JET PUMP

Abstract: A jet pump comprises a nozzle (1) for a high speed primary flow, a mixing tube (2) into which the primary flow is directed by the nozzle (1), and an inlet (3) to the mixing tube (2) for a secondary flow, the inlet (3) surrounding the primary flow nozzle (1). Means (2′) is provided for changing the cross section of the mixing tube (2) abruptly in order to produce a rise in static pressure immediately downstream, thereby increasing mixing of the primary and secondary flows, stabilizing the mixing process and enabling significant noise reduction when used in engine testing apparatus.

Description: The present invention relates to jet pumps and, more particularly, to a means of stabilizing the process of mixing which takes place between primary and secondary flow through the pump. Jet pumps have been known and used for many years and operate utilizing the entraining properties of a high speed jet of primary fluid in order to pump a secondary fluid. A simplified example of a jet pump is shown, in Figure 1, to comprise a primary nozzle 1 through which a high pressure primary fluid accelerates up to a high velocity into a mixing tube 2 which is located coaxially with the nozzle 1. The mixing tube 2 has a secondary inlet 3 surrounding the primary fluid nozzle 1 through which the secondary fluid is induced to enter. The inlet 3 usually comprises an aerodynamically faired inlet designed to reduce any pressure loss which might be incurred by the entrained fluid as it enters the mixing tube 2. The mixing tube 2 is of constant cross-sectional area and of sufficient length to enable adequate mixing of the primary and secondary fluids such that the velocity distribution at the exit end of the tube is substantially uniform. Typically the length of the mixing tube will be equal to at least six times its diameter when the configuration is a cylindrical one. A diffuser 4 is located at the exit end of the mixing tube 2 so that at least part of the kinetic energy at the end of the mixing tube can be converted into an increase in static pressure before the fluid is finally delivered from the apparatus. As a result of this action the diffuser creates a region of low pressure at its inlet which, in turn, is propagated upstream to the inlet of the mixer tube and so assists in the entrainment of the secondary stream of fluid. One particular use of jet pumps is in the testing of gas turbine engines where the outlet of the engine provides a primary flow of fluid to a jet pump apparatus, ambient air being drawn in as a secondary fluid in order both to reduce the velocity of the exit gases from the engine and to reduce the temperature of the exhaust jet, enabling testing to take place in relatively confined areas. However, the problem of instability in the jet pump creates a large noise problem. Although there is a wide range of possible uses for such jet pumps the mixing process between the primary and secondary fluids is relatively inefficient so that they have not achieved wide-scale use. The inefficiency of the mixing process reduces driving pressure, requires the jet pump to have a considerable length in order to achieve adequate mixing, and can give rise to noise and flow instability, particularly when the primary stream is supersonic. The present invention is directed to overcoming the problems associated with inefficient mixing of the primary and secondary flows through the jet pump. According to the present invention there is provided a jet pump which comprises a nozzle for a high speed primary flow, a mixing tube into which the primary flow is directed by the nozzle, and an inlet to the mixing tube for a secondary flow, the inlet surrounding the primary flow nozzle, characterized by means in the mixing tube for changing the cross section of the mixing tube abruptly in order to produce a rise in static pressure immediately downstream, thereby increasing mixing of the primary and secondary flows and stabilizing the mixing process. The means for changing the cross-section of the mixing tube is preferably located towards the inlet end of the mixing tube and may comprise a stepped or ramped increase in the cross-sectional area of the tube proper, but may also or alternatively include an orifice plate or fence. In a further alternative the diameter of the wall of the mixing tube is increased over a short longitudinal distance to provide an annular groove in the wall of the mixing tube. It has been found that the velocity distribution towards the end of the mixing tube can be made considerably more uniform and that fluctuations in pressure are reduced, i.e. stability is increased, and that mixing of the primary and secondary flows is enhanced. This enables the length of the mixing tube to be reduced from the conventional length and enables a reduction in noise to be achieved as well. This is particularly significant in engine testing applications where high noise levels can be a major environmental nuisance. Four examples of jet pumps constructed in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a conventional jet pump in longitudinal section; Figures 2a-c show portions of jet pumps, again in longitudinal section; Figure 3 shows a fourth example according to the invention, in greater detail; Figures 4A and 4B show trace recordings of static pressure in the jet pump of Figure 4 and a prior art jet pump respectively; and, Figure 5 illustrates in graph form the fluctuations in pressure along the wall of the mixing tube of the example shown in Figure 4 in comparison with a conventional jet pump of identical dimensions. In Figure 2a is shown a mixing tube 2 formed with a means 2′, for changing the cross-section of the mixing tube, which comprises an annular step 5 between the wall 6 of the mixer inlet and the wall 7 of the tube, so as to provide a stepped abrupt increase in the diameter of the mixing tube. Preferably the step 5 will have a height of about 10% of the mixing tube diameter, but the precise dimensions will depend on the ratio of the diameters of the nozzle 1 and the tube 2 and on the designed driving pressure of the primary fluid. Figure 2b shows a construction similar to Figure 2a, but having a frusto-conical wall portion 8 providing a less abrupt increase in the diameter of the mixing tube, but preferably of similar diametrical domensions. Figure 2c shows a wall 7 of the mixing tube 2 being formed with an annular groove 10 over a relatively short longitudinal distance, the depth of the groove being of the order of 10% of the diameter of the mixing tube. Figure 3 shows a jet pump having a primary nozzle 1 of 8mm diameter emitting a primary jet into a mixing tube of diameter 28mm and length 235mm and containing an orifice plate 9 positioned closely adjacent the inlet 3, thus providing a reduced cross-sectional area for the combined flow. The diameter of the orifice 9′ shown is 22.5mm. The mixing tube extends into a diffuser 4 having a length of 240mm and an outlet diameter of 45mm. The orifice plate protrudes only part of the way towards the high velocity jet of primary fluid and it is important to ensure that the primary jet does not impinge on the orifice plate. In a comparison test with a jet pump having the same dimensions, but without the orifice plate, it was found that with a primary jet of air driven through the nozzle 1 at a driving pressure of 30 psi, the fluctuation of the static pressure about the mean at a series of positions along the wall of the mixing tube was dramatically reduced. Figures 4A and 4B show trace recordings of static pressure P against time t measured under identical conditions at a location on the wall of the mixing tube downstream of the inlet 3, (A) when an orifice plate as shown in Figure 3 is in position in the mixing tube 2 and (B) when it is not present in the mixing tube, Figures 4A and 4B clearly illustrating the smoothing in pressure variation which is achieved. A series of tapping points n (not shown in Figure 3) were also used to measure static pressure, the tapping points being spaced at intervals of 20mm along the length of the mixing tube starting from the orifice plate. It can be seen from Figure 5 that although the level of pressure fluctuation δP (measured in kPa) about the mean reduces in the downstream direction in the conventional jet pump (curve A), the level of fluctuation in the example of the invention (curve B) is significantly reduced all along the tube, to a level less than about half that of the fluctuation in the conventional pump. It will be appreciated that other formations within the mixing tube 2 may be provided within the scope of the invention.

