Numerical investigation of heat and mass transfer of an evaporating sessile drop on a horizontal surface (original) (raw)
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International Journal of Heat and Mass Transfer, 2005
A numerical model for studying the evaporative cooling processes that take place in a new type of cooling tower has been developed. In contrast to conventional cooling towers, this new device called Hydrosolar Roof presents lower droplet fall and uses renewable energy instead of fans to generate the air mass flow within the tower. The numerical model developed to analyse its performance is based on computational flow dynamics for the two-phase flow of humid air and water droplets. The Eulerian approach is used for the gas flow phase and the Lagrangian approach for the water droplet flow phase, with two-way coupling between both phases. Experimental results from a full-scale prototype in real conditions have been used for validation. The main results of this study show the strong influence of the average water drop size on efficiency of the system and reveal the effect of other variables like wet bulb temperature, water mass flow to air mass flow ratio and temperature gap between water inlet temperature and wet bulb temperature. Nondimensional numerical correlation of efficiency as a function of these significant parameters has been calculated.
Experimental investigation of droplet dynamics and heat transfer in spray cooling
Experimental Thermal and Fluid Science, 2003
Experimental investigation on droplet dynamics and heat transfer in spray cooling was conducted. Water and water solutions with different surfactant additions (sodium dodecyl sulfate) were used to cool a 10 mm in diameter, horizontal copper surface. A multi-nozzle spray system was constructed for studying the effects of mass flux on spray cooling. This multi-nozzle system provides a variable mass flux spray from 0.156 to 1.20 kg/m 2 s with a diameter and velocity variation less than 20%. The incoming and outgoing droplets were characterized in situ with a newly developed laser phase-Doppler anemometry (PDA). It was found that the heat transfer process in spray cooling can be divided into four regimes using the expulsion rate defined as the ratio of local outgoing to incoming mass fluxes. The advantage of the surfactant addition in spray cooling has also been investigated. Besides adding surfactant results in relative small diameters for both impinging and expulsing droplets, the slope of the droplet expulsion rate at the transition to the critical heat flux (CHF) regime also changes sharply with surfactant addition which results in a almost constant heat removal rate near the CHF regime. This character may provide an additional safety mechanism for a heat transfer device to avoid burnout.
Sensible Heat Transfer during Droplet Cooling: Experimental and Numerical Analysis
Energies
This study presents the numerical reproduction of the entire surface temperature field resulting from a water droplet spreading on a heated surface, which is compared with experimental data. High-speed infrared thermography of the back side of the surface and high-speed images of the side view of the impinging droplet were used to infer on the solid surface temperature field and on droplet dynamics. Numerical reproduction of the phenomena was performed using OpenFOAM CFD toolbox. An enhanced volume of fluid (VOF) model was further modified for this purpose. The proposed modifications include the coupling of temperature fields between the fluid and the solid regions, to account for transient heat conduction within the solid. The results evidence an extremely good agreement between the temporal evolution of the measured and simulated spreading factors of the considered droplet impacts. The numerical and experimental dimensionless surface temperature profiles within the solid surface and along the droplet radius, were also in good agreement. Most of the differences were within the experimental measurements uncertainty. The numerical results allowed relating the solid surface temperature profiles with the fluid flow. During spreading, liquid recirculation within the rim, leads to the appearance of different regions of heat transfer that can be correlated with the vorticity field within the droplet.
