A model for droplet evaporation near Leidenfrost point

The droplet evaporation process after impinging on a solid wall near Leidenfrost point is theoretically analyzed. Considering the change of heat transfer effective in the evaporation process, it is divided into recoil stage and spherical stage, and the heat transfer models in these two stages are built, respectively. The effect of initial Weber number, initial droplet diameter, solid surface superheat and wettbility are included in the models. A correlation for predicting evaporation lifetime is obtained based on the theoretical analysis and experimental results. By comparing analysis results with experimental data, it is concluded that the evaporation process can be predicted by present model. The results imply that Leidenfrost point may be not the turning point of heat transfer mechanism. The effect of drop size and Weber number are also analyzed.     

Resident time of a compound drop impinging on a hot surface

The resident time of a water-in-diesel compound drop impinging on a hot surface at a temperature higher than the Leidenfrost temperature was investigated experimentally. Past experimental evidence suggested that the resident time of a pure liquid drop was independent of the impact velocity. And this independency could also be seen for compound drops. For both pure drops and compound drops, the resident time became longer with increasing outer diameter of the drop. For water-in-diesel compound drops of a given outer diameter, the resident time decreased as the volume of the core water drop increased. By using a modified Weber number which took into account of the two interfaces of the compound drop, a correlation of the non-dimensional resident time was obtained and was in good agreement with the experimental data.     

Pattern analysis of a single droplet impinging onto a heated plate

The time scale for measuring the heat transfer phenomena, such as the Leidenfrost phenomenon, is much longer than the hydrodynamic transient period of a droplet impinging onto a heated surface. However, the Leidenfrost temperature has been long since taken as the criterion to classify characteristic regimes for not only heat transfer but also hydrodynamic impact patterns of liquid‐solid‐interface problems in the literature. The impingement phenomena of a single droplet onto a heated plate for We <200 were experimentally classified as five different characteristic patterns in this paper. They are [I] completely wet, [II] wet film boiling, [III] transition, [IV] dry rebounding, and [V] satellite dry rebounding impact patterns. The former two patterns belong to wet impact, while the latter two are dry impact. This work reveals that the hydrodynamic impact parameter does influence the classification of characteristic impact patterns, especially for the dry impact patterns. For high impact Weber numbers, the dry impact happens to the plate surface temperature much lower than the commonly believed Leidenfrost temperature due to the squeeze film effect. The evolutions that impact changes from one to another characteristic patterns are also discussed. This paper offers a systematic and reliable database for future numerical simulations. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 579–594, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20089     

Leidenfrost Droplets on Microstructured Surfaces

The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary between structured sections, the droplet becomes asymmetric and may move or spin. An axisymmetric computational fluid dynamics (CFD) model is also presented that shows qualitative agreement with experimental observations.     

Leidenfrost boiling of methanol droplets on hot porous/ceramic surfaces

An experimental and analytical study of film boiling methanol droplets on a porous/ceramic surface is reported. Droplet evaporation times in the wetting and film boiling regimes were measured on a polished stainless-steel surface and three ceramic/alumina surfaces of 10%, 25% and 40% porosity. It was found that the Leidenfrost temperatures increased as surface porosity increased. The Leidenfrost point of the 10% and 25% porous surfaces were nearly 100 K higher and 200 K higher, respectively, than that of the polished stainless-steel surface; methanol droplets could not be levitated on the 40% porous surface at surface temperatures as high as 620 K, which was the maximum surface temperature which could be imposed on this particular material with our apparatus. The evaporation time of liquid deposited on this surface was thus almost two orders of magnitude lower than for levitated droplets on the three other surfaces tested at the same temperature. In the Leidenfrost regime droplets evaporated faster on the porous surfaces than on the stainless-steel surface, and the evaporation time decreased with increasing surface porosity at the same surface temperature. The reduced evaporation times were thought to have their origin in a decrease of the vapor film thickness separating the droplet from the ceramic surface due to vapor absorption and flow within the ceramic material. An analysis of flow in a horizontal channel bounded by an impermeable ẇall above and a permeable wall of finite thickness below was used to model the film boiling process. The results provided a basis for correlating our evaporation time measurements.     

An experimental study of bouncing Leidenfrost drops: Comparison between Newtonian and viscoelastic liquids

The effect of polymer additives on the dynamic Leidenfrost phenomenon (rebound of liquid drops impacting on very hot walls, where a thin vapour cushion separates the liquid from the surface) is studied experimentally by high-speed imaging. Drops of a dilute solution (200 ppm) of Polyethylene Oxide (PEO), with equilibrium diameters of 2.66 and 3.32 mm, were compared with drops of pure water (diameters of 2.76 and 3.49 mm) during the impact on an aluminium surface at a temperature of 400 °C and impact Weber numbers between 7 and 160. The additive causes a slight reduction of the maximum spreading diameter and of the retraction velocity of the drop after impact and, within a certain range of Weber numbers, enhances significantly the maximum height of the drop center of mass during rebound. These results, obtained for a non-wetting case, are different from those previously obtained for impacts on dry surfaces, where polymer additives hardly change the maximum spreading diameter but reduce the retraction velocity of nearly one order of magnitude and completely suppress drop rebound.     

