Study On The Mechanism Of Fouling In Stainless Steel Pipe And The Effect Of Heat Transfer On The Surface Of Stainless Steel Pipe

Fouling is one of the major factors affecting energy efficiency.Therefore investigating the fouling process and growth mechanism of fouling can effectively improve the energy using efficiency,ease the energy crisis,save energy and reduce emissions,and protect the environment.In this paper,experimental and numerical simulations are used to study the growth of calcium carbonate crystals fouling on the heat transfer surface and investigate their effects on heat transfer characteristics.The morphology,size and distribution of calcium carbonate crystals fouling on the heat transfer surface in different growth stages of fouling are observed.The growth law of calcium carbonate fouling on the heat transfer surface is analyzed.The experimental study is carried out using the double pipe heat exchanger with 316L stainless steel tube.100 mg/L calcium carbonate solution is used as the working medium,the working fluid inlet temperature is kept at 55±0.5°C and the hot water inlet temperature is kept at 77±0.5°C,and the flow rate in the tube is maintained at 0.13 m/s?Re=1800?.The experimental results show that the change of the total heat transfer coefficient with run time likes:firstly increase and then decrease and followed by an increase,while the change of the fouling heat resistance is exactly the opposite to the heat transfer coefficent with run time.In the whole fouling process,the thermal resistance can be divided into three stages:positive thermal resistance stage,negative thermal resistance stage,and fouling progressive growth stage.In the fouling process,the number and size of crystals gradually increase with time and this increase is over until the crystal completely covers the heat transfer surface,especially in the gravitational growth stage.Second nucleation occurs on the surface of the fouling layer which increases the fouling rate rapidly.The removal rate increases slowly with time,the deposition rate and removal rate eventually reach a dynamic equilibrium,and in this case the thermal resistance of the fouling is asymptotic change.In fouling process,the calcium carbonate morphology is transformed from initially unstable particles to stable calcite.Based on the statistical results of the distribution of crystallization fouling on the heat transfer surface,a physical model and a mathematical model are established to study the effect of calcium carbonate fouling on the heat transfer characteristics.The numerical simulation results show that the presence of calcium carbonate crystals increases the disturbance of the fluid and enhances the convective heat transfer.The larger the crystal size,the greater the disturbance effect,and the stronger the effect of heat transfer enhancement;at the same time,due to the poor thermal conductivity of calcium carbonate crystal,heat transfer is deteriorated with increasing the size of the crystal,the surface area occupied by the crystals increases,and the resistance on heat transfer gradually increases.The time-varying Nu/Nu₀ of the simulation is the same as the experimentally measured K/K₀ variation over time.