The Heat Transfer And Flow Characteristics Analysis Of Helically Coiled Tube Heat Exchanger

Due to the compact structure and high transfer coefficient, non-fouling, helically coiled tube heat exchanger has been widely used in industrial applications. In this thesis, water as the working medium, the reliability of numerical simulation method was proved by experimental results. The goal driven optimization study of helically coiled tube heat exchanger(HCTHE) shell side was carried out by using the proved numerical simulation method, and the optimization of the entirety was investigated numerically. In order to improve the heat transfer and flow performance, a new type HCTHE, helically coiled tube heat exchanger with corrugation, was proposed innovatively, and the turbulent heat transfer and flow characteristic of helically coiled tube with corrugation were numerically investigated and the heat transfer enhancement mechanism was analyzed. The main work and results are as follows:(1) The numerical simulation method of HCTHE was determined and numerical calculation of shell side and tube side of HCTHE were carried out. A laboratory scale heat exchanger platform was built and investigated experimently. The results show that the maximum experimental error of Qc, ΔTm, h, Nu, ΔT, ΔP and f are less than 5%, that is to say, the experiment is reliable. To verify the reliability of numerical simulation, the simulation results were compared with experimental results. It indicated that when the steam flow rate is 2.5m3/h in tube and the cold water flow rate is in the range of 1.9 m3/h~7.2m3/h in shell side, the relative error of cold water pressure drop is-27.65%~-7.49%, the relative error of Nu is-0.607%~2.196%, and when the steam flow rate is 2.5m3/h in shell sied and the cold water flow rate is in the range of 2.1 m3/h~7.2m3/h in tube, the relative error of cold water pressure drop is-25.69%~1.55%, the relative error of Nu is 0.0137%~0.87%, which are within the allowable range. The feasibility and reliability of numerical simulation method is proved.(2) The proved numerical simulation method was used and multi-objective driven optimization with genetic algorithm in ANSYS Workbench was applied. It can be find that d and R have a significant impact on heat transfer and flow characteristics, and the influence of S on that is less. The influence of d, R and S on heat transfer and flow characteristics has been analyzed. The optimal structures of each working condition are obtained, the optimal structure, d=8mm,R=50mm,S=80mm, which are applied to each working condition(in a given range of this thesis). The obtained optimal structure results are analyzed and the correlation equations about Nu and f are acquired.(3) The same calculation model is applied to the coupled multi-objective driven optimization with genetic algorithm of the heat exchanger entirety, the results indicate that d and R have a significant impact on heat transfer and flow characteristics. Three groups optimal structures, d=8mm,R=50mm,S respectively is 80 mm, 85 mm, 90 mm, which make the heat transfer and resistance performance of HCTHE shell side and tube side synchronously achieve the best, are obtained.(4) In order to improve the heat transfer performancement, a new type HCTHE, helically coiled tube heat exchanger with corrugation which was combined with the corrugated tube, was proposed. The turbulent heat transfer and flow characteristic of helically coiled tube with corrugation in the new type HCTHE were numerically investigated and the heat transfer enhancement mechanism was analyzed. The simulation results indicate that the centrifugal force coursed by the helical movement of fluid leads to secondary flow in the vertical flow direction, meanwhile, back flow in main flowing direction is caused by the periodic expansion and contraction of tube cross section, which destroys the flow boundary layer, enhances the turbulence intensity and strengthens the heat transfer process greatly. The overall heat transfer performance of HCTC is much better than that of smooth helically coiled tube in the same working condition, and heat transfer enhancement effect near the outside wall at the downstream side section of corrugation is much higher than that of other position.