Method for determining condensation temperature of nitrogen under low vacuum pressure

Abstract

The invention discloses a method for determining the condensation temperature of nitrogen under low vacuum pressure, which comprises the following steps of: in one test, controlling and keeping the total pressure of a spray pipe inlet stable and unchanged through a pressure regulating system, regulating the total temperature of the spray pipe inlet to a higher total temperature through a temperature regulating system, and gradually reducing the total temperature of the spray pipe inlet according to a certain temperature interval, in the operation process of a wind tunnel, the total pressure of a nozzle inlet, the total temperature of the nozzle inlet and the pitot pressure of a nozzle outlet are monitored in real time, and the condensation temperature of nitrogen under low vacuum pressure is determined by comprehensively analyzing the data. By means of the method, the condensation temperature of the nitrogen under the low vacuum pressure can be accurately determined, and the defects of theoretical formula obtaining and traditional test obtaining methods are overcome. By means of the result, the operation total temperature of the hypersonic low-density wind tunnel is reasonably determined, the design difficulty of the heater can be reduced, and the overall energy matching scale of the wind tunnel can be reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of hypersonic wind tunnels, and in particular relates to a method for determining the condensation temperature of nitrogen under low vacuum pressure. Background Art

[0002] In a hypersonic wind tunnel, the test gas expands through the nozzle. When it reaches the test section, the temperature and pressure of the gas drop sharply. Under a certain pressure, if the static temperature of the gas is lower than the static saturation temperature, i.e., the condensation temperature, at the pressure, a phase change will occur, and the test gas will change from gas to liquid. If liquefaction occurs, it will affect the test measurement. In order to prevent the test gas from liquefying during the operation of the wind tunnel, it is usually necessary to heat it. At this time, it is necessary to determine the minimum heating total temperature of the wind tunnel. At present, the determination of the total temperature of a hypersonic wind tunnel is to first query the static saturation temperature of the gas under a certain pressure, and at the same time, according to the operating Mach number and isentropic relationship, the minimum operating total temperature is calculated from the static saturation temperature. The total temperature should be within a reasonable range and cannot be too low or too high. If it is too low, condensation will occur, and too high a total temperature will affect the construction scale and difficulty of the equipment. Since the Mach number of a low-density wind tunnel is relatively high, if the static saturation temperature, i.e., the condensation temperature, is too high, it will cause a sharp increase in the total temperature, and the total temperature required by the wind tunnel will be too high, which will cause a waste of investment and difficulties in construction. Therefore, the determination of the condensation temperature of nitrogen under low vacuum pressure is crucial to the operation of the wind tunnel.

[0003] The static saturation temperature of existing nitrogen at different pressures is given by a theoretical formula. This theoretical formula is used for pressures ≥ 100Pa. When the pressure is greater than 100Pa, the test value is in good agreement with the theoretical value, so the total temperature calculated is in line with the actual situation. However, when the pressure is very low, the error of the theoretical formula is too large, and the required static saturation temperature is too conservative. If the static saturation temperature is still calculated based on the theoretical formula, the static saturation temperature is too high, and there is a large error with the actual situation. Since the operating pressure of hypersonic low-density is very low, the static pressure of the flow field is lower than 10Pa, and the Mach number is very large (10-25). The static saturation temperature calculated according to the existing theory is too high, and a very high total temperature is required before the airflow expands. This requires the heater to have high-temperature heating capabilities, which will increase the difficulty of equipment construction and investment scale. It can be seen that the solution of determining the static saturation temperature of nitrogen at different pressures through a theoretical formula, in a near-vacuum environment, the calculated static saturation temperature is too high and cannot be used accurately.

