scholarly journals Effect of Functional Surfaces with Gradient Mixed Wettability on Flow Boiling in a High Aspect Ratio Microchannel

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 239
Author(s):  
Vahid Ebrahimpour Ahmadi ◽  
Akam Aboubakri ◽  
Abdolali Khalili Sadaghiani ◽  
Khellil Sefiane ◽  
Ali Koşar
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Reference Sample ◽  
Hydrophobic Surface ◽  
Heat Fluxes ◽  
Mixed Wettability

Flow boiling is one of the most effective phase-change heat transfer mechanisms and is strongly dependent on surface properties. The surface wettability is a crucial parameter, which has a considerable effect on the heat transfer performance, particularly in flow boiling. The contact angle determines the number of nucleation sites as well as bubble dynamics and flow patterns. This study introduces three new generation mixed wettability surfaces and compares them with a wholly hydrophobic surface reference sample, in flow boiling in a high aspect ratio microchannel. The mixed wettability substrates have five regions as fully Al2O3, (hydrophobic zone) region, three different patterned configurations with various A* values, and fully SiO2 (hydrophilic zone) region, where A* is defined as A Al2O3/A total (hydrophobicity ratio). Boiling heat transfer results were obtained for each surface at various wall heat fluxes and three different mass fluxes. According to the obtained results, significant enhancements in heat transfer (by up to 56.7%) could be obtained with biphilic surfaces compared to the reference sample (hydrophobic surface). Performed flow visualization proves that the tested biphilic surfaces enhance heat transfer by reducing the bubbly flow regime and extending the slug regime.

scholarly journals Effect of Surface Biphilicity on FC-72 Flow Boiling in a Rectangular Minichannel

2021 ◽  
Vol 7 ◽  
Author(s):  
Akam Aboubakri ◽  
Vahid Ebrahimpour Ahmadi ◽  
Suleyman Celik ◽  
Abdolali K. Sadaghiani ◽  
Khellil Sefiane ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Flow Regime ◽  
Boiling Heat Transfer ◽  
Heat Transfer Performance ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Hydrophobic Surface ◽  
Heat Fluxes ◽  
Transfer Performance ◽  
Transfer Coefficients

Flow boiling is one of the most effective mechanisms in heat transfer thanks to the latent heat of vaporization. Surface modifications such as mixed-wettability have a considerable effect on the boiling heat transfer performance in terms of enhancement in boiling heat transfer as well as critical heat flux. This study introduces a new method of fabrication of biphilic surfaces, where C4F8 (Octafluorocyclobutane) islands are surrounded by silicon. Two different biphilic surfaces were fabricated and compared with the entirely uniform hydrophobic surface taken as a reference,. Each of the biphilic surfaces has three different sections, namely inlet, middle and outlet regions. The first region is mainly hydrophobic (inlet), while the third region is mainly hydrophilic (outlet). The heat transfer coefficients were obtained at different heat fluxes. Compared to the entirely uniform hydrophobic surface, the results show that biphilic surfaces enhance the boiling heat transfer performance by up to 50%. The visualization results revealed that the biphilic surfaces lead to more nucleation sites in the bubbly flow regime and break up the elongated bubbles in the slug flow regime.

Confined Bubble and Heat Transfer during Flow Boiling in a High Aspect Ratio Mini-Channel

2011 ◽  
Vol 312-315 ◽  
pp. 548-553 ◽  
Author(s):  
Yuan Wang ◽  
Khellil Sefiane
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Bubble Growth ◽  
Flow Boiling ◽  
Critical Time ◽  
Heat Fluxes ◽  
Equivalent Radius ◽  
Mass Fluxes ◽  
Transfer Mechanisms ◽  
Working Liquid

Single vapour bubble growth and heat transfer mechanism during flow boiling in a rectangular horizontal mini-channel were experimentally investigated. The hydraulic diameter of the channel was 1454 μm, with an aspect ratio (Win/din) of 10. Degassed FC-72 was used as the working liquid. In this paper, bubble equivalent radius was found to increase linearly till a critical time, beyond which the growth turned into exponential. Bubble growth rate increases with increasing heat flux. Heat transfer mechanisms of the bubble growth at different heat fluxes and mass fluxes were discussed. In addition, the relation between thermal and flow conditions with bubble temporal geometry was explored.

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.

