Suresh Garimella

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 …

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 …

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 …

Heat pipes and vapor chambers are passive two-phase heat transport devices that are used for thermal management in electronics. The passive operation of a heat pipe is facilitated by capillary wicking of the working fluid through a porous wick, and thus is subject to an operational limit in terms of the maximum pressure head that the wick can provide. This operational limit, often termed as the capillary limit, is the maximum heat input at which the pressure drop in the wick is balanced by the maximum capillary pressure head; operating a heat pipe or a vapor chamber above the capillary limit at steady-state leads to dryout. It thus becomes important to predict the performance of heat pipes and vapor chambers and explore the parametric design space to provide guidelines for minimized thermal resistance while satisfying this capillary limit. An increasingly critical aspect is to predict the transient thermal response of vapor chambers. Moreover, heat pipes and vapor chambers are extensively being used in electronic systems where the power input is dictated by the end-user activity and is expected to even exceed the capillary limit for brief time intervals. Thus, it is imperative to understand the behavior of heat pipes and vapor chambers when operated at steady and transient heat loads above the capillary limit as dryout occurs. However, review of the literature on heat pipe performance characterization reveals that the regime of dryout operation has been virtually unexplored, and thus this thesis aims to fill this critical gap in understanding. The design for minimized thermal resistance of a vapor chamber or a heat pipe is guided by the relative …

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 …

Heat pipes and vapor chambers are passive thermal management devices used for efficient heat transport 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 capillary 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 …

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 …

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 two-phase 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 …

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 …

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 unimodal 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 limited 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 …

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 …

(*) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 USC 154 (b) by 55 days. Garcia-Cordero J., et al., Sessile droplets for chemical and biologi cal assays. Lab Chip, 2017, 17, 2150-2166. Regnault C., et al., Microfluidic Devices for Drug Assays. High Throughput 2018, 7, 18; doi: 10.3390/ht7020018.

We propose and experimentally demonstrate the fiber optic sensor (FOS) combined with the strain free fiber Bragg grating (FBG) and the intensity-based FOS for multipoint sensing of the temperature and the strain simultaneously.

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 …

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 …

Decades of prior study has yet to fully disentangle the complex transport mechanisms that are attributed to highly effective heat transfer during boiling. Rational design of enhanced surfaces to maintain lower surface temperatures during boiling requires improved insight into the individual heat transfer processes and their dependence on surface characteristics. This study seeks to advance the understanding of the fundamental role that surface wettability plays in determining the relative contributions of different heat transfer mechanisms and on the overall heat transfer efficacy during bubble growth. Two-phase, diabatic simulations of single bubble growth considering interfacial phase change and a custom dynamic contact angle framework are employed to investigate how the distinct contact-line and bubble dynamics that are experienced on hygrophilic, hygrophobic, and ambiphilic surfaces impact heat transfer. The …

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m 2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent …

Flow boiling in microchannel heat sinks is capable of providing the high-heat-flux dissipation required for thermal management of next-generation wide bandgap power electronics at low pumping power and uniform surface temperatures. One of the primary issues preventing implementation of these technologies is the presence of flow boiling instabilities, which may reduce the heat transfer performance. However, the effect of individual instabilities, such as the parallel channel instability or pressure drop oscillations, on the overall heat transfer coefficient and critical heat flux in microchannel heat sinks has not been fully quantified. The primary cause of these dynamic flow boiling instabilities is the interaction between the inertia of a two-phase mixture in a heated channel and sources of compressibility located upstream of the inlet. In order to isolate the effect of pressure drop oscillations on flow boiling heat transfer …

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 …

Surface wettability is known to have a major influence on the ebullition characteristics of a bubble growing from a solid surface. Yet, simplistic static characterization of the wetting behavior is still relied upon to indicate performance characteristics during boiling. In this study, a theoretical framework is developed for the wetting and dewetting processes occurring during bubble growth based upon the dynamic contact angles. This framework is incorporated into adiabatic volume-of-fluid simulations to capture the influence of the surface wettability on contact line and contact angle dynamics during bubble growth and departure. The simulations span a large range of dynamic wetting behaviors and fluid properties. The receding contact angle is shown to govern the early stages of bubble growth as the contact line recedes outward from the bubble center and is the dominant wetting characteristic that determines the …

