Study On Rolling-ploughing Extrusion Composite Forming Mechanism And Heat Transfer Performance Of Three-dimensional Inner Finned Tube

With the development of economy and society,China faces many challenges such as energy security and environmental pollution.Energy conservation is an important means to alleviate energy security.Shell and tube heat exchangers are widely used in the industrial fields and their heat transfer performance directly affects the energy consumption level in this field.Finned tubes are the core components of heat exchangers,and their thermal performance directly affects the heat transfer performance of the heat exchangers.The three-dimensional inner finned tube has excellent heat transfer performance,which has attracted wide attention from academia and industry.However,how to efficiently manufacture three-dimensional innner finned tubes is still a problem.Therefore,in order to solve this problem,a novel rolling-ploughing extrusion composite forming method is proposed.A new ploughing extrusion tool with arc-shaped main cutting edge and arc-shaped extrusion curved surface is developed to realize efficient and controllable fabricating of three-dimensional inner finned tubes.The main research contents are as follows:Rolling-ploughing extrusion forming method was proposed to fabricate three-dimensional inner finned tubes: on the basis of two-dimensional internal spiral fins formed by roll pressing,three dimensional inner finned tubes are formed by ploughing extrusion method.A new ploughing extrusion tool had been developed,which has the characteristics of arc-shaped main cutting edge and arc-shaped extrusion surface.The experimental results show that the ploughing extrusion tool breaks through the classical enveloping principle of the main and secondary cutting edges of the traditional tools,so that the inner spiral fins of the inner surface are not removed and extruded to form three-dimensional inner fins.Critical depth of ploughing extrusion for three-dimensional inner finned tube is studied and the formula for calculating the maximum allowable ploughing extrusion depth is derived.The ploughing extrusion process of three-dimensional inner finned tubes was studied by experiments and finite element analysis methods.The three-dimensional inner finned tube can be divided into three stages: initial ploughing stage,extrusion stage and stable forming fin.The physical field of three-dimensional inner finned tube in each stage of forming process and the influence of tool parameters on the physical field are analyzed.The results show that the stress concentration exists at the root of the three-dimensional inner fin during the forming process.With the increase of the radius of the arc,the equivalent stress of the three-dimensional inner fin at the root of the inner fin increases,and the internal fin is inclined in an axial direction.Results also show that large extrusion angle is beneficial to formation of vertical three-dimensional inner fins.The effects of ploughing extrusion depth and feed on the geometry of three-dimensional inner finned tubes were analyzed.It is found that the height of 3-D inner fin increases with the increase of feed and ploughing extrusion depth,which is consistent with the theoretical prediction.The fin thickness increases with the increase of feed.However,the ploughing extrusion depth has no effect on the fin spacing,which is in agreement with the theoretical prediction formula.At the same time,the influences of the tool's main cutting edge angle,extrusion angle and arc radius on the geometric structure of the inner fin were also studied.The results show that the fin height decreases with the increase of the main cutting edge angle,and the slant angle of the inner fin increases with the increase of the main cutting edge angle.The fin height and slant angle increases with the increase of extrusion angle.The larger the arc radius of the ploughting extrusion tool is,the smaller the fin height and inclination angle are.The heat transfer performance of the three-dimensional inner finned tube was studied.Compared with the heat transfer performance of the internal spiral finned tube,corrugated tube and dimple tube reported in literature,the maximum overall thermal performance of three-dimensional inner finned tube increased by 6.9%-30%.Compared with the traditional internal spiral finned tube,the overall thermal performance of the three-dimensional inner finned tube is improved by 13.1%,and has been successfully applied to the air source heat pump of a famous enterprise.To further improve the heat transfer performance of three-dimensional inner finned tubes,three-dimensional inner finned tube equipped with Y-branch insert and three-dimensional inner finned tube equipped with porous fiber insert were proposed.Results show that compared with single 3-D inner finned tube,the overall thermal performance of three-dimensional inner finned tube equipped with Y-branch insert is increased by 15.7-86.2%;The overall thermal performance of three-dimensional inner finned tube equipped with Y-branch insert is 2.06 times higher than that of the spiral grooved tube equipped with twisted strips.Compared with the conical inserts and multiple twisted tape inserts reported in the literature,the overall thermal performance of three-dimensional inner finned tube with porous fiber insert is improved by 33.9%-78.9%.