A finite volume coaxial heat exchanger model with moving boundaries and modifications to correlations for two-phase flow in fluted annuli

Coaxial Heat Exchangers (CHXs) are now used extensively in heat pump (HP) and refrigeration systems. The design of such systems requires estimation of CHX's thermal and hydraulic performance. This paper presents a generalized finite volume CHX model that is capable of simulating single-phase and two-phase flow with a smooth or fluted inner tube. The concepts of segment insertion and subdivision (moving boundary within the segment) are adopted in this model to track the phase change point along the flow channel. This allows for reliable model accuracy even with a lower number of discretized finite volumes, thereby reducing computation time. For the segment insertion and subdivision functions, four different approaches are provided in order to achieve the best accuracy and least computational effort under various flow configurations and fluid conditions. The simulation model can be applied to straight and helical tubes with choices of smooth, grooved and fluted tube surface. Empirical single-phase and two-phase correlations from the literature are adopted in the proposed model for the applicable surfaces. At present, there are no correlations for two-phase flow in fluted tube annuli, in the open literature. This paper proposes modifications to existing two-phase fluted surface heat transfer and pressure drop correlation formulations by applying empirical two-phase flow multipliers onto existing fluted tube single phase correlations. The solving methodology of this model requires the heat exchanger geometry, flow configuration, inlet states, as well as mass flow rates of the fluids to solve for the outlet conditions, heat load, pressure drop and charge of the CHX. Coupled with non-linear equation solver, the model can also provide the required mass flow rate for a specific superheat/subcooling on either side of the CHX. The model and modified correlations are validated against experimental data of brine-to-refrigerant fluted tube condenser and evaporator used in a heat pump application.