Experimental Investigation into Performance of Laval Nozzles for Reaction Turbines

Large resources of thermal and industrial waters at a temperature of 100–150°С can be employed to generate electricity in both a single-circuit thermal cycle with direct usage of natural steam in condensing turbines and a double-circuit cycle with a low-boiling working fluid. Both options require high capital expenditures for their implementation. A new power-generation method on the basis of low-grade heat is to install a full-flow hydro-steam turbine (HST), which utilizes the heat of separated liquid from a geothermal station or the heat of industrial waters, in a single-circuit thermal cycle. Capital expenditures for these units are much lower since special equipment for preparation of the working fluid is not required. The efficiency of a hydro-steam turbine is directly related to the efficiency of its main element, which is the Laval nozzle. Designing this turbine requires reliable data on the performance of a Laval nozzle with a high expansion ratio when it operates on highly subcooled water. Such information can only be obtained experimentally since this region of operation of Laval nozzles has been poorly studied as of yet. To determine the required characteristics of the nozzles, an experimental setup was designed and manufactured with a set of Laval nozzles, including nozzles with different opening angles of the diverging section and nozzles with an oblique cut. The working fluid was water under conditions close to the operating conditions in an HST. The results are presented of the experimental study of Laval nozzles in a wide range of expansion ratios for operation on boiling water. The experiments were performed at different pressures downstream of the nozzle, and their results were compared with previously obtained experimental data. The experimental setup and design features of the nozzle are described. The experimental data on the flow coefficient, velocity coefficient, expansion ratio, initial conditions, nozzle shapes, and size effect, on which the conclusions made are based, have been analyzed. Characteristics have been found that enable us to predict the efficiency of a reaction hydro-steam turbine and properly calculate its output.