Can-type Gas Turbine Combustor Research Articles

Numerical Investigations on the Effects of Dome Cooling Air Flow on Combustion Characteristics and Emission Behavior in a Can-Type Gas Turbine Combustor

To meet the requirements of achieving higher efficiency and lower NOx pollution, the flame temperature in gas turbine combustors increases continually; thus, the effusion-cooling technology has been used to ensure the combustor liner remains within the allowed temperature, by which the combustion characteristics and emission behavior are possibly influenced. In order to investigate the effects of dome cooling air flow on combustion characteristics and NOx emissions, three-dimensional combustion simulations for a swirl-stabilized can-type gas turbine combustor are carried out in this work by using the computational fluid dynamics (CFD) method. Through adjusting the ratio of the dome cooling air flow and the dilution cooling air flow, the characteristics of flow field, temperature distribution and NOx emissions under each work condition are analyzed. At different ratios of the dome-cooling air flow to the total air flow, the flow velocity field in the region near the center of the combustion chamber is not changed much, while the velocity field near the chamber wall shows a more significant difference. The temperature in the outer recirculation zone within the combustion chamber is effectively reduced as the dome cooling air flow increases. By analyzing the distribution characteristics of the concentration of OH*, it is demonstrated that the dome cooling air flow does not have a direct effect on the reaction of combustion. It is also found that as the ratio of the dome cooling air flow to the total air flow increases from 0 to 0.15, the value of the NOx emissions drops from 28.4 to 26.3 ppmv, about a 7.4% decrease. The distribution of the NOx generation rate in the combustion chamber does not vary significantly with the increasing dome cooling air flow. Furthermore, by calculating the residence time in different stages, when the the ratio of the dome cooling air flow to the total air flow varies from 0 to 0.15, the residence time in the pilot stage decreases obviously, from 42 ms to 18 ms. This means that reduction in residence time is the main factor in the decrease of NOx emissions when the dome cooling air flow increases.

CFD Investigation of Methane Combustion with Excess Air in Can-Type Gas Turbine Combustor

This paper presents an investigation on the effect of excess air to combustion characteristics in a full-scale, gas turbine combustor, commonly used in power plant. The investigation was carried out using Computational Fluid Dynamics (CFD), and prior validation was made with the actual operation data of a power station, as well as the adiabatic flame temperature of methane. The mass fraction of CO<sub>2</sub>, O<sub>2</sub> and NO<sub>x</sub> emissions for blended methane with different percentages of excess air explosions was also investigated. The stoichiometric excess air varies from 0% to 30% with air-fuel mixture of 2.7 kg/s. The geometric model of the combustor is extracted from actual gas turbine combustor using 3D scanner and converted into CAD model for simulation. The Navier-Stokes equations were solved using commercial CFD code, ANSYS Fluent, with RNG k-ε chosen to close the turbulence. For reacting species, the species transport model is assumed. Results showed that the addition of excess air during combustor has little effect on the velocity and temperature distribution, both at the combustor interior, as well as at the exit. For the emission of CO<sub>2</sub> and O<sub>2</sub>, though there was no clear trend on the relations between the emission of these species and the excess air, the impact was quite significant. The production of NO<sub>x</sub> was also found to be independent on the excess air ratio, but instead, was a strong function of combustion and exhaust gas temperature.

Investigation on combustion characteristics and emissions of biogas/hydrogen blends in gas turbine combustors

In the present work, numerical investigations are performed to study the combustion characteristics of biogas fuel blended with hydrogen at various compositions for a non-premixed swirling flame in a can-type gas turbine combustor. The amount of hydrogen enrichment varies from 0 to 50% by volume. A numerical approach using the non-premixed flamelet model, turbulent standard (k–ε) model, and P-1 radiation model was adopted for simulating the can-type combustor power at a fixed operating power of 60 kW. The steady laminar flamelet model was used to analyze the effect of hydrogen enrichment, global equivalence ratio with different swirl numbers on a stable flame operation, temperature distribution and contours, velocity streamline contours, NO emissions, and species concentrations. The results indicate that hydrogen enrichment and the variation of the equivalence ratio and the swirl numbers significantly impacted the flame macrostructure. Hydrogen enrichment in the fuel intensifi combustion, leading to higher flame temperature and wider flammability than bure biogas. Maximum NO emissions in the outlet chamber have been dropped by 43 and 78 (ppm @15 % by volume of O2) for the biogas and biogas-50% H2, respectively, due to the reduced flame temperature leading to reduction in thermal NOx formation with reduction equivalence ratio from 0.5 to 0.2. The flame temperature and NO emissions at ϕ=0.2 with a high rate of hydrogen (50% H2) are close to the results of pure biogas (0% H2) at the same equivalence ratio. The results show that CO and CO2 emissions decrease with increasing hydrogen addition and decreasing the equivalence ratio; due to a decrease in the amount of carbon, the cooling effect, and an increase in the OH concentration.

