Due to the development of modern technology, the application of stealth technology in aircraft is essential. Infrared signals of aircraft are mainly emitted from propulsion engines and exhaust gas, and infrared guided missiles are specialized in tracking them. To avoid infrared signal detection, an active infrared signal reduction method includes spraying water or solid particles around the exhaust gas. In this study, an experimental study was conducted to understand the characteristics of the infrared signal of exhaust gas depending on the flowrate of water and the angle of injection.
A mirco turbo-jet engine was used to generate exhaust gas, and a water spray system was constructed to spray water around the exhaust nozzle. Eight water spray nozzles were installed in the circumferential direction around the exhaust nozzle. In the water spray system, water was supplied to water spray nozzles by operating a water pump and the water supply pressure was gradually increased from one bar to six bar using a bypass valve. In addition, water was sprayed parallel to the exhaust gas or into the exhaust gas. The infrared signal was measured in the range of one to ten times the diameter of the exhaust nozzle(D) from the exhaust nozzle outlet with the water mass flow rate. The exhaust gas temperature and the water infrared transmittance was measured at 10D.
When spraying water parallel to the exhaust gas, the infrared signal decreased as the water mass flow rate increased. The reason for the decrease in infrared signals is that the transmittance decreases due to the water surrounding the exhaust gas. But when spraying water into the exhaust gas, the infrared signal increased as the water mass flow rate increased. The reason for this increase in infrared signals is that the magnitude of infrared signals emitted from high-temperature H2O vapor generated by the mixing of exhaust gas and water is very large.