A Numerical and Experimental Investigation of a Gas Propelled Liquid Droplet Impinging Onto a Heated Surface

2011 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega
Keyword(s):  
Kinetic Energy ◽  
Contact Area ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Spray Cooling ◽  
High Heat Flux ◽  
Solid Contact ◽  
Droplet Spreading ◽  
Air Jet ◽  
The Impact

Due to the higher rates of heat transfer and the spatial homogeneity of heat removal that can be achieved with spray cooling, these systems have been widely proposed for cooling high heat flux electronics. In particular, gas-assisted spray cooling systems, in which a vapor phase jet propels the liquid phase droplets to a target surface, have been shown to be even more efficient in removing heat than sprays consisting of droplets alone. However, in all the studies found in the literature, in which the basic problem has been approached as a single-droplet event, only the behavior of a free falling droplet has been studied. To date, there is no fundamental investigation of the physics of gas or vapor-assisted spray cooling. To study this problem an experimental and numerical investigation of the deformation process of a liquid droplet transported by a gas stream impinging on a heated surface was performed. A preliminary study [1] has shown that increasing air jet velocities leads to an augmentation in liquid-solid contact area. Nevertheless, for low We*, the increase in droplet spreading diameter is only a consequence of the increase in droplet kinetic energy before the impact rather than the pressure and shear stress imposed by the gas during the spreading. An order of magnitude analysis showed that shear effects are negligible compared to the normal pressure of the jet. A first order analytical model of the droplet spreading behavior indicated that the jet stagnation pressure acting on the droplet surface becomes important at relatively low Weo and higher We* by contributing to the reduction in liquid film thickness and to the augmentation in liquid-solid contact area. It was shown that the work done by the gas stream in deforming the liquid droplet must be at least 10% of the initial kinetic energy of the droplet to start having a significant effect on the droplet deformation during the early stage of impact.

Time-dependent measurements of length and area of the contact line in contact-boiling regime

2021 ◽  
Vol 926 ◽  
Author(s):  
Mohammad Khavari ◽  
Tuan Tran
Keyword(s):  
Contact Line ◽  
Contact Area ◽  
High Speed ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Bubble Nucleation ◽  
High Speed Imaging ◽  
Wide Range ◽  
Length And Area ◽  
The Impact

During the impact of a liquid droplet on a sufficiently heated surface, bubble nucleation reduces the contact area between the liquid and the solid surface. Using high-speed imaging combined with total internal reflection, we measure and report how the contact area decreases with time for a wide range of surface temperatures and impact velocities. We also reveal how formation of the observed fingering patterns contributes to a substantial increase in the total length of the contact line surrounding the contact area.

Numerical Simulation of Heat Transfer Mechanisms in Spray Cooling

2010 ◽  
Author(s):  
Eelco Gehring ◽  
Mario F. Trujillo
Keyword(s):  
Heat Transfer ◽  
Heated Surface ◽  
Spray Cooling ◽  
Impingement Zone ◽  
Ongoing Work ◽  
Efficient Suppression ◽  
A Cell ◽  
Cooling Behavior ◽  
Free Surface Capturing ◽  
The Impact

A primary mechanism of heat transfer in spray cooling is the impingement of numerous droplets onto a heated surface. This mechanism is isolated in the present and ongoing work by numerically simulating the impact of a single train of FC-72 droplets employing an implicit free surface capturing methodology. The droplet frequency and velocity ranges from 2000–4000 Hz, and 0.5–2 m/s, respectively, with a fixed drop size of 239 μm. This gives a corresponding Weber and Reynolds range of 10–170 and 330–1300, respectively. Results show that the impingement zone is largely free of phase change effects due to the efficient suppression of the local temperature field well below the saturated value. Due in part to the relatively high value of the Prandtl number and the compression of the boundary layer from the impingement flow, a cell size on the order of 1 μm is necessary to adequately capture the heat transfer dynamics. It is shown that the cooling behavior increases in relation to increasing frequency and impact velocity, but is most sensitive to velocity. In fact, for sufficiently low velocities the calculations show that the momentum imparted on the film is insufficient to maintain a near stationary liquid crown. The consequence is a noticeable penalty on the cooling behavior.

