Research on Vacuum Heat Test Method for Accelerating Cooling of Specimen

Research on Vacuum Heat Test Method for Accelerating Cooling of Specimen
Core Tips: 1 Foreword The Mars Pathfinder aircraft launched by the Delta launch vehicle on December 4, 1996 was the second launch of the NASA Mars exploration program. The Mars Pathfinder was under the goal of “Fast, Good, and Provincial”. It was developed in less than US$150 million over three years. by"

1 Foreword The Mars Pathfinder aircraft launched by the Delta launch vehicle on December 4, 1996 was the second launch of the NASA Mars exploration program. The Mars Pathfinder spent three years under the goal of “fast, good, and saving”. Time, less than $150 million in funding was developed.

Through the introduction of "Mars Pathfinder system-level solar thermal vacuum", it is learned that a method of shortening the test period is adopted in the Mars Pathfinder test. The application of this method saves time and money for the test.

The large-capacity test specimens of the lunar module are required to have relatively low temperatures in the cold environment. In the cooling process, the pressure of the container is properly lifted, so that the heat exchange between the specimen and the heat sink is mainly radiative heat exchange. The main purpose is to convert convection heat to achieve accelerated cooling.

2 Mars Pathfinder's thermal vacuum test The model's limited budget and time allow the thermal vacuum test project to be carried out under the “fast, good, and saving” guidelines. It is important to adopt a method that enhances convection heat transfer and shortens the test cycle. .

The time allocated for the trial was limited to 15 days, and 5 days of incident handling time.

Since the Mars Pathfinder thermal equilibrium temperature is very low, designing a test that accomplishes all goals within 11 days can be difficult. In order to shorten the non-critical test time (such as cooling and temperature rise), it can be done by not simulating the actual The thermal process, instead, uses a method of accelerating the cooling and warming of the aircraft.

The purpose of accelerating cooling and warming is to reduce the time it takes for the aircraft to reach its steady state condition. In this experiment, simulations were not performed in the time sequence from the thermal environment near the Earth to the cold environment near Mars. Instead, the experiment began with the simulation of the cold Mars environment and gradually warmed up to the near-surface thermal environment. In the evacuation phase, the accelerated cooling method is used to reduce the temperature; and between the two stable conditions, that is, the low temperature to the helium temperature, the heater and the infrared lamp are used to accelerate the heating.

The test is to vacuum the container to better than IXl (r3Pa, then filled with pure nitrogen, maintain the pressure in the container is 1000Pa, the heat sink temperature is kept at -135ac, the principle is to increase the convection heat transfer between the sample and the heat sink In order to accelerate the cooling of the test piece, the temperature of the test piece is reduced to about -ir, and the pressure of the vessel is restored to better than 1×l (T3Pa) by using the vacuum system's pre-stage and cryopump, and then the cryogenic holding stage, heating stage, and temperature stage Test.

3 Theoretical Analysis of Accelerated Cooling Method 3.1 Calculation of Temperature of Vessel Wall Accelerated cooling method will also affect the vacuum vessel while cooling the specimen. The low temperature of the vessel wall is a problem. Because 1000Pa of nitrogen allows the conduction and convection heat of the gas to significantly increase the heat transfer between the heat sink and the vessel wall, the ice formed after cooling the outside of the vessel becomes the heat between the cold vessel and the warm air in the test chamber. Resistance. The material of the vacuum vessel wall is austenitic stainless steel, and its low temperature performance is no problem, but the ribs of the vessel are carbon steel and may become brittle at temperatures below -2 (TC. These effects are analyzed by the following calculations.

The heat exchange with the container wall consists of three parts: radiation heat exchange between the heat sink and the container wall; convection heat exchange between the heat sink and the container wall; convection heat exchange between the container wall and the test chamber air.

3.1.1 Analysis of convection heat transfer between the heat sink and the container wall The nitrogen gas is filled in the vacuum container. When the pressure is 1000 Pa, the molecular free path or gas molecular diameter is first calculated, m, the pressure of the gas is 3.75 P, Pa, Current considerations, 1000 Pa; Thermodynamic temperature of T gas, K. At this point pi is a constant, so thermal conductivity is independent of pressure.

Calculation of gas thermal conductivity 1 gamma-6 The following calculation of the convection heat transfer coefficient k thermal conductivity, ff / mK, with the container wall temperature changes; l feature size, take 3m for KM3. Nu meets 7U)/2, 71 For the heat sink temperature 100K, L is the container wall temperature, K'; l feature size, take 3m for KM3; v kinematic viscosity, for the characteristic temperature 50*C, 9.23Xl (TmVs. Therefore Gr brings the parameters into Mm The heat flux generated by convective heat transfer is 3.1.2 The radiant heat flux density of the outer surface of the heat sink and the inner surface of the container * Absorption ratio of the inner surface of the container, taking 0.2; T, the temperature of the heat sink, taking 100K; the temperature of the inner surface of the L container , K. 3.1.3 Convection heat transfer between the container wall and the test chamber air 7L* - vessel wall temperature, K. Heat flux generated by convection heat transfer To sum up the above, one can derive the vessel wall temperature equation A* - unit area, lm2 The density of the P container wall material is 8000 kg/m3 for stainless steel, and the relationship between the temperature of the container wall and the time can be obtained by the above formula, and the temperature of the container wall can be seen to change with time.The effect of natural convection on the temperature of the container wall varies with the container wall. The decrease in temperature decreases.

After accelerated cooling for 460 min, the container wall temperature reaches -2 (TC, a low line is determined for the container. After the temperature is lower than this limit, the pressure in the container is immediately restored to a high vacuum (better than lX103Pa), and the temperature of the container is increased to A safe area.

The temperature curve of the container wall 3.2 The heat sink between the heat sink and the test specimen The temperature of the specimen placed in the heat sink is 7 and the heat sink temperature is 7; for 100K, the heat exchange between the two meets F, the surface area of ​​the specimen; F2 Heat sink surface area; e! Emissivity of the test piece, taking 0.8:e2 heat sink internal surface emissivity, taking 0.9; T, piece of temperature, K: L heat sink temperature, 100K. Take the windsurfing test in KM3 as an example. For 16.3m2, 61.23m2, the parameters were brought in: the corresponding calculation of the heat transfer coefficient between the sample and the heat sink is still taking the situation of the windsurfing board as an example, the temperature equation of the cooling process is listed, only the radiation The situation of heat exchange meets the requirements of 4.536+0.37 China Institute of Science and Technology Information, 9902222. Yang Shiming. Heat transfer. Yi and other education publishers, 1987

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