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Article
Publication date: 3 July 2017

M. Sabour, Mohammad Ghalambaz and Ali Chamkha

The purpose of this study is to theoretically analyze the laminar free convection heat transfer of nanofluids in a square cavity. The sidewalls of the cavity are subject to…

Abstract

Purpose

The purpose of this study is to theoretically analyze the laminar free convection heat transfer of nanofluids in a square cavity. The sidewalls of the cavity are subject to temperature difference, whereas the bottom and top are insulated. Based on the available experimental results in the literature, two new non-dimensional parameters, namely, the thermal conductivity parameter (Nc) and dynamic viscosity parameter (Nv) are introduced. These parameters indicate the augmentation of the thermal conductivity and dynamic viscosity of the nanofluid by dispersing nanoparticles.

Design/methodology/approach

The governing equations are transformed into non-dimensional form using the thermo-physical properties of the base fluid. The obtained governing equations are solved numerically using the finite element method. The results are reported for the general non-dimensional form of the problem as well as case studies in the form of isotherms, streamlines and the graphs of the average Nusselt number. Using the concept of Nc and Nv, some criteria for convective enhancement of nanofluids are proposed. As practical cases, the effect of the size of nanoparticles, the shape of nanoparticles, the type of nanoparticles, the type of base fluids and working temperature on the enhancement of heat transfer are analyzed.

Findings

The results show that the increase of the magnitude of the Rayleigh number increases of the efficiency of using nanofluids. The type of nanoparticles and the type of the base fluid significantly affects the enhancement of using nanofluids. Some practical cases are found, in which utilizing nanoparticles in the base fluid results in deterioration of the heat transfer. The working temperature of the nanofluid is very crucial issue. The increase of the working temperature of the nanofluid decreases the convective heat transfer, which limits the capability of nanofluids in decreasing the size of the thermal systems.

Originality/value

In the present study, a separation line based on two non-dimensional parameters (i.e. Nc and Nv) are introduced. The separation line demonstrates a boundary between augmentation and deterioration of heat transfer by using nanoparticles. Indeed, by utilizing the separation lines, the convective enhancement of using nanofluid with a specified Nc and Nv can be simply estimated.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 27 no. 7
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 14 March 2016

Yanzhong Wang, Wentao Niu, Song Wei and Guanhua Song

This paper aims to improve the cooling performance of the impinging jet to the machining and power transmissions, and provides more parameters to the design of the cooling system…

Abstract

Purpose

This paper aims to improve the cooling performance of the impinging jet to the machining and power transmissions, and provides more parameters to the design of the cooling system.

Design/methodology/approach

A multiphase flow model with heat transfer terms is established to calculate the convective heat transfer coefficient. The computational fluid dynamics method is used to simulate the jet flow. The convective heat transfer coefficients with different spray parameters are calculated and their variations are obtained. Temperatures are tested to reflect the cooling performance (convective heat transfer coefficients) with different spray parameters.

Findings

The results show that the higher convective heat transfer coefficient can be obtained with the same flow rate by decreasing nozzle diameter while increasing either the number of nozzles or the oil supply pressure. The spray distance was found to have little influence on convective heat transfer; however, the more the spray is directed parallel to the surface, the higher the convective heat transfer coefficient. The computational results coincide well with the experimental results.

Originality/value

The research presented here leads to a design reference guideline that could be used in machining and power transmissions to reduce the temperature, thus improving their quality and efficiency, and preventing failure at high speeds and/or under heavy loads.

Details

Industrial Lubrication and Tribology, vol. 68 no. 2
Type: Research Article
ISSN: 0036-8792

Keywords

Article
Publication date: 1 July 2014

Mehdi Bahiraei, Seyed Mostafa Hosseinalipour and Morteza Hangi

The purpose of this paper is to attempt to investigate the particle migration effects on nanofluid heat transfer considering Brownian and thermophoretic forces. It also tries to…

Abstract

Purpose

The purpose of this paper is to attempt to investigate the particle migration effects on nanofluid heat transfer considering Brownian and thermophoretic forces. It also tries to develop a model for prediction of the convective heat transfer coefficient.