Numerical Investigation of a Liquid Droplet Impinging on a Heated Surface

2009 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega ◽  
Ryan Anderson
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Fundamental Problem ◽  
Water Drop ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Heat Removal ◽  
Droplet Spreading ◽  
Droplet Shape ◽  
Spreading Dynamics

Previous studies, most of them experimental, reveal that the cooling effectiveness of a water drop impinging on a heated surface depends on the wall temperature, droplet shape and velocity. All previous studies focus on the behavior of a droplet falling in a quiescent environment, such as still air. Evidence in the literature also shows that gas assisted droplet sprays, in which a gas phase propels the droplets, are more efficient in heat removal than sprays consisting of droplets alone. It is conjectured that this is due to an increase in the maximum droplet spreading diameter upon impact, a thinner film, and consequently an increase in the overall heat transfer coefficient. Recent experiments in the author’s group [1, 2] show that the carrier gas jet strongly influences droplet spreading dynamics by imposing normal and shear forces on the liquid surface. The heat transfer is greatly augmented in the process, compared to a free falling droplet. To date, there has been no fundamental investigation of the physics of gas assisted spray cooling. To begin to understand the complicated process, this paper reports on a fundamental problem of a single liquid droplet that impinges on a heated surface. This paper contributes a numerical investigation of the problem using the volume of fluid (VOF) technique to capture droplet spreading dynamics and heat transfer in a single drop event. The fluid mechanics is investigated and compared to the experimental data. The greatest uncertainty in the simulation is in the specification of the contact angle of the advancing or receding liquid front, and in capturing the onset of the three-dimensional fingering phenomena.

Experiments and Modeling of a Liquid Droplet Transported by a Gas Stream Impinging on a Heated Surface: Evaporative Regime

2009 ◽  
Author(s):  
Ryan P. Anderson ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Single Phase ◽  
Heat Dissipation ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Thin Liquid Film ◽  
Spray Cooling ◽  
High Speed Photography ◽  
Transfer Coefficients

Understanding the transport mechanisms involved in a single droplet impinging on a heated surface is imperative to the complete understanding of droplet and spray cooling. Evidence in the literature suggests that gas assisted sprays and mist flows are more efficient than sprays consisting only of liquid droplets. There has been few if any fundamental studies on gas-assisted droplets or spray cooling, in which a carrier gas or vapor stream propels the droplet to the target surface. The current work extends previous studies of a droplet impinging on a heated surface conducted by the same group from the single phase regime into the evaporative regime. For both regimes, understanding the transport physics due to the heat transfer from the heated surface to the droplet and then by convection and evaporation to the airflow is of fundamental importance. High-speed photography was used to capture the spreading process and yielded results that correlated well with previously published isothermal and single-phase results. The heat transfer was measured with a fitting approach by which the instantaneous temperature profile was matched to an analytic solution to determine the instantaneous value of the centerline heat transfer coefficient. A very large increase in the heat dissipation was observed when compared to previously published single-phase results. Heat transfer was optimized at Reynolds numbers that produced an optimally thin liquid film and high heat and mass transfer coefficients on the surface of the film.

