Specific features of heat transfer at the stagnation point of an impact axisymmetric jet at low Reynolds numbers

The average and pulsating thermal characteristics at the stagnation point of a plate at inflow of an impact axisymmetric air jet have been studied. The influence of the Reynolds number $(100 < \rm Re < 12000)$ on heat transfer has been investigated for a jet flowing from a long tube (diameter $d = 5$ mm, relative length $h/d = 200$) with the output located at a distance $h/d = 20$ from the obstacle. The measurements have been carried out using a heat-flow sensor with high spatial and temporal resolutions. A nonmonotonic change in heat transfer having a maximum is found in the range $\rm Re < 4000$ (in contrast to the known monotonic increase in heat transfer). A significant increase $(200$–$600\%)$ in the Nusselt number is observed for outflow from a tube in comparison with jet outflow from a nozzle. At $\rm Re > 4000$, the difference in heat transfers for two cases of jet formation (from a tube and a nozzle) decreases asymptotically.