Study Of Heat Transfer Enhancement And Mechanical Properties Of Twisted Tube Heat Exchangers

Twisted tube is an efficient heat transfer enhancement tube. In this thesis, twisted tubes as well as twisted tube heat exchangers are studied for their heat transfer enhancement and mechanical properties.(1) Finite element analysis is performed to calculate the axial stiffness of the twisted tube. Experiments are carried out to verify the calculation. Effects of the twist ratio, lead and thickness on the axial stiffness are investigated. It is found that with increasing the twist ratio, the axial stiffness of the twisted tube is decreased. But a larger lead will give a higher axial stiffness. A so-called stiffness reduction coefficient for the twisted tube is given based on the straight tube, which is helpful for the engineering design of the twisted tube heat exchangers.(2) Twisted tube is a non-circular heat exchange tube. Strictly speaking, it is not appropriate to simply consider the twisted tube as an ordinary tube in the design of twisted tube heat exchangers based on GB151-1999Shell-and-Tube Heat Exchanger. In this thesis, stiffness weakening factor Kf is defined and formulated for the twisted tube, and a so-called equivalent elastic modulus method is proposed for the design of twisted-tube heat exchangers. The idea is when applying GB151-1999Shell-and-Tube Heat Exchanger to the design of twisted-tube heat exchangers, the twisted tubes are considered to be ordinary tubes with the same size but different elastic modulus. As an application example of the equivalent elastic modulus method, strength calculation for the tubesheet of a twisted tube heat exchanger was performed and analyzed for the stress changing with the stiffness of the twisted tube. This work indicates that the equivalent elastic modulus method is reasonable and applicable in engineering.(3) Finite element analysis with the software ANSYS was performed on the tubesheet of a twisted tube heat exchanger under three loading cases. Effects of the axial stiffness of the twisted tubes on the strength of the tubesheet were investigated. It is found that as the axial deformation compensation of the twisted tubes is better than that of the plain straight tube, stresses at the tubesheet are decreased when using twisted tubes under the thermal loading, but the stresses are increased under the pressure loadings. Although the average axial stress at the twisted tubes is almost the same as that at the straight tubes, the stress distribution at the cross section of the twisted tubes is not uniform and the maximum stress is much larger than the average stress, which implies that the ability of the twisted tubes to resist fatigue or stress corrosion cracking is much weaker than the straight tubes. For different thickness of the tubesheet, the effect of the axial stiffness of the twisted tubes is different. The smaller the thickness of the tubesheet, the stronger the effect is.(4) Fluid simulation is performed with the computational fluid dynamics analysis software FLUENT on the double-pipe heat exchangers with twisted inner tube or straight inner tube. A comparison is made between these two heat exchanger for the flow field distribution, heat transfer coefficient and pressure drop on both tube side and shell side. The influence of distortion ratio and lead on heat transfer and flow resistance is analyzed. It indicates that the more twisted the tube is, the larger the heat transfer coefficient is, and so is the pressure drop. Therefore, it is necessary to comprehensively consider the effect on the heat transfer coefficient and pressure drop when choosing twisted tubes. According to the simulation results, the correlation equations for heat transfer enhancement coefficient and pressure loss coefficient of twisted tube double-pipe heat exchanger are fitted in order to facilitate the process calculation with twisted tubes.(5) A software for heat exchanger process calculation is developed using Microsoft Visual Studio2010based on the correlation equations of the heat transfer coefficient and the pressure loss coefficient obtained in this thesis, which is helpful for the engineering design and application of twisted tube heat exchangers.