Characterization of Phase Change Heat and Mass Transfers in Monoporous Silicon Wick Structures

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Steve Q. Cai ◽  
Avijit Bhunia
Keyword(s):  
Phase Change ◽  
Microelectromechanical Systems ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Maximum Phase ◽  
Liquid Transport ◽  
Maximum Heat ◽  
Pillar Diameter ◽  
Viscous Loss ◽  
Wick Structure

Silicon is the primary material of integrated circuit (IC) manufacturing in microelectronic industry. It has high thermal conductivity and superior thermomechanical properties compatible to most semiconductors. These characteristics make it an ideal material for fabricating micro/mini heat pipes and their wick structures. In this article, silicon wick structures, composed of cylindrical pillars 320 μm in height and 30–100 μm in diameter, are developed for studies of phase change capability. Fabrication of the silicon wick structures utilizes the standard microelectromechanical systems (MEMS) approach, which allows the precise definition on the wick dimensions, as well as the heated wick area. On these bases, experimental characterizations of temperature variations versus input heat fluxes, associated with simultaneous visualization on the liquid transport and the dryout, are performed to investigate the wick dimensional effects on the maximum phase change capability. On the wick structure with the pillar diameter/pores of 100 μm and a heated wick area of 2 mm × 2 mm, the phase change reached a maximum heat flux of 1130 W/cm2. Despite of the liquid bottom-feed approach, interactions between liquid and vapor phases enables the heated wick structure absorb liquid from its surrounding wick area, including from its top side with a longer liquid transport path. In contrast, a wick structure with fine pillars (10 μm in diameter) inhibited the generation of nucleate boiling. Evaporation on the meniscus interface becomes the major phase change mechanism. A large heated wick area (4 mm × 4 mm) increases the viscous loss in transporting liquid to wet the entire wick, advancing the dryout at 135 W/cm2. Mass transfer analysis, as well as discussion of the experimental results, indicates that a dimensional ratio r/l (pillar diameter/characteristic length of the heated wick area) is a key parameter in determining the maximum phase change capability. A low r/l ratio enhances heat and mass transport capability, as well as heat transfer coefficient.

High Heat Flux Phase Change on Silicon Wick Structures

2012 ◽  
Author(s):  
Qingjun Cai ◽  
Avijit Bhunia ◽  
Yuan Zhao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Test Chamber ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Wick Structure ◽  
Silicon Pillars

Silicon is the major material in IC manufacture. It has high thermal conductivity and is compatible with precision micro-fabrication. It also has decent thermal expansion coefficient to most semiconductor materials. These characteristics make it an ideally underlying material for fabricating micro/mini heat pipes and their wick structures. In this paper, we focus our research investigations on high heat flux phase change capacity of the silicon wick structures. The experimental wick sample is composed of silicon pillars 320μm in height and 30 ∼ 100μm in diameter. In a stainless steel test chamber, synchronized visualizations and measurements are performed to crosscheck experimental phenomena and data. Using the mono-wick structure with large silicon pillar of 100μm in diameter, the phase change on the silicon wick structure reaches its maximum heat flux at 1,130W/cm2 over a 2mm×2mm heating area. The wick structure can fully utilize the wick pump capability to supply liquid from all 360° directions to the center heating area. In contrast, the large heating area and fine silicon pillars 10μm in diameter significantly reduces liquid transport capability and suppresses generation of nucleate boiling. As a result, phase change completely relies on evaporation, and the CHF of the wick structure is reduced to 180W/cm2. An analytical model based on high heat flux phase change of mono-porous wick structures indicates that heat transfer capability is subjected to the ratio between the wick particle radius and the heater dimensions, as well as vapor occupation ratio of the porous volume. In contrast, phase change heat transfer coefficients of the wick structures essentially reflect material properties of wick structure and mechanism of two-phase interactions within wick structures.

Heat Transfer Enhancement in Compact Phase Change Microchannel Heat Exchangers for High Flux Laser Diodes

2018 ◽  
Author(s):  
Jensen Hoke ◽  
Todd Bandhauer ◽  
Jack Kotovsky ◽  
Julie Hamilton ◽  
Paul Fontejon
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Flow Boiling ◽  
Laser Diodes ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Average Maximum

