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 fluid dynamics within a transmitter, focusing on vane and duct characteristics with accompanying diagrams.
| Identifier | ExFiles\Box 156\4\ scan0092 | |
| Date | 31th May 1939 guessed | |
| 6 stream, and passes from one element to another virtually without shock. At the right of Fig. 3 is a simple epicyclic gear, which is used to reverse the car, and which comprises also a gear ratio forward for use in descending abnormally steep gradients in exceptional circumstances, when "engine braking" effect may be required superior to that provided by the transmitter. It may be mentioned that when the car overruns the engine, there is no "free-wheel" effect, the engine being driven by the transmitter at the ratio of 1 to 1 under all normal driving conditions. For normal forward running, starting from rest either on the level or on the steepest inclines on which a car is likely to be found, the transmitter supplies all the torque required and does not need the aid of a mechanical gear. Fig. 4. Fig. 5. Fig. 6. Vane and Duct Characteristics.—The receiving ends of the vanes of all the elements of the transmitter are of bulbous formation, the thickness of the bulbs bearing appropriate relation to their pitch. The virtue of this construction is that it enables the vanes to receive without shock liquid impinging upon them at any angle within a fairly wide range. The ducts between the vanes of all the elements are designed on the principle of the gradually convergent nozzle, a conformation conducive to steadiness of flow in a duct at relatively high velocities, as will be seen from the laboratory tests described below. In this connection, it must be mentioned that mere reduction of area does not suffice to ensure steadiness of motion within a duct; the reduction must be effected by means excluding angular divergence of opposite faces of the ducts. Thus, in a duct of rectangular cross-section, for example, one pair at least of opposite faces must be convergent; the other may be parallel, but it must not be divergent.* In a radial inward flow turbine, the arrangement of the ducts according to this rule presents no difficulty, the geometry of the machine is such that the requisite arrangement results as a matter of course. But in a vaned wheel in which the radial flow is outward, such as a centrifugal pump, the position is reversed and a difficulty arises which has hitherto not been overcome. A gradual reduction of the cross-sectional area of the ducts has been provided by convergent disposition of the faces forming the axial depth of the ducts, but the vane faces forming the width of the ducts have been invariably divergent, so that, notwithstanding the gradual reduction of area, the ducts of a centrifugal pump have always had a conformation which is *per se* conducive to unsteadiness of the stream in any duct. Even in the absence of angular motion of the duct, even with angular motion at high speed, however, such as obtains in centrifugal pumps, the divergence of the vane faces introduces another and much graver cause of eddy motion, namely, the tendency of the stream within the ducts of a centrifugal pump to follow a path along the trailing duct faces, leaving bodies of dead liquid along the leading faces.† This tendency increases as the liquid approaches the duct outlets, and the disturbance which it causes attains its maximum * See A.{Mr Adams} H.{Arthur M. Hanbury - Head Complaints} Gibson, Proc. Roy.{Sir Henry Royce} Soc., A, vol. 83 (1910). † See J.{Mr Johnson W.M.} A.{Mr Adams} Smith, ENGINEERING, vol. lxxiv, page 762 (1902); see also C. B. Stewart, Bulletin of the University of Wisconsin, No. 173 (1907). (6424 D.{John DeLooze - Company Secretary}) 7 value at the particular point at which, in an hydro-kinetic power transmitter, disturbance of the flow is most detrimental, i.e., at the critical point at which the streams issuing from the ducts of the driving element become merged into one annular stream which has to bridge the gap between the driving element and the driven. With inordinate eddy motion at this point, control of the rotational velocity of the annular stream emerging from the driving element, and an automatic co-ordination with the variable peripheral velocity obtaining at the receiving ends of the vanes of the driven element, is an impossibility. In the transmitter dealt with in this paper an attempt has been made to eliminate these causes of unsteadiness of motion. The problem has been solved by disposing the vanes of the driving element not wholly in radial planes as in hydraulic couplings, or in uniform curves backwards, as has been the practice in