A comparative study of elliptical and round scramjet combustors by Improved Delayed Detached Eddy Simulation

Abstract

To explore the combustion performance of non-rectangular type supersonic combustors, the flow and combustion characteristics in round and round-to-elliptic shape-transition (RdEST) supersonic combustors under the same configurations of flight Mach number and fuel equivalence ratio were compared based on modeling results. The fuel equivalence ratio is maintained the same as 0.8, while two inlet Mach numbers of 2.5 and 3.0 both corresponding to a real flight Mach number of 6.5 are tested. To alleviate the strict requirements on wall-normal and -parallel grid spacing, Improved Delayed Detached Eddy Simulation (IDDES) is employed in this study to enable an automatic choice of RANS or LES mode depending on the local boundary layer thickness and turbulent viscosity. To reduce the computational cost of stiff kerosene oxidation chemistry, a total of four versions of skeletal mechanisms (respectively 48s/197r, 39s/153r, 28s/92r and current 19s/54r) have been developed based on the detailed 2815s/8217r Dagaut mechanism by using a highly efficient and reliable directed relation graph with error propagation and sensitivity analysis (DRGEPSA) method together with manual path analysis. Although the mechanism size has been significantly reduced, key kinetic properties such as adiabatic flame temperature, heat release rate, ignition delay and laminar flame speed all agree well with the original detailed mechanism. The static pressure along streamwise direction is compared with the measurement to validate the modeling results. Two key aspects are well predicted, i.e. the pressure ratio and the initial pressure rise location, indicating that the flame anchoring location and the distribution of wave structures inside the combustor are close to the actual situation. Then the aerodynamic fields are analyzed for the round and elliptic combustors to compare their flow, mixing and combustion related flow structures. The three-dimensional wave structures inside the elliptic combustor are firstly shown to reveal the influence of nonaxisymmetric cross-section on the shock train and Mach field. Especially the time evolution of the flame region is analyzed, and dominant flame modes are extracted by the aid of proper orthogonal decomposition (POD) method.

Development and Fidelity Evaluation of a Skeletal Ethylene Mechanism under Scramjet-Relevant Conditions

“…The time-integration is advanced by the second-order Crank-Nicholson scheme . Details of the numerical methods can refer to Wu et al The code has been extensively validated for nonreactive highly underexpanded jets and supersonic combustions. , …”

“…For example, Hui et al and Wang et al compared the capability of the skeletal mechanism in predicting the pressure profiles in HCCI-like engine and compression ignition engine simulation. For scramjet combustor modeling, Yao et al validated the second level of mechanism fidelity by comparing with the experimental pressure distribution which is often the only available data from scramjet experiments. The above levels of fidelity validations care about only the steady statuses, while the third level of fidelity validation cares about the accurate predictions of intermediate species when the modeling focuses on unsteady combustion processes, such as the ignition and flame stabilization.…”

“…For example, Hui et al 8 and Wang et al 9 compared the capability of the skeletal mechanism in predicting the pressure profiles in HCCI-like engine and compression ignition engine simulation. For scramjet combustor modeling, Yao et al 10 validated the second level of mechanism fidelity by comparing with the experimental pressure distribution which is often the only available data from scramjet experiments. The above levels of fidelity validations This study will elaborate the integrated process of skeletal mechanism development for scramjet combustor modeling along with its fidelity evaluation on basic kinetic properties as well as its accuracy in realistic scramjet modeling.…”