Abstract:
Most heat transfer tubes are designed for either fully uniform wall temperature or fully uniform wall
heat flux boundary conditions under forced convection. Several applications, including but not limited
to the solar collectors of renewable energy systems, do however operate with non-uniform boundary
conditions. Limited research has been conducted on non-uniform wall heat flux heat transfer
coefficients in circular tubes, especially for mixed convection conditions. Such works are normally
numerical in nature and little experimental work is available. In this experimental investigation the
effects of the circumferential heat flux distribution and heat flux intensity on the single phase (liquid)
internal heat transfer coefficient were considered for a horizontal circular tube. Focus was placed on
the laminar flow regime of water within a stainless steel tube with an inner diameter of 27.8 mm and
a length to diameter ratio of 72. Different outer wall heat flux conditions, including fully uniform and
partially uniform heat fluxes were studied for Reynolds numbers ranging from 650 to 2 600 and a
Prandtl number range of 4 to 7. The heat flux conditions included 360˚ (uniform) heating, lower 180˚
heating, upper 180˚ heating, 180˚ left and right hemispherical heating, lower 90˚ heating, upper 90˚
heating and slanted 180˚ heating. Depending on the angle span of the heating, local heat fluxes of 6
631 W/m2
, 4 421 W/m2
, 3 316 W/m2
, 2 210 W/m2
and 1 658 W/m2 were applied. Results indicate that
the local and average steady state Nusselt numbers are greatly influenced by the applied heat flux
position and intensity. Highest average heat transfer coefficients were achieved for case where the
applied heat flux was positioned on the lower half (in terms of gravity) of the tubes circumference,
while the lowest heat transfer coefficients were achieved when the heating was applied to the upper
half of the tube. Variations in the heat transfer coefficient were found to be due to the secondary
buoyancy induced flow effect. The relative thermal performance of the different heating scenarios
where characterised and described by means of newly developed heat transfer coefficient
correlations for fully uniform heating, lower 180° heating, and upper 180° heating.
