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.

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

2018 ◽  
Author(s):  
Kan Zhou ◽  
Junye Li ◽  
Zhao-zan Feng ◽  
Wei Li ◽  
Hua Zhu ◽  
...  
Keyword(s):  
Heat Flux ◽  
Flow Pattern ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Lower Mass ◽  
Subcooled Flow Boiling ◽  
Mass Fluxes ◽  
Subcooled Flow

For improving the functionality and signal speed of electronic devices, electronic components have been miniaturized and an increasing number of elements have been packaged in the device. As a result there has been a steady rise in the amount of heat necessitated to be dissipated from the electronic device. Recently microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging, In the present study an experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel with gap depth of 0.52 mm and width of 5 mm was conducted with deionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m2s and imposed heat flux from 3 to 20 W/cm2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, flow pattern and onset of nucleate boiling of subcooled flow boiling were investigated through instrumental measurements and a high speed camera. It was found that the slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, and the slope was greater for lower mass fluxes, with lower superheat required for boiling incipience. As for the visualization images, for relatively lower mass fluxes the bubbles generated were larger and not easy to depart from the vertical upward placed narrow microchannel wall, giving elongated bubbly flow and reverse backflow. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewetting occurred. Meanwhile the initiative superheat and heat flux of onset of nucleate boiling were compared with existing correlations in the literature with good agreement.

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.

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.

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.

Subcooled Flow Boiling Inception and Heat Transfer of Water in a Circular Tube Under Pulsatile Flow

2018 ◽  
Author(s):  
Hongsheng Yuan ◽  
Sichao Tan ◽  
Kun Cheng ◽  
Xiaoli Wu ◽  
Chao Guo ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Steady Flow ◽  
Flow Rate ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Subcooled Flow Boiling ◽  
Average Heat ◽  
Subcooled Flow

The flow rate can fluctuate in offshore nuclear power systems which are exposed to wind and waves, as well as in loops where flow instabilities occur, resulting in different thermal-hydraulic characteristics compared with that under steady flow. Among the thermal-hydraulic characteristics, onset of nucleate boiling (ONB) model determines whether the fluid is boiling, and boiling heat transfer is crucial to equipment performance and safety, both being key issues in subcooled flow boiling. Therefore, an experimental study was conducted to investigate how an imposed periodic flow oscillation affects the boiling inception and heat transfer of subcooled flow boiling of water in a vertical tube. The experiments were conducted under atmospheric pressure with the average flow rate ranging from 96kg/m2s to 287kg/m2s and heat flux ranging from 10kW/m2 to 197kW/m2. The relative pulsatile amplitude range is 0.1–0.3 and pulsatile period range is 10s-30s. Photographic images and thermal parameters such as temperatures and flow rate were recorded. The lack of nucleation site on the heated surface of the test section results in high wall superheat at ONB. The effects of pulsatile amplitude and period on superheat at boiling onset and average heat transfer were analyzed. The results show that the superheat at boiling inception is decreased when the average heat flux is lower than the heat flux at boiling inception of the corresponding steady flow, and the superheat at boiling onset is increased when the average heat flux is higher than the heat flux at boiling onset of the corresponding steady flow. The above effect of flow rate pulsation on superheat increases with increasing amplitude and decreasing period, and the mechanism can be explained by boiling nucleation theory. The lack of large active nucleation site also affects the boiling heat transfer. By comparing the contribution of nucleate boiling to heat transfer with the widely used Cooper’s pool boiling correlation, the subcooled flow boiling was found suppressed by convection. The average heat transfer of both the intermittent flow boiling and the single phase flow is influenced by flow oscillation.

Flow Pattern Analysis and Heat Transfer Characteristics During Subcooled Flow Boiling in a Rectangular Microchannel on ZnO Microrod Surface

2021 ◽  
Vol 143 (12) ◽  
Author(s):  
Yuhao Lin ◽  
Junye Li ◽  
Jia Sun ◽  
Wei Li ◽  
Yanlong Cao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Pattern ◽  
Titanium Surface ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Heat Transfer Characteristics ◽  
Rectangular Microchannel ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow

