Research On The Characteristics Of Combined Fouling Of Particulate And Precipitation Fouling In Internally Helical Enhanced Tubes

Fouling of heat transfer tubes has become one of the major concerns in industry.It causes diminution of heat transfer coefficients and service lifespan,and potential safety troubles as well as ultimately leads to significant increases in energy consumption and operation costs,which has already received users' concern.Compared with smooth tubes,internally helical enhanced tubes,with the advantages of high heat transfer coefficient,have become one of the most widely used units of heat transfer tubes in shell-and-tube segments.For years,as the enhancing techniques are used,it is urgent to adopt new technologies to test and study the formation mechanism of fouling on enhanced tubes,so as to acquire comprehensive cognition of fouling and build the anti-fouling basic theories.To this end,this study focuses on the common combined fouling of particulate and precipitation on surface of the condenser,and reveals interaction mechanism in the combined fouling from both macroscopic and mesoscopic perspectives.Firstly,the author designed a shell-and-tube condenser for testing fouling on heat transfer tubes,based on which the experimental set-up is established.Moreover,the effect of water velocity and heat flux on the total heat transfer coefficient of heat transfer tubes was studied.Based on that,the author established the mathematical relation,and put forward the modified formula of fouling thermal resistances.The results show that the modified formula can reduce the impact of two influence factor on fouling thermal resistance which thus improves the accuracy of the test,and lays the foundation for the future series of studies.Secondly,the experimental researches of the particulate fouling,precipitation fouling and combined fouling under different heat transfer tubes have been conducted.Results show that particulate foulant in the cooling water is used as “slow-releasing”composition,which could extend the induction period in the formation process of the combined fouling.Also,once the fouling grows,particulate foulant and precipitation foulant promote each other in the fouling process,which produces a higher asymptotic fouling resistance than that of any single precipitation or particulate fouling.After scaling down,the variation profile of the combined fouling quite resembles the fouling process in a real system.Thus,this could be considered as a research method for qualitative analysis instead of the long-term test of combined fouling.Thirdly,a fouling model of internally helical enhanced tubes which contains multiple parameters was developed in this study based on the long-term fouling data,aiming to improve previous fouling models.This new model is expressed in the form of continuous function,which overcomes the shortcoming of piecewise formulation of previous model.With clear physical symbols,it provides a better modeling method for the follow-up study in fouling thermal resistance of enhanced tubes.After that,based on the experimental fouling thermal resistance data,the sticking probability and deposit bond strength of fouling were obtained by calculation and analysis.For the first time,the interaction mechanism of combined fouling was analyzed based on the specific data of two parameters,and the influence of deposition and denudation process in three kinds of heat transfer tubes was explained.The results indicate that particles of Si O2 provide extra nucleation sites for precipitation of Ca CO3 in the cooling water.The enhanced tubes appeared to be neither easy to stick nor to denude after fouling than the plain tubes.The enhanced tube with small interval between two ribs was more likely to form particulate fouling than precipitation fouling.And then,the motion characteristics of continuous phase of working water and discrete phase of particle were supplemented to analyze the formation of combined fouling from both macroscopic and mesoscopic perspectives.The computationalfluid-dynamics method was applied to establish the numerical model of the internally helical enhanced tubes,and the flow and heat transfer process of liquid-solid twophase working water in tubes was simulated.Based on the experimental conditions,the flow characteristics in the heat transfer tubes were studied.Also,the differences of fouling are explained by the velocity field,temperature field and concentration field caused by particle motion in different kinds of heat transfer tubes.The results indicate that fouling is more likely to deposit on the back of the rib and between two adjacent ribs due to the influence of the spiral internal ribs.In the near-wall area,the temperature of plain tube is higher than that of enhanced tubes due to its poor heat transfer performance,which leads to the more foulant precipitation.Also,gravity plays a greater role in the deposition of particulate foulant in plain tube.For internally helical enhanced tubes,the trajectory of particles is consistent with the spiral internal ribs and contracts to the central axis of the heat transfer tubes.As a result,the proportion of particulate fouling in the combined fouling of internally helical enhanced tubes is smaller than that of plain tube.Finally,A performance evaluation criterion considering the influence of fouling was proposed,which combined the enhancing techniques and fouling that were supposed to be one research field.The accuracy and feasibility of this criterion were proved in many directions with theoretical analysis and experimental data.Also,some limitations of this performance evaluation criterion in practical application were proposed.In practical engineering,this performance evaluation criterion can be provided for performance research of the internally helical enhanced tubes.In this study,through the combination of experiments,theoretical analyses and numerical simulations,the growth characteristics and interaction mechanisms of the binary combined fouling on the liquid-side of the internally helical enhanced tube have been studied and discussed in depth from macroscopic and mesoscopic aspects.These achievements can enrich the research content of the fouling in heat transfer tubes,and provide theoretical support for solving the fouling problem discovered by the long-term limit of enhancement techniques at the same time.