The design challenges of micro gas turbines, compared to large gas turbines, lies in the size of Athe components. Firstly, the small components cannot achieve a high-pressure ratio which cause the mass flow rate to be relatively high for the engine size in order to meet the thrust requirements. This leads to a low combustor residence time and a poor turbine inlet temperature profile. Secondly, the small components make achieving the manufacturing tolerances in the tip gaps difficult which can lead to component mismatches. Complete gas turbine CFD simulations are used to determine component mismatches as a final step before manufacturing an engine by simulating the full flow path of the fluid through the engine. Due to the size and complexity of a gas turbine, the mesh used to model the geometry in CFD packages is large and, consequently, takes a long time to converge. Because time is money, methods to reduce the simulation time are developed and tested. Lower fidelity models than 3D CFD were not considered as they would not be able to provide the required insight into the fluid solution to, if proven to be accurate, determine component mismatches in future projects.
The objective of this study was to develop a CFD model using Numeca to simulate the full, main gas flow path of a micro gas turbine. The key limitation was that the chemical process of combustion would not be modelled and the simulation had to run on a single i7 computer with 32 GB of RAM. Furthermore, the accuracy of the developed method had to be assessed. This was achieved, firstly, by determining how well the performance parameters of the simulated engine compared to experimental results and, secondly, the accuracy of the temperature distributions in the combustor. This study modelled combustion as a heat source rather than using complex combustion models built into modern CFD packages which are computationally more expensive. Methods used to simulate gas turbines were found in the open literature, but none of them modelled combustion with a heat source. Two methods for applying the heat source to the combustion domain, namely primary zone heating and global heating were investigated. Primary zone heat addition introduced heat only in the primary combustion zone in the inner combustor annulus. Global heating introduced heat throughout the entire combustor domain from its inlet up to its outlet.
The simulated results compare well with experimental results with most variables being within approximately 5% for the BMT 120 KS engine. The mass flow rate results were underpredicted on average by 21%. This should be investigated by, firstly calibrating the bell mouth inlet, limiting the temperature in the combustor and secondly, increasing the distance between the trailing edge of the deswirler and the interface between the domains of the compressor and the combustor. The results produced by the primary zone heat source model were closer to the experimental results than those produced by the global heat source model, but the former took more than Department of Mechanical and Aeronautical Engineering University of Pretoria six times longer to converge. The temperature contours in the combustor were also more realistic when using primary zone heating since this method predicted the location where the inner liner of the combustor was burnt during the experiments. The location of the heating zone was found to be important because the temperature distributions are sensitive to recirculation zones. It can be concluded that the simulation methods of this study can be used to perform performance predictions when changes are made to engine components. Identifying component mismatches can also be investigated with these methods when the mass flow deficit has been addressed.
Dissertation (MEng)--University of Pretoria, 2018.