Experiments on Flame Flashback in a Quasi-2D Turbulent Wall Boundary Layer for Premixed Methane-Hydrogen-Air Mixtures

Premixed combustion of hydrogen-rich mixtures involves the risk of flame flashback through wall boundary layers. For laminar flow conditions, the flashback mechanism is well understood and is usually correlated by a critical velocity gradient at the wall. Turbulent transport inside the boundary layer considerably increases the flashback propensity. Only tube burner setups have been investigated in the past and thus turbulent flashback limits were only derived for a fully-developed Blasius wall friction profile. For turbulent flows, details of the flame propagation in proximity to the wall remain unclear. This paper presents results from a new experimental combustion rig, apt for detailed optical investigations of flame flashbacks in a turbulent wall boundary layer developing on a flat plate and being subject to an adjustable pressure gradient. Turbulent flashback limits are derived from the observed flame position inside the measurement section. The fuels investigated cover mixtures of methane, hydrogen and air at various mixing ratios. The associated wall friction distributions are determined by RANS computations of the flow inside the measurement section with fully resolved boundary layers. Consequently, the interaction between flame back pressure and incoming flow is not taken into account explicitly, in accordance with the evaluation procedure used for tube burner experiments. The results are compared to literature values and the critical gradient concept is reviewed in light of the new data.

Emissions and Noise Pollution Diagnosis of a Turbogas Power Plant During Commissioning Service

This work is focused on the diagnosis of behavior, from the point of view of control emissions and noise level, of a power Turbogas plant during the process of commissioning, to guarantee that its operation complies with national and international standards. The environmental diagnosis of the power plant was developed as part of the performance evaluation of the unit. The conditions of the unit evaluation include operation at base load and partial load, as well as time periods for load changes. The evaluated power plant consists of an aeroderivative gas turbine installed in a simple cycle, operating with a cooling system (chiller) installed in the urban zone of Mexico City. Therefore, it should comply with the legislation and regulations of the city concerning air pollution and allowed noise, besides the international standards established by the manufacturer. The study includes emissions measurements using a Continuous Emissions Monitoring System installed in-situ, previously calibrated and checked during and after the test which was found inside the permissible deviation of 3%. Measurements were recorded at intervals of 5 minutes during test periods of 110 minutes for each load and 45 minutes for load changes. On the other hand, noise pressure evaluation was carried out in near field as well as far field produced by the power plant during operation. Measurements were carried out by using precision instruments installed specifically for it. A temporary system for obtaining data was used to monitoring the environmental conditions every 30 seconds. It was possible to verify that the turbogenerator complies with all noise levels and contaminant emissions requirements and regulations according to the limits established by the manufacturer and national and international standards.

Effects of Fuel Nozzle Condition on Gas Turbine Combustion Chamber Exit Temperature Distributions

The effects of fuel nozzle condition on the temperature distributions experienced by the nozzle guide vanes have been investigated using an optical patternator. Average spray cone angle, symmetry, and fuel streaks were quantified. An ambient pressure and temperature combustion chamber test rig was used to capture exit temperature distributions and to determine the pattern factor. The rig tests matched representative engine operating conditions by matching Mach number, equivalence ratio, and fuel droplet size. It was observed that very small deviations (± 10° in spray cone angle) from a nominal distribution in the fuel nozzle spray pattern correlated to increases in pattern factor, apparently due to a degradation of mixing processes, which created larger regions of very high temperature core flow and smaller regions of cooler temperatures within the combustion chamber exit plane. The spray cone angle had the most measureable influence while the effects of spray roundness and streak intensity had slightly less influence. Comparisons were made with published studies conducted on the combustion chamber geometry, and recommendations were made for fuel nozzle inspections.

