Date of Award


Degree Type


Degree Name

Doctor of Philosophy


An experimental study on combined free and forced convection heat transfer in the entrance region of horizontal concentric annuli has been completed. A Mach-Zehnder interferometer was used to determine the temperature distributions and the average heat transfer coefficients. The light beam was parallel to the flow direction with the velocity and temperature profiles developing simultaneously in a three dimensional flow pattern.;Two diameter ratios of 0.5036 and 0.6353 were studied with the length to equivalent diameter ratio varying from 12 to 66. Air, at atmospheric pressure, was used as the fluid in this study.;The Rayleigh numbers, based on the equivalent diameter, varied from 324 to 1900. The Reynolds numbers varied from 50 to 1265 and the Graetz numbers ranged from 1.1 to 78.5.;The free convection intensity was found to start at the inlet of the annuli and increase to a steady state value at a larger axial distance. The free convection increases the average Nusselt number by a factor of 3 for the rod and 2 for the tube of the annuli when compared to the predictions of forced convection only.;The heat transfer data in the entrance region of the annuli were divided into three different regions: (i) Region 1, near the inlet, the forced convection heat transfer is dominant over the free convection heat transfer. (ii) Region 2, this is the region in which the free convection flow is developing. (iii) Region 3, the flow is fully developed and any further increase in axial distance will not change the heat transfer rate.;The Schlieren method demonstrated that the secondary flow due to the buoyancy forces was found only in the upper two thirds of the annular cavity.



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