Heat transfer and flow structure in a plane diverging channel

Abstract

Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼f(Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed.