Unsteady flamelet modeling for N2H4/N2O4 flame accompanied by hypergolic ignition and thermal decomposition

In this study, the applicability of the flamelet approach to numerical simulations of hydrazine (N₂H₄)/nitrogen tetroxide (NTO, NO) combustion, in which hypergolic ignition and thermal decomposition occur, is investigated in terms of two-dimensional numerical simulations of two types of N₂H₄/NTO jet flames, namely, the gaseous N₂H₄/NTO jet flame and the N₂H₄ spray jet flame in the gaseous NTO stream. In case of the gaseous jet flame, the numerical simulation is performed employing the unsteady flamelet/progress variable (UFPV) approach. In case of the spray jet flame, on the other hand, a non-adiabatic unsteady flamelet/progress variable (NAUFPV) approach, which combines the UFPV approach and the non-adiabatic flamelet/progress variable (NAFPV) approach, is proposed. The validity of these and some other existing flamelet approaches is investigated by comparison with the results obtained using the exact approach, in which detailed chemical reactions are directly solved. The results show that the UFPV and NAUFPV approaches drastically improve predictions of the N₂H₄/NTO gaseous and spray combustion behavior, respectively, including the ignition process, the flame lift-off height, and the distributions of temperature and chemical species concentrations. This indicates that self-decomposition flame of fuel is successfully captured by the UFPV approach, and that the NAUFPV approach can additionally take into account the heat loss effect due to evaporation of droplets

Surface density function evolution and the influence of strain rates during turbulent boundary layer flashback of hydrogen-rich premixed combustion

The statistical behavior of the magnitude of the reaction progress variable gradient [alternatively known as the surface density function (SDF)] and the strain rates, which govern the evolution of the SDF, have been analyzed for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow. The non-reacting part of the channel flow is representative of the friction velocity based Reynolds number Reτ = 120. A skeletal chemical mechanism with nine chemical species and twenty reactions is employed to represent hydrogen-air combustion. Three definitions of the reaction progress variable (RPV) based on the mass fractions of H2, O2, and H2O have been considered to analyze the SDF statistics. It is found that the mean variations of the SDF and the displacement speed Sd depend on the choice of the RPV and the distance away from the wall. The preferential alignment of the RPV gradient with the most extensive principal strain rate strengthens with an increase in distance from the cold wall, which leads to changes in the behaviors of normal and tangential strain rates from the vicinity of the wall toward the middle of the channel. The differences in displacement speed statistics for different choices of the RPV and the wall distance affect the behaviors of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of the effective normal strain rate experienced by the progress variable isosurfaces for different wall distances and choices of RPVs

Influences of liquid fuel atomization and flow rate fluctuations on spray combustion instabilities in a backward-facing step combustor

Combustion instabilities occurring in spray combustion fields inside a backward facing step combustor have been investigated by performing large-eddy simulations (LES). In this study, the influence of fluctuations in the incoming oxidizer air velocity (caused by drastic pressure oscillations in the combustor during combustion instability) on the droplet diameter distribution (due to atomization) of the injected liquid fuel spray, as well as the influence of pressure oscillations on the fuel flow rate have been taken into consideration using appropriate models. For the temporal fluctuations in fuel droplet diameter distribution, a model for the Sauter Mean Diamter (SMD) of atomized droplets, obtained as a function of spray injection parameters and gas/liquid properties, is incorporated in the LES. Additionally, to consider the temporal fluctuations in fuel flow rate along with its phase difference with the pressure oscillations, a model derived from Bernoullis principle is proposed and employed in the LES. The objective is to examine in detail, the impacts of the fluctuations in fuel droplet diameter distribution and the fluctuations in fuel injection rate individually, as well as the impact of the mutual interaction of these two fluctuations, on the spray combustion instability characteristics. Results of the LES reveal that the temporal fluctuations in fuel droplet diameter distribution resulting from combustion instability, lead to a reduction in the intensity of pressure oscillations and hence the combustion instability’s strength. Additionally, the temporal fluctuations in liquid fuel flow rate strongly influence the intensity of spray combustion instability, and it is observed that the combustion instability intensity increases with the increase in phase difference between the fuel flow rate fluctuations and pressure oscillations. Furthermore, the effect of the temporal fluctuations in fuel droplet diameter distribution resulting in the reduction of combustion instability intensity, becomes more pronounced as the phase shift between the fuel flow rate fluctuations and pressure oscillations becomes larger. It is clarified that the above-mentioned behavior of spray combustion instability, results from the change in the correlation between heat release rate fluctuations and pressure oscillations near the combustor’s dump plane, which is caused by the change in the local residence time of fuel droplets and the local fuel droplet evaporation rate

