CN112816518B - Method and device for testing supercooling degree thermal boundary in solidification process in circular tube - Google Patents
Method and device for testing supercooling degree thermal boundary in solidification process in circular tube Download PDFInfo
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Abstract
The invention discloses a method and a device for testing supercooling degree thermal boundary in solidification process in a circular tube. A refrigerating device and a stirring device are arranged in the phase-change material constant temperature container. Also disclosed is a method for testing supercooling thermal boundaries during solidification within a tubular. The invention can solve the problems by adopting the method for testing the natural convection heat transfer coefficient when the phase change material on the outer side of the wall surface coexists with solid and liquid under the condition of no external impact, vibration and other strong disturbance. The method can avoid the inconvenience caused by using the external heat insulation material and the influence of temperature fluctuation caused by heat exchange with the outside.
Description
Technical Field
The invention relates to a method for testing a solidification supercooling degree thermal boundary in a circular tube, and also relates to a method for testing the solidification supercooling degree thermal boundary in the circular tube.
Background
Supercooling refers to the difference between the theoretical crystallization temperature (freezing point) of a substance (e.g., metal, alloy, crystal) and the actual crystallization site temperature. Many phase change materials have supercooling during crystallization, i.e., crystallization cannot proceed when the temperature is lowered to the freezing point. Crystallization occurs only after the temperature continues to decrease below a certain theoretical freezing point. The supercooling degree of the phase-change material and influencing factors thereof have very important influence on the performance of the phase-change material in practical application, but the difficulty of the supercooling degree test and theoretical research is greatly increased because the influencing factors are more and the duration is very short in the crystal nucleus formation process.
The research method of the supercooling degree comprises experiments, theoretical analysis and numerical simulation. At present, when a theoretical analysis and numerical simulation method is adopted for research, a mathematical model can be established and solved only by accurately obtaining a thermal boundary condition through an experimental method. There are three common thermal boundary conditions: the first type of thermal boundary condition is wall temperature; the second type of thermal boundary wall heat flow; the third type of thermal boundary is the convective heat transfer coefficient of the outer wall surface and the ambient temperature.
Numerous studies have found that wall temperatures are difficult to measure directly and accurately. If the forced convection heat transfer mode is adopted outside the wall surface, larger speed fluctuation is generated in the liquid flowing process, impact and vibration are generated on the wall of the test tube, and external disturbance such as impact and vibration has great influence on the supercooling degree measurement precision. And the heat flow in the heating process is obtained by directly testing the current and voltage of the heating belt. But the heat flow in the cooling process is difficult to directly obtain by adopting the method.
Disclosure of Invention
The invention provides a method and a device for testing supercooling heat boundary in the solidification process in a circular tube, which can solve the problems by adopting a method for testing natural convection heat transfer coefficient when phase change materials outside a wall surface coexist in solid-liquid under the condition of no external impact, vibration and other strong disturbance. The method can avoid the inconvenience caused by using the external heat insulation material and the influence of temperature fluctuation caused by heat exchange with the outside.
The aim of the invention is achieved by the following technical scheme:
the device for testing the supercooling degree thermal boundary of the solidification process in the circular tube comprises a circular tube of liquid to be tested, wherein the circular tube of liquid to be tested is placed in a constant temperature container of phase change material, a fourth thermometer is arranged in the constant temperature container of phase change material, a second thermometer is arranged on the outer wall of the circular tube of liquid to be tested in a close fit manner, and a first thermometer is arranged in the circular tube of liquid to be tested. A refrigerating device and a stirring device are arranged in the phase-change material constant temperature container.
According to the device for testing the supercooling degree thermal boundary in the circular tube during solidification, the phase change material constant temperature container is a constant temperature water bath, the fourth thermometer is arranged in the constant temperature water bath, the second thermometer is tightly attached to the outer wall of the circular tube of the liquid to be tested, and the first thermometer is arranged in the circular tube of the liquid to be tested.
The device for testing the supercooling degree thermal boundary in the circular tube in the solidification process comprises a constant-temperature water bath, wherein a solid-liquid phase-change material container is sleeved in the constant-temperature water bath, a liquid circular tube to be tested is sleeved in the solid-liquid phase-change material container, a first thermometer is arranged in the middle of the circular tube to be tested, a second thermometer is arranged at the position, close to the circular tube to be tested, of the solid-liquid phase-change material container, a third thermometer is arranged at the position, close to the constant-temperature water bath, of the inner wall of the solid-liquid phase-change material container, a fourth thermometer is arranged in the constant-temperature water bath, and a refrigerating device and a stirring device are arranged in the constant-temperature water bath.
