CFD Modeling of Turbulent Flows in Industrial Applications with Emphasis on Premixed Combustion in Gas Turbines

Increasing restrictions on hazardous emissions combined with demands for good profitability from the operators in the energy market have contributed strongly to the development of innovations in which the emphasis is on low emissions and high efficiency. For gas turbine-based power plants, emission of NOx in particular is governed by stringent legislation. To meet these restrictions, most of the modern gas turbines for power production are equipped with a lean premixed combustion system. Increased awareness of global warning has helped to focus development on concepts that reduce the emission of CO2 into the atmosphere, since this gas is believed to be a major contributor to the problem. The maintenance of an existing power plant plays an important role in keeping emissions at a constant low level over time, since the performance of a power plant starts to decline directly after it has been put into operation. A major contributor to this decline is fouling of the compressor. The Evaporative gas turbine cycle is one example of new concepts that focus on high efficiency and low emissions. Other examples are cycles focusing on CO2 emissions, such as bio-fuel based cycles and cycles involving CO2 removal. The combustion process is an essential part of these new concepts. In contrast to the traditional cycles, the majority of these new high-efficiency and low-emission power cycles operate with combustion atmospheres that differ from the traditional hydrocarbon-air mixtures. Detailed investigations of such combustion systems are necessary for further development and/or modification of existing systems. Investigations of this type are often based on both experiment and numerical modeling, the latter studies being based on computational fluid dynamics codes and/or chemical kinetics codes. This thesis is based on computational fluid dynamics modeling of turbulent flows mainly related to premixed combustion. The methodology in this thesis was first to establish a method by which it would be possible to investigate premixed turbulent combustion in combustors of industrial character. Here, computations with different turbulence and combustion models have been applied and these computations have been validated against experimental data. The emphasis has been on accurate prediction of emissions, especially NOx and CO, in gas turbine combustors using the level-set flame library approach. Later, the method that was established was applied to combustion processes of relevance concepts dealing with low emission, such as the EvGT cycle. The influence of dilution on the emissions has been investigated for combustors under different operating conditions. Two different flame configurations have been investigated, namely a bluff body stabilized-flame and a swirl-stabilized flame. The two-phase flow in the compressor inlet of an industrial gas turbine during off-line washing conditions has also been investigated in this thesis, since a fouled compressor results in an efficiency drop which may lead to an increase in emissions

Investigation of two-equation turbulence models applied to a confined axis-symmetric swirling flow

The modeling of industrial combustion applications today is almost exclusively based on two-equation turbulence models. Despite its known limitations, the most the widely used model is still the standard k-ε model. The objective of this paper is to investigate the performance of two-equation turbulence models applied to a confined swirling flow. Numerical modeling of an axis-symmetric confined sudden expansion, followed by a contraction with the assumption of steady flow and an incompressible fluid, has been conducted. The flow field is what can be expected in simplified dump gas turbine combustor geometry. In this investigation, three different swirl cases were considered: no swirl, moderate swirl (no central re-circulation zone) and strong swift (a central recirculation zone occurring). The models investigated were: the standard k-ε model, a curvature-modified k-ε model, Chen's k-ε model, a cubic non-linear k-ε model, the standard k-ω model and the Shear Stress Transport (SST) k-ω model. The results show that almost all models were able to predict the major impact of the moderate swirl: reduced outer re-circulation lengths and retardation of the axial velocity on the center-line. However, the Chen k-ε model and the SST k-ω model were found to better reproduce the mean velocity field and the turbulent kinetic energy field from the measurements. For a strong swift, a large re-circulation zone is formed along the center-line, which the standard k-ε model and the modified k-ε model fail to predict. However, the shape and size of the re-circulation zone differ strongly between the models. At this swirl number, the performances of all models were, without exception, worse than for the lower swift numbers. The SST k-ω model achieved the best agreement between computations and experimental data

Investigation of turbulence models applied to premixed combustion using a level-set flamelet library approach

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by all external code, which employs a detailed chemical mechanism. However a model for the turbulent flame speed is required, which, among other considerations, depends on the turbulence intensity, i.e., these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-Cd, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-epsilon model, a cubic nonlinear k-e model, the standard k-omega model and the shear stress transport (SST) k-omega model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the nonlinear k-epsilon model offers no advantage over the standard model, regardless of flame speed model. The k-omega based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Muller flame speed model shows the least sensitivity to the choice of turbulence model

Reacting Boundary Layers in Homogeneous Charge Compression Ignition (HCCI) Engine

