Nicolas Galanis

Heat transfer enhancement in turbulent tube flow using Al2O3 nanoparticle suspension

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‐e 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 co...

Analytical Solution for Fully Developed Mixed Convection Between Parallel Vertical Plates With Heat and Mass Transfer

Exact analytical solutions for fully-developed, steady-state laminar mixed convection with heat and mass transfer between vertical parallel plates are presented. The thermal boundary conditions are UWT or UHF while the concentration at each wall is assumed to be uniform but not necessarily the same. The solution for the UWT case depends on a single parameter which combines the effects of thermal and solutal buoyancy. In the UHF case it depends on three independent parameters, the ratio of the thermal Grashof number to the Reynolds number, the ratio of the solutal Grashof number to the Reynolds number and the wall heat flux ratio

Low Reynolds number mixed convection in vertical tubes with uniform wall heat flux

Abstract Upward mixed convection of air in a long, vertical tube with uniform wall heat flux has been studied numerically for Re =1000, Re =1500 and Gr ⩽10 8 using a low Reynolds number k – e model. The results for the fully developed region identify two critical Grashof numbers for each Reynolds number, which correspond to laminar–turbulent transition and relaminarization of the flow. They also distinguish the Re – Gr combinations that result in a pressure decrease over the tube length from those resulting in a pressure increase. A correlation expressing the fully developed Nusselt number in terms of the Grashof and Reynolds numbers is proposed. It is valid for laminar and turbulent flows in the range 1000⩽ Re ⩽1500, Gr ⩽5×10 7 .

Effects of axial diffusion on laminar heat transfer with low Peclet numbers in the entrance region of thin vertical tubes

Laminar upward and downward flows with mixed convection in a thin vertical tube with a short uniformly heated section were investigated numerically. Calculations were performed by solving the elliptic Navier-Stokes and energy equations for slow flows of air (Pr = 0.7) corresponding to low Reynolds numbers from 20 to 500 and a wide range of Grashof numbers. Results reveal that axial diffusion plays a significant role in preheating the fluid upstream from the entrance of the heat transfer region for both aiding and opposing buoyancy, with a stronger effect for the latter, and can lead to flow reversal. By mapping the conditions that correspond to significant preheating and flow reversal on a Grashof-Reynolds plane, it has become possible to establish criteria that determine (1) when the upstream boundary conditions can be applied at the entrance of the heated section and (2) when the elliptical formulation is necessary to describe the flow field accurately. Applications in which this regime of mixed convection occur include shell-tube heat exchangers, and nuclear reactors.

Optimal Design of ORC Systems with a Low-Temperature Heat Source

A numerical model of subcritical and trans-critical power cycles using a fixed-flowrate low-temperature heat source has been validated and used to calculate the combinations of the maximum cycle pressure (Pev) and the difference between the source temperature and the maximum working fluid temperature (DT) which maximize the thermal efficiency (ηth) or minimize the non-dimensional exergy losses (β), the total thermal conductance of the heat exchangers (UAt) and the turbine size (SP). Optimum combinations of Pev and DT were calculated for each one of these four objective functions for two working fluids (R134a, R141b), three source temperatures and three values of the non-dimensional power output. The ratio of UAt over the net power output (which is a first approximation of the initial cost per kW) shows that R141b is the better working fluid for the conditions under study.

Developing laminar mixed convection of nanofluids in an inclined tube with uniform wall heat flux

Purpose – The aim is to study the conjugate problem of developing laminar mixed convection flow and heat transfer of water‐Al2O3 nanofluid inside an inclined tube submitted to a uniform wall heat flux.Design/methodology/approach – The set of non‐linear, coupled and fully elliptic governing equations has been solved using a “finite‐control‐volume” numerical method, the classical power‐law scheme for computing heat and momentum fluxes staggered and non uniform grids for spatial discretization of various regions of the tube.Findings – Numerical results have shown that the presence of nanoparticles slightly intensifies the secondary flow due to buoyancy, in particular in the developing region. It also increases the average Nusselt number and decreases slightly the product ReCf with respect to those of water. For the horizontal inclination, two new correlations have been proposed to calculate these two variables in the fully developed region, for Grashof number ranging from 103 to 105 and particle volume conce...

Developing mixed convection with aiding buoyancy in vertical tubes: a numerical investigation of different flow regimes

Abstract Laminar upward flows with mixed convection in a vertical tube with a uniformly heated zone preceded and followed by adiabatic zones were investigated numerically. Calculations were performed by solving the elliptic Navier–Stokes and energy equations for air and a wide range of heating lengths, Reynolds and Richardson numbers. Different combinations of these parameters establish the existence of five types of flow fields: developing with or without flow reversal, developing followed by a fully developed region both without flow reversal, and developing with flow reversal followed by a fully developed region with or without flow reversal. The conditions leading to flow reversal as well as significant upstream diffusion of heat and momentum have been mapped on the Peclet–Richardson plane for different lengths of the heated zone.

Buoyancy effects on upward and downward laminar mixed convection heat and mass transfer in a vertical channel

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 ...

Coupled conduction, convection, radiation heat transfer with simultaneous mass transfer in ice rinks

ABSTRACT This article presents numerical predictions of velocity, temperature, and absolute humidity distributions in an indoor ice rink with ventilation and heating. The computational fluid dynamics (CFD) simulation includes the effects of radiation between all inside surfaces of the building envelope, turbulent mixed convection, and vapor diffusion, as well as conduction through the walls and condensation on the ice. The net radiative fluxes for each element of the envelopes inside surfaces are calculated with a modified version of Gebharts method. This modification reduces the calculation time and the memory required to store the radiation view factors for the discretized elements of the inside surfaces of the envelope. The predicted temperatures show very good agreement with measured data. The CFD code also calculates the heat fluxes toward the ice due to convection from the air, to condensation of vapor, and to radiation from the walls and ceiling. It is shown that a low emissivity ceiling reduces the sum of these fluxes and the risk of vapor condensation on the ceiling.

Developing laminar mixed convection with heat and mass transfer in horizontal and vertical tubes

Abstract The effects of the solutal and thermal Grashof numbers on the flow, temperature and concentration fields in tubes with uniform heat flux and concentration at the fluid-solid interface have been investigated numerically using a three-dimensional axially parabolic model. Results show a complex development of the flow field which is strongly influenced by the values of the two Grashof numbers and by the tube inclination. For vertical tubes the flow field is also influenced by the relative direction of the flow and the buoyancy forces. In general, very close to the tube inlet forced convection boundary layer development dominates. Further downstream, the effects of solutal buoyancy predominate while those of thermal origin determine the flow field far downstream and, in particular, the fully developed conditions. The axial evolution of the wall shear stress τ z , the Nusselt number Nu z and the Sherwood number Sh z in both horizontal and vertical tubes are presented for different combinations of the two Grashof numbers. For horizontal tubes and vertical tubes with upward flow these three variables are greater than the corresponding ones for forced convection. The opposite is true for downward flow in vertical tubes.