Dynamics of Contact Angle During Growth and Liftoff of Bubbles at a Single Nucleation Site in Flow Boiling

2004 ◽  
Author(s):  
Wangcun Jia ◽  
Vijay K. Dhir
Keyword(s):  
Contact Angle ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleation Site ◽  
Contact Angles ◽  
Working Fluid ◽  
Wafer Surface ◽  
Photographic Images ◽  
Flow Boiling Heat Transfer ◽  
Processing Program

Contact angle is a critical parameter needed in the mechanistic models for the prediction of flow boiling heat transfer. In this paper, variations of upstream and downstream contact angles for a single vapor bubble in flow boiling on horizontal and vertical surfaces were investigated experimentally. The nucleation site is a well-characterized cavity, which was etched on a highly polished silicon wafer surface using micro-fabrication techniques. Water at one atmosphere pressure was used as the working fluid. Photographic images of the bubble were recorded during its inception, growth and liftoff by using high-speed video system and analyzed by an image-processing program. The results provide clean data on the dependence of upstream and downstream contact angles on surface orientation and flow velocity.

The Effect of Surface Wettability on Viscous Film Deposition

2009 ◽  
Author(s):  
Alexandru Herescu ◽  
Jeffrey S. Allen
Keyword(s):  
Contact Angle ◽  
Film Thickness ◽  
Capillary Number ◽  
High Speed ◽  
Glass Tube ◽  
Contact Angles ◽  
Film Deposition ◽  
Surface Wettability ◽  
Working Fluid ◽  
Uniqueness Of The Solution

The viscous deposition of a liquid film on the inside of a capillary has been experimentally investigated with a focus on the relationship between the film thickness and surface wettability. With distilled water as a working fluid tests were run in a 622 microns diameter glass tube with contact angles of 30° and 105°, respectively. In the first set of experiments the tube was uncoated while in the second set a fluoropolymer coating was applied to increase the contact angle. A film thickness dependence with the contact angle θ (surface wettability) as well as the Capillary number in the form hR ∼ Ca2/3/cosθ is inferred from scaling arguments. For partial wetting it may explain the existence of a thicker film for nonzero contact angle. It was further found that the non-wetting case of 105° contact angle deviates significantly from the existing theories, the film thickness presenting a weak dependence with the Capillary number. This deviation as well as the apparent non-uniqueness of the solution is thought to be caused by the film instability (rupture) observed during the tests. The thickness of the deposited film as a function of the Capillary number was estimated from the liquid mass exiting the capillary and the gas-liquid interface (meniscus) velocity, and compared to Bretherton’s data and a correlation proposed by Quere. The film thickness measurements as well as the meniscus velocity were determined with the aid of a Photron high speed camera with 10000 frames per second sampling capability coupled with a Nikon TE-2000 inverted microscope and a Precisa electronic balance.

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

2015 ◽  
Author(s):  
Maritza Ruiz ◽  
Claire M. Kunkle ◽  
Jorge Padilla ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Heat Sink ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Contact Angles ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Maximum Heat ◽  
Flow Boiling Heat Transfer

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling levels, and mass fluxes are considered in this type of heat sink for use in applications with high fluxes up to 300 W/cm2. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 μm and 1 cm radius with a single inlet and a single outlet. The channel is heated on one side through a copper conducting surface, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. Flow boiling heat transfer and pressure drop data were obtained for this heat sink device using water at near atmospheric pressure as the working fluid for inlet subcooling levels from 20 to 81°C and mean mass flux levels ranging from 184 to 716 kg/m2s. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface with water equilibrium contact angles in the range of 14–40°, typical of common metal surfaces. The other was a surface coated with zinc oxide nanostructures that are superhydrophilic with equilibrium contact angles measured below 10°. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter as indicated by reduced temperature oscillations during boiling and achieving higher maximum heat flux without dryout. Reducing inlet subcooling levels was also found to reduce the magnitude of oscillations in the oscillatory boiling regime. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm2 with an average surface temperature of 134 °C and requiring a pumping power to heat rate ratio of 0.01%. The hydrophilic surface retained wettability after a series of boiling tests. Recommendations for use of this heat sink design in high flux applications is also discussed.

