High Heat Flux Applications Research Articles

Experimental investigation of heat transfer characteristics and performance of smooth and wicking surfaces in spray cooling for high heat flux applications

In this work, studies have been conducted on heat transfer performance of both smooth and modified wicking surfaces in a spray cooling experiment for high heat flux applications. The heat transfer performance of the two surfaces in terms of critical heat flux (CHF) and heat transfer coefficient (HTC) were determined in an experiment using a two-phase thermal loop, operating with de-ionized water as the working fluid in a pressure range of 101.45–165.80 ​kPa and working fluid mass flow rates of 0.84–1.26 ​g per second. The result shows that the modified wicking surface has higher CHF of 225 ​W/cm2 at cooling temperature difference of 130 ​°C compared to 160 ​W/cm2 at cooling temperature difference of 180 ​°C for the smooth surface. Further analysis of the results indicates that, maximum HTC enhancement of 8500 ​W/m2-K is achieved at cooling temperature difference of about 100 ​°C for the modified wicking surface over the smooth surface. Using a high speed camera set at 10,000 frames per second, it was observed that there was increased surface wettability on the modified surface due to fluid wicking and spreading, with reduced Leidenfrost effect at the onset of CHF. This effect resulted in lower surface temperatures and higher CHF for the modified surface compared to the smooth surface.

Development of Nanostructured-based Composites as Advanced Thermal Interface Materials

Thermal management is one of the most critical issues in electronics due to increasing power densities. This problem is getting even worse for small and sophisticated devices due to air gaps present between the heat source and heat sink. Thermal interface materials (TIM) are used to reduce the air gaps and significantly increase the heat transfer capability of the system. A high-thermal-performance, cost-effective and reliable TIM would be needed to dissipate the generated heat, which could enable significant reductions in weight, volume and cost of the thermal management system. In this study a number of different nanostructured materials are reviewed for potential use as a filler material in our effort to develop advanced TIM composite. Some of the candidate filler materials considered is Carbon Nanotubes, Graphene and Few Layer Graphene (FLG), Boron Nitride Nanotubes (BNNT) and Boron Nitride Nanomesh (BNNM) and Boron Arsenide (BAs). Objective is to identify composition of boron arsenide as filler in polymer-nanostructured material composite TIM for high heat flux applications. In order to design boron-arsenide-based TIM composite with enhanced effective thermal conductivity, a number of metallic and nonmetallic base-filler material composites are considered with varying filler fractions. Empirical mixture models based on effective medium theories (EMT) are evaluated for estimating effective conductivity of the two-component boron arsenide-filler composite TIM structure.

Optimization of stepwise varying width microchannel heat sink for high heat flux applications

The semiconductor devices are widely employed for many applications. A significant amount of the heat must be dissipated properly to keep the devices' performance stable. Additionally, under high operation temperatures, a physical damage is possible due to thermal stresses that threaten the safety components and increase the failure rate. The development in the direction of more tightly packed electronic devices increases the challenges of offering an efficient cooling for these devices under high heat fluxes in a very restricted space. Hence, an advanced cooling technique is compulsory to attain the appropriate performance along with extending the lifespan of microelectronic devices. Stepwise varying width microchannel heat sink is considered one of the innovative cooling system from these microelectronic devices. In the current study, different designs of stepwise varying width microchannel heat sink were optimized, and their thermal performance was studied for more uniform cooling of microelectronics devices.

Study of a Hybrid of Pulsating Heat Pipe and Distributed Jet Array

Pulsating heat pipes (PHPs) are effective devices for transferring a large amount of heat. For the existing PHP design, the U-turn geometry makes it difficult to use on small heat spots, which are very common in electronic cooling and many high heat flux applications. Additionally, the pipe flow in PHPs is not an optimum form for convective heat transfer. So far little research has attempted to resolve these issues. In this study, a hybrid design is introduced incorporating the characteristics of the PHP for its passive operation and the distributed jet array for its efficient convection. Based on the proposed structure, test articles are fabricated and experimentally studied. The design is proved successful. The heat transfer performance is evaluated by parametric studies and compared with the closed-loop PHPs. The flow visualization results are analyzed with different operational stages, flow patterns, and dimensionless numbers. The investigation concludes that the proposed design not only provides a new configuration for passive oscillatory heat transfer but also offers enhanced performance and applicability over the existing PHPs.

