Marc Bellenoue

Experimental and Numerical Study of the Influence of Temperature Heterogeneities on Self-Ignition Process of Methane-Air Mixtures in a Rapid Compression Machine

The self-ignition of methane-air mixtures is analyzed in conditions relevant for Homogeneous Charge Compression Ignition (HCCI) engines from both (i) experimental, and (ii) numerical modeling points of view. On the one hand, a Rapid Compression Machine (RCM) is retained as a pertaining experimental setup to carry out the first part of the study. On the other hand, the PDF method is the modeling framework chosen to describe the coupling between detailed chemical description and turbulent fluctuations. The emphasis is on the influence of the temperature heterogeneities induced by heat transfer during the compression stroke on the subsequent ignition process. Such temperature heterogeneities have already been observed in RCMs (Mittal and Sung, 2006), and they are qualitatively assessed by schlieren imaging in the present study. The simultaneous analysis of the pressure traces and direct visualizations allows to discriminate two distinct modes of ignition exhibiting different levels of coherence of the exothermic process. It is shown that the appearance of one or the other of these two modes is directly related to the temperature distribution at the onset of ignition. Then, the information gathered through the experimental results is used to check the ability of a simplified model to reproduce both qualitatively and quantitatively the observed results. The comparison with the experimental data suggests that the proposed modeling approach provides a well suited framework for further modeling studies of HCCI combustion.

Blind hood effects on the compression wave generated by a train entering a tunnel

Abstract The running of high-speed trains on railway networks creates strong transient flows in tunnels. A large number of solutions have been proposed to reduce the amplitude of the pressure gradients in tunnels and one of the most efficient solutions consists in the addition of a hood before the tunnel entrance. The present paper provides a detailed experimental study on the effects of blind hood (hood without any perforation) with constant section on the generation of a compression wave. An axisymmetric reduced scale (1/140th) experimental apparatus was used and the train Mach number was in the range from 0.06 to 0.15. Results showed that the initial compression front can be split into several smaller fronts in the presence of a blind hood. The number of fronts depends on the respective length of the hood to the train nose length and only slightly on the train Mach number. The characteristics of each of these fronts (amplitude and gradient) can be predicted for given train, tunnel and hood. Furthermore the present analysis showed that, for a fixed train/tunnel blockage ratio, there exists a unique optimum hood for which the pressure gradient can be minimized. For the usual train/tunnel blockage ratio (around 0.2), the maximum reduction of the pressure gradient was proved not to exceed 3. The exact value of the optimum pressure gradient depends on both the train/tunnel blockage ratio and the train nose geometry.

Experimental Study of Single-Wall Flame Quenching at High Pressures

Measurements of wall heat flux and ionization current for the case of cold wall and unburned gas temperature 850 K were carried out with a stoichiometric methane-air mixture in a head-on quenching regime in a pressure range of 0.8–16 MPa. It was found that maximum wall heat flux density increases as P0.5 while the dimensionless maximum wall heat flux (maximum wall heat flux normalized to the rate of energy release in flame) slightly decreases with a pressure rise. The authors evaluated quenching distance independently from the maximum wall heat flux and from the electrical probe current. In both cases the thermal flame quenching model and simplified model of combustion plasma-probe interaction was applied. The accuracy of the ionization probes method was improved taking into account the influence of pressure and temperature effects on the electrical conductivity of flame. A good correlation between results obtained with two methods was found in all range of pressure.

Syngas Application to Spark Ignition Engine Working Simulations by Use of Rapid Compression Machine

© 2012 Monteiro et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Syngas Application to Spark Ignition Engine Working Simulations by Use of Rapid Compression Machine

Influence of Operating Conditions and Residual Burned Gas Properties on Cyclic Operation of Constant-Volume Combustion

The pressure-gain combustion concept is a solution envisioned to increase the thermodynamic efficiency of gas turbines. This article addresses the behaviour of piston-less constant-volume combustion in relevant conditions of engine application. For this purpose, a lab-scale combustion vessel (0.3 L) is run in cyclic operation (10 Hz) with an improved control over the boundary conditions. This facility features the spark-ignited, turbulent combustion of n-decane directly injected in preheated air (423 K, 0.4 MPa), with an overall equivalence ratio of 0.9. Solenoid valves are used to perform the air intake and burnt gas exhaust. A 0D analysis is developed and used to compute the gas thermodynamic evolution based on the experimental pressure traces. The effect of the main operating parameters on the combustion process is discussed: ignition delay, exhaust pressure and wall temperature. The vessel is operated without scavenging, hence the exhaust pressure drives the amount and the temperature of residual burnt gas (16–39% according to the 0D analysis). Highly diluted cycles (exhaust pressure 0.2 MPa) exhibit a higher combustion efficiency, but have a longer combustion duration (3 times more) than those of low dilution (exhaust pressure 0.07 MPa). For a higher wall temperature representative of engine combustor (1000 K), the heat losses are directly reduced, which affects the residual burnt gas properties. This also influences the residual gas temperature (870–1030 K) as well as dilution (10–26%).

Characterization of Liquid Impinging Jet Injector Sprays for Bi-Propellant Space Propulsion: Comparison of PDI and High-Magnification Shadowgraphy

Impinging jet sprays are investigated in the reference case of like-doublet injector, for application to bi-propellant combustion. Green propellants are considered, namely ethanol as a fuel and hydrogen peroxide as an oxidizer, that is well represented by water. This study reports original comparisons between standard spray characterization (PDI) and high-magnification shadowgraphy of the spray (2.5 x 3.2 mm, 2.5 μm per pixel) based on short laser backlight illumination (5 ns). Shadowgraphy images describe accurately the inner spray structure and provide the size and velocity of droplets. This diagnostic is used to analyse the influence of jet momentum (driven by injection pressure) on impinging jet atomization, as well as the evolution of spray topology, drop size distribution and average diameter along the spray centreline. The application of shadowgraphy to the dense region of water and ethanol sprays shows the different atomization behaviour of these two fluids with respect to their surface tension. Elliptical droplets are characterized inside the spray, which confirms the interest of a direct visualization of droplets in such dense sprays.

Syngas Laminar Burning Velocity: Comparison Between the Schlieren Photography and Spherical Bomb Methods

Gasification is a thermochemical process which can be defined as the conversion of an organically derived, carbonaceous feedstock by partial oxidation[1]. Typical gas product (syngas) includes mainly H2, CO, CO2, CH4 and N2, when air is used as oxidizing agent.Copyright