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).
Analysis of the mechanical design and efficiency shortcomings of low and high-speed impellers, limiting supercharging development.
| Identifier | WestWitteringFiles\V\December1930-February1931\ Scan070 | |
| Date | 23th December 1930 guessed | |
| - 4 - This means that in an actual impeller a continuous equalisation of pressure must take place between the different flow lanes and consequently a bad disturbance is continuously created and maintained with a detrimental effect on good efficiency. The vanes of the high speed type impeller hitherto used cannot be anything else but radial at the discharge if destructive centrifugal bending forces are to be eliminated. The radial discharge means a high entrance velocity into the diffuser which causes the latter to become burdened with the creation of a larger percentage of the total pressure rise. Since the pressure rise is always obtained less efficiently in the diffuser than in the impeller itself, the overall efficiency consequently suffers. This fact in addition to the fact that radial discharging impellers are for other reasons less efficient than backward discharging impellers prevents 95% of all impeller manufacturers (pump manufacturers included) from using radially discharging vanes. MECHANICAL DESIGN OF THE PRESENT IMPELLER. Since the impeller has ceased to be a mere mixing device and is used to give appreciable manifold pressure, it has become necessary to run it at high peripheral speeds at which high centrifugal forces are developed, requiring an adequate mechanical design to obtain a reasonable margin of safety against bursting. In this respect it is important to remember that the strength and the carrying capacity of a rotating disc depends to a large extent on the diameter of its centre bore. The smaller the centre bore can be made, the lower are the stresses in the disc for a given speed. A comparatively small decrease in the diameter of this bore results in a large increase in strength and maximum safe running speed. The hub construction used in the present impeller necessitates an extra large bore to receive the steel hubs and the impeller is considerably weakened thereby. A further weakening is caused by the six small off-centre holes used to accomodate the through rivets. Around these small holes serious stress concentration is very likely to occur at high speeds. If this impeller has not given trouble at 1 to 14 gear ratio, it must be considered a tribute to the quality of the material used. CONCLUSION. Obviously, these shortcomings of the low speed type of impeller when used in high speed service present a very definite limit to real progress in supercharging development. contd. | ||