Research On Heat Transfer And Flow Characteristics Of A Heat Pipe Heatsink With Longitudinal Vortex Generators

As a form of various types of fin-and-tube heat exchangers, heat pipe heat sinks find wide application in air cooling of electrical equipments such as CPUs for their good heat transfer performance. Air-side thermal resistance is the dominant part of total thermal resistance of a heat pipe heat sink, which makes heat transfer enhancement on the air side an important subject. Longitudinal vortex generator (LVG) is a form of passive technique, which enhances heat transfer by manipulation of secondary flow distribution at the expense of relatively small pressure loss penalty. This thesis carries out numerical calculation on the flow and heat transfer characteristics of a heat pipe and fin heat sink with LVGs embeded on the fin surface, and employes particle velocity image (PIV) method to study the flow field distribution of a flow about a circular cylinder with rectangular winglet LVGs placed downstream.Effects on fluid flow structure and heat transfer characteristics of parameters with respect to fin-and-tube position (transverse tube spacing, fin length, longitudinal tube offset) are investigated by numerical simulation, the results of which show that: proper transverse tube spacing is necessary for satisfactory fin heat dissipating capacity; decreasing fin length is benefical for heat emission intensification per fin unit area; moderate tube setoff downstream is helpful for improvement of total heat transfer coefficient to some extent, while the improvement trend is more apparent for fins with shorter length.Three-dimensional numerical simulation is employed to investigate flow structure and heat transfer characteristics of a heat pipe and fin heat sink with rectangular winglet LVGs. Effects of LVG parameters (angle of attack, location on the fin, winglet length, common flow up/down layout) are studied on flow and heat transfer performance of the heat sink. The results show that the average j factor is augmented by 15%～19%, while corresponding f factor increases by 30%-42%, in comparison with the heat pipe and fin heat sink without LVGs, for Re number ranging from 375 to 1125. Analyses on isovel distribution, velocity vector distribution and isothemal distribution of flow passage sections show that longitudinal vortexes change flow structure of the flow passage, intensify the mixing of fluid in main region and near the wall, thin the thermal boundary near the wall and increase the temperature gradient, and LVGs placed downstream of tubes help inhibit and weaken the wake region of the cylinder where heat transfer is quite poor. The angle of attackα=30°makes the optimum comprehensive heat transfer and pressure loss performance; heat transfer enhancement performance is best for LVGs placed downstream of the tube, moderate for LVGs placed side of the tube and worst for LVGs placed upstream of the tube; both j and f factors grow uniformly with the increase of winglet length; for the case of angle of attack a=30°, the performance of heat transfer for common-flow-down layout of LVGs is much better than that of common-flow-up layout.An approximate model experiment is arranged under the principle of similitude, using water to simulate the motion of air. Particle image velocimetry method is employed to study the flow structure of a flow about a cylinder without and with rectangular winglet LVGs (a=30°,60°respectively) placed downstream in a lucite water tunnel. The experiment results show that LVGs not only bring forth a pair of longitudinal vortexes with opposite rotation direction and bilateral symmetry going downstream with the main stream, but also can help inhibit or eliminate the wake region of the cylinder; with the increase of LVG angle of attack, the axis of longitudinal vortex gradually deviates from that of the main stream, accompanying the decrease of lasting intensity of longitudinal vortex.