From: firstname.lastname@example.org (DMacDoug) Organization: America Online, Inc. (1-800-827-6364) Date: 31 Oct 95 00:31:28 References: 1
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You are correct in your assumption that the mass of the water itself increases power of the engine--it does indeed increase the total mass flow thru the engine, and thereby increases engine thrust. The largest effect however is cooling of the airflow through the compressor via evaporative effect of the "water". This increases the aerodynamic efficiency of the compressor. In turboprop systems I am familiar with, the liquid is a water-methanol mixture (typically 30%-40% methanol) such that the heating value of the alcohol offsets the thermal energy required to evaporate the water. In this way, the turbine temperature of the engine is not greatly affected when the water-methanol injection is turned on. It is primarily used to boost takeoff performance under hot/high conditions, and can provide a power boost on the order of 20% over "dry" performance. The aircraft typically has a regulated flow system which is actuated by the pilot, and enough water-methanol tank capacity to provide several minutes of augmentation (enough for 2-3 takeoffs). The water injection systems on the early turbojets used on 707's or DC-8's may be different, I am not familiar with them.