Hot Solid Surface Research Articles

Dynamic Leidenfrost Effect: Relevant Time and Length Scales.

When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate.

衝突液滴のライデンフロスト現象

When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire or a (non-isothermal) glass surface using high-speed frustrated total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on the substrates.

Determining Contact Angle and Spreading Velocity of a Droplet Impacted Hot Solid Surface

Spray cooling is a method to lower the solid surface temperature by splashing droplets. The evaporation of droplet will take the heat from solid surface. One of the factors that contributes to the evaporation rate and droplet spreads on the surface is the wettability of fluid on surface. Contact angle is a parameter in categorizing the wettability of fluid at a solid surface. Many methods have been developed to measure the contact angle on solid surface. This paper will explain how to measure the contact angle by analyzing the image taken when droplet fluid attached to a surface. The method use MATLAB codes as tools for processing image and compute the result. There are four steps in processing an image of droplet on a surface: convert image to binary image, detect the boundary of the surface, determine the correlation of x-y pixel, and take the tangent of a line at the outer point of contact between fluid and surface. Validation is taken by comparing the result with “standard image” with various resolutions. This method also explains droplet dynamics when impacting a hot surface.

Droplet First and Second Consecutive Impacts and Droplet–Droplet Collision on a Hot Surface in the Film Boiling Region

We studied experimentally the behavior of a single cold water droplet that falls onto a hot horizontal flat solid surface in the film boiling region. We found that when the droplet hits the surface (for the first time), three different regimes may occur. These regimes depend on the ratio of fluid's inertia and its surface tension (the Weber number, indicated as We) and on the surface temperature. For relatively low We numbers or surface temperatures, the droplet completely bounces back from the surface and no breakup occurs. For intermediate We numbers or surface temperatures, the spreading stage is faster and the droplet undergoes spreading and partial recession before it breaks up into bouncing small secondary droplets that leap inward and successively coalesce above the surface to form a single droplet once again. For high We numbers or surface temperatures, the spreading velocity is higher, the contact area with the surface is greater, and the liquid film thickness is smaller. Thus, during the expansion of the spreading stage, the droplet breaks up into bouncing small secondary droplets that uninterruptedly leap outward and travel independently. We also present the limiting conditions differentiating between the different behaviors found. This work shows droplet film boiling behaviors that are essentially different than droplet levitation on top of a thin vapor layer, as mainly assumed in theoretical models. We also observed that when a droplet hits the surface for the second, consecutive time (and on), the droplet behaves somewhat differently due to its preheating, very low impact velocity, different shape, spin, orientation, and the surface temperature. At the second impact on the surface (and on), the droplet can continue its bounce in a unique and different manner than in the first impact or it can explode violently to small secondary droplets. Both are unique and differentiating mainly by the droplet's shape and orientation at the exact moment of impact on the surface. Additionally, a rare and unique view of droplet–droplet collision during film boiling is presented. This type of collision behaves in a different way than other droplet–droplet collisions and compared to the adiabatic case of droplet–droplet collision on a nonheated surface. The behaviors found, presented, and discussed in this study change our view of the droplet–surface and droplet–droplet interactions that occur in spray cooling in the film boiling region.

Measurement and Observation of Elementary Transition Boiling Process after Sudden Contact of Liquid with Hot Surface

