Cooling eectiveness of a water drop impinging on a hot surface (original) (raw)
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Cooling effectiveness of a water drop impinging on a hot surface
International Journal of Heat and Fluid Flow, 2001
We studied, using both experiments and a numerical model, the impact of water droplets on a hot stainless steel surface. Initial substrate temperatures were varied from 50°C to 120°C (low enough to prevent boiling in the drop) and impact velocities from 0.5 to 4 m/s. Fluid mechanics and heat transfer during droplet impact were modelled using a``Volume-of-Fluid'' (VOF) code. Numerical calculations of droplet shape and substrate temperature during impact agreed well with experimental results. Both simulations and experiments show that increasing impact velocity enhances heat¯ux from the substrate by only a small amount. The principal eect of raising droplet velocity is that it makes the droplet spread more during impact, increasing the wetted area across which heat transfer takes place. We also developed a simple model of heat transfer into the droplet by one-dimensional conduction across a thin boundary layer which gives estimates of droplet cooling eectiveness that agree well with results from the numerical model. The analytical model predicts that for ®xed Reynolds number (Re) cooling eectiveness increases with Weber number (We). However, for large Weber numbers, when We) Re 0:5 , cooling eectiveness is independent of droplet velocity or size and depends only on the Prandtl number.
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.
Study on the behavior of small droplet impinging onto a hot surface
2011
The effects of droplet diameter, surface roughness, and impinging velocity on the behavior of droplet impinging onto a hot surface have been studied. The surface samples used in the experiment were cylinder blocks of stainless steel having four different degrees of roughness, i.e., Ra 0.04, 0.2, 3, and 10. The diameter and impinging velocity were controlled independently by using a micro-jet dispenser. Their values were in the ranges of 300–700 μm and 1.0–4.0 m/s, respectively. The contact time was found to increase with an increase in the surface roughness and was of the order of the self-oscillation of the water droplet. The maximum spread of droplet decreased with increasing impinging velocity. The cooling curve was obtained for the range of surface temperatures from 500C to 100C, and it was found that the cooling time decreased with an increase in the surface roughness of stainless steel. Moreover, the cooling effectiveness of each droplet increased with an increase in the sur...
World Academy of Research in Science and Engineering, 2020
The purpose of this research is to investigate the Critical Heat Flux (CHF) and its relationship with thermal inertia during impact of a droplet on hot horizontal surface. In the study, three (3) different types of material were used which were Aluminum, Brass and Stainless Steel (304). The materials dimension were 50.0 mm in diameter and 30.0 mm in height. The materials were polished until they became a mirror polished surface. Ethanol was used as the test liquid. The droplet diameter was approximately 3.528 mm. The impact height was approximately 65.0 mm corresponding to impact velocity of 1.129 m/s. The evaporation lifetime was measured in seconds by using a digital stopwatch. As a result, it was observed that the CHF occurred at the surface temperatures of 105, 120 and 160 °C for aluminum, brass and stainless steel, respectively. It was also observed that all CHF data showed an evaporation lifetime below 1 sec order which is similar with other literatures.
The dynamics of the water droplet impacting onto hot solid surfaces at medium Weber numbers
Heat and Mass Transfer, 2017
List of symbols A Surface area (m 2) D, d Diameter (m) h Latent heat of evaporation (kJ/kg) q Heat flux (W/m 2) T Temperature (°C, K) Q Heat (kJ) u Droplet velocity (m/s) We Weber number Greek symbols β Spreading ratio ρ Density (kg/m 3) σ Surface tension (N/m) θ Contact angle (°) τ Evaporation time (s) ∞ Surrounding/ambient Subscripts corr Correction drop Drop g Gas KY Kurabayashi-Yang l Liquid Leid Leidenfrost max Maximum o Origin s Solid; surface, spreading w Wall * Deendarlianto
A comprehensive numerical investigation on the impingement and spreading of a non-isothermal liquid droplet on a solid substrate with heterogeneous wettability is presented in this work. The time-dependent incompressible Navier-Stokes equations are used to describe the fluid flow in the liquid droplet, whereas the heat transfer in the moving droplet and in the solid substrate is described by the energy equation. The arbitrary Lagrangian-Eulerian (ALE) formulation with finite elements is used to solve the time-dependent incompressible Navier-Stokes equation and the energy equation in the time-dependent moving domain. Moreover, the Marangoni convection is included in the variational form of the Navier-Stokes equations without calculating the partial derivatives of the temperature on the free surface. The heterogeneous wettability is incorporated into the numerical model by defining a space-dependent contact angle. An array of simulations for droplet impingement on a heated solid substrate with circular patterned heterogeneous wettability are presented. The numerical study includes the influence of wettability contrast, pattern diameter, Reynolds number and Weber number on the confinement of the spreading droplet within the inner region, which is more wettable than the outer region. Also, the influence of these parameters on the total heat transfer from the solid substrate to the liquid droplet is examined. We observe that the equilibrium position depends on the wettability contrast and the diameter of the inner surface. Consequently, the heat transfer is more when the wettability contrast is small and/or the diameter of inner region is large. The influence of the Weber number on the total heat transfer is more compared to the Reynolds number, and the total heat transfer increases when the Weber number increases.