Distributed combustion mode in a can-type gas turbine combustor – A numerical and experimental study

A novel combustor design assisted with increased hot air jet momentum is used to enhance the internal recirculation. Geometric modifications are incorporated in terms of altered air injection scheme to improve the mixing and internal recirculation of hot combustion products. The combustor operates with liquefied petroleum gas in a distributed combustion mode, and thermal intensities vary in the range of 20–40 MWm−3atm−1. Numerical calculations of the reacting flow field show the presence of large recirculation zones, and they are quantified using reactant dilution ratio, Rdil. It is greater than 2.5 for the most part of the combustor, thereby creating a sufficiently heated and diluted environment for sustaining the distributed combustion at high thermal intensities. OH* chemiluminescence studies are carried out for the first time to observe the combustion zone structure for the conventional and distributed combustion regime in a gas turbine combustor. The distribution of OH* becomes increasingly uniform with reduced maximum OH* intensity in this combustion mode. The measured NOx is less than 8 ppm, and CO emission is below 25 ppm for all the operating conditions (ϕ = 0.2–1). Acoustic emissions drop significantly, as the combustor switches its operation to a distributed combustion mode. The novelty of this study lies in the application of this combustion mode in a gas turbine engine. This article also sheds light on the importance of the internal recirculation of combustion products, to achieve a distributed combustion mode in a generic can combustor at gas turbine relevant conditions.

A novel air injection scheme to achieve MILD combustion in a can-type gas turbine combustor

This article presents the design and development of a can-type gas turbine combustor operating in flameless combustion mode with liquid fuels. The combustor operates with kerosene and thermal intensities varying from 5.1 to 7.5 MW/m3. A novel air-injection scheme is proposed, in which air is supplied from different injection holes namely, swirl air near fuel injection, primary, secondary and dilution air in the downstream. These air injection holes are arranged in a way to help create strong recirculation of hot combustion products leading to increased mixing and dilution of incoming fresh reactants. Direction, orientation and mass fraction of swirl, primary, secondary and dilution air are optimized through a series of reacting flow simulations aimed at maximizing the reactant dilution ratio, a key parameter to achieve flameless combustion. It is observed that if the momentum of the air is increased beyond a critical value, the combustor switches its operation into flameless mode due to increased mixing and dilution of fresh reactants with hot combustion products. Combustion occurs in a well-distributed reaction regime within the combustor volume. Measured NOx emissions are less than 5 ppm and acoustic emissions are significantly reduced during the combustor operation in flameless combustion mode.

Thermal and emission characteristics of reverse air flow CAN combustor

Relative assessment of thermal and emission characteristics of conventional and reverse air flow CAN type gas turbine combustor is reported. Large Eddy Simulations (LES) are carried out with Wall Adaptive Local Eddy (WALE) viscosity model for sub grid scale stresses. Turbulence-chemistry interaction is modelled using presumed shape Probability Density Function (PDF) approach. Discrete Ordinates (DO) model is used for radiative heat transfer. Experimental measurement of temperature and NOx emission level at the exit of combustors are reported. Flow pattern and flame shape obtained from numerical analysis for conventional and reverse air flow combustor are correlated to combustor liner wall heating, exit temperature characteristics and NOx emission. The liner wall region of conventional combustor is dominated by hot combustion gases whereas the location of hot flame in reverse air flow combustor is in the vicinity of centreline. The exit temperature measurement shows that the wall side temperature in reverse air flow combustor is relatively low compared to conventional combustor. Reverse air flow arrangement in combustor eliminates the hot spots of high temperature gradient from primary region of combustor which results into significant reduction in NOx emission level in reverse air flow combustor as compared to conventional combustor. Thermal imaging of outer wall of combustors demonstrates that the radiative energy transfer from liner wall to outer wall is small in reverse air flow combustor compared to conventional combustor. This depicts that the wall-cooling requirement decreases in reverse air flow combustor.