A Fast response surface temperature measurement technique has been developed to investigate transient transition boiling phenomena after ethanol droplet impact on a nickel hot surface. Our motivation is to make clear the mechanism of boiling transition between nucleate and film boiling which is closely related the recovery of wetting on the hot surface being higher than the Leidenfrost temperature. In this study transient boiling phenomena during sudden contact of an ethanol sessile droplet on a hot solid surface has been investigated to understand what governs whether the solid surface keeps wet or not. The droplet impact boiling system is very simple and easier to observe but it includes the elementary transition boiling process. Since the transient transition took place within a couple ten milliseconds, observation with high speed video system and fast response surface temperature measurement technique are essential for better understanding of the transition boiling phenomena.The experiments were conducted for single and multiple droplet impacts. The multiple droplet impacts simulates liquid and solid contact situations on a hot surface during spray or laminar jet quench. The nickel disk of 50mm in diameter and 5mm in thickness was used as the hot surface. The hot surface was inclined from 0 deg (horizontal) to 40 deg. The fast-response film thermocouple with a typical minimum response time of 80 microseconds was developed and fabricated on the nickel disk at the depth of 3 micrometers from the surface. The temperature histories just beneath the surface were recorded at the sampling frequency of 200kHz. The boiling phenomena beneath a sessile droplet impacted on the hot surface were observed by using a microscope equipped with the high speed video camera at the maximum frame rate of 22.5 kfps. The surface temperature and surface heat flux were estimated with 1D inverse heat conduction analysis. Histories of local surface temperature and local heat flux were compared with the boiling video image. The experiments were done for different initial wall temperatures up to 250°C, different liquid subcoolings from 33 to 53K. The multiple impact frequency was changed from 280 to 840Hz.In case of the single droplet impact tests, the observation results showed that wetting situation was maintained for very short time even though the surface temperature was beyond the liquid superheat limit temperature. We defined the vapor film generation time as the characteristic boiling transition time scale from wetted nucleation boiling regime to dry film boiling regime. The film generation time were measured for the extensive initial surface temperature range, different subcoolings and different impact velocities. As the initial surface temperature increased, the film generation time decreased to order of microsecond and film boiling situation seemed to be established via spontaneous nucleation (vapor explosion) process. In case of the multiple droplets impact tests, film generation times were evaluated every droplet impact during continuous cooling from the initial solid temperature. As long as the generation time was less than 1 millisecond and much shorter than the impact period, stable film boiling was observed. Then the film generation time increased with the droplet impact times and approached to the droplet impact period as the cooling time elapsed. This region was recognized as the transition boiling region. After the generation time reached to the droplet impact period and then stable wetted situation, namely nucleate boiling was observed. As compared with the single impact tests, the generation time for the multiple impact became shorter than that for the single droplet impact at same initial surface temperature and the lower limit surface temperature observed vapor film generation was reduced. This fact may imply the effect of transient heat conduction on the wetting temperature shifts to higher wall superheat temperature beyond liquid superheat limit temperature during quenching with jets or sprays.

Photographic study of hydrodynamics of drops of aqueous polymer solution impinging on hot solid

Spray-cooling methods are utilized in quench hardening and throughout the metal-forming industry. Aqueous solutions of water-soluble polymers are the typical quenchant. Although the impact behavior of the droplets has a great influence on the heat transfer between the hot metal surface and quenchant, the hydrodynamics of drops of aqueous polymer solutions are rarely studied. In this study, the collision of drops of an aqueous solution of polymer with a hot sapphire solid surface was investigated using strobe photography. A solution of 10wt% polyoxyethylene polyoxypropylene glycol, with an average molecular weight of approximately 20,000, was used as the test polymer quenchant. Experiments were conducted with drop diameters in the range 2.09–2.42mm, impact velocities from 0.83 to 1.25m/s, and surface temperatures of 200, 350, and 500°C. The effect of varying the temperature and the dimensionless Weber number on the collision behavior and drop evolution was investigated. Microscopic observations revealed that polymer residue remained on the surface when the temperature of the solid was equal to or less than 350°C. At 500°C, drops impacting on the surface at low Weber numbers deformed into a thin disc, recoiled, and finally rebounded off the solid in a spray of mist. No polymer residue remained. The residence times of the rebounding drops after impact increased with the Weber number. In addition, the measured residence times were slightly longer than some experimentally determined formulae for simple compound liquid drops predict.

Modelling of the interaction between a falling n-heptane droplet and hot solid surface