The method for testing the supercooling thermal boundary in the solidification process in the circular tube comprises the following steps: a. injecting the liquid to be measured into the circular tube of the liquid to be measured, and keeping the initial temperature of the liquid to be measured higher than the theoretical freezing point and lower than the theoretical boiling point;
d. the method comprises the steps that a phase change material thermostatic container is filled with a secondary refrigerant, a refrigerating device and stirring equipment of the phase change material thermostatic container are started, the secondary refrigerant is cooled until the phase change material thermostatic container is in a solid-liquid coexisting state, namely, when the temperature of a fourth thermometer is constant, the refrigerating arrangement and the stirring arrangement are closed, and the count is recorded;
e. c, placing the liquid round tube to be measured in the step a into a constant temperature container made of phase change materials, and simultaneously recording the temperature values of the first thermometer and the second thermometer along with the time;
f. stopping the continuous time until the temperatures of the first thermometer and the second thermometer are kept stable and unchanged, monitoring the fourth thermometer, and ensuring that the count of the fourth thermometer is unchanged during a cooling experiment;
g. the temperature profile at supercooling degree test is divided into three parts: a liquid region, a solid-liquid coexisting region and a solid region.
The method for testing the supercooling thermal boundary in the solidification process in the circular tube comprises the following steps:
a. injecting the liquid to be measured into the circular tube of the liquid to be measured, and keeping the initial temperature of the liquid to be measured higher than the theoretical freezing point and lower than the theoretical boiling point;
b. the constant-temperature water bath is filled with liquid with the solidifying point lower than that of the liquid to be measured as a secondary refrigerant, the constant-temperature water bath is started, the temperature is reduced to be lower than the actual crystallization temperature of the liquid to be measured, and the constant-temperature water bath temperature is kept;
c. placing the solid-liquid phase change material container into a constant-temperature water bath, and filling the phase change material container with the phase change material to enable the phase change material to be a solid-liquid mixture; the temperature of the solid-liquid mixture is consistent with the temperature of the secondary refrigerant in the constant-temperature water bath;
d. starting a constant-temperature water bath of the constant-temperature water bath tank to cool the secondary refrigerant until the phase change material in the solid-liquid phase change material container is in a solid-liquid coexisting state, namely, when the temperatures of the third thermometer and the fourth thermometer are consistent, closing refrigeration setting and stirring setting, and recording the counts of the first thermometer, the second thermometer, the third thermometer and the fourth thermometer;
e. c, placing the liquid round tube to be measured in the step a into a solid-liquid phase change material container, and simultaneously recording the temperature values of the first thermometer and the second thermometer along with the time change;
f. stopping the process when the duration time reaches the time when the temperatures of the first thermometer and the second thermometer are kept stable and unchanged, monitoring the third thermometer, and ensuring that the count of the third thermometer is unchanged during a cooling experiment;
g. the time-dependent curve of the supercooling degree test is divided into three parts: a liquid region, a solid-liquid coexisting region and a solid region.
According to the method for testing the supercooling heat boundary in the solidification process in the circular tube, the calculation formula of the average heat flow density of the three areas and the calculation of the average convective heat transfer coefficient in the period of time are disclosed as follows:
(1) Pure liquid and pure solid regions:
a certain time period t of the pure liquid and pure solid cooling zone i Average heat flux density Q in i Calculation formula of (W) and average convection heat exchange coefficient h in the period i (W/m 2 DEG C) is calculated as follows:
Q i ＝m.c p,j .(T 0,i -T l,i )/t i ； (1)
H i ＝Q 1,i /A/(T w1,i -T m )； (2)
wherein T is 0,i ,T l,i Respectively t is i Average initial temperature and average final temperature of the test cross section in the sample tube in the time period; the calculation method of the two average temperatures is as follows: the center temperature T of the cylinder obtained by the test of the first thermometer cent And an inner wall surface temperature T at a radius calculated by a transient heat conduction method R Is used for the arithmetic mean of (a),
i.e. T 0,i ＝(T 0,i,cent +T 0,i,R )/2,T l,i ＝(T l,i,cent +T l,i,R )/2
Subscripts cent and R respectively represent the positions of the center and the radius of the test section;
analytical solution of temperature at cylinder radius can be obtained according to transient heat conduction process
T R ＝(T cent -T m ).cos(β i )+T m (3)
β i Is an overrunning equation tan beta i ＝Bi/β i Is used for the root of (a),
specific number of schlieren Bi=h at radius i .R/λ j (4)
Because of being a hidden function, solving for h i When the temperature is measured, the initial temperature and the final temperature of the outer wall of the sample tube are first read by a second thermometer in a time period i, and a value T slightly larger than the initial temperature and the final temperature is assumed 0 , i,R ' and T l,i,R ' then calculate T 0,i And T l,i And according to equation (1) and equation (2), respectively calculate Q i ' and h i 'A'; then re-apply h i ' after bringing in equation (4), a new T is found by equation (3) 0,i,R "sum T l,i,R "; then comparing the initial assumption value with the calculated difference between the two parameters, i.e. |T 0,i,R ’-T 0,i,R "|and|T l,i,R ’-T l,i,R "|", if the difference is greater than 0.01, T is used 0,i,R "sum T l,i,R "replace the value T of the initial hypothesis respectively 0,i,R ' and T l,i,R ' re-calculating until the difference is less than 0.01, and finally obtaining hi; in addition, at the initial time when the liquid sample is just put into the system to start the test, since the whole initial sample temperature is uniform, T 0,i Directly equal to the center temperature T of the sample 0,i,cent ；
