Bubble Growth During Nucleate Boiling in Microchannels

2010 ◽  
Author(s):  
David M. Christopher ◽  
Xipeng Lin
Keyword(s):  
Heat Transfer ◽  
Bubble Growth ◽  
High Speed ◽  
Stokes Equations ◽  
Electronics Cooling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Vof Model

The flow and heat transfer in microchannels has been of great interest for some years now due to the significantly higher heat transfer coefficients useful for enhancing the heat transfer in very small but high heat flux applications such as electronics cooling. Nucleate boiling heat transfer in microchannels is also of great interest for generating even higher heat transfer rates; however, numerous studies have shown that the bubble formation immediately fills the entire microchannel with vapor significantly reducing the heat transfer since the bubble size is normally of the same size as the microchannel. The bubble growth process is very fast and difficult to study experimentally, even with high speed cameras. This study numerically analyzes the flow and bubble growth in a microchannel for various conditions by solving the Navier-Stokes equations with the VOF model with an analytical microlayer model to provide the large amount of vapor produced by the curved region of the microlayer. As each bubble forms, the large pressure drop around the bubble causes the bubble to quickly break away from the nucleation site and move quickly downstream. The bubbles are quite small with the size depending on the bulk flow velocity, subcooling and the heating rate. The numerical results compare quite well with preliminary experimental observations of bubble growth on a microheater embedded in the channel wall for FC-72 flowing in a microchannel.

Enhanced Flow Boiling Over Open Microchannels With Uniform and Tapered Gap Manifolds

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
Satish G. Kandlikar ◽  
Theodore Widger ◽  
Ankit Kalani ◽  
Valentina Mejia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Performance ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Flow boiling in microchannels has been extensively studied in the past decade. Instabilities, low critical heat flux (CHF) values, and low heat transfer coefficients have been identified as the major shortcomings preventing its implementation in practical high heat flux removal systems. A novel open microchannel design with uniform and tapered manifolds (OMM) is presented to provide stable and highly enhanced heat transfer performance. The effects of the gap height and flow rate on the heat transfer performance have been experimentally studied with water. The critical heat fluxes (CHFs) and heat transfer coefficients obtained with the OMM are significantly higher than the values reported by previous researchers for flow boiling with water in microchannels. A record heat flux of 506 W/cm2 with a wall superheat of 26.2 °C was obtained for a gap size of 0.127 mm. The CHF was not reached due to heater power limitation in the current design. A maximum effective heat transfer coefficient of 290,000 W/m2 °C was obtained at an intermediate heat flux of 319 W/cm2 with a gap of 0.254 mm at 225 mL/min. The flow boiling heat transfer was found to be insensitive to flow rates between 40–333 mL/min and gap sizes between 0.127–1.016 mm, indicating the dominance of nucleate boiling. The OMM geometry is promising to provide exceptional performance that is particularly attractive in meeting the challenges of high heat flux removal in electronics cooling applications.

scholarly journals Boiling Heat Transfer and Flow Regimes in Microchannels—A Comprehensive Understanding

2011 ◽  
Vol 133 (1) ◽  
Author(s):  
Tannaz Harirchian ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Electronics Cooling ◽  
Flow Regimes ◽  
Operating Conditions ◽  
Comprehensive Understanding ◽  
Transfer Coefficients ◽  
Cross Sectional

Flow boiling in microchannels has been investigated extensively over the past decade for electronics cooling applications; however, the implementation of microchannel heat sinks operating in the two-phase regime in practical applications has lagged due to the complexity of boiling phenomena at the microscale. This has led to difficulties in predicting the heat transfer rates that can be achieved as a function of the governing parameters. From extensive experimental work and analysis performed in recent years, a clear picture has emerged that promises to enable prediction of flow boiling heat transfer over a wide parameter space. Experiments have been conducted to determine the effects of important geometric parameters such as channel width, depth, and cross-sectional area, operating conditions such as mass flux, heat flux, and vapor quality, as well as fluid properties, on flow regimes, heat transfer coefficients, and pressure drops in microchannels. A detailed mapping of flow regimes occurring under different conditions has been facilitated with high-speed flow visualizations. In addition, quantitative criteria for the transition between macro- and microscale boiling behaviors have been identified. In this paper, these recent advances toward a comprehensive understanding of flow boiling in microchannels are summarized.

