Droplet Heat Transfer Research Articles

Droplet on oil impregnated surface: Temperature and velocity fields

The water droplet dynamics on silicon oil impregnated polydimethylsiloxane (PDMS) replicated and inclined surface is considered and the droplet sliding velocity incorporating force and energy balance is formulated. Since silicon oil film is kept at 308 K and water droplet temperature is initially at 300 K, the droplet heat transfer due to temperature disturbance is examined during droplet sliding. Velocity and temperature fields are computed using the experimental conditions. The droplet sliding velocity resulted from the analytical formulation is compared with the high-speed camera data. In addition, the predictions of the droplet surface temperature are validated with the thermal camera data. The findings reveal that the droplet sliding velocity obtained from the analytical formulation is in agreement with the high-speed camera data. The predictions of droplet temperature also agree well with the thermal camera data. Droplet heat transfer results in two contour rotating circulating structures in the droplet, which contribute to convection heat transfer in the droplet fluid. Silicon oil cloaking on water droplet surface alters the heating rates and flow structure in the droplet. As the distance along the silicon oil surface increases, a unique circulation structure is developed inside the droplet, which enhances the Nusselt and the Bond numbers. Formation of a single circulation is attributed to increased silicon oil ridge height in the frontal region of the droplet.

Numerical investigation of droplet-droplet collisions in a water and milk spray with coupled heat and mass transfer

Large scale simulation models can aid in improving the design of spray dryers. In this work an Eulerian-Lagrangian model with coupled gas phase and droplet heat and mass transfer balances is used to study airflow dynamics, temperature and humidity profiles at different positions in the spray. The turbulent gas flow is solved using large eddy simulation (LES). A turbulent dispersion model accounts for the stochastic subgrid fluid velocity fluctuations along the droplet trajectory. The dispersed phase is treated with Lagrangian transport of droplets, and collisions between droplets which are detected with a stochastic Direct Simulation Monte Carlo (DSMC) method. The outcome of a binary collision is described by collision boundary models for water and milk concentrates. The drying of droplets is modeled by the reaction engineering approach (REA). The effect of the inlet air conditions and of droplet viscosity on the temperature and humidity distributions are analyzed. Most of the heat and mass transfer occurs in the first 10-20 cm from the nozzle where the slip velocities and temperature and humidity driving forces are higher. The droplets size increases, both in the axial and radial direction, because of the dominance of coalescence over separation in the droplet spray studied here. Because the spray domain considered in this work is relatively small, the droplet residence time is small, and consequently the amount of evaporation is still low. The droplet size distributions of milk concentrates are affected by the predominance of coalescence over separation events. The coalescence dominated regime increases when the droplet viscosity is higher.

High vapour pressure nanofuel droplet combustion and heat transfer: Insights into droplet burning time scale, secondary atomisation and coupling of droplet deformations and heat release

Combustion characteristics of ethanol-water (EW) droplets laden with ceria nanoparticles are investigated. The present experimental study focuses on three facets of droplet combustion (i) burning time scale of droplets with and without NPs, (ii) pathways of secondary atomisation due to interface deformations and (iii) coupling of droplet shape deformations and flame heat release. A theoretical vaporisation timescale is advocated which considers natural convection-based evaporation, mass loss due to daughter droplet ejections, and flow through porous media. Droplets seeded with ceria nanoparticles, exhibit arrested surface undulations although internal ebullition is discernibly enhanced as compared to EW droplets without NPs. Deformations and formation of surface craters in EW droplets are traced to the imbalance between local vapour recoil (due to rapid ethanol vaporisation) and surface tension. Such craters collapse and form high-speed ligaments which eventually break at the tip through Rayleigh–Plateau mechanism. This pathway of secondary atomisation of EW droplets has been elucidated using a modified local weber number. On the contrary, for nanofuels, bubble rupture is the mechanism behind the surface crater formation. Proper orthogonal decomposition (POD) technique is utilised for investigating the droplet shape and flame heat release coupling. EW droplet shape and HR are found to be a synced system with a phase lag arising from the flame response timescale. However, a weak coupling is detected for nanofuel droplets.