Abstract The combination of microstructured surface and microchannel flow boiling is expected to solve the thermal management problems of high-heat-flux devices. In this study, the experimental investigation of subcooled flow boiling in a high aspect ratio, one-sided heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods were synthesized on the titanium surface to be used as the heated surface compared with the bare titanium surface. A facile image tool is utilized to process the flow patterns photographed by a high-speed camera, which is analyzed with the heat transfer characteristics. The flow pattern of isolated bubbly flow reveals the large number of nucleation sites formed on the microrod surface but the heat transfer performance deteriorates with increasing mass flux because of the smaller bubble area and weaker nucleation. With increasing heat flux, the flow pattern changes from isolated bubbly flow to alternating bubbly/slug flow and alternating slug/annular flow. The latter flow pattern is confirmed to bring a higher heat transfer coefficient due to the larger area of thin-film evaporation. Compared with the bare surface, a higher heat transfer coefficient is achieved on the ZnO microrod surface for up to 37% due to the more nucleate sites and strengthened convective evaporation. Therefore, this surface might be suitable for heat dissipation in the watercraft or aerospace industry considering the low density, strong intensity, and corrosion resistance of titanium.

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.

scholarly journals An experimental study on the subcooled flow boiling pressure drop of R141b in the system of two microchannels

2021 ◽  
Vol 2119 (1) ◽  
pp. 012053
Author(s):  
A. S. Shamirzaev
Keyword(s):  
Experimental Data ◽  
Experimental Study ◽  
Heat Flux ◽  
Pressure Drop ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Copper Block ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Boiling Incipience

Abstract An experimental study of the pressure drop under subcooled flow boiling of the refrigerant R141b in a system with two slotted microchannels was carried out. A copper block with two microchannels 2 mm wide, 0.4 mm deep, and 16 mm long was used as an experimental section for testing. The mass flow rate varied in the range from 1 to 4 g/s, the initial subcooling from 20°C to 50°C. Experimental data show a significant decrease in the pressure drop when the critical heat flux is reached. The experimental data are compared with the model known from the literature. Experimental data show that the occurrence of nucleate boiling incipience at subcooled boiling corresponds to a larger heat flux than that given by the recommended correlation.

Onset of Nucleate Boiling and Active Nucleation Site Density During Subcooled Flow Boiling

2002 ◽  
Vol 124 (4) ◽  
pp. 717-728 ◽  
Author(s):  
Nilanjana Basu ◽  
Gopinath R. Warrier ◽  
Vijay K. Dhir
Keyword(s):  
Heat Flux ◽  
Contact Angle ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Subcooled Flow Boiling ◽  
Site Density ◽  
Wall Superheat ◽  
Subcooled Flow ◽  
Nucleation Site Density

The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within ±30 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C.

scholarly journals Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber

2021 ◽  
Vol 11 (13) ◽  
pp. 5883
Author(s):  
Edgar Santiago Galicia ◽  
Yusuke Otomo ◽  
Toshihiko Saiwai ◽  
Kenji Takita ◽  
Kenji Orito ◽  
...  
Keyword(s):  
Heat Flux ◽  
Porous Body ◽  
Flow Boiling ◽  
Heating Surface ◽  
Working Fluid ◽  
High Porosity ◽  
Subcooled Flow Boiling ◽  
Wall Superheat ◽  
Subcooled Flow ◽  
Bare Surface

Passive methods to increase the heat flux on the subcooled flow boiling are extremely needed on modern cooling systems. Many methods, including treated surfaces and extended surfaces, have been investigated. Experimental research to enhance the subcooled flow boiling using high sintered fiber attached to the surface was conducted. One bare surface (0 mm) and four porous thickness (0.2, 0.5, 1.0, 2.0 mm) were compared under three different mass fluxes (200, 400, and 600 kg·m−2·s−1) and three different inlet subcooling temperature (70, 50, 30). Deionized water under atmospheric pressure was used as the working fluid. The results confirmed that the porous body can enhance the heat flux and reduce the wall superheat temperature. However, higher porous thickness presented a reduction in the heat flux in comparison with the bare surface. Bubble formation and pattern flow were recorded using a high-speed camera. The bubble size and formation are generally smaller at higher inlet subcooling temperatures. The enhancement in the heat flux and the reduction on the wall superheat is attributed to the increment on the nucleation sites, the increment on the heating surface area, water supply ability through the porous body, and the vapor trap ability.

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