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Research Papers: Natural and Mixed Convection

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

[+] Author and Article Information
Oronzio Manca

Dipartimento di Ingegneria Aerospaziale e Meccanica, Seconda Università degli Studi di Napoli, Real Casa dell’Annunziata via Roma 29, 81031 Aversa Caserta (CE), Italyoronzio.manca@unina2.it

Sergio Nardini

Dipartimento di Ingegneria Aerospaziale e Meccanica, Seconda Università degli Studi di Napoli, Real Casa dell’Annunziata via Roma 29, 81031 Aversa Caserta (CE), Italy

J. Heat Transfer 131(6), 062503 (Apr 09, 2009) (10 pages) doi:10.1115/1.3084212 History: Received February 02, 2008; Revised October 04, 2008; Published April 09, 2009

## 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. 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 $10≤2L/b≤20$.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 6

Flow visualization for b=20.0 mm and qΩ=60 W/m2(Ra=5.38×103). Longitudinal section at (a) z=0 mm; transversal sections at (b) x=20 mm, (c) x=100 mm, (d) x=150 mm, and (e) x=200 mm.

Figure 7

Flow visualization for b=40.0 mm and qΩ=60 W/m2(Ra=8.92×104). Longitudinal section at (a) z=0 mm; transversal sections at (b) x=20 mm, (c) x=100 mm, (d) x=150 mm, and (e) x=200 mm.

Figure 8

Flow visualization for b=40.0 mm and qΩ=120 W/m2(Ra=1.92×105). Longitudinal section at (a) z=0 mm; transversal sections at (b) x=20 mm, (c) x=100 mm, (d) x=150 mm, and (e) x=200 mm.

Figure 9

Air temperatures versus time for b=40.0 mm, z=0 mm, and qΩ=120 W/m2(Ra=1.92×105) at several y locations and four axial coordinates: (a) x=20 mm, (b) x=60 mm, (c) x=100 mm, and (d) x=200 mm

Figure 10

Time averaged air temperature versus y-coordinate at several x locations and z=0 mm

Figure 11

Comparison between different correlations: (a) average Nusselt numbers, Nu and Nu∗, versus average Rayleigh number and aspect ratio, Ra (b/2L); (b) dimensionless maximum wall temperatures, T+ and T+∗ versus average Rayleigh number and aspect ratio, Ra (b/2L).

Figure 1

(a) View of the test section; (b) sketch of the heated sandwiched plate

Figure 2

Air temperature measurement grid in the longitudinal section: (a) b=20.0 mm; (b) b=40.0 mm

Figure 3

Sketch of the test section with visualization and air temperature measurement arrangements: (a) longitudinal section; (b) three dimensional view

Figure 4

Sketch of the main flow in the cavity

Figure 5

Surface temperature versus longitudinal x-coordinate for three Ohmic wall heat fluxes: (a) b=20.0 mm (Ra= (●) 2.26×103, (▽) 5.38×10, (◼) 8.91×103); (b) b=32.3 mm (Ra= (●) 1.64×104, (▽) 3.86×104, (◼) 6.45×104); (c) b=40.0 mm (Ra= (●) 3.71×104, (▽) 8.92×104, (◼) 1.92×105)

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