Heat Transfer Enhancement By Fiber Fins And Fiber Inserts

In the field of heat transfer enhancement, attention has been focused on the increase of heat transfer rate with less flow resistance. The method of analog between convective heat transfer and heat conduction with internal heat sources has been further developed in the present work. The uniformity of velocity and temperature fields is pointed out to be an importment factor in convective heat transfer, and uniformity factor is defined in order to describe the field uniformity quantitatively. With this concept, some known heat transfer phenomena are re-analyzed. The velocity and temperature profiles will become more uniform with the increasing angle of wedge for wedge-type flow. Consequently, the temperature gradient on the wall is increased and heat transfer is enhanced. In the parallel channel rotation problem, secondary flow will uniform the velocity and temperature profile and lead to heat transfer enhancement.Fiber inserts are proposed to uniform temperature profile of fluid in the duct and then enhance heat transfer. Numerical investigation shows that the high thermal conductivity of the fiber insert will make the tempreature distribution more uniform and consequently cnhance heat transfer. The highest heat flux appears on the region near the tip of insert and a weakened heat transfer region appears at the downstream of the finber insert. Some factors, such as geometric dimension, thermal conductivity, Reynolds and Prandtl number, are found to affect heat transfer cnhancement. Thermal performance of fiber inserts is better than other kinds of inserts for the given pumping power.Simplified analysis on a single fiber fin in the duct flow demonstrates that the heat transfer can be enhanced with the increasing length-to-diameter ratio and thermal conductivity ratio. Numerical investigation indicates that heat flux on the base of fiber fins are far greater than that of duct wall, a buttlyfly shapeenhanced region appears around the base of fiber fin, while, a heat transfer reduced region occurs at the downstream of fiber fin. The presence of reduced region yields a better thermal performance for cross arrangement than for inline arrangement of fiber fin. PEC (performance evaluation criteria based on a given pumping power) is increased for higher Reynolds and Prandtl number as well as cross arrangement. For the equal volume porosity (ratio of volume occupied by fins), The thinner the fiber is, the higher the PEC value.A novel enhanced tube with fiber fins is developed and experimentally investigated under different structure of fins and inserts. Experimental results show that in the Reynolds number range of 400 to 4000, the value of PEC for enhanced tube is 1.3~4.6, which means a considerable enhancement in heat transfer for the given pumping power. The experimental results qualitatively agree with numerical simulation. Compared with the experimental results of 5 kinds pin fin configuration reported by Kays and London, the present enhanced tubes show much better thermal performance for given pumping power.