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1 – 10 of over 28000Karthikeyan Paramanandam, Venkatachalapathy S, Balamurugan Srinivasan and Nanda Kishore P V R
This study aims to minimize the pressure drop across wavy microchannels using secondary branches without compromising its capacity to transfer the heat. The impact of secondary…
Abstract
Purpose
This study aims to minimize the pressure drop across wavy microchannels using secondary branches without compromising its capacity to transfer the heat. The impact of secondary flows on the pressure drop and heat transfer capabilities at different Reynolds numbers are investigated numerically for different wavy microchannels. Finally, different channels are evaluated using performance evaluation criteria to determine their effectiveness.
Design/methodology/approach
To investigate the flow and heat transfer capabilities in wavy microchannels having secondary branches, a 3D conjugate heat transfer model based on finite volume method is used. In conventional wavy microchannel, secondary branches are introduced at crest and trough locations. For the numerical simulation, a single symmetrical channel is used to minimize computational time and resources and the flow within the channels remains single-phase and laminar.
Findings
The findings indicate that the suggested secondary channels notably improve heat transfer and decrease pressure drop within the channels. At lower flow rates, the secondary channels demonstrate superior performance in terms of heat transfer. However, the performance declines as the flow rate increased. With the same amplitude and wavelength, the introduction of secondary channels reduces the pressure drop compared with conventional wavy channels. Due to the presence of secondary channels, the flow splits from the main channel, and part of the core flow gets diverted into the secondary channel as the flow takes the path of minimum resistance. Due to this flow split, the core velocity is reduced. An increase in flow area helps in reducing pressure drop.
Practical implications
Many complex and intricate microchannels are proposed by the researchers to augment heat dissipation. There are challenges in the fabrication of microchannels, such as surface finish and achieving the required dimensions. However, due to the recent developments in metal additive manufacturing and microfabrication techniques, the complex shapes proposed in this paper are feasible to fabricate.
Originality/value
Wavy channels are widely used in heat transfer and micro-fluidics applications. The proposed wavy microchannels with secondary channels are different when compared to conventional wavy channels and can be used practically to solve thermal challenges. They help achieve a lower pressure drop in wavy microchannels without compromising heat transfer performance.
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Karthikeyan Paramanandam, Venkatachalapathy S. and Balamurugan Srinivasan
The purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and…
Abstract
Purpose
The purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and width of the ribs on heat transfer enhancement, secondary flows, flow distribution and temperature distribution are examined at different Reynolds numbers. The effectiveness of each heatsink is evaluated using the performance factor.
Design/methodology/approach
A three-dimensional solid-fluid conjugate heat transfer numerical model is used to study the flow and heat transfer characteristics in microchannels. One symmetrical channel is adopted for the simulation to reduce the computational cost and time. Flow inside the channels is assumed to be single-phase and laminar. The governing equations are solved using finite volume method.
Findings
The numerical results are analyzed in terms of average Nusselt number ratio, average base temperature, friction factor ratio, pressure variation inside the channel, temperature distribution, velocity distribution inside the channel, mass flow rate distribution inside the secondary channels and performance factor of each microchannels. Results indicate that impact of rib width is higher in enhancing the heat transfer when compared with its length but with a penalty on the pressure drop. The combined effects of secondary channels, ribs and cavities helps to lower the temperature of the microchannel heat sink and enhances the heat transfer rate.
Practical implications
The fabrication of microchannels are complex, but recent advancements in the additive manufacturing techniques makes the fabrication of the design considered in this numerical study feasible.
Originality/value
The proposed microchannel heatsink can be used in practical applications to reduce the thermal resistance, and it augments the heat transfer rate when compared with the baseline design.
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A numerical study is performed to investigate flow instability phenomena in a square channel with steady, laminar throughflow. The channel rotates around an axis perpendicular to…
Abstract
A numerical study is performed to investigate flow instability phenomena in a square channel with steady, laminar throughflow. The channel rotates around an axis perpendicular to the channel longitudinal axis. The flow field extends from the channel entrance to a distance of 120 to 600Dh. The range of Reynolds number is Re = 300−2000. The inlet flow velocity is assumed uniform. Surface vorticity intensity is introduced to indicate the variation of vortices. It is revealed that at intermediate Reynolds numbers (680 > Re > 300), the flow is characterized by three vortex patterns: at slow rotation there is one vortex pair; at intermediate rotation a secondary vortex, in addition to the original vortex, emerges near the trailing wall and then breaks down downstream; and at rapid rotation the secondary vortex does not exist with the flow being restabilized to form a single‐pair vortex pattern. At low Reynolds numbers (Re ≤ 300), the flow exhibits a single‐pair vortex pattern, while at high Reynolds numbers (Re ≥ 680), the flow experiences the emergence and breakdown of a secondary vortex, but no restabilization is found with an increase in the rotational speed. It is also disclosed that the variation of the vortices is related to the distance from the inlet.
