Turbulent Spray Research Articles

Joint Gas-Phase Velocity-Scalar PDF Modeling for Turbulent Evaporating Spray Flows

The article proposes a joint gas-phase velocity-scalar probability density function (PDF) model for turbulent spray flows. The joint gas-phase velocity-mixture fraction PDF transport equation for turbulent spray flows is derived and modeled. The simplified Langevin model and interaction-by-exchange-with-the-mean (IEM) model are extended to account for spray evaporation, and they close the PDF transport equation, which is solved by a Lagrangian Monte-Carlo particle method, and the droplet characteristics including heating, vaporization, and motion are solved by a Lagrangian droplet parcel method using the point-source approximation. A steady, two-dimensional, axi-symmetric, turbulent methanol/air dilute non reacting spray flow without swirl is simulated using the present method. The results of the gas velocity obtained by the joint PDF method are in good agreement with experimental data, and the PDF method improves the simulation results obtained using a moment closure model.

NOx emissions from high swirl turbulent spray flames with highly oxygenated fuels

Combustion of fuels with fuel bound oxygen is of interest from both a practical and a fundamental viewpoint. While a great deal of work has been done studying the effect of oxygenated additives in diesel and gasoline engines, much less has been done examining combustion characteristics of fuels with extremely high mass fractions of fuel bound oxygen. This work presents an initial investigation into the very low NOx emissions resulting from the combustion of a model, high oxygen mass fraction fuel. Glycerol was chosen as a model fuel with a fuel bound oxygen mass fraction of 52%, and was compared with emissions measured from diesel combustion at similar conditions in a high swirl turbulent spray flame. This work has shown that high fuel bound oxygen mass fractions allow for combustion at low global equivalence ratios with comparable exhaust gas temperatures due to the significantly lower concentrations of diluting nitrogen. Despite similar exhaust gas temperatures, NOx emissions from glycerol combustion were up to an order of magnitude lower than those measured using diesel fuel. This is shown to be a result not of specific burner geometry, but rather is influenced by the presence of higher oxygen and lower nitrogen concentrations at the flame front inhibiting NOx production.

LES/probability density function approach for the simulation of an ethanol spray flame

A consistent probability density function (PDF) based combustion modeling approach for large eddy simulation of turbulent spray combustion is formulated. A Lagrangian Monte-Carlo approach for solving the PDF transport equation in the context of low-Mach number LES formulations is developed. The LES/PDF approach is used to simulate an experimental pilot-stabilized ethanol spray flame. In this particular flame, droplet evaporation occurs away from the flame front, leading to separation of the two processes. A high-temperature pre-flame zone with a stratified mixture is found to exist throughout the length of the flame. The high flow rate of the droplet-laden air prevents the flame front from propagating through this evaporated, but well-mixed fuel/air mixture. Comparison with experiments displayed large discrepancies concerning the droplet inflow conditions. In particular, the inflow turbulence is significantly modified by droplet-turbulence interactions in the injection pipe.

Lagrangian conditional statistics of turbulent n-heptane spray combustion in different injection conditions

3D DNS is performed for n-heptane sprays that go through ignition and combustion in different injection conditions. Conditional statistics are obtained for multiple Lagrangian groups of sequentially evaporating fuel in the group and the collective combustion regime. Ignition occurs close to but leaner than the most reactive mixture fraction for fuel of the longest residence time and low scalar dissipation rates. Combustion propagates with strong interaction among neighboring groups after ignition of the preceding flame group in the given conditions. Multiple flame groups are required for accurate description of combustion of a spray over long injection duration or multiply injected sprays. Reasonable agreement is shown between DNS and model predictions of conditional evaporation and scalar dissipation rates in the range of meaningful probabilities. Budgets are checked for all component terms in the balance equation of conditional sensible enthalpy with evaporation source terms.