Experimental study of spreading characteristics of droplet impacting on canopy fabric surface

2017 ◽  
Vol 31 (34) ◽  
pp. 1750325 ◽  
Author(s):  
Han Cheng ◽  
Chao Qiu ◽  
Changchun Zhou ◽  
Xuebin Sun ◽  
Rui Yang
Keyword(s):  
Experimental Study ◽  
Kinetic Energy ◽  
Impact Velocity ◽  
Working Conditions ◽  
Experimental Results ◽  
Initial Kinetic Energy ◽  
Droplet Spreading ◽  
Visualization Technology ◽  
Fabric Surface ◽  
The Impact

A new experiment based on visualization technology is designed to study the spreading characteristics of droplet impacting on canopy fabric. The processes of droplet impacting on 66 type polyamide grid silk are captured. The experimental results show that the spreading characteristics are also affected by fabric pretension and fabric permeability. The pretension is favorable for the droplet to reach the final equilibrium stage. The impact velocity determines the initial kinetic energy and plays a major role in the droplet spreading. The fabric permeability determines the wettability and has different effects on spreading characteristics under different working conditions. In addition, the above factors can enhance the two competitive processes of spreading and imbibing at the same time. The spreading characteristics depend on which process is the dominant one.

Droplet Oscillation After Impact on a Solid Surface

2016 ◽  
Author(s):  
Yina Yao ◽  
Shuai Meng ◽  
Cong Li ◽  
Xiantao Chen ◽  
Rui Yang
Keyword(s):  
Contact Angle ◽  
Spray Deposition ◽  
Liquid Droplet ◽  
Droplet Diameter ◽  
Dynamic Contact Angle ◽  
Dynamic Contact ◽  
Oscillation Period ◽  
Droplet Spreading ◽  
Oscillation Phenomenon ◽  
The Impact

Droplet spreading and oscillation occur when a liquid droplet impacts on the solid surfaces. This process is vital in many industrial applications, such as ink-jet printing technologies, spray coating and agricultural spray deposition. However, the researches that have been done mainly focused on the spreading process, and less attention has been paid to the droplet oscillation phenomenon, which has influence on the solidification and evaporation process. Therefore, the study on droplet oscillation phenomenon after the impact is necessary and valuable. This paper aims at analyzing the droplet oscillation phenomenon using VOF method. Since the contact angle varies dramatically in the dynamic process, a dynamic contact angle model is introduced to improve the simulation accuracy. The dynamic contact angle model has been verified by comparing the numerical results with experimental and theoretical results. In order to study the factors that may influence the droplet oscillation period, different droplet diameters and impact velocities are utilized in this simulation. The results show that the oscillation period presents a positive relationship with droplet diameter. However, the impact velocity has no apparent influence on the oscillation period, which agrees well with the theoretical analysis.

Experimental Investigation of Heat Transfer Characteristics of Inkjet Assisted Spray Cooling

Volume 3 ◽  
2004 ◽  
Author(s):  
Ratnesh K. Sharma ◽  
Cullen E. Bash ◽  
Chandrakant D. Patel
Keyword(s):  
Heat Transfer ◽  
Power Density ◽  
Heated Surface ◽  
Spray Cooling ◽  
Operating Conditions ◽  
Heat Transfer Characteristics ◽  
Subsequent Formation ◽  
Transfer Characteristics ◽  
The Impact ◽  
Volumetric Flux

Increases in microprocessor power density along with an accompanying spatial variation in power density has been well documented in recent years. These combined factors pose a severe challenge for the provisioning of cooling resources at the microprocessor level. The use of thermal inkjet technology to precisely supply coolant onto the surface of a microprocessor has the potential to address this problem in a chip-scale form factor. By providing coolant when and where it is needed on the surface of a chip or package, very high critical heat fluxes can be obtained in an energy efficient manner in a minimum of physical space. In this paper, the unique heat transfer characteristics of inkjet assisted spray cooling of a heated surface are investigated. Sprays of water are used to cool heated surfaces ranging from 281mm2 to 35mm2. Several experiments are conducted at different nozzle-to-surface distances to measure critical heat flux (CHF) at different flow rates and firing frequencies. The impact of volumetric flux variation on CHF is studied. CHF data, measured over broad range of operating conditions is correlated to volumetric flux and liquid properties. Flow visualization studies are also conducted to understand the vapor-liquid interaction at the heater surface and the intermediate region. Jet breakup length studies are carried out to understand the propagation of Rayleigh instabilities in the spray jets and, subsequent, formation of liquid drops. CHF data combined with fluid flow studies have been used to optimize the nozzle-to-surface clearance. Results obtained from these experiments are invaluable for the design of micro scale spray cooling devices for chips.

