Weiguang Huang

An Effective Turbine Blade Parameterization and Aerodynamic Optimization Procedure Using an Improved Response Surface Method

This paper describes a procedure for a rapid and accurate 3D aerodynamic optimization of high performance turbine blades. This procedure has been developed to account for the complicated geometrical aspects and the complex nature of the associated fluid flow, while remaining simple, practical and demanding less computing power. The focus has been placed on the blade geometrical representation using a set of simple algebraic equations (blade parameterization) and on the aerodynamic optimization methodology based on the numerical computations by a N.S. solver. The turbine blade, including thickness distribution and camber line for each section of the blade span and radial stacking line, has been defined by polynomials, allowing investigation of the influence of any single-parameter change on blade performance. An improved response surface method, by incorporating a simulated annealing algorithm (RS-SAM), has been found to improve the accuracy and to strengthen the optimum-searching ability. A multi-objective response surface method (MORSM) has also been included for testing. One example is given here to demonstrate the effectiveness of the procedure.Copyright

The Effects of Fuel Dilution With Steam on Gas Turbine Combustor Performance

The formation of nitrogen oxides (NOx ) in combustion systems is a significant pollutant source in the environment, and How to control NOx emission is a worldwide concern as the utilization of fossil fuels continues to increase. Syngas is produced from variety of fossil fuels through a gasification process. Though syngas is regarded as one kind of clean fuel, its NOx emission control techniques in combustion systems still remain many problems for further development. Steam dilution of fuel is an effective method for NOx reduction in practical gas turbine systems. This paper describes the study focusing on the influence about steam dilution of fuel for reducing the NOx emission. Experimental investigations are conducted on a 40MW gas turbine fired with syngas. The pollutant emissions, combustor dynamic pressure, metal temperature distribution of liner and combustion efficiency are analyzed at the base load of gas turbine with different steam injection rate to fuel flow. Three-dimensional CFD numerical simulation of combustor according to experiment parameters is applied to investigate the influence of steam dilution on NOx emission and the combustion liner wall temperature. Comparisons are made between experiment data and CFD simulation results for further understandings about NOx formation characteristics in steam diluted syngas and its influence to gas turbine combustion system. The investigation of this paper’s work shows tha steam dilution is an effective method of NOx emission control technique in practical gas turbine combustion systems fired with syngas. The CFD simulation results show that steam in flame can remarkably reduce temperature of the flame for its high thermal capacity and it can remarkably reduce formation of NOx in gas turbine combustor. Moreover, as the steam flowrate added to fuel increase the wall temperature of some zone of liner may increase because of the convection heat transfer is strengthened and the combustion oscillation will be weaken.Copyright

Numerical Study on the Response of Tip Leakage Flow Unsteadiness to Micro Tip Injection in a Low-Speed Isolated Compressor Rotor

A numerical study on the unsteady tip leakage flow with discrete micro tip injection from casing shroud in a low-speed isolated axial compressor rotor is presented. The main target is to clarify the flow mechanism of how the stall control measures act on the tip leakage flow typified by its self-induced unsteady flow characteristics. At operating condition near stall point, a series of calculations have been carried out for different axial position of injector and different injected mass flow rate. The computation results of flow field near rotor tip region show that under the influence of injected flow, the transient pressure distribution fluctuates along blade chord on both pressure and suction sides with respect to the relative position of injector and rotor. The pressure difference across the pressure and suction sides of compressor blade changes correspondingly, thus introduces a forced flow unsteadiness interacting with the unsteady tip leakage flow. When the injection is relatively strong and able to meet the tip leakage flow at its origination, the self-induced unsteadiness of tip leakage flow can be suppressed completely. In most cases, both frequency components of the self-induced unsteadiness and forced-induced unsteadiness are co-existing. The corresponding transient flow contours show that a local high pressure spot appears near blade pressure side, which moves downstream and shifts the tip leakage flow trajectory with less or without touching the neighboring pressure surface of the blade. Based on this understanding of discrete tip injection as force-induced flow unsteadiness, the numerical results are also analyzed to optimize the effect of injection in changing the route of tip leakage flow trajectories and therefore the chance of stability improvement of the compressor rotor.Copyright

The Effects of Pressure on Gas Turbine Combustor Performance: An Investigation via Numerical Simulation