Aircraft engines’ needs from combustion science and engineering

Turbulent multiphase combustion plays an important role in both nature (e.g., volcanos and pool fires) and industry (e.g., industrial furnaces, aeroengines, and internal combustion engines). It is a highly complex multiscale and multi-physicochemical process in which interactions between the dispersed and continuous phases, phase change, droplet collisions, evaporation, mixing, heat transfer, and chemical reactions occur simultaneously. In recent years, significant progress has been made in understanding the mechanisms of spray flames and their behaviors in combustion engines. This paper covers key and representative developments in the area of turbulent spray combustion with a focus on spray–chemistry–turbulence interactions. The effects of turbulence–chemistry, spray–turbulence, and spray–chemistry interactions on the spray process, ignition, flame stabilization and emission are comprehensively discussed at elevated pressures and temperatures. Furthermore, spray–radiation and spray flame–wall interactions, which are important to engine performance and emission characteristics, are scrutinized. Supercritical spray flames and turbulent spray flames in dual-fuel engines are also discussed. Finally, outlooks and further challenges for the research field are outlined.

CFD Investigation into Swirl Spray Turbulence Combustion

CFD Investigation into Swirl Spray Turbulence Combustion

The Structure of the Auto-Ignition Region of Turbulent Dilute Methanol Sprays Issuing in a Vitiated Co-flow

This paper presents planar imaging of laser induced fluorescence (LIF) from key reactive species in the auto-ignition region of dilute turbulent spray flames of methanol. High-speed (5 kHz) LIF-OH imaging as well as low speed (10 Hz) imaging of joint LIF-OH-CH2O is performed. The product of the OH and CH2O signals is used as a qualitative indicator of local heat release. The burner is kept intentionally simple to facilitate computations and the spray is formed upstream of the jet exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The studied flames are all lifted but differ in the shape of their leading edge and heat release zones. Similarities with auto-ignition of gaseous fuels, as well as differences, are noted here. Formaldehyde is detected earlier than OH implying that the former is a key precursor in the initiation of auto-ignition. Growing kernels of OH that are advected from upstream, close in on the jet centreline and ignite the main flame. The existence of double reaction zones in some flames may be due to ignitable mixtures formed subsequent to local evaporation of droplets and subsequent mixing. When air is used as spray carrier, reaction zones broaden with distance, possibly due to increased partial premixing and regions of intense heat release occur near the flame centreline further downstream. With nitrogen as carrier, the flame maintains a nominal diffusion-like structure with reaction zones of uniform width and substantially less concentration of heat release on the flame centreline.

Modeling and Simulation of Evaporating Spray Flows with Coalescence in an Eulerian Framework

AbstractThe study concerns the modeling and simulation of gas‐droplet and droplet‐droplet interactions on spray dynamics and droplet size distribution in turbulent spray flows where the spray is described in an Eulerian framework. The primary focus is to develop a suitable mathematical model that can describe the underlying physics including the polydispersion of the spray, evaporation, drag, coalescence, and interaction with the convective gas flow. This is achieved by applying the direct quadrature method of moments, which is extended to account for evaporation, drag force and coalescence by implementing well‐established standard models available in literature. The model is developed and formulated in three‐dimensional physical space, whereas the computations are carried out in a one‐dimensional geometric configuration since the reference experimental setup is only one‐dimensional. A water spray carried by nitrogen was injected through a hollow cone nozzle into a cylindrical spray chamber. The results reveal that evaporation and coalescence have a significant effect on the droplet size distribution whereas the droplet velocity is influenced by drag force as anticipated. Computational and experimental results agree very well when coalescence is considered.

