Nominal divergence ratio

Keenan classified his nozzles according to the nominal divergence ratio, the ratio of a nominal exit area to the throat area. The curves in Figure A6.1 are labelled with these ratios, and we shall continue to use these labels as identifiers. 

Keenan classified the nozzles according to their nominal divergence ratio , a figure described as arbitrary... 

Nominal divergence ratio Physical divergence ratio... 

In attempting to explain his measurements, Keenan eschewed the nominal divergence ratio, but chose instead the assumed divergence ratio , determined from the pressure ratio at which the nozzle efficiency reached its maximum value. Keenan did not detail the equations he used, but we will outline the type of approach possible. 

Table A8.4 Principal features for convergent-divergent nozzle with nominal divergence ratio of 1.38. Bold type indicates the values taken from the experimental data...

The pressure ratio at minimum efficiency is calculated to be somewhat below the measured value for the nozzles with nominal divergence ratios of 1.38 and 2.34 (with discrepancies of 0.094 and 0.063, respectively). But the measured behaviour of the nozzles with nominal divergences of 3.19 and 7.93 is captured well in this regard. 

The nozzle efficiency at pressure ratios above the calculated upper critical point is overestimated for nozzles with 2.34 and 3.19 nominal divergence, but is reproduced reasonably well for the nozzle of 1.38 nominal divergence. However, the calculated upper critical pressure ratio at 0.69 is 0.09 lower than the measured value for this last nozzle. Data does not exist for the upper critical pressure for the nozzle of 7.93 nominal divergence ratio, but the match between... 

Figure A6.6 Measured versus calculated efficiency for the convergent-divergent nozzle of 2.34 nominal divergence ratio.

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