# A Review on Two-Phase Volumetric Expanders and Their Applications
## 5 Two-Phase Twin Screw Expander
### Literature Review of Two-Phase Twin Screw Expander  
The Swedish company SRM carried out pioneering work concerning two-phase twin screw expanders, which led to profiles with effective sealing and a large flow area through which the working fluid could flow. Indeed, the rotors profiles must form a good seal between each other and between the casing at all rotational positions in order to minimize internal leakages, resulting in maximization of the isentropic efficiency. Moreover, direct contact between the lobe surfaces must be avoided to prevent seizures. Most screw rotor designs used today are derivatives of the SRM rotor profiles [46]. In the early 2000s, a new family of rotor profiles (called \"N\" profiles) of two-phase twin screw expanders was developed. The main advantage was the low rotor contact stresses between the male and female rotors and almost pure rolling motion along with the contact band between them. Thus, timing gears were not required, provided that a low viscosity working fluid was present in the working chamber. The working fluid circulated fairly freely through the bearings using drilled passages, and internal seals were absent. Experimental tests were carried out with R-113 as a working fluid containing less than 1% lubricating oil. After several hundred hours of tests, there was no detectable wear found on bearings or rotors. The maximum isentropic efficiency was 0.76 [46]. The analysis of the two-phase scroll expanders mainly addressed developing their applications in power cycles, such as direct expansion of a geothermal fluid, TFC, and a wet  
Figure 13: Two-phase twin screw expander, with architecture (**a**) [12] and principle of operation (**b**) [45]. vapor Rankine cycle. The review in this field is described as follows, and the main results are in Table 2.  
The field of study related to the two-phase screw expanders in power cycles began in the 1970s thanks to [PERSON] [25; 47]. He conceived a two-phase twin screw expanders for the direct expansion of a geothermal fluid. The screw expander was of a Lysholm type, and it achieved maximum adiabatic efficiencies of only 0.53 in small-scale tests and 0.68 on a 1 MW machine. Moreover, the large size of screw expanders needed for large-scale power generation presented serious manufacturing problems. [PERSON] et al. provided a numerical analysis of the earlier two-phase screw expanders working with water to explain their poor performances and improve their operation [48]. For this aim, they conducted an experimental campaign to validate the model by testing several machines working with R113. These experimental results provided the pressure-volume diagram and revealed an adiabatic efficiency between 0.20-0.80 with an inlet temperature of 90-120 \({}^{\circ}\)C. These authors pointed out that a higher liquid fraction leads to a greater fluid density, thus, causing greater pressure drops across the ports in the intake. In the remaining lines of their study, they proposed a numerical case study to test two stages of a two-phase twin screw expander working with n-butane. They determined an adiabatic efficiency of 0.82 for an expansion drop between 137 \({}^{\circ}\)C and 35 \({}^{\circ}\)C.  
In 2005, [PERSON] et al. [49] theoretically investigated the application of a two-phase twin screw expander for direct expansion of a geothermal fluid. They found that the BVR is independent of operating conditions and exclusively depends on the size and shape of the inlet conduit geometry that can be changed with moderate costs. Low values of BVR imply higher working fluid flow rates, resulting in lower adverse effects of leakages. Nevertheless, BVR values cannot be too low, otherwise the under-expansion phenomenon takes place, resulting in a reduction in the isentropic efficiency. On the other hand, a compromise between leakages and friction losses determines the optimum rotational velocity, which also depends on the working fluid type. In particular, the velocity increase implies the decrease in the leakages and the increase in the friction losses.