Experimental Comparison of Flow Boiling Heat Transfer in High Aspect Ratio Microchannels With Plain and Grooved Walls

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Xiao Cheng ◽  
Huiying Wu
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Liquid Film ◽  
High Aspect Ratio ◽  
Heat Sink ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Experimental Comparison ◽  
Dry Out

Abstract The dry-out easily occurs on high-aspect ratio microchannel sidewalls due to the decreasing of liquid film thickness. In this paper, the triangular microgrooves possessing the characteristic of evaporating meniscus were designed on the microchannel sidewalls. The heat sink consisted of 33 parallel microchannels, having a hydraulic diameter of 100 μm and an aspect ratio of 4. A platinum film heater and platinum resistance temperature detectors (RTDs) were integrated on the backside of the heat sink to realize uniform heating and precise temperature measurement, respectively. Flow boiling visualization experiments were carried out by high-speed camera in triangular groove-wall and plain-wall microchannels at mass fluxes of 148–490 kg/m2·s and inlet temperatures of 42 °C and 60 °C. The boiling curve, heat transfer coefficient (HTC), pressure drop, and two-phase flow boiling instability were systematically investigated to assess the flow boiling performances. Thin liquid film was observed in the triangular grooves during the dry-out process, compared to the dry-out in plain-wall microchannels. The oscillations of wall temperature, inlet temperature, and pressure drop were significantly suppressed in triangular groove-wall microchannels. Moreover, the earlier onset of nucleate boiling, improved heat flux, and HTC were realized in triangular groove-wall microchannels compared to plain-wall microchannels. Therefore, triangular groove design on the sidewalls is a promising solution to enhance boiling heat transfer and suppress flow boiling instabilities for high-aspect ratio microchannels.

Heat Transfer during Flow Boiling of Water in Short Microchannel with High Aspect Ratio

2021 ◽  
Vol 30 (2) ◽  
pp. 200-206
Author(s):  
A. S. Shamirzaev ◽  
A. S. Mordovskoi ◽  
V. V. Kuznetsov
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Flow Boiling

Effects of Heat Flux, Mass Flux, Inlet Fluid Subcooling, and Channel Orientation on Subcooled Flow Boiling in a High-Aspect-Ratio Rectangular Microchannel

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Wei Li ◽  
Zengchao Chen ◽  
Junye Li ◽  
Kan Zhou ◽  
Zhaozan Feng
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Aspect Ratio ◽  
Mass Flux ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Rectangular Microchannel ◽  
Subcooled Flow Boiling

Abstract An experimental investigation of subcooled flow boiling in a high-aspect-ratio, one-sided heating, silicon-based rectangular microchannel was conducted utilizing de-ionized water as the working fluid. The microchannel was 5.01 mm wide and 0.52 mm high, having a hydraulic diameter of 0.94 mm and an aspect ratio (AR) of 10. The heat flux, mass flux, and inlet fluid subcooling were in the ranges of 0–30 W/cm2, 200–500 kg/m2 s, and 5–20 °C, respectively, while the orientations were vertical and horizontal. Parametric study on heat transfer characteristics including the onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) was carried out combined with flow visualization. Significant appearance of ONB without boiling hysteresis was observed in the boiling curve, accompanied with bubble nucleation. Nucleate boiling occurred first near the exit, where the HTC increased more sharply, while easier bubble nucleation was found near the sides. Unique time-dependent flow pattern consisting of isolated bubbly flow, elongated bubbly flow, partial dry-out, and rewetting process was observed. More nucleation sites were activated at higher heat flux, while higher initial heat flux and wall superheat for ONB as well as higher CHF value were obtained at higher mass flux and inlet subcooling. Compared to the vertical channel, higher wall temperature and pressure drop with larger oscillation amplitudes were found for the horizontal counterpart, where the merged bubbles agglomerated in the heating section, resulting in earlier dry-out which deteriorated heat transfer.

Pressure Drop and Two Phase Flow During Boiling of FC-72 in a High Aspect Ratio Micro-Channel

2010 ◽  
Author(s):  
Yuan Wang ◽  
Khellil Sefiane ◽  
Rachid Bennacer
Keyword(s):  
Pressure Drop ◽  
Aspect Ratio ◽  
High Aspect Ratio ◽  
High Speed ◽  
Flow Boiling ◽  
Bubble Nucleation ◽  
Heat Fluxes ◽  
Micro Channel ◽  
Growth Interface ◽  
Mass Fluxes

Liquid-vapour interface behaviours and the pressure drop fluctuation during flow boiling in a single rectangular micro-channel with high aspect ratio were experimentally investigated in the present study. A micro-channel with the hydraulic diameter of 727 μm was chosen, the aspect ratio of which was 10. FC-72 was used as the working liquid. Wide ranges of mass fluxes and heat fluxes were tested to study bubble dynamics and hydraulic instabilities during flow boiling. With the aid of high speed camera, various liquid-vapour interface behaviours during flow boiling were captured, such as bubble nucleation and growth, interface deformation, recoiling and rewetting. Based on the interface movement, the evaporation rate of the liquid was discussed. Evaporation rates of bubble nose and bubble tail at different mass fluxes and heat fluxes were compared. Besides, the effects of thermal and flow conditions on bubble geometry were discussed. Moreover, the pressure fluctuations resulting from boiling instabilities was remarkable. Fluctuation frequency was largely affected by the heat and mass fluxes.

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