D. R. Ballal

Gas Turbine Combustion : Alternative Fuels and Emissions, Third Edition

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Design and Development of a Research Combustor for Lean Blow-Out Studies

In a modern aircraft gas turbine combustor, the phenomenon of lean blow-out (LBO) is of major concern. To understand the physical processes involved in LBO, a research combustor was designed and developed specifically to reproduce recirculation patterns and LBO processes that occur in a real gas turbine combustor. A total of eight leading design criteria were established for the research combustor. This paper discusses the combustor design constraints, aerothermochemical design, choice of combustor configurations, combustor sizing, mechanical design, combustor light-off, and combustor acoustic considerations that went into the final design and fabrication. Tests on this combustion reveal a complex sequence of events such as flame lift-off, intermittency, and onset of axial flame instability leading to lean blowout. The combustor operates satisfactorily and is yielding benchmark quality data for validation and refining computer models for predicting LBO in real engine combustors.

Studies of Jet Fuel Thermal Stability in a Flowing System

A flowing, single-pass heat exchanger test rig, with a fuel capacity of 189 liters, has been developed to evaluate jet fuel thermal stability. This “Phoenix Rig” is capable of supplying jet fuel to a 2.15 mm i.d. tube at a pressure up to 3.45 MPa, fuel temperature up to 900 K, and a fuel-tube Reynolds number in the range 300–11,000. Using this test rig, fuel thermal stability (carbon deposition rate), dissolved oxygen consumption, and methane production were measured for three baseline jet fuels and three fuels blended with additives. Such measurement were performed under oxygen-saturation or oxygen-starved conditions. Tests with all of the blended fuel samples showed a noticeable improvement in fuel thermal stability. Both block temperature and test duration increased the total carbon deposits in a nonlinear fashion. Interestingly, those fuels that need a higher threshold temperature to force the consumption of oxygen exhibited greater carbon deposits than those that consume oxygen at a lower temperature. These observations suggested a complicated relationship between the formation of carbon deposits and the temperature-driven consumption of oxygen. A simple analysis, based on a bimolecular reaction rate, correctly accounted for the shape of the oxygen consumption curve for various fuels. This analysis yielded estimates of global bulk parameters of oxygen consumption. The test rig yielded quantitative results, which will be very useful in evaluating fuels additives, understanding the chemistry of deposit formation, and eventually developing a global chemistry model.

Aerodynamics of bluff-body stabilized confined turbulent premixed flames

Detailed information on the influence of geometric and flow parameters on the structure and properties of recirculation zone in confined combusting flows is not available. In this paper, recirculation zone structure and turbulence properties of methane-air mixtures downstream of several conical flameholders were measured using LDA. These tests employed different blockage ratios (13 and 25 percent), cone angles (30, 45, 60, and 90 deg), equivalence ratios (0.56, 0.65, 0.8, and 0.9), mean annular velocities (10, 15, and 20 m/s), and approach turbulence levels (2, 17, and 22 percent). It was found that increasing the blockage ratio and cone angle affected the recirculation zone size and shape only slightly. Also, these parameters increased the shear stress and turbulent kinetic energy (TKE) moderately. Increasing the equivalence ratio or approach turbulence intensity produced a recirculation zone shape very similar to that found in the cold flow. TKE decreased due to turbulent dilatation produced by increased heat release. These observations are discussed from the viewpoint of their importance to practical design and combustion modeling.

Turbulent Combustion Properties Behind a Confined Conical Stabilizer

This paper reports that turbulence properties were investigated in and around the recirculation zone produced by a 45 deg conical flame stabilizer of 25 percent blockage ratio confined in a pipe supplied with a turbulent premixed methane-air mixture at a Reynolds number of 5.7 {times} 10{sup 4}. A three-component LDA system was used for measuring mean velocities, turbulence intensities, Reynolds stresses, skewness, kurtosis, and turbulent kinetic energy. It was found that wall confinement elongates the recirculation zone by acceleration the flow and narrow it by preventing means streamline curvature. For confined flames, turbulence production is mainly due to shear stress-mean strain interaction. In the region of maximum recirculation zone width and around the stagnation point, the outer stretched flame resembles a normal mixing layer and gradient-diffusion closure for velocity holds. However, an in the absence of turbulent heat flux data, counter gradient diffusion cannot be ruled out. Finally, and because of the suppression of mean streamline curvature by confinement, in combustion flow, the production of turbulence is only up to 33 percent of its damping due to dilatation and dissipation.

