A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

International audienceData obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterized by different density ratios σ are analyzed in order to study the influence of thermal expansion on flame surface area and turbulent burning rate. Obtained results show that, on the one hand, pressure gradient induced within the flame brush due to heat release in flamelets significantly accelerates unburned gas that deeply intrudes into combustion products in a form of an unburned mixture finger, thus, causing large-scale oscillations of the turbulent burning rate and flame brush thickness. Under conditions of the present simulations, contribution of this mechanism to creation of flame surface area is substantial and is increased by the density ratio, thus, implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal to one another. Apparent inconsistency between these results implies existence of another thermal expansion effect that reduces the influence of the density ratio on the flame surface area and burning rate. Investigation of the issue shows that the axial flow acceleration by the combustion-induced pressure gradient not only straightforwardly creates flame surface area by pushing a finger tip into products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to a high-speed (at σ = 7.53) axial 1 flow (a jet) of unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces a residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened due to a shorter residence time

Comparison Of Cavitation Phenomena In Transparent Scaled-Up Single-Hole Diesel Nozzles

The structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole inclined at 90, 85, 80 and 0 degree to the nozzle axis has been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. Observations revealed that at the inception stage, cavitation bubbles were not seen at the same locations in all the four nozzles. Cavitation bubbles grew intensively and developed into cloud-like structures. Shedding of the cloud cavitation was observed. When the flow was increased further the cloud-like cavitation bubbles developed into a dense large-scale cavitation cloud extending downstream of the hole. Under this condition the cavitation started mainly as a glassy sheet at the entrance of the hole. Until this stage the spray appeared to be symmetric. When the flow was increased beyond this stage, a sheet of cavitation covered a significant part of the hole on one side, extending to the hole exit. This non-symmetric distribution of cavitation within the hole resulted in a jet, which atomized on the side where more cavitation was distributed and non-atomizing on the side with less cavitation. The distribution of cavitation in the hole was different for different nozzles

Molecular transport effects on turbulent flame propagation and structure

Abstract:Various experimental and DNS data show that premixed combustion is affected by the differences between the coefficients of molecular transport of fuel, oxidant, and heat not only at weak but also at moderate and high turbulence. In particular, turbulent flame speed increases with decreasing the Lewis number of the deficient reactant, the effect being very strong for lean hydrogen mixtures. Various concepts; flame instability, flame stretch, local extinction, leading point, that aim at describing the effects of molecular transport on turbulent flame propagation and structure are critically discussed and the results of relevant studies of perturbed laminar flames (unstable flames, flame balls, flames in vortex tubes) are reviewed. The crucial role played by extremely curved laminar flamelets in the propagation of moderately and highly turbulent flames is highlighted and the relevant physical mechanisms are discussed

Evaluation of the Results of Initial Engine Tests with STID Equipment

The report provides an evaluation of selected cylinder pressure records of a W\ue4rtsil\ue4 320 engine operating according to the STID principle with early, low pressure steam injection, and a discussion of the effects of the steam injection on emissions. It also contains an evaluation of the application potential of low pressure steam injection on engines.Due to pressure limitations of the available boiler, only tests with steam pressures up to 50 bar and steam temperatures up to 320\ub0C could be performed. The steam pressure limitation caused that only injection during the intake stroke and early during the compression stroke was feasible which strongly restricted the scope of the study. However, the research has contributed considerably to the understanding of the STID process and in particular to evaluation of the steam effects on emissions and combustion. The low pressure steam injection is also one of the possible STID application versions to the normal, not thermally upgraded (hot), engines which requires only minor modifications of the steam generating facilities used in current ship installations.It is shown that the ignition and combustion processes in the engine are practically unaffected by injection of the required mass of steam, which typically shall not be larger than 3.5 times the mass of fuel, at which soot, unburned hydrocarbon and CO emissions start to grow. Within this range the NOx emissions can be described by a simple formula NOx =NOx,o(1-0.2m), where m is the steam to fuel mass ratio and NOx,o the emissions of the engine with out steam injection.The optimal way of using low pressure steam is to inject the steam into the inlet manifold of the engine which eliminates the complications of direct steam injection apparatus and eliminates the growth of maximum pressure. The steam temperature shall be as low as possible to minimize the reduction of volumetric efficiency of the engine and to reduce heat losses. The surplus steam enthalpy shall be used to additionally supercharge the engine using an ejector or tangential injection into the impeller of the compressor. It is recommended to utilize the low pressure steam injection on efficiency optimized engines by which both fuel consumption, NOx and soot emission reduction can be achieve