Bingham fluid simulations using a physically consistent particle method

The Bingham fluid simulation model was constructed and validated using a physically consistent particle method, i.e., the Moving Particle Hydrodynamics (MPH) method. When a discrete particle system satisfies the fundamental laws of physics, the method is asserted as physically consistent. Since Bingham fluids sometimes show solid-like behaviors, linear and angular momentum conservation is especially important. These features are naturally satisfied in the MPH method. To model the Bingham feature, the viscosity of the fluid was varied to express the stress-strain rate relation. Since the solid-like part, where the stress does not exceed the yield stress, was modeled with very large viscosity, the implicit velocity calculation was introduced so as to avoid the restriction of the time step width with respect to the diffusion number. As a result, the present model could express the stopping and solid-like behaviors, which are characteristics of Bingham fluids. The proposed method was verified and validated, and its capability was demonstrated through calculations of the two-dimensional Poiseuille flow of a Bingham plastic fluid and the three-dimensional dam-break flow of a Bingham pseudoplastic fluid by comparing those computed results to theory and experiment

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H2/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H2/air flames. Next, LESs of a lean-premixed H2/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure

A flamelet LES of turbulent dense spray flame using a detailed high-resolution VOF simulation of liquid fuel atomization

A numerical framework used to model dense spray flames is proposed. In this framework, the liquid fuel (acetone) atomization is solved by a detailed high-resolution VOF simulation, and the Eulerian components of liquid droplets are transformed into Lagrangian droplets, which are stored in a database at a certain downstream cross-section. Then, the combustion process is solved by a LES/FPV (flamelet progress variable) adopting the pre-stored database of Lagrangian droplets (i.e., the position, size, and velocity of each droplet) as the inlet boundary conditions. This framework is a one-way coupling between a VOF simulation and a combustion simulation. The validity of this approach is investigated by comparing the computations with the experiments of the Sydney Piloted Needle Spray Burner. The VOF simulation shows that the volume flux of the droplets at the nozzle exit fluctuates both temporally and spatially and the larger droplets tend to be located away from the center axis compared to the small droplets. The computed breakup length is in good agreement with the empirical correlation. In the database of the Lagrangian droplets for the LES/FPV of spray flames, the location of the sampling cross-section, the sampling time, and the threshold value for Eulerian–Lagrangian (E-L) transformation strongly affect the properties of the Lagrangian droplets, and are critical for the successful use of the LES/FPV. Two spray flames with different recess distances are computed using their optimal pre-stored droplets databases and both show generally good agreement with the experiments in terms of the gas temperature and droplet size distributions. The spray flame with a longer recess distance, which is more representative of a dilute spray, is considered to have a longer and wider premixed core than that with a shorter recess distance representing a dense spray. The discrepancy in the prediction of denser spray flames becomes more evident leading to over-predictions of gas temperature further downstream. Reasons for this behavior are discussed in the text

Numerical analysis of heat transfer characteristics of spray flames impinging on a wall under CI engine-like conditions

