Pool Boiling Using Thin Enhanced Structures Under Top-Confined Conditions

2006 ◽  
Vol 128 (12) ◽  
pp. 1302-1311 ◽  
Author(s):  
Camil-Daniel Ghiu ◽  
Yogendra K. Joshi
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Pool Boiling ◽  
Single Layer ◽  
Temperature Limit ◽  
Heat Fluxes ◽  
The Other ◽  
Vapor Flow ◽  
Maximum Heat ◽  
Maximum Heat Flux

An experimental study of pool boiling using enhanced structures under top-confined conditions was conducted with a dielectric fluorocarbon liquid (PF 5060). The single layer enhanced structures studied were fabricated in copper and quartz, had an overall size of 10×10mm2, and were 1mm thick. The parameters investigated in this study were the heat flux (0.8-34W∕cm2) and the top space S(0-13mm). High-speed visualizations were performed to elucidate the liquid/vapor flow in the space above the structure. The enhancement observed for plain surfaces in the low heat fluxes regime is not present for the present enhanced structure. On the other hand, the maximum heat flux for a prescribed 85°C surface temperature limit increased with the increase of the top spacing, similar to the plain surfaces case. Two characteristic regimes of pool boiling have been identified and described: isolated flattened bubbles regime and coalesced bubbles regime.

Subcooled Boiling in the Ultrasonic Field (on Cause of MEB Generation)

2009 ◽  
Author(s):  
Fumio Inagaki ◽  
Koichi Suzuki ◽  
Chungpyo Hong
Keyword(s):  
Heat Flux ◽  
Aqueous Solutions ◽  
Ultrasonic Vibration ◽  
Pool Boiling ◽  
Atmospheric Condition ◽  
Heating Surface ◽  
Ultrasonic Field ◽  
Subcooled Boiling ◽  
Maximum Heat ◽  
Maximum Heat Flux

Subcooled quasi-pool boiling for water, ethanol aqueous solutions of 10wt% and 50wt% and ethanol in ultrasonic field is performed for the upward flat heating surface of copper block with 10mm in diameter under the atmospheric condition. Tested liquid subcooling is 15K, 20K and 25K for water and aqueous solutions of ethanol and 20K, 30K and 40K for 100wt% ethanol. At 20K of liquid subcooling for water and ethanol aqueous solutions, no microbubble emission boiling (MEB) has been observed in quasi-pool boiling. Even if MEB occurred, the heat flux does not increase and it turns easily to film boiling. In ultrasonic field, MEB occurs remarkably and the heat flux increases higher than the ordinary critical heat flux as observed in highly subcooled boiling. The experimental results show that the ultrasonic vibration introduces the instability of interface of liquid and vapor and accelerate MEB at 20K of liquid subcooling for water and aqueous solutions of ethanol. At 15K of liquid subcooling for water and aqueous solutions, no effect of ultrasonic vibration is observed. At 25K of liquid subcooling, the ultrasonic vibration extends MEB region to higher superheat of heating surface for aqueous solutions of ethanol. The maximum heat flux in MEB decreases with increasing of ethanol concentration and becomes CHF for 100wt% ethanol. No effect of ultrasonic vibration on boiling is observed for the 100wt% ethanol in the present experiments.

scholarly journals Boiling of FC-72 on Surfaces with Open Copper Microchannel

Energies ◽  
2021 ◽  
Vol 14 (21) ◽  
pp. 7283
Author(s):  
Robert Kaniowski ◽  
Robert Pastuszko
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Pool Boiling ◽  
Fold Increase ◽  
Heat Transfer Process ◽  
Cross Sectional ◽  
Maximum Heat ◽  
Maximum Heat Flux

The paper presents the results of experimental research on pool boiling heat transfer of dielectric liquid FC-72. Measurements were made at atmospheric pressure on open surfaces with microchannels. Heat transfer surfaces, in the form of parallel milled microchannels, were made of copper. The rectangular cross-sectional microchannels were 0.2 to 0.5 mm deep and 0.2 to 0.4 mm wide. The surfaces, compared to a smooth flat surface, provided a five-fold increase in the heat transfer coefficient and a two-fold increase in the critical heat flux. The article analyses the influence of the width and height of the microchannel on the heat transfer process. The maximum heat flux was 271.7 kW/m2, and the highest heat transfer coefficient obtained was 25 kW/m2K. Furthermore, the experimental results were compared with selected correlations for the nucleate pool boiling.

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.

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.