Evaporation of an oil‐in‐water type emulsion droplet on a hot surface

Evaporation characteristics of an Oil‐in‐Water (O/W) emulsion droplet were examined experimentally. The evaporation time per unit of initial surface area of a droplet τ^* was used to estimate the evaporation characteristics of droplets with different diameters and to compare a water droplet and an emulsion droplet. Results show that τ^* of an O/W emulsion droplet is shorter than a water droplet in the Leidenfrost film boiling regime. The four evaporation modes of O/W type emulsion droplets were observed. These depended on the mixing ratio of water and oil, G_S, and hot surface temperature, T_W. Increasing G_S increases the emulsion droplet's Leidenfrost temperature when the droplet is used as a die‐cast releasing agent. Microexplosions were observed during Leidenfrost film boiling when T_W was greater than 250^°C. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 527–537, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20081     

Temperature fluctuation under bubbles of pool boiling in decompression

Experiments on pool boiling of water were performed under decompression. Temperature fluctuation was measured with three thermocouples which were equipped on the copper block surface by plating. Rapid temperature drops in this fluctuation arise at the same times. This shows the existence of evaporation of the microlayer under bubbles. Evaluation of this evaporation of the microlayer and rapid temperature drops revealed the existence of a new evaporation factor in obedience to the liquid surface overheating. Temperature drops are of two types at decompression. One is rapid drop due to cavity existence and the other is slow drop due to generation in the superheated bulk liquid on the mirror surface. This rapid temperature drop is gradual at the first stage. For verification of gradual drops, we introduced an equation concerning micro film thickness under the bubble on the basis of the viscosity and the preservation of momentum. This equation has an exponential factor for microfilms. Numerical calculations showed good agreement with the experimental data. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 567–581, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10102     

Characteristics of condensate drop movement with application of bulk surface temperature gradient in Marangoni dropwise condensation

The characteristics of the spontaneous movement of condensate drops when a bulk temperature gradient is applied to a horizontal condensing surface in Marangoni dropwise condensation of a water?ethanol vapor mixture were experimentally investigated over a wide range of bulk temperature gradients and for various mass fractions. Drops were observed to move from the low-temperature side to the high-temperature side of the heat transfer surface. When the initial drop distance was adopted as a parameter for the Marangoni force acting on the condensate drop together with the surface tension gradient corresponding to the surface temperature of the condensing surface, the drop velocity was highly correlated with both the surface tension gradient and the initial drop distance over a wide range of parameters. At relatively large initial drop distances, the condensate drop velocity increases as the initial drop distance is reduced and it subsequently decreases after the velocity reaches its maximum value under an almost constant bulk surface tension gradient. The drop velocity increases linearly with increasing bulk surface tension gradient for a constant initial drop distance.     

Molecular dynamics simulation of vaporization of an ultra-thin liquid argon layer on a surface

We performed molecular dynamics simulations of the vaporization phenomenon of an ultra-thin layer (2 nm) of liquid argon on a platinum surface. The simulation started from a molecular system of three phases (liquid argon, solid platinum and argon vapor) in equilibrium at 110 K. The platinum wall was then suddenly heated to a higher temperature (a moderately higher temperature of 150 K and a much higher temperature of 300 K were investigated). Features of our simulation model include a fast algorithm based on a tree data structure and a constant temperature solid wall model based on a 3-D Langevin equation. The entire vaporization process was successfully simulated. The results reveal trends that agree with our knowledge of vaporization of a similar macroscopic system. For example, for the high surface temperature the vaporization process is reminiscent of the Leidenfrost phenomenon and after the formation of a vapor region between the surface and the liquid mass, the latter deforms and tends to approximately acquire a spherical “droplet” shape, as one would have expected from macroscopic considerations. Contrary to this, a gradual evaporation process occurs at moderate wall temperatures. After complete evaporation and upon reduction of the wall temperature, condensation takes place leading to reconstruction of the initial liquid layer.     

