Flow, Turbulence and Combustion最新文献

A transport equation for the flame displacement speed evolution in premixed flames is derived from first principles, and the mean behaviours of the terms of this equation are analysed based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames with a range of different Karlovitz numbers. It is found that the regime of combustion (or Karlovitz number) affects the statistical behaviour of the mean contributions of the terms of the displacement speed transport equation which are associated with the normal strain rate and curvature dependence of displacement speed. The contributions arising from molecular diffusion and flame curvature play leading order roles in all combustion regimes, whereas the terms arising from the flame normal straining and reactive scalar gradient become leading order contributors only for the flames with high Karlovitz number values representing the thin reaction zones regime. The mean behaviours of the terms of the displacement speed transport equation indicate that the effects arising from fluid-dynamic normal straining, reactive scalar gradient and flame curvature play key roles in the evolution of displacement speed. The mean characteristics of the various terms of the displacement speed transport equation are explained in detail and their qualitative behaviours can be expounded based on the behaviours of the corresponding terms in the case of 1D steady laminar premixed flames. This implies that the flamelet assumption has the potential to be utilised for the purpose of any future modelling of the unclosed terms of the displacement speed transport equation even in the thin reaction zones regime for moderate values of Karlovitz number.

The Partially Stirred Reactor (PaSR) model is carried out for the ammonia-air combustion system by means of stochastic modeling, namely by solving the transport equation for the joint Probability Density Function (PDF). The turbulent mixing is accounted for by the Linear Mean-Square Estimation (LMSE) mixing model. Notwithstanding the simplified nature of the PaSR modeling, the transported-PDF method enables capturing the effect of mixing frequency on the combustion system, especially the NOx emission. Since the chemical source term is in a closed form in the transported-PDF method, it allows us to apply different chemical mechanisms to explore, whether the set of elementary reactions that are identified as important for the prediction of NOx in the PaSR model is sensitive to the choice of chemical mechanisms. Furthermore, the effect of the residence time in the PaSR model has also been studied, and compared with those in the Perfectly Stirred Reactor (PSR) model (infinite large mixing frequency). Moreover, since the ammonia under oxygen enrichment shows some similar combustion behaviors in terms of e.g. laminar burning velocity as the ammonia under hydrogen enrichment, how large the difference of thermo-kinetic states (e.g. temperature and NOx emission) predicted by PaSR models and in laminar premixed flame configuration is also investigated. A further discussion focuses on the effect of thermal radiation, where the radiative heat loss roles in the prediction of NOx for the turbulent simulation is examined. By using the optically thin approximation model, it is shown that the thermal radiation exhibits little effect on the considered combustion systems within a typical turbulent time-scale.

High Reynolds transonic ideal and non-ideal gas flows around a smooth circular cylinder are investigated by means of Large Eddy Simulations over a range of Mach numbers encompassing the drag divergence. The global aerodynamic performance of the cylinder in both air and a dense vapor are compared, as well as the influence of the thermodynamic behavior of the working fluid on the wake development. The drag divergence is delayed in the dense vapor flow compared to air, and the overall pressure drag is increased due to the lower back pressure. Loss generation mechanisms are also studied via entropy production in the boundary layer and by means of a loss breakdown analysis commonly used in turbomachinery. The specific entropy production rate is found to be lower in the dense gas flow compared to air. Finally, the momentum loss coefficient is reduced upon suppressing unsteady transonic vortex shedding.

We present results of implicit large eddy simulation (LES) and different Reynolds-averaged Navier–Stokes (RANS) models of the MTU 161 low pressure turbine at an exit Reynolds number of (90,000) and exit Mach number of 0.6. The LES results are based on a high-order discontinuous Galerkin method and the RANS is computed using a classical finite-volume approach. The paper discusses the steps taken to create realistic inflow boundary conditions in terms of end wall boundary layer thickness and freestream turbulence intensity. This is achieved by tailoring the input distribution of total pressure and temperature, Reynolds stresses and turbulence length scale to a Fourier series based synthetic turbulence generator. With this procedure, excellent agreement with the experiment can be achieved in terms of blade loading at midspan and wake total pressure losses at midspan and over the channel height. Based on the validated setup, we focus on the discussion of secondary flow structures emerging due to the interaction of the incoming boundary layer and the turbine blade and compare the LES to two commonly used RANS models. Since we are able to create consistent setups for both LES and RANS, all discrepancies can be directly attributed to physical modelling problems. We show that both a linear eddy viscosity model and a differential Reynolds stress model coupled with a state-of-the-art correlation-based transition model fail, in this case, to predict the separation induced transition process around midspan. Moreover, their prediction of secondary flow losses leaves room for improvement as shown by a detailed discussion of turbulence kinetic energy and anisotropy fields.

The effects of hydrogen enrichment on flame and flow dynamics of a swirl-stabilised partially premixed methane-air flame are studied using large eddy simulation. The sub-grid reaction rate is modelled using unstrained premixed flamelets and a presumed joint probability density function approach. Two cases undergoing thermoacoustic oscillations at ambient conditions are studied. The addition of hydrogen modifies both thermoacoustic and fluid dynamical characteristics. The amplitude of the fundamental thermoacoustic mode increases with the addition of 20% hydrogen by volume. A second pressure mode associated with the chamber mode is also excited with the hydrogen addition. Intermittent single, double and triple helical instabilities are observed in the pure methane case, but are suppressed substantially with hydrogen addition. The results are analysed in detail to shed light on these observations. The feedback loop responsible for the thermoacoustic instability is driven by mixture fraction perturbations resulting from the unequal impedances of the fuel and air channels. It is shown that hydrogen addition increases the flame’s sensitivity to these perturbations, resulting in an increase in amplitude. This higher amplitude thermoacoustic oscillation, along with a higher local heat release rate in the presence of hydrogen, is shown to considerably modify the flow structures, leading to a suppression of the helical instabilities.

