Sudden-expansion Combustor Research Articles

A Numerical Study on variation of Flow Pattern of Fluid Passing through three Different Modified Dump Combustors

In this paper, a numerical study on flow characteristics of fluid passing through a sudden expansion plain dump combustor and modified dump combustors having a central restriction of three different shapes at a certain distance ahead the throat has been carried out. The two dimensional, steady differential equations for conservation of mass, momentum and the governing realizable k-e equations are solved for the Reynolds number 1.2x105, aspect ratio (AR) of 3 and 100% central restriction. The variations of streamline contours, re-circulating zones and axial velocities along the radial distance at various axial positions have been studied. After overall study, it is observed that the adverse pressure gradient between central and wall region at the outlet casing is more. Re-circulation zone (RZ) formation in modified dump combustors is more compared to a plain sudden expansion combustor. The fluctuation of radial profile of axial velocity is more at any location for the fillet shape restriction compared to other two shapes of restrictions (rectangular and hemispherical). The modified combustors may perform in efficient manner in terms of complete combustion and less pollutant emission due to formation of more re-circulating bubbles throughout the outlet casing of the combustor.

Investigations on the influence of swirl intensity on solid-fuel ramjet engine

In this paper, the influence of swirl intensity on solid fuel regression rate and combustion phenomena in a solid fuel ramjet has been investigated numerically and experimentally. First, an in-house code has been developed to solve axisymmetric Reynolds-averaged Navier–Stokes equations of unsteady turbulent swirling compressible flow field with chemical reactions. Second, experiments have been conducted in a solid fuel ramjet without swirl to validate the developed code for solid fuel decomposition; then the predictive capability of the code is validated by using; swirling flow passes through sudden expansion combustor, shock-induced combustion case, and a semi-infinite plate. Third, unsteady simulations are carried out for reacting turbulent flows with and without swirl in a solid fuel ramjet using high-density polyethylene (HDPE) solid fuel. Then, the computational results are analyzed and discussed. The results show that, the presence of swirl is more effective in enhancing the regression rate and the turbulent mixing throughout the ramjet. Increase swirl number increases the heat and mass transport at the fuel surface and hence increases the regression rate.

Numerical investigation of NOx reduction in a sudden-expansion combustor with inclined turbulent air jet

Axisymmetric sudden-expansion geometry of a co-flowing methane-air diffusion flame is considered to investigate the effect of air inlet conditions on NOx formation, flow field and temperature distribution using the k-ɛ turbulence and β-PDF combustion model. The predicted results are in acceptable agreement with the published experimental and numerical data. The obtained results show that increasing air turbulence intensity results in considerable decrease in NO formation. Increasing the inlet angle of the air causes the NO formation to decrease due to raising vorticity strength. As a new index, the mass-averaged integral of vorticity magnitude is introduced to investigate the effect of altering inlet angle of the air on the flow field.

Vortical Structure of Reacting Flow in a Sudden-Expansion Combustor with Solid Fuel

Vortical Structure of Reacting Flow in a Sudden-Expansion Combustor with Solid Fuel

Transient flame spread during convective ignition of solid fuel in a sudden-expansion combustor

The convective ignition of solid fuel (PMMA) in a sudden-expansion combustor ( Re h = 6200, u 0 = 22 m/s, [O 2] ∼ 11.7%, T 0 = 810 °C) is investigated from the perspective of flame–vortex interactions. Three phases of the transient flame spread are identified via the diagnostics of flow visualization and particle image velocimetry (PIV). The dominance of small/large vortices is revealed, respectively, in the pre-/post-ignition regimes, which demonstrates the small-to-large vortex transformation due to heat release. Attributed to the decreased characteristic reaction time and enhanced mixing, the first ignition is observed at the downstream end of the fuel, after which a primitive flame is formed and initiates the opposed flame spread. During the spread, the rolling behavior of flame kernels are considered to be dominated by the small eddies. The combined effects of broken vortices and continuing pyrolysis introduce the periodical extinction–reignition around the reattachment region. At the final phase, the entrainment of flame kernels into the shear layer is facilitated by the large shedding vortices, and a sustained diffusion flame is established. The study not only provides novel insights into the convective ignition of solid fuel in the separation–reattachment flow, but also serves as a basis for the advancement of ignition control.

