Summary

Radiative heat transfer between two or more surfaces can be approximated using the total, normal emissivity. This article provides empirically determined total normal emissivities for a number of materials including metals, metal oxides, common building materials and paints.

Definitions

Introduction

Treatment of radiative emission can be simplified by assuming that the emissivity is spectrally independent i.e. emissivity is constant with respect to wavelength. In this case the simplified emissivity is called the directional emissivity and represents the ‘spectral average’ of emission and hence the common name of this assumption; the grey surface approximation (wavelength of radiation determines colour and the ‘average’ of all colours is grey).

Although the total emissivity is averaged over all wavelengths it still maintains directional dependence which is a source of complexity in radiative heat transfer calculations. However, as described in the emissivity article and shown in the figure below, both conductors and non-conductors alike can be reasonably approximated using the emissivity in the normal direction.

In the subsequent sections the typical total, normal emissivities are listed for a range of materials.

Emissivities of Selected Materials

Further Reading

• Chemical Engineering Volume 1, Sixth Edition: Fluid Flow, Heat Transfer and Mass Transfer
• Heat Transfer Handbook
• Heat Transfer A Practical Approach
• Radiative Heat Transfer, Third Edition