When ratio of environmental pressure to total pressure increases from design state to third limit condition (normal shock at the outlet of nozzle)

there are different shock wave phenomena outside the Laval nozzle. According to extant theory about shock wave reflection

the working range between design condition and the third limit condition was subdivided into several sections: strong oblique shock wave condition

Mach reflection condition

regular reflection and dual solution domain. Three critical ratios of environmental pressure to total pressure were proposed and determined to replace flow-deviation angle for estimating different Laval nozzle working statuses. Varying tendency of these critical ratios of Laval nozzle in various aspect ratios were also given with a figure. The pressure ratios corresponding regular reflection

Mach reflection and strong oblique wave in a planar Laval nozzle with aspect ratio 5 were 0.0841

0.0959 and 0.2005 respectively

and then the accuracy of these values were verified with numerical simulation.