1.  Technical Field
The present disclosure relates to an ejector including a single-fluid atomization nozzle and a heat pump apparatus including the ejector. 
2.  Description of the Related Art
Atomization technologies are applied to energy-related technologies, such as combustion of liquid fuels; and to various industrial fields, such as spray painting, spray drying, moisture adjustment, spraying of agricultural chemicals, and fire extinguishing.  Performance required for a spray nozzle varies depending on the use of the spray nozzle.  Various atomization methods for spray nozzles have been developed.  Examples of such methods include turbulent atomization, atomization including breaking of a thin film formed by spraying, centrifugal atomization, atomization including forming and breaking a liquid thread, and atomization using interaction between two fluids. 
Ejectors are used as decompression means of various apparatuses, such as vacuum pumps and refrigeration cycle apparatuses.  As illustrated in FIG.  9, a refrigeration cycle apparatus 200 described in Japanese Patent No.  3158656 includes a compressor 102, a condenser 103, an ejector 104, a separator 105, and an evaporator 106.  The ejector 104 receives a refrigerant liquid as a drive flow from the condenser 103, sucks in and pressurizes a refrigerant vapor supplied from the evaporator 106, and ejects the refrigerant liquid and the refrigerant vapor toward the separator 105.  The separator 105 separates the refrigerant liquid and the refrigerant vapor from each other.  The compressor 102 sucks in the refrigerant vapor pressurized by the ejector 104.  Thus, the compression work to be done by the compressor 102 is reduced and the COP (coefficient of performance) of a refrigeration cycle is improved. 
As illustrated in FIG.  10, the ejector 104 includes a nozzle 140, a suction port 141, a mixer 142, and a pressurizer 143.  A plurality of connection ports 144, through which the inside of the nozzle 140 is connected to the outside of the nozzle 140, are disposed near the outlet of the nozzle 140.  The refrigerant vapor is sucked into the ejector 104 through the suction ports 141.  A part of the refrigerant vapor sucked into the ejector 104 flows to the inside of the nozzle 140 through the connection ports 144.  The nozzle 140 of the ejector 104 has a reduced-diameter portion near the outlet thereof.  In the reduced-diameter portion, the flow velocity of the refrigerant increases and the pressure of the refrigerant decreases.  Accordingly, the phase of the refrigerant (drive flow), which is supplied to the nozzle 140, changes from a liquid phase to a vapor-liquid two-phase in the reduced-diameter portion.  However, when a supercooled liquid is used as a drive flow, the drive flow cannot be atomized because the phase change does no occur. 
As illustrated in FIG.  11, an ejector 300 described in International Publication No.  2015/019563 includes a first nozzle 301, a second nozzle 302, an atomization mechanism 303, and a mixer 304.  A working fluid in a liquid phase is supplied to the first nozzle 301.  A working fluid in a vapor phase is sucked into the second nozzle 302.  The atomization mechanism 303 is disposed at an end of the first nozzle 301 and atomizes the working fluid in the liquid phase while maintaining the liquid phase.  The atomized working fluid generated by the atomization mechanism 303 and the working fluid in the vapor phase sucked into the second nozzle 302 are mixed in the mixer 304, and thereby a merged fluid flow is generated. 
As illustrated in FIG.  12, the atomization mechanism 303 includes an ejection section 306 and a collision surface forming section 307.  The ejection section 306 is attached to the end of the first nozzle 301.  The ejection section 306 has a plurality of orifices 308.  The orifices 308 extend through a bottom part of the ejection section 306, which has a tubular shape, so as to connect the first nozzle 301 to the mixer 304.  Through the orifices 308, a refrigerant liquid is ejected from the first nozzle 301 toward the collision surface forming section 307.  The collision surface forming section 307 has a collision surface 309, with which a jet from the ejection section 306 is to collide.  The collision surface forming section 307 includes a shaft portion 310 and a flared portion 311.