Combustion and Oxidation of Lube Oils at Gas Turbine Conditions: Experimental Methods

Because of the high temperatures involved, undesirable ignition events can happen during gas turbine operation, often necessitating expensive down time and repairs. The ignition events are frequently linked to the lubricant, a flammable mixture of large hydrocarbons with a very low vapor pressure. To understand better the role of the lubricant in such ignition events, increased understanding of the fundamental thermal and oxidation characteristics of such oils is needed. To this end, a suite of different tests has been set up and demonstrated at the TEES Turbomachinery Laboratory at Texas A&M University (TAMU) to study various aspects of lubrication oil breakdown and oxidation at elevated temperatures, mostly those related to their coking and ignition behaviors. Five types of tests have been implemented: ignition delay time measurements using a shock tube; hot surface ignition (HSI); autoignition temperature (AIT) determination; thermal cook-off under controlled heating; and a high-temperature coking experiment. Such tests can be used both for fundamental understanding of how lube oils burn and for comparing the reactivity of various types and grades of oil. Each technique at TAMU is briefly described in this paper as they pertain to gas turbine lube oils, and sample results are presented for a common lubrication oil, Mobil DTE 732. For this oil, the HSI tests produced a lowest temperature without ignition of 510°C, and in shock-tube measurements, lower-temperature ignition kinetics are observed below about 1300 K, even at 1 atm. Typical AIT values for oils have been found to be around 370°C but do vary amongst brands, types, and level of degradation. The measured temperatures for the exothermic and boiling events were measured as 166±2 °C and 277±4 °C using the cook-off rig.