CORRESPONDENCE Notes on Centrifugal down the impeller tip speed for a given work input. There are no losses specifically associated Compressors with slip. (7) In the section on 'design of inlet guide vanes' the 'impeller tip speed' is referred to. The Comments on the Article by Mr Thompson impeller tip speed is constan t and in any case not directly relevant to the intake design except inso far as that the eye speed is a proportion of it, dependent on the radius ratio. (8) In the first paragraph on diffusers it is To the Editor true that the centrifugal is always much lighter stated that 'the air-speed in the (combustion) than the axial—in large sizes with modern Dear Sir, chambers must not be greater than 200-300 ft./sec. methods of construction of axials there may not I wish to make some comments on the article to ensure flame stabilization (if the air-speed be a great deal of difference. in your August issue entitled 'Some Notes on exceeds the velocity of propagation of the fiame (3) In discussing the frontal area of the two Centrifugal Compressors' by L. N. Thompson. front, obviously combustion cannot be main types, the difference between the single- and This is a distinct contrast to the article by Pro tained)'. The fiame speed with fuels and tem double-sided types of centrifugal should be borne fessor Baxter in the previous issue comparing peratures normally used is less than 10 ft./sec, in mind. The meaning of 'air is drawn into the axial and centrifugal types, which was excellent, so that this does not bear any direct relation to eye of the impeller by combined vacuum and high being objective and making all the important the mean air velocity in the combustion chamber. rotation induction' is not apparent. points most fairly and clearly. The present article There must be a region of much lower velocity (4) The resultant velocity of the air leaving the is very loosely worded and is, in fact, positively for flame stabilization—elsewhere higher velo impeller tip is in fact usually somewhat lower than misleading in parts. cities can be used at the expense of high pressure the impeller tip speed. For example, with a slip In particular I should like to make the follow losses. factor of 0-9, the radial velocity must be greater ing remarks: (9) An adverse pressure gradient tends to than about 0-44x the impeller tip speed, in (1) I doubt if the reciprocating engine super cause boundary layer breakaway, with attendant order that the resultant velocity shall be greater charger influenced the choice of compressor type than the tip speed, which is unlikely. Commonly energy losses, in any circumstances. The log a great deal, or if experience with it was of very the radial velocity will be nearer 0-3 x impeller spiral which it is stated would be the flow path great value, at least in the early stages. At that in a vaneless diffuser with incompressible flow, tip speed. time the supercharger pressure ratios used were would only of course occur with parallel side (5) It is stated that 'general present practice is much lower than those required for gas turbines walls and no losses. Diffuser throat area is never to design for 50 per cent (pressure increase?) in and there were many other problems of a different made actually equal to the calculated value, but the impeller and the other 50 per cent in the nature from those commonly encountered in always somewhat larger—the actual amount de diffuser'. superchargers. For confirmation of this view see pending on circumstances, chiefly the maximum Sir Frank Whittle's original paper to the Institu This is a loose statement of the fact that in the operating pressure ratio. tion of Mechanical Engineers in 1945 'Th e Early ideal case with radial vanes (used in high-speed (10) Values of eye Mach number of well over History of the Whittle Jet Propulsion Gas machines for the reasons stated except that it 0-8 have been used, without serious losses Turbine'. would be bending which did the damage), no pre- occurring. Impeller channel Mach numbers whirl, no slip and no losses, 50 per cent of the (2) It is stated that the 'ultimate' isentropic cannot easily be calculated properly, but only energy input is converted to pressure within the efficiency of a centrifugal is about 78 per cent. mean values at various radii. These should cer impeller, and the remainder in the diffusing The meaning of the word 'ultimate' in this con tainly not exceed about 0-6—in fact considerable system. This is not a matter of design but is un text is not altogether clear. At high pressure ratios care has t o be taken to ensure that this is the case avoidable. Deviations from this are due to pre- of about 4i, efficiencies of 80-81 per cent are as higher values can easily occur, which introduce whirl, losses and slip. currently obtainable and at low pressure ratios considerable losses. (See Cheshire's paper to the of about 2, 87 per cent or so can be reached. As The fact that about 50 per cent of the energy Institution of Mechanical Engineers, Proceedings, to the future, there is no basic reason why con imparted to the air appears as pressure energy at 1945, Vol. 153, p. 430.) siderably higher values cannot be achieved given the impeller tip, does not mean that 50 per cent Yours faithfully, the necessary development. The different methods of the pressure rise occurs within the impeller. of definition of efficiencies should also be re The air does not, of course, necessarily enter the J. HODGr.. membered—the axials' 'flange to flange' total impeller axially—prewhirl is often used. Power Jets (Research and head efficiency is generally a good bit lower than (6) The only reason why it is desirable to have Development) Ltd., Farn- its 'over blading' efficiency. It is not necessarily the slip factor as near unity as possible is to keep borougli, Hants October 1952 319
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Oct 1, 1952
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