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1 – 10 of over 2000Zhipeng Duan, Peng Liang, Hao Ma, Niya Ma and Boshu He
The purpose of this paper is to numerically investigate the flow characteristics and extend the data of friction factor and Reynolds number product of hydrodynamically developing…
Abstract
Purpose
The purpose of this paper is to numerically investigate the flow characteristics and extend the data of friction factor and Reynolds number product of hydrodynamically developing laminar flow in three-dimensional rectangular microchannels with different aspect ratios.
Design/methodology/approach
Using a finite-volume approach, the friction factor characteristics of Newtonian fluid in three-dimensional rectangular ducts with aspect ratios from 0.1 to 1 are conducted numerically under no-slip boundary conditions. A simple model that approximately predicts the apparent friction factor and Reynolds number product fappRe is referenced as a semi-theoretical fundamental analysis for numerical simulations.
Findings
The accurate and reliable results of fappRe are obtained, which are compared with classic numerical data and experimental data, and the simple semi-theoretical model used and all comparisons show good agreement. Among them, the maximum relative error with the classic numerical data is less than 3.9 per cent. The data of fappRe are significantly extended to other different aspect ratios and the novel values of fappRe are presented in the tables. The characteristics of fappRe are analyzed as a function of a non-dimensional axial distance and the aspect ratios. A more effective and accurate fourth-order fitting equation for the Hagenbach's factor of rectangular channels is proposed.
Originality/value
From the reliable data, it is shown that the values of fappRe and the model can be references of pressure drop and friction factor for developing laminar flow in rectangular channels for researchers and engineering applications.
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Abstract
The paper adopts a simplified two‐dimensional approach to deal with convective heat and mass transfer in laminar flows of humid air through wavy finned‐tube exchangers. The computational domain is spatially periodic, with fully developed conditions prevailing at a certain distance from the inlet section. Both the entrance and the fully developed flow region are investigated. In the fully developed region, periodicities in the flow, temperature and mass concentration fields are taken into account. The approach is completely general, even if the finite element method is used for the discretizations. In the application section, velocity, temperature, and mass concentration fields are computed first. Then apparent friction factors, Nusselt numbers, Colburn factors for heat and mass transfer, and goodness factors are evaluated both in the entrance and in the fully developed region.
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Abstract
By neglecting the influence of tubes, this paper adopts a simplified two‐dimensional approach to deal with laminar convection of air through wavy finned‐tube exchangers. Pressure drop and heat transfer characteristics are investigated in the fully developed region of the flow channels between adjacent fins. The solutions are presented for several space ratios (height over length of a module) and two corrugation angles. They concern laminar flows both below and above the onset of the self‐sustained oscillations that precede the transition to turbulence. Fully developed velocity and thermal fields are computed by imposing anti‐periodic conditions at inlet/outlet sections of a single calculation cell. In the range of Reynolds numbers investigated, Nusselt numbers and friction factors first increase with space ratios (up to a value depending on the corrugation angle), then start decreasing with increasing space ratios.
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Carolina P. Naveira-Cotta, Jian Su, Paulo Lucena Kreppel Paes, Philippe R. Egmont, Rodrigo P.M. Moreira, Gabriel Caetano G.R. da Silva and André Sampaio Monteiro
The purpose of this paper is to investigate the impact of semi-circular zigzag-channel printed circuit heat exchanger (PCHE) design parameters on heat transfer and pressure drop…
Abstract
Purpose
The purpose of this paper is to investigate the impact of semi-circular zigzag-channel printed circuit heat exchanger (PCHE) design parameters on heat transfer and pressure drop of flows under high Reynolds numbers and provide new thermal-hydraulic correlations relevant to conditions encountered in natural gas processing plants.
Design/methodology/approach
The correlations were developed using three-dimensional steady-state computational fluid dynamics simulations with varying semicircular channel diameter (from 1 to 5 mm), zigzag angle (from 15° to 45°) and Reynolds number (from 40,000 to 100,000). The simulation results were validated by comparison with experimental results and existing correlations.
