Heat transfer has been investigated in the presence of hydrodynamic cavitation instigated by 20-μm wide inlet micro-orifices entrenched inside 227-μm hydraulic diameter microchannels. Average surface temperatures, heat transfer coefficients, and pressure drops have been obtained over effective heat fluxes ranging from 39 to 558W∕cm2 at mass flux of 1814kg∕m2s under noncavitating and three cavitating conditions. Significant heat transfer enhancement has been recorded during supercavitating flow conditions in comparison to noncavitating flows with minimal pressure drop penalty. Once supercavitating conditions were reached, no apparent heat transfer augmentation was detected with the reduction of the cavitation index. Visualization of the flow morphology and the heat transfer coefficient characteristics aided in the evaluation of the dominant heat transfer mechanism under various thermal-hydraulic conditions.

Critical heat flux (CHF) of water in circular stainless steel microchannels with inner diameters ranging from ∼127μm to ∼254 μm was investigated. Forty-five CHF data points were acquired over mass velocities ranging from 1,200 kg/m2s to 53,000 kg/m2s, heated lengths from 2 cm to 8 cm, and exit qualities from −0.2 to 0.15. Most of the exit qualities fell below 0.1. It was found that CHF conditions were more dependent on mass velocity and heated length than on exit thermal condition. The results were also compared to six CHF correlations, with a mean average error ranging from 22% to 261.8%. A new correlation was proposed to better predict the critical heat flux data under the thermal-hydraulic conditions studied in this investigation. In developing the correlation, 319 data points were added from two previous studies.

No abstract

Microchannels and minichannels have been shown to have many potential applications for cooling high-heat-flux electronics over the past 3 decades. Synthetic jets can enhance minichannel performance by adding net momentum flux into a stream without adding mass flux. These jets are produced because of different flow patterns that emerge during the induction and expulsion stroke of a diaphragm, and when incorporated into minichannels can disrupt boundary layers and impinge on the far wall, leading to high heat transfer coefficients. Many researchers have examined the effects of synthetic jets in microchannels and minichannels with single-phase flows. The use of synthetic jets has been shown to augment local heat transfer coefficients by 2–3 times the value of steady flow conditions. In this investigation, local heat transfer coefficients and pressure loss in various operating regimes were experimentally measured. Experiments were conducted with a minichannel array containing embedded thermocouples to directly measure local wall temperatures. Flow regimes ranged from laminar to turbulent. Local wall temperature measurements taken directly beneath the synthetic jet in a laminar flow regime indicated that when a synthetic jet was used, the heat transfer coefficient was increased as much as 2.8 times the value as when synthetic jets were not used. Significant heat transfer coefficient augmentation also propagated to the upstream location, where heat transfer was increased to 2.2 times the value as when the synthetic jets were not used. Additional measurements show that synthetic jets significantly altered the pressure loss coefficient of the minichannels and that this effect was more pronounced in laminar flow than in turbulent flow. The effect of operating frequency on heat transfer and pressure loss is also presented. It was shown that the optimal operating point for the synthetic jet within a minichannel was in transitional to weakly turbulent flow (2600<Re<4500) to maximize the increase in heat transfer coefficient and minimize the increase in pressure loss.

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%.