Khawar J. Syed

CFD Simulation of the Flow Within and Downstream of a High-Swirl Lean Premixed Gas Turbine Combustor

CFD analysis of the flow within a high-swirl lean premixed gas turbine combustor and over the 1 st row nozzle guide vanes is presented. The focus of the investigation is the fluid dynamics at the combustor/turbine interface and its impact on the turbine. For the configuration in question, temperature indicating paint observations of the nozzle guide vanes, acquired during engine development tests, show features consistent with the presence of a highly rotating vortex core emerging from the combustor. The configuration was modelled by a fully compressible reacting CFD analysis, whose domain stretched from the exit of the combustor swirl generator to downstream of the 1 st row nozzle guide vanes. The CFD analysis, when using a Reynolds stress turbulence model, predicted a highly rotating vortex core. The predicted interaction between the core and the nozzle guide vanes were consistent with the temperature indicating paint observations. The interaction is dominated by the vortex core being attracted to the locus of lowest static pressure.

The Nature of NOx Formation Within an Industrial Gas Turbine Dry Low Emission Combustor

The NOx formation within a practical lean premixed gas turbine combustor concept has been investigated. The effects of chemical kinetics and fuel/air mixing have been isolated, by adopting an approach, which combines high pressure combustion testing, CFD and chemical reactor modelling. Given the complexities of the underlying fluid dynamic and chemical processes and their interactions, consistency has been sought between experimental and numerical approaches, prior to drawing any conclusions. Two variants of Siemens Industrial Turbomachinery’s dry low emissions combustor have been investigated, one exhibiting near-ideally premixed combustion over a wide range of combustor pressure drop. Perfectly Stirred Reactor analysis, utilising the GRI 3.0 NOx mechanism, shows that NOx formation is dominated by the N2O and Zeldovich routes, with the N2O route being the larger at flame temperatures below 1800-1900K, for systems operating at 14bars, 400°C inlet temperature and at residence times of interest. Other reactions involving H-N-O chemistry are also significant, however the CH-N-O chemistry has a negligible impact.

Active Control of Fuel Splits in Gas Turbine DLE Combustion Systems

This paper presents a system for the active control of the fuel split within a two-stream Dry Low Emissions (DLE) gas turbine. The system adjusts the fuel split based upon the amplitude of combustor pressure fluctuations and burner metal temperature. The active control system, its implementation and its performance during engine tests on Siemens SGT-200 is described. The paper describes the active fuel split control algorithm. Engine test results are then presented for steady and transient loads with different rates of change of the engine operation temperature, including rapid load acceptance and load shedding. Additionally, cycling operating conditions were tested to evaluate the performance of the algorithm in typical island mode and mechanical drive applications. The active control algorithm was successful in providing stable and reliable control of the turbine allowing very low emissions levels to be attained without manual intervention. In fact it allows areas to be reached that until now were excluded. The impact of operational parameter changes (e.g. load change, ambient temperature, fuel composition etc.) on the engine operability proved the active control software’s ability to respond seamlessly. In addition, it prevented flameout and/or high pressure fluctuation while keeping burner temperatures within limits. Recorded emissions showed a reduction in NOx was achieved when the fuel split was controlled by the algorithm compared to standard operation. This was a direct result of the algorithm successfully identifying the lean stability limit and operating close to it.Copyright

DES Analysis of Confined Turbulent Swirling Flows in the Sub-critical Regime

DES are applied to confined turbulent swirling flows exhibiting vortex breakdown. The DES results are compared with measurements obtained on a water test rig operating at a Reynolds number of 4600, and with further predictions obtained via different modelling strategies such as the Reynolds Stress Model (RSM) based URANS and LES. In the LES, the standard Smagorinsky model, and its modification by Voke are used as the subgrid scale models. DES and LES results are observed to be in better agreement with the experiments, compared to the URANS-RSM predictions. The overall performance of DES is observed to be similar to that of LES computations, though, a slight superiority of DES compared to LES can still be claimed, for the considered test case.

