Dynamics and temperature of droplets impacting onto a heated wall

This paper presents results of an experimental investigation of water droplet impacts onto a smooth heated plate made of nickel. A high-speed camera is used to observe the impact regimes (rebound, splashing, and deposition of a liquid film) for a large set of impact conditions (wall temperature, droplet incidence angle, velocity and size). The observations help specifying the conditions of existence of the regimes. In a second part of the paper, the emphasis is placed on the droplet heating during an interaction with the heated wall. The droplet temperature is measured with the help of the two-colour laser-induced fluorescence thermometry in the case of the different impact regimes. The droplet change in temperature during an impact is found to be dependent on the normal velocity but not on the wall temperature when it exceeds the Leidenfrost temperature.     

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     

Discussions of transient heat transfer to a water droplet on heated surfaces under atmospheric pressure

This paper discusses the transient heat transfer to a water droplet on heated surfaces considering a surface temperature change, in the contact period when the droplet is in contact with them. The initial surface temperature ranges from the lowest limit of the nucleate boiling region to the Leidenfrost point. Attention is paid to the two parts of the contact period : the waiting period before the onset of the first bubbling and the successive boiling period after this onset. Our proposal of transient heat transfer is based on some assumptions backed up by theoretical and experimental considerations—a theory of the first bubbling by Michiyoshi and a few experimental results from our previous papers and other works.     

Analysis of the spray field development on a vertical surface during water spray-quenching using a flat spray nozzle

The aims of this study were (i) to conduct experimental spatially and time-resolved measurements of flow development on large heated surfaces during transient spray cooling operations and (ii) to investigate and discuss the influence of spray cooling mechanisms such as bubble formation and the flow field development of the cooling fluid and how this affects heat transfer. Quartz plates were heated to above 500 °C and then sprayed with pressurised water subcooled to 80 K. High speed images of the quench process were collected at a rate of 3000 Hz making it possible to track the movement of the quench front as the plate cools below the Leidenfrost temperature of the fluid at that location. Observations showed that the relative importance of droplet-surface interactions decreases once the Leidenfrost temperature is reached on the plate: It was found that once the water contacts the surface, a water pool develops rapidly which grows larger as the pool floods the heated surface. Comparisons between the spatial flow development and heat transfer on the plate are made in order to describe these interactions more accurately. This information not only provides crucial input into process simulations, but is also useful to develop theoretical models of fluid–solid interaction describing the wetting of a heated component due to water spraying.     

A three-region conduction-controlled rewetting analysis by the Heat Balance Integral Method

Conduction-controlled rewetting of two-dimensional objects is analyzed by the Heat Balance Integral Method (HBIM) considering three distinct regions: a dry region ahead of wet front, the sputtering region immediately behind the wet front and a continuous film region further upstream. The HBIM yields solutions for wet front velocity, sputtering length and temperature field with respect to wet front. Employing this method, it is seen that heat transfer mechanism is dependent upon two temperature parameters. One of them characterizes the initial wall temperature while the other specifies the range of temperature for sputtering region. Additionally, the mechanism of heat transfer is found to be dependent on two Biot numbers comprising a convective heat transfer in the wet region and a boiling heat transfer in the sputtering region. The present solution exactly matches with the one-dimensional analysis of K.H. Sun, G.E. Dix, C.L. Tien [Cooling of a very hot vertical surface by falling liquid film, ASME J. Heat Transf. 96 (1974) 126–131] for low Biot numbers. Good agreement with experimental results is also observed.     

