
The Meredith effect is a phenomenon whereby the
aerodynamic drag
In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
produced by a
cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced by the expansion of the hot air in the duct. The effect was discovered in the 1930s and became more important as the speeds of piston-engined aircraft increased over the next decade.
The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid. Typically the fluid is a coolant carrying
waste heat from an internal combustion engine.
[Becker, J.; "''The high-speed frontier: Case histories of four NACA programs, 1920-1950''," SP-445, NASA (1980), Chapter 5: High-speed Cowlings, Air Inlets and Outlets, and Internal-Flow Systems: The ramjet investigation]
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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 intake, ram air effect. As the air flows through the radiator it is heat
In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
ed, raising its temperature slightly and 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.[Meredith, F. W]
Cooling of Aircraft Engines With Special Reference To Ethylene Glycol Radiators Enclosed In Ducts
Aeronautical Research Council R&M 1683, 1936. 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 Meredith effect inspired the early American work on the aero-thermodynamic duct or ramjet, due to the similarity of their principles of operation. In more recent times the phenomenon has been utilised in racing cars by mounting the engine cooling radiators in tunnels.
History
F. W. Meredith was a British engineer working at the Royal Aircraft Establishment
The Royal Aircraft Establishment (RAE) was a British research establishment, known by several different names during its history, that eventually came under the aegis of the Ministry of Defence (United Kingdom), UK Ministry of Defence (MoD), bef ...
(RAE), Farnborough. Reflecting on the principles of liquid cooling, he realized that what was conventionally regarded as waste heat, to be transferred to the atmosphere by a coolant in a radiator, need not be lost. The heat adds energy to the airflow and, with careful design, this may be used to generate thrust. The work was published in 1936.
The phenomenon became known as the "Meredith effect" and was quickly adopted by the designers of prototype fighter aircraft then under development, including the Supermarine Spitfire
The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and other Allies of World War II, Allied countries before, during, and after World War II. It was the only British fighter produced conti ...
and Hawker Hurricane
The Hawker Hurricane is a British single-seat fighter aircraft of the 1930s–40s which was designed and predominantly built by Hawker Aircraft Ltd. for service with the Royal Air Force (RAF). It was overshadowed in the public consciousness by ...
whose Rolls-Royce PV-12 engine, later named the Merlin, was cooled by ethylene glycol. An early example of a Meredith effect radiator was incorporated in the design of the Spitfire for the first flight of the prototype on 5 March 1936.[Gingell, G. (Ed.); "''The Supermarine Spitfire - 40 years on''," Royal Aeronautical Society, 1976, Page 13.]
Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine
The radial engine is a reciprocating engine, reciprocating type internal combustion engine, internal combustion engine configuration in which the cylinder (engine), cylinders "radiate" outward from a central crankcase like the spokes of a wheel. ...
would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949.
See also
* Brayton cycle
References
{{reflist
1930s aircraft piston engines
Aircraft aerodynamics
Aerospace engineering