Analysis of counterflow heat exchanger for non-newtonian fluids

Abstract

ANALYSIS of convective heat transfer in channel flow involves the solution of the appropriate energy equation with specified conditions at the channel wall. The conditions normally used are those of uniform wall temperature, uniform wall heat flux or the boundary condition of the third kind. Considerable work has appeared in the literature on these problems. In the last decade, attention was directed towards problems associated with countercurrent heat exchange with assigned temperatures of each stream at the inlet and coupling conditions in the form of temperature and flux continuity at the boundary of the two streams. Nunge and Gill [ 1 ] analysed the convective heat transfer during laminar Newtonian flow in parallel plate channels for counter flow of the two streams assuming the same physical properties in both the streams. This analysis was extended to laminar flows in double pipe heat exchangers by Nunge and Gill [2], Blanco et al. [3] and Stein [4], and for turbulent flow by Blanco and Gill [5]. For a class of countercurrent systems characterized by the feature that the resistance to transfer in one of the phases is negligibly small, Safonov and Potapov [6, 7~ developed a method for obtaining the local and asymptotic Nusselt numbers and applied it to countercurrent heat or mass transfer during laminar flow in parallel plate channels and in circular tubes. This analysis is particularly suitable for systems in which the two streams differ sharply in their physical properties. Hitherto all the studies have considered that both the fluid streams are Newtonian. In practice, counter flow heat exchangers where one of the stream fluids is non-Newtonian find wide applicability in polymer processing operations. It is the objective of the present work to provide a simplified analysis of the counter flow heat exchanger problem with one non-Newtonian fluid stream in which lies the major resistance to the transfer and negligible resistance in the annular stream. The non-Newtonian fluid considered is of the power law type and it will be shown that the results of Safonov and Potapov [7] for Newtonian flow and the Graetz-Nusselt problem for power law flow [8] are par. ticular cases of the present work.