Parallel Numerical Simulations Of The Whole Scramjet Engine Flowfields On Unstructured Grids

Supersonic Combustion Ramjet (Scramjet for short) is integrated with the airbreathing hypersonic vehicle which aims mainly for the flight in the high altitude environment at Mach number above 6.0. There are complex phenomena such as supersonic combustion and the interactions between the shock and the boundary layer in the internal flowfield of the Scramjet combustor. As one of the important research methods, Computaional Fluid Dynamics(CFD) can get the detail of the flowfield in a short time and it can offer the basis for the choice of an appropriate engine configuration especially in the process of the preliminary design.However, it has become more difficult to generate the structured grid when the geometry of the vehicle is more complex, so the design process has been bottlenecked by the time consuming and laborious generation of the grid. Freed from the structural limitation, it is much easier for the unstructured grid to control the size and the density of the mesh and the generation of the unstructured grid can be finished quickly and automatically once the geometry is determined. On the other hand, the data structure of the unstructured grid provides convenience to the adaptation of the grid and the parallelization. Unlike the structured grid, the neighbor points of the unstructured grid are not explicitly known, so the information about the connectivity of the grid etc must be recorded in the preprocessing process and it results in more memory taken than the structured grid. Some numerical methods such as LU decomposition can not be applied directly to the CFD computation on the unstructured grid. Now many technologies which have been maturely developed on the structured grid have been successfully used in the unstructured grid and the unstructured grid technology has wide applications in all kinds of flow simulations, but up to now there is scarcely any literature about the parallel simulations of the complex combustion flowfield of the Scramjet combustor by the independently developed numerical software on the unstructured grid at home.A numerical software platform applicable to the supersonic turbulently reacting flow simulation named AHL_UNS3D has been developed by the Airbreathing Hypersonic Laboratory of CARDC on unstructured grids focusing on the research of the Scramjet reacting flow. After a lot of verifications and validations, numerical simulations have been conducted on the internal flowfields of the Scramjet.To simulate the turbulent flow of the Scramjet combustor more correctly, the hexahedral or prismatic semi-structured grid is extruded perpendicularly from the wall and tetrahedral grid are used for the rest space. Different kinds of grids are connected by the pyramidal grid.AHL_UNS3D achieves parallelization based on MPI and discretizes the integral governing equations by the cell-vertex finite volume method. It can simulate two-dimensional(2D), axisymmetrical and three-dimensional(3D) steady and unsteady invicid, laminar and turbulent flows of the perfect gas and the multispecies mixture. The time evolution can use Runge-Kutta or LU-SGS method. Four schemes including those of Steger-Warming,Van Leer,AUSMDV and AUSMPW+ can be used for the inviscid flux vector calculation. There are several chemical kinetic models for the hydrogen or the hydrocarbon fuel ethylene and there are two types of turbulence models: Spalart-Allmara(S-A) one-equation turbulence model and k-ωtype two-equation turbulence models (including the original Wilcox k-ω, Kok's TNT, Menter's BSL and SST), and the two-equation turbulence models can be solved coupled or uncoupled with the main govening equations.14 steady and 2 unsteady benchmark cases are simulated to validate the capabilities of AHL_UNS3D for the inviscid, laminar, turbulent, multispecies chemically reacting flows and unsteady flows and the results indicate that the numerical software has good resolution and reliability. Considering the characteristics of the internal flow of the Scramjet combustor, typical flows such as Lehr's shock induced combustion, the cavity flow, the compression corner flow and the perpendicular injection flow of the hydrogen from a circular hole are calculated and the appropriate chemical kinetic model, numerical scheme for the inviscid flux computation and turbulence model are chosen for the reacting flow of the Scramjet by the comparisons with the experimental results.To validate the ability of AHL_UNS3D for the chemically reacting flow of the Scramjet, the diffusing and reacting cases of the 2D parallel injection of a well known benchmark, ie. the hydrogen flow of Burrows & Kurkov are simulated. 2D and 3D jet-to-jet simulations are conducted respectively for the Scramjet models of the Hyshot scheme of the hypersonic center of Queesland University of Australia and the National Aerospace Laboratory(NAL) of Japan. Then the numerical simulations for the cold flow and reacting flows of three fuel/air equivalence ratios of a whole Scramjet engine of CARDC fueled by the hydrogen in slice region are conducted. Finally the reacting flow of Taha's combustor without the piloted ethylene in the half-width region is simulated and the influence of the cavity to the ignition and combustion of the ethylene is analyzed.This paper consists of seven chapters.Chapter one is the introduction. The background of this paper and the status of the researches of the Scramjet at home and abroad, the characteristics and the development of the unstructured grid technology, the functions as well as applications of numerical softwares abroad able to simulate the Scramjet flowfield on unstructured grids are introduced and comparisons in seven different aspects with AHL_UNS3D are made. Finally, the main work of this paper is outlined briefly. Chapter two is about numerical methods, including the data structure and the control volume, governing equations, turbulence models, the discretization method of the integral governing equations, numerical schemes for the inviscid flux computation and parallel algorithms used in the software. Chapter three is validations of the numerical software. Chapter four to six are the simulations of the flowfields of the Scramjet combustor and the whole engine fueled by the hydrogen or the ethylene. Chapter seven is the conclusion. The main researches and innovative points are reviewed and the outlook of the future development of the software is given.