Pool boiling heat transfer on copper foam covers with water as working fluid

Pool boiling heat transfer with porous media as the enhanced structure is attractive due to its simple geometry and easy operation. However, the available studies focus on low porous porosities. Metallic foams provide large porous porosities that have been less studied in the literature. In this paper a set of copper foam pieces were welded on the plain copper surface to form the copper foam covers for the pool boiling heat transfer enhancement. Water was used as the working fluid. Enhancement of pool boiling heat transfer compared with plain surface depends on the increased bubble nucleation sites, extended heat transfer area, and resistance for vapor release to the pool liquid. Effects of pores per inch (ppi) of foam covers, foam cover thickness, and pool liquid temperatures are examined. It is found that temperatures at the Onset of Nucleate Boiling (ONB) are significantly decreased on copper foam covers compared with on plain surfaces. Heat transfer coefficients of foam covers are two to three times of the plain surface. A large ppi value provides large bubble nucleation sites and heat transfer area to enhance heat transfer, but generates large vapor release resistance to deteriorate heat transfer. Therefore an optimal ppi value exists, which is 60 ppi in this paper. Generally small ppi value needs large foam cover thickness, and large ppi value needs small foam cover thickness, to maximally enhance heat transfer. Effect of pool liquid temperature on the heat transfer enhancement depends on the ppi value. For small ppi value such as 30 ppi, lower pool liquid temperature can dissipate higher heat flux at the same wall superheat. However, the heat transfer performance is insensitive to the pool liquid temperatures when large ppi values such as 90 ppi are used.     

Modulated wick heat pipe

In heat pipes, modulation of evaporator wick thickness provides extra cross-sectional area for enhanced axial capillary liquid flow and extra evaporation surface area, with only a moderate increase in wick superheat (conduction resistance). This modulated wick (periodic stacks and grooves over a thin, uniform wick) is analyzed and optimized with a prescribed, empirical wick superheat limit. A thermal-hydraulic heat pipe figure of merit is developed and scaled with the uniform wick figure of merit to evaluate and optimize its enhancement. The optimal modulated wick for the circular and flat heat pipes is found in closed-form expressions for the viscous-flow regime (low permeability), while similar results are obtained numerically for the viscous-inertial flow regime (high permeability which is also gravity sensitive). The predictions are compared with the experimental result of a prototype (low permeability, titanium/water pipe with the optimal design) heat pipe which gives a scaled figure of merit of 2.2. Good agreement is found between the predicted and measured performance. The maximum enhancement is limited by the pipe inner radius (tapering of the stacks), the wick effective thermal conductivity, and the prescribed wick superheat limit.     

Enhanced Pool Boiling Performance of Microchannel Patterned Surface with Extremely Low Wall Superheat through Capillary Feeding of Liquid

ABSTRACT

The pool boiling performance plays a key role in the development of high heat flux dissipating applications. The high critical heat flux and low wall superheat are two of the critical factors that affect the long-term life of devices. In this paper, enhanced pool boiling performance can be achieved by well-designed microchannels in copper surfaces using a precision diamond dicing method. The microchannel patterned surface with the channel length of 0.4 mm obtains a critical heat flux of 169.8 W/cm², which has a 193% enhancement compared to the plain surface. Besides, the extremely low wall superheat of 3 K has been achieved, and thus the heat transfer coefficient reaches 51.8 W/cm²·K, about 738% larger than that of the plain surface. Herein, the microcavity has increased the nucleation site, the surface can promote the bubbles escape, and then the channel can continuously supply the liquid. Hence, the extremely low wall superheat at high heat flux occurs due to the rapid bubble departure and enhanced capillary feeding of liquid replenishment to active nucleation sites on the surface. The above results provide an effective way for the realization of high-performance two-phase microchannel patterned heat sinks via optimizing the microstructure geometry.     

