Measurements of Bubble Nucleation Characteristics in Pool Boiling of a Wetting Liquid on Smooth and Roughened Surfaces

2008 ◽  
Author(s):  
John P. McHale ◽  
Suresh V. Garimella
Keyword(s):  
High Speed ◽  
Electrical Discharge Machining ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Ccd Camera ◽  
Terminal Velocity ◽  
Nucleation Site ◽  
Bubble Nucleation ◽  
Working Fluid ◽  
Nucleation Behavior

Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (Ra) are 0.03 and 5.89 micrometers for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, bubble terminal velocity, and active nucleation site density are measured from the monochrome movies, which are recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported.

Bubble Dynamics on Nanostructured Cu Surfaces

2008 ◽  
Author(s):  
Chen Li ◽  
Nikhil Koratkar ◽  
G. P. Peterson
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Bubble Diameter ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Working Fluid ◽  
Visualization System ◽  
Boiling Process ◽  
Order Of Magnitude

Nucleate boiling performance was enhanced up to an order of magnitude through direct deposition of Cu nanorods on a planar Cu surface. The methodology that enables order of magnitude improvement in boiling performance without fabricating complicated surface structures or changing the working fluid will have broad impact on metal-liquid type two-phase heat exchangers. In this study, discussion was focused on bubble dynamics on the nanostrucured Cu surfaces. We observed striking differences in bubble dynamics through nucleation boiling process for the nanostructured surface including smaller bubble diameters, higher release frequencies and nucleation site density, and large fluctuations in bubble diameter prior to release. These differences during the boiling process are responsible for the enhanced heat transfer. High quality images were captured through a well-designed visualization system, which comprises of a high-speed charge-coupled device (CCD) camera, microscope and data acquisition system. This visualization study aims to quantitatively study the bubble dynamics on the nanostructured Cu surfaces.

scholarly journals Lattice Boltzmann Simulation of Nucleate Pool Boiling in Saturated Liquid

2011 ◽  
Vol 9 (5) ◽  
pp. 1347-1361 ◽  
Author(s):  
Yoshito Tanaka ◽  
Masato Yoshino ◽  
Tetsuo Hirata
Keyword(s):  
Lattice Boltzmann ◽  
Solid Wall ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Immiscible Fluids ◽  
Nucleate Pool Boiling ◽  
Two Phase ◽  
Boiling Process

AbstractA thermal lattice Boltzmann method (LBM) for two-phase fluid flows in nucleate pool boiling process is proposed. In the present method, a new function for heat transfer is introduced to the isothermal LBM for two-phase immiscible fluids with large density differences. The calculated temperature is substituted into the pressure tensor, which is used for the calculation of an order parameter representing two phases so that bubbles can be formed by nucleate boiling. By using this method, two-dimensional simulations of nucleate pool boiling by a heat source on a solid wall are carried out with the boundary condition for a constant heat flux. The flow characteristics and temperature distribution in the nucleate pool boiling process are obtained. It is seen that a bubble nucleation is formed at first and then the bubble grows and leaves the wall, finally going up with deformation by the buoyant effect. In addition, the effects of the gravity and the surface wettability on the bubble diameter at departure are numerically investigated. The calculated results are in qualitative agreement with other theoretical predictions with available experimental data.

Direct Measurement of Electrode Movement During Electrical Discharge Machining by Means of Automatic Discharge Gap Controller

2010 ◽  
Vol 4 (6) ◽  
pp. 552-561 ◽  
Author(s):  
Masahiko Kita ◽  
◽  
Tohru Ishida ◽  
Yoshimi Takeuchi
Keyword(s):  
Shape Memory Alloy ◽  
Shape Memory ◽  
Power Supply ◽  
Research Group ◽  
Electrical Discharge ◽  
High Speed ◽  
Electrical Discharge Machining ◽  
Working Fluid ◽  
Method Of Measurement ◽  
Discharge Gap

