The gas turbine combustor and turbine blade walls serve to facilitate the flow of hot gases. These walls must be structurally strong to withstand the pressure differential across the walls. They must also have sufficient thermal resistance to withstand continuous and cyclic high temperature operation. These requirements can be achieved through the use of high- temperature, oxidation resistant materials combined with the effective use of cooling techniques.
An air jet impingement or impingement/effusion heat transfer cooling system may look simple, but the aerodynamics are complex, which also affects the cooling heat transfer and wall thermal gradients. The application of CHT CFD is a useful tool in visualising the air jet cooling flow fields, estimating the heat transfer and wall thermal effects. At present there are relatively few three dimensional (3D) CHT CFD studies concerned with impingement and impingement/effusion cooling techniques. This Ph. D. research investigations aims to use CHT CFD in understanding the aerodynamics and associated heat transfer effects of air jet cooling systems that are applicable to gas turbine combustor and turbine blade walls.
(a) Impingement jet single sided flow exit [25]
(b) Impingement jet four sided flow exit [47]
(c) Impingement/effusion flow exit through effusion hole
Experimental investigations were carried out in other work [3-5, 19, 22, 23, 25, 43-46] flat wall cooling with several geometrical variables and flow characteristics, on jet impingement and impingement/effusion cooling heat transfer applied to gas turbine combustor wall cooling which is also applicable to turbine blade cooling. The investigations, were performed in two different test configurations which will be referred to as the conventional test rig (or low temperature rig) and the combustion test rig (or high temperature rig). It is a primary objective of the present work, to reproduce the experimental results that have been previously published using CHT CFD methodology. The CHT CFD investigations reported here were carried out using the commercial CFD codes ANSYS ICEM (a mesh generation tool) and ANSYS Fluent (a flow solver). These CFD tools help in understanding and explaining the aerodynamics and coupled effects of conjugate heat transfer that exist in the experimental geometries that were investigated, which can also lead to new designs.
It transpire that the key parameters that significantly influence GT impingement jet and impingement/effusion heat transfer cooling are the geometrical and flow variables as shown in Figure 1.7 (a - c) and they include: the dimensionless geometrical hole pitch to diameter, impingement gap to diameter and hole length to diameter ratios X/D, Z/D and L/D respectively. By varying the number of holes N or pitch X, the number of holes/unit surface area (hole density) n or X-2 (m-2) is also varied. The main flow variable is the gas turbine coolant mass flux G (kg/sm2bar), which influences the impingement jet hole and gap velocities as well as the effusion hole flow aerodynamics and also affects the Reynolds number Re.
The effects of X/D, Z/D, L/D, n (m-2) and G (kg/sm2bar) for the range of hole diameter D, hole pitch X, gap Z and hole length L on impingement, effusion and impingement/effusion cooling heat transfer systems will be investigated. Changing Re implies changing either the geometrical or flow variables and these changes in values will be investigated using CHT CFD. By carrying out these investigations, the major GT aerodynamics and heat transfer components, the pressure loss ∆P and heat transfer coefficients (HTC) h (kW/m2K) are determined. Experiments [22, 25, 46] show that for smaller X/D at high G values, the pressure loss ∆P is low and this is significant to low NOx combustions applicable in industrial gas turbine combustor, but the heat transfer values are lower due to cross-flow effects. Impingement jet geometries with high X/D values, implies that the pressure loss ∆P and heat transfer are also high. This application of high X/D or ∆P with low G values is more appropriate for use in impingement/effusion cooling systems. This present Ph. D. research work will investigates these geometries and associated flow conditions.
Experimental investigations [4, 44] show that the number of holes/square meter (or hole density) of impingement or effusion surfaces n (m-2) does not influence the design in
principle, but it does influence cross-flow for impingement jet cooling. The importance of varying n experimentally is usually ignored and only little work has been done on this, CHT CFD work reported here investigates the influence of n for which experimental data are available for validation of the CFD predictions. Validation of the CHT CFD predictions with existing experimental data leads to a better understanding and explanation of the data. This present CHT CFD investigations will concentrate on testing the range of turbulence models, grids and influence of y+ values, these are discussed in Chapter 3. Once agreement is reached between the CFD prediction and the experimental work, other test geometries relevant to the application of gas turbine components walls cooling will be investigated using the optimised turbulence model and grid. The specific objectives of the study are to:
1. Develop model geometries with adequate grid resolution that are capable of predicting and explaining the experimental data and the associated cooling effectiveness.
2. Investigate the aerodynamic interactions that result from the deterioration of impingement heat transfer with axial distance, which was found experimentally. This similar aerodynamic effect that result into reversed jet flow onto jet plate of an impingement/effusion or four sided impingement jet flow exit cooling systems will also be investigated.
3. Understand the heating of the impingement jet walls caused by flow recirculation within the impingement gap that has been shown experimentally.
4. Predict the thermal gradients in the target walls in order to estimate the thermal stresses that occur during GT cooling processes.
5. Understand the influence of impingement gap cross-flow velocity interaction with the trailing edge high velocity air jets and the deflection of this jets.
6. Use optimised CHT CFD calculations to predict the best air jet cooling design procedures that are capable of effectively cooling gas turbine combustor and turbine blades. This will also apply to impingement/effusion cooling designs.
A further problem is that when X/D and the impingement wall pressure loss ∆P are small the pressure gradient along the discharge duct creates a flow-maldistribution (defined as the unequal distribution of coolant air mass flow in the jet holes). This is undesirable because it leads to uneven heat transfer along the duct. A further aim of this Ph. D. work is to predict and explain the influence of the flow-maldistribution as the X/D values are varied. Other effects of jet deflection, which have deleterious effects on heat transfer will also be investigated. For example, heat transfer and thermal gradients deterioration caused by cross- flow in the axial direction on the trailing edge target walls.