Flow, Turbulence and Combustion最新文献

Combustion applications such as internal combustion engines are a major source of power generation. Renewable alternative fuels like hydrogen and ammonia promise the potential of combustion in future power applications. Most power applications encounter flame wall interaction (FWI) during which high heat losses occur. Investigating heat loss during FWI has the potential to identify parameters that could lead to decreasing heat losses and possibly increasing the efficiency of combustion applications. In this work, a study of FWI (CH₄-air mixture) in a constant volume chamber, with a head-on quenching configuration, at high pressure in both laminar and turbulent conditions is presented. High-speed surface temperature measurement using thin junction thermocouples coupled with high-speed flow field characterization using particle image velocimetry (PIV) are used simultaneously to investigate the effect of pressure during FWI (P_int) and turbulence intensity (q) on the heat flux peak (Q_P). In laminar combustion regimes, it is found that Q_P is proportional to P_int^0.35. The increase in q is shown to affect both P_int and Q_P. Finally, comparing Q_P versus P_int for both laminar and turbulent combustion regimes, it is found that an increase in q leads to an increase in Q_P (b = 0.76).

In the present study, a recently proposed extended population balance equation (PBE) model for aggregation and sintering is incorporated into a large eddy simulation-probability density function (LES-PDF) modelling framework to investigate synthesis of silica nanoparticles in a turbulent diffusion flame. The stochastic field method is employed to solve the LES-PBE-PDF equations, characterising the influence of the unresolved sub-grid scale motions and accounting for the interactions between turbulence, chemistry and particle dynamics. The models for gas-phase chemistry and aerosol dynamics are the same as those recently used by the authors to simulate silica synthesis in a laminar flame (Tsagkaridis et al. in Aerosol Sci Technol 57(4):296–317, 2023). Thus, by retaining the same kinetics without any adjustments in parameters, we focus on the modelling issues arising in silica flame synthesis. The LES results are compared with experimental in-situ small-angle X-ray scattering (SAXS) data from the literature. Good agreement is found between numerical predictions and experimental data for temperature. However, the LES model underestimates the SAXS data for the primary particle diameter by a factor of two. Possible reasons for this discrepancy are discussed in view of the previous laminar flame simulations.

Rapid compression machines (RCM) are well-known tools to study the autoignition phenomenon under engine-relevant conditions. Covering a wide range of pressure and temperature at the top dead center (TDC), it can be employed with different types of mixtures and thermal stratification. Creating a homogeneous hot core region after compression in the combustion chamber is one of the challenges to overcome for RCM studies. The objective of the present work is to characterize from aerodynamic and thermal points of view a new configuration in the optical RCM of Pprime Institute. The latter aims at ensuring a wider adiabatic core region in terms of time and space through the installation of a creviced piston, specifically adapted to the square cross-section cylinder of this particular RCM. For this purpose, the internal flow has been qualified using high-frequency Particle Image Velocimetry with different laser sheet locations. Temperature variation during and after compression is measured at several positions with respect to the cylinder head, using thermocouples with wire diameter of 7.6 µm. It is observed that the piston cavity is able to collect the boundary layer created during compression and maintain a wide region at low velocity after the top dead center. Furthermore, it is demonstrated that different temperature gradient values can be generated and quantified within the adiabatic core region through differential heating of the chamber. This feature is promising for future works devoted to the analysis of combustion regimes. More generally, the thin wire thermocouples are shown to be accurate and reliable sensors to measure temperature in severe and transient pressure and temperature conditions specific to RCM internal flows.

This paper aims to evaluate the prediction accuracy of various porous bleed models on two flow cases of particular interest for supersonic applications: turbulent boundary layer bleeding and control of shock-boundary layer interactions. A thorough literature review was conducted to select the most relevant models. The models were then implemented as suction/blowing boundary conditions in the in-house compressible RANS solver elsA with a flexible approach based on source terms. Reference simulations with the holes considered in the computational domain were conducted in order to assess the models’ prediction capabilities. Two kinds of comparison were performed. In the first step, physical data (e.g., pressure, bleed mass flux) were extracted from the reference simulation and compared to the model predictions for the same conditions. In the second step, we performed RANS simulations using the various models as boundary conditions on a porous patch. Significant discrepancies between reference data and model predictions are highlighted, particularly for the bleed mass flux and velocity profiles, for which too high levels of momentum are predicted in the wall vicinity. This effect is supposedly attributed to the continuous application of transpiration over the patch. This has been shown to lead to an overestimation of the bleed effectiveness for the shock-boundary interaction flow case. In addition, reference simulations conducted in this study show that the diameter of the bleed holes influences the flow, which the existing porous bleed models do not consider. The outcome of this work, which highlights the deficiencies of state-of-the-art models, suggests the need to elaborate more advanced modeling for accurate prediction of both porous bleed performance and effect on the controlled flow.