Evaporation and condensing augmentation of water droplets in flue gas

The unsteady heat and mass transfer of sprayed water in the flue gas is modelled according to the iterative method of numerical research. The complex “droplet problem” covers the analysis of combined energy transfer in a semitransparent droplet, also combined heating and evaporation of the droplet. The surface temperature of the evaporating droplet is determined, at which the balance of energy fluxes taken to the surface and taken from the surface is reached. The thermal state mode of an evaporating droplet depends on the way of droplet heating as well. The change of thermal state and phase transformations parameters of water droplets warming in flue gas is analysed in the universal time scale. The initial evaluation of heat energy accumulated in exhaust flue gas utilization by water injection is presented.     

The peculiarities of hot liquid droplets heating and evaporation

The change of the thermal state of a sprayed liquid droplet is calculated using the method of combined analytical and numerical research, which requires the balance of energy flows, incoming to the droplet and outgoing from it. The method evaluates unsteadiness of heat and mass transfer processes and interaction, which occurs under the influence of the Stefan’s hydrodynamic flow, radiant flow absorbed in semitransparent liquid and the Knudsen layer, which surrounds the droplets. The expedience of the thermal state’s evaluation of dispersed liquid is verified using the parameter, expressed by the ratio of liquid’s initial temperature on equilibrium evaporation temperature of droplets. As the above-mentioned parameter is less than 1, liquid is offered to be called “cold”; “warm”, as the parameter equals 1 and “hot”, as it exceeds 1. In each case the peculiarities of the thermal state change of sprayed liquid droplets are individual during an unsteady phase transformation mode. The characteristic curves, representing the change of transfer parameters, are determined for conductively heated droplets and when the Knudsen layer’s influence is neglected. These curves join together variation of the parameters of the thermal state change and phase transformation for droplets of infinitesimal set of diameters, but with the same initial liquid temperature, as the droplets evaporate in gas with constant parameters. Deviations from the characteristic curves allow evaluating the influence of more complicated boundary conditions on the interaction of transfer processes.     

Interaction of transfer processes during unsteady evaporation of water droplets

The change of water droplets state is modelled numerically under various heat and mass transfer conditions during their unsteady evaporation. The modelling is performed using the method of combined analytic–numeric research of heat and mass transfer in a two-phase “droplets–gas” flow. The algorithm of an iterative research is constructed for the analytically obtained system of integral equations. Regularities of heat transfer process interaction are examined. The dependence of the droplet state change on its heating manner is determined. Unsteadiness and interaction of transfer processes, as well as selectivity of radiant absorption in water droplets are evaluated. It is indicated that cognition of the droplet state change regularities in the case of conductive heating is very important in determination of two-phase flow and in construction of an engineering research method.     

Analysis of evaporating mist flow for enhanced convective heat transfer

Enhancement of forced convective heat transport through the use of evaporating mist flow is investigated analytically and by numerical simulation. A two-phase mist, consisting of finely dispersed water droplets in an airstream, is introduced at the inlet of a longitudinally-finned heat sink. The latent heat absorbed by the evaporating droplets significantly reduces the sensible heating of the air inside the heat sink which translates into higher heat-dissipation capacities. The flow and heat transfer characteristics of mist flows are studied through a detailed numerical analysis of the mass, momentum and energy transport equations for the mist droplets and the airstream, which are treated as two separate phases. The coupling between the two phases is modeled through interaction terms in the transport equations. The effects of inlet mist droplet size and concentration on the thermal performance of the heat sink are analyzed parametrically. The results provide insight into the complex transport processes associated with mist flows. The simulations indicate that significantly higher heat transfer coefficients are obtained with mist flows as compared to air flows, highlighting the potential for the use of mist flows for enhanced thermal management applications.     

