Interaction of Heat Transfer by Conduction, Convection, and Radiation in a Radiating Fluid

1963 ◽  
Vol 85 (4) ◽  
pp. 318-328 ◽  
Author(s):  
R. Viskanta
Keyword(s):  
Heat Transfer ◽  
Energy Transport ◽  
Relative Role ◽  
Heat Transfer Characteristics ◽  
Temperature Variations ◽  
System Parameters ◽  
Heating And Cooling ◽  
Finite Distance ◽  
Parallel Surfaces

Consideration is given to the interaction of conduction, convection, and radiation in a fully developed laminar flow. The flat duct consists of two diffuse, nonblack, isothermal parallel surfaces a finite distance apart; the fluid between them emits and absorbs thermal radiation. The problem is formulated in terms of a nonlinear integro-differential equation, and the solution is obtained by a method employed by Barbier. Numerical examples show the influence of the system parameters such as the optical thicknesses, the ratio which determines the relative role of energy transport by conduction to that by radiation, the emissivity of the duct walls as well as the differences between heating and cooling on the temperature variations across the duct and on the heat-transfer characteristics. Two methods for obtaining approximate temperature distributions for optically transparent and opaque radiating media are outlined and the results discussed.

Heat Transfer by Conduction and Radiation in Absorbing and Scattering Materials

1965 ◽  
Vol 87 (1) ◽  
pp. 143-150 ◽  
Author(s):  
R. Viskanta
Keyword(s):  
Heat Transfer ◽  
Radiant Energy ◽  
Dimensional System ◽  
Parallel Plates ◽  
Heat Transfer Characteristics ◽  
One Dimensional ◽  
System Parameters ◽  
Finite Distance ◽  
Radiant Energy Transfer ◽  
Scattering Material

Heat transfer by simultaneous conduction and radiation in thermal radiation absorbing, emitting, and scattering materials is investigated theoretically. Consideration is given to a one-dimensional system consisting of two diffuse, nonblack, isothermal parallel plates separated by a finite distance. The space between the two plates is filled with an isotropically scattering material. The problem is formulated exactly in terms of integrodifferential and integral equations. The results define as well as illustrate several mechanisms of radiant energy transfer and show how one mode of heat transfer influences the other. The numerical results reveal the effect of the system parameters on the heat transfer characteristics. In particular, it is shown that the effect of albedo on the heat transfer is small. Albedo being the parameter which represents the fraction of the incident pencil of radiation which has been scattered.

Heat Transfer Characteristics of Baffled Channel Flow

2011 ◽  
Author(s):  
Changwoo Kang ◽  
Kyung-Soo Yang
Keyword(s):  
Heat Transfer ◽  
Channel Flow ◽  
Large Scale ◽  
Heat Removal ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow Heat ◽  
Two Parameters ◽  
Time Periodic

Heat transfer characteristics of baffled channel flow, where thin baffles are mounted on both channel walls periodically in the direction of the main flow, have been numerically investigated in a laminar range. In baffled channel flow, heat transfer characteristics are significantly affected by large-scale vortices generated due to flow separation at the tips of the baffles. In this investigation, a parametric study has been carried out to identify the optimal configuration of the baffles to achieve the most efficient heat removal from the channel walls. Two key parameters are considered, namely baffle interval (L) and Reynolds number (Re). We elucidate the role of the primary instability, a Hopf bifurcation from steady to a time-periodic flow, in the convective heat transfer in baffled channel flow. We also propose a contour diagram (“map”) of averaged Nusselt number on the channel walls as a function of the two parameters. The results shed light on understanding and controlling heat transfer mechanism in a finned heat exchanger, being quite beneficial to its design.

