Ejector mixer

Abstract

An ejector mixer has a convergent section and a downstream divergent section downstream of the convergent section. The downstream divergent section has a divergence half angle of 0.1-2.0 over a first span of at least 3.0 times a minimum diameter of the mixer.

Claims

1. An ejector (200; 300; 400; 600) comprising: a primary inlet (40); a secondary inlet (42); an outlet (44); a primary flowpath from the primary inlet to the outlet; a secondary flowpath from the secondary inlet to the outlet; a mixer having a convergent section (204) downstream of the secondary inlet; a diffuser downstream of the mixer; and a motive nozzle (100) surrounding the primary flowpath upstream of a junction with the secondary flowpath and having an exit (110), wherein: the mixer comprises a downstream divergent section (206) downstream of the convergent section and having a divergence half angle (.sub.2) of 0.1-2.0 over a first span of at least 3.0 times a minimum diameter (D.sub.MIN) of the mixer; and the diffuser has a divergence half angle of greater than 2.0 over a second span of at least 3.0 times the minimum diameter of the mixer.

2. The ejector (200; 300; 400; 600) of claim 1 wherein: the downstream divergent section divergence half angle is 0.5-1.5 over said first span.

3. The ejector (200; 300; 400; 600) of claim 2 wherein: there is no mixer straight portion of more than 5.0 times the minimum diameter of the mixer.

4. The ejector (200; 300; 400; 600) of claim 1 wherein: the downstream divergent section divergence half angle is 0.8-1.0 over said first span.

5. The ejector (200; 300; 400; 600) of claim 4 wherein: there is no mixer straight portion of more than 5.0 times the minimum diameter of the mixer.

6. The ejector (200; 300; 400; 600) of claim 1 wherein: there is no mixer straight portion of more than 5.0 times the minimum diameter of the mixer.

7. The ejector (200; 300; 400; 600) of claim 1 wherein: a boundary between the downstream divergent section and the diffuser is a distance downstream of the motive nozzle exit 3-6 times the minimum diameter of the mixer.

8. The ejector (200; 300; 400; 600) of claim 7 wherein: the downstream divergent section divergence half angle and the diffuser divergence half angle continuously progressively increase over said first span and second span.

9. The ejector (200; 300; 400; 600) of claim 7 wherein: there is no mixer straight portion of more than 5.0 times the minimum diameter of the mixer.

10. The ejector (200; 300; 400; 600) of claim 1 wherein: the downstream divergent section divergence half angle and the diffuser divergence half angle continuously progressively increase over said first span and second span.

11. The ejector (200; 300; 400; 600) of claim 1 wherein: the motive nozzle is a convergent-divergent nozzle having said exit within the mixer convergent portion.

12. The ejector (200; 300; 400; 600) of claim 11 wherein: there is no mixer straight portion of more than 5.0 times the minimum diameter of the mixer.

13. A vapor compression system comprising: a compressor (22); a heat rejection heat exchanger (30) coupled to the compressor to receive refrigerant compressed by the compressor; the ejector (200; 300; 400; 600) of claim 1; a heat absorption heat exchanger (64); and a separator (48) having: an inlet (50) coupled to the outlet of the ejector to receive refrigerant from the ejector; a gas outlet (54); and a liquid outlet (52).

14. A method for operating the system of claim 13 comprising: compressing the refrigerant in the compressor; rejecting heat from the compressed refrigerant in the heat rejection heat exchanger; passing a flow of the refrigerant through the primary ejector inlet; and passing a secondary flow of the refrigerant through the secondary inlet to merge with the primary flow.

15. The method of claim 14 wherein: the refrigerant comprises at least 50% CO.sub.2 by weight.

16. An ejector comprising: a primary inlet (40); a secondary inlet (42); an outlet (44); a primary flowpath from the primary inlet to the outlet; a secondary flowpath from the secondary inlet to the outlet; a convergent section (114) downstream of the secondary inlet; a motive nozzle (222) surrounding the primary flowpath upstream of a junction with the secondary flowpath and having: a throat (106); and an exit (110); and means for limiting efficiency sensitivity to off-design operating conditions by preventing a shock in a diffuser, wherein: the means comprises a diverging mixing section; and the diverging mixing section comprises a zone having a divergence half angle of 0.1-2.0 over a first span of at least 3.0 times a minimum diameter (D.sub.MIN) of the mixing section.

