(612f) Multi-Scale Thermal Analysis for Compact Plate-Type Heat Exchangers

This paper presents thermal analysis methods and results for compact plate-type heat exchangers for high temperature heat transfer, using the effective porous media (EPM) approximation. For nuclear hydrogen applications an intermediate heat exchanger (IHX) is required to transfer heat from high temperature high-pressure primary helium to an intermediate heat transfer fluid, and process heat exchangers are required to transfer heat from the intermediate fluid to the process fluid. Under these conditions, plate-type and other compact heat exchangers with small flow channels provide a good candidate approach for this application, because they can achieve high power densities with small amounts of material. However, these types of heat exchangers can be susceptible to very large stresses during thermal transients, for example when the flow of one fluid is abruptly stopped.

The evaluation of the detailed flow, temperature and stress distributions in a compact heat exchanger with computational fluid dynamics (CFD) and finite element methods (FEM), at the resolution scale of the flow channels, requires prohibitive computational effort. Previous work at UC Berkeley [1] developed an alternative method to obtain approximate stresses that greatly reduces computational effort. This method is composed of three steps. First, the heat exchanger is broken down into several regions. Unit cell models are built based on each region that captures all of the most important features of that region. The effective mechanical and thermal properties for each unit cell are then determined using FEM simulations. Second, global stress distributions based on the volume-averaged unit-cell-equivalent model are computed by using the effective mechanical and thermal properties. Third, the values from the stress distributions are then applied to the unit cells to find localized points of high stress.

In the work reported here, the methods developed to predict global and local stress distributions are extended to predict thermal response, using an effective porous media (EPM) approach. In this approach, CFD analysis (or experiments) at the subchannel level are used to predict effective permeabilities and heat transfer coefficients, which are then used to analyze global transient flow and temperature distributions, which can in turn be used to predict global and local stress distributions. Derivations for the new method are provided, along with a summary of the finite-difference numerical implementation and examples of results.

References

1) W. Huang, H. Zhao, and P.F. Peterson, ?Multi-scale Stress Analysis for Compact Plate Heat Exchangers,? 2006 International Congress on Advances in Nuclear Power Plants (ICAPP '06), Reno, NV, June 4-8, 2006.