Findings
The results revealed that the thermal-hydraulic performance was mostly affected by the zigzag angle, followed by the ratio of the zigzag channel length to the hydraulic diameter. Overall, smaller zigzag angles favored heat transfer intensification while keeping reasonably low pressure drops.
Originality/value
This study is, to date, the only one providing thermal-hydraulic correlations for PCHEs with zigzag channels under high Reynolds numbers. Besides, the broad range of parameters considered makes the proposed correlations valuable PCHE design tools.
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Three‐dimensional laminar forced convective heat transfer in ribbed square channels is investigated. In these channels, transverse and angled ribs are placed on one or two of the…
Abstract
Three‐dimensional laminar forced convective heat transfer in ribbed square channels is investigated. In these channels, transverse and angled ribs are placed on one or two of the walls to form a repetitive geometry. After a short distance from the entrance, also the flow and the dimensionless thermal fields repeat themselves from module to module allowing the assumption of periodic, or anti‐periodic, conditions at the inlet/outlet sections of the calculation cell. Prescribed temperature boundary conditions are assumed at all solid walls, including the ribs. Pressure drop and heat transfer characteristics are compared for rib angles ranging from 90° (transverse ribs) to 45°, and different values of the Reynolds number. The influence of rib geometries is investigated below and above the onset of the self‐sustained flow oscillations that precede the transition to turbulence. Numerical simulations are carried out employing an equal order finite‐element procedure based on a projection algorithm.
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K. Jeevan, G.A. Quadir, K.N. Seetharamu, I.A. Azid and Z.A. Zainal
To determine the optimal dimensions for a stacked micro‐channel using the genetic algorithms (GAs) under different flow constraints.
Abstract
Purpose
To determine the optimal dimensions for a stacked micro‐channel using the genetic algorithms (GAs) under different flow constraints.
Design/methodology/approach
GA is used as an optimization tool for optimizing the thermal resistance of a stacked micro‐channel under different flow constraints obtained by using the one dimensional (1D) and two dimensional (2D) finite element methods (FEM) and by thermal resistance network model as well (proposed by earlier researcher). The 2D FEM is used to study the effect of two dimensional heat conduction in the micro‐channel material. Some parametric studies are carried out to determine the resulting performance of the stacked micro‐channel. Different number of layers of the stacked micro‐channel is also investigated to study its effect on the minimum thermal resistance.
Findings
The results obtained from the 1D FEM analysis compare well with those obtained from the thermal resistance network model. However, the 2D FEM analysis results in lower thermal resistance and, therefore, the importance of considering the conduction in two dimensions in the micro‐channel is highlighted.
Research limitations/implication
The analysis is valid for constant properties fluid and for steady‐state conditions. The top‐most surfaces as well as the side surfaces of the micro‐channel are considered adiabatic.
Practical implications
The method is very useful for practical design of micro‐channel heat‐sinks.
Originality/value
FEM analyses of stacked micro‐channel can be easily implemented in the optimization procedure for obtaining the dimensions of the stacked micro‐channel heat‐sinks for minimum thermal resistance.
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Abstract
Most compact heat exchangers and heat dissipating components rely on convection enhancement mechanisms that reduce the continuous growth of boundary layers. Usually surface irregularities, in the form of interruptions and/or vortex generators, are introduced in the flow passages. The resulting geometric configurations are periodic in space and, after a short distance from the entrance, induce velocity and thermal fields that repeat themselves from module to module. The numerical models presented here consider the space‐periodicity and allow flows that are stationary at sub‐critical values of the Reynolds number, but become time‐periodic, or quasi periodic, above the critical value of the Reynolds number. Space discretizations are achieved by an equal order finite element procedure based on a projection algorithm. Two‐dimensional schematizations are employed to analyze the effects of surface interruptions and transverse vortex generators, while three‐dimensional schematizations are employed for longitudinal vortex generators.
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M.A. Delgado, J.M. Franco and E. Kuhn
The aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.
Abstract
Purpose
The aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.
