Experimental distribution of phases and pressure drop in a two-phase offset strip fin type compact heat exchanger

Abstract Uniform distribution of fluids is crucial to obtain high performance in compact heat exchangers. Maldistribution has been studied by many authors, especially for parallel channels heat exchangers. But theoretical models and experimental studies for predicting flow maldistribution in offset strip fins exchangers are scarce. Offset strip fins, besides their higher thermal hydraulic performances, favour lateral distribution due to their geometry. In this work, an experimental investigation has been carried out for this type of heat exchanger. The experimental set-up consists in a flat vertical compact heat exchanger (1 m × 1 m area and 7.13 mm thickness) equipped with offset strip fins with a hydraulic diameter of 1.397 mm. Air and water are the working fluids. The flow rates of each phase in seven zones regularly distributed at the outlet have been measured as well as the pressures at the inlet, the outlet and two intermediate positions. These measurements were completed with visualisations using a high-speed camera. First, the single-phase flow has been investigated. A correlation for friction factor has been derived from experiments covering laminar, transition and turbulent regimes. CFD simulations of the single-phase flow have been performed. The numerical results were compared with the determined correlation and with correlations available in the literature. In single-phase flow, a uniform distribution was experimentally observed. Then, the two-phase hydrodynamics was characterised. A flow regime map was established and the influences of phases inlet directions (co-current and counter-current inlets of the phases) and of superficial velocities on the distribution were studied. The gas superficial velocity has more effect on the distribution than the liquid one. Comparison between pressure drop profiles and flow rate distribution profiles shows that information about pressure drop can provide information about phase distribution. It must be noticed that the nonuniform distribution of phases can entail the coexistence of several flow regimes in the heat exchanger.

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