Numerical Study Of Falling Film Flow And Heat Transfer In Transverse Tubes Containing Non-Condensable Gases

The shortage of freshwater resources is a major problem facing the survival and development of human society today.Seawater desalination technology can use the abundant seawater resources on the earth to alleviate this problem.Low temperature multi-effect horizontal tube falling film evaporation is widely regarded as one of the most promising seawater desalination technologies.It has the advantages of high heat transfer efficiency,low heat source requirements,large operating flexibility and low power consumption.Through numerical simulation of the falling film flow and heat transfer process of the seawater horizontal tube,the influence mechanism of different factors on the falling film flow and heat transfer of the horizontal tube is explored,which can provide a reference for the development of low temperature multi-effect horizontal tube falling film evaporation technology.In this paper,the Fluent software in the ANSYS16.1 series is used to analyze and study the falling film flow and heat transfer process of seawater horizontal tubes containing noncondensable gases.In the process of this study,the physical properties of seawater and the changes of gas solubility with temperature are considered.In this paper,a non-condensable gas precipitation model in seawater is established,and numerical simulation is carried out with the Volume of Fluid model in Fluent,and the flow of the liquid film outside the transverse tube and the precipitation of non-condensable gas under different conditions are obtained.Numerical simulation was carried out on the falling film flow and heat transfer process of seawater under different conditions such as spray density,heat exchange tube temperature,heat exchange tube diameter and liquid distribution height in the laminar flow range.The distribution of thermal coefficient was analyzed,and the interaction between the evolution of non-condensable gas in the working medium and the falling film flow and heat transfer in the transverse tube was analyzed.The results show that the non-condensable gas evolution process can be divided into four stages: bubble adsorption,bubble floating,bubble collapse and bubble insignificant.The precipitation of non-condensable gas will increase the temperature of the liquid in the container more rapidly,increasing the local heat transfer coefficient of the heating wall;increasing the temperature of the heating wall will increase the coefficient of non-condensable gas,but when the temperature of the liquid in the container is high,the non-condensable gas will increase.The precipitation rates of the samples are not much different;increasing the salinity will weaken the precipitation of non-condensable gases.The process of falling film flow in a horizontal tube containing non-condensable gas can be divided into four stages: dripping,spreading,fully developing and leaving.In the range of spray density from 0.016kg/(m·s)to 0.055kg/(m·s),the size of the bubbles at the bottom of the horizontal tube increases continuously with the increase of spray density.Larger size bubbles will separate out small bubbles to maintain If its own volume is within a certain range,there will be a large bubble stably in the liquid at the bottom of the horizontal tube.Beyond this range,a stable large bubble cannot be formed in the liquid at the bottom of the horizontal tube.After raising the temperature of the outer wall of the heat exchange tube,the thickness of the liquid film outside the tube decreases,the precipitation rate of non-condensable bubbles becomes faster,the liquid film outside the tube fluctuates more violently,and the local heat transfer coefficients at the top and bottom of the transverse tube increase.Within the diameter range of the heat exchange tube studied in this paper,increasing the diameter of the heat exchange tube will reduce the thickness of the liquid film outside the transverse tube and the degree of fluctuation of the liquid film,but the local heat transfer coefficient does not change with the increase of the diameter of the heat exchange tube.some kind of trend.After increasing the liquid distribution height of the falling film in the horizontal tube,the distribution of the liquid film outside the tube is more uniform,the thickness of the liquid film is smaller,the fluctuation of the liquid film in the upper half of the horizontal tube is more severe,and the local heat transfer coefficient at the top and bottom of the horizontal tube is greatly improved.