Microgap Channel Research Articles

Experimental study of flow boiling characteristics in minigap channels over a wide heat flux range

The flow boiling characteristics in minigap channels were investigated by observing the two-phase flow patterns, measuring the two-phase pressure drops, and calculating the two-phase heat transfer coefficients and critical heat fluxes in minigap channels with heights of 0.5–2 mm. The working fluid was deionized water with mass fluxes of 200–400 kg m−2 s−1. The results show that bubbly flow, sweeping flow, churn flow, and churn-annular flow occur in sequence with increasing heat fluxes. An unstable two-phase flow regime was observed during the sweeping flow. The pressure drop varied in three regimes that were related to the flow patterns. The maximum pressure drops are 12.2, 3.8 and 1.6 kPa, respectively in 0.5 mm, 1 mm and 2mm channels at 400 kg m−2 s−1. The effect of gap height on the heat transfer coefficient first increases and then decreases with increasing heat flux. The heat transfer coefficients of all channels are at a high level when the heat flux is high. The maximum heat transfer coefficients are 7.3, 6.9 and 5.8 W cm−2 K−1 in the 0.5 mm, 1mm and 2mm channels at 400 kg m−2 s−1, which occurred before critical condition. The critical heat flux increased with increasing mass flux and gap height. In addition, a pressure drop correlation was developed to more accurately predict the pressure drop in microgap channels with an accuracy of 10.8%. The Liu-Winterton correlation and the Shah correlation give the best predictions for the subcooling boiling heat transfer coefficient. The Sun-Mishima correlation and the Kandlikar correlation provide the most accurate predictions for the saturated boiling heat transfer coefficients.

Simultaneous measurement of gas-liquid interface motion and temperature distribution on heated surface using temperature-sensitive paint

We developed a simultaneous measurement technique for temperature distribution on a heated surface and gas-liquid interface motion of two-phase flows in microgap channel using temperature-sensitive paint (TSP). Since the TSP layer is optically transparent, the proposed method enables us to simultaneously measure the temperature distribution and the motion of gas-liquid interface through the TSP layer using an in-house stereoscope-like device. Since the refractive index of gas-phase differs from that of liquid-phase, the luminescent intensity of TSP in gas-phase varies from that in liquid-phase even though the temperatures are same in both phases. Then, we proposed a correction method for temperature distribution in gas-phase. To validate the proposed method, the temperature distribution measurements were conducted in liquid single-phase flow. The Nusselt numbers calculated from the measured temperatures were in good agreement with the Sieder-Tate equation. Then, we measured the temperature distributions for the boiling flows under several heated temperature conditions. The temperature distributions at growing bubble and bubble coalescence were successfully measured. TSP is useful method for investigating the temperature distribution in two-phase flows.

Experimental study on convective boiling flow and heat transfer in a microgap enhanced with a staggered arrangement of nucleated micro-pin-fins

The boiling flow and heat transfer of FC-72 in a microgap channel are experimentally addressed. The heated surface of the microgap with a test area of 10 mm × 10 mm was enhanced using a staggered array of 613 columnar micro pin fins. Inside the micro pin-fins are nucleated with a 60 μm pore and opening of 45 μm. The test surface of the channel is a square of 10 mm, and the height of the channel is 100 μm. The saturation working temperatures of FC-72 is 50 °C, the surface heat flux was up to 60 kW/m2, and the mass flux was selected in the range of 94–275 kg/m2 s. The on nucleated boiling temperatures (ONB), convective heat transfer coefficient, outlet dryness, and pressure drop are measured and reported. The boiling behavior of FC-72 is also recorded using an image capturing device and reported. A comparison between the superheat temperature of nucleated pin fins and plane pin fins (with no nucleation) shows that nucleation of pin fins reduces the superheat temperature. The outcomes reveal that for a high mass flux, 196–275 kg/m2 s, the superheat temperature is high, and the degree of superheat increases as the surface heat flux increases. When the surface heat flux is high (q″ > 30 kW/m2), the convective heat transfer is almost independent of mass flux. At the initiation flow boiling, a notable jump in the pressure drops can be seen. After that, the pressure drop increases gradually by the increase in surface heat flux. The boiling images reveal that the boiling flow initiated inside the nucleation pores of micro pin-fins.