Experimental Studies of Nanofluid Droplets in Spray Cooling

Spray cooling is used in cooling of electronic devices to remove large heat fluxes. Heat transfer to droplets impinging on a heated surface and boiling off has been studied. Most work is on a well-controlled system of a single drop falling onto a horizontal heated plate from a fixed height. These have revealed the droplet impingement mechanics to be a function largely of Weber number and excess temperature, and a range of regimes is observed similar to those in pool boiling, with a clearly identifiable critical heat flux. Nanofluids exhibit enhanced boiling heat transfer in pool boiling. The effect of nanoparticles on droplet boil-off was studied in this work. Nanofluid drops were let fall onto a surface at temperature greater than the saturation temperature, and behavior and heat flux were recorded and contrasted to that of a pure fluid. The working fluids used were pure water, ethanol, and dimethyl sulfoxide (DMSO) and ethanol– or DMSO–nanoparticle solutions (the nanoparticles were aluminum, with concentrations of up to 0.1% by weight in DMSO and 3.2% by weight in ethanol). High-speed photographic images of droplet evolution in time were obtained and indicate that there are differences in the behavior of nanofluid droplets as they boil off the surface, compared to pure fluids. Increasing nanoparticle concentration decreases the receding droplet breakup on rebound after impingement and appears to reduce the maximum spreading of a droplet as well. Maximum recoil height is reduced with increasing nanoparticle concentration. Experimental measurements of the heat fluxes associated with the pure and nanofluid droplets did not show significant enhancement, though there was noticeable improvement in the DMSO nanofluids.     

Analytical Model for Describing Experimental Results on Parameters Influencing Heat Transfer in Film Boiling with Spray Quenching

An analytically solvable mathematical model is developed to estimate heat transfer quantities in the film boiling region of metal quenching with water sprays. The model is based on the hydrodynamic of a single droplet which is separated from the metal by a vapor film. The temperature profile within the droplet is calculated as semi-infinite body because of the short contact time. It is validated with own experimental results and those from the literature. The influence of size and velocity of the droplet, spray flux, surface temperature, temperature of the cooling water and the salinity level are discussed. The droplet size and velocity play a less significant influence on the heat transfer. The heat transfer coefficient is found to increase linearly with the spray flux. The heat flux is proportional to the difference of boiling and water temperature. With the model it is shown, that even for the high impingement densities the droplet covered area is very small.     

Characterization of the heat transfer accompanying electrowetting or gravity-induced droplet motion

Electrowetting (EW) involves the actuation of liquid droplets using electric fields and has been demonstrated as a powerful tool for initiating and controlling droplet-based microfluidic operations such as droplet transport, generation, splitting, merging and mixing. The heat transfer resulting from EW-induced droplet actuation has, however, remained largely unexplored owing to several challenges underlying even simple thermal analyses and experiments. In the present work, the heat dissipation capacity of actuated droplets is quantified through detailed modeling and experimental efforts. The modeling involves three-dimensional transient numerical simulations of a droplet moving under the action of gravity or EW on a single heated plate and between two parallel plates. Temperature profiles and heat transfer coefficients associated with the droplet motion are determined. The influence of droplet velocity and geometry on the heat transfer coefficients is parametrically analyzed. Convection patterns in the fluid are found to strongly influence thermal transport and the heat dissipation capacity of droplet-based systems. The numerical model is validated against experimental measurements of the heat dissipation capacity of a droplet sliding on an inclined hot surface. Infrared thermography is employed to measure the transient temperature distribution on the surface during droplet motion. The results provide the first in-depth analysis of the heat dissipation capacity of electrowetting-based cooling systems and form the basis for the design of novel microelectronics cooling and other heat transfer applications.     

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.     

Heat transfer from plastic film heated by thermal radiation

The heat transfer characteristics of a plastic film were experimentally studied using a two‐dimensional plane model apparatus of a double bubble tubular blown film process, which is one of the most important methods of polymer processing to produce a biaxially oriented film. In the model apparatus, a polyethylene film was heated by infrared heaters and cooled by plane air jets. The radiation absorptivity of the film was estimated on the basis of the temperature and irradiation measurements of the film that was cooled by natural convection, i.e., without the air jets. The convective heat transfer coefficient on the film cooled by the air jets was evaluated using the estimated absorptivity of the film. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 265–278, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20059     

Flow and heat transfer of natural convection around an upward‐facing horizontal plate with a vertical plate at the edge

