Flow Boiling on Micropin Fins Entrenched Inside a Microchannel—Flow Patterns and Bubble Departure Diameter and Bubble Frequency

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Santosh Krishnamurthy ◽  
Yoav Peles
Keyword(s):  
High Speed ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Nucleate Boiling ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
High Speed Photography ◽  
Bubble Frequency ◽  
Pin Fins ◽  
Parallel Microchannels

Flow boiling of HFE 7000 in five parallel microchannels of 222 μm hydraulic diameter, each containing a single row of 24 in-line 100 μm pin fins, was investigated. High speed photography revealed the dominant flow patterns, namely, the bubbly flow, the multiple flow, and the wavy-annular flow. The interaction of the bubble with the pin fins during nucleate boiling from G=350 kg/m2 s to G=827 kg/m2 s and wall heat fluxes from 10 W/cm2 to 110 W/cm2 is detailed.

Flow Morphologies and Bubble Dynamics in Microchannels With Entrenched Micro-Pin Fins

2008 ◽  
Author(s):  
Santosh Krishnamurthy ◽  
Yoav Peles
Keyword(s):  
Annular Flow ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
Pin Fins ◽  
Micro Pin Fins

Flow boiling of HFE 7000 on micro pin fins entrenched inside microchannels was experimentally studied for G = 350 kg/m2s to G = 827 kg/m2s and wall heat fluxes from 10 W/cm2 to 110 W/cm2. Bubbly, multiple, and wavy-annular flow patterns were observed. The interaction of the bubble with the pin fins during nucleate boiling is detailed.

Heat Transfer and Pressure Drop for Flow Boiling of Water in Narrow Vertical Rectangular Channels

2003 ◽  
Author(s):  
Jianyun Shuai ◽  
Rudi Kulenovic ◽  
Manfred Groll
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
High Heat ◽  
Flow Patterns ◽  
Industrial Applications ◽  
Heat Fluxes ◽  
Experimental Conditions ◽  
Rectangular Channels

Flow boiling in small-sized channels attracted extensive investigations in the past two decades due to special requirements for transfer of high heat fluxes from narrow spaces in various industrial applications. Experiments on various aspects of flow boiling in narrow channels were carried out and theoretical attempts were undertaken. But these investigations showed large differences, e.g. up till now the knowledge on the development of flow patterns in small non-circular flow passages is very limited. This paper deals with investigations on flow boiling of water in two rectangular channels with dimensions (width×depth) 2.0×4.0 mm2 and 0.5×2.0 mm2 (corresponding hydraulic diameters are 2.67 mm and 0.8 mm). The pressure at the test section exit is atmospheric. For steady-state experimental conditions the effects of heat flux, mass flux and inlet subcooling on the boiling heat transfer coefficient and the pressure drop are investigated. Flow patterns and the transition of flow patterns along the channel axis are visualized and documented with a video-camera. Bubbly flow, slug flow and annular flow are distinguished in both channels. Preliminary flow pattern maps are generated.

Flow Boiling Patterns of Liquid Nitrogen in Micro-Tubes

2008 ◽  
Author(s):  
P. Zhang ◽  
X. Fu ◽  
R. Z. Wang
Keyword(s):  
Liquid Nitrogen ◽  
High Speed ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Superconducting Devices ◽  
Surface Tension Force ◽  
Flow Patterns ◽  
Factors Affecting ◽  
Major Factors ◽  
Churn Flow

Application of liquid nitrogen to cooling is widely used in such fields as cooling of the high temperature superconducting devices, cryosurgery and so on. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical flow boiling patterns of liquid nitrogen in micro-tubes in the experiments. The main flow patterns were bubbly flow, slug flow, churn flow and annular flow. And the confined bubbly flow and mist flow were also observed. These flow patterns were characterized on the flow regime maps. And the surface tension force and the size of the tube diameter were found to be the major factors affecting the flow pattern transitions.

