Experimental Study Of Heat Transfer Characteristics For Hydrocarbon Refrigerants Condensation In Shell Side Of Helically Baffled Shell-and-tube Heat Exchanger

Helically baffled shell-and-tube heat exchanger(HBHX)has the advantages of good comprehensive performance,little fouling and small fluid-induced vibration,and it is widely used as the main condenser in pre-cooling cycle of liquefied natural gas(LNG)plants.In the HBHX for LNG,the hydrocarbon refrigerants flows along the spiral channel,leading to the complex flow pattern in the shell side.The complex flow pattern makes it difficult to predict the of the heat exchanger performance,which may result in the design failure of the heat exchanger.Understanding the mechanism of condensation heat transfer in the shell side of heat exchanger can provide theoretical guidance for the independent research and development of this heat exchanger.However,due to the lack of the published theories,the development of optimization and design of HBHX for LNG is hindered.The objective of this study is to develop the identification criterions of flow patterns and the heat transfer correlations for engineering application.The heat transfer characteristics of hydrocarbons flow condensation in the HBHXs are systematically investigated,and the main results and findings are summarized as follows.(1)An experimental rig is designed and built for testing the heat transfer performance of hydrocarbons flow condensation in the shell side of HBHX,and the accurate measurement of the local shell-side heat transfer coefficient for HBHX is realized under safe and explosion-proof condition.The accurate adjustment of the test conditions is realized by the design of precooling condenser and the bypass cycle;the test section reflecting the structural characteristics of HBHX element are established by adopting the same parameters of baffle helical angle and pipe diameter as the actual HBHX;direct measurement of tube wall temperature is realized by these thermocouples installed in the laser-drilled axial holes,resulting in more accurate measurement of heat transfer coefficient than that obtained by thermal resistance separation method;the efficient gas diffusion scheme in experimental environment is designed and the detectors for local combustible gas concentration are arranged to ensure the operation safety of the experimental devices.(2)The two-phase flow patterns of hydrocarbons in the shell side of HBHXs are observed,the flow pattern map and the identification criterions of flow patterns for horizontal HBHX are established.The experimental objects include horizontal and vertical test sections,and the experimental working fluids contain pure propane and ethane/propane mixtures(ethane mass fraction of 0%~50%).The experimental conditions cover vapor quality of 0.1~0.9 and mass flux of 20~50 kg/m~2 s.The experimental results show that,the distribution of liquid in vertical HBHX is more even than that in the horizontal one,and the stratified flow,stratified-spray flow and spray flow are observed in the horizontal HBHX;the critical vapor quality for flow pattern transition decreases with the increase of mass flux;the critical vapor quality for flow pattern transition increases as the component proportion of volatile component(ethane)increases.The identification criterions of flow patterns by Grant are not available to identify the flow patterns in the horizontal HBHX,and new flow pattern identification criterions are developed.(3)The heat transfer coefficients of hydrocarbons flow condensation in the shell side of horizontal HBHX are experimentally measured.The working fluids include pure propane and ethane/propane mixture(ethane mass fraction of 0%~50%),and the experimental conditions cover vapor quality of 0.1~0.9,mass flux of 20~50 kg/m~2 s and heat flux of 3.0~7.0kW/m~2.The results show that,the heat transfer coefficient of two-phase ethane/propane mixture firstly increases and then decreases with the increase of vapor quality,and the maximum value appears around 0.8;the heat transfer coefficients of two-phase propane and ethane/propane mixture decreases and increases with the increase of heat flux,respectively;as the component proportion of ethane increases from 0%to 15%,the heat transfer coefficient decreases sharply,while as the component proportion of ethane increases from 15%to 50%,the heat transfer coefficient rarely changes.The existing correlations for condensation on horizontal tubes are not available for horizontal HBHX.A new heat transfer correlation is developed based on Belghzai correlation,and the predicting deviations are within±10%for 91%experimental data of propane and within±25%for95%experimental data of ethane/propane mixture,respectively.(4)The heat transfer coefficients of hydrocarbons flow condensation in the shell side of vertical HBHX are experimentally measured.The working fluids include pure propane and ethane/propane mixture(ethane mass fraction of 0%~50%),and the experimental conditions cover vapor quality of 0.1~0.9,mass flux of 20~50 kg/m~2 s and heat flux of 3.0~7.0kW/m~2.The results show that,the average heat transfer coefficient of two-phase propane in vertical heat exchanger is 15%~55%lower than that of horizontal one,while the heat transfer coefficients of two-phase ethane/propane mixture are close in vertical and horizontal heat exchanger;the heat transfer coefficient of two-phase ethane/propane mixture firstly increases and then decreases with the increase of vapor quality,and the maximum value appears around 0.8;the heat transfer coefficient increases with the increase of heat flux for both two-phase propane and ethane/propane mixture;as the ethane mass fraction increases from 0%to50%,the heat transfer coefficient of ethane/propane mixture rarely changes at vapor quality of 0.2~0.6,while it decreases at vapor quality of 0.8~0.9.In this thesis,an experimental rig is designed and built,and the investigation on the heat transfer characteristics of hydrocarbons flow condensation in the shell side of HBHXs is performed.The flow pattern identification criterion and the heat transfer correlations are established to provide theoretical basis for the design of HBHX used in LNG plants.It is helpful to improve the energy utilization efficiency of LNG condensation equipment,and to reach the goal of energy saving and emission reduction.