Emil Rahim

Modeling and Prediction of Two-Phase Microgap Channel Heat Transfer Characteristics

A detailed analysis of microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids, gathered from the open literature and sorted by the Taitel and Dukler flow regime mapping methodology, is performed. Annular flow is found to be the dominant regime for this thermal transport configuration and to grow in importance with decreasing channel diameter. A characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity) for the flow of refrigerants and dielectric liquids in miniature channels is identified. The inflection points in this M-shaped curve are seen to equate approximately with flow regime transitions, including a first maximum at the transition from Bubble to Intermittent flow and a second maximum at moderate qualities in Annular flow, just before local dryout begins. The predictive accuracy of five classical two-phase heat transfer correlations for miniature channel flow is examined. Selecting the best fitting of the classical correlations for each of the flow regime categories is seen to yield predictive agreement with regime-sorted heat transfer coefficients that does not depart significantly from the agreement found in large pipes and channels.

Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels

This keynote lecture will open with a brief review of the primary two-phase flow regimes and their impact on thermal transport phenomena in tubes and channels. The Taitel and Dukler flow regime mapping methodology will then be described and applied to the two-phase flow of refrigerants and dielectric liquids in microgap channels. The effects of channel diameter, as well as alternative transition criteria, on the prevailing flow regimes in microgaps will be explored along with available criteria for microchannel behavior. Available microgap data will then be shown to reflect the dominance of annular flow and to display a characteristic heat transfer coefficient curve in such configurations. It is found that the heat transfer coefficients in the low-quality annular flow segment of this locus can be predicted by available, microtube correlations, but that the moderate-quality transition to the axially-decreasing segment occurs at substantially.

Direct Submount Cooling of High-Power LEDs

Rapidly increasing light emitting diode (LED) heat fluxes necessitate the development of aggressive thermal management techniques that can intercept the dissipated heat directly in the submount. Microgap coolers, which eliminate solid-solid thermal interface resistance and provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, offer a most promising approach for cooling high-power LEDs. This paper focuses on the two-phase thermofluid characteristics of a dielectric liquid, FC-72, flowing in an asymmetrically heated chip-scale microgap channel, 10 mm wide × 37 mm long, with channel heights varying from 110 μm to 500 μm and channel wall heat fluxes of 200 kW/m2. The experimental two-phase, area-averaged heat transfer coefficients of FC-72 reached 10 kW/m2·K, significantly higher than the single-phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single-phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique. It is shown that this two-phase cooling approach could be used to dissipate in excess of 600 kW/m2 in the submount of high-power LEDs.

Thermal management of high heat flux nanoelectronic chips

Driven by the continuing Moore’s Law evolution in chip technology, power dissipation of nanoelectronics chips could exceed 300W, with heat fluxes above 150W/cm2, within the next few years, along with localized, sub-millimeter zones with heat fluxes in excess of 1kW/cm2. New and novel cooling techniques, with the ability to selectively cool sub-millimeter “sun spots” while providing effective global cooling for high heat flux chips are needed. Several promising approaches, including the application of miniaturized silicon and BiTe thermoelectric coolers and direct cooling with dielectric liquids through thin film evaporation, will be described.

Thermofluid characteristics of two-phase flow in micro-gap channels

Microgap coolers provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, thus eliminating the significant interface thermal resistance associated with thermal interface materials and/or solid-solid contact between the component and a microchannel cold plate. This study focuses on the two-phase thermo-fluid characteristics of a dielectric liquid, FC- 72, flowing in an asymmetrically-heated chip-scale micro-gap channel, 10 mm wide by 37 mm long, with channel heights varying from 110 mum to 500 mum and channel wall heat fluxes of 200 kW/m2 (20 W/cm2). The two-phase, area-averaged heat transfer coefficients of FC-72 reached 15.5 kW/m2-K, significantly higher than the single phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique.

Two-phase microgap cooling of a thermally-simulated microprocessor chip

Forced flow of refrigerants and dielectric liquids, undergoing phase change in a microgap channel above an active chip is a promising candidate for the thermal management of advanced semiconductor devices. This paper presents two-phase heat transfer and pressure drop results for a chip-scale, uniformly heated, microgap channel using HFE-7100 and FC-87 as the working fluids. Results for two channel configurations are presented: a chip-size, short channel, representative of a product configuration and a longer channel, representing a laboratory prototype. Each microgap channel was tested with three nominal gap heights: 100, 200, and 500 micrometer. An inverse computation technique is used to determine the heat flow into the wetted surface of the microgap channel, and subsequently, the local heat transfer coefficients on the surface. A detailed analysis of the microgap heat transfer data for two-phase flow of HFE-7100 and FC-87, sorted by the Taitel and Dukler flow regime mapping methodology, is performed. Sections of the characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity) for the flow of dielectric liquids in miniature channels are identified. The inflection point in this inverse parabolic curve is seen to equate approximately with flow regime transition from Intermittent to Annular. The predictive accuracy of Chen (1966) and Shah (1976) classical two-phase heat transfer correlations for miniature channel flow is examined for both average and local heat transfer coefficients.

Parametric Dependence of Annular Flow Heat Transfer in Microgaps

Forced flow of refrigerants and dielectric liquids, undergoing phase change in a heated microgap channel between chips or in parallel microchannels in a compact cooler, is a promising candidate for the thermal management of advanced semiconductor devices. It has been found that Annular flow is the dominant flow regime in such miniature channels and that relatively high heat transfer coefficients are encountered in the moderate-to-high quality sections of such channels. Following a discussion of flow regimes and thermal characteristics of miniature channels, attention turns to exploring the parametric dependence of annular flow thermal transport in microgaps including the effects of channel diameter, mass flux, and working fluid on the two-phase heat transfer coefficients.Copyright

Thermal Characteristics of a Chip-Scale Two-Phase Microgap Cooler

Two-phase heat transfer and pressure drop results for a chip-scale, uniformly heated, microgap channel, with nominal gap heights of 100, 200, and 500 μm and using HFE-7100 and FC-87 as the working fluids, are reported. Average heat transfer coefficients in the range of 5 to 30 kW/m2-K were observed, for exit qualities up to 60%. Local heat transfer coefficients, obtained through an inverse computational technique, are found to vary strongly with thermodynamic quality and to fall within ±30% of the predictions of the venerable Chen correlation.

Thermofluid Characteristics of Two-Phase Microgap Coolers

The thermofluid characteristics of a chip-scale microgap cooler, including single-phase flow of water and FC-72 and flow boiling of FC-72, are explored. Heat transfer and pressure drop results for single phase water are used to validate a detailed numerical model and, together with the convective FC-72 data, establish a baseline for microgap cooler performance. Experimental results for single phase water and FC-72 flowing in 120 μm, 260 μm and 600 μm microgap coolers, 31mm wide by 34mm long, at velocities of 0.1 – 2 m/s are reported. “Pseudo-boiling” driven by dissolved gas and flow boiling of FC-72 are found to provide significant enhancement in heat transfer relative to theoretical single phase values.Copyright