Application of aluminum flat heat pipe for dry cooling near the hot spot of a radar array with a multiscale structure

Power amplifiers used for high-speed radio frequency (RF) consume a considerable amount of power and create thermal burden. The heat flux per module of power amplifiers that are based on gallium nitride (GaN) component devices, which have recently entered maturity in the market and shown excellent RF power output, is approximately 190 W/cm2, thereby causing thermal management burden. When the number of arrays increases, and the power consumption is large, liquid cooling methods, such as maximizing cooling performance by optimizing the liquid flow path design or designing the cooling liquid line as close as possible to the heat source in the power package module, are mainly considered. However, the close contact of the liquid lines to the heat source may form a dewdrop in the package module, thereby decreasing reliability, especially in a radar system with power array modules. As an alternative, dry cooling methods that use heat pipes have been investigated. However, radar array package modules have not elicited much research attention. In this study, an aluminum flat heat pipe (AFHP) was developed for a 1 × 4 3D array module for the RF power amplifier of the next-generation X-band radar. This module is lightweight and is based on aluminum material, has a flat shape, and facilitates antenna interconnection through its hole structures. Subsequently, an AFHP with an antenna interconnection structure was designed and fabricated. Moreover, the temperature limit in the GaN power device and the temperature distribution in the entire package module were evaluated by testing the 1 × 4 antenna array module structure at 193 W/cm2 of heat flux per module. The results obtained in this study could confirm the possibility of using AFHP as a dry cooling method to high heat flux application.

Droplet boiling on heated surfaces with various wettabilities

Single droplet boiling is a basic problem for spray cooling which is an effective technology for high heat flux applications. Visualizations were used to investigate the effect of surface wettability on single droplet boiling with measurements of the droplet evaporation times. Four boiling regimes were identified on the hydrophilic surface and the bare aluminum surface, including nucleate boiling, nucleate boiling with violent atomization, transition boiling and film boiling. The nucleate boiling with violent atomization regime was not seen on the hydrophobic surface and the superhydrophobic surface. The transition boiling regime was not seen on the superhydrophobic surface. The Leidenfrost temperatures were approximately 220 °C on the hydrophilic surface, 200 °C on the bare aluminum surface, 170 °C on the hydrophobic surface and 170 °C on the superhydrophobic surface. The effect of surface wettability on the Leidenfrost temperature was discussed using the potential barrier existing at the transition to film boiling. A droplet film boiling model was proposed to relate the droplet evaporation time, initial droplet size and surface superheat with a semi-empirical correlation developed for the droplet evaporation time. The correlation agrees well with the current measured droplet evaporation times for film boiling and with data in the literature.

The enhancement of dropwise and spray evaporative cooling by using extract of Reetha added water as a coolant

Although the use of synthetic surfactant in various quenching process significantly enhances the heat removal rate, the corrosive nature of the synthetic surfactants and their interaction with the metal surface during cooling does not allow to use. The replacement of the synthetic surfactant by another surfactant which is free from the abovementioned disadvantages can mitigate the requirement of various industries dealing with high heat flux applications. In this regard, the literature does not reveal any information. Therefore, in the current work, an attempt has been made to replace the synthetic surfactant by preparing a surfactant which does not depict the disadvantage of the synthetic surfactant. In this work, the surfactant is prepared from the extract of Reetha (Sapindus mukorossi). The measurement of various physical properties of the extract of Reetha and also different characterizations such as FT-IR and EDX analysis confirm the existence of surfactant properties in the extract of Reetha. Furthermore, the corrosion analysis clearly indicates insignificant corrosion in comparison with synthetic surfactants. The post quenching analysis of the heat treated steel plate reveals that the extract of Reetha added water does not depict the deposition characteristics. The thermal performance of the extract of Reetha is determined by conducting dropwise evaporative cooling and spray quenching experiments. The result reveals that the extract of Reetha has an excellent heat removal capacity (CHF = 1.3 MW/m2) in the critical heat flux region.

Liquid impingement cooling of cold plate heat sink with different fin configurations: High heat flux applications

The objective of this study is to investigate the cooling performance of the liquid jet impingement of a cold plate heat sink with different fin geometries using water and nanofluids as working fluids. In experiments, the monitored parameters mainly focus on the effects of different heat flux, the water mass flow rate, and heat source area on its thermal resistance. In the numerical study, a three-dimensional of a single-phase and two-phase Eulerian turbulent flow models are used to visualize the temperature and fluid flow behaviors of the cold plate heat sinks. The initial and the boundary conditions of the model based on the experimental conditions. It indicated from the experiments that the increasing of Reynolds number, heat source area, and power input results in decreased thermal resistance. Using nanofluids as working fluid gives the thermal resistance lower than de-ionized water as the working fluid. Additionally, the predicted results have verified with the measured data. It found that good agreement is obtained and gives an average error of 2.69%, 11.77% for CP base temperature and thermal resistance, respectively.

