Heat transfer from boundary layers undergoing an acceleration induced reverse transition

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Abstract
An experimental and analytical investigation of heat transfer in an acceleration induced transitional flow regime is presented. A previously developed criterion for the onset of reverse transition of turbulent boundary layers is combined with a theory of forward transition to derive the effect of free‐stream turbulence intensity on the critical value of an acceleration parameter for two initial velocity profiles within the turbulent boundary layer. The results are consistent with experimental observations.Local heat transfer measurements were obtained along converging walls of rectangular flow channels having convergence angles of 45, 60, and 70 deg. Free‐stream turbulence (1.6 to 11%) was induced by grids upstream of the start of convergence, and the local Reynolds number were varied by variation in channel size and stagnation pressures (200 to 500 lb./sq.in. abs.) at a gas stagnation temperature of 1,500°R.The results indicate a two‐stage reverse transition process along the converging wall with a forward transition between the two stages of reverse transition. The local Nusselt numbers obtained over a wide range of an acceleration parameter are correlated with the local Reynolds number, the acceleration parameter at the start of convergence, and the local flow area contraction ratio. The corrlation is in the form of laminarization paths having negative slopes for (NNu)r/(NPr)0.4 vs. (NRe)r at common area ratios. Local values of free‐stream turbulence intensity are required for proper application of the reverse transition criterion in conjunction with the heat transfer correlation presented.