After fogging process in water injected gas turbine systems

In gas turbine system with after fogging, water droplets are injected after compressor. After fogging could have more significant potential for enhancement of specific power production compared to inlet fogging alone, since a larger water injection rate is possible. Transient analysis of after fogging process is carried out by using a heat and mass transfer modeling on water droplet evaporation. Transient variables such as droplet diameter and air temperature are evaluated as the droplet evaporation proceeds for different values of initial droplet diameter, pressure ratio of compressor, and water injection ratio. The evaporation time for injected droplets are also estimated. Present results show that the evaporation time decreases sensitively with increasing pressure ratio or initial droplet diameter. However, the effect of water injection ratio on evaporation time is relatively insignificant unless water injection ratio is near the critical ratio.     

Evaporation phenomenon past a rotating hydrocarbon droplet of ternary components

A numerical study of heat and mass transfer from an evaporating fuel droplet rotating around its vertical axis was performed in forced convection only on the side opposite to the flow. The flow was assumed to be laminar, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on the abovementioned assumption, the conservation equations in a general curvilinear coordinate were solved numerically. The behavior of rotating droplet evaporation in the forced convection flow can be investigated by analyzing the effects of the rotation of the droplet on the evaporation process of multi-component hydrocarbons droplet. The droplet is simulated to behave as a hard sphere. The transfer equations are discretized using an implicit finite difference method. Thomas algorithm is used to solve the system of algebraic equations. Moreover, dimensionless parameters of heat and mass transfer phenomena around a rotating hydrocarbon droplet were determined. The thickness of the boundary layer is unknown for this model and therefore, it was determined in function of time. Additionally, the study concerns “Dgheim dimensionless number” which is the ratio of the rotation forces over the viscosity forces. Dgheim dimensionless number is correlated to Nusselt and Sherwood numbers for multi-component hydrocarbon droplets in evaporation by taking into account the effect of heat and mass Spalding, Prandtl and Schmidt numbers respectively. Also, correlations for Nusselt and Sherwood numbers in terms of Reynolds, Prandtl and Schmidt numbers are proposed. These correlations consider the rotation phenomenon and advance the variation of the thermophysical and transport properties in the vapor phase of multi-component blends.     

Mathematical modeling of near-wall flows of two-phase mixture with evaporating droplets

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The Effect of Droplets Evaporation on Turbulence Modification and Heat Transfer Enhancement in a Two-Phase Mist Flow Downstream of a Pipe Sudden Expansion

Turbulent droplet-laden flow downstream of a sudden pipe expansion is numerically studied using an Eulerian two-fluid model. The model is used to investigate the effect of droplet evaporation on the particle dispersion and on the gas phase turbulence modification. Turbulence suppression in the case of evaporating droplets is hardly observed near the wall, and the level of turbulence tends to the corresponding value for the single-phase flow regime. In the flow core, where evaporation is insignificant, a decrease in the level of gas turbulence (to 20 % as compared to a single-phase flow) can be observed. The maximal effect of droplet evaporation is obtained in the wall region of the tube. A considerable increase in the maximal value of heat exchange on adding the evaporating droplets to the separated flow is shown (more than 1.5-fold as compared to the single-phase flow at a small value of droplet mass concentration of M _L1≤ 0.05). The addition of the solid non-evaporating particles causes a slight increase in the maximum value of heat transfer in the case of small particles and a decrease in heat transfer in the case of large particles.     