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.

scholarly journals Boiling Heat Transfer Performance of Parallel Porous Microchannels

Energies ◽  
2020 ◽  
Vol 13 (11) ◽  
pp. 2970
Author(s):  
Donghui Zhang ◽  
Haiyang Xu ◽  
Yi Chen ◽  
Leiqing Wang ◽  
Jian Qu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Boiling Process

Flow boiling in microporous layers has attracted a great deal of attention in the enhanced heat transfer field due to its high heat dissipation potential. In this study, flow boiling experiments were performed on both porous microchannels and a copper-based microchannel, using water as the coolant. As the heat flux was less than 80 W/cm2, the porous microchannels presented significantly higher boiling heat transfer coefficients than the copper-based microchannel. This was closely associated with the promotion of the nucleation site density of the porous coating. With the further increase in heat flux, the heat transfer coefficients of the porous microchannels were close to those of the copper-based sample. The boiling process in the porous microchannel was found to be dominated by the nucleate boiling mechanism from low to moderate heat flux (<80 W/cm2).This switched to the convection boiling mode at high heat flux. The porous samples were able to mitigate flow instability greatly. A visual observation revealed that porous microchannels could suppress the flow fluctuation due to the establishment of a stable nucleate boiling process. Porous microchannels showed no advantage over the copper-based sample in the critical heat flux. The optimal thickness-to-particle-size ratio (δ/d) for the porous microchannel was confirmed to be between 2–5. In this range, the maximum enhanced effect on boiling heat transfer could be achieved.

Nucleate Boiling Heat Transfer on Plain and Microporous Surfaces in Subcooled Water

2017 ◽  
Vol 139 (8) ◽  
Author(s):  
Seongchul Juna ◽  
Jinsub Kima ◽  
Hwan Yeol Kimb ◽  
Seung M. Youa
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Average Particle Size ◽  
Nucleate Boiling ◽  
Average Particle ◽  
Transfer Coefficients ◽  
Sem Image ◽  
Subcooled Water ◽  
Plain Surface ◽  
Bubble Sizes

The growth of hovering bubbles on Copper, High-Temperature Thermally-Conductive Microporous Coating (Cu-HTCMC) and plain surface were compared at 1,000 kW/m2 in nucleate boiling with different subcoolings. Images obtained by a high speed camera operating at 2,000 frames per second were used. The Cu-HTCMC was created by sintering copper powders with the average particle size of 67 μm and ∼300 μm thickness, which showed the optimized nucleate boiling and critical heat flux enhancement. The hovering bubble size became smaller as subcooling increased for both Cu-HTCMC and plain surface due to condensation by surrounding subcooled water. At 30 K subcooling, big hovering bubbles disappeared on both surfaces. Small bubbles were shown on plain surface and mists were shown on Cu-HTCMC surface. The hovering bubble sizes were close and the growth times were comparable for both surfaces in saturated and 10 K subcooling cases. However, the bubbles on Cu-HTCMC surface were smaller than those of plain surface at 20 K and 30 K subcoolings. This is believed to be due to the microporous structures shown in the SEM image (top left figure). The heat transfer coefficients of Cu-HTCMC were ∼300 kW/m2K for various subcoolings, about 6 times higher than those of plain surface (top right figure). The figure indicates slightly increasing trend of the heat transfer coefficient with subcooling. This is believed to be the result of the disappearance of relatively big size bubbles in Cu-HTCMC case.