Numerical Simulation of Combustion Induced Noise Using LES and Computational Aeroacoustics

Many technical combustion devices are susceptible to thermoacoustic instabilities. In this work, the noise emission by a turbulent jet flame is analyzed by means of a hybrid LES/CAA (Large Eddy Simulation/Computational Aero Acoustics) approach as a first step towards a numerical investigation of combustion instability. The hybrid LES/CAA approach is based on a LES of the reactive flow utilizing a low Mach number formulation. Within the CAA part of the simulations, linearized Euler equations (LEE) are solved. A simplified formulation to describe the thermoacoustic sound sources is extracted from the reactive LES. For the present study, the CFD code FASTEST is coupled with the aeroacoustic simulation tool PIANO. The two solvers are combined to a single tool for the description of the acoustics of reacting flows. Both codes make use of geometry flexible grids enabling the simulation of complex geometries commonly used within technical combustion systems.

Investigations of Gaseous Alternative Fuels at Atmospheric and Elevated Temperature and Pressure Conditions

Increasing interest in alternative fuels for gas turbines stimulates research in gaseous fuels other than natural gas. Various gas mixtures, based on methane as the main component, are considered as possible fuels in the future. In particular, methane enrichment with hydrogen or dilution with carbon dioxide is of considerable interest. Some experiments and numerical calculations have been undertaken to investigate methane-hydrogen and methane-carbon dioxide gas flames, however most of these investigations are limited by particular pressure or temperature conditions. This paper presents the investigation of the combustion of methane–carbon dioxide mixtures at atmospheric and elevated temperature and pressure conditions. Two experimental rigs were used, a Bunsen burner and swirl burner. Bunsen burner experiments were performed in the High Pressure Optical Chamber, which is located within the Gas Turbine Research Centre of Cardiff University — at 3 bara and 7 bara pressure, and 473 K, 573 K and 673 K temperature conditions for lean and rich mixtures. Planar Laser Tomography (PLT) was applied to investigate turbulent burning velocity. Burning velocity of the gas mixture was calculated using two different image processing techniques and the difference in the results obtained using these two techniques is presented and discussed. Laser Doppler anemometry (LDA) was utilised to define turbulence characteristics such as turbulence intensity and integral length scale. Due to the variability of the velocity flow field and turbulence intensity across Bunsen burners, the importance of measuring position and conditions is discussed. The sensitivity of this variance on the flame regime as defined in the Borghi diagram is evaluated. In the second part of the study, a generic swirl burner was used to define the flame flashback limits for methane–carbon dioxide mixtures at atmospheric conditions. The gas mixture stability graphs are plotted, and the effect of CO2 addition are discussed.

Active Control of Combustion Instabilities in a Matrix Burner Using Primary and Pilot Fuel and Acoustic Forcing

Lean premixed flames applied in modern gas turbines leads to reduce NOx emissions, but at the same time they are more susceptible to combustion instabilities than diffusion flames. These oscillations cause pressure fluctuations with high amplitudes and unacceptable noise as well as the risk of component or even engine failure. They can lead to pockets of fuel being formed in the mixing chamber and to bad mixing, which leads to increase in emissions. This paper reports the successful decoupling of the pressure and heat release inside the combustion chamber of a matrix burner using two actuation techniques. This led to the successful attenuation of the dominant instability modes occurring inside the combustor of the matrix burner. In the first case, acoustic forcing was used to decouple the pressure and the heat release inside the combustor. This was achieved by using a loudspeaker to modulate the primary air mass flow. This was followed by using acoustic forcing in CFD to decouple the pressure and heat release inside the combustor. For the action of the loudspeaker, sinusoidal forcing was used to mimic the modulation action of the diaphragm of the loudspeaker. In the second case, a fast gaseous “on-off” injector was used to modulate the primary fuel mass flow. After this, pilot fuel modulation was used to stabilize the flame. The control law governing the primary and pilot fuel modulation is discussed in details. The effect of open loop control on NOx emissions in the burner is also reported and discussed.

