Mechanistic Study of Subatmospheric Pressure, Subcooled, Flow Boiling of Water on Structured-Porous Surfaces

2012 ◽  
Vol 134 (11) ◽  
Author(s):  
S. J. Penley ◽  
R. A. Wirtz
Keyword(s):  
Heat Flux ◽  
Reynolds Number ◽  
Surface Area ◽  
High Speed ◽  
Flow Boiling ◽  
Mechanistic Study ◽  
Surface Geometry ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Porous Surfaces

Subcooled flow boiling experiments with water at 0.2-atm pressure assess the utility of fine filament screen laminate enhanced surfaces as high-performance boiling surfaces. Experiments are conducted on vertically oriented, multilayer copper laminates in distilled water. The channel Reynolds number is varied from 2000 to 20,000, and subcooling ranges from 2 to 35 K. Boiling performance is documented for ten different porous surfaces having pore hydraulic diameters ranging from 39 μm to 105 μm, and surface area enhancement ratios ranging from 5 to 37. Heat flux of up to 446 W/cm2 is achieved at 35 K subcooling at a channel Reynolds number of 6000, which represents a 3.5-fold increase in critical heat flux (CHF) over that of the saturated pool boiling on the same surface. Results show that CHF is strongly correlated with subcooling, and the effect of subcooling is more pronounced as the channel Reynolds number is increased. It is found that CHF enhancement due to subcooling and channel Reynolds number is intrinsically linked to the surface area enhancement ratio, which has an optimum that depends on the degree of subcooling. High-speed video imagery (up to 8100 fps) and long-range microscopy are used to document bubble dynamics. Boiling mechanisms inherent to subcooling, enhanced surface geometry, and CHF are discussed.

Sub-Atmospheric Pressure, Subcooled, Flow Boiling of Water on Screen Laminate Enhanced Surfaces

2010 ◽  
Author(s):  
S. J. Penley ◽  
R. A. Wirtz
Keyword(s):  
Heat Flux ◽  
Reynolds Number ◽  
Surface Area ◽  
High Speed ◽  
Flow Boiling ◽  
Fold Increase ◽  
Surface Geometry ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Flow Intensity

Subcooled flow boiling experiments with water at 0.2atm pressure assess the utility of fine filament screen-laminate enhanced surfaces as high performance boiling surfaces. Experiments are conducted on vertically oriented, multi-layer copper laminates in distilled water. The channel Reynolds number is varied from 2,000 to 20,000; and, subcooling ranges from 2K to 35K. Boiling performance is documented for ten different surfaces having pore hydraulic diameters ranging from 39μm to 105μm, and surface area enhancement ratios ranging from 5 to 37. Heat flux of up to 453W/cm2 is achieved at 35K subcooling at a channel Reynolds number of 6,000, which represents a 3.5-fold increase in Critical Heat Flux (CHF) over that of saturated pool boiling on the same surface. Results show that CHF is strongly correlated with subcooling; and the effect of subcooling is more pronounced as flow intensity is increased. It is found that CHF enhancement due to subcooling and flow intensity is intrinsically linked to the surface area enhancement ratio, which has an optimum that depends on the degree of subcooling. High speed video imagery (1200fps) and long range microscopy are use to document bubble dynamics. Boiling mechanisms inherent to subcooling, enhanced surface geometry, and CHF are discussed.

scholarly journals Enhancement of Subcooled Flow Boiling Heat Transfer with High Porosity Sintered Fiber Metal

2021 ◽  
Vol 11 (3) ◽  
pp. 1237
Author(s):  
Yusuke Otomo ◽  
Edgar Santiago Galicia ◽  
Koji Enoki
Keyword(s):  
Heat Flux ◽  
Mass Flux ◽  
High Speed ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Working Fluid ◽  
High Porosity ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Porous Surfaces

We conducted experimental research using high-porosity sintered fiber attached on the surface, as a passive method to increase the heat flux for subcooled flow boiling. Two different porous thicknesses (1 and 0.5 mm) and one bare surface (0 mm) were compared under three different inlet subcooling temperatures (30, 50 and 70 K) and low mass flux (150–600 kg·m−2·s−1) using deionized water as the working fluid under atmospheric pressure. The test section was a rectangular channel, and the hydraulic diameter was 10 mm. The results showed that the heat flux on porous surfaces with a thickness of 1 and 0.5 mm increased by 60% and 40%, respectively, compared to bare surfaces at ΔTsat = 40 K at a subcooled temperature of 50 K and mass flux of 300 kg·m−2·s−1. An abrupt increase in the wall superheat was avoided, and critical heat flux (CHF) was not reached on the porous surfaces. The flow pattern and bubble were recorded with a high-speed camera, and the bubble dynamics are discussed.

