Pulse compression detonation system

A pulse compression detonation system (PCD-system) is a hybrid of pulse detonation and compression systems. It has been designed to solve the problem of a high-frequency efficient initiation of a detonation in fuel-air mixtures. Instead of the Shchelkin spiral, U-bend tubes and an electrical treatment of detonable mixture, a technique of an ultra-fast pressurized filling of a detonation tube with a preheated detonable gaseous mixture is applied to reduce a time and a length of deflagration to detonation transition (DDT) in the tube. A prototype of the PCD-system has been made at National Technical University «Kharkiv Polytechnic Institute» in Ukraine in 2017. The measurements of the prototype with the detonation tube diameter of 20 mm and its length of 600 mm were taken. The device was operated using the mixture of atmospheric air and LPG fuel. The shock wave velocity at the open end of the tube reached 1700 m/s. The frequency of device pulsations was 23-24 H¹. The DDT occurred due to the mixture heating and the compression of it ². A cooperation between National Technical University “Kharkiv Polytechnic Institute” and University of Warmia and Mazury in Olsztyn started to investigate an efficiency of the PCD-system as a detonation gun for coating technology.

Problem statement

Efficiency of pulse detonation systems such as pulse detonation engines or detonation tubes depends on types of an oxidizer–fuel mixture. Use of an oxygen-fuel mixture instead of an air-fuel mixture allows increasing in the efficiency. For example, the efficiency of an air-breathing pulse detonation engine calculated by Lenoir engine cycle is η = 0.29. The efficiency of engines fed by an oxygen-fuel mixture is η = 0.38. We assumed in the calculation that a pressure ratio is λ = 8 for an air case and a pressure ratio of λ = 15 corresponds to an oxygen case. The ratio of specific heats was k = 1.35. Efficiency of air-breathing pulse detonation systems can be raised by adaptation of Humphrey cycle to such systems. For example, when a compression ratio is n = 3.5, the efficiency of the air-breathing engine exceeds η = 0.49. Thus, it requires compressing an air-fuel mixture into a detonation tube before detonation initiation.

Design of a detonating impulse compression system

Construction

PCD-system includes the piston compressor 1 with the cylinder 2. The crankshaft 4 connected to the external drive is used for the reciprocal motion of the piston 3. The intake valve 7 arranged inside the intake port 6 of the cylinder head. The air supply system 8 is connected to the port 6. The fuel can be supplied both immediately into the cylinder 2 of the compressor and into the inlet port 6. The detonation tube 9 is connected to the cylinder 2 through the exhaust port 10.

 Principle of operation

PCD-system uses a phenomenon of a choked flow where a detonation tube is applied as a gas outlet. A pulsed mixture compression at closed end of the detonation tube is realized by a piston compressor in such a manner that a mass flow rate at a “closed” end of the tube exceeds a mass flow rate at an open end of the tube. It causes a compression of an air-fuel mixture into the detonation tube. Detonation is initiated when the mixture is compressed.

PCD-system operates in the following way: The crankshaft 4 starts moving in a circular motion by the external drive. During the motion of the piston 3 from the top dead center to the bottom dead center the intake valve 7 is opened, and the detonable gas mixture is pumped into the cylinder 2 of the compressor 1 through the intake port 6 using the supply system 8. As soon as the BDC is reached the valve 7 is closed. Due to the further motion of the piston 3 from the BDC to the TDC the compression of combustible mixture occurs in the cylinder 2 and in the detonation tube 9. This results in an increase of the density, the temperature and the pressure of combustible mixture at a closed end of the detonation tube 9 and inside the tube itself. As the piston approaches the TDC the combustible mixture is self-ignited due to the compression of it. Then, the deflagration to the detonation transition occurs in the detonation tube 9. The output of detonation products from the tube 9 happens in a short period of time, when the piston is near the TDC. Then, the process is repeated.

Potential use of pulse compression detonation system

PCD-system is applied in techniques of a generation of pulsed high-speed hot gas flows and also the acceleration of solid particles and drop-liquid medium. The PCD-system can be used by pulse detonation engines to initiate the detonation, for the detonation coating, to solve the problems related to crushing of minerals, abrasive or water blasting, to produce aerosols, for gas detonation lasers and as a vibrator machine.

Notes

¹  The maximal frequency can exceed 100 Hz per one detonation tube.

² The critical tube diameter at which the deflagration-to-detonation transition happens is equal to the detonation cell size λ. For the propane –air mixture λ≈ 50 mm at normal temperature and pressure.

References

• Senderowski C, Bojar Z. (2009) Influence of Detonation Gun Spraying Conditions on the Quality of Fe-Al Intermetallic Protective Coatings in the Presence of NiAl and NiCr Interlayers. Journal of Thermal Spray Technology. 18(3): 435.
• Korytchenko KV. (2014) High-Voltage Electro-Discharge Technique Used for the Generation of Shock Waves and the Heating of Reactive Gases. Dr.Sc. thesis.
• Pawlowski A, Czeppe T, Major L, Senderowski C. (2009) Structure Morphology of Fe-Al Coating Detonation Sprayed onto Carbon Steel Substrate. Archives of Metallurgy and Materials. 54(3): 783.