Ultra-Cooling Heat Transfer Characteristics Using Cryogenic Micro-Solid Nitrogen Spray

2012 ◽  
Author(s):  
U. Oh ◽  
Jun Ishimoto ◽  
Naoki Harada ◽  
Daisuke Tan
Keyword(s):  
Heat Transfer ◽  
Heated Surface ◽  
Spray Cooling ◽  
High Heat ◽  
Heat Transfer Process ◽  
Solid Particles ◽  
High Heat Flux ◽  
Heat Transfer Characteristics ◽  
Solid Nitrogen ◽  
New Type

The fundamental characteristics of heat transfer and cooling performance of micro-solid nitrogen particulate spray impinging on a heated substrate were numerically investigated and experimentally measured by a new type of integrated computational-experimental technique. The employed CFD based on the Euler-Lagrange model is focused on the cryogenic spray behavior of atomized particulate micro-solid nitrogen and also on its ultra-high heat flux cooling characteristics. Based on the numerically predicted performance, a new type of cryogenic spray cooling technique for application to a ultra-high heat power density device was developed. In the present integrated computation, it is clarified that the cryogenic micro-solid spray cooling characteristics are affected by several factors of the heat transfer process of micro-solid spray which impinges on heated surface as well as by atomization behavior of micro-solid particles.

scholarly journals The Impact of Nanofluids on Droplet/Spray Cooling of a Heated Surface: A Critical Review

Energies ◽  
2020 ◽  
Vol 14 (1) ◽  
pp. 80
Author(s):  
Yunus Tansu Aksoy ◽  
Yanshen Zhu ◽  
Pinar Eneren ◽  
Erin Koos ◽  
Maria Rosaria Vetrano
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Heated Surface ◽  
Spray Cooling ◽  
Heat Transfer Process ◽  
Transfer Enhancement ◽  
Physical Mechanisms ◽  
The Subject ◽  
The Impact ◽  
Droplet Spray

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified.

Computer Modeling of Liquid Droplet Impact on Heat Transfer During Spray Cooling

2005 ◽  
Author(s):  
R. Panneer Selvam ◽  
Sandya Bhaskara ◽  
Juan C. Balda ◽  
Fred Barlow ◽  
Aicha Elshabini
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Vapor Bubble ◽  
Liquid Droplet ◽  
Heat Removal ◽  
Spray Cooling ◽  
Droplet Impact ◽  
System Level ◽  
High Heat Flux ◽  
Micro Level

Spray cooling is a high flux heat removal technique considered for systems dissipating high power within small areas such as advanced lasers. Recently Selvam and Ponnappan (2004 & 2005) identified the importance of modeling heat transfer in a thin liquid film on a hot surface at the micro level and illustrated how this micro level modeling could help to improve the macro level spray cooling. The goal of this research is to advance the theoretical understanding of spray cooling to enable efficient system level hardware designs. Two-phase flow modeling is done using the level set method to identify the interface of vapor and liquid. The modifications made to the incompressible Navier-Stokes equations to consider surface tension and phase change are presented. The equations are solved using the finite difference method. The effect of liquid droplet impact on a 40 μm thick liquid film containing vapor bubble and the consequent heat removal is explained with a sequence of temperature vs. time contours. From that, the importance of fast transient conduction in the liquid film leading to high heat flux in a short time is illustrated. The optimum positioning of the droplet with respect to the vapor bubble for effective heat removal is also systematically investigated. This information is expected to help in proper positioning of the droplet in three-dimensional modeling.

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