DA-00-02-2 -- Evaporative Heat Transfer in Mesoscale Heat Exchangers

An application for an integrated array of small (mesos-cale) cooling devices is introduced and a survey of related research is provided. A test apparatus appropriate for experiments with small-scale, low-capacity evaporators is described. Two-phase pressure drop and heat transfer data are presented for R-134a in a heat exchanger consisting of an inlet manifold, 52 parallel channels, and an exit manifold. Each individual channel has a cross-sectional flow area 800 µm by 800 µm and is 74 µm long. Experiments are conducted over a range of conditions, with flow rates up to 0.48 g/s, inlet qual-ities from 7% to 15%, and an evaporating temperature of approximately 10°C. The heat exchanger operated with a pres-sure drop of less than about 8 kPa and provided a heat transfer coefficient greater than 8000 W/(m²•K). The heat transfer data suggest that nucleate boiling dominates for flow rates below an equivalent Reynolds number (Re_eq) of about 40 in a channel. A comparison of the pressure drop and heat transfer results to related data from the literature shows general agreement and supports these promising results for mesoscale heat exchangers.

Units: SI