Local heat transfer coefficients at the inlet of an annular flow passage

Annular flow geometries are commonly found in several heat exchange devices, however, relatively little experimental work has been done specifically to determine the local heat transfer coefficients in developing flow in annuli. In this experimental study circumferentially averaged local heat transfer coefficients were determined with water for a uniform heat flux boundary condition on the inner wall in a horizontal concentric annular test section with a length of 1.05 m, a hydraulic diameter of 14.8 mm, and a diameter ratio of 0.648. Local inner wall temperatures were obtained via liquid crystal thermography with the aid of an automated camera position system. Conventional on-centre and off-centre annular inlet configurations with three inlet sizes were evaluated for Reynolds numbers ranging from 2000 to 7500. It was found that the local heat transfer coefficients were significantly higher at the inlet, and decreased as the boundary layers developed. Local maximum and minimum heat transfer coefficients were identified which may be due to flow impingement caused by the inlet geometry. A new local heat transfer correlation for the turbulent flow in regions close to the inlet of an annular flow passage was developed.