Liquid-vapor phase change heat transfer in microchannels offers a number of significant advantages for thermal management of high heat flux laser diodes, including reduced flow rates and near constant temperature heat rejection. Modern laser diode bars can produce waste heat loads >1 kW cm−2, and prior studies show that microchannel flow boiling heat transfer at these heat fluxes is possible in very compact heat exchanger geometries. This paper describes further performance improvements through area enhancement of microchannels using a pyramid etching scheme that increases heat transfer area by ∼40% over straight walled channels, which works to promote heat spreading and suppress dry-out phenomenon when exposed to high heat fluxes. The device is constructed from a reactive ion etched silicon wafer bonded to borosilicate to allow flow visualization. The silicon layer is etched to contain an inlet and outlet manifold and a plurality of 40μm wide, 200μm deep, 2mm long channels separated by 40μm wide fins. 15μm wide 150μm long restrictions are placed at the inlet of each channel to promote uniform flow rate in each channel as well as flow stability in each channel. In the area enhanced parts either a 3μm or 6μm sawtooth pattern was etched vertically into the walls, which were also scalloped along the flow path with the a 3μm periodicity. The experimental results showed that the 6μm area-enhanced device increased the average maximum heat flux at the heater to 1.26 kW cm2 using R134a, which compares favorably to a maximum of 0.95 kw cm2 dissipated by the plain walled test section. The 3μm area enhanced test sections, which dissipated a maximum of 1.02 kW cm2 showed only a modest increase in performance over the plain walled test sections. Both area enhancement schemes delayed the onset of critical heat flux to higher heat inputs.

On Development of a Semi-Mechanistic Wall Boiling Model

2013 ◽  
Author(s):  
Saurish Das ◽  
Hemant Punekar
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Thermal Stresses ◽  
Cooling System ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Wall Heat Transfer ◽  
Cooling Systems ◽  
Transfer Phenomenon

In modern cooling systems the requirement of higher performance demands highest possible heat transfer rates, which can be achieved by controlled nucleate boiling. Boiling based cooling systems are gaining attention in several engineering applications as a potential replacement of conventional single-phase cooling system. Although the controlled nucleate boiling enhances the heat transfer, uncontrolled boiling may lead to Dry Out situation, adversely affecting the cooling performance and may also cause mechanical damage due to high thermal stresses. Designing boiling based cooling systems requires a modeling approach based on detailed fundamental understanding of this complex two-phase heat and mass transfer phenomenon. Such models can help analyze different cooling systems, detect potential design flaws and carry out design optimization. In the present work a new semi-mechanistic wall boiling model is developed within commercial CFD solver ANSYS FLUENT. A phase change mechanism and wall heat transfer augmentation due to nucleate boiling are implemented in mixture multiphase flow framework. The phase change phenomenon is modeled using mechanistic evaporation-condensation model. Enhancement of wall heat transfer due to nucleate boiling is captured using 1D empirical correlation, modified for 3D CFD environment. A new method is proposed to calculate the local suppression of nucleate boiling based on the flow velocity, and hence this model can be applied to any complex shaped coolant passage. For different wall superheat, the wall heat fluxes predicted by the present model are validated against experimental data, in which 50-50 volume mixture of aqueous ethylene glycol (a typical anti-freeze coolant mixture) is used as working fluid. The validation study is performed in ducts of different sizes and shapes with different inlet velocities, inlet sub-cooling and operating pressures. The results are in good agreement with the experiments. This model is applied to a typical automobile Exhaust Gas Recirculation (EGR) system to study boiling heat transfer phenomenon and the results are presented.

Statistical Modelling of Bubble Nucleation and Heat Transfer Using Interface Tracking in TransAT CMFD Code

2010 ◽  
Author(s):  
Chidambaram Narayanan ◽  
Siju Thomas ◽  
Djamel Lakehal
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Statistical Modelling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Bubble Nucleation ◽  
Heat Fluxes ◽  
Conductive Heat Transfer ◽  
Transfer Model ◽  
Conductive Heat

This paper presents results of numerical simulations of various processes that demonstrate phase change heat transfer at high heat fluxes using the level-set method. The model used for the purpose has been first validated for the growth of an evaporating bubble in infinite medium, and fim boiling in 2D and 3D. It has then been applied to simulate the nucleation and departure of a single bubble from a solid body subject to conductive heat transfer. Unlike our previous investigations where phase change induced evaporation rate was incorporated like a sub-grid scale heat transfer model applied to the triple contact line, the present work reports simulations with direct phase change modelling by integrating energy fluxes at the interface. The effect of the conductive heat transfer in the solid from which the bubble departs is also taken into account. Comparison with visual images suggests that accounting for conjugate heat transfer is important to capturing micro-hydrodynamics in nucleate boiling, at least qualitatively.

