V. Ovcharenko

Extension of the Combustion Stability Range in Dry Low NOx Lean Premixed Gas Turbine Combustor Using a Fuel Rich Annular Pilot Burner

The present work is concerned with improving combustion stability in lean premixed (LP) gas turbine combustors by injecting free radicals into the combustion zone. The work is a joint experimental and numerical effort aimed at investigating the feasibility of incorporating a circumferential pilot combustor, which operates under rich conditions and directs its radicals enriched exhaust gases into the main combustion zone as the means for stabilization. The investigation includes the development of a chemical reactors network (CRN) model that is based on perfectly stirred reactors modules and on preliminary CFD analysis as well as on testing the method on an experimental model under laboratory conditions. The study is based on the hypothesis that under lean combustion conditions, combustion instability is linked to local extinctions of the flame and consequently, there is a direct correlation between the limiting conditions affecting combustion instability and the lean blowout (LBO) limit of the flame. The experimental results demonstrated the potential reduction of the combustion chamber’s LBO limit while maintaining overall NOx emission concentration values within the typical range of low NOx burners and its delicate dependence on the equivalence ratio of the ring pilot flame. A similar result was revealed through the developed CHEMKIN-PRO CRN model that was applied to find the LBO limits of the combined pilot burner and main combustor system, while monitoring the associated emissions. Hence, both the CRN model, and the experimental results, indicate that the radicals enriched ring jet is effective at stabilizing the LP flame, while keeping the NOx emission level within the characteristic range of low NOx combustors. [DOI: 10.1115/1.4026186]

Study and Field Tests of the Novel Low Pressure Fogger System for Industrial Gas Turbine

Based on three patented innovations (air-assist atomizer, wetness sensor, and closed loop programmable logic controller (PLC)), a new low-pressure power gas turbine augmentation system was developed. Two modifications of air-assist atomizer were tested in the Jet Engine Laboratory of the Technion. The tests were performed to investigate influence of thermodynamic parameters on spray characteristics, as droplet size and velocity distribution of the spray. The system as a whole has passed field test in a gas turbine of a power station. Droplet characteristics, conceptual design aspects, and test results are described. It was found that the droplet sauter mean diameter was 20 μm. The field tests demonstrated that the power augmentation system operates safely and reliably. Wetness sensors and closed loop PLC proved to be a safe method for power augmentation, which prevents droplet penetration into the compressor inlet.

Experimental Study of Air-Assist Atomizers for Fogging Systems

It is known that the temperature of the ambient air significantly affects the power output of gas turbines. To decrease inlet air temperature water injection at the intake of the turbine is commonly used. Existing air-assist atomizers consume significant more energy in the form of high-pressure air (about 6 bars) as compared to the jet impinging ones. Two new designs of the air-assist atomizers for fogger system are developed and studied. These atomizers differ from the existing ones by special air vortex chamber. It enables to achieve high tangential air velocities and reduce air input pressure. It is shown that at air pressure drop of 0.75–1 bar, the Sauter Mean Diameter of droplets is in the range from 20 to 25 μm when air-to-liquid flow rate ratio is equal to 4:1. Water supply pressure was in the range of 0.5–1 bar during the tests and its value did not markedly affected droplets diameter. It is expected that during optimization of the design and operating condition (air and liquid pressure), the size of the droplets would be further reduced. When air pressure was above 1 bar the droplets diameter was almost not affected by this parameter. Droplet velocity and droplet flux distribution along spray radius were also measured. It was found that the droplets distribution along radius is almost uniform. Spray cone was 30° for one device and 90° for the other.Copyright

