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Numerical Investigation of Second-Law Characteristics of Ramjet Throttling

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Master's thesis

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A numerical study of a generic axisymmetric ramjet operating at conditions corresponding to flight Mach 3.0 and a standard altitude of 10 km is presented. The study includes both modeling of steady-state flowfields in the ramjet as well as transient throttling maneuvers in which the throttle is decreased or increased from maximum or minimum throttle positions. The results presented here focus on entropy generation and performance characteristics. Combustion-generated exothermic heat release is modeled using simple volumetric energy addition to the flow within a defined heat release zone. The study utilizes two levels of wall boundary modeling, corresponding respectively to inviscid and viscous walls in the ramjet. The second level of modeling with viscous walls presents many challenges due to the inherent tendency of the no-slip boundary condition to cause reverse flow to develop in the ramjet, particularly along the wetted surfaces of the inlet where the adverse pressure gradient associated with the deceleration and heat release in the ramjet has the largest initial impact. This separated flow results in eventual unstart of the ramjet due to large-scale propagation of the separation upstream there is also inherent unsteadiness due to boundary layer effects. To address the challenges presented by the no-slip boundary condition, a bleed boundary condition specified at the inlet throat is incorporated. This bleed extracts approximately 10 of the mass flow. As an alternative to bleeding mass from the flow path of the ramjet, a generic alternative model of a ramjet dump combustor is also studied. This configuration has a geometry in which a constant area heat addition zone is located downstream of a large step at the exit of the ramjet inlet. This configuration is analyzed and compared to the original configuration without the dump combustor. It is found that both the bleed boundary condition and the dump combustor are extremely effective at preventing the normal

Subject Categories:

  • Combustion and Ignition
  • Jet and Gas Turbine Engines

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