Energetic budget on an evaporating monodisperse droplet stream using combined optical methods: Evaluation of the convective heat transfer

Aerothermal properties in a fuel spray is a central problem in the field of the design of the combustion chambers of automotive engines, turbojets or rocket engines. Heat and mass transfer models are necessary in the predictive calculation schemes used by the motorists. Reliable experimental data must be obtained for both the validation and development of new physical models linked to heat transfer and evaporation in sprays, where aerodynamic interactions have a key role. This paper proposes an experimental study of the energetic budget of a monodisperse ethanol droplet stream, injected in the thermal boundary layer of a vertical heated plate. The droplet size reduction is measured using a light scattering technique (interferential method) in order to characterize the evaporation, as the droplet mean temperature is monitored using the two colors laser-induced fluorescence technique. The convection heat transfer coefficient and the Nusselt number are inferred from the overall energetic budget, as a function of the inter-droplet distance, characterizing the interaction regime. The results are compared to physical models combined with numerical simulations available in the literature, for moving, evaporating isolated droplets and for three droplets arrangement in linear stream.     

Numerical analysis of the interactions between evaporating droplets in a monodisperse stream

A numerical simulation of evaporation in a monodisperse droplet stream is proposed, taking into account the transient state of the evaporation, and the non-uniform mass and heat transfer coefficients on the droplet surface. These investigations emphasize the strong interaction effects between closely spaced droplets in a dense spray, reducing significantly the transfer coefficients. Moreover, the Marangoni force becomes more significant than the viscous force, driving the internal motion of the droplet and affecting the temperature fields. Otherwise, a better understanding of the evaporation phenomenon around closely spaced droplets will help to refine the existing models used in dense sprays.     

蒸发液滴内部非稳态流动的数值计算

在气液两相流VOF(volume of fluid,VOF)模型的基础上耦合CSF(continuum surface force,CSF)表面张力模型,建立了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中内部非稳态流动模型,对液滴蒸发过程中内部非稳态流动进行了研究。基于相变理论,采用用户自定义函数将流体相变模型加入非稳态流动模型中进行耦合计算,获得了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中的内部流动及变化过程。液滴蒸发过程中非稳态内部流动由液滴表面的温度梯度引发,Marangoni流动在液滴内部形成的时间非常短,流体从液滴表面高温区域流向低温区域。计算结果表明:高温平板上随着液滴蒸发的进行,液滴内部一直保持两个对称的涡流,Marangoni流动比较稳定;高温空气环境中随着液滴蒸发的进行,液滴内部四个涡流逐渐转变成两个对称的涡流;液滴内部温度分布因Marangoni流动加强传热而变得均匀,同时由于温度分布变得均匀,Marangoni流动被削弱。     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要.为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5％)液滴的蒸发特性.结果 表明:加入少量正丁醇溶液并不影...     

Heat and mass transfer of combusting monodisperse droplets in a linear stream

Heat and mass transfer phenomena in fuel sprays is a key issue in the field of the design of the combustion chambers where the fuel is injected on a liquid form. The development and validation of new physical models related to heat transfer and evaporation in sprays requires reliable experimental data. This paper reports on an experimental study of the energy budget, i.e. internal flux, evaporation flux and convective heat flux for monodisperse combusting droplets in linear stream. The evaporation flux is characterized by the measurement of the droplet size reduction by the phase Doppler technique, and the droplet mean temperature, required for the internal and convective heat flux evaluation, is determined by two-color, laser-induced fluorescence. The Nusselt and Sherwood numbers are evaluated from the heat and mass fluxes estimation, as a function of the inter-droplet distance. The results are compared to physical models available in the literature, for moving, evaporating and isolated droplets. A correction factor of the isolated droplet model, taking into account drop-drop interaction on the Sherwood and Nusselt numbers, is proposed.     

Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers

The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement.     