Heat and mass transfer of a fuel droplet evaporating in oscillatory flow

A numerical study of the heat and mass transfer from an evaporating fuel droplet in oscillatory flow was performed. The flow was assumed to be laminar and axisymmetric, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on these assumptions, the conservation equations in a general curvilinear coordinate were solved numerically. The behaviors of droplet evaporation in the oscillatory flow were investigated by analyzing the effects of flow oscillation on the evaporation process of a n-heptane fuel droplet at high pressure.The response of the time history of the square of droplet diameter and space-averaged Nusselt numbers to the main flow oscillation were investigated in frequency band of 1–75 Hz with various oscillation amplitudes. Results showed that, depending on the frequency and amplitude of the oscillation, there are different modes of response of the evaporation process to the flow oscillation. One response mode is synchronous with the main flow oscillation, and thus the quasi-steady condition is attained. Another mode is asynchronous with the flow oscillation and is highly unsteady. As for the evaporation rate, however, in all conditions is more greatly enhanced in oscillatory flow than in quiescent air.To quantify the conditions of the transition from quasi-steady to unsteady, the response of the boundary layer around the droplet surface to the flow oscillation was investigated. The results led to including the oscillation Strouhal number as a criteria for the transition. The numerical results showed that at a low Strouhal number, a quasi-steady boundary layer is formed in response to the flow oscillation, whereas by increasing the oscillation Strouhal number, the phenomena become unsteady.     

Measurements of multicomponent microdroplet evaporation by using Rainbow Refractometer and PDA

The evaporation process of a multi-component droplet has been studied utilizing optical diagnostic techniques. Investigation focused on the measurements of the different volatility mixtures and the variation of the mass fraction within a droplet of different components. The method for determining the temperature of a multicomponent droplet containing nonvolatile and volatile compositions has been proposed. For a droplet with two volatile compositions, such as water and ethanol mixture, the model of evaporation for droplet temperature measurements was also suggested based on the experimental data. Droplets with different mass fractions of each component under different temperature conditions were measured utilizing a primary rainbow refractometer and a recently improved phase-Doppler analyzer. It was also found that the droplet temperature could hardly reach its saturation temperature within the droplet unlike explained by previous researchers. The effects of temperature gradient within an evaporating microdroplet and the temperature discontinuity on the evaporating surface have been discussed.     

Material removal associated with condensation on a droplet in motion

This paper treats a three-phase, multicomponent fluid mechanics-heat and mass transfer problem. Solutions to the nonlinear, coupled boundary layer equations that govern laminar condensation heat and mass transfer in the vicinity of the forward stagnation point of a spherical cold water drop translating in a saturated mixture of five components are presented. The environment surrounding the drop is composed of a condensable (steam), a noncondensable and nonabsorbable (air), a noncondensable but absorbable (e.g. elemental iodine), a chemically reactive component (e.g. methyl iodide), and a particulate substance. The dispersed and the continuous phases have been treated simultaneously. The effect of chemical reaction between a reactive component in the continuous phase and an additive (e.g. hydrazine) in the droplet liquid has also been considered. The droplet sizes, the thermodynamic range, and the nature of chemical constituents chosen for the illustrative calculations are closely related to the operating conditions that are likely to prevail in the containment spray atmosphere of a nuclear reactor following a loss of coolant accident. The choice of elemental iodine and methyl iodide has an added feature. The mass transfer resistance for elemental iodine is almost entirely in the gaseous phase while the absorption of methyl iodide could be regarded as a liquid-phase resistance-controlled process. Mass transport, in the presence of condensation, is seen to depend in a rather complicated manner on droplet radius.     

Modeling and Simulation of Effects of Turbulence on Vaporization, Mixing and Combustion of Liquid-Fuel Sprays

The objective of this work is twofold. Firstly, the effects of turbulence intensity variations on the turbulent droplet dispersion, vaporization and mixing for non-reacting sprays (with and without swirl) are pointed out. Secondly, the effects of the coupling of the turbulence modulation with external parameters, such as swirl intensity, on turbulent spray combustion are analyzed in configurations of engineering importance. This is achieved by using advanced models for turbulence, evaporation and turbulence modulation implemented into FASTEST-LAG3D-codes: (1) To highlight the influence of turbulence modulation on some spray properties, a thermodynamically consistent modulation model has been considered besides the standard assumption and the well known Crowe's model. For turbulent droplet dispersion, we rely on the Markov-sequence formulation. (2) In order to characterize phase transition processes ongoing on droplets surfaces, a non-equilibrium evaporation model shows better agreement with experiments in comparison with the quasi-equilibrium-based evaporation models often used. (3) The results of turbulence intensity variations reveal the existence of a limited range out of which the increase or decrease of the turbulence intensity affects no more the efficiency of the heat and mass transfer. A derived characteristic number, a vaporization Damkhöler number, possesses a critical value which separates two different behavior regimes with respect to the turbulence/droplet vaporization interactions. (4) Under reacting conditions, it is shown how the evaporation characteristics, mixing rate and combustion process are strongly influenced by swirl intensity and turbulence modulation. In particular, the turbulence modulation modifies the evaporation rate, which in turn influences the mixing and the species concentration distribution. In the case under investigation, it is demonstrated that this effect cannot be neglected for low swirl intensities (Sw.Nu. ≤ 1) in the region far from the nozzle, and close to the nozzle for high swirl number intensities. In providing these particular characteristics, a reliable control of the mixing of gaseous fuel and air in evaporating and reacting sprays, and a possible optimization of the mixing process can tentatively be achieved.     

