A method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry

Abstract

The present invention discloses a method for measuring the phase change enthalpy of a fluid material based on distributed flow calorimetry. The present invention uses a distributed flow calorimeter as an experimental device, passes the fluid into a test pipeline, and accurately controls the experimental conditions such as the flow rate of the fluid, the pipeline heating power, and the pressure inside the pipe, and scans and obtains the temperature data on the surface of the pipeline, thereby deriving the phase change enthalpy of the fluid material. Compared with the traditional static measurement method, the present invention can perform continuous measurement under the flow state, and is particularly suitable for the dynamic performance evaluation of phase change materials under different pressure states. At the same time, the method also greatly simplifies the operation process, reduces the interference of external factors, and greatly improves the measurement accuracy and applicability.

Description

Technical Field

[0001] The present invention relates to the technical field of thermodynamic parameter measurement of fluid materials, and more specifically to a method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry. Background Art

[0002] Phase change enthalpy describes the heat exchanged between a substance and the environment when it undergoes a phase change at a constant temperature and pressure. It is one of the important parameters that reflect the thermal effect of a material's phase change process. This parameter is of great significance for understanding the thermal stability, thermal management efficiency, phase change latent heat storage, and the development of new functional materials. Therefore, it is particularly important to develop an efficient, accurate, and widely applicable method for measuring phase change enthalpy.

[0003] At present, the methods for measuring phase change enthalpy mainly include differential scanning calorimetry (DSC), synchronous thermal analysis (STA), etc. Among them, differential scanning calorimetry is one of the most commonly used methods for measuring phase change enthalpy. This method determines the phase change enthalpy of the sample by measuring the change of heat flow rate (or power difference) between the sample and the reference material with time or temperature at a program-controlled temperature. However, the traditional DSC method is easily affected by various factors such as sample mass, heating rate, cooling rate, etc. during the measurement process, resulting in large errors in the measurement results. In addition, the enthalpy change of the fluid in the entire reaction tube can be obtained using a traditional flow calorimeter. However, it is difficult for a traditional flow calorimeter to obtain continuous temperature data on the surface of the pipeline, which makes it difficult to accurately obtain the interval where the fluid undergoes phase change in the pipeline, so the phase change enthalpy of the fluid material cannot be measured. Summary of the invention

[0004] In view of the problems existing in the traditional phase change enthalpy measurement method mentioned in the background technology, the present invention proposes a method for measuring the phase change enthalpy of fluid materials based on distributed flow calorimetry.

[0005] The specific steps of the present invention include the following:

[0006] Step 1: Under set pressure conditions, estimate the temperature range where the fluid undergoes phase change, and determine the experimental conditions that can cause the fluid to undergo phase change;

[0007] Step 2: Determine the time required for the test system to reach a stable state under the air traffic control state under the experimental conditions;

[0008] Step 3: After the test system reaches a stable state, calibrate the horizontal distance between the temperature probe and the pipeline;

[0009] Step 4: Pass fluid into the test pipe, and after the test system reaches a stable state, perform scanning and temperature measurement according to the distance calibration result;

[0010] Step 5: Establish a phase change enthalpy calculation equation, and obtain the fluid phase change enthalpy based on the data obtained from the scanning temperature measurement.

[0011] Compared with the existing fluid phase change enthalpy measurement method, the present invention has the following beneficial effects:

[0012] (1) The present invention precisely controls the experimental conditions such as the fluid mass flow rate, pipeline heating power, and pipe pressure, and measures the relevant parameters after the system reaches a stable state, thereby deriving the phase change enthalpy of the fluid material. This method reduces the interference of external factors such as sample mass, heating rate, and cooling rate on the measurement of the phase change enthalpy of the fluid material, greatly improving the accuracy and applicability of the measurement. In addition, this method uses a relatively large amount of sample during the measurement process, which is more accurate than methods such as DSC and STA.

[0013] (2) The present invention can obtain the phase change enthalpy of the fluid under different pressure states by setting the pressure conditions.

