Cooling Performance of Microjet Arrays
CapeSym's MicroJet Cooling Arrays (MJCAs) can be used with three forms of impingement cooling for different levels of performance. With the measured heat transfer coefficients, a micro-jet impingement cooling system can easily handle heat fluxes in excess of 1kW per square centimeter.
Liquid Impingement
Heat transfer coefficients up to 50 W/cm2 K have been obtained with water as the coolant. The microjets in these experiments were 300µm in diameter, returns 250µm in diameter, and the array was placed about 360µm away from the target.
Two sets of experimental results are shown in the adjacent figure, one for an unmodified target surface (red) and another with conical cavities fabricated under each jet (green). The modified target surface significantly increased the heat transfer coefficient, confirming our theoretical prediction. Fundamental considerations indicate that by varying the geometry of the cavities we can improve the performance of MJCAs further.
MJCAs have also been tested with fluids other than water. The effect of fluid properties is well characterised by the laminar correlation for Nusselt number for laminar jets: Nu ~ Re0.5 Pr0.3
Air Impingement
Air cooling has a number of advantages over liquid cooling: Air is readily available; leakage does not offer any hazards and does not damage neighboring devices; in some applications air can be exhausted into the environment obviating the need for recycling equipment; and air has a much wider operating temperature range than most liquids whose performance is limited to the range spanned by their freezing and boiling point temperatures.
The thermophysical properties of air render it unsuitable for removing high heat fluxes with traditional approaches, but with impingement using MJCAs we have demonstrated heat transfer coefficients as high as 2 W/cm2 K. This capability significantly expands the levels of heat flux that can be handled by air cooling.
Boiling Impingement
In general, heat removal systems that rely on boiling of a flowing stream of coolant, the so-called forced convective boiling, are characterized by very high heat transfer coefficients. Thus, for a given thermal load they can be made more compact (smaller surface area) and require smaller coolant flow rates than single-phase systems. The limiting factor in convective, as well as pool, boiling is the burnout (also called Critical Heat Flux, CHF) limit. This upper limit corresponds to the condition where boiling is so vigorous and the velocity of departing bubbles is so high that the liquid coolant cannot reach the hot wall and a vapor blanket is formed over the hot surface. The very low thermal conductivity of the vapor blanket results in rapid and dramatic increase in the temperature of the hot surface. In most physical systems the temperature increase can be as high as several hundred degrees, resulting in catastrophic failure.
A typical value of CHF for nucleate boiling is about 100 W/cm2. In forced convective systems, this value can be increased with increasing flow rate. We have demonstrated that by using microjet impingement, the value of CHF can be increased substantially to above 1 kW/cm2 at extremely low flow rates. These results are shown below and indicate that impingement boiling can be effectively used for removing very high heat fluxes at fairly small flow rates.
