gsphelp  Con/Di exhaust nozzle

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gsphelp  Con/Di exhaust nozzle

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In the convergent-divergent nozzle component type isentropic expansion is calculated to ambient pressure if the nozzle is not choked. Expansion to the nozzle throat in the convergent part is similar to the convergent nozzle calculation. The expansion beyond the throat becomes supersonic if the nozzle throat is choked. Determined is wether the flow is under- or over-expanded. For over-expanded flow, the location of the normal shock is determined (in- or outside the divergent part of the nozzle). If the flow is over-expanded with the shock inside the nozzle, shock losses have to be accounted for and expansion to ambient pressure in the nozzle exit is calculated.

 

A nozzle exit discharge coefficient CD (usually 'CD9') represents the relation between geometric and effective exit (end of divergent part) flow cross areas:

 

 A_geom = CD * A_eff
 

The effective area is lower than geometric area due to boundary layer effects on the flow (reduced velocity due to friction).

Either design nozzle exit A_eff or A_geom can be user specified using a user specified CD, or directly user specified depending on selected Exhaust nozzle options.

 

The graph below shows the 3 different operating regimes of a con-di nozzle of a turbojet example (sample project TJET_condi.mxl) model. Nozzle pressure ratio is 4 and ambient conditions are ISA SL:

con-di ratio 1 - 1.155

Here the nozzle is underexpanded up to 1.155 where the nozzle is fully expanded and maximum gross thrust is obtained. There are only oblique shocks downstream outside the nozzle.

con-di ratio 1.155 - ~4.7

Here the nozzle is overexpanded with the shock outside the divergent nozzle

con-di ratio ~4.7 -

Here the nozzle is overexpanded with the shock inside the divergent nozzle.

 

With the Ideal con-di nozzle complete expansion option checked, shocks are ignored and ideal complete expansion to con-di nozzle exit is assumed, usually resulting in the maximum theoretical nozzle gross thrust FG. The effect of CV can be seen comparing the blue and red curves in the graph.