Shchelkin spiral
Encyclopedia
The Shchelkin spiral is a device that assists the transition from deflagration
Deflagration
Deflagration is a term describing subsonic combustion that usually propagates through thermal conductivity; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration...

 (subsonic combustion) to detonation
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...

 in a pulse detonation engine
Pulse detonation engine
A pulse detonation engine, or "PDE", is a type of propulsion system that uses detonation waves to combust the fuel and oxidizer mixture. The engine is pulsed because the mixture must be renewed in the combustion chamber between each detonation wave initiated by an ignition source. Theoretically, a...

. The spiral is named after Kirill Ivanovich Shchelkin
Kirill Ivanovich Shchelkin
Kirill Ivanovich Shchelkin was a Russian scientist involved in the development of Soviet nuclear weapons, in particular the physics of combustion and explosion...

, a Russian physicist who described it in his 1965 book Gas Dynamics of Combustion.

In pulse detonation engines, direct detonation of the combustible mixture
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 can be relatively straightforward, but require more energy than when detonation is preceded by deflagration. The deflagration to detonation transition
Deflagration to detonation transition
Deflagration to detonation transition refers to a phenomenon in ignitable mixtures of a flammable gas and air when a sudden transition takes place from a deflagration type of combustion to a detonation type of combustion...

(DDT) must however occur within the length of the detonation tube, which is a restriction provided by the Shchelkin spiral. The Shchelkin spiral acts as an obstacle by creating a partial blockage of the detonation tube, effectively shortening the distance along the detonation tube in which the transition occurs.
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