[0004] The static saturation temperature under a certain pressure can also be measured by traditional static tests. That is, in a fixed container, evacuate to a certain pressure, gradually reduce the temperature of the gas, and when it is reduced to a certain temperature and the gas is observed to condense, the pressure and temperature are recorded. Then the container pressure is reduced again, the gas temperature is further reduced, and when the gas is observed to condense again, the pressure and temperature data are recorded again, and the above process is repeated to obtain the pressure and static saturation temperature curve. However, this method relies on manual observation and has large errors. This method is easy to implement to reduce the pressure of the container, but after reducing the temperature of the gas in the container to -190°C, it is difficult to continue to reduce the temperature by traditional liquid nitrogen cooling or other low-temperature liquefied medium cooling methods, that is, it is very difficult to cool down to a low temperature close to absolute zero. Moreover, this test method takes a long time, has large errors, and is difficult to achieve due to high temperature. However, due to the high Mach number and low total pressure of the hypersonic low-density wind tunnel, the static pressure of the test gas drops below 10Pa, and its static saturation temperature is lower than 10K, close to absolute zero. It is very difficult to obtain the saturation relationship under low vacuum pressure and temperature using traditional test methods, and there is no ready-made test data.

[0005] In summary, the above theoretical methods and traditional experimental methods are unable to accurately establish the saturation relationship between low vacuum pressure and temperature, and thus are unable to accurately determine the minimum total operating temperature of the wind tunnel. They can only be operated in an amplified margin manner, and are unable to accurately determine the condensation temperature of nitrogen under low vacuum pressure. Summary of the invention

[0006] The technology of the present invention solves the problem: overcome the shortcomings of the prior art, provide a method for determining the condensation temperature of nitrogen under low vacuum pressure, accurately determine the condensation temperature of nitrogen under low vacuum pressure, and make up for the shortcomings of theoretical formula acquisition and traditional experimental acquisition methods. Using the results, the total operating temperature of the hypersonic low-density wind tunnel can be reasonably determined, which can not only reduce the design difficulty of the heater itself, but also reduce the overall energy supporting scale of the wind tunnel.

[0007] In order to solve the above technical problems, the present invention discloses a method for determining the condensation temperature of nitrogen under low vacuum pressure, comprising:

[0008] Adjust the nozzle inlet total pressure to P0 and the nozzle inlet total temperature to T0, and measure the nozzle outlet pitot pressure to P 02 ; Among them, the values ​​of P0 and T0 satisfy: when the pressure is P0 and the temperature is T0, the airflow will not condense;

[0009] Keep the total pressure P0 at the nozzle inlet stable, perform the first cooling adjustment on the total temperature at the nozzle inlet, and measure the nozzle outlet pitot pressure P after the first cooling adjustment. 02,1 ;

[0010] P 02,1 With P02 Compare and get P 02,1 With P 02 The comparison results of

[0011] If P 02,1 With P 02 The comparison result of P meets the set criterion, then it is determined that the airflow has condensed. 02,1 The static pressure P1 and static temperature T1 at the nozzle outlet after the first cooling adjustment are calculated;

[0012] If P 02,1 With P 02 If the comparison result does not meet the set criterion, it is determined that the airflow has not condensed, the total pressure P0 at the nozzle inlet is kept stable, and the total temperature at the nozzle inlet is adjusted for the second time; until the nozzle outlet Pitot pressure P is measured after the total temperature at the nozzle inlet is adjusted for the nth time. 02,n With P 02 The comparison result meets the set criterion, and it is determined that the airflow condenses after the nth cooling adjustment. 02,n The static pressure P at the nozzle outlet after the nth cooling adjustment is calculated. n and nozzle outlet static temperature T n ; Where n≥2;

[0013] According to the calculated nozzle outlet static pressure P1 and nozzle outlet static temperature T1 after the first cooling adjustment, or the nozzle outlet static pressure P after the nth cooling adjustment n and nozzle outlet static temperature T n , determine the condensation temperature of nitrogen at pressure P0 as T1, or determine the condensation temperature of nitrogen at pressure P n The condensation temperature is T n ;

[0014] The total pressure at the nozzle inlet was changed and the above process was repeated to obtain the condensation temperature of nitrogen at different low vacuum pressures.