Forced Convective Boiling in Microchannels for kW/cm2 Electronics Cooling

2003 ◽  
Author(s):  
Daniel J. Faulkner ◽  
Reza Shekarriz
Keyword(s):  
Heat Transfer ◽  
Cooling System ◽  
Electronics Cooling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
Design Solution ◽  
High Heat Flux ◽  
Convective Boiling ◽  
High Heat Transfer

This paper reports some of the results of our tests for the development of a high heat flux cooling system for thermal management of high power electronics. Our objective is to develop a practical design solution for achieving 1000 W/cm2 cooling. To achieve such high heat transfer rates, we have pursued and combined design advantages of a microchannel heat exchanger, high heat fluxes associated with forced convective nucleate boiling, and the use of a nanoparticles laden fluid for enhancement of heat transfer. A laboratory test module was designed, built, and tested to verify its performance. The experimental system employed sub-cooled as well as saturated forced convection boiling heat transfer in a high aspect ratio parallel microchannel heat sink. The working fluids tested were water and a selection of ceramic-based nanoparticle suspensions (nanofluids). The system was observed to readily dissipate heat fluxes in excess of 275 W/cm2 of substrate, while maintaining the substrate at or below 125°C. For optimized fin geometry, the current conditions would result in greater than 500 W/cm2. While the use of nanofluids was intended for boiling enhancement to push the envelop beyond 1000 W/cm2, we discerned limited improvement in the overall heat transfer rate. Future studies are planned for further exploitation of nanoparticles for enhancement of convective nucleate boiling.

Flow Visualization in Boiling Water Flows Leading Up to Departure From Nucleate Boiling in an Electrically-Heated Duct

2009 ◽  
Author(s):  
Helene A. Krenitsky ◽  
Evan T. Hurlburt ◽  
Larry B. Fore ◽  
Paul T. McKeown ◽  
Richard B. Williams
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Visualization ◽  
Test Section ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat Flux ◽  
Wall Region ◽  
Data Set

A fundamental departure from nucleate boiling (DNB) flow visualization experiment was designed to obtain a better understanding of flow boiling by visually capturing the mechanisms leading up to and including DNB for subcooled vertical flow boiling. At the critical heat flux (CHF) the heat transfer coefficient between the wall and fluid is greatly reduced, entering an inefficient heat transfer region that can cause a rapid increase in wall temperature. Most of the visual data on DNB in the open literature comes from experiments conducted with refrigerants or with water at relatively low pressure. One goal of this test was to capture high-quality photographs leading up to DNB for water at higher pressures, higher mass fluxes, and larger inlet subcooling than most of the data in the open literature. The fundamental DNB experiment consisted of three different run stages: incipient boiling, subcooled boiling, and CHF runs, which were intended to capture the behavior leading up to and including a departure from nucleate boiling crisis. At high heat flux conditions, the steep temperature and refractive-index gradients in the water near the wall act like lenses and bend the light away from the wall, which is the region of interest for discerning the DNB mechanism. By frosting the inner surface of the window on the light source side, the nearly collimated light was diffused as it entered the test section and enabled better visualization near the wall region. A high speed camera was used in testing. A typical run consisted of a 2 second image data set, with a resolution of 512 by 160 pixels, at 10,000 frames per second. Three excursive CHF runs were achieved, the last of which melted the test section. The trigger function on the camera captured images from before and after the power trip for the last CHF run. A trend can be seen of an increasing two-phase friction factor with power that begins to increase more rapidly at test section powers greater than 64.5kW. The 1995 Groenevel, et al. (1996) look-up table proved to be a good estimate of the heat flux at DNB.

A Near-Wall Interfacial Area Concentration Model to Predict Departure From Nucleate Boiling Critical Heat Flux Based on High Speed Video From Boiling Water Flows

2009 ◽  
Author(s):  
Evan T. Hurlburt ◽  
Helene A. Krenitsky ◽  
Richard C. Bauer
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
High Speed ◽  
Interfacial Area ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Interfacial Area Concentration ◽  
High Speed Video ◽  
Near Wall

