From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Notes explaining the advantages of a hyperboloid impeller as a centrifugal supercharger.
| Identifier | WestWitteringFiles\V\December1930-February1931\ Scan067 | |
| Date | 23th December 1930 guessed | |
| COPY. SOME NOTES EXPLAINING THE HYPERBOLOID IMPELLER AND ITS ADVANTAGES AS A CENTRIFUGAL SUPERCHARGER. (1) This impeller is specially designed for ultrahigh speed supercharger service and is not a mere adaptation from commercial practice. (2) The high efficiency of the impeller is partly due to the excellent flow through it, which takes place from the inlet to the discharge in a perfectly straight path in which the eddy and loss-producing deflections the air must undergo when passing through the ordinary impeller are completely avoided. (3) The air passages of the impeller are gradually and smoothly diverging in contrast to the abrupt changes in area found in the present impeller passages. (4) The inlet angles are correct at every point throughout the radial extent of the inlet edge. (5) All particles of air regardless of the location of their point of entrance receive the same amount of energy from the impeller, which is not true in the case of the present type of impeller. (See analysis of latter, Page 2, Paragraph 1.) (6) The peculiar shape of the vanes permits the use of a discharge angle of less than 90 degrees, or in other words of backward discharging vanes, which for reasons of mechanical strength is entirely impossible in the case of the present high speed impeller. It is a well known fact that a backward discharging impeller is inherently more efficient than a radially discharging impeller. For this reason the latter design is shunned whenever possible. Since this is the case in low speed practice, over 95% of the world's centrifugal impeller manufacturers use backwards discharging vanes exclusively. (7) It is characteristic of all backward discharging impeller designs that compared with a radial impeller, a greater portion of the total pressure rise through the compressor is obtained in the impeller itself and less is left to be created in the diffuser by a conversion of velocity into pressure. The overall supercharger efficiency is therefore less dependent on the efficiency of the diffuser which inherently is not very high even for ordinary commercial designs where space, weight and other considerations are not a factor as in the case in an aircraft engine. contd. | ||