GB676371A - Improvements in multi-stage cased screw-propeller fans, compressors, pumps and the like - Google Patents

Abstract

676,371. Screw fans &c. MACARD SCREWS, Ltd., and TAYLOR, E. S. W. July 8, 1949 [July 13, 1948; June 25, 1949], Nos. 18164/49. Drawings to Specification. Class 110(i). The reaction is of the order of 100 per cent or greater, and the inlet guide vanes impart positive whirl (i.e. opposite to direction of rotation) to the fluid; each set of rotor blades reverses the direction of whirl to a negative value, the interstage guide vanes again reverse the direction of whirl to a positive value, and the outlet guide vanes discharge the fluid axially; the geometric pitch of the rotor blades of at least the first stage is constant from tip to hub, with the mean geometric pitch coefficient greater than 0.5. The inlet guide vanes are of the impulse type, the interstage guide vanes provide reaction at their entry ends and impulse at their discharge ends, and the outlet guide vanes are of the reaction type. The whirls at entry to and discharge from each set.of rotor blades are substantially equal in magnitude, but opposite in direction, varying inversely with the radial distance from the axis of rotation. The rotor blades and interstage guide vanes are such as to produce constant effective pitch (which is infinite in the case of the interstage guide vanes) from tip to hub, which may necessitate a variation of geometric pitch from tip to hub to compensate for an increasing angle of attack. The inlet and outlet guide vanes are advantageously of decreasing geometric pitch from tip to hub. The width and crosssectional area of the rotor blades may increase from tip to hub, except at the high pressure end, where they may be constant. To maintain a constant aspect ratio, the width of the rotor blades may decrease from inlet to discharge, the number of rotor blades per stage increasing inversely to this decrease to prevent undue leakage at the blade tip clearance. Alternatively, the flow passage may be cylindrical at the discharge end or even over its entire length, the mean geometric pitch of the rotor blades decreasing along the cylindrical portion in inverse relation to the increase of fluid density or fluid acoustic velocity, whichever increase is the smaller. This reduction of geometric pitch is preferably such that the mean value of the ratio of axial velocity to peripheral speed is limited to a range of say, 1.6 to 1.7 at the suction end to 0.2 to 0.3 at the discharge end. The relative curvature and relative thickness of the rotor blade profiles may be constant or increase from tip to hub. Some or all of the guide vanes may be rotatably mounted. Specifications 512,487, 515,469, 587,518 and 644,319 are referred to.