Orientation Effects in Two-Phase Microgap Flow

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for aerospace applications, but the lack of acceptable models and correlations for orientation- and gravity-independent operation has limited their use. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity in two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation, mass flux, and heat flux on flow boiling of HFE7100 in a 1.01 mm tall × 13.0 mm wide × 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes (CHFs), heat transfer coefficients (HTCs), and flow regimes across orientations, was achieved for mass fluxes of 400 kg/m2 s and greater (corresponding to a Froude number of about 0.8). The present results are compared to published criteria for achieving orientation- and gravity-independence.

Experimental study of flow boiling in a hybrid microchannel-microgap heat sink

A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.

Experimental investigation of the effect of heat sink orientation on subcooled flow boiling performance in a rectangular microgap channel

Investigation on the effect of heat sink orientation on the flow boiling performance in a rectangular microgap channel of width (w) 20 mm, length (L) 30 mm and depth (d) 220 µm with a hydraulic diameter (Dh) of 435 µm were performed. Experiments were conducted in different orientations such as horizontal downward facing (HD), horizontal upward facing (HU), horizontal with heating area vertically aligned (HV), vertical up flow orientation (VUF) and vertical downflow orientation (VDF) with mass fluxes (G) ranging from 193 to 911 kg/m2 s using deionized water as the working fluid. The experimental results were compared with the results of 31 parallel ‘U’ shaped microchannel study conducted by the authors earlier. The experimental results in microgap channel showed more or less similar critical heat flux (CHF) and heat transfer coefficients across all the orientations tested, indicating that the effect of gravity is negligible on the flow boiling performance in microgap channel. Even though the CHF and heat transfer coefficients for different orientations are comparable, the total pressure drop for vertical downflow orientation (VDF) was higher compared to other orientations. Significant pressure fluctuations were observed for the VDF orientation during flow boiling at low mass fluxes in the microgap channel. Comparison of flow boiling in the parallel microchannels with microgap channel depicts that the bubble growth pattern is entirely different. The bubble grows or elongates in both spanwise and axial directions in the case of the microgap channel which reduces the interaction between the flowing liquid and the vapour enabling the microgap channel to perform consistently at all orientations.

Waves, Instabilities, and Rivulets in High Quality Microgap Two-Phase Flow

Two-phase flow in sub-millimeter microgap channels offers highly potent thermal management capability and is the foundation for the emerging "embedded cooling" paradigm of electronic cooling. While the thermofluid characteristics and operational limits of such microcoolers are intimately tied to distinct forms of vapor-liquid aggregation in the microgap channel, insufficient attention has been paid to the formation of distinct wave patterns and instabilities on the thin liquid films associated with high-quality microgap channel flow. This paper focuses on the results of visualization and heat transfer studies of such two-phase flows, under both adiabatic and diabatic conditions, for FC-72 flowing in a 184 micron microgap channel at a mass flux of 230 kg/(m2.s). The study has revealed the existence of a post-annular, high-quality Rivulet flow regime, in which the liquid film breakdown and local wall dryout drives large surface anisothermalities and limits the heat transfer rate from the wall.As predicted by the prevailing flow regime models, annular flow is found to be the dominant flow regime for this microgap configuration.. For the adiabatic conditions, flow qualities ranged between 27% and 81%, and widely spaced, 3D waves, with a wavelength that decreases with increasing flow quality, were observed on the liquid-vapor interface. For the diabatic condition, the inlet flow quality was maintained at 36% and the exit flow quality varied between 47% and 97%. For exit qualities greater than 61%, the liquid film would periodically rupture into rivulet of varying width and length. The spacing, length, and width of the rivulets varied considerably, and can easily stretch well into dryout region. The axial variation of the wall heat transfer coefficient was found to reflect and confirm the expected axial propagation of the two- phase flow regimes and the onset of local dryout associated with the newly-defined Rivulet regime.

