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Corresponding Author

Hamza, Mohammed

Subject Area

Mechanical Power Engineering

Article Type

Original Study

Abstract

This paper presents an experimental study of laminar forced convection heat transfer for cold water flows through annular tube. The test section consisted of two annular spaces the inner of which for the tested cold water while the outer for the heating water flow that heated in a tank by electrical heaters. The inner tube of the annulus is rotated with rotational speeds which varied from 175 to 650 p.m. The outer tube of the annulus have a constant diameter of 54.5mm but the diameter of inner tube of the annulus is equal to 21, 26.7 and 33.5 mm so as to give diameter ratios of 0.62, 0.49 and 0.386. Variable numbers of semi-circle of buffles are used circumferentially around the rotating tube. The diameter of buffle is equal to 52 mm and 2 mm thickness. Cold water flowed axially through the annular space with velocities that ranged from (0.0192 to 0.0666 m/s), to give axial Reynolds numbers in the range (612 ≤ Re ≤ 2175), to cover the laminar flow regime. In the present work the effects of radius ratio, axial Reynolds number and rotational Reynolds number on the heat transfer are investigated. The results show that, the rate of heat transfer of the rotational annular tubes is higher than that of the stationary one of the same radius ratio and axial Reynolds numbers, and the rate of heat transfer of rotational annular tubes with baffles higher than rotational annular tubes without baffles of the same radius ratio and axial Reynolds numbers. 1- Rotation without buffle, Produces a maximum increase in Nusselt number of the order of 79% at Rea= 2080, Reω= 21545 and rotational speed= 480 r.p.m, (increase of only 11% in Nusselt number at Rea=646.2, Reω=7808 ,( rotational speed of 175 r.p.m), compared to the results of the stationary annular tube of the same radius ratio . 2- Rotation with buffle, Produces a maximum increase in Nusselt number of 200% at Rea= 2175, Reω= 21884 and rotational speed= 480 r.p.m, and increase of only 84% in Nusselt number at Rea=634, Reω=21778, ( rotational speed of 175 r.p.m) compared to the results of the stationary annular tube of the same radius ratio . Two correlations are made to describe the relation between Nusselt numbers and axial Reynolds number, diameter ratio, pitch to diameter ratio P/D and rotational Reynolds number and Reynolds number for stationary, rotating and buffled rotating annuli.

Keywords

Laminar forced convection; heat transfer; annular tube; rotating inner tube; radius ratio

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