Non-premixed Spray Research Articles

NEW EXPERIMENTAL DATA OF CHILD DROPLETS IDENTIFICATION AFTER TWO-LIQUID DROPLET BREAKUP

New experimental data of child droplets characteristics (times to puffing/microexplosion, distribution of sizes, and ratio of the evaporation surface area before and after puffing/microexplosion) of heated two-liquid droplets in puffing and microexplosion modes are presented. Different droplet configuration effects (fuel liquid (rapeseed oil) - shell, water-core, vice versa) are discussed. The experiments were carried out with two-liquid droplets in which coal particles were added to meet nonpremixed spray technologies into the combustion chambers. Breakup conditions of water/fuel droplets in monodisperse and polydisperse aerosols are determined. The difference in secondary atomization effects of water and fuel is described. The effect of gas temperature in the range 720-1070 K on child droplets characteristics is discussed.

A DNS evaluation of mixing and evaporation models for TPDF modelling of nonpremixed spray flames

In the present work, nonpremixed temporally evolving planar spray jet flames are simulated using both direct numerical simulation (DNS) and the composition transported probability density function (TPDF) method. The objective is to assess the performance of various mixing and evaporation source term distribution models which are required to close the PDF transport equation in spray flames. Quantities which would normally be provided to the TPDF solver by spray models and turbulence models are provided from the DNS: the mean flow velocity, turbulent diffusivity, mixing frequency, and cell-mean evaporation source term. Two cases with different Damköhler numbers (Da) are considered. The low Da case (Da-) features extinction followed by reignition while extinction in the high Da case (Da+) is insignificant. The TPDF modelling considers two mixing models: interaction by exchange with the mean (IEM) and Euclidean minimum spanning trees (EMST). Three models for distribution of the evaporation source terms are considered: EQUAL which distributes them in proportion to notional particles’ mass weight, NEW which creates new particles of pure fuel, and SAT which distributes the sources preferentially to notional particles close to saturation. It is found that the IEM model overpredicts the extinction when used with any evaporation model for both Da- and Da+ cases. The EMST model captures well the trend for extinction and reignition for the Da- case when it is coupled with the EQUAL evaporation model, but it overpredicts the extinction when coupled with the NEW or SAT evaporation model. For the Da+ case, all evaporation models reasonably capture the flame dynamics when coupled with EMST. The flame temperature in the mixture fraction space was examined to further assess the model performance. In general the EMST model results in narrow PDFs with little conditional fluctuation, while the IEM model produces bimodal PDFs with burning and partial extinction branches.

The role of separation of scales in the description of spray combustion

The present paper deals with the description of the interacting multiscale processes governing spray vaporization and combustion downstream from the near-injector atomization region in liquid-fueled burners. One of the main objectives is to emphasize the progress made in the mathematical description and understanding of reactive spray flows by incorporation of rationally derived simplifications based on the disparity of length and time scales present in the problem. In particular, we aim to show how the disparity of the scales that correspond – with increasing values of their orders of magnitude – to the droplet size, interdroplet spacing, and width of the spray jets, ensures the validity of their homogenized description. The two-way coupling associated with exchanges of mass, momentum, and energy between the gas and the liquid phases is dominated by the homogenized exchanges with the gas provided collectively by the droplets, and not by the direct interaction between neighboring droplets. The formulation is used as a basis to address nonpremixed spray diffusion flames in the Burke-Schumann limit of infinitely fast chemical reactions, with the conservation equations written in terms of chemistry-free coupling functions that allow for general nonunity Lewis numbers of the fuel vapor. Laminar canonical problems that have been used in the past to shed light on different aspects of spray-combustion phenomena are also discussed, including spherical spray clouds and structures of counterflow spray flames in mixing layers. The presentation ends with a brief account of some open problems and modeling challenges.

