1 Introduction
The performance of products and components is generally affected and restricted by their internal temperature, which is determined by the heat generated by themselves and the surrounding environmental conditions. Whenever there is a temperature gradient in the system formed by the product and its surroundings, there is a heat transfer process in between. This section includes low and high temperature tests, tests with sudden temperature changes and temperature gradient tests, heat dissipation test samples and non-heat dissipation test samples.
The test equipment is (chamber or chamber) can be used with and without forced air circulation of high and low temperature equipment.
1.1. Benchmark environmental conditions
The actual environmental conditions in which the product will operate in the future are often not accurately predicted or prescribed. Therefore, it is generally not possible to design, manufacture or test on the basis of actual environmental conditions. Therefore, it is necessary to consider the following factors and to specify some commonly used benchmark environmental conditions.
1.2. Non-heat dissipation products
If the ambient temperature is uniform and no heat is generated in the product, the direction of heat flow is: when the ambient temperature is high, heat is transferred to the product from the surrounding air; Conversely, if the temperature of the product is high, the heat is transferred from the product to the surrounding air. This heat transfer process continues until the temperature of all parts of the product reaches the ambient air temperature. Thereafter, the heat transfer process will cease unless the ambient temperature changes. In this case, it is simple to determine the baseline ambient temperature, the only condition is that it should be evenly distributed and constant. However, when the product does not reach the ambient air temperature, the determination of the benchmark ambient temperature is more complicated, and the following 13 (Heat Dissipation Products).
1.3. Products for heat dissipation
If there is heat generated in the product, but no heat is transferred to the surrounding air, the temperature of the product will continue to rise. In fact, the heat generated by the product is constantly diverging to the ambient air, and finally, the heat generated by the product is balanced with the heat dissipated in the surrounding cooling air, so that the temperature of the product reaches a stable level. Only when the ambient temperature rises (or falls) does the temperature inside the product rise (or fall) further until a new equilibrium is reached.
In this case, the reference ambient temperature should be determined in such a way that simple and reproducible heat transfer conditions are obtained. Since heat transfer is carried out by three different modes, convection, radiation, and conduction, clear conditions must be obtained for each mode separately and at the same time.
If multiple test samples are tested at high temperature in the same test chamber, it should be ensured that all test samples are at the same ambient temperature and have the same installation conditions. However, in the case of low-temperature tests, it is not necessary to distinguish between a single test sample and multiple test samples.
1.4. Ambient temperature
In general, the user of the product requires the maximum and minimum values of the allowable ambient temperature for the operation of the product, and this should also be specified for the purpose of testing.
Since heat transport is associated with a temperature gradient, the temperature of the medium around the product must change all the time, which makes it difficult to determine the "ambient temperature". Therefore, "ambient temperature" should be specifically defined.
1.5. Surface temperature
The main influence on the performance of a product is its own temperature. Therefore, it is appropriate to monitor and adjust the test equipment with reference to the temperature of some key points on the surface of the test sample or even inside it.
2. Basis of different test procedures
2.1. Principle of heat transfer
2.1.1. Thermal convection
2.1.1.1. When the test is carried out in the test chamber, convection heat dissipation plays an extremely important part in the heat exchange of heat dissipation test samples.
The coefficient of heat transfer from the surface of the test sample to the surrounding air is affected by the velocity of the surrounding air. The higher the air velocity, the more efficient the heat exchange. Therefore, the higher the air velocity at the same ambient temperature, the lower the surface temperature of the test sample.
In addition to the surface temperature of the test sample at any location, the air flow also affects the temperature distribution on the surface of the test sample.
2.1.1.2 There is no simple relationship between the surface temperature and temperature distribution of the test sample for different air velocities and directions. It is also evident that if the test is to be carried out in accordance with the actual conditions, a specific airflow velocity and direction of the airflow will be specified for the test chamber, which will involve many problems in the design of the test chamber. In order to facilitate the comparison of the test results with the actual environmental conditions, it is necessary to specify a clear and reproducible test conditions, which leads to the use of "free air conditions".
2.1.1.3. "White by Air Condition" uses air conditions in an infinite space. In this case, the air movement in this space is only affected by the heat dissipation test sample itself, and the energy radiated by the test sample is absorbed in this space. Therefore, it is impractical to attempt to reproduce free air conditions in a test chamber, and the use of free air conditions usually does not result in the use of expensive or impractical large test chambers. Since the free air condition has certain technical advantages and is easier to achieve than the prescribed forced air conditions, it is the preferred method for low and high temperature tests on heat dissipation test specimens, and in some cases it may be difficult to perform the test using the non-forced air circulation method. Therefore, two alternative methods are given for situations where low-velocity air is allowed to be used for forced air circulation: the first method is suitable for situations where the size of the test chamber is large enough to meet the requirements of Appendix A, but forced air circulation is required for the heating or cooling of the test chamber.
The second method is suitable for situations where the chamber is too small to meet the requirements of Appendix A, or where the first method cannot be used for other reasons.
2.1.2. Thermal radiation
2.1.2.1. When discussing the conditions of the test chamber used for the test heat dissipation test sample, the radiative heat transfer cannot be ignored, and in the case that the test sample and the test chamber wall are hot black (the emissivity coefficient is about 1), about half of the heat transfer from the test sample to the test chamber wall is transferred by thermal radiation. If the heat dissipation test sample is subjected to a certain temperature test in a test chamber with a hot white wall or a hot black wall, the surface temperature of the test sample 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.
