| Spiral wound heat exchangers(SWHE)have the characteristics of high heat transfer coefficient,good thermal compensation performance,and wide operating range,usually utilized as the main cryogenic heat exchangers(MCHE)by large LNG plants.The performance of SWHE has a significate influence on the liquefaction efficiency of natural gas.The precise design of SWHE is the premise for the natural gas liquefaction plant to operate normally.SWHE consists of a tube bundle and a steel shell.In the heat transfer process,gas streams condense on the tube side in the upward flow,and mixed refrigerants evaporate on the shell side in the falling film flow.Because the shell-side thermal resistance is dominant in the SWHE,the study of SWHE focuses on the flow and heat transfer characteristics in the shell side.The heat transfer performance of SWHE is influenced by the fluid distributed to the tube bundle.In order to make the fluid distributed uniformly on the bundle,the fluid distributor is installed at the entrance of SWHE.The liquid distribution performance of the distributor has a direct influence on the heat transfer performance of the SWHE.Therefore,the optimization design of distributors is a critical problem in the design of SWHE.In addition,the structure of SWHE is very complicated,and the fluid flows in multiple forms.The heat transfer mechanism of SWHE is not fully understood.The geometrical parameters and flow forms significantly influence the flow and heat transfer characteristics.The effect of structural parameters on flow and heat transfer characteristics of different flow forms need to be further studied.It is challenging to conduct a full-scale experiment on SWHE due to its enormous size.Therefore,numerical simulation and prototype research are effective methods to study the flow and heat transfer performance of SWHE.In this paper,a 3-D numerical model is adopted to study the influence of distribution structure parameters on falling film flow.Furthermore,according to the tube bundle structure of SWHE,a numerical model of spiral tubes is established to explore the influence of structure parameters on the flow and heat transfer characteristics of SWHE.Based on the structure of SWHE and the numerical simulation results,a small test prototype was designed,and the flow and heat transfer performance of the prototype was tested.The main research contents of this paper are as follows:1)The numerical model of falling film flow outside horizontal pipe is established,and the distributors distribute the fluid.Based on the principle that the axial opening section of the distributor is the same,different types of distributors are designed.Numerical simulation is performed for each distributor,and the effects of distribution hole diameter,distribution hole spacing,and liquid distribution height on falling film flow were analyzed.Furthermore,the correlation and interaction among distribution parameters are also analyzed.2)The evaluation index of liquid distribution performance is developed.The uniformity of liquid column between tubes and the uniformity of liquid film outside tube were adopted to evaluate the liquid distribution performance.The calculation methods of the uniformity of liquid column between tubes and liquid film outside tube were proposed.The influence of distributing hole diameter,distributing hole spacing,and distributing liquid height on distributing liquid uniformity was analyzed.When the axial cross-sectional area of distribution holes is determined,the optimum solution of the liquid distributor is given and verified.3)The numerical model of falling film flow outside the spiral tubes is built.And the numerical method of spiral tubes with different winding angles is suggested.The distribution states of the liquid film under the drop flow,column flow,and sheet flow are observed.The distribution rules of film thickness under the different patterns are obtained,and the liquid film uniformity of different winding angles is analyzed.The film thickness is measured in the axial and circumferential directions when the Reynolds number is less than 2000,and the effect of the Reynolds number on film thickness is analyzed.4)The heat transfer numerical model is built to investigate the heat transfer characteristics outside the spiral tube in the liquid falling film flow.The heat transfer characteristics and mechanism of liquid falling film flow are analyzed.Moreover,the heat transfer states are simulated under different inlet temperatures,tube wall temperatures,working pressure,and flow medium,and the heat transfer coefficient is calculated.The heat transfer correlation considering the influence of winding angle is developed based on the results.5)According to the results of numerical analysis and the structural characteristics of SWHE,a small prototype is designed.The design and structure parameters are confirmed with the working condition and the structure parameters of the real spiral wound heat exchangers.And the visualization of the shell side flow process is realized.According to the design conditions,the structure and process design of the test prototype is completed,and the prototype manufacturing is completed.6)The testing process of the small prototype is designed.Based on the original test bench,the test platform was reformed to realize the adjustment,control,and measurement of fluid flow,pressure,temperature,and other parameters with propane on one side and glycol solution on the other side.7)The flow and heat transfer performance of the small prototype is tested.The experiments proceeded with the liquid propane on the shell side and glycol solution on the tube side.The test process meets the safety and measurement accuracy requirements.The flow state is observed in the adiabatic condition,and the heat transfer coefficient is tested in the heat transfer state. |
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