Experimental investigation on flow characteristics in a narrow annulus

Experiments are carried out to investigate the flow characteristics with/without heat exchange in a narrow annulus. In the experiments, directions of flow include horizontal, upstream and downstream flow. Experimental results show that the flow characteristics of water through the narrow annulus are different from those in normal tubes. Flow directions have little influences on the flow friction for the fluid flow in the narrow annulus with/without heat exchange. The flow characteristics in the narrow annulus have relations to the liquid temperature difference at the inlet and outlet of the annulus. Their influences on the flow characteristics are relatively obvious in the laminar flow area. When the Reynolds number is larger than 10⁴, there are little differences between the flow friction factors with/without heat exchange. It is also found that the asymmetrical flow can make the friction factor increase, whereas the symmetrical flow can reduce the flow friction. In the experiments, the transition from laminar to turbulent flow is carefully observed. In the narrow annulus, the flow transition is initiated earlier than that in normal pipes at a Reynolds number range from 1,100 to 1,500, which is different from the heat transfer transition. The results are gained to provide bases for the further investigations on the two-phase flow in narrow annuli.