Effects of Oxygen and Additives on the Thermal Stability of Jet Fuels

A previously developed flowing single-pass heat-exchanger test rig (Phoenix rig) has been used to evaluate the effectiveness of various additives and the kinetic mechanism of both deposit formation and oxygen consumption. The Phoenix rig has been modified to include not just a heated single tube, but also a cooling test section and both hot and cold filters. The effects of flow conditions, antioxidants, and metal deactivator additives on the location and amount of the deposit are discussed. In general antioxidants were effective at reducing the deposits on the hot test section, but almost invariably caused increased plugging of cool downstream filters. Downstream plugging of cool filters also increased with decreased temperatures in the heated section or with increased flow. Tests with both oxygen-saturated and oxygen-depleted fuels have shown that the solubility of oxygen is linearly related to the fraction of oxygen in a sparge gas, and that the amount of deposit is linearly related to the total quantity of dissolved oxygen passed. Finally, in contrast to initial modeling efforts, the consumption of oxygen is shown to be significantly more complex than a simple bimolecular, pseudo-first-order in oxygen, process. It is found to be much closer to pseudo-zero-order in the early stages, decaying to pseudo-firt-order when the oxygen nears depletion

Lean Blowout in a Research Combustor at Simulated Low Pressures

A propane-fueled research combustor has been designed to represent the essential features of primary zones of combustors for aircraft gas turbine engines in an investigation of lean blowouts. The atmospheric pressure test facility being used for the investigation made it difficult to approach the maximum heat release condition of the research combustor directly. High combustor loadings were achieved through simulating the effects on chemical reaction rates of subatmospheric pressures by means of a nitrogen diluent technique. A calibration procedure is described, and correlated experimental lean blowout results are compared with well-stirred reactor calculations for the research combustor to confirm the efficacy of the calibration.

Isothermal Flow Fields in a Research Combustor for Lean Blowout Studies

This paper reports on a propane-fueled research combustor which has been designed and developed to investigate lean blowouts in a simulated primary zone of the combustors for aircraft gas turbine engines. To understand the flow development better and to ensure that the special provisions in the combustor for optical access did not introduce undue influence, measurements of the velocity fields inside the combustor were made using laser-Doppler anemometry. These measurements were made in isothermal, constant density flow to relate the combustor flow field development to known jet behavior and to backward-facing step experimental data in the literature. The major features of the flow field appear to be consistent with the expected behavior, and there is not evidence that the provision of optical access adversely affected the flows measured.

Studies of Jet Thermal Stability in a Flowing System

A flowing, single-pass heat exchanger test rig, with a fuel capacity of 189 litres, has been developed to evaluate jet fuel thermal stability. This so called, “Phoenix Rig” is capable of supplying jet fuel to a 2.15 mm I.D. tube at a pressure up to 3.45 MPa, fuel temperature up to 900K, and a fuel-tube Reynolds number in the range 300–11,000. Using this test rig, fuel thermal stability (carbon deposition rate), dissolved oxygen consumption, and methane production were measured for three baseline jet fuels and three fuels blended with additives. Such measurement were performed under oxygen-saturation or oxygen-starved conditions.Tests with all of the blended fuel samples showed a noticeable improvement in fuel thermal stability. Both block temperature and test duration increased the total carbon deposits in a nonlinear fashion. Interestingly, those fuels that need a higher threshold temperature to force the consumption of oxygen exhibited greater carbon deposits than those that consume oxygen at a lower temperature. These observations suggested a complicated relationship between the formation of carbon deposits and the temperature-driven consumption of oxygen. A simple analysis, based on a bi-molecular reaction rate, correctly accounted for the shape of the oxygen consumption curve for various fuels. This analysis yielded estimates of global bulk parameters of oxygen consumption. The test rig yielded quantitative results which will be very useful in evaluating fuel additives, understanding the chemistry of deposit formation, and eventually developing a global chemistry model.Copyright

Design and Development of a Research Combustor for Lean Blowout Studies

In a modern annular aircraft gas turbine combustor, the phenomenon of lean blow out (LBO) is of major concern. To understand the physical processes involved in LBO, a research combustor was designed and developed to specifically reproduce recirculation patterns and LBO processes that occur in a real gas turbine combustor.A total of eight leading design criteria were established for the research combustor. This paper discusses the combustor design constraints, aerothermochemical design, choice of combustor configurations, combustor sizing, mechanical design, combustor light-off, and combustor acoustic considerations that went into the final design and fabrication. Tests on this combustor reveal a complex sequence of events such as flame lift-off, intermittency, and onset of axial flame instability leading to lean blowout. The combustor operates satisfactorily and is yielding benchmark quality data for validating and refining computer models for predicting LBO in real engine combustors.© 1990 ASME