Influence Of Behavior Of Metallic Heat Transfer Surface On The Formation Characteristics Of Crystalline Fouling

Fouling would often be referred to as "the major unsolved problem in heat transfer field". Its complicacy and uncertainty, especially lacks of consensus at some common problems, have hampered the development of the fouling research to a great extent. Therefore, in the introduction, via literature review of the research status both at home and abroad, this thesis expressly summarized four vague but fundamental questions in the literature of the recent twenty years, and expounded on them from the more original scientific questions. Respectively, they are the questions including the crystalline nucleation route, the debates on the classification between crystalline fouling and particle fouling, the components of the adsorption force of crystalline fouling, and the limitation of surface free energy used to evaluate the surface anti-fouling effect. From the global properties of the fouling formation, this thesis focused on the research gap neglected by these four fundamental problems, that is, the effect of surface behavior on the fouling properties. Through fouling experiments and further analysis, this thesis systematically discussed the effect of surface roughness, surface modification, surface free energy and surface corrosion on the formation and adhesion of the calcium carbonate crystalline fouling on the metallic heat transfer surfaces.This thesis first studied the effect of roughness on fouling adhesion on the AISI 304 stainless steel surface; found that the adhesion of fouling does not have a simple linear relationship with the surface roughness of samples. The sample with the middle roughness has the slight poor anti-fouling property, while the polished surface with lowest surface roughness can not obviously improve anti-fouling property. The effect of surface roughness on the fouling adhesion is essentially its effect on the formation of "transitional interface" via corrosion or oxidation. The anti-fouling property of surface is related to the corrosion resistance of the matrix. The surface that has poor corrosion resistance also has poor anti-fouling property. But this effect can only occur in the induced period of fouling formation, and this effect is limited for improving the anti-fouling property.And then in the respect of the behavior of the most common metal materials, measurement of surface free energy and anti-corrosion behavior of carbon steel, copper and 304 stainless steel was performed respectively. The phase constitutions of the precipitation at different time were tracked by using X-ray Diffraction (XRD) method during the 28 h rapid fouling adhering experiments. The change of surface morphology was also observed before and after the fouling adhering experiment. Through weighting method, the 28 h fouling weight graph was drew, and the reasons for different performances of these three most common metal materials were discussed respectively. The results showed the long-term fouling growth performance is not necessarily related to metal surface free energy under clean conditions, but is closely associated with surface corrosion. The effect of surface corrosion on the formation and adhesion of crystalline fouling can be considered from two key aspects, one is the anti-corrosion behavior, which decides the depth and size of corrosion pits, the performance of the "transitional interface", and the amount of metal ions released from the surface; the other is these released metal ions which will influence the crystal phases and the mechanical properties of the fouling, and ultimately on the fouling adhesion on the surface. The analysis found that the weighting method is not sutiable to carbon steel because the anti-corrosion behavior of carbon steel is too bad that the corrosion weight loss should not be neglected and hence the precise fouling weight gain could not be obtained. For the copper and stainless steel, the latter gained more weight in the first half of the fouling experiment, but the former increased the fouling growth speed substantially in the second half. The proposed theory in this thesis thought that this phenomenon isrelated to the different ions e released from the metal matrixes, which would influnece the fouling properties.In order to identify how significant the released metal ions exert influence on crystallographic characteristics of the fouling, this thesis studied the effect of different sorts and different concentrations of metal ions on the micro-morphology, the crystalline structure, the grain size and the strength of the calcium carbonate fouling through rapid fouling experiment.by adding some surface metal ions to the reaction solution in The ions included copper ions, nickel ions, ferrous ions and ferric ions. On one hand, the analysis of crystalline phases indicated that the these metal ions have the same role roughly, that is, the relative content of aragonite increases and that of calcite decreases with the increase of the concentration of the metal ions. On the other hand, the grain size decreases successively in the order of iron ions, nickel ions and copper ions., The decrease of grain size can induce two effect, the one is that the crystalline growth speed slowly and the fouling is harder to accumulate successively, the other is the yield stress and the hardness increase and the denudation of the fouling becomes more difficult successively.In other words, if these metal materials were used for a long term, fouling on the latter one in this order would grow more quickly than that on the fronter materials.On the whole, nickel ions play a positive role in the anti-fouling effect of the surface materials. This finding prompted us to develop electroless plating nickel-based alloy coatings on the carbon steel matrix to prevent corrosion and inhibit fouling adhesion. So far, we have exploited a series of high-efficiency and long-life formulas and technological process. Three formulas were selected to study their performances in this thesis. Measurement of surface free energy and anti-corrosion behavior of these three nickel-based alloy coatings was performed respectively. The components of the precipitation at different time were tracked by using XRD method during the 28 h rapid fouling adhering experiments. The change of surface morphology was also observed before and after the fouling adhering experiment. The 28 h fouling weight variation graph was also drawn. The results showed the long-term fouling growth performance is not necessarily related to the metal surface free energy under clean conditions, but is closely associated with surface corrosion, that is, the anti-fouling property of these nickel-based coatings increases with their anti-corrosion property. In conclusion, the influence of the metal surface behavior on the fouling formation characteristics is not an isolated issue. Compared with the surface free energy, the effect of surface corrosion is more important. The anti-corrosion property and the released ions from metal surface can directly affect the interfacial bonds between fouling and the metal matrix, as well as the crystalline structures, strength and the denudation property of the fouling. These factors can ultimately affect the long-term performance of the fouling growth on the metallic heat transfer surfaces.