Performance tests of a gas turbine combustor are usually conducted at atmospheric or medium pressure which is quite different from its real operating condition. The effects of pressure on the performance of a gas turbine combustor for burning medium-heating-value syngas are researched by numerical simulation in this paper. The geometry of the combustor is modeled by coupling all its components including nozzle, combustor liner and sealant tube. In the simulation a laminar flamelet model and P-1 radiation model are adopted. The numerical results show that at the same fuel and air inlet temperature and the same equivalence ratio, the operation pressure has less effect on the flow fields, but its effect on the temperature distribution is obvious. Both the highest temperature in the combustor and the outlet temperature increase with increasing operating pressure because of the weakening of the dissociation of the H2 O, CO2 and so on. Moreover, as pressure increases, the concentration of H2 O and CO2 in the combustor increase, and so to does the absorption coefficient and the emissivity of gas inside the liner. As a result, the radiation heat transfer between the gas and the combustion liner wall is enhanced, and the wall temperature of the liner increases. The NOx emissions of the combustor are also distinctly higher at high pressure than at low pressure.Copyright

Experimental Investigation of Thermoacoustic Oscillations in Syngas Premixed Multi-Swirler Model Combustors

This paper presents the experimental results of the thermoacoustic oscillations in several premixed syngas multi-swirler model combustors. Multi-swirler lean premixed combustion technology has been successfully applied to achieve an anticipative stability, lower noise and high efficiency in industrial gas turbines that burn natural gas or distillated oil. However, some critical operational issues, including combustion oscillations, flashback, blowout and autoignition, should be considered and balanced when burning the coal-derived syngas mainly composed of H2 and CO. In this paper several multi-swirler combustors are tested on an atmospheric-pressure downscaled test rig. Each multi-swirler combustor includes several elemental swirling nozzles with the equal Swirl Number and different array of port configurations. The dynamic pressure, dynamic heat release and critical flashback equivalence ratio are tested in these model combustors burning several kinds of simulated syngases with a similar low heat value. Firstly, the critical equivalence ratios of flashback are shown and compared with those in single-swirler combustors. Secondly, the paper presents the analysis of the temporal and spectral features of dynamic pressure oscillations using many data-processing methods. Thirdly, we describe the bifurcation and retardation phenomenon when the combustion transforms between stable and unstable operations. We also discuss how the equivalence ratio, the fuel composition and the combustor inlet velocity play important roles in determining the amplitudes, the frequencies, the bifurcation and retardation of the thermoacoustic oscillations. Finally, we use a wavelet transformation with a higher resolution in time domain than that with a PSD estimation by the AR model. The processes of amplitude “jump” and flashback are analyzed in details. The results in this paper could improve the current understanding of the nonlinear self-excited and combustion driven thermoacoustic oscillations in gas turbines and give us some references to the development of lean premixed syngas turbines for coal-based IGCC and co-generation systems.Copyright

Numerical Investigation of a Stagnation Point Reverse Flow Combustor

Flameless combustion is characterized by ultra-low NOx emissions, high combustion efficiency and very stable flame, which is able to operate stably at very lean fuel-air mixtures without problems of combustion oscillation and flashback. Stagnation Point Reverse Flow (SPRF) combustors, as an important application of flameless combustion, have been experimentally studied by various optical diagnostic techniques. In this paper, Eddy Dissipation Concept (EDC) model with detailed chemical reaction mechanisms of natural gas GRI 2.11 is used to investigate the flame characteristics of a SPRF combustor operating at premixed mode with various mass flow rates and equivalence ratios of CH4 and air mixtures. The numerical results indicate that as the fuel and air mixture injection velocities increase, there are no distinct changes in the jet penetrations. However, flame temperatures and NOx emissions decrease, CO emissions increase, and OH is distributed in wider area and more evenly. For turbulent flow, intense reactions take place in the shear layer and the stagnation zone and they gradually shift to the combustor outlet as the jet velocities increase. As the equivalence ratios increase from 0.5 to 1, the NOx emissions always increase, although they are very low when equivalence ratio is below 0.7. However, the CO emissions decrease firstly, reaching the minimum value at equivalence ratio of 0.58, and then increase. The numerical results are compared with experimental data and it is verified that EDC model can capture the important flow field characteristics and flame structure and is appropriate for modeling SPRF combustor.Copyright

Unsteady Tip Clearance Flow in a Low-Speed Axial Compressor Rotor With Upstream and Downstream Stators

In the course of advancing the understanding of the unsteady flow nature of compressor tip clearance flows, the present paper investigates the unsteady tip clearance flow in the second rotor of a two-stage low-speed axial compressor and its interaction with upstream and downstream stators. Numerical methods were adopted in the present study and the research focused on clarifying the unsteadiness of tip clearance flow behavior and its link to the change of rotor performance, subjected to the variables of axial gap sizes between the rotor and upstream and downstream stators. The result shows how and why the tip leakage vortex trajectory changes its shape with the change of gap size, and its impact on the rotor pressure rise characteristic. Within all the computed operating range, the pressure rise increases monotonically with the decrease of upstream axial gaps, but no monotonic variation was observed with the change of downstream axial gaps. This trend of performance change could be explained by the unsteady effect of upstream stator wakes, and the overall result is that the rotor performance was found to be more influenced by the upstream interaction than the downstream interaction. The frequency characteristic of the tip clearance vortex, under the influence of gap size and compressor operating condition, was also analyzed to provide a quantified estimation of its periodic flow behavior and a comparison with the recent results of other researchers.Copyright