centrifugal-pump design, but in radial planes from a point at or near their receiving ends up to a point near to the major radius of the circuit, the vanes being curved backwards from the latter point to their delivery ends. The construction is illustrated in Figs. 4 to 16, also in the diagram, Fig. 20. With such a construction, a gradual decrease of cross-sectional area being provided in the usual manner, i.e., by convergent disposition of the faces forming the axial depth of the ducts, the thickness of the radial portion of the vanes can be increased gradually to the extent required to avoid divergence of adjacent vane faces, or, indeed, to provide convergence of such faces, the curve backwards enabling the thickness of the vanes to be reduced gradually to a sharp edge at the delivery ends without involving angular divergence of adjacent vane faces at the outlet. The tendency to unsteadiness of flow which results from angular divergence of the solid boundaries of any stream is thus eliminated. Furthermore, due to the absence of divergence, coupled with the provision of a substantial rate of reduction of the cross-sectional area of the ducts towards their outlets, the tendency of the stream to leave dead liquid along the leading faces of the ducts is effectively counteracted along the radial portion of the ducts, while in the portion which immediately precedes the outlet, where a high degree of steadiness is of particular importance, this tendency is removed, since in the latter portion the liquid is subjected to angular deceleration in space. Thus a very high degree of steadiness of motion, similar to that obtaining at the nozzle of a Pelton wheel, is established at the critical point at which the streams issuing from adjoining ducts become merged into one annular stream. The liquid is thus enabled to bridge the gap between the driving and the driven elements in a uniformly steady annular jet. In relation to the tendency of a stream in a pump to leave dead liquid along the leading faces of the ducts, the fundamental difference between the conditions prevalent in a transmitter having its driving element constructed as above described and the conditions which have hitherto obtained in hydro-kinetic transmitters and in centrifugal pumps, will be appreciated by reference to the paper of Mr. J.{Mr Johnson W.M.} A.{Mr Adams} Smith, of Melbourne, and to the very interesting diagrams published in it which were prepared by him from photographs which he was able to take of the actual course which the stream follows in the ducts of a centrifugal pump. Fig. 17 and 18 are reproductions of these diagrams,‡ Fig. 17 shows the course of the stream in a pump having radial vanes, as in an hydraulic coupling, while Fig. 18 shows the course of the stream in a pump in which the vanes are curved uniformly backwards, as has been hitherto the practice in centrifugal pump design. Lines 1, 2 and 3 in Figs. 17 and 18 indicate the limits of the regions occupied by dead liquid along the leading duct faces at three different angular speeds. Figs. 19 and 20 are diagrams of centrifugal-pump wheels having ducts constructed on the principle followed in the driving element of the transmitter referred to in this paper. Fig. 19 shows a wheel designed to discharge in a radial direction, a construction intended for cases where high velocities of flow prevail under all working conditions, as, for example, in certain pumps for lifting liquid. Fig. 20 (see also Figs. 4 to 7) shows a wheel designed to discharge in an axial direction, a construction which is essential to the proper application of the principle in all cases where, as in any hydro-kinetic transmitter capable of functioning at the ratio of 1 to 1, the velocity of flow varies within wide limits. In considering these diagrams account should be taken of the fact that, whereas Figs. 17 and 18 show the path followed by the liquid in a wheel having ducts of constant axial depth, in the transmitter illustrated in this paper the faces limiting the depth of the ducts converge gradually towards the outlet by an angle of about 6 deg. As can be seen in Figs. 8 to 16, this angle of convergence results in a very substantial reduction of the cross-sectional area of the ducts towards the outlets. In this connection it should be borne in mind that in an hydro-kinetic transmitter the extent to which the ducts can be made to converge in axial depth is strictly limited by the necessity of maintaining a reasonably close relation between the Fig. 7. ENGINEERING ‡ Loc. cit. The diagrams are reproduced from Professor A.{Mr Adams} H.{Arthur M. Hanbury - Head Complaints} Gibson’s treatise, Hydraulics and its Applications. | ||