Bubble Behavior and Mean Diameter in Subcooled Flow Boiling

1996 ◽  
Vol 118 (1) ◽  
pp. 110-116 ◽  
Author(s):  
O. Zeitoun ◽  
M. Shoukri
Keyword(s):  
Heat Flux ◽  
Mass Flux ◽  
High Speed ◽  
Flow Boiling ◽  
Bubble Diameter ◽  
Image Processing Technique ◽  
Bubble Behavior ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
The Mean

Bubble behavior and mean bubble diameter in subcooled upward flow boiling in a vertical annular channel were investigated under low pressure and mass flux conditions. A high-speed video system was used to visualize the subcooled flow boiling phenomenon. The high-speed photographic results indicated that, contrary to the common understanding, bubbles tend to detach from the heating surface upstream of the net vapor generation point. Digital image processing technique was used to measure the mean bubble diameter along the subcooled flow boiling region. Data on the axial area-averaged void fraction distributions were also obtained using a single-beam gamma densitometer. Effects of the liquid subcooling, applied heat flux, and mass flux on the mean bubble size were investigated. A correlation for the mean bubble diameter as a function of the local subcooling, heat flux, and mass flux was obtained.

Subcooled Flow Boiling in a Minichannel

2009 ◽  
Author(s):  
Akira Oshima ◽  
Koichi Suzuki ◽  
Chungpyo Hong ◽  
Masataka Mochizuki
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Flow Boiling ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
High Heat Flux ◽  
Subcooled Flow Boiling ◽  
Maximum Heat ◽  
High Speed Video ◽  
Subcooled Flow

It has been considered that the dry-out is easy to occur in boiling heat transfer for a small channel, a mini or microchannel because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric condition for a horizontal rectangular channel of which size is 1mm in height and 1mm in width with a flat heating surface of 10mm in length and 1mm in width placed on the bottom of the channel. The heating surface is a top of copper heating block and heated by ceramics heaters. In the high heat flux region of nucleate boiling, about 70 ∼ 80 percent of heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux (CHF) by a high speed video observation. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at higher liquid subcooling than 30K. The maximum heat flux obtained was 8MW/m2 (800W/cm2) at liquid subcooling of higher than 40K and the liquid velocity of 0.5m/s. However, the surface temperature was extremely higher than that of centimeter scale channel. The high speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.

Heat Transfer Characteristics and Flow Pattern Visualization for Flow Boiling in a Vertical Narrow Microchannel

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Kan Zhou ◽  
Hua Zhu ◽  
Wei Li ◽  
Junye Li ◽  
Kuang Sheng ◽  
...  
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Heat Dissipation ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Rectangular Microchannel ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow

Recently, microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging and integration. In this study, an experimental investigation of subcooled flow boiling in a high-aspect-ratio rectangular microchannel was conducted with de-ionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m2s and the imposed heat flux from 3 to 20 W/cm2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, onset of nucleate boiling (ONB), and flow patterns of subcooled flow boiling were investigated with the aid of instrumental measurements and a high-speed camera. The slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, with lower superheat required of boiling incipience for lower mass fluxes. Meanwhile, the initiative superheat and heat flux of onset of nucleate boiling were compared with the existing correlations in the literature with good agreement. As for the flow visualization images, slug flow and reverse backflow were observed, where transient local dryout as well as rewetting occurred. A facile image processing tool was developed to profile the transient development and progression of the liquid–vapor interface and partial dryout patches in microchannels, which proved that the physical quantities of bubble dynamics for the elongation period during subcooled boiling could be well detected and calculated.

Dependence of Vapor Void Fraction on Fundamental Bubble Parameters in Subcooled Flow Boiling

2006 ◽  
Author(s):  
Hayato Kubota ◽  
Tatsuhiro Ishida ◽  
Tomio Okawa ◽  
Isao Kataoka ◽  
Michitsugu Mori
Keyword(s):  
Heat Flux ◽  
Void Fraction ◽  
Bubble Size ◽  
High Speed ◽  
Flow Boiling ◽  
Bubble Diameter ◽  
Nucleate Boiling ◽  
Vapor Generation ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow

A visual study of water subcooled flow boiling was conducted to clarify the mechanism of triggering the net vapor generation (NVG). The test section was a transparent sapphire grass tube of 20 mm in inside diameter; a high-speed camera was used to capture the behavior of vapor bubbles. In the present experiments, the vapor void fraction in the heated tube was expressed as the function of the following bubble parameters: nucleation site density, frequency of bubble release, bubble lifetime, and bubble size. Among these four bubble parameters, the bubble size had a particularly strong influence on the vapor void fraction: the void fraction was approximately proportional to the forth power of mean bubble diameter. Consequently, mean bubble diameter should be large enough for the vapor void fraction to increase rapidly with the wall heat flux. In low flowrate experiments, bubbles generated at nucleation sites were relatively large at the onset of nucleate boiling (ONB). The heat flux at ONB hence appeared the reasonable approximation of that at NVG. Whereas, in high flowrate experiments, bubbles were small at ONB and much higher heat flux was necessary to obtain large bubbles. Thus, the heat flux required to trigger NVG was much higher than that at ONB. It was concluded in the present experimental conditions that accurate evaluation of mean bubble diameter was of significant importance in predicting the onset of net vapor generation.

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