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. Initial work focused on two-phase flow regimes and distribution characteristics of air-water mixtures in plate-type heat exchanger headers. The results quantified the effects of inlet mass flux, inlet quality, header flow regime and header pressure drop on distribution. An experimental study of more realistic microchannel condenser manifolds was designed based on these initial conclusions. In these tests, saturated refrigerant was 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 was designed, fabricated, and calibrated. The flow visualization and distribution results from this study will be used to develop a mechanistic model, and insights from these experiments will be used to develop header designs that improve the distribution in microchannel heat and mass transfer components.