An experimental study on the heat dissipation of LED lighting module using metal/carbon foam

In this study, thermal performance of the cooling modules applicable for Par 38 light bulb is investigated. A total of six heat sink modules, including the basic reference design, metal foam, and carbon foam are tested and compared. Tests are performed and analyzed using the transient test method based on JESD51-1 standard. It is found that the thermal resistance from junction to die attach is quite small. By contrast, the thermal resistance of the heat sink dominates the total resistance, and it comprises 55% of the total resistance for the standard heat sink module. With some slight opening on the base plate, the thermal resistance can be improved by approximately 12%. The thermal resistance for the carbon foam having an embedded metal plate shows the least thermal resistance of 1.14 K/W, followed by the carbon foam, and then the metal foam. The lower thermal resistance of carbon foam in association with copper metal foam is due to its higher emissivity. In addition to better heat transfer performance as compared to the standard plate heat sink, the utilization of carbon foam and metal foam can also significantly reduce the weight of the heat sink. In this study, the weight of the heat sink can be reduced as much as 33%.     

Study on high performance evaporator with micro‐grooves (Measurement of capillary force in a single groove and modeling of heat and mass transfer)

A micro‐grooved evaporator is composed of µm‐wide grooves on a heat transfer plate in which the inter‐line regions at the liquid–vapor meniscus of coolant become identifiable. The high‐heat performance of the evaporator is realized by this inter‐line region (ILR) where the liquid thin film reduces the thermal resistance on the heat transfer surface. In this report, we propose a numerical simulation model of heat and mass transfer in a single groove to predict its capillary force and heat flux. The capillary force performance (capillary‐rise length in a groove) of a single groove was measured for samples of varying width, superheat, and inclination. The performance was found to be a maximum at a specific groove width of 200–400 µm, which is in good agreement with the predicted results calculated by the proposed model. For a better prediction of capillary‐rise length, the effective capillary force and the effective flow resistance were considered. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20257     

Boiling Heat Transfer Enhancement Using Micro-Machined Porous Channels for Electronics Cooling

Boiling heat transfer enhancement for a passive electronics cooling design is presented in this paper. A novel pool boiling enhancement technique is developed and characterized. A combination of surface modification by metallic coating and micro-machined porous channels attached to the modified surface is tested and reported. An experimental rig is set up using a standard BGA package with 12 mm × 12 mm thermal die as a test surface. The limiting heat flux for a horizontally oriented silicon chip with fluorocarbon liquid FC-72 is typically around 15 W/cm². Boiling heat transfer with the designed enhancement techniques is investigated, and the factors influencing the enhancement are analyzed. The metallic coated surface at 10°C wall superheat has a heat flux six times larger than an untreated chip surface. Micro-machined porous channels with different pore sizes and pitches are tested in combination with the metallic coated surface. The boiling heat flux is seven times larger at lower wall superheat compared to the plain chip surface. Maximum critical heat flux (CHF) of 38 W/cm² is obtained with 0.3 mm pore diameter and 1 mm pore pitch. A ratio of pore diameter and pore pitch is found to correlate well with the heat transfer enhancement obtained by experiments. Structures with smaller pore diameter to pitch ratio and larger pore opening are found to have higher heat transfer enhancement in the tested combination.     

One dimensional thermal analysis of silicon carbide ceramic foam used for solar air receiver

Using air as heat transfer fluid for electricity generation offers some significant advantages for the development of Concentrated Solar Power (CSP): high conversion efficiency, low environmental impact and being used in deserts or other areas scarce of water resources. Silicon carbide ceramic foams have the characteristics of light weight, high strength, large specific surface areas, high porosity and excellent thermal shock resistance performance which make them particularly fit for absorber material in CSP. In this paper, thermal performance of silicon carbide ceramic foam as solar air receiver is investigated analytically based on the one dimensional physical model. The analytical results show that the air flow resistance increases obviously with increasing air outlet temperature, the air flow resistance while the air outlet temperature is equal to 1000 °C is nearly 3 times the one while the air outlet temperature is equal to 20 °C with air velocity range is between one and six meters per second. The results of one dimensional analysis of flow and heat transfer process of ceramic foams suggest that there exists an input solar energy flux limit for the unpressurized system, which will lead to limit the power capacity and the outlet air temperature enhancement.     