Influence of fuel injection method on performance of upward swirl can-type combustor

A new concept of reverse fuel injection is introduced to enhance the fuel-burning rate in upward swirl can-type gas turbine combustor. The existing combustor has conical shaped injector where fuel jets are arranged on 90° cone. In reverse fuel injection, the injectant exits the fuelling device in reverse axial direction towards the wall of hemispherical dome. The dome wall deaccelerates the fuel jet first and then fuel flows in all directions. The injector length is chosen as design variable for analysis of reverse fuel injection flow field. Combustion experiments are carried out for existing conical injector and different reverse fuel injectors. Large Eddy Simulations (LES) are carried out to simulate non-premixed turbulent combustion flow field and it is combined with Discrete Ordinates (DO) method for the radiation modelling. The turbulence-chemistry interaction is modelled using presumed shape Probability Density Function (PDF) approach. Results obtained show that reverse fuel injection provides rapid near field mixing and better fuel penetration. Improved mixing results into release of large amount of heat energy during combustion, which increases combustion efficiency and reduces CO emission level without adversely affecting the NOx emission level. The obtained results show that injector length influences mixing efficiency, residence time and emission level. There exists an optimal penetration height for efficient mixing and total pressure recovery efficiency and it is achieved when injector length is kept at 5 mm in the present study. Thermal imaging of outer wall of combustor demonstrates that reverse fuel injection does not adversely affect the wall temperature.

Three-component particle image velocimetry in a generic can-type gas turbine combustor

A stereoscopic particle image velocimetry system was used to obtain the velocity field of a can-type combustor in the non-reacting condition. In order for these measurements to be taken, an optically accessible can-type forward flow combustor was manufactured. The combustor has a 10-vane swirler in the dome as well as a primary zone with six 9.5 mm holes, a secondary zone with eight 5 mm holes and a dilution zone with ten 11.8 mm holes. The two cooling rings have 30 and 50 × 1.2 mm holes and are placed between the three zones. The main flow features were captured such as the recirculation zones and jets. The more subtle features such as flow entering the swirler, entering the dilution holes from the annulus, and converging into smaller annuli around the secondary and dilution zones were also evident in some sections of the data. An unexpected flow recirculation was observed in the dilution zone. The departure of the flow from the recirculation regions to join the bulk flow in the dilution zone was also shown.

Large eddy simulation of a model gas turbine combustor

Large eddy simulation (LES) has been used to predict temperature and species concentrations in a model can-type gas turbine combustor operating in a non-premixed combustion regime. The subgrid scale stresses have been modelled adopting the standard Smagorinsky–Lilly model, whilst combustion has been accounted for using a conserved scalar approach. The dependence of the thermodynamical status of the mixture on the conserved scalar has been described invoking a flamelet assumption, and the shape of the scalar probability density function (pdf) assigned to be a β function. The subgrid scalar variance, needed to parameterise the scalar pdf, has been modelled according to local equilibrium arguments. The complex flow pattern developing from the interaction of a strongly swirling flow in the primary zone with the impinging primary jets has been captured by the present simulations in great detail. In particular, it has been observed that an accurate description of the inlet section of the combustor plays a fundamental role in the prediction of the fuel placement and, hence, of the temperature distribution in the primary region.

A Computational Model for the Study of Gas Turbine Combustor Dynamics

A dynamic combustor model is developed for inclusion into a one-dimensional full gas turbine engine simulation code. A flux-difference splitting algorithm is used to numerically integrate the quasi-one-dimensional Euler equations, supplemented with species mass conservation equations. The combustion model involves a single-step, global finite-rate chemistry scheme with a temperature-dependent activation energy. Source terms are used to account for mass bleed and mass injection, with additional capabilities to handle momentum and energy sources and sinks. Numerical results for cold and reacting flow for a can-type gas turbine combustor are presented. Comparisons with experimental data from this combustor are also made.