Nucleate Boiling Bubble Dynamics in a PDMS Microchannel With a Single Nucleation Site

2011 ◽  
Author(s):  
Haojie Wang ◽  
Xipeng Lin ◽  
David M. Christopher
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Working Fluid ◽  
Heater Surface ◽  
Short Period ◽  
Pdms Microchannel ◽  
Very High

The bubble dynamics for flow boiling in a single microchannel was experimentally studied in the present work. A platinum heater was used as the nucleation site in a 0.1 mm hydraulic diameter PDMS (Polydimethylsiloxane) microchannel with FC72 as the working fluid. A high speed camera was used to visualize the bubble dynamics. The results show that the bubbles grow much slower than predicted by standard correlations due to the very large convective heat transfer to the liquid flowing around the bubble in the microchannel. The results also show that the bubble departure frequency, heat flux and bubble departure diameter are well correlated by two dimensionless parameters that also include the effect of the properties. Finally, the results suggest very high speed dryout and rewetting of the heater surface during the bubble growth with a very short period of more complete rewetting of the heater surface when a bubble separates from the main vapor stem.

Two-Phase Flow in Microchannels

2001 ◽  
Author(s):  
G. Hetsroni ◽  
A. Mosyak ◽  
Z. Segal
Keyword(s):  
Heat Sink ◽  
High Speed ◽  
Single Phase ◽  
Flow Boiling ◽  
Electronics Cooling ◽  
Working Fluid ◽  
Microchannel Heat Sink ◽  
Two Phase ◽  
Infrared Radiometry ◽  
Phase Water

Abstract Experimental investigation of a heat sink for electronics cooling is performed. The objective is to keep the operating temperature at a relatively low level of about 323–333K, while reducing the undesired temperature variation in both the streamwise and transverse directions. The experimental study is based on systematic temperature, flow and pressure measurements, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250μm. According to the objectives of the present study, Vertrel XF is chosen as the working fluid. Experiments on flow boiling of Vertrel XF in the microchannel heat sink are performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the two-phase results to a single-phase water flow.

A Fundamental View of the Flow Boiling Heat Transfer Characteristics of Nano-Refrigerants

2012 ◽  
Author(s):  
Lorenzo Cremaschi
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Phase ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Working Fluid ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow Boiling Heat Transfer

Driven by higher energy efficiency targets and industrial needs of process intensification and miniaturization, nanofluids have been proposed in energy conversion, power generation, chemical, electronic cooling, biological, and environmental systems. In space conditioning and in cooling systems for high power density electronics, vapor compression cycles provide cooling. The working fluid is a refrigerant and oil mixture. A small amount of lubricating oil is needed to lubricate and to seal the sliding parts of the compressors. In heat exchangers the oil in excess penalizes the heat transfer and increases the flow losses: both effects are highly undesired but yet unavoidable. This paper studies the heat transfer characteristics of nanorefrigerants, a new class of nanofluids defined as refrigerant and lubricant mixtures in which nano-size particles are dispersed in the high-viscosity liquid phase. The heat transfer coefficient is strongly governed by the viscous film excess layer that resides at the wall surface. In the state-of-the-art knowledge, while nanoparticles in the refrigerant and lubricant mixtures were recently experimentally studied and yielded convective in-tube flow boiling heat transfer enhancements by as much as 101%, the interactions of nanoparticles with the mixture still pose several open questions. The model developed in this work suggested that the nanoparticles in this excess layer generate a micro-convective mass flux transverse to the flow direction that augments the thermal energy transport within the oil film in addition to the macroscopic heat conduction and fluid convection effects. The nanoparticles motion in the shearing-induced and non-uniform shear rate field is added to the motion of the nanoparticles due to their own Brownian diffusion. The augmentation of the liquid phase thermal conductivity was predicted by the developed model but alone it did not fully explain the intensification on the two-phase flow boiling heat transfer coefficient reported in previous work in the literature. Thus, additional nano- and micro-scale heat transfer intensification mechanisms were proposed.

Experimental Verification of Analytical Prediction of Pool Boiling CHF Incorporating the Effects of EHD and Contact Angle

2015 ◽  
Author(s):  
Ichiro Kano ◽  
Takahiro Sato ◽  
Naoki Okamoto
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Electric Field ◽  
Boiling Heat Transfer ◽  
Pool Boiling ◽  
Contact Angles ◽  
Dielectric Liquid ◽  
Working Fluid ◽  
Analytical Prediction ◽  
Compound Effect

Boiling heat transfer enhancement via compound effect of Electro-Hydro-Dynamic (EHD) and contact angle has been experimentally and analytically investigated. A fluorinated dielectric liquid (Asahi Glass Co. Ltd, AE-3000) was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. In order to change the contact angle between the boiling surface and the dielectric liquid, the different materials Cu, Cr, NiB, Sn, and mixture of 5 and 1.5 micro meter diamond particles were electrically deposited on a boiling surface. The critical heat flux (CHF) for different contact angles showed 20.5 ∼ 26.9 W/cm2 which was −7 ∼ 25 % of that for a non-coated Cu surface (21.5 W/cm2). Upon application of a −5 kV/mm electric field to the micro structured surface (the mixture of 5 and 1.5 micro meter particles), a CHF of 99 W/cm2 at a superheat of 33.5 K was obtained. The previous theoretical equation of pool boiling predicted the CHF with the electric field and without the electrode.