T w1,i For t is i Average temperature of the wall surface of the sample tube in the time period; c when the sample is in a pure liquid state p,j ，λ j Specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) of liquid, respectively, C when the sample is in pure solid state p,j ，λ j Specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) in solid state, respectively;
(2) Average heat flux density Q of liquid cooling area in phase change process 2 Calculation formula of (W) and average convection heat exchange coefficient h in the period 2 (W/m 2 DEG C) is calculated as follows:
Q 2 ＝L t.m /t ls ；
H 2 ＝Q 2 /A/(T w,ls -T m )；
the material of the slender straight tube (cylinder) is thin-wall stainless steel, the outer diameter D (m) and the inner diameter D (m) of the straight tube are adopted, the liquid level height of the liquid to be measured after the liquid is poured into the straight tube is L (m), the heat conductivity coefficient lambda (W/m.K) of the metal tube is adopted, and the temperature of the third thermometer is the temperature T of solid-liquid coexistence m ；
Density ρ (kg/m) of liquid sample 3 ) Initial temperature T of sample 0 Too muchMinimum temperature point T of cold stage c The theoretical solidifying point of the sample solution is T s Lt (J/kg) is the latent heat of liquid-solid phase change; in the solid-liquid coexisting stage, the time period from the lowest point to the completely solidified sample point is t l,s The average temperature of the outer side wall of the sample tube in the time period is T w,ls The method comprises the steps of carrying out a first treatment on the surface of the The final stable temperature in the solid state stage is T e The method comprises the steps of carrying out a first treatment on the surface of the All temperature units involved are in degrees celsius and the time units are seconds;
volume of liquid to be measured V (m 3 )：V＝π(d/2) 2 L；
Mass m (kg) of liquid to be measured: m=ρv;
area of convective heat transfer A (m) 2 )：A＝πDL。
By adopting the technical scheme, the invention has the beneficial effects that:
the technology can solve the technical problem that the strong original speed fluctuation generated in the forced convection heat transfer mode generates impact and vibration on the test tube wall by testing the natural convection heat transfer coefficient when the phase change materials outside the wall coexist in solid-liquid under the condition of no external impact, vibration and other strong disturbance. The invention can avoid the use of external heat insulating material on the outer wall surface, thereby avoiding the inconvenience caused by the use of external heat insulating material and avoiding the influence of temperature fluctuation caused by heat exchange with the outside.
Drawings
Fig. 1 is a schematic structural view of the device of the present invention.
FIG. 2 is a graph showing temperature versus time for a supercooling test.
Detailed Description
The structure and method of the present invention are described in detail below with reference to fig. 1 and 2.
The device for testing the supercooling degree thermal boundary of the solidification process in the circular tube comprises a circular tube of liquid to be tested, wherein the circular tube of liquid to be tested is placed in a constant temperature container of phase change material, a fourth thermometer is arranged in the constant temperature container of phase change material, a second thermometer is arranged on the outer wall of the circular tube of liquid to be tested in a close fit manner, and a first thermometer is arranged in the circular tube of liquid to be tested. A refrigerating device and a stirring device are arranged in the phase-change material constant temperature container. In the invention, the phase change material constant temperature container is a constant temperature water bath, the fourth thermometer is arranged in the constant temperature water bath, the second thermometer is closely attached to the outer wall of the liquid circular tube to be measured, and the first thermometer is arranged in the liquid circular tube to be measured.
The method for testing the supercooling thermal boundary of the solidification process in the circular tube by using the device for testing the supercooling thermal boundary of the solidification process in the circular tube comprises the following steps:
a. injecting the liquid to be measured into the circular tube of the liquid to be measured, and keeping the initial temperature of the liquid to be measured higher than the theoretical freezing point and lower than the theoretical boiling point;
d. the method comprises the steps that a phase change material thermostatic container is filled with a secondary refrigerant, a refrigerating device and stirring equipment of the phase change material thermostatic container are started, the secondary refrigerant is cooled until the phase change material thermostatic container is in a solid-liquid coexisting state, namely, when the temperature of a fourth thermometer is constant, the refrigerating arrangement and the stirring arrangement are closed, and the count is recorded;
e. c, placing the liquid round tube to be measured in the step a into a constant temperature container made of phase change materials, and simultaneously recording the temperature values of the first thermometer and the second thermometer along with the time;
f. stopping the continuous time until the temperatures of the first thermometer and the second thermometer are kept stable and unchanged, monitoring the fourth thermometer, and ensuring that the count of the fourth thermometer is unchanged during a cooling experiment;
g. the temperature profile at supercooling degree test is divided into three parts: a liquid region, a solid-liquid coexisting region and a solid region.