Electrowetting-based enhancement of droplet growth dynamics and heat transfer during humid air condensation

Condensation heat transfer can be significantly enhanced by condensing vapor as droplets (instead of a film), which rapidly roll-off. This work studies the use of electrowetting to enhance coalescence, growth and roll-off dynamics of condensed droplets, thereby enhancing the condensation rate and associated heat transfer. This enhancement depends on the nature of fluid motion (translation of droplets, oscillations of the three phase line), which in turn depends on the magnitude and frequency of the applied electrical actuation waveform. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation under the influence of an electric field. It is seen that droplet growth is enhanced as the voltage and frequency of AC electric fields is increased, with AC electric fields seen to be more effective than DC electric fields. Roll-off dynamics also depends on the frequency of the AC field. Overall, electric fields alter the droplet size distribution and move the condensate to more favorable states for removal from the surface. The condensation rate depends on the roll-off diameter of the droplet, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. An analytical heat transfer model is utilized to relate the measured condensation rate with condensation heat transfer. It is noted that this study deals with condensation of humid air, and not pure steam. Overall, this study reports more than 30% enhancement in condensation rate resulting from the applied electric field, which highlights the attractiveness of electrowetting for condensation heat transfer enhancement.

A 1-D model of spraying performance for wet flue gas desulfurization scrubber based on predicted slurry temperature

To predict the slurry temperature in wet flue gas desulfurization system, a theoretical model was put forward and the calculation method was set up, the correlation between the parameters of inlet flue gas and the slurry temperature was first found. Based on the predicted slurry temperature, a 1-D coupled model of droplets motion, heat and mass transfer and pressureloss of liquid-gas two phases was established, and the numerical solutions were acquired by Runge-Kutta method, meanwhile, field tests were made to validate its reliability. Additionally, the distribution characteristics of droplet velocity, temperature and relative humidity of flue gas, and pressure loss over the height of spraying zone were discussed. The effect of drop diameter and other factors on the performance of spraying process were analyzed. The results show that the slurry temperature and pressure loss agree well with the operation values with errors of less than 5% and 7%. Outlet flue gas is near saturation state under discussed range of parameters. The drop diameter is the main factor that affects the performance of wet flue gas desulfurization system.

Thermal and Flow Behavior of a Droplet Fluid Wetted by Parallel Hydrophobic Walls

Thermal analysis of a locally heated droplet located in two parallel plates with hydrophobic surfaces is considered. The spacing between the parallel plates is altered to modify the three-phase contact line of the droplet on both hydrophobic surfaces. The thermal state of the walls is altered via changing temperature difference between the parallel plates while initiating the droplet heat transfer. In this case, the bottom plate temperature is increased, while the top plate temperature is kept constant. The thermal field developed inside the droplet is obtained numerically incorporating the conditions used in the experiments. The influence of the droplet volume on the Nusselt and the Bond numbers is examined for the cases of droplets formed (i) on a hydrophobic plate and (ii) in between two parallel plates with hydrophobic wetting state. It is found that the droplet wetted by the parallelly located hydrophobic plates alters the circulation cell structure formed inside the droplet. Temperature field extends further inside the droplet as the wetted diameter is extended on the hydrophobic surfaces. The Nusselt number attains larger values for the two-plate case than the free droplet on a single plate case. The Bond number remains less than unity for the free and two-plate wetted droplets.