Effect of Evaporation of Liquid Droplets on the Distribution of Parameters in a Two–Species Laminar Flow

A calculation model was developed, and the heat– and mass–transfer characteristics in a laminar air—vapor—droplet flow moving in a round tube were studied numerically. The distributions of parameters of the two–phase flow over the tube radius were obtained for varied initial concentrations of the gas phase. The calculated heat and mass transfer is compared to experimental data and calculations of other authors. It is shown that evaporation of droplets in a vapor—gas flow leads to a more intense heat release as compared to a one–species vapor—droplet flow and one–phase vapor flow     

An experimental study of the evaporation characteristics of emulsified liquid droplets

This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.List of Symbols h_fg latent heat of vaporization, KJ/kg - m mass of the droplet, gm - T_b boiling temperature, °C - T_c critical temperature, °C - T_L Leidenfrost temperature, °C - T_s initial surface temperature of the hot surface, °C     

Dynamics of and heat transfer to a butane droplet evaporating in water

The dynamics and the associated heat transfer process of butane droplets evaporating in water are investigated experimentally. New data are presented for the instantaneous growth, rise velocity and the heat transfer coefficient. The behaviour of the bubble-droplet, from the initial to the final stages of evaporation, is divided into four regions and is described with reference to similarities with the behaviour of a spherical droplet, spheroidal bubble-droplet, large spheroidal bubble and spherical cap bubble. The equations which represent the results for the heat transfer coefficient are given.     

Accurate analysis of multicomponent fuel spray evaporation in turbulent flow

The aim of this paper is to perform an accurate analysis of the evaporation of single component and binary mixture fuels sprays in a hot weakly turbulent pipe flow by means of experimental measurement and numerical simulation. This gives a deeper insight into the relationship between fuel composition and spray evaporation. The turbulence intensity in the test section is equal to 10%, and the integral length scale is three orders of magnitude larger than the droplet size while the turbulence microscale (Kolmogorov scales) is of same order as the droplet diameter. The spray produced by means of a calibrated droplet generator was injected in a gas flow electrically preheated. N-nonane, isopropanol, and their mixtures were used in the tests. The generalized scattering imaging technique was applied to simultaneously determine size, velocity, and spatial location of the droplets carried by the turbulent flow in the quartz tube. The spray evaporation was computed using a Lagrangian particle solver coupled to a gas-phase solver. Computations of spray mean diameter and droplet size distributions at different locations along the pipe compare very favorably with the measurement results. This combined research tool enabled further investigation concerning the influencing parameters upon the evaporation process such as the turbulence, droplet internal mixing, and liquid-phase thermophysical properties.     

Droplet heat transfer on micro-post arrays: Effect of droplet size on droplet thermal characteristics

Heat transfer towards a water droplet from hydrophobic micro-post array surface is considered while mimicking the environmental temperatures. Micro-post arrays are created on a silicon wafer surface via lithography technique. The textured surfaces are replicated by polydimethylsiloxane (PDMS) to achieve an optical transmittance. The droplet adhesion on micro-post array surface is presented and the influence of droplet size on the heat transfer and droplet internal flow characteristics is examined. The flow predictions are validated via the particle image velocimetry data. It is found that adhesion force between the water droplet and the micro-post arrays surface depends on the geometric size and the orientation of the micro-post arrays on the surface. Temperature and flow fields are influenced by the droplet size. The Nusselt and the Bond numbers increase with the droplet volume; however, the Bond number remains less than unity indicating that the Marangoni current dominates over the buoyancy current in the droplet. The Nusselt number attains larger values for micro-post array surface than that of the plain surface. This is because of temperature and velocity oscillations along the contact lines at the droplet bottom due to the pitches of the micro-post arrays.     