The second structure of the device for testing supercooling thermal boundary in the solidification process in the circular tube is as follows: the device comprises a liquid circular tube 3 to be measured, wherein the liquid circular tube to be measured is placed in a phase change material constant temperature container, a fourth thermometer 7 is arranged in the phase change material constant temperature container, a second thermometer 5 is arranged on the outer wall of the liquid circular tube to be measured in a clinging manner, and a first thermometer is arranged in the liquid circular tube to be measured. A refrigerating device and a stirring device are arranged in the phase-change material constant temperature container. The constant temperature container of the phase change material comprises a constant temperature water bath 1, a solid-liquid phase change material container 2 is sleeved in the constant temperature water bath, a round pipe sleeve 3 of liquid to be measured is arranged in the solid-liquid phase change material container, a first fourth thermometer is arranged in the middle of the round pipe of the liquid to be measured, a second thermometer 5 is arranged at the position, close to the round pipe of the liquid to be measured, of the solid-liquid phase change material container, a third thermometer 6 is arranged at the position, close to the constant temperature water bath, of the inner wall of the solid-liquid phase change material container, a fourth thermometer 7 is arranged in the constant temperature water bath, and a refrigerating device and a stirring device are arranged in the constant temperature water bath.
The method for testing the supercooling thermal boundary of the solidification process in the circular tube by using the device for testing the supercooling thermal boundary of the solidification process in the circular tube comprises the following steps:
a. injecting the liquid to be measured into the circular tube of the liquid to be measured, and keeping the initial temperature of the liquid to be measured higher than the theoretical freezing point and lower than the theoretical boiling point;
b. the constant-temperature water bath is filled with liquid with the solidifying point lower than that of the liquid to be measured as a secondary refrigerant, the constant-temperature water bath is started, the temperature is reduced to be lower than the actual crystallization temperature of the liquid to be measured, and the constant-temperature water bath temperature is kept;
c. placing the solid-liquid phase change material container into a constant-temperature water bath, and filling the phase change material container with the phase change material to enable the phase change material to be a solid-liquid mixture; the temperature of the solid-liquid mixture is consistent with the temperature of the secondary refrigerant in the constant-temperature water bath;
d. starting a constant-temperature water bath of the constant-temperature water bath tank to cool the secondary refrigerant until the phase change material in the solid-liquid phase change material container is in a solid-liquid coexisting state, namely, when the temperatures of the third thermometer and the fourth thermometer are consistent, closing refrigeration setting and stirring setting, and recording the counts of the first thermometer, the second thermometer, the third thermometer and the fourth thermometer;
e. c, placing the liquid round tube to be measured in the step a into a solid-liquid phase change material container, and simultaneously recording the temperature values of the first thermometer and the second thermometer along with the time change;
f. stopping the process when the duration time reaches the time when the temperatures of the first thermometer and the second thermometer are kept stable and unchanged, monitoring the third thermometer, and ensuring that the count of the third thermometer is unchanged during a cooling experiment;
g. the time-dependent curve of the supercooling degree test is divided into three parts: a liquid region, a solid-liquid coexisting region and a solid region.
