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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-474-8

ISBN Online: 978-1-56700-473-1

International Heat Transfer Conference 16
August, 10-15, 2018, Beijing, China

FORCED CONVECTION THERMAL ENTRANCE LENGTH FOR SIMULTANEOUSLY HYDRODYNAMICALLY AND THERMALLY DEVELOPING LAMINAR FLOW AT A CONSTANT HEAT FLUX

Get access (open in a dialog) DOI: 10.1615/IHTC16.cov.023020
pages 3069-3075

Abstract

Although the thermal entrance length has been extensively investigated in the past, no clear distinction has been made between the coefficients for hydrodynamically fully developed and thermally developing, or simultaneously hydrodynamically and thermally developing flow. This leads to uncertainty about the value of the coefficient in the thermal entrance length correlation. The purpose of this paper was to experimentally investigate the laminar forced convection thermal entrance for simultaneously hydrodynamically and thermally developing flow in smooth horizontal tubes heated at a constant heat flux. An experimental set-up was designed, built and validated against literature. A smooth circular test section with an inner diameter of 4.0 mm and maximum length-to-diameter ratio of 1 373 was used. Heat transfer measurements were taken at Reynolds numbers between 900 and 2 000 at heat fluxes of 0.5 kW/m2 and 1.0 kW/m2. Water was used as the test fluid, therefore the Prandtl number was approximately seven. It was found that a longer thermal entrance length was required for simultaneously hydrodynamically and thermally developing flow than for hydrodynamically fully developed and thermally developing flow. Therefore, a coefficient of at least 0.12 was suggested for simultaneously hydrodynamically and thermally developing flow heated at a constant heat flux, and not 0.05 as suggested in most subject literature.