固着液滴蒸发研究进展及展望

固着液滴是指附着于壁面上的液滴,其蒸发行为及传热传质特性是喷雾冷却、喷墨打印等相变传热传质领域的基础问题之一。文中重点针对固着液滴蒸发过程所涉及的自身形态演变规律、气液固三相耦合传热/传质/流动特性进行了综述。结合毫微尺度固着液滴基本蒸发模式、热质传递形式、气液两相流动特征和界面输运行为,分析了液滴性质、壁面条件、气相环境条件等关键因素对固着液滴蒸发过程的内在作用机制和影响规律,提出了微纳尺度固着液滴（群）热质传递过程与机理的相关研究展望。     

Investigation of heat and mass transfer between the two phases of an evaporating droplet stream using laser-induced fluorescence techniques: Comparison with modeling

Heat and mass exchanges between the two phases of a spray is a key point for the understanding of physical phenomena occurring during spray evaporation in a combustion chamber. Development and validation of physical models and computational tools dealing with spray evaporation requires experimental databases on both liquid and gas phases. This paper reports an experimental study of evaporating acetone droplets streaming linearly at moderate ambient temperatures up to 75 °C. Two-color laser-induced fluorescence is used to characterize the temporal evolution of droplet mean temperature. Simultaneously, fuel vapor distribution in the gas phase surrounding the droplet stream is investigated using acetone planar laser-induced fluorescence.Temperature measurements are compared to simplified heat and mass transfer model taking into account variable physical properties, droplet-to-droplet interactions and internal fluid circulation within the droplets. The droplet surface temperature, calculated with the model, is used to initiate the numerical simulation of fuel vapor diffusion and transport in the gas phase, assuming thermodynamic equilibrium at the droplet surface. Influence of droplet diameter and droplet spacing on the fuel vapor concentration field is investigated and numerical results are compared with experiments.     

New approaches to numerical modelling of droplet transient heating and evaporation

New approaches to numerical modelling of droplet heating and evaporation by convection and radiation from the surrounding hot gas are suggested. The finite thermal conductivity of droplets and recirculation in them are taken into account. These approaches are based on the incorporation of new analytical solutions of the heat conduction equation inside the droplets (constant or almost constant h) or replacement of the numerical solution of this equation by the numerical solution of the integral equation (arbitrary h). It is shown that the solution based on the assumption of constant convective heat transfer coefficient is the most computer efficient for implementation into numerical codes. This solution is applied to the first time step, using the initial distribution of temperature inside the droplet. The results of the analytical solution over this time step are used as the initial condition for the second time step etc. This approach is applied to the numerical modelling of fuel droplet heating and evaporation in conditions relevant to diesel engines, but without taking into account the effects of droplet break-up. It is shown to be more effective than the approach based on the numerical solution of the discretised heat conduction equation inside the droplet, and more accurate than the solution based on the parabolic temperature profile model. The relatively small contribution of thermal radiation to droplet heating and evaporation allows us to take it into account using a simplified model, which does not consider the variation of radiation absorption inside droplets.     

Heat convection within evaporating droplets in strong aerodynamic interactions

The temperature field within evaporating ethanol droplets is investigated, relying on the two-color laser induced fluorescence (LIF) measurement technique and on a Direct Numerical Simulation (DNS). The configuration studied corresponds to a monodisperse droplet stream in a diffusion flame sustained by the droplet vapor. An experimental probe volume, small compared to the droplet size, is used to characterize the temperature field within the droplets, whereas DNS takes into account key aspects of the droplet heating and evaporation such as the non-uniform and transient stress, and the mass and heat transfer coefficients at the droplet surface. These investigations reveal that the frictional stresses are strongly reduced due to the small spacing between the droplets. They also show that the Marangoni effect has a significant influence on the internal motion and hence on the internal temperature field.     

Comparative Investigation on Droplet Evaporation Models for Modeling Spray in Cross-Flow

The coupling model of flow and heat and mass transfer for gas-spray droplet two-phase flow has been developed to simulate the evaporating spray in cross-flow. The correlations used for describing the droplet evaporation and motion in convective flow have been compared. The comparisons of calculated results show that the different correlations for determining Nusselt number and Sherwood number impose a significant influence on the lifetime of droplet. The modification of Nusselt number and Sherwood number with regard to the heat and mass boundary around the droplet is of great importance, while different mixing laws for mixture properties and different drag coefficient equations only demonstrate a slight effect on the evaporation characteristics of droplet. The characteristics of spray droplets and cross-flow in terms of both evaporation and motion are obtained. The secondary flow phenomenon is observed in the simulation results and contributes to achieving a more even distribution of temperature and an improved mixing effect of the vapor and cross-flow.     