According to the method for testing the supercooling heat boundary in the solidification process in the circular tube, the calculation formula of the average heat flow density of the three areas and the calculation of the average convective heat transfer coefficient in the period of time are disclosed as follows:
(1) Pure liquid and pure solid regions:
a certain time period t of the pure liquid and pure solid cooling zone i Average heat flux density Q in i Calculation formula of (W) and average convection heat exchange coefficient h in the period i (W/m 2 DEG C) is calculated as follows:
Q i ＝m.c p,j .(T 0,i -T l,i )/t i ； (1)
H i ＝Q 1,i /A/(T w1,i -T m )； (2)
wherein T is 0,i ,T l,i Respectively t is i Average initial temperature and average final temperature of the test cross section in the sample tube in the time period; the calculation method of the two average temperatures is as follows: the center temperature T of the cylinder obtained by the test of the first thermometer cent And an inner wall surface temperature T at a radius calculated by a transient heat conduction method R Is used for the arithmetic mean of (a),
i.e. T 0,i ＝(T 0,i,cent +T 0,i,R )/2,T l,i ＝(T l,i,cent +T l,i,R )/2
Subscripts cent and R respectively represent the positions of the center and the radius of the test section;
analytical solution of temperature at cylinder radius can be obtained according to transient heat conduction process
T R ＝(T cent -T m ).cos(β i )+T m (3)
β i Is an overrunning equation tan beta i ＝Bi/β i Is used for the root of (a),
specific number of schlieren Bi=h at radius i .R/λ j (4)
Because of being a hidden function, solving for h i When the temperature is measured, the initial temperature and the final temperature of the outer wall of the sample tube are first read by a second thermometer in a time period i, and a value T slightly larger than the initial temperature and the final temperature is assumed 0 , i,R ' and T l,i,R ' then calculate T 0,i And T l,i And according to equation (1) and equation (2), respectively calculate Q i ' and h i 'A'; then re-apply h i ' after bringing in equation (4), a new T is found by equation (3) 0,i,R "sum T l,i,R "; then comparing the initial assumption value with the calculated difference between the two parameters, i.e. |T 0,i,R ’-T 0,i,R "|and|T l,i,R ’-T l,i,R "|", if the difference is greater than 0.01, T is used 0,i,R "sum T l,i,R "replace the value T of the initial hypothesis respectively 0,i,R ' and T l,i,R ' re-calculating until the difference is less than 0.01, and finally obtaining hi; in addition, at the initial time when the liquid sample is just put into the system to start the test, since the whole initial sample temperature is uniform, T 0,i Directly equal to the center temperature T of the sample 0,i,cent ；
T w1,i For t is i Average temperature of the wall surface of the sample tube in the time period; c when the sample is in a pure liquid state p,j ，λ j Specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) of liquid, respectively, C when the sample is in pure solid state p,j ，λ j Specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) in solid state, respectively;
(2) Average heat flux density Q of liquid cooling area in phase change process 2 Calculation formula of (W) and average convection heat exchange coefficient h in the period 2 (W/m 2 DEG C) is calculated as follows:
Q 2 ＝L t.m /t ls ；
H 2 ＝Q 2 /A/(T w,ls -T m )；
the material of the slender straight tube (cylinder) is thin-wall stainless steel, the outer diameter D (m) and the inner diameter D (m) of the straight tube are adopted, the liquid level height of the liquid to be measured after the liquid is poured into the straight tube is L (m), the heat conductivity coefficient lambda (W/m.K) of the metal tube is adopted, and the temperature of the third thermometer is the temperature T of solid-liquid coexistence m ；
Density ρ (kg/m) of liquid sample 3 ) Initial temperature T of sample 0 Minimum temperature point T of supercooling stage c The theoretical solidifying point of the sample solution is T s Lt (J/kg) is the latent heat of liquid-solid phase change; in the solid-liquid coexisting stage, the time period from the lowest point to the completely solidified sample point is t l,s The average temperature of the outer side wall of the sample tube in the time period is T w,ls The method comprises the steps of carrying out a first treatment on the surface of the The final stable temperature in the solid state stage is T e The method comprises the steps of carrying out a first treatment on the surface of the All temperature units involved are in degrees celsius and the time units are seconds;
volume of liquid to be measured V (m 3 )：V＝π(d/2) 2 L；
Mass m (kg) of liquid to be measured: m=ρv;
area of convective heat transfer A (m) 2 )：A＝πDL。
In the invention, when the experiment is performed, water is used as the liquid to be tested, and the specific experiment is as follows:
in order to test the supercooling degree of water during icing, water is firstly injected into a clean and dry slender cylinder as a liquid to be tested (the cylinder needs to keep a larger length-diameter ratio so as to reduce the influence of natural convection on radial heat transfer when the inner wall surface of the cylinder is cooled), and the initial temperature is kept above the theoretical freezing point for a period of time so as to achieve thermodynamic equilibrium. The initial temperature of the liquid water to be measured is higher than 0 ℃ of the theoretical freezing point under normal pressure and lower than 100 ℃ of the boiling point, and the specific initial temperature can be determined according to actual conditions so as to avoid the situation that the liquid with too high temperature evaporates or the heat is too large to be reduced below the temperature required by supercooling.
(2) The water tank of the constant temperature water bath is filled with liquid with low freezing point as a secondary refrigerant, such as water and glycol mixed liquid with the freezing point of minus 30 ℃. And (3) starting the constant-temperature water bath to enable the highest temperature of the secondary refrigerant in the water bath to be at least reduced below the actual crystallization temperature of the liquid to be detected, wherein the temperature is selected according to experimental conditions, such as-14 ℃. The secondary refrigerant can be crystallized, namely a solid-liquid mixed state, the latent heat of phase change is used for providing cold energy, and the external environment temperature far away from the outer wall side of the sample cylinder to be tested can be kept constant for a long time after the refrigerating unit is shut down and the experiment is finished.
