Indian Missile News And Discussion

[Arun_S]

SSridhar

SaiK wrote:looking at the nozzle exhaust, what is the science behind :-



1. The flame is visible only from meters away from the nozzle?

2. The dia or size of flame is much narrower than the nozzle dia?

The plume signature is determined by a number of things such as propulsion systems, propellants, nozzle geometry etc. The plume exits the nozzle at supersonic velocity. There are two parts of the plume which are of interest, the periphery and the central part (aka inviscid inner core). They both hit a shock wave subsequently. The periphery hits an oblique shock wave and the flow direction is bent. The central core part passes through a shock wave that is perpendicular to the incoming supersonic flow that results in no change in direction. This boundary is known as Mach Disk. These cause the typical diamond shape.



The inviscid inner core which is also fuel rich had not yet mixed with the air unlike the peripheral flow which burns with the oxygen in the air. At Mach Disk, the inner core reduces in velocity but increases in temperature and pressure and burns with the oxygen in the air. Hence we see the flame a few feet/metres away from the nozzle. The flow beyond Mach Disk becomes subsonic and again increases to supersonic. This pattern is repeated.

IIRC SSridhar is mixing the inner and peripheral flow in the explanation above. The core flow is optimally mixed (Stoichiometric ratio of fuel and exidizer) and burm at maximum efficiency while peripheral flow is intentionally fuel rich (excess fuel as used as coolenat to save the nozzle throat material from melting/ablating).