Abstract:
An ejector ( 200; 300; 400; 600 ) has a primary inlet ( 40 ), a secondary inlet ( 42 ), and an outlet ( 44 ). A primary flowpath extends from the primary inlet to the outlet. A secondary flowpath extends from the secondary inlet to the outlet. A mixer convergent section ( 114 ) is downstream of the secondary inlet. A motive nozzle ( 100 ) surrounds the primary flowpath upstream of a junction with the secondary flowpath. The motive nozzle has an exit ( 110 ). The mixer has a downstream divergent section down-stream of the convergent section and having 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. Patent Application Ser. No. 61/501,448, filed Jun. 27, 2011, and entitled “Ejector Mixer”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to refrigeration. More particularly, it relates to ejector refrigeration systems. 
         [0003]    Earlier proposals for ejector refrigeration systems are found in U.S. Pat. No. 1,836,318 and U.S. Pat. No. 3,277,660.  FIG. 1  shows one basic example of an ejector refrigeration system  20 . The system includes a compressor  22  having an inlet (suction port)  24  and an outlet (discharge port)  26 . The compressor and other system components are positioned along a refrigerant circuit or flowpath  27  and connected via various conduits (lines). A discharge line  28  extends from the outlet  26  to the inlet  32  of a heat exchanger (a heat rejection heat exchanger in a normal mode of system operation (e.g., a condenser or gas cooler))  30 . A line  36  extends from the outlet  34  of the heat rejection heat exchanger  30  to a primary inlet (liquid or supercritical or two-phase inlet)  40  of an ejector  38 . The ejector  38  also has a secondary inlet (saturated or superheated vapor or two-phase inlet)  42  and an outlet  44 . A line  46  extends from the ejector outlet  44  to an inlet  50  of a separator  48 . The separator has a liquid outlet  52  and a gas outlet  54 . A suction line  56  extends from the gas outlet  54  to the compressor suction port  24 . The lines  28 ,  36 ,  46 ,  56 , and components therebetween define a primary loop  60  of the refrigerant circuit  27 . A secondary loop  62  of the refrigerant circuit  27  includes a heat exchanger  64  (in a normal operational mode being a heat absorption heat exchanger (e.g., evaporator)). The evaporator  64  includes an inlet  66  and an outlet  68  along the secondary loop  62 . An expansion device  70  is positioned in a line  72  which extends between the separator liquid outlet  52  and the evaporator inlet  66 . An ejector secondary inlet line  74  extends from the evaporator outlet  68  to the ejector secondary inlet  42 . 
         [0004]    In the normal mode of operation, gaseous refrigerant is drawn by the compressor  22  through the suction line  56  and inlet  24  and compressed and discharged from the discharge port  26  into the discharge line  28 . In the heat rejection heat exchanger, the refrigerant loses/rejects heat to a heat transfer fluid (e.g., fan-forced air or water or other fluid). Cooled refrigerant exits the heat rejection heat exchanger via the outlet  34  and enters the ejector primary inlet  40  via the line  36 . 
         [0005]    The exemplary ejector  38  ( FIG. 2 ) is formed as the combination of a motive (primary) nozzle  100  nested within an outer member  102 . The primary inlet  40  is the inlet to the motive nozzle  100 . The outlet  44  is the outlet of the outer member  102 . The primary refrigerant flow  103  enters the inlet  40  and then passes into a convergent section  104  of the motive nozzle  100 . It then passes through a throat section  106  and an expansion (divergent) section  108  through an outlet (exit)  110  of the motive nozzle  100 . The motive nozzle  100  accelerates the flow  103  and decreases the pressure of the flow. The secondary inlet  42  forms an inlet of the outer member  102 . The pressure reduction caused to the primary flow by the motive nozzle helps draw the secondary flow  112  into the outer member. The outer member includes a mixer having a convergent section  114  and an elongate throat or mixing section  116 . The outer member also has a divergent section or diffuser  118  downstream of the elongate throat or mixing section  116 . The motive nozzle outlet  110  is positioned within the convergent section  114 . As the flow  103  exits the outlet  110 , it begins to mix with the flow  112  with further mixing occurring through the mixing section  116  which provides a mixing zone. Thus, respective primary and secondary flowpaths extend from the primary inlet and secondary inlet to the outlet, merging at the exit. In operation, the primary flow  103  may typically be supercritical upon entering the ejector and subcritical upon exiting the motive nozzle. The secondary flow  112  is gaseous (or a mixture of gas with a smaller amount of liquid) upon entering the secondary inlet port  42 . The resulting combined flow  120  is a liquid/vapor mixture and decelerates and recovers pressure in the diffuser  118  while remaining a mixture. Upon entering the separator, the flow  120  is separated back into the flows  103  and  112 . The flow  103  passes as a gas through the compressor suction line as discussed above. The flow  112  passes as a liquid to the expansion valve  70 . The flow  112  may be expanded by the valve  70  (e.g., to a low quality (two-phase with small amount of vapor)) and passed to the evaporator  64 . Within the evaporator  64 , the refrigerant absorbs heat from a heat transfer fluid (e.g., from a fan-forced air flow or water or other liquid) and is discharged from the outlet  68  to the line  74  as the aforementioned gas. 
