Meredith effect in Supermarine Spitfire > .
Rolls-Royce Merlin - one of the most important engines of WW2 > .
The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid such as ethylene glycol. Typically the fluid is a coolant carrying waste heat from an internal combustion engine.
The duct must be travelling at a significant speed with respect to the air for the effect to occur. Air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect. As the air flows through the radiator it is heated, raising its temperature slightly and further increasing its volume. The hot, pressurised air then exits through the exhaust duct which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. The air expands and decreases temperature as it passes along the duct, before emerging to join the external air flow. Thus, the three processes of an open Brayton cycle are achieved: compression, heat addition at constant pressure and expansion. The thrust obtainable depends upon the pressure ratio between the inside and outside of the duct and the temperature of the coolant. The higher boiling point of ethylene glycol compared to water allows the air to attain a higher temperature increasing the specific thrust.
If the generated thrust is less than the aerodynamic drag of the ducting and radiator, then the arrangement serves to reduce the net aerodynamic drag of the radiator installation. If the generated thrust exceeds the aerodynamic drag of the installation, then the entire assemblage contributes a net forward thrust to the vehicle.
The North American P-51 Mustang, which first flew in 1940, adopted both the Merlin engine and the Meredith principle.
Around this time the Meredith effect also inspired early American work on the aero-thermodynamic duct or ramjet, due to the similarity of their principles of operation.
The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid such as ethylene glycol. Typically the fluid is a coolant carrying waste heat from an internal combustion engine.
The North American P-51 Mustangmakes significant use of the Meredith effect in its belly radiator design.
The duct must be travelling at a significant speed with respect to the air for the effect to occur. Air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect. As the air flows through the radiator it is heated, raising its temperature slightly and further increasing its volume. The hot, pressurised air then exits through the exhaust duct which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. The air expands and decreases temperature as it passes along the duct, before emerging to join the external air flow. Thus, the three processes of an open Brayton cycle are achieved: compression, heat addition at constant pressure and expansion. The thrust obtainable depends upon the pressure ratio between the inside and outside of the duct and the temperature of the coolant. The higher boiling point of ethylene glycol compared to water allows the air to attain a higher temperature increasing the specific thrust.
If the generated thrust is less than the aerodynamic drag of the ducting and radiator, then the arrangement serves to reduce the net aerodynamic drag of the radiator installation. If the generated thrust exceeds the aerodynamic drag of the installation, then the entire assemblage contributes a net forward thrust to the vehicle.
The North American P-51 Mustang, which first flew in 1940, adopted both the Merlin engine and the Meredith principle.
Around this time the Meredith effect also inspired early American work on the aero-thermodynamic duct or ramjet, due to the similarity of their principles of operation.
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