A. Campo, O. Manca and B. Morrone
To address the impact of adding insulated plate extensions at the entrance of an isoflux vertical parallel‐plate channel on the thermal performances of natural convection in air…
Abstract
Purpose
To address the impact of adding insulated plate extensions at the entrance of an isoflux vertical parallel‐plate channel on the thermal performances of natural convection in air for these systems.
Design/methodology/approach
The model relies on the full elliptic conservation equations which are solved numerically in a composite three‐part computational domain by means of the finite‐volume method.
Findings
Results are reported in terms of wall temperatures, induced mass flow rates, as well as velocity and temperature profiles of the air for various thermal and geometric parameters. The wall temperatures increase when the extensions are appended at the inlet of the channel. Wall temperature profiles strongly depend on the Rayleigh number and the dependence of the heated channel aspect ratio is weaker than the extension ratio. Velocity and temperature profiles modify inside the heated channel due to the thermal development. In addition, correlation equations for main engineering quantities, such as the induced mass flow rate, average Nusselt number and dimensionless maximum wall temperature in terms of the channel Rayleigh number, channel aspect ratio and extension ratio are presented.
Research limitations/implications
The investigation has been carried out in the following ranges: 103‐105 for the Rayleigh number, 5.0‐15.0 for the channel aspect ratio and 1.0‐5.0 for the extension ratio. The hypotheses on which the present analysis is based are: two‐dimensional, laminar and steady‐state flow, constant thermophysical properties with the Boussinesq approximation.
Practical implications
Thermal design of heating systems in manufacturing processes, evaluation of heat convective coefficients and maximum attained wall temperatures.
Originality/value
Evaluation of the thermal and velocity fields and correlation equations for the Nusselt number and maximum dimensionless temperatures in natural convection in air for vertical channels. The paper is useful to thermal designers.
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Assunta Andreozzi, Bernardo Buonomo and Oronzio Manca
The purpose of this paper is to evaluate the thermal and fluid dynamic behaviors of natural convection in a vertical channel‐chimney system heated symmetrically at uniform heat…
Abstract
Purpose
The purpose of this paper is to evaluate the thermal and fluid dynamic behaviors of natural convection in a vertical channel‐chimney system heated symmetrically at uniform heat flux in order to detect the different fluid motion structures inside the chimney, such as the cold inflow from the outlet section of the chimney and the reattachment due to the hot jet from the channel, for different extension and expansion ratios of the adiabatic extensions.
Design/methodology/approach
The model is constituted by two‐dimensional steady‐state fully elliptic conservation equations which are solved numerically in a composite three‐part computational domain by means of the finite‐volume method.
Findings
Stream function and temperature fields in the system are presented in order to detect the different fluid motion structures inside the chimney, for different extension and expansion ratios of the adiabatic extensions. The analysis allows to evaluate the effect of the channel aspect ratio on the thermal and fluid dynamic behaviors on a channel‐chimney system and thermal and geometrical conditions corresponding to a complete downflow. Guidelines to estimate critical conditions related to the beginning of flow separation and complete downflow are given in terms of order of magnitude of Rayleigh and Froude numbers.
Research limitations/implications
The hypotheses on which the present analysis is based are: two‐dimensional, laminar and steady‐state flow, constant thermophysical properties with the Boussinesq approximation. The investigation is carried out in the following ranges: from 100 to 100,000 for the Rayleigh number, from 5.0 to 20 for the aspect ratio, from 1.0 to 4.0 for the expansion ratio and from 1.5 to 4 for the extension ratio.
Practical implications
Thermal design of heating systems in different technical fields, such as in electronic cooling and in building ventilation and houses solar components, evaluation of heat convective coefficients and guidelines to estimate critical conditions related to the beginning of flow separation and complete downflow.
Originality/value
The paper is useful to thermal designers because of its evaluation of the thermal and velocity fields, correlation for the Nusselt number and guidelines criteria in terms of Rayleigh and Froude numbers to evaluate conditions of flow separation and complete downflow in natural convection in air for vertical channels‐chimney systems.
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Assunta Andreozzi, Oronzio Manca and Vincenzo Naso
Research on natural convection in open channels is very extensive due to its role in many engineering applications such as thermal control of electronic systems. In this paper, a…
Abstract
Research on natural convection in open channels is very extensive due to its role in many engineering applications such as thermal control of electronic systems. In this paper, a parametric analysis is carried out in order to add knowledge of heat transfer in air natural convection for a symmetrically heated vertical parallel plate channel with a central auxiliary heated or adiabatic plate. The two‐dimensional steady‐state problem is solved by means of the stream function–vorticity approach and the numerical solution is carried out by means of the control volume method. Results are obtained for both a heated and unheated auxiliary plate, for a Rayleigh number in the range 103–106, for a ratio of the auxiliary plate height to the channel plate height equal to 0, 0.5 and 1 and for a ratio of the channel length to the channel gap in the range 5–15. Correlations for maximum wall temperatures and average channel Nusselt numbers are proposed.