Characteristics of Bi-metallic Interfaces Formed During Direct Energy Deposition Additive Manufacturing Processing

Various additive manufacturing processes are being evaluated to reduce the time and cost for fabrication of low volume, complex, and multifunctional assemblies. This study evaluated two direct energy deposition processes for the fabrication of large bi-metallic structures. The materials evaluated were Inconel 625 and copper alloy C18150, which are used in various high heat flux applications. Inconel was deposited onto the C18150 substrate using blown powder and wire-fed processes. Complete bonding was obtained in both processes and the resulting interfaces were evaluated using microscopy and indentation testing. Differences were observed in the interface region suggesting the kinetic energy of the blown powder process resulted in more residual stress at the interface, promoting recrystallization and enhanced diffusion. This created a broader interface in the blown powder specimens compared to a narrower mechanically mixed interface with the wire-fed process.

Evidence of electron heating at different radial locations on Proto-MPEX

This paper reports on the observation of controlled underdense electron heating for high heat flux applications in the Prototype Material Plasma Exposure eXperiment. The goal of this experiment is to demonstrate the source and heating concepts to create a high intensity plasma at material targets to study plasma-material interactions in similar plasma conditions to magnetic fusion divertors. A 80 kW helicon source at 13.56 MHz is used for the plasma density production and a 15 kW microwave source at 28 GHz is used for electron heating. For electron densities ∼5 × 1018 m−3, the electron temperature at the target location increases from 2 eV to 5 eV and the core heat flux increases by a factor of 5. It is shown that the radial profile of the heat flux delivered at the target can be controlled by either the background plasma density or the magnetic field. Analysis suggests that the electron heating is due to heating near the upper-hybrid resonance layer and that this heating scenario may scale up to divertor-relevant electron densities ∼5 × 1019 m−3 by increasing the microwave driving frequency to 105 GHz.

200 MeV proton irradiation of the oxide-dispersion-strengthened copper alloy (GlidCop-Al15)

In search to identify extremely stable materials to perform the collimation function of the 7-TeV proton beam halo at the Large Hadron Collider (LHC), oxide dispersion strengthened (DS) copper alloys were explored. These internally oxidized copper alloys known as GlidCop are also leading candidates for high heat flux applications in fusion reactors and as divertor and first wall structure in ITER. These considerations have led to an extensive body of research on neutron-induced changes in the microstructure and physio-mechanical properties. This study focused primarily on the damage induced by 200 MeV protons on as wrought and cold-worked GlidCop Al15 DS copper ally selected as a candidate material of the LHC beam collimation structure to damage levels up to ∼10 displacements-per-atom (dpa) and irradiation temperatures up to ∼600 °C. For reference, low dose spallation neutron damage at sub-zero irradiation temperature was included in the study. Proton irradiation effects on dimensional stability, mechanical behavior, electrical resistivity and X-ray diffraction-based microstructural changes are presented in the paper. GlidCop Al15 exhibited excellent thermal stability and superior to pure Cu resistance to conductivity loss. Proton irradiation-induced hardening and embrittlement at distinct irradiation temperature regimes were shown to follow similar trends to what was reported under neutron exposure to even higher dpa damage. At Tirr ∼600 ± 20 °C the cold-worked DS copper alloy exhibited softening accompanied by elongation reduction while at Tirr ∼200 ± 10 °C the as-wrought experienced hardening with elongation increase. High resolution X-ray diffraction revealed that even low-dose neutron irradiation has a strong influence on the size and distribution of dispersed Al2O3 particles.

Thin film evaporative cooling system for high heat flux applications

One of the promising ways of removing large heat fluxes from the surface of heat-stressed elements of electronic devices is the use of evaporating thin layer of liquid film, moving under the action of the gas flow in a flat channel. In this work, a prototype of evaporative cooling system for high heat flux removal with forced circulation of liquid and gas coolants, capable to remove heat flux of up to 1 kW/cm2 and higher is presented. The peculiarity of the test section used in the present work is that the width of the channel is equal to the width of the heating element (1 cm). It was found that the configuration of the test section allows to get higher values of the heat flux compared with the case when the channel width is higher than the width of the heater, since in the latter case some portion of liquid is deviating from the heater due to the thermocapillary forces.

