Direct numerical simulations of a planar jet laden with evaporating droplets

A direct numerical simulation (DNS) study is conducted on the various aspects of phase interactions in a planar turbulent gas-jet laden with non-evaporative and evaporative liquid droplets. A compressible computational model utilizing a finite difference scheme for the carrier gas and a Lagrangian solver for the droplet phase is used to conduct the numerical experiments. The effects of droplet time constant, mass-loading and mass/momentum/energy coupling between phases on droplet and gas-jet fields are investigated. Significant changes in velocity, temperature, density and turbulence production on account of the coupling between the liquid and gas phases are observed in non-isothermal jets with evaporating droplets. Most of these changes are attributed to the density stratification in the carrier gas that is caused by droplet momentum and heat transfer.     

Effect of temperature on the vorticity field of an evolving water droplet

Vorticity in liquid droplets and the impact of temperature on the evolution of dripping droplets have received little attention in the published literature. Here, an investigation is conducted to determine the vorticity field within a droplet as it evolves from the onset to the breakup point and to show the impact of temperature on the vorticity field. The work is performed both experimentally and computationally. It was observed that the water leaving the nozzle grows into a cylindrical shape, forms a throat, contracts near the throat, and finally breaks up and forms an independent nearly spherical droplet. Presence of a flow recirculation within the droplet was noticed. Vorticity iso-surfaces were circumferentially asymmetric, and indicated a three-dimensional droplet behavior with rotational (non-zero vorticity) effects. Temperature enhanced the vorticity-related asymmetry and affected the pressure and velocity within the droplet.     

Transient convective burning of interactive fuel droplets in double-layer arrays

The transient convective burning of n-octane droplets interacting within double-layer arrays in a hot gas flow perpendicular to the layers is studied numerically, with considerations of droplet surface regression, deceleration and relative movement due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature and surface tension. Each layer in the double-layer array is a periodic droplet array aligned orthogonal to the free stream direction. The droplets in different layers are arranged either in tandem or staggered. Several different flame structures are found for the double-layer arrays. The transient behaviors of the droplets in both upstream and downstream layers are studied and compared, for various initial relative stream velocity and initial transverse droplet spacing. The average surface temperature and vaporization rate for the front (or upstream) droplets and back (or downstream) droplets are influenced by the flame structure. The front droplets in a double-layer array behave similarly to the droplets in a single-layer array for the streamwise droplet spacing considered in this study. The back droplets approach the front droplets because they generally have lower drag.     

Numerical Simulation of Vortex Engine Flow Field： One Phase and Two Phases

Aiming at improving efficiency in combustion systems, the study on droplet behavior and its trajectory is of crucial importance. Vortex engine is a kind of internal combustion engine which uses swirl flow to achieve higher combustion efficiency. One of the important advantages of designing vortex engine is to reduce the temperature of walls by confining the combustion products in the inner vortex. The scopes of this investigation are to study vortex engine flow field as well as effective parameters on fuel droplet behavior such as droplet diameter, droplet initial velocity and inlet velocity of the flow field. The flow field is simulated using Reynolds Stress Transport Model (RSM). The Eulerian-Lagrangian method and the one-way coupling approach are employed to simulate two phase flow and dispersed phase in the chamber, respectively. A new method, based on computing pressure force exerted on the droplet surface, is introduced to determine the distinction between using one-way and two-way coupling approaches. The results showed that the droplets with smaller diameter are more likely to follow the flow stream lines than bigger droplets, thus evaporate completely in the chamber. Moreover, droplets with greater initial velocity have higher evaporation rate, yielding the existence of evaporation and combustion in the inner vortex. Additionally, the higher inlet velocity of continuous phase results in higher centrifugal force, leads droplets in question to deviate towards the wall faster.     

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.     