Accurate prediction of the interactions between evaporating liquid droplets and solids are critical for many industrially important processes. A model based on coupled Level Set-Volume of Fluid approach was developed to simulate the interaction of evaporating liquid droplets with hot solid surfaces. The model incorporates appropriate source terms in the multiphase calculations to account for the heat and mass transfer. Accurate and stable numerical procedure was developed and incorporated in open source solver OpenFOAM. A brief discussion on the model development along with several key issues that are associated with this process was presented.The resulting numerical model was validated through the experimental data of Chandra and Avedisian (Chandra, S., Avedisian, C.T., 1991. Proc. R. Soc. Lond., Ser. A 432, 13–41). Although some discrepancies were found between the numerical results and experimental data, the model was found to be capable of reproducing the reduced droplet spreading rate as the temperature of the surface is increased away from the saturation temperature. The decrease in rate of surface wetting results from the combined effects of surface tension, viscous forces and evaporation at the liquid-solid-vapour contact line. Further, the effects of increased pressure at the solid-liquid interface resulting from the rapid evaporation of the liquid, which in some cases can be quite severe such that the liquid gets lifted-off from the surface, were also captured, in good agreement with experimental observations. Finally, the effects of the solid temperature on the evaporation and heat transfer rates of the droplets were presented and analysed.

Leidenfrost Dynamics

This review discusses how drops can levitate on a cushion of vapor when brought in contact with a hot solid. This is the so-called Leidenfrost phenomenon, a dynamical and transient effect, as vapor is injected below the liquid and pressed by the drop weight. The absence of solid/liquid contact provides unique mobility for the levitating liquid, contrasting with the usual situations in which contact lines induce adhesion and enhanced friction: hence a frictionless motion, and the possibility of bouncing after impact. All these characteristics can be combined to create devices in which self-propulsion is obtained, using asymmetric textures on the hot solid surface.

Heat transfer enhancement of an electric air heating furnace by inserting silicon carbide ceramic foam panels

Silicon carbide ceramic foams are used to achieve high performances in many industrial heat exchange systems. This paper experimentally investigated the heat transfer characteristics of air flowing through high-temperature silicon carbide ceramic foams in an electric air heating furnace heated by resistance wires and silicon–carbon sticks. It was found that for an air inlet flow rate of 200m3/h, the air outlet temperature reached 981°C after about 2h when five silicon carbide ceramic foam panels were inserted inside the furnace, while it only reached about 650°C when no ceramic foam was inserted. The heat transfer enhancement was due to that the ceramic foams enlarged the heat transfer area between air and hot solid surfaces. The results also showed that the position of the ceramic foam in the furnace played an important role in its effect on the heat transfer. This study well validated that silicon carbide ceramic foam can be used as radiant energy absorbers and heat exchangers especially in volumetric solar receivers in concentrated solar power (CSP) plants.

Heat Transfer and Dynamics of Impinging Droplets on Hot Horizontal Surfaces

In fluidised bed olefin polymerisation reactors, a liquid monomer is added for enhancing heat removal (super-condensed mode). This broadens the operating window and can substantially increase the capacity of a given reactor hardware. Design and location of liquid injection nozzles play a key role in dictating the performance of condensed mode operations. An understanding of the fundamental characteristics of droplet impingement onto solid particles is critical for proper design. In this paper, we study the interactions between liquid droplets and hot solid surfaces across various boiling regimes. High-speed digital imaging is used to capture the droplet vaporisation process. Water droplets impacting on a solid wall with different Weber numbers are investigated. The effect of various parameters such as surface and initial droplet diameter, liquid velocity and surface tension is studied. Experimental data of dynamic drop impact on a flat hot surface are reported. A phenomenological model is developed to describe the droplet vaporisation phenomena in single phase and nucleate boiling regimes. The results from the phenomenological model are compared with the experimental data presented here and the experimental results reported in the literature. The study provides useful clues for understanding the real case when solid particles are much larger than the injected liquid droplets.

Three Dimensional Modeling of the Hydrodynamics of Oblique Droplet-Hot Wall Interactions During the Reflood Phase After a LOCA