17. The ejector of claim 13 wherein: the diverging mixing section does not have a straight portion more than 5.0 times the minimum diameter of the mixing section.

18. The ejector of claim 17 wherein: a diffuser, downstream of the mixing section, has a divergence angle of greater than 2.0 over a span of at least 3.0 times the minimum diameter of the mixing section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a prior art ejector refrigeration system.

(2) FIG. 2 is an axial sectional view of a prior art ejector.

(3) FIG. 3 is a partially schematic axial sectional view of a first ejector.

(4) FIG. 4 is a CFD simulation of the ejector of FIG. 3.

(5) FIG. 5 is a CFD simulation of a prior art ejector.

(6) FIG. 6 is a schematic axial sectional view of a second ejector.

(7) Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

(8) FIG. 3 shows an ejector 200. The ejector 200 may be formed as a modification of the ejector 38 and may be used in vapor compression systems (e.g., FIG. 1) where conventional ejectors are presently used or may be used in the future. An exemplary ejector is a two-phase ejector used with CO.sub.2 refrigerant (e.g., at least 50% CO.sub.2 by weight). To differentiate from the corresponding portions of the ejector 38, the ejector 200 has a mixer 202 having a convergent section 204 in place of the convergent section 114 and a slightly divergent section 206 in place of the mixing section 116 (discussed further below). The divergent diffuser 208 replaces the diffuser 118. As is discussed below, use of a slightly divergent section 206 is believed to limit sensitivity to off-design operation. For example, the ejectors may be optimized for performance at a given operating condition. Their efficiency will drop with departures from the design condition. Relative to a straight mixer, the slightly divergent section 206 reduces the efficiency loss for a given departure from design conditions.

(9) FIG. 3 further shows a transition location 210 between the convergent section 204 and the section 206 and a transition location 212 between the section 206 and the diffuser 208. The mixer has a length L between these locations. The section 204 has a convergence half angle .sub.1. The slightly divergent section 206 has a divergence half angle .sub.2. The diffuser 208 has a divergence half angle .sub.3. In the FIG. 3 implementation, each of these half angles is essentially constant. Accordingly, in the exemplary FIG. 3 embodiment, a minimum cross-sectional area of the mixing section is found at the location 210 and has a diameter shown as D.sub.MIN. A diameter at the location 212 is shown as D.sub.T. As is discussed further below, by replacing the baseline straight mixing section 116 with the slightly divergent section 206 (e.g., less divergent than a conventional diffuser) performance sensitivity to the flow rate may be reduced. Whereas exemplary prior art and present diffuser half angles .sub.3 are in the vicinity of 3 or greater (e.g., at least >2.0, more narrowly, at least >2.5 or at least >3.0), exemplary mixing section divergence half angles are smaller than 3 (e.g., 0.1-2.0, more narrowly, 0.5-1.5 or 0.8-1.0). Such a mixing section angle may exist over a longitudinal span similar to the length of an existing mixer straight section (e.g., at least 3.0 times D.sub.MIN or an exemplary 3.0-6.0D.sub.MIN). Exemplary diffuser length may also be greater than 3.0 times D.sub.MIN.

(10) This exemplary configuration may be distinguished from a hypothetical configuration that has a conventional straight mixer and a shallow diffuser in several ways. First, there is the presence of the steeper diffuser. Second, there may be the absence of any straight mixer. For example, the exemplary mixer would lack any straight or nearly straight portion (e.g., less than 0.1 half angle) over a longitudinal span of more than 5.0 times a minimum diameter of the mixer (more narrowly, 3.0 times or 2.0 times).

(11) The pressure recovery performance of a typical ejector depends greatly on the mixer diameter. For a given operating condition (i.e. motive and suction mass flows) there exists an optimum mixer entrance diameter. A mixer diameter smaller than the optimum value results in the acceleration of the flow within the mixer which is followed by a lossy shock through the diffuser resulting in a poor pressure-rise performance. On the other hand, if the mixer is too big for the flow-rate, the entrainment of the suction flow at the entrance would be suppressed, leading to a drop in the performance.

(12) FIG. 4 shows a flow through an ejector having a diverging mixer whereas FIG. 5 shows a baseline ejector having a conventional/straight mixer. In the FIG. 5 baseline: L/D=4.4 optimized for a given condition. FIG. 5 shows a flow rate slightly greater than the design value. The flow shocks to subsonic upon entering the diffuser, creating losses.