The structure of an acoustically forced, reacting two-phase jet

An experimental study of the structure of an acoustically forced, reacting two-phase jet was performed. The jet was acoustically forced to control the formation and evolution of large-scale structures in the near field of the jet. Phase-locked data acquisition techniques were used to correlate droplet statistics and dynamics with features of the large-scale structures. Phase Doppler interferometry was used to acquire droplet statistics. Planar imaging techniques were applied to document the distribution of droplets within the jet. The results show that the interaction between droplets and large-scale structures leads to a nonuniform distribution of droplets in the reacting jet. The combination of transport effects and droplet evaporation leads to the formation of droplet clusters. The group combustion behavior of the droplet field was evaluated by estimating the group combustion number from experimental data. External sheath burning is present in the early portion of the flame followed by a transition to external group combustion as clusters begin to be the dominant feature. Late in the cluster lifetime there is a shift to internal group combustion.     

Monodisperse droplet heating and evaporation: Experimental study and modelling

Results of experimental studies and the modelling of heating and evaporation of monodisperse ethanol and acetone droplets in two regimes are presented. Firstly, pure heating and evaporation of droplets in a flow of air of prescribed temperature are considered. Secondly, droplet heating and evaporation in a flame produced by previously injected combusting droplets are studied. The phase Doppler anemometry technique is used for droplet velocity and size measurements. Two-colour laser induced fluorescence thermometry is used to estimate droplet temperatures. The experiments have been performed for various distances between droplets and various initial droplet radii and velocities. The experimental data have been compared with the results of modelling, based on given gas temperatures, measured by coherent anti-stokes Raman spectroscopy, and Nusselt and Sherwood numbers calculated using measured values of droplet relative velocities. When estimating the latter numbers the finite distance between droplets was taken into account. The model is based on the assumption that droplets are spherically symmetrical, but takes into account the radial distribution of temperature inside droplets. It is pointed out that for relatively small droplets (initial radii about 65 μm) the experimentally measured droplet temperatures are close to the predicted average droplet temperatures, while for larger droplets (initial radii about 120 μm) the experimentally measured droplet temperatures are close to the temperatures predicted at the centre of the droplets.     

A thermal immiscible multiphase flow simulation by lattice Boltzmann method

The lattice Boltzmann (LB) method, as a mesoscopic approach based on the kinetic theory, has been significantly developed and applied in a variety of fields in the recent decades. Among all the LB community members, the pseudopotential LB plays an increasingly important role in multiphase flow and phase change problems simulation. The thermal immiscible multiphase flow simulation using pseudopotential LB method is studied in this work. The results show that it is difficult to achieve multi-bubble/droplet coexistence due to the unphysical mass transfer phenomenon of “the big eat the small” – the small bubbles/droplets disappear and the big ones getting bigger before a physical coalescence when using an internal energy based temperature equation for single-component multiphase (SCMP) pseudopotential models. In addition, this unphysical effect can be effectively impeded by coupling an entropy-based temperature field, and the influence on density fields with different energy equations are discussed. The findings are identified and reported in this paper for the first time. This work gives a significant inspiration for solving the unphysical mass transfer problem, which determines whether the SCMP LB model can be used for multi-bubble/droplet systems.     

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.     

A molecular dynamics simulation of droplet evaporation

A molecular dynamics (MD) simulation method is developed to study the evaporation of submicron droplets in a gaseous surrounding. A new methodology is proposed to specify initial conditions for the droplet and the ambient fluid, and to identify droplet shape during the vaporization process. The vaporization of xenon droplets in nitrogen ambient under subcritical and supercritical conditions is examined. Both spherical and non-spherical droplets are considered. The MD simulations are shown to be independent of the droplet and system sizes considered, although the observed vaporization behavior exhibits some scatter, as expected. The MD results are used to examine the effects of ambient and droplet properties on the vaporization characteristics of submicron droplets. For subcritical conditions, it is shown that a spherical droplet maintains its sphericity, while an initially non-spherical droplet attains the spherical shape very early in its lifetime, i.e., within 10% of the lifetime. For both spherical and non-spherical droplets, the subcritical vaporization, which is characterized by the migration of xenon particles that constitute the droplet to the ambient, exhibits characteristics that are analogous to those reported for “continuum-size” droplets. The vaporization process consists of an initial liquid-heating stage during which the vaporization rate is relatively low, followed by nearly constant liquid-temperature evaporation at a “pseudo wet-bulb temperature”. The rate of vaporization increases as the ambient temperature and/or the initial droplet temperature are increased. For the supercritical case, the droplet does not return to the spherical configuration, i.e., its sphericity deteriorates sharply, and its temperature increases continuously during the “vaporization” process.     