Waves and instabilities in high quality adiabatic flow in microgap channels

Two-phase flow in microgap channels offers highly potent thermal management capability and is the foundation for the emerging “embedded cooling” paradigm of electronic cooling. While heat transfer and pressure drop in such flows are intimately tied to their distinct forms of vapor–liquid aggregation, insufficient attention has been paid to characterizing the wave patterns and sub-regimes in high-quality microgap channel flow. The present visualization study focuses on two-phase flow in an adiabatic 184μm microgap channel operating at three mass fluxes of FC-72; 220, 420, and 620 kg/m2-s, with flow qualities ranging from approximately 40% to 90%. As predicted by a modified Taitel–Dukler flow regime map, annular flow is found to be the dominant flow regime for the present microgap configuration. Within the annular flow regime, unique 3-D wave patterns are observed at the liquid–vapor interface. The wavelength of these interfacial waves is observed to decrease with increasing flow quality and mass flux. Linear stability analysis of the liquid–vapor interface is found to yield strong agreement in predicted wavelength and wave growth rate distribution with the experimental results.

Confinement Effects on FKE-774 Critical Heat Flux in Buoyancy-Driven Microgap Channels

Critical heat flux (CHF) of the fluoroketone fluid FKE-774 in vertical microgap channels is explored, with a focus on submillimeter spacings. Experiments were conducted using a 20 mm × 20 mm heated aluminum surface. Microgap channel spacings were decreased down to 0.3 mm, providing channel aspect ratios (height/spacing) as high as 67. In the limit where channel spacing is large, CHF was found to be 140 kW/m2 for saturated boiling at atmospheric pressure. A reduction in CHF of 55% was observed for the largest channel aspect ratio investigated. Results for degradation of the CHF limit with decreasing microgap spacing are compared to a correlation available in the literature and show a roughly hyperbolic dependence on channel aspect ratio (height/spacing) for aspect ratios larger than 10.

THERMOFLUID CHARACTERISTICS OF HIGH-QUALITY FLOWIN CHIP-SCALE MICROGAP CHANNELS

Two-phase flow in chip-scale microgap channels offers highly potent thermal management capability and is the foundation for the emerging ”embedded cooling” paradigm of electronic cooling. While heat transfer and pressure drop in such flows are intimately tied to distinct forms of vapor-liquid aggregation, insufficient attention has been paid to characterizing the behavior of high-quality, short channel, microgap flow. This paper focuses on the results of simultaneous photographic and infrared visualization of such two-phase flows, under diabatic conditions, in a single 10 mm × 12 mm× 200 μm microgap channel cooled by low mass fluxes of FC-72. Churn flow, displaying pulsatile flow of confined vapor bubbles and slugs, is found to be the dominant flow regime for this short microgap channel configuration, yielding heat transfer coefficients that are an order-of-magnitude higher than all-liquid flow but are relatively insensitive to flow quality and heat flux. With increasing heat flux, local dryout and regions of elevated wall temperature grow and, periodically, expand to cover a majority of the microgap channel area. The average heat transfer coefficient is found to be in approximate agreement with the Shah correlation for low qualities and the Chen correlation for midlevel flow qualities, but both correlations are found to considerably overestimate the heat transfer coefficient at higher qualities, especially when local dryout becomes more frequent.

1023 マイクロギャップ内流動沸騰における薄板伝熱面温度分布の感温塗料を用いた計測

1023 マイクロギャップ内流動沸騰における薄板伝熱面温度分布の感温塗料を用いた計測

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.

