The Experimental Investigation And Numerical Simulation Of The Heat Transfer Characteristic In An LNG Submerged Combustion Vaporizer

As a major demand for national economic development,the trade of liquefied natural gas(LNG)is increasing,making its supporting facilities attracted wide attention.Compared with any other types of LNG vaporize,the submerged combustion vaporizer(SCV)is an ideal basic load type and emergency peak shaving type vaporizer because of its advantages of higher thermal efficiency,more compact structure,and ability to start up fast.From the present academy researches,the study on heat transfer characteristics remain as a single-factor test,synthetic evaluation between the influence relationships of tube side and shell side are still lacking.This paper focus on the experimental investigation and numerical simulation of the flow and heat transfer characteristic in SCV in order to supply the theoretical fundamental of the structure design.The main tasks are as follows:The flow pattern and temperature distribution in the shell side of SCV are the key factors affecting the heat transfer performance.The factors such as the inlet temperature of the tube,the bath temperature,the hot air temperature,the air flux and the initial liquid height are the key factors to determine the distribution of the flow field and the temperature distribution of the shell.In order to evaluate the comprehensive action mechanism of various factors,a cold cross model experiment with multiple factors was designed.It can be concluded:(1)By increasing the inlet temperature and bath temperature,the thickness of the boundary layer near the wall and the heat transfer resistance in the boundary layer can be reduced,thus enhancing heat transfer;(2)The influence of air flux and initial liquid height on heat transfer coefficient is mainly reflected in its influence on flow velocity and void fraction in the shell side flow field.The increase of the air flux and the decrease of the initial liquid height will cause the increase of the void fraction,which makes the gas with lower thermal conductivity become the leading medium of heat transfer,weakening the heat transfer;However,the increase of air flux and the decrease of initial liquid height will aggravate the turbulence of the two-phase flow and enhance the heat transfer to a certain extent.Therefore,there is an optimal ratio between the intake and the initial water level,which makes the heat transfer of the shell process most intense.When the air flux is 76 Nm~3/h,the heat transfer performance under the initial liquid height of 435 mm is the best in the experimental parameter range;(3)Since the specific heat of the gas is small,the hot air is cooled rapidly to the water bath temperature after entering the water bath,so its influence on the heat transfer coefficient is relatively small.For the trans-critical heat transfer process of SCV tube,the harsh requirements for experimental conditions increase the difficulty of experimental research.In contrast,numerical simulation is more convenient for the control of heat transfer conditions and can monitor the thermal properties and heat transfer characteristics in tube.In this paper,the heat transfer process of natural gas in a serpentine heat exchanger tube is numerically simulated.It can be concluded:(1)With the gradual increase of natural gas temperature,the heat transfer coefficient increases first and then decreases,and reaches the peak value at the pseudo critical temperature of natural gas.With the increase of the mass flow in the tube,the critical position of natural gas is moved along the tube direction and the overall heat transfer performance in the tube is enhanced.However,when the temperature of the water bath or the inlet temperature of the tube increases,the critical position of the natural gas moves forward along the direction of the tube,and the heat transfer performance in the tube will be reduced.Therefore,in order to maintain the high heat transfer performance in the tube,the water bath temperature should consider the reasonable range of the heat transfer performance inside and outside the tube under the premise of meeting the production needs.(2)The heat transfer enhancement is enhanced by the thinning effect of the two flow on the boundary layer at the elbow,and the heat transfer enhancement effect is increased by 11%when the curvature radius of the elbow is reduced by 50%.The difference of the heat transfer coefficient of the heat transfer tube is derived from the uneven distribution of the tube cross section under the action of the floating lift,so that the heat transfer coefficient is divided according to the law of top<side<bottom.(3)The model fitting of the heat transfer coefficient obtained by numerical simulation shows that the Jackson&Hall model can accurately predict the cross critical heat transfer characteristics of natural gas in the a serpentine heat exchanger tube,with an average error of less than 5%.