Boundary layer and heat transfer in a two-phase gas-evaporating droplet medium

An asymptotic model of the flow in the laminar boundary layer of a gas-evaporating droplet mixture is constructed within the framework of the two-continuum approximation. The case of evaporation of the droplets into an atmosphere of their own vapor is examined in detail with reference to the example of longitudinal flow over a hot flat plate. Numerical and asymptotic solutions of the boundary layer equations constructed are found for a number of limiting situations (low droplet concentration, no droplet deposition, significant droplet deposition). The development of the flow with respect to the longitudinal coordinate is studied and it is shown that in the absence of droplet deposition a region of pure vapor may be formed near the surface. Similarity criteria are established and the mechanism of surface heat transfer enhancement is studied for a low evaporating droplet concentration in the boundary layer. In the inertial deposition regime the results of calculating the integral heat transfer coefficient are found to correspond with the experimental data [1].Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.3, pp. 42–50, May–June, 1992.     

On the calculation of wall temperatures in the post dryout heat transfer region

A non-equilibrium post dryout heat transfer model for calculating the wall temperature distribution in vertical upflows is presented in this study. The model is based upon the three path heat transfer formulation developed by MIT researchers (Laverty & Rohsenow 1964, Forslund & Rohsenow 1968, Hynek et al. 1969 and Plummer et al. 1974) that involves heat transfer from wall to vapor, from wall to droplets in contact with the wall and from vapor to liquid droplets in the vapor core. Downstream gradients for the bulk vapor temperature, vapor quality, droplet size and vapor velocities are identical to those used by Hynek et al. (1969) and Plummer et al. (1974). Conditions at the dryout location are calculated using a modified version of a technique developed by Hynek et al. (1969).A procedure for determining an average droplet diameter based on a size distribution is introduced. Migration of droplets through the boundary layer and droplet deposition flux are predicted with the model of Gani? & Rohsenow (1979). Heat transfer from the wall to the impinging liquid droplets is calculated with a correlation by Holman & McGinnis (1969). Mechanisms contributing to wall to droplet heat transfer are identified as (a) droplet-wall contact, (b) intensive droplet evaporation inside the boundary layer, and (c) destruction of the boundary layer due to droplet migration to, and rebound from, the hot surface. The significance of the average droplet size and size distribution is demonstrated through its control over the free stream evaporation and droplet deposition rates.Predicted uniform heat flux wall temperature profiles for water, nitrogen and freon 12 are in good agreement with the data of Era et al. (1966), Bennett et al. (1967), Forslund & Rohsenow (1968), Ling et al. (1971), Groeneveld (1972) and Janssen & Kervinen (1975).     

Heat flux characteristics of spray wall impingement with ethanol,butanol, iso-octane,gasoline and E10 fuels