(3) And (3) placing a solid-liquid phase change material container filled with a phase change material in the constant-temperature water bath tank (if the secondary refrigerant is in a solid-liquid mixed state in the second step, the solid-liquid phase change material container is not arranged, and a liquid round tube to be measured is directly placed in the constant-temperature water bath tank), wherein the proportion of the phase change material needs to be adjusted according to specific experimental conditions so as to generate a solid-liquid mixture, the temperature of the initially stabilized solid-liquid mixture is kept consistent with the stable temperature of the secondary refrigerant in the external water bath, then latent heat is released through phase change so as to compensate the loss of external heat dissipation, and finally, the temperature of an accessory on the inner wall side of the solid-liquid phase change material container is kept unchanged. The amount of the solid-liquid coexisting phase change material in the container and the amount of the solid generated by crystallization thereof need to be in a solid-liquid mixed state all the time in the whole experimental period, when the external temperature changes, the stable external temperature environment is ensured for the experimental liquid to be tested, and the phenomenon that the external environment temperature of the liquid to be tested changes greatly due to complete melting of the solid cannot occur. For example, a mixed solution of water and glycol of about 28% can be filled, so that enough solid ice particles appear at-14 ℃ to prevent the temperature rise after the solid in the solid-liquid mixture is completely melted when the external refrigerating cycle of the secondary refrigerant is stopped for avoiding vibration and impact in the experimental process.
(4) And when the experiment is carried out, a constant-temperature water bath is started, so that the temperature of the refrigerating medium at the outermost layer is reduced to a certain proper low temperature (the temperature can be adjusted according to the experiment requirement), and after a period of heat transfer is stable, the phase change material in the solid-liquid phase change material container is in a solid-liquid coexisting state with proper solid content. When the solid-liquid phase change material container is far away from the third thermometer of the liquid round tube to be measured (can be near the inner side wall of the solid-liquid phase change material container, but does not need to be closely attached to the inner side wall, and the solid-liquid coexistence area is guaranteed), and the temperature of the fourth thermometer of the constant-temperature water bath is stable, the readings are recorded. The cooling and stirring system of the thermostatic water bath is then turned off to eliminate external vibration. And then placing the liquid round tube to be tested with stable initial temperature into a solid-liquid phase change material container with solid-liquid coexisting state, and simultaneously recording the temperature changes of the first thermometer of the liquid round tube to be tested and the second thermometer which is tightly attached to the outer side wall of the cylinder wall along with time. And stopping the experiment until the temperatures of the two thermometers 1 and 2 are kept basically stable and unchanged, monitoring the temperature of the third thermometer, and ensuring that the temperature of the third thermometer is in a solid-liquid coexisting region in the cooling experiment process and is unchanged.
(6) The calculation method of the heat flux density in a certain period of time comprises the following steps: the temperature change curve with time during supercooling degree test is shown in the following graph, and is generally divided into three parts, namely a liquid region, a solid-liquid coexisting region and a solid region, as shown in fig. 2.
The material of the slender straight tube (cylinder) is thin-wall stainless steel, the outer diameter D (m) and the inner diameter D (m) of the straight tube are adopted, the liquid level height of the liquid to be measured after the liquid is poured into the straight tube is L (m), the heat conductivity coefficient lambda (W/m.K) of the metal tube is adopted, and the temperature of the third thermometer is the temperature T of solid-liquid coexistence m ；
Density ρ (kg/m) of liquid sample 3 ) Initial temperature T of sample 0 Minimum temperature point T of supercooling stage c The theoretical solidifying point of the sample solution is T s Lt (J/kg) is the latent heat of liquid-solid phase change. In the solid-liquid coexisting stage, the time period from the lowest point to the completely solidified sample point is t l,s The average temperature of the outer side wall of the sample tube in the time period is T w,ls The method comprises the steps of carrying out a first treatment on the surface of the The final stable temperature in the solid state stage is T e . All temperature units involved are in degrees celsius and time units are seconds.
Volume of liquid to be measured V (m 3 )：V＝π(d/2) 2 L；
Mass m (kg) of liquid to be measured: m=ρv;
area of convective heat transfer A (m) 2 )：A＝πDL；
(1) Pure liquid and pure solid regions:
a certain time period t of the pure liquid and pure solid cooling zone i Average heat flux density Q in i Calculation formula of (W) and average convection heat exchange coefficient h in the period i (W/m 2 DEG C) is calculated as follows:
Q i ＝m.c p,j .(T 0,i -T l,i )/t i ； (1)
H i ＝Q 1,i /A/(T w1,i -T m )； (2)
wherein T is 0,i ,T l,i Respectively t is i Average initial and average final temperatures of the test cross-sections in the sample tubes over a period of time. The calculation method of the two average temperatures is as follows: the center temperature T of the cylinder obtained by the test of the first thermometer cent And an inner wall surface temperature T at a radius calculated by a transient heat conduction method R Is used for the arithmetic mean of (a),
i.e. T 0,i ＝(T 0,i,cent +T 0,i,R )/2,T l,i ＝(T l,i,cent +T l,i,R )/2
Subscripts cent and R denote the location of the center and radius, respectively, of the test section.
