| Fly-ash deposition and erosion exist widely in the heating surface of boiler.The essence of deposition and erosion is the particle collision,and they often occur at the same time,and inhibit each other.Ignoring their relationship will affect the reliability of the research results on deposition and erosion,resulting in failing to locate the deposition and erosion of the heating surface accurately.This will bring some difficulties to the design and operation of the boiler.Based on the dynamic theory of particle impact,the models of particle deposition and erosion are established respectively,and the coupling mechanisms between deposition and eroison of heating surface under cold and hot states are studied.The main research contents are described as follows:(1)A new soft-particle DEM model of mesoscopic particle impact is proposed,which incorporates adhesive,elastoplastic and microslip behaviors.The elastic-plastic transition zone is introduced into the mesoscopic particle impact,and the normal force model of the elastic-plastic loading and the elastic-plastic unloading are established by analyzing the continuity,smoothness and monotony of the normal force.Then the tangential force model is established by using the microslip theory.Based on the model,it is found that the model predictions,such as the restitution coefficient,the angular velocity,the rebound angle and the other motion characteristics,are basically consistent with the experimental results,which are better than that of other models.The model can well reflect the variation of normal pressure in the process of collision.In the loading stage,after the pressure of the contact center reaches the elastic compression threshold,the increasing speed of pressure in the contact zone gradually slows down,and the pressure distribution gradually tends to be uniform.In the unloading,when the pressure gradually decreases,the pressure distribution will change the uniform distribution to the parabolic distribution gradually.The model can accurately predict the deposition characteristics of particles,and the critical adhesion velocity decreases linearly with the increase of particle size in the logarithmic coordinate.(2)A new mechanistic model for abrasive erosion is proposed using the soft sphere model.Based on the trace method,the cutting removal and the deformation damage removal are calculated by using the crater sizes.Then,by comparing with the existing experimental results,the irrelevance between the empirical coefficient and the impact conditions in the model is verified,and the relationship between the empirical coefficient and the hardness ratio is further analyzed.The influence of particle breakage on the wear is mainly studied.Based on the model,it is found that the present model has the high applicability and accuracy in predicting the maximum erosion angle.The empirical coefficients has nothing to do with impact conditions such as the impact angle and the impact velocity,but has a great correlation with the hardness ratio.When H~~<0.15,the coefficient n in the model decreases with the increase of hardness ratio,and when H~~>0.15,the coefficient n is stable at 0.5.The cutting erosion factor _Cc～increases with the increase of hardness ratio in double logarithmic coordinates,and the deformation erosion factor c～_D also increases with the increase of hardness ratio.Due to the influence of particle breakage,the tangential velocity exponent f _Cand the normal velocity exponent f _D show two kinds of regular distribution.When H~~<0.15,f _C shows a linear growth trend,while H~~>0.15,f _C remained unchanged,and f _C ranges from2.0 to 2.7.When H~~<0.15,f _D is almost constant at 2.8,while H~~>0.15,f _D decreases.In addition,the correlation formulas of n,_Cc～,and c～_D are established,which can be applied to the prediction of erosion rate..(3)A dual-morphology prediction method of"particle cluster+moving grid"is proposed,and the DEM parcel is used to accelerate the deposition calculation time.Based on Computational Fluid Dynamics(CFD)and Discrete Element Method(DEM),the fly-ash deposition,erosion and morphologies of the tube under the cold condition are predicted.Then,the effects of inflow velocity and particle size on deposition and erosion are analyzed.It is found that the morphology prediction can realize the synchronous simulation of fly-ash deposition and erosion,which is in good agreement with the experimental results.Using a certain particle size to fill the DEM particle bag not only does not change the rebound characteristics of the sedimentary body,but also saves the calculation time of deposition.The erosion rate is almost proportional to the 3.4 power of particle size,and the 2.7 power of particle velocity.The deposition amount on the tube surface increases with the increase of time,but this increase trend gradually slows down.Meanwhile,the deposition time decreases with the increase of particle impact velocity,while the erosion amount on the tube surface increases linearly with the increase of time.(4)By modifying the material properties of the existing model,the theory of particle deposition and erosion under thermal state is developed.Then,the coupling mechanism of deposition and erosion on the tube surface under thermal state are studied.It is found that the model can accurately predict the rebound characteristics and critical adhesion velocity under the action of temperature,and has a high accuracy for predicting the particle deposition and erosion characteristics on the heating surface.With the increase of inlet temperature and surface temperature,the deposition on the tube surface increases,but the erosion characteristics is in contrast.With the increase of inlet temperature,the material loss decreases.With the increase of the tube surface temperature,the material loss increases.The coupling effect of particle deposition and erosion is obvious at medium velocity,and there is a competitive relationship between particle deposition and erosion.Reducing the temperature of the heating surface,as a way of temperature control,should be the priority to reduce the ash deposition and erosion.(5)By calculating the erosion morphology,the influence of long time flying-ash erosion on the morphology of heat tube is studied,and the influence of morphology on the thermal characteristics of the heat tube is analyzed.It is found that when the collision position is relatively small,the collision frequency and the impact angle decline as time going on.The results for the collision position are in contrast,with strong impact frequency and impact angle on the rise.With time incresed,the peak value becomes smaller at the saddle-shaped erosion rate curve,and the position of peak value moves towards the higher angle.Meanwhile,the outside of the saddle-shaped curve expands towards the higher angle,namely the higher erosion rate at the higher angle,or that the erosion loss becomes more serious.With the increase of time,the total erosion rate increases,but the total erosion quantity is decreases.The Nusselt number grows slightly and the trend grows faster as time goes on.The analysis of tube temperature characteristics will be helpful to reveal the mechanism of tube leakage under the long-term erosion. |
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