Local Thermal Non-equilibrium Analysis of Cu-Al2O3 Hybrid ‎Nanofluid Natural Convection in a Partially Layered Porous ‎Enclosure with Wavy Walls

A numerical study is performed to investigate the local thermal non-equilibrium effects on the natural convection in a two-dimensional enclosure with horizontal wavy walls, layered by a porous medium, saturated by Cu-Al2O3/water hybrid nanofluid. It is examined the influence of the nanoparticle volume fraction, varied from 0 to 0.04, the Darcy number (10-5 ≤ Da ≤ 10-2), the modified conductivity ratio (0.1 ≤ ϒ ≤ 1000), the porous layer height (0 ≤ Hp ≤ 1), and the wavy wall wavenumber (1 ≤ N ≤ 5) on natural convection in the enclosure. Predictions of the steady incompressible flow and temperature fields are obtained by the Galerkin finite element method, using the Darcy-Brinkman model in the porous layer. These are validated against previous numerical and experimental studies. By resolving separately the temperature fields of the working fluid and of the porous matrix, the local thermal non-equilibrium model exposed hot and cold spot formation and mitigation mechanisms on the heated and cooled walls. By determining the convection cell strength, the Darcy number is the first rank controlling parameter on the heat transfer performance, followed by N, Hp and γ. The heat transfer rate through the hybrid nanofluid and solid phases is highest when N = 4 at a fixed value of nanoparticle volume fraction

Off-design performance of a liquefied natural gas plant with an axial turbine of novel endwall design

A design optimization workflow for the casing of a 1.5 stage axial turbine is implemented through a novel endwall surface definition, towards improving the turbine efficiency. The new non-axisymmetric casing design compares favourably to an established diffusion design technique. The workflow uses an axial turbine three-dimensional Reynolds Averaged Navier-Stokes model built in OpenFOAM Extend 3.2 with the k-ω Shear Stress Transport turbulence closure. Computer-based optimization of the surface topology using a Kriging surrogate model automates the design process. The designs are optimized using the total pressure loss across the full stage as the target function. Axial turbine performance gains are obtained from the workflow, which persist both at the design condition and off-design. These gains are used to project the impact of equivalent design improvements to the power turbine of a representative Natural Gas liquefaction plant cycle. Cycle Coefficient of Performance enhancements between 2.05% and 2.923% are obtained, at design and at off design conditions. Implementing these performance improvements has the potential to reduce carbon dioxide emissions by 165.54 tonnes per year at design and by 108.18 tonnes per year at off design, in a representative Natural Gas liquefaction plant

An adaptive sampling technique for optimizing the design of axial turbine endwalls

Computer-based optimization is demonstrated for the design of a 1.5 stage axial turbine casing endwall. It uses a recursive implementation of the Optimal Latin Hypercube (OLH) technique for sampling the design space and Kriging as the surrogate model for performance. The latter is evaluated by Computational Fluid Dynamics (CFD). At design and at off design conditions, reductions in the stage total pressure loss coefficient verify the effectiveness of the optimization method

Design optimization workflow and performance analysis for contoured endwalls of axial turbines

Advances in computer-based optimization techniques can be used to enhance the efficiency of energy conversions processes, such as by reducing the aerodynamic loss in thermal power plant turbomachines. One viable approach for reducing this flow energy loss is by endwall contouring. This paper implements a design optimization workflow for the casing geometry of a 1.5 stage axial turbine, towards mitigating secondary flows. Two different parametric casing surface definitions are used in the optimization process. The first method is a new non-axisymmetric casing design using a novel surface definition. The second method is an established diffusion design technique. The designs are tested on a three-dimensional axial turbine RANS model. Computer-based optimization of the surface topology is demonstrated towards automating the design process. This is implemented using Automated Process and Optimization Workbench (APOW) software. Kriging is used to accelerate the optimization process. The optimization and its sensitivity analysis give confidence that a good predictive ability is obtained by the Kriging surrogate model used in the prototype design process tested in this work. A flow analysis confirms the positive impact of the optimized casing groove design on the stage isentropic efficiency compared to the diffusion design and compared to the benchmark axisymmetric design

Perspectives on the treatment of secondary flows in axial turbines

A review is presented of the main end-wall treatments for the secondary flows of axial turbines. This encompasses the use of axisymmetric and non-axisymmetric contoured end-walls, the use of fences, and of air injection and blowing. Experimental and numerical results show promise in all these techniques. Interest seems to be drawing towards the use of non-axisymmetric contoured end-walls, due to their good compatibility with current turbine passage designs and their appealing stage pressure loss reduction at design conditions and off design. An insight is provided into the flow changes from non-axisymmetric end walls and on their effect on secondary flow losses