Natural convective flows around an upward‐facing horizontal heated plate with a vertical plate at the edge were investigated experimentally. Of particular concern were the influences of the vertical plate on the fluid flow and the heat transfer of the horizontal plate. The flow and temperature fields adjacent to the horizontal plate were visualized with dye and a liquid‐crystal thermometry. The results show that the vertical plate obstructs the flow from the top of the vertical plate, while the flow from the open edge of the horizontal plate covers the whole horizontal surface when the height of the vertical plate exceed H/W = 0.14 for adiabatic vertical plate and H/W = 0.1 for the heated vertical plate. The local heat‐transfer‐coefficients of the horizontal plate were also measured. It was found that the vertical adiabatic plates deteriorate the heat transfer, while the heated vertical plates enhance the heat transfer from the horizontal plates. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 527–539, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20033     

Enhancement of natural convection heat transfer from a vertical heated plate using inclined fins

An enhancement technique is developed for natural convection heat transfer from a vertical heated plate with inclined fins, attached on the vertical heated plate to isolate a hot air flow from a cold air flow. Experiments are performed in air for inclination angles of the inclined fins in the range of 30° to 90° as measured from a horizontal plane, with a height of 25 to 50 mm, and a fin pitch of 20 to 60 mm. The convective heat transfer rate for the vertical heated plate with inclined fins at an inclination angle of 60° is found to be 19% higher than that for a vertical heated plate with vertical fins. A dimensionless equation on the natural convection heat transfer of a vertical heated plate with inclined fins is presented. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 334–344, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20168     

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.     

Interfacial temperature measurements,high-speed visualization and finite-element simulations of droplet impact and evaporation on a solid surface

The objective of this work is to investigate the coupling of fluid dynamics, heat transfer and mass transfer during the impact and evaporation of droplets on a heated solid substrate. A laser-based thermoreflectance method is used to measure the temperature at the solid–liquid interface, with a time and space resolution of 100 μs and 20 μm, respectively. Isopropanol droplets with micro- and nanoliter volumes are considered. A finite-element model is used to simulate the transient fluid dynamics and heat transfer during the droplet deposition process, considering the dynamics of wetting as well as Laplace and Marangoni stresses on the liquid–gas boundary. For cases involving evaporation, the diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet–air interface. High-speed visualizations are performed to provide matching parameters for the wetting model used in the simulations. Numerical and experimental results are compared for the transient heat transfer and the fluid dynamics involved during the droplet deposition. Our results describe and explain temperature oscillations at the drop–substrate interface during the early stages of impact. For the first time, a full simulation of the impact and subsequent evaporation of a drop on a heated surface is performed, and excellent agreement is found with the experimental results. Our results also shed light on the influence of wetting on the heat transfer during evaporation.     

A coupled level set and volume-of-fluid simulation for heat transfer of the double droplet impact on a spherical liquid film

A coupled level set and volume-of-fluid method is applied to investigate the double droplet impact on a spherical liquid film. The method focuses on the analysis of surface curvature, droplet diameter, impact velocity, double droplets vertical spacing, the thickness of the liquid film of two liquid droplets after the impact on a spherical liquid film, and the influence of flow and heat transfer characteristics. The results indicate that the average wall heat flux density of the double liquid droplet impact on a spherical liquid film is greater than that of a flat liquid film. Average wall heat transfer coefficient increases with the increase in the liquid film’s spherical curvature. When the liquid film thickness is smaller, the average wall heat flux density of the liquid film is significantly reduced by the secondary droplets generated from the liquid film. When the liquid film thickness is larger, the influence of liquid film thickness on the average wall heat flux density gradually decreases. The average wall heat flux density increases with the increase in impact velocity and the droplet diameter; it also decreases with the increase in double droplets vertical spacing.     

Natural convection heat transfer of water in a horizontal circular gap

An experimental study on the natural convection heat transfer on a horizontal downward facing heated surface in a water gap was carried out under atmospheric pressure conditions. A total of 700 experimental data points were correlated using Rayleigh versus Nusselt number in various forms, based on different independent variables. The effects of different characteristic lengths and film temperatures were discussed. The results show that the buoyancy force acts as a resistance force for natural convection heat transfer on a downward facing horizontal heated surface in a confined space. For the estimation of the natural convection heat transfer under the present conditions, empirical correlations in which Nusselt number is expressed as a function of the Rayleigh number, or both Rayleigh and Prandtl numbers, may be used. When it is accurately predicted, the Nusselt number is expressed as a function of the Rayleigh and Prandtl numbers, as well as the gap width-to-heated surface diameter ratio; and uses the temperature difference between the heated surface and the ambient fluid in the definition of Rayleigh number. The characteristic length is the gap size and the film temperature is the average fluid temperature.     

