نوع مقاله : مقاله پژوهشی
دانشکده مهندسی مکانیک- دانشگاه فردوسی مشهد
عنوان مقاله [English]
In the subject of heat transfer enhancement, there has been considerable interest in developing heat exchangers that have high efficiency, low cost, light weight, and the smallest size possible. So, energy prices and environmental considerations are being undertaken to support attempts to create a better performance than that of existing designs. The air side heat transfer coefficient is low, so, by enhancement of the boundary layer, it must be increased. The role of inserts in internal forced convection has been widely acknowledged as a passive device in heat transfer enhancement. The study follows the heat transfer enhancement caused by insersian of an obstacle into a boundary layer inside a horizontal tube. The flow of air is heated by electrical current. The presence of an obstacle was found to increase heat transfer, sometimes without inducing turbulence, but also increases pressure drop. The results demonstrate that the model could be a useful tool for optimization of heat exchanger performance in the presence of obstacles. The use of obstacle inserts leads to a considerable increase in heat transfer and pressure drop over the smooth tube. In this study, experimental and numerical heat transfer coefficients and friction in turbulent pipe flow were carried out with different obstacles. To evaluate the effect of barriers, air as fluid, with a Reynolds number range of 5000 to 20000, has been investigated. The mainstream barrier was stimulated with four cones, discs, rings and an O-ring to prevent cross, with the same (40\%) barriers for various steps. The results show that O-rings with P/D=2 have the most effect on stimulation of the main flow and the increase in heat transfer. It is also shown that by increasing the Nusselt and friction factor average ratio, respectively, 2.6-3.2 and 20-22, that the overall enhancement efficiency performance increased 16\% for Reynolds numbers above 12000; enhanced by stimulation of the ainstream. The numerical results agree well with experimental results.