Channel diffusers of turbo-expansion engines

Abstract

Channel diffusers (CD) were experimentally obtained for the first time in the investigation of multiple bladeless diffusers (BD). It is known that an indispensable element of the design of BD is a sudden narrowing of the steam at the outer formed by the so-called ledge [1]. The angle of the stream at the outlet from the BD is increased to values that are usually found in vaned diffusers (VD). An experimental investigation of the influence of the width of the ledge on the losses in the BD showed that the minimum of losses corresponds to the alternative of the diffuser without a ledge [2]. Such BD became later known as channel BD.It follows from Fig. 1 that the through-flow part of the CD is formed by three geometric elements: the nozzle (O-Or), of which there may be four to ten in dependence on the scale of the stage; the oblique section of the nozzle by the curved wall (Or-ed( which may also be regarded as volute; the radial gap between the CD and the rotor (ed-1).The total opening cross-sectional area of the orifice of the CD determining (together with the regime parameters) the flow rate of the working gas is calculated by the formula For: Znaorb a,where z n is the number of nozzles; aor is the height of the nozzle orifice; b a is the width of the diffuser.The mean geometric angle at the outlet from the volute, viz., at the inlet to the gap, i.e., on the circumference of the trailing edges of the vanes, can be found from the expression sin ~ed = (zn/2n) (aor/Rea) :where Red is the radius of the circle formed by the edges.The presented geometric relations of CD make it possible to evaluate the characteristic traits of the geometry of CD compared with VD that have the same total opening cross-sectional area of the orifice. The substantially smaller number of nozzles and the geometric angle %d = 5-9* in CD compared with VD (z n = 19-27; sea = 12-16") increase the width of the diffuser and the height of the nozzle orifice. The last is due to the fact that the number of nozzles was reduced more than the angle. It should also be noted that technologically the CD is considerably simpler than the VD.We investigated experimentally CD of different scale: small nonregulated diffusers with rotor diameter D 1 = 30-60 ram; medium nouregulated and regulated diffusers with D 1 = 30-60 ram; medium nonregulated and regulated diffusers with D l = 100-150 mm; large regulated diffusers with D t = 250 rnm. The investigations were carried out on test stands with static scavenging as part of the experimental ("hot") stages and as part of industrial ("cold") stages. The angular momentum of the stream behind the diffuser was measured with a trap wheel. Figure 2 presents the loss factors ~1, the angles of th~ stream ct t depending on the reduced speed his = Cls/Cerit of the channel and vaned diffusers of different scale (here Cerit is the critical speed, cls is the absolute speed of the isentropic stream at the entry to the rotor). Small CD with D 94 = 50 mm and For = For/Rot 2 have the number of nozzles z n = 6 which is optimal for the ...