Boiling in Enhanced Surface Microchannels

2005 ◽  
Author(s):  
Chih-Jung Kuo ◽  
Ali Kosar ◽  
Michael K. Jensen ◽  
Yoav Peles
Keyword(s):  
Large Scale ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Heat Fluxes ◽  
Bubble Frequency ◽  
Large Scale Systems ◽  
Site Density ◽  
Enhanced Surface ◽  
Nucleation Site Density

An experimental investigation is presented on flow boiling of de-ionized water in 227-μm hydraulic diameter microchannels with reentrant type cavities. Key features of nucleate boiling are discussed. Active nucleation site density, bubble frequency and departure diameter, and flow patterns over mass velocities ranging from 41 kg/m2-s to 302 kg/m2-s and heat fluxes ranging from 28 to 445 W/cm2 are studied. Similarities and differences with results obtained on large-scale systems and unenhanced microchannels are discussed.

An Experimental Investigation of Sub-Cooled Flow Boiling in Hypervapotron Cooling Configuration

2004 ◽  
Author(s):  
Peipei Chen ◽  
Barclay G. Jones ◽  
Ty A. Newell
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Experimental Studies ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Subcooled Boiling ◽  
Reduced Pressure

This work reports on experimental studies to visualize nucleate boiling on the enhanced heat transfer surface of the hypervapotron for with application in the International Thermonuclear Experiment Reactor [ITER]. This research uses the simulant fluid Freon (R134A) instead of prototypic water to model the system performance. This results in much lower thermophysical conditions to represent the prototypic phenomena. By using reduced pressure, temperatures, etc, based on the critical physical properties of both working fluids, Freon and water, the dramatic drop in the level of these quantities with Freon allows the use of modest test conditions. The experiment was conducted for both saturated and subcooled boiling with different heat fluxes (from 50 to 300 kW/m2). A comparison of the heat transfer performance of finned structures and flat surfaces were examined under particular fluid conditions. The uniqueness of this work is the visualization method that allows direct observation of the subcooled boiling process of the Hypervapotron surfaces. Working with a high speed (12,000 frames per second), high fidelity digital camera with variable magnifications (from 1×–25×), the sub-cooled boiling phenomena was observed in detail. A major conclusion of this work is the existence of two separate zones linked to different energy removal efficiency in hypervapotron. Under high heat flux condition, enhanced boiling heat transfer (about 20–30% higher than flat surface) was observed for hypervapotron effect, while saturated boiling happened in the cavity, and a large portion of the region was vapor filled. The process of vapor bubble rotation in the slot appeared to be helpful to enhance energy transfer, as evidenced by an improved wetting condition on the heating surfaces.

Flow Boiling of Coolant (HFE-7000) Inside Structured and Plain Wall Microchannels

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
C.-J. Kuo ◽  
Y. Peles
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Side Walls ◽  
Parallel Microchannels ◽  
The Heat Transfer Coefficient

Flow boiling was experimentally studied using coolant HFE-7000 for two types of parallel microchannels: a plain-wall microchannel and a microchannel with structured reentrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G=164 kg/m2 s to G=3025 kg/m2 s and mass qualities (energy definition) ranging from x=−0.25 to x=1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

scholarly journals High heat flux flow boiling of refrigerant R236fa in parallel microchannels

2019 ◽  
Vol 196 ◽  
pp. 00062
Author(s):  
Vladimir Kuznetsov ◽  
Alisher Shamirzaev ◽  
Alexander Mordovskoy
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Flux Flow ◽  
Microchannel Heat Exchanger ◽  
Parallel Microchannels

This paper presents the results of an experimental study of the heat transfer during flow boiling of refrigerant R236fa in a horizontal microchannel heat sink. The experiments were performed using closed loop that re-circulates coolant. Microchannel heat exchanger that contains two microchannels with 2x0.4 mm cross-section was used as the test section. The dependence of average heat flux on wall superheat and critical heat flux were measured in the range of mass fluxes from 600 to 1600 kg/m2s and in the range of heat fluxes from 5 to 120 W/cm2. For heat flux greater than 60 W/cm2, nucleate boiling suppression has significant effect on the flow boiling heat transfer, and this leads to decrease of the heat transfer coefficient with heat flux grows.