A vapor chamber using extended condenser concept for ultra-high heat flux and large heater area

We proposed an extended vapor chamber (EVC), consisting of an evaporator part and an extended condenser part. A layer of compressed copper foam was sintered on the inner evaporator surface. The extended condenser includes a circular-straight groove network and a fin region. The groove network distributes generated vapor everywhere in the internal volume of EVC. A set of capillary holes are machined within fins. A sliced copper foam bar is inserted in each of capillary hole. The peaks of copper foam bar are tightly contacted with the evaporator copper foam piece. Water is used as the working fluid with a heater area of 0.785 cm². A minimum thermal resistance of 0.03 K/W is reached for the bottom heating. The heat flux is up to 450 W/cm² without reaching dryout. The transition point of thermal resistances versus heat fluxes is significantly delayed with the heat flux exceeding 300 W/cm², beyond which thermal resistances are only slightly increased. EVC not only improves temperature uniformity on the evaporator and fin base surfaces, but also evens the temperature distribution along the fin height direction to increase the fin efficiency. Inclination angles and charge ratios are combined to affect the thermal performance of EVC. An optimal charge ratio of 0.3 was recommended. EVC can be used for ultra-high heat flux and larger heater area conditions.     

Effects of wall slip and temperature jump on heat and mass transfer characteristics of an evaporating thin film

A new mathematical model is developed to predict heat and mass transport characteristics of the evaporating thin film. The model considers effects of velocity slip and temperature jump at the solid-liquid interface, disjoining pressure, and surface tension. Three-dimensional nonequilibrium molecular dynamics simulations for coupling between the momentum and heat transfer at the nanoscale solid-liquid interface are performed to obtain the slip length and interfacial thermal resistance length. It is found that both slip length and interfacial thermal resistance length decrease significantly with the decreasing interface wettability of the liquid to the wall. Velocity slip and temperature jump at the solid-liquid interface intend to reduce the superheat degree of the evaporating thin film, and thus result in a sharp decrease of the heat and mass transport characteristics of the evaporating thin film. It is also noted that velocity slip and temperature jump at the solid-liquid interface show a more pronounced effect as the superheat degree increases.     

Modeling and optimization of composite thermal insulation system with HGMs and VDMLI for liquid hydrogen on orbit storage

The passive thermal insulation system for liquid hydrogen (LH₂) on orbit storage mainly consists of foam and variable density multilayer insulation (VDMLI) which have been considered as the most efficient and reliable thermal insulation system. The foam provides main heat leak protection on launch stage and the VDMLI plays a major role on orbit stage. However, compared with the extremely low thermal conductivity of VDMLI (1 × 10⁻⁵ W/(m·K)) at high vacuum, the foam was almost useless. Recently, based on hollow glass microspheres (HGMs) we have proposed the HGMs-VDMLI system which performs better than foam-VDMLI system. In order to improve insulation performance and balance weigh and environmental adaptability of passive insulation system, the HGMs-VDMLI insulation system should be configured optimally. In this paper, the thickness of HGMs and the number and arrangement of spacers of VDMLI were configured optimally by the “layer by layer” model. The effective thicknesses of HGMs were 25 mm for 60 layers MLI and 20 mm for 45 layers VDMLI. Compared with 35 mm foam and 45 layers VDMLI system, the heat flux of 20 mm HGMs and 45 layers VDMLI system was reduced by 11.97% with the same weight, or the weight of which was reduced by 9.91% with the same heat flux. Moreover, the effects of warm boundary temperature (WBT) and vacuum pressure on thermal insulation performance of the system were also discussed.     