Experimental investigation on ice accretion on a rotating aero-engine spinner with hydrophobic coating

Ice accretion on aero-engine inlets is a serious threat to the inflight safety. As an important part of the engine inlet components, the rotating spinner has special icing phenomena compared with the stationary ones. An experimental study is conducted to investigate the ice accretion on a rotating aero-engine spinner in the icing wind tunnel. A full-scale engine spinner model is used in the experiments operated under different conditions. The hydrophobic nanoparticle-polymer composite coating is applied to cover on the aluminum-based spinner and then to examine its anti-icing performance. The dynamic icing processes and the final ice shapes on the rotating spinner surface are captured. The experimental results reveal that the ice growth and ice shape are basically determined by the airflow temperature and slightly affected by the rotating speed when the airflow velocity and icing cloud parameters are constant. The ice fracture and shedding are greatly dependent on the rotating speed when the icicle length is large enough. The freezing delay on the hydrophobic coating is inconspicuous and there is even no effect on the ice growth. However, the ice shedding on the coating happens, especially in the mixed icing condition with a higher rotating speed, demonstrating its icephobicity. Meanwhile, the dynamic icing processes and the effects of the airflow velocity and rotating speed on the ice growth are analyzed based on the mechanism of the supercooled droplet impingement characteristics, heat and mass transfer, force analysis on the rotating surface. The surface wettability is introduced to evaluate the freezing delay and icephobicity on the hydrophobic coating.

Effect of droplet characteristics on heat transfer of mist/air cooling in a pin-finned channel

Effects of droplet characteristics of mist/air cooling on heat transfer for three pin-fin structures are investigated. The round-tip pin-fin structure is newly proposed with partial detachment from one endwall with a round-shaped tip structure. A flat-tip pin-fin with partial detachment and a traditional pin-fin with full attachment serve as references. Reynolds-averaged Navier-Stokes equations and the shear-stress-transport turbulence model are applied. Influences of initial mist temperature, initial mist diameter and initial mist velocity are analyzed in the Reynolds number range 15,000 to 50,000. The round-tip pin-finned channel has highest heat transfer coefficient and lowest pressure loss among the structures. Heat transfer enhancement increases first gradually and then decreases sharply with increasing initial mist diameter but an optimal diameter exists for the highest Nusselt numbers. Nusselt number decreases monotonically with increasing initial mist temperature. Droplet movement and heat transfer are nearly independent of initial mist velocity.

Macrotextures-induced jumping relay of condensate droplets

Self-propelled droplet jumping plays a crucial role in numerous applications such as condensation heat transfer, self-cleaning, and water harvesting. Compared to individual droplet jumping, the coalescence-induced droplet jumping in a domino manner has attracted more attention due to its potential for the high performance of droplet mobility and heat transfer. However, there is an apparent gap in the current literature regarding the demonstration of the advantage of this preferred droplet transport in a well-controlled way. In this study, we report the attainment of droplet jumping relay by designing a nanosheet-covered superhydrophobic surface with V-shaped macrogrooves (Groove-SHS). We find that the macrogroove arrays can significantly modify the droplet dynamics in the presence of a non-condensable gas (NCG) by coupling rapid droplet growth and efficient droplet removal by jumping relay. The condensate droplets formed through the NCG diffusion layer on top of the cones and between the grooves serve as more efficient conduits for heat transfer. The droplets with higher mobility formed on the bottom of the grooves can undergo a series of coalescence which results in the preferred droplet jumping relay. Such a droplet jumping relay can induce a considerable vibration for triggering the removal of droplets on top of the cones. The condensation performance of the Groove-SHS is increased by 60% compared to that of the flat superhydrophobic surface due to the synergistic effect of rapid droplet growth and efficient droplet removal facilitated by the integration of the droplet jumping relay. The mechanisms revealed in this work pave the way for dropwise condensation enhancement.

Simulation of Heat and Mass Transfer of Droplets in Drying an Overheated Liquid Under Conditions of Combined Energy Effect

The results of simulation of heat and mass transfer of droplets in drying an overheated liquid under conditions of convective and infrared energy effect are presented. The influence of various parameters, in particular, of the liquid overheating temperature, infrared radiation flux density, and of the carrier phase velocity oscillations on the drop evaporation kinetics has been analyzed. The possibilities of reducing the duration of droplet drying by means of energy effect are shown.

