SUMMARY

Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of energy systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. In the present study, flow distribution in mini- and microchannel heat exchanger manifolds is investigated. Two-phase flow regimes and distribution characteristics of air-water mixtures in plate-type heat exchanger headers are investigated first. These investigations quantified the effects of inlet mass flux, inlet quality, header flow regime and header pressure drop on distribution. The insights gained from these air-water mixture studies are used to investigate flow distribution in microchannel condenser manifolds. Saturated refrigerant is supplied to the header and distributed into ten parallel 1-mm diameter channels. Accurately measuring distribution in these experiments is critical; therefore, a novel compact refrigerant flow rate sensor is designed, fabricated, and calibrated. These flow visualization and distribution data are used to develop a distribution model that predicts the liquid and vapor flow rates entering each heat exchanger channel. The results from this study provide insights into the factors that affect two-phase flow distribution and offer potential methods to evenly distribute liquid and vapor in alternative header geometries.