Numerical simulation of fuel sprays at high ambient pressure: the influence of real gas effects and gas solubility on droplet vaporisation

This paper deals with the numerical simulation of the vaporisation of an unsteady fuel spray at high ambient temperature and pressure solving the appropriate conservation equations. The extended droplet vaporisation model accounts for the effects of non-ideal droplet evaporation and gas solubility including the diffusion of heat and species within fuel droplets. To account for high-temperature and high-pressure conditions, the fuel properties and the phase boundary conditions are calculated by an equation of state and the liquid/vapour equilibrium is estimated from fugacities. Calculations for an unsteady diesel-like spray were performed for a gas temperature of 800 K and a pressure of 5 MPa and compared to experimental results for droplet velocities and diameter distribution. The spray model is based on an Eulerian/Lagrangian approach. The comparison shows that the differences between the various spray models are pronounced for single droplets. For droplet sprays the droplet diameter distribution is more influenced by secondary break-up and droplet coagulation.     

高温气流中双液滴蒸发过程实验研究

建立了液滴蒸发的实验系统,采用悬挂液滴法对高温气流中单、双液滴的蒸发特性进行研究.实验结果表明：双液滴实验时的液滴蒸发过程与单液滴蒸发过程类似;液滴间相互作用使液滴周围蒸汽的浓度增大,气液传质浓度差减小,液滴与周围环境的传质速度降低,使蒸发速率减小;在纯辐射环境中液滴间相互作用对蒸发过程的影响较强,在辐射对流环境中液滴间相互作用对蒸发过程的影响较弱.     

Heat and mass transfer in evaporating droplets in interaction: Influence of the fuel

The prediction of heat and mass transfer in fuel sprays is a key issue in the design of combustors where the fuel is injected in a liquid form. The development and validation of new physical models requires reliable experimental data. This paper reports on an experimental study to characterize the Nusselt and Sherwood numbers of monodisperse droplets made of fuels having different volatilities and evaporating into flowing hot air. Simultaneous measurements of the droplet size and mean temperature allowed evaluating the heat fluxes that take part in the evaporation. The experimental Nusselt and Sherwood numbers are then compared to the case of an isolated droplet. It appears that these numbers are particularly dependent on the interactions between the droplets in a way that depends on the fuel nature.     

高频感应等离子场中液滴运动蒸发过程模拟

建立在高频感应热等离子体环境下单个溶液液滴的运动蒸发模型,采用数值计算的方法模拟了液滴在等离子体射流中的运动和传热过程,分析了不同操作参数对液滴运动蒸发过程的影响.结果表明:液滴初始入射尺寸越小,表面溶质质量分数达到饱和状态所用时间越短;初始入射速度越快,表面溶剂蒸发速度越快,溶质结晶析出时间越短;入射角较大时,液滴会被反向涡流卷吸,表面浓度达到饱和状态的时间较长.     

Experimental and Numerical Analysis of Thermal Interaction Between Two Droplets in Spray Cooling of Heated Surfaces

Dropwise cooling is a subject of interest for numerous industrial applications, which fosters fundamental research on the related mechanisms. The present work is focused on studying the cooling effect of 2 water droplets gently released onto a heated solid surface. The nominal initial temperature of the substrate was lower than 100 °C, thereby referring to evaporation regime. Heat-transfer phenomena were analyzed by an experimental and numerical approach at the solid/liquid interface and over non-wetted regions, thus evaluating mutual interaction between droplets. Infrared thermography was employed in a facility built to measure surface temperature from below through a fully non-intrusive approach. An infrared-transparent disk served as the substrate; its black-painted upper surface allowed heating and droplet deposition to occur on a blackbody. A numerical code was developed to model heat transfer within all bodies and at all interfaces by the finite-volume discretization method. Numerical results showed very good agreement with experimental temperature profiles and heat-flux distribution was predicted over the whole sampling region. Cooling effect was determined quantitatively together with the extent of the mutual-interaction region, where the influence of 2 sequentially-released droplets was proved higher and longer than that of a single-droplet configuration with the same amount of deposited water.