Guillaume Ribert

Self-similar behavior and chemistry tabulation of burnt-gas diluted premixed flamelets including heat-loss

In many combustion systems, the reactive gases feeding the reaction zones are diluted by burnt products, to favor flame stabilization, homogenize the temperature distribution and reduce pollutant emission. The objective of this paper is to discuss a premixed flamelet detailed chemistry tabulation strategy for vitiated and non-adiabatic combustion. Dilution by burnt products is parameterized here with two controlling quantities: the amplitude of the heat-loss in the burnt gases, for instance at walls, and the level of reactant vitiation. The chemical response of premixed flames to variations of these parameters is studied and it is shown that most chemical properties of burnt-gas diluted flames feature self-similar behavior, which can be used to dramatically downsize chemical tables based on canonical flamelets. The self-similar behavior of the flamelets is studied for both molecular diffusion and chemical source budgets in a progress variable composition space. It is found that two different scaling relations are needed to ensure self-similar behavior of both major and radical species.

Direct Numerical Simulation and Large-Eddy Simulation of Supersonic Channel Flow

Compressible isothermal-wall channel flows are studied with direct numerical simulation and large-eddy simulation tools. Computations are carried out using a high-order, low-dissipation, bandwidth-optimized weighted essentially nonoscillatory numerical scheme to describe the hyperbolic terms of the Navier–Stokes equations. Periodic supersonic channel flow direct numerical simulation (M=1.5, Reτ=221, Tw=500  K, and Tc=700  K) is used to validate the procedure and the numerical scheme; a new subgrid term contribution based on pressure drop is proposed for the driving term required in momentum and energy equations for large-eddy simulation. Coherent structures of the flowfield are analyzed with scatter plots, Q criterion, and vorticity fields. As expected, the strong Reynolds analogy is not valid for this nonadiabatic flow. Streaks and horseshoe-like structures are highlighted and detailed. The authors propose a scenario for the formation of horseshoe-like structures. With large-eddy simulation tools, a dyna...

Supercritical Fluid Behavior in a Cooling Channel

The present study is dedicated to the simulation of supercritical uids owing inside small cooling channels. In a context of rocket motor engine, a strong heat ux coming from the combustion chamber (locally 80 MW/m) may lead to very steep gradients close to the wall. These density gradients have to be thermodynamically and numerically caught to really understand the mechanism of heat transfer from the wall to the uid. In this study, the shock-capturing WENO numerical scheme, usually used with the perfect gas law, is extended to real gases and applied to the simulation of the EH3C con guration. The WENO scheme is compared with a fourth-order central-di erence (4CD) scheme on a temperature slot and sinus function, and a non-periodic supersonic channel ow. For the last case, a better behavior of the WENO scheme is noticed away from the inlet, in accordance with the results of the temperature slot convection. For the EH3C con guration, only the WENO scheme achieves a substantial computation.

Supercritical Combustion of Liquid Oxygen (LOX) and Methane Stabilized by a Splitter Plate

A sys tematic numerical analysis has been conducted to explore supercritical mixing and combustion dynamics of liquid oxygen and methane separated by a splitter plate. The unified theoretical/numerical frameworks for the treatment of general fluid thermodynamics have been extended to accommodate turbulence/flame interactions. Different turbulent combustion models have been implemented. The applicability of those models is carefully assessed by comparing the chemical time to the characteristic turbulence time scales. The results indicate that the flamelet assumption is valid and the combustion process is mixing dominant throughout the entire flowfield under the present simulation conditions. The direct -losure approach over-predicts the reaction rate and gives rise to a thickened flame. The flow dynamics and combustion process in the vicinity of the splitter plate are quantified. Simulations of bot h the flamelet model and direct-closure approach confirm that the flame is stabilized by the wake recirculation zone with hot product right behind the splitter plate.

