The paper presents an experimental study of laminar forced convection heat transfer in different rotational speeds varies from 100 to 628 r.p.m to give rotational Reynolds number in the range 2000 ≤ Reω≤ 29400. The inner tube of the annulus is varied so as to give radii ratios of 0.3879, 0.614 and 0.8842. Water is flowed axially through the annular space with velocities ranged from 0.00414 to 0.27 m/s to give axial Reynolds number in the range 80 ≤ Reh ≤ 2700 to cover the laminar flow regime. The outer tube of the annulus is cooled under uniform temperature via the evaporation of refrigerant R 22 flowing through a refrigeration circuit while the inner tube of the annulus is thermally insulated. In the present work the effect of radius ratio, axial Reynolds number and rotational Reynolds number on the heat transfer are subjects of major inertest.
The experimental results of this work show that the heat transfer of the rotational as well as the stationary annular tubes increase with the increase of the radius ratio of the annulus. The results show also that heat transfer of the rotational annular tubes is higher than this of the stationary ones at the same radius ratio and axial Reynolds number. There are peak values of heat transfer at a rotational Reynolds number of nearly 12725 (rotational speed of 270 rpm nearly) for different radius ratio of the annulus. An increase as much as 44 % in the heat transfer is reported for annular tube of radius ratio of 0.8842 and rotational speed of 270 rpm. Excellent correlation is established between Nusselt number and axial Reynolds number, rotational Reynolds number and radius ratio of the annulus for both rotational and stationary annular tubes.