Two-phase passive heat transport devices such as vapor chambers, loop heat pipes, and capillary pumped loops utilize porous evaporators for phase change and to drive fluid transport. Nucleate boiling can occur within such capillary-fed porous evaporators, especially under high-heat-flux operation, as has been visually observed in various experimental studies in the literature. However, prior modeling efforts have typically only considered single-phase flow of liquid through a completely saturated porous medium for characterizing the dryout limit and thermal performance. The present work offers a new semi-empirical model for prediction of thermal resistance and dryout during boiling in capillary-fed evaporators. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer …

Flow boiling in a network of heated parallel channels is prone to instabilities that can cause uneven flow distribution, thereby degrading the heat transfer performance of the system and limiting predictability. This study experimentally investigates flow maldistribution between two parallel microchannels that arises due to the Ledinegg instability. The channels are heated uniformly and are thermally isolated from each other, such that both channels are subjected to the same input power regardless of the flow distribution. The channels are hydrodynamically connected in parallel and deionized water is delivered at a constant total flow rate shared by both channels. Direct measurements of the flow rate, wall temperature, and pressure drop in individual channels are performed simultaneously with flow visualization. At low power levels, when both channels remain in the single-phase liquid regime, the flow is evenly …

Systems and methods that utilize enhanced boiling surfaces to promote the efficiency of boiling. Such a system has a surface that is hydrophobic and exhibits a sufficiently low receding contact angle to a liquid such that vapor spreading during bubble growth and premature transition to film boiling is mitigated.

The Intrachip Enhanced Cooling Fundamentals (ICECool Fun) effort was launched by the Defense Advanced Research Projects Agency (DARPA) under the leadership of Dr. Avram Bar-Cohen during 2012–2015 to target an order of magnitude improvement in chip level and hot spot heat fluxes, compared to the then state-of-the-art (SOA). Evaporative cooling technologies to achieve potential targets of 1 kW/cm 2 at the chip level and 5 kW/cm 2 at the hot spot level were targeted. A key goal was to improve fundamental understanding of the evaporative cooling physics at the relevant scales, and a numerical modeling capability to enable the co-design of such solutions in emerging computing and communications systems. A summary of the five projects pursued under this effort is provided, including the key accomplishments and developed capabilities.

In this paper, the effect of three different types of nanostructured reinforcements on the mechanical and tribological properties of nanocomposites are investigated. Carbon nanotube, nanoclay and nanographene oxide with equal weight percentages and under similar environmental conditions are added into the epoxy resin. To achieve uniformly dispersed nanoparticles within the epoxy matrix, mechanical stirring with ultra-sonication is utilized. After degassing process, tensile and wear test specimens were made according to the relative standards. Three samples of each nanocomposite were prepared and tested. Young's modulus, ultimate tensile strength, strain at break point and toughness of the specimens were extracted from the stress-strain curve using the tensile test. Dry wear test was performed at 20N, 60N and 100N loads using a disk on pin testing machine at room temperature. The results showed that in the sample containing carbon nanotubes, the ultimate stress increased by 16% and the strain at the break point increased by 27% compared to pure epoxy. Also, carbon nanotube/epoxy and nanoclay/epoxy nanocomposites showed the highest wear resistance compared to other samples, so that in the sample containing nanoclay reinforcement, a 60% decrease in the amount of wear rate was observed compared to the pure epoxy sample.

An easy-to-use representation of vapor chambers is developed in terms of effective anisotropic properties. This approach enables accurate simulation of the vapor chamber represented as a solid conduction block by assigning appropriate values to its effective density, specific heat, in-plane thermal conductivity, and through-plane thermal conductance. These effective properties are formulated such that the vapor chamber operation in terms of steady-state and transient thermal responses matches a full, physical simulation of phase change and energy transport in the vapor core; they are intrinsic properties that can be applied independently of the boundary conditions and heat input.