Heat-Transfer Enhancement Incorporating Fin-Like Structures Inside Droplet on Hydrophobic Surface

Enhancement of droplet heat transfer on a hydrophobic surface is examined via introducing the fin-like structures inside the droplet without altering the wetting state of the surface. A solution crystallization of polycarbonate surface is carried out and the functionalized silica particles are deposited onto the crystallized surface to create the hydrophobic surface characteristics. The ferrous particles (Fe2O3) are locally spread onto the hydrophobic surface and, later, manipulated by an external magneto-static force generating various configurations of fin-like structures inside the droplet. The droplet with fin-like structures is heated from the hydrophobic surface through introducing a constant temperature heat source. Flow and temperature fields inside the droplet are simulated in line with the experimental conditions. It is found that changing the configuration of the fin-like structures in the droplet modifies significantly the flow and temperature fields inside the droplet. The Bond number remains less than unity for all configurations of the fin-like structures while demonstrating the importance of the Marangoni current over the buoyancy current in the flow field. The presence of the fin-like structures lowers the difference between the fluid bulk and the minimum temperatures inside the droplet and improves considerably the heat transfer rates and the Nusselt number.

Investigation of mass and heat transfer transitional processes of water droplets in wet gas flow in the framework of energy recovery technologies for biofuel combustion and flue gas removal

In this paper, complex processes of water droplet heat and mass transfer are analyzed in a cycle of condensing, transitional evaporation and equilibrium evaporation regimes during phase change which occurs on a droplet's surface. The dynamics of a heated droplet's surface temperature is directly related to the change in the regimes. The definition of the dynamics is based on a numerical iterative scheme which depends on the balance of a droplet surface's heat flux. In this scheme, the energy of phase change and external heat transfer are combined as well as the internal heat transfer occurring in droplets. The numerical investigation results of the water droplets' phase change were used as a basis while defining the inputs provided by the droplet slipping and the radiation absorbed in the flue gas within the interactions between the processes of complex transitional transfers. For this investigation, the conditions have been set to be typical for heat utilization technologies and biofuel furnaces used in flue gas removal.

Bulk Dynamic Spray Freeze-Drying Part 2: Model-Based Parametric Study for Spray-Freezing Process Characterization

Spray freeze-drying is an evolving technology that combines the benefits of spray-drying and conventional lyophilization techniques to produce drug substance and drug product as free-flowing powders. The high surface-to-volume ratio associated to the submillimeter spray-frozen particles contributes to shorter drying and reconstitution times. The formation of frozen particles is the most critical part of this dehydration technique because it defines the properties of final product. Based on a previously proposed and validated model, the current goal is to understand the role of various controllable parameters in the spray-freezing process. More specifically, given a set of spraying conditions, the model is used to predict the minimum distance required to cool and freeze the droplets below a temperature that prevents coalescence and product agglomeration. A parametric study is carried out to map the operational limit conditions of the actual spray-freezing column apparatus under consideration. For the spray freeze-drying conditions of interest, model simulations indicate that convection contributes to at least 80% of the total droplet heat transfer and, consequently, that freezing column gas temperature and droplet diameter are the most important process parameters affecting the freezing distance.

