HEAT TRANSFER ON A SPHERE AT DIFFERENT INFLOW TURBULENCE
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: heat transfer, sphere, flow turbulence
Heat transfer process from solid surface to external flow is a function of many parameters. For forced convection of heat from surface the main parameter which stimulated this process is the average flow velocity. Nevertheless many literature data indicated that increase of turbulence level of external flow cause increase of heat transfer coefficient even when average velocity doesn’t change. This cause that in many application accuracy of estimation of heat transfer intensity can be not sufficient, because it is very difficult to determine the level of turbulence in external flow generated by different flow sources. In this case average velocity can be the some but level of turbulence, as the simplest parameter of turbulent flow structure, can be different because turbulent flow generated by different flow sources can be characterised by different flow structures.. Very often the average flow velocity is not good enough to describe turbulent flow structures when they influence on heat transfer processes on surfaces.
In the paper results of experimental investigations of influence of the flow turbulence level on average heat transfer on spheres of different diameters will be presented.
To estimate influence of turbulence level of external flow on average heat transfer coefficient on a simple surface geometry, sphere, the experimental tests were performed. The flow was generated by nozzle. For different distances from nozzle outlet it was possible to get turbulence level in free jet from about 0,5% till about 20% with no change of the average flow velocity. The constant temperature anemometer was used to measure level of flow turbulence. The average heat transfer coefficient on spheres was measured in steady-state thermal conditions by means of an electrical heating system. Heat transfer convection from spheres diameter of 0.01m, 0.02m, 0.03m and 0.05m was measured at different turbulence level of inflow. The constant temperature anemometer (CTA) TSI 1050 with hot wire probe was used to measure level of flow turbulence. For measurements of distribution of local heat transfer coefficient the surface probe connected to CTA bridge was used. All data were recorded by digital acquisition system.
Investigations indicated that increase of turbulence level of flow cause increase of average heat transfer from isothermal surface of sphere to ambient air even when the average inflow velocity doesn’t change i.e. Reynolds number of average flow remain constant. For sphere diameter of d = 0.03m increasing of flow turbulence to 22% cause increase of average heat transfer coefficient about 30%. Heat transfer process on spheres of larger diameter is more sensitive on increasing of external flow turbulence. There is no evidence that diameter of nozzles generated flow (i.e. scale of turbulent flow structures) influence on heat transfer process in tested range of sphere to nozzle diameters rate. Comparison between trend lines for different diameters of spheres indicated that for smaller spheres influence of intensity of turbulence is noticeable lower.
Analysis of local heat transfer distributions around sphere surfaces indicated that increasing of heat transfer process is not uniform and depend on external flow turbulence intensity.
Performed measurements show that level of turbulence intensity of external flow can influence in essential manner on heat transfer process on solid surfaces of different kinds thermal equipments. This effect introduce inaccuracy in heat transfer calculations because estimation of turbulence intensity in advance as a results of flow channel construction is very difficult.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
