Investigation of flow characteristics in heat exchangers of various geometries

INVESTIGATION OF FLOW CHARACTERISTICS IN HEATEXCHANGERS OF VARIOUS GEOMETRIESNurhan Adil ÖZTÜRKABSTRACTThe aim of the present study is firstly to investigate details of flow structure inthe upstream and downstream of a circular cylinder base mounted on a flat surface indeep water flow for the Reynolds number ranging from 750 to 9600. The analysis ofthe instantaneous and time-averaged flow data demonstrates that the horseshoevortices take place in the base of the cylinder in three dimensional form. Thesevortices magnify the scour and entrainment processes.Secondly, experimental investigation of flow structure in the passage of arectangular duct with a built-in single circular cylinder was conducted. A duct with anarrow gap was designed to simulate fin-tube heat exchanger containing a singlecircular cylinder. Development of the entrainment process between main flow andwake flow regions is magnified by immigrating helical horseshoe vortex systememerging from upstream of the cylinder. The occurrence and development ofboundary layer separation and formation of horseshoe vortices upstream of thecylinder in close region of both upper and lower plate surfaces and merging of thesedeveloping horseshoe vortices which form a primary vortex system are alsoobserved.Finally, the flow characteristics of multiple circular cylinder arrangements inthe passage of rectangular duct were examined in order to reveal the mechanisms ofhydrodynamics which are mostly responsible for the enhancement of the heattransfer rate. In the case of staggered cylinder arrangements, the flow is restless, i.e.,accelerations and decelerations of flow continuously happens to cause a high rate offluctuations. The entrainment process of flow the particularly in the junction regionsis significant.Keywords: PIV, horseshoe vortex, cylinder, turbulent flow, water flow