[0015] The above method for determining the condensation temperature of nitrogen under low vacuum pressure also includes: selecting a nozzle that meets the Mach number requirements, preparing all wind tunnel systems, pumping the vacuum pressure to below 5 Pa, and keeping the wind tunnel running; wherein the wind tunnel test gas medium is nitrogen.

[0016] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, if P 02,1 With P 02 The comparison result is: P 02,1 ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02 The comparison result meets the set criterion; if P 02,1With P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02 The comparison result does not meet the set criteria.

[0017] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, if P 02,n With P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02,n With P 02 The comparison result meets the set criteria.

[0018] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, the total temperature at the nozzle inlet is reduced according to the set temperature difference ΔT through the temperature control system, thereby achieving temperature reduction regulation of the total temperature at the nozzle inlet.

[0019] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, the static pressure P1 and static temperature T1 at the nozzle outlet during the first cooling are calculated by the following formula:

[0020]

[0021]

[0022] Among them, γ represents the specific heat ratio of nitrogen; Ma1 represents the Mach number of the nozzle after the first cooling adjustment.

[0023] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, the static pressure P at the nozzle outlet after the nth temperature reduction adjustment is calculated by the following formula: n and nozzle outlet static temperature T n :

[0024]

[0025]

[0026] Among them, Ma n It represents the Mach number of the nozzle after the nth cooling adjustment.

[0027] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, Ma1 and Ma2 are calculated by the following formula: n :

[0028]

[0029]

[0030] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, ΔT=10K.

[0031] In the above method for determining the condensation temperature of nitrogen under low vacuum pressure, the total pressure at the nozzle inlet is regulated by the pressure regulating system.

[0032] The present invention has the following advantages:

[0033] The present invention discloses a method for determining the condensation temperature of nitrogen under low vacuum pressure, which overcomes the shortcomings of existing theoretical methods and traditional test methods, can accurately determine the condensation temperature of nitrogen under low vacuum pressure, and thus can accurately determine the lowest operating total temperature of the wind tunnel under different Mach numbers and different total pressures, provide accurate temperature input for the design of the heater and the operation of the wind tunnel, ensure the anti-condensation requirements of the airflow while meeting the wind tunnel operation requirements, so that the wind tunnel can operate at a higher Mach number, and overcome the existing problems of too high total temperature, too large design margin, and high equipment investment scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of a method for determining the condensation temperature of nitrogen under low vacuum pressure in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments disclosed in the present invention will be further described in detail below with reference to the accompanying drawings.

[0036] One of the core ideas of the present invention is: in order to reduce the scale of equipment investment and operation risk, a method for determining the condensation temperature of nitrogen under low vacuum pressure is proposed, which provides accurate temperature input for the design of the heater and the operation of the wind tunnel, meets the use requirements of the wind tunnel, and can ensure the anti-condensation requirements of the airflow, so that the wind tunnel can operate at a higher Mach number, and can overcome the problems of the existing heater design temperature being too high, the design margin being too large, and the equipment investment scale being high. The method of the present invention utilizes the existing wind tunnel equipment, and determines the condensation temperature of nitrogen under low vacuum pressure by analyzing these three values ​​by real-time monitoring of the total pressure at the nozzle inlet, the total temperature at the nozzle inlet, and the pitot pressure at the nozzle outlet. In an experiment, the total pressure at the nozzle inlet is kept stable by controlling the pressure regulating system, and the total temperature at the nozzle inlet is first adjusted to a higher total temperature by the temperature regulating system, and then the total temperature at the nozzle inlet is gradually reduced according to a certain temperature interval. During the operation of the wind tunnel, the total pressure at the nozzle inlet, the total temperature at the nozzle inlet, and the pitot pressure at the nozzle outlet are monitored in real time, and the condensation temperature of nitrogen under low vacuum pressure is determined by comprehensive analysis of the above data.

