Search

Acoustic characteristics of a thrust chamber with quarter-wave resonators are numerically studied based on the unsteady Reynolds-averaged Navier-Stokes (URANS) method. Organized pressure disturbance model and constant-volume bomb model are applied as artificial disturbances to excite pressure oscillations in the chamber. Eigenfrequencies and amplitudes of acoustic modes of the chamber are obtained by fast fourier transform (FFT) analysis, while damping characteristics are evaluated by the half-power bandwidth method. Predicted damping capacities of the chamber with and without quarter-wave resonators agree well with experimental results. Pressure oscillations can be controlled by a quarter-wave resonator mainly through reducing the amplitude of target acoustic mode, rather than increasing damping capacity of the chamber. Major damping mechanism of the resonator is cutting down pressure peak of target acoustic mode and raising up its pressure trough (CPRT); therefore the amplitude of target acoustic mode is reduced significantly. Moreover, acoustic energy can be dissipated by vortex at the orifice and by viscosity on the surface of a resonator, which increase damping capacity of the chamber slightly. Under the condition with multi-modes pressure oscillations, a resonator can still suppress pressure oscillations of target acoustic mode through CPRT. However, it may enhance pressure oscillations of other modes due to redistribution of oscillation energy among all acoustic modes.

Following an order analysis of key parameters, a decoupled procedure for simulation of convection-radiation heat transfer problems in supersonic combustion ramjet (scramjet) engine was developed. The radiation module of the procedure consisted of Perry 5GG weighted sum gray gases model for spectral property calculation and discrete ordinates method S4 scheme for radiative transfer computation, while the flow field was computed using the Favre average conservative Navier-Stokes (N-S) equations, in conjunction with Menter’s k-ω SST two-equation model. A series of 2D supersonic nonreactive turbulent channel flows of radiative participants with selective parameters were simulated for validation purpose. Radiative characteristics in DLR hydrogen fueled and NASA SCHOLAR ethylene fueled scramjets were numerically studied using the developed procedure. The results indicated that the variations of spatial distributions of the radiative source and total absorption coefficient are highly consistent with those of the temperature and radiative participants, while the spatial distribution of the incident radiation spreads wider. It also demonstrated that the convective heating is significantly affected by the complexity of the flow field, such as the shock wave/boundary layer interactions, while the radiative heating is simply an integral effect of the whole flow field. Although the radiative heating in the combustion chambers reaches a certain level, an order of magnitude of 10 kW/m2, it still contributes little to the total heat transfer (

The supersonic mixing layer flow, consisting of a relatively cold, slow diluted hydrogen stream and a hot, faster air stream, is numerically simulated with detailed transport properties and chemical reaction mechanisms. The evolution of the combustion process in the supersonic reacting mixing layer is observed and unsteady phenomena of ignition, flame propagation and extinction are successfully captured. The ignition usually takes place at the air stream side of braid regions between two vortexes due to much higher temperature of premixed gases. After ignition, the flame propagates towards two vortexes respectively located on the upstream and downstream of the ignition position. The apparent flame speed is 1569.97 m/s, which is much higher than the laminar flame speed, resulting from the effects of expansion, turbulence, vortex stretching and consecutive ignition. After the flame arrives at the former vortex, the flame propagates along the outer region of the vortex in two branches. Then the upper flame branch close to fuel streamside distinguishes gradually due to too fuel-riched premixed mixtures in the front of the flame and the strong cooling effect of the adjacent cool fuel flow, while the lower flame branch continues to propagate in the vortex.

A particle-laden turbulent channel flow is investigated to study particle clusters in large-scale turbulent coherent structures. The fluid phase is calculated by large eddy simulation and particles are tracked using Lagrangian trajectory method. The flow Reynolds number is 180 based on the friction velocity and half-width of the channel. The particle is lycopodium with St=0.93 which may well follow the fluid phase. The mean and fluctuating velocities of both two phases are obtained, which are in good agreement with previous data. The strongest accumulations of particles in low-speed streak structures are observed at y+=11.3. Moreover, once particles are captured in low-speed streaks, most of them will reside there for a long period. Particles clustered in low-speed streaks obtain smaller instantaneous wall-normal and spanwise velocities than those out of there, which induce a larger particle flux into low-speed streaks than that out of there. The study is important for understanding particle dispersion mechanisms in gas-particle turbulent channel flows.

Turbulent two-phase reacting flow in the chamber of LOX/RP-1 bipropellant liquid rocket engine is numerically investigated in this paper. The predicted pressure and mean axial velocity are qualitatively consistent with the experimental measurements. The self-excited pressure oscillations are obtained without any disturbance introduced through the initial and boundary conditions. It is found that amount of abrupt pressure peaks appear frequently and stochastically in the head regions of the chamber, which are the important sources to drive and strengthen combustion instability. Such abrupt pressures are induced by local constant volume combustion, because local combustible gas mixtures with high temperature are formed and burnt out suddenly due to some fuel droplets reaching their critical state in a rich oxygen surrounding. A third Damkohler number is defined as the ratio of the characteristic time of a chemical reaction to the characteristic time of a pressure wave expansion to measure the relative intensity of acoustic propagation and combustion process in thrusters. The analysis of the third Damkohler number distributions in the whole thrust chamber shows that local constant volume combustion happens in the head regions, while constant pressure combustion presents in the downstream regions. It is found that the combustion instability occurs in the head regions within about 30 mm from the thruster head.

A three-dimensional large eddy simulation (LES) of a spatially developing round jet is carried out in cylindrical coordinates using a dynamic subgrid model with strong inflow instability. Evolutions of large-scale vortex structures represented by tangential vortices are obtained and compared with flow visualization. Also presented are three-dimensional spatial evolutions of coherent structure, which are of quasi two-dimensional Kelvin-Helmholtz instability and vortex rings as well as breaking up of the vortex rings with fully three-dimensional characteristics. Predicted results of mean velocity and turbulent intensity agree well with experiments. They are also compared with the results predicted by LES using standard Smagorinsky model and show good self-similarity. Turbulence spectrum of the predicted velocity shows the −5/3 decay for higher wave number, as expected for turbulent round jet flows. In addition, β-test and γ-test are carried out for the instantaneous velocity, showing that the present LES method can successfully predict the hierarchical structure of round jet.

A multi-lattice deterministic trajectory (MLDT) model is developed to simulate dense gas-particle flow in a vertical channel. The actual inter-particle collision and particle motion are treated by a Lagrangian model with three sets of lattices to reduce computational time. Cluster formation and motion near the wall are successfully predicted with mean particle volume fraction and velocity, showing quantitatively agreement with experimental results. The mechanism of particles concentrated near the wall is investigated by considering effects of gravity, particle-wall collisions, inter-particle collisions and velocity profiles of the gas phase. It is shown that the inter-particle collision and gas-phase velocity distribution are the essential factors for cluster formation near the wall, while gravity and particle-wall collision only have minor effects on particle concentration near the wall. Particles are unable to remain in the high velocity region due to the strong inter-particle collisions, while they tend to stay in the low velocity region for weak inter-particle collisions. In addition, the effects of channel width and particle sizes on cluster formation are also investigated and it is found that particle concentration near the wall reduces with the decrease of channel width and increase of particle size.