LES of a methanol spray flame with a stochastic sub-grid model

This paper describes the Large Eddy Simulation (LES) of a methanol/air turbulent nonpremixed spray flame. An Eulerian stochastic field method is employed for the turbulence-chemistry interaction of the gas phase while a Lagrangian formulation is used for the liquid phase. A reduced reaction mechanism (18 species and 14 reactions) is adopted and stochastic models are used to account for the influence of sub-grid scale (sgs) motions on droplet dispersion and evaporation. Comparisons of the predicted gas phase and droplet statistics with measurements show a good agreement confirming that the droplet dispersion and evaporation models used in this work are adequate. The general features of the spray flame such as the occurrence of external group combustion and its development into separate combusting islands are well captured.

An experimental study of well-defined turbulent nonpremixed spray flames

An experiment system was designed for the study of well-defined turbulent nonpremixed spray flames. Particular emphasis was placed on minimizing the influence of the injector design and on maximizing turbulence within the spray flames. A comprehensive description of the structure of such flames was obtained by applying a variety of complementary diagnostic techniques, including: broadband chemiluminescence imaging, CH∗ emission imaging, phase Doppler interferometric techniques, and spontaneous Raman spectroscopy. Two methanol spray flames were examined in detail, with Reynolds number ranging from 2.1 × 10 4 to 2.8 × 10 4. Flame appearance and detailed measurements confirmed the occurrence of group combustion. Near the burner mouth, a dense column of drops enveloped by a common flame was observed. Further up, large corrugated structures were visualized which eventually developed into separate “islands.” A significant fraction of the spray escaped unburned, which implies that droplet evaporation is slow in this configuration. Detailed scanning of the flames provided an extensive database of average and fluctuating components of gas velocity and temperature, as well as spray and droplet size–classified properties. Key conclusions from such measurements include: the evidence of two-way coupling between the two phases along the centerline near the burner mouth; a velocity acceleration in the densest areas of the spray flames, as a result of momentum addition through vaporization, followed by deceleration farther downstream as the jet spreading predominates; and the droplet inertial behavior, especially for the large size classes, as confirmed by estimates of some relevant Stokes numbers. The average flame height was found to correlate with an overall equivalence ratio and with the initial concentration of droplets at the burner mouth.

An experimental investigation of non-premixed turbulent spray flames and their self-similar behavior

An experimental investigation was conducted in well-defined, turbulent non-premixed spray flames of methanol. An ultrasonic atomizer was used to disperse the liquid into droplets with a small velocity slip with respect to the gas. As a result, the influence of the liquid injector on the flowfield was deliberately minimized. Turbulence was enhanced by the use of a combination of perforated plate and contoured geometry. Close to the burner exit, where the liquid loading was high, the spray flames had a dense droplet core providing the vapor source, and an outer envelope flame. Two components of the velocity and temperature of the gaseous phase, as well as size, velocity, and concentration of the droplet phase, along with the relevant statistical quantities, were measured by phase Doppler anemometry and spontaneous Raman thermometry. The characterization of the temperature field, and, indirectly, of the gaseous density field, via equation of state, allowed for the mapping of the experimental data to a constant density space, by using a Howarth transformation. In this transformed space, the spray flames could be described as a combination of point source of vaporization and point source of momentum. In particular, detailed characterization of two spray flames showed that the vapor source in the gas-phase mass governing equation and the axial momentum behave in a self-similar fashion, the former throughout the spray flame. Close to the burner exit, where the droplet concentration was large, the scaling of point source of momentum did not apply because of the two-way coupling between the phases.

Bipropellant combustion in a liquid rocket combustion chamber

A comprehensive computer simulation code is developed for the combustion-flow analysis of a bipropellant liquid rocket engine combustor. A bipropellant group combustion model is built in the computer code to facilitate the determination of various combustion modes, including the combustion of oxidizer droplets, fuel droplets and gas mixture. Several physical submodels are also adopted to account for the interaction between bipropellant droplets and the gas-phase flow. An axisymmetric cylindrical liquid rocket engine combustion chamber installed with two annular-ring injectors is selected to demonstrate the capabilities of the present computer simulation code. Two types of injection processes are analyzed. One is characterized by a non-premixed spray process and the other is a premixed spray process. Selected numerical computations show that the premixed spray results in a better utilization of combustion chamber space and gives a higher combustion efficiency in general. But finer-droplet sprays do not necessarily lead to a higher combustion efficiency as traditionally speculated.