Heat transfer enhancement by air injection in upward heated mixed-convection flow of water

The present paper reports on the possibility of enhancing the heat transfer rate in diabatic pipe water upflow using injection of air at the inlet of the heated channel. In turbulent upward mixed convection, the laminarization effect in the near-wall region is the main responsible of heat transfer reduction. Under these conditions, air injection was proved to enhance the heat transfer coefficient up to a factor of 10 in mixed-convection flow, simply as a consequence of turbulence increase and related suppression of the laminarization effect by air bubbles injected in the water flow. A specific series of experiments has been carried out with the aim of determining the flow pattern in the pipe using a Plexiglas tube. High speed movies of the flow allowed to evaluate the slip ratio, and then the void fraction, together with the identification of the flow pattern, which was mostly slug flow. The observed flow pattern showed how, in all cases, the gas phase caused a disturbance in the velocity profile in the near-wall region. The turbulence increase locally produced by air bubbles suppresses the laminarization effect, thus greatly enhancing the heat transfer coefficient. (C) 1999 Elsevier Science Ltd. All rights reserved.