# A Review on Two-Phase Volumetric Expanders and Their Applications
## 4 Two-Phase Scroll Expander
### Literature Review of Two-Phase Scroll Expander  
The analysis of the two-phase scroll expanders mainly addressed developing their applications in vapor compression refrigeration cycles with CO\({}_{2}\) as working fluid. The investigations in this field are described as follows. The field of study related to the two-phase scroll expanders in the CO\({}_{2}\) vapor compression refrigeration cycle began in the early 2000s thanks to [PERSON] et al. [36]. In particular, they performed numerical simulations to investigate the performances of the expander for directly driving the compressor. The assumed displacement volume was 7.93 cm\({}^{3}\) and the assumed operating conditions were an inlet pressure of 8.56 MPa, outlet pressure of 4.17 MPa, and a shaft speed of 3300 rpm. The calculated volumetric and isentropic efficiencies of the machine were 0.87-0.64 and 0.73-0.56 at leakage gap size of 5-10 \(\upmu\)m, respectively. Moreover, the same authors [37] have developed a simulation code of different types of two-phase positive displacement expanders (including the scroll type), including the irreversibilities (valve losses, internal fluid leakages, and heat transfer losses). Requested input data to the simulation code are the geometry of the machine, working fluid properties, and the operating conditions (e.g., suction pressure, suction density, discharge pressure, and angular speed). The results of the simulation code are the volumetric and isentropic efficiencies of the machine, the thermodynamic state of the fluid during the process, the mass flow rate of the internal fluid leakages and heat transfer during the process, net mass flow rate, power output, and the forces and moments exerted by the working fluid on the machine. In the same period, [PERSON] et al. [38; 39] carried out the first pioneering studies in the field of two-phase scroll expanders in CO\({}_{2}\) vapor compression refrigeration cycle. In particular, they built the proof-of-concept prototype of the machine (with a displacement volume of 2.3 cc and volume expansion ratio of 2) based on the design concept developed by CFD and FEM analysis. Testing was carried out in the presence of lubricating oil (PAG type), at an inlet pressure of 101 bar, inlet temperature of 53.6 \({}^{\circ}\)C, outlet pressure of 41 bar, shaft speed of 3300 rpm, and shaft power of 637 W. The quantification of experimental performances was ongoing. Figure 12: Two-phase scroll expander, with geometry (**a**) [12] and principle of operation (**b**) [10]. In 2006, [PERSON] et al. [33] performed both theoretical and experimental investigations on a prototype of a two-phase scroll expander derived from a scroll compressor manufactured by Matsushita Electric Industrial Co. Ltd. The machine worked in the presence of lubricating oil and with a chamber volume of 1.54 cm\({}^{3}\) at the beginning of the expansion process and BVR of 2.18. The theoretical volumetric and total efficiencies of the two-phase scroll expander were 0.85 and 0.60, respectively at inlet pressure of 10 MPa, inlet temperature of 40 \({}^{\circ}\)C, outlet pressure of 4 MPa and 3600 rpm. The experimental volumetric and total efficiencies were 0.80 and 0.55, respectively at an inlet pressure of 9 MPa, inlet temperature of 40 \({}^{\circ}\)C, outlet pressure of 4 MPa, and a shaft speed of 3500 rpm. In 2008, [PERSON] et al. [34] developed a two-phase scroll expander prototype implementing techniques to reduce the radial and flank leakages. The machine had a displacement volume of 0.5 cm\({}^{3}\) and worked with an inlet pressure of 10 MPa, inlet temperature of 20 \({}^{\circ}\)C, and outlet pressure in the range 4-7 MPa. In particular, the decrease in the radial leakages was obtained by using the over-expansion. Indeed, this solution the separation of the orbiting spiral from the fixed one to be avoided, even in the case of a change in operating conditions. The decrease in the flank leakages was obtained by a mechanism using the oil-film pressure on the shaft bearing to force the orbiting spiral to the fixed spiral.