This study deals with the development of a new method of directly measuring the movement of an electrode during normal electrical discharge machining (EDM) and the movement of an electrode during EDM by means of an automatic discharge gap controller (ADGC) devised by our research group. The ADGC, which mainly consists of a bidirectional actuator using a shape memory alloy (SMA) and an electrode and power supply for EDM, can sustain stable EDMby autonomously and automatically controlling the position of the electrode to keep the discharge gap appropriate. However, the movement of the electrode being controlled by the ADGC cannot be directly measured due to itsminute, high-speed, vibration-like movements inside the working fluid during EDM. This means that there is no way to prove that the ADGC actually controls the position of the electrode so as to maintain a suitable discharge gap for continuing stable EDM. This also means that there is no way to evaluate the movement of the electrode quantitatively and to design or optimize the structure of an ADGC so as to give the ADGC the desired or best performance. Therefore, a method to directlymeasure the electrodemovement by an ADGC is devised in this study. The results obtained in the measurement experiments using this method of measurement prove that the ADGC actually moves its electrode to achieve stable EDM, and they allow the movement of the electrode to be evaluated quantitatively.

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.

scholarly journals Study on Surface Evenness of Super-high-thickness Cutting in High-speed Wire Electrical Discharge Machining

2021 ◽  
Author(s):  
Cong Deng ◽  
Zhidong Liu ◽  
Ming Zhang ◽  
Hongwei Pan ◽  
Mingbo Qiu
Keyword(s):  
Electrical Discharge ◽  
High Speed ◽  
Electrical Discharge Machining ◽  
Point Of View ◽  
Working Fluid ◽  
Wire Electrical Discharge Machining ◽  
Wire Electrode ◽  
Positional Accuracy ◽  
Vibration Absorption ◽  
The Wire

Abstract Surface machined by high-speed wire electrical discharge machining (HS-WEDM) at super-high thickness (more than 1000 mm) cutting suffers from uneven surface, a major problem that has been investigated in this paper. According to the analysis, as wire frame span increases, the rigidity of the wire electrode decreases, and under the action of discharge explosive force, wire electrode vibration intensifies. As a result, the machining stability inevitably decreases. However, the core problem is whether there is enough working fluid in the slit to dampen and absorb the vibration of the wire electrode so as to ensure the positional stability of the wire electrode. To verify the above point of view: first, the wire guide and gravity take-up with bidirectional tension in the wire feeding system were installed to improve the positional accuracy of the wire electrode; second, to improve the flow of the working fluid into the slit, the slit width was increased by improving the working fluid and a medium carrier with a higher melting point and vaporization point can reduce the vaporization of the working fluid in the slit as much as possible. The experiment showed that the outlet flow of the improved working fluid is 56.72% higher than that of the original working fluid when cutting a 750 mm thick workpiece, which increases the damping and vibration absorption effect of the working fluid on the wire electrode in the long and narrow gap. After the above measures were implemented, super-high thickness cutting can be carried out continuously and steadily, the surface evenness was significantly improved, and the workpiece with a thickness of 2000 mm was cut successfully.

Enhanced Macroconvection Mechanism With Separate Liquid–Vapor Pathways to Improve Pool Boiling Performance

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Nucleation Site ◽  
Enhancement Mechanism ◽  
Performance Improvements ◽  
Wall Superheat ◽  
Significant Performance ◽  
Multiscale Structures ◽  
Microlayer Evaporation

Understanding heat transfer mechanisms is crucial in developing new enhancement techniques in pool boiling. In this paper, the available literature on fundamental mechanisms and their role in some of the outstanding enhancement techniques is critically evaluated. Such an understanding is essential in our quest to extend the critical heat flux (CHF) while maintaining low wall superheats. A new heat transfer mechanism related to macroconvection is introduced and its ability to simultaneously enhance both CHF and heat transfer coefficient (HTC) is presented. In the earlier works, increasing nucleation site density by coating a porous layer, providing hierarchical multiscale structures with different surface energies, and nanoscale surface modifications were some of the widely used techniques which relied on enhancing transient conduction, microconvection, microlayer evaporation, or contact line evaporation mechanisms. The microconvection around a bubble is related to convection currents in its immediate vicinity, referred to as the influence region (within one to two times the departing bubble diameter). Bubble-induced convection, which is active beyond the influence region on a heater surface, is introduced in this paper as a new macroconvection mechanism. It results from the macroconvection currents created by the motion of bubbles as they grow and depart from the nucleating sites along a specific trajectory. Directing these bubble-induced macroconvection currents so as to create separate vapor–liquid pathways provides a highly effective enhancement mechanism, improving both CHF and HTC. The incoming liquid as well as the departing bubbles in some cases play a major role in enhancing the heat transfer. Significant performance improvements have been reported in the literature based on enhanced macroconvection contribution. One such microstructure has yielded a CHF of 420 W/cm2 with a wall superheat of only 1.7 °C in pool boiling with water at atmospheric pressure. Further enhancements that can be expected through geometrical refinements and integration of different techniques with macroconvection enhancement mechanism are discussed here.

