Transition from the turbulent to the laminar regime for internal convective flow with large property variations

The results of a primarily experimental study of the transition from turbulent flow to laminar flow as a consequence of high heating rates are presented. Results are reported for hydrodynamically fully developed, low Mach number flows of air and helium through a vertical, circular tube. The electrically heated section was 100 diameters in length; entering Reynolds numbers ranged from 1700–40,000, and maximum wall-to-bulk temperature ratios reached 4.4. As a means of predicting the occurrence of a transition from turbulent flow to laminar flow, the experimental results are compared to the acceleration parameter suggested by Moretti and Kays and to a modified form of the parameter that is appropriate to a circular tube. It is suggested that the variable property turbulent flow correlations do not provide acceptable predictions of the Nusselt number and the friction factor if the value 4μq′′G2DTcp≃1.5×10−6 based on bulk properties, is exceeded for an initially turbulent flow situation. It is further suggested that Nusselt numbers and friction factors at locations down-stream from the point xDlaminar≃(2×10−8)(Tinlet)(Reb,inlet)2TwTbmax−1 for bulk temperatures in degrees Rankine may be obtained from the laminar correlation equations even though the flow is initially turbulent.