Radiation Effects on Natural Convection Between Horizontal Walls With Heated Upper Plate

2000 ◽  
Author(s):  
Giuseppe Guidotti ◽  
Oronzio Manca ◽  
Sergio Nardini ◽  
Biagio Morrone
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Air Temperature ◽  
Radiation Effects ◽  
Fluid Layer ◽  
Radiation Heat Transfer ◽  
Mean Value ◽  
Radiative Heating ◽  
Temperature Measurements ◽  
Secondary Motion

Abstract Radiation heat transfer affects natural convection of air inside an open ended cavity with a heated horizontal upper plate and an unheated lower parallel plate. The influence is mainly due to radiative heating of the lower plate, and plane fluid layer secondary motion could arise. In this paper an experimental study is carried out to describe and to detect the influence of radiation on air flow and on heat transfer coefficient by means of wall temperature profiles, smoke visualization, and air temperature measurements. The analysis is obtained for an emissivity of the horizontal plates equal to 0.8, for distances between the plates of 20.0, 32.3, and 40.0 mm. By means of flow visualization and local air temperature measurements in the cavity as a function of time, remarkable secondary motion in the cavity is observed when qΩ is equal to 120 W/m2. Measurement of the air temperature in the cavity also shows that radiation causes and damps secondary motion at the same time. Profiles of the mean value of the air temperature as a function of the x and y coordinates confirm both the main flow path inside the cavity and radiation effects on convective heat transfer. Finally, correlations related to average Nusselt number are proposed for natural convection as well as for heat transfer as a whole, that is convection along with radiation.

Experimental Investigation of Radiation Effects on Natural Convection in Horizontal Channels Heated From Above

2009 ◽  
Vol 131 (6) ◽  
Author(s):  
Oronzio Manca ◽  
Sergio Nardini
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Air Temperature ◽  
Radiation Effects ◽  
Fluid Layer ◽  
Radiation Heat Transfer ◽  
Temperature Measurements ◽  
Temperature Profiles ◽  
Vertical Coordinate ◽  
Aspect Ratios

Radiation heat transfer affects natural convection of air inside an open-ended cavity with a heated horizontal upper plate and an unheated lower parallel plate. Its effect is the heating of the lower plate, which heats the adjacent fluid layer and could determine secondary motions. In this paper, an experimental study is carried out to describe the effect of high value of surface emissivity on air flow in an open-ended cavity. The investigation is performed by means of wall temperature profiles, smoke visualization, and air temperature measurements. Results are obtained for an emissivity of the horizontal plates equal to 0.8, for aspect ratios between 10.0 and 20.0. By means of flow visualization and local air temperature measurements in the cavity as a function of time, remarkable secondary motions in the cavity are observed for the highest considered surface heat flux (Ra=8.91×103, 6.45×104, and 1.92×105). Measurement of the air temperature in the cavity also shows that radiation causes and damps secondary motions at the same time. Mean air temperature profiles as a function of the vertical coordinate, at different locations along the longitudinal axis, confirm both the main flow path inside the cavity and radiation effect on convective heat transfer. Finally, correlations for average Nusselt numbers and dimensionless maximum wall temperatures, in terms of Rayleigh number and channel aspect ratio, are proposed for natural convection with or without radiative heat transfer contribution for 2.26×103≤Ra≤1.92×105 and 10≤2L/b≤20.

Numerical Simulations of Natural Convection With Radiation in an Open Cavity Containing a Conducting and Centered Solid Body

2014 ◽  
Author(s):  
A. Lugarini de Souza ◽  
A. T. Franco ◽  
S. L. M. Junqueira ◽  
J. L. Lage
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Solid Body ◽  
Radiation Effects ◽  
Radiation Heat Transfer ◽  
Block Size ◽  
Heat Transfer Process ◽  
Open Cavity ◽  
Convection Flow ◽  
Solid Block

Although of relevance to a variety of engineering applications, the study of natural convection within an open cavity containing a conducting solid body is rarely found in the literature. Moreover, previous studies have pointed out that radiation heat transfer rates are at least of the same order of the laminar natural convection rates in cavities, making the inclusion of radiation effects and important step toward obtaining more realistic and practical results. The present study considers then a square cavity, with one wall heated and the other opened to an adjacent fluid reservoir, having a square conducting solid block centered in it and accounting for natural convection and radiation effects. Notice, for a large block size, the geometric configuration of the resulting flow channel is similar to that of a fracture along a reservoir wall. The resulting natural convection flow is simulated numerically for performing a nondimensional parametric study seeking to unveil the effects of block dimension, surface emissivity and Rayleigh number into the heat transfer process. The cavity filling fluid is assumed to have constant and uniform properties, as is the solid block, and the fluid-to-solid conductivity ratio is set as unity in the present study. The screening (radiation) effect caused by the presence of the solid block is discussed, as well as the convective and radiative drop phenomena. The convection and radiation Nusselt numbers are evaluated and compared for each simulated case.

scholarly journals Influence of Varying Pressure on Natural Convection and Radiation Heat Transfer in an Enclosure

PAMM ◽  
2005 ◽  
Vol 5 (1) ◽  
pp. 575-576 ◽  
Author(s):  
Jan Langebach ◽  
Stephan Senin ◽  
Christian Karcher
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Radiation Heat Transfer ◽  
Radiation Heat

2D Natural Convection and Radiation Heat Transfer Simulations of a PWR Fuel Assembly Within a Constant Temperature Support Structure

2006 ◽  
Author(s):  
Pablo E. Araya Go´mez ◽  
Miles Greiner
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Fuel Assembly ◽  
Heat Generation ◽  
Temperature Difference ◽  
Wall Temperature ◽  
Radiation Heat Transfer ◽  
Radiation Heat ◽  
Fuel Rods ◽  
Water Reactor

Two-dimensional simulations of steady natural convection and radiation heat transfer for a 14×14 pressurized water reactor (PWR) spent nuclear fuel assembly within a square basket tube of a typical transport package were conducted using a commercial computational fluid dynamics package. The assembly is composed of 176 heat generating fuel rods and 5 larger guide tubes. The maximum cladding temperature was determined for a range of assembly heat generation rates and uniform basket wall temperatures, with both helium and nitrogen backfill gases. The results are compared with those from earlier simulations of a 7×7 boiling water reactor (BWR). Natural convection/radiation simulations exhibited measurably lower cladding temperatures only when nitrogen is the backfill gas and the wall temperature is below 100°C. The reduction in temperature is larger for the PWR assembly than it was for the BWR. For nitrogen backfill, a ten percent increase in the cladding emissivity (whose value is not well characterized) causes a 4.7% reduction in the maximum cladding to wall temperature difference in the PWR, compared to 4.3% in the BWR at a basket wall temperature of 400°C. Helium backfill exhibits reductions of 2.8% and 3.1% for PWR and BWR respectively. Simulations were performed in which each guide tube was replaced with four heat generating fuel rods, to give a homogeneous array. They show that the maximum cladding to wall temperature difference versus total heat generation within the assembly is not sensitive to this geometric variation.

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