Two-phase Heat Sink Research Articles

Hybrid microchannel/multi-jet two-phase heat sink: A benchmark and geometry optimization study of commercial product

In this paper, an experimental study of two-phase cooling heat sink using the dielectric working fluid Novec/HFE-7000 is performed. The liquid flow is directed into microchannel fins through multiple impinging jets. A copper microchannel heat sink with a fin height of 3mm, fin thickness of 0.1 mm and channel width of 0.1 mm was used for removing heat flux from the heated surface. A copper block was used to mimic computer chip with a surface area of 1” x 1” (6.45 cm2). The experiment was performed at heat flux from 5 W/cm2 to 80 W/cm2, subcooling of 5 °C, 15 °C, 25 °C, and flow rates 1, 1.25 and 1.75 lpm. The results in the two-phase regime for high heat fluxes are compared with analytical correlations. Reduced subcooling can results in enhancement of heat transfer coefficient by 25 % but will increase the pressure drop through the heat sink. Heat transfer coefficient increases with flow rate in the single-phase region and in the convective boiling region. In nucleate boiling region, heat transfer coefficient increases with heat flux rather than flow rate. Geometry optimization of heat sink was carried out considering effects of fin height on thermal and hydraulic performance. It is found that thermal resistance variation with fin height shows a parabolic trend and an improvement of 46 % in thermal performance can be obtained. A new correlation was also developed to predict results of experimental data with mean absolute error of 7 %. The inclusion of Reynolds number and hydraulic diameter was found to considerably reduce error when compared to values obtained with the previously available correlation, in which only the subcooling and heat flux were the parameters for modelling the heat transfer.

Time-resolved characterization of microchannel flow boiling during transient heating: Part 2 – Dynamic response to time-periodic heat flux pulses

Flow boiling in microchannels is an effective method for dissipating high heat fluxes. However, two-phase heat sink operation during transient heating conditions remains relatively unexplored. In Part 1 of this two-part study, the dynamic response of flow boiling to a single heat flux pulse was experimentally studied. In this Part 2, the effect of heating pulse frequency on microchannel flow boiling is explored when a time-periodic series of pulses is applied to the channel. HFE-7100 is driven through a single 500 μm-diameter glass microchannel using a constant pressure reservoir. A thin indium tin oxide layer on the outside surface of the microchannel enables simultaneous transient heating and flow visualization. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to the flow visualizations to characterize the boiling process. A square-wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m2). Three different time-periodic flow boiling fluctuations were observed for the heat flux levels and pulse frequencies investigated in this study: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heat flux pulses between 15 and 75 kW/m2 and heating pulse frequencies above 1 Hz, time-periodic flow regime transitions between single-phase and two-phase flow are reported. For heating profiles involving 150 kW/m2 heat flux pulses, fluid in the microchannel is always boiling and thus the flow regime transitions are eliminated. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. Outside this heating pulse frequency range, the pressure drop instability occurs at the intrinsic frequency of the system. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.

Microscale layering of liquid and vapor phases within microstructures for a new generation two-phase heat sink

In this study, a new heat sink architecture is introduced that operates at two different phase change heat transfer modes. At low wall superheat temperatures, the heat sink operates under the thin film evaporation heat transfer mode and then transitions to the boiling heat transfer mode when the wall superheat temperature increases. This unique function is enabled through constraining the liquid and vapor phases into separate domains using a capillary-controlled meniscus formed within a hierarchical 3D structure. The structure is designed to form thin liquid layers between vertically oriented menisci across which the liquid is evaporated into neighboring vapor channels. The entire structure is then capped by a hydrophobic vapor-permeable membrane to fully confine the liquid layers while allowing vapor to pass through the membrane. At low wall superheats, the liquid layers directly evaporate into the vapor channels. In this operation mode, the heat flux was linearly increased to a maximum of 54W/cm2 at approximately 8°C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 62kW/m2K. The heat transfer coefficient only slightly declined with increasing the wall superheat temperature but substantially improved as the liquid supply pressure was increased. Increasing the superheat temperature beyond 7–9°C resulted in transition to the boiling heat transfer mode with a pronounced increase in surface temperature fluctuations. This transition to boiling results in a decline in the heat transfer coefficient because the meniscus formed between the liquid and vapor spaces breaks down. However, the heat removal capacity is significantly increased, and a critical heat flux of about 300W/cm2 is reached at <30°C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 100kW/m2K.

