Non-linear temperature distribution of gas convection through commercial microtubes

The characteristics of nitrogen convective heat transfer through commercial stainless steel microtubes with inner diameter of 0.175 mm and 0.750 mm are investigated both experimentally and numerically. An analysis of the total uncertainty on the Nusselt number is carried out in order to quantitatively identify the contribution introduced by the different operative parameters in experiment. The problems related to the estimation of the right outlet temperature measurement for gas micro-flows and to the determination of the axial trend of the bulk gas temperature along the microtube are deeply discussed for both the tested micro-tubes. It is highlighted that the axial local gas bulk temperature distribution can present a strong non-linearity along the tube especially for microtubes having a low inner diameter and a thick solid wall. It has been 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 determined in the previous experimental works conducted with gas flows through microtubes where the trend of the gas bulk temperature between the inlet and the outlet of the microtube was assumed linear without verifying this hypothesis.