Intermediate Heat Exchanger Dynamic Thermal Response

This report presents UCB progress in developing a comprehensive thermal and fluid dynamics model for the NGNP intermediate heat exchanger (IHX) and other compact heat exchangers. For nuclear hydrogen applications, an IHX is required to transfer heat from high temperature and high-pressure primary helium coolant to a hydrogen production process. An intermediate heat transfer loop is used for the purpose. Under these conditions, plate-type heat exchangers with small flow channels, such as the well known Heatric designs, are a major candidates because they can achieve high power densities with small amounts of material, and can be fabricated using a diffusion bonding process so that the entire heat exchanger has the strength of the base 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 interrupted abruptly. UCB has proposed a capillary shell and tube IHX configuration that could have lower susceptibility to thermal shock. For all IHX options accurate analysis of global and local thermal stresses are critical to evaluating the heat exchanger reliability and safety. In order to estimate the stresses in compact heat exchangers a comprehensive thermal and hydraulic model is needed. The model developed here uses an effective porous media (EPM) approach because the evaluation of the detailed global flow with computational fluid dynamics (CFD) as well as finite element methods (FEM) for the mechanical analysis, at the resolution scale of the flow channels involves prohibitive computational time. The EPM fluid dynamics and heat transfer computational code developed at UCB is called the compact heat exchanger thermal and hydraulics (CHEETAH) code. CHEETAH solves for the transient temperature-distribution in the IHX. This temperature distribution can then be imported into a commercial finite element analysis (FEA) code for mechanical stress analysis using the EPM methods developed earlier by UCB for global and local stress analysis [2]. These simulation tools will also allow the designer to optimize the heat exchanger design, to minimize the pressure drop while maximizing the IHX’s thermal effectiveness, as well as to optimize the mechanical performance of the IHX particularly as it relates to creep deformation and transient thermal stresses.