Supercritical CO2 (sCO2) cycles are considered as a promising technology for next generation concentrated solar thermal, waste heat recovery, and nuclear applications. Particularly at small scale, where radial inflow turbines can be employed, using sCO2 results in both system advantages and simplifications of the turbine design, leading to improved performance and cost reductions. This paper aims to provide new insight toward the design of radial turbines for operation with sCO2 in the 100–200 kW range. The quasi-one-dimensional mean-line design code topgen is enhanced to explore and map the radial turbine design space. This mapping process over a state space defined by head and flow coefficients allows the selection of an optimum turbine design, while balancing performance and geometrical constraints. By considering three operating points with varying power levels and rotor speeds, the effect of these on feasible design space and performance is explored. This provides new insight toward the key geometric features and operational constraints that limit the design space as well as scaling effects. Finally, review of the loss break-down of the designs elucidates the importance of the respective loss mechanisms. Similarly, it allows the identification of design directions that lead to improved performance. Overall, this work has shown that turbine design with efficiencies in the range of 78–82% is possible in this power range and provides insight into the design space that allows the selection of optimum designs.

Pulmonary surfactant, a unique developmentally regulated, phospholipid-rich lipoprotein, is synthesized by the type II epithelial cells (AECII) of the pulmonary alveolus, where it is stored in organelles termed lamellar bodies. The synthesis of pulmonary surfactant is under multifactorial control and is regulated by a number of hormones and factors, including glucocorticoids, prolactin, insulin, growth factors, estrogens, androgens, thyroid hormones, and catecholamines acting through beta-adrenergic receptors, and cAMP. While there is increasing evidence that microRNAs (miRNAs) are involved in the regulation of almost every cellular and physiological process, the potential role of miRNAs in the regulation of pulmonary surfactant synthesis remains unknown. miRNA-26a (miR-26a) has been predicted to target SMAD1, one of the bone morphogenetic protein (BMP) receptor downstream signaling proteins that plays a key role in differentiation of lung epithelial cells during lung development. In this study, we explored the regulation role of miR-26a in the synthesis of pulmonary surfactant. An adenoviral miR-26a overexpression vector was constructed and introduced into primary cultured fetal AECII. GFP fluorescence was observed to determinate the transfection efficiency and miR-26a levels were measured by RT-PCR. MTT was performed to analyze AECII viability. qRT-PCR and Western blotting were used to determine the mRNA and protein level of SMAD1 and surfactant-associated proteins. The results showed that miR-26a in fetal AECII was overexpressed after the transfection, and that the overexpression of miR-26a inhibited pulmonary surfactant synthesis in AECII. There was no significant change in cell proliferation. Our results further showed that overexpression of miR-26a reduced the SMAD1 expression both in mRNA and protein level in fetal AECII. These findings indicate that miR-26a regulates surfactant synthesis in fetal AECII through SMAD1.

The foil bearing is an enabling technology for turbomachinery systems, which has the potential to enable cost efficient supercritical CO2 cycles. The direct use of the cycle's working fluid within the bearings results in an oil-free and compact turbomachinery system; however, these bearings will significantly influence the performance of the whole cycle and must be carefully studied. Moreover, using CO2 as the operating fluid for a foil bearing creates new modeling challenges. These include highly turbulent flow within the film, non-negligible inertia forces, high windage losses, and nonideal gas behavior. Since the flow phenomena within foil bearings is complex, involving coupled fluid flow and structural deformation, use of the conventional Reynolds equation to predict the performance of foil bearings might not be adequate. To address these modeling issues, a three-dimensional flow and structure simulation tool has been developed to better predict the performance of foil bearings for the supercritical CO2 cycle. In this study, the gas dynamics code, eilmer, has been extended for multiphysics simulation by implementing a moving grid framework, in order to study the elastohydrodynamic performance of foil bearings. The code was then validated for representative laminar and turbulent flow cases, and good agreement was found between the new code and analytical solutions or experiment results. A separate finite difference code based on the Kirchoff plate equation for the circular thin plate was developed in Python to solve the structural deformation within foil thrust bearings, and verified with the finite element analysis from ansys. The fluid-structure coupling algorithm was then proposed and validated against experimental results of a foil thrust bearing that used air as operating fluid. Finally, the new computational tool set is applied to the modeling of foil thrust bearings with CO2 as the operating fluid.

Please cite this article as: Qin K, Jacobs PA, Keep JA, Li D, Jahn IH, A fluid-structure-thermal model for bump-type foil thrust bearings, Tribology International (2018),• Implementation of a computational framework for fluid-structure-thermal simulations.• The fluid-thermal coupling is validated.• A third of the generated heat is advected with the fluid in the CO2 case, compared to only 3% for the air case.• Heat transfer to the stator is similar for both air and CO2 cases. AbstractThis paper presents a multi-physics multi-timescale computational framework for the three-dimensional and two-way coupled fluid-structure-thermal simulation of foil thrust bearings. Individual solvers for the transient fluid flow, structural deformation, heat conduction and the coupling strategy are discussed. Next, heat transfer models of the solid structures within foil thrust bearings are also described in detail. The result is a multi-physics computational framework that can predict the steady state and dynamic performance of foil thrust bearings. Numerical simulations of foil thrust bearings with air and CO 2 are then performed. It is found that the centrifugal pumping that naturally occurs in CO 2 bearings due to the high fluid density provides a new and effective cooling mechanism for the CO 2 bearing.