Two-phase Microchannel Heat Sinks Research Articles

Improved flow boiling performance and temperature uniformity in counter-flow interconnected microchannel heat sink

Microchannel flow boiling is one of the most effective solutions to the problem of heat dissipation caused by high power electronic devices. However, inherent drawbacks such as low critical heat flux due to dry-out of the liquid film near the channel exit, relatively high two-phase flow pressure drop, severe temperature nonuniformity and boiling instability at high heat flux still prevent widespread commercial application. In this study, based on the idea of uniform vapor quality along the flow direction, a copper Counter-Flow Interconnected Microchannel (CFIM) was proposed to address the main issues currently faced by two-phase microchannel heat sink. A comprehensive comparative study of CFIM, Co-Current Microchannel (CCM) and Counter-Flow Microchannel (CFM) was carried out from the aspects of boiling stability, boiling heat transfer characteristics and temperature uniformity. Three slots of 0.2 mm, 0.5 mm and 0.8 mm in width, denoted as IM0.2, IM0.5 and IM0.8, respectively, are considered. Experiments are conducted at mass fluxes of 291–618 kg/m2·s, saturation temperature of 40 °C and effective heat fluxes of 3–293 W/cm2, using the dielectric fluid R1233zd(E) as the working fluid. The results show that the CHF and average two-phase heat transfer coefficient (HTC) of CFIM are increased by 40.6 ∼ 100.5 % and 70.4–83.6 %, respectively, while the two-phase pressure drop is decreased by 17–32.6 % compared to CCM. The design of the counter-flow interconnected structure can manipulate the void fraction and two-phase flow pattern, resulting in a near uniform void fraction and flow pattern. The structure of these connecting slots promotes nucleate boiling and fluid mixing of neighboring channels. More importantly, the flow boiling instability and temperature non-uniformity in CFIM are well suppressed. In addition, IM0.2 has higher heat dissipation than IM0.5 and IM0.8 due to enhanced bubble nucleation, reducing superheat requirements and periodic rewetting to the center of the channel. The results of this research reveal that utilizing a microchannel heat sink with a counter-flow interconnected configuration can effectively improve heat transfer performance during flow boiling. Furthermore, it helps to mitigate issues such as flow boiling instability and temperature non-uniformity.

On the importance of fluidic manifold design and orientation on flow boiling instability in microchannel heat sinks

This study proposes simple modifications in the design of inlet and outlet plenum and inlet and outlet port orientations to suppress the instabilities in two-phase microchannel heat sinks. We investigate three designs which include small-volume inlet and outlet plenums, large-inlet small-outlet volume plenums, and small-inlet large-outlet volume plenums, at four orientations, i.e., upward facing horizontal flow (+0°), downward facing horizontal flow (-0°), and vertically aligned upward (+90°) and downward flows (-90°). The inlet-outlet port orientation is perpendicular to the plane of the microchannel footprint area. It is found that the large inlet plenum decreases the backflow of vapour for all orientations. An increase in the size of the outlet plenum in the upward facing horizontal flow configuration helps to accommodate sudden volume increase due to vapour to minimise the fluctuations/instability. Conversely, the opposite directions of flow and buoyancy force in the case of a large volume outlet manifold significantly increase the tendency of backflow of vapour for the case of downward facing horizontal flow and vertically aligned downward flow orientations. The study uses these insights to propose simple guidelines for the design of inlet and outlet plenum and port orientations to suppress the instability in two-phase microchannel heat sinks at high heat fluxes and under unfavourable orientations.

