Bruno Renou

Local scalar flame properties of freely propagating premixed turbulent flames at various Lewis numbers

Abstract Local scalar flame properties of freely propagating turbulent premixed flames including the flame curvature h, and local displacement speed relative to the fresh gases Sdu, have been measured simultaneously for methane, propane, and hydrogen/air flames at Lewis numbers varying from 0.33 to 1.4 and ratios of rms turbulent velocity to unstretched laminar burning velocity u′/SL,0u varying from 0 to 3.1. Three different mixtures were separately spark-ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field-imaging technique based on high-speed laser tomography measured the temporal evolution of local flame properties. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (pdfs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame speed variations. The temporal evolution of the mean flamelet radius of curvature converges towards half of the integral length scale measured in the cold flow. Flame response in terms of displacement speed to curvature is found to be statistically dependent, and a linear relationship is observed. For propane/air flames, the displacement speed can be assumed to be independent of local flame curvature at each stage of flame propagation, whereas a very strong increase of displacement speed with positive curvatures can be observed along the wrinkled flame contour for hydrogen/air flames.

Effects of stretch on the local structure of preely propagating premixed low-turbulent flames with various lewis numbers

An experimental investigation of the flame response to strain rate in the case of unsteady premixed low-turbulent flames is presented. In order to point out the fundamental aspects of the mutual interaction between combustion and turbulence, measurements of local flame properties (curvature, displacement speed) and tangential strain rate were performed under varying conditions of Lewis number and turbulence. Three different mixtures (methane/air, propane/air, and hydrogen/air) were successively spark ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field imaging technique coupling high-speed laser tomography and cross-correlation particle image velocimetry (PIV) was used to measure the temporal evolution of local flame stretch exerted by the turbulent cold flow. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (PDFs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame-speed variations. Tangential strain rate was determined by using the velocity field in the neighborhood of the flame front and appears to be independent of the Lewis numbers. A strong correlation between local flame curvature and tangential strain rate was demonstrated, underlining the cold flow effects on the local flame structure. The influences of turbulence and Lewis number were evaluated and compared with numerical simulations. Then, local flame stretch distributions were determined versus time, indicating that a significant proportion of the flame was under compression.

Multi-scale energy injection: a new tool to generate intense homogeneous and isotropic turbulence for premixed combustion

Highly turbulent flow, generated by an original multi-scale device and nearly homogeneous and isotropic, is experimentally investigated. This multi-scale device is made of three perforated plates shifted in space such that the diameter of their holes and their blockage ratio increase with the downstream distance. The multi-scale turbulence injection (MuSTI) is compared with a mono-scale turbulence injection (MoSTI), the latter being only constituted of the last plate of the MuSTI. This comparison is done for both cold and reactive flows. For the cold flow the following are shown, in comparison with the classical mono-scale device, for the MuSTI device in the near-field: (i) The turbulent kinetic energy is larger, and the kinetic energy supply is distributed over the whole range of scales. This is emphasised by second- and third-order structure functions. (ii) The shear-stresses are enhanced. (iii) The homogeneity and isotropy are reached earlier (≈50%). (iv) The jet-merging distance is the relevant scalin...

Laser-Induced Spark Ignition of Premixed Confined Swirled Flames

Optimization of the ignition location is of crucial importance for many combustion systems and requires advanced knowledge of the ignition process for both fundamental and applied configurations. In this article, a premixed swirl burner was designed to experimentally study the impact of the spark location on successful ignition and to detail the scenario from ignition to flame stabilization. Two swirl numbers were investigated to evaluate their impact on the ignition process. Particular attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic particle image velocimetry) as well as on ignition conditions. Ignition probability maps were obtained for a constant level of deposited energy. Contrary to previous studies, no correlation between local turbulent kinetic energy and ignition probability was observed, and a deeper analysis of the temporal evolution of the flame kernel within the combustion chamber is required. Coupling fast flame visualization with the corresponding pressure signal demonstrated that the efficiency of the ignition location was not only controlled by the local flow properties, but also by the early flame kernel development, linked by its typical trajectories within the combustion chamber.

