When discussing the conditions of the high and low temperature test chamber for the test heat dissipation test sample, the radiant heat transfer cannot be ignored. In the case where the test sample and the test box wall are hot black (radiation coefficient is about 1), from the test sample to the test box wall About half of the heat transfer is transferred by heat radiation. If the heat dissipation test sample is subjected to a certain temperature test in the high and low temperature test box with the box wall hot white or the box wall hot black, the surface temperature of the test sample will be It will be significantly different, so if you want to obtain reproducible test results, the relevant specifications should limit the emissivity and temperature of the test chamber wall.
If there are other test samples, heating or cooling components, mounting brackets, etc. between the test sample and the box wall, the heat radiation between the test sample and the box wall will be affected, and the heat color and temperature of the test box wall will be affected. Does not meet the requirements, the percentage of the box wall part that can be "seen" at a certain point on the test sample determines the viewing angle factor of the change point, and the "view factor" of each point of the test sample is not subject to certain Interference with temperature-required devices.
Under ideal "free air" conditions, the heat transferred from the test sample to the surrounding air is completely absorbed by the surrounding air. This is because the heat of free convection and radiation exchange is completely absorbed, usually most devices (including equipment and components ) Is operated in an environment very similar to hot black rather than hot white. It is easier to make the inner wall of the high and low temperature test chamber similar to hot black than to make hot white, because most paints and (unpolished) materials It is closer to hot black than hot white. At the same time, due to the aging effect of the material over time, it is particularly difficult to maintain the hot white in the box for a long time.
If the temperature change of the box wall is within 3% of the specified test temperature, and the emissivity of the box wall changes between 0.7-1.0, the change in the surface temperature of the test sample is usually less than 3K. The factor radiation heat transfer is the same as the test sample The difference between the fourth power of the surface temperature and the fourth power of the wall temperature is proportional. The radiant heat transfer at low temperature is less significant than that at high temperature, so the color and temperature of the wall are not so strict during the low temperature test. .
The heat exchange through heat radiation mainly depends on the temperature of the test chamber wall. This dependence is why when the difference between the surface temperature of the test sample and the ambient air temperature is large, the temperature of the test sample is not corrected according to the standard procedures. The main reason why the test cannot be performed with forced air circulation.
Note: The heat transfer principle of the high and low temperature test chamber is divided into three parts: heat convection, heat radiation, and heat conduction. There are corresponding articles in this station. The article title is: Heat Convection of the High and Low Temperature Test Chamber Heat Transfer Principle, and High and Low Temperature Test Chamber Heat Transfer For the thermal radiation of the thermal principle and the thermal conduction of the high and low temperature test chamber, please click to browse!
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