Research On Shell-Side Two-Phase Flow And Heat Transfer In FLNG Spiral-Wound Heat Exchanger Under Sloshing Conditions

Spiral-wound heat exchanger（SWHE）,with compact structure and good heat exchange performance,is usually used as the main low-temperature heat exchanger in natural gas liquefaction.When used in LNG floating production storage and offloading（FLNG）,the heat exchanger needs to operate under complex sea conditions.Sloshing conditions are complex and changeable,the flow and heat transfer under the sloshing conditions are more complex.The relevant optimization and design are blind because of the lack of mechanism.In this thesis,the shell side of the SWHE is taken as the research object,and the influence of sloshing conditions on the shell side of the SWHE is studied by CFD numerical calculation.Research contents and conclusions are as followed.（1）Horizontal tube model is established to study the influence of different mass flow,heat exchange diameter,tube spacing and refrigerant dryness on falling film flow outside the tube.The results show that Re=120 is the critical Reynolds number for flow pattern transition,the flow pattern is droplet flow when Re<120,and pattern transition are observed when Re>120.The influence of tube spacing and diameter on the flow outside the tube is multifaceted,and appropriate sizes should be selected to achieve maximum heat transfer coefficient.The vapor quality reflects the proportion of the liquid phase,and the smaller the vapor quality,the greater the heat transfer coefficient.（2）The effects of the winding diameter,spiral angle,and sloshing form on the two-phase flow of a single-layer tube bundle are studied by establishing a tube bundle model and the sloshing conditions are achieved with UDF code.The results show that the spiral-wound structure can cause the refrigerant on the shell side to migrate laterally away from the mainstream,resulting in an increase in flow resistance;As the winding diameter decreases and the helix angle increases,the heat transfer coefficient significantly decreases.When Re=6000,the heat transfer coefficient of the winding tube bundle with winding diameter D_wound=0.12 m decreases by 41.5%compared to the horizontal straight tube bundle,and with helix angleφ=16°decreases by 44.42%;Compared to the static condition,the heat transfer coefficient decreases by 64.44%,86.67%,and 72.22%under axial,radial,and circumferential sloshing conditions,respectively.（3）A model of multi-layer tube bundles is established based on a single-layer tube bundle,studying the influence of interlayer structure on flow and heat transfer outside the tube bundles,and further investigating the influence of sloshing forms and parameters.The results show that the refrigerant in the interlayer structure undergoes radial migration under the influence of adjacent tubes,resulting in a significant increase in flow resistance and no significant enhancement in the corresponding heat transfer effect.Compared with the static working condition,different forms of sloshing have different effects on the flow and heat transfer outside the multi-layer heat exchange tube bundle.The axial sloshing has little effect,while radial sloshing weakens heat transfer,while circumferential sloshing enhances heat transfer;When the sloshing form is the same,larger amplitude and higher frequency sloshing can enhance the heat transfer.（4）An experimental system of the pre-distribution section is established and corresponding models are established to study the influence of gas-liquid ratio on flow distribution through experiments and simulations.The results indicate that the size of the imported hydraulic pressure determines the average liquid phase outlet flow rate of the distribution device;The intake air and liquid pressure ratio affect the height of the liquid level in the distribution chamber.When in the middle section,the distribution uniformity is good,with a dispersion coefficient of only 0.015.When the liquid level exceeds the critical positions at both ends,the distribution performance is extremely poor,and the distribution device fails.A heat exchange tube section distribution plate model and an improved model are established to study the distribution uniformity of the aluminum distribution plate in the groove before and after the improvement.The results show that the refrigerant flow in the distribution plate of the wound tube section is disordered,with a dispersion coefficient of 0.4;The improved slot aluminum distribution plate has added baffles that hinder the migration of refrigerant between layers,weakening the interlayer influence,improving distribution uniformity,and reducing the dispersion coefficient to 0.16.In summary,the geometric structure of the wound heat exchanger affects the liquid phase distribution on the shell side,with the interlayer structure having the greatest impact,resulting in an increase in flow resistance of an order of magnitude.Compared to the static condition,the sloshing condition makes the liquid phase refrigerant outside the tube bundle more dispersed,and the overall heat transfer performance decreases significantly.High parameter sloshing can appropriately enhance heat transfer at the cost of increasing external power consumption.The lateral migration deviating from the mainstream is the main reason for the uneven distribution of refrigerant on the shell side,and weakening the lateral migration can increase the uniformity of flow distribution.The results can provide theoretical support for efficient enhanced heat transfer technology based on complex ocean conditions,and promote the development of the FLNG industry.