ESDU 81045 derives, from a wide range of data sources, a correlation giving the variation of Stanton number with Reynolds number and Prandtl number for a complete range of annulus diameter ratios from zero (plain circular tube) to unity (parallel plates). The basic calculation is derived for a uniform heat flux at one surface with the other surface insulated. Data and methods for dealing with other situations are also provided. These include (i) uniform heat flow, of equal or differing magnitudes, at both surfaces, (ii) varying heat flux at one or both surfaces, or (iii) constant surface temperature at one or both surfaces. In addition, some qualitative guidance is provided on the influence of effects such as free-convection, eccentricity, and developing temperature profile.