An Experimental Ignition Delay Study of Alkane Mixtures in Turbulent Flows at Elevated Pressure and Intermediate Temperatures

Autoignition delay times of mixtures of alkanes and natural gas were studied experimentally in a high pressure and intermediate temperature turbulent flow reactor. Measurements were made at pressures between 7 and 15 atm and temperatures from 785 to 935K. The blends include binary and ternary mixtures of methane, ethane and propane; along with various natural gas blends. Based on these data, the effect of higher hydrocarbons on the ignition delay time of natural gas type fuels at actual gas turbine engine conditions has been quantified. While the addition of higher hydrocarbons in quantities of up to 30% were found to reduce the ignition delay by up to a factor of four, the delay times were still found to be greater than 60 milliseconds in all cases which is well above the residence times of most engine premixers. The data were used to develop simple Arrhenius type correlations as a function of temperature, pressure and fuel composition for design use.

CFD Aerodynamic and Reactive Study of an Innovative Lean Combustion System in the Frame of the NEWAC Project

This paper deals with the very last activities carried out by EnginSoft in the frame of the EU funded research programme NEWAC. The work regards the pre-production numerical tests performed on the single annular combustor with the purpose of verify its performance in reactive frame. The core of this study is the innovative lean-burn injection system technology, developed by University of Karlsruhe and AVIO for medium OPR. Such device has been widely investigated in previous activities in order to optimise the combustor layout and the numerical procedure for this work [1].

A Study on the Flowfield of an Innovative Z-Flowpath Gas Turbine Combustor

Reverse flow combustors were widely used in small and micro gas turbine engines. The wall area of this type of combustors was quite large. And there were two flow turning points in their flow-path. Thus the wall cooling and main flow dilution were two intrinsic problems for them. Apart from that, their high pressure losses and heavy weight were also two problems which seriously deteriorate the performance of the engines. Moreover, their primary hole jets on opposite walls were non-symmetrical, which would affect the stability and intensity of the recirculation flows. In order to improve the combustion performance, a new conceptual Z-flowpath combustor was proposed. The new combustor consisted of two 45 degree yawing instead of returning in the main flow-path. The flowfield of the new combustor was predicted by the commercial code FLUENT, after a validation for the flowfield in a model reverse flow combustor with previous experimental results. The prediction showed that the flowfield of the primary zone in the Z-flowpath combustor was highly symmetrical, the size and the intensity of the recirculation zone were about 10 and 2 times greater than the normal reverse flow combustor, respectively, while the pressure loss and the total area of the flame tube wall of the Z-flowpath combustor were decreased dramatically to be 69.4% and 51% of that in the reverse flow combustor, respectively.

Combustion Dynamics in a Gas Turbine Single Annular Combustor Sector

The occurrence of combustion instability dynamics known, as “screech, howl and growl,” in the combustors of gas turbine engines is a very difficult challenge for engineers. The very high amplitude pressure oscillations caused by combustion dynamics, are not only detrimental to the operation of the engine and combustor, but the difficulty in predicting and remedying these problems can lead to significant costs and delays in engine development. The coupling of the unsteady heat release in the flame with the natural acoustic resonance modes of the combustor duct causes the phenomena of combustion dynamics. To improve our understanding of stability characteristics in such complex systems, encountered in many industrial applications, the flame structure of an atmospheric swirl-stabilized burner, containing dilution and cooling air holes and fed with natural gas fuel, was systematically investigated for various inlet temperatures, pressure drops and air-fuel ratios. Experiments were also designed and conducted with the goal to understand better the phenomena of combustion dynamics that were experienced. More specifically, six acoustic pressure transducers were incorporated in the combustor and in the upstream duct to measure the acoustic field and the acoustic impedance characteristics at specified locations of interest. A one-dimensional wave propagation model is presented to predict the acoustic frequencies and damping of resonance modes, based on the geometry of the test rig, the flow conditions, and the acoustic impedance characteristics of the terminations of the combustor. This paper will present the acoustic analysis of the test data in the light of the above-mentioned theoretical modeling. The limitations of the current test rig are pointed out and changes in the rig design are discussed for future research.