The Feasibility of Water Injection Into the Turbine Coolant to Permit Gas Turbine Contingency Power for Helicopter Application

Abstract

A system which would allow a substantially increased output from a turboshaft engine for brief periods in emergency situations with little or no loss of turbine stress rupture life is proposed and studied analytically. The increased engine output is obtained by turbine overtemperature; however, the temperature of the compressor bleed air used for hot section cooling is lowered by injecting and evaporating water. This decrease in cooling air temperature can offset the effect of increased gas temperature and increased shaft speed and thus keep turbine blade stress rupture life constant. The analysis utilized the Navy NASA Engine Program or NNEP computer code to model the turboshaft engine in both design and off-design modes. This report is concerned with the effect of the proposed method of power augmentation on the engine cycle and turbine components. A simple cycle turboshaft engine with a 16:1 pressure ratio and a 1533 K (2760° R) turbine inlet temperature operating at sea level static conditions was studied to determine the possible power increase and the effect on turbine stress rupture life that could be expected using the proposed emergency cooling scheme. The analysis showed a 54 percent increase in output power can be achieved with no loss in gas generator turbine stress rupture life. A 231 K (415° F) rise in turbine inlet temperature is required for this level of augmentation. The required water flow rate was found to be 0.0109 kg water per kg of engine air flow. For a 4.474 MW (6000 shp) engine this would require 32.26 kg (71.13 lbm) of water for a 2.5 min transient. At this power level, approximately 25 percent of the uncooled power turbine life is used up in a 2 1/2-min transient. If the power turbine were cooled, this loss of stress-rupture life could be reduced to zero. Also presented in this report are the results of an analysis used to determine the length of time a ceramic thermal barrier coating would delay the temperature rise in hot parts during operation at elevated temperatures. It was hoped that the thermal barrier could be used as a scheme to allow increased engine output while maintaining the life of hot section parts during short overtemperature transients. The thermal barrier coating was shown to be ineffective in reducing blade metal temperature rise during a 2.5-min overtemperature.