二甲醚液滴高压蒸发的数值模拟

在分析燃油液滴高压蒸发规律的基础上,考虑液滴内部的热传导过程、内部环流和非理想气体效应,建立了高压蒸发模型,并利用该模型对二甲醚(DME)单液滴的蒸发过程进行了数值模拟分析。采用状态方程法计算了DME-N2体系的气液相平衡。结果表明:高压有利于燃料液滴蒸发;即使环境压力超过燃油的临界压力,其平衡蒸发温度也未必能达到临界温度。     

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...     

An experimental study on the spray and thermal characteristics of R134a two-phase flashing spray

Flashing spray of volatile liquids is a common phenomenon observed in many industrial applications such as fuel injection of engines, accidental release of flammable and toxic pressure-liquefied gases, failure of a vessel or pipe in the form of a small hole in chemical industry, and cryogenic spray cooling in laser dermatology, etc. In flashing spray, the volatile liquid is depressurized rapidly at the exit of a nozzle (or a hole in a vessel) and becomes superheated. Such superheated liquid (in the form of either a jet or droplets) will lead to explosive atomization with fine droplet and a short spray distance. This paper presents an experimental investigation to the spray and thermal characteristics of flashing spray using cryogen R134a. A photographic study of the spray is firstly conducted to visualize the spray formation and the dynamic characteristics of the spray. Afterwards, the spray characteristics are measured by the phase Doppler Particle Analyzer (PDPA). The distributions of the diameter reveals the dramatic dynamic variation of the liquid droplets due to explosive atomization of large droplets in the region near the exit of nozzle, while the self-similar velocity profiles are fitted by two empirical correlations to describe the non-dimensional axial and radial velocities, respectively. The temperature field within the spray is measured by a small thermocouple. The temperature measurements provide detailed quantitative information of both radial and axial temperature distributions of droplets within the spray. These experimental results provide deep understanding into the whole characteristics of two-phase flashing spray of volatile liquids.     

Numerical solution for film evaporation of a spherical liquid droplet on an isothermal and adiabatic surface

A numerical solution for the problem of film evaporation of a liquid droplet on a horizontal surface is presented. The droplets are small enough to be assumed spherical. Two principal cases are considered: (1) the horizontal surface is maintained at a constant temperature (case I), and (2) the surface is insulated while the ambience is hot (case II). The complete set of equations governing this problem were solved under the following assumptions : (1) evaporation is quasi-steady, (2) no internal liquid circulation, (3) constant properties, and (4) the droplet temperature is spatially uniform but temporally varying. The Lewis number is not assumed to be unity; gas phase viscous effects, Stefan type convection, and gas phase inertia are included in the analysis. The total droplet evaporation time was found to decrease with increasing plate (I) or ambient (II) temperature as expected, and the droplet progressively moves away from the plate as it evaporates. The numerical results agree with the analytical solution for film evaporation of a droplet above an adiabatic surface in a hot ambience in the limit of large effective Reynolds number (i.e. potential flow).     

