This paper explores the subcooled nucleate boiling and critical heat flux (CHF) characteristics of a hybrid cooling module that combines the cooling attributes of micro-channel flow and jet impingement. A test module was constructed and tested using HFE-7100 as working fluid. Increasing the coolant's flow rate and/or subcooling shifted both the onset of boiling (ONB) and CHF to higher heat fluxes and higher wall temperatures. The hybrid module yielded heat fluxes as high as 1127 W/cm2, which is the highest value ever achieved for a dielectric coolant at near atmospheric pressure. It is shown the hybrid cooling configuration involves complex interactions between circular jets and micro-channel flow, and unusual spatial variations of void fraction and liquid velocity. These variations are ascertained using the Developing Homogeneous Layer Model (DHLM) in which the micro-channel flow is described as consisting of a homogeneous two-phase layer along the heated wall and a bulk liquid layer. CHF is determined by a superpositioning technique that consists of dividing the heated wall into two portions, one dominated by jet impingement and the other micro-channel flow. This technique is shown to be highly effective at predicting the CHF data for the hybrid cooling configuration.

This study examines the single-phase and two-phase cooling performance of a hybrid micro-channel/micro-jet impingement cooling scheme using HFE 7100 as working fluid. This scheme consists of supplying coolant from a series of jets that deposit liquid into the micro-channels. A single-phase numerical scheme that utilizes the k–ε turbulent model and a method for determining the extent of the laminarized wall layer shows very good predictions of measured wall temperatures. It is shown jet velocity has a profound influence on single-phase cooling performance. High jet velocities enable jet fluid to penetrate the axial micro-channel flow and produce a strong impingement effect at the wall. On the other hand, the influence of jets at low jet velocities is greatly compromised compared to the micro-channel flow. During nucleate boiling, vapor layer development along the micro-channel in the hybrid module is fundamentally different from that encountered in conventional micro-channels. Here, subcooled jet fluid produces repeated regions of bubble growth followed by bubble collapse, rather than the continuous growth common to conventional micro-channel flow. By reducing void fraction along the micro-channel, the hybrid scheme contributes greater wall temperature uniformity. Increasing subcooling and/or flow rate delay the onset of boiling to higher heat fluxes and higher wall temperatures, and also increase critical heat flux considerably. A nucleate boiling heat transfer coefficient correlation is developed that fits the present data with a mean absolute error of 6.10%.

This study examines the cooling performance of two hybrid cooling schemes that capitalize upon the merits of both micro-channel flow and jet impingement to achieve the high cooling fluxes and uniform temperatures demanded by advanced defense electronics. The jets supply HFE 7100 liquid coolant gradually into each micro-channel. The cooling performances of two different jet configurations, a series of circular jets and a single slot jet, are examined both numerically and experimentally. The single-phase performances of both configurations are accurately predicted using 3D numerical simulation. Numerical results point to complex interactions between the jets and the micro-channel flow, and superior cooling performance is achieved by optimal selection of micro-channel height. The two-phase cooling performance of the circular jet configuration is found superior to that of the slot jet, especially in terms of high-flux heat dissipation. Unprecedented cooling fluxes as high as 1,127 W/cm2 are achieved with the circular jets without incurring CHF.

This paper explores the two-phase cooling performance of a hybrid cooling scheme in which a linear array of micro-jets deposits liquid gradually along each channel of a micro-channel heat sink. The study also examines the benefits of utilizing differently sized jets along the micro-channel. Three micro-jet patterns, decreasing-jet-size (relative to center of channel), equal-jet-size and increasing-jet-size, were tested using HFE 7100 as working fluid. It is shown feeding subcooled coolant into the micro-channel in a gradual manner greatly reduces vapor growth along the micro-channel. Void fraction increased between jets but decreased sharply beneath each jet, creating a repeated pattern of growth followed by coalesce, and netting only a mild overall increase in void fraction along the flow direction with predominantly liquid flow at outlet. Unlike most flow boiling situations, where pressure drop increases with increasing heat flux, pressure drop in the hybrid configurations actually decreased and reached a minimum just before CHF. This behavior is closely related to the low void fraction and predominantly liquid flow. Pressure drop in the two-phase region is highest for the equal-jet-size pattern, followed by the decreasing-jet-size and increasing-jet-size patterns, respectively. Low void fraction increased the effectiveness of the hybrid cooling schemes in utilizing bulk liquid subcooling and therefore helped achieve high CHF values. The decreasing-jet-size pattern, which had the highest outlet subcooling, achieved the highest CHF. A single correlation was constructed for the three jet patterns, which relates the two-phase heat transfer coefficient to heat flux and wall superheat.