Thermal performance of aluminium‐foam CPU heat exchangers

This study investigates the performance of existing central processing unit (CPU) heat exchangers and compares it with aluminium‐foam heat exchangers in natural convection using an industrial set‐up. Kapton flexible heaters are used to replicate the heat produced by a computer's CPU. A number of thermocouples are connected between the heater and the heat sink being used to measure the component's temperature. The thermocouples are also connected to a data‐acquisition card to collect the data using LabVIEW program. The values obtained for traditional heat exchangers are compared to published data to validate experiments and set‐up. The validated set‐up was then utilized to test the aluminium‐foam heat exchangers and compare its performance to that of common heat sinks. It is found that thermal resistance is reduced more than 70% by employing aluminium‐foam CPU heat exchangers. The results demonstrate that this material provides an advantage on thermal dissipation under natural convection over most available technologies, as it considerably increases the surface‐area‐to‐volume ratio. Furthermore, the aluminium‐foam heat exchangers reduce the overall weight. Copyright © 2005 John wiley & Sons, Ltd.     

Evaporation heat transfer of liquid nitrogen on microstructured surface at high superheat level

Liquid nitrogen, as a coolant, is generally applied in cell vitrification cryopreservation. It takes heat from the carrier with cell samples through its violent evaporation on the carrier surface. As a result, the temperature of the carrier plunges dramatically. This article focuses on the unsteady evaporation heat transfer characteristics of liquid nitrogen on a microstructured surface etched into the frozen carrier surface at a high superheat level. The heat flux and evaporation heat transfer coefficient of liquid nitrogen were investigated using a lumped capacitance method. The experimental results showed that the cooling rate of the thin film evaporation on the microstructured surface is obviously higher than that of pool boiling, which is currently being used for cell cryopreservation. The heat flux and the evaporation heat transfer coefficient work together to present a parabolic trend with the superheat decreasing during this heat transfer process. Besides, the microstructure of the surface has an important effect on the evaporation heat transfer of liquid nitrogen. The larger the thin film evaporation zone is, the higher the heat transfer coefficient is. The current investigation results in a cell cryopreservation method through vitrification with relatively low concentrations of cryoprotectants.     

Theoretical analysis of startup of a pulsating heat pipe

A theoretical analysis is conducted to determine the primary factors affecting the startup characteristics of a pulsating heat pipe. It is found that the wall surface condition, evaporation in the heating section, superheat, bubble growth, and vapor bubbles trapped in cavities at the capillary inner wall affect the startup of oscillating motion in the pulsating heat pipe. The required superheat and heat flux level for the startup of oscillating motions in a pulsating heat pipe depend on the cavity size of the inner wall surface and the naturally-formed vapor bubbles and their shapes. When the capillary inner surface is coated or fabricated with cavities or roughness, the pulsating heat pipe can be readily started up. And it is found that the working fluid significantly affects the startup characteristics of a pulsating heat pipe. The results presented here can result in a better understanding of the startup operation of a pulsating heat pipe.     

Heat transfer characteristics of submerged jet impingement boiling of saturated FC-72

Global heat transfer characteristics of submerged jet impingement boiling of a highly wetting dielectric fluid (FC-72) on a heated copper surface are presented. The effect of variation of the jet exit Reynolds number (Re) on boiling incipience, fully developed nucleate boiling, and critical heat flux (CHF) are documented. The jet exit Re is varied by variations of the jet exit velocity and the jet nozzle diameter for a fixed surface diameter. High-speed visualization is used to supplement trends observed in the heat transfer data. Scenarios of low and high incipience wall superheat are identified, corresponding to partially or fully developed nucleate boiling condition upon initiation of boiling. For the high incipience wall superheat scenario, the time of spread of boiling activity over the heated surface during temperature overshoot is found to be inversely proportional to the wall superheat temperature at boiling incipience. The incipient boiling wall superheat temperature is found to be uncorrelated with jet Re and jet diameter. A cumulative probability distribution function is used to characterize the onset of boiling with wall superheat temperature. At a fixed Re, CHF increases with increasing jet velocity and with decreasing jet diameter, indicating that the jet kinetic energy is a key parameter in CHF enhancement. The CHF data are compared with available jet impingement CHF correlations from literature on free surface and confined jets. The free surface jet CHF correlation by Monde and Katto (1978) [1] is seen to best capture the experimental data trends for Re greater than 4000.     