[0037] In this embodiment, the method for determining the condensation temperature of nitrogen under low vacuum pressure includes:

[0038] Step 1: Select a nozzle that meets the Mach number requirements, prepare all wind tunnel systems, pump the vacuum pressure to below 5Pa, and keep the wind tunnel running.

[0039] In this embodiment, a nozzle with a high Mach number is generally selected; the wind tunnel test gas medium is nitrogen.

[0040] Step 2: Adjust the nozzle inlet total pressure to P0 and the nozzle inlet total temperature to T0, and measure the nozzle outlet pitot pressure to P 02 ,like Figure 1 shown.

[0041] In this embodiment, the total pressure at the nozzle inlet can be adjusted to P0 and kept stable by the pressure regulating system; the total temperature at the nozzle inlet can be adjusted to a higher temperature by the temperature regulating system to ensure that the airflow will not condense at this time. That is, the values ​​of P0 and T0 satisfy: when the pressure is P0 and the temperature is T0, the airflow will not condense.

[0042] Step 3: Keep the total pressure P0 at the nozzle inlet stable and adjust the total temperature at the nozzle inlet for the first time, and measure the nozzle outlet pitot pressure P after the first temperature adjustment. 02,1 .

[0043] In this embodiment, the temperature adjustment system can be used to reduce the total temperature at the nozzle inlet according to the set temperature difference ΔT to achieve temperature reduction regulation of the total temperature at the nozzle inlet. Generally, ΔT=10K.

[0044] Step 4: 02,1 With P 02 Compare and get P 02,1 With P 02 The comparison results.

[0045] Step 5: If P 02,1 With P 02 The comparison result of P meets the set criterion, then it is determined that the airflow has condensed. 02,1 The nozzle outlet static pressure P1 and nozzle outlet static temperature T1 after the first cooling adjustment are calculated.

[0046] In this embodiment, if P 02,1 With P 02 The comparison result is: P 02,1 ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02The comparison result meets the set criterion. The nozzle outlet static pressure P1 and nozzle outlet static temperature T1 during the first cooling can be calculated by the following formula:

[0047]

[0048]

[0049] Wherein, γ represents the specific heat ratio of nitrogen; Ma1 represents the Mach number of the nozzle after the first cooling adjustment,

[0050] Furthermore, it can be determined that the condensation temperature of nitrogen at a pressure of P0 is T1.

[0051] Step 6: If P 02,1 With P 02 If the comparison result does not meet the set criterion, it is determined that the airflow has not condensed, the total pressure P0 at the nozzle inlet is kept stable, and the total temperature at the nozzle inlet is adjusted for the second time; until the nozzle outlet Pitot pressure P is measured after the total temperature at the nozzle inlet is adjusted for the nth time. 02,n With P 02 The comparison result meets the set criterion, and it is determined that the airflow condenses after the nth cooling adjustment. 02,n The static pressure P at the nozzle outlet after the nth cooling adjustment is calculated. n and nozzle outlet static temperature T n ; where n≥2.

[0052] In this embodiment, if P 02,1 With P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02 The comparison result does not meet the set criterion, indicating that the airflow has not condensed and the airflow condensation point has not been found.

[0053] Furthermore, if P 02,n With P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02,n With P 02 The comparison result meets the set criteria, indicating that condensation has occurred. Because when the gas condenses, the Mach number at the nozzle outlet will decrease and the pitot pressure value at the nozzle outlet will increase.

[0054] Furthermore, the static pressure P at the nozzle outlet after the nth cooling adjustment can be calculated by the following formula: nand nozzle outlet static temperature T n :

[0055]

[0056]

[0057] Among them, Ma n represents the Mach number of the nozzle after the nth cooling adjustment,

[0058] Furthermore, it can be determined that nitrogen at pressure P n The condensation temperature is T n .

[0059] At this time, the calculated (P n , T n ) is a state point.

[0060] Step 7, changing the total pressure at the nozzle inlet, repeating the above process, and obtaining the condensation temperature of nitrogen at different low vacuum pressures.

