Two-phase gas-liquid flows are commonly encountered in various industrial processes. The slug flow regime, characterized by alternating streamwise segments of gas bubbles and liquid slugs, is frequently observed in microscale channels where capillary forces play an important role. Accurate knowledge of the liquid film thickness surrounding bubbles during slug flow is crucial for predicting various hydrodynamic, interfacial, and thermal transport characteristics of interest, such as heat transfer during microchannel flow boiling. In this study, we experimentally and numerically investigate the effect of bubble length and velocity on liquid film thickness in an air-water slug flow in a single microchannel of circular cross-section. In the experiments, an open-loop flow facility is used to generate air-water slug flow in a circular microchannel with independently varying bubble lengths and velocities. A laser confocal displacement meter is used to measure the liquid film thickness, while the bubble length and velocity are extracted from high-speed visualizations. The liquid film thickness is observed to increase with increasing bubble velocity and length; while this effect of velocity has been reported in the literature, this is the first reporting of the influence of bubble length on liquid film thickness. Additionally, numerical simulations that replicate the experimental boundary conditions are performed using a two-phase volume-of-fluid approach to corroborate this trend. Furthermore, as the bubble length increases for a given bubble velocity, the film thickness asymptotically approaches a maximum value that agrees with the semiinfinite bubble approximation previously reported in the literature. A new empirical correlation is developed that is valid for all bubble lengths and offers accurate predictions [9 % mean absolute error (MAE)] of the liquid film thickness as a function of the dimensionless bubble length and dimensionless capillary number.

Air-cooled heat sinks remain a simple and reliable technology for heat dissipation in high-power-density servers, computing, and electrified transportation applications. The power dissipation capabilities of air cooling can be improved by utilizing advanced heat-spreading techniques to increase the finned surface area, often by incorporating vapor chambers in the heat sink base. Vapor chambers offer phase-change-based heat spreading at lower thermal resistances compared to solid metallic heat spreaders, thereby allowing heat transfer from small, concentrated heat sources (> 100 W over 100 mm(2)) to a large rejection area. Accurate thermal performance and dryout limit prediction in such vapor-chamber-embedded heat sinks is critical for their design. However, models that describe vapor chamber transport typically do not account for nucleate boiling in the evaporator wick, which is increasingly likely to occur as applications trend toward higher heat fluxes. Due to the substantial increase in two-phase flow pressure drop in the presence of nucleate boiling, model-based predictions of dryout limits considering only single-phase flow of liquid in the evaporator wick can be significantly off the mark. This paper introduces a physics-based modeling approach for predicting the thermal resistance and dryout heat flux for vapor chambers that accounts for the occurrence of boiling in the evaporator wick. The heat transfer model considers the thermal coupling between a solid heat spreader representing the vapor chamber wall and the transport in the core of the vapor chamber to calculate the temperature fields. The onset of boiling is characterized using a wall superheat criterion that determines the area of the wick undergoing boiling. Using the mass fluxes from the heat transfer model, the pressure fields in the wick are solved. In the boiling region of the wick, fluid transport is modeled using the Darcy-Ergun equation corrected for the relative permeabilities of the liquid and vapor. A case study illustrating the usage of the model is demonstrated for an example vapor chamber of dimensions 50 mm x 50 mm x 5 mm, with a sintered copper wick structure. A purely evaporation-based pressure drop prediction for such a vapor chamber with the assumption of single-phase flow in the wick leads to large overprediction of the dryout heat flux. The modeling framework developed herein highlights the need to account for boiling in the evaporator wick for vapor chambers used in such high-heat-flux applications.

