| Heat transfer phenomenon is widely existed in the industrial fields, especially in all kinds of heat exchangers. People have made great efforts to improve the heat transfer efficiency by way of changing the heat exchager design. However, fouling and film condensation also affect the heat transfer efficiency. The formation of fouling on the heat transfer surfaces deteriorates the heat exchanger performance, causes enormous economic losses by decreasing operating efficiency, increases the energy consumption and maintenance costs. The film condensation appears commonly in lots of heat exchangers and weakens the heat transfer efficiency because of its higher heat resistance. Today, fouling formation and film condensation are still the problems in heat exchanger research field that are not solved sufficiently.In this paper, the fouling types and compositions of Shandong provience were investigated firstly and ascertained the calcium carbonate fouling as basic research objects. Furthermore, it was found that the calcium carbonate fouling including calcite and aragonitein for all the testing samples. The fouling composition contents were affected not only by the different areas but also by the thickness of the fouling layer of same sample. The research on fouling adhesion interface for typical fouling layer indicated that there was a "transitional interface" which liked a "bridge" to connect the matrix and fouling. The fouling adhesion was related to the formation and microstructure of the "transitional interface". In generally, the fouling adhesion will be easy if the "transitional interface" and fouling are alike in crystal structure and lattice parameter. The lattice of the "transitional interface" layer may easily match with the similar lattice of fouling. The transitional interface is the corrosive or oxidative product, and its crystalline structure is different from the matrix.According to the results above, this study put forword prolonging the fouling induction period by inhibiting the formation of "transitional interface". The Ni-P deposit has attracted much attention due to its unique properties, such as corrosion resistance, wear resistance, paramagnetic characteristic, hardness and electro catalytic activity of hydrogen evolution. Another potential application of Ni-P deposit about anti-fouling and enhancing heat transfer property used in heat exchanger were studied in this paper. It is reported that the Ni-P deposits have different microstructure in despite of the similar composition in the previous literatures. By studying the electroless plating processing, the effect of electroless plating processing parameters on the deposit composition and microstructure was clarified. The experimental results indicated that the composition of Ni-P deposits was decided mainly by main salt (nickel sulphate) and reducing agent (sodium hypophosphite), the amount of complexing agents did not have important influence on phosphorus content. However, the amount of complexing agents could affect the surface morphology, plating rate and microstructure of electroless Ni-P deposits. With the increasing of the amount of complexing agents, nanocrystalline phase content became increased, thus increased the microhardness of the deposit. The electroless Ni-P deposits with the various microstructures could be obtained by further altering the phosphorous contents of the deposits. The more attention was payed to the effect of different microstructure on the physical property, anti-fouling property and heat transfer property of these Ni-P deposits. The main research conclusions in this paper were as follows.(1) Three different kinds of microstructure, such as nano-crystalline, amorphous and co-existence of both, could be obtained by adjusting the process. The content of phosphorus of these samples was 12.7wt.%, 10.6wt.% and 6.9wt.%, respectively. The corresponding nanocrystalline contents were 5mass%, 17mass% and 92mass%, respectively. The microhardness of the deposit became increased with the increase of nanocrystalline phase. For the sample which contains most nanocrystalline phase, the value of hardness was about 800Hv, which was the value that most Ni-P deposits were heat treated. This indicated the strengthen action of the nanocrystalline phase. The differential thermal analysis results also indicated that the formation of nanocrystalline phase could improve the thermal stability of deposit and crystallization temperature shifted higher value. Further results about interfacial tensiometer analysis indicated that these deposit were hydrophobic with lower surface energy, and no linear corresponding relation between the surface energy and the microstructure.(2) As for the effect of heat treatment on property of the Ni-P deposit, the heat treatment at 200°C for one hour did not change obviously the deposit microstructure except for the structure relaxation, and the hardness and friction coefficient did not have significant variations than that of the as-deposited sample. After being heat treatment at 400°C for one hour, the hardness and wear resistance of the deposit were improved due to the precipitation of Ni3P. The SEM observation indictaed that the wear mechanism was typical adhesive wear for the samples of as-deposited and being heat treated at 200°C. However, the wear mechanism was changed to the abrasive wear plus little adhesive wear when the Ni-P deposits were heat treated at 400°C for a hour. The deposits showed better wear resistance property after being heat treated at 400°C for one hour. The strengthening mechanism for Ni-P deposit was precipitation strengthening and solid solution strengthening, which was depended on phosphorus content and heat treatment process.(3) The results of anti-fouling experiments showed that Ni-P deposits could greatly inhibit the adhesion of crystallization fouling in comparison with uncoated copper, stainless steel and carbon steel surfaces. The further experiments indicated that the anti-fouling properties had a decreasing tendency with increase of the amount of nanocrystalline phase in the Ni-P deposit. When the structure of the whole matrix of Ni-P deposit was almost amorphous phase, the Ni-P deposit exhibited the best anti-fouling properties. Electrochemical corrosion experiment resulted that all the Ni-P deposits had better corrosion resistance property than that of uncoated carbon steel and the corrosion resistance property showed the decreasing tendency with increase of the amount of nanocrystalline phase in the Ni-P deposit. The amorphous deposits had the best corrosion resistance.The trend about the effect of microstructure on the anti-fouling property was consistent with that of corrosion resistance. Anti-fouling and corrosion resistance were intrinsically linked, that was, the easy eroded surface was also easily adhered by fouling through forming the "transitional interface" which connected the fouling and surface of deposit. The formation of amorphous phase in deposit was beneficial to the improvement of anti-fouling and corrosion resistance properties in comparison with that of nanocrystalline phase. The fouling accumulation model of induction period was regressed.(4) The effect of Ni-P deposit on promoting condensation heat transfer was investigated by the condensation heat transfer testing. By regulating the steam pressure and flow velocity of cooling water, the steady dropwise condensation could be obtained. The experimental results revealed that dropwise condensation was formed on the Ni-P deposits which have lower surface free energy. It was found that the dropwise and film coexisting condensation on the Ni-P surface, but only film condensation was observed on the carbon steel surafce. Experimental results also revealed that the improvement of deposit on heat transfer was related to the amount of amorphous phase. That is, the heat transfer efficiency was improved with the increase of amorphous phase because of the decrease of surface free energy value. The overall heat transfer coefficient of Ni-P deposit was increased by 40-97% than that of uncoated surface. |
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