Mohamed Omri and Nicolas Galanis
The purpose of this paper is to evaluate the capacity of two equation turbulence models to reproduce mean and fluctuating quantities in the case of both natural convection and…
Abstract
Purpose
The purpose of this paper is to evaluate the capacity of two equation turbulence models to reproduce mean and fluctuating quantities in the case of both natural convection and isothermal flows.
Design/methodology/approach
Numerical predictions of mean velocity profiles, air and wall temperatures as well as turbulent kinetic energy by three different two equation models (standard k‐ε, renormalisation group k‐ε and shear‐stress transport‐k‐ω) are compared with corresponding experimental values.
Findings
The prediction of mean velocities and temperatures is in all cases satisfactory. On the other hand, the prediction of turbulent quantities is less precise.
Originality/value
The three models under consideration in this paper can be used for engineering applications such as HVAC calculations.
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Thierry Maré, Nicolas Galanis, Sylvie Prétot and Jacques Miriel
To determine the axial evolution of the hydrodynamic and the thermal fields for mixed convection in inclined tubes and to investigate the presence of flow reversal.
Abstract
Purpose
To determine the axial evolution of the hydrodynamic and the thermal fields for mixed convection in inclined tubes and to investigate the presence of flow reversal.
Design/methodology/approach
The elliptical, coupled, steady state, three‐dimensional governing partial differential equations for heated ascending laminar mixed convection in an inclined isothermal tube were solved numerically using a finite volume staggered grid approach.
Findings
The axial evolution of the velocity profiles and fluid temperatures show that upstream diffusion has an important effect near the inlet of the heating region. As a result, both the wall shear stress and the Nusselt number are affected upstream of the heating zone. Flow reversal occurs of GF≥9 × 105. The shape and size of the region with negative velocities depends strongly on the value of the Grashof number. The effect of the Grashof number on the axial evolution of the wall shear stress and the Nusselt number is shown to be very important in the region of developing flow.
Research limitations/implications
The results have been calculated for one Reynolds number (Re=100), a single fluid (air) and one tube inclination (45°).
Practical implications
Further results of this type can be mapped and would be useful for heat exchanger design.
Originality/value
This is the first time that flow reversal has been calculated numerically for inclined tubes. Most previous studies concern horizontal or vertical tubes and use axially parabolic equations which are easier to solve but can not calculate the flow field in the region with backflow.
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Youssef Azizi, Brahim Benhamou, Nicolas Galanis and Mohammed El‐Ganaoui
The objective of the present study is to investigate numerically the effects of thermal and buoyancy forces on both upward flow (UF) and downward flow (DF) of air in a vertical…
Abstract
Purpose
The objective of the present study is to investigate numerically the effects of thermal and buoyancy forces on both upward flow (UF) and downward flow (DF) of air in a vertical parallel‐plates channel. The plates are wetted by a thin liquid water film and maintained at a constant temperature lower than that of the air entering the channel.
Design/methodology/approach
The solution of the elliptical PDE modeling the flow field is based on the finite volume method.
Findings
Results show that buoyancy forces have an important effect on heat and mass transfers. Cases with evaporation and condensation have been investigated for both UF and DF. It has been established that the heat transfer associated with these phase changes (i.e. latent heat transfer) may be more or less important compared with sensible heat transfer. The importance of these transfers depends on the temperature and humidity conditions. On the other hand, flow reversal has been predicted for an UF with a relatively high temperature difference between the incoming air and the walls.
Originality/value
Contrary to most studies in channel heat and mass transfer with phase change, the mathematical model considers the full elliptical Navier‐Stokes equations. This allows one to compute situations of flow reversal.
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Sidi El Bécaye Maïga, Cong Tam Nguyen, Nicolas Galanis, Gilles Roy, Thierry Maré and Mickaël Coqueux
To study the hydrodynamic and thermal behaviors of a turbulent flow of nanofluids, which are composed of saturated water and Al2O3 nanoparticles at various concentrations, flowing…
Abstract
Purpose
To study the hydrodynamic and thermal behaviors of a turbulent flow of nanofluids, which are composed of saturated water and Al2O3 nanoparticles at various concentrations, flowing inside a tube submitted to a uniform wall heat flux boundary condition.
Design/methodology/approach
A numerical method based on the “control‐volume” approach was used to solve the system of non‐linear and coupled governing equations. The classical κ‐ε model was employed in order to model the turbulence, together with staggered non‐uniform grid system. The computer model, satisfactorily validated, was used to perform an extended parametric study covering wide ranges of the governing parameters. Information regarding the hydrodynamic and thermal behaviors of nanofluid flow are presented.
Findings
Numerical results show that the inclusion of nanoparticles into the base fluid has produced an augmentation of the heat transfer coefficient, which has been found to increase appreciably with an increase of particles volume concentration. Such beneficial effect appears to be more pronounced for flows with moderate to high Reynolds number. In reverse, the presence of nanoparticles has induced a rather drastic effect on the wall shear stress that has also been found to increase with the particle loading. A new correlation, Nufd=0.085 Re0.71 Pr0.35, is proposed to calculate the fully‐developed heat transfer coefficient for the nanofluid considered.
