Heat Transfer Research On The Dilatant Closed-loop Pulsating Heat Pipe Using Nano-fluids

Closed-loop pulsating heat pipe (CLPHP), which is a research hot spot at present in heat transfer area, is investigated and researched in this dissertation. By carrying out visualization experiments and chaotic dynamic calculations on CLPHP, new structures are developed and new working fluids are tested. The conclusions gained in this dissertation can provide directions for engineering applications of the CLPHP.Characteristic temperature curve is gained by experimental phenomena observation and heat transfer calculation, and then, it is compared with the time-temperature curve which is measured during its start-up. The characteristic temperature curve embodies the states of the working fluids and the heat transfer status inside the heat pipes, which can be used to judge the operation situation of CLPHP in practical application.The causes of failure in start-up of the traditional CLPHP when its evaporating section was slowly heated are analyzed. In order to enhance the pressure fluctuation inside the pipe, the dilatant closed-loop pulsating heat pipe is introduced and investigated. And the experiment results show that this new kind of CLPHP possesses better starting performance than the common type.The heat transfer performances of the dilatant closed-loop pulsating heat pipe are researched in this dissertation and some important conclusions are gained. When the heating power increases, the starting temperature decreases. But when the heating power is too high, dry burning occurs. There is obvious promotion of the gravity to its operation. When the charging ratio is low, spot heating is necessary to its starting and operation, which qualitatively proves that the Butterfly Effect exists in the operation of CLPHP. By enlarging the dilatant room, the heat transfer performances of the dilatant closed-loop pulsating heat pipe could be improved, meanwhile, the starting temperature could be further decreased. But when the heating power is too high, more working fluids appear in the dilatant room, which leads to the heat transfer deterioration.By using alcohol as the working fluid, it is found that the initial charging distribution is determined by the surface tension of the working fluid while the starting temperature are determined by the working fluid's latent heat of vaporization and its saturation temperature. And the latent heat of vaporization and the specific heat of the working fluid affect the heat transfer performances greatly. Based on these conclusions, it is believed that the working fluids with low latent heat of vaporization, low vaporization temperature and high specific heat are fit for the dilatant closed-loop pulsating heat pipe.By using nano-fluids (TiO₂-water) as the working fluids, it is found that, the nano-fluid is much more stable when the dispersant is added in, and the heat transfer performance is improved greatly. In the temperature range 60℃-65℃, by using TiO₂-water of 1.5%, the heat transfer rate increases about 20% and the equivalent thermal conductivity increases about 50%. In the temperature range 70℃-75℃, by using TiO₂-water of 0.7%, the heat transfer rate increases about 20% and the equivalent thermal conductivity increases about 50%. A good dispersion of the nano-particles in the based fluid is the key to improve the heat transfer. When the agglomeration and sedimentation occur, the heat transfer enhancement turns to the heat transfer deteriorationThe heat transfer process of the dilatant closed-loop pulsating heat pipe is analyzed by using chaotic dynamic method and phase space reconstruction. It is found that, when the operation of CLPHP is stable, the attractor morphology is a compactly spherical distribution. When the CLPHP is at the start-up stage, the attractor distributes symmetrically in the phase space. When the CLPHP is at the dry burning stage, the attractor morphology is a long-narrow zonal distribution. By calculating the characteristic parameters, the heat transfer process of the CLPHP is found to be chaotic dynamic. With the increase of the equivalent thermal conductivity, the maximum Lyapunov index and the optimum delay time decrease while the optimum dimension increases.