Venkateswaran Sankaran

Modeling of Turbulent Mixing Layer Dynamics in Ultra-High Pressure Flows

*† ‡ § The dynamics of air/nitrogen mixing layers under high pressure are studied using a computational approach, which embodies real gas equations of state, an advanced flux formulation for accurate unsteady solutions and detached eddy simulations (DES) of the turbulent dynamics. Real-fluid properties are obtained through efficient interpolation of adaptive property tables. Advanced preconditioned flux formulations are employed to reduce the inherent artificial dissipation in second-order finite-volume schemes. The DES method is employed within a ω − k turbulence model. The effectiveness of our computational approach is demonstrated using an exact solution for Taylor vortices and by computing decay of isotropic turbulence in a box. The paper focuses on the dynamics of air-nitrogen mixing layers for various Reynolds numbers and momentum thicknesses under high pressures. Both 2D and 3D DES simulations are obtained and the results are compared with 2D unsteady-RANS simulations to highlight the differences.

Coupling between hydrodynamics, acoustics, and heat release in a self-excited unstable combustor

The unsteady gas dynamic field in a closed combustor is determined by the nonlinear interactions between chamber acoustics, hydrodynamics, and turbulent combustion that can energize these modes. These interactions are studied in detail using hybrid RANS/large eddy simulations (RANS = Reynolds Averaged Navier-Stokes) of a non-premixed, high-pressure laboratory combustor that produces self-excited longitudinal instabilities. The main variable in the study is the relative acoustic length between the combustion chamber and the tube that injects oxidizer into the combustor. Assuming a half-wave (closed-closed) combustion chamber, the tube lengths approximately correspond to quarter-, 3/8-, and half-wave resonators that serve to vary the phasing between the acoustic modes in the tube and the combustion chamber. The simulation correctly predicts the relatively stable behavior measured with the shortest tube and the very unstable behavior measured with the intermediate tube. Unstable behavior is also predicted for the longest tube, a case for which bifurcated stability behavior was measured in the experiment. In the first (stable) configuration, fuel flows into the combustor uninterrupted, and heat release is spatially continuous with a flame that remains attached to the back step. In the second (unstable) configuration, a cyclic process is apparent comprising a disruption in the fuel flow, subsequent detachment of the flame from the back step, and accumulation of fuel in the recirculation zone that ignites upon arrival of a compression wave reflected from the downstream boundary of the combustion chamber. The third case (mixed stable/unstable) shares features with both of the other cases. The major difference between the two cases predicted to be unstable is that, in the intermediate length tube, a pressure wave reflection inside the tube pushes unburnt fuel behind the back step radially outward, leading to a post-coupled reignition mechanism, while in the case of the longest tube, the reignition is promoted by vortex convection and combustor-wall interaction. Other flow details indicated by the simulation include the relative phase between flow resonances in the tube and the combustor, increased mixing due to baroclinic torque, and the presence of an unsteady triple flame.

Prediction of Combustion Instability with Detailed Chemical Kinetics

Abstract : Combustion instability in an unstable single element rocket chamber using methane as the fuel is computationally studied. Effects of the kinetics mechanism are examined by comparing the results using a single step global mechanism and a detailed mechanism with 32 species and 177 reactions. Significant differences between the two predictions are identified including the amplitude of the unsteady pressure oscillations, and more importantly, the underlying mechanisms responsible for driving the combustion instabilities.

Combustion Instability Mechanisms in a Pressure-coupled Gas-gas Coaxial Rocket Injector

Abstract : An investigation of the instability mechanism present in a laboratory rocket combustor is performed using computational fluid dynamics (CFD) simulations. Three cases are considered which show different levels of instability experimentally. Computations reveal three main aspects to the instability mechanism, the timing of the pressure pulses, increased mixing due to the baroclinic torque, and the presence of unsteady tribrachial flame. The stable configuration shows that fuel is able to flow into the combustor continuously allowing continuous heat release. The unstable configuration shows that a disruption in the fuel flow into the combustor allows the heat release to move downstream and new fuel to accumulate in the combustor without immediately burning. Once the large amounts of fuel in the combustor burn there is rapid rise in pressure which coincides with the timing of the acoustic wave in the combustor. The two unstable cases show different levels of instability and different reignition mechanism.

