Autoignition of Diesel Water Emulsions at Conditions Relevant to Gas Turbine Engine Operation

Venkatasubramanian Athmanathan, Purdue University

Abstract

Lean premixed prevaporixed (LPP) combustion systems are commonly used in gas turbine engines. The advantages of using an LPP system are many including lower NOx emissions and better combustion efficiency. However, LPP systems pose the risk of autoignition and flashback in the premixer. Therefore understanding the autoignition characteristics of fuels used by modern-day fuel flexible stationary gas turbine engines is imperative for the safe operation of land-based gas turbine engines. In a gas turbine premixer, when the fuel-air mixture temperature is sufficiently low, the combustion reaction occurs via a low-temperature chemical reaction pathway called cool flames. When the mixture temperature is sufficiently raised, autoignition occurs and conventional combustion reaction occurs via the high-temperature chemical pathway. Both cool flames and conventional combustion are initiated by ignition processes. In this investigation, the characteristics of the two-stage ignition process in Water-in Diesel Emulsion (WiDE) fuels are studied at gas turbine engine relevant conditions. A well instrumented, high pressure and temperature apparatus, capable of operating at pressures up to 30 bar, at inlet air temperatures up to 920K with an air flow rate of up to 2.26 kg/s, has been designed and built to study combustion processes relevant to gas turbines. In this work, autoignition and cool flames have been studied at temperatures, bulk velocities, water-fuel-ratios, global equivalence ratios of relevance to gas turbines, for a base reactor pressure of Pb. The transition from cool flame to autoignition has also been studied for pressures Pb, 1.33Pb and 1.50Pb. A variety of cool-flames and autoignition events that occur at engine relevant conditions have been observed. Cool-flames typically show a gradual but sustained pressure rise of 5–10% above the operating pressure while a sudden pressure rise of two or three times the operating pressure occurs during autoignition events. For a range of operating conditions, cool flames are sustainable through the maximum testing time (5–7 seconds) of the current experiments. A flange housing radially symmetric cartridge heaters, designed to emulate realistic local hot spots is placed at several axial locations downstream of the injector to study their effects on ignition propensity. Experiments were carried out with and without wall heating. At each operating condition, a test was repeated at least 3 times, to ensure reliability and repeatability of the observed phenomena. At each condition, the air temperature was gradually increased until a cool-flame was observed or autoignition occurred. Sustainable cool-flames in the premixer were observed for the first time in this investigation. These cool flames produce partial oxidation products for extended periods of time (5-7 seconds), which when combusted, can lead to reduced emissions from the combustion device.

Degree

M.S.M.E.

Advisors

Gore, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering|Energy

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