Experimental studies of effect of tube location and arrangement on heat transfer characteristics for plate-finned tube heat exchanger

Abstract

Flow and heat transfer in a plate-finned circular-tube assembly is examined using naphthalene sublimation technique to scrutinize the microscopic flow and heat transfer phenomenon and also to evaluate the overall performance to find out the optimal tube location and tube arrangement. Flow visualization using smoke generation technique is also made. The examined parameters are the gap(δ) to tube diameter(D) ratio δ/D, the Reynolds number $Re_D$, the tube location l/D for single tube and the offset of two tube centers s/D for two-tube arrangement. The tubes are simulated by acrylic disks between two parallel naphthalene molds. Flow visualization past a single tube has shown the existence of a large recirculating twin vortex behind the tube and its growth with the increase of the Reynolds number. It also has revealed the occurrence of vortex shedding behind the tube when $Re_D$is increased to as high as 2,660 for δ/D=0.2. If the offset in the two-tube case is small (s/D=0.25), the first tube hinders the flow around the second tube, lowering the flow velocity there. When it is moderate (δ/D)0.5), the flow rate between the two tubes becomes large and the twin vortices behind the second tube become more asymmetric. It accelerates the vortex shedding when the Reynolds number is increased to about 1,770. Friction factor f is obtained for various tube arrangements. For a single tube arrangement, the friction factor is almost independent on the tube location. The in-line tube arrangement gives smaller f} than the staggered two-tube case. The friction factor gradually increases with the increase of the tube offset, since the surface area exposed to high skin friction is increased. It increases steadily with s/D due to increase of the flow velocity between the tubes and remains nearly constant beyond s/D=1.0. The uncertainty of the Sherwood number $Sh_D$ in the naphthalene sublimation method is analyzed by examining the simplified flow between two parallel p...