A study of forced convection heat transfer : Constant heat flux

Abstract

In this paper, the force convection heat transfer with different cross section area ducts was studied experimentally and theoretically. Three different duct shapes with an identical area of 0.002 m2 were manufactured for experiment. They were a circular duct, a square duct and a triangle duct were investigated. The constant heat flux assumption was applied for each experimental tested case. A wired heater with heat rate up to 920 W was used for applying heat to the test unit. During the test, the heat flux was increased from 16 to 250 W/m2. The Reynold Number (Re), Nusselt Number (Nu) and Stanton Number (St) were determined for discussion. The air velocity flowing through the duct was varied between 8 and 24 m/s. It was found that the theoretical results agreed well with the experimental results. It was indicated that Re is proportional to the Nu but it is inversely proportional to St. With an identical heat flux, it showed that the Nu at high air speed is higher than those at low air speed. In case of using the different duct shape, a circular pipe has the highest average Nu which is different from that of the theoretical Nu. With three different duct shapes, it indicated that the heat transfer performance provided by a triangle duct was slightly different from that provided by a circular duct while there was a larger difference when they were compared to a square duct. However, a circular duct still provided a highest performance. In comparison of theoretical results with experimental results, the error is around 15% (low heat flux) and 38% (high heat flux). The St at high air speed is lower than those at low air speed. The circular duct also provides the highest average St. The error of 2.3% (low heat flux) and 16% (high heat flux) was obtained.