Heat pipes are capillary-pumped two-phase devices that transport heat from localized sources in electronics to a heat sink through continuous evaporation and condensation of an internal working fluid. Given the reliance of a heat pipe on capillary transport of liquid through internal wicking structures to the evaporator section where heat is applied, operating it at a power exceeding the so-called capillary limit can lead to dryout at the evaporator and subsequent device failure. However, in response to highly dynamic workloads in the electronics being cooled, heat pipes may more typically be exposed to heat loads above the capillary limit only intermittently over brief time intervals. Understanding the heat pipe response to these transient workloads is critical, as designing heat pipes for steady-state operation at the peak transient heat load would represent an expensive overdesign. Our previous work has experimentally characterized the transient heat pipe response to power pulses exceeding the capillary limit. It was demonstrated that a pulse must be sustained for a minimum duration termed the timeto-dryout before dryout is initiated. Once a pulse-induced transient dryout does occur in a heat pipe, its thermal resistance does not necessarily recover back to the pre-dryout performance even after the power input drops below the capillary limit. This behavior, termed thermal hysteresis, can be circumvented if the power is lowered (or throttled) to a sufficient threshold below the capillary limit for an extended time interval. In the current work, a first-of-its-kind transient heat pipe model is developed to predict the salient features of heat pipe response to pulse-load-induced dryout as well as recovery from dryout. The model uniquely considers spatiotemporal variations in local liquid saturation in the wick (i.e., the fraction of pore volume occupied by the liquid). Experiments are performed using commercial heat pipe samples that span a range of sizes and wick types to validate the model predictions. It is shown that the model can predict the transient thermal response, including thermal hysteresis, of the heat pipe during pre-dryout, dryout (pulse load), and post-dryout (recovery) stages with good accuracy. The model results are also validated against experiments for heat pipes spanning a range of wick types, heat pipe lengths, and heat pipe thicknesses. The capability to accurately predict crucial temporal events during dryout and recovery is key to establishing power expenditure strategies in electronics and designing heat pipes with improved dryout and recovery performance.

WSPC Series in Advanced Integration and PackagingEmbedded Cooling of Electronic Devices, pp. 333-380 (2024) No AccessChapter 9: Recent Experimental and Modeling Advances in Two-Phase Embedded Microfluidic CoolingTodd A. Kingston, Justin A. Weibel, and Suresh V. GarimellaTodd A. KingstonCenter for Multiphase Flow Research and Education, Department of Mechanical Engineering, Iowa State University, Ames, IA, USA, Justin A. WeibelCooling Technologies Research Center, School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA, and Suresh V. GarimellaUniversity of Vermont, Burlington, VT, USAhttps://doi.org/10.1142/9789811279379_0009Cited by:0 (Source: Crossref) PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: Advances in the packaging approaches and semiconductor materials used for high-power electronic devices will usher in a paradigm shift in the thermal management strategies employed to dissipate extremely high heat fluxes at reasonable operating temperatures. Traditional "remote cooling" systems that have been commonly used to manage thermal loads generated by these devices suffer from parasitic interfacial, conduction, and spreading resistances, which lead to large temperature gradients. The next generation of "embedded cooling" systems will bring coolant very close to the heat source, eliminating these thermal resistances but requiring a solution that can directly manage high local heat fluxes without an intermediate heat spreader. This work highlights recent advances in the experimental characterization and modeling of two-phase embedded-cooling systems. Specifically, it summarizes the development of two-phase hierarchical manifold microchannel heat sinks, a promising high-performance embedded-cooling solution, as well as experimental and numerical studies investigating the underlying microchannel flow boiling phenomena. Static and dynamic flow boiling instabilities among parallel channels, along with the potential for coupling between transient heating conditions and these two-phase flow dynamics, offer unique implementation challenges that may arise in embedded-cooling systems. With continued progress in the understanding of these phenomena and predictive high-fidelity numerical modeling of microchannel flow boiling, embedded cooling systems will alleviate the thermal challenges that currently limit the power and performance of many electronic devices, enabling technological advancements across many industries. Keywords: Embedded coolingflow boilingflow instabilitymicrochannelmicrofluidictwo-phase FiguresReferencesRelatedDetails Recommended Embedded Cooling of Electronic DevicesMetrics History KeywordsEmbedded coolingflow boilingflow instabilitymicrochannelmicrofluidictwo-phasePDF download

