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.

Cryogenic Cavitation Mitigation in a Liquid Turbine Expander of an Air-Separation Unit through Collaborative Fine-Tuned Optimization of Impeller and Fairing Cone Geometries

“…Such a model was justified by some previous investigators in the studies of both conventional hydraulic machines [13,52], and cryogenic turbomachines [7][8][9], with the cavitation models having been verified by the prese t team of authors in our previous publications [42,44]. The cryogenic hydrofoil flow was simulated and the predicted surface static pressure and temperature were consistent with the test data [53] (Figure 3).…”

“…Intensive CFD callings are required for the evaluation of cavitation performance in the optimization process and to establish the objective functions [13,44,47]. To have an insight into cavitating flow physics prior to commencing the optimization, two-phase flow analysis is conducted for the originally designed expander geometry, from which the influential geometric factors are identified.…”

“…The optimization algorithms proposed by the present authors has been successfully applied in axial flow compressors [58] and in cryogenic liquid turbine expander design optimization [44]. The incorporation of such optimization algorithms into the present turbine expander design optimization is briefed below, and a detailed description of the optimization algorithms can be found in our previous publications [44,58]. Figure 9 presents the optimization framework for cryogenic cavitation mitigation in an ASU liquid turbine expander, which is driven by the newly established cavitation-attenuation objective function that allows for the collaborative fine-tuning of the impeller and fairing cone geometric shapes.…”

“…Notably, cavitation of conventional hydraulic turbines/pumps is mainly associated with local pressure, but cavitation of cryogenic liquid turbine expanders is much more complex, and depends on both the variations of static pressure and static temperature due to the significant thermodynamic effect of cryogenic fluids. Over the years, significant work on cryogenic liquid turbine expanders and the cavitating flow mechanism have been explored by the present team of authors [42][43][44]. As demonstrated in [44,45], the swirling flow in liquid turbine expanders has significant influence over impeller and diffuser tube cavitation, and both swirling flow and cavitation are sensitive to the geometric variations of impeller blades [44].…”

“…Over the years, significant work on cryogenic liquid turbine expanders and the cavitating flow mechanism have been explored by the present team of authors [42][43][44]. As demonstrated in [44,45], the swirling flow in liquid turbine expanders has significant influence over impeller and diffuser tube cavitation, and both swirling flow and cavitation are sensitive to the geometric variations of impeller blades [44]. In our previous work [46], a fairing cone was introduced to control impeller cavitation.…”