         [0006]    Use of an ejector serves to recover pressure/work. Work recovered from the expansion process is used to compress the gaseous refrigerant prior to entering the compressor. Accordingly, the pressure ratio of the compressor (and thus the power consumption) may be reduced for a given desired evaporator pressure. The quality of refrigerant entering the evaporator may also be reduced. Thus, the refrigeration effect per unit mass flow may be increased (relative to the non-ejector system). The distribution of fluid entering the evaporator is improved (thereby improving evaporator performance). Because the evaporator does not directly feed the compressor, the evaporator is not required to produce superheated refrigerant outflow. The use of an ejector cycle may thus allow reduction or elimination of the superheated zone of the evaporator. This may allow the evaporator to operate in a two-phase state which provides a higher heat transfer performance (e.g., facilitating reduction in the evaporator size for a given capability). 
         [0007]    The exemplary ejector may be a fixed geometry ejector or may be a controllable ejector.  FIG. 2  shows controllability provided by a needle valve  130  having a needle  132  and an actuator  134 . The actuator  134  shifts a tip portion  136  of the needle into and out of the throat section  106  of the motive nozzle  100  to modulate flow through the motive nozzle and, in turn, the ejector overall. Exemplary actuators  134  are electric (e.g., solenoid or the like). The actuator  134  may be coupled to and controlled by a controller  140  which may receive user inputs from an input device  142  (e.g., switches, keyboard, or the like) and sensors (not shown). The controller  140  may be coupled to the actuator and other controllable system components (e.g., valves, the compressor motor, and the like) via control lines  144  (e.g., hardwired or wireless communication paths). The controller may include one or more: processors; memory (e.g., for storing program information for execution by the processor to perform the operational methods and for storing data used or generated by the program(s)); and hardware interface devices (e.g., ports) for interfacing with input/output devices and controllable system components. 
       SUMMARY 
       [0008]    One aspect of the disclosure involves an ejector having a primary inlet, a secondary inlet, and an outlet. A primary flowpath extends from the primary inlet to the outlet. A secondary flowpath extends from the secondary inlet to the outlet. A mixer convergent section is downstream of the secondary inlet. A motive nozzle surrounds the primary flowpath upstream of a junction with the secondary flowpath. The motive nozzle has an exit. The mixer has a downstream divergent section downstream of the convergent section and having 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. 
         [0009]    In various implementations, there may be essentially no normal mixture straight portion (e.g., no straight portion of length more than 5.0 times the minimum diameter of the mixer, more narrowly, no more than 2.0 times). There may be a diffuser downstream of the mixer (e.g., having a divergence half angle of greater than 2.5° over a span of at least 3.0 times the minimum diameter of the mixer. A needle may be mounted for reciprocal movement along the primary flowpath between a first position and a second position. A needle actuator may be coupled to the needle to drive the movement of the needle relative to the motive nozzle. 
         [0010]    Other aspects of the disclosure involve a refrigeration system having a compressor, a heat rejection heat exchanger coupled to the compressor to receive refrigerant compressed by the compressor, a heat absorption heat exchanger, a separator, and such an ejector. An inlet of the separator may be coupled to the outlet of the ejector to receive refrigerant from the ejector. 
         [0011]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic view of a prior art ejector refrigeration system. 
           [0013]      FIG. 2  is an axial sectional view of a prior art ejector. 
           [0014]      FIG. 3  is a partially schematic axial sectional view of a first ejector. 
           [0015]      FIG. 4  is a CFD simulation of the ejector of  FIG. 3 . 
           [0016]      FIG. 5  is a CFD simulation of a prior art ejector. 
           [0017]      FIG. 6  is a schematic axial sectional view of a second ejector. 
       
    
    
       [0018]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0019]      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 2  refrigerant (e.g., at least 50% CO 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. 
         [0020]      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 θ 1 . The slightly divergent section  206  has a divergence half angle θ 2 . The diffuser  208  has a divergence half angle θ 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 MIN . A diameter at the location  212  is shown as D 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 θ 3  are in the vicinity of 3° or greater (e.g., at least&gt;2.0°, more narrowly, at least&gt;2.5° or at least&gt;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 MIN  or an exemplary 3.0-6.0D MIN ). Exemplary diffuser length may also be greater than 3.0 times D MIN . 
         [0021]    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). 
         [0022]    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. 
         [0023]      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. 
         [0024]    In  FIG. 4 , the mixer length and the minimum diameter are preserved from the baseline: L/D MIN ˜4.4 and L/D T ˜3.9. The flow decelerates in the mixer and enters the diffuser without shock. 
         [0025]    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. 
         [0026]    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. 
         [0027]    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 MIN  may initially be chosen as the diameter of the baseline straight mixing portion. D T  will be slightly greater based upon the chosen angle θ 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 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 T  may be slightly greater than the baseline diameter (e.g., 101-110%, more narrowly, 102-104%). 
         [0028]    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. 
         [0029]    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 0 ≡0. 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 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 2 . The location at the downstream end of the diffuser (where it stops diverging) is Z 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 1 ′ which may be the same as Z 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 3 ′. In the exemplary implementation, Z 3 ′ is shown as the same as Z 3 .  FIG. 6  further shows a partially arbitrarily chosen transition location  312  between the mixer and diffuser at a position Z 2 ′. The exemplary position location  312  is defined as the location wherein a half angle θ has a value of 1°. The exemplary Z 2 ′ is shown as being essentially the same as Z 2 . 
         [0030]    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. 
         [0031]    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.