[0014] (3) The present invention can continuously measure various relevant parameters under the fluid flow state, and is particularly suitable for dynamic performance evaluation of the phase change process of fluid materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Fluid material phase change enthalpy measurement flow chart;

[0016] Figure 2 The core structure diagram of the distributed flow calorimetry system;

[0017] Figure 3 The relationship between temperature and enthalpy of water in NIST;

[0018] Figure 4 Pipe wall temperature and position relationship diagram;

[0019] Figure 5 The temperature corresponding to the measuring point. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] The present invention aims to provide a phase change enthalpy measurement method which is efficient, accurate and widely applicable. The method uses a distributed flow calorimeter as an experimental device. By passing a fluid into a test pipeline and accurately controlling experimental conditions such as the flow rate of the fluid, the pipeline heating power, and the pressure inside the pipe, the temperature data of the pipeline surface is scanned and obtained, thereby deducing the phase change enthalpy of the fluid material.

[0022] The present application discloses a method for measuring the phase change enthalpy of a fluid material based on distributed flow calorimetry. Figure 1 As shown, the specific steps are as follows:

[0023] Step 1: Under certain pressure conditions, estimate the temperature range in which the fluid undergoes phase change, and determine the experimental conditions that can cause the test fluid to undergo phase change;

[0024] Step 2: Determine the time required for the test system to reach a stable state in an empty pipe state (without fluid loaded) under experimental conditions;

[0025] Step 3: After the test system reaches a stable state, calibrate the horizontal distance between the temperature probe and the pipeline;

[0026] Step 4: Fluid is introduced into the test pipe, and after the system reaches a stable state, scanning and temperature measurement are performed according to the distance calibration result;

[0027] Step 5: Establish a phase change enthalpy calculation equation, and obtain the fluid phase change enthalpy based on the experimental data of the scanning temperature measurement.

[0028] The core structure of the distributed flow calorimeter for measuring the phase change enthalpy of fluid materials used in the embodiments of the present invention is as follows: Figure 2 As shown. During the experiment, a fluid sample is drawn at a constant flow rate, and the fluid is made to flow through the test portion of the pipe clamped by the two electrodes 5, and then the operating pressure in the pipe is adjusted by the pressure regulating device. A constant current source is used to apply current to the pipe clamped by the two electrodes, thereby heating the fluid.

[0029] In order to measure the phase change enthalpy, it is necessary to obtain multiple parameters such as mass flow, pressure, temperature, etc. The mass flowmeter 2 is used to collect the mass flow of the fluid; thermocouples are installed at the entrance and exit of the test part pipeline to measure the fluid temperature; pressure transmitters 3 are installed at the entrance and exit to measure the pressure in the pipe, and the sensor and the pipeline are connected by a switching valve 4. The movable infrared temperature measuring device 1 is used to scan and obtain the axial temperature of the pipe wall. The movable infrared temperature measuring device includes a thermal imager 1-1, an infrared temperature measuring probe 1-2, a longitudinal motion guide rail 1-3 and a transverse motion guide rail 1-4 (the longitudinal motion guide rail 1-3 and the transverse motion guide rail 1-4 constitute a two-dimensional mobile platform); the temperature is measured by the thermal imager 1-1 and the infrared temperature measuring probe 1-2, and the longitudinal motion guide rail 1-3 and the transverse motion guide rail 1-4 are used to track and scan the pipeline.

[0030] In a certain example, the step 1 specifically includes, first, adjusting the operating pressure in the pipe by connecting a pressure regulating device at the outlet of the pipeline flow loop, and installing pressure transmitters at the outlet and inlet of the pipeline test section to measure whether the pressure is accurate. Secondly, the determination of the experimental working conditions for the phase change of the test fluid includes the determination of the fluid mass flow rate and the excitation current applied to the pipeline. The selection of these two parameters requires attention to two factors: one is to ensure that the phase change can occur, and the other is to ensure that the jitter of the pipeline test section is small when selecting this parameter for the experiment, so as to facilitate scanning temperature measurement to obtain accurate pipe wall temperature data.

[0031] Specifically, the test fluid is introduced into the test system at a small mass flow rate, and the mass flow rate data of the fluid is obtained in real time. If the mass flow rate data of the test fluid is unstable, the mass flow rate of the fluid is appropriately increased to obtain an experimental condition with a relatively stable mass flow rate. However, the mass flow rate cannot be too large, as excessive mass flow rate will cause severe jitter in the test part of the pipeline.

