Flow, Turbulence and Combustion最新文献

Ammonia combustion is gaining interest as a feasible alternative to traditional fossil fuels because of to the low environmental impact and as hydrogen and energy carrier. This study used Computational Fluid Dynamics (CFD) simulations to compare various turbulence models for premixed ammonia/hydrogen combustion in a swirl-stabilized burner. The primary aim was to identify the best turbulence model for accurately predicting the flow dynamics, combustion behaviour, and emissions profiles of ammonia/hydrogen fuel blends. The turbulence models evaluated were Large Eddy Simulation (LES), Realizable k-(epsilon), Renormalization Group (RNG) k-(epsilon), k-(omega) SST, and Reynolds Stress Model (RSM). On the LES side, a further comparison of two subgrid models (Smagorinsky-Lilly and WALE) was investigated. The Flamelet Generated Manifold (FGM) method was utilized with a detailed chemistry scheme taking into consideration all (hbox {NO}_x) reactions. To improve the prediction of (hbox {NO}_x) emissions, additional scalar transport equations for NO and (hbox {NO}_2) were included. This methodology aimed to be a balance between computational efficiency and the accuracy expected of detailed chemistry models. Validation was done with a swirl burner from Cardiff University’s Gas Turbine Research Centre. Results showed that all turbulence models accurately captured flame characteristics in terms of exhaust temperature and axial velocity with minor differences in the recirculation zones, where only the RSM model can predict the velocity trend as the LES simulation while other RANS models differ by at least 7 m/s. The temperature reached by the LES resulted 100 K higher than the other models in the flame zone. LES simulation can predict the emission value with an error of less than 10(%). Moreover, the error related to emissions derived from the RANS simulations was not negligible, underestimating (hbox {NO}_x) emissions by about 35(%). However, RSM model produced results that were closer to those derived from the high-fidelity LES when compared to the others RANS models, particularly in terms of flame thickness and emissions. It was concluded that it is mandatory to perform an unsteady analysis to reach reasonable results.

The work constructs the Gas kinetic solver (GKUA) to solve the Boltzmann model equation. Then the solver is respectively confirmed by NS, DSMC and experiments in typical conditions during reentry. Furthermore, the Maxwellian gas-surface interaction model is utilized to study the effects of reflected gas molecules state ((alpha_{e})) on flow field and aerodynamic properties at various extent of gas rarefaction. Results reveal the temperature is more susceptible to the state of reflected gas molecules compared with pressure. And the larger gas rarefaction tends to weaken the effects. As for surface heat flux, it just increases with (alpha_{e}) in lower gas rarefaction, while it behaves as the opposite trend with larger gas rarefaction. Freestream condition (H = 50km,Ma = 8.0,AOA = 60^{o}) is set for booster model in practical application. We experience the shrinks of aerodynamic pitch moment coefficient with more (alpha_{e}). These results are valuable for the construction of expired spacecraft forecasting platform which integrates exterior ballistics with aerothermodynamic computations to obtain tracks of spacecraft fragments in advance.

Direct numerical simulations (DNS) have been utilised to investigate the impact of different thermal wall boundary conditions on premixed V-flames interacting with walls in a turbulent channel flow configuration. Two boundary conditions are considered: isothermal walls, where the wall temperature is set either equal to the unburned mixture temperature or an elevated temperature, and adiabatic walls. An increase in wall temperature has been found to decrease the minimum flame quenching distance and increase the maximum wall heat flux magnitude. The analysis reveals notable differences in mean behaviours of the progress variable and non-dimensional temperature in response to thermal boundary conditions. At the upstream of the flame–wall interaction location, higher mean friction velocity values are observed for the case with elevated wall temperature compared to the other cases. However, during flame–wall interaction, friction velocity values decrease for isothermal walls but initially rise before decreasing for adiabatic walls, persisting at levels surpassing isothermal conditions. For all thermal wall boundary conditions, the mean scalar dissipation rates of the progress variable and non-dimensional temperature exhibit a decreasing trend towards the wall. Notably, in the case of isothermal wall boundary condition, a higher scalar dissipation rate for the non-dimensional temperature is observed in comparison to the scalar dissipation rate for the progress variable. Thermal boundary condition also has a significant impact on Reynolds stress components, turbulent kinetic energy, and dissipation rates, showing the highest magnitudes with isothermal case with elevated wall temperature and the lowest magnitude for the isothermal wall with unburned gas temperature. The findings of the current analysis suggest that thermal boundary conditions can potentially significantly affect trubulence closures in the context of Reynolds averaged Navier–Stokes simulations of premixed flame–wall interaction.