Vaporization of polydisperse fuel sprays in a laminar boundary layer flow: A sectional approach

The effect of the presence of a cold wall on the downstream changes in size distribution of a spray of fuel droplets undergoing vaporization and combustion is theoretically analyzed. The fuel is considered to be in the form of discrete liquid droplets which have an arbitrary range of sizes and differ in their rates of vaporization. In fact, the total number of discrete droplet sizes needed to simulate actual fuel sprays can be immense. To avoid the dimensionality problem associated with the discrete form of population balance equations of an ensemble of individual burning or evaporating particles, “sectional conservation equations” are used. The method, based on dividing the droplet size domain into sections and dealing only with one integral quantity in each section (e.g., number, surface area of droplets, or volume), has the advantage that the integral quantity is conserved within the computational domain and the number of conservation equations required is simply equal to the number of sections. Employing known solutions for the boundary layer flow field, the “sectional size conservation equations” are solved assuming that droplets follow streamlines. New solutions for the changes in size distributon of droplets as a function of temperature and distance from the wall are presented. Since the present analysis uses an arbitrary droplet size distribution as an initial condition, it may be used to evaluate the performance of various atomizers, as demonstrated in the present study.     

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.     

Combustion characteristics of droplets of coal/oil and coal/oil/water mixtures

The utilization of coal/oil mixtures in furnaces and boilers appears to be a viable short-to-medium range solution to partially alleviate the demand on petroleum supply with minimum combustor modifications. The present investigation first demonstrates that the combustion characteristics of the coal/oil mixture droplets depend intimately on the intensity of internal circulation. Rapid internal motion favors batch-distillation-type vaporization which causes the coal particles to agglomerate. On the other hand, slow internal motion traps the volatile components in the inner core of the rapidly-heated droplet and may eventually lead to internal boiling and subsequently fragmentation of the droplet. Experimental results reveal a mixed behavior, although the degree of agglomeration is significant even at moderate levels of coal loading. It is subsequently shown that the addition of small quantities of water significantly enhances the potential and intensity of droplet explosion. Practical implications of the present results are also discussed.     

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.     

Transient vaporization and burning in dense droplet arrays

Three-dimensional droplet-array combustion with an unsteady liquid-phase and a quasi-steady gas-phase is studied computationally by a generalized approach using a mass-flux potential function. Symmetric and asymmetric droplet arrays with non-uniform droplet size and non-uniform spacing are considered. Burning rates are computed and correlated with the number of droplets, an average droplet size, and an average spacing for the array through one similarity parameter for arrays as large as 1000 droplets. Total array vaporization rates are found to be maximized at a specific droplet number density that depends on liquid volume within the array. An unsteady liquid-phase model with either a uniform or a radially varying temperature distribution is coupled with the quasi-steady gas-phase solution for decane, heptane, and methanol fuels. Droplet interactions and liquid-phase heating have been shown to almost double the lifetime when compared to an isolated droplet. Depending on fuel type, initial temperature, and array geometry, droplets may initially burn with individual flames, transition to a single group flame, and transition back to individual flames as vaporization progresses. In most cases, group combustion occurs upon ignition and is the dominant mode of combustion.     

3D multiphase lattice Boltzmann simulations for morphological effects on self-propelled jumping of droplets on textured superhydrophobic surfaces

Coalescence induced droplets self-propelled jumping on textured superhydrophobic surfaces (SHS) is numerically simulated using multiple–relaxation–time (MRT), and three dimensional (3D) multiphase isothermal lattice Boltzmann method. Symmetric boundary conditions and parallel computation with OpenMP algorithm are used to accelerate computational speeds. Simulation results for velocity field show that the downward velocity of the droplet is reverted to upward direction due to the counter action of the wall to the contact base of the droplet during the period of droplet deformation on the texture. For a fixed droplet diameter, the spacing of the microstructure is found to play a key role on jumping velocity of the coalescence droplet, and an optimal spacing of the microstructure exists for a maximum jumping velocity. For a texture with small spacings, the adhesion force due to surface tension is large because of the large contact area which results in a decrease of its jumping velocity. On the other hand, for a texture with large roughness spacings, the lower contour of the droplet will fall into the texture, which will also decrease droplet jumping velocity. Simulation results for jumping velocities are used to explain large differences in measured jumping velocities of small droplets (with radius less than 20 μm) on hierarchical textured and nanostructured surfaces in existing experiments.