Numerical analysis of oil injection effects in a single screw expander

The pursuit of higher efficiency for compression and micro-power generation systems has pushed the researchers to an in-depth analysis of positive displacement machines. Single-screw machines, among the others, are gaining attention in the Organic Rankine Cycle (ORC) systems as expanders, thanks to their extended maintenance intervals and compactness. The performances of such devices are strongly affected by the working conditions, and especially the presence of oil has major effects on the operability. The main advantage of adopting an oil-injected device consists in the lube sealing effect, which permits better performance (greater shaft power for assigned boundary conditions) as well as higher reliability. The choice of whether using an oil-free configuration or not is related to the working fluid cleanness, system complexity (oil separator, filters, recovery pump), flow rate and pressure ratio. In this paper, the full 3D numerical simulation of an oil-injected single-screw expander operating with R245fa refrigerant is presented. Oil is injected together with the working fluid at the inlet of the machine. Oily droplets are tracked over the admission duct to show how the oil droplets reach the inlet ports of the screw machine. Different behaviors related to different oil droplet diameters in the range of (0.5 - 50) μm are studied, for the same operating point. The proper distribution of the oil droplets on the screw inlet ports are directly related to the single screw expander performance. In addition, a particular screw position is analyzed for studying the effects of leakages on the oil injection and oil film evolution over the time.