The statistical behaviours of wall heat flux and wall shear stress and their interdependence during unsteady head-on quenching of statistically planar turbulent premixed flames within turbulent boundary layers due to heat loss through the cold wall have been analysed using three-dimensional Direct Numerical Simulation data with friction Reynolds numbers of (Re_tau =110) and 180. In both cases, the mean wall shear stress decreases during flame-wall interaction, whereas the mean wall heat flux magnitude increases with time as the flame approaches the wall and eventually assumes a maximum value before decreasing with the progress of flame quenching. The integral length scales of wall heat flux in both streamwise and spanwise directions have been found to grow with time after the maximum mean heat flux magnitude is obtained for the two (Re_tau) cases considered. However, the integral length scale of wall shear stress in the streamwise direction grows but the integral length scale of wall shear stress in the spanwise direction decreases with time after the maximum mean heat flux magnitude is reached. Moreover, the correlation coefficient between the wall heat flux magnitude and wall shear stress becomes increasingly negative while the mean wall heat flux increases with time, but this negative correlation weakens with the progress of flame quenching. The first few (i.e., most energetic) Proper Orthogonal Decomposition (POD) modes of wall shear stress and the wall heat flux magnitude have been found to capture the qualitative nature of the correlation between these quantities and their spatial variations. It is found that tens of most energetic POD modes are needed to capture the mean and variances of wall heat flux and wall shear stress. The number of most energetic modes, which contribute significantly to the statistics of both wall heat flux and wall shear stress, decreases with decreasing (Re_tau) and also with the progress of flame quenching due to the weakening of turbulence effects.

A comprehensive understanding of the mechanisms of intrinsic flame instability, including hydrodynamic and thermodiffusive instabilities, is becoming more important with the move towards greater reliance on hydrogen as a zero-carbon fuel. While intrinsic flame instabilities have been studied extensively both numerically and experimentally, certain important features, including their onset, have been defined mainly by qualitative measures. This work proposes a quantitative marker to identify the onset of intrinsic flame instabilities derived from the statistics of the entropy equation. Direct numerical simulations were carried out for two-dimensional laminar premixed planar methane-air flames, with varying amounts of hydrogen addition up to 100% by volume. Entropy generation mechanisms were analysed based on contributions resulting from heat conduction, viscous dissipation, mass diffusion, and chemical reaction. Instability onset was shown to be characterised by increased data dispersion in all entropy generation terms. The dispersion was quantified by the statistical range, which increased for all locations within the flame as the flame transitioned into instability. Increasing hydrogen addition resulted in a delayed instability onset attributed to the decreasing hydrodynamic instability growth rate. The entropy generation rate due to viscous dissipation was found to be smaller in magnitude compared to other mechanisms, but it was found to be the most sensitive indicator of instability onset. This quantity is readily computed using data from numerical simulations and can be estimated from experimental data, suggesting its potential use as a marker of intrinsic flame instability.

Hydrogen may play an important role in gas turbine engines for achieving carbon neutrality and performing high-altitude missions. Hydrogen influence on the flame speed of aviation kerosene at low pressures was investigated using a constant-volume bomb. The laminar flame speed of aviation kerosene at atmospheric pressure exhibited a linear relationship with increasing hydrogen mass fraction, with a more pronounced promoting effect under fuel-rich conditions. Hydrogen promotion effects on normalized kerosene laminar flame speed are weaker at low pressures than those at atmospheric pressures. The addition of hydrogen and low pressure suppresses flame instability of aviation kerosene especially under fuel-rich conditions, thereby reducing the promoting effect of turbulence on fuel-rich flame propagation. A scaling law that accounted for the influence of flame stability was successfully constructed to characterize the turbulent flame speed of hydrogen-rich aviation kerosene under different conditions.

The present experimental study reports first observations of stability, blowout, and blowoff characteristics of ammonia–hydrogen–nitrogen fuel blend flames with varying volumetric ammonia fractions (({x}_{{NH}_{3}})) in a coaxial combustor. The ({x}_{{NH}_{3}}) is varied from 20 to 80%. For flames of ammonia fraction equal to 70% (({x}_{{NH}_{3}}=0.7)), three types of flame transitions are observed within fuel flow Reynolds number (({Re}_{f})) of 40–575 as a coflow Reynolds number (({Re}_{a})) is increased in steps. Initially, the coflow air remains laminar and ({Re}_{a}) is increased gradually from laminar to turbulent limit. Different flame stabilization modes are characterized as burner-attached and lifted flame. The flame extinction modes are classified as lifted-blowoff, attached-blowoff and attached-blowout types. These flame transitions and stabilization characteristics are shown to be similar to methane flames. However, the flame height and liftoff height are shown to be different. The flames of fuel blends with ammonia fraction less than or equal to 60% (({x}_{{NH}_{3}}le 0.6)) are shown to behave fundamentally different from that of flames with ({x}_{{NH}_{3}}>0.6) (and also methane flames). Specifically, within the tested ({Re}_{f}) range, only one type of flame transition is observed as ({Re}_{a}) is systematically varied in the former as compared to three types observed in the latter. Also, with a decrease in ammonia fraction (and a corresponding increase in hydrogen percentage), the liftoff limit, reattachment limit, and blowout limits all are observed to increase. The effect of ammonia composition on flame height and liftoff height is also elaborated. The present study also provides empirical correlations (particularly for the low power flames) for predicting blowout and blowoff limits in both lifted and attached conditions for ammonia-hydrogen–nitrogen fuel blend flames.

