title: Numerical Study of Combustion of Hydrogen and Ethylene in a Two-Dimensional Dual-Mode Scramjet Combustor
Creators: name: Carson, Robert A.
Publisher: name: Old Dominion University
publisherIdentifier: https://ror.org/04zjtrb98
publisherIdentifierScheme: ROR
Year: 2004

description: This thesis presents computational fluid dynamics results for two-dimensional flow geometries utilizing two fuels, hydrogen (H2) and ethylene (C2H4). Upstream interaction was evident with the use of both fuels due in part to recirculation in the combustor, the inlet pressures and the combustion process. Results were obtained for hydrogen using a reduced chemical kinetic model while results obtained for ethylene employed a reduced chemical kinetic model as well as an expanded chemical kinetic model.

Initially, an examination of grid composition was conducted in order to select the most numerically efficient and accurate grid for reacting flow in the current combustor geometry. Previous research had been conducted using an unstructured 30,000 cell grid. For this study, a similar coarse unstructured 30,000 cell grid and a fine unstructured 118,000 cell grid was created. A comparison was made and the upstream interaction and characteristic flowfield was best modeled by the 118,000 cell grid. This grid was used for all subsequent cases in this study. Following the selection of the grid, a verification of symmetry in the flowfield was made without fuel present for hydrogen-air inlet conditions. When reactions were enabled and hydrogen fuel injected, the equivalence ratio, ø, was set at = 1.13 and ø= 1.44 to calculate the most efficient combustion reaction and evaluate how the upstream interaction and flowfield symmetry were affected. The most efficient equivalence ratio was ø = 1.13 which resulted in the largest degree of upstream interaction. Symmetry loss occurred in both cases for ø = 1.13 and ø=1.44. These equivalence ratios were obtained using temperature changes at the air inlet. The final analysis for the hydrogen case was the application of a fully developed flow at the air inlet. The application of this full)'developed flow adversely affected the flowfield, allowing the leading oblique shock to propagate upstream to the isolator inlet. In an experimental case, this flow application would cause an 'unstart' condition leading to no combustion occurring in the engine geometry.

The use of hydrocarbon fuels is of particular interest to research in scramjet engines. This study uses ethylene as the fuel of interest. Grid selection was considered first. The previous fine unstructured 118,000 cell grid was used and a slightly modified 131,000 cell grid was used in the geometry change. This geometry change incorporates a larger backward facing step at the combustor region entrance. The previous height was 3.2 mm which was increased to 6.4 mm. The airflow with ethylene inlet conditions shows symmetric results for both geometries. When injectors are active and the flow is nonreacting, there is minimal upstream interaction and excellent mixing. When reactions are enabled, upstream interaction increases, loss of symmetry occurs and the combustion efficiency is calculated., The step height of 3.2 mm has the most efficient reaction and constitutes the largest degree of upstream interaction. When the chemical kinetic model is expanded, the combustion process is much more difficult to model numerically. An evaluation of increasing iterations shows similar results to early iterations of the reduced chemical kinetic model results.