Enhanced Flow Boiling Using Radial Open Microchannels With Manifold and Offset Strip Fins

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Alyssa Recinella ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Flow Rate ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Valuable Insight ◽  
Reduced Pressure ◽  
Maximum Heat ◽  
Wall Superheat ◽  
Maximum Heat Flux

The increasing demand for designing effective cooling solutions in high power density electronic components has resulted in exploring advanced thermal management strategies. Over the past decade, phase-change cooling has received widespread recognition due to its ability to dissipate large heat fluxes while maintaining low temperature differences. In this paper, a radial flow boiling configuration through a central inlet was studied. This configuration is particularly suited for chip cooling application. Two heat transfer surfaces with (a) radial microchannels, and (b) offset strip fins were fabricated and their flow boiling performance with distilled water was obtained. Furthermore, the effect of the liquid flow rate on the boiling performance and enhancement mechanisms was also investigated in this study. At a flow rate of 240 mL/min, a maximum heat flux of 369 W/cm2 at a wall superheat of 49 °C and a pressure drop of 59 kPa was achieved with the radial microchannels, while the offset strip fins achieved a maximum heat flux of 618 W/cm2 at a wall superheat of 20 °C. Increasing the flow rate to 320 mL/min resulted in a heat flux of 897 W/cm2 demonstrating the potential of using a radial configuration for enhancing the boiling performance. The increase in flow cross-sectional area was shown to be responsible for the reduced pressure drop when compared to straight microchannel configurations. The high-speed imaging incorporated in each test provided valuable insight and understanding into the flow patterns and underlying mechanism in these geometries. With the ease of implementation, highly stable flow, and further optimization possibilities with different microchannel and taper configurations, the radial geometry is expected to provide significant performance enhancement well beyond a critical heat flux (CHF) of 1 kW/cm2.

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

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Akshat Negi ◽  
Aniket M. Rishi ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Frequency Region ◽  
Heat Fluxes ◽  
Bubble Collapse ◽  
Fluid Temperature ◽  
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 low surface to fluid temperature differences. The maximum heat flux dissipated during boiling is limited by the critical heat flux (CHF). Difficulties in visualizing the boiling process and monitoring surface temperature make it difficult to identify the impending CHF condition. As a result, larger factors of safety are employed, and the equipment is operated at considerably lower heat fluxes. This study focuses on identifying acoustic signatures of different nucleate boiling regimes. The bubble nucleation and coalescence along with bubble collapse lead to variation in acoustic emission patterns during boiling. In this work, acoustic signatures representative of the impending CHF condition are identified during pool boiling of water over plain and enhanced copper substrates. It was observed that sound was dominant in two frequency regions (400–500 Hz dominant throughout the 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. This combination of the amplitude trend has potential to predict the approaching CHF condition. This is the first study that correlates high-speed images with the acoustic trends at different stages of nucleate boiling and compares experimental frequency with Minnaert frequency for coalescing bubbles.

Performance of a Flexible Evaporator for Loop Heat Pipe Applications

2008 ◽  
Author(s):  
Mukta S. Limaye ◽  
James F. Klausner
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Fluxes ◽  
Loop Heat Pipe ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Maximum Heat Flux

A flat and flexible evaporator, which conforms to contoured surfaces, has been developed for loop heat pipe applications. A loop heat pipe (LHP) is a passive, two phase heat transfer device that uses a porous membrane in the evaporator to circulate fluid. A number of flexible membranes have been tested as evaporator wicks that have a length of 12.7 cm and heated area of 50.6 cm2. For cellulose, polyethylene, and blotting paper membranes, maximum heat fluxes of 0.43, 1.5 and 2.9 W/cm2 have been observed, respectively. The maximum heat transfer coefficients measured for these membranes are 551, 876, and 2100 W/m2-K, respectively. The best performance was observed by a membrane made of a fibrous cotton matrix, typically used as gauze. This material has a large pore size and high wettability with water. When tested in a rigid, brass evaporator, the maximum heat flux observed is 5.95 W/cm2, and the maximum heat transfer coefficient is 2865 W/m2-K. A flexible evaporator is fabricated using a heat sealable, flexible barrier pouch, and the cotton matrix membrane is sealed inside. The maximum measured heat flux for the flexible evaporator is 3.2 W/cm2 and maximum measured heat transfer coefficient is 1165 W/m2-K. The observed reduction in heat transfer as compared to the rigid evaporator is due to the poor contact between the evaporator and membrane. It is concluded that for the flexible evaporator membranes considered, the heat transfer mechanism is boiling and the maximum heat flux is limited by the wicking rate of the membrane. For a given membrane, the wicking rate increases with a reduction in the wicking length and decreases with an increasing rate of evaporation. To further improve the performance of the flexible evaporator, it is necessary to ensure efficient vapor removal from the evaporator as well as maintaining good contact between the membrane and the evaporator surface.