[0032] Specifically, a small excitation current is first applied to both ends of the test pipe, and the fluid temperature at the pipe outlet is measured. After the pipe outlet temperature stabilizes, the excitation current applied to the pipe is increased in a certain step size. If the fluid temperature at the pipe outlet changes little after the excitation current is changed, that is, the change is less than a set threshold, the excitation current is continued to be increased. If the fluid temperature at the outlet changes dramatically compared to the previous measurement result, it is determined that the test fluid can undergo a phase change under this excitation current.

[0033] In a certain example, step 2 specifically involves that when different excitation currents are applied to the test portion of the pipeline, the time required for the pipeline surface temperature field to reach a stable state is different, so it is necessary to determine the time required for the system to reach a stable state when the empty pipe is heated with a determined excitation current when no fluid is passing through the system.

[0034] In one example, step 3 specifically involves calibrating the horizontal distance between the infrared temperature probe and the pipeline in an empty pipe state. When different excitation currents are applied to the pipeline, the pipeline will bend and deform, which will cause the distance between the pipeline and the thermal imager and the infrared temperature probe to change. Therefore, after the test system reaches a stable state, the horizontal distance between the infrared temperature probe and the pipeline needs to be calibrated.

[0035] The calibration in this example is to align the temperature measuring spot of the infrared temperature measuring probe with the center of the pipeline, record which pixel of the thermal imager the center of the pipeline is on at this time, and then calibrate the pixel position every certain distance. Subsequent scans are performed based on the calibrated pixel position.

[0036] In one example, step 4 specifically includes, after completing the horizontal distance calibration of the electrically heated empty pipe, waiting for the pipe to cool, then passing the fluid, and then applying the excitation current for heating. After waiting for the system to reach a stable state, scanning and measuring the temperature are performed according to the distance calibration result.

[0037] In one example, step 5 is specifically that, according to the energy conservation principle, when the electric heating of the empty pipe reaches a stable state, all the input energy of the system is lost through convection and radiation on the pipe surface. Therefore, the function of the heat loss power of the pipe wall can be obtained by the electric heating of the empty pipe.

[0038] When fluid is passed into the electric heating pipe, the energy conservation equation of the system in a stable state is equation (1), and the enthalpy change of the fluid in the entire test pipe can be calculated according to equation (2).

[0039] (1)

[0040] Among them, P Heater is the system heating power, in W; P Loss is the heat loss power of the pipe wall, unit is W; P Fluid is the heat absorption power of the fluid, in W.

[0041] (2)

[0042] Where △H is the enthalpy change of the fluid, in kJ / kg; m is the mass flow rate of the fluid, in g / s.

[0043] During the phase change of the fluid in the pipe, the fluid continuously absorbs heat, but the temperature of the fluid remains constant. Therefore, during the phase change of the fluid, the temperature of the outer wall of the corresponding position of the pipe also remains constant. Based on this constant temperature characteristic, the length of the interval where the pipe wall temperature remains constant can be used as an important basis for calculating the phase change enthalpy of the fluid.

[0044] Furthermore, to obtain the phase change enthalpy of the fluid, it is necessary to obtain the pipe wall temperature data, the electric heating power data, the fluid mass flow rate, and the length of the pipeline test section. The phase change enthalpy of the fluid material is calculated according to formula (3).

[0045] (3)

[0046] Among them, △H x is the phase change enthalpy of the fluid, in kJ / kg; The pipe surface temperature is The corresponding heat loss power; x is the length of the pipeline with constant surface temperature, unit is m; L is the total length of the test section of the pipeline, unit is m.

[0047] Water phase change enthalpy measurement experiment:

[0048] In order to verify the reliability of the method for measuring the phase change enthalpy of fluid materials based on distributed flow calorimetry, water was selected for the experiment to measure the phase change enthalpy of water at a standard atmospheric pressure of 0.1 MPa. Figure 3 This is the relationship curve between water temperature and enthalpy found by NIST (National Institute of Standards and Technology). It can be seen that the phase transition temperature of water under standard atmospheric pressure is 372.76 K and the phase transition enthalpy is 2257.4 kJ / kg.