Experimental and Numerical Investigations of Surge Extension on a Centrifugal Compressor with Vaned Diffuser Using Steam Injection

This paper presents the first report on surge extension with steam injection through both experimental and numerical simulation. The experimental section covers the test facility, instrumentation, and prestall modes comparison with and without steam injection. It is found that surge extension is not in proportion to injected steam. There exists an upper bound above which deteriorates the margin. Injection of less than 1% of the designed mass flow can bring about over 10% margin improvement. Test results also indicated that steam injection not only damps out prestall waves, but also changes prestall modes and traveling direction. At 90% speed, injection changed the prestall mode from spike to modal, while at 80% speed line, it made the forward traveling wave become backward. Through numerical simulation, location and number of injectors, molecular weight, and temperature of injected gas are modified to explore their influences on surge margin. Similar to the test results, there exists an upper bound for the amount of steam injected. The flow field investigation indicates that this bound is caused by the early trigger of flow collapse due to the injected steam which is similar to the tip leakage flow spillage caused spike stall in axial compressors.

Dynamic and Flashback Characteristics of the Syngas Premixed Swirling Combustors

Flashback and combustion thermoacoustic oscillation are two of the major problems in the lean premixed combustors of gas turbine, especially for hydrogen enriched syngas fuels. This paper introduced the experimental results regarding the characteristics research of thermoacoustic oscillation, dynamic heat release and flashback based on a premixed combustion experimental system. The critical flashback equivalence ratio and three dynamic parameters were tested in four types of combustor using eleven kinds of simulated syngas which have the same LHV. Partial flashback and full flashback are observed. With the increase of the hydrogen concentration, the critical equivalence ratio is decreasing and full flashback occurs more easily. The change of nozzle’s geometry structure has a consequent effect on the flashback characteristics. Flashback occurs more easily when the thermoacoustic oscillation is greater, and the corresponding critical equivalence ratio is smaller. Both flashback status and normal status are relative stable combustion within a certain range of equivalence ratio. As the generation and disappearance of the thermoacoustic oscillation and its amplitude-frequency characteristics somehow could be in some randomicity, the critical equivalence ratio is correspondingly not a constant. The dynamic chemiluminescence intensity which indicates the dynamic heat release has a good corresponding relationship in terms of amplitude and frequency characteristics with the dynamic pressure which indicates the thermoacoustic oscillation. Through power spectral density analysis, dynamic pressure and chemiluminescence intensity signals both reflect the dynamic characteristics of the combustor and can be applied in the research of the combustion instability, including the thermoacoustic oscillation characteristics, fluctuation of the chemical heat release rate and flashback characteristics and so on. The results and methods could give some reference to the development of hydrogen enriched gas turbines for future IGCC and poly-generation systems.Copyright

The Stability-Limiting Flow Mechanisms in a Subsonic Axial-Flow Compressor and Its Passive Control With Casing Treatment

The phenomenon of flow instability in the compression system such as fan and compressor has been a long-standing “bottle-neck” problem for gas turbines/aircraft engines. With a vision of providing a state-of-the-art understanding of the flow field in axial-flow compressor in the perspective of enhancing their stability using passive means. Two topics are covered in this paper. The first topic is the stability-limiting flow mechanism close to stall, which is the basic knowledge needed to manipulate end-wall flow behavior for the stability improvement. The physical process occurring when approaching stall and the role of complex tip flow mechanism on flow instability in current high subsonic axial compressor rotor has been assessed using single blade passage computations. The second topic is flow instability manipulation with casing treatment. In order to advance the understanding of the fundamental mechanisms of casing treatment and determine the change in the flow field by which casing treatment improve compressor stability, systematic studies of the coupled flow through a subsonic compressor rotor and various end-wall treatments were carried out using a state-of-the-art multi-block flow solver. The numerically obtained flow fields were interrogated to identify complicated flow phenomenon around and within the end-wall treatments and describe the interaction between the rotor tip flow and end-wall treatments. Detailed analyses of the flow visualization at the rotor tip have exposed the different tip flow topologies between the cases with treatment casing and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by end-wall treatments is a result of the tip flow manipulation. Compared to the smooth wall case, the treated casing significantly dampen or absorb the blockage near the upstream part of the blade passage caused by the upstream movement of tip clearance flow and weakens the roll-up of the core vortex. These mechanisms prevent an early spillage of low momentum fluid into the adjacent blade passage and delay the onset of flow instability.Copyright