Experimental Characterization of a Two-Phase Heatsink

Experimental Characterization of a Two-Phase Heatsink

Two-Phase Heat Sinks with Microporous Coating

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy based and was sprayed on heat sink channels, while the second coating was formed by sintering copper particles on heat sink channels. Minichannel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and 12 rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for minichannel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from ∼6°C to slightly more than 1°C. No increase in pressure drop or pumping power due to the microporous coating was measured. The minichannel heat sinks with porous coating had on average 1.5 times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had on average 2.5 times higher heat transfer coefficient and the critical heat flux was 1.5 to 2 times higher compared with the uncoated heat sinks.

Measurement and prediction of pressure drop in a two-phase micro-pin-fin heat sink

This study concerns pressure drop in a two-phase heat sink containing an array of staggered square micro-pin-fins having a 200 × 200 μm 2 pin cross-section by a 670 μm pin height. Three inlet temperatures of 30, 60 and 90 °C, and six maximum mass velocities for each inlet temperature, ranging from 183 to 420 kg/m 2 s, were tested. Frictional pressure drop in the boiling region is deemed the dominant pressure drop component. The Lockhart–Martinelli correlation for laminar liquid–laminar vapor combination in conjunction with a previous single-phase friction factor correlation can adequately predict the data. Micro-pin-fins offer better flow stability than parallel micro-channels.

An experimental study of two phase flow and boiling heat transfer in bi-dispersed porous channels

The results of an experimental study of two-phase flow and boiling heat transfer in channels packed with sintered copper bi-dispersed porous media are presented. For comparison, the same channels were packed with sintered copper mono-dispersed porous media. Experiments were conducted using distilled water as the working fluid. The absolute permeabilities of the sintered copper mono- and bi-dispersed porous media were firstly determined by measuring pressure drops at different inlet velocities of the isothermal flow of subcooled water. Experiments were then carried out to characterize flow boiling of water through both mono- and bi-dispersed porous channels. The pressure drops under the boiling condition were also measured. The results indicated that the porous material has a highly effective two-phase heat sink, especially for bi-dispersed porous materials which have a lower flow resistance than that of the mono-dispersed porous materials having the same pore diameter as the micro-pore diameter of the bi-dispersed porous material.

Two-Phase Heat Dissipation Utilizing Porous-Channels of High-Conductivity Material

The results of an experimental study of two-phase heat dissipation in high-conductivity porous channel heat sinks are presented. Porous channels of various sizes were fabricated using sintered copper particles inside rectangular copper channels with base dimensions of 25 mm by 25 mm, either 3 or 10 mm in height. The experiments were conducted using subcooled water as the working fluid and test conditions ranged from an inlet temperature of 85 to 95°C, inlet pressures of 1.062 to 1.219 bars, flow rates of 22.5 to 150 ml/min, and heat fluxes of 10 to 25 W/cm2. The experimental results were compared to the results predicted using a previously developed numerical model. For water with inlet subcooling in the range of 6.6 to 10.8°C, heat transfer coefficients for open channel flow were increased from 1.25 to 1.94 W/cm2°C to 1.79 to 3.33 W/cm2°C, or a 43 to 142 percent improvement through the use of porous channels with mean particle diameters of 0.97, 0.54, 0.39, or 0.33 mm. The results indicate that the high thermal conductivity of the porous material and the large solid-fluid contact area combine to create a highly effective, two-phase heat sink, which may provide an effective mechanism for cooling high heat flux microelectronics.