Doctor of Philosophy, The Ohio State University, 2022, Aerospace Engineering

Gas turbine engines are an essential technology in aviation and power generation. One of the challenges associated with increasing the efficiency of gas turbines is the thermal loading experienced by the engine components downstream of the combustors especially the high-pressure turbine blades. High temperatures and rotational velocities can cause blade failures in numerous ways such as creep or stress rupture. Technologies like film cooling are implemented in these components to lower the thermal loading and reduce the risk of failure. However, these introduce complexities into the flow which in turn increases the difficulty of predicting the performance of film cooled turbines. Accurately predicting the capabilities of these components is essential to prevent failure in gas turbine engines. Engineers use a combination of experiments and computational simulations to understand how these technologies perform and predict the operating conditions and lifespan of these components. A combined experimental and numerical program is performed on a single stage high-pressure turbine to increase understanding of film cooling in gas turbines and improve computational methods used to predict their performance. The turbine studied is a contemporary production model from Honeywell Aerospace with both cooled and uncooled turbine blades. The experimental work is performed at The Ohio State University Gas Turbine Laboratory Turbine Test Facility, a short duration facility operating at engine corrected conditions. The experiments capture heat flux, temperature, and pressure data across the entire blade, but this work will focus on the turbine blade tip data. Tip temperature data are captured using a high-speed infrared camera providing a unique data set unseen in the current literature. In addition to the experiments, transient conjugate heat transfer simulations of a single turbine passage are performed to recreate the experiments and give insight into the flow field in the tip (open full item for complete abstract)

Committee: Randall Mathison (Advisor); Sandip Mazumder (Committee Member); Michael Dunn (Committee Member); Jeffrey Bons (Committee Member) Subjects: Aerospace Engineering; Mechanical Engineering

Master of Science, The Ohio State University, 2020, Mechanical Engineering

In order to keep turbine blade surface temperature below melting point in gas turbine engines, internal passages in blades must be used to route cooler air through the blade. Design optimization of cooling passages necessitates an understanding of heat transfer patterns to minimize cooling mass flow. This project compares two approximations used to determine the heat transfer rate inside cooling channels in both computational and experimental investigations. The two approximations used in this project are constant surface temperature and transient heating. In an operating engine, the accuracy of both these conditions are not guaranteed. During steady state operation, the blade can cycle through many different flow paths which will impart different temperatures across the surface, and at no time will a blade be under completely uniform temperature except for the starting cycle. However, to make measurements of heat transfer easier, the two assumptions mentioned beforehand are utilized extensively. The constant surface temperature method uses a heater attached to the back of a thin copper plate to hold the surface temperature at a constant value in air flow. In the transient full-field method, thermochromic liquid crystals, which change colors with temperature, are applied to flat plate and turbulated geometries to capture the change in wall temperature during heating and cooling processes. Heat transfer rates are then derived from the transient temperature data using a semi-infinite solid model. The constant temperature approach is better established than the transient method and produces significantly higher Nusselt numbers, but the transient method provides better spatial resolution. A numerical conjugate heat transfer model is used to further investigate the discrepancy between the methods. The experimental geometry is replicated for both methods to gain an understanding of the fluid dynamics in each setup and how they differ.

Committee: Randall Mathison Ph.D (Advisor); Michael Dunn Ph.D (Committee Member) Subjects: Aerospace Engineering; Mechanical Engineering

MS, University of Cincinnati, 2022, Engineering and Applied Science: Aerospace Engineering

The gas phase pyrolysis method, also called the floating catalyst method, has the potential to provide carbon nanotube (CNT) materials such as sheet, tapes, and yarn at an industrial scale. However, refinement and scale up of the process is still needed to put CNT materials into everyday applications. Experimentation is being performed around the world to increase the properties of CNT materials and to increase the efficiency and output of the floating catalyst process. This experimentation is based mostly on trial and error. But trial and error have limitations in optimizing the process because many variables are involved in the process, experimentation is expensive, and setting up the synthesis reactor is a manual and time-consuming process. Many experiments are required to try to determine the flow characteristics, temperature profiles, reactor size, tube material, and many process variables that will improve the quality and the yield of the CNT web or sock produced. Moreover, different research groups around the world report somewhat different approaches and end results from their specific processes, and it is difficult to rationalize all the different results. Clearly, a more efficient approach is needed to improve the gas phase synthesis method. Especially upscaling the production process will require accurate predictive analysis before companies invest large funding into mass production of CNT materials. Ideally, a way of trying various flowrates, changing the configuration of the hot zone, the injector position, and many other variables without manually reconfiguring the reactor setup each time is needed. Therefore, this thesis developed a computational simulation solution, through the conjugate heat transfer approach, without modelling of the catalyst or nanotube particles, to optimize the gas phase reaction to synthesize CNT materials. Using CFD solvers such as Ansys-Fluent, the design of the reactor was modeled and modified in a comprehensive manner, ther (open full item for complete abstract)

Committee: Shaaban Abdallah Ph.D. (Committee Member); Mark Schulz Ph.D. (Committee Member); Kelly Cohen Ph.D. (Committee Member) Subjects: Aerospace Materials

PhD, University of Cincinnati, 2015, Engineering and Applied Science: Aerospace Engineering

Despite the significant advancements in computational fluid dynamics, modeling turbomachinery flows remains extremely difficult. The challenges include complex unsteady blade row interactions, large thermal gradients, and complex geometries. During the design process, simplifications and approximations are necessary to reduce the computational cost. Two common simplifications are the use of adiabatic boundary conditions and steady methods for resolving the flow field in multistage turbomachinery. The Harmonic Balance (HB) method is an efficient way to simulate periodic unsteady phenomena. Compared to traditional time-marching methods, the HB method reduces the computational cost by considering only the dominant frequencies of the solution field. Using a Fourier series representation of the solution variables, an unsteady governing equation transforms into a series of steady-like equations. The cost is further reduced when considering multistage turbomachinery. Unlike a time-marching method, which requires periodic boundaries, the HB method models a single blade passage per blade row. A phase lag condition is applied instead of a periodic condition. At the junction between blade rows, an interface resolves the relative motion and any passage mismatch. Assuming the heat transfer of wall boundary conditions is, by definition, non-physical. Assuming no heat transfer (an adiabatic wall) is certainly wrong for turbomachinery flows because of the large thermal gradients. An constant temperature wall can provide a better approximation, but for complex geometries, the temperature is not known a priori. The most accurate approach involves modeling both the fluid and solid domains, which is called Conjugate Heat Transfer (CHT). This can be performed in several ways, but the most stable method is one in which the fluid and solid are strongly coupled. This strong coupling is achieved by using the same discretization in both domains. This dissertation details an approach (open full item for complete abstract)

Committee: Mark Turner Sc.D. (Committee Chair); John Benek Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member) Subjects: Aerospace Materials