Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L17: Reacting Flows III: Engines, Sprays, and Soot |
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Chair: Douglas Schwer, Naval Research Laboratory Room: 320 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L17.00001: The flow field in a rotating detonation-wave engine Kazhikathra Kailasanath, Douglas Schwer Rotating detonation-wave engines (RDE) are a form of continuous detonation-wave engine. They potentially provide further gains than an intermittent or pulsed detonation--wave engine (PDE). However, significantly less work has been on this concept when compared to the PDE. In this talk, we present the detailed flow field in an idealized RDE, primarily consisting of two concentric cylinders. A premixed detonable mixture is injected into the annulus between the two concentric cylinders. Once a detonation is initiated, it keeps travelling around in the annulus as long as there is fresh detonable mixture ahead of it. Hence, the injection process is critically important to the stability and performance of the RDE. Furthermore, we show that the flow field is quite complex consisting of multiple shock waves and the outflow is primarily axial, although the detonation-wave is travelling around circumferentially. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L17.00002: Fluid dynamics in a Rotating-Detonation-Engine with micro-injectors Douglas Schwer Rotating detonation engines (RDE's) represent a natural extension of the extensively studied pulse detonation engines (PDE's) for obtaining propulsion from the high efficiency detonation cycle. RDE's require fuel and oxidizer under high pressure to be injected through micro-nozzles from one or two plenums (for premixed and non-premixed). This injection process is critically important to the stability and performance of the RDE. This paper studies the effect of this injection process on the detonation wave within the combustion chamber, with an emphasis on how the fluid dynamics are affected. Both two-dimensional and three-dimensional simulations are done using well proven numerical methods for both the combustion chamber and mixture plenums of an idealized RDE. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L17.00003: LES Of A Model Aircraft Combustor: Spray Model Assumptions And Pollutant Formation Edward Knudsen, Shashank, Heinz Pitsch Large eddy simulations of NASA's Lean Direct Injection spray combustor are performed. Fuel spray is described using Lagrangian particles, and combustion is modeled using a multi-regime flamelet approach. Particular emphasis is placed on analyzing how spray model assumptions affect pollutant predictions. This analysis is performed by running a series of cases that employ different spray evaporation models, different spray cone angles, and different bulk air flow rates. Results demonstrate that both CO and NO are leading order sensitive to the details of the spray formulation. Temperature is also shown to be leading order sensitive to the spray formulation, while major species such as CO$_2$ and H$_2$O are less sensitive. These results highlight the difficulty of validating pollutant models with liquid fueled experiments that are subject to spray modeling uncertainty. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L17.00004: Theory of vaporization and combustion of fuel sprays in strained laminar mixing layers Javier Urzay, Antonio Sanchez, Heinz Pitsch, Amable Linan The vaporization and combustion of a monodisperse fuel spray in a laminar counterflow mixing layer is investigated under conditions such that the droplet Stokes number is smaller than 1/4, so that the droplets do not cross the stagnation plane, but instead tend to accumulate there because of their inertial slip motion. Vaporization is confined to the thin strained mixing layer separating the spray from the hot stream, which can be described with an Eulerian description for the liquid phase. The numerical integration of the resulting boundary-value problem provides the structure of the reactive mixing layer, including the standoff distance from the stagnation plane where droplets disappear and the flame location. Limiting asymptotic solutions for extreme values of the controlling parameters are also determined, and a generalized vaporization law for droplets in the presence of thermal gradients is derived. The results provide increased understanding of laminar spray flamelets. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L17.00005: A multi-scalar PDF approach for LES of turbulent spray combustion Venkat Raman, Colin Heye A comprehensive joint-scalar probability density function (PDF) approach is proposed for large eddy simulation (LES) of turbulent spray combustion and tests are conducted to analyze the validity and modeling requirements. The PDF method has the advantage that the chemical source term appears closed but requires models for the small scale mixing process. A stable and consistent numerical algorithm for the LES/PDF approach is presented. To understand the modeling issues in the PDF method, direct numerical simulation of a spray flame at three different fuel droplet Stokes numbers and an equivalent gaseous flame are carried out. Assumptions in closing the subfilter conditional diffusion term in the filtered PDF transport equation are evaluated for various model forms. In addition, the validity of evaporation rate models in high Stokes number flows is analyzed. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L17.00006: DNS of aerosol evolution in a turbulent jet Kun Zhou, Antonio Attili, Fabrizio Bisetti The effects of turbulence on the evolution of aerosols are not well understood. In this work, the interaction of aerosol dynamics and turbulence are studied in a canonical flow configuration by numerical means. The configuration consists of a hot nitrogen stream saturated with dibutyl phthalate (DBP) vapor mixing with cool air in a shear layer. A direct numerical simulation (DNS) for the momentum and scalar fields is coupled with the direct quadrature method of moments (DQMOM) for the condensing liquid phase. The effects of turbulent mixing on aerosol processes (nucleation, condensation, and coagulation) are quantified by analyzing the statistics of number density and droplet sizes. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L17.00007: Soot formation in unstrained diffusion flames Etienne Robert, Nils-Erik Olofsson, Jonathan Johnsson, Henrik Bladh, Per-Erik Bengtsson The formation of soot particles has been investigated in CH$_4$/O$_2$ diffusion flames using a burner which allows the creation of a nearly unstrained planar reaction sheet. The sooting limits, soot volume fraction and particle size were measured as a function of bulk flow across the flame mixture strength and transport properties of the reactants. Mass spectrometry was used to measure the effective mixture composition close to the flame and Laser Induced Incandescence (LII)for the soot volume fraction and particle size. The parameter space was mapped as follows: Starting from a stable non-sooting baseline flame, the mixture strength was progressively increased by raising the fuel volume fraction while keeping other parameters constant (bulk flow across the flame, oxidant and inert composition). As the mixture strength was increased, the soot volume fraction and particle size increased up to a point where very big soot particle aggregates became visible to the naked eye on the flame side of the sooting layer. The exact mechanism by which these super aggregates arise is unknown but it is postulated that the absence of strain in the flow field and the thermophoretic effect allows soot particles to remain in a region of the burning chamber suitable for growth for an extended period of time. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L17.00008: Characterization of Mixing and Ignition Effects in Flow-Reactors Using a Particle Method Simon Weiher, Matthias Ihme Recent investigations have indicated discrepancies between measurements and simulations of the ignition delay for syngas-mixtures at high-pressure/low- temperature conditions. While relevant sources for these discrepancies have been identified in the context of rapid compression machines and shock tubes, the underlying mechanisms and nonidealities in flow-reactor experiments have not yet been quantified. The objective of this investigation is to characterize effects of turbulence and flow-field inhomogeneities on the mixing and ignition-dynamics in flow-reactors. To this, an idealized flow-reactor is considered, in which the unsteady and three-dimensional flow-field is obtained from the solution of a large-eddy simulation. A particle method is used to describe the mixing, induction, and subsequent ignition of the reactants. Utilizing this model, parametric studies are performed to systematically quantify effects of initial mixture-preparation, flow-rate modulation, and turbulence-levels on the ignition-process over a range of practically relevant temperature and pressure conditions. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L17.00009: Quantification of the uncertainties in the prediction of extinction of hydrogen-air diffusion flames Nicolas Kseib, Javier Urzay, Gianluca Iaccarino The study of the physical processes that lead to extinction of flames in gaseous hydrogen-air non-premixed combustion is of paramount importance for the reliable design of power plants and advanced propulsion systems in automobiles and hypersonic aircrafts. However, there remain several uncertainties in the experimental quantification of reaction rates of elementary steps in most of hydrogen-air mechanisms, which can produce hazards in hydrogen manipulation and engine malfunction. In this study, the effects of aleatory uncertainties in the chemical reaction-rate constants induced in hydrogen-air counterflow diffusion-flame extinction processes are addressed, with a probabilistic representation of the uncertain parameters sampled with a Markov-Chain Monte Carlo algorithm. Measurements of the reaction-rate constants and their associated uncertainty factors, reported earlier for the Stanford hydrogen-air detailed chemical mechanism, are used to study the propagation of uncertainties in the calculation of scalar dissipation rates at extinction. Non-intrusive methods are used to analyze the variablities, with the probability density function of the scalar dissipation rate being sampled around regions involving flame extinction and global sensitivity indices being computed by Monte Carlo sampling. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L17.00010: Regression Rate Enhancement of Hybrid Rocket Motors using Mixed Hybrid Concept Palani Kumar Chidambaram, Amit Kumar Low regression rates have been a major problem for hybrid rocket motors. In the present study, the effect on regression rate by adding ammonium perchlorate (AP) in solid fuel is studied numerically. AP mixed with HTPB is used as solid fuel and gaseous oxygen (GOX) is used as oxidizer. Solid fuel compositions are chosen such that the rocket motor retains start-stop capability. A reduced three step mechanism proposed in the literature is utilized to simulate the combustion. In the combustion chamber, two distinct flame fronts are captured. AP decomposition reaction forms a premixed flame front near the fuel surface. The AP decomposed products also react with HTPB. Heat released in these reactions improves the heat transferred to solid fuel and the regression rate significantly. Un-burnt fuel in the products further reacts with GOX forming a diffusion flame front farther from fuel surface. The presence of premixed flame front thus overcomes the low-regressing nature of hybrid combustion. It is found that 50{\%} AP in solid fuel increases the regression rate by as much as 3 times. [Preview Abstract] |
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