Investigation results on heat transfer in low-Prandtl helium-xenon gas mixture and air, flowing in a small diameter tube, are presented. New experimental data on the heat transfer coefficient in the flow of helium-xenon mixture are obtained; results of numerical simulation are compared with the experimental data and known empirical correlations. Based on the simulation data it is shown that in a heated tube an increase in the Reynolds number due to an increase in the flow rate intensifies the heat transfer, and it may be due to the flow acceleration. It is shown that the high flow velocity and significant acceleration have a considerable effect on heat transfer in a tube, and the use of mean-mass stagnation temperatures as the determining one for generalization of data on "heat transfer is insufficient. For the studied conditions, the known correlations give a significant error in determination of the heat transfer coefficient: the lower the Prandtl number and gas density, the higher the error. (C) 2017 Elsevier Ltd. All rights reserved.