The water droplets dynamics and phase transformations in biofuel flue gases flow

This work presents the phase transformations and movement dynamics in a case of complex transfer processes self-interaction on sprayed water droplets in biofuel furnace flue gases flow. The droplet phase transformation cycle is defined according to a numerical iterative scheme which is created by heat flow balance condition onto the droplet surface, where combined solutions for the tasks of external heat transfer and phase change as well as internal heat transfer in a droplet, are designed. The flue gases velocity, temperature and droplet dispersity are named as defining factors of droplets trace to full evaporation in flue gases at biofuel furnace. Relevant secondary factors are recognized as sprayed water temperature, initial velocity provided for droplets, humidity of flue gases and specifics of droplets heat transfer. It is based on, that boundary conditions, which ensures the fastest and the slowest water droplets evaporation, do not define the marginal trace of droplets till evaporation. In biofuel flue gases flow at droplet phase transformation regimes cycle two, practically equivalent periods are distinguished. In the first period significant is droplet growth due to influence of condensation, warming water expansion and droplets slipping decrease in flue gases. In the second period the trace of droplets is already defined by flue gases flow velocity. The combined droplet heating process is more significant for larger droplets, while for smaller than 70 µm diameter droplets impact of thermal radiation is neglect.

Direct numerical simulations of droplet condensation

We present in this work well-resolved and accurate Direct Numerical Simulations (DNS) of droplet condensation. Despite the great scientific and industrial interest, to this day, there is not an extensive knowledge of the different processes involved in droplet condensation. Consequently, DNS should be considered as a promising tool to investigate on this phenomenon. A preliminary validation of our simulations is carried out by direct comparison with the quasi-static theory of the droplet condensation in an infinite vapour medium. Next, more complex configurations have been considered: the condensation of a moving droplet in a subcooled vapour flow and the condensation of a hemispherical droplet deposed on an isothermal flat surface. The latter represents a first step towards the understanding of the more demanding DropWise Condensation. In both configurations, the effects of the Jakob number, Ja, have been thoroughly analysed to understand how the condensation impacts on the droplet heat flux and dynamics. This has led to the definition of a particular condensation regime for the lower Ja values, hereinafter called low condensation rate regime, where the droplet heat transfer is independent of the Ja. By increasing the Ja, instead, the effects due to condensation start to grow exponentially. This regime is referred as the high condensation rate regime in this paper. Finally, some general trends for correlations on the Nusselt number and drag coefficient accounting for condensation are proposed in this study.

Numerical analysis of droplet impact and heat transfer on an inclined wet surface

A numerical model based on the coupled level set and volume of fraction (CLSVOF) method is developed to investigate the dynamics and heat transfer of a droplet impacting on an inclined wet surface. Numerical results show that the droplet shows spreading, edge jets and splashing upon impact with high velocities. The front spreading factor and liquid velocity are larger than the back-spreading ones. As the impact velocity increases, both the front and back spreading factors and velocities increase. Significant air entrapment is observed when the droplet approaches the liquid film on the solid surface. The pressure difference between the trapped air and the liquids leads to the formation of an air film underneath the droplet. The entrapped air also shows a dynamic process. It becomes a small bubble at the initial stage of droplet impact and reduces heat transfer from the liquid droplet to the surface. This effect diminishes when the trapped air escapes the liquids. The average wall heat flux is found to be closely correlated to the impact velocity, but that correlation weakens as the impact velocity increases. Results from this study provide better understandings of the fluid dynamics and heat transfer during droplet oblique impact on inclined wet surfaces.

Convective dropwise condensation out of humid air inside a horizontal channel – Experimental investigation of the condensate heat transfer resistance