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The purpose of this paper is to address various works on mixed convection and proposes 10 unified models (Models 1–10) based on various thermal and kinematic conditions of the…
Abstract
Purpose
The purpose of this paper is to address various works on mixed convection and proposes 10 unified models (Models 1–10) based on various thermal and kinematic conditions of the boundary walls, thermal conditions and/ or kinematics of objects embedded in the cavities and kinematics of external flow field through the ventilation ports. Experimental works on mixed convection have also been addressed.
Design/methodology/approach
This review is based on 10 unified models on mixed convection within cavities. Models 1–5 involve mixed convection based on the movement of single or double walls subjected to various temperature boundary conditions. Model 6 elucidates mixed convection due to the movement of single or double walls of cavities containing discrete heaters at the stationary wall(s). Model 7A focuses mixed convection based on the movement of wall(s) for cavities containing stationary solid obstacles (hot or cold or adiabatic) whereas Model 7B elucidates mixed convection based on the rotation of solid cylinders (hot or conductive or adiabatic) within the cavities enclosed by stationary or moving wall(s). Model 8 is based on mixed convection due to the flow of air through ventilation ports of cavities (with or without adiabatic baffles) subjected to hot and adiabatic walls. Models 9 and 10 elucidate mixed convection due to flow of air through ventilation ports of cavities involving discrete heaters and/or solid obstacles (conductive or hot) at various locations within cavities.
Findings
Mixed convection plays an important role for various processes based on convection pattern and heat transfer rate. An important dimensionless number, Richardson number (Ri) identifies various convection regimes (forced, mixed and natural convection). Generalized models also depict the role of “aiding” and “opposing” flow and combination of both on mixed convection processes. Aiding flow (interaction of buoyancy and inertial forces in the same direction) may result in the augmentation of the heat transfer rate whereas opposing flow (interaction of buoyancy and inertial forces in the opposite directions) may result in decrease of the heat transfer rate. Works involving fluid media, porous media and nanofluids (with magnetohydrodynamics) have been highlighted. Various numerical and experimental works on mixed convection have been elucidated. Flow and thermal maps associated with the heat transfer rate for a few representative cases of unified models [Models 1–10] have been elucidated involving specific dimensionless numbers.
Originality/value
This review paper will provide guidelines for optimal design/operation involving mixed convection processing applications.
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L'istruzione professionale nell'industria alberghiera si trova oggi in piena trasformazione. Le forme finora praticate non sono più sufficienti. Si devono trovare e seguire nuove…
Abstract
L'istruzione professionale nell'industria alberghiera si trova oggi in piena trasformazione. Le forme finora praticate non sono più sufficienti. Si devono trovare e seguire nuove vie. Il tempo preme. Si devono prendere rapidamente nuove misure se non si vuole che si formino lacune pericolose.
Zhiguo Tang, Feng Deng, Yongtao Ji and Jianping Cheng
The purpose of this paper is to improve the overall heat transfer performance and the temperature uniformity of the heat sink and to explore the effects of the jet Reynolds number…
Abstract
Purpose
The purpose of this paper is to improve the overall heat transfer performance and the temperature uniformity of the heat sink and to explore the effects of the jet Reynolds number and the nanoparticle volume fraction of the nanofluids on the flow and heat transfer performance.
Design/methodology/approach
A heat sink with discontinuous arc protrusions in the wall jet region is proposed for confined slot jet impingement. A sloping upper cover plate is added to improve the heat transfer effect in this area. An Al2O3–water nanofluid is selected as the working fluid of the jet for better heat transfer. The Standard k-e turbulence model is used for numerical calculation. The key structural parameters of the heat sink are optimized by the response surface method and a genetic algorithm. The effects of the jet Reynolds number (Re) and the nanofluid concentration (ϕ) on the flow and heat transfer performance of the optimized heat sink are investigated.
Findings
The average Nusselt number of the optimal heat sink is 8.2% higher and the friction resistance is 5.9% lower than that of the initial flat plate heat sink when ϕ = 0.02 and Re = 8,000. The discontinuous arc protrusions and the sloping upper cover plate substantially enhance the heat transfer in the later stage of jet development, improving the temperature uniformity of the heat sink. The maximum temperature difference of the optimal heat sink is 28.1% lower than that of the flat plate heat sink at the same nozzle height. As the jet Reynolds number and the nanofluid particle concentration increase, the Nusselt number of the optimized heat sink and the friction coefficients increase, resulting in a decrease in the evaluation coefficient. However, the overall temperature uniformity of the heat sink is improved under all conditions.
Originality/value
The novel heat sink structure provides a new way to enhance the heat transfer and temperature uniformity of confined slot jet impingement. The flow and heat transfer performance of the heat sink impinged by confined slot jet of nanofluids are obtained. The combination of response surface method and genetic algorithm can be applied to the multi-objective optimization of heat resistance and flow resistance of heat sink.
