Effects of flow maldistribution on the thermal performance of cross-flow micro heat exchangers

C Nonino,S Savino

doi:10.1088/1742-6596/745/3/032099

Abstract

The combined effect of viscosity- and geometry-induced flow maldistribution on the thermal performance of cross-flow micro heat exchangers is investigated with reference to two microchannel cross-sectional geometries, three solid materials, three mass flow rates and three flow nonuniformity models. A FEM procedure, specifically developed for the analysis of the heat transfer between incompressible fluids in cross-flow micro heat exchangers, is used for the numerical simulations. The computed results indicate that flow maldistribution has limited effects on microchannel bulk temperatures, at least for the considered range of operating conditions.

Highlights

In heat exchanger design, calculations are usually based on the assumption of uniform velocity distributions in the flow passages even if this hypothesis is not realistic under actual operating conditions
An improved version has been used to study the effects of the viscosity-induced flow maldistribution, i.e., the lack of uniformity in microchannel average velocity stemming from the temperature dependence of fluid viscosity [14]
The effects of geometry-induced flow maldistribution are analyzed with reference to two of the micro heat exchanger geometries, three of the mass flow rates and the three materials considered in [14]

Summary

Introduction

Calculations are usually based on the assumption of uniform velocity distributions in the flow passages even if this hypothesis is not realistic under actual operating conditions. An improved version has been used to study the effects of the viscosity-induced flow maldistribution, i.e., the lack of uniformity in microchannel average velocity stemming from the temperature dependence of fluid viscosity [14]. 2. Physical model The effects of flow maldistribution in cross-flow micro heat exchangers are studied using a finite element procedure developed for the analysis of fluid flow and heat transfer in micro heat exchangers of that type [13,14]. With reference to the portion of the core away from the external surfaces parallel to the microchannel layers, it is possible to exploit the existing symmetries to study the thermal performance of a cross-flow micro heat exchanger by solving numerically the energy equation in a domain corresponding to a repetitive portion of the core that includes two half-layers of microchannels (one for the cold and one for the hot fluid) and the solid wall in between (shown in yellow in figure 1). Governing equations The steady-state incompressible flow in each microchannel is governed by the Navier-Stokes equations which can be solved in their parabolised form, provided that the Reynolds number is larger than about 50 so that diffusion of momentum in the axial direction can be neglected [15]

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Results and discussion

Conclusions

Methods