The Use of Methanol as an Alternative Fuel: Droplet Formation and Evaporation

Methanol, produced from natural may be considered as an alternative fuel for fossil based liquid fueled gas turbines, especially for land based systems. In the present work, the effect of physical properties of methanol and kerosene on atomization and evaporation are compared. The spray’s liquid flux, droplet sizes and droplet velocities, cone angle were measured using Phase Doppler Particle Analyzer/Laser Doppler Velocimeter (TSI PDPA/LDV) system. Water, kerosene and ethanol (ethanol instead of methanol was used due to the toxicity of methanol) were used and tested at the same input liquid pressures. Analytical analysis of evaporation time for a single droplet of kerosene and methanol showed that the evaporation time is about the same for two fuels with the same droplet diameters. However, due the methanol’s lower calorific value and the fact that its volume flux must be more than twice as much (for similar thermal power), its corresponding evaporation time is longer than for kerosene. The evaporation time for kerosene and methanol, which took into accounts that more methanol should be evaporated, was simulated by CFD. The simulation results showed that methanol spray requires significantly longer distance than kerosene. Thus, the spray of methanol has larger droplet diameter than kerosene and prolonged evaporation time.Copyright

Increasing Operational Stability in Low NOx GT Combustor by a Pilot Flame

The need for NOx reduction in gas turbine (GT) stimulates research for new combustion methods. Lean combustion is a method in which combustion takes place under low equivalence ratio and relatively low combustion temperatures. As such, it has the potential to lower the effect of the relatively high activation energy nitrogen-oxygen reactions which are responsible for substantial NOx formation during combustion processes. Moreover, lowering temperature reduces the reaction rate of the hydrocarbon-oxygen reactions and deteriorates combustion stability. The objective of the present study is to reduce the lower equivalence ratio limit of the stable combustion operational boundary in lean GT combustors. A lean premixed gaseous combustor was equipped with a surrounding concentric pilot flame operating under rich conditions, thus generating a significant amount of reactive radicals. The main combustor’s mixture composition was varied from stoichiometric to lean mixtures. The pilot’s mixture composition varied by changing the air flow rate, within a limited reach mixtures range. The pilot gas flow rate was always lower than five percent of the total gas supply at the specific stage of the experiments. The experiments and simulation showed that despite the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection by the pilot combustion has the potential to lower the limit of the global equivalence ratio (and temperatures) while maintaining stable combustion. Therefore the amount of generated NOx is expected to be significantly reduced as compared to a similar combustor of identical inlet and exit temperatures. In order to study the relevant mechanisms responsible for combustion stabilization, CFD and CHEMKIN simulations were performed to reveal the detailed flow characteristics and their spatial distribution within the combustor. Based on the CFD results, the CHEMKIN model was developed. The CHEMKIN simulations for atmospheric pressure showed satisfactory agreement with experimental results. Further simulation confirmed the advantageous of the technique also at elevated pressures. It is therefore important to understand the relevant mechanisms responsible for combustion stabilization and their spatial distribution within the combustor. The present work discusses an experimental- CFD-CHEMKIN combined approach aimed at studying the influence of radicals generated in the pilot ring combustion on the processes taking place in the main combustor.Copyright

CFD Assisted Design of Micro GT Combustor

The work presents the development of a micro-combustor design, where the combustion process was simulated by CFD and tested experimentally. The inner diameter of the first model was 5.5 mm, the exit diameter 2.5 mm, and the length 24.5 mm. The designed heat release was 200W. Some modifications of the microcombustor were studied. Three-dimensional model for combustion simulations was used. The ‘conjugate heat transfer’ methodology, based on a simultaneous solution of the heat transfer equations for gas and combustor walls, coupled with equations for the working fluid, enabled the prediction of the combustor wall temperatures. To check model convergence 2 simulations with different number of cells were carried out. Effect of heat radiation was also studied by the CFD simulation. The fuel is methane and stoichiometric ratio was simulated. Reactive flow calculations were carried out with a two-step reaction. The analysis of the simulated results was based on the obtained velocity profiles, concentration and temperature distributions within the liner. Preliminary simulations showed that the first combustor design had inefficient combustion. The reason was poor mixing of methane and air inside the mixing chamber and deterioration of the combustion by dilution holes. Consequently, the combustor design was modified and simulated. The simulation showed that the modification significantly improved mixing and combustion process and better combustion was provided. Due to complexity associated with performing combustion experiments in such small dimensions, only limited data could be recorded. A small combustor was manufactured and tests and demonstrated its successful operation. Measurements of temperature and optical UV-VIS-IR - emissions at the combustor exit were obtained. The experimental and simulation results are compared and a good qualitative agreement was found between the experiments and the predicted values.Copyright

