D. R. Ballal

Bio: D. R. Ballal is an academic researcher from Cranfield University. The author has contributed to research in topics: Extinction (optical mineralogy). The author has an hindex of 1, co-authored 1 publications receiving 92 citations.

Combustors for micro-gas turbine engines

Abstract: The development ofa hydrogen-air microcombustor is described. The combustor is intended for use in a 1 mm 2 inlet area, micro-gas turbine engine. While the size of the device poses several difficulties, it also provides new and unique opportunities. The combustion concept investigated is based upon introducing hydrogen and premixing it with air upstream of the combustor. The wide flammability limits of hydrogen-air mixtures and the use of refractory ceramics enable combustion at lean conditions, obviating the need for both a combustor dilution zone and combustor wall cooling. The entire combustion process is carried out at temperatures below the limitations set by material properties, resulting in a significant reduction of complexity when compared to larger-scale gas turbine combustors. A feasibility study with initial design analyses is presented, followed by experimental results from 0.13 cm 3 silicon carbide and steel microcombustors. The combustors were operated for tens of hours, and produced the requisite heat release for a microengine application over a range of fuel-air ratios, inlet temperatures, and pressures up to four atmospheres. Issues of flame stability, heat transfer, ignition and mixing are addressed. A discussion of requirements for catalytic processes for hydrocarbon fuels is also presented.

High Power Density Silicon Combustion Systems for Micro Gas Turbine Engines

Abstract: As part of an effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, fabrication, experimental testing, and modeling of the combustion system. Two radial inflow combustor designs were examined; a single-zone arrangement and a primary and dilution-zone configuration. Both combustors were micromachined from silicon using deep reactive ion etching (DRIE) and aligned fusion wafer handing. Hydrogen-air and hydrocarbon-air combustion were stabilized in both devices, each with chamber volumes of 191 mm 3 . Exit gas temperatures as high as 1800 K and power densities in excess of 1100 MW/m 3 were achieved. For the same equivalence ratio and overall efficiency, the dual-zone combustor reached power densities nearly double that of the single-zone design. Because diagnostics in microscale devices are often highly intrusive, numerical simulations were used to gain insight into the fluid and combustion physics. Unlike large-scale combustors, the performance of the microcombustors was found to be mole severely limited by heat transfer and chemical kinetics constraints. Important design trades are identified and recommendations for microcombustor design are presented.

Lean blowout model for a spray-fired swirl-stabilized combustor

Abstract: In aero-engine applications, the lean blowoff (LBO) limit plays a critical role in the operational envelope of the engine. The geometry of the combustion chamber primary zone plays a critical role in establishing LBO limits. This is especially true for advanced lean burn concepts which introduce the majority of the combustion air in a manner designed to enhance the rate of mixing with the fuel. In the present study, specific mixer hardware has been designed to develop a systematic, statistically sound test matrix to study the effect of mixer components (primary swirl vane, secondary swirl vane, Venturi, and co-and counterswirl) on LBO. A strategy is employed to develop, based on an existing model, a new predictive model for LBO which accounts for a heterogeneous swirl-stabilized reaction and explicitly relates the geometry of the hardware to the LBO limit. The model predicts the LBO fuel/air ratio at three operating temperatures to within 14% of the measured value. The multivariate experiments used to relate LBO to geometry were also further analyzed to establish the main hardware parameters affecting LBO. Specifically, the Venturi and swirl sense (co- versus counter-swirl) were found to impact LBO at lower air inlet temperatures (294 and 366 K). The Venturi and counter-swirl enhance the atomization and mixing processes which are more rate limiting at lower temperatures, and, as a result, improve stability. At a higher inlet air temperature (477 K), the secondary swirl vane angle also plays a role in determining LBO, with larger angles (75°) generating better stability, which is associated with a stronger recirculation zone. The hardware configuration with the best LBO performance over the different conditions was identified (45° primary swirler, 55° secondary swirler, counter-swirl with Venturi).