Chaotic bubble interactions during vigorous boiling have foiled attempts at an accurate mechanistic understanding of this important industrial transport process. Prediction of the boiling crisis (dryout) that occurs due to the spontaneous merger of bubbles into an insulating vapor film at critical heat flux remains an unsolved challenge. This work diagnoses the dryout process using synchronized high-resolution spatiotemporal heat flux and phase data, obtained via through-substrate infrared and visual inspection. We examine prevailing theories for the boiling crisis and provide evidence to rule out all but one. The boiling crisis is found to be a consequence of a peak in the nucleate boiling curve, past which the degradation of boiling heat transfer and concomitant increase in superheat are caused by replacement of the thermally efficient contact line region with the inefficient vapor-covered region for the surface-fluid combination studied. This results in substrate thermal runaway and dryout. Rather than seeking a separate dryout trigger mechanism, nucleate boiling models must instead inherently capture this peak. The heat flux partitioning employed demonstrates that critical heat flux can indeed be predicted by capturing this peak. A generalized framework is suggested for predicting the boiling curve as part of a multidimensional surface, a boiling manifold.

Heat transfer during boiling involves a variety of transport mechanisms. Available experimental techniques cannot yet fully delineate these mechanisms which has contributed to a long-standing, persistent challenge of constructing accurate mechanistic models for boiling. In this work, we develop a method to identify and distinguish between the individual heat transfer mechanisms that occur during boiling using synchronous, through-substrate, high-speed visual and infrared measurements. Local heat fluxes are deduced from temperature measurements and a synchronized set of binarized phase maps are obtained from processing high-speed visual measurements. Experimental pool boiling investigation of HFE-7100 fluid on an indium tin oxide surface revealed four distinct heat transfer signatures in the heat flux maps corresponding to liquid convection, contact line evaporation, vapor convection, and local microconvection due to rewetting during boiling. To classify these regions, pixel-wise binary and morphological operations are performed on the phase maps of the entire surface. In contrast with prior partitioning techniques which use standalone heat flux measurements, our synchronous high-resolution visualization enables the region classification around fine bubble footprints that otherwise would not be detected with heat flux maps alone. The heat fluxes and superheats are then partitioned into their underlying mechanisms using classified regions from the phase maps. Analysis of the nucleate boiling regime data shows that different mechanisms contribute in varying degrees to the overall heat transfer at different points along the boiling curve: for the surface-fluid combination studied, single-phase liquid heat transfer is the primary contributor at low heat fluxes while at high heat fluxes, contact line evaporation contributed the most. We further employ the experimental approach to partitioning dynamic processes resulting from step increases in heat input demonstrating its potential for investigating transient phenomena such as dryout. The experimental and post-processing method introduced in this study is the first to delineate partitioning between different heat transfer mechanism regions in the presence of multiple interacting bubbles over the entire surface throughout the boiling curve in both steady and transient operating conditions.

Heat pipes and vapor chambers are passive heat spreaders driven by capillary pumping of an internal working fluid via a porous wick. The capillary limit is the maximum steady-state heat input at which the fluid pressure drop can be supported by the capillary pressure head generated in the wick. However, heat pipes and vapor chambers often find application in devices where the heat input is highly transient and can exceed the capillary limit for brief time intervals. Operating heat pipes briefly above the capillary limit will not result in a dryout if the operating time interval does not exceed a characteristic time-to-dryout. Operation over a duration that exceeds this time-to-dryout can induce transient dryout and may lead to thermal hysteresis, that is, the original heat pipe thermal resistance may not be recovered even after the heat input is lowered back below the capillary limit. To fully recover the heat pipe performance after a transient dryout event, our recent experiments have shown that the heat input must be lowered (or throttled) significantly below the capillary limit. Due to the highly transient nature of power dissipation from electronic devices, it becomes imperative to characterize the throttling power level and duration required to ensure full recovery of a heat pipe from dryout under transient operations. This work experimentally characterizes recovery of heat pipes from dryout by power throttling under transient conditions, where 'power throttling' is the act of reducing the operating power level significantly below the capillary limit to eliminate post-dryout thermal hysteresis. We deduce from the experiments that the power must be throttled for longer than a minimum throttling time interval, defined as the time-to-rewet, in order to eliminate dryout-induced thermal hysteresis. The dependence of this time-to-rewet on the throttling power level is explored, and guidelines are presented on the need for throttling and the choice of throttling power under transient conditions.