Incipient-boiling superheat for sodium in turbulent,channel flow: Effects of heat flux and flow rate

An experimental study was carried out in which the effects of heat flux and velocity on the incipient-boiling superheat were determined for turbulent flow of sodium in an annular channel. The heating surface was polished type-316 stainless steel having a profilometer roughness of 14–18 μin. (rms). The rate of temperature rise of the heating surface was maintained constant in each incipient-boiling run, by gradually increasing the inlet sodium temperature to the test section, while the heat flux on the heater was held constant. In this way, the independent variables of heat flux and rate of temperature rise were separated.For a finite rate of temperature rise, it was found that the greater the heat flux, the greater was the incipient-boiling superheat, other things being equal. It was also found that the greater the rate of temperature rise, the greater was the effect of heat flux. The flux was varied over the range 25 000–300 000 $\text{Btu}\text{hft}{}^2$.In general agreement with published results of previous investigators, the incipient-boiling superheat was found to have a strong dependence on the flow rate, falling off exponentially as the flow rate was increased.The axial location of boiling inception was determined by means of a series of voltage taps spaced along the outer wall of the test section; and the results presented herein represent superheat values for, and nucleations at, the upper end of the heater, or at the highest heating-surface temperature in the testsection channel.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

Numerical investigation of the bubble growth in horizontal rectangular microchannels

To explore the mechanism of flow boiling in microchannels, the processes of a single-vapor bubble evaporating and two lateral bubbles merging in a 2D microchannel are investigated. The temperature recovery model based on volume of fluid method is adopted to perform the flow boiling phenomena. The effects of wall superheat, Reynolds number, contact angle, surface tension, and two-bubble merger on heat transfer are discussed. Wall superheat dominates the bubble growth and is roughly proportional to wall heat flux. The update of velocity and temperature fields’ distribution in the channel increases with increasing inflow Reynolds number, which improves the wall heat flux markedly. Besides, the area of thin liquid film between the wall and the bubble is enlarged by reducing the contact angle, thus, expanding the wall heat flux several times compared with the single-phase cross section. However, variation of surface tension (0.0589, 0.1?N/m) is found to be insignificant.     

Numerical thermal analysis of nanofluid flow through the cooling channels of a polymer electrolyte membrane fuel cell filled with metal foam

Improvement in the cooling system performance by making the temperature distribution uniform is an essential part in design of polymer electrolyte membrane fuel cells. In this paper, we proposed to use water-CuO nanofluid as the coolant fluid and to fill the flow field in the cooling plates of the fuel cell stack by metal foam. We numerically investigated the effect of using nanofluid at different porosities, pore sizes, and thicknesses of metal foam, on the thermal performance of polymer electrolyte membrane fuel cell. The accuracy of present computations is increased by applying a three-dimensional modeling based on finite-volume method, a variable thermal heat flux as the thermal boundary condition, and a two-phase approach to obtain the distribution of nanoparticles volume fraction. The obtained results indicated that at low Reynolds numbers, the role of nanoparticles in improvement of temperature uniformity is more dominant. Moreover, metal foam can reduce the maximum temperature for about 16.5 K and make the temperature distribution uniform in the cooling channel, whereas increase in the pressure drop is not considerable.     

High heat flux thermal management through liquid metal driven with electromagnetic induction pump

In this paper, a novel liquid metal-based minichannel heat dissipation method was developed for cooling electric devices with high heat flux. A high-performance electromagnetic induction pump driven by rotating permanent magnets is designed to achieve a pressure head of 160 kPa and a flow rate of 3.24 L/min, which could enable the liquid metal to remove the waste heat quickly. The liquid metal-based minichannel thermal management system was established and tested experimentally to investigate the pumping capacity and cooling performance. The results show that the liquid metal cooling system can dissipate heat flux up to 242 W/cm² with keeping the temperature rise of the heat source below 50°C. It could remarkably enhance the cooling performance by increasing the rotating speed of permanent magnets. Moreover, thermal contact resistance has a critical importance for the heat dissipation capacity. The liquid metal thermal grease is introduced to efficiently reduce the thermal contact resistance (a decrease of about 7.77 × 10⁻³ °C/W). This paper provides a powerful cooling strategy for thermal management of electric devices with large heat power and high heat flux.     

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.