【作者】 程延海；

【导师】 程林；

【作者基本信息】 山东大学 ， 机械设计及理论， 2009， 博士

【摘要】 换热普遍存在于生活及各种工业领域中,特别是各种热交换器中,这其中如何提高换热效率一直是人们孜孜不倦追求的目标。除了与换热器的设计方式有关外,污垢及膜状凝结也都不同程度地影响着换热效率。污垢的形成恶化了换热设备的性能,降低了设备的运行效率,增大了能源消耗以及设备维护成本,从而严重地影响了设备的正常运行,造成了巨大的经济损失;而膜状凝结也广泛存在于各种汽水换热器中,对有效传热有着不同程度的影响。这两个问题是传热研究领域中没有得到很好解决的问题。在研究中对于污垢的形成,首先从研究污垢的种类与组成入手,对山东省各地的污垢展开大量调查研究,确定了污垢的主要组成是碳酸钙类污垢的基础研究点。进一步的分析中发现,每个地区的碳酸钙类污垢都是由方解石和文石两大类组成的,不但不同地区的两类碳酸钙组成比例不一样,而且同一块污垢沿厚度方向的组成比例也是不一样的。对典型污垢附着界面的研究表明,污垢与换热面相连接的部分存在一过渡层,过渡层的形成难易及晶体结构决定了污垢附着的难易。一般而言,具有类似晶体结构及晶格参数的过渡层可以加快相似结构污垢的附着,相近的晶格之间更易于相互匹配,该过渡层表现为换热面被氧化或者腐蚀的产物。在污垢的防护方面,根据以上的研究结果,本研究提出了以控制过渡界面形成来延长污垢诱导期形成的思路。具有良好耐蚀性的Ni-P镀层由于具有独特的性能,例如耐蚀性、耐磨性、顺磁性、良好的硬度以及自催化性能而备受关注。在本研究中,则主要研究了化学镀Ni-P镀层的另一潜在的应用,即抗垢性能与改善传热性能。首先针对以前Ni-P镀层成分近似但微观结构不同的报道,通过研究化学镀工艺对镀层组织性能的影响,解明了化学镀工艺参数在控制镀层成分及微观结构方面的影响规律。镀层的化学成分仅与主盐以及还原剂的浓度有关,而与络合剂浓度无关:保持主盐及还原剂浓度不变,改变络合剂浓度直接影响了镀层的微观形貌与镀速,并对镀层的晶体结构产生了影响。随着络合剂数量的增加,镀层中纳米相的含量增加,在微观硬度上表现为硬度的增加。进一步地调整化学镀Ni-P镀层中P的含量,获得了具有不同微观结构的镀层。重点研究了这类镀层中不同微观结构对镀层物性及抗垢性、传热性能影响的规律。主要研究结果总结如下:（1）通过调整工艺参数,采用化学镀的方法可以获得结构形态分别为非晶、混合晶及纳米晶的镀层,其磷含量分别为12.7wt%、10.6wt%以及6.9wt%,对应的纳米晶含量分别为5%、17%以及92%。镀层的硬度随纳米相的增加而增大,对于几乎全部为纳米晶的镀层,最高硬度达到了大约800Hv,这是以前很多Ni-P镀层研究中必须使用热处理才能达到的硬度值,说明了纳米相的强化作用。进一步的热分析表明,纳米晶由于具有更高的稳定性,它的存在可以提高镀层的热稳定性,使晶化温度移向较高的温度。进一步的表面接触角测试表明,这些镀层均不润湿水,具有较低的表面自由能,且不同结构形态镀层表面的表面自由能与镀层结构形态之间没有线性的对应关系。（2）在对镀层进行热处理的过程中,在200℃保温一小时进行热处理后,镀层没有明显的相变,仅有结构驰豫的变化,在硬度及摩擦系数上,与镀态下的镀层变化不大。而400℃保温一小时热处理后,镀层结构形态发生了明显的变化,硬度得到一定程度的提高,所有试样都有明显的Ni₃P析出。在磨损机理上,镀态以及200℃保温一小时热处理时,镀层表现为纯粘着磨损。而400℃热处理时,镀层则以粘着磨损为主,同时出现了部分磨粒磨损,表现出更好的耐磨性。Ni-P镀层的强化机理为析出强化和固溶强化,这取决于镀层中P的含量以及热处理温度的不同。（3）抗垢性能试验表明,与未经表面处理的碳钢、不锈钢以及铜换热表面相比,它们单位面积污垢沉积速度远高于经过镀层表面改性后污垢的沉积速度。而对于改性后的镀层换热表面而言,随着镀层中纳米相含量的增加,抗垢性能有减弱的趋势,非晶镀层表现出最佳的抗垢性能。对于镀层的耐蚀性能,电化学腐蚀实验表明,三种不同纳米相含量镀层的耐蚀性均优于普通碳素钢,随着镀层中纳米相含量的增加,耐蚀性能降低,非晶镀层的耐蚀性能最好。结构形态对耐蚀性能的影响趋势与对抗垢性能的影响趋势是一致的。抗垢性能与耐蚀性能有内在的联系,易于被腐蚀的表面也容易通过“过渡层”的形成而易于污垢的附着。镀层中非晶相的形成比纳米相的形成更有利于改善抗垢性能以及耐蚀性能。在此研究基础上回归出的“起始阶段”污垢诱导期的积聚模型对于预测换热表面生垢的诱导期具有指导作用。（4）镀层表面改性对促进高效传热方面的影响。通过搭建凝结传热试验台,调整蒸汽压力与冷却水流量匹配关系得到了稳定的珠状凝结。结果表明,这些低能表面的镀层具有明显促进珠状凝结的效果,在凝结过程中表现为珠膜共存状态,镀层对换热性能的改善与镀层中非晶相的含量相关,随着非晶相含量的增加,换热效率得到增强,这归咎于表面能的减小。与没有镀层的碳钢管相比,平均换热系数提高了（0.4-0.97）倍。更多还原

【Abstract】 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.更多还原