Fuel Spray Characteristics of Gas Turbine Fuel Injectors at Cold Start Conditions

Engine starting at extremes of cold ambient and fuel temperatures are a concern to both military and commercial operators of jet engines, but the military in particular needs the capability to quick start under very cold conditions. The ongoing worldwide conversion to JP8 fuel is compromising this ability for older engines developed for use on wide-cut JP4 fuel. The greatly reduced volatility and increased viscosity of JP8 fuel significantly impacts fuel spray angle and droplet size at start conditions resulting in over-long and aborted starts in cold climates. A program was carried out by Pratt & Whitney and Textron Fuel Systems to investigate and improve the spray qualities of a dual orifice pressure atomizing fuel injector of a commonly used military jet engine in the start to low power range. The primary fuel injector portion was modified to produce a finer droplet size and wider spray angle. A hybrid design with an aerating secondary was also tested. Droplet diameter and spray angles were measured to −40°F (−40°C) temperatures. Variations in fuel flow rate and fuel type were also investigated. It was found that cold fuel had an adverse effect on spray qualities for the all pressure atomizing nozzles, but the hybrid injector was less affected and produced superior sprays at all conditions on the primary portion. Improvements in the all pressure atomizing design resulted in lesser but still significant benefits.