Analytical solution of temperature at cylinder radius can be obtained according to transient heat conduction process
T R ＝(T cent -T m ).cos(β i )+T m (3)
β i Is an overrunning equation tan beta i ＝Bi/β i Is used for the root of (a),
specific number of schlieren Bi=h at radius i .R/λ j (4)
Because of being a hidden function, solving for h i When the temperature is measured, the initial temperature and the final temperature of the outer wall of the sample tube are first read by a second thermometer in a time period i, and a value T slightly larger than the initial temperature and the final temperature is assumed 0 , i,R ' and T l,i,R ' then calculate T 0,i And T l,i And according to equation (1) and equation (2), respectively calculate Q i ' and h i 'A'; then re-apply h i ' after bringing in equation (4), a new T is found by equation (3) 0,i,R "sum T l,i,R "; then comparing the initial assumption value with the calculated difference between the two parameters, i.e. |T 0,i,R ’-T 0,i,R "|and|T l,i,R ’-T l,i,R "|", if the difference is greater than 0.01, T is used 0 , i,R "sum T l,i,R "replace the value T of the initial hypothesis respectively 0,i,R ' and T l,i,R ' re-calculation until the iterative calculation with the difference less than 0.01 ends, thereby finally obtaining hi. In addition, at the initial time when the liquid sample is just put into the system to start the test, the liquid sample is initiallyThe whole temperature of the initial sample is uniform, so T 0,i Directly equal to the center temperature T of the sample 0,i,cent 。
T w1,i For t is i Average temperature of the sample tube wall surface over a period of time. When the sample is in a pure liquid state Cp, j ，λ j the specific heat capacity (J/kg. ℃) and the heat conductivity coefficient (W/m.K) of the liquid, respectively, cp when the sample is in a pure solid state, j ，λ j specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) in the solid state, respectively.
(2) Average heat flux density Q of liquid cooling area in phase change process 2 Calculation formula of (W) and average convection heat exchange coefficient h in the period 2 (W/m 2 DEG C) is calculated as follows:
Q 2 ＝L t.m /t ls ；
H 2 ＝Q 2 /A/(T w,ls -T m )。
while only the preferred embodiments of the present invention have been described above, it should be noted that it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the general inventive concept, and these should also be regarded as the scope of the invention, which is not to be limited to the effects of the invention in its practice or the application of the patent.
Claims (1)
1. The utility model provides a device of solidification process supercooling degree thermal boundary in test pipe, includes liquid pipe that awaits measuring, its characterized in that: the liquid round tube to be measured is placed in a phase change material constant temperature container, a fourth thermometer is arranged in the phase change material constant temperature container, a second thermometer is arranged on the outer wall of the liquid round tube to be measured in a close fit manner, and the first thermometer is arranged in the liquid round tube to be measured; the constant-temperature container of the phase-change material comprises a constant-temperature water bath, a solid-liquid phase-change material container is sleeved in the constant-temperature water bath, a liquid circular tube to be tested is sleeved in the solid-liquid phase-change material container, a third thermometer is arranged at the position, close to the constant-temperature water bath, of the inner wall of the solid-liquid phase-change material container, and a fourth thermometer is arranged in the constant-temperature water bath;
the method for testing the supercooling thermal boundary of the solidification process in the circular tube by using the device for testing the supercooling thermal boundary of the solidification process in the circular tube comprises the following steps:
a. injecting the liquid to be measured into the circular tube of the liquid to be measured, and keeping the initial temperature of the liquid to be measured higher than the theoretical freezing point and lower than the theoretical boiling point;
b. the constant-temperature water bath is filled with liquid with the solidifying point lower than that of the liquid to be measured as a secondary refrigerant, the constant-temperature water bath is started, the temperature is reduced to be lower than the actual crystallization temperature of the liquid to be measured, and the constant-temperature water bath temperature is kept;
c. placing the solid-liquid phase change material container into a constant-temperature water bath, and filling the phase change material container with the phase change material to enable the phase change material to be a solid-liquid mixture; the temperature of the solid-liquid mixture is consistent with the temperature of the secondary refrigerant in the constant-temperature water bath;
d. starting a constant-temperature water bath of the constant-temperature water bath tank to cool the secondary refrigerant until the phase change material in the solid-liquid phase change material container is in a solid-liquid coexisting state, namely, when the temperatures of the third thermometer and the fourth thermometer are consistent, closing refrigeration setting and stirring setting, and recording the counts of the first thermometer, the second thermometer, the third thermometer and the fourth thermometer;