Confined and submerged two-phase jet impingement offers a compact and effective heat transfer technology for thermal management. In contrast to free-surface jets, few studies have investigated the critical heat flux phenomenon during confined and submerged jet impingement. In this study, the impact of jet velocity on critical heat flux is examined during confined and submerged impingement of a water jet. A single jet issues through a 3.75 mm-diameter orifice and impinges on a circular 25.4 mm-diameter heated surface. The jet velocity is varied between 0.30–1.62 m/s (Re ≈ 3500–20,000) with the height of the confinement gap held equal to the jet diameter at 3.75 mm. The two-phase flow morphology is observed via high-speed visualization from the top of the confinement gap, and the transient pressure drop across the jet orifice and flow gap is measured simultaneously. Critical heat flux increases monotonically with velocity from 104 W/cm2 to 275 W/cm2 over the tested velocity range. As with free-surface jet impingement, two regimes can be identified: a low-velocity regime and a velocity-dominated regime. However, the observed critical heat fluxes for the confined and submerged jet are 32 % to 16 % lower than those predicted for a free-surface jet configuration. This curtailment of critical heat flux is related to disturbances in the incoming liquid flow induced by dynamics of the two-phase flow in the confinement gap, as indicated by large, recorded pressure drop oscillations. These flow and pressure oscillations are attenuated by the jet momentum with increasing velocity, as corroborated by high-speed flow visualizations. In the low-velocity regime, there are significant effects of these two-phase interactions in the confinement gap. Close to critical heat flux, brief periods occur when vapor completely obstructs the jet orifice, during which a significant portion of the heated surface dries out. In the velocity-dominated regime, at higher jet velocities, the two-phase flow disturbances cause observable instances of mild reduction in the flow rate of the incoming liquid jet, which are followed by dry out of small regions on the periphery of the heated surface. Flow visualizations during the transition to film boiling indicate that premature dry out of the heated surface is induced by these flow and pressure oscillations.

The efficient heat transfer resulting from flow boiling in microchannel heat sinks can help dissipate high heat fluxes from high-density electronic devices across a small temperature difference. However, practical implementation challenges unique to two-phase flow boiling, as compared to single-phase liquid cooling, have prevented its widespread adoption. A primary challenge is the occurrence of dynamic twophase flow instabilities, such as pressure drop oscillations (PDO), that have the potential to degrade heat transfer performance or trigger premature critical heat flux (CHF) under some conditions. Under other conditions, PDOs are observed to have little to no impact on performance. One factor proposed by modeling studies to be responsible for this discrepancy in observations of the effect of dynamic instabilities on performance is the thermal capacitance of the heat sink, though this has not been confirmed by experiments. In this study, the effect of thermal capacitance on the transient thermal response of a heat sink experiencing PDOs is examined through use of a dynamic two-phase flow model and experiments. Flow boiling experiments are performed with a controlled compressible volume upstream of parallelmicrochannel heat sinks having either a large or a small thermal capacitance. In accordance with the behavior predicted by our model, when thermal capacitance is reduced, the pressure drop oscillation frequency is found to decrease and temperature swings in the heat sink become more severe. Additionally, the experimentally measured CHF limit is diminished in the heat sink at smaller thermal capacitance. These results reveal thermal capacitance as a critical parameter that determines how much dynamic instabilities degrade flow boiling performance in a microchannel heat sink. & COPY; 2023 Elsevier Ltd. All rights reserved.

