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Nili-Ahmadabadi, Mahdi,Nematollahi, Omid,Cho, Dae Seung,Kim, Kyung Chun Elsevier 2018 Applied thermal engineering Vol.137 No.-
<P><B>Abstract</B></P> <P>A real dense gas such as R245fa is mostly used in organic-Rankine-cycle turbine expanders. The dense gas effects should be taken into account, especially in the transonic and supersonic flow regimes. Oblique shock and the interaction of shock and separation on the turbine blades are phenomena that have little deviation between a real gas and an ideal gas. This research numerically simulated the flow passing through a cascade of simple straight blades with keen edges considered for an ideal gas (air) and dense R245fa gas in a supersonic flow regime. The blade geometry was selected so that the deviations between the dense and ideal gas flows would be clearer than that with actual blades. The AUSM density-based method and NIST real gas model were used to model the ideal and dense gas, respectively. A second-order scheme was used for discretization, and the shear stress transport (SST) model was for the turbulence. The results show that an oblique shock is created on the leading edge when the inlet Mach number is 2.18 in dense gas. In ideal gas, a bow shock is created at the front of the leading edge. Moreover, for a wall pressure coefficient distribution, the separation point in dense gas is posterior than that in ideal gas.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Flow passing through a cascade has numerically simulated. </LI> <LI> Simple straight blades with keen edges are considered. </LI> <LI> Ideal gas (air) and R245fa gas in a supersonic flow regime are compared. </LI> <LI> Oblique shock is created in dense gas while a bow shock is in ideal gas. </LI> </UL> </P>
Optimization of a seven-stage centrifugal compressor by using a quasi-3D inverse design method
Mahdi Nili-Ahmadabadi,Farzad Poursadegh 대한기계학회 2013 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.27 No.11
This paper focuses on performance improvement of a centrifugal compressor. An inverse design method for 3D design approaches is formulated to address this concern. The design procedure encompasses two major steps. First, with the use of ball spine algorithm, which is an inverse design algorithm, on the meridional plane of impeller, the hub and shroud of impeller are computed based on a modified pressure distribution along them. Second, an original and progressive algorithm is developed for design of blade camber line profile on the blade-to-blade planes of impeller based on blade loading improvement. Full 3D analysis of the current and designed compressor is accomplished by using a Reynolds-averaged Navier–Stokes equations solver. A comparison between the analysis results of the current and designed compressor shows that the total-to-total isentropic efficiency and pressure ratio of the designed compressor under the same operating conditions are enhanced by more than 4.5% and 5%, respectively.
Centrifugal compressor shape modification using a proposed inverse design method
Mahdi Nili-Ahmadabadi,Farzad Poursadegh 대한기계학회 2013 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.27 No.3
This paper is concerned with a quasi-3D design method for the radial and axial diffusers of a centrifugal compressor on the meridional plane. The method integrates a novel inverse design algorithm, called ball-spine algorithm (BSA), and a quasi-3D analysis code. The Euler equation is solved on the meridional plane for a numerical domain, of which unknown boundaries (hub and shroud) are iteratively modified under the BSA until a prescribed pressure distribution is reached. In BSA, unknown walls are composed of a set of virtual balls that move freely along specified directions called spines. The difference between target and current pressure distributions causes the flexible boundary to deform at each modification step. In validating the quasi-3D analysis code, a full 3D Navier-Stokes code is used to analyze the existing and designed compressors numerically. Comparison of the quasi-3D analysis results with full 3D analysis results shows viable agreement. The 3D numerical analysis of the current compressor shows a huge total pressure loss on the 90° bend between the radial and axial diffusers. Geometric modification of the meridional plane causes the efficiency to improve by about 10%.
Shape memory effect in nanocrystalline NiTi alloy processed by high-pressure torsion
Shahmir, H.,Nili-Ahmadabadi, M.,Huang, Y.,Myun Jung, J.,Seop Kim, H.,Langdon, T.G. Elsevier Sequoia 2015 Materials science & engineering. properties, micro Vol.626 No.-
A NiTi alloy was processed by high-pressure torsion for 10 turns followed by post-deformation annealing at 673K for various times. An anneal for 60min gave a nanocrystalline microstructure with a superior shape memory effect and an improvement of more than 40% over the initial state.
Shahmir, Hamed,Nili-Ahmadabadi, Mahmoud,Huang, Yi,Jung, Jai Myun,Kim, Hyoung Seop,Langdon, Terence G. Elsevier 2018 Materials science & engineering. properties, micro Vol.734 No.-
<P><B>Abstract</B></P> <P>A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19' phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19' phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition.</P>