Re: Water injection ?

From: (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.