An experimental and computational study of the nearwall combustion in a Homogeneous Charge Compression Ignition (HCCI) engine has been conducted by applying laser based diagnostic techniques in combination with numerical modeling. Our major intent was to characterize the combustion in the velocity- and thermal boundary layers. The progress of the combustion was studied by using fuel tracer LIF, the result of which was compared with LDA measurements of the velocity boundary layer along with numerical simulations of the reacting boundary layer. Time resolved images of the PLIF signal were taken and ensemble averaged images were calculated. In the fuel tracer LIF experiments, acetone was seeded into the fuel as a tracer. It is clear from the experiments that a proper set of backgrounds and laser profiles are necessary to resolve the near-wall concentration profiles, even at a qualitative level. Partial resolution of the velocity boundary layer was enabled by using a slightly inclined LDA probe operated in back-scatter mode. During these conditions, it was possible to acquire velocity data within 0.2 mm from the wall. A one-dimensional model of the flow field was devised to make the connection between the thermal and the velocity boundary layer. The investigations suggest that wall interaction is not the responsible mechanism for the rather high emissions of unburned hydrocarbons from HCCI engines. It is believed that the delayed oxidation, indicated by the fuel tracer LIF experiments and numerical simulations, is due to the thermal boundary layer. From the data at hand, it is concluded that the thermal boundary layer is on the order of 1 mm thick. In this boundary layer the reactions are delayed but not quenched

A reduced-order through-flow program for choked and cooled axial turbines

A reduced-order through-flow program has been developed at the Lund Institute of Technology. The goal of the work was to develop and verify a program suitable for scientific studies of coaxial turbines. The program has built-in capability for multiple choked stages, and turbine cooling, as well as flexible modules for state, losses, deviation, and blockage. The code uses Matlab [trademark] as a platform for numerics, pre- and post-processing. The turbine modules are available from the Institute free of charge for scientific use. Matlab is a commercially available mathematical package and is used as a numerical tool by many universities and companies. Today, several codes are available for turbine analysis at different levels. Most of the codes are proprietary and not available outside the companies that have developed them. There are, however, commercially available codes, but the user does not normally have accesses to the source code. This poses serious problems when such codes are used for scientific studies, and open easily modified code is indeed a desirable feature. The present paper describes in detail the calculation methods used to simulate performance of a cooled and choked turbine at off-design conditions. The algorithms necessary for finding the turbine choke point will, for example, be described in detail. The way in which the loss, deviation (including secondary and tip clearance), and blockage are included in a quasi 1-D calculation environment is also presented. The code is suitable for further development (e.g. streamline curvature through-flow), since it is based on modules for e.g. state, combustion, loss, deviation, diffuser, numerics, and in and output data processing. It is fairly easy to transform the Matlab program into e.g. Fortran. However, the use of the original platform simplifies plotting of turbine characteristics, velocity triangles, etc. The program is validated against test-rig data from an industrial two-stage power turbine. Copyrigh

Investigation of the two-phase flow field of the GTX100 compressor inlet during off-line washing

A modern gas turbine compressor, with its highly aerodynamically loaded blades, is sensitive to changes in profile shape and to surface roughness. Fouling is inevitable, despite highly efficient filtration systems. The remedy to this problem is washing. There are two different approaches, online or off-line washing. The off-line wash is the most effective one, whilst on-line washing only prolongs the interval between off-line washes. Most findings in this field are highly empirical, being based on some 50 years of industrial gas turbine operation. This paper is an investigation of the two-phase flow in the bellmouth of the compressor during off-line washing conditions. The unit under study was the GTX100 turbo-set. Computational fluid dynamics (CFD) is used in this paper to perform a detailed study of the flow field. The main emphasis has been on studying the characteristics of the injected spray used for cleaning of the compressor. The benefit of heating this fluid is of special interest, since if this heating can be avoided, the outage time for the off-line compressor wash can be shortened. To provide the CFD computations with accurate boundary conditions for the spray, laser-based measurements of a spray, originating from an authentic wash nozzle, have been conducted. The commercial CFD program Star-Cd has been used for all computations. The computations show that the water injected, regardless of its inlet temperature, is cooled down to ambient air temperature well before the spray reaches the inlet guide vanes. This indicates that heating of the wash fluid can be abolished. The airflow seems not to be to influenced by the injected fluid to any great extend

CFD investigation of the effects of different diluents on the emissions in a swirl stabilized premixed combustion system

