Methodology to develop off-design models of heat exchangers with non-ideal fluids

Methodology to develop off-design models of heat exchangers with non-ideal fluids

Supercritical CO2 (sCO2) closed Brayton cycles are promising heat engines for next-generation thermal power plants since they are efficient, highly scalable, and compatible with a variety of heat sources. A potential application for these cycles is load-following concentrating solar power plants with thermal storage, which will frequently operate at off-design conditions. Accurate and computationally efficient models of the cycle’s heat exchangers and turbomachinery are required to assess and optimise its off-design performance. The printed circuit heat exchangers (PCHEs) used in the sCO2 closed Brayton cycle are challenging to model since they exhibit non-ideal-gas effects and typically use zigzag channels, for which flow patterns and heat transfer mechanisms are not completely understood. Moreover, heat transfer correlations that capture all effects relevant to a given geometry and flow conditions are often unavailable.

We present a methodology to develop accurate and computationally efficient on- and off-design models of heat exchangers that exhibit complex nonlinear behaviours. This methodology involves fitting a 1D discretised heat exchanger model to experimental data using nonlinear least-squares optimisation. Unknown internal heat exchanger geometric parameters are used as fitting parameters.

We demonstrate the proposed methodology by developing numerical models for two PCHEs: (1) an sCO2–sCO2 PCHE operating far from CO2’s critical point and (2) an oil–sCO2 PCHE operating close to CO2’s critical point, where non-ideal fluid property variations are significant. Test data spans heat loads from 6–48% and 15–39% of name-plate duty for heat exchangers (1) and (2) respectively. Across these operating ranges, the maximum and mean heat transfer prediction residuals are 0.91% and 0.36% for heat exchanger (1) and 3.04% and 1.24% for heat exchanger (2). Additionally, we show that good accuracy can be obtained when using only the channel hydraulic diameter as a fitting parameter. Due to their low computational cost and high accuracy, models developed using the proposed methodology are eminently suitable for off-design modelling and optimisation of the sCO2 closed Brayton cycle and other power cycles or industrial processes where heat exchangers exhibit complex nonlinear behaviour.

Files associated with this item cannot be displayed due to copyright restrictions.