Fractal characterization of heat exchange tube surfaces and numerical simulation on ash deposition characteristics
Objective The surfaces of heat exchanger tube bundles become increasingly rougher due to prolonged use.To explore the deposi-tion characteristics of fly ash particles on heat exchanger tube surfaces with varying roughness and the mechanisms affecting this process,the deposition behavior of fly ash particles of different sizes under actual working conditions is simulated.The study aims to reveal the intrinsic relationship between particle deposition and surface roughness.Methods Tube bundle surface models with different roughness levels were developed using fractal theory and the improved Weierstrass-Mandelbrot(W-M)function.The surface models were generated using Matlab,and mesh generation was con-ducted using ICEM software.Numerical simulations were performed in Fluent,combined with a user-defined function(UDF).The renormalization group(RNG)k-epsilon(k-ε)turbulence model was employed to simulate the incompressible turbulent flow,and the computational fluid dynamics-discrete phase model(CFD-DPM)was applied to track and analyze the motion of fly ash particles of different sizes in gas-solid two-phase flow.Particle deposition behavior was modeled using energy conservation and critical velocity theory.Results and Discussion Increasing surface roughness of the heat exchanger tube led to a significant increase in both inlet pres-sure and pressure drop,indicating increased energy loss during fluid flow.This was attributed to the enhanced internal friction caused by higher surface roughness.Greater surface roughness also intensified fluid velocity and turbulent kinetic energy.Reducing near-wall fluid velocity would in turn promote particle deposition.Rough surface peaks influenced fluid flow,generat-ing vortex structures on both windward and leeward sides,which further enhanced fluid-wall interactions.As surface roughness increased,the particle deposition rate rose significantly,especially for small particles with diameters of 1-3 µm.These particles showed higher deposition rates and wall capture efficiency due to their low inertia.As particle size increased,the number of particle-wall collisions and the overall collision enhancement rate first increased and then decreased.Compared to smooth sur-faces,the rough surfaces with an Rq of 1.50 mm exhibited a higher number of particle collisions,indicating the significant impact of surface roughness on particle collision behavior.Conclusion The deposition of fly ash particles and the formation of rough surfaces on tube bundles exhibit a positive feedback loop.Particle deposition gradually makes the tube wall surfaces rougher,which in turn further accelerates particle deposition.Vortices generated by rough surfaces increase fluid-wall friction,resulting in greater energy loss,increased turbulent kinetic energy,and reduced flow velocity,thereby intensifying pipe wear and corrosion,ultimately shortening the equipment's lifes-pan.Understanding the deposition mechanisms of fly ash particles under actual operating conditions allows for the prediction of deposition rates and distribution.This study provides a theoretical basis for optimizing tube bundle design and maintenance strat-egies,and offers new data and theoretical support for gas-solid two-phase flow and particle deposition theory.
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