The effect on the Nusselt number of the non-linear axial temperature distribution of gas flows through commercial microtubes

Abstract

The characteristics of nitrogen convective heat transfer through commercial stainless steel microtubes with inner diameter of 172 μm and 750 μm are investigated both experimentally and numerically. In this work is highlighted that the axial local gas bulk temperature distribution can present a strong nonlinearity along the flow direction especially for microtubes having a small inner diameter and a thick solid wall. It is also demonstrated that the trend of the experimental Nusselt numbers as a function of the Reynolds number can be considered in good agreement with the conventional correlations if the average bulk temperature is calculated by taking into account the axial non-linearity of the gas bulk temperature. This fact explains the low values of the Nusselt numbers obtained in the previous experimental works appeared in literature where the convective heat transfer for gas flows through microtubes has been investigated assuming the gas bulk temperature distribution between the inlet and the outlet of the microtube as linear without verifying this hypothesis.