Condensate drop movement in Marangoni condensation by applying bulk temperature gradient on heat transfer surface

The spontaneous movement of condensate drops by application of a bulk temperature gradient on the heat transfer surface in Marangoni condensation was investigated, with consideration for applications to heat transfer devices. In the Marangoni condensation process, the removal of condensate on the heat transfer surface is important to maintain good heat transfer. A heat transfer device, in which the liquid movement occurs without external forces such as gravity and vapor shear force, may be useful in various applications. As a result of experiments using a water–ethanol vapor mixture, the movement of droplets from the low‐temperature side to high‐temperature side could be observed on a horizontally arranged heat transfer surface. The relation between the velocity of drop movement and the gradient of surface tension was studied for different concentrations. Furthermore, the effect of inclination of the condensing surface was examined. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 387–397, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20218     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

The effect of capillary wicking action of micro/nano structures on pool boiling critical heat flux

It is well known that nanoparticles deposited on a heating surface during nanofluids boiling can change the characteristics of the heating surface and increase the critical heat flux (CHF) dramatically. We considered a new approach to investigate the nanoparticle surface effect on CHF enhancement using surfaces modified with artificial micro, nano, and micro/nano structures similar to deposited nanoparticle structures through the anodic oxidation on the zirconium alloy heater. We examined the effect of the capillary wicking action ability on the CHF enhancement due to the micro, nano, and micro/nano structured surfaces. The results demonstrated that the CHF enhancement on the modified surfaces was a consequence of the capillary wicking action ability of the artificial micro/nano structures through the high-speed visualization of the capillary wicking action.     

Effect of contact angle on wetting limit temperature

The effect of surface wettability on evaporation of a water drop has been examined experimentally using surfaces with various contact angles. To greatly change the surface wettability, TiO₂ superhydrophilicity, plasma irradiation, and super‐water‐repellent surface are adopted as the heating surface. The range in contact angle achieved by these methods was between 0^° and 170^°. The relationship between the contact angle and the wetting limit temperature was obtained and it was found that the lifetime of a water drop dramatically decreases with contact angle in the lower temperature region, and that the wetting limit temperature increases with the contact angle. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 513–526, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20128     

Comparative study of electrode voltage drops in MHD generators with and without ash in the combustion products

For estimation of the influence of slag layer on the electrode voltage drop, we have considered two identical open-cycle MHD generators with and without ash in the combustion products. Using the simple model of a slag layer with ohmic condution of current, we have evaluated the electrode voltage drop in a finitely segmented Faraday-type MHD generator with slagging electrodes. The electrode voltage drops in the presence of slag at different values of slag/plasma surface temperatures with and without slag for various values of Hall parameter and electrode wall temperature have been compared. It has been found that the presence of slag layer over the electrode surface with high slag/plasma surface temperature is more advantageous for lower electrode wall temperature and higher Hall parameter.     

Measurements of critical heat flux and liquid–vapor structure near the heating surface in pool boiling of 2-propanol/water mixtures

Saturated pool boiling of 2-propanol/water mixtures on a 12 mm diameter horizontal disk under atmospheric pressure was investigated. The CHF of the mixtures increased up to 1.7 times the CHF of water at 3.0–4.7 mol% concentrations of 2-propanol. To examine the mechanism of the CHF enhancement in the mixtures, liquid–vapor structures close to the heating surface were measured using a conductance probe. It was found that in the boiling of the mixtures, liquid–vapor structures show strong non-uniformity in the radial direction of the heating surface. The void fractions at 0.1–1 mm above the heating surface are small at the central region and large near the periphery of the heating surface. The liquid layer between the vapor mass and the heating surface is considerably thicker than that of water at the central region and becomes thinner near the periphery of the heating surface. This thicker liquid layer is likely to be the cause of the CHF enhancement in the 2-propanol/water mixtures.     

Heat transfer and flow characteristics of flow boiling of CO2‐oil mixtures in horizontal smooth and micro‐fin tubes

Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO₂ and CO₂‐oil mixtures in horizontal smooth and micro‐fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro‐fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m²·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro‐fin tube. Compared with the flow patterns in the case of pure CO₂, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO₂‐oil mixtures. With pure CO₂, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro‐fin tube were higher than those of the smooth tube. With CO₂‐oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO₂, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro‐fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross‐sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20287     

3-D modeling of the dynamics and heat transfer characteristics of subcooled droplet impact on a surface with film boiling

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 solid–vapor/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.     

Wettability Improvement by Plasma Irradiation and Its Applications to Phase-Change Phenomena

Plasma irradiation is one of the techniques to improve surface wettability. This technique can be used to enhance heat transfer of liquid–vapor phase change. For instance, evaporation of a water droplet can be enhanced by plasma irradiation. The relation between plasma irradiation time and contact angle was examined first for three metals and then the lifetime of a water drop on a hot surface was measured changing the surface wettability by plasma irradiation. The lifetime of the water drop decreased and the wetting limit temperature increased with the increasing irradiation time of plasma. Hydrophilicity by plasma irradiation is not a permanent effect but it will be useful for enhancement of cooling of hot metal.     

The effect of pressure and surface material on the leidenfrost point of discrete drops of water

The maximum evaporation time and Leidenfrost point for discrete drops of water deposited on smooth surfaces of stainless steel, brass and Monel, at pressures ranging to 75 lb/in2 are obtained and compared. The results suggest that, contrary to expectation, thermal diffusivity of the hot surface is not the controlling factor. The evaporation time-surface temperature correlation due to Baumeister et al. is substantially confirmed.