Recently, new cycles for power generation, such as wet cycles and cycles for CO2 capture, have gained increasing interest. These new cycles use some sort of dilution in the air/fuel mixture, e.g. steam or CO2. Gas turbine cycles using LCV gases can also be said to fit this description. Almost all modern gas turbines use a lean premixed combustion system, since it combines low NOX emissions with high combustion efficiency. The main objective of this paper is to study the influence of different diluents on the NOX and CO emissions at different inlet temperature, equivalence ratio, pressure and mass flow. The studied combustor was a premixed swirl stabilized combustor with optical access and emission sampling equipment. The combustor uses Danish natural gas as its main fuel. Computational fluid dynamics (CFD) has been employed to perform the investigations. It is common knowledge that turbulence models based on the Buissinesq assumption are not generally capable of handling a highly swirling flow in a correct way. Therefore, a differential Reynolds stress model (DRSM) has been employed for modeling of the turbulence. The turbulent combustion has been modeled with the level-set flamelet library approach (FLA). In this approach a laminar flamelet is linked to turbulent flow field via a non-reacting scalar G and its variance. The laminar flamelet is modeled with separate code. This code solves the combustion development with a detailed reaction mechanism for a laminar, non-stretched and premised one-dimensional flame. This is of great importance when emissions are to be predicted. All fluid dynamics computations were performed with the commercial CFD code Star-CD, version 3.20, where the FLA combustion model was implemented through Fortran based user subroutines. The computed flow field was validated against experimental data during non-reaction flow conditions. The computations showed good agreement with the experimental data. The computed CO and NOX emissions showed the same trends as the experimental data for the reacting case with an undiluted flame, when the equivalence ratio was altered. The computed emissions were used to build up an emission map for different dilutions during different operation conditions. Copyrigh

Reacting boundary layers in a homogeneous charge compression ignition (HCCI) engine

An experimental and computational study of the near-wall combustion in a Homogeneous Charge Compression Ignition (HCCI) engine has been conducted by applying laser based diagnostic techniques in combination with numerical modeling. Our major intent was to characterize the combustion in the velocity- and thermal boundary layers. The progress of the combustion was studied by using fuel tracer LIF, the result of which was compared with LDA measurements of the velocity boundary layer along with numerical simulations of the reacting boundary layer. Time resolved images of the PLIF signal were taken and ensemble averaged images were calculated. In the fuel tracer LIF experiments, acetone was seeded into the fuel as a tracer. It is clear from the experiments that a proper set of backgrounds and laser profiles are necessary to resolve the near-wall concentration profiles, even at a qualitative level. Partial resolution of the velocity boundary layer was enabled by using a slightly inclined LDA probe operated in back-scatter mode. During these conditions, it was possible to acquire velocity data within 0.2 mm from the wall. A one-dimensional model of the flow field was devised to make the connection between the thermal and the velocity boundary layer. The investigations suggest that wall interaction is not the responsible mechanism for the rather high emissions of unburned hydrocarbons from HCCI engines. It is believed that the delayed oxidation, indicated by the fuel tracer LIF experiments and numerical simulations, is due to the thermal boundary layer. From the data at hand, it is concluded that the thermal boundary layer is on the order of 1 mm thick. In this boundary layer the reactions are delayed but not quenched

Investigation of turbulent combustion in humid air using a level-set flamelet library approach

To meet the demands of high electrical efficiencies in combination with low emissions for gas turbine-based power plants, wet cycles have gained new interest. The combustion chamber is a key component in such a power plant. Detailed investigations are necessary to gain insight into the formation of hazardous emissions such as CO and NOx. In this paper, computational fluid dynamics (CFD) has been employed to perform the investigations. To examine the influence of humidity content in the inlet air stream on the mean flame position and CO and NOx emissions, the humidity content was varied from 0 to 31%. The CO emissions are a measure of incomplete combustion. The inlet temperature and the maximum flame temperature were held constant for all computations. The influence of different combustor loads was also investigated. This will influence the flame position, which is important for emission formation. The flame under investigation is a lean premixed propane flame, stabilized by a bluff body. The turbulent combustion has been modeled with the level-set flamelet library approach (FLA). This model includes a detailed chemical mechanism, which is of great importance when emissions are to be predicted. To illustrate this importance, the FLA model is compared with a simple eddy break-up model and validated against known experimental data at dry condition. The level-set flamelet library method shows very good agreement with the experimental data for both the temperature profile and the CO and NOx emissions. The simple eddy breakup model is only able to predict the temperature profile fairly well while the important emissions are being greatly overestimated. The computations have shown that the flame position, and hence the residence time, is most sensitive to the change in combustor load - while the degree of humidification appears to be of less importance. The CO emissions rise with an increase in the degree of humidification at constant load, while the opposite behavior is true for NOx emissions. At a constant level of humidification, the CO emissions fall with an increase in load and again, the opposite behavior is observed for NOx emissions. A map of emission indices for CO and NOx under different load conditions as a function of humidification can be generated. This map shows an operation window within which both the CO and NOx emissions are quite low