Cavitation-Enhanced Microchannel Heat Exchanger Demonstration and Heat Transfer Correlation Development Using R-134a

Abstract

Electronics of all types, particularly those in the military aviation arena, are decreasing in size while at the same time increasing in power. As a result, newer high-heat-flux electronic components are exceeding the cooling capabilities of conventional single-phase military aviation coldplates and coolants. It is for this reason that we have been investigating new methods to cool the next generation of high-heat-flux military aviation electronics. In this work, a novel method of inducing two-phase conditions within a microchannel heat exchanger has been developed and demonstrated. Micro-orifices placed upstream of each microchannel in a microchannel heat exchanger not only cause an improvement in flow distribution, but can also induce cavitation in the incoming subcooled refrigerant and result in favorable two-phase flow regimes for enhanced heat transfer. In this study, R-134a is used as the coolant in the cavitation enhanced microchannel heat exchanger (CEMC-HX) which has been integrated into a vapor compression refrigeration system. Multiple micro-orifice geometries combined with a fixed microchannel geometry (nominally 250 μm × 250 μm) were investigated over a range of applied base heat fluxes (10–100 W/cm2) and mass fluxes (500–1000 kg/m2-s). Two-phase heat transfer coefficients exceeding 100,000 W/m2-K at refrigerant qualities of less than 5% have been demonstrated due to the achievement of preferential, cavitation-induced, flow regimes such as annular flow. To the author’s knowledge, this is the highest heat transfer coefficient ever reported in the literature for R-134a. Additionally, a four term two-phase heat transfer correlation was developed that achieved a mean absolute error (MAE) of 25.5%.

Heat transfer characteristics of R-134a in a converging-Diverging nozzle

“…Figure 9 indicates that a number of points are as high as Nu = 10,000, corresponding to h = 285kW m −2 K −1 . This is higher than 100 kW m −2 K −1 cited by [22] as the highest recorded heat transfer coefficient for R-134a. Thus these nozzles have a significant potential for application to high heat density…”

“…Cole et al [21] explore whether this phenomena might be useful in a thermal management system. Sole et al [22] study a similar application of cavitation to microchannel heat transfer enhancement for R-134a, showing heat transfer coefficients as high as 100 kW m −2 K −1 for mass fluxes in the range of 500-1000 kg m −2 s −1 . Yuan et al [23] also study this enhancement, documenting an increase in heat transfer with an increase of heat flux and a decrease in quality.…”