e. c, placing the liquid round tube to be measured in the step a into a solid-liquid phase change material container, and simultaneously recording the temperature values of the first thermometer and the second thermometer along with the time change;
f. stopping the process when the duration time reaches the time when the temperatures of the first thermometer and the second thermometer are kept stable and unchanged, monitoring the third thermometer, and ensuring that the count of the third thermometer is unchanged during a cooling experiment;
g. the temperature change curve with time during supercooling degree test is divided into three parts: a liquid region, a solid-liquid coexisting region and a solid region;
the calculation formula of the average heat flux density of the three areas and the calculation of the average convective heat transfer coefficient in the period of time are disclosed as follows:
(1) Pure liquid and pure solid regions:
a certain time period t for cooling the pure liquid region and the pure solid region i Average heat flux density Q in i Calculation formula of (W) and average convection heat transfer coefficient H in the period i (W/m 2 DEG C) calculationThe formula is as follows:
Q i =m.c p,j .(T 0,i -T l,i )/t i ； （1）
H i =Q 1,i /A/(T w1,i -T m )； （2）
wherein T is 0,i , T l,i Respectively t is i Average initial temperature and average final temperature of the test cross section in the sample tube in the time period; the calculation method of the two average temperatures is as follows: the center temperature T of the cylinder obtained by the test of the first thermometer cent And an inner wall surface temperature T at a radius calculated by a transient heat conduction method R Is used for the arithmetic mean of (a),
i.e. T 0,i =(T 0,i,cent +T 0 , i,R )/2, T l,i =(T l,i,cent +T l , i,R )/2
Subscripts cent and R respectively represent the positions of the center and the radius of the test section;
analytical solution of temperature at cylinder radius can be obtained according to transient heat conduction process
T R =(T cent -T m ) . cos(β i )+T m （3）
β i Is an overrunning equation tan beta i =Bi/β i Is used for the root of (a),
specific number of schlieren bi=h at radius i .R/λ j （4）
Because of being a hidden function, solving for H i When the temperature is measured, the initial temperature and the final temperature of the outer wall of the sample tube are firstly read in a time period i by referring to a second thermometer, and a first is assumedA value T slightly greater than the reading 0 , i,R ' and T l , i,R ' then calculate T 0,i And T l,i And according to equation (1) and equation (2), respectively calculate Q i ' and H i 'A'; then re-apply H i ' after bringing in equation (4), a new T is found by equation (3) 0 , i,R ' and T l , i,R ''s; then comparing the initial assumption value with the calculated difference between the two parameters, i.e. |T 0 , i,R ’- T 0 , i,R ' and T l , i,R ’- T l , i,R ' if the difference is greater than 0.01, T is used 0 , i,R ' and T l , i,R ' replaces the initially assumed value T respectively 0 , i,R ' and T l , i,R ' re-calculating until the difference is less than 0.01, and finally obtaining Hi; in addition, at the initial time when the liquid sample is just put into the system to start the test, since the whole initial sample temperature is uniform, T 0,i Directly equal to the center temperature T of the sample 0,i,cent ；
T w1,i For t is i Average temperature of the wall surface of the sample tube in the time period; when the sample is in a pure liquid stateC p,j ，λ j The specific heat capacity (J/kg. ℃) and the heat conductivity coefficient (W/m.K) of the liquid are respectively, when the sample is in a pure solid state,C p,j ，λ j specific heat capacity (J/kg. ℃) and thermal conductivity (W/m.K) in solid state, respectively;
(2) Average heat flux density Q of pure liquid region of phase transition process 2 Calculation formula of (W) and average convection heat transfer coefficient H in the period 2 (W/m 2 DEG C) is calculated as follows:
Q 2 =L t.m /t ls ；
H 2 =Q 2 /A/(T w,ls -T m ) ；
the cylinder material of the slender straight tube is thin-wall stainless steel, the outer diameter D (m) and the inner diameter D (m) of the straight tube are adopted, the liquid level height of the liquid to be measured after the liquid to be measured is poured into the straight tube is L (m), the heat conductivity coefficient lambda (W/m.K) of the metal tube is the temperature T of solid-liquid coexistence m ；
Density ρ (kg/m) of liquid sample 3 ) Initial temperature T of sample 0 Minimum temperature point T of supercooling stage c The theoretical solidifying point of the sample solution is T s Lt (J/kg) is the latent heat of liquid-solid phase change; in the solid-liquid coexisting stage, the time period from the lowest point to the completely solidified sample point is t l,s The average temperature of the outer side wall of the sample tube in the time period is T w,ls The method comprises the steps of carrying out a first treatment on the surface of the The final stable temperature in the solid state stage is T e The method comprises the steps of carrying out a first treatment on the surface of the All temperature units involved are in degrees celsius and the time units are seconds;
volume of liquid to be measured V (m 3 ）：V=π(d/2) 2 L；
Mass m (kg) of liquid to be measured: m=ρv;
area of convective heat transfer A (m) 2 )：A=πDL。
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