自然散热情况下辐射和对流哪个占主导作用

根据斯蒂芬波尔兹曼公式，定义一个热辐射传热系数（类似于对流传热系数）：

假设辐射率=0.8 ，Ts比Ta高1℃。则辐射传热系数与环境温度的关系如下：

典型的环境温度为50℃时，辐射系数为6 W/m2-K 。

针对机箱内一个水平放置PCB来讲，其自然对流传热方程如下：

（此公式从何而来？）

假设环境温度已知，为50℃（323K），PCB辐射率为0.95.

那么自然对流和辐射传热的系数分别计算如下：

从图中可以看出，在温差<20℃（K）时，辐射传热系数大于自然对流系数。温差超过20℃时，两种传热系数几乎相等。所以在温差较小时，辐射传热一定不能被忽略。

当然，实际情况千差万别，但这个简单的例子可以帮助建立一些基本的概念。

Don't underestimate radiation in electronic cooling

February 1, 2001 Bruce GueninCalculation Corner, Design, Number 1, Volume 7Heat Transfer Coefficient, Stephan-Boltzmann Constant, Thermal Radiation

don’t underestimate radiation in elec tronics cooling

Bruce M. Guenin, Ph.D., Associate Editor, Amkor Technology, Inc.

It is easy to underestimate the role of thermal radiation as a significant contributor to electronics cooling in environments without forced air flow. By its very nature it is invisible. The proper

treatment of it can be intimidating due to the complicated nature of the

phenomenon in environments in which localized hot regions are in the view of other localized hot regions.

However, it is possible to get a basic understanding of radiation without even worrying about such complications as view factors.The first thing to do is to respond to the basic engineering urge to

linearize anything possible. Hence, Equation 1 is a recasting of the familiar Stephan-Boltzman equation, dividing it by the temperature difference between a surface (assumed isothermal) and the facing surface

(assumed to be at the air temperature). The result is a heat transfer coefficient, which represents the effect of radiation at a given temperature.

The numerical factor is the Stephan-Boltzmann constant and is the emissivity. The emissivity is in

the range 0.8 – 0.9 for dielectrics and 0.1 – 0.2 for commercial metals. The temperatures are expressed in absolute temperature Kelvin units.

Figure 1. Temperature dependence.

Even though we have linearized the S-B equation, the resultant heat transfer coefficient is still highly

temperature-dependent. In fact, it is proportional to the third power of the absolute temperature. Figure 1 illustrates this temperature dependence, where we have assumed an emissivity of 0.8 and a

temperature difference between the surface and the air of 1°C.

The lower x axis indicates absolute temperature.The upper x axis indicates degrees centigrade in the

range of interest to electronics cooling. At a typical ambient temperature range, say around 50°C, h RAD is approximately 6 W/m2-K.

It is useful to compare the radiation heat transfer coefficient to the heat transfer coefficient applicable

to a horizontal printed circuit board in a large enclosure. This expression represents an average for heat transfer from the top and bottom surfaces of the board [1].

The following graph, Figure 2, compares the magnitude of the radiation and natural convection heat transfer coefficients as a function of the temperature difference between the surface and air

temperature, where the air temperature is assumed to be 50°C.