Examination of Mode Shapes in an Unstable Model Rocket Combustor

** † ‡ Experimental data from an unstable model rocket combustor are analyzed using a linear one-dimensional acoustic model. The focus is on determining the influence of the oxidizer tube of the swirl-coaxial injector element on the acoustics and stability of the system at varying chamber lengths. The most significant effects of the oxidizer tube are seen in the system resonance frequencies and amplitude of acoustic pressure oscillations in the combustion chamber. Resonance frequencies are shown to be non-integer multiples, unlike traditional longitudinal modes, at certain combustion chamber lengths and are verified by comparisons with experimental data. In addition, certain modes calculated by the acoustic model to be highly damped in the combustion chamber are found to be unstable during the experiment, thereby indicating that the unsteady combustion process is dominating the stability response. Further, comparisons of the acoustic model with detailed CFD analysis verify the mode shape predictions and indicate the additional influence of mean flow Mach number and nonlinear effects on the mode shapes and frequencies.

Analysis of self-excited combustion instabilities using decomposition techniques

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Application of a Generalized Instability Model to a Longitudinal Mode Combustion Instability

An analytic model is used to study combustion insta bilities via the solution of an inhomogeneous pressure wave equation by a modified Galerkin method. This generalized instability model (GIM) is part of a testbed develo ped to study combustion instabilities in liquid rocket engines and augmentors. This model represents the lower fidelity spectrum of the testbed, and is compared with its higher order counterparts to asses its accuracy as well as its limitations. The model is used to study spec ial problems in acoustics, such as ducts with discontinuous changes in flow properties, continuous and discontinuous changes in mean flow Mach numbers, as well as their coupling with unsteady heat addition. The model is then used to determine the longitudinal stability charac teristics of a model rocket combustor and compared with relevant experimental results.

Experimental and Computational Investiagtion of Combustor Acoustics and Instabilities, Part II: Transverse Modes

A combined experimental-computational study of transverse acoustic modes and combustion instabilities in a rectangular liquid rocket chamber is presented. Experimental results show that transverse modes can be spontaneously excited in the rectangular chamber. The amplitudes of the acoustic response are governed by the number and location of the injector elements. In general, stronger response of the 1W mode is observed when the injector element is positioned near a pressure anti-nodal location. Companion CFD solutions of the Euler and Navier-Stokes solutions are also performed and compared with the experimental measurements. Good qualitative agreement of the acoustic chamber response is obtained. Further, the computational studies are utilized to perform parametric studies of the eects of non-linear forcing and viscous eects due to the presence of side-wall boundary layers.

Analysis of Self-Excited Combustion Instabilities Using Decomposition Techniques

Proper orthogonal decomposition and dynamic mode decomposition are evaluated for the study of self-excited longitudinal combustion instabilities in laboratory-scaled single-element gas turbine and rocket combustors. Since each proper orthogonal decomposition mode comprises multiple frequencies, specific modes of the pressure and heat release are not related, which makes the analysis more qualitative and less efficient for identifying physical mechanisms. On the other hand, dynamic mode decomposition analysis generates a global frequency spectrum in which each mode corresponds to a specific discrete frequency so that different dynamics can be correlated. In addition, proper orthogonal decomposition results are found to be inaccurate when only a limited amount of spatial information is provided in contrast with dynamic mode decomposition results, which provide more reliable results. Overall, dynamic mode decomposition analysis proves to be a robust and systematic method that can give consistent interpretati...

Computational and Experimental Investigation of Transverse Combustion Instabilities

Abstract : Concurrent experiments and computations are used to analyze combustion instabilities in a transverse mode combustion chamber. The experiments employ a shear-coaxial injector element, positioned within a rectangular chamber. The reacting flow portion of the study element is optically accessible and the chamber is extensively instrumented with high-frequency pressure transducers. High amplitude transverse acoustics modes are driven by unstable injector elements located near the chamber end-walls. Different levels of instability are obtained by varying the operation of these driving elements. High-fidelity computational fluid dynamics simulations are used to model this set-up, although only the study element is fully represented and the transverse acoustics modes are generated by vibrating the side walls at the appropriate frequencies. The computational results are compared quantitatively with the high frequency pressure measurements, and qualitatively by using the CH* chemiluminescence signal from the experiment. The combustion response of the first, second and third transverse modes obtained using a dynamic modal decomposition procedure show excellent agreement between the experiments and simulations. The overall approach shows significant promise for screening the combustion response of candidate injector configurations for rocket applications.