DNS and LES of wall-bounded compressible turbulent flows in narrow cooling channels

This paper describes the numerical study of a compressible turbulent channel flow with isothermal walls using direct numerical and large eddy simulations. Computations are carried out using a high-order low-dissipation bandwidth-optimized WENO method to describe the hyperbolic terms of Navier-Stokes equations. The DNS results are compared to another DNS data in order to validate the procedure. Coherent structures of the flow field is analyzed with the Q criterion, the vorticity field, and scatter-plots. The Strong Reynolds Analogy is not valid for such non-adiabatic flows. In the case of large eddy simulations, the impact of a dynamic procedure for the turbulent Prandtl number modeling is evaluated. Results are found more accurate and computations are more stable.

Experimental investigation of Ethane and Propane injection under sub- and super-critical conditions

This work was funded by the ANR (Agence Nationale de la Recherche) research project REFINE (Real-gas Effects on Fluid Injection: a Numerical and Experimental study), Grant No. ANR-13-BS09-0007.

Simulation of high-pressure methane flames

Combustion in liquid rocket engines happens under severe thermodynamical conditions: pressure exceeds the critical pressure of injected propellants and temperature is cryogenic. Such a situation requires an important effort of modeling: real gas effects are incorporated through cubic equations of state along with pressure-correction terms, and transport properties follow specific rules. Modeling for turbulent combustion is also an issue. A way to introduce finite rate chemistry and real gas effects into LES is the use of tabulated thermochemistry because the number of transported variables is then reduced compared to a multi species formulation. This model is used to perform the large-eddy simulation of a co-axial CH4-LOx injector operating at supercritical pressures. The computed flame length is in the range of the experimental observations and it is observed that chemical equilibrium is reached well before the outlet of the combustion chamber.

Boundary conditions treatment for supercritical flows with tabulated thermochemistry

Combustion in liquid rocket engines happens under severe thermodynamical conditions: pressure exceeds the critical pressure of injected propellants and temperature is cryogenic. Such a situation requires an important effort of modeling: real gas effects are incorporated through cubic equations of state along with pressure-correction terms, and transport properties follow specific rules. Modeling for turbulent combustion is also an issue and is presently considered in the compressible sole SiTComB with tabulated chemistry thus reducing the number of transported variables. In this study, a framework is provided to deal with real-gas compressible reacting flows with tabulated thermochemistry. As a consequence, a cubic equation of state for tabulated thermochemistry is derived with the adopted thermodynamic relations. The boundary conditions are expressed following the Navier-Stokes Characteristic Boundary Conditions formalism. Two-dimensional nonreactive theoretical test cases have been performed with success to demonstrate the capacity of the new method. The simulation of a three-dimensional non-reactive single injector derived from Mayer’s experiment, that consists of a supercritical nitrogen jet injection into a warm nitrogen atmosphere, is also performed with success. The comparison between the tabulated approach and a fully coupled reference solution leads to similar results for the density values along the jet axis and for the spreading rate. The REFINE (Real-gas Effects on Fluid Injection: a Numerical and Experimental study) project is detailed. The low-Mach numerical code YALES2 is evaluated.

Large-Eddy Simulation of a Supersonic Burner

Large eddy simulations of a supersonic hydrogen-air burner have been performed with the SiTComB numerical code. A reduced kinetic scheme is used and chemical source terms are evaluated based on resolved quantities. Three mesh resolutions have been considered: 2, 30 and 113 million of points (MP). With 2MP the flame is unstable and no comparison with experimental data becomes possible. However, the flame lift-off height is over predicted for the case 30MP. Refining the mesh (113MP) improves the capture of mixing and the flame lift-off height gets close to the experimental results. However this case must be be further converged to get accurate and final conclusions. Scatterplots of temperature and species mass fractions follows the trends already observed in past studies. A lookup table of auto-ignition is built for different levels of pressure and composition corresponding to the values found in the large-eddy simulation. Delays of auto-ignition are found of the order of magnitude of the time required to convect a pocket of pure fuel at a constant speed equal to the fuel inlet velocity.