Design/methodology/approach

A modified form of the single-phase approach was used in which an equation for mass conservation of particles, proposed by Buongiorno, has been added to the other conservation equations. Due to the importance of temperature in particle migration, temperature-dependent properties were applied. In addition, neural network was used to predict the convective heat transfer coefficient.

Findings

At greater volume fractions, the effect of wall heat flux change was more significant on nanofluid heat transfer coefficient, whereas this effect decreased at higher Reynolds numbers. The average convective heat transfer coefficient raised by increasing the Reynolds number and volume fraction. Considering the particle migration effects, higher heat transfer coefficient was obtained and also the concentration at the tube center was higher in comparison with the wall vicinity. Furthermore, the proposed neural network model predicted the heat transfer coefficient with great accuracy.

Originality/value

A review of the literature shows that in the single-phase approach, uniform concentration distribution has been used and the effects of particle migration have not been considered. In this study, nanofluid heat transfer was simulated by adding an equation to the conservation equations to consider particle migration. The effects of Brownian and thermophoretic forces have been considered in the energy equation. Moreover, a model is proposed for prediction of convective heat transfer coefficient.

Details

Engineering Computations, vol. 31 no. 5
Type: Research Article
ISSN: 0264-4401

Keywords

Article
Publication date: 15 November 2011

Martin Hettegger, Bernhard Streibl, Oszkár Bíró and Harald Neudorfer

For an accurate simulation of the temperature distribution inside an electrical machine a method for deriving the convective heat transfer coefficient numerically would be…

Abstract

Purpose

For an accurate simulation of the temperature distribution inside an electrical machine a method for deriving the convective heat transfer coefficient numerically would be desirable. The purpose of this paper is to present a reliable simulation setup, which is able to reproduce the measured convective heat transfer coefficient at certain spots on the end windings of an electric machine.

Design/methodology/approach

The heat flux density on certain spots on the end windings of an induction motor have been measured with heat flux sensors, in order to find out the convective heat transfer coefficient. To identify the air mass flow inside a cooling duct of an encapsulated cooling circuit during the operation of the motor, the pressure loss inside the duct has been measured. The measured data for temperature and air mass flow have been used as boundary conditions for the identification of the convective heat transfer coefficient with a commercial software for computational fluid dynamics (CFD).

Findings

The measured data for the local convective heat transfer coefficients have been compared to the results of the numerical simulation for various rotational velocities. The quality of the simulated convective heat transfer coefficient depending on the rotational velocity meets the measured values. Owing to the used simplified model, the quantity of the measured values differ strongly around the simulated coefficient for the convective heat transfer.

Originality/value

The derivation of the convective heat transfer is a challenging subject in CFD but has become more reliable with the invention of the SST and the SAS‐SST turbulence model. In the present work, measurements on the end windings have been compared to simulation results derived with the SAS‐SST turbulence model.

Details

COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, vol. 30 no. 6
Type: Research Article
ISSN: 0332-1649

Keywords

Article
Publication date: 4 December 2017

Hasan Celik, Moghtada Mobedi, Oronzio Manca and Unver Ozkol

The purpose of this study is to determine interfacial convective heat transfer coefficient numerically, for a porous media consisting of square blocks in inline arrangement under…

Abstract

Purpose

The purpose of this study is to determine interfacial convective heat transfer coefficient numerically, for a porous media consisting of square blocks in inline arrangement under mixed convection heat transfer.

Design/methodology/approach

The continuity, momentum and energy equations are solved in dimensionless form for a representative elementary volume of porous media, numerically. The velocity and temperature fields for different values of porosity, Ri and Re numbers are obtained. The study is performed for the range of Ri number from 0.01 to 10, Re number from 100 to 500 and porosity value from 0.51 to 0.96. Based on the obtained results, the value of the interfacial convective heat transfer coefficient is calculated by using volume average method.

