Tah-teh Yang

AIR EXTRACTION IN A GAS TURBINE FOR INTEGRATED GASIFICATION COMBINED CYCLE (IGCC): EXPERIMENTS AND ANALYSIS

This paper presents an investigation of extracting air from the compressor discharge of a heavy-frame gas turbine. The study aimed to verify results of an approximate analysis: whether extracting air from the turbine wrapper would create unacceptable nonuniformity in the flow field inside the compressor discharge casing. A combined experimental and computational approach was undertaken. Cold flow experiments were conducted in an approximately one-third scale model of a heavy-frame gas turbine; a closely approximated three-dimensional computational fluid dynamic analysis was also performed. This study substantiated the earlier prediction that extracting air from the turbine wrapper would be undesirable, although this method of air extraction is simple to retrofit. Prediffuser inlet is suggested as an alternate location for extracting air. The results show that not only was the problem of flow nonuniformity alleviated with this alternate scheme, but the frictional power loss in the compressor discharge casing was also reduced by a factor of two.

An Experimental/Computational Study of Airflow in the Combustor–Diffuser System of a Gas Turbine for Power Generation

This paper presents an experimental/computational study of cold flow in the combustor-diffuser system of industrial gas turbines employing can-annular combustors and impingement-cooled transition pieces. The primary objectives were to determine flow interactions between the prediffuser and dump chamber, to evaluate circumferential flow nonuniformities around transition pieces and combustors, and to identify the pressure loss mechanisms. Flow experiments were conducted in an approximately one-third geometric scale, 360-deg annular test model simulating practical details of the prototype including the support struts, transition pieces, impingement sleeves, and can-annular combustors. Wall static pressures and velocity profiles were measured at selected locations in the test model. A three-dimensional computational fluid dynamic analysis employing a multidomain procedure was performed to supplement the flow measurements. The complex geometric features of the test model were included in the analysis. The measured data correlated well with the computations. The results revealed strong interactions between the prediffuser and dump chamber flows. The prediffuser exit flow was distorted, indicating that the uniform exit conditions typically assumed in the diffuser design were violated. The pressure varied circumferentially around the combustor casing and impingement sleeve. The circumferential flow nonuniformities increased toward the inlet of the turbine expander. A venturi effect causing flow to accelerate and decelerate in the dump chamber was also identified. This venturi effect could adversely affect impingement cooling of the transition piece in the prototype. The dump chamber contained several recirculation regions contributing to the losses. Approximately 1.2 dynamic head at the prediffuser inlet was lost in the combustor-diffuser, much of in the dump chamber where the fluid passed though narrow pathways. A realistic test model and three-dimensional analysis used in this study provided new insight into the flow characteristics of practical combustor-diffuser systems.

Use of Subdomains for Inverse Problems in Branching Flow Passages

For inverse problems in complex flow passages, a calculation procedure based on a multizone Navier-Stokes method was developed. A heuristic approach was employed to derive wall shape corrections from the wall pressure error. Only two subdomains sharing a row of control volumes were used. The grid work in the common region was identical for both subdomains. The flow solver, inverse calculation procedure, multizone Navier-Stokes method and subdomain inverse calculation procedure were validated independently against experimental data or numerical predictions. Then, the subdomain inverse calculation method was used to determine the wall shape of the main duct of a branching flow passage. A slightly adverse pressure gradient was prescribed downstream of the sidebranch. Inverse calculations resulted in a curved wall diffuser for which the wall pressure distribution matched the design (prescribed) wall pressure distribution. The present method was illustrated for laminar, incompressible flows in branching passages. However, the method presented is flexible and can be extended for turbulent flows in multiply connected domains.

Viscous Flow Computations for Compressor/Combustor Diffuser Design to Allow Air Extraction for IGCC Systems

A computational procedure based on the solution of fully elliptic Navier-Stokes equations on a body-fitted non-orthogonal grid was used to obtain flow fields in annular diffusers with a suction slot at the inner and outer walls. The turbulence effects were simulated by high Reynolds number form of the k-e model. The calculation method was used to modify an industrial gas turbine (GE MS · 7001F) compressor/combustor annular diffuser to allow extraction of compressed airflow for coal gasification in simplified IGCC Systems. The air for gasification was extracted through a suction slot on the outer wall of the diffuser which was curved to improve the overall performance and to avoid flow separation; both of these insured by providing accelerated flow through the suction slot and nearly constant wall pressure downstream of the slot. Suction slot and outer wall geometries to result in the above conditions were determined by a trial and error procedure. The diffuser’s performance was further improved by extracting 6% of the compressed air through a slot at the inner wall, kept straight due to structural constraints. The resulting diffuser arrangement was relatively insensitive to the upstream disturbances.Copyright

