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Jiang, HuaEffect of Changes in Flow Geometry, Rotation and High Heat Flux on Fluid Dynamics, Heat Transfer and Oxidation/Deposition of Jet Fuels
Doctor of Philosophy (Ph.D.), University of Dayton, 2011, Mechanical Engineering

Jet fuel is used in high-performance military flight vehicles for cooling purposes before combustion. It is desirable to investigate the influence of the flow and heating conditions on fuel heat transfer and thermal stability to develop viable mitigation strategies. Computational fluid dynamics (CFD) simulations and experiments can provide the understanding of the fuel physical phenomena which involves the fluid dynamics, heat transfer and chemical reactions. Three distinct topics are studied: The first topic considers the effect of flow geometry on fuel oxidation and deposition. Experiments and CFD modeling were performed for fuels flowing through heated tubes which have either a sudden expansion or contraction. It was found that the peak deposition occurs near the maximum oxidation rate and excess deposition is formed near the step. This study provides information for the fuel system designer which can help minimize surface deposition due to fuel thermal oxidation.

In the second area of study, the fuel passed heated rotational test articles to investigate the effect of rotation on fuel heat transfer. The coupled effects of centrifugal forces and turbulent flow result in fuel temperatures that increase with rotational speed. This indicates that the convective heat transfer is enhanced as rotational speed increases. This work can assist the understanding of using jet fuel to cool the turbine engine.

In the third segment of research, the fuel was exposed to “rocket-like” conditions. This investigation is to explore the effect of high heat flux and high flow velocity on fuel heat transfer and oxidation/deposition. Simulations show a temperature difference over several hundred degrees in the radial direction within the very thin thermal boundary layer under rapid heating. The fuel contacting the interior wall is locally heated to a supercritical state. As a result, the heat transfer is deteriorated in the supercritical boundary layer. Both simulated and measured deposit profiles show a peak deposit near the end of the heated section. These observations may eventually have an application to the design of high speed supersonic vehicles with improved cooling capabilities.

Committee:

Jamie S. Ervin, PhD (Advisor); Steven Zabarnick, PhD (Committee Co-Chair); Timothy J. Edwards, PhD (Committee Member); Kevin P. Hallinan, PhD (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

jet fuel; heat transfer deterioration; high heat flux; temperature peak; supercritical; fuel properties; nozzle; sudden expansion/contraction in flow path; fuel deposition; turbulence models; rotation passage; recirculation flow; excess deposition

Gopalakrishnan, Raj NarayanCFD Analysis of Turbulent Twin Impinging Axisymmetric Jets at Low Reynolds Number
MS, University of Cincinnati, 2017, Engineering and Applied Science: Aerospace Engineering
CFD analysis of turbulent twin-impinging round jets was performed to establish the growth profile and velocity characteristics of resultant jet. To ensure that a high degree of confidence can be assigned to the planned multi-jet impingement simulations, a single axisymmetric jet was first numerically resolved and compared to published jet experiments results. This required the definition of inlet boundary condition of the jet to be accurately documented. To that end, a turbulent flow exiting a long circular pipe was first modelled and analyzed to ensure that the inlet boundary condition to a single axisymmetric jet was in good agreement with the experiments. Once, the development length based on mean velocity was calculated, the pipe flow at a Reynolds number of 7,500 was analyzed and compared with in house and published experimental data. It was observed that the solution using the SST turbulence model performs better than the solution obtained using the Realizable k-e model in the pipe domain. Once the analysis was completed, velocity and turbulence components at the outlet of the pipe were extracted and used as input for the single jet flow simulations. Single axi-symmetric round jet flow was analyzed using computational techniques and validated with experimental results to establish the suitable turbulence model for simulation of low Reynolds number jets exiting from fully developed pipe. It was observed that although all the turbulence models studied could closely predict the mean velocity field, they were not able to accurately predict the turbulence intensity distributions. From the models studied, it was concluded that SST k-? model was the best turbulence model for simulating low Reynolds number jet flow exiting from fully developed pipe. Based on the insight gained from single jet analysis, CFD analysis on turbulent impinging jets was performed. Multiple Reynolds numbers and impingement angles were considered for the study to gain better understanding of the parameters affecting resultant jet growth and distribution. Based on the mesh obtained from grid sensitivity study, jets impinging at 30o, 45o and 60o at constant Reynolds number of 7500, and jets impinging at 30o angle at Reynolds number of 5000, 7500 and 10000 were numerically analyzed. It was observed that the profile of the resultant jet closely matched with the prediction of elliptical profile predicted by past researchers. It was also seen that higher jet growth was predicted in case of jets impinging at higher angle and higher momentum of jets were predicted in case of jets impinging with higher Reynolds number. Hence, it was concluded that based on the application required; ideal impinging strategy can be applied for obtaining optimal results.

Committee:

Peter Disimile, Ph.D. (Committee Chair); Shaaban Abdallah, Ph.D. (Committee Member); Milind Jog, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Impinging Jets;CFD Analysis;Low Reynolds Number;Turbulence Models;Round Jets;Fully developed pipe

Deng, DingfengA NUMERICAL AND EXPERIMENTAL INVESTIGATION OF TAYLOR FLOW INSTABILITIES IN NARROW GAPS AND THEIR RELATIONSHIP TO TURBULENT FLOW IN BEARINGS
Doctor of Philosophy, University of Akron, 2007, Mechanical Engineering
The relationship between the onset of Taylor instability and appearance of what is commonly known as “turbulence” in narrow gaps between two cylinders is investigated. A question open to debate is whether the flow formations observed during Taylor instability regimes are, or are related to the actual “turbulence” as it is presently modeled in micro-scale clearance flows. This question is approached by considering the viscous fluid flow in narrow gaps between two cylinders with various eccentricity ratios. The computational engine is provided by CFD-ACE+, a commercial multi-physics software. The flow patterns, velocity profiles and torques on the outer cylinder are determined when the speed of the inner cylinder, clearance and eccentricity ratio are changed on a parametric basis. Calculations show that during the Taylor and wavy vortex regime velocity profiles in the radial direction are sinusoidal with pressure variations in the axial direction even for the case of the “long journal bearing” (L/D>2). Based on these findings, a new model for predicting the flow behavior in long and short journal bearing films in the transition regime is proposed. Unlike the modified turbulent viscosity of the most accepted models (Constantinescu, Ng-Pan, Hirs and Gross et al.), the viscosity used in the new model is kept at its laminar value. Experimental torque measurements and flow visualization are performed for three kinds of oils with different viscosities. It is shown that in general there is a good agreement between the numerical and experimental torques except those in turbulent regime. Comparison between numerical and experimental flow patterns is also made and it shows that they match well in the Couette, Taylor and Wavy regimes. In general there is a good agreement between the numerical and experimental results including torque measurements and flow patterns. The new model for predicting the flow behavior in journal bearing films in the transition regime is justified.

Committee:

Minel Braun (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Taylor Instability; Flow Patterns; Velocity Profiles; New Model; Transition Flow; Narrow Gaps; Long Journal Bearings; Transition Reynolds Equation; Turbulence; Turbulence Models; Torque Measurements; Flow Visualizations