In nucleate boiling as the heat flux from the wall to the fluid is increased the heat transfer coefficient initially increases. At a sufficiently high heat flux called the critical heat flux (CHF) the heat transfer mechanism suddenly becomes less effective resulting in a rapid jump in wall temperature. In bubbly subcooled (or near-subcooled) conditions the CHF mechanism is referred to as departure from nucleate boiling. Departure from nucleate boiling (DNB) refers to the transition from nucleate boiling where liquid contacts the wall to film boiling in which a vapor layer contacts the wall. Various hypotheses have been used in modeling and predicting CHF. High speed video images of boiling water flows taken at Bettis Laboratory at the critical heat flux visually captured sufficient evidence of the DNB mechanism that improved insight into DNB modeling may be possible. This paper summarizes high speed video image analysis and the development of a new DNB critical heat flux model based on the image analysis findings. Using short window averages of image data, a significant increase in transmitted light intensity is seen near the wall just prior to CHF. The increase suggests that at CHF there is a transient reduction in the interfacial area concentration, ai, or bubble number density near the wall. This is believed to be the result of a sudden increase in bubble coalescence rates near the wall. The increase in coalescence rates results in a reduction in the interfacial area concentration causing it to reach a maximum at CHF. This near-wall maximum in ai at CHF under flow boiling conditions is consistent with recent pool boiling data in the literature. The image based observations motivated development of an interfacial area based CHF model to predict the maximum in the interfacial area concentration at CHF. The model predicts that a critical nucleation site density or a near-wall critical void fraction can be used as a DNB CHF criterion. This is a valuable simplification that can be directly implemented in three-dimensional thermal hydraulic codes. The critical nucleation site density result was used as an input to a simple wall heat transfer partition model to predict the critical heat flux. The model relies on correlation based estimates for the superheat temperature, bubble departure diameter, and bubble departure frequency. Model predictions are compared to CHF values taken from Groeneveld’s 2006 CHF look-up table.

Experimental Investigation of Nanofluid Flow Boiling in a Single Microchannel

2012 ◽  
Author(s):  
Zachary Edel ◽  
Abhijit Mukherjee
Keyword(s):  
Heat Transfer ◽  
Bubble Growth ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Forced Flow ◽  
Nanofluid Flow ◽  
Two Phase ◽  
Nanoparticle Suspension ◽  
Nanoparticle Deposition

The trends of decrease in size and increase in power dissipation for micro-electronic systems present a significant challenge for thermal management of modern electronics. The preferable cooling solution could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated for 0.001 vol% aluminum oxide nanofluids in a brass microchannel and compared to results for regular water. For the case of nanofluid flow boiling, high speed images were taken after boiling durations of 25, 75, 125, and 150 min. Bubble growth rates were measured and compared for each case. Flow regime oscillation was observed and regime duration was split into two periods: single-phase liquid and two-phase. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). The addition of nanoparticles was shown to stabilize bubble growth as well as the transition of flow regimes between liquid, two-phase, and vapor.

An Experimental Study on Flow Boiling Characteristics of pHEMA Nano-Coated Surfaces in a Microchannel

2016 ◽  
Author(s):  
Abdolali Khalili Sadaghiani ◽  
Yağmur Şişman ◽  
Gözde Özaydın İnce ◽  
Ali Koşar
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Transfer Coefficients ◽  
Rectangular Microchannel ◽  
Camera System ◽  
Coated Surfaces

In this study, the effect of pHEMA (Polyhydroxyethylmethacrylate) nanostructure coated surfaces on flow boiling was investigated in a rectangular microchannel. Experiments were conducted using deionized water as the working fluid to investigate flow boiling in a microchannel with dimensions of 14 cm length, 1.5 cm width, and 500 μm depth. The effect of pHEMA coatings (coated on 1.5 × 1.5 cm2 silicon plates) on heat transfer coefficients and flow patterns was assessed and supported using a high speed camera system. Although the contact angle decreases on nano-coated surfaces, due to surface porosity, boiling heat transfer coefficient increases. Furthermore, visualization results indicated that uncoated surfaces experienced a smaller nucleate boiling region. It was also observed that dryout occurs at higher heat fluxes for coated surfaces.

Sign in / Sign up

Export Citation Format

Share Document