The settlement of barnacles on a ship hull is a common form of marine biofouling. In this study, the Reynolds number dependency of turbulent flow over a surface partially covered by barnacle clusters is investigated using direct numerical simulations of turbulent channel flow at friction Reynolds numbers ranging from 180 to 720. Mean flow, Reynolds and dispersive stress statistics are evaluated and compared to the corresponding results for a generic irregular rough surface with a Gaussian height distribution. For the barnacle surface, distinctive features emerge in the velocity statistics due to the interplay between the barnacle clusters and the large, connected smooth-wall sections surrounding them. This aspect is further investigated by applying a rough-smooth decomposition to the local time-averaged flow statistics for the barnacle surface. Using this decomposition, the partial recovery of smooth-wall behaviour over the smooth sections of the barnacle surface can be observed in the Reynolds stress statistics with the streamwise Reynolds stresses exhibiting a similar behaviour as previously found for boundary layers over surfaces with a rough to smooth transition.

The stabilization of high Karlovitz number (Ka) jet flames is challenging due to the strong mean shear, and the role of entrainment on high Ka flame stabilization is not well understood. In the present work, a direct numerical simulation study of fluid entrainment and its effect on the flame stabilization in a three-dimensional turbulent high Ka premixed jet flame with a strong mean shear was carried out. The global entrainment characteristics in the turbulent jet flame was analyzed, which shows that the mass flow of the jet increases almost linearly with the streamwise distance. The turbulent/non-turbulent (T/NT) interface was investigated and the conditional statistics near the T/NT interface were analyzed. It was found that the enstrophy transport is generally balanced by the vortex stretching term and the viscous dissipation term. In the region close to the interface, the enstrophy generation from the viscous diffusion term is dominant, which has significant impact on the T/NT interface propagation. Overall, the T/NT interface propagates towards the non-turbulent region. Therefore, the species in the coflow of the non-turbulent region are entrained into the turbulent region across the T/NT interface. Various terms of species transport equations conditioned on the T/NT interface were analyzed. It was concluded that the entrainment of species such as OH plays an important role in flame stabilization in the upstream region.

The paper presents large eddy simulations of a turbulent line burner and studies the influence of turbulence modelling, for various levels of flame extinction. The classical Smagorinsky model, as well as a static and dynamic version of a one-equation model are applied to model sub-grid scale turbulence. Within this context, two different combustion models are considered: the eddy dissipation model (EDM) with infinitely fast chemistry and the eddy dissipation concept (EDC) with simplified finite-rate chemistry. The model assessment is made through comparison to experimental data by considering both first and second order statistics. For the cases without extinction, the results indicate that the use of the dynamic one-equation turbulence model performs poorly with either of the combustion models. The analysis suggests that the dynamically determined turbulence model parameters have a significant effect in the mixing time scales and the resulting reaction rates. For the extinction cases, the use of EDC with finite-rate chemistry is able to predict fairly well the combustion efficiency in conditions far from extinction and during complete extinction. The onset to flame extinction is predicted less satisfactorily, with the discrepancies attributed to radiation modelling and the use of a simplified reaction mechanism.

Large Eddy Simulations (LESs) use Sub-Grid Scale (SGS) models to account for the effects of the unresolved scales of turbulence. The complex processes that occur in the small scales make the development of SGS models challenging. This complexity is even compounded in the presence of multiphase physics due to the mutual interactions between the small-scale hydrodynamics and the dispersed phase distribution and behaviour. In this study, we propose to avoid using an SGS model and demonstrate a novel technique to use a Periodic Box (PB) Direct Numerical Simulation (DNS) solver to find and represent the local SGS turbulence for supplementing a LES. This technique involves matching the local characteristic strain rate in the LES with the large-scale characteristic strain rate in the PB DNS. For simplicity, we assume Homogeneous Isotropic Turbulence (HIT) to be a good representation of SGS turbulence. For a test case, viz. HIT, we compare the averaged turbulence spectra from the LES and the PB DNS with the exact solution from a full DNS simulation. The results show an almost seamless coupling between the large and small scales. As such, this model is more accurate than the common Smagorinsky model in describing the properties of small scales while working within the same assumptions. Further, the effective Smagorinsky constant predicted by our model and the DNS simulation agree. Finally, a two-way coupling is introduced where an effective viscosity is computed in the PB DNS and supplied back to the LES. The results show a definitive improvement in the LES while maintaining stability. The findings showcase the capability of a PB DNS to support a LES with a near-exact simulation of the SGS turbulence.

The geometrically complex mechanisms of energy transfer in the compound space of scales and positions of wall turbulent flows are investigated in a temporally evolving boundary layer. The phenomena consist of spatially ascending reverse and forward cascades from the small production scales of the buffer layer to the small dissipative scales distributed among the entire boundary layer height. The observed qualitative behaviour conforms with previous results in turbulent channel flow, thus suggesting that the observed phenomenology is a robust statistical feature of wall turbulence in general. An interesting feature is the behaviour of energy transfer at the turbulent/non-turbulent interface, where forward energy cascade is found to be almost absent. In particular, the turbulent core is found to sustain a variety of large-scale wall-parallel motions at the turbulent interface through weak but persistent reverse energy cascades. This behaviour conforms with previous results in free shear flows, thus suggesting that the observed phenomenology is a robust statistical feature of turbulent shear flows featuring turbulent/non-turbulent interfaces in general.