Pool Boiling Studies on Nano-Structured Surfaces

2007 ◽  
Author(s):  
S. R. Sriraman ◽  
D. Banerjee
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Test Fluid ◽  
Wafer Surface ◽  
Test Surface ◽  
Maximum Heat ◽  
Structured Surfaces ◽  
Nano Structures ◽  
Maximum Heat Flux ◽  
Rectangular Pattern

The effect of nano-structures on pool boiling heat transfer is explored in this study. The silicon nano-structures are fabricated on 4″ double-side polished silicon wafers using Step and Flash Imprint Lithography (SFIL) process. An array of “nano-fins” consisting of 200 nm diameter pillars of 100 nm height, lateral pitch of 1 μm and transverse pitch of 0.9 μm are created in a rectangular pattern on the wafer surface. The test surface is used for pool boiling experiments using PF-5060 as the test fluid (Manufacturer - 3M Co., boiling point - 56 °C). Experiments are performed under saturation and liquid sub-cooling conditions. The heat flux measurements on the nanostructures are compared with that of bare surfaces. The test rig is of constant heat flux type. Results are reported at the maximum heat flux point. The nano-structured test surface showed a 41% increase in heat flux compared to the bare test surface under the saturation test condition. The 10-degree sub-cooling and 20-degree sub-cooling test conditions showed a 19% and 27% enhancement at the maximum heat flux point respectively.

Pool Boiling Heat Transfer Enhancement Through Nanostructures on Silicon Microchannels

2012 ◽  
Vol 3 (3) ◽  
Author(s):  
Z. Yao ◽  
Y.-W. Lu ◽  
S. G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Silicon Nanowires ◽  
Boiling Heat Transfer ◽  
Bubble Dynamics ◽  
Pool Boiling ◽  
Hierarchical Structures ◽  
Heat Fluxes ◽  
Average Height ◽  
Maximum Heat ◽  
Maximum Heat Flux

Uniform silicon nanowires (SiNW) were successfully fabricated on the top, bottom, and sidewall surfaces of silicon microchannels by using a two-step electroless etching process. Different microchannel patterns with the channel width from 100 to 300 μm were first fabricated in a 10 mm × 10 mm silicon chip and then covered by SiNW with an average height of 10–20 μm. The effects of the microchannel geometry, micro/nano-hierarchical structures on pool boiling were studied and the bubble dynamics on different sample surfaces were compared. It was found that the combination of the micro/nanostructures promoted microbubble emission boiling under moderate heat fluxes, and yielded superior boiling heat transfer performance. At given wall superheats, the maximum heat flux of the microchannel with SiNW was improved by 120% over the microchannel-only surface, and more than 400% over a plain silicon surface. These results provide a new insight into the boiling mechanism for micro/nano-hierarchical structures and demonstrate their potential in improving pool boiling performance for microchannels.

The Mechanism of and Stability Criterion for Nucleate Pool Boiling of Sodium

1969 ◽  
Vol 91 (3) ◽  
pp. 315-328 ◽  
Author(s):  
I. Shai ◽  
W. M. Rohsenow
Keyword(s):  
Heat Flux ◽  
Liquid Metals ◽  
Bubble Formation ◽  
Pool Boiling ◽  
Heat Fluxes ◽  
Nucleate Pool Boiling ◽  
Thermal Layer ◽  
Minimum Heat ◽  
Bubble Growth And Departure ◽  
Recorded Temperature

Experimental data for sodium boiling on horizontal surfaces containing artificial cavities at heat fluxes of 20,000 to 300,000 Btu/ft2 hr and pressures between 40 to 106 mm Hg were obtained. Observations are made for stable boiling, unstable boiling and “bumping.” Some recorded temperature variations in the solid close to the nucleating cavity are presented. It is suggested that for liquid metals the time for bubble growth and departure is a very small fraction of the total bubble cycle, hence the delay time during which a thermal layer grows is the most significant part of the process. On this basis the transient conduction heat transfer is solved for a periodic process, and the period time is found to be a function of the degree of superheat, the heat flux and the liquid thermal properties. A simplified model for stability of nucleate pool boiling of liquid metals is postulated from which the minimum heat flux for stable boiling can be found as a function of liquid-solid properties, liquid pressure, the degree of superheat, and the cavity radius and depth. At relatively low heat fluxes, convection currents have significant effects on the period time of bubble formation. An empirical correlation is proposed, which takes into account the convection effects, to match the experimental results.

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