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Seyyed Mostafa Hoseinalipour, Hamidreza Shahbazian and Bengt Ake Sunden
The study aims to focus on rotation effects on a ribbed channel of gas turbine blades for internal cooling. The combination and interaction between secondary flows generated by…
Abstract
Purpose
The study aims to focus on rotation effects on a ribbed channel of gas turbine blades for internal cooling. The combination and interaction between secondary flows generated by angled rib geometry and Coriolis forces in the rotating channel are studied numerically.
Design/methodology/approach
A radially outward flow passage as an internal cooling test model with and without ribs is used to perform the investigation. Aspect ratio of the passage is 1:1. Square ribs with e/Dh = 0.1, p/e = 10 and four various rib angles of 90°, 75°, 60° and 45° are configured on both the leading and trailing surfaces along the rotating duct. The study covers a Reynolds number of 10,000 and Rotation number in the range of 0-0.15.
Findings
Nusselt numbers in the ribbed duct are 2.5 to 3.5 times those of a smooth square duct, depending on the Rotation number and rib angle. The maximum value is attained for the 45° ribbed surface. The synergy angle between the velocity and temperature gradients is improved by the angled rib secondary flows and Coriolis vortex. The decrease of the synergy angle is 8.9, 13.4, 12.1 and 10.1 per cent for the 90°, 75°, 60° and 45° ribbed channels with rotation, respectively. Secondary flow intensity is increased by rotation in the 90° and 75° ribbed ducts and is decreased in 45° and 60° ribbed cases for which the rib-induced secondary flow dominates.
Originality/value
The primary motivation behind this work is to investigate the possibility of heat transfer enhancement by vortex flow with developing turbulence in the view point of the field synergy principle and secondary flow intensity.
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Xing Xie, Zhenlin Li, Baoshan Zhu and Hong Wang
The purpose of this study is to suppress secondary flows and improve aerodynamic performance of a centrifugal impeller.
Abstract
Purpose
The purpose of this study is to suppress secondary flows and improve aerodynamic performance of a centrifugal impeller.
Design/methodology/approach
A multi-objective optimisation design system was described. The optimization design system was composed of a three-dimensional (3D) inverse design, multi-objective optimisation and computational fluid dynamics (CFD) analysis. First, the control parameter ΔCp for the secondary flows was derived and selected as the optimisation objective. Then, aimed at minimising ΔCp, a 3D inverse design for impellers with different blade loading distributions and blade lean angles was completed and multi-objective optimisation was conducted. Lastly, the improvement in the distribution of secondary flows and aerodynamic performance of the optimal impeller was demonstrated by CFD analysis.
Findings
The study derived the control parameter ΔCp for the secondary flows. ΔCp can indicate the distribution of secondary flows both near the blade pressure and suction surfaces. As ΔCp decreased, secondary flows decreased. The blade loading distribution with fore maximum blade loading at the shroud and aft maximum blade loading at the hub, coupled with a small negative blade lean angle, could help suppress secondary flows and improve aerodynamic efficiency.
Originality/value
A direct control method on internal flow field characteristic-secondary flows by optimisation design was proposed for a centrifugal impeller. The impeller optimisation design process saves time by avoiding substantial CFD sample calculations.
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Elham Mohammadi and Alireza Toloei
The purpose of this paper is to analyze the flowfield structure and performance of dual secondary injection system for thrust vectoring in a convergent‐divergent nozzle and to…
Abstract
Purpose
The purpose of this paper is to analyze the flowfield structure and performance of dual secondary injection system for thrust vectoring in a convergent‐divergent nozzle and to compare it with a single secondary injection system.
Design/methodology/approach
Dual secondary injection for thrust vectoring in a convergent‐divergent nozzle is studied by solving three‐dimensional Reynolds‐averaged Navier‐Stokes equations by the means of Fluent. Realizable k‐ε turbulent model with enhanced wall‐treatment approach is used for viscous model. Density‐based solver and explicit scheme are employed in the computational model. In order to study the effect of injection location on the flowfield, distance between ports is considered as the key variable.
Findings
Results show that under some circumstances, dual secondary injection system is more effective than a single injection system with the same mass flow rate. The study shows that when the distance between two ports is 8.5 times of the injection port's diameter (or more) and in the same time the first injection port is at least 1 throat diameter far from the nozzle throat, this system will show a better performance. In addition, this system reduces the probability of bow shock impingement to the opposite wall and consequently, the side force production has less limitation.