Future fuel stocks for spark-ignition engines are expected to include a significant portion of bio-derived components with quite different chemical and physical properties to those of liquid hydrocarbons. State-of-the-art high-pressure multi-hole injectors for latest design direct-injection spark-ignition engines offer some great benefits in terms of fuel atomisation, as well as flexibility in in-cylinder fuel targeting by selection of the exact number and angle of the nozzle’s holes. However, in order to maximise such benefits for future spark-ignition engines and minimise any deteriorating effects with regards to exhaust emissions, it is important to avoid liquid fuel impingement onto the cylinder walls and take into consideration various types of biofuels. This paper presents results from the use of heat flux sensors to characterise the locations and levels of liquid fuel impingement onto the engine’s liner walls when injected from a centrally located multi-hole injector with an asymmetric pattern of spray plumes. Ethanol, butanol, iso-octane, gasoline and a blend of 10% ethanol with 90% gasoline (E10) were tested and compared. The tests were performed in the cylinder of a direct-injection spark-ignition engine at static conditions (i.e. quiescent chamber at 1.0 bar) and motoring conditions (at full load with inlet plenum pressure of 1.0 bar) with different engine temperatures in order to decouple competing effects. The collected data were analysed to extract time-resolved signals, as well as mean and standard deviation levels of peak heat flux. The results were interpreted with reference to in-cylinder spray formation characteristics, as well as fuel evaporation rates obtained by modelling. In addition, high-speed images of single droplets of fuel impinging onto the array of the heat flux sensor were acquired with simultaneous sampling of the heat flux signal in an attempt to provide further interpretation. The single droplet tests showed ability of the signals to quantify droplet mass impinged on the sensor. Analysis of the peak heat flux at static engine conditions quantified values of fuel temperature at impingement in agreement with the wet bulb temperatures predicted by the droplet evaporation model. Comparison of the static and motoring engine heat flux signals around the bore showed the effect of the intake flow on the spray’s pattern at impingement and demonstrated fuel presence on the liner that survived at exhaust valve open timing. The general behaviour was different for the alcohols to that of the hydrocarbons, with ethanol exhibiting the effect of its high latent heat on the signals and butanol exhibiting effects related to poor atomization and slow evaporation.     

Investigation on Heat and Mass Transfer in a Evaporator of a Capillary-Pumped Loop with the Lattice Boltzmann Method: Pore Scale Simulation

A two-dimensional transient numerical model based on the lattice Boltzmann method (LBM) for the global evaporator of a capillary-pumped loop (CPL) is proposed to describe heat and mass transfer with evaporation in the porous wick, heat conduction in the cover plate, and heat transfer in the vapor groove. To indicate the stochastic phase distribution characteristics of most porous wick, the quartet structure generation set (QSGS) is introduced for generating more realistic microstructures of porous media. By using the present lattice Boltzmann algorithm along with the porous structure, the heat and mass transfer of an evaporator on pore scale can be predicted without resorting to any empirical parameters determined case by case. The energy equations for entire evaporator are solved as a conjugate problem, which are solved by means of a spatially varying relaxation time in the lattice Boltzmann model and the liquid flow is driven via the interfacial mass flux. A convective boundary condition considering the latent heat during the evaporation on the interface is introduced into the lattice Boltzmann model based on the nonequilibrium extrapolation rule. Especially, the bounce-back rule and the equilibrium rule of the LBM are, respectively, introduced to deal with the momentum boundary conditions inside the porous wick and on the evaporation interface in order to ensure the stability and the efficiency of the LBM model. Numerical results corresponding to different working conditions and different working fluids are presented, which provide guidance for the evaporator design of a CPL system.     

Towards the problem of droplet injection in a vapor with the aim of reducing the pressure

As applied to the analysis of sprinkler systems which inject droplets into a vapor in the case of emergency pressure increases, the process of vapor condensation on a single droplet is considered. For the specification of the intensity of interphase heat and mass transfer, the solution of an unsteady heat conduction problem is used. Approximate formulas describing the laws of the pressure drop in a vapor-droplet system due to the condensation of the vapor phase are obtained.     

The influence of thermal expansion flow on droplet evaporation

It has been demonstrated recently that it follows from conservation of mass that unsteady temperature fields create flow in an incompressible fluid with a temperature-dependent density even in the absence of gravity. The paper studies the influence of thermal expansion flow on spherically symmetric evaporation of an isolated droplet. A model problem of a droplet evaporating at a constant rate is first considered. In this idealized situation one can use the assumption of a thin thermal boundary layer to solve analytically the unsteady moving-boundary heat conduction problem to find the temperature field inside the droplet both with and without the thermal expansion flow. Next evaporation of a fuel droplet in a diesel engine is studied numerically. The heat diffusion equation is solved in the liquid phase while the standard quasi-steady model is used for the gas phase. The results of the calculation show that for high ambient temperatures the influence of the thermal expansion flow on the droplet lifetime can be considerable.     

Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.