Experimental Investigation of the CHF Condition During Flow Boiling of Water in Microtubes

2007 ◽  
Author(s):  
Anand P. Roday ◽  
Michael K. Jensen
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Flow Boiling ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Two Phase ◽  
Flow Boiling Heat Transfer ◽  
C 50 ◽  
Different Diameters

The critical heat flux (CHF) condition sets an upper limit on the flow-boiling heat transfer process. With the growing demand for the use of two-phase flow in micro and nano-sized devices, there is a strong need to understand the CHF phenomenon in channels of such small dimensions. This study experimentally investigates the critical heat flux condition during flow boiling in a single stainless steel microtube of two different diameters—0.427mm, and 0.286 mm. Degassed water is the working fluid. The effects of various parameters—diameter, mass flux (350–1500 kg/m2s), inlet subcooling (2°C–50°C), and length-to-diameter ratio (75–200) on the CHF condition are studied for the exit condition being nearly atmospheric pressure. The CHF increases with an increase in mass flux. The effect of the inlet subcooling on the CHF condition is more complex. With a decreasing inlet subcooling, the CHF decreases until saturated liquid is reached; thereafter, the CHF increases with quality.

The Effect of Inlet Orifice on Critical Heat Flux and Flow Boiling Heat Transfer in Single Horizontal Microtube

2013 ◽  
Author(s):  
Yanfeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Critical Heat Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Saturation Pressure ◽  
Working Fluid ◽  
Flow Boiling Heat Transfer

Flow oscillation is a crucial issue for the development of flow boiling heat transfer in the applications. Inlet orifice has been proven be an option to eliminate the oscillation. However, the effects of inlet orifice on critical heat flux and flow boiling heat transfer coefficient are lack of study. In this work, the effects of inlet restriction on critical heat flux and heat transfer coefficient in single horizontal microtube under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two smaller microtubes are assembled at the inlet of main microtube to achieve the restriction configurations of 50% and 20% area ratios. The experimental measurement is carried out at mass fluxes ranging from 160–870 kg/m2·s and heat fluxes varying from 6–170 kW/m2. Two saturation pressures, 10 and 45 kPa, are tested. The experimental results of critical heat flux and two phase heat transfer coefficient obtained in the microtube without orifice are compared with the existing correlations. The addition of an orifice does not enhance the normal critical heat flux but increases the premature critical heat flux. In aspect of heat transfer, the orifice shows improvement on heat transfer coefficient at low mass flux and high saturation pressure.

On the Gas Entrapment and Nucleation Site Density During Pool Boiling of Saturated Water

1993 ◽  
Vol 115 (3) ◽  
pp. 670-679 ◽  
Author(s):  
C. H. Wang ◽  
V. K. Dhir
Keyword(s):  
Contact Angle ◽  
Nucleation Site ◽  
Contact Angles ◽  
Heating Process ◽  
Heater Surface ◽  
Site Density ◽  
Uniform Temperature Field ◽  
The Stability ◽  
Free Energy Analysis ◽  
Nucleation Site Density

A model to describe the effect of wettability on nucleation site density is presented. First, from Helmholtz free energy analysis, a criterion for the entrapment condition in a uniform temperature field is developed. Second, the stability condition of preexisting gas/vapor nuclei during the heating process and the minimum superheat required to initiate nucleation are determined. The prediction of the entrapment condition and the incipient temperature are consistent with the experimental observations made on surfaces having naturally existing cavities. Third, a naturally formed cavity on a heater surface is modeled as a spherical cavity. The cumulative active nucleation site density for a specified contact angle is expressed in terms of the cumulative density of cavities existing on the surface as Na = Pas · Nas where Nas is the heater surface cumulative cavity density with cavity mouth angles less than a specified value and Pas is a function of contact angle and cavity mouth angle. The model successfully predicts active site densities for different contact angles.

Flow Boiling Heat Transfer Characteristics of a Minichannel Up to Dryout Condition

2009 ◽  
Author(s):  
Rashid Ali ◽  
Bjo¨rn Palm ◽  
Mohammad H. Maqbool
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Working Fluid ◽  
Flow Boiling Heat Transfer ◽  
The Heat Transfer Coefficient

In this paper the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.7mm and a uniformly heated length of 220mm. R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 °C and 32 °C. Mass flux was varied from 50 kg/m2 s to 600 kg/m2 s and heat flux ranged from 2kW/m2 to 156kW/m2. The test section was heated directly using a DC power supply. The direct heating of the channel ensured uniform heating and heating was continued until dry out was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.

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