During the reflood phase, following a loss-of-coolant-accident (LOCA), the main mechanism for the precursory cooling of the fuel is by convective heat transfer to the vapor, with the vapor being cooled by the evaporation of the entrained saturated droplets. However, it is believed that the droplets that reach the rod could have an effect on this cooling process. Despite the fact that those droplets do not actually wet the fuel rod due to the formation of a vapor film that sustains them and prevents them from touching the wall, the temperature drop caused by the impingement of such water droplets on a very hot solid surface (whose temperature is beyond the Leidenfrost temperature (1966, “A Track About Some Qualities of Common Water,” Int. J. Heat Mass Transfer, 9, pp. 1153–1166)) is of the order of 30–150°C (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liquid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The associated heat flux is of the order of 105–107 W/m2 and the heat extracted is in the range of 0.05 J over the time period of the interaction (a few ms) (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liquid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The hydrodynamic behavior of the droplets upon impingement is reported to affect the heat transfer effectiveness of the droplets. In the dispersed flow regime the droplets are more likely to impinge on the hot surface at a very small angle sliding along the solid wall, still without actually touching it, and remaining in a close proximity for a much larger time period. This changes the heat transfer behavior of the droplet. Here, we investigate numerically the hydrodynamics of the impingement of such droplets on a hot solid surface at various incident angles and various velocities of approach. For our simulations, we use a computational fluid dynamics (CFD), finite-volume computational algorithm (TransAT©). The level set method is used for the tracking of the interface. We present three-dimensional results of those impinging droplets. The validation of our simulation is done against experimental data already available in the literature. Then, we compare the findings of those results with previous correlations.

Dropwise cooling: Experimental tests by infrared thermography and numerical simulations

In this paper, infrared thermography is used to measure the transient contact temperature between impinging droplets and hot solid surfaces. Droplets are released onto the heated solid surface of a barium fluoride (BaF 2) disk, which has a high transmittance (about 90%) in the 8–12 μm range (typical of longwave infrared cameras). The interface temperature is measured from below, through the solid material, by infrared thermography. Since the solid is IR-transparent, a black coating layer is used to allow radiative heating of the surface and provide a method to measure the liquid–solid interface temperature. A numerical code is then presented, which simulates the evaporation of water droplets on hot solid surfaces. At the present stage of development, single-phase evaporation is addressed. The three-dimensional energy diffusion equation, discretized using the finite volume method, is employed to model the transient within both the droplets and the solid substrate. The numerical results are validated by comparison with the experimental data.

Comparison of measured and modelled droplet–hot wall interactions

In this study CFD simulations of both sessile droplets resting upon a vapour cushion and droplets bouncing off a hot solid surface are presented. As a droplet approaches a hot surface the vapour layer formed by evaporation from the droplet acts like a cushion and can prevent contact between the liquid and the hot surface. Rather than hitting and wetting the surface, the droplet can rebound from the vapour film. For the tracking of the interface between the two fluids a one-fluid Level Set method is used, embodied in the TransAT © finite-volume two-phase flow computational code. Inter alia, this incorporates a full Navier–Stokes solution in the region of the thin film. The method is used to analyse the experiments conducted by Wachters et al. [L.H.J. Wachters, H. Bonne, H.J. Van Nouhuis, The heat transfer from a horizontal plate to sessile water drops in the spheroidal state, Chemical Engineering Science 21 (1966) 923–936] and Biance et al. [A.-L. Biance, F. Checy, C. Clanet, G. Lagubeau, D. Quere, On the elasticity of an inertial liquid shock, Journal of Fluid Mechanics 554 (2006) 47–66]. Good agreement with the experimental observations is obtained.

Research on stability of liquid film on hot solid surface impinged by small droplets

A heat and mass transfer model was proposed for the thin liquid film on the hot solid surface cooled by the impinging small droplets, with consideration of the effect of the droplet impact, surface tension, thermocapillary, evaporation/condensation, and van der Waals attraction. The nondimensional equation for predicting the evolution of the interface of the thin liquid film was derived in the presented model with the relevant boundary conditions. The stability of the thin liquid film impacted by cool small droplets is discussed.

Multi-scale simulation of oblique collisions of a droplet on a surface in the Leidenfrost regime

This work illustrates the phenomena of oblique impact of a water droplet on a hot solid surface in the Leidenfrost regime using a multi-scale model. The flow and heat transfer behavior on the droplet scale as described by the macroscopic model is solved using the finite volume method, together with the level set and the immersed boundary methods which quantify the variations of the gas–liquid and fluid–solid interfaces. A micro-scale vapor layer model is used to account for the resistance effect of the vapor layer generated by the film-boiling evaporation. These two models are coupled in each time step, and solved concurrently. Based on this multi-scale model, the effect of the impact velocity is investigated numerically. The droplet shape and impact parameters such as momentum loss and contact time calculated from the present model are compared with the experimental results obtained from the literature. A good agreement is seen between the simulated and the experimental results.