A new hybrid cooling scheme is proposed for high-flux thermal management of electronic and power devices. This scheme combines the cooling benefits of micro-channel flow and micro-jet impingement with those of indirect refrigeration cooling. Experiments were performed to assess single-phase cooling performance using HFE 7100 as working fluid. Excellent predictions were achieved using the standard k–ε model. The proposed cooling scheme is shown to involve complex interactions of impinging jets with micro-channel flow. Increasing jet velocity allows jets to penetrate the micro-channel flow toward the heated surface, especially in shallow micro-channels, greatly decreasing wall temperature. Despite the relatively poor thermophysical properties of HFE 7100, the proposed cooling scheme facilitated the dissipation of 304.9W/cm2 without phase change; further improvement is possible by increasing jet velocity and/or decreasing coolant temperature. In addition to the numerical predictions, a superpositioning technique is introduced that partitions the heat transfer surface into zones that are each dominated by a different heat transfer mechanism, and assigning a different heat transfer coefficient value to each zone. Using this technique, a new correlation is developed that fits the data with a mean absolute error of 6.04%.

: This report examines the pressure drop and heat transfer characteristics of two-phase micro-channel heat sinks. New models and correlations are proposed for both saturated and subcooled boiling conditions. These tools are shown to provide accurate predictions of pressure drop, two-phase heat transfer coefficient and critical heat flux. Also discussed in this report is a new hybrid cooling system that combines the cooling attributes of micro-channel flow and jet impingement. It is shown this system is very effective at meeting the cooling demands of high-flux defense electronics, including the ability to dissipate in excess of 1000 W/sq cm. Detailed predictive tools are provided to tackle the design of a hybrid cooling module.

This paper explores the single-phase and two-phase cooling performance of a hybrid micro-channel/slot-jet module using HFE-7100 as working fluid. Three-dimensional numerical simulation using the k–ε turbulent model is used to both assess the single-phase performance and seek a geometry that enhances heat removal capability and surface temperature uniformity while decreasing mean surface temperature. This geometry is then tested experimentally to validate the numerical findings and aid in the development of correlations for both the single-phase and two-phase heat transfer coefficients. The hybrid module is shown to maintain surface temperature gradients below 2°C for heat fluxes up to 50W/cm2. Even without phase change, the hybrid module is capable of dissipating heat fluxes as high as 305.9W/cm2. Highly accurate single-phase correlations are developed using a superpositioning technique that consists of assigning a different heat transfer coefficient for each portion of the heat transfer area based on the dominant heat transfer mechanism for that portion. Increasing subcooling and/or flow rate is shown to delay the onset of nucleate boiling to a higher heat flux and higher surface temperature, as well as enhance critical heat flux (CHF). A correlation previously developed for hybrid micro-channel/micro-circular-jet module is deemed equally effective at predicting two-phase heat transfer data for the present hybrid module.

Experimental and numerical methods were used to explore the cooling performance of a new hybrid device consisting of a slot jet impinging into a micro-channel, thus capitalizing upon the merits of both cooling configurations. The three-dimensional heat transfer characteristics of this device were analyzed using the standard k–ε turbulent model. Numerical predictions for liquid PF-5052 show excellent agreement with experimental measurements. Vorticity effects are shown to greatly influence cooling performance outside the impingement zone. Higher jet Reynolds numbers yielded stronger attachment to the heated surface and lower surface temperatures. The model was also used to optimize the cooling performance for a water-cooled device. Lower surface temperatures were achieved by decreasing jet width and micro-channel height. These findings are used to recommend a simplified hybrid cooling geometry in pursuit of both lower surface temperatures and smaller temperature gradients across the heated surface.

Experiments were performed to investigate the two-phase cooling characteristics of a new hybrid cooling scheme combining the cooling attributes of slot jets and micro-channel flow. A test module was constructed in which dielectric PF-5052 liquid was introduced through five 0.48 mm wide and 12.7 mm long slot jets, each leading to a 1.59 mm wide and 1.02 mm deep channel. Increases in flow rate and subcooling yielded similar trends of delaying the inception of boiling and increasing critical heat flux (CHF). A previous channel flow correlation predicted CHF values far smaller than measured, while those for slot jets yielded closer predictions. This proves the cooling performance of the hybrid configuration is dominated more by jet impingement than by micro-channel flow. By dividing the test surface into a portion that is dominated by jet impingement and another by micro-channel flow, and applying the appropriate CHF correlation for each portion, the CHF data for this hybrid cooling configuration are predicted with a mean absolute error of 8.42%.