Research On Characteristics Of Flow And Heat Transfer And Characteristic Parameters For Open-Rack Vaporizer (ORV)

Environmental and energy issues are increasingly becoming the focus of world economic development.As one of the three pillars of primary energy in the world,natural gas is recognized as a clean energy source because of its high calorific value and low environmental pollution.Now,natural gas is increasingly being developed and utilized by people.Liquefied natural gas(LNG)has become a rapidly emerging industry.LNG gasifier is heat exchange equipment specially used for gasification of liquefied natural gas.Open-Rack Vaporizer(ORV)is an important large-scale gasification equipment in the LNG receiving terminal,its gasification performance and efficiency directly affect the load capacity of the LNG terminal,and it is also one of the bottlenecks of the localization of the equipment.In this paper,combining with the practical application of gasifier and the literatures,through the analysis of the structure and working principle of ORV,the flow and heat transfer law in the heat exchange tube of of the ORV is investageted by theoretical analysis,numerical simulation and experimental research.The distribution characteristics of the liquid film on the outer wall of the heat exchange tube were measured by experiments.On this basis,the distribution of temperature field and the mechanism of freezing inside the liquid film on the outer wall of the heat exchange tube are analyzed.Finally,the stress and strain characteristics of heat exchange tube in the ORV under the coupling action of large temperature difference and high internal pressure are tested by experimental method.The purpose of this paper is to explore more reasonable structural and operational parameters,improve the gasification rate of seawater ORV and provide data support for practical engineering applications,so as to improve the localization engineering application of seawater ORV.The following conclusions are drawn:1.The Numerical models of flow and heat transfer of the heat exchange tube without spoiler and with spoiler is established respectively,the fluid flow and heat transfer process in the heat exchange tube are calculated numerically,and the factors affecting the gasification rate are analyzed.The numerical simulation shows that the LNG temperature,gas holdup and mixture flow velocity at the bottom entrance of heat exchange tube are the lowest and the pressure is the highest;As the gasification process progresses,the temperature,gas content,and flow rate increase continuously along the tube process,the pressure gradually decreases.2.Based on the numerical calculation model of flow and heat transfer in heat transfer tube without spoiler,the relationship between gasification and flow rate,inlet temperature,roughness of heat transfer tube is analyzed.The results show that: Under different flow rates and temperature conditions,the LNG gasification process is basically the same,but the gasification speed is different.As the intake speed and temperature decrease,the heating time of the LNG in the tube becomes relatively long,and the gasification process proceeds more fully.There is a section with a steep rise in gas holdup between 2m and 3m,the temperature and gas holdup are basically smooth transitions in each section between 3m and 6m,and the gasification process has been basically stable.The gas holdup under different working conditions eventually tends to a fixed value,indicating that the gasification capacity of the heat exchange tube is certain,and changing the inlet LNG speed can only change the reaction degree of the gasification process.With All other things being equal,the gasification rate distributions at different sections along the tube path in the two heat exchange tube models with different inner wall roughness were compared.With the increase of inner wall roughness,the gasification effect in the evaporation section became better.The finned tube model with spoiler has a slight advantage in the heating section,but the roughness of the inner wall has little effect on the gas holdup.3.Based on the numerical model of flow and heat transfer inside the heat exchanger tube with a spoiler,the relationship between gasification process and characteristic parameters such as temperature,velocity and pressure drop in the heat transfer is obtained,and the results of the heat transfer process are compared and verified by the experimental method.Under the same boundary conditions,compared with the calculation results of the heat exchange tube without disturbing rod,the existence of spoiler rod improves the gasification efficiency of LNG in the heat exchange tube,and the gas content at the outlet is similar,with low speed and temperature.In the evaporation section,the gasification effect of the heat exchange tube without spoiler is better than that with spoiler.In the heating section,the fluid in the annular clearance and inner tube are mixed.Under the guidance of the spoiler,the velocity of the internal medium increases and the mixing degree increases.Then the degree of turbulence is enhanced.The high-temperature medium from the annular gap is fully mixed with the low-temperature medium in the inner tube and the heat exchange continues,so the degree of LNG gasification in the inner tube is enhanced,the gas holdup in the heat exchange tube with spoiler rod is higher than that without spoiler rod corresponding,and the gasification process is enhanced,making the total gas holdup rise rapidly.The flow of the fluid in the inner tube with spoiler rod moves from the original straight along the axis to the spiral running along the spoiler rod,therefor the turbulence degree of the internal fluid is enhanced,then the internal fluid speed is increased which can cause the increase of the loss along the way.4.An experiment was conducted to test the water film characteristics on the outer wall of the heat exchanger tube of the ORV,and a tool for measuring the water film thickness of the seawater distributor was developed.The film thickness distribution on the outer wall of the heat exchange tube was tested experimentally.The results show that the change trend of liquid film between the fins of different heat exchange is basically consistent with the change of the vertical distance and horizontal distance of the distributor,and the liquid film thickness increases with the increase of flow rate.The horizontal distance between the distributor and the heat exchange tube has a greater influence on the liquid film thickness than the horizontal distance between the distributor and the heat exchange tube,and the liquid film thickness decreases with the increase of flow channel size.5.The mechanism of thermal and mass turbulence transport effectheat and phase transition processes such as local ice-gasification on the transport process of ORV are solved under actual conditions.It can be seen from the unsteady state numerical simulation of two kinds of liquid film thickness under different inlet temperatures the temperature field in the liquid film is a gradient from the outside to the inside.When the inlet seawater temperature is 273.15 K,freeze formation will begin inside the liquid film.The 5mm liquid film freezes at the interface between solid and liquid,and there is also melting ice phenomenon.The dynamic change of the freezing situation shows little change in the position of the interface between solid and liquid.When the 10 mm liquid film starts to consider the freezing process,there is an obvious interface between the solid and the liquid that is likely to freeze at first.The temperature difference in the whole region is about 1K.6.The strain of the gasifier under the two conditions of liquid nitrogen injection and icing outside the tube was tested by replacing LNG with liquid nitrogen.The relationship between the strain of gasifier and liquid nitrogen inlet temperature is analyzed.It can be seen from the experimental results that the heat exchanger tube bundle of the gasifier shrinks after being injected with liquid nitrogen and the strain along the liquid nitrogen flow direction is greater than that in the vertical direction,which is mainly determined by the length of the pipe.Under the same condition,the closer to the liquid nitrogen inlet,the greater the strain.With the gasification of liquid nitrogen,the temperature increases and the strain decreases.Near the boiling point of liquid nitrogen,strain tends to zero.The strain values of the header and manifold are obviously larger than the strain variables of the heat exchange tube and maintain a higher strain level.Icing on the outside of the tube will affect the heat exchange between the inside and outside of the heat exchange tube and increase the strain variable.Icing will not only affect the heat exchange effect of the gasifier,but also affect the structural safety of the gasifier.