Research On Heat Transfer Characteristics Of Steam-air Mixture Flowing Longitudinally Over Vertical Coating Tubes

The waste heat is a heat resource that can not be fully utilized in industrial production and has been paid more and more attention to in recent years.The waste heat is directly discharged into the natural environment,which not only can cause great loss of energy,but also will bring some negative effects to the environment.The waste heat in flue gas is one of the most important types of waste heat resources,and the low-temperature flue gas waste heat is usually ignored because of its small heat transfer temperature difference and low temperature corrosion problems.For the low-temperature flue gas,using a new type anti-corrosion coating layer on the heat ex-changer surface,on the one hand,can improve the boiler heat efficiency by recovering the large latent heat of condensate,on the other hand,can alleviate the the acid corrosion behavior on heat ex-changer tube surface that will ensure the safe operation of equipment and increase the service life of the heat ex-changer,which has great significance to realize the goal of energy saving and emission reduction.Proceed from the theoretical analysis and engineering practical consideration,the convection condensation heat transfer characteristics of steam-air mixture flowing longitudinally over copper tube,Ni-P coating tube and PTFE(polytetrafluoroethylene)coating tube were studied theoretically,experimentally and simulatively in this paper.The mass fraction of water vapor in steam-air mixture is about 6%?16%.Firstly,the heat and mass transfer of steam-air mixture was analyzed on the basis of Colburn-Hougen convection condensation heat transfer model,which pointed out that at the gas-liquid interface,the non-condensable gas layer is the main resistance of heat and mass transfer process.In addition,from the perspective of molecular diffusion,the influence factors of convection condensation heat transfer coefficient were deduced and made dimensionless to acquire the experimental correlation.Secondly,the convection condensation heat transfer composite experiment platform was designed to simulate the process of flue gas waste heat recovery and study the copper tube,Ni-P coating tube and PTFE coating tube's convection condensation heat transfer characteristics.The condensation states of water vapor on the different tube surfaces were also studied.Based on multiple linear regression analysis of the experimental data,the experimental correlations about convection condensation heat transfer of the three tubes were obtained respectively.The relative errors between theoretical values from the experimental correlations and experimental values are within± 15%,which indicates that the experimental correlations have high relevance and accuracy.Finally,the simulation about convection condensation heat transfer process was achieved by using the Component Transportation Model and UDF condensation program in Fluent software.The result agrees well with the experiment and the maximum relative error between simulative value and experimental value of convection condensation heat transfer coefficients is around 25%.The experimental results show that the heat transfer coefficients increase with increasing of water vapor mass fraction and mixed gas flow rate.Compared with the copper tube,the condensation heat transfer coefficients of Ni-P coating tube and PTFE coating tube increase by 1.9%to 25.9%and 4.4%to 29.2%,and the convection condensation heat transfer coefficients increase by 12.2%to 24.2%and 16.4%to 35.9%.The PTFE coating tube has the best heat transfer performance.Film-wise condensation occurs on copper tube surface,but for PTFE coating tube,the condensation state is drop-wise,and the condensation state on Ni-P coating tube surface is the coexistence of both film-wise and drop-wise.It is worth noting that,compared with the lower water vapor concentrations,the enhancement on the heat transfer of coating layer shows a marginal effect under the condition of higher water vapor mass fraction.Therefore,in practice,it is necessary to determine whether to use surface modification heat transfer enhancement technology or not according to the specific conditions and techno-economic comparison.In addition,it can be seen from the simulation results that the heat transfer process and condensation process are mainly concentrated in the first half of the heat ex-changer.In the second half,the temperature drop of the steam-air mixture is reduced,and the water vapor mass fraction tends to be stable.