HELP NASA CEA CODE

I am using Nasa CEA code for calculating Isp for different Ae/At
ratios with chamber pressure of 1000 psi. Problem is Isp should have a
peak at optimum expansion condition i.e. when Pexit= Patm but with
increasing Ae/At ratios Isp's are increasing although the PexitPatm.
Can anybody who has used the code help me ?

Warm Regards
Vaibhav

Vaibhav wrote...
I am using Nasa CEA code for calculating Isp for different Ae/At
ratios with chamber pressure of 1000 psi. Problem is Isp should have a
peak at optimum expansion condition i.e. when Pexit= Patm but with
increasing Ae/At ratios Isp's are increasing although the PexitPatm.
Can anybody who has used the code help me ?

If you overexpand a convergent-divergent nozzle beyond ideal conditions
(i.e., when Pexit Patm), the exit velocity will indeed be greater than the
ideal exit velocity. However, you need to account for the "base pressure"
thrust loss, Aexit * (Patm - Pexit). This drag term grows with
overexpansion and it prevents overexpanded thrust from exceeding the ideal
thrust.

Note: There are limits to how much viscous attachment effects allow a
nozzle to be overexpanded. Eventually, the flow will separate from the
divergent wall.

Regards,
Jeff Berton