Simulation of spray development and turbulent combustion processes in low and high speed diesel engines by the CMC-ISR model

Simulation is performed to analyse the characteristics of turbulent spray combustion in conventional low and high speed diesel engine conditions. Turbulence–chemistry interaction is resolved by the Conditional Moment Closure (CMC) model in the spatially integrated form of an Incompletely Stirred Reactor (ISR). After validation against measured pressure traces, characteristic length and time scales and dimensionless numbers are estimated at the locations of sequentially injected fuel groups. Conditional flame structures are calculated for sequentially evaporated fuel groups to consider different available periods for ignition chemistry. Injection overlaps the combustion period in the high rpm engine, while most combustion occurs after injection and evaporation are complete in the low rpm engine. Ignition occurs in rich premixture with the initial peak temperature at the equivalence ratio around 2–4 as observed in Dec [2]. It corresponds to the most reactive mixture fraction of the minimum ignition delay for the given mixture states. Combustion proceeds to lean and rich sides in the mixture fraction space as a diffusion process by turbulence. The mean scalar dissipation rates (SDRs) are lower than the extinction limit to show stability of diffusion flames throughout the combustion period.

Spray algorithm without interface construction

This research is aimed to create a new and robust family of convective schemes to capture the interface between the dispersed and the carrier phases in a spray without the need to build up the interface boundary. The selection of the Weighted Average Flux (WAF) scheme is due to this scheme being designed to deal with random flux scheme which is second-order accurate in space and time. The convective flux in each cell face utilizes the WAF scheme blended with Switching Technique for Advection and Capturing of Surfaces (STACS) scheme for high resolution flux limiters. In the next step, the high resolution scheme is blended with the WAF scheme to provide the sharpness and boundedness of the interface by using switching strategy. In this work, the Eulerian–Eulerian framework of non-reactive turbulent spray is set in terms of theoretical proposed methodology namely spray moments of drop size distribution, presented by Beck and Watkins [1]. The computational spray model avoids the need to segregate the local droplet number distribution into parcels of identical droplets. The proposed scheme is tested on capturing the spray edges in modelling hollow cone sprays without need to reconstruct two-phase interface. A test is made on simple comparison between TVD scheme and WAF scheme using the same flux limiter on convective flow hollow cone spray. Results show the WAF scheme gives a better prediction than TVD scheme. The only way to check the accuracy of the presented models is by evaluating the spray sheet thickness.

Numerical Investigation of Dense Turbulent Sprays with an Eulerian Approach and DQMOM

AbstractThe spray characteristics of the injected liquid fuel predominantly influence the combustion and emissions in IC‐engines and gas turbines. This is predetermined by a dense spray region in which droplet‐droplet interaction processes play a significant role. In order to accurately describe and control the dense spray behavior in modern engines, an appropriate numerical modeling tool is needed. This contribution aims at including droplet‐droplet interactions into an Eulerian approach coupled to the Direct Quadrature Method of Moments (DQMOM) in order to describe evaporating droplet polydispersity and analyze dense turbulent sprays phenomena. Among the advantages of the Eulerian approach are a lower computational cost through optimal parallel computing and a straightforward liquid‐gas phase coupling. To assess the designed tool, numerical results are compared to Phase Doppler Anemometry (PDA) measurements of a hollow‐cone water spray. The experiment provides comprehensive validation data that include gas velocities, droplet size distribution, droplet mass fluxes and droplet velocities. Turbulence is captured by two different k‐ε based models. Preliminary results show that the designed tool is able to capture the process under study in a satisfactory way. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A reduced mechanism for biodiesel surrogates with low temperature chemistry for compression ignition engine applications

Biodiesel is a promising alternative fuel for compression ignition (CI) engines. It is a renewable energy source that can be used in these engines without significant alteration in design. The detailed chemical kinetics of biodiesel is however highly complex. In the present study, a skeletal mechanism with 123 species and 394 reactions for a tri-component biodiesel surrogate, which consists of methyl decanoate, methyl 9-decanoate and n-heptane was developed for simulations of 3-D turbulent spray combustion under engine-like conditions. The reduction was based on an improved directed relation graph (DRG) method that is particularly suitable for mechanisms with many isomers, followed by isomer lumping and DRG-aided sensitivity analysis (DRGASA). The reduction was performed for pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2 for both extinction and ignition applications. The initial temperatures for ignition were from 700 to 1800 K. The wide parameter range ensures the applicability of the skeletal mechanism under engine-like conditions. As such the skeletal mechanism is applicable for ignition at both low and high temperatures. Compared with the detailed mechanism that consists of 3299 species and 10806 reactions, the skeletal mechanism features a significant reduction in size while still retaining good accuracy and comprehensiveness. The validations of ignition delay time, flame lift-off length and important species profiles were also performed in 3-D engine simulations and compared with the experimental data from Sandia National Laboratories under CI engine conditions.