Theoretical and experimental research on flow boiling heat transfer in microchannels for IGBT modules

Two-phase microchannel heat sink has received extensive attention due to its small size, high heat transfer efficiency, good temperature uniformity, and low working fluid charge. Heat flux and mass flow rate are important parameters that affect the heat transfer performance of two-phase microchannel heat sink. For different heat fluxes, if the mass flow rate is not reasonable, dry-out may occur, which will significantly reduce the heat transfer performance. In order to further explore the influence of mass flow rate and heat flux on heat transfer characteristics, the flow boiling heat transfer performance and flow regimes under different mass flow rates and heat fluxes were studied for a two-phase microchannel heat sink using R245fa as the working fluid. The results showed that when the mass flow rate was 100 kg/(m2·s), the boiling heat transfer coefficient of R245fa showed a trend of increasing firstly and then decreasing with the increase of the heat flux. At a higher heat flux, the local dry-out in the microchannels began to appear, and the heat transfer performance decreased significantly. And the higher the heat flux was, the more serious the dry-out was. There was no obvious dry-out phenomenon when the mass flow rate was 150–300 kg/(m2·s) and the heat flux was below 500 kW/m2. The research provided a valuable reference for exploring the influence of the mass flow rate and the heat flux on the flow boiling heat transfer characteristics of the microchannel heat sink.

Pressure drop of R134a in mini channels with micro pin fins during flow boiling

Flow boiling in mini channel with micro pin fin arrays is a promising technology for thermal management in various fields such as microelectronics, energy utilization and national defence. Understanding its flow characteristics is the pave stone for further structure optimization of two-phase microchannel heat sinks. However, previous studies paid few attentions to the effect of different structural parameters on its flow performance, especially for the application with long channels for practical evaporators in aircraft environmental control systems. In this paper, an experimental system was set up to investigate the flow boiling pressure drops of refrigerant R134a in a 300 mm long channel with micro pin fin arrays. The flow characteristics of staggered diamond pin fin arrays with different channel widths of 1.0, 1.2 and 1.4 mm and fin angles of 30°, 60°and 90° were comprehensively investigated. The pin fins with a height of 0.5 mm were arranged in the rectangular channel through 3D printing. Two-phase pressure drops in the test section were then measured. The test range of vapor quality, mass flux, heat flux and saturation pressure were 0–1, 200–500 kg/m2s, 10.0–37.5 kW/m2 and 390–700 kPa respectively. Experimental results revealed that the smaller channel width and the greater diamond angle resulted in the greater two-phase pressure drops. The heat flux showed an insignificant influence on the two-phase pressure drops. The existing empirical correlations for pressure drops were evaluated and failed to predict the present experimental data. A new one with the consideration of pin fin structure parameters was then developed to improve the prediction accuracy with the maximum mean absolute deviation of 5.4 %. The proposed correlation can illuminate future study on flow boiling in mini channel with pin fin arrays and be beneficial for the practical design of two-phase microchannel heat sinks.

Study of Two-Phase Microchannel Heat Sink Fabricated by A.M. Technology for Energy Reuse in the Electronic Device

Data centers’ electricity energy consumption accounts for 1% of global electricity demand and 0.3% of all global CO2 emissions. Energy reuse as a core of a net zero carbon data center, as a macro goal benefiting to mankind, needs micro innovations from thermal engineers to reclaim the distributed and low-grade thermal energy from diversified electronic equipment. This article presents the attempt to combine the advantages of high-density heat transferring technology by two-phase microchannels and the agility of Additive Manufacturing (A.M.) technology into a heat sink by which thermal energy can be collected in premium quantity and quality. The heat sink prototype adopted the two-layer microchannel design by leveraging the unique capability of A.M. technology to form complicated spatial geometric features, such as the functional channel profiles with diverged cross-sections along the flow direction, intermittent channels, and curved channels. It was fabricated at one-time processing by AlSi10Mg powder SLS/SLM, had an exterior base area of 25 cm2, and interior micro-fins with a minimal thickness of 0.2 mm and fin pitch of 0.38 mm. A test rig had been built to validate the thermal dynamic and hydraulic performance of this microchannel heat exchanger working with HFE 7100 as the coolant. The heat flux under certain wall superheat and pressure drop catches the equivalent grade of microchannels made by conventional micro-cutting approaches on copper or aluminum. The maximum inlet coolant temperature could be elevated up to 60.0 °C with less than 90.0 °C CPU case temperature, which provides the feasibility of high-grade heat recovery. The test results present the promising prospects of this design and A.M. technology in the field of two-phase microchannel heat exchanging, by which the electronic devices in megawatt hyperscale data center can be changed from energy consumers to energy generators for the greenhouse, district heating, and hot water system.

