Study On Convective Heat Transfer Of Hypothermic Seawater Flow Across A Tube

With the exploration and development of oil and gas resources in the deep sea,a large amount of heat exchange is carried out between the marine riser and seawater in low temperature environment.The thermodynamic properties of the fluid in the tube changed,but because of the low temperature and complexity heat exchanger,a lot of expressions are no longer applicable.Now for the study of heat transfer characteristics of mixed flow across a tube is not sufficient,so the the study on convective heat transfer of hypothermic seawater has an important significance.In this paper,seawater thermophysical properties is tested by densitometer,viscometer,thermal conductivity measurement and constant temperature water-bathing.The results show that the density and viscosity decrease with an increase in temperature,but the thermal conductivity and specific heat capacity are on the opposite,and the empirical formula of seawater thermophysical properties are given between 0~20?.Seawater flow process in the tank is simulated by FLUENT.The major parameters which influence the uniformity of the flow field are discussed.Among the major parameters which influence the uniformity of the flow field,the shape and layers have the greatest effect,then is the location and hole diameter,and the thickness is the smallest.The optimal perforated panel is as follow: the hole diameter is 2cm,the thickness is 20 cm,the first panel is 10 cm away from the entrance,and the second 60 cm.Based on similarity theory,the laboratory physical model is establishment.And the convective heat transfer experiment and numerical simulations are carried out.The results show that Nu increases with the increase of Re and Gr.With the smaller of Re,the influence of Gr on Nu is more obvious.Nu decreases with the increase of diameter so that the small diameter tube is beneficial to enhance the convective heat transfer intensity.The flow field in the tank is numerically simulated,and the fluid near the riser is spiral upward flow.Considering the influence of natural convection and forced convection,experimental value is comparied with pure forced convection and the add of forced convection and natural convection.The results show that the influence of natural convection decreases with the increase of Re,and increases with the increase of Gr.The influence of natural convection is stronger with the increase of tube and with the decrease of Re,the influence of diameter to natural convection is more obvious.Finally,the experimental correlation formula of mixed convective heat transfer is obtained.