The internal structure of igniting turbulent sprays as revealed by complex chemistry DNS

A parametric study of spark ignition in a uniform monodisperse turbulent spray is performed with complex chemistry three-dimensional Direct Numerical Simulations in order to improve the understanding of the structure of the ignition kernel. The heat produced by the kernel increases with the amount of fuel evaporated inside the spark volume. Moreover, the heat sink by evaporation is initially higher than the heat release and can have a negative effect on ignition. With the sprays investigated, heat release occurs over a large range of mixture fractions, being high within the nominal flammability limits and finite but low below the lean flammability limit. The burning of very lean regions is attributed to the diffusion of heat and species from regions of high heat release, and from the spark, to lean regions. Two modes of spray ignition are reported. With a relatively dilute spray, nominally flammable material exists only near the droplets. Reaction zones are created locally near the droplets and have a non-premixed character. They spread from droplet to droplet through a very lean interdroplet spacing. With a dense spray, the hot spark region is rich due to substantial evaporation but the cold region remains lean. In between, a large surface of flammable material is generated by evaporation. Ignition occurs there and a large reaction zone propagates from the rich burned region to the cold lean region. This flame is wrinkled due to the stratified mixture fraction field and evaporative cooling. In the dilute spray, the reaction front curvature pdf contains high values associated with single droplet combustion, while in the dense spray, the curvature is lower and closer to the curvature associated with gaseous fuel ignition kernels.

Large eddy simulation of fuel injection and mixing process in a diesel engine

The large eddy simulation (LES) approach implemented in the KIVA-3V code and based on one-equation sub-grid turbulent kinetic energy model are employed for numerical computation of diesel sprays in a constant volume vessel and in a Caterpillar 3400 series diesel engine. Computational results are compared with those obtained by an RANS (RNG k-ɛ) model as well as with experimental data. The sensitivity of the LES results to mesh resolution is also discussed. The results show that LES generally provides flow and spray characteristics in better agreement with experimental data than RANS; and that small-scale random vortical structures of the in-cylinder turbulent spray field can be captured by LES. Furthermore, the penetrations of fuel droplets and vapors calculated by LES are larger than the RANS result, and the sub-grid turbulent kinetic energy and sub-grid turbulent viscosity provided by the LES model are evidently less than those calculated by the RANS model. Finally, it is found that the initial swirl significantly affects the spray penetration and the distribution of fuel vapor within the combustion chamber.

Modeling of nonreacting and reacting turbulent spray jets using a fully stochastic separated flow approach

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– ε 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. Successful implementation of the FSSF approach to turbulent nonreacting and reacting spray jets is demonstrated. Results are compared against experimental measurements as well as with predictions using the CSSF approach for both nonreacting and reacting spray jets. The FSSF approach shows little difference from the CSSF predictions for nonreacting spray jets but differences are significant for reacting spray jets. In general, the FSSF approach gives good predictions of the flame length and structure but further improvements in modeling may be needed to improve the accuracy of some details of the predictions.

