Optimisation techniques for combustor design

For gas turbines, the demand for high-performance, more efficient and longer-life turbine blades is increasing. This is especially so, now that there is a need for high-power and low-weight aircraft gas turbines. Thus, the search for improved design methodologies for the optimisation of combustor exit temperature profiles enjoys high priority. Traditional experimental methods are found to be too time-consuming and costly, and they do not always achieve near-optimal designs. In addition to the above deficiencies, methods based on semi-empirical correlations are found to be lacking in performing three-dimensional analyses and these methods cannot be used for parametric design optimisation. Computational fluid dynamics has established itself as a viable alternative to reduce the amount of experimentation needed, resulting in a reduction in the time scales and costs of the design process. Furthermore, computational fluid dynamics provides more insight into the flow process, which is not available through experimentation only. However, the fact remains that, because of the trial-and-error nature of adjusting the parameters of the traditional optimisation techniques used in this field, the designs reached cannot be called “optimum”. The trial-and-error process depends a great deal on the skill and experience of the designer. Also, the above technologies inhibit the improvement of the gas turbine power output by limiting the highest exit temperature possible, putting more pressure on turbine blade cooling technologies. This limitation to technology can be overcome by implementing a search algorithm capable of finding optimal design parameters. Such an algorithm will perform an optimum search prior to computational fluid dynamics analysis and rig testing. In this thesis, an efficient methodology is proposed for the design optimisation of a gas turbine combustor exit temperature profile. The methodology involves the combination of computational fluid dynamics with a gradient-based mathematical optimiser, using successive objective and constraint function approximations (Dynamic-Q) to obtain the optimum design. The methodology is tested on three cases, namely: (a) The first case involves the optimisation of the combustor exit temperature profile with two design variables related to the dilution holes, which is a common procedure. The combustor exit temperature profile was optimised, and the pattern factor improved, but pressure drop was very high. (b) The second case involves the optimisation of the combustor exit temperature profile with four design variables, one equality constraint and one inequality constraint based on pressure loss. The combustor exit temperature profile was also optimised within the constraints of pressure. Both the combustor exit temperature profile and pattern factor were improved. (c) The third case involves the optimisation of the combustor exit temperature profile with five design variables. The swirler angle and primary hole parameters were included in order to allow for the effect of the central toroidal recirculation zone on the combustor exit temperature profile. Pressure loss was also constrained to a certain maximum. The three cases show that a relatively recent mathematical optimiser (Dynamic-Q), combined with computational fluid dynamics, can be considered a strong alternative to the design optimisation of a gas turbine combustor exit temperature profile. This is due to the fact that the proposed methodology provides designs that can be called near-optimal, when compared with that yielded by traditional methods and computational fluid dynamics alone.Thesis (PhD)--University of Pretoria, 2009.Mechanical and Aeronautical Engineeringunrestricte

Optimal mixing of multiple reacting jets in a gas turbine combustor

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.This paper addresses the design optimisation methodology used to optimise a gas turbine combustor exit temperature profile. The methodology uses computational fluid dynamics and mathematical optimisation to optimise the combustor exit temperature profile. The studies from which the results were derived, investigated geometric variations of a complex three-dimensional flow field in a gas turbine combustor. The variation of geometric parameters impacts on mixing effectiveness, of which the combustor exit temperature profile is a function. The combustor in this study is an experimental liquid-fuelled atmospheric combustor with a turbulent diffusion flame. The computational fluid dynamics simulations use the Fluent code with a standard k-ε model. The optimisation is carried out with the Dynamic-Q algorithm, which is specifically designed to handle constrained problems where the objective and constraint functions are expensive to evaluate. All the optimisation cases investigated led to an improved combustor exit temperature profile as compared to the original one.vk201

Thermal analysis of a kitchen natural ventilation system with computational fluid dynamics: a case study

A numerical study was conducted to perform the design analysis of a novel kitchen natural ventilation system of Madirelo Hotel School. The analysis was performed by using Computational Fluid Dynamics to assess the performance of natural ventilation system. The design is expected to be energy efficient and cost effective. The parameters of interest are temperature distribution and air flow velocities within the kitchen. These two parameters provide the fluid dynamics in the kitchen for both human comfort and evacuation of fumes. The ventilation system utilized concepts such as displacement ventilation, buoyancy effect and green house effect. The boundary conditions were derived from the assumed loading of the kitchen. The results have revealed that the flow phenomena in the kitchen cannot provide both human comfort and extraction of fumes in a satisfactory manner. It would be necessary to optimize the positioning and flow rates of the fluid flow components

Optimization of gas turbine combustor mixing for improved exit temperature profile

In this article, a design optimization technique for mixing in a gas turbine combustor is presented. The technique entails the use of computational fluid dynamics and mathematical optimization to optimize the combustor exit temperature profile. Combustor geometric parameters were used as optimization design variables. This work does not intend to suggest that combustor exit temperature profile is the only performance parameter important for the design of gas turbine combustors. However, it is a key parameter of an optimized combustor that is related to the power output and durability of the turbine. The combustor in this study is an experimental liquid-fuelled atmospheric combustor with a turbulent diffusion flame. The computational fluid dynamics simulations use a standard k-ε model. The optimization is carried out with the Dynamic-Q algorithm, which is specifically designed to handle constrained problems where the objective and constraint functions are expensive to evaluate. The optimization leads to a more uniform combustor exit temperature profile than with the original one