Radiative Heat Transfer Coefficient in Double-Pipe Heat Exchanger with Isothermal Inner Pipe and Insulated Outer Pipe

Abstract

Analysis of a double-pipe heat exchanger that considers only forced convection may yield inaccurate results due to the fact that there is also thermal radiation, especially when the gas flowing in the heat exchanger has a higher temperature. This article presents a method of analysis that takes into
account thermal radiation in order to determine the radiation heat transfer coefficient, which should be used in conjunction with convective heat transfer coefficient in order to obtain more accurate results. An inner pipe of the heat exchanger is assumed to be at a constant temperature, whereas the outer pipe is insulated. The fluid is air that is transparent to radiation. The surfaces of the inner and outer pipes are divided into small elements that have uniform temperatures. The nonlinear system of equations of radiative exchange and energy balance equations is solved for temperature distributions, from which the radiative heat transfer coefficient is determined. The radiative heat transfer coefficient is expressed in terms of the average gas temperature and temperature of the inner pipe. It is found that a correction factor must be inserted into this expression. In addition, the correction factor is not a constant, but varies with Reynolds number and emissivities of the inner and outer pipes.