Design of Compression Ignition (CI) engines with improved thermal efficiencies needs better understanding of the heat transfer mechanism from spray flame to the combustion chamber wall. In this regard, heat transfer occurring during the interaction between impinging spray flame and wall, under CI engine-like conditions, is investigated in this study using 3-Dimensional numerical simulations based on an Eulerian–Lagrangian framework. Simulations are performed for different fuel spray injection velocities (which are representative of different fuel injection pressures in CI engines), to examine their influence on the heat transfer between impinging spray flame and wall. To couple the convective and radiative heat transfer at the wall surface with the conduction heat transfer occurring within the finite thickness wall, Conjugate Heat Transfer (CHT) is incorporated in the simulations. A Non-Adiabatic Flamelet/Progress Variable (NA-FPV) approach is employed as the combustion model of n-dodecane, which is considered to be the fuel for liquid spray. Dynamics of the liquid film formed on the wall surface by impinging fuel droplets are captured using a particle-based approach. Contribution of radiative heat flux is taken into consideration using the Discrete Ordinates (DO) method. Results indicate that the total heat flux (sum of convective and radiative heat fluxes) at the wall surface increases with the fuel injection velocity. It is observed that the total wall heat flux is largest in the stagnation zone where the spray flame impinges directly on the wall surface, while the radiative heat flux at the wall surface becomes larger as the distance from this stagnation zone increases. Additionally, it is found that the influence of fuel injection velocity on the radiative heat flow rate at the wall surface is rather small. This radiative heat flow rate when expressed as a percentage of the total wall heat flow rate, ranges from ≈ 18% to 30% (depending on the 3 cases investigated), indicating that its contribution cannot be neglected for the CI engine-like conditions under which the present simulations are performed. Furthermore, to characterize the heat transfer occurring during spray flame-wall interaction process, correlations between the Nusselt number Nu (corresponding to the wall heat loss) and Reynolds number Re (of the flow field) of the form Nu ∝ Re, are analysed and compared with that of a previous experimental study to assess their applicability. It is found that, depending on how the Nusselt number Nu is defined (either using the total wall heat flux or the convective heat flux), the value of the correlation index n changes. When Nu is calculated based on the total wall heat flux (which includes the contribution from the radiative heat flux), the value of n is found to be 0.49 which is close to the correlation index value of n = 0.4 reported in the recent experiments performed at Toyota Central R&D Labs., Inc

Investigation of combustion noise generated by an open lean-premixed H₂/air low-swirl flame using the hybrid LES/APE-RF framework

An open lean-premixed hydrogen/air low-swirl (LPHALS) turbulent flame exhibiting a pronounced peak in its combustion noise spectra, is investigated numerically using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. Under this framework, the reacting flow-field of the flame is computed via Large-Eddy Simulation (LES), while the direct combustion noise it produces is captured by solving the Acoustic Perturbation Equations for Reacting Flows (APE-RF). Flame configuration and simulation conditions correspond to those of an experimental study on an open lean-premixed H₂/air flame stabilized using a Low-Swirl Burner (LSB). LES results are validated against experimental data. The CAA simulation is able to predict a pronounced sharp peak in the computed combustion noise spectra, similar to one of the two characteristic peaks observed in the measured combustion noise spectra. Frequency of this spectral peak predicted by the CAA simulation is 840 Hz, which is close to that of the higher frequency secondary spectral peak at 940 Hz measured in the experiment. Upon examining the hybrid LES/APE-RF results, the noise generation mechanism at 840 Hz is found to be the intense local heat release rate fluctuations, caused by strong interaction between the flame and the periodically generated vortical flow structures in the shear layers, downstream of the LSB exit. Additionally, analysis of the spectral content and directivity of the noise generated by different acoustic source terms is performed, in order to investigate their impact on the radiated acoustic field, and hence the characteristics of direct combustion noise produced by the open LPHALS flame

Improved Measurement Characteristics of Elemental Compositions Using Laser-Induced Breakdown Spectroscopy

Rapid detection of coal and fly ash is significant to improve the efficiency of thermal power plants and reduce environmental pollution. Given its fast response, high sensitivity, real-time, and noncontact features, laser-induced breakdown spectroscopy (LIBS) has a great potential for on-line measurement in these applications. The direct measurement of particles and gases using LIBS was studied, and the method was shown to be effective for this application

Predictions of NO and CO emissions in ammonia/methane/air combustion by LES using a non-adiabatic flamelet generated manifold

A large-eddy simulation (LES) employing a non-adiabatic flamelet generated manifold approach, which can account for the effects of heat losses due to radiation and cold walls, is applied to NH3/CH4/air combustion fields generated by a swirl burner, and the formation mechanisms of NO and CO for ammonia combustion are investigated in detail. The amounts of NO and CO emissions for various equivalence ratios, are compared with those predicted by LES employing the conventional adiabatic flamelet generated manifold approach and measured in the bespoke experiments. The results show that the amounts of NO and CO emissions predicted by the large-eddy simulations with the non-adiabatic flamelet generated manifold approach agree well with the experiments much better than the ones with the adiabatic flamelet generated manifold approach. This is because the NO and CO reactions for NH3/CH4/air combustion are quite susceptible to H and OH radicals’ concentrations and gas temperature. This suggests that it is essential to take into account the effects of various heat losses caused by radiation and cold walls in predicting the NO and CO emissions for the combustion of ammonia as a primary fuel