Swirler Flow Field Characteristics in a Sudden Expansion Combustor Geometry

The present study investigates the flow field characteristics of a gas turbine swirler in a model combustion chamber, using particle image velocimetry. Detailed mean and RMS velocities, vorticity, Reynolds shear stress, and pseudoturbulent kinetic energy were obtained at various cross sections downstream of the swirler and in a plane along the inlet flow direction. The experiments were performed in a sudden expansion square geometry. A central toroidal recirculation zone and corner recirculation zone was observed and characterized. Another instability caused by swirl, called precessing vortex core, has been observed far downstream of the swirler, in the plane located at Z/D = 2.5 and 1.25 (D, diameter of the swirler) depending on the pressure drop across the swirler. High RMS velocity magnitudes are observed in several cross-sectional planes indicating high levels of turbulence generated by the swirling effect which promotes rapid mixing. The structure of the complex swirling flow field has been investigated both qualitatively and qualitatively.

A pure Eulerian model for simulating dilute spray combustion

A pure Eulerian model is developed to simulate steady-state dilute spray combustion. This model is based on a fundamental description of various interacting processes which occur during spray combustion including gas-phase and spray droplet-phase turbulent flow, gas-phase turbulent combustion, radiation heat transfer and spray droplet evaporation. Both gas-phase and spray droplet-phase conservation equations are described using Eulerian coordinates. A comprehensive mathematical model is used to simulate isothermal flow, combusting flow and kerosene spray combusting flow in an axisymmetric sudden-expansion combustor. Validity of the model and the simulation scheme are established from the good agreement of the prediction with experimental data in gas-phase isothermal flow and propane combusting flow. Simulation of kerosene spray combustion shows that small droplets evaporate rapidly and do not enter the recirculation zone. For larger droplets, the length of recirculation zone is almost the same as that of the gas-phase. Simulated velocity, temperature and oxygen concentration for kerosene combustion also agrees well with experimental results.

Particle behaviors in a pulverized coal-fired sudden-expansion combustor with coaxial jets

This paper presents a numerical simulation of particle motion and combustion in a pulverized coal-fired sudden-expansion combustor with coaxial jets. The computations are conducted for both bituminous and lean coal flames. The obtained results show the trajectories and the mass and temperature variations with time for coal particles of different initial sizes. It is seen that the interactions between the particles and the recirculating flow have pronounced effects on particle motion and combustion. Particle motion should match the gas-flow pattern and the penetration of the coal particles into the hot reverse flow is favorable for their achieving sufficient heating and devolatilization. On the other hand, the gas recirculating flow pattern has to be properly arranged to meet the needs of particle heating and devolatilization for different grades of coal.

99/02182 Second-order moment turbulence-chemistry model for simulating NO x formation : Guo, Y. et al. Gongcheng Rewuli Xuebao, 1998, 19, (3), 382–386. (In Chinese)

Thermal NO and fuel NO formation models with a second-order moment closure totreat the turbulence chemistry interaction are incorporated into the comprehensive modelfor simulating pulverized coal combustion. Simulation of three dimensional pulverized coalcombustion and NO formation has been made in a sudden-expansion combustor with highvelocity jets. The predicted results show that the values of thermal NO are smaller thanthat of fuel NO, the amounts of thermal NO produced is only about 2% of the total NO.Compared with the predicted results using the Arrhenius model, it can be seen that theNO concentration predicted by the Arrhenius model are larger than that predicted by thesecond-order moment turbulence chemistry model, so the turbulence effect is beneficial toreduce the NO forn1ation under the modeling condition in this combustor.

