GB872228A - Improvements in and relating to turbines and like machines - Google Patents

Abstract

872,228. Boundary layer control. GENERAL DYNAMICS CORPORATION. March 5,1959, No. 7669/59. Class 4. [Also in Group XXVI] A turbine or blower blade of aerofoil section has a slot connecting the high and low pressure faces thereof, the minimum width of the slot lying in the range 2 to 2À66 times the thickness of the boundary layer at the location for the slot exit on the low pressure face at the design operating condition of the blade. Preferably in a blade of the laminar-flow type, the slot opens into the low pressure face along a line located in the transitional region between laminar and turbulent boundary layer flow, or located between the forward and rearward lines of maximum frictional intensity (the product of the viscosity and the velocity gradient in the boundary layer). The slot inlet on the high pressure surface should be in the transitional region between laminar and turbulent boundary layer floor on that surface. In a non-laminar flow blade, the slot exit in the low pressure face lies within the range from 5 to 7% of the blade chord, downstream of the line of maximum static pressure increase following the line of minimum static pressure at the design operating conditions of the blade. The boundary layer on the high pressure surfaces is thus removed at the slot, and that on the low pressure surface energized at the slot, resulting in higher efficiency. In the Figure the slot divides the blade, of chord c, into leading and trailing aerofoil portions. The entry width m of the slot should be greater than the exit width s, the latter being between one half and five% of the chord c. Similar limits are given for the length o of the slot, which may increase from tip to root of the blade. The maximum distance n y of the mean camber line from the chord line should be at a station n x near the rear part of the leading portion. The angle #F between the chord line c<SP>1</SP> of the leading portion and the overall chord line should be smaller than the angle #R<SP>1</SP> between the line joining the trailing edges of the two portions and the overall chord line. The angle between the trailing edge of the high pressure surface of the forward portion and the chord line of that portion should be greater than the angle formed between the tangent to the lower pressure surface of the rearward portion at the slot exit and the chord line of the forward portion. The camber angle # of the blade should increase from tip to root. The two portions may be connected across the slot by a stay of small radial dimension. The slot may be formed in rotor or stator blades. The power consumption required by a blower at flow capacities less than the design capacity, and the amount of airborne noise produced, are stated to be reduced if the minimum width of the slot lies between two and four% of the total blade chord, the slot being located at between 55 and 70% of the chord from the leading edge.