Internal convective heat transfer to gases in the low-Reynolds-number “turbulent” range

•Addresses flow in tubes with strong heating rates at low turbulent Reynolds numbers.•Aims to explain why local Nu varies roughly as square of decreasing local Re.•Hypothesis: caused by thermal boundary layer within growing “laminar” layer.•DNS demonstrates laminar Leveque similarity analysis reasonable.•Improves basic understanding of “relaminarization” due to heating gases. For internal vertical gas flow in tubes with strong heating rates at low turbulent Reynolds numbers, a typical experimental observation is that the local Nusselt number varies roughly as the square of the decreasing local Reynolds number. An aim of the present note is to examine this situation. This examination leads to the hypothesis that the behavior results from the evolution of the thermal boundary layer developing within the primarily molecular transport layer which is also growing from the wall. Comparisons to direct numerical simulations demonstrate that reasonable predictions are provided by an extension of the Leveque similarity analysis for laminar thermal boundary layers. The present observations modify and improve our fundamental understanding of the process called “relaminarization” in these flows.