Simultaneous Hydrodynamic and Thermal Development in Mixed Convection in a Vertical Annulus With Fluid Property Variations

This paper concerns a theoretical investigation of forced and mixed convection heat transfer in a vertical concentric annulus. An implicit finite difference technique is developed to study the effects of temperature-dependent fluid properties, which are represented by power law relations. The fluid under consideration in this study is air, which is an ideal gas with Pr = 0.72. Computations are made with a radius ratio of 0.25. The inner wall is heated at UHF, and the outer wall is heated at UHF or is insulated. The axial distribution of the wall-to-bulk temperature ratio is found to undergo a maximum or minimum, depending on whether the wall is heated or insulated. Fluid property variations enhance the local Nusselt numbers. It is shown that, depending on the length of the duct, heat transfer can be appreciably affected by fluid property variation. For the UHF cases studied here, free convection effects enhance the local Nusselt numbers at intermediate axial distances.

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Applicability of Uniform Heat Flux Nusselt Number Correlations to Thermosyphon Heat Exchangers for Solar Water Heaters

Journal of Solar Energy Engineering ◽

10.1115/1.2888151 ◽

1999 ◽

Vol 121 (2) ◽

pp. 85-90 ◽

Cited By ~ 4

Author(s):

S. Dahl ◽

J. Davidson

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Mixed Convection ◽

Heat Exchangers ◽

Convection Heat Transfer ◽

Uniform Heat Flux ◽

Convection Heat ◽

Solar Water ◽

Nusselt Numbers ◽

Uniform Heat

Nusselt numbers are measured in three counterflow tube-in-shell heat exchangers with flow rates and temperatures representative of thermosyphon operation in solar water heating systems. Mixed convection heat transfer correlations for these tube-in-shell heat exchangers were previously developed in Dahl and Davidson (1998) from data obtained in carefully controlled experiments with uniform heat flux at the tube walls. The data presented in this paper confirm that the uniform heat flux correlations apply under morerealistic conditions. Water flows in the shell and 50 percent ethylene glycol circulates in the tubes. Actual Nusselt numbers are within 15 percent of the values predicted for a constant heat flux boundary condition. The data reconfirm the importance of mixed convection in determining heat transfer rates. Under most operating conditions, natural convection heat transfer accounts for more than half of the total heat transfer rate.

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

Periodica Polytechnica Mechanical Engineering ◽

10.3311/ppme.17563 ◽

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.

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Forced Convection Heat Transfer in a Finitely Conducting Externally Finned Pipe

Journal of Heat Transfer ◽

10.1115/1.3250530 ◽

1988 ◽

Vol 110 (3) ◽

pp. 571-576 ◽

Cited By ~ 5

Author(s):

F. Moukalled ◽

S. Acharya

Keyword(s):

Heat Transfer ◽

Forced Convection ◽

Numerical Study ◽

Convection Heat Transfer ◽

Axial Direction ◽

Bulk Temperature ◽

Convection Heat ◽

Forced Convection Heat Transfer ◽

Nusselt Numbers ◽

Wall Conductivity

A numerical study to determine the influence of axial wall conduction on forced convection heat transfer in an externally finned pipe has been made. The effects of wall conductivity, interfin spacing, and external heat transfer coefficient are examined by comparing the results with the corresponding solutions obtained assuming negligible wall conduction. Results indicate that the axial conduction in the pipe walls has a significant influence on the heat transfer behavior. The bulk temperature or the heat transferred to the fluid is underestimated when wall conduction is ignored. At high wall conductivity values, the wall temperatures and Nusselt numbers exhibit a monotonic variation in the axial direction, with the behavior becoming increasingly nonmonotonic as the wall conductivity value is decreased.

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