A new facility feasible for the investigation of dropwise condensation at a vertical polymer surface (9 × 6 mm, L × H) is described in detail. The droplets are liquefied out of humid air, which flows through a rectangular channel (12 × 32 mm, W × H) and passes the sub-cooled surface of a sensor. The latter one is flush mounted into one of the vertical channel walls beyond a hydrodynamic entry length of 550 mm determining perpendicular heat fluxes by measuring temperature differences. Furthermore the droplet covered surface is observed through the opposite channel wall by an infrared camera. The variable experimental parameters include temperature, water content and flow velocity of the humid air, as well as the surface temperature of the sensor.The scope of the experimental study is to investigate the mean heat transfer resistance of the condensate droplets which form on the substrate surface. The procedure is based on the determination of heat fluxes and the evaluation of the temperature differences between the substrate surface and the liquid/gas interface by means of thermography, both of them carried out synchronously. The experimental droplet heat transfer coefficient, hd, is evaluated as an average value and it considers both heat conduction through the droplets and the augmentation of heat transfer due to inner convection. The parameter variation covers Reynolds numbers between 7900 and 21400, relative humidities between 56% and 95% and air temperatures between 30 °C and 46 °C for a broad range of cooling temperatures. The latter one influences the droplet surface temperature and together with the water content of the bulk, it determines the driving force for mass transfer. hd was found to increase with the Reynolds number and the driving force for mass transfer. However, increasing the latter one beyond a certain value causes massive accumulation of noncondensable gases at the liquid/gas interface inhibiting the vapour mass flux and leading to invariant droplet heat transfer coefficients. The experimental data is correlated providing the basis for the experimental investigation of the heat transfer at the liquid/gas interface which will be done in future.

Ferro-Liquid Droplet Heat Transfer on Water Surface: Effect of Droplet Volume on Droplet Fluidity

Heat transfer analysis of a transformer oil–based, a sessile ferro-liquid droplet formed on a stagnant water surface is carried out. Two counterrotating circulation cells are formed inside water and the droplet. Increasing the droplet volume enhances the maximum velocity and circulation cell sizes inside the droplet. The convection current and heat diffusion from the droplet bottom toward the droplet tip enhances temperature increase in the central region of the droplet. The Bond number remains greater than unity for all droplet volumes, indicating the significant contribution of the buoyancy current to the flow field. The Nusselt number increases with increasing droplet volume.

Droplet size distributions in dropwise condensation heat transfer: Consideration of droplet overlapping and multiple re-nucleation

Evolution and distribution of condensate droplets are the most important information in dropwise condensation. For surfaces with larger contact angles, droplet interactions and distributions become more complicated due to their three dimensional profiles. In this study, the effect of droplet overlapping and multiple re-nucleation on droplet size distribution and heat transfer are analyzed quantificationally by numerical simulations. Experimental droplet images obtained by environmental scanning electronic microscope are also presented to support the analyses. The simulated droplet size distributions show that droplet overlapping and multiple re-nucleation can improve the spatial distribution of small droplets for larger contact angle surfaces, and the difference in droplet size distributions decreases as droplet radius is increased. The distributions of larger droplets with radii larger than the critical coalescence size are also affected by the overlapping and multiple re-nucleation effect. The difference between droplet size distributions finally diminish for different contact angles when droplet radius is increased to dozens of microns and the effect of overlapping and multiple re-nucleation become less important in that size range. As contact angle is increasing, surface coverage decreases, while the number density of small droplets is greatly increased, and the overall effect leads to an improved heat transfer performance. The present simulations reveal the effect of droplet overlapping and multiple re-nucleation quantificationally from the perspective of three dimensional droplet profiles, which provide insight into the better understanding of droplet size distributions and extend the dropwise condensation theory.

The influence of key factors on the heat and mass transfer of a sessile droplet

Heat transfer, temperature and velocity fields of droplets of water and aqueous salt solutions have been studied. The initial concentration of salt solutions C01 = 30% in accordance with advanced technological applications. It is shown that the heat flux q for a water droplet is several times higher than for salt solutions. During the droplet evaporation, this difference increases, since q for aqueous salt solutions is many times reduced. While maintaining the wall temperature under the drop quasi-constant (Tw = 83 °C), the heat transfer coefficient α (for the time t < 150 s) for a drop of water 1.5–2 times exceeds α for the salt solution, and is 7 times higher for water than for the salt solution at t = 280 s. Using PIV measurements the liquid velocity fields have been obtained; and for the first time it has been shown that only for very small times and at high ΔT, the important role is played by thermo-gravitational convection in the droplet (the Rayleigh number Ra). But already for t > 10 s, the thermal and solutal Marangoni flows have a greater influence on the flow in a drop of the salt solution.