scholarly journals Prandtl Number Effects on the Entropy Generation During the Transient Mixed Convection in a Square Cavity Heated from Below

2021 ◽  
Author(s):  
Nawal Ferroudj ◽  
Hasan Koten ◽  
Sacia Kachi ◽  
Saadoun Boudebous
Keyword(s):  
Heat Transfer ◽  
Prandtl Number ◽  
Mixed Convection ◽  
Entropy Generation ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Working Fluid ◽  
Square Cavity ◽  
Nusselt Numbers ◽  
The Impact

This numerical study considers the mixed convection, heat transfer and the entropy generation within a square cavity partially heated from below with moving cooled vertical sidewalls. All the other horizontal sides of the cavity are assumed adiabatic. The governing equations, in stream function–vorticity form, are discretized and solved using the finite difference method. Numerical simulations are carried out, by varying the Richardson number, to show the impact of the Prandtl number on the thermal, flow fields, and more particularly on the entropy generation. Three working fluid, generally used in practice, namely mercury (Pr = 0.0251), air (Pr = 0.7296) and water (Pr = 6.263) are investigated and compared. Predicted streamlines, isotherms, entropy generation, as well as average Nusselt numbers are presented. The obtained results reveal that the impact of the Prandtl number is relatively significant both on the heat transfer performance and on the entropy generation. The average Nusselt number increase with increasing Prandtl number. Its value varies thereabouts from 3.7 to 3.8 for mercury, from 5.5 to 13 for air and, from 12.5 to 15 for water. In addition, it is found that the total average entropy generation is significantly higher in the case of mercury (Pr«1) and water (Pr»1) than in the case of air (Pr~1). Its value varies approximately from 700 to 1100 W/m3 K for mercury, from 200 to 500 W/m3 K for water and, from 0.03 to 5 W/m3 K for air.    

Numerical Study on Fluid Flow and Heat Transfer of Mixed Convection to a Stagnation Region From an Upward-Facing Horizontal Plate

2011 ◽  
Author(s):  
Akihiko Mitsuishi ◽  
Kenzo Kitamura
Keyword(s):  
Heat Transfer ◽  
Mixed Convection ◽  
Numerical Study ◽  
Infinite Plate ◽  
Convection Heat Transfer ◽  
Constant Heat Flux ◽  
Working Fluid ◽  
Stagnation Region ◽  
Typical Structure ◽  
Recent Experimental Study

Mixed convection heat transfer from an upward-facing horizontal semi-infinite plate to a stagnation region is studied by means of direct numerical simulation. All the cases studied are simulated under constant heat flux condition on the plate. Assuming that the working fluid is air at room temperature and pressure, the Prandtl number is kept at 0.71. The Reynolds and the modified Grashof numbers are in the ranges of 102−103 and 107−108, respectively. Longitudinal vortical structure, which was discovered in the recent experimental study, is successfully simulated. The typical structure appears as a pair of counter-rotating vortices being elongated over the plate. The relationship between this structure and the heat transfer rate is clarified. Characteristics of the vortices are investigated in detail.

Numerical Study on Mixed Convection Heat Transfer Enhancement in a Long Horizontal Channel Using Periodically Distributed Rotating Blades

2019 ◽  
Author(s):  
Mahmudul Islam ◽  
Shahriar Alam ◽  
Md. Shajedul Hoque Thakur ◽  
Mohammad Nasim Hasan ◽  
M. Ruhul Amin
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Prandtl Number ◽  
Mixed Convection ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Horizontal Channel ◽  
Convection Heat ◽  
Rotating Blades ◽  
Mixed Convection Heat Transfer

Abstract A numerical study has been conducted on mixed convection heat transfer enhancement in a long horizontal channel provided with periodically distributed rotating blades. The upper wall of the channel is maintained at a constant low temperature (Tc) while the lower wall is kept hot at a constant high temperature (Th). A series of rotating blades having negligible thickness in comparison to its length is placed periodically along the centerline of the channel with the spacing between two successive blades’ rotational axes being equal to the height of the channel under consideration. The mathematical model of the present problem is governed by two-dimensional laminar transient continuity, momentum and energy equations. The governing equations are transformed to non-dimensional forms and then the moving mesh problem due to blade motion is solved by implementing Arbitrary Lagrangian-Eulerian (ALE) finite element formulation with triangular discretization scheme. Three different working fluids have been considered such as water, air and liquid Gallium that essentially cover a wide range of Prandtl Number (Pr) from 0.026 to 7.1. The dynamic condition of the rotating blades has been represented by Reynolds Number (Re) that is varied in the range of 1 to 500 and its effect on fluid flow and heat transfer has been investigated for the case of pure mixed convection heat transfer, characterized by Richardson number (Ri) of unity. Numerical results have been presented and analyzed in terms of the distribution of streamline and isotherm patterns, spatially averaged Nusselt number and normalized average Nusselt number variation along the hot wall for different parametric system configurations. The results of the present study show that, presence of rotating blades increases the heat transfer significantly in the channel. Heat transfer increases with increasing Prandtl Number (Pr) and the enhancement becomes more significant at higher Reynolds Numbers (Re).Power Spectrum analysis in frequency domain obtained from the FFT analysis indicates that, the rotating blade oscillation frequency and the oscillation frequency of Nusselt number differ at higher range of Reynolds Number (Re) and Prandtl Number (Pr). Therefore, dynamic condition of the rotating blades together with the thermophysical properties of working fluid play vital role in modulating the heat transfer characteristics and fluid flow behavior within the long horizontal channel.

