This paper evaluates the effect of cooling air mass flow on turbine efficiency in the AL-31F engine using an energy balance and the turbine stage work equation. The analysis covers corrected low-pressure spool speeds from 70 to 105 percent and compares an uncooled reference with cooling cases labeled 1 and 3 percent. The 3 percent case produces a larger efficiency penalty throughout the investigated range. Approximate relative reductions read from the calculated curves are 6⁠–9 percent and 1⁠–2 percent for secondary efficiency in the 3 and 1 percent cases, respectively. The corresponding reductions in effective efficiency are approximately 7⁠–9 percent and 1.5⁠–2 percent. The efficiency curves show local variations with speed, including a recovery near the 100 percent operating point. Including the available energy of the coolant lowers effective efficiency relative to secondary efficiency. The results describe aerodynamic and energy penalties associated with cooling; determining an optimum coolant flow additionally requires a validated heat transfer model and material temperature limits.

Huu Noi Truong, Van Huong Ngo, Trong Son Phan,"Effect of Cooling Air Mass Flow on Turbine Efficiency in the AL-31F Engine" International Journal of Advanced Multidisciplinary Research and Educational Development, V2(5): Page(817-819) September - October 2026. ISSN: 3107-6513. www.ijamred.com. Published by Scientific and Academic Research Publishing.