Heat transfer enhancement strategies in a swirl flow minichannel heat sink based on hydrodynamic receptivity

The swirl flow minichannel heat sink has shown to be a promising alternative for thermal management of high heat flux applications, such as electronics and concentrated photovoltaics. Effective heat transfer enhancement strategies for this device are identified by studying the receptivity of temperature disturbances to a momentum forcing input. Steady state laminar flow is calculated numerically and experimental measurements are carried out to validate the results for subcritical Reynolds numbers. Using the framework of nonmodal stability theory, a harmonically driven linear perturbation problem is formulated, and the methodology to apply the local and parallel flow approximations based on order of magnitude arguments is detailed. The input-output response of temperature perturbations to forcing of the radial, azimuthal, and wall-normal momentum components is calculated for a range of wavenumbers, waveangles and temporal frequencies. The largest amplification is presented by streamwise vortices and streaks, followed by axisymmetric inward travelling waves, and then by streamwise propagating waves. Micromachining the channel walls with streamwise spiral grooves is proposed as a heat transfer enhancement technique. Excitation of streamwise independent structures in the wall-normal direction is expected, therefore maximum amplification should be obtained. Due to its simple implementation, we also propose using a pulsating flow rate as a heat transfer enhancement technique. Receptivity results for streamwise propagating waves of radial forcing show a response curve with moderate amplification for a wide range of actuation frequencies. Experimental work is conducted to measure the performance of the swirl flow channel heat sink using flow pulsations at the range of forcing frequencies suggested by the receptivity study. Compared to the unforced case, a lower wall temperature (up to 5°C cooler) was observed with pulsations, at the same imposed heat flux and flow rate. To get the same wall temperature as in the unforced case, a pumping power reduction of up to 26.6% was observed, and using the same pumping power resulted in up to a 10.3% Nusselt enhancement. Hydrodynamic receptivity was successfully used to identify effective heat transfer enhancement strategies, resulting in a significant performance improvement for the swirl flow channel heat sink. This physics based approach can be extended to other techniques, for instance, to select the wavelength of a wavy surface, the periodicity of surface roughness elements, or the frequency of acoustic vibrations.

Heat Transfer Characteristics of Al2O3-Kerosene and Fe3O4-Kerosene Nanofluids Flowing in a Minitube at Supercritical Pressures

Regenerative cooling system is thought to be an effective and practical solution to better thermal management for high heat flux applications. In this paper, the potential of nanofluids as regenerative coolants at supercritical pressures was evaluated. Two-step method was applied to prepare Al₂O₃-kerosene and Fe₃O₄-kerosene nanofluids.

Development of a heat sink module for a near-term DEMO divertor

A heat sink module has been developed for high-heat flux applications in the divertor region of a near term DEMO divertor. The concept employs water jet impingement, in a cascading flow path, together with CuCrZr as the structural material. This fluid-flow architecture maximises the heat transfer coefficient obtained for a given coolant flow rate. Combined with high operational pressures, it facilitates increased coolant outlet temperatures. A conjugate CFD study was performed to compare its performance to the ITER divertor under a representative heat flux boundary condition of 10 MW/m2. A transient thermochromic liquid crystal technique was used to experimentally validate the CFD heat transfer data, using air as the working fluid at matched Reynolds numbers. These results showed that area averaged heat transfer coefficients could be accurately predicted by the CFD to within ∼12%.

Textile preforms for tungsten fibre-reinforced composites

Demanding high heat flux applications, as for example plasma-facing components of future nuclear fusion devices, ask for the development of advanced materials. For such components, copper alloys are currently regarded as heat sink materials while monolithic tungsten is foreseen as directly plasma-facing material. However, the combination of these materials in one component is problematic since they exhibit different thermomechanical characteristics and their optimum operating temperatures do not overlap. In this context, an improvement can be achieved by applying composite materials that make use of drawn tungsten fibres as reinforcement. For the manufacturing processes of these composites, suitable tungsten fibre preform production methods are needed. In the following, we will show that tungsten fibres can be processed to suitable preforms by means of well-established textile techniques as studies regarding the production of planar weavings (wire distances of 90–271 µm), circular braidings (multilayered braidings with braiding angle of 60° and 12°) as well as multifilamentary yarns (15 tungsten filaments with 16 µm diameter) are presented. With such different textile preforms tungsten fibre-reinforced tungsten (W f/W) with a density of over 99% and pore-free tungsten fibre-reinforced copper W f/Cu composites were produced which proves their applicability with respect to a composite material production processes.

