Horia C. Flitan and Paul G. A. Cizmas

Department of Aerospace Engineering, Texas A&M University College Station, Texas 77843-3141 horiaf@plano. tamu. edu and cizmas@tamu. edu

Thomas Lippert and Dennis Bachovchin

Siemens Westinghouse Power Corporation Pittsburgh, Pennsylvania

Dave Little

Siemens Westinghouse Power Corporation Orlando, Florida

Abstract This paper presents a numerical investigation of the unsteady transport phe­nomena in a turbine-combustor. The fbw and combustion are modeled by the Reynolds-averaged Navier-Stokes equations coupled with the species conserva­tion equations. The chemistry model used herein is a two-step, global, finite rate combustion model for methane and combustion gases. The governing equations are written in the strong conservation form and solved using a fully implicit, finite difference approximation. This numerical algorithm has been used to in­vestigate the airfoil temperature variation and the unsteady blade loading in a four-stage turbine-combustor. The numerical simulations indicated that in situ reheat increased the turbine power by up to 5.1%. The turbine combustion also increased blade temperature and unsteady blade loading. Neither the tempera­ture increase nor the blade loading increase exceeded acceptable values for the turbine investigated.

Keywords: Turbine-combustor, in situ reheat, unsteady flow, turbine flow

1. Introduction

In the quest to increase the thrust-to-weight ratio and decrease the thrust specific fuel consumption, turbomachinery designers are facing the fact that


K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 551-566. © 2006 Springer. Printed in the Netherlands.

the combustor residence time can become shorter than the time required to complete combustion. As a result, combustion would continue in the turbine, which up to recently has been considered to be undesirable. A thermody­namic cycle analysis demonstrates performance gains for turbojet engines with turbine-burner [Sirignano and Liu, 1999]. Even better performance gains for specific power and thermal efficiency are predicted for power generation gas – turbine engines when the turbine is coupled with a heat regenerator.

The process of combustion in the turbine is called in situ reheat and the turbine in which combustion takes place is called turbine-burner. The fuel is commonly injected in the turbine-burner through the vanes. Several challenges are, however, associated with the combustion in the turbine-burner: mixed sub­sonic and supersonic fews; fews with large unsteadiness due to the rotating blades; hydrodynamic instabilities and large straining of the flow due to the very large three-dimensional acceleration and stratified mixtures [Sirignano and Liu, 1999]. The obvious drawback associated with the strained fbws in the turbine-burner is that widely varying velocities can result in widely vary­ing residence time for different flow paths and as a result there are flammabil­ity difficulties for regions with shorter residence times. In addition, transverse variation in velocity and kinetic energy can cause variations in entropy and stagnation entropy that impact heat transfer. The heat transfer and mixing may be enhanced by increasing interface area due to strained flows.

Turbine aerodynamics might be drastically modified by strong exothermic combustion processes in a turbine-burner. Thermal expansion due to combus­tion could significantly change the pressure variation and the shock strength and location. As a result, the blade loading would be modified. There is evidence on a low pressure turbine without in situ reheat, that the tempera­ture non-uniformities can generate strong entropic and vortical waves. These waves produced excitations large enough to generate unsteady loadings and stresses on the 5th stage of alow pressure turbine, sufficient to cause high-cycle fatigue failures of a disk/blade/tip-shroud system mode crossing [Manwaring and Kirkeng, 1997]. The danger of high-cycle fatigue is even more imminent for a turbine-burner because larger temperature non-uniformities are likely to produce stronger entropic and vortical waves.

Experimental data for conventional (i. e., without in situ reheat) gas-turbines have shown the existence of large radial and circumferential temperature gra­dients downstream of the combustor [Dills and Follansbee, 1979, Elmore et al., 1983]. These temperature non-uniformities, called hot streaks, have a signifi­cant impact on the secondary flow and wall temperature of the entire turbine. Since the combustor exit few may contain regions where the temperature ex­ceeds the allowable metal temperature by 260-520°C [Butler et al., 1989], un­derstanding the effects of temperature non-uniformities on the flow and heat transfer in the turbine is essential for increasing vane and blade durability. It is estimated that an error of 55oC in predicting the time-averaged tempera­ture on a turbine rotor can result in an order of magnitude change in the blade life [Graham, 1980, Kirtley et al., 1993].

Temperature non-uniformities generated by the upstream combustor can be amplified in a turbine-burner. Consequently, it is expected that not only the secondary ft>w and wall temperature be affected but also the blade loading due to the modified pressure distribution. Temperature non-uniformities in a turbine-burner can also affect the location of hot spots on airfoils and as a result can impact on the internal and film cooling schemes.

There are extensive experimental [Whitney et al., 1980, Schwab et al., 1983, Stabe et al., 1984, Butler et al., 1989, Sharma et al., 1992, Shang et al., 1995] and numerical [Rai and Dring, 1990, Krouthen and Giles, 1988, Taka – hashi and Ni, 1991, Shang and Epstein, 1996, Dorney et al., 2000, Dorney et al., 1999] results for the infhence of temperature non-uniformities on the flow and heat transfer in a conventional turbine. To the best knowledge of the authors, however, there are no data available in the open literature for the effect of in situ reheat on turbine-burners. The objective of this paper is to investigate the effects of in situ reheat on the unsteady aerothermodynamics in a multi-stage turbine-combustor. This numerical simulation is crucial for the development of turbine-burners which, in spite of their challenges, can pro­vide significant performance gains for turbojet engines and power generation gas-turbine engines.

The next section presents the physical model used for the simulation of flow and combustion in the turbine-combustor. The governing equations and the chemistry model are presented. The third section describes the numerical model. This section includes information about the grid generation, boundary conditions, numerical method and parallel algorithm. The results are presented in the fourth section.