Computational Investigation of Turbulent Swirling Flows in Gas Turbine Combustors

In the first part of the paper, Computational Fluid Dynamics analysis of the combusting flow within a high-swirl lean premixed gas turbine combustor and over the 1 st row nozzle guide vanes is presented. In this analysis, the focus of the investigation is the fluid dynamics at the combustor/turbine interface and its impact on the turbine. The predictions show the existence of a highly-rotating vortex core in the combustor, which is in strong interaction with the turbine nozzle guide vanes. This has been observed to be in agreement with the temperature indicated by thermal paint observations. The results suggest that swirling flow vortex core transition phenomena play a very important role in gas turbine combustors with modern lean-premixed dry low emissions technology. As the predictability of vortex core transition phenomena has not yet been investigated sufficiently, a fundamental validation study has been initiated, with the aim of validating the predictive capability of currently-available modelling procedures for turbulent swirling flows near the sub/supercritical vortex core transition. In the second part of the paper, results are presented which analyse such transitional turbulent swirling flows in two different laboratory water test rigs. It has been observed that turbulent swirling flows of interest are dominated by low-frequency transient motion of coherent structures, which cannot be adequately simulated within the framework of steady-state RANS turbulence modelling approaches. It has been found that useful results can be obtained only by modelling strategies which resolve the three-dimensional, transient motion of coherent structures, and do not assume a scalar turbulent viscosity at all scales. These models include RSM based URANS procedures as well as LES and DES approaches.

A Novel Approach to Predicting NOx Emissions From Dry Low Emissions Gas Turbines

An empirical modelling concept for the prediction of NOx emissions from Dry Low Emissions (DLE) gas turbines is presented. The approach is more suited to low emissions operation than are traditional approaches. The latter, though addressing key operating parameters, such as temperature and pressure drop, do not address issues such as variation in fuel/air distribution through the use of multi fuel stream systems, which are commonly applied in DLE combustors to enable flame stability over the full operating range. Additionally the pressure drop dependence of NOx in such systems is complex and the exponent of a simple pressure drop term can vary substantially. The present approach derives the NOx model from the equations that govern the NOx chemistry, the fuel/air distribution and the dependence of the main reaction zone upon its controlling parameters. The approach is evaluated through comparing its characteristics with data obtained from high pressure testing of a DLE combustor fuelled with natural gas. The data were acquired at a constant pressure and preheat temperature (14 Bara and 400°C) and a range of flame temperatures and flow rates. Though the model is configured to address both relatively fast and slow NOx formation routes, the present validation is conducted under conditions where the latter is negligible. The model is seen to reproduce key features apparent in the data, in particular the variable pressure drop dependence without any ad-hoc manipulation of a pressure drop exponent.Copyright

Development of an Oil Injection System Optimised to the ABB Double Cone Burner

Present day land-based gas turbine combustors, operating on oil, must meet strict requirements for emissions (CO, unburned hydrocarbons, particulates, smoke and NOx) and burn stabily without pulsations over a wide range of operating conditions. In addition many engines, such as those produced by ABB, operate with both oil and natural gas fuels either together or independently. This paper concentrates on the development of an oil injection system which is optimised for ABB’s double cone burner (Figure 1) and which does not affect the operation of this burner on natural gas. The development procedure, which involved a coupling of numerical and experimental techniques, is described. The results of the application of this procedure indicate that a simple plain jet atomiser in conjunction with a small quantity of unswirling air admitted at the head of the burner is the best option for this burner.Copyright

TRANSIENT COMBUSTION MODELING OF AN OSCILLATING LEAN PREMIXED METHANE/AIR FLAME

The main objective of the present study is to demonstrate accurate low frequency transient turbulent combustion modeling. For accurate flame dynamics some improvements were made to the standard TFC combustion model for lean premixed combustion. With use of a 1D laminar flamelet code, predictions have been made for the laminar flame speed and the critical strain rate to improve the TFC (Turbulent Flame Speed Closure) combustion model. The computational fluid dynamics program CFX is used to perform transient simulations. These results were compared with experimental data of Weigand et al [1]. Two different turbulence models have been used for predictions of the turbulent flow.