2.1.2.2. Between the test sample and the wall of the box, if there are other test samples, heating or cooling components, mounting brackets, etc., the thermal radiation between the test sample and the wall of the box will be affected. The thermal color and temperature of the chamber will not meet the requirements, and the percentage of the wall that can be "seen" at a particular point on the test sample determines the viewing angle factor at that point. The "angle factor" of each point of the test sample should not be disturbed by some devices that do not meet the requirements for the thermal color and temperature of the box wall.
2.1.2.3 Under the ideal "free air" condition, the heat transferred from the test sample to the surrounding air is completely absorbed by the surrounding air, which occurs due to the complete absorption of the heat exchanged by free convection and radiation.
Typically, most installations, including equipment and components, operate in environments that closely resemble hot black rather than hot white.
It is easier to make the inner wall of the test chamber similar to hot black than hot white. Because most paints and (unpolished) materials are closer to hot black than hot white. At the same time, due to the aging effect of the material over time, it will be particularly difficult to keep the wall of the box (chamber) hot white for a long time.
If the wall temperature change is within 3% of the specified test temperature (calculated at the Kelvin temperature) and the emissivity coefficient of the wall is 0Between 7 and 1, the change in the surface temperature of the test sample is usually less than 3K. Because the radiative heat transfer is proportional to the difference between the fourth power of the surface temperature of the test sample and the fourth power of the wall temperature, the radiative heat transfer at low temperature is not so significant compared with that at high temperature, so the requirements for the color and temperature of the box wall are not very strict in the low temperature test.
2.1.2.4 Heat exchange by thermal radiation depends mainly on the temperature of the test chamber wall, and this dependence is why the test sample is used.
When the difference between the surface temperature and the ambient air temperature is large, the main reason why forced air circulation cannot be used to perform the test without correcting the temperature of the test sample in accordance with Appendix E.
2.1.3. Heat conduction
2.1.3.1. The heat dissipation of heat conduction depends on the thermal characteristics of the mounting bracket and other connectors.
2.1.3.2 There are many heat dissipation devices and components, which are prescribed to be installed on endothermic or other devices with good thermal conductivity. As a result, a certain amount of heat is effectively dissipated through heat conduction.
Therefore, the relevant standard should specify the thermal characteristics of the mounting frame, and it is best to reproduce these thermal characteristics of the mounting frame when conducting the test.
2.1.3.3If the equipment or component is installed in more than one method with different thermal conductivity values, the worst-case scenario should be considered when testing. Depending on the application, the worst-case scenario will be different, such as:
a) High temperature test of heat dissipation test samples. Since the direction of heat transfer during the test is from the test sample to the mount, the heat transfer of the mount is the smallest, that is, the transmission direction when the thermal conductivity of the mount is the smallest (adiabatic).
b) High temperature test of non-heat dissipation test samples. As long as the test sample has not yet reached thermal stability, the heat is transferred from the box wall to the test sample through the mounting frame. Then, in the worst case, when the thermal conductivity of the mounting bracket is large, in order to avoid the heating time of the mounting bracket being too long, thereby delaying the heat transfer from the box wall to the test sample. The heat capacity of the mounting bracket should be small.
c) Low temperature test of heat dissipation test samples and non-heat dissipation test samples. Since the heat during the test is transmitted by the test sample to the wall of the box through the mounting frame, the worst-case scenario (the lowest temperature of the test sample) is when the heat transfer efficiency is the highest, i.e. when the thermal conductivity of the mounting frame is high.
2.1.4. Forced air circulation
2.1.4.1 The volume of the test chamber is large enough to fully meet the requirements of Appendix A, but the heating and cooling of the chamber may require forced air circulation. In this case, the test sample should be placed in a test chamber with room temperature for inspection, so that the temperature of the representative points on the surface of the test sample will not be excessively affected by the forced air circulation in the chamber. If the surface temperature at any point on the test sample is not reduced by more than 5 due to the forced air circulation in the test chamber, as in the test chamber without forced air circulation, the cooling effect of the forced air circulation can be considered to be quite small and negligible.
2.1.4.2 If the test chamber is too small to meet the test requirements in Appendix A, or as per Section 2 of this section1.4.1. When the measured surface temperature difference exceeds 5, it is advisable to carry out an exploratory test outside the test chamber, first place the test sample in the test chamber where the test chamber is placed, apply the load conditions specified by the relevant standard for the test temperature, and measure the temperature of a number of representative points on the surface of the test sample, so as to give the reference point for calculating the surface temperature under the specified test conditions.
For a small temperature difference between the ambient temperature and the surface temperature, as long as the ambient temperature changes t. hours, it can be assumed that the temperature difference t: it is the same at different ambient temperatures. If it is t:<25and t:<30, then the error is within 3.
The relationship between the surface temperature of the test sample at different ambient temperatures is shown in Appendix E. If the surface temperature at a certain ambient temperature is known, the surface temperature at any ambient temperature can be calculated using the calculation diagram in Appendix E, so that when the surface temperature of the test sample at room temperature is known, the surface temperature range at the specified test conditions can be extended by the use of the calculation chart in Appendix E. The calculation diagram in Appendix E can be used at least t:=80 and t=65,21.4.3When using either of the first and second methods in this part, the selection of representative points to be examined should be carried out in detail about the situation of the test sample (e.g., temperature distribution, thermal limit point, etc.), because the selection of this point is mainly a matter of training judgment. Therefore, it is recommended to give preference to the test method without forced air circulation.
For exploratory tests, it may be necessary to test the performance of the chamber so that a series of similar tests (e.g., tests of similar components) can be performed, while in other cases (e.g., for different products), the chamber needs to be evaluated before each test.