NOxPrediction in 3-D Turbulent Diffusion Flames by Using Implicit Multigrid Methods

Abstract Modeling of the nitric oxide formation in the turbulent nonpremtxed methane and syngas (CO/H2/N2) diffusion flame is studied using an implicit time-stepping and multigrid technique. The chemical kinetic model for both methane-air in a sudden-expansion combustor and syngas-air combustion in a laboratory combustor as well as in a gas turbine combustor is assumed to have 49 species and 229 finite-rate, reversible reaction steps. The standard k - e turbulence model and the algebraic correlation closure model are applied to close the time-averaged Navier-Stokes and species equations (Liao et al., 1995) respectively. The computation requires about 250 time steps to reduce the residual by 3 orders of magnitude for the 3-D turbulent methane-air diffusion flame case on a 34 × 18 × 18 grid, which shows convergence rate is much faster than conventional iterative methods. Computational results with detailed chemistry are exhibited and some of them are compared with experimental data. Qualitative agreement be...

Controlling mechanisms of ignition of solid fuel in a sudden-expansion combustor

Ignition of solid fuel by a hot oxidizing flow in a sudden-expansion combustor was investigated experimentally. The controlled variables of the experiments were concentration of oxygen (12-25%), gas temperature (750-850 C), and flow velocity (19-46 m/s). The step height was 29 mm. The corresponding Reynolds numbers based on the flow velocity and the step heights were 12 x 10(sup 4)-31 x 10(sup 4). The controlling mechanisms of ignition in the flow with abundant oxygen were distinct from those with little oxygen. The initial flame kernels formed near the reattachment point and adjacent to the surface of solid fuel when the oxygen concentration was large. The process was controlled by diffusion and the ignition delay decreased with increased flow velocity. For the flow containing oxygen at a small concentration, the initial flame kernels formed within the recirculation zone and away from the surface of the solid fuel. The process was then controlled by the chemical kinetics and the ignition delay increased with increased flow velocity. 15 refs.

Ignition transient of a polymethylmethacrylate slab in a sudden-expansion combustor

Ignition and the subsequent flame development of a polymethylmethacrylate (PMMA) slab in a sudden-expansion combustor was observed experimentally. The solid fuel was ignited by a hot oxidizing flow stream that was supplied by a wind tunnel. The controlled variables of the experiment were: step height (17–29 mm), inlet flow velocity (15–30 m/s), and inlet gas temperature (700–850°C). The oxygen concentration of the gas stream was 13%. The ignition process generally initiated in the recirculation zone, but occasionally occurred near the end of the fuel slab at high temperature of the gas and small velocity of the flow. The results show that high temperature and a high step height favored ignition, broadened the ignition limits and abbreviated the ignition delay. The interval of flame development was about 0.1 s when ignition occurred in the recirculation zone but 0.9 s for ignition near the end of the fuel slab. As the step height increased, the opposed flame spread rate increased, but the rate of concurrent flame spread decreased. In this work, the rate of opposed flame spread was 0.2–7.7 m/s and that of concurrent flame spread was 9.6–18.3 m/s.

Study of two-phase flow for a ramjet combustor

A reactive two-phase flow is studied experimentally by a special laser Doppler anemometry (LDA) technique and numerically by solving the elliptic governing equations. The relation between the liquid fuel injection and the aerodynamic flameholding mechanism of a dump combustor was investigated. The flow geometry chosen was an axisymmetric sudden expansion with small diameter ratio. The measurements included pressure, temperature, gas species concentration, as well as velocities of both phases and droplet size. A special processing technique enabled the simultaneous measurement of the gas velocity represented by the smallest droplets and of the size-velocity correlation of the droplets in the range of 5-500 |xm. The specific geometry of the sudden expansion combustor creates an annular flame stabilized just downstream of the step. The geometry of the flame depended very much on the equivalence ratio, with larger values creating longer and thicker flames. The combustion shortened the recirculation length from about 11 times the step height to about four times. There was no direct influence of the droplets on the average gas velocity in cold flow, whereas the size distribution of the droplets had significant influence on the combusting flow pattern. Two computational case studies were performed for isothermal and reactive two-phase flow situations. The numerical results indicated the fast dynamic and thermal response of the small droplets to the surrounding gas flow. Moreover, the droplets did not penetrate into the fuel-rich recirculation zone. The shear layer, which separates it from the core, was characterized by a local equivalence ratio close to unity. The numerical predictions and the experimental results exhibited fairly good agreement, provided that the initial conditions of the spray and the thermal boundary conditions are determined experimentally.