Pumpless Loop for Narrow Channel and Micro-Channel Boiling

Abstract

A compact cooling system is examined which capitalizes upon fluid density differences between two vertical, parallel, interconnected tubes to achieve a pumpless cooling loop. A heat-dissipating device is incorporated into a boiler at the bottom of the hot tube. The large density differences between the two tubes produces a substantial nonequilibrium in hydrostatic pressure, drawing liquid downwards through the cold tube as a two-phase mixture is released upwards in the hot tube. Cooling with this pumpless loop is fundamentally different from, and far superior to, pool boiling thermosyphons because of the former’s ability to separate the path of replenishment liquid from that of the released vapor. Experiments were performed to explore the effects of boiler gap (separation distance between the boiling surface and opposite insulating wall) on cooling performance and critical heat flux (CHF) for water and FC-72. The gap, which is the primary measure of boiler miniaturization, was varied from 0.051 to 21.46 mm. For large gaps, CHF showed insignificant dependence on the gap for both fluids. However, small gaps produced CHF variations that were both drastic and which followed opposite trends for the two fluids. Decreasing the gap below 3.56 mm produced a substantial rise in CHF for FC-72. For water, CHF was fairly insensitive down to 0.51 mm, below which it began to decrease sharply. These trends are shown to be closely related to the small surface tension and contact angle of FC-72 producing very small bubbles which can easily pass through narrow gaps in FC-72, while much larger bubbles in water obstruct liquid replenishment in narrow gaps. A numerical model is constructed to determine how the gap influences the various components of pressure drop, velocities, coolant flow rate, and hence system response to heat input.

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

“…In fact, annular flow was the dominant regime for most medium-and high-flux conditions. This drastic difference between two-phase flow patterns for R134a and water is explained by the low surface tension of R134a producing far smaller bubbles than in water [20]. Very small bubbles are far more apt to travel discretely along a micro-channel before coalescing into long bubbles, thus enabling the bubbly and slug flow regimes to be more prevalent in refrigerant microchannel flow.…”

Thermal design methodology for high-heat-flux single-phase and two-phase micro-channel heat sinks

“…The model has been successful at predicting flow boiling pressure drop of R-113 in a heat sink containing 510 circular micro-channels [17], [19]. On the other hand, a recent study by Mukherjee and Mudawar [30] revealed several fundamental differences in flow boiling behavior in micro-channels between water and FC-72; the latter is a fluorochemical coolant with thermophysical properties fairly similar to those of R-113. Their results reveal the low surface tension and small contact angle of fluorochemicals produce bubble departure diameters that are one to two orders of magnitude smaller than those for water.…”

“…The boundary condition for Wall A of the unit cell is (30) and all other unit cell boundaries are assumed adiabatic.…”

Two-Phase Microchannel Heat Sinks: Theory, Applications, and Limitations

“…Mukherjee and Mudawar [160,161] developed a new passive cooling technique that can yield far better cooling rates than thermosyphons. The system they developed utilizes fluid density 041002-24 / Vol.…”

“…To assess the effectiveness of a microchannel boiler at both overcoming these pressure gradients and achieving superior cooling performance compared to thermosyphons, Mukherjee and Mudawar [160,161] tested this concept with both FC-72 and water by varying the "boiler gap" from 0.051 to 21.46 mm as well as using boiler surfaces with vertical microchannels. For large gaps, CHF was fairly insensitive to gap width for both fluids.…”