Wärme - und Stoffübertragung

, Volume 27, Issue 8, pp 481–488 | Cite as

Free convection from horizontal screened plates

  • W. M. Lewandowski
  • H. Bieszk
  • J. Cieśliński
Article

Abstract

Experimental investigations of laminar free convection from horizontal isothermal surfaces screened by cylindrical vertical walls are presented. Screen diameters (D) were equal to the diameter of a heating plate while their heights (H) were varied. Results obtained for several heating fluxes and two different liquids indicate that depending on the (H/D) ratio of cylindrical screens, the heat flux transferred from the heating plate to the liquid varies. Three subranges of screening effect have been distinguished: primary inhibition, intensification and secondary inhibition. For primary inhibition, a theoretical model has been proposed and critical ratio of (H/D) has been predicted. The results of flow visualization are also discussed.

Nomenclature

a

thermal diffusivity [m2/s]

D

external diameter of the plate, internal diameter of screens [m]

g

gravitational acceleration [m/s2]

H

height of screen [m]

Nu=α D/λ

Nusselt number

Nu(H/D)=0

Nusselt number for plate without screens

NuH/D

Nusselt number for plate with screen of heightH

Ra=g β (Tw1TF)D3/(ν a)

Rayleigh number

RaH=g β (Tw−Tw1)H3/(ν a)

Rayleigh number inside screen

q

heat flux density [W/m2]

Tw

surface temperature of the heating plate [°C]

Tw1

boundary temperature of the motionless liquid [°C]

TF

bulk temperature of the liquid [°C]

α

heat transfer coefficient [W/(m2·K)]

β

coefficient of volumetric expansion [1/K]

ν

kinematic viscosity

λ

thermal conductivity [W/(m·K)]

Freie Konvektion von horizontalen, seitlich abgeschirmten Platten

Zusammenfassung

Es werden experimentelle Untersuchungen der laminaren freien Konvektion an horizontalen, isothermen Flächen mit vertikalen zylindrischen Abschirmflächen beschrieben. Die Schirmdurchmesser (D) waren gleich dem einer Heizplatte, während die Schirmhöhen (H) variiert wurden. Die Resultate, welche für mehrere Wärmeflüsse und mit zwei verschiedenen Flüssigkeiten gewonnen wurden, zeigen, daß — abhängig von VerhältnisH/D der zylindrischen Schirme — der von der Heizplatte an das Fluid übergehende Wärmestrom stark variiert. Drei Bereiche konnten bezüglich des Abschirmeffektes unterschieden werden: erste Hemmung, Verstärkung und zweite Hemmung. Für den Fall der ersten Hemmung wird ein theoretisches Modell vorgeschlagen und das kritischeH/D-Verhältnis vorausberechnet. Abschließend erfolgt die Diskussion von Ergebnissen, welche aus der sichtbaren Darstellung des Strömungsfeldes gewonnen wurde.

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Copyright information

© Springer-Verlag 1992

Authors and Affiliations

  • W. M. Lewandowski
    • 1
  • H. Bieszk
    • 1
  • J. Cieśliński
    • 1
  1. 1.Technical University of GdańskGdańskPoland

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