Dropwise Condensation on Superhydrophobic Microporous Wick Structures

Previous research in dropwise condensation (DWC) on rough microtextured superhydrophobic surfaces has demonstrated evidence of high heat transfer enhancement compared to smooth hydrophobic surfaces. In this study, we experimentally investigate the use of microporous sintered copper powder on copper substrates coated with a thiol-based self-assembled monolayer to attain enhanced DWC for steam in a custom condensation chamber. Although microtextured superhydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. Sintered copper powder diameters from 4 μm to 119 μm were used to investigate particle size effects on heat transfer. As powder diameter decreased, competing physical factors led to improved thermal performance. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a superhydrophobic 4 μm diameter microporous copper powder surface compared to a smooth hydrophobic copper surface. For the smallest powders observed, this improvement is primarily attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. Interestingly, the contact angle hysteresis of sessile water droplets measured in air is in contradiction with the departing droplet size observations made during condensation of saturated steam. It is evident that the specific design of textured superhydrophobic surfaces has profound implications for enhanced condensation in high heat flux applications.

Modeling and optimization of condensation heat transfer at biphilic interface

Biphilic surfaces with heterogeneous wettability and hierarchical topography can significantly enhance the condensation heat transfer performance due to the mutual benefits of the hydrophilic patterns and global superhydrophobicity, which facilitate fast nucleation and frequent droplet departure. To investigate the underlying physics of the condensation process on a biphilic surface and further explore the optimal design to achieve preferable heat transfer performance, we develop a comprehensive model that can capture the recurrent transition of filmwise-to-dropwise condensation on biphilic topographies by considering the dynamics of droplet morphology for individual droplet growth and droplet departure, and the associated droplet size distribution. The numerical results indicate that the varying droplet contact angle and the stepwise increase of the droplet contact base area substantially influence the droplet growth and the coalescence-induced droplet departure during the condensation process, and therefore contribute directly to the overall heat transfer enhancement. The model is validated by comparing the results with the heat flux measured in a custom-designed environment chamber. The developed model not only reveals the physics of condensation on biphilic surfaces but also provides important guidelines for the design and optimization of biphilic topographies for high heat flux applications.

Numerical study of the geometrically graded micro-channel heat sink for high heat flux application

With the rapid development of the micro-scale and nano-scale electronic systems, the requirement of the operating temperature becomes more stringent to ensure good reliability and safety. Hence, thermal management technologies for dissipating the heat generated become an important agenda for researchers over the world. Micro-channel heat sink has been widely investigated by researchers and it is known that by increasing the total heat transfer area to volume ratio could enhance overall heat transfer performance. There are various types of micro-channel available such as straight channel, pin fin and wavy channel. Straight channel is the most common used of micro-channel in the market due to ease of manufacturing and low cost. There are several drawbacks associated with the straight channel design especially cooling performance is deteriorated along the flow direction. This will lead to the development of high temperature gradient from upstream to downstream and resulted uneven cooling performance which in turn shortens the device life expectancy. In this study, a novel geometrically graded micro-channel heat sink is introduced to improve the thermal performance of the conventional straight channel heat sink. Computational fluid dynamic simulations were conducted to investigate the performance of the novel fin micro-channel and straight channel heat sink. Under heat load of 2000 W, the geometrically graded micro-channel heat sink could reduce the average temperature to 69.6 °C and variation of temperature to 2.7 °C.

Vapor chambers with hydrophobic and biphilic evaporators in moderate to high heat flux applications

Two new types of vapor chambers were fabricated to investigate the influence of engineering the wettability of evaporators for possibility of simultaneously reducing the thermal resistances and increasing the critical heat flux (CHF). One vapor chamber was fabricated by pressing a hydrophilic screen mesh wick structure into a hydrophobic evaporator through support posts integral to the condenser portion of the vapor chamber. The second vapor chamber included a biphilic evaporator without an evaporator wick, as such, the superhydrophilic zone surrounding the central hydrophobic area directly above the heater provided the capillary forces. Despite their lower thermal performances compared with those of the baseline vapor chamber fabricated by sintering the hydrophilic copper particles onto a hydrophilic copper substrate, both novel vapor chambers exhibited promising performances, as such, the vapor chamber with a hydrophobic evaporator approached CHF at high heat fluxes around 105 W/cm2, and the one with a biphilic evaporator did not approach the incipience of CHF until 162 W/cm2. The mechanical contact between the screen mesh and the hydrophobic evaporator, and the patterning of the biphilic evaporator were identified as the key roles in thermal performances of the tested vapor chambers. Suggestions are given to improve the thermal performances of the proposed vapor chambers in this study. If successful, the fabrication cost of these vapor chambers will be reduced by skipping high-temperature sintering process on the evaporators, the method that is used to fabricate the commonly used sintered-powder vapor chambers.