In-Seuck Jeung

Numerical Investigation of Transverse Hydrogen Jet into Supersonic Crossflow Using Detached-Eddy Simulation

A three-dimensional unsteady reacting flowfield that is generated by transverse hydrogen injection into a supersonic mainstream is numerically investigated using detached-eddy simulation and a finite-rate chemistry model. Grid refinement with the grid-convergence-index concept is applied to the instantaneous flowfield for assessing the grid resolution and solution convergence.Validation is performed for the jet penetration height, and the predicted result is in good agreement with experimental trends. The results indicate that jet vortical structures are generated as the interacting counter-rotating vortices become alternately detached in the upstream recirculation region. Although the numerical OH distribution reproduces the experimental OH–planar-laser-induced fluorescence well, there are some disparities in the ignition delay times due to the restricted availability of experimental and numerical data. The effects of the turbulence model on combustion are identified by a comparative analysis of the Reynolds-averaged Navier–Stokes and detached-eddy simulation approaches. Their effects are quantified by the production ofH2O, which is the primary species of hydrogen combustion.

Numerical Study of Scram Accelerator Starting Characteristics

A numerical study is carried out to investigate the ignition and the detonation initiation process in a scram accelerator operating at a superdetonative mode. To simulate the scram accelerator launching process, a conical projectile is considered, injected with an initial velocity of 2500 m/s from the 1 atm air into a 25 atm 2H 2 + O 2 + mN 2 mixture. As a dilution gas, nitrogen is selected and assumed to be inert. The time accurate solutions of Reynolds averaged Navier-Stokes equations for chemically reacting flows are obtained by using a point-implicit method and an upwind-biased third-order scheme with a steady-state solution for airflow as an initial condition. To examine combustion characteristics and ram-accelerator operation limits, mixture compositions are varied from 2H 2 + O 2 + 3.76N 2 to 2H 2 + O 2 + 9N 2 by changing the amount of N 2 . The flowfield results show the detailed ignition mechanism, the initiation process of the oblique detonation, and the staring characteristics of the scram accelerator. The results also identify clearly the combustion characteristics of the operational failures at lower and upper dilution limits that have been observed in experiments

Numerical Study of Mixing Enhancement by Shock Waves in Model Scramjet Engine

Anumerical study hasbeenconductedto investigatetheeffectofshockwaveson thesupersonichydrogen ‐airjet e ame stabilized in a Mach 2.5 circularcross-section combustor. Thenumerical model utilizes multispecies Navier ‐ Stokes equationswith detailed chemical reaction modelsand employsa k‐! shear stresstransport model. A wedge is mounted on the side wall of the combustor in order to e nd the interaction of the oblique shock waves with the hydrogen‐air jetlike e ame. The interaction between the shock waves and the mixing layer is classie ed according to the increasing tendency of the growth rate of the mixing layer downstream of the shock waves. It is found that the shock wavescreatea radially inward/outward aire owto thee ame and elongatea e ame-holding recirculation zone, and thus fuel ‐air mixing is enhanced signie cantly, resulting in improved combustion efe ciency. Also, the overall performance is investigated by changing the shock position and considering the mixing/combustion efe ciency and total pressure loss in a model scramjet combustor. Because there exists a tradeoff between the enhanced mixing/combustion efe ciency and the decreased total pressure recovery, it is suggested that the optimized shock position needs to be determined in order to obtain the maximum overall combustor performance using the overall performance index.

Robust HLLC Riemann solver with weighted average flux scheme for strong shock

Many researchers have reported failures of the approximate Riemann solvers in the presence of strong shock. This is believed to be due to perturbation transfer in the transverse direction of shock waves. We propose a simple and clear method to prevent such problems for the Harten-Lax-van Leer contact (HLLC) scheme. By defining a sensing function in the transverse direction of strong shock, the HLLC flux is switched to the Harten-Lax-van Leer (HLL) flux in that direction locally, and the magnitude of the additional dissipation is automatically determined using the HLL scheme. We combine the HLLC and HLL schemes in a single framework using a switching function. High-order accuracy is achieved using a weighted average flux (WAF) scheme, and a method for v-shear treatment is presented. The modified HLLC scheme is named HLLC-HLL. It is tested against a steady normal shock instability problem and Quirks test problems, and spurious solutions in the strong shock regions are successfully controlled.

Nitrogen oxides emissions in turbulent hydrogen jet non-premixed flames: Effects of coaxial air and flame radiation

An experimental study was performed to investigate the effects of fuel-air mixing on NO x emissions in hydrogen non-premixed flames with coaxial air. The major parameters used to modify the fuel-air mixing were fuel jet velocity and coaxial air velocity. Measurements of NO x emission, flame length, and volume were made to investigate the relationship between flame residence time, global strain rate, and NO x emission scaling. Global strain rate and flame residence time from measured flame length and flame volume were used as parameters for the analysis of the experimental data to identify the relevant parameters that lead to the observed NO x scaling law in coaxial air flames. The overall half-power scaling was observed in coaxial air flames, irrespective of coaxial air conditions, but the degree of deviation from the half-slope curve differed in each case. Comparison of the results of pure hydrogen flames with those of helium-diluted hydrogen flames showed that flame radiation played a significant role in 100% hydrogen flames with coaxial air, and the deviation from half-power scaling was due to the difference in the flame radiation. Global strain rate rather than flame residence time is a more appropriate scaling parameter to predict the emission index of NO x /τ B in the turbulent hydrogen non-premixed flames with and without coaxial air. The half-power scaling law previously observed in hydrogen non-premixed jet flames with no coaxial air is applicable to NO x emission in hydrogen non-premixed jet flames with coaxial air, if flame radiation is taken into consideration.

