| The traditional plate radiator of ONAN(oil nature air nature)transformer has the disadvantages of large area and low heat dissipation efficiency,its temperature rise often exceeds the temperature limit by national standards when the transformer runs at full load or over load status during summer heat peaks.Recent year’ research indicated that three-dimensional finned tube with discrete raised ribs,which is one of representative heat exchange component,can destroy the stability of flow boundary layer and improve heat transfer performance effectively,hence the future of it has wide applications in the field of heat dissipation of power transformer.However,there is very lack of the study on natural convection heat transfer characteristics of the oil side and air side of 3-D finned tube,and very little information on the effects of tube arrangement and tube row spacing on the oil flow distribution and heat transfer performance.Therefor,aiming at the 3-D finned tube radiators for ONAN power transformer,the heat transfer characteristics of oil side and air side of single 3-D finned tubes were investigated firstly.Then based on the results of aforesaid study,the 3-D finned tube radiators for ONAN transformer were designed and processed.Finally,the natural convection heat transfer characteristics of the radiators were studied and the heat dissipation performances of3-D finned tube radiators were comparative analysed with the traditional plate radiator.The main results were as follows:(1)Aiming at the characteristics of oil nature convection heat transfer in 3-D internally finned tubes,the effects of fin height,fin width,axial fin pitch and circular fin pitch were investigated by orthogonal experiments,the influence laws of structural parameters on the heat transfer performance were obtained and an empirical formula for oil natural convection heat transfer was determined by multiply linear regression.The results showed that the higher the top oil temperature,the highter would be the heat transfer performance compared with smooth tube.When the top oil temperature was between 90 °C and 100 °C,the influence of fin height on heat transfer performance accounted for about 37.2 %,axial fin pitch accounted for about 25.6 %,and fin width and circular fin pitch accounted for 19.7 % and 17.5 %.The optimal combination of fin structure parameters in the experimental range was obtained with a fin height 2.0 mm,fin width 2.0 mm,axial fin pitch 2.0 mm and circular fin pitch 10.0 mm.(2)Aiming at the characteristics of air nature convection heat transfer around vetical 3-D finned tubes,the effects of external fin structure parameters on heat transfer were investigated experimentally.Based on the numerical method,the reasons for the effects were explained.The empirical formula for air natural convection heat transfer was determined by multiply linear regression.The results show that the heat transfer performance of the 3-D external fin tube with aligned fin is better than that of the 3-D crossing-finned tube and the smooth tube.Within the scope of the study the maximum Nusselt numbers of the 3-D aligned-finned tubes was 2.97 times greater than that of the smooth tube.Nusselt numbers increased with the increasing of fin height and the decreasing of axial fin pitch,but minimally affected by fin width and circular fin pitch.(3)The effects of the change of tube pitch on oil flow distribution,inlet oil temperature and heat transfer coefficient of the radiators were numerically studied.Based on the results of aforesaid study,two 3-D finned tube radiators with different tube row spacing were designed and manufactured,a heat dissipation experimental platform was extablished,and the heat dissipation performances of the 3-D finned tube radiator were comparative analysed with the traditional plate radiator under the condition of equivalent volume.The results showed that the heat dissipation capacity of the 3-D finned tube radiator increased about 13.27% compared with the plate radiator when the temperature rise was 55 K.It could be seen that the 3-D finned tube radiator has a good application potential for heat dissipation of oil-immersed transformer. |