Two-phase modeling of micro-channel critical flows with inlet sub-cooling: A review and benchmark study

Micro-channel two-phase critical flow is encountered in the chemical industry and power engineering, such as critical flow leakage/release and two-phase micro-channel heat sinks. Realistic modelling of micro-channel two-phase critical flow deserves enough attention. The paper aims to review the existing two-phase critical flow models and to evaluate the application to simulate the micro-channel critical flow. Choking and critical flow criteria of the single-phase flow and two-phase flow are discussed, and the pressure gradient near the critical cross-section is compared. The development as well as the key characteristic of the critical flow model is drawn on. A benchmark of critical flow models against the experimental data for the micro-channel critical flow, especially for the pressure profile, has been carried out to check the ability of the models to accurately reproduce the micro-channel critical flow, as well to reasonably describe the fluid flow and interfacial transfer. The constitutive relations of existing separated flow models are further compared and evaluated depending on the analysis of the fluid flow and interfacial transfer for two-phase flow to assess constitutive relations. Additionally, the bubble nucleation and metastable liquid property are included in separated flow models to explore the boiling delay in the micro-channel critical flow.

Achieving ultra-high coefficient of performance of two-phase microchannel heat sink with uniform void fraction

Thermal management of high-power electronics/devices has been a very challenging issue for data centers, flourishing electric vehicle revolution as well as cooling of the first wall of a Tokamak fusion reactor. As a prominent cooling strategy, two-phase flow in microchannels has received extensive research over decades. However, inherent defects such as relatively low critical heat flux due to the dry out of liquid film near the exit of the channel and relatively high two-phase flow pressure drop at high heat flux still hinder extensive commercial applications. The present work proposes a robust microchannel heat sink design toward uniform void fraction distribution along the flow direction. The two-phase flow pressure drop will not increase significantly with increase in heat flux. This is made possible by an innovative and unique combination of diverging microchannel and counter-flow manifold, which enables extensive channel-to-channel heat transfer, especially near the end or inlet of the channels. Moreover, flow visualization reveals that nearly uniform void fraction distribution along the channel with relatively high bubble slug ratio is possible. For a relatively large area of 12 cm2 and under the limitation of wall temperature of 140 °C, a heat flux as high as 3525 kW/m2 is achieved without sign of reaching the critical heat flux with nearly negligible pressure drop increment comparing to single-phase convection for a relatively low mass flux of 600 kg/m2s. An ultra-high coefficient of performance of 75,675 is attained under a low inlet temperature and low mass flux.

Experimental investigation and analysis of parametric trends of instability in two-phase micro-channel heat sinks

This study explores Density Wave Oscillation (DWO) and Parallel Channel Instability (PCI) in a micro-channel heat sink containing 38 parallel channels having a hydraulic diameter of 316 μm. Experiments are executed using FC-72 as working fluid with mass velocities from 171.2 to 1124 kg/m2s and a fairly constant inlet subcooling of ~15°C. The flow instabilities are reflected in pressure fluctuations detected mostly in the heat sink's upstream plenum. Both inlet pressure and pressure drop signals are analyzed in pursuit of amplitude and frequency characteristics for different mass velocities and over a range of heat fluxes. The analysis is complemented by detailed visualization of interfacial features along the parallel channels using high-speed video. Appreciable confinement of bubbles in individual channels is shown to promote rapid axial bubble growth. The study shows significant variations in the amount of vapor generated and dominant flow patterns among channels, a clear manifestation of PCI, especially for low mass velocities and high heat fluxes. It is also shown effects of the heat sink's instabilities are felt in other components of the flow loop. The parametric trends for PCI are investigated with the aid of three different types of stability maps which show different abilities at demarcating stable and unstable operation.

