| Strengthening heat exchange technology is of great significance to the rational use of energy and environmental protection and even the sustainable development of mankind.Based on the theory of perturbation or destruction boundary layer-reinforced heat exchange,a new type of reinforced heat exchange tube is proposed——an intermittent spiral inner rib heat exchanger with upstream curved surface,and the flow and heat transfer characteristics of the intermittent helical inner rib heat exchanger with five cross-sectional shapes of rectangle,semi-ellipse,triangle,dome rectangle and trapezoid in the Reynolds range of 3200 to 9600 are numerically simulated.The movement of fluid in the intermittent helix rib heat exchanger tube is illustrated by combining the trace graph and the velocity vector diagram.The effects of structural parameters such as rib base,rib height,helix angle,rib number and rib length on heat exchange and resistance characteristics are analyzed by numerical calculation method.The results show that:1)It is found that when the Reynolds number is less than 4800,the structure of the upstream curved surface has little effect on heat exchange and resistance characteristics.However,when the Reynolds number is greater than 6400,the presence of upstream curved surface can effectively reduce the local resistance at the beginning and end of the fluid flow through the intermittent helix ribs,and the resistance coefficient is 60%compared to the non-upstream curved surface structure.2)When the Reynolds number is high(more than 5000),the dome rectangular cross-section breaks the inner ribs of the spiral for the best heat exchange effect,Nussel number is 1.31 times the worst effect of the triangular ribs,followed by Rectangular and trapezoidal,semi-elliptical cross-sectional ribs,rectangular and trapezoidal Nussel number are similar,1.15 times more than triangular ribs,and semi-elliptical ribs are 1.06 times more than triangular ribs.When the Reynolds number is less than 5000,the combined heat exchange performance coefficient of rectangular and trapezoidal cross-section ribs is the largest,at 2.9 to 3.0.The enhanced heat exchange effect of the inner ribs of the triangular cross-section is the worst in the Range of Reynolds 3200 to 9600,and the increase of Nussel number is not as good as that of rectangular,trapezoidal and dome rectangular cross-section ribs,while the resistance coefficient is large,the comprehensive performance coefficient is only 2.0 to 2.2,which is about 24%lower than that ofthe other cross-sectional shapes.3)The simulated trace and velocity vector plots show that when fluid flows into the ribbed segment,both the incoming and backflow surfaces of the ribs effectively induce the generation of vortex-enhanced heat transfer.When the fluid flows from the ribbed segment into the smooth section,it can continue to maintain the axial rotation speed and strengthen the heat transfer of the smooth segment.The next set of ribbed segments can be arranged as the fluid rotation speed gradually turns to zero.4)In the study of structural parameters affecting heat exchange and resistance characteristics,it is found that the number of ribs and the angle of the spiral have an important effect on the heat exchange effect.When the number of ribs is too many,the spiral angle is too large,the inter-rib flow channel narrow,the flow resistance increases,the Nussel number and the comprehensive performance coefficient are showing a downward trend。The rib head number is 28 heads of the dome rectangular break spiral inner rib heat exchange tube Nusser number and the comprehensive performance coefficient is 40 head of the ribs heat exchanger increased by 10%and 13%.The helix angle is 18°～30° range for the best improvement of heat exchange performance,the comprehensive performance factor of 2.9 to 3.1,continue to increase the helix angle of the combined effect is not great.5)The numerical calculation of the structural parameters results in the arrangement of a dome rectangular spiral with upstream curved surface in the heat exchange tube with a pipe diameter of 8.96mm,and the more suitable structural parameters are:recommended rib base 0.3mm to 0.35mm;rib height 0.3mm to 0.7mm(large Reynolds should choose shorter rib),inner rib head 28 head,spiral angle 18°～30°.Selecting the appropriate structural parameters is calculated,and the comprehensive performance coefficient of the intermittent spiral rib heat exchanger can reach 3.0 to 3.4 in the Reynolds range of 3200 to 9600.In this paper,the intermittent spiral inner ribs with upstream curved surface structure are studied,the effect of strengthening heat exchange is good and the resistance loss is relatively small,which has a good promotion effect on the comprehensive heat exchange performance in the tube,and provides a basis for the development of a new type of reinforced heat exchange tube. |
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