On the similarity of variable viscosity flows

Turbulent mixing is ubiquitous in both nature and industrial applications. Most of them concern different fluids, therefore with variable physical properties (density and/or viscosity). The focus here is on variable viscosity flows and mixing, involving density-matched fluids. The issue is whether or not these flows may be self-similar, or self-preserving. The importance of this question stands on the predictability of these flows; self-similar dynamical systems are easier tractable from an analytical viewpoint. More specifically, self-similar analysis is applied to the scale-by-scale energy transport equations, which represent the transport of energy at each scale and each point of the flow. Scale-by-scale energy budget equations are developed for inhomogeneous and anisotropic flows, in which the viscosity varies as a result of heterogeneous mixture or temperature variations. Additional terms are highlighted, accounting for the viscosity gradients, or fluctuations. These terms are present at both small and large scales, thus rectifying the common belief that viscosity is a small-scale quantity. Scale-by-scale energy budget equations are then adapted for the particular case of a round jet evolving in a more viscous host fluid. It is further shown that the condition of self-preservation is not necessarily satisfied in variable-viscosity jets. Indeed, the jet momentum conservation, as well as the constancy of the Reynolds number in the central region of the jet, cannot be satisfied simultaneously. This points to the necessity of considering less stringent conditions (with respect to classical, single-fluid jets) when analytically tackling these flows and reinforces the idea that viscosity variations must be accounted for when modelling these flows.

Variable Viscosity Jets: Entrainment and Mixing Process

Turbulent jets with viscosity stratification are very important from both fundamental and practical viewpoints. In this study, we carry out a comparison between Constant Viscosity Flows and Variable Viscosity Flows, in a round jet, on the basis of the same initial conditions (same jet momentum and/or the same initial Reynolds number). The two fluids are density-matched. A propane jet issues into a slight nitrogen coflow for which the kinematic viscosity ratio is \(R_{v} \equiv \nu _{N_{2}}/\nu _{\mathrm{propane}} = 3.5\). The Reynolds number of the jet (based on the diameter, the initial velocity and the propane viscosity) is of 8000. The direct interactions between the velocity and the scalar fields reflect the need to perform simultaneous measurements of these two physical quantities. The stereo-Particle Image Velocimetry (stereo-PIV) and the Planar Laser Induced Fluorescence have been chosen for the velocity and the concentration measurements, respectively. Experimental results are discussed, for both velocity and scalar fields, in the axial plane of the turbulent axisymmetric jet. It is shown that the presence of a strong viscosity discontinuity across the jet edge results in an increase in both the scalar spread rate and the turbulent fluctuations.

Experimental Study of Aeronautical Ignition in a Swirled Confined Jet-Spray Burner

Aeronautical gas turbine ignition is still not well understood and its management and control is mandatory for new lean-burner designs. The fundamental aspects of swirled confined two-phase flow ignition are addressed in the present work. Two facilities enable the analysis of two characteristic phases of the process. The KIAI-Spray single-injector burner was investigated in terms of local flow properties, including the air velocity and droplet fuel (n-heptane) size-velocity characterization by phase Doppler anemometry (PDA), and the study of local equivalence ratio by means of planar laser induced fluorescence (PLIF) on a tracer (toluene). The initial spark location inside the chamber is vital to ensure successful ignition. An ignition probability map was elaborated varying the location of a 532 nm laser-induced spark in the chamber under ultra-lean nominal conditions (phi=0.61). The outer recirculation zone (ORZ) was found to be the best region for placing a spark and successfully igniting the mixture. A strong correlation was found between the ignition probability field and the airflow turbulent kinetic energy and velocity fields. Local equivalence ratio enhances the importance of the ORZ. Once a successful ignition is accomplished on one injector, the injector-to-injector flame propagation must be examined. Highspeed visualization through two synchronized perpendicular cameras was applied on the KIAI-Spray linear multi-injector burner. Four different injector-to-injector distances and four fuels of different volatilities (n-heptane, n-decane, n-dodecane and jet-A1 kerosene) were evaluated. Spray branches and inter injector regions changed with the inter-injector distance. Two different flame propagation mechanisms were identified: the direct radial propagation and the arc propagation mode. Ignition delay times were modified with the injector-to-injector distance and with the different fuels.