Mechanism Analyese And Performance Study Of Flow And Heat Transfer In Shell-side Of Shell-and-Tube Heat Exchanger With Helical Baffles

As universal heat transfer equipments, shell-and-tube heat exchangers (STHXs) are widely applied in various industrial sectors, such as refining, chemical industry, environmental protection, power generation, refrigeration, food processing, utilization of new energy, etc. Benefiting from its importance, related performance improvement and efficiency enhancement become feasible solutions for energy saving and emission reduction, and therefore it can bring significant social and economic benefits.In this paper, methods integrating theoretical analysis, numerical simulation and experiment are adopted to research heat transfer mechanism and performance of a novel STHXs with helical baffles (STHXsHB). Researches are mainly focusing on the following aspects.The mathematical model of full-developed ideal helical flow is established in cylindrical coordinates based on some simplifications of shell-side actual helical flow of STHXsHB. The basic relationship between each velocity component is obtained through mathematical derivation. The inherent mechanism of ideal helical flow is studied in detail. Integrating results of ideal helical flow analyses and simultaneously considering more practical factors, shell-side actual helical flow is then analyzed. In reference to the study on obliquely cross flow circle tubes and cross flow elliptic tubes, the research method of shell-side actual helical flow are proposed in this thesis.The complete shell-side flow domain models of STHXs with continuous and quarter-ellipse non-continuous helical baffles are established in order to simulate shell-side flow and heat transfer characteristics. Viscosity dependence on temperature of the working fluid is considered. RNG k-s turbulence model and mesh adaptive technology is used. New unified calculation methods of shell-side Reynolds number are put forward which can describe shell-side flow pattern of STHXsHB with different structures. The resistance factor for calculation of the pressure drop is also given. Mechanical energy dissipation rate which include turbulent dissipation and viscous dissipation are adopted to describe shell-side local flow resistance. Numerical results show that:Under the same shell inner diameter, the larger the helix angle, the longer the inlet and outlet regions. When the helix angle is relatively small, convective heat transfer coefficient and mechanical energy dissipation rate of the import and export sectors is less than that of fully developed regions, and the case with relatively large is just the opposite.Shell-side radial distributions of axial velocity component, mechanical energy dissipation rate and convective heat transfer coefficient are ununiform, and the helix angle is smaller, the distribution is more ununiform.On condition of same shell-side Reynolds number, on the one hand heat transfer performance decreases slightly as the increase of helix angle, on the other hand friction factor decreases remarkably as the decrease of helix angle when helix angle is large enough.Comparing continuous and non-continuous helical baffles, the leakage from triangle zone between two adjacent quarter-ellipse non-continuous baffles causes shell-side flow pattern to deviate from continuous helical flow. Its shell-side radial distributions of axial velocity component, mechanical energy dissipation rate and convective heat transfer coefficient are more ununiform. The structure of staggered overlap helical baffles forces the shell-side flow pattern further deviate from continuous helical flow. The structure of double helical baffles improves shell-side performance and uniforms heat transfer between different tubes.A novel double shell-pass STHXs with continuous helical baffles is proposed in this thesis. There are two sets of continuous helical baffles to form two separate helical channels, which are also used as shell-pass partitions. If the shell inner diameter, inlet flow rate and helix angle are all same, structure of double shell-pass can make the bundles span and flow cross-sectional area half, and hence double the shell-side fluid velocity. The defects corresponding to continuous helical baffles with large helix angle can be overcome. Its shell-side performance is improved significantly. Finally, experiments are performed and results show that shell-side comprehensive performance of STHXs with continuous helical baffles is much better than that of STHXs with segmental baffles. The numerical method is validated by experiment data.