A high efficiency parallel unstructured solver dedicated to internal combustion engine simulation

IFP-C3D, a hexahedral unstructured parallel solver dedicated to multiphysics calculation is being developed at IFP to compute the compressible combustion in internal engines. IFP-C3D uses an unstructured formalism, the finite volume method on staggered grid, time splitting, SIMPLE loop, subcycled advection, turbulent and Lagrangian spray and a liquid film model. Original algorithms and models such as the conditional temporal interpolation methodology for moving grids, the remapping algorithm for transferring quantities on different meshes during the computation enable IFP-C3D to deal with complex moving geometries with large volume deformation induced by all moving geometrical parts (intake/exhaust valve, piston). Large super-scalar machines up to 1000 processors are being widely used and IFP-C3D has been optimized for running on these Cluster machines. IFP-C3D is parallelized using the Message Passing Interface (MPI) library to distribute a calculation over a large number of processors. Moreover, IFP-C3D uses an optimized linear algebraic library to solve linear matrix systems and the METIS partitionner library to distribute the computational load equally for all meshes used during the calculation and in particular during the remap stage when new meshes are loaded. Numerical results and performance timing are presented to demonstrate the computational efficiency of the code.

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH ∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH ∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH ∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH ∗ are seen to appear in this region and grow as they are advected further downstream.

A mixture fraction framework for the theory and modeling of droplets and sprays

A mixture fraction is carefully defined for evaporation and combustion of droplets and sprays. The definition is valid at points in either the liquid or gas phases and care is taken to distinguish between definitions based on conserved scalars appropriate for heat transfer and those for mass transfer. Results are presented for Spalding B numbers and values of the mixture fraction at the droplet surface for the fast chemistry case and for the case where the droplet cannot sustain an envelope flame. The classical theory for an isolated droplet with spherical symmetry yields simple formulae when expressed in mixture fraction terms. New results are then readily obtained for several quantities of interest in spray modeling. The formulation provides a seamless unification of droplet evaporation processes with gas-phase mixing and reaction. Mixing in a turbulent spray jet is identified as a model problem that clarifies the role of large scale structures in the overall mixing process. Important constraints on the parameter space for sprays are shown to be greatly clarified when expressed in the mixture fraction framework. It is shown how the classical approach for segregated flow with Eulerian/Lagrangian modeling of dispersion and transfer processes in turbulent sprays can be upgraded to include fluctuations in the temperature and composition surrounding the droplets on top of those coming from the turbulent velocity fluctuations. Such preliminary calculations that assume a simple chemically reacting system can readily be upgraded using flamelet functions derived from counterflow experiments or computations: these can then form the starting point for full chemistry calculations using such approaches as conditional moment closure.

Complex chemistry simulations of spark ignition in turbulent sprays

Three-dimensional DNS of two-phase flows with the point-source approximation and with complex chemistry for n-heptane has been used to extract physical information on the structure of igniting kernels following localised heat deposition in turbulent monodisperse sprays. Consistent with experiment, small sparks fail to ignite and sprays ignite later than premixed gaseous mixtures. Reaction rates are intense in spherical zones near droplets and much lower in the interdroplet spacing, resulting in a highly wrinkled flame surface. The propagation of these reaction zones was observed. The flame shows a locally non-premixed character, with reactions proceeding at a wide range of mixture fractions, which increases as evaporation progresses. The distribution of various chemical species is presented. The results constitute a database for model validation and physical analysis.

Analysis of combustion regimes and conditional statistics of autoigniting turbulent n-heptane sprays

DNS is performed for a statistically one dimensional layer of a spray region resembling diesel engine conditions. The group and collective combustion regimes are identified according to the ratio of the chemical and transport time scales for a single droplet. The statistics in group combustion are similar with those in gas phase combustion. The collective combustion regime involves interspersed rich regions with different dissipation characteristics. Reasonable agreements are shown with the scaled AMC model and the linear evaporation model in the ranges of meaningful probability. Initially the evaporation terms are dominant in the budgets of the conditional enthalpy equation. After ignition the chemical reaction term becomes dominant to be balanced by the time rate of change term. For modeling turbulent spray combustion it may not be essential to consider detailed micro structures around each droplet, unless in the droplet combustion regime.