EXPERIMENTAL INVESTIGATION OF THE EFFECT OF SYNTHETIC JETS ON LOCAL HEAT TRANSFER COEFFICIENTS IN MINICHANNELS DURING LAMINAR AND TURBULENT FLOW CONDITIONS

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.