Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . iddle range of pressure, and tend toward a a b ~ _c ^- £ 120 100 8 60 0 12 3 4 5 6 Inches from the end of the Nozzle Fig. 26. Experiments with an Expand-ing Nozzle Showing the Effect of Vary-ing the Final Pressure. NOZZLE DESIGN 57 minimum when the lower pressure approaches a perfectvacuum. In the divergent nozzle, however, there appear to be no internaloscillations of pressure after those at the throat have died out. The size and most likely also the shapeof the externa
Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . iddle range of pressure, and tend toward a a b ~ _c ^- £ 120 100 8 60 0 12 3 4 5 6 Inches from the end of the Nozzle Fig. 26. Experiments with an Expand-ing Nozzle Showing the Effect of Vary-ing the Final Pressure. NOZZLE DESIGN 57 minimum when the lower pressure approaches a perfectvacuum. In the divergent nozzle, however, there appear to be no internaloscillations of pressure after those at the throat have died out. The size and most likely also the shapeof the external space has a considerableeffect on these oscillations of pressure. Jude states in this connection thatthere is a greater loss in velocity, dueto oscillations or eddies, in a square orrectangular nozzle than in a circularone. Recent experience with nozzlesof this type does not bear out thisstatement, except in the case probablyof square or rectangular nozzles withno rounding at the edges. An efficiencyof 97 per cent, is not unusual for prop-erly designed square and rectangular Fig. nozzles without any square. 012 Inches from Mouth Experiments witha Non-expanding NozzleShowing the Effect of edges; and circular nozzles have certainly Varying the Final Pressure>never given 99 per cent, efficiency. Under- and Over-Expansion. The best efficiency of a nozzle isobtained when the expansion required is that for which the nozzlewas designed, or when the expansion ratio for the condition of thesteam corresponds with the ratio of the areas of the mouth andthroat of the nozzle. A little under-expansion is far better, how-ever, than the same amount of over-expansion, meaning that anozzle that is too small for the required expansion is more efficientthan one that is correspondingly too large.* Fig. 28 shows a * It is a very good method, and one often adopted, to design nozzles so thatat the rated capacity the nozzles under-expand at least 10 per cent., and maybe20 per cent. The
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