Two-phase flow, Numerical simulation, Impulse turbine
A two-phase impulse turbine used to replace the classic expansion valve in a refrigeration system needs a nozzle/expander to transform the flow stored enthalpy into kinetic energy, and a runner that comes afterwards to transform the flow kinetic energy into torque. That process transforms the isenthalpic expansion of the refrigerant into, ideally, an isentropic one. Replacing a classic isenthalpic expansion with a nearly isentropic one increases the cycle cooling capacity by 8% up to 20 % for the same compressor input power and generates an additional electric power by the two-phase turbine. During the expansion in the nozzle, the static pressure of the flow decreases and a phase change occurs, for the flow is slightly subcooled, by 2 to 3 K, at the nozzle entrance. A Turgo turbine runner design is presented. The runner makes use of the kinetic energy of a two-phase flow and transforms it into torque. The geometry and material of the runner components are chosen to provide mechanical integrity and high efficiency. The design takes into consideration the number of buckets, the flow behavior inside a bucket, the power loss due to drag force, the flow impingement angle with the bucket and many other factors. The variation of the efficiency of the runner with the refrigerant quality and the gas physical properties is also studied. FEM (finite element method) is used to calculate the mechanical stress on the structure of the runner that is supposed to withstand static and fatigue stress, and erosion due to the two-phase nature of the flow. FEM is also used to visualize the flow inside the bucket and to estimate the power loss due to drag force. The calculated efficiency of the runner is then compared to the efficiency of a similar tested runner.