Findings

It was found that at low porosities (such as 0.51), the interfacial Nusselt number does not considerably change with Ri and Re numbers. However, for porous media with high Ri number and porosity (such as 10 and 0.51, respectively), secondary flows occur in the middle of the channel between rods improving heat transfer between solid and fluid, considerably. It is shown that the available correlations of interfacial heat transfer coefficient suggested for forced convection can be used for mixed convection for the porous media with low porosity (such as 0.51) or for the flow with low Ri number (such as 0.01).

Originality/value

To the best of the authors’ knowledge, there is no study on determination of interfacial convective heat transfer coefficient for mixed convection in porous media in literature. The present study might be the first study providing an accurate idea on the range of this important parameter, which will be useful particularly for researchers who study on mixed convection heat transfer in porous media, macroscopically.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 27 no. 12
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 1 August 2016

Hsien-Hung Ting and Shuhn-Shyurng Hou

The purpose of this paper is to numerically investigate the convective heat transfer of water-based CuO nanofluids flowing through a square cross-section duct under constant heat

Abstract

Purpose

The purpose of this paper is to numerically investigate the convective heat transfer of water-based CuO nanofluids flowing through a square cross-section duct under constant heat flux in the turbulent flow regime.

Design/methodology/approach

The numerical simulation is carried out at various Peclet numbers and particle concentrations (0.1, 0.2, 0.5, and 0.8 vol%). The finite volume formulation is used with the semi-implicit method for pressure-linked equations algorithm to solve the discretized equations derived from the partial nonlinear differential equations of the mathematical model.

Findings

The heat transfer coefficients and Nusselt numbers of CuO-water nanofluids increase with increases in the Peclet number as well as particle volume concentration. Also, enhancement of the heat transfer coefficient is much greater than that of the effective thermal conductivity at the same nanoparticle concentration.

Research limitations/implications

Simulation of nanofluids turbulent forced convection at very high Reynolds number is worth for further study.

Practical implications

The heat transfer rates through non-circular ducts are smaller than the circular tubes. Nevertheless, the pressure drop of the non-circular duct is less than that of the circular tube. This study clearly presents that the nanoparticles suspended in water enhance the convective heat transfer coefficient, despite low volume fraction between 0.1 and 0.8 percent. Adding nanoparticles to conventional fluids may enhance heat transfer performance through the non-circular ducts, leading to extensive practical applications in industries for the non-circular ducts.

Originality/value

Few papers have numerically studied convective heat transfer properties of nanofluids through non-circular ducts. The present numerical results show a good agreement with the published experimental data.

Details

Engineering Computations, vol. 33 no. 6
Type: Research Article
ISSN: 0264-4401

Keywords

Article
Publication date: 19 January 2021

Fengxia Lu, Meng Wang, Weizhen Liu, Heyun Bao and Rupeng Zhu

This paper aims to propose a numerical method to calculate the convective heat transfer coefficient of spiral bevel gears under the condition of splash lubrication and to reveal…

Abstract

Purpose

This paper aims to propose a numerical method to calculate the convective heat transfer coefficient of spiral bevel gears under the condition of splash lubrication and to reveal the lubrication and temperature characteristics between the gears and the oil-air two-phase flow.

Design/methodology/approach

Based on computational fluid dynamics, the multiple reference frames (MRF) method was used to simulate the rotational characteristics of gears and the motions of their surrounding fluid. The lubrication and temperature characteristics of gears were studied by combining the MRF method with the volume of the fluid multiphase flow model.

Findings

The convective heat transfer coefficient can be improved by increasing the rotational speed and the oil immersion depth. Moreover, the temperature of the tooth surface having a large convective heat transfer coefficient is also found to be low. A large convection heat transfer coefficient could lead to a good cooling effect.

Originality/value

This method can be used to obtain the convective heat transfer coefficient values at different meshing positions, different radii and different tooth surface positions. It also can provide research methods for improving the cooling effect of gears under the condition of splash lubrication.