[0049] Measurement conditions: pressure 0.1 MPa, volume flow rate 10 mL / min, current 50 A. It has been verified that water can complete phase change in the pipeline under this condition.

[0050] Experimental results: The measured water mass flow rate is 0.186 g / s, the electric heating power is 700 W, Figure 4 The wall temperature corresponding to different axial positions is scanned and measured. The average wall temperature measured in the phase change position interval corresponds to a total heat loss power of 28.86 W. Figure 5 There are a total of 1858 temperature measurement points, of which 1128 temperature measurement points were obtained in the phase change temperature change interval.

[0051] The phase change enthalpy of water calculated by formula (3) is 2190.60 kJ / kg. Compared with the NIST result, the relative error of the phase change enthalpy of water obtained by the method is -0.48%, which shows that the measurement result of the present invention is relatively reliable.

[0052] In summary, the present invention can perform continuous measurement in a flowing state, thereby obtaining the phase change enthalpy of a fluid material. At the same time, the method also greatly simplifies the operation process, reduces the interference of external factors, and greatly improves the measurement accuracy and applicability. The present invention has broad prospects in the research and development and application of phase change materials.

[0053] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry, characterized in that The method comprises the following steps: Step 1: Under set pressure conditions, estimate the temperature range where the fluid undergoes phase change, and determine the experimental conditions that can cause the fluid to undergo phase change; Step 2: Determine the time required for the test system to reach a stable state under the air traffic control state under the experimental conditions; Step 3: After the test system reaches a stable state, calibrate the horizontal distance between the temperature probe and the pipeline; Step 4: Pass fluid into the test pipe, and after the test system reaches a stable state, perform scanning and temperature measurement according to the distance calibration result; Step 5: Establish a phase change enthalpy calculation equation, and obtain the fluid phase change enthalpy based on the data obtained by the scanning temperature measurement; the phase change enthalpy calculation equation is: ； in ∆H x is the phase change enthalpy of the fluid, P Heater Heating power for the system, The pipe surface temperature is T ( x ) corresponds to the heat loss power, x is the length of the pipe where the surface temperature is constant, L is the total length of the tested section of the pipeline, m is the mass flow rate of the fluid.

2. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1, characterized in that: In step 1, it also includes adjusting the operating pressure in the pipe by connecting a pressure regulating device at the outlet of the pipeline flow loop, and installing pressure transmitters at the outlet and inlet of the pipeline test section to measure whether the pressure is accurate.

3. A method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1 or 2, characterized in that: In step 1, the determination of the experimental conditions under which the test fluid undergoes phase change includes the determination of the mass flow rate of the fluid and the excitation current applied to the pipeline.

4. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 3 is characterized in that: Pass the fluid into the test system and obtain the mass flow data of the fluid in real time. If the mass flow data of the fluid is unstable, increase the mass flow of the fluid to obtain an experimental condition with a relatively stable mass flow.

5. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 3, characterized in that: Apply the excitation current to both ends of the test pipe and measure the fluid temperature at the pipe outlet. After the pipe outlet temperature stabilizes, increase the excitation current applied to the pipe in a certain step size. If the fluid temperature change at the pipeline outlet is lower than the threshold value after changing the excitation current, the excitation current continues to be increased; If the fluid temperature at the outlet changes dramatically compared to the previous measurement result, it is determined that the fluid can undergo a phase change under this excitation current.

6. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1, characterized in that: In step 2, the time required for the test system to reach a stable state is the time required for the test system to reach stability when an empty pipe is heated with a determined excitation current in a state where no fluid flows through the test system.

7. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1, characterized in that: In step 3, the calibration is to align the temperature measuring spot of the temperature measuring probe with the center of the pipeline, record that the center of the pipeline is on a certain pixel of the thermal imager at this time, and then calibrate the pixel position once every certain distance.

8. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1, characterized in that: In step 4, the scanning temperature measurement utilizes a two-dimensional moving platform disposed above the pipeline.

9. The method for measuring phase change enthalpy of fluid materials based on distributed flow calorimetry according to claim 1, characterized in that: In step 5, the phase change enthalpy is determined based on the pipe wall temperature data, the electric heating power data, the fluid mass flow rate, and the length of the pipeline test section.

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