Evolution and heat transfer after droplet impact on heated liquid film with vapor bubbles inside

Abstract

To better understand the droplet impact on the liquid film with vapor bubbles in spray cooling, a two-dimensional symmetric numerical model is set up using the Coupled Level Set and Volume of Fluid method (CLSVOF). Three simulative cases are taken, considering the effects of film thickness and the presence of vapor bubbles or not. The main purposes of this paper are to investigate the evolution of vapor bubbles during droplet impact and to identify the effect of vapor bubbles on convection heat transfer. The results indicate that vapor bubbles will detach from the wall and break up at the surface of the liquid film during droplet impact, for a thinner film, later a “sawtooth” liquid film appears at the non-impact region. However, for a thicker film, no bubbles rupture and the detached bubbles will flow inside the liquid film and then some will merge into larger bubbles. In the presence of vapor bubbles, the crater radius is larger for a thicker liquid film. The presence of vapor bubbles will facilitate the subcooled droplet to spread to the heated wall, leading to a substantial increase in surface heat flux.     

Simultaneous measurement of local film thickness and temperature distribution in wavy liquid films using a luminescence technique

Heat transfer in falling liquid film systems is enhanced by waviness. Comprehension of the underlying kinetic phenomena requires experimental data of the temperature field with high spatiotemporal resolution. Therefore a non-invasive measuring method based on luminescence indicators is developed. It is used to determine the temperature distribution and the local film thickness simultaneously. First results are presented for the temperature distribution measurement in a laminar-wavy water film with a liquid side Reynolds number of 126 flowing down a heated plane with an inclination angle of 2°. The measured temperature distributions are used to calculate the local heat transfer coefficient and the convective heat flux perpendicular to the wall for different points in the development of a solitary wave.     

Mixed convective heat transfer of water in a pipe under supercritical pressure

Because of the rapid properties variation of fluid under supercritical pressure, there is a violent secondary flow in a heated pipe, which will certainly complicate the heat transfer of fluid in a pipe under supercritical pressure. In this paper, a numerical study is conducted for the laminar developing mixed convective heat transfer of water under supercritical pressure. The velocity field and temperature field are given, and the influence of different parameters on flow and heat transfer is investigated in detail. The results show that secondary flow has a great influence on velocity and temperature distributions and thus affects the friction factor and the Nusselt number remarkably. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 608–619, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20079     

Investigation of catalyst particle hydrodynamic and heat transfer in three phase flow circulating fluidized bed

Vaporization of gas oil droplets has significant effects on the gas-solid flow hydrodynamic and heat transfer characteristic. A three dimensional CFD model of the riser section of a CFB have been developed considering three phase flow hydrodynamic, heat transfer and evaporation of the feed droplets. Several experiments were performed in order to obtain the data needed to evaluate the model using a pilot scale CFB unit. The Eulerian approach was used to model both gas and catalyst particle phases comprising of continuity, momentum, heat transfer and species equations as well as an equation for solid phase granular temperature. The flow field and evaporating liquid droplet characteristics were modeled using the Lagrangian approach. The catalyst particle velocity and volume fraction were measured using a fiber optic probe. The comparison between model predictions of catalyst particle velocity and volume fraction with the experimental data indicated that they were in good agreements and the Syamlal-O'Brien was the most accurate drag equation. The CFD model was capable of predicting the main characteristic of the complex gas-solids flow hydrodynamic and heat transfer, including the cluster formation of the catalyst particles near the reactor wall. In addition, the simulation results showed droplet vaporization caused reduction of catalyst particle residence time. Moreover, the higher ratios of the feed to catalyst flow rates led to the lower values of the catalyst temperature profile minimum.