Bubble Ebullition on a Hydrophilic Surface

2015 ◽  
Vol 137 (2) ◽  
Author(s):  
Aritra Sur ◽  
Yi Lu ◽  
Carmen Pascente ◽  
Paul Ruchhoeft
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
High Speed ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Working Fluid ◽  
Hydrophilic Surface ◽  
Nucleate Boiling Heat Transfer ◽  
Nucleate Bubble

Nucleate boiling heat transfer depends on various aspects of the bubble ebullition, such as the bubble nucleation, growth and departure. In this work, a synchronized high-speed optical imaging and infrared (IR) thermography approach was employed to study the ebullition process of a single bubble on a hydrophilic surface. The boiling experiments were conducted at saturated temperature and atmospheric pressure conditions. De-ionized (DI) water was used as the working fluid. The boiling device was made of a 385-um thick silicon wafer. A thin film heater was deposited on one side, and the other side was used as the boiling surface. The onset of nucleate boiling (ONB) occurs at a wall superheat of ΔTsup= 12 °C and an applied heat flux of q" = 35.9 kW/m2. The evolution of the wall heat flux distribution was obtained from the IR temperature measurements, which clearly depicts the existence of the microlayer near the three-phase contact line of the nucleate bubble. The results suggest that, during the bubble growth stage, the evaporation in the microlayer region contributes dominantly to the nucleate boiling heat transfer; however, once the bubble starts to depart from the boiling surface, the microlayer quickly vanishes, and the transient conduction and the microconvection become the prevailing heat transfer mechanisms.

Visualization of Pool Boiling From Plain and Enhanced Structures

2005 ◽  
Author(s):  
Camil-Daniel Ghiu ◽  
Yogendra K. Joshi
Keyword(s):  
Atmospheric Pressure ◽  
Pool Boiling ◽  
Single Layer ◽  
Main Tool ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometric Complexity ◽  
Boiling Process

A visualization study of pool boiling at atmospheric pressure from plain and enhanced structures was conducted with PF 5060 as working fluid. The single layer enhanced structures were fabricated in copper and were 1 mm thick. The parameters investigated in the present study are heat flux, width of microchannels and overall structure width. A monochrome CCD camera with attached magnifying lens served as the main tool for observation of the boiling process from the structures. The nucleate boiling regime for a plain surface is usually divided into two sub-regimes: the isolated bubbles regime and the coalesced bubbles regime. For enhanced structures, the increase in geometric complexity leads to different flow regimes that may establish under different heat fluxes. This study evaluates these regimes using movies and still photographs. A comparison with the plain case is made and the differences highlighted.

Effect of Heat Flux on Bubble Coalescence Phenomena and Sound Signatures During Pool Boiling

2020 ◽  
Author(s):  
Akshat Negi ◽  
Aniket M. Rishi ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Frequency Region ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Bubble Collapse ◽  
Maximum Heat ◽  
Acoustic Signatures

Abstract Boiling heat transfer is extensively used in various industrial applications to efficiently dissipate a large amount of heat by maintaining lower surface temperatures. The maximum heat flux dissipated during boiling is limited by the critical heat flux (CHF) and limited visualization of the boiling surface limits the identification of the impending CHF condition to rely on temperature monitoring alone. The study presented here focuses on developing a method for analyzing and identifying acoustic signatures throughout the nucleate boiling regimes that are indicative of the boiling state of the heater surface. The bubble nucleation and coalescence along with bubble collapse at the liquid-vapor interface leads to variation in acoustic emission patterns during boiling. These sound waves are studied and acoustic signatures that are representative of the impending CHF are identified over plain and enhanced copper substrates with water as the working fluid. During pool boiling study, it was observed that sound was dominant in two frequency regions (400–500 Hz dominant throughout nucleate boiling and 100–200 Hz dominant at heat fluxes > 100 W/cm2). However, just before CHF, a sudden drop in amplitude was observed in the high frequency region (400–500 Hz), while the amplitude in low frequency region (100–200 Hz) continued to rise. It was concluded that this acoustic study can be used as a tool to predict the approaching CHF condition.

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