(296a) Flow Visualization of the Horizontal Tube Reactor with an Open Outlet

Horizontal tube reactor has been widely used for the synthesis of nanomaterials or for the deposition of thin films. It has also been used for the synthesis of CNT fibers, with the modification of an open outlet. The overall flow rate of gases in this case is generally low, and typical Reynolds number is in the order of tens~hundreds. Therefore, most of previous researchers assumed that the flow inside the tube reactor was a laminar flow. However, existence of flow circulations inside the tube reactor was predicted in the recent publication based on the computational fluid dynamics (CFD) simulations. Although the simulation revealed important circulation phenomenon, experimental verification was incomplete. Here, we demonstrated that the flow patterns inside the horizontal tube reactor (with an open end) is rather complex using a simple flow visualization technique. Two flow circulations were observed at the furnace inlet and outlet, and the in-flow of external air from the tube outlet was confirmed in the flow visualization experiment (Short movies will be presented at the conference presentation.). Complex flow patterns are caused by the buoyancy force due to the temperature difference in the cross-sectional direction. There were large temperature differences across the cross-sections at the furnace inlet (80^oC) or the outlet (60^oC), while there was negligible temperature difference across the cross-section at the middle of the reactor. Important characteristics of these complex flow patterns could be accurately predicted using CFD calculations. The effect of buoyancy force would become greater with the increase of the diameter of the tube reactor, so that it should be considered seriously for the scale-up of the reactor.