Peer review

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2020-0233/

Details

Industrial Lubrication and Tribology, vol. 73 no. 3
Type: Research Article
ISSN: 0036-8792

Keywords

Article
Publication date: 1 August 1995

Himadri Chattopadhyay and Sukanta K. Dash

The conception of a heat function, just like the stream function used ina laminar two dimensional incompressible flow field visualization, has beenintroduced to visualize the…

Abstract

The conception of a heat function, just like the stream function used in a laminar two dimensional incompressible flow field visualization, has been introduced to visualize the convective heat transfer or the flow of energy around a sphere when the sphere is either being cooled or heated by a stream of fluid flowing around it. The heat function is developed in a spherical polar coordinate and is used to generate the heat lines around the sphere. The heat lines essentially show the magnitude and direction of energy transfer around the sphere with and without the existence of a finite radial velocity at the surface. The steady state hydrodynamic field around the sphere is numerically obtained up to a maximum Reynolds number of 100 and the corresponding thermal field has been obtained by solving the steady state energy equation. The field properties thus obtained are utilized to form the heat function, which becomes an effective tool for visualization of convective heat transfer.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 5 no. 8
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 1 September 1995

Koki Kishinami, Hakaru Saito and Jun Suzuki

Combined free and forced laminar air convective heat transfer from avertical composite plate with isolated discontinuous surface heating elementshas been studied numerically and…

Abstract

Combined free and forced laminar air convective heat transfer from a vertical composite plate with isolated discontinuous surface heating elements has been studied numerically and experimentally. The problem has been simplified by neglecting heat conduction in unheated elements of the plate to accomplish a better understanding of the complicated combined/complicated convection problem. In this study, it is most important in explaining the heat transfer behaviour to clarify the interactions between buoyancy and inertia forces in the convective field and also the coupling effects of unheated elements upon the combined flow fields. Therefore, the temperature distributions of the wall surface and local Nusselt number, obtained by numerical calculations and experiments, have been discussed based on the various parameters associated with the present convection problem, i.e., Grashof number GrL, Reynolds number ReL, geometry factor D/L and stage number N. Heat transfer characteristics Nut/Re1/2L of this combined and coupled convection of air are presented as a function of a generalized coupling dimensionless number GrL/Re2L, and stage number N for certain values of the geometry factor of D/L.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 5 no. 9
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 7 September 2015

M. Schrittwieser, O. Bíró, E. Farnleitner and G. Kastner

The purpose of this paper is to approximate the convective heat transfer using a few non-dimensional parameters in the design process of large synchronous machines. The computed…

Abstract

Purpose

The purpose of this paper is to approximate the convective heat transfer using a few non-dimensional parameters in the design process of large synchronous machines. The computed convective wall heat transfer coefficient can be used in circuit models or can be defined in numerical heat conduction (HC) models to compute the thermal field in the solid domains without the time consuming computation of the fluid domain.

Design/methodology/approach

Computational fluid dynamics (CFD) has been used to include a wide range of different designs, operating conditions and cooling schemes to ensure accurate results for a wide range of possible machines. Neural networks are used to correlate the computed heat transfer coefficients to various design parameters. The data set needed to define the weights and bias layers in the network has been obtained by several CFD simulations. A comparison of the evaluated solid temperatures with those obtained using the conjugate heat transfer (CHT) method has been carried out. The results have also been validated with calorimetric measurements.

Findings

The validation of the HC model has shown that this model is capable of yielding accurate results in a few minutes, in contrast to the several hours needed by the CHT solution. The workflow to determine the convective heat transfer in a specific part of an electrical machine has been also been established. The approximation of the convective wall heat transfer coefficient is shown to be obtainable in sufficient detail by using a neural network.

Research limitations/implications

The paper describes a method to approximate the convective heat transfer accurately in a few seconds, which is very useful in the design process. The heat convection can then be characterized in a HC model including the solid domains only. The losses can be defined as sources in the solid domains, e.g. copper and iron, obtained by electromagnetic calculations and the thermal field can hence be easily computed in these parts. This HC model has the main advantage that the time consuming computation of the fluid domain is avoided.

Originality/value

The novelty in this work is the approximation of the convective heat transfer by using a neural network with an accuracy of less than 5 percent as well as the development of a HC model to compute the temperature in the solid domains. The investigations presented pinpoint relevant issues influencing the thermal behavior of electrical machines.

Details

COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, vol. 34 no. 5
Type: Research Article
ISSN: 0332-1649

Keywords

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