To meet stringent emission norms and achieve enhanced engine performance in spark-ignition engines, in-cylinder charge motion is one of the most important factors for fuel–air mixture preparation and proper combustion. However, in small-bore spark-ignition engines, the development of tumble motion is hindered by an anticlockwise vortex located beneath the intake valve, leading to an early tumble decay during compression. Moreover, the intensity of the tumble directly depends on the intake mass flow rate, regulated by throttle valve openings. Therefore, understanding the impact of throttle openings on flow evolution in small-bore engines is essential. This study employs computational fluid dynamics (CFD) simulations, validated against experimental data of in-cylinder pressure traces and ensemble-averaged flow fields, to analyze the influence of throttle openings on flow fields. Flow evolution on multiple planes is discussed in-depth, along with the jet emanating from the intake valve curtain area, which is correlated with the formation of in-cylinder flow structures. Additionally, it is found that both intake mass flow rate and backflow intensity significantly affect the flow fields. While backflow during intake valve opening (IVO) is more pronounced under 25% throttle opening (TO) condition, it minimally impacts the flow fields on the symmetric tumble plane during the intake stroke for both the TO conditions. Conversely, backflow during intake valve closing (IVC) is more prominent under 100% TO, resulting in earlier tumble decay compared to 25% TO. Also, the effect of backflow is found to have minimal effects on the flow fields of the cross-tumble plane and offset tumble plane.

Low dimensional subspaces are extracted out of highly complex turbulent pipe flow at (Re_{tau }=181) using a Characteristic Dynamic Mode Decomposition (CDMD). Having lower degrees of freedom, the subspaces provide a more clear basis to detect events which meet our understanding of large-scale coherent structures. To this end, a temporal sequence of state vectors from direct numerical simulations are rotated in space-time such that persistent dynamical modes on a hyper-surface are found travelling along its normal in space-time, which serves as the new time-like coordinate. The main flow features are captured with a minimal number of modes on a moving frame of reference whose velocity matches that of the most energetic scale. Reconstruction of the candidate modes in physical space gives the low rank model of the flow. The structures living in this subspace have long lifetimes, posses wide range of length-scales and travel at group velocities close to that of the moving frame of reference. The modes within this subspace are highly aligned, but are separated from the remaining modes by larger angles. We are able to capture the essential features of the flow like the spectral energy distribution and Reynolds stresses with a subspace consisting of about 10 modes. The remaining modes are collected in two further subspaces, which distinguish themselves by their axial length scale and degree of isotropy.

As the main by-product of converter steelmaking process, converter gas has significant potential for energy recovery due to its high calorific value. However, there is a significant risk of explosion during the recycling process. In order to ensure the process safety of converter gas recovery and achieve efficient energy utilization, it is necessary to study the process of CO deflagration in the tube and prevent it. This article combines experiments and numerical simulations to study the effects of obstacles inside tube, water content in the air, and the length of the smooth section on CO deflagration characteristics. The results show that the propagation characteristics of flames in the smooth section are related to the flow field and have periodicity. The length of the smooth section does not significantly affect the maximum deflagration pressure. During the propagation of flames in the obstacle section, the acceleration effect of each obstacle on the flame is similar, and the deflagration becomes more and more intense as the number of obstacles increases. The peak value is reached at the last obstacle, about 0.72 MPa, and the flame speed can reach 672 m/s. The water content in the air has a significant impact on the maximum deflagration pressure of CO, as H₂O triggers a series of chain branching reactions. When the water content increases to 0.39%, the maximum deflagration pressure reaches its peak. In terms of numerical simulation, the reliability of the open-source combustion solver XiFoam was verified. The combustion, transport, and thermodynamic property parameters for premixed gas of CO and humid air were provided using Cantera. Finally, in order to avoid the occurrence of deflagration during the converter gas recovery process, it is necessary to strictly control its moisture content.