| The oil-immersed transformers with plate radiator have some disadvantages such as low heat dissipation efficiency and easy aging.The transformer’s temperature rise often exceeds the national standard requirements when transformer is working at full load or overload operation during summer heat peaks,which is a safety risk.Recent studies have shown that the three-dimensional finned tube with discrete fins,which is one of the representatives of passive heat transfer enhancement can effectively destroy the flow boundary layer and improve the heat dissipation capacity.It is expected to replace the traditional plate radiator as the next generation heat dissipation technology for oil-immersed transformers.Oil-immersed transformers dissipate heat by natural cooling for most of the year.However,there is still a lack of research on the natural convection heat transfer enhancement characteristics of three-dimensional finned tubes,and the heat transfer performance of the vertical 3-D finned tube radiators for power transformer.In the paper,aiming at the vertical 3-D finned tube radiators for power transformer,the natural convection heat transfer characteristics of the air side of vertical three-dimensional finned tube are studied firstly.Based on the numerical results,the 3-D finned tube radiator for transformer is designed and manufactured,and the natural cooling performance and forced convection cooling performance of the 3-D finned tube radiator are evaluated.A forced convection calculation model based on the porous medium method is established to analyze the influence of the tubular arrangement of vertical 3-D finned tube radiator on the heat dissipation capacity of the radiator.Based on the research results,a 3-D finned tube radiator group for pilotscale experiment is designed and installed on an actual power transformer for temperature rise test to evaluate the top oil temperature and the wind temperature rise characteristics of the transformer that installed with the 3-D finned radiator.The main research results of this paper are as follows:(1)According to the natural convection heat transfer characteristics of the air side of the vertical three-dimensional finned tube,the influence of the structural parameters of the outer wall of the three-dimensional ribbed tube on the heat transfer performance is numerically studied.The empirical correlation of natural convection heat transfer is obtained by multivariate nonlinear fitting.In the study range,the Nusselt number of natural convective heat transfer of 3-D finned tube is up to 1.77 times that of smooth tube.Under the same condition,with the increase of the fin height and the increase of the fin width,the heat transfer performance of the finned tube is enhanced,and with the increase of the axial and circumferential fin pitch,the heat transfer performance of the finned tube is weakened.(2)The 3-D finned tube radiator for power transformer is designed and processed,and the heat transfer performance test platform for transformer is also setted up.The results show that:At the oil natural air natural(ONAN)cooling mode,when the temperature rise is 55K,the heat dissipation of 3-D finned radiator increases by 11.35%compared with plate radiator.At the oil natural air forced(ONAF)cooling mode,for a3-D finned tube radiator,the fan top arrangement is better than the side arrangement,and the side arrangement is better than the bottom arrangement.When the fan totoal pressure increases from 16Pa to 144Pa,the heat dissipation of 3-D finned tube radiator is 1.72～2.11times of that at ONAN cooling mode when fan is side arrangement,and 2.22～2.58 times of that at ONAN cooling mode when fan is top arrangement.And the advantage of the 3-D finned tube radiator is not significant compared with the plate radiator.At the oil forced air natural(OFAN)cooling mode,when the temperature rise is 50K,the oil flow rate increases from 2.2m~3/h to 6.8 m~3/h,the heat dissipation of the 3-D finned tube radiator is2.13～2.28 times of that at ONAN mode.When the oil side of the radiator changes from natural circulation to forced circulation,the average heat transfer temperature difference on the air side is increased,and the heat transfer performance of the radiator is improved.At the oil forced air forced(OFAF)mode,when the temperature rise is 40K,the fan total pressure is 64Pa,and the oil flow rate is 4.0m~3/h,the heat dissipation of the 3-D finned tube radiator can reach 5.79 times of that at ONAN mode.(3)Based on the theory of porous media,a numerical solution model of 3-D finned tube radiator based on the method of heat transfer in porous media is established.The heat transfer model based on the porous medium heat transfer method is used to simulate the radiator with different tubular arrangement.The results show that in terms of the flow characteristics of 3-D finned radiator,the flow pressure drop of the radiator increases and the inlet flow rate decreases with the decrease of transverse tube pitch(S1)and longitudinal tube pitch(S2),and the influence of S1 is more significant.As for the heat transfer characteristics of the radiator,in the range studied in this paper,No.10 heat sink with S1=35.5mm and S2=30.4mm has the best heat dissipation capacity,whose average surface temperature decreased by 6.61K compared with the initial tubular arrangement.From the perspective of engineering application,in the range of S1 and S2 studied in this paper,reducing S1 and S2 can effectively reduce the average temperature of the heat sink,and the adverse effects such as the increasing flow resistance and decreasing inlet flow rate caused by the increasement of the number of heat exchange tubes can be completely offset by the increase of heat exchange area of the radiator.Considering the importance and cost of the radiator,reducing S1 takes precedence over reducing S2.(4)A 3-D finned tube radiator group for pilotscale experiment is designed and installed on an actual transformer for transformer temperature rise test.The results show that when the transformer worked at ONAN cooling mode,the top oil temperature and average winding temperature of the transformer that installed with 3-D finned tube radiator reduced about 10℃and 9℃,respectively compared with that of the transformer that installed with plate radiator.Compared with the transformer installed with plate radiator under rated load,the top oil temperature rise of the transformer installed with 3-D finned tube radiator under 1.1 times load is still reduced by about 7℃and the average winding temperature rise is still reduced by about 5℃.When the transformer worked at ONAF cooling mode under rated load,the top oil temperature rise of the transformer installed with 3-D finned tube radiator is about 5℃lower and the average winding temperature rise is about 5℃lower than that of the transformer installed with plate radiator.It can be seen that the 3-D finned tube radiator has a good application prospect in the field of oil-immersed transformer. |