Vapor chambers provide highly effective heat spreading to assist in the thermal management of electronic devices. Although there is a significant body of literature on vapor chambers, most prior research has focused on their steady-state response. In many applications, electronic devices generate inherently transient heat loads and, hence, it is critical to understand the transient thermal response of vapor chambers. We recently developed a semi-analytical transport model that was used to identify the key mechanisms that govern the thermal response of vapor chambers to transient heat inputs (Int. J. Heat Mass Trans. 136 (2019) 995–1005). The current study utilizes this understanding of the governing mechanisms to develop design guidelines for improving the performance of vapor chambers under transient operating conditions. Two key aspects of vapor chamber design are addressed in this study: first, a …

Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux electronic devices that are often operated in transient modes. In Part 1 of this two-part study, the dynamic response of a heated 500 μm channel undergoing flow boiling of HFE-7100 is experimentally investigated for a single heat flux pulse. Three heat flux levels exhibiting highly contrasting flow behavior under constant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m2). Transient testing is conducted by pulsing between these three heat flux levels and varying the pulse duration. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are …

It is an everyday observation that water droplets condensing on a rigid substrate coalesce immediately upon touching. Soft surfaces, however, possess unique properties that create an illusionary effect in this situation, making it appear that droplets can touch and form a dense pattern instead of coalescing. In article number 2000731 Rishav Roy, Suresh V. Garimella, and coworkers demonstrate that this reluctance to coalesce is a consequence of deformation of the soft surfaces to form intervening ridges between the droplets. The cover art illustrates the topography of droplet ‘footprints’ after they are removed from a cryogenically fixed soft substrate.

Vapor chambers developed for high-heat-flux operation require advanced evaporator wick designs that can sustain capillary flow when boiling occurs over the heater region. A two-layer evaporator wick integrates a thin base wick layer that is supplied with liquid from a thick cap layer through an array of vertical feeding posts distributed over the heated area. This design allows boiling to occur within the thin base layer, while separating the incoming liquid feeding and outgoing vapor venting pathways. In our prior work, boiling in two-layer wicks was experimentally demonstrated to provide high-heat-flux dissipation over larger heater areas and at low thermal resistance. The current study experimentally explores the effect of two-layer wick design parameters, specifically the dimensions that alter the area available for liquid feeding and vapor venting, on the thermal performance and dryout limit of the wick, using water …

Manifold microchannel heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels afford a large enhancement in heat transfer area; however, the flow morphology in manifold microchannels during two-phase operation, as well as the resulting thermal performance, are not well understood. In this work, a single manifold microchannel representing a repeating unit in a heat sink is fabricated in silicon with a bonded glass viewing window. Samples of different channel lengths (750 μm and 1500 μm) and depths (125 μm, 250 μm, and 1000 μm) are considered; channel and fin widths are both maintained at 60 μm. Subcooled fluid (HFE-7100) is delivered to the channel at a constant flow rate such that the fluid velocity at the inlet is ~1.05 m/s in all cases. A high-speed camera is used to visualize the two-phase flow in the channel through …

Microscale interactions with deformable substrates are of fundamental interest for studying self‐assembly processes and the mobility of cells on soft surfaces, with applications in traction force microscopy. The behavior of microscale water droplets on a soft polymer substrate is investigated. Droplets formed by condensation on the soft substrate are reluctant to coalesce, which leads to coverage of the surface with clusters of droplets assembled in a honeycomb‐like pattern. Cryogenically fixed in this state, scanning electron microscopy of these droplets reveals the presence of an intervening wetting ridge of the polymer that acts as a barrier between neighboring droplets and prevents coalescence. A linear elastic deformation model is developed to predict this surface profile and corroborate the observed behavior.

