Normal Emissivity of an Isothermal, Diffusely Reflecting Cylindrical Cavity (With Top) as a Function of Inside Radius

The normal emissivity of an isothermal cylindrical cavity (with top), with a diffusely reflecting interior of reflectivity much less than 1, is calculated approximately as a function of the inside radius of the cylinder by the DeVos method. The calculation is analytical, and considers the singly and doubly reflected radiation escaping from the cavity aperture. The results of the analysis indicate that, for cylinders whose length-to-lid aperture ratio is much larger than 1: (a), for a given cylinder length and lid aperture, the configuration with the inside diameter approximately 0.64 the length has the smallest normal emissivity; (b), as the inside diameter increases or decreases from the configuration of smallest normal emissivity, the normal emissivity increases monotonically.