An experimental test protocol for simulating the air-side fouling of heat exchangers, as well as metrics to characterize the extent of fouling undergone by the heat exchanger and its performance in clean and fouled conditions, were proposed in a companion paper. In this study, the air-side fouling of a finned microchannel heat exchanger is experimentally investigated according to the proposed protocol. Key test parameters influencing heat exchanger fouling are identified based on studies in the literature, and their impact is experimentally investigated. The effectiveness of in situ cleaning methods is also experimentally evaluated. Transient measurement data and photographs taken during the fouling process reveal the nature of fouling, while steady-state data quantify the degradation in heat exchanger performance due to fouling. This two-part study defines a generalized experimental approach to enable characterization and comparison of heat exchanger surfaces on a standardized basis, and provides detailed experimental data for modeling heat exchanger fouling that includes all necessary information to allow for model validation.

Understanding salt crystallization due to evaporation of a salt solution from a porous medium is critical in applications ranging from saltwater distillation to preservation of historical monuments. Efflorescence is the crystallization of salt on the exposed surface of the porous medium. In this work, efflorescence patterns are visually observed and characterized on sintered copper particle wicks with spatially uni-modal and bimodal compositions of different particle sizes. Efflorescence is found to form earlier and spread readily over a wick made from smaller particles, owing to their lower porosity, while it is lim-ited to certain areas of the surface for wicks composed of the larger particles. A scaling analysis explains the observed efflorescence patterns in the bimodal wicks caused by particle size-induced nonuniform porosity and permeability. The non-uniformity reduces the advective flux in a high-permeability region by diverting flow towards a low-permeability region resulting from different pressure drops along the regions. This reduction in advective flux manifests as an exclusion distance surrounding a crystalliza-tion site where efflorescence is not expected to occur. The dependence of this exclusion distance on the porosity and permeability of the porous medium and the operating conditions is investigated. A large exclusion distance associated with the regions with bigger particles in the bimodal wicks explains pref-erential efflorescence over the regions with smaller particles. Our novel scaling analysis coupled with the introduction of the exclusion distance provides guidelines for designing heterogeneous porous media that can localize efflorescence.& COPY; 2023 Elsevier Ltd. All rights reserved.

Heat pipes and vapor chambers are passive thermal management devices used for efficient heat trans-port by phase change. Their passive operation is enabled by capillary pumping of the working fluid in a porous wick, which is operationally limited by the maximum pressure head it can provide. This cap-illary limit marks the maximum heat input at which the capillary pressure generated can overcome the pressure drop in the wick; operating above the capillary limit at steady state leads to dryout. Heat pipes and vapor chambers are increasingly being used in electronics systems where end-user activity dictates the transient power input which can therefore be highly variable and time-dependent. It was recently shown that heat pipes can withstand a power pulse exceeding the capillary limit for brief time intervals. Under such operating conditions, the heat pipe will experience dryout only if the duration of the pulse load is longer than a certain characteristic time interval. The pulse-load-induced dryout may result in an increased thermal resistance when the power is reduced back down to pre-dryout levels, thus exhibiting a hysteresis in heat pipe thermal performance. In this work, we experimentally characterize the recovery from pulsed-load-induced dryout. We further propose that the observed change in steady-state thermal performance before and after dryout results from contact angle hysteresis at the three-phase contact line of the wick-liquid interface. A model is developed based on this proposed mechanism to predict the na-ture of recovery from dryout-induced thermal hysteresis, as well as to identify that a given heat pipe has a maximum possible hysteresis. The experiments illustrate the trends inferred from the model for the re-covery process and confirm the existence of a "maximum hysteresis line," which identifies the worst-case scenario for thermal hysteresis after heat pipe dryout. Based on these mechanistic learnings, a new test -ing protocol is proposed for experimentally characterizing this post-dryout maximum hysteresis signature for a heat pipe.(c) 2023 Elsevier Ltd. All rights reserved.

