Study On The Heat Transfer Characteristics Of Micro-grooved Oscillating Heat Pipes With Self-rewetting Fluids

Oscillating heat pipe(OHP)is a high efficiency heat transfer device normally made of capillary tube and provides advantages of simple structure,low fabrication cost,fast response to high heat loads and good environmental adaptability.As a new member in the wickless heat pipe family,OHPs have received a continuing and growing interest in the past two decades and considered as a promising solution in many industrial and commercial fields,such as heat recovery,drying,solar energy collecting,and electronics cooling.But the two-phase system which embodies muti-disciplinary physics is so complicated that the investigation on OHPs is still at a primary stage,to further strengthen the thermal characteristics and application of OHP remains to be studied.In this thesis,microgroove(micro-fin)structures were introduced to OHPs to improve the thermal performance.The thermal performance of OHPs at different operating conditions were studied with different working fluids and structure designs.The main conclusions are as follows:(1)The heat transfer characteristics of micro-grooved OHPs was experimentally investigated and compared with smooth-tube OHPs at vertical and horizontal orientations.All of these OHPs were made from copper tubes and deionized water was used as the working fluid at a volumetric filling ratio of 50%.The internal diameters(IDs)of three smooth-tube OHPs are 3.4,4.0 and 4.8 mm,respectively,and the internal hydraulic diameters of two micro-grooved OHPs are about 2.0 and 2.8 mm.Experimental results show that the microgroove structure enable a profound improvement of OHP performance and reduce the startup power input.The addition of microgroove structure make the OHP to remarkably outperform smooth-tube OHPs in both effective thermal conductivity and thermal resistance,and the average evaporator temperature could also be reduced significantly.It is found that the heat transfer enhancement of micro-grooved OHPs is highly associated with the combined effect derived from the phase-change intensification both at the heating and cooling sections,the helical microgroove induced rotary motion ofworking fluid,and enhanced liquid backflow to the evaporator due to microgroove-induced capillarity.(2)Self-rewetting fluids with different mass fractions were prepared due to the worse thermal performance of OHPs at horizontal orientations,of which main thermophysical properties were measured.In the vertical bottom heating mode,self-rewetting fluids is not benefit to the start-up and thermal performance of micro-grooved OHP compared with the deionized water.Meanwhile,the evaporator temperature of OHP increased obviously with deteriorated thermal performance under low heating power input.But in the horizontal orientation,self-rewetting fluids could substantially improve the dry-out characteristics and wall temperature uniformity of micro-grooved OHP.When the 0.15wt% 1-heptanol aqueous solution was used as working fluids,dry-out power of OHP increased over 4 times than the deionized water.(3)A flexible oscillating heat pipe(FOHP)with the adiabatic section made of fluororubber 246 and heating/cooling sections made of micro-grooved copper tubes was also designed and fabricated.Deionized water was used as the working fluid.The thermal performance of the FOHP featured by four different structural styles was experimentally investigated at different filling ratios and length of adiabatic sections.Results show that the FOHP could function well and exhibited highly spatial flexibility and acceptable heat transfer performance.The bending of adiabatic section partially degrades the start-up and heat transfer characteristics of FOHP,largely depend on the deformation extent.The thermal performance of the FOHP varied from high to low in light of the structural style follows such an order,i.e.,“I” shape,“stair-step” shape,“inverted-U” shape,and “N”shape.And a lower start-up power input or evaporator temperature of the FOHP is largely determined by a smaller filling ratio from 50% to 70%.The FOHP,characterized by a hot/cold-end length of about 1 m,provides a promising solution for the thermal management of some spatial complicated energy utilization systems.