The influence of recycle on double-pass heat and mass transfer through a parallel-plate device

The influence of recycle on double-pass parallel-plate heat and mass exchan gers with uniform wall temperature has been studied analytically. The heat or mass transfer problem is solved for fully developed laminar velocity pro files in parallel-plate channels by ignoring axial conduction, and with flu id properties of temperature independence. Analytical results show that rec ycle can effectively enhance the heat or mass transfer rate compared with t hat in an open conduit (without an impermeable plate barrier inserted for d ouble-pass and recycle operation). The desirable preheating effect and the undesirable effect of decreasing residence time are the two conflict effect s produced by the recycle operation. It was found that the increase in preh eating effect by increasing the reflux ratio can generally compensate for t he decrease of residence time, leading to improved performance, exceptional ly for very small Graetz numbers. (C) 1998 Elsevier Science Ltd. All rights reserved.