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

2015 ◽  
Author(s):  
Maritza Ruiz ◽  
Claire M. Kunkle ◽  
Jorge Padilla ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Heat Sink ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Contact Angles ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Maximum Heat ◽  
Flow Boiling Heat Transfer

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling levels, and mass fluxes are considered in this type of heat sink for use in applications with high fluxes up to 300 W/cm2. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 μm and 1 cm radius with a single inlet and a single outlet. The channel is heated on one side through a copper conducting surface, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. Flow boiling heat transfer and pressure drop data were obtained for this heat sink device using water at near atmospheric pressure as the working fluid for inlet subcooling levels from 20 to 81°C and mean mass flux levels ranging from 184 to 716 kg/m2s. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface with water equilibrium contact angles in the range of 14–40°, typical of common metal surfaces. The other was a surface coated with zinc oxide nanostructures that are superhydrophilic with equilibrium contact angles measured below 10°. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter as indicated by reduced temperature oscillations during boiling and achieving higher maximum heat flux without dryout. Reducing inlet subcooling levels was also found to reduce the magnitude of oscillations in the oscillatory boiling regime. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm2 with an average surface temperature of 134 °C and requiring a pumping power to heat rate ratio of 0.01%. The hydrophilic surface retained wettability after a series of boiling tests. Recommendations for use of this heat sink design in high flux applications is also discussed.

Effect of Heat Flux on Bubble Coalescence Phenomena and Sound Signatures During Pool Boiling

2020 ◽  
Author(s):  
Akshat Negi ◽  
Aniket M. Rishi ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Frequency Region ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Bubble Collapse ◽  
Maximum Heat ◽  
Acoustic Signatures

Abstract Boiling heat transfer is extensively used in various industrial applications to efficiently dissipate a large amount of heat by maintaining lower surface temperatures. The maximum heat flux dissipated during boiling is limited by the critical heat flux (CHF) and limited visualization of the boiling surface limits the identification of the impending CHF condition to rely on temperature monitoring alone. The study presented here focuses on developing a method for analyzing and identifying acoustic signatures throughout the nucleate boiling regimes that are indicative of the boiling state of the heater surface. The bubble nucleation and coalescence along with bubble collapse at the liquid-vapor interface leads to variation in acoustic emission patterns during boiling. These sound waves are studied and acoustic signatures that are representative of the impending CHF are identified over plain and enhanced copper substrates with water as the working fluid. During pool boiling study, it was observed that sound was dominant in two frequency regions (400–500 Hz dominant throughout nucleate boiling and 100–200 Hz dominant at heat fluxes > 100 W/cm2). However, just before CHF, a sudden drop in amplitude was observed in the high frequency region (400–500 Hz), while the amplitude in low frequency region (100–200 Hz) continued to rise. It was concluded that this acoustic study can be used as a tool to predict the approaching CHF condition.

Thermal Performance of Carbon Nanotube Enhanced Vapor Chamber Wicks

2010 ◽  
Author(s):  
Sungwon S. Kim ◽  
Justin A. Weibel ◽  
Timothy S. Fisher ◽  
Suresh V. Garimella
Keyword(s):  
Carbon Nanotubes ◽  
Thermal Resistance ◽  
Thermal Performance ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Positive Bias ◽  
Maximum Heat ◽  
Capillary Action ◽  
Wick Structure ◽  
Copper Mesh

Vapor chambers are often used as spreaders to dissipate high heat fluxes by taking advantage of liquid-vapor phase change. Wicking of the working fluid in vapor chambers is accomplished through capillary action, which is strongly affected by the wick structure. Traditionally, copper meshes with micrometer-scale pore sizes have been used as wicking structures, but it is expected that heat fluxes in the next generation of high-power electronic devices will cause boiling in these devices and lead to dryout with conventional wick materials. With a goal of increasing maximum heat dissipation and reducing thermal resistance, a wick structure composed of both conventional copper mesh and carbon nanotubes has been developed and characterized. The high-permeability mesh provides for a low-resistance bulk flow path while the carbon nanotubes, with their high thermal conductivity and high surface area, modify the wick surface for enhanced capillary action. CNT-enhanced integrated wicks were fabricated by sintering a copper mesh on Cu-Mo-Cu substrates, on which CNTs were grown. A thin layer of copper was evaporated onto the CNTs to improve wicking and wettability with water, the working fluid of interest. Samples grown under varying degrees of positive bias voltage and varying thicknesses of post-CNT-growth copper evaporation were fabricated, so that the surface morphology of the samples could be varied. The resultant boiling curves and associated wick thermal resistances indicate that micro/nano integrated wicks fabricated with higher positive bias voltages during CNT synthesis, and thicker copper coatings, lead to improved thermal performance and lower wick thermal resistance. Notably, heat fluxes at the heater surface of greater than 500 W/cm2 were observed without reaching a critical heat flux condition.

Investigations of Biporous Wick Structure Dryout

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Qingjun Cai ◽  
Ya-Chi Chen
Keyword(s):  
Heat Flux ◽  
Heat Pipe ◽  
High Speed ◽  
Heat Fluxes ◽  
Large Temperature ◽  
Vapor Flow ◽  
High Heat Flux ◽  
Temperature Hysteresis ◽  
Liquid Transport ◽  
Wick Structure

Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures.

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