(13) In FIG. 4, the mixer length and the minimum diameter are preserved from the baseline: L/D.sub.MIN4.4 and L/D.sub.T3.9. The flow decelerates in the mixer and enters the diffuser without shock.

(14) If, however, flow rate drops below the design point, the diverging mixer will have slightly worse (more lossy) performance than the straight mixer. However, it will be worse by much less than its high flow performance is better. Thus, integrated over time, the performance of the diverging mixer will be more efficient.

(15) Thus, in the divergent mixer, the small entrance diameter reduces the deterioration of suction entrainment at low flow rates while the divergence suppresses the flow acceleration inside the mixer for high flow rate operating conditions.

(16) In one basic implementation, the ejector may be implemented from a conventional baseline ejector (or configuration thereof) replacing the straight mixing portion with the slightly divergent portion. For example, D.sub.MIN may initially be chosen as the diameter of the baseline straight mixing portion. D.sub.T will be slightly greater based upon the chosen angle .sub.2. The diffuser divergence angle may be preserved from the baseline. Further experimental variations may refine such ejector or configuration. For example, it has been determined that D.sub.MIN may be modified to be slightly less than the diameter of the baseline straight mixing portion. For example, it may be 95-100% of the baseline diameter (more narrowly, 98-99%). In distinction, D.sub.T may be slightly greater than the baseline diameter (e.g., 101-110%, more narrowly, 102-104%).

(17) Alternatively, or additionally, a computational fluid dynamics (CFD) program may be used to model ejector performance while the various parameters are varied. For example, as discussed above, FIG. 4 shows an ejector having such a slight divergence in the mixing section 206. By way of contrast, FIG. 5 shows a similar plot for a baseline ejector. The simulated conditions involve a slight off-design operation. In baseline nominal operating conditions, the efficiencies of the prior art and FIG. 3 ejectors are both 48%. With an off-design condition of slightly higher flow, the baseline prior art ejector drops to 39% estimated efficiency whereas the ejector of FIG. 3 retains 44% efficiency.

(18) As an alternative variation, FIG. 6 shows an ejector 300 having a continuously curving longitudinal profile downstream of the minimum diameter location 310. To conveniently reference the longitudinal/axial positions of various locations to compare with the FIG. 3 embodiment, one possible reference is to use the motive nozzle exit as the origin of a Z axis pointing centrally downstream. Thus, this arbitrarily defines Z.sub.00. A location of the minimum mixer cross-sectional area (or the beginning of any straight zone at said minimum cross-sectional area) has a position Z.sub.1. In the exemplary FIG. 3 embodiment, this is also the beginning of the mixer divergent portion. In the exemplary embodiment, a location of the junction between the mixer and diffuser is at a position Z.sub.2. The location at the downstream end of the diffuser (where it stops diverging) is Z.sub.3. In the exemplary implementation, upstream of the location 310, the ejector is otherwise the same as the ejector 200 and, therefore, other than identifying the convergent section 304 instead of 204 other portions are not distinctly numbered. The exemplary minimum diameter location 310 is at a position Z.sub.1 which may be the same as Z.sub.1. In the exemplary implementation, an ejector outlet diameter at the outlet 44 is the same in the ejector 300 as in the ejector 200. This outlet diameter may be associated with the size of piping used. FIG. 6 further shows the outlet of the ejector 300 at position Z.sub.3. In the exemplary implementation, Z.sub.3 is shown as the same as Z.sub.3. FIG. 6 further shows a partially arbitrarily chosen transition location 312 between the mixer and diffuser at a position Z.sub.2. The exemplary position location 312 is defined as the location wherein a half angle has a value of 1. The exemplary Z.sub.2 is shown as being essentially the same as Z.sub.2.

(19) The ejectors and associated vapor compression systems may be fabricated from conventional materials and components using conventional techniques appropriate for the particular intended uses. Control may also be via conventional methods. Although the exemplary ejectors are shown omitting a control needle, such a needle and actuator may, however, be added.

(20) Although an embodiment is described above in detail, such description is not intended for limiting the scope of the present disclosure. It will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, when implemented in the remanufacturing of an existing system or the reengineering of an existing system configuration, details of the existing configuration may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.