A Review On Transient Thermal Management of Electronic Devices

Abstract Much effort in the area of electronics thermal management has focused on developing cooling solutions that cater to steady-state operation. However, electronic devices are increasingly being used in applications involving time-varying workloads. These include microprocessors (particularly those used in portable devices), power electronic devices such as IGBTs, and high-power semiconductor laser diode arrays. Transient thermal management solutions become essential to ensure the performance and reliability of such devices. In this review, emerging transient thermal management requirements are identified, and cooling solutions reported in the literature for such applications are presented with a focus on time scales of thermal response. Transient cooling techniques employing actively controlled two-phase microchannel heat sinks, phase change materials (PCM), heat pipes/vapor chambers, combined PCM-heat pipes/vapor chambers, and flash boiling systems are examined in detail. They are compared in terms of their thermal response times to ascertain their suitability for the thermal management of pulsed workloads associated with microprocessor chips, IGBTs, and high-power laser diode arrays. Thermal design guidelines for the selection of appropriate package level thermal resistance and capacitance combinations are also recommended.

Experimental and analytical investigation of flow loop induced instabilities in micro-channel heat sinks

Two-phase flow instabilities, both static and dynamic, are commonplace in closed two-phase loops. For two-phase micro-channel heat sinks, past studies have reported different types of instability, including density wave oscillation (DWO), pressure drop oscillation (PDO), and parallel channel instability (PCI). In the present study, emphasis is placed on identifying and suppressing dominant instabilities for flow boiling of R134a in a micro-channel heat sink having 609.6-mm long and 203.2-mm wide base area, and containing 100 of 1 × 1 mm2 channels. A dominant charge transition instability (CTI) is identified as causing appreciable oscillations in both mass velocity and pressure drop. Discussed is how PID control could be used to suppress CTI. But PCI was also prevalent within individual CTI oscillation cycles, taking the form of comparatively small amplitude, high frequency oscillations. It is shown CTI and PDO are fundamentally different in that ensuing fluctuations in mass velocity and pressure drop are synchronized for the former, but out of phase for the latter. Additionally, CTI occurs in the positive slope region of the heat sink’s pressure drop versus mass velocity characteristics, while PDO occurs in the negative slope region. A detailed analytical model is also presented to predict onset of CTI. Overall, this study shows how loop instrumentation signals may be used in the model to both predict and suppress instability.

Frequency analysis of pressure oscillations in large length-to-diameter two-phase micro-channel heat sinks

Two-phase micro-channel heat sinks are prime candidates for incorporation into thermal control systems (TCSs) of future space vehicles and planetary habitats. Unlike small heat sinks employed in the electronics industry, TCS heat sinks are characterized by large length-to-diameter ratio, for which limited information is presently available. This study employs a 609.6-mm long by 203.2-mm wide heat sink containing 100 of 1×1mm2 micro-channels and uses R134a as working fluid. The large length-to-diameter ratio of 609.6 is especially instrumental to capturing detailed axial variations of flow pattern and corresponding variations in local heat transfer coefficient. High-speed video analysis of the inlet plenum shows appreciable vapor backflow under certain operating conditions, which is also reflected in periodic oscillations in the measured pressure drop. In fact, the backflow frequency captured by video matches closely the frequency obtained from Fourier analysis of the pressure drop signal. While density-wave oscillations are encountered in individual channels, the phenomena observed are more closely related to parallel-channel instability. It is shown the periodic oscillations and vapor backflow are responsible for initiating intermittent dryout and appreciable drop in local heat transfer coefficient in the downstream regions of the channels. A parametric study of oscillation frequency shows a dependence on four dimensionless parameters that account for amount of vapor generation, subcooling, and upstream liquid length, in addition to Weber number. A new correlation for oscillation frequency is constructed that captures the frequency variations relative to these individual parameters.

