measured to be about 0.1-0.2 m/S.

S3, the electric heating wire 6 is turned on, and heating is performed with constant power, resulting in a boundary condition of uniform heat flux density.

S4, observing the temperature of the fluid and the wall temperature of a plurality of measuring points, wherein when the difference between the fluid temperature and the wall temperature of all the measuring points does not change with time, namely T_w,i-T_i(1<i<n) does not change with time, and the difference between the fluid temperature and the wall temperature at all the measurement points is the same, namely T_w,i-T_i＝T_w,j-T_j(1<i<n,1<j<n, i ≠ j), it can be assumed that the fluid between the measurement points has entered a steady state while the flow entered a fully developed segment. And recording the fluid temperature and the wall surface temperature of the measuring point at a certain moment and the current and voltage values of the electric heating wire.

S5, calculating the value of the measured fluid heat conductivity coefficient according to the formula 1.17:

the invention uses methanol as the measured fluid as an embodiment, the circulating pipeline adopts a circular pipeline, the diameter of the pipeline is 152 mu m, the temperature of the fluid entering the measuring section is 25 ℃, the length L of the measuring section is 7.2cm, and L is the same as that of the measured section_f4cm, 4.57W of power applied by the electric heating wire and 13.3W/cm of heat flow density²The circulation flow rate of methanol was 300ml/h, and the wall surface temperature and fluid temperature measurement points were arranged in 5. The first measuring point of the wall surface temperature is 3cm away from the inlet of the measuring section, then temperature measuring points are arranged at intervals of 1cm, the wall surface temperature measuring points are arranged on the outer wall of the pipeline, and a K-type thermocouple is used for measuring; the distance between the first measuring point of the fluid temperature and the inlet of the measuring section is 3cm, then, one temperature measuring point is arranged every 1cm, the fluid temperature measuring points are arranged in the pipeline, and the armored thermocouple is used for measuring.

The reynolds number of methanol was calculated to be 1182.51<2300, so the flow of methanol was laminar. By measurement, when a steady state was reached, the average wall temperature at each measurement point was 62.25 ℃ and the fluid temperature was 38.51 ℃. The final calculated value of the methanol heat transfer coefficient is:

calculated with Refprop software 197.63 × 10^-3The relative error of W/(m.K) is only 1.32% (if the optimum measurement conditions are achieved, the error can be further reduced), and therefore the test method and the test apparatus have high accuracy.