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

Abstract

An air-separation unit (ASU) uses atmospheric air to produce essential pure gaseous and liquid products for many industrial sectors but requires intensive power consumption. In recent years, cryogenic liquid turbine expanders have been used to replace the traditional J-T valves in air-separation units to save energy. In this paper, an effective design optimization method is proposed to suppress swirling flow and mitigate cavitation in liquid turbines. A flexible tuning of the impeller and fairing cone geometries is simultaneously realized, where the optimization variables are identified via a geometric sensitivity study. A novel objective function is deliberately established by allowing both swirling flow and cavitation characteristics, driving the optimizer to search for deswirling and cavitation-resistant geometries. A kriging surrogate model with an adaptive sampling strategy and a cooperative co-evolution algorithm (CCEA) are incorporated to solve the highly nonlinear optimization problem, where the former reduced the costly evaluations but simultaneously maintained the model prediction accuracy and enabled the aim-oriented global searching (the latter decomposes the problem into several readily solved sub-problems that could be solved in parallel at a high-convergence rate). The optimized impeller and fairing cone geometries were quite favorable for suppressing swirling flow and mitigating cavitation. The impeller cavitation was significantly reduced, with the maximal vapor volume fraction reduced from 0.365 to 0.17 at the blade surface; the diffuser tube high-swirl flow was significantly deswirled and the intensive vapor fraction around the centerline largely reduced, with the maximal vapor volume fraction in the diffuser tube reduced from 0.387 to 0.121. As a result, the isentropic efficiency of the liquid turbine expander was improved from 88.4% to 91.43%. Highlights •Objective function considers both the swirling flow and cavitation characteristics •The developed optimization method is proven to be efficient for cavitating flow mitigation • Collaborative fine-tuned impeller and fairing cone geometries diminish the swirling flow and then suppress cavitation Energies 2020, 13, 50 2 of 21In large-scale internal compression ASUs, high-pressure liquefied air (up to 60-75 MPa) needs to be throttled to a low level (around 0.5-0.6 MPa) so as to meet the technical requirements of the downstream distillation column. Conventionally this has been done with a Joule-Thomson valve, but this can cause severe problems. For example, the high-level pressure head is wasted in the cryogenic ASU, and it unexpectedly raises the cryogenic system temperature, which can compromise the ASU's energy efficiency by 2.5%-3.5%. In recent years, liquid turbine expanders have been adopted in place of traditional Joule-Thomson valves, as they have a proven capacity for enhancing ASU energy efficiency [1][2][3]. Liquid turbine expanders reduce the pressure head of the working medium, while simultaneously converting it into output shaft pow...

Performance and flow characteristics of the liquid turbine for supercritical compressed air energy storage system

“…He et al [32] presented steady and unsteady performance of the liquid turbine and used Zwart-Gerber-Belamri model [33] to simulate the cryogenic cavitation. Song et al [34][35][36][37][38] developed optimization methods to design the geometry of the liquid turbine and suppress the cavitation. Wang et al [39] studied the influence of the impeller faring cone on cavitation.…”