Two-phase Turbulent Combustion Research Articles

Numerical investigation on the combustion characteristics of powder fuel under different regulation parameters

A comprehensive numerical study is undertaken to investigate the operating feature in a powder fueled scramjet combustor. The effect of regulation parameters on the flow field and combustion mechanism is examined systematically. The numerical simulation method is developed for multi-species gas-solid two-phase turbulent combustion in the Euler-Lagrange frame. The particle surface reaction model and the Eddy-dissipation Concept model are utilized for powder fuel combustion. The combustor flow field, flame structure, and species distribution in the powder fueled scramjet combustor are revealed in detail. The numerical simulation results show that the combustion organization scheme of tandem double-cavity can provide an excellent flame stabilization effect. The powder fuel achieves continuous combustion and sufficient release heat in the supersonic airflow. The combustion flow field features and flame distribution are strongly influenced by the transport solid-gas ratio and the equivalent ratio. The increase of both regulation parameters improves the combustion efficiency of powder fuel in the region from the injection hole to the downstream cavity. Nevertheless, the large equivalent ratio significantly leads to higher total pressure losses in the combustor.

Studies of a combined way of flame stability in ramjet combustor

Abstract numerical simulation was conducted to study the influence of bleeding. The Euler-Lagrange method was used to investigate the two-phase turbulent combustion flow. Standard k-e turbulent model was adopted in the continuous phase simulation and particle-trajectory model was adopted in the dispersed phase simulation. The results demonstrates: air bleeding can improve the flow field after the strut and the stability of trapped vortex in the cavity; change of bleeding temperature has little effect on the total pressure recovery coefficient and significant effect on combustion efficiency; When fuel-air ratio changes, the combustor performs better in a lean oil state.

On the development and application of a droplet flamelet-generated manifold for use in two-phase turbulent combustion simulations

A droplet flamelet-generated manifold (DFGM) is developed to account for the effects of finite-rate chemistry of individual droplet combustion during simulations of a turbulent combustion environment. A spherically symmetric droplet model is developed for methanol using the hydrocarbon and nitrogen kinetic mechanisms developed at UC San Diego to account for chemical reaction rates. The inclusion of finite-rate chemistry allows for the capturing of the transition from diffusion to kinetically controlled combustion as the droplet diameter decreases. The droplet model is used to create a DFGM by successively solving the 1D flame equations at varying drop sizes, where the source terms for energy, mixture fraction (Z), and progress variable are cataloged as a function of Z and droplet diameter. A unique coupling of the spherical and planar FGMs is developed and is used to account for individual and group combustion processes simultaneously. Three combustion models are considered when modeling a methanol spray flame; (i) an evaporative model coupled with an unsteady planar flamelet-generated manifold (UFGM), (ii) using only the DFGM method, and (iii) the DFGM coupled with the planar UFGM. The models are compared against one another as well as experimental data for a methanol spray flame with an annular air jet. The DFGM model is shown to agree well with burn rates and normalized flame radii from individual droplet burning experiments. Good overall agreement is observed between the experimental data and the DFGM and coupled UFGM models for temperature, particle size distribution, and OH concentration.

A two-phase turbulent combustion model and its validation for spray flames

A two-phase turbulent combustion model (FSM) with the inclusion of individual droplet burning behavior was applied in a methanol air spray flame. The FSM model results were compared with the experimental data and results of the traditional two-phase turbulent combustion model (TM). The comparison of the results showed that in most regions the FSM model results were in good agreement with the experimental data. In all the three cases, the FSM model predictions showed the same tendency as the experimental data, which was better than the TM model results. So in the liquid methanol spray flame cases simulated in this paper under atmospheric condition, the FSM model is reasonable for temperature profiles prediction, which could be applied in other liquid fuel combustion simulation in the future.

Autoignition of monodisperse biodiesel and diesel sprays in turbulent flows

A new experimental configuration has been developed to examine the effects of flow on the autoignition of dilute diesel and biodiesel sprays, where the spray is injected in the form of monodisperse individual droplets at right angles to a hot air turbulent flow. The ignition location has been measured by monitoring the OH* chemiluminescence. A qualitative comparison of the flame behaviour between ethanol, acetone, heptane and biodiesel as fuels has also been carried out. With decreasing volatility of the fuel, the flame showed progressively a higher number of individual droplets burning, with the first autoignition spots appearing at random locations but in general earlier than the intense droplet-flame emission. The time-averaged autoignition length increased with increasing air velocity and with increasing intensity of the turbulence, while it decreased with the temperature and the droplet size. The data can be used for validating models for two-phase turbulent combustion.

Numerical Simulation of the Two-Phase Turbulent Combustion Flow in the Multicomponent Propellant Rocket Engine

The numerical simulation, based on computational fluid dynamics methodology, has been performed to study the two-phase turbulent combustion flow in rocket engine using non-metallized multicomponent propellant. A reduced reaction mechanism is developed for modelling combustion of fuel droplets in the absence of metal. Gas governing equations are two dimensional axisymmetric N-S equations in Eulerian coordinates. The trajectory model is adopted to analyse the droplet-phase including the droplet collision, breakup and evaporation. The gas flow is influenced by the droplets by adding source term to N-S equations. The reliability of the simulation programme is validated by comparing numerical simulation result with engine test data.