Isothermal mass flow measurements in microfabricated rectangular channels over a very wide Knudsen range

Measurement and modeling of gas flows in microelectromechanical systems (MEMS) scale channels are relevant to the fundamentals of rarefied gas dynamics (RGD) and the practical design of MEMS-based flow systems and micropumps. We describe techniques for building robust, leak-free, rectangular microchannels which are relevant to micro- and nanofluidic devices, while the channels themselves are useful for fundamental RGD studies. For the first time, we report the isothermal steady flow of helium (He) gas through these channels from the continuum to the free-molecular regime in the unprecedented Knudsen range of 0.03–1000. On the high end, our value is 20-fold larger than values previously reported by Ewart et al (2007 J. Fluid Mech. 584 337–56). We accomplished this through a dual-tank accumulation technique which enabled the monitoring of very low flow rates, below 10−14 kg s−1. The devices were prebaked under vacuum for 24 h at 100 °C in order to reduce outgassing and attain high Kn. We devised fabrication methods for controlled-depth micro-gap channels using silicon for both channel ceiling and floor, thereby allowing direct comparisons to models which utilize this simplifying assumption. We evaluated the results against a closed-form expression that accurately reproduces the continuum, slip, transition, and free-molecular regimes developed partly by using the direct simulation Monte Carlo method. The observed data were in good agreement with the expression. For Kn > ∼100, we observed minor deviations between modeled and experimental flow values. Our fabrication processes and experimental data are useful to fundamental RGD studies and future MEMS microflow devices with respect to extremely low-flow measurements, model validation, and predicting optimal designs.

Experimental investigation of local flow boiling heat transfer and pressure drop characteristics in microgap channel

Two-phase microgap channel cooling concept has been recently proposed for cooling the heat sources directly in application of electronic devices thermal management. This concept is relatively new and more research should be carried out systematically to investigate the size effects of microgap channel on heat transfer and pressure drop mechanisms. In this study, local flow boiling phenomenon in different microgap sizes has been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap of depth 190μm, 285μm and 381μm, using deionized water with inlet temperature of 86°C. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using three different mass fluxes 420kg/m2s, 690kg/m2s and 970kg/m2s and effective heat flux varying from 0 to 110W/cm2. An array of integrated micro-temperature sensors are used in this study to obtain the local temperatures and subsequently local heat transfer coefficients are determined. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap channel. The results of this study show that flow boiling heat transfer coefficient is dependent on gap size, and the lower the gap size, higher the heat transfer coefficient. Moreover, it has been observed that confined slug and annular boiling are the dominant heat transfer mechanisms in microgap channels after the onset of nucleate boiling. Hence, local heat transfer coefficient increases significantly because of thin film evaporation during confined boiling at high heat flux. This study also evaluates the effectiveness of microgap cooling technology, to eliminate temperature gradient and hotspots.

Two-Phase Thermal Transport in Microgap Channels—Theory, Experimental Results, and Predictive Relations

A comprehensive literature review and analysis of recent microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids is presented. The flow regime progression in such a microgap channel is shown to be predicted by the traditional flow regime maps. Moreover, Annular flow is shown to be the dominant regime for this thermal transport configuration and to grow in importance as the channel diameter decreases. The results of heat transfer studies of single miniature channels, as well as the analysis and inverse calculation of IR images of a heated microgap channel wall, are used to identify the existence of a characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity), with inflection points corresponding to transitions in the two-phase cooling modalities. For the high-quality, Annular flow conditions, the venerable Chen correlation is shown to yield predictive agreement for microgap channels that is comparable to that attained for macrochannels and to provide a mechanistic context for the thermal transport rates attained in microgap channels. Results obtained from infrared imaging, revealing previously undetected, large surface temperature variations in Annular flow, are also reviewed and related to the termination of the favorable thin-film evaporation mode in such channels.

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.

Evaporation and flow dynamics of thin, shear-driven liquid films in microgap channels

Thin and ultra-thin shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Such flows experience complex, and as yet poorly understood, two-phase flow phenomena requiring significant advances in fundamental research before they could be broadly applied. This paper focuses on the results obtained in experiments with locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film flow of water and FC-72 have been obtained for a 2 mm channel operating at room temperature. While the water film can be smooth under certain liquid/gas flow rates, the surface of an intensively evaporating film of FC-72 is always distorted by a pattern of waves and structures. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to 10 times higher than that for a falling film, which makes shear-driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.

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.