Numerical Re-examination of Chilton–Colburn Analogy for Variable Thermophysical Fluid Properties

2017 ◽  
Vol 139 (7) ◽  
Author(s):  
Rajan Kumar ◽  
Shripad P. Mahulikar
Keyword(s):  
Heat Transfer ◽  
Energy Transport ◽  
Momentum Transport ◽  
Flow Friction ◽  
Steep Temperature Gradient ◽  
Fluid Properties ◽  
Conservation Of Momentum ◽  
Energy Equations ◽  
Fanning Friction Factor

The Chilton–Colburn analogy is very helpful for evaluating the heat transfer in internal forced flows. The Chilton–Colburn analogy between the Chilton–Colburn j-factor for heat transfer, jH (St·Pr2/3) and the Fanning friction factor (cf) is popularly considered to hold when St·Pr2/3 equals to cf/2, for constant fluid properties. The physical fluid properties, namely, viscosity and thermal conductivity, are generally a function of temperature for microconvective water flow due to a quite steep temperature gradient. Therefore, in present investigation, the validity of Chilton–Colburn analogy between St·Pr2/3 and cf is re-examined for laminar microconvective flow with variable thermophysical fluid properties. It is observed that the Chilton–Colburn analogy is valid only for that portion of the flow regime, where St·Pr2/3 decreases with decreasing cf. The validity of Chilton–Colburn analogy is also verified by the inverse dependence of Reynolds number (Re) with cf. Two modified nondimensional parameters “ΠSμ and ΠSk” are emerged from the nondimensional form of 2D, steady-state, incompressible, pure continuum-based, laminar conservation of momentum and energy equations, respectively. These modified nondimensional parameters show the significance of variable fluid properties in momentum transport and energy transport. Additionally, the role of ΠSμ and ΠSk in flow friction is also investigated. The higher values of ΠSμ and ΠSk indicate the stronger influence on microconvection due to large variations in fluid properties.

CO2-Filled Vertical Geothermal Boreholes: Modeling and Application

2013 ◽  
Author(s):  
Parham Eslami-Nejad ◽  
Mohamed Ouzzane ◽  
Zine Aidoun
Keyword(s):  
Heat Transfer ◽  
Carbon Dioxide ◽  
Thermal Performance ◽  
Heat Pumps ◽  
Superior Performance ◽  
Fluid Temperature ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Transfer Characteristics ◽  
Heating And Cooling

In the context of extremely cold or hot climates, ground coupled heat pumps offer several environmental and energy efficiency advantages over conventional methods for heating and cooling of buildings and they are increasingly used in most near-zero and net-zero energy buildings. However, the high initial cost of the ground loop portion (geothermal boreholes) has often raised the question on the economical competitiveness of the system. In the present study, a geothermal borehole is proposed in which two-phase carbon dioxide (mixture of liquid and vapor) exchanges heat with the ground to improve the thermal performance of the borehole and thus to reduce borehole length. Carbon dioxide shows several cost and environmental advantages. Moreover, it offers superior thermophysical properties and heat transfer characteristics. A numerical model has been developed to study the complex thermal behavior of a two-phase CO2-filled vertical geothermal borehole. The model can handle both two-phase and single-phase conditions along the borehole length. An explicit solution for fully coupled conservation equations of mass, momentum and energy as well as an equation of state are applied. The model accounts for the thermal interaction among the pipes and it predicts the fluid temperature, pressure and two-phase quality profiles. It is used to assess the thermal performance of the CO2-filled secondary loop geothermal borehole operating in heating mode. Results indicate that the proposed borehole offers superior performance due to the relatively high two-phase heat transfer characteristics of CO2.

Effect of Variable Sidewall Temperatures on the Combined Surface Radiation—Convection in a Discretely Heated Enclosure

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
S. Saravanan ◽  
N. Raja
Keyword(s):  
Heat Transfer ◽  
Transfer Rate ◽  
Surface Radiation ◽  
Computational Results ◽  
Heat Transfer Characteristics ◽  
Total Heat Transfer ◽  
Flow And Heat Transfer ◽  
Volume Method ◽  
Made In

This paper reports the changes made in the flow and heat transfer characteristics of a closed enclosure in the presence of sidewalls with symmetrical linear heating. The flow inside the enclosure is primarily driven by a centrally placed discrete heater with thermal radiation included at all surfaces involved. Finite volume method-based computational results corresponding to the resulting steady-state were obtained. The factors causing augmentation and suppression of heat transfer are discussed for two types of sidewall heating. Moreover, it is found that the role of radiation is well stronger than convection in determining the total heat transfer rate when the sidewall heating is decreasing with height.

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