Numerical Simulation of the Two-Phase Turbulent Combustion Flow in the Multicomponent Propellant Rocket Engine

The numerical simulation, based on computational fluid dynamics methodology, has been performed to study the two-phase turbulent combustion flow in rocket engine using non-metallized multicomponent propellant. A reduced reaction mechanism is developed for modelling combustion of fuel droplets in the absence of metal. Gas governing equations are two dimensional axisymmetric N-S equations in Eulerian coordinates. The trajectory model is adopted to analyse the droplet-phase including the droplet collision, breakup and evaporation. The gas flow is influenced by the droplets by adding source term to N-S equations. The reliability of the simulation programme is validated by comparing numerical simulation result with engine test data.

Large-eddy simulation of two-phase spray combustion for gas turbine combustors

In three-dimensional arbitrary curvilinear coordinates, an Eulerian–Lagrangian formulation is applied to large-eddy simulation (LES) of instantaneous gas–liquid two-phase turbulent combustion flows in gas turbine combustors. Three dimensional block-structured grids are generated by zone method and solving a system of elliptic partial differential equations. The k-equation sub-grid scale model is used to simulate the sub-grid eddy viscosity and the EBU combustion sub-grid scale model is employed to predict the chemical reaction rate. The gas-phase governing equations are solved with SIMPLE algorithm and hybrid scheme in non-staggered grid system. A stochastic separated flow formulation is used to track the droplet trajectories velocities, size and temperature history by Lagrangian equations of motion and thermal balance. Multi-zone coupling method is employed to transport data between interfaces. The influences of two different primary hole positions and three different fuel–air ratios on turbulent two-phase reacting flows are calculated. Predictions are in reasonable agreement with the measured velocity using PIV system and temperature, species concentration measurements at the exit. It is shown that the present approach may be used to study spray combustion flow fields for guiding the design of advanced gas turbine combustors.

Flame propagation through aluminum particle cloud in a combustion tube

The propagation of a flame is investigated experimentally and theoretically for a large, horizontal combustion tube containing a mixture of air and aluminum powder with pre-existing turbulence. One end of the tube is closed and the other is connected to a large dump-tank. Twenty dispersion systems are used on the tube to produce a uniform suspension of aluminum dust in the tube with a mean diameter of 6 μm. The characteristics of a flame front from the ignitors at the closed end are measured using photodiodes and the development of pressure is monitored by transducers. Experimental results revealed the entire process of an accelerating flame and the development of shock waves. A set of conservation equations for two-phase turbulent combustion flow is derived, using the two-fluid model, k–ε model, Hinze–Tchen model and EBU-Arrhenius model for turbulent combustion. The SIMPLE scheme usually applied to the homogeneous turbulent combustion is extended to fit this two-phase, reactive behavior. The results of calculations show the positive feedback coupling among combustion, expansion and turbulence during flame propagation. Computed and measured results are generally in good agreement.

Direct Numerical Simulation of Statistically Stationary One- and Two-Phase Turbulent Combustion: A Turbulent Injection Procedure

A new turbulent injection procedure dedicated to fully compressible direct numerical simulation (DNS) or large eddy simulation (LES) solvers is proposed. To avoid the appearance of spurious acoustic waves, this method is based on an accurate tracking of the turbulent structures crossing the boundary at the inlet of the domain. A finite difference DNS solver has been coupled with a spectral simulation in which a statistically stationary homogeneous turbulence evolves to provide fluctuating boundary conditions.

Combustion of a submerged gaseous oxidizer jet in a liquid metal

This investigation is concerned with the combustion of a gaseous oxidizer jet discharging through an injector submerged in a bath containing a molten metal fuel. Conditions were limited to high injector Reynolds numbers, which generates a turbulent two-phase forced jet as opposed to a stream of individual bubbles. Experiments were conducted using chlorine as the oxidizer with sodium as the fuel. The extent of the gaseous portion of the flow, which corresponds to the flame length in more conventional systems, was observed with x-rays for a range of bath operating conditions. Temperature measurements were made along the centerline of the jet, in the region downstream of the active combustion zone. The two-phase turbulent combustion process was analyzed in a manner similar to conventional diffusion flames. A variable density single-fluid model was used to represent the two-phase mixture, in conjunction with an entrainment assumption appropriate for systems with large density differences between the jet and the ambient fluid. The theory provided an expression which correlated the present combustion length results; earlier measurements of flame lengths for gaseous forced turbulent diffusion flames; and, earlier measurements of vapor penetration length in two-phase condensing jets (by analogy). A similar correlation was also obtained between the centerline enthalpy decrement of the present combustion process, and scalar quantities in more conventional turbulent jet systems. In regard to both combustion lengths and centerline quantities, the large density difference between the injected and ambient materials in the present system, results in a substantial extension of the range of data for these correlations. Consideration of the thermodynamics of the metal combustion process also indicates conditions of operation where the extent of the gaseous portion of the flow is controlled by the recondensation of combustion product vapors, rather than consumption of the oxidizer.