Improvement in performance of multi-pass laminar counterflow heat exchangers with external refluxes

A study on improvement in performance of multi-pass rectangular heat exchangers with uniform wall temperature and with external refluxes has been carried out under counterflow operations. An orthogonal expansion technique for solving such a conjugated Graetz problem is developed and applied to investigate a substantially improving heat transfer. A description of the average Nusselt number and the outlet dimensionless temperature is given. It is shown that for asymptotic average Nusselt numbers exit in counterflow as in single-flow operations. Numerical values of asymptotic average Nusselt numbers are reported for a wide range of Graetz numbers. Comparisons of analytical results are made with single-flow operations of the same size (without sheets inserted and without recycle). Considerable improvement in the heat transfer rate for large Graetz numbers is obtainable by introducing the recycle-effect concept in designing such double-pass operations. The effect of sheet location on the enhancement of heat transfer efficiency as well as on the increment of power consumption has been also discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.