Degree

Doctor of Philosophy (PhD)

Department

Mechanical and Industrial Engineering

Document Type

Dissertation

Abstract

Liquefying solid fuels are preferred in hybrid propulsion for their higher regression rate. A liquefying solid fuel undergoes melting, and the liquid film formed over the fuel surface is expected to affect the fuel regression rate. The current study conducted Single-Phase (SP), Two-Phase (TP), and MultiPhase (MP) simulations for paraffin wax (liquefying) and HTPB (Non-liquefying) in gaseous oxygen (GOX) cross-flow to investigate the transport effects of the liquid film. A vaporization algorithm was developed and implemented in the parallel framework of ANSYS-Fluent. Additionally, the turbulence model was updated to account for the presence of solid fuel in the flow field, and gas combustion was modeled using a hybrid, finite-rate Eddy Dissipation Model (EDM) with temperature-dependent properties.

Three heat balance models at the vaporizing interface were examined for SP simulations, and an appropriate model was chosen to best match the experimental data. As a parametric study, the oxidizer mass flux was varied over the range of 20-40 kg/m2-s, and the mass transport mechanisms were examined to understand the significance of melting in liquefying fuels. Shear-driven liquid flow enhances the fuel regression rate by 50 − 65% for the tested mass flux range. In the framework of SP simulations, the liquid viscosity of paraffin wax was varied to examine the influence of liquid film hydrodynamics and fuel properties on regression rate. In the high viscosity limit, the mean regression rate tends to have a constant value of approximately half that predicted for paraffin wax, which is attributable to the low bulk flow within the liquid film. Also, the influence of Reynolds number (Re) and transport properties on regression rate were examined. It is found that the regression rate can be expressed as a function of oxidizer mass flux over the tested Re range, and the Reynolds analogy for transport properties underpredicts the vaporization rate by approximately 40%. The results demonstrate the importance of mass transport by liquid bulk flow and temperature-dependent properties for accurately modeling the combustion of liquefying fuels.

Subsequently, MP simulations were conducted, varying the inlet mass flux for the paraffin wax to analyze the adequacy of an SP simulation. The MP simulations predicted regression rates comparable with the experimental data, albeit higher than the SP predictions and experimental correlation. The regression rate difference is linked to an SP simulation’s simplified melt film model and non-regressing fuel surface. In contrast, the test duration difference may partly explain the regression rate difference with the experiments. The flow field differences were also analyzed by comparing the pertinent scalar fields. The MP simulations accentuated the impact of the melt film model and fuel surface regression on the mean regression rate. The TP simulation of paraffin wax, in the high liquid viscosity limit, and the TP simulation of HTPB emphasized the potential influence of non-uniform fuel regression on the flow field and turbulence parameters. In summary, the simulation efforts present improvements to the SP simulations and provide a numerical approach for MP simulations. SP simulation can be an effective tool, in terms of computational speed and accuracy, to evaluate the sensitivity of regression trends to problem parameters in a quasi-steady condition. For transient hybrid combustion problems, the research advocates for the MP framework, highlighting its capability and necessity.

Date

4-12-2024

Committee Chair

Keith Gonthier

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Available for download on Friday, December 31, 2027

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