Performance Testing and Modeling of Printed Circuit Heat Exchangers for Advanced Nuclear Reactor Applications

Abstract

The primary mission of very-high-temperature reactors (VHTRs) is to generate electricity and provide high-temperature process heat for industrial applications with high efficiency, which relies on an effective intermediate heat exchanger (IHX) that transfers heat from the primary fluid (i.e., helium) to a secondary fluid. A printed circuit heat exchanger (PCHE) is one of the leading IHX candidates to be employed in VHTRs due to its compactness and capability for high-temperature, high-pressure applications with high effectiveness.

In this study, the thermal-hydraulic performance of a fabricated zigzag-channel PCHE was investigated. New pressure drop and heat transfer correlations were developed based on the experimental data. Local thermal-hydraulic performance of the PCHE indicated that fully-developed flow conditions were never achieved, which was attributed to the wavy nature of zigzag flow channels as well as the large temperature variations along the flow direction. This study concluded that heat transfer discrepancies between the hot side and cold side were caused by the differences in both thermal boundary conditions and thermophysical properties. Several effects on the PCHE’s thermal-hydraulic performance were also studied, including the fluid and solid thermophysical properties, radiuses of curvature at zigzag bends, channel configurations, channel pitch lengths in the fluid flow direction, and zigzag pitch angles. It was found that the mean pressure loss factors and mean Nusselt numbers in zigzag channels with sharp bends were 4–6% larger than those in zigzag channels with bends with a curvature radius of 4 mm. A multi-objective optimization for the PCHE’s geometry was conducted based on the numerically obtained correlations. A total number of 142 points on the Pareto front were obtained. Users can select the optimal geometrical parameters for the zigzag-channel PCHE designs from the obtained Pareto front based on their needs. In addition, the stress field of a simplified three-dimensional geometry was obtained. It was observed that the highest stress occurred at diffusion-bonded interfaces and that it was less than the maximum allowable stress of the structural material.

Furthermore, a computer code was developed to predict both the steady-state and transient behaviors of both straight-channel and zigzag-channel PCHEs. Comparisons of the numerical results with the experimental data indicated that the dynamic model was successful in predicting the experimental transient scenarios. The numerical results could also provide useful insight on control strategy development for an integrated high-temperature reactor system with process heat applications, for instance, using the helium mass flow rates or helium inlet temperatures variations to adjust the heat exchanger effectiveness and heat transfer rate.

Finally, the heat exchanger model was implemented into a system dynamic code to simulate the transient behavior of a 20-MWth Fluoride-salt-cooled High-temperature Reactor (FHR). Results were obtained for three initiating events: a positive reactivity insertion, a step increase of the helium flow rate, and a step increase of the helium inlet temperature to the secondary heat exchanger (SHX). The results demonstrated that the FHR reactor, for the three transient scenarios analyzed, had inherent safety features. The results also showed that the intermediate loop consisting of the IHX and SHX played a significant role in the transient progression of the integral system.

This study provides critical insights into the thermal-hydraulic performance of PCHEs that can be applied to nuclear power, offshore industry, solar power, dual cycles for process heat applications, and cooling of electronics and fuel cells.