A Numerical Study of Unsteady Mixed Convection in a Square Cavity: Transition From Laminar to Turbulent Flow

2013 ◽  
Author(s):  
K. M. Akyuzlu
Keyword(s):  
Heat Transfer ◽  
Mixed Convection ◽  
Turbulent Flow ◽  
Turbulence Model ◽  
Numerical Study ◽  
Working Fluid ◽  
Heat Transfer Characteristics ◽  
Square Cavity ◽  
Transfer Characteristics ◽  
Unsteady Mixed Convection

A study was conducted to simulate the circulation patterns and heat transfer characteristics of flows in a square cavity during transition from laminar to turbulent mixed convection conditions using numerical techniques. The cavity under study is assumed to be filled with a compressible fluid. The bottom of the cavity is insulated and stationary where as the top of the cavity (the lid) is assumed to be stationary initially and then pulled at constant speed for times greater than zero. The vertical walls of the cavity are kept at constant but unequal temperatures. A two-dimensional, physics based mathematical model is adopted to predict the momentum and heat transfer inside this rectangular cavity. A standard two equation turbulence model is used to model the turbulent flow inside the enclosure and the compressibility of the working fluid is represented by an ideal gas relation. The numerical solution techniques adopted in this study is a hybrid one (implicit-explicit) where the conservation equations for the velocity, temperature, and pressure are solved using an implicit technique (Coupled Modified Strongly Implicit Procedure -CMSIP) whereas the equations for the standard K-ε turbulence model are solved using an explicit (MacCormack) technique. In both techniques, a second order accurate finite difference technique is used to discretize the governing equations. Then numerical experiments were carried out to simulate the unsteady flow and heat transfer characteristics of mixed convection flow inside a square cavity filled with air (Pr = 0.72) for different Richardson numbers in the range of 0.00868–0.03470; corresponding to Reynolds numbers ranging from 2000 to 4000, respectively, when the Rayleigh number was kept constant at 105. Vertical and horizontal temperature and velocity profiles were generated while the flow goes through transition from laminar to turbulent. Changes in wall heat flux were calculated and average Nusselt numbers were determined for each parametric study.

Numerical study of mixed convection heat transfer inside a vertical microchannel with two-phase approach

2018 ◽  
Vol 135 (2) ◽  
pp. 1119-1134 ◽  
Author(s):  
Mohammad Reza Tavakoli ◽  
Omid Ali Akbari ◽  
Anoushiravan Mohammadian ◽  
Erfan Khodabandeh ◽  
Farzad Pourfattah
Keyword(s):  
Heat Transfer ◽  
Mixed Convection ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Convection Heat ◽  
Two Phase ◽  
Phase Approach ◽  
Mixed Convection Heat Transfer ◽  
Vertical Microchannel

scholarly journals Numerical Study of Mixed Convection in a Channel Filled with a Porous Medium

2019 ◽  
Vol 9 (2) ◽  
pp. 211 ◽  
Author(s):  
Filiz Ozgen ◽  
Yasin Varol
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Mixed Convection ◽  
Nuclear Waste ◽  
Numerical Study ◽  
Temperature Fields ◽  
Dimensionless Form ◽  
Geothermal Resources ◽  
Nusselt Numbers ◽  
The Finite Difference Method

The heat transfer of mixed convection in a horizontal channel filled with a porous medium has been studied in this article, given that it plays an extensive role in various technical applications, such as flow of fluid in geothermal resources, formations in chemical industries, the storage of radioactive nuclear waste material, and cooling. Those equations written in a dimensionless form have been solved using the finite difference method for different values of the parameters. The results obtained from the study have been presented through streamlines, isotherms, and both local and average Nusselt numbers. It has been observed that parameters such as the Rayleigh and Peclet numbers have an effect on flow and temperature fields.

Periodic Fluid Flow and Heat Transfer in a Square Cavity Due to an Insulated or Isothermal Rotating Cylinder

2009 ◽  
Vol 131 (11) ◽  
Author(s):  
Y.-C. Shih ◽  
J. M. Khodadadi ◽  
K.-H. Weng ◽  
A. Ahmed
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
Rotating Cylinder ◽  
Flow Fields ◽  
Square Cavity ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Rotating Objects

The periodic state of laminar flow and heat transfer due to an insulated or isothermal rotating cylinder object in a square cavity is investigated computationally. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square, and equilateral triangle) with different sizes are placed in the middle of a square cavity. A combination of a fixed computational grid and a sliding mesh was utilized for the square and triangle shapes. For the insulated and isothermal objects, the cavity is maintained as differentially heated and isothermal enclosures, respectively. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr=5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta<1750). The periodic flow field, the interaction of the rotating objects with the recirculating vortices at the four corners, and the periodic channeling effect of the traversing vertices are clearly elucidated. The simulations of the dynamic flow fields were confirmed against experimental data obtained by particle image velocimetry. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to the conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. Transient variations of the average Nusselt numbers of the respective systems show that for high Re numbers, a quasiperiodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt numbers of the insulated and isothermal object systems were correlated with the rotational Reynolds number and shape of the object. For high Re numbers, the performance of the system is independent of the shape of the object. On the other hand, with lowering of the hydraulic diameter (i.e., bigger objects), the triangle and the circle exhibit the highest and lowest heat transfers, respectively. High intensity of the periodic channeling and not its frequency is identified as the cause of the observed enhancement.

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