Experimental tests of dropwise cooling on infrared-transparent media

The present work is aimed at analyzing the cooling of hot solid surfaces induced by liquid droplets. In particular, the study is focused on the non-intrusive measurement of the transient contact temperature between impinging droplets and hot solid surfaces. An experimental apparatus was built and set up in order to approach the non-trivial problem of the measurement of a solid–liquid interface temperature after droplet impingement. The solid–liquid interface temperature was monitored from below through a transparent-to-infrared material. That material had been coated with a very thin layer of high-emissivity, opaque paint on its upper side, so that it could effectively respond to the infrared camera located below. The paper reports the main results that have been collected to date, with particular regard to the approaches used to coat the transparent solid. Some considerations are also expressed about the effectiveness of the proposed method and about the improvements that are currently being implemented to get new and more accurate interface temperature measurements.

액적 충돌에 동반된 열전달에 관한 수치적 연구

Numerical analysis of the heat transfer associated With droplet Impact on a hot solid surface is performed by solving the equations governing conservation of mass, momentum and energy In the liquid and gas phases The deformed droplet shape is tracked by a level set method winch is modified to achieve volume conservation and to include the effect of contact angle at the wall. The numerical method is validated through the calculations for the cases reported in the literature. Based on the numerical results, the heat transfer rate is found to depend strongly on the droplet spread radius. Decreased advancing/receding contact angles enlarge the splat radius and m turn enhance the wall heat flux The effect of impact velocity on the droplet spread is reduced as the droplet Size decreases. Also, droplet atomization is observed to significantly enhance the heat transfer rate and the effect IS pronounced for a smaller Size of droplet. An existing model equation to predict the maximum spread radius is improved for application to a micro dropletbr/

Experimental Study on Cooling a Hot Solid Surface with Water Mist

In this paper, a series of experiments are performed to investigate the hot solid surface cooling with water mist under different conditions, such as different initial surface temperatures and mist characteristics (droplet size, droplet velocity, etc.), while these parameters are simultaneously varied. A 150 150 10mm 3chrome-plated stainless steel plate is heated from below by a 1500-W electric cooker continuously. A downward pressure nozzle is positioned on a square steel plate, 1000mm above the surface to inject water mist, the 0.5mm K-type thermocouples are jointed on the surface to obtain its temperature, and a TVS-2000ST Thermography is used to visualize the transient thermal behavior of a water mist and obtain the temperature distribution of the surface. The characteristics of the water mist are obtained by a modified PIVS (Particle Image Velocimetry and Sizing) technique previously. Based on the experiments, the effects of mist parameters and the initial surface temperature on droplet evaporative cooling are analyzed. Further discussion is carried out by considering the relationship between these parameters and the droplet Weber number, the results show that an inverse relationship between the cooling efficiency and mist droplet Weber number existed in this study.

Evaporation of Water Droplets Placed on a Heated Horizontal Surface

A numerical analysis of the evaporation process of small water droplets with diameters of 1 mm or less that are gently deposited on a hot isothermal solid surface has been performed. This study considers the internal fluid motion that occurs as a result of the thermocapillary convection in the droplet and it determines the effect of fluid motion on the heat transfer between the drop and the solid surface. This study is particularly relevant because the internal fluid motion has not been considered in previous numerical and analytical models presented in the literature. To assess the effects of internal fluid motion, the model results are compared to numerical results provided by a heat conduction model that neglects the fluid motion. The Navier-Stokes and Thermal Energy equations are solved using the Artificial Compressibility Method with Dual Time Stepping. Boundary-fitted grids are used to track the changes in the droplet surface shape during the evaporation process. The numerical simulations have demonstrated that the internal fluid motion provides vastly different temperature distributions in the drop compared to the results from the heat conduction model that neglects fluid motion. The evolution of the droplet geometry was simulated from an initial spherical-shaped cap until the contact angle was close to the receding contact angle.

Gas spreading on a heated wall wetted by liquid.

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.