A Novel Gas-Assist Atomizer for Power Stations

A novel high flow rate gas-assist atomizer for liquid atomization was developed. The method of liquid supply in the zone of maximal air velocity is used. It is shown that it is possible to achieve fine atomization as the relative velocity between gas and liquid is very high. However actual sprays have droplets with larger size due to the rapid decrease of the difference between air and liquid velocities. So droplets disintegrate mainly due to the turbulent velocity fluctuations of the air flow. The experimental study included two stages: laboratory tests and field tests inside a full size boiler of a 220 MW power station. At the first stage, several atomizer modifications were tested using water and compressed air. Droplet size was measured by a special Laser Light Scattering method. Liquid flow rate was equal to 3500 kg/hr. The liquid atomization quality at each cross-section of the spray was estimated by measuring the liquid-droplets sizes at several stations across the spray. The tests were carried out for two distances, 30 and 40 cm, downstream of the nozzle. The tests show that for the proposed atomizer droplets SMD was reduced from 135 to 67 microns. Droplets SMD maintains constant value when liquid flow rate is reduced by 50%. The spray angle was kept as in a standard atomizer and equal to 110 degrees under all operating conditions. It was found that to obtain this angle, the pressure downstream of the nozzle core should be atmospheric. The atomizer with the best performances was selected for the field tests. It was assumed that the atomizer which shows the best results for air-water mixture would be superior also for steam-fuel mixture. Field tests of the atomizer within the burner of an actual power station in Israel (boiler by Babcock Borsig Company), demonstrated a significant reduction in NOx content, from 540 to 270 ppmv as well as better service conditions.Copyright

Study of Two Miniature Air-Assist Atomizers With Radial and Axial Air Swirlers

The performances of two types of miniature air-assist atomizers were investigated; one with air being directed to the liquid spray through radial-tangential air channels and the other with air supplied through a small axial swirler. The study has shown that droplet size is reduced significantly when the air velocity increases up to about 50 m/s. However, further increase in air velocity has only a weak effect on the droplet size. In the absence of air supply, elevating the liquid pressure causes a reduction in the droplet diameter. The maximum values of the droplet mass flux shifts to the spray periphery with increasing of air velocity. In the air-assist operational regime, the liquid pressure has a slight effect on SMD however; the spray cone angle is increased significantly and can achieve values of up to 120 degrees for low liquid pressure drop. The larger spray angle at comparable droplet size distribution makes the atomizers with the radial air swirlers more favorable for small jet engines.Copyright

Airblast Swirl Atomizer for Small Jet Engines

The current work presents a study of an air blast simplex atomizer which supplies air to the fuel spray through radial channels for liquid film dispersing. The performance of two atomizer configurations using either co-rotating or counter-rotating liquid and air flows was investigated. The change in the direction of liquid swirl was achieved by replacing the inner swirler with a similar one having the holes drilled on the opposite side. Atomizer flow number FNSI in the experiments was 0.74·10−7 . The study has shown that droplet size reduces significantly when the air velocity grows up to 60 m/s but further increase in air velocity has almost no effect on the droplet size. In the absence of air supply the increase in liquid pressure leads to the decrease of Sauter mean diameter (SMD) of droplets from 85 to 60 μm. However, in the air blowing regime the liquid pressure has only slight effect on SMD rising it from 30 to 40 μm. Better atomization performance has been obtained in the case of co-rotation atomizer.Copyright

Low Pressure Fogger System for Power Augmentation of Industrial Gas Turbine

Based on three patented innovations (air-assist atomizer, wetness sensor, closed loop PLC), a new low-pressure power GT augmentation system was developed. Two modifications of air-assist atomizer were tested in the Jet Engine Laboratory of the Technion. The tests were performed to investigate influence of thermodynamic parameters on spray characteristics, as droplet size and velocity distribution of the spray. The system as a whole has passed field test in a gas turbine of a power station. Droplet characteristics, conceptual design aspects and tests results are the subject of this paper.Copyright