Air Extraction in a Gas Turbine for Integrated Gasification Combined Cycle (IGCC): Experiments and Analysis

This paper presents an investigation of extracting air from the compressor discharge of a heavy-frame gas turbine. The study was aimed to verify results of an approximate analysis: whether extracting air from the turbine wrapper would create unacceptable nonuniformity in the flow field inside the compressor discharge casing. A combined experimental and computational approach was undertaken. Cold flow experiments were conducted in an approximately one-third scale model of a heavy-frame gas turbine; a closely approximated 3-D computational fluid dynamic analysis was also performed. This study substantiated the earlier prediction that extracting air from the turbine wrapper would be undesirable although this method of air extraction is simple to retrofit. Prediffuser inlet is suggested as an alternate location for extracting air. The results show that not only the problem of flow non-uniformity was alleviated with this alternate scheme, but the frictional power loss in the compressor discharge casing was also reduced by a factor of two.Copyright

Calculations of Fuel NO Formation in a Gas Turbine Combustor

PCGC-2, a two-dimensional combustion code for pulverized coal gasification and combustion, and PHOENICS, a general purpose fluid dynamics code, were adapted for use in simulating the conversion of fuel nitrogen to nitric oxide, NO, in a gas turbine combustor using low-Btu fuel. A two-reaction global mechanism was used to describe the oxidation of fuel nitrogen. PCGC-2 is limited to two-dimensional, axisymmetric calculations. Both two- and three-dimensional simulations were made with PHOENICS. A parametric study was conducted to determine the variation of fuel nitrogen conversion with changes in the input variables including the inlet fuel nitrogen concentration and swirl numbers. The fuel nitrogen conversion predicted with both codes is similar to those reported in experimental studies on gaseous fuels. The conversion decreased with increasing fuel nitrogen inputs as shown in experimental data. The fuel conversion predicted in three-dimensional simulations for an industrial gas turbine was slightly higher than those in simplified two-dimensional simulations.Copyright

Comparative Evaluation of Combustion Codes for Low-Btu Gas Applications

Four computer codes (PHOENICS, PCGC, FLUENT and INTERN) representing a spectrum of existing combustion modeling capabilities were evaluated for low-Btu gas applications. In particular, the objective was to identify computer code(s) that can be used effectively for predictions of (a) the flow field to yield efficient combustion, (b) the temperature field to ensure structural integrity and (c) species concentrations to meet environmental emission standards in a gas turbine combustor operating on low-Btu coal gas. Detailed information on physical models, assumptions, limitations and operational features of various codes was obtained through a series of computational runs of increasing complexity and grouped as (a) experimental validation, (b) code comparison and (c) application to coal gas combustion.INTERN is not suitable for the present application since it has been tailored to model combustion process of premixed hydrocarbon fuels. FLUENT is easy to use and has detailed combustion models (in Version 3), however, it is not favored here because the user is unable to alter, modify or change the existing model(s). While PCGC-2 has the most comprehensive models for combustion, it is not user friendly and is inherently limited to axisymmetric geometry. PCGC-3 is expected to overcome these drawbacks. Built in combustion models in PHOENICS are similar to those in FLUENT. However, the user can implement advanced models on PHOENICS leading to a flexible and powerful combustion code.Copyright

Numerical Simulations of Low-Btu Coal Gas Combustion in a Gas Turbine Combustor

A numerical model for turbulent reacting flow is described and applied for predictions in an industrial gas turbine combustor operating on low-Btu coal gas. The model, based on fast-reaction limit, used Favre averaged conservation equations with the standard k-e model of turbulence. Effects of turbulent fluctuations on chemistry are described statistically in terms of the mean, variance and probability density function (assumed to be β-distribution) of the mixture fraction. Two types of geometric approximations, namely axisymmetric and three-dimensional, were used to model the combustor. Computations were performed with (a) no swirl (b) weak swirl and (c) strong swirl at the fuel and primary air inlets. Essentially, the same bulk mean temperature distributions were obtained for axisymmetric and three-dimensional calculations while the computed pattern factors and the liner wall temperatures for the two differed significantly. Complete combustion was predicted with strong swirl, a result supported by the available test data. The maximum liner wall temperature predicted for three-dimensional calculations compared favorably with the experimental data while the predicted maximum exhaust gas temperature differed by ≈120 K. The difference was attributed to measurement uncertainties, model assumptions and lack of accurate data at the inlets. The maximum flame temperature was below 1,850 K indicating that thermal NOx may be insignificant.Copyright