Study Of Heat Transfer Enhancement And Axial Load-Bearing Capability Of Corrugated Tubes

With the rapid development of modern industries and in the light of world energy crisis, more and more attention has been put on technologies for effective use of all kinds of energy. As having heat transfer enhancement effects, corrugated tubes are more and more widely used to construct the shell-and-tube heat exchangers. However, at the present time, the mechanism of the heat transfer enhancement of corrugated tubes is still not very clear, accurate heat transfer coefficient calculation formulas are not available and no specific standards or codes can be readily used for the strength design of the corrugated shell-and-tube heat exchangers. Therefore, it is of great value both theoretically and practically to systematic study heat transfer enhancement behaviors and the axial load-bearing and expansion compensation capability of corrugated tubes.Flow and heat transfer of fluids with different viscosities inside a corrugated tube were numerically and experimentally studied with stress on the mechanism and effect of heat transfer enhancement compared with a straight tube. By relating the flow phenomena under different flow conditions and the local heat transfer coefficient, it is found that the vortices located in the wave crests of the corrugated tube plays key roles for heat transfer enhancement. The critical Reynolds Number for flow transition from laminar to turbulent in a corrugated tube is obtained which is much lower than that in a straight tube. For the effect of the heat transfer enhancement, an optimal enhanced region is found in terms of ratio of heat transfer coefficient of the corrugated tube to that of a straight tube under different flow conditions. With considering the influences of structural parameters and fluid properties on heat transfer and flow resistance, two fitted formulas for calculating Nusselt Number and friction coefficients respectively are given for engineering use.In order to investigate the heat transfer enhancement on a fluid flowing outside a corrugated tube, experiments and numerical simulations were carried out on a double-pipe heat exchanger with its inner tube being corrugated. Results show that for water under the same flow conditions (same Reynolds Numbers), the average heat transfer coefficient outside the corrugated tube is much lower than that inside the tube. This means that the heat transfer of a corrugated double-pipe heat exchanger is dominated by the heat transfer outside the tube. It is also found that corrugated tubes are very effective in enhancing the shell-side heat transfer, and the average convection film coefficient outside the tube is about three times of that outside a straight tube under same conditions. Results also show that higher heat transfer effect can be obtained with smaller inner diameter of the outer tube and smaller wave radius of the inner corrugated tube. To facilitate engineering design and application of corrugated double-pipe heat exchangers, a formula considering the influence of structural parameters of heat exchangers is developed for calculating average Nusselt Number outside the corrugated tube.In addition, axial tensile strength of corrugated tube is also explored through experimental stress analysis and numerical simulation. Distributions of axial and hoop stresses in a corrugated tube under tensile loading are investigated and compared with those in a straight tube. It turns out that as a result of structure discontinuity, stress distributions in a corrugated are complicated and axially changed greatly. Specifically, the hoop stress in the corrugated tube becomes significant even for an axial loading. By performing stress linearization along wall thickness, it is found that bending stress comes out and even exceeds the membrane or average axial stress. The bending stress varies axially while the average axial stress changes a little. After performing parameter analysis, a simple fitted formula for the calculation of the bending stress is given in the aim of facilitating strength evaluation of the engineering application of the corrugated tubes.Finally, the axial deformation of corrugated tubes under an axial tensile or compressed load was studied experimentally and numerically with the purpose of determining the axial stiffness of the tubes. All related structural parameters are considered. By defining a stiffness weakening factor K_f, a simple formula for calculating the axial stiffness of corrugated tubes is put forward which may be used to evaluate the axial deformation of corrugated tubes in the design of the corrugated shell-and-tube heat exchangers.