Fluid Flow And Fouling Characteristics In A Helical Coil Exchanger

In recent years,helical coil heat exchangers have been widely used in petro-chemistry,air separation,chemical and other fields due to its compact structure and high heat exchange efficiency.However,the research on the helical coil heat exchanger has not yet been deepened because of its complicated structure,special manufacturing methods,and the difficulty to detect the microscopic flow state inside the tube Therefore,this work conducted a comprehensive study on the fluid flow and fouling characteristics in a helical coil exchanger through experimental and simulated methods.It will show guidance for the design of efficient helical coil heat exchangers.The main contents are as follows1.Taking water as the medium,effects of structure parameters including coiling diameter,coiling angle and tube diameter on the pressure drop were investigated experimentally.The results showed that under the same conditions,the pressure drop of the helical coil was larger if the coiling diameter or the tube diameter of the helical coil was smaller.The coiling diameter and the tube diameter had different effects on the pressure drop at different flow rate ranges.At low flow rates(u<0.6 m/s),the influence of the coiling diameter and the tube diameter on the pressure drop was small But at high flow rates(u>0.6 m/s),the coiling diameter and the tube diameter had a significant influence on pressure drop.Under the experimental conditions used in this work,the influence of the coiling angle on the pressure drop was small.By developing a characteristic value ?=(1/Re)0.5(d)0.4(D)0.2,a sectional prediction model was also established for the pressure drop in the helical coil:?P/?L=mRe"(d/D)1(pu2/d).When?>0.001,m=2.8,n=-0.41,l=0.42.When ?<0.001,m=3.88,n=-0.38,l=0.60.Based on this model,the relative deviation of the pressure drop between the calculated and the measured was less than ± 10%.2.A quantitative method was proposed for the characterization of secondary flow intensity in the helical coil.The larger the variance o2 calculated from residence time distribution,or the smaller the ratio S calculated from axial velocity to the total at the exit of the helical coil,the stronger the secondary flow.The influence of structure parameters including coiling diameter,coiling angle and tube diameter on the secondary flow was further investigated by this method.It was found that the secondary flow intensity became greater as the coiling diameter or the tube diameter decreased,and the coiling angle had no significant effect on the fluid flow in the helical coil.There were boundary layer action zone,secondary flow action zone,and secondary flow limit zone with the increasing flow rate,which was corresponded to the phenomenon that ?2 decreased firstly,then increased and finally kept stable with the increase of flow rate.The smaller coiling diameter or the tube diameter would induce a smaller transition flow rate from the boundary layer action zone to the secondary flow action zone,and the secondary flow limit zone was more likely to occur in a helical coil with the smaller tube diameter.The CFD simulation results showed that the S in the boundary layer action area was about 1,and the S decreased sharply in the secondary flow action area.3.Based on the principle that the solubility of calcium sulfate solution decreases when heating,the CFD simulation method of the crystal fouling process in the helical coil was established.The influence of the structural parameters and operating parameters on the fouling was investigated.The results showed that the fouling was easier to appear when the coiling diameter and tube diameter was smaller under the same heat transfer area.The coiling angle almost had no influence on the fouling inside the tube.When increasing the flow rate,the erosion of the fluid on the fouling was intensified,thus it can reduce the deposition of fouling in the helical coil effectively when increasing the flow rate.Compared with the straight tube,the fluid temperature in the helical coil was higher,then the fouling rate was larger,when the heat exchange area was the same and the wall temperature or heat flow was the same.But when the heat exchange area and the heat exchange amount were the same,the fouling rate in the helical coil was smaller due to the higher erosion rate induced by secondary flow...