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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.
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Ali Akbar Abbasian Arani and Reza Moradi
Using turbulators, obstacles, ribs, corrugations, baffles and different tube geometry, and also various arrangements of these components have a noticeable effect on the shell and…
Abstract
Purpose
Using turbulators, obstacles, ribs, corrugations, baffles and different tube geometry, and also various arrangements of these components have a noticeable effect on the shell and tube heat exchangers (STHEs) thermal-hydraulic performance. This study aims to investigate non-Newtonian fluid flow characteristics and heat transfer features of water and carboxyl methyl cellulose (H2O 99.5%:0.5% CMC)-based Al2O3 nanofluid inside the STHE equipped with corrugated tubes and baffles using two-phase mixture model.
Design/methodology/approach
Five different corrugated tubes and two baffle shapes are studied numerically using finite volume method based on SIMPLEC algorithm using ANSYS-Fluent software.
Findings
Based on the obtained results, it is shown that for low-mass flow rates, the disk baffle (DB) has more heat transfer coefficient than that of segmental baffle (SB) configuration, while for mass flow rate more than 1 kg/s, using the SB leads to more heat transfer coefficient than that of DB configuration. Using the DB leads to higher thermal-hydraulic performance evaluation criteria (THPEC) than that of SB configuration in heat exchanger. The THPEC values are between 1.32 and 1.45.
Originality/value
Using inner, outer or inner/outer corrugations (outer circular rib and inner circular rib [OCR+ICR]) tubes for all mass flow rates can increase the THPEC significantly. Based on the present study, STHE with DB and OCR+ICR tubes configuration filled with water/CMC/Al2O3 with f = 1.5% and dnp = 100 nm is the optimum configuration. The value of THPEC in referred case was 1.73, while for outer corrugations and inner smooth, this value is between 1.34 and 1.57, and for outer smooth and inner corrugations, this value is between 1.33 and 1.52.
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Mohsen Izadi, Nemat M. Maleki, Ioan Pop and S.A.M. Mehryan
This paper aims to numerically investigate the natural convection heat transfer of a hybrid nanofluid into a porous cavity exposed to a variable magnetic field.
Abstract
Purpose
This paper aims to numerically investigate the natural convection heat transfer of a hybrid nanofluid into a porous cavity exposed to a variable magnetic field.
Design/methodology/approach
The non-linear elliptical governing equations have been solved numerically using control volume based finite element method. The effects of different governing parameters including Rayleigh number (Ra = 103 − 106), Hartman number (Ha = 0 − 50), volume fraction of nanoparticles (φ = 0 − 0.02), curvature of horizontal isolated wall (a = 0.85 − 1.15), porosity coefficient (ε = 0.1 − 0.9) and Darcy number (Da = 10−5 − 10−1) have been studied.
Findings
The results indicate that at low Darcy numbers close to 0, the average Nusselt number Nua enhances as porosity coefficient increases. For a = 1 and a = 1.15 in comparison with a = 0.85, the stretching of the isothermal lines is maintained from the left side to the right side and vice versa, which indicates increased natural convection heat transfer for this configuration of the top and bottom walls. In addition, at higher Rayleigh numbers, by increasing the Hartmann number, a significant decrease is observed in the Nusselt number, which can be attributed to the decreased power of the flow.
Originality/value
The authors believe that all the results, both numerical and asymptotic, are original and have not been published elsewhere.
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Emad H. Aly and Ioan Pop
The purpose of this study is to present both effective analytic and numerical solutions to MHD flow and heat transfer past a permeable stretching/shrinking sheet in a hybrid…
Abstract
Purpose
The purpose of this study is to present both effective analytic and numerical solutions to MHD flow and heat transfer past a permeable stretching/shrinking sheet in a hybrid nanofluid with suction/injection and convective boundary conditions. Water (base fluid) nanoparticles of alumina and copper were considered as a hybrid nanofluid.
Design/methodology/approach
Proper-similarity variables were applied to transform the system of partial differential equations into a system of ordinary (similarity) differential equations. Exact analytical solutions were then presented for the dimensionless stream and temperature functions. Further, the authors introduce a very nice analytic and numerical solutions for both small and large values of the magnetic parameter.
Findings
It was found that no/unique/two equal/dual physical solutions exist for the investigated boundary value problem. The physically realizable practice of these solutions depends on the range of the governing parameters. For a stretching/shrinking sheet, it was deduced that a hybrid nanofluid works as a cooler on increasing some of the investigated parameters. Moreover, in the case of a shrinking sheet, the first solutions of hybrid nanofluid are stable and physically realizable rather than the nanofluid, while those of the second solutions are not for both hybrid nanofluid and nanofluid.
Originality/value
The present results for the hybrid nanofluids are new and original, as they successfully extend (generalize) the problems previously considered by different authors for the case of nanofluids.