This chapter highlights the multiscale concentrated solar power thrust, which focused on developing new low-cost manufacturable technologies for both high- and moderate-temperature thermal cycles. In the high-temperature range, the focus was on the supercritical carbon dioxide (s-CO2) Brayton cycle. Research involved developing low-cost heliostats coupled with novel bladed receivers and a novel CO2 test loop. A key focus was developing a functional testbed to evaluate and optimize the Brayton cycle as a cost-shared effort with the Indian Institute of Science. The project also investigated developing a novel helical receiver to heat the CO2. Extensive computational modeling of the thermal flow and gradients was conducted to develop the novel CO2 cycle. The program also pursued developing low-cost mirrors, absorbers, and troughs for Rankine cycle solar thermal parabolic trough technology. A new …

The evaporation of water droplets placed on heated hydrophilic, hydrophobic, and superhydrophobic substrates is numerically investigated. Simplified analytical models for droplet evaporation only include vapor diffusion transport in the surrounding gas domain and assume an isothermal droplet interface at the substrate temperature. The comprehensive model developed in this study accounts for all of the pertinent transport mechanisms. The interface is cooled via absorption of latent heat during evaporation, and the saturated vapor concentration is coupled to local temperature at the droplet interface. Conjugate heat and mass transfer are solved throughout the system using temperature-dependent physical properties. Buoyancy-driven convective flows (induced by both species concentration and temperature gradients) in the droplet and gas domains are also simulated. The evaporation rates predicted as a …

The literature abounds in flow visualization methods and their application to a very large range of situations. This chapter presents techniques for the visualization of air and water (liquid) flows. The principle for each technique is provided in addition to implementation details, equipment used, sources for procurement, and example visualizations. The chapter presents the holographic interferometry technique and example visualizations. Smoke entrainment is the most common visualization technique for laminar air flows, but has somewhat limited use in turbulent flows due to its rapid diffusion by turbulent mixing. Flow visualization is usually easier to perform in water, and also yields better quality results. As a result, water-flow tunnels are frequently used to study air flows by testing scale models at low velocity. As with smoke visualization, dye entrainment is successful mostly in laminar flow. The enhanced mixing in …

Thin film evaporation yields high local heat fluxes that contributes significantly to the total heat transfer rate during various two-phase transport processes including pool boiling, flow boiling, and droplet evaporation, among others. Recent studies have shown a strong correlation between the roughness of a surface and its two-phase heat transfer characteristics, but the underlying role of nanoscale surface roughness in thin film evaporation is not fully understood. In the present work, a thin film evaporation model is developed that accounts for the role of the roughness-affected disjoining pressure and flow permeability in determining the film thickness profile and heat transfer rate. Nanoscale surface roughness leads to a flatter evaporating meniscus profile when the effect of disjoining pressure is more pronounced of the two and promotes evaporation, consistent with previous experimental observations. However, our …

An understanding of the factors that affect the flow freezing process in microchannels is important in the development of microfluidic ice valves featuring well-controlled and fast response times. This study explores the effect of channel diameter on the flow freezing process and the time to achieve channel closure. The freezing process is experimentally investigated for a pressure-driven water flow (0.3 ml/min) through three glass microchannels with inner diameters of 500 μm, 300 μm, and 100 μm, respectively, using channel-wall temperature measurements synchronized with high-magnification, high-speed imaging. Freezing invariably initiates in supercooled water as a thin layer of dendritic ice that grows along the inner channel wall, followed by the formation and growth of a thick annular ice layer which ultimately causes complete channel closure. The growth time of the annular ice layer decreases monotonically …

The balance between the capillary pressure provided by the wick in a heat pipe or vapor chamber and the flow resistance to liquid resupply at the evaporator determines the maximum heat load that can be sustained at steady state. This maximum heat load is termed as the capillary limit; operation at steady heat loads above the capillary limit will result in dryout at the evaporator wick. However, different user needs and device workloads can lead to highly transient heat loads in applications ranging from consumer electronic devices to server processors. In these instances, the operation of heat pipes must be assessed in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed …

This chapter covers various enhancement techniques including the use of geometric adjustments such as staggering chip-arrays; surface enhancements in the form of a large variety of fins and surface enhancements in the form of a large variety of fins. It deals with forced convection. A large number of choices exist for the enhancement of forced-convection cooling of electronic equipment. Electronic components themselves often act as protrusions that enhance mixing in the flow. While the layout of the components on the board is mostly governed by nonthermal hardware considerations, the strategic placement of additional barriers could be used to significant advantage. Air cooling of heated surfaces with impinging jets has been studied extensively. The chapter discusses some of the enhancement techniques used in combination with each other. Air cooling is often supplemented by liquid cooling as the ultimate …