Computational investigation of shock-enhanced mixing and combustion

A computational investigation of shock-enhanced mixing and combustion is presented. To understand the influences of the mixing process on the combustion process, the mixing characteristics of the reacting case are compared with those of the nonreacting case. Parametric studies varying the conditions of fuel injection are conducted to find the trends of the mixing and combustion processes. Three-dimensional Navier-Stokes equations with a chemical reaction model and κ-ω turbulence model are used. The upwind method of Roes flux difference splitting scheme is adopted. It is shown that the mixing process has a strong influence on the combustion process, whereas the combustion process does not have any significant effect on the mixing process. The combustion process is divided into two mixing regimes: a convection-dominated regime, where the burning rate increases with distance from the injection plane, and a diffusion-dominated regime as one moves downstream, where burning rate is constant. In the parametric studies, varying the fuel pressure with the fuel density held fixed makes little difference, whereas varying the fuel density makes a significant difference in mixing rate and burning rate. A prediction of minimum combustor length for complete combustion is made.

Flow Characteristics of Small-Sized Supersonic Inlets

The flow characteristics were investigated for two types of small-sized supersonic inlet models: rectangular and axisymmetric inlets. The twomodels were designed by the samemethod and are of similar size, with a total length of about 160 mm. The flow phenomena that were generated at the stable and unstable operating conditions of both models are visualized and discussed in detail. The main flow feature for stable operation is the existence of a shock train induced by the shock boundary-layer interactions. The results show that the shock-train shape and position are varied along with the upstream and area ratios, which strongly impact the downstream flow and total pressure recovery. For unstable operation, various buzz phenomena are observed for different area ratios. The results indicate that the buzz phenomenon of the small-sized inlet is identical to that of a large inlet, but the small inlet can be easily affected by the presence of a separation bubble. Although the shapes of the inlets are different, the base frequencies of a big buzz for bothmodels are very similar. The role of the separation on the compression surface in the buzz process of the small-sized inlet is shown.

Unsteady-State Simulation of Model Ram Accelerator in Expansion Tube

Steady- and unsteady-state numerical simulations have been carried out to investigate the ram accelerator flowfield that had been studied experimentally using an expansion tube facility at Stanford University. Navier-Stokes equations for chemically reactive flows were used for the modeling with a detailed hydrogen-air combustion mechanism. The governing equations were analyzed using a fully implicit and time-accurate total variation diminishing scheme. As a result, steady-state simulation reveals that the near-wall combustion regions are induced by aerodynamic heating in the separated flow region. This result agrees well with experiments in the case of the 2H 2 + O 2 + 17N 2 mixture but fails to reproduce the centerline combustion in the case of the 2H 2 + O 2 + 12N 2 mixture. To investigate the reason for this disagreement in the flow establishment process, unsteady-state simulations have been carried out, and the results show the detailed process of flow stabilization. The centerline combustion is revealed to be an intermediate process during flow stabilization. It is induced behind a Mach stem formed by the intersection of strong oblique shock waves at an early stage of the flow stabilization process

Computational Investigation of Shock-Enhanced Mixing: Application to Circular Cross Section Combustor

A computational investigation of shock-enhanced mixing applied to circular cross section combustors is presented. In a circular cross section combustor, there are shock wave intersections at the center axis that produce adverse effects on mixing characteristics. The purpose of this investigation is to analyze the adverse effects due to the shock wave intersections and to develop new combustor geometry minimizing the adverse effects and improving the mixing characteristics. Three-dimensional Navier-Stokes equations are used to calculate the motion of the mean flow. A κ-ω turbulence model is used to calculate the turbulent viscosity. It is shown that, in a circular cross section combustor, the shock waves intersecting at the center axis reduce the strength of the longitudinal vorticity and push the fuel downward. A change in the shape of the cross section of the combustor to an octagonal cross section results in a reduction of the adverse effects and an improvement of the mixing characteristics.

Scaling effect of the combustion induced by shock-wave boundary-layer interaction in premixed gas

A numerical study is conducted to investigate the combustion phenomena associated with hypersonic propulsion devices, such as a ram accelerator and an oblique detonation wave engine, operating at super detonative speed. For the close examination of the phenomena, the combustion problem is modeled as a shock-wave-boundary-layer interaction problem in premixed combustible gas. Flow Mach number of 5 and the incidence shock wave angle of 20° are selected, and inflow values of pressure and temperature are set to 1 bar and 293 K. The overall conservation equations are used as governing equations with a detailed chemistry mechanism of hydrogen-air mixture. Because the Reynolds number in the propulsion devices is very high, the entire flow field is assumed being fully turbulent and is analyzed by two-equation SST (shear stress transport) turbulence modeling. The governing equations are discretized by a high-order accurate upwind scheme and solved in a fully coupled manner with a fully implicit time accurate integration method. Two regimes of combustion are identified: a steady boundary-layer flame held by the separation bubble at the shock impinging point and an unstable oblique detonation wave that propagates forward. The behavior with respect to the fluid dynamic length scale may be attributed to the different values of Damkohler number defined as a ratio of flow residence time to the chemical induction time.