Optimization of the non-axisymmetric stator casing of a 1.5 stage axial turbine

The interaction of secondary flows with the main passage flow in turbomachines results in entropy generation and in aerodynamic loss. This loss source is most relevant to low aspect ratio blades. One approach for reducing this flow energy loss is by endwall contouring. However, limited work has been reported on using non-axisymmetric endwalls at the stator casing and on its interaction with the tip leakage flow. In this paper, a non-axisymmetric endwall design method for the stator casing is implemented through a novel surface definition, towards mitigating secondary flow losses. This design is tested on a three-dimensional axial turbine RANS model built in OpenFOAM 3.2 Extend, with k−ω SST turbulence closure. The flow analysis confirms the foundations of the new surface definition approach, which is implemented using Alstom Process and Optimization Workbench (APOW) software. Computer-based optimization of the surface topology is demonstrated towards automating the design process of axial turbines in an industrial design workflow. The design is optimized using the total pressure loss across the first stator and across the full stage, as the target function. Numerical predictions of the 1.5 stage axial turbine show the positive impact of the optimized casing design on the efficiency that increases by 0.69% against the benchmark axisymmetric stage from RWTH Aachen, which is validated against experiment. The new non-axisymmetric casing is also beneficial at off-design condition. The effective mitigation of the secondary flows is predicted to give a 0.73% efficiency gain off-design

Improving the Performance of Gas Turbine Power Plant by Modified Axial Turbine

Computer-based optimization techniques can be employed to improve the efficiency of energy conversions processes, including reducing the aerodynamic loss in a thermal power plant turbomachine. In this paper, towards mitigating secondary flow losses, a design optimization workflow is implemented for the casing geometry of a 1.5 stage axial flow turbine that improves the turbine isentropic efficiency. The improved turbine is used in an open thermodynamic gas cycle with regeneration and cogeneration. Performance estimates are obtained by the commercial software Cycle – Tempo. Design and off design conditions are considered as well as variations in inlet air temperature. Reductions in both the natural gas specific fuel consumption and in CO2 emissions are predicted by using the gas turbine cycle fitted with the new casing design. These gains are attractive towards enhancing the competitiveness and reducing the environmental impact of thermal power plant

The Performance of a 1.5 stage Axial Turbine with a Non-Axisymmetric Casing at Off-Design Conditions

Advances in manufacturing techniques allow greater freedom in designing axial turbine stage passages, including non-axisymmetric end walls. A non-axisymmetric end wall design method for the stator casing is implemented through a novel surface definition, towards mitigating secondary flow losses. The new design is compared with a diffusion design from the literature. Off-design operations are considered. umerical predictions of a 1.5 stage axial turbine show a reduction in the rotor row total pressure loss of 1.69 % against the benchmark axisymmetric stage from RTWH Aachen, which is validated against experiment. Flow analysis gives an insight into the effectiveness of the new surface definition approach, which is implemented using Alstom Process and Optimization Workbench (APOW). software at design conditions. The numerical predictions show that performance is retained at off-design conditions

Numerical study of the flow past an axial turbine stator casing and perspectives for its management

The interaction of secondary flow with the main passage flow results in entropy generation; this accounts for considerable losses in turbomachines. Low aspect ratio blades in an axial turbine lead to a high degree of secondary flow losses. A particular interest is the reduction in secondary flow strength at the turbine casing, which adversely affects the turbine performance. This paper presents a selective review of effective techniques for improving the performance of axial turbines by turbine end wall modifications. This encompasses the use of axisymmetric and non-axisymmetric end wall contouring and the use of fences. Specific attention is given to non-axisymmetric end walls and to their effect on secondary flow losses. A baseline three-dimensional steady RANS k-ω SST model, with axisymmetric walls, is validated against experimental measurements from the Institute of Jet Propulsion and Turbomachinery at the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen, Germany, with comparative solutions generated by ANSYS Fluent and OpenFOAM. The predicted performance of the stator passage with an axisymmetric casing is compared with that from using a contoured casing with a groove designed using the Beta distribution function for guiding the groove shape. The prediction of a reduced total pressure loss coefficient with the application of the contoured casing supports the groove design approach based on the natural path of the secondary flow features. This work also provided an automated workflow process, linking surface definition in MATLAB, meshing in ICEM CFD, and flow solving and post-processing OpenFOAM. This has generated a casing contouring design tool with a good portability to industry, to design and optimize new turbine blade passages

Thermo-hydraulic performance of a circular microchannel heat sink using swirl flow and nanofluid

Increasing the power and reliability of microelectronic components requires heat sinks with greater heat transport performance. This study investigates the hydraulic and heat transport performance of a silicon heat sink under a constant heat flux of 100 W/cm2 with liquid coolant running through parallel microchannels. The coupled heat transfer through the silicon walls and the coolant, modelled as a single-phase fluid, is examined over the Reynolds number range for micro-channels of circular cross-section, with a straight tape, and with 100 ≤ ≤ 350a twisted tape that induces swirl in the flow. Al2O3 nanofluid at nanoparticle volume fractions = 0, 1, 2 and 3% is used as the coolant. The microchannel heat sink with swirl flow and with the highest nanoparticle volume fraction concentration provides the lowest thermal resistance and contact temperature. Whilst it has a higher flow resistance than the micro-channel with no tape cooled by pure water, it has a positive trade-off between the gains in cooling performance and in flow resistance. This makes these configurations attractive for designing more performing heat sinks for temperature limited or temperature sensitive micro-electronics