Claims: 1. A jet pump which comprises a nozzle (1) for a high speed primary flow, a mixing tube (2) into which the primary flow is directed by the nozzle (1), and an inlet (3) to the mixing tube (2) for a secondary flow, the inlet (3) surrounding the primary flow nozzle (1), characterized by means (2′) for changing the cross section of the mixing tube (2) abruptly in order to produce a rise in static pressure immediately downstream, thereby increasing mixing of the primary and secondary flows and stabilizing the mixing process. 2. A jet pump according to claim 1, characterized in that the means (2′) for changing the cross-section of the mixing tube (2) is located towards the inlet end of the mixing tube (2). 3. A jet pump according to claim 1 or claim 2, characterized in that the means (2′) for changing the cross-section of the mixing tube (2) comprises a stepped or ramped increase in the cross-sectional area of the tube (2). 4. A jet pump according to claim 1 or claim 2, characterized in that the means (2′) for changing the cross-section of the mixing tube (2) comprises an orifice plate or fence. 5. A jet pump according to claim 1 or claim 2, characterized in that the means (2′) for changing the cross-section of the mixing tube (2) comprises an annular groove in the wall of the mixing tube (2) formed by an increase in the diameter of the wall of the mixing tube (2) over a short longitudinal distance. 6. An engine testing apparatus in which, in use, an engine produces a flow of exhaust gas as a primary flow to a jet pump into which air is drawn as a secondary flow, the jet pump being constructed in accordance with any of claims 1 to 5.

Inventors: ADKINS, RICHARD CYRIL
Applicants: BRITISH AEROSPACE PLC
Application year: 1987
Main classification: F04F5
IPC codes: G01M15, F04F5