Turbojet Engine Altitude Relight Studies

Transportation & Aerospace

Dr M.F. Bardon, BEng, MEng, PhD, PEng

Jet engine altitude relight studies are being conducted at the Department of Mechanical Engineering's Jet Test Cell. In a specially designed sector rig, which includes a section from an actual General Electric J85 jet engine combustion chamber, conditions of reduced pressure and high velocity air flow are attainable, simulating high altitude engine flame out scenarios. Working at such conditions in the test sector rig, parameters linked to the restarting of gas turbines at high altitudes following a flame out condition are studied. As such, relight envelopes are determined for various altitude simulations as a function of fuel type, fuel flow, ignition energy, etc. The study is chiefly aimed at evaluating the impact of the NATO single fuel (Jet A-1) on ignition processes for marginal (high altitude) conditions. To this end, both stock and enhanced ignition circuitry are utilized in the experimentation within the test rig.

The sector rig is a component of a larger ejector system, and as such provides a secondary flow to another channel exterior to the test cell building through which compressed air derived from two gas turbine auxiliary power units (APUs) is flowing. As air is drawn from the laboratory space through the sector rig into this primary ejector flow, a pressure reduction within the sector results. This sector rig/ejector system has been constructed and tested at RMC. It is currently capable of creating an inlet total pressure of 0.44 atm and a combustor static pressure below 0.43 atm, which corresponds to an altitude of over 21,000 feet (6.4 km).

Following detailed combustion chamber pressure vs. air flow rate calibration procedures, tests to identify the limits of ignitability of Jet A-1 and JP 4 fuel in the sector rig were conducted. By systematically adjusting the fuel flow rate, the altitude relight limits and lean blowout fuel flow rates were determined for various airflow settings. These tests were conducted using stock gas turbine ignition circuitry. These steps will be repeated using custom programmable ignition circuitry. Variations in ignition limits will be determined as a function of ignition energy levels, as well as of spark current waveforms. Alternative technologies may also be investigated, such as plasma jet ignition.

Relationships between fuel droplet distributions and mixture ignitability will be studied using laser-based methods including phase/doppler anemometry (PDA). The use of high speed (up to 8000 frames per second) digital video equipment will also be used to study flame kernel development within the sector rig. Methods for simulating higher altitudes with or without corresponding lower temperatures are also being considered. The incorporation of additional APUs to the ejector system would boost primary flow and allow a greater pressure drop within the sector rig.

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