Numerical investigation of the cooling effectiveness of a droplet impinging on a heated surface
International Journal of Heat and Mass Transfer, 2008
Computational fluid dynamics numerical simulations for 2.0 mm water droplets impinging normal onto a flat heated surface under atmospheric conditions are presented and validated against experimental data. The coupled problem of liquid and air flow, heat transfer with the solid wall together with the liquid vaporization process from the droplet's free surface is predicted using a VOF-based methodology accounting for phase-change. The cooling of the solid wall surface, initially at 120°C, is predicted by solving simultaneously with the fluid flow and evaporation processes, the heat conduction equation within the solid wall. The range of impact velocities examined was between 1.3 and 3.0 m/s while focus is given to the process during the transitional period of the initial stages of impact prior to liquid deposition. The droplet's evaporation rate is predicted using a model based on Fick's law and considers variable physical properties which are a function of the local temperature and composition. Additionally, a kinetic theory model was used to evaluate the importance of thermal non-equilibrium conditions at the liquid-gas interface and which have been found to be negligible for the test cases investigated. The numerical results are compared against experimental data, showing satisfactory agreement. Model predictions for the droplet shape, temperature, flow distribution and vaporised liquid distribution reveal the detailed flow mechanisms that cannot be easily obtained from the experimental observations.
International Journal of Heat and Mass Transfer, 1980
The problem of liquid droplet vaporization in a hot convective gaseous environment is analyzed. A new gas-phase viscous, thermal and species concentration boundary layer analysis is developed using an integral approach. The gas-phase analysis is coupled with a modified form of a previous liquid-phase analysis for the internal motion and heat transfer [S. Prakash and W. A. Sirignano, Inr. J. Heat Mass Transfer 21, 885-895 (1978)]. The coupled problem is solved for three hydrocarbon fuels (n-hexane, n-decane, and nhexadecane). The results show that the droplet vaporization is unsteady, and that the tem~ra~ure distribution within the droplet is nonuniform for a significant part of the droplet lifetime. Some of the results are compared with the already existing correlations after correcting them for the heat flux into the liquid phase.
Numerical study of the drift and evaporation of water droplets cooled down by a forced stream of air
Applied Thermal Engineering, 2018
• A numerical simulation of water droplets falling in a forced air stream was performed. • Suitable size of water droplets for reducing drift and evaporation was estimated. • Mass evaporated was between 0.2 and 1.2% of the total droplet mass. • Droplet diameters between 4 and 10 mm are suitable for reducing water losses. • Diameter higher than 3 mm and air velocities lower than 5 m/s avoid drifting.
Evaporative cooling of water in a natural draft cooling tower
A mathematical model of the performance of a cooling tower is presented. The model consists of two interdependent boundary-value problems, a total of 9 ODE, and the algorithm of self-consistent solution. The first boundary-value problem describes evaporative cooling of water drops in the spray zone of a cooling tower; the second boundary-value problem describes film cooling in the pack. Simulation of the boundary-value problems has been made. The comparison between experimental data and calculated results showed that the model correctly describes the basic regularities of the cooling tower performance. In the effective droplet-radius approximation the difference in the thermal efficiency between calculated and experimental results does not exceed 3%. The limits of applicability of the developed mathematical model and its possibilities are discussed.
CFD simulation of wet cooling towers
Applied Thermal Engineering, 2006
Heat and mass transfer inside a natural draft wet cooling tower (NDWCT) have been investigated numerically under different operating and crosswind conditions. The three-dimensional CFD model has utilized the standard k-e turbulence model as the turbulence closure. The current simulation has adopted both the Eulerian approach for the air phase and the Lagrangian approach for the water phase. The film nature of the water flow in the fill zone has been approximated by droplets flow with a given velocity. The required heat and mass transfer have been achieved by controlling the droplet velocity. At that specific droplet velocity, effects of the following operating parameters on the thermal performance of the NDWCT have been investigated: droplet diameter, inlet water temperature, number of nozzles, water flow rate and number of tracks per nozzle. As a result, the effect of crosswind velocity on the thermal performance has been found to be significant. Crosswinds with velocity magnitude higher than 7.5 m/s have enhanced the thermal performance of the NDWCT. (M. Behnia). 1 Tel.: +61 2 9036 9518; fax: +61 2 9036 9519; Mobile: +61 414 369 518. www.elsevier.com/locate/apthermeng Applied Thermal Engineering 26 (2006) 382-395
Modeling of heat transfer through a liquid droplet
Heat and Mass Transfer
In dropwise condensation, the released latent heat passes through the static and sliding droplets to the condensing surface at a rate limited by various thermal resistances. In the present work, numerical simulation of heat transfer through a droplet is carried for one under static and sliding condition. 3-D governing equations with appropriate boundary conditions are solved for the surface, promoter layer and droplet included within the computational domain. Simulations are carried out using an in-house CFD solver. The simulation results are validated against the available data and are found in good agreement. The observations of the present work are: (a) heat transfer through the droplet achieves steady state over a timescale of micro-seconds, (b) the heat fluxes of deformed and equivalent spherical-cap droplet are found to be equal, (c) Marangoni convection is significant for Ma ≥ 2204, (d) convection is the dominant mode of heat transfer during drop slide-off (e) constriction resistance is insignificant for a copper surface of thickness ≤ 2 mm, (f) average heat flux increases with increasing contact angle, interfacial heat transfer coefficient, degree of subcooling and Reynolds number; however, it decreases with increasing Prandtl number of the liquid. These results are useful for sensitivity analysis of various thermal resistances in the mathematical modeling of dropwise condensation underneath inclined surfaces.