International Journal of Thermal Sciences, 2011
The conjugate problem of fluid flow and heat transfer during the impact of water droplets onto a heated surface is studied numerically using the Volume of Fluid (VOF) methodology; adaptive grid refinement is used for increased resolution at the droplet moving interface. The phenomenon is assumed to be 2D-axisymmetric and the wall temperature is moderated to prevent the onset of nucleate boiling. Parametric studies examine the effect of Weber number, droplet size, wall initial temperature and liquid thermal properties on the cooling process of the heated plate during the impaction period. The main variables describing the evolution of the phenomenon are non-dimensionalised with expressions arising from the transient conduction theory. It is proved that for all cases examined, these non-dimensional expressions can be grouped together for describing the hydrodynamic and thermal behavior in a similar manner. Additionally, semi-analytic expressions are derived, which, for a given range of variation, describe the spatial distribution and the temporal evolution of the temperature of the wall as well also the heat flux absorbed from the droplet, cooling effectiveness and mean droplet temperature.
International Journal of Heat and Mass Transfer, 2006
The impact of a subcooled water and n-heptane droplet on a superheated flat surface is examined in this study based on a three-dimensional model and numerical simulation. The fluid dynamic behavior of the droplet is accounted for by a fixed-grid, finite-volume solution of the incompressible governing equations coupled with the 3-D level-set method. The heat transfer inside each phase and at the solidvapor/liquid-vapor interface is considered in this model. The vapor flow dynamics and the heat flux across the vapor layer are solved with consideration of the kinetic discontinuity at the liquid-vapor and solid-vapor boundaries in the slip flow regime. The simulated droplet dynamics and the cooling effects of the solid surface are compared with the experimental findings reported in the literatures. The comparisons show a good agreement. Compared to the water droplet, it is found that the impact of the n-heptane droplet yields much less surface temperature drop, and the surface temperature drop mainly occurs during the droplet-spreading stage. The effects of the droplet's initial temperature are also analyzed using the present model. It shows that the droplet subcooling degree is related closely to the thickness of the vapor layer and the heat flux at the solid surface.
International Journal of Heat and Mass Transfer, 2014
The effect of surface wettability on the collision dynamics and heat transfer phenomena of a single water droplet impacting upon a heated solid surface has been studied experimentally. To modify the surface wettability, two modules of stainless steel coated by TiO 2 were employed. The first module was induced by ultraviolet irradiation to produce the hydrophilic surface, while the second one was not. The diameter and the depth of coating surface were 30 mm and 200 nm, respectively. The droplet size was varied from 1.90 to 2.90 mm and substrate temperature raised up to 340°C. The interaction of an impact water droplet with a heated solid surface was investigated using a high-speed video camera. As a result, it was found that; (1) in the lower surface temperature region the evaporation time decreases as the static contact angle decreases, (2) the wetting limit temperature decreases with the increase of static contact angle, (3) the ultraviolet irradiation on the TiO 2 surface does not change the qualitative behavior of the evolution of wetting diameters, and (4) the maximum wetting diameter increases with the decrease of static contact angle below the wetting limit temperatures.
The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet's free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet's evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet's evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate's temperatures, which have been varied between 20-90°and 60-100°C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement.