Spray Combustion

The two-phase filtered mass density function (FMDF) method is employed for large eddy simulation (LES) of high speed evaporating and combusting n-heptane sprays using simple (global) and complex (skeletal) chemical kinetic mechanisms. The resolved fluid velocity and pressure fields are obtained by solving the filtered compressible Navier–Stokes equations with high-order Eulerian finite difference methods. The liquid spray and gas scalar (temperature and species mass fractions) fields are both obtained by Lagrangian stochastic models. The chemistry calculation is accelerated by incorporating the parallel in situ adaptive tabulation (ISAT) method. There are two-way interactions among Eulerian and Lagrangian fields. Simulations of evaporating sprays with and without combustion indicate that the two-phase LES/FMDF results are consistent and compare well with the available experimental data at different gas temperatures and oxygen concentrations. The spray controlled flame tends to move away from a diffusion flame structure toward a premixed one as the oxygen concentration decreases and/or the ambient gas temperature increases because of changes in spray-induced turbulence and mixing. The LES/FMDF results for ignition delay show more sensitivity to the chemical kinetic model at lower gas temperatures due to slower reaction and stronger turbulence–chemistry interactions. The liftoff length is less sensitive to the kinetics.

To achieve uniform and efficient combustion in a rocket engine, a fine uniform spray is needed. The same is achieved by designing an injector with good atomization characteristics. Gas-centered swirl coaxial (GCSC) injector elements have been preferred recently in liquid rocket engines because of an inherent capability to dampen the pressure oscillations in the thrust chamber. The gas-centered swirl coaxial injector chosen for this study is proposed to be used in a semi-cryogenic rocket engine operating with oxidizer rich hot exhaust gases from the pre-burner and liquid kerosene as fuel. In this paper, nine different configurations of gas-centered swirl coaxial injector, sorted out by studying the spray angle and coefficient of discharge with swirl number varying from 9 to 20 and recess ratio of 0.5, 1, and 1.5 are investigated for their atomization characteristics. Spray uniformity, spray cone angle, and droplet size in terms of Sauter mean diameter and mass median diameter are studied at various momentum flux ratios for all configurations. Sauter mean diameter is almost independent of recess ratio, whereas cone angle was inversely proportional to the recess ratio. A finer atom-ization was observed for injectors of high swirl number but the pressure drop also increased to achieve the same flow rate. An injector of medium swirl number and recess ratio of 1.5 is deemed most fit for above-mentioned application.

A high–fidelity two-phase large eddy simulation
(LES)/filtered mass density function (FMDF) model is
developed and used for detailed simulations of turbulent spray
breakup, evaporation and combustion. The spray is simulated
with Lagrangian droplet transport, stochastic breakup, wake,
collision/coalescence and finite rate heat and mass transfer
submodels. The spray model is used together with the
compressible, Eulerian LES gas flow model for velocity and
pressure fields and the two-phase Lagrangian FMDF for the
scalar (species mass fraction and enthalpy) field. There are
two-way couplings between all Lagrangian and Eulerian
models. The numerical results for non-reacting and reacting
sprays are compared with the available experimental data for
global spray variables such as the spray penetration length,
ignition delay and flame lift-off lengths. It is shown that the
two-phase LES/FMDF results are consistent and compare well
with the experimental data.

In this work the effect of flash boiling on liquid break up and atomization is first reviewed and then described in detail for the sprays produced by two high pressure multistream gasoline direct injectors. They were operated at 120bar and 200bar pressure with a fuel injection duration of 0.8ms into sub-atmospheric pressures between 1 and 0.1bar and four temperatures between 20°C and 100°C. The sprays were injected into an optically accessed chamber whose temperature and pressure were independently controlled to recreate the conditions found in an engine likely to cause flash boiling. Shadowgraph imaging with a single shot CCD camera was applied to obtain images of the spray morphology under a matrix of the above conditions. The interaction between spray streams is particularly complex under flash boiling conditions being a function of the nature of the fuel and the geometry of the injector. In this investigation, three different fuels, regular RON-95 gasoline and single components fuels, n-heptane and iso-octane were used with two different injectors, one with a 60 degree and the other with a 90 degree nominal external cone angle. In total, five variables were investigated: fuel pressure, ambient pressure, ambient temperature, fuel composition and injector geometry. Once processed, the recorded images allowed measurement of spray tip 2 penetration and cone angle. Qualitative data on the change in shape of the spray was also available. The results showed that flash boiling has potential to reduce droplet diameters and improve fuel vaporisation, however, the associated change in spray shape must be taken into account to avoid problems with spray impingement.

This paper presents the experimental and numerical results for a two stage combustor capable of achieving flameless combustion with liquid fuels for different thermal heat inputs of 20, 30, 40 and 60 kW and heat release density of 5-15 MW/m 3 . Combustion characteristics and pollutant emissions are studied for three different fuels, kerosene, diesel and gasoline. The influence of droplet diameter on pollutant emissions at all conditions is studied. The fuel and oxidizer are supplied at ambient conditions. The concept of high swirl flows has been adopted to achieve high internal recirculation rates, residence time and increased dilution of the fresh reactants in the primary combustion zone, resulting in flameless combustion mode. Air is injected through four tangential injection ports located near the bottom of the combustor and liquid fuel is injected through a centrally mounted pressure swirl injector. Computational analysis of the flow features shows that decrease in the exit port diameter of the primary chamber increases the recirculation rate of combustion products and helps in achieving the flameless combustion mode. Based on preliminary computational studies, a 30 mm primary chamber exit port diameter is chosen for experimental studies. Detailed experimental investigations show that flameless combustion mode was achieved with evenly distributed combustion reaction zone and uniform temperature distribution in the combustor. Pollutant

Atomization, vaporization, turbulent mixing, and the chemical kinetics of a liquid fuel impact the combustion efficiency, stability, and emissions within IC engines. When fuels with varying physical properties are used in the same fuel injection system, they can exhibit very different atomization, represented by droplet size (SMD) and spray droplet distribution (SDD), which can then potentially influence the flame stability. In the current paper, the effect of the atomization process on flame stability is investigated by using a single nozzle size with three different hydrocarbon fuels, i.e., n-heptane, n-dodecane, and toluene. This study also investigates the flame stability for the same fuels using a unique approach in which nozzle sizes are selectively varied for each fuel to control the atomization process and minimize differences in the spray characteristics. The results show that the atomization process plays a major role in the flame liftoff heights and blowout limits. n-Dodecane, which produces the largest droplets and exhibits the lowest volatility, experiences highest flame liftoff heights and the greatest resistance to flame blowout compared to the other tested fuels. When the atomization process is controlled, more similar liftoff heights and blowout limits for all fuels were observed. The differences in liftoff height was shown to be a result of two-phase (spray) flame speed (i.e., pre-vaporized fuel reactivity (SL), and time scales associated with droplet evaporation). Differences in blowout was shown to be largely tied to fuel volatility, droplet size (i.e., the liquid loading into the flame), and the heat of combustion.

In the work, we have successfully developed practical surface tracking methods to calculate the erosive volume and the associated burning areas which are the important parameters to solve a nonlinear, pressurization-rate dependent combustion ballistics. Three methodologies, namely the front tracking, the emanating ray and the least distance methods, are proposed. The front tracking method is based on the Lagrangian point of view; while both the emanating ray and the least distance methods are formulated from the Eulerian viewpoint. Two two-dimensional test problems have been examined to compare with the programming complexity, simulation accuracy and required CPU time of the proposed methods. It is found that the least distance method performs superior to the other two methods in numerical respects. The least distance method is implemented with tetrahedron grids to track the outward propagation of a three-dimensional cubic. Comparison between the predicted erosive volume and corresponding theoretical result yields satisfactory agreement.

In this work the effect of flash boiling on liquid break up and atomization is first reviewed and then described in detail for the sprays produced by two high pressure multistream gasoline direct injectors. They were operated at 120bar and 200bar pressure with a fuel injection duration of 0.8ms into sub-atmospheric pressures between 1 and 0.1bar and four temperatures between 20°C and 100°C. The sprays were injected into an optically accessed chamber whose temperature and pressure were independently controlled to recreate the conditions found in an engine likely to cause flash boiling. Shadowgraph imaging with a single shot CCD camera was applied to obtain images of the spray morphology under a matrix of the above conditions. The interaction between spray streams is particularly complex under flash boiling conditions being a function of the nature of the fuel and the geometry of the injector. In this investigation, three different fuels, regular RON-95 gasoline and single components fuels, n-heptane and iso-octane were used with two different injectors, one with a 60 degree and the other with a 90 degree nominal external cone angle. In total, five variables were investigated: fuel pressure, ambient pressure, ambient temperature, fuel composition and injector geometry. Once processed, the recorded images allowed measurement of spray tip 2 penetration and cone angle. Qualitative data on the change in shape of the spray was also available. The results showed that flash boiling has potential to reduce droplet diameters and improve fuel vaporisation, however, the associated change in spray shape must be taken into account to avoid problems with spray impingement.

Structures of both gaseous and liquid ethanol flames in different oxidizing gas environments in the axisymmetric counterflow configuration at atmospheric pressure are studied. Initially, ethanol/air gas flames are considered where pure ethanol is directed against air at initial temperatures of 400 K, and N 2 is successively removed to obtain structures of ethanol/O 2 gas flames. Furthermore, the addition of CO 2 to the oxidizer side is carried out. Then, an ethanol spray is carried by air and directed against an air stream, and the same procedure is performed as described for the gas flames. The gas strain rate at the fuel side of the configuration is increased from low values of 55/s up to extinction, and the initial droplet diameter is varied. For the combustion of gaseous ethanol in air and in pure oxygen, the nitrogen removal results in an increase in the maximum flame temperature from 2010 K to 2920 K at a gas strain rate of 55/s on the fuel side of the configuration, and the extinction strain rates are 630/s and 26,000/s, respectively. It is confirmed that ethanol spray flames in air show two reaction zones at low strain whereas the lean ethanol spray flames in pure oxygen exhibit a single reaction zone in all situations studied. For increased liquid fuel mass flow rate to a global equivalence ratio of unity, two reaction zones are retrieved. An analysis regarding the addition of CO 2 in both the ethanol/oxygen gas and spray flames is also discussed and is found that CO 2 dilution of the carrier gas the spray is much more efficient than diluting the opposed gas stream in the counterflow configuration for the generation of MILD combustion conditions in oxy-fuel flames.

We propose a two-phase-fluid model for a turbulent full-cone high speed atomizing liquid jet that describes its dynamics in a simple but comprehensive manner with only the apex angle of the cone being a disposable parameter. The basic assumptions are that (i) the jet is statistically stationary and that (ii) it can be approximated by a mixture of a liquid and a gas with its phases in dynamic equilibrium. To derive the model, we impose conservation of the liquid volume and total momentum fluxes. Our model equation admits analytical solutions for the composite density and velocity of the two-phase fluid, both as functions of the distance from the nozzle, from which the dynamic pressure and gas the entrainment rate coefficient are calculated. Assuming a far-field approximation, we theoretically derive a constant gas entrainment rate coefficient solely in terms of the cone angle. Moreover, we carry out experiments for a single-phase turbulent air jet and show that the predictions of our...

To achieve uniform and efficient combustion in a rocket engine, a fine uniform spray is needed. The same is achieved by designing an injector with good atomization characteristics. Gas-centered swirl coaxial (GCSC) injector elements have been preferred recently in liquid rocket engines because of an inherent capability to dampen the pressure oscillations in the thrust chamber. The gas-centered swirl coaxial injector chosen for this study is proposed to be used in a semi-cryogenic rocket engine operating with oxidizer rich hot exhaust gases from the pre-burner and liquid kerosene as fuel. In this paper, nine different configurations of gas-centered swirl coaxial injector, sorted out by studying the spray angle and coefficient of discharge with swirl number varying from 9 to 20 and recess ratio of 0.5, 1, and 1.5 are investigated for their atomization characteristics. Spray uniformity, spray cone angle, and droplet size in terms of Sauter mean diameter and mass median diameter are stu...

The main role in air/fuel mixture formation at the IC diesel engines has the energy introduced by fuel into the IC engine that is the characteristics of spraying fuel into the combustion chamber. The characteristic can be defined by the spray length, the spray cone angle, the physical and the chemical structure of fuel spray by different sections. Having in mind very complex experimental setups for researching in this field, the mentioned characteristics are mostly analyzed by calculations. There are two methods in the literature, the first based on use of the semi-empirical expressions (correlations) and the second, the calculations of spray characteristics by use of very complex mathematical methods. The second method is dominant in the modern literature. The main disadvantage of the calculation methods is a correct definition of real state at the end of the nozzle orifice (real boundary conditions). The majority of the researchers in this field use most frequently the coefficient of total losses inside the injector. This coefficient depends on injector design, as well as depends on the level of fuel energy and fuel energy transformation along the injector. Having in mind the importance of the real boundary conditions, the complex methods for calculation of the fuel spray characteristics should have the calculation of fuel flows inside the injector and the calculation of spray characteristics together. This approach is a very complex numerical problem and there are no existing computer programs with satisfactory calculation results. Analysis of spray characteristics by use of the semi-empirical expressions (correlations) is presented in this paper. The special attention is dedicated to the analysis of the constant in the semi-empirical expressions and influence parameters on this constant. Also, the method for definition of realistic boundary condition at the end of the nozzle orifice is presented in the paper. By use of this method completely avoid a use of the coefficient of total losses inside the injector. At the same time, semi-empirical expressions have the universal constant that does not depend on the injector design.

Numerical modeling of several turbulent nonreacting and reacting spray jets is carried out using a fully stochastic separated flow (FSSF) approach. As is widely used, the carrier-phase is considered in an Eulerian framework, while the dispersed phase is tracked in a Lagrangian framework following the stochastic separated flow (SSF) model. Various interactions between the two phases are taken into account by means of two-way coupling. Spray evaporation is described using a thermal model with an infinite conductivity in the liquid phase. The gas-phase turbulence terms are closed using the k-e model. A novel mixture fraction based approach is used to stochastically model the fluctuating temperature and composition in the gas phase and these are then used to refine the estimates of the heat and mass transfer rates between the droplets and the surrounding gas-phase. In classical SSF (CSSF) methods, stochastic fluctuations of only the gas-phase velocity are modeled.