Air-side fouling of heat exchangers has received much attention in the literature through both experimental and modeling studies. However, no standards currently exist that define a test procedure to experimentally foul heat exchangers and evaluate their performance in a fouled condition. This is evidenced by the variation in test procedures, fouling agents, metrics, and operating conditions reported. Based on a detailed review of past experiments, a standardized, repeatable test procedure is proposed that allows cross-comparison of data acquired in different laboratory facilities. The procedure generates time-resolved information on foulant buildup on the finned heat exchanger and the consequent impact on heat exchanger performance. Various fouling agents used in the literature and fouling deposits observed on field-installed heat exchangers are compared, and a representative fouling agent is recommended. A method is proposed to quantify heat exchanger fouling and metrics identified to quantify heat exchanger performance before and after fouling. This is the first of two companion papers on an experimental investigation of air-side fouling of heat exchangers. The companion paper presents experimental data obtained by implementation of the proposed procedure to investigate fouling of a finned microchannel heat exchanger.

Microchannel flow boiling heat sinks that leverage the highly efficient heat transfer mechanisms associated with phase change are a primary candidate for cooling next-generation electronics in electric vehicles. In order to design flow boiling heat sinks for such practical applications, one key obstacle is an understanding of the conditions for occurrence of dynamic two-phase flow instabilities, to which microscale flow boiling is particularly susceptible, as well as their impact on heat transfer performance. While mapping the operational regimes of these instabilities has been well-studied, with numerous stability criteria available, their impact on the heat transfer performance of heat sinks in practical applications is not understood. This work seeks to assess the impact of pressure drop oscillations and parallel channel instabilities on the surface temperature and critical heat flux in parallel microchannel heat sinks. This is achieved through measurement of time-averaged steady-state temperatures and pressures, combined with high-frequency pressure signals and high-speed flow visualization. These data are compared across three controlled flow configurations that comprise a condition of stable flow boiling, a condition where only parallel channel instabilities can occur, and a third where both pressure drop oscillations and parallel channel instabilities can occur. Experiments are performed using the dielectric refrigerant HFE-7100 in 2 cm long parallel microchannel heat sinks with square-cross-section channels (0.25, 0.5, 0.75, and 1 mm widths) at three mass fluxes (100, 400, and 1600 kg/m2s). Across this range of conditions, the time-averaged surface temperature and critical heat flux were remarkably insensitive to the occurrence of these instabilities despite the significant hydrodynamic events and transient flow patterns observed.

For micro-electronic components and systems, reliability under thermomechanical stress is of critical importance. Experimental characterization of hotspots and temperature gradients, which can lead to deformation in the component, relies on accurate mapping of the surface temperature. One method of noninvasively acquiring this data is through infrared (IR) thermography. However, IR thermography is often limited by the typically low resolution of such cameras. Additionally, the unique surface finish preparations required to infer physical deformation using digital image correlation (DIC) generally interfere with the ability to measure the temperature with IR thermography, which prefers a uniform high emissivity. This work introduces a one-shot technique for the simultaneous measurement of surface temperature and deformation using multiframe super-resolution-enhanced IR imaging combined with DIC analysis. Multiframe super-resolution processing uses several subpixel shifted images, interpolating the image set to extract additional information and create a single higher-resolution image. Measurement of physical deformation is incorporated using a test sample with a black background and low-emissivity speckle features, heated in a manner that induces a nonuniform temperature field and stretched to induce physical deformation. Through processing and filtering, data from the black surface regions used for surface temperature mapping are separated from the speckle features used to track deformation with DIC. This method allows DIC to be performed on the IR images, yielding a deformation field consistent with the applied tensioning. While both the low- and super-resolution data sets can be successfully processed with DIC, super-resolution helps to reduce noise in the extracted deformation fields. As for temperature measurement, using super-resolution is shown to allow for better removal of the speckle features and reduce noise, as quantified by a lower mean deviation from the spatial moving average.

