The Effect on the Nusselt Number of the Nonlinear Axial Temperature Distribution of Gas Flows Through Microtubes

The characteristics of nitrogen convective heat transfer through commercial stainless-steel microtubes with inner diameters of 0.172 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 the experiments. The problems related to the estimation of the correct outlet temperature measurement for gas microflows and to the determination of the axial trend of the bulk gas temperature along the microtube are deeply discussed for both microtubes tested. It is highlighted that the axial local gas bulk temperature distribution is strongly nonlinear along the tube, especially for microtubes having a small 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 that predicted by the conventional correlations if the average bulk temperature is calculated while taking into account the axial nonlinearity of the gas bulk temperature distribution and the accurate measurement of the outlet gas temperature. The results presented in this paper give a physical explanation of the unexpected low values of the Nusselt numbers determined in the previous experimental works on gas flows through microtubes