| The twisted tube is a highly efficient enhanced heat transfer tube, the ellipticalcross section of it forms spiral flow channel by continuously twisting, the spiralchannel induces spiral flow and generates a secondary flow normal to the mainstreamin the tube. The twisted tube heat exchanger which consists of twisted tubes generatesself-supporting structure in the shell side, fluid flows longitudinally and spirally,which enhances the flow mixture and intensity of turbulence, so the engineeringapplication of twisted tube heat exchanger has a high value. The heat transfer andflow resistance characteristics in the tube side and shell side are studied by numericalsimulation method. Finally other forms of twisted tube are proposed, the effects andmechanisms of enhanced heat transfer of them are studied. The following is the mainwork and results:(1) Studying twisted tube with numerical simulation and optimizing theparameters. The optimization design goal and variable constraint space is determined.The structure of twisted tube is optimized by using simulation platform AnsysWorkbench, the results show that the influence of variable a on the comprehensiveevaluation index is larger than pitch S, when the value of pitch S is between78～105mm and the value of variable a is between10.2～10.68mm, the heat transferenhancement performance is better. According to flow filed, temperature filed,secondary flow, local Nusselt number, field synergy angle, the enhanced heat transfermechanisms are analyzed. According to the540numerical simulation results, thegeneral criterion correlations for Nusselt number and friction factor are fitted.(2) The heat transfer and flow resistance characteristics of the shell side of thetwisted tube heat exchanger are studied. The shape of shell of the twisted tube heatexchanger is hexagon, the results which are obtained from the characteristics of flowfiled and temperature filed in shell side show that both periodicity rotational flow andspoiler at the support of the long axises play the role of enhanced heat transfer, it alsocan ensure that draft tube is the effective way to prevent short-circuiting in shell side. The effects of geometry on the heat transfer and flow resistance are compared; theresults show that when pitch S reduces or variableA0increases, both the Nusseltnumber and friction factor are increased. According to the128numerical simulationresults, the general criterion correlations for Nusselt number and friction factor arefitted.(3) Two kinds of special-shaped twisted tube have been proposed，and the heattransfer and flow resistance characteristics of them are studied with numericalsimulation. For the bidirectional twisted tube, the effects of geometry of bidirectionaltwisted tube on η are analyzed, and compared with the twisted tube. The resultsindicate that the comprehensive evaluation index of bidirectional twisted tube isbetter than the twisted tube. Bidirectional twisted tubes also have a goodenhancement effect when the values of Reynolds number is less than20000. Thestructure of bidirectional twisted tube is optimized, when the value of pitch S isbetween100～130mm and the value of variable a is between10.15～10.45mm,the heat transfer enhancement performance is better. The direction of rotating flow isperiodic change, which is the key of enhanced heat transfer. For the spiral ribs twistedtube, the effects of rib height, rib number, variable a and pitch S of the spiral ribstwisted tube are analyzed, it is induced that the low rib height and less rib number andvariable a were better, pitch S has the biggest effect on η. Compared with thetwisted tube, the spiral ribs twisted tube has higher values of η when the Reynoldsnumber ranges from2300to50000, which greatly expands the use of twisted tube. |
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