Flow Boiling of Deionized Water in Ultrahigh-Aspect-Ratio Microchannel Heat Sink

2021 ◽  
Author(s):  
Peilin Cui ◽  
Zhenyu Liu
Keyword(s):  
Heat Flux ◽  
Aspect Ratio ◽  
Heat Sink ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Nucleate Boiling ◽  
Flow Patterns ◽  
Deionized Water ◽  
Inlet Temperature ◽  
High Heat Flux

Abstract Microchannel heat sinks have attracted much attention due to its superiority of removing high heat flux with a very small size, which recently have been applied in the electronic cooling. The flow boiling of deionized water in the ultrahigh-aspect-ratio copper microchannels was experimentally investigated in this work. The heat sink consists of 20 rectangular straight microchannels (5000μm × 200μm) with the hydraulic diameter of 385 μm and the aspect ratio of 25. The experiment was conducted with the heat flux range of 39.7–1368 kW/m2 and the mass flow range of 21.3–41.6 kg/(m2·s) (the inlet temperature of 30°C). The flow patterns in the ultrahigh-aspect-ratio microchannels were captured using the visualization technique and four flow regimes (bubbly flow, slug flow, churn flow and annual flow) were clearly observed. The nucleate boiling sites increase significantly in the depth direction on the large-area side wall at bubbly flow. An obvious delay for flow patterns transformation in the upper regions compared with the lower region of microchannel was found. The bubble sliding was observed with a large sliding distance compared to the conventional low-aspect-ratio microchannels, which leads to an enhanced the heat transfer. It shows that the higher effective heat flux and the lower pressure drop were achieved in ultrahighaspect-ratio microchannel due to its special flow patterns in the channel and large surface area in the limited volume.

Cross Flow Boiling in Micro-Pin Fin Heat Sinks

2007 ◽  
Author(s):  
Santosh Krishnamurthy ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Cross Flow ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Convective Boiling ◽  
Surface Heat Fluxes ◽  
Pin Fins

Flow boiling of water across a bank of circular staggered micro pin fins, 250 μm long and 100 μm diameter with pitch-to-diameter ratio of 1.5, was experimentally studied for mass fluxes ranging from 346 kg/m2s to 794 kg/m2s and surface heat fluxes ranging from 20 W/cm2 to 350 W/cm2. The local two-phase heat transfer coefficients were measured using thermistors located along the flow path of the channel. The flow was visualized and classified as vapor slug and annular flow patterns. Based on the observed flow patterns, the dominant heat transfer mechanism during boiling process was assumed to be convective boiling.

Onset of Boiling, Heat Transfer and Flow Patterns of Flow Boiling On the Superhydrophobic Porous Copper Surface in a Microchannel

2021 ◽  
Author(s):  
Yuhao Lin ◽  
Junye Li ◽  
Sun Jia ◽  
Wei Li ◽  
Yanlong Cao
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Flow Patterns ◽  
Copper Surface ◽  
Bubble Nucleation ◽  
Heat Fluxes ◽  
Porous Surface ◽  
Mass Fluxes ◽  
Porous Copper

Abstract To explore the effect of micro-structured porous surface on enhancing the heat transfer of flow boiling in a microchannel, the micro-porous copper surface was fabricated with microscale pores in ranges of 1-5 µm, which presented super-hydrophobicity. Subcooled flow boiling experiments were carried out to study the hydraulic and thermal transport performance in a single narrow rectangular microchannel. High-speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux and mass flux on heat transfer performance and flow patterns for originally hydrophilic bare copper surface and superhydrophobic micro-porous structured surface. The onset of boiling (ONB) characteristics of both test surfaces were compared with predictive correlations with a good agreement and verified by the captured flow pattern images. Benefit from the superhydrophobic wettability provided by its micro-scale porous structures and a large number of potential nucleation sites, the required wall superheats, and imposed heat fluxes of onset of boiling both decreased for the modified surface. The flow patterns on the two surfaces varied, which resulted in the different trends of heat transfer coefficient with mass fluxes and heat fluxes. Because of the strengthened bubble departure process, the enhancement of the porous surface compared to the original bare surface gradually increased with mass fluxes. The average two-phase heat transfer coefficients of the superhydrophobic porous copper surface were enhanced for up to 74.84%, due to the earlier onset of boiling, higher frequency of bubble nucleation, and strengthened boiling intensity.

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