Pressure drop characteristics of large length-to-diameter two-phase micro-channel heat sinks

Unlike prior published two-phase micro-channel studies that concern mostly miniature heat sinks, the present study addresses transport characteristics of a heat sink containing large length-to-diameter micro-channels. The copper heat sink has a 609.6-mm long by 203.2-mm wide base area, and contains 100 parallel micro-channels having a 1×1mm2 cross-section, and a length-to-diameter ratio of 609.6 to 1. The study addresses pressure drop characteristics of R134a for subcooled inlet conditions with inlet pressures of 689.4–731.3kPa, mass velocities of 75.92–208.79kg/m2s, and base heat fluxes of 4036–28,255W/m2. The data are compared to predictions of the Homogeneous Equilibrium Model (HEM) in conjunction with six different two-phase mixture viscosity relations, correlations based on the Separated Flow Model (SFM) and intended for macro-channels, and correlations based on SFM and intended for micro-channels. Overall, fairly good predictions are achieved with HEM, compared to poor predictions with the macro-channel SFM correlations. The micro-channel SFM correlations fared better, yielding mean absolute error values as low as 6.66%. Flow visualization results show appreciable periodic fluctuations in two-phase flow patterns, which include four primary patterns: bubbly/slug, slug, transition, and annular, along with two other patterns associated specifically with downstream dryout: transition and annular. These observations point to the need for more research to address the role of flow instabilities and fluctuations in flow pattern development.

Transient Analysis of Nonuniform Heat Input Propagation Through a Heat Sink Base

For thermal management architectures wherein the heat sink is embedded close to a dynamic heat source, nonuniformities may propagate through the heat sink base to the coolant. Available transient models predict the effective heat spreading resistance to calculate chip temperature rise, or simplify to a representative axisymmetric geometry. The coolant-side temperature response is seldom considered, despite the potential influence on flow distribution and stability in two-phase microchannel heat sinks. This study solves three-dimensional transient heat conduction in a Cartesian chip-on-substrate geometry to predict spatial and temporal variations of temperature on the coolant side. The solution for the unit step response of the three-dimensional system is extended to any arbitrary temporal heat input using Duhamel's method. For time-periodic heat inputs, the steady-periodic solution is calculated using the method of complex temperature. As an example case, the solution of the coolant-side temperature response in the presence of different transient heat inputs from multiple heat sources is demonstrated. To represent a case where the thermal spreading from a heat source is localized, the problem is simplified to a single heat source at the center of the domain. Metrics are developed to quantify the degree of spatial and temporal nonuniformity in the coolant-side temperature profiles. These nonuniformities are mapped as a function of nondimensional geometric parameters and boundary conditions. Several case studies are presented to demonstrate the utility of such maps.

Suppressing high-frequency temperature oscillations in microchannels with surface structures

Two-phase microchannel heat sinks are attractive for thermal management of high heat flux electronic devices, yet flow instability which can lead to thermal and mechanical fatigue remains a significant challenge. Much work has focused on long-timescale (∼seconds) flow oscillations which are usually related to the compressible volume in the loop. The rapid growth of vapor bubbles which can also cause flow reversal, however, occurs on a much shorter timescale (∼tens of milliseconds). While this high-frequency oscillation has often been visualized with high-speed imaging, its effect on the instantaneous temperature has not been fully investigated due to the typical low sampling rates of the sensors. Here, we investigate the temperature response as a result of the high-frequency flow oscillation in microchannels and the effect of surface microstructures on this temperature oscillation with a measurement data acquisition rate of 1000 Hz. For smooth surface microchannels, fluid flow oscillated between complete dry-out and rewetting annular flow due to the short-timescale flow instability, which caused high-frequency and large amplitude temperature oscillations (10 °C in 25 ms). In comparison, hydrophilic surface structures on the microchannel promoted capillary flow which delayed and suppressed dry-out in each oscillation cycle, and thus significantly reduced the temperature oscillation at high heat fluxes. This work suggests that promoting capillary wicking via surface structures is a promising technique to reduce thermal fatigue in high heat flux two-phase microchannel thermal management devices.

