Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EH: Reacting Flows II |
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Chair: Noel Clemens, University of Texas Room: Hilton Chicago Williford B |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EH.00001: The recirculation dynamics of bluff body stabilized premixed combustion. Marios Soteriou, Robert Erickson, Prashant Mehta Bluff body stabilized premixed combustion is present in many power generation and propulsion systems such as gas turbines and afterburners. In this environment, flow recirculation behind the bluff body provides low speed, hot products that act as an ignition source for the incoming reactants and help anchor the flame. Beyond this coarse phenomenological description, however, understanding of bluff body flame stabilization via recirculation is rather limited. This is particularly so for non-swirling systems which are the focus of this work. To investigate this problem we use numerical simulation. The computational model is Lagrangian, employing the vortex element method for the simulation of the low Mach number exothermic flow field and a G-equation flamesheet for the reacting field. Numerical results are contrasted to experimental and analytical findings to demonstrate the ability of the model to reproduce the flow unsteady behavior in both the reacting and non-reacting environments. Analysis focuses on the differences in the recirculation dynamics between the reacting (symmetric shedding) and non reacting (asymmetric shedding) flow. It is shown that these differences are sensitive to both internal flow parameters such as heat release and flame speed but also to boundary conditions such as inlet temperature and confinement. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EH.00002: Mixing and Flow-field Characteristics of Strongly-forced Transitional / Turbulent Jets and Jet Flames Krishna Lakshminarasimhan, Noel Clemens, Ofodike Ezekoye Strong pulsations of the fuel flow rate have previously been shown to dramatically alter the flame length and luminosity of nonpremixed jet flames. The mechanisms responsible for such changes are explored experimentally in nonreacting and reacting strongly pulsed jets by using cinematographic PIV and acetone PLIF. The large amplitude forcing was obtained by pulsing the flow using a solenoid valve at the organ-pipe resonance frequency of the fuel delivery tube. The velocity fluctuations in the flow produced by the resonant pulsing of the jet can reach to about 8 times that of the mean flow. The jet characteristics were studied for Reynolds numbers based on mean flow velocity ranging between 800 and 2400. The PIV shows that with strong pulsations the jet exhibits significant reverse flow into the fuel delivery tube and an increase in turbulence in the near-field region. The acetone PLIF imaging was performed inside and outside the fuel tube in order to study the effects of pulsations on the mixing. These measurements showed significant in-tube partial premixing due to the reverse flow near the nozzle exit as well as enhanced mixing due to coherent vortical structures and increased turbulence. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EH.00003: Nonpremixed Combustion in an Accelerating Transonic Flow Undergoing Transition Felix Cheng, Feng Liu, William Sirignano The flow through a turbine passage is modelled by a mixing layer with fuel and oxidizer streams flowing through a channel with imposed streamwise pressure gradients due to varying cross- sectional area. Due to the strong favorable pressure gradients, the flow accelerates from low subsonic to low supersonic speed and at the same time undergoes transition from laminar flow to turbulence. In this study, we focus on the transitional stage of this unsteady, accelerating, reacting, and compressible mixing layer. The full Navier-Stokes equations coupled with multiple reacting species equations and chemical reactions are solved numerically. No turbulence model is employed since the transitional flow is fully deterministic. Inlet perturbations determined from linear stability analysis are introduced at the inlet to excite the mixing layer. The production of both positive and negative vorticity due to the exothermic chemical reactions is identified, and the interactions between regions of unlike vorticity are characterized. The instability produces a strain field that results in the tearing of the flame. The effects of the streamwise pressure gradient and the amplitude of the inlet disturbances on the flame structures are investigated. Grid- and domain- independencies are performed to ensure the accuracy of the numerical solutions. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EH.00004: Development and ignition of a strong under-expanded jet Matei Radulescu, Chung K. Law The high pressure storage of a combustible gas presents several explosion hazards in the case of an accidental tank rupture or puncture. Although the establishment and ignition of a steady jet was studied extensively in the past, the starting process has attracted much less attention. In the present study, we focus on the initial transient accidental release of a light combustible gas from a tank at high initial pressure. When the gas is light, such hydrogen, a strong hemi-spherical blast wave is driven in the surrounding air, compressing the air to high temperatures. The hot air exchanges mass and heat with the cold expanded hydrogen at the jet head interface via diffusion and can lead to local ignition. Simulations of the non-reactive release of under-expanded jets are used to elucidate the transient gas-dynamics of the jet establishment, involving the decay of the hemispherical shock and contact surface and the establishment of the barrel shock system of steady under-expanded jets. The results are used to validate a semi-empirical theory for the time history profiles of shock and contact surface decay. The ignition problem is treated separately from the gasdynamics problem as a non-steady diffusion layer, where the evolution of the diffusion layer is given analytically. Estimates of the critical tank pressures and jet sizes are derived for conditions at which a diffusion layer will be ignited. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EH.00005: Study of Combustion Dynamics in a Swirl Gas Combustor Devkinandan Tokekar, Urmila Ghia, Karman Ghia Combustion in a lean pre-mixed (LPM) combustor may become unstable due to small changes in geometry and the manner in which reactants are introduced. This may lead to excessive thermal loads and possible off-design operation. A comprehensive understanding of combustion instability is therefore needed. Hence, the present study aims to analyze the flow and flame dynamics in a model LPM gas turbine combustor and investigate the causes for combustion instabilities arising in LPM combustion. Fluent is used as the flow solver for the present study. The 3-D Navier-Stokes equations are solved along with finite-rate chemical reaction equations and variable thermophysical properties. Large-eddy-simulation technique is used to model turbulence. The dynamic version of the Smagorinsky-Lilly model is employed to describe subgrid-scale turbulent motions and their effect on large-scale structures. A non-reacting flow simulation is performed first, and the results show good agreement with published experimental and numerical work. Presently, the reacting flow analysis is in progress to determine the effect of equivalence ratio and inlet flow temperature on the stability characteristic of the combustor. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EH.00006: Large-eddy simulation of a coaxial-jet combustor with convective heat-losses Lee Shunn, Parviz Moin In this study numerical simulations of non-premixed methane-air combustion are conducted to investigate the effects of convective heat-losses in a coaxial-jet combustor. The turbulent flow field is simulated via large-eddy simulation (LES) on a structured, orthogonal mesh using a conservative discretization of the transport equations. The effects of thermal-losses on the combustor are evaluated by comparing the results from simulations with adiabatic and isothermal wall-conditions, respectively. In the adiabatic simulations, turbulence/chemistry interactions are described using the flamelet/progress-variable approach of Pierce and Moin (J. Fluid Mech. 504, 73-97, 2004) in which filtered transport equations are solved for the mixture fraction and a reaction progress variable. For the heat-transfer case, the flamelet/progress-variable method is extended by a thermally-quenched flamelet library and a filtered energy equation to describe heat transfer to the confinement. The resulting velocity, species concentration, and temperature fields are compared to the experimental values of Spadaccini, et al. (U.S. EPA Rep. EPA-600/2-76-247a, 1976). [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EH.00007: Large eddy simulations of a bluff-body stabilized hydrogen-methane jet flame Tomasz Drozda, Reza Sheikhi, Peyman Givi, Stephen Pope Large eddy simulation (LES) is conducted of the turbulent bluff-body stabilized hydrogen-methane flame as considered in the experiments of the Combustion Research Facility at the Sandia National Laboratories and of the Thermal Research Group at the University of Sydney [1]. Both, reacting and non-reacting flows are considered. The subgrid scale (SGS) closure in LES is based on the scalar filtered mass density function (SFMDF) methodology [2]. A flamelet model is used to relate the chemical composition to the mixture fraction. The modeled SFMDF transport equation is solved by a hybrid finite-difference (FD) / Monte Carlo (MC) scheme. The FD component of the hybrid solver is validated by comparisons of the experimentally available flow statistics with those predicted by LES. The results via this method capture important features of the flames as observed experimentally.\newline [1] A. R. Masri, R. W. Dibble, and R. S. Barlow. The structure of turbulent nonpremixed flames revealed by Raman-Rayleigh-LIF measurements. \textit{Prog. Energy Combust. Sci.}, 22:307--362, 1996. \newline [2] F. A. Jaberi, P. J. Colucci, S. James, P. Givi, and S. B. Pope. Filtered mass density function for large eddy simulation of turbulent reacting flows. \textit{J. Fluid Mech.}, 401:85--121, 1999. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EH.00008: Auto-Ignition Delay Times and Nozzle Velocity Profiles of an Ethane Jet Gerald Fast, Dietmar Kuhn, Andreas G. Class The direct injection combustion of hydrocarbons in IC engines represents a complex process. To provide a fundamental understanding of individual processes we study an idealized injection process. At current stage our focus is on auto- ignition of gaseous ethane in a transient free jet. The fuel- specific chemical kinetics is coupled to the non-stationary turbulent mixing of fuel and oxidizer. Our experiment provides the input for a theoretical study which aims to predict auto ignition criteria for a prescribed concentration, temperature, pressure and velocity field. We determine the ignition delay times and the progress of the non-stationary ignition in a turbulent flow field in terms of joint probability density functions (JPDF). The test facility provides oxidizer pressures up to 40 bars at 500°C temperature. The injection system consists of a high speed valve which injects the fuel gas into the combustion chamber. The pipe outlet is located in the field of view of the windows to admit optical measurements techniques. Present results of ignition delay times and time-resolved velocity profiles measured with LDV are presented. This work was supported by DFG grant SFB-606. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EH.00009: Tunable Chemical Plumes Michael Rogers, Stephen Morris Buoyant plumes are typically studied in the laboratory by injecting fluid into a denser medium. Plumes produced in this way entrain less buoyant fluid from their surroundings, dampening the buoyancy of the ascending fluid. Here, we consider a new type of plume that is produced by an autocatalytic chemical reaction - the iodate-arsenous acid reaction. The reaction occurs at a sharp front which separates reactants from less dense products. Using this reaction, buoyancy-driven chemical plumes are created by allowing an ascending front to escape from a capillary tube into a large tank. In a plume created in this way, entrainment assists the reaction, producing new buoyancy by delivering reactant into the plume. The behavior of chemical plumes is `tuned' by altering both the viscosity of the system, and the ratio of chemical reactants. Chemical plumes may be tuned in such a way that overturning vortical flow is produced in the plume head. Such vortical motion stirs in fresh reactant solution and can drive the detachment of the plume head, which subsequently forms a free, accelerating vortex ring. Depending on the viscosity and concentration of the reactant solution, successive detachments may form multiple generations of chemical plume heads from a single initiation event. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EH.00010: Flow and Reaction in a Porous Rotating Disk Electrode Bomi Nam, Nicolas Mano, Adam Heller, Roger Bonnecaze We study experimentally and numerically the flow and reaction in a porous rotating disk electrode (PRDE). While the mass transport and electrochemical reaction rate on rotating disk electrodes are well defined by the Koutetskii-Levich theory, the PRDE exhibits much richer behavior controlled by the thickness, radius and permeability of the porous disk as well as the rotation rate. From experiments we find that the current generated in the PRDE generally exhibits three regimes separated by two sharp transitions, which are a function of these parameters. The fundamental cause of this behavior is hydrodynamic, and it is explored by numerically solving the flow and reaction in a PRDE. The model consists of the Navier-Stokes equation in the ambient fluid, a modified Darcy's law accounting for the rotation in the porous medium, and a convection-diffusion equation with a first order reaction all coupled through boundary conditions. A range of flow rates, geometries, porous medium and fluid properties, and reaction rates have been investigated. In general it is found that more fluid and reactant perfuse the porous disk as the rotation rate, thickness to radius ratio and permeability increase. The behavior of the current generated in the PRDE is explained through a ratio of the reaction time and the residence time. It is found that the appropriate dimensionless reaction time is sufficient to characterize the system. The simulation results agree very well with experimental data. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
EH.00011: Ignition of Hydrocarbon Fuels by a Low Temperature Repetitively Pulsed Nanosecond Discharge Plasma Sivaram Gogineni, Ainan Bao, Guofeng Lou, Saurabh Keshav, Munetake Nishihara, Igor Adamovich The results of nonequilibrium RF plasma ignition experiments demonstrate that the highest fuel conversion efficiency is achieved in lean air-fuel mixtures, including conditions when there is no flame generated in the test section. In the latter case, fuel oxidation occurs in plasma chemical reactions, which are not related to combustion. Since the net fuel oxidation process is exothermic, heat release during the plasma chemical reactions results in achieving thermal ignition, as the equivalence ratio is increased. The RF plasma also stabilizes the flame, without using flameholders. The results demonstrated the use of nonequilibrium, high-voltage (15-20 kV), short pulse duration (20-30 nsec), high repetition rate (40 kHz) pulsed discharge for ignition. Lean premixed ethylene-air flows at P=0.1 atm are ignited by the uniform and stable repetitively pulsed discharge. The plasma temperature before adding the fuel is rather low, about 200$^{0}$ C. Ignition of hydrocarbon fuels by using volume filling, high voltage, high repetition rate nanosecond pulsed discharge plasma has been achieved for the first time. This demonstrates that this type of nonequilibrium discharge can be used as a large volume, energy efficient ignition source and flame stabilizer in lean fuel-air mixtures, at conditions when conventional ignition sources are ineffective. [Preview Abstract] |
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