In the August 2003 issue, the Technical Data column was devoted to emissivity in practical temperature measurements. But what about the emissivity value that should be entered in your numerical model? Can we use the measured values as discussed in the column mentioned? Let me summarize the main points of it:
- Emissivity is not only a material property but also a surface property, at least for opaque materials. Consequently, all kinds of coatings (oxides, grease, water film) may influence the value that has been measured under pristine conditions. For example, the emissivity of a copper surface covered with 2 �m oxide increases from 0.03 to 0.2. In practice, very often surfaces that are initially shiny are covered with oxide and dust after one year of operation. Additionally, surface texture can influence the emissivity because of a strong dependence on the angle.
- For heat transfer calculations we need the so-called total hemispherical emissivity, found by integration over all wavelengths and all angles. What is being measured with an IR camera is the normal spectral emissivity.
In summary, to use an IR camera to measure the emissivity is not recommended because it returns the normal emissivity restricted to the wavelength band of the detector.
As a rule of thumb: for unpolished metals the ratio of hemispherical to total emissivity is 1.1-1.3, and for non-conductors 0.95-0.97. Another interesting angle-dependent difference between metals and insulators is the fact that under a very shallow angle with the plane the emissivity of metals tends to one and of insulators to zero.
To make things more complex, the emissivity of many materials of interest is strongly dependent on the wavelength. In other words, there is a big difference between the visible band and most common IR bands (0.8-3 �m, 3-8 �m, 8-14 �m). Si and Ge are notorious examples. It is very relevant for our rather low-temperature applications because the bulk of the radiation is in the long-wavelength region: 8-14 �m. This is the main reason why the color of paints is not relevant for heat sinks and covers. All colors are black. Henry Ford would have loved to live in an IR world.
How do we deal with this problem in practice? Apart from the fact that it is simply impossible to account for all the physics of radiation in a practical situation, the problem is only relevant when radiation is an important contribution to the total heat transfer. For natural convection cases, this contribution might well be 40%. When the choice is between an emissivity of 0.05 and 0.1, the radiation contribution is 2 to 4%. Not something to worry about. When the choice is 0.7 to 0.9, matters are different. One should realize that most heat transfer computer codes that support radiation heat transfer start from the following set of simplified assumptions:
- Surfaces diffuse, grey
(sometimes specular [mirror-like]), opaque - Surfaces isothermal
- Surfaces uniformly irradiated
- Medium is transparent for all relevant wavelengths
In summary, you should not be surprised to find differences between your experimental data and your simulation results when radiation is a major factor, even when all other data are known to within 1%.
The Table shows typical normal total emissivity values @ 20 �C, unless stated otherwise. Note that other references may quote different values.
Table 1. Normal Total Emissivity Values @ 20�C
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