[0061] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0062] The contents not described in detail in the specification of the present invention belong to the common knowledge of the professionals in this field.

Claims

1. A method for determining the condensation temperature of nitrogen under low vacuum pressure, characterized in that: include: Adjust the nozzle inlet total pressure to P0 and the nozzle inlet total temperature to T0, and measure the nozzle outlet pitot pressure to P 02 ; Among them, the values ​​of P0 and T0 satisfy: when the pressure is P0 and the temperature is T0, the airflow will not condense; Keep the total pressure P0 at the nozzle inlet stable, perform the first cooling adjustment on the total temperature at the nozzle inlet, and measure the nozzle outlet pitot pressure P after the first cooling adjustment. 02,1 ; P 02,1 With P 02 Compare and get P 02,1 With P 02 The comparison results of If P 02,1 With P 02 The comparison result of P meets the set criterion, then it is determined that the airflow has condensed. 02,1 The static pressure P1 and static temperature T1 at the nozzle outlet after the first cooling adjustment are calculated; If P 02,1 With P 02 If the comparison result does not meet the set criterion, it is determined that the airflow has not condensed, the total pressure P0 at the nozzle inlet is kept stable, and the total temperature at the nozzle inlet is adjusted for the second time; until the nozzle outlet Pitot pressure P is measured after the total temperature at the nozzle inlet is adjusted for the nth time. 02,n With P 02 The comparison result meets the set criterion, and it is determined that the airflow condenses after the nth cooling adjustment. 02,n The static pressure P at the nozzle outlet after the nth cooling adjustment is calculated. n and nozzle outlet static temperature T n ; Where n≥2; According to the calculated nozzle outlet static pressure P1 and nozzle outlet static temperature T1 after the first cooling adjustment, or the nozzle outlet static pressure P after the nth cooling adjustment n and nozzle outlet static temperature T n , determine the condensation temperature of nitrogen at pressure P0 as T1, or determine the condensation temperature of nitrogen at pressure P n The condensation temperature is T n ; The total pressure at the nozzle inlet was changed and the above process was repeated to obtain the condensation temperature of nitrogen at different low vacuum pressures.

2. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 1, characterized in that: Also includes: A nozzle that meets the Mach number requirements is selected, all wind tunnel systems are ready, and the vacuum pressure is pumped below 5Pa to keep the wind tunnel running; among them, the wind tunnel test gas medium is nitrogen.

3. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 1, characterized in that: If P 02,1 With P 02 The comparison result is: P 02,1 ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02 The comparison result meets the set criterion; if P 02,1 With P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02,1 With P 02 The comparison result does not meet the set criteria.

4. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 1, characterized in that: If P 02 ,n and P 02 The comparison result is: P 02,n ≥(P 02 +5%P 02 ), then determine P 02 ,n and P 02 The comparison result meets the set criteria.

5. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 1, characterized in that: The temperature control system is used to reduce the total temperature at the nozzle inlet according to the set temperature difference ΔT, thereby achieving cooling regulation of the total temperature at the nozzle inlet.

6. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 5, characterized in that: The nozzle outlet static pressure P1 and nozzle outlet static temperature T1 during the first cooling are calculated by the following formula: Among them, γ represents the specific heat ratio of nitrogen; Ma1 represents the Mach number of the nozzle after the first cooling adjustment.

7. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 6, characterized in that: The static pressure P at the nozzle outlet after the nth cooling adjustment is calculated by the following formula n and nozzle outlet static temperature T n : Among them, Ma n It represents the Mach number of the nozzle after the nth cooling adjustment.

8. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 7, characterized in that: Ma1 and Ma are calculated by the following formula n :

9. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 5, characterized in that: ΔT=10K.

10. The method for determining the condensation temperature of nitrogen under low vacuum pressure according to claim 1, characterized in that: The total pressure at the nozzle inlet is regulated by the pressure regulating system.

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