| The value of study on the pulsating flow showed two aspects.Firstly,A new technology of composite enhanced heat transfer was proposed,which benefited development of new high-effective heat exchangers.Secondly,with academic development on pulsating flow researches such as summary of flow and heat transfer phenomena,study of energy dissipation and exploration of drag reduction methods,the field of heat and mass transfer could be impoved constantly.This work employed theoretical,experimental and numerical methods to study the flow and heat transfer performance of the triangle groove mini-channels for low Reynolds number pulsating flow and verify the applicability on the drag reduction of flexible wall.Based on theories of hydromechanics and hemodynamics,new models were constructed for various objects to analysis the interactions among pulsating parameters,geometry,channel properties and flow resistance.Experiments were also carried out to verify the theoretical results and to illuminate flow and heat transfer performace.Subsequently,the mechanisms on pulsating flow were revealed by using numerical simulation.In the following sections,the conclusions were presented.(1)Flow field structure and vortex evolution under the impacts of pulsating parameters and channel geometries for the pulsating flow in various triangle groove mini-channels were studied in this section.A mathematical model of vorticity-transport was proposed to find out the main influence factors of vorticity field,which was verified by PIV experiment.Subsequently,numerical simulation was carried out to confirm the mechanism of vortex motion.The results indicated that the repeating sequence of vortex generation,growth,expansion and ejection from the groove to the main stream were performed with variation of pulsating velocity,which was resulted from adverse pressure gradient.Otherwise,the path of vortex motion was mainly affected by the pulsating frequency,and the pulsating amplitude had influence on the vortex pattern.(2)According to the definition of the secondary flow strength,pulsating flow strength was proposed to describle the intensity of pulsating flow quantificationally,whose rationality was affirmed by experimental and numerical results.(3)Based on the mechanical equilibrium principle,pressure drop equation,introducing eddy quasi-“energy”,was established to illustrate the relation between flow resistance and vortex montion.It was found from the mathematical model that the average pressure drop of pulsating flow is inversely proportional to the height of the channel,but positively proportional to the eddy quasi-" energy ".Besides,the eddy quasi-" energy " could determined the value of pressure drop under the condition of the same flow channel.Meanwhile,the decrease of the channel height not only directly caused the increase of the pressure drop,but also indirectly affected the vortex quasi-"energy" at the same pulsating parameters.(4)By using experimental and numerical methods,the heat transfer characteristics of pulsating flow in the triangle groove mini-channel,and the influence of pulsating parameters and geometry on the heat transfer performance were discussed respectively.And then,from the viewpoint of "heat transport",combining with field synergy theory and entropy production analysis,the process of heat transfer enhancement by pulsating flow in the triangular groove mini-channel was analyzed.The results showed that the heat transfer and flow state of pulsating flow can be optimized efficiently by coordinating the control of different physical quantities.(5)By using the coupled bionics method and refering to the "elastic cavity model" in the field of hemodynamics,a mathematical model of pressure drop coupling pulsating flow with the flexible wall was proposed,which involved nonlinear deformation characteristics of the flexible wall.According to the model,it could be found that the flexible wall exhibited superior drag reduction performance as compared to rigid wall at the same flow parameters and geometry.The effect of drag reduction mainly depend on the deformation length of the flexible wall,which was affected by the amplitude of velocity and pressure.Subsequently,the above mathematical model was verified through experiments.Moreover based on dimensional analysis,an experimental correlation was established to predict the drag reduction rate of the flexible channel for pulsating flow,which fits 76.4 % of 195 data within ±25 %.(6)The comprehensive heat transfer performance of pulsating flow in the flexible triangular groove mini-channel was studied experimentally.The results showed that the flexible triangular groove mini-channel was in favor of drag reduction for pulsating flow,while the heat transfer couldn’t be weakened,compared with steady flow in the rigid channel.However,comprehensive heat transfer performance of pulsating flow in the rigid channel pulsating flow was still superior to that in the flexible channel.In addition,through the visual experiment,the deformation of the flexible wall by stress were described graphically to present the pulsation characteristics of the flexible wall. |
PDF Full Text Request