Swirling and cavitating flow suppression in a cryogenic liquid turbine expander through geometric optimization

Abstract

A single-stage cryogenic liquid turbine expander is developed as a replacement of Joule–Thompson valve in the internal compression air separation unit for energy-saving purpose. Flow analysis and optimization is conducted for the turbine expander. With the original geometry, static pressure drops gradually from the nozzle to impeller together with a 2.7 K temperature drop, which exhibits simultaneously a smooth throttling characteristic and cryogenic refrigeration effect. However, similar to the conventional hydraulic turbine, a vortex-rope is apparently formed around the draft tube centerline. It leads to considerable mechanical energy dissipation and, subsequently, a local pressure drop and temperature rise, which have made the turbine expander vulnerable to cavitation. The draft tube vortex swirling flow has been found to be sensitive to exit geometric shape of rotating impeller. To suppress the swirling flow and cavitation, design optimization of impeller geometric shape is further conducted with an efficient global optimization method developed by the authors, where in particular, an innovative optimization objective function and a simultaneous tuning of both impeller meridian profile and blade shape are incorporated. The former is a linear combination of the draft tube loss factor and normalized impeller exit static pressure. It depicts the draft tube swirling flow behavior and also captures somehow the cavitation flow physics. The latter permits a very flexible variation of the impeller geometry. Such a highly nonlinear problem is solved by the global optimization algorithm, in which the Kriging surrogate model is used but updated through adaptive sampling. It is demonstrated that with the optimized geometry, the vortex-rope like characteristics has diminished apparently and both scale and intensity of swirling region are reduced significantly. As a result, the low static pressure region has shrunk and the local temperature rise is reduced and, subsequently, the cavitation is effectively suppressed.