The Flow Inside a Model Gas Turbine Combustor: Calculations

The present study assesses the ability of the k-ε turbulence model to calculate the flow inside gas turbine combustors. Results of calculations using a cylindrical system of coordinates, hybrid differencing, and a mesh with about 40,000 nodes are compared with velocity measurements of the flow inside a perspex model of can-type gas turbine combustor. The larger discrepancies between measurements and predictions were found in the primary region. The complexity of the flow near the primary jet impingement led to underprediction of the maximum negative axial velocity and turbulence kinetic energy by about 35 and 20 percent, respectively. The calculated results exhibited higher levels of momentum diffusion compared to the experiments and did not show the two contrarotating vortices created between the primary jets; no qualitative agreement with the azimuthal velocity downstream of the primary jets could be achieved. Despite these deficiencies, the model gave acceptable results in other regions of the combustor and correct prediction of the main features of the combustor flow was possible.

Combustion process in a gas turbine combustor when using H 2, NH 3 and LPG fuels

Combustion characteristics of gaseous hydrogen fuel in a can-type gas turbine combustor are presented. Combustion performance of hydrogen is compared with that of ammonia and liquefied petroleum gaseous fuels using the same combustion hardware. The experiments have been directed to study the influence of the swirling angle of the primary air, secondary air ratio and equivalence ratio on combustion performance. Four different swirl angles namely, 0, 32, 52 and 72° are used. The secondary air ratios are varied from 10 to 50% from the total air supplied. Results obtained are similar for all fuels used. Changes in primary air swirl angle, equivalence ratio and secondary air ratios cause significant changes in combustion performance. Hydrogen combustion is characterized by high combustion efficiency, ease of ignition and good flame stability. However, it can also result in relatively high metal temperature which can impact on liner durability and high NO x emissions.

Velocity Characteristics of Isothermal and Combusting Flows in a Model Combustor

Abstract Mean and rms values of the axial and swirl velocity components obtained by a laser Doppler anemometer in a model can-type gas turbine combustor under isothermal and combusting flow conditions are reported. The results show that the combustor swirl is characterised by a solid body rotation near the axis and a free vortex flow in regions close to the combustor wall. Combustion strengthens and contracts the primary vortex, accelerates the flow in stream-wise direction and increases the rms of velocity fluctuations. Although the effect of air-fuel ratio on velocity characteristics is relatively weak in the primary zone where the interaction of fuel momentum with the swirler flow causes some changes in primary vortex structure, it becomes steadily more pronounced with downstream distance.

Velocity, temperature, and species characteristics of the flow in a gas-turbine combustor

The isothermal and combusting flow characteristics of a model can-type gas-turbine combustor are reported. In isothermal flow a laser-Doppler velocimeter was used to measure the longitudinal and tangential velocity characteristics, and the mean concentration of a tracer of helium gas, injected through the fuelling device, was used to obtain the mixture fraction distribution. In combusting flow, with propane as fuel, the velocimeter measured density weighted velocity characteristics, thin digitally compensated bare-wire thermocouples measured unweighted temperature characteristics, and a sampling probe transported gas samples to various instruments which measured near density weighted concentrations of O 2, UHC, H 2, CO 2, CO, and NO x . The mean velocity results show that, relative to isothermal flow, combustion increases the strength and decreases the width of the primary vortex and, further downstream, attenuates the magnitude and strength of the swirl by the longitudinal acceleration of the flow. Turbulence measurements indicate that the production of turbulent kinetic energy in the dilution zone of the combustor is due to “conventional” mechanisms by the interaction of shear stress with shear strain. In the recirculation zone, however, gradient transport is negligible and the effect of the mean pressure gradient is likely to be important in the balance of turbulent kinetic energy. The scalar field is dominated by the primary jets. An increase of 34% in the air-fuel ratio causes a similar increase in the pattern factor at the exit plane and a 14% decrease in the combustion efficiency. The pattern factor is improved by 15% and optimal combustion efficiency is obtained by increasing the inlet air temperature by 432K. In the primary zone, combustion is controlled more by physical than chemical kinetic processes and the formation of pollutants is well described by a partial equilibrium model. Downstream of the primary holes the fuel breakdown and the CO oxidation are reaction-rate limited, and the scalar field may be described by a constrained equilibrium model.