Design/methodology/approach
Lubricating greases having the same composition but differing in processing protocols have been manufactured and characterized in order to isolate the rheological behaviour from the formulation.
Findings
Some successful empirical correlations between rheological (viscous and viscoelastic) and technological standardized parameters, with the friction factor obtained from a ball‐disc tribometer, have been established in order to elucidate the role of the rheological behaviour of lubricating greases on the friction process. In addition to this, an energetic evaluation of the structural degradation of greases during the friction process has been carried out by performing stress‐growth experiments. Thus, the storage energy density, which is related to the grease capacity to accumulate energy in the elastic deformation, and the limiting energy density, which represents the dissipation of energy in the flow process, have been satisfactorily correlated with the friction factor.
Research limitations/implications
The complex rheological behaviour of lubricating greases, the extreme deformations and the high‐shear stresses resulting in a tribological contact imply that it is difficult to develop a model to describe their behaviour in an elastohydrodynamic lubricating contact.
Originality/value
This paper provides a resource of practical data to be applied in tribological systems.
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Fakhrodin Lalegani, Mohammad Reza Saffarian, Ahmadreza Moradi and Ebrahim Tavousi
According to very small dimensions of the microchannels, producing a microchannel with smooth surfaces is approximately impossible. The surface roughness can have a specific…
Abstract
Purpose
According to very small dimensions of the microchannels, producing a microchannel with smooth surfaces is approximately impossible. The surface roughness can have a specific effect on microchannel performances. This paper aims to investigate the changes in friction and pressure drop in the microchannels by considering the different roughness elements on microchannel wall and changes in elementary geometry and flow conditions. Results show a significant effect of roughness on the pressure drop and friction.
Design/methodology/approach
Two-dimensional fluid flow in the rough microchannels is analyzed using FLUENT. Microchannels have a height of 50 µm. Water at room temperature (25°C) has been used as working fluid. The Reynolds numbers are considered in laminar flow range and from 50 to 300.
Findings
The results show that the value of friction factor reduces nonlinearly with an increase in Reynolds number. But, the pressure drops and the Poiseuille number in the microchannels increase with an increase in Reynolds number. The values of the pressure drop and the friction factor increase by increasing the height and size of the roughness elements, but these values reduce with an increase in the distance of roughness elements.
Originality/value
The roughness elements types in this research are rectangular, trapezoidal, elliptical, triangular and complex (composed of multiple types of roughness elements). The effects of the Reynolds number, roughness height, roughness distance and roughness size on the pressure drop and friction in the rough microchannels are investigated and discussed. Furthermore, differences between the effects of five types of roughness elements are identified.
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J. Paulo Davim and Rosária Cardoso
PEEK is an abbreviation for polyetheretherketone, a high performance engineering semicrystalline thermoplastic. This material can operate at higher temperatures and has excellent…
Abstract
Purpose
PEEK is an abbreviation for polyetheretherketone, a high performance engineering semicrystalline thermoplastic. This material can operate at higher temperatures and has excellent friction and wear properties, which are optimised in the specially formulated tribological grade PEEK‐CF30. The purpose of this work was to develop a thermo‐mechanical model to predict the tribological behaviour of the composite PEEK‐CF30/steel pair, in dry sliding, related to friction and wear with the pv factor, the temperature and the sliding distance, using multiple regression analysis (MRA).
Design/methodology/approach
This paper presents a new thermo‐mechanical model to predict the tribological behaviour of the composite PEEK‐CF30/steel pair, in dry sliding, using MRA. A plan of experiments was performed on a pin‐on‐disc machine PLINT TE67HT®.
Findings
The objective was to establish a thermo‐mechanical model to predict the PEEK‐CF30 behaviour related to friction and wear with the pv factor (product of apparent pressure of contact and sliding velocity), the temperature and the sliding distance. This model was obtained by multiple linear regression. Finally, confirmation tests were performed to make a comparison between the obtained results from the mentioned model and the experimental results.
Originality/value
The novel element of this paper is the application of design experiments and MRA in tribological model behaviour in an advanced material – PEEK‐CF 30.
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