Thermal design and operational limits of two-phase micro-channel heat sinks

While the vast majority of published studies on two-phase micro-channel heat sinks have been focused on determination of pressure drop and heat transfer coefficient, very few studies have addressed the operational limits of these devices. This study provided a comprehensive methodology for thermal design of micro-channel heat sinks with saturated inlet conditions. This includes predictive methods for pressure drop and heat transfer coefficient using universal correlations that rely on large databases amassed from numerous sources, and which encompass many working fluids, and very broad ranges of hydraulic diameter, mass velocity, inlet pressure, and inlet quality. This is followed by predictive tools for thermal limits associated with dryout incipience and premature critical heat flux, as well as two-phase critical flow limit. The three limits are combined to define an envelope for acceptable heat sink performance. Using these tools, a parametric study is performed to determine the variation of maximum heat flux with total volumetric flow rate for different combinations of the channel’s geometrical parameters for three working fluids, HFE-7100, R134a, and water. Then, the values of maximum heat flux are used to assess corresponding variations of pressure drop and maximum bottom wall temperature of the heat sink. It is shown that maximum heat flux is dominated by different limits for different flow rate ranges, and may be increased significantly, while decreasing bottom wall temperature, by using a large number of small channels. Furthermore, using deeper micro-channels is shown to increase maximum heat flux and decrease pressure drop, while producing a relatively weak adverse effect on bottom wall temperature.

Surface Structure Enhanced Microchannel Flow Boiling

We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.

Boiling Two Phase Flow in Microchannels: a Review

The studies in boiling water in microchannel that are shaped along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses: (a) dissipation high heat flux by two phase flow (b) how the geometry of micro channels affects their activity i.e. of circular and non-circular geometry, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase microchannel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. There are a lot of complications for the development of these tools because of insufficient databases and also the drastic differences in boiling behavior of different fluids in microchannel. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nanofluids and carbon nanotube coatings, with proven merits to single-phase macrosystems, may not offer similar advantages to microchannel heat sinks. The boiling flow regimes in microchannels were categorized as the confined bubble, slug flow, and annular flow depending mainly on the length of the bubble. Based on the dominant flow regimes in microchannels, a steady slug-annular flow model is proposed to predict the pressure drop and heat transfer characteristics. As a correlation method, the homogeneous flow model with empirical correlations was applied to the boiling flow in microchannels. In addition, this review concludes that boiling flow in microchannel can be an effective way to eliminate heat from the heat sinks.

Numerical model of a two-phase microchannel heat sink electronics cooling system

Microchannel heat sinks using two-phase flow boiling have excellent potential for cooling high heat flux electronic devices. A numerical model of a two-phase cooling system using microchannel heat sinks is presented. This is supplemented with transient heat conduction models of the heat sink and heat exchanger and with various state of the art empirical correlations to close the equations. Finite volume discretization and the SIMPLE algorithm are used to solve the mixture conservation equations of mass, momentum and energy. The numerical code is verified using the method of manufactured solutions. This method reveals that the numerical order in space and time is consistent with the expected values from theory, second order and first order, respectively. Further some illustrative results of the model are validated using experimental measurements. These results agree well with each other and indicate good predictive capability of the system model.

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

Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase microchannel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase microchannel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macrochannels. Conversely, other fluids may exhibit considerable “confinement” even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nanofluids and carbon nanotube coatings, with proven merits to single-phase macrosystems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of microchannel flow with those of jet impingement.

Design of a Microbreather for Two-Phase Microchannel Heat Sinks

The design of a vapor breather that uses microstructures and surface chemistry to improve two-phase microchannel heat sink performance was investigated. Test breather devices were designed, fabricated, and characterized that enabled imaging of bubble-breather interactions and understanding of geometry on breather performance. A model was subsequently developed to interpret the experimental results, which incorporates the effect of both liquid inlet flow rate and breather port vacuum level on the maximum air removal rate. This work offers insights and guidelines for the development of optimized breather structures to improve two-phase microchannel performance for thermal management applications.