Study On The Heat Transfer And Flow Characteristics Of The Three-Dimensional Oscillating Heat Pipe

With the trend of high integration,compactification and miniaturization of all kinds of equipment,heat accumulation,heat uneven distribution and local overheat spot occurred frequently.To ensure the equipment works well,higher requirement for thermal management system was put forward.Currently,the widely used active thermal management system relied on the system design and extra energy input to achieve temperature uniformity.While the passive thermal management device which could response to the uneven heat distribution automatically was only applied to the flat plate heat source.By focusing on the uneven heat distribution of bulk heat source,this paper mainly investigated the heat transfer and multi-phase flow characteristics of the threedimensional oscillating heat pipe(3D-OHP)wth multi-layer structure.The machnism of how three-dimensional affect the thermal and temperature uniformity performance was revealed.Then,the battery thermal management system based on 3D-OHP and 3DOHP/phase change material(PCM)coupled thermal management system were developed,and the temperature control and uniformity performance of these systems were studied.The main conclusions are as follows:(1)3D-OHP with multi-layer structure(2 layers,3 layers,4 layers and 5layers)had been designed,and the heat transfer and flow characteristics of 3D-OHP with different layers,inclination angle,filling ratio,working fluid and tube diameter was studied.The results showed that: The 3D-OHP had good thermal resistance uniformity during stable oscillation stage when vertical placed.Comparing with 3D-OHP with single layer,2 layers,3 layers and 5 layers,4 layers 3D-OHP has the smallest start-up temperature,power and thermal resistance with both vertically and horizontally placed.With the increase of filling ratio,the start-up temperature,power and thermal resistance decreased at first and then increased,and the best filling ratio was 50%.For different working fluid and tube diameter,the capillary resistance force was dominated when the inner diameter was 1mm,while the viscosity took the main position when the inner diameter was bigger.The specific and latent heat was also very important to help startup and avoid dry-out.Therefore,the start-up temperature of 3D-OHPs with deionized water and self rewetting working fluid(SRWF)as working fluid decreased with the increase of tube diameter,while the start-up temperature and power of 3D-OHP with ethanol as working fluid was the smallest when the tube diameter was 1mm.The Marangoni effect could help backflow of SRWF to avoid dry-out,especially in a small inner diameter tube.(2)For the 4-layers 3D-OHP,the temperature uniformity performance with uneven heat source and different inclination angle was studied.The mechanism of temperature uniformity was revealed.The results showed that: When the heating power was small,the 3D-OHP was in incomplete oscillating stage,the limit of temperature uniformity was small.When the heating power was big,3D-OHP reached complete oscillating stage,the limit of temperature uniformity was big.Compared to that with M2 heating mode,3D-OHP with M1 heating mode showed higher temperature uniformity limit.The inclination angle affected the temperature uniformity limit.With the decrease of inclination angle,the temperature limit uniformity of 3D-OHP with M1 heating mode increased,and that of 3D-OHP with M2 heating mode decreased.With the increase of heating power in one layer of 3D-OHP,the oscillation flow velocity of liquid-vapor slugs increased.With the further increase of heating power,the flow pattern transferred from plug flow pattern to annular flow,which help to achieve temperature uniformity.When the heating power was too big,local dry-out occurred.However,changing the inclination angle could change the distribution of liquid-vapor slugs and help to change the temperature uniformity performance with different heating modes.(3)The one-dimensional numerical model was proposed.The effect of the tube diameter,length of evaporation and condensation sections and the length ratio between two evaporation section on the flow characteristics and the oscillating flow characteristics inside one layer of 3D-OHP with different heating power were studied.The results showed that: With the same heating power,the 1mm inner diameter OHP had the smallest oscillating amplitude,and the 2mm inner diameter OHP had the biggest amplitude.With the increase of inner diameter,the oscillating amplitude decreased,and duration of liquid slugs reached stable oscillation stage increased.When the evaporation section and condensation section have the same length,the liquid-vapor slugs could reach stable oscillation stage.With the increase of the length of condensation section,the oscillation amplitude of liquid-vapor slugs decreased.With the increase of length of evaporation section,the pressure of vapor slugs increased,and the oscillation amplitude and velocity increased.When the length of two evaporation section were the same,the liquid-vapor slugs could reach stable oscillation stage and had a big oscillation amplitude.When the difference between the lengths of two evaporation sections increased appropriately,the frequency of oscillation flow would increase with the amplitude barely changed.However,when further inctease the difference,the amplitude and frequency would decrease.The pressure of vapor slugs and the flow velocity inside one layer increased with the increase of heating power,which indicated the atomatic velocity increase of oscillating flow may help achieve temperature uniformity.(4)Temperature control and uniformity performance of 3D-OHP based battery thermal management system(BTM)and PCM/3D-OHP coupled system were studied.The main conclusions were shown as follows: The 3D-OHP based BTM system could decrease the maximum temperature rise under different working angels obviously(smaller than 25?)and could maintain a reasonable maximum temperature difference(smaller than 4?).When the heating power was lower than 20 W for the unusal heat generation of one battery,the temperature difference was still reasonable(smaller than6?),which indicated that the BTM system had a good cooling and temperature uniformity performance.For the two coupled systems,paraffin wax around the OHP tube cannot melt under 80W-120 W because of the good heat transfer performance of OHP.The paraffin wax/3D-OHP system needs longer time than paraffin wax/OHPs system for completely melting under the same heat input.The former system also has lower wall temperature and paraffin wax temperature,which indicates better cooling performance.During the solidification process,both the two systems have better performance than paraffin wax only.The solidification time of the latter system was about 48% of paraffin wax and the former system only 29%.This paper has 100 figures,12 tables and 180 references.