Interpreting the experimental results for convergent-divergent nozzles

4 Interpreting the experimental results for convergent-divergent nozzles 

The curve of velocity coefficient, C /, for this nozzle shows a maximum value in the supersonic region of 0.96 at a pressure ratio of 0.27, implying t Ncrit2 = rjf/p = 0.96 = 0.922. The velocity coefficient falls with increasing pressure ratio, then shows a sharp rise to a value of 0.92 at a pressure ratio of 0.78 then it rises very gradually to 0.925 as the pressure ratio falls to 0.95. We will assume that the upper critical pressure ratio occurs at the end of the sharp rise, i.e. at a value of 0.78. Its associated efficiency value is nucrii I = 0.92 = 0.846. 

We may estimate the efficiency of the convergent section of the nozzle at choking using equation (14.88)  

We may now apply equations (A6.15) to (A6.18) and solve iteratively for the two unknowns, divergence ratio, At/A and pressure ratio at minimum efficiency. P /poT resulting in the values  

Once the divergence ratio has been found, an iterative solution of equation (14.75) for the conditions at the nozzle end gives the upper and lower critical pressure 

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