Influence of turbulence on the transfer of heat from plates with and without a pressure gradient

Abstract

The work described in this paper is a continuation of that done by Kestin and Maeder on the influence of free-stream turbulence on the coefficient of heat transfer from cylinders in cross-flow. The present work is concerned exclusively with the flat plate and deals at length with the case of zero incidence, i.e. of zero pressure gradient. The effect of adding a favorable pressure gradient was investigated in a preliminary way. Kestin and Maeder have demonstrated the existence of two effects produced by an increase in the turbulence intensity of the free-stream in the case of cross-flow past a circular cylinder. (1) An increase in turbulence intensity causes earlier transition, and, generally, affects the flow pattern about the body. (2) An increase in turbulence intensity causes local changes in the coefficients of heat transfer and, presumably, in the flow pattern in the boundary layer. The existence of the local effect has also been observed by Kestin, Maeder and Sogin, Giedt, Sato and Sage, Sage et al., Seban, and van der Hegge Zijnen. The present investigation shows that the local effect is completely absent in the case of a flat plate at zero incidence, in good agreement with the work due to Edwards and Furber and Kline et al., but in contradiction to the findings of Sugawara and Sato. This is a remarkable difference between the present case and that of a cylinder. A qualitative explanation of this difference was suggested by Kestin et al. The main conclusion reached by them was that large effects from changes in the free-stream turbulence can be produced only in the presence of pressure gradients. This conclusion was tested by imposing a favorable pressure gradient on the plate. It was found that small changes in the turbulence intensity of the free stream cause large changes in the coefficient of heat transfer in the laminar range. Experiments with a pressure gradient were carried out in a preliminary way, and no exhaustive measurements were undertaken at this stage. In particular, the effect on turbulent boundary layers as well as the effect of adverse pressure gradients were not investigated. The experimental arrangement and the techniques used in measurement are described in detail; the accuracy is carefully examined and detailed check-measurements, in particular, measurements of velocity profiles were undertaken in order carefully to correlate the heat transfer measurements with the different flow regimes which are possible in the boundary layer.