Understanding the dynamics of precipitation and crystallization as salt solutions evaporate from porous media is of importance in the context of preservation of historical monuments, understanding soil nutrient content, and design of porous evaporators for use in distillation plants. Transient advection-diffusion equations govern the salt mass fraction profile of the solution inside the porous medium. These governing equations are solved to obtain the solute mass fraction profile within the porous medium as well as the effloresced salt crust. Further accounting for precipitation allows a study of the formation and growth of efflorescence and subflorescence. Crystallization experiments are performed by allowing a NaCl solution to evaporate from a porous medium of copper particles and the subflorescence trends predicted by the model are validated. The modeling framework offers a comprehensive tool for predicting the spatiotemporal solute mass fraction profiles and subsequent precipitation in a porous medium. (c) 2022 Elsevier Ltd. All rights reserved.

Enhancement of the rate of boiling heat transfer, a critically significant need across a range of industrial transport processes, can be achieved by the introduction of surface microstructures. However, the precise mechanism of such enhancement is not definitively understood. We establish microlayer evaporation from the imbibed liquid layer underneath the growing vapor bubbles as the key mechanism of enhancement in boiling heat transfer coefficient for microstructured surfaces. We experimentally characterize nucleate boiling heat transfer performance on silicon surfaces custom-fabricated with controlled microstructures using HFE-7100 as the working fluid. We then undertake an analytical prediction of the microlayer evaporation from the microstructured surface. A clear dependence of the measured boiling heat transfer coefficients from microstructures of different dimensions on the predicted evaporation heat transfer coefficients allows us to conclude that microlayer evaporation governs the boiling enhancement from microstructured surfaces.

Evaporation from porous structured surfaces is encountered in a variety of applications including electronics cooling, desalination, and solar energy generation. Of major interest in the design of thermal systems for such applications is a prediction of the heat and mass transfer rates during evaporation from these surfaces. The present study develops a figure of merit (FOM) that characterizes the efficacy of evaporative heat transfer from microstructured surfaces. Geometric quantities such as the contact line length per unit area, porosity, and contact angle that are independent of details of the surface structure are utilized to develop the FOM, allowing for flexibility in its application to a variety of structured surfaces. This metric is calibrated against an evaporative heat transfer model and further benchmarked with evaporation heat transfer data from the literature. The FOM successfully captures the variation in evaporation heat transfer coefficient across different structures as well as the optimum dimensions for a given structure, and therefore can serve as a tool to survey available structures and also optimize their dimensions for heat and mass transfer enhancement.

As the size, weight, and performance requirements of electronic devices grow increasingly demanding, their packaging has become more compact. As a result of thinning or removing the intermediate heat spreading layers, nonuniform heat generation from the chip-scale and component-level variations may be imposed directly on the attached microchannel heat sink. Despite the important heat transfer performance implications, the effect of uneven heating on the flow distribution in parallel microchannels undergoing boiling has been largely unexplored. In this study, a two-phase flow distribution model is used to investigate the impact of uneven heating on the flow distribution behavior of parallel microchannels undergoing boiling. Under lateral uneven heating (i.e., the channels are each heated to different levels, but the power input is uniform along the length of any given channel), it is found that the flow is significantly more maldistributed compared to the even heating condition. Specifically, the range of total flow rates over which the flow is maldistributed is broader and the maximum severity of flow maldistribution is higher. These trends are assessed as a function of the total input power, degree of uneven heating, and the extent of thermal connectedness between the channels. The model predictions are validated against experiments for a representative case of thermally isolated and coupled channels subjected to even heating and extreme lateral uneven heating conditions and show excellent agreement.