Computational Analysis of Single Drops and Sprays for Spray Cooling Applications
Spray cooling is a key technology in the thermal management of the next generation electronic, aircraft and spacecraft systems. There have been relatively fewer computational studies of spray cooling because simulating all the detailed physics and dynamics of a spray consisting of millions of drops per second is computationally very expensive. In this study, computational approaches have been used to analyze single drops and sprays for spray cooling applications. The commercially available Computational Fluid Dynamics (CFD) code ANSYS Fluent (versions 14, 14.5, 15) has been used to perform single drop and spray simulations on two desktop workstations and the High Performance Computing (HPC) cluster at West Virginia University (WVU). Single drop impingement on wet surfaces has been studied using the 2D axisymmetric Volume of Fluid (VOF) model in ANSYS Fluent 14 and 14.5. The free surface shape and hydrodynamics of single drops after they impact on wet surfaces have been validated with the experiments performed by members of the WVU Spray Cooling team in the Mechanical and Aerospace Engineering Department (MAE) at WVU. Initial film thickness, initial drop diameter, initial drop shape and gravity effects have been investigated for water at room temperature. It has been concluded that gravity has significant effects on the drop and film dynamics while drop shape does not have any significant effects. The 2D axisymmetric Discrete Phase Model (DPM) with the wall film submodel in ANSYS Fluent 14 has been used to perform simulations of spray impact on flat surfaces. The effects of the nozzle-to-surface distance, spray half angle, spray coolant, spray mass flow rate and gravity on spray variables (e.g. average drop diameters, drop velocities, etc.) have been analyzed for a full cone spray based on the Spraying System 1/8 G nozzle operating at 40 psi which has been used in the spray experiments performed by members of the WVU Spray Cooling team. Full cone 40 psi water spray cooling simulations with phase change have been performed in 3D coordinates using the DPM, Eulerian Wall Film (EWF) and the Species Transport Model (STM) in ANSYS Fluent 15. The free surface shape and hydrodynamics of the film have been analyzed. The film thickness results have been compared with experiments. The effects of the surface temperature, spray temperature and air temperature on the film characteristics (e.g. film thickness, film velocity magnitude) and heat transfer (e.g. surface heat flux) have been studied. It has been concluded that air temperature does not have a significant effect on the film characteristics and heat transfer whereas spray temperature has significant effects. Increasing the spray temperature 50 K (from 300 K to 350 K) causes a 62% decrease in the surface heat flux. Full cone 40 psi water spray cooling simulations have been also performed in 2D axisymmetric coordinates using the Eulerian Multiphase (EM) model in ANSYS Fluent. The computed average surface heat flux value was 8% different compared to the 3D DPM-EWF-STM model. However, there has been a large discrepancy in the film characteristics between these two models and also between the EM model and experiments. In conclusion, the 3D DPM-EWF-STM model is the preferred method in order to analyze spray cooling at the present time. i TABLE OF CONTENTS