Practical implications
Dual secondary injection for thrust vector control (SITVC) needs less secondary flow and therefore it makes less reduction in the primary thrust. It means that for a specific primary thrust, less mass fuel is needed which makes it more economic regarding the traditional SITVC systems.
Originality/value
The paper's value lies in using a three‐dimensional model to study the effect of two ports distance on SITVC performance and comparison among the performance of dual and single injections when there is an impingement.
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S. Jafari, M. Rahnama and E. Jahanshahi Javaran
– The present work aims to deal with simulation of turbulent duct flow using generalized lattice Boltzmann equation (GLBE) in which large eddy simulation was employed.
Abstract
Purpose
The present work aims to deal with simulation of turbulent duct flow using generalized lattice Boltzmann equation (GLBE) in which large eddy simulation was employed.
Design/methodology/approach
The sub-grid scale turbulence effects were simulated through a shear-improved Smagorinsky model (SISM) which is capable of predicting turbulent near wall region accurately without any wall function. Computations were done for fully developed turbulent square duct flow at Ret=300, based on duct width and average friction velocity.
Findings
Results obtained for turbulent duct flow reveal that the GLBE in conjunction with SISM is able to correctly predict the existence of secondary flows and the computed detailed structure of first- and second-order statistics of main and secondary motions. The methodology is validated by comparing with previously published data. It is concluded that such framework is capable of predicting accurate results for turbulent duct flow. In addition, the operations in the present method are local; it can be easily programmed for parallel machines.
Originality/value
The numerical method, including generalized lattice Boltzmann method with forcing term and implementation of SISM in GLBE, is used for the first time to simulate turbulent duct flow.
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To investigate the effect of aspect ratio on the quantitative analogy between developing laminar flows in orthogonally rotating straight ducts and stationary curved ducts
Abstract
Purpose
To investigate the effect of aspect ratio on the quantitative analogy between developing laminar flows in orthogonally rotating straight ducts and stationary curved ducts
Design/methodology/approach
A fractional step method is used to obtain the numerical solution of the governing equations by decoupling the solution of the momentum equations from the solution of the continuity equation. In order to clarify the similarity of the two flows, dimensionless parameters KLR and Rossby number, Ro, in a rotating straight duct were used as a set corresponding to Dean number, KLC, and curvature ratio, λ, in a stationary curved duct.
Findings
Under the condition that the aspect ratio was larger than one and that the magnitude of Ro or λ was large enough to satisfy the “asymptotic invariance property” the quantitative analogy between the two flows was established clearly.
Research limitations/implications
As the aspect ratio decreased below one, the difference between the secondary flow intensities of these two flows increased, and therefore, the analogy between the two flows was not as evident as that for the larger aspect ratios.
Practical implications
Based on this methodology, the characteristics of the developing flow in orthogonally rotating ducts of higher aspect ratio can be predicted by considering the flow in stationary curved ducts, and vice versa.
Originality/value
The results obtained in this study will suggest an optimal criterion for the application of this approach to the flow similarity analysis in rectangular ducts with arbitrary aspect ratios.
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Jitendra Kumar Singh, Gauri Shenker Seth and Saikh Ghousia Begum
The purpose of this paper is to present an analytical study on an unsteady magnetohydrodynamic (MHD) boundary layer flow of a rotating viscoelastic fluid over an infinite vertical…
Abstract
Purpose
The purpose of this paper is to present an analytical study on an unsteady magnetohydrodynamic (MHD) boundary layer flow of a rotating viscoelastic fluid over an infinite vertical porous plate embedded in a uniform porous medium with oscillating free-stream taking Hall and ion-slip currents into account. The unsteady MHD flow in the rotating fluid system is generated due to the buoyancy forces arising from temperature and concentration differences in the field of gravity and oscillatory movement of the free-stream.
Design/methodology/approach
The resulting partial differential equations governing the fluid motion are solved analytically using the regular perturbation method by assuming a very small viscoelastic parameter. In order to note the influences of various system parameters and to discuss the important flow features, the numerical results for fluid velocity, temperature and species concentration are computed and depicted graphically vs boundary layer parameter whereas skin friction, Nusselt number and Sherwood number at the plate are computed and presented in tabular form.
Findings
An interesting observation is recorded that there occurs a reversal flow in the secondary flow direction due to the movement of the free stream. It is also noted that a decrease in the suction parameter gives a rise in momentum, thermal and concentration boundary layer thicknesses.
Originality/value
Very little research work is reported in the literature on non-Newtonian fluid dynamics where unsteady flow in the system arises due to time-dependent movement of the plate. The motive of the present analytical study is to analyse the influences of Hall and ion-slip currents on unsteady MHD natural convection flow of a rotating viscoelastic fluid (non-Newtonian fluid) over an infinite vertical porous plate embedded in a uniform porous medium with oscillating free-stream.
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