Practical implications
This study has provided an interesting insight into the nanofluid thermal behaviors in the context of a confined tube flow. The results found can be easily exploited for various practical heat transfer and thermal applications.
Originality/value
The present study is believed to be an interesting and original contribution to the knowledge of the nanofluid thermal behaviors.
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Hussein Bazzi, Cong Tam Nguyen and Nicolas Galanis
This work is concerned with the problem of the transient behaviours of the axisymmetric thermocapillary laminar flow occurring inside a half zone subjected to a variable thermal…
Abstract
This work is concerned with the problem of the transient behaviours of the axisymmetric thermocapillary laminar flow occurring inside a half zone subjected to a variable thermal boundary condition during a heating process. The molten liquid with its deformable free surface is considered incompressible with constant physical properties except for its density in buoyancy forces where Boussinesq’s approximation has been applied. The system of governing equations has been successfully solved by using the modified‐SIMPLE method, while the instantaneous position of the free surface was determined by employing a special procedure. Numerical simulations have been carried out for both NaNO3 and Silicon float zones operating under 1‐g and μ‐g conditions. The transient behaviours as well as the influence of the Marangoni number and the aspect ratio have been investigated.
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Syed Saqib Shah, Hakan F. Öztop, Rizwan Ul-Haq and Nidal Abu-Hamdeh
The purpose of this paper is to analyse the buoyancy flow, mass and heat transfer in coaxial duct under Soret and Dufour effect. The combined effects of the thermal-diffusion and…
Abstract
Purpose
The purpose of this paper is to analyse the buoyancy flow, mass and heat transfer in coaxial duct under Soret and Dufour effect. The combined effects of the thermal-diffusion and diffusion-thermo coefficients, as well as the Schmidt number, on natural convection in a heated lower coaxial curve were explored using the proposed physical model. The Dufour and Soret effects are taken into consideration in the energy and concentration equations, respectively.
Design/methodology/approach
The dominating mathematical models are converted into a set of non-linear coupled partial differential equations, which are solved using a numerical approach. The controlling non-linear boundary value problem is numerically solved using the penalty finite element method with Galerkin’s weighted residual scheme over the entire variety of essential parameters.
Findings
It was observed that different parameters were tested such as heat generation or absorption coefficient, buoyancy ratio, Soret coefficient, Dufour coefficient, Lewis number and Rayleigh number. Effect of Rayleigh number, absorption/generation and Dufour coefficient on isotherm are significantly reported. For greater values of Lewis number, maximum mass transfer in duct in the form of molecular particles is produced. Buoyancy ratio parameter decreases the average rate of heat flow and increases its mass transfer.
Originality/value
The main originality of this work is to make an application of Soret and Dufour effects in a coaxial duct in the presence of source sink.
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Paris Tsartas, Harry Coccossis and Magdalena Vasileiou
The aim of this chapter is to provide a holistic overview of issues and topics regarding tourism geography in Greece: from its origins to its current situation. By following a…
Abstract
The aim of this chapter is to provide a holistic overview of issues and topics regarding tourism geography in Greece: from its origins to its current situation. By following a historical tracking of tourism activities in this country and the growth of tourism geography as an academic domain at the university level, the main goal is to analyze the shift of academic research on tourism from geographical perspective, as presented in both the Greek and English language literature. The chapter concludes that there is a need for practical orientation and redefinition of typologies of tourism geographies in order to apply a more sustainable and cross-disciplinary approach in the academic discourse on Greek tourism.
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Travel and tourism have had a long history in the Nordic countries, but research on tourism has a relatively short tradition in the region. Recently, academic interest in the…
Abstract
Travel and tourism have had a long history in the Nordic countries, but research on tourism has a relatively short tradition in the region. Recently, academic interest in the Nordic tourism space has grown and diversified especially as a result of increasing numbers of academics and institutions involved with tourism geographies and studies and education in the region. The Nordic context has provided thematic focus areas for empirical studies that characterize tourism geographies in the region, with topics including nature-based tourism, utilization of wilderness areas, second-home and rural developments, impacts in peripheries, and tourism as a tool for regional development. In addition, there are emerging research themes outside of the traditional core topics, such as urban, events, and heritage tourism.
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Nicolai Scherle and Hans Hopfinger
This chapter aims to familiarize the reader with some of the important aspects of tourism geography in the German-speaking countries. It starts with a primarily historical-genetic…
Abstract
This chapter aims to familiarize the reader with some of the important aspects of tourism geography in the German-speaking countries. It starts with a primarily historical-genetic perspective on tourism development and the theoretical traditions associated with them. The second section describes the structure of the discipline, with a focus on the institutionalization of the field in the universities including their research specialization. The chapter maintains that tourism geography plays a marginal role compared with other subdisciplines of geography, though this is reflected primarily in its institutionalization and less so in the research undertaken. The last section deals with the current challenges and future prospects in German-speaking geographies of tourism from a problem-centered perspective.