| Steam condensing heat exchanger is an indispensable part of the steam circulation system and the steam power plant, whose service life as well as the work efficiency is of great importance to the reliable and economic operation of the thermal system. Kinds of tubes for heat transfer assembled the main component of the steam condensing heat exchanger, its operating performance directly determined by heat transfer coefficient. It means that research on heat transfer enhancement of steam condensation tube has great theoretical value and practical significance.Nowadays, the study on heat transfer of water vapor condensation as an important field of research and application of the heat transfer has got more and more attention of the experts and scholars. With deeper knowledge and understanding of the mechanism for condensation process, plenty of calculation and research on condensation model which made a qualitative and quantitative description of the process of condensation have been brought out. However, most study on enhancing heat transfer in condensation process of steam is confined in the tube outside condensation, while research on in-tube heat transfer characteristics is much less. In terms of heat transfer enhancement measure, most of the researches at home and abroad apply methods of physical modification or external disturbing of the tube, which are difficult in application of engineering practice and thus have few examples for engineering. Until today, systematic study and report on modifying condensation surface (especially the nner surface of the tube) to enhance heat transfer has not much conducted。In view of this, surface modified technique based on the technology of high temperature fusion welding inside furnace is adopted in this article to modify the inner and outer surface of ordinary carbon steel tube and a new type of steam condensation heat transfer element, surface fusion welding tube, is constructed. In this article, the promotion of tube condensation heat transfer performance by fusion welding surface is researched in terms of condensation experiment of inner and outer surface of signal tube and tube bundle, numerical calculation and mechanism analysis of heat transfer enhancement. The main work and the corresponding conclusions are as follows:(1) The outer surface condensation experimental platform of surface fusion welding tube was set up, and the outer surface condensation heat transfer contrast experiment between the surface fusion welding tube and ordinary steel tube was conducted. In the experiment, experimental section of signal tube and tube bundle were built respectively, with the circulating water in the tube as cold source, the water vapor (hereinafter referred to as steam) condensed on the outer surface of two kinds of tubes respectively. Through the calculation of experimental data, the overall heat transfer coefficient and outer surface condensation heat transfer coefficient of surface fusion welding tube and ordinary steel tube is got, and the relation curve of overall heat transfer coefficient k and flow rate of circulating water (0.6-1.3m/s) in tube is obtained, which shows that the overall heat transfer coefficient of surface fusion welding tube is over20%higher than that of ordinary steel tube, the value having a tendency to increase with the increase of flow rate of circulating water. The calculation results also shows that the outer surface condensation heat transfer coefficient h0of surface fusion welding tube is3times of ordinary steel tube, showing the surface fusion welding tube has a significant enhanced effect to the performance of condensation heat transfer. The wall temperature distribution curve of circumferential and length direction of tube is obtained through the method of measuring the wall temperature by welding the thermocouple. The analysis of the curve shows that the surface fusion welding tube has a higher surface heat flux density and is able to withstand the heat load more evenly.(2) The condensation heat transfer experimental platform of tube bundles was set up, the tube bundles was constructed for establishing the condensation heat transfer experiment outside the tube under the vacuum level in four kinds of tube respectively (including the surface fusion welding tube), and the curves of the overall heat transfer coefficient U with the vacuum level in typical Re of different kinds of tubes are established. It is discovered that the heat transfer characteristic of the tube becomes better as the vacuum level gets higher, and the surface fusion welding tube’s heat transfer characteristic is much better than that of the stainless steel tube. The conclusion indicates that it is very potential and worthwhile for the surface fusion welding tube to supersede the various ordinary stainless tube in engineering practice.(3) By combing the experimental data with calculation in the way of Wilson plot technique the change law of the condensation heat transfer coefficient outside the tube with the vacuum level is obtained of the experimental section, finding that the high vacuum level can enhance the heat transfer is the condensation heat transfer coefficient will be improved as the vacuum level improves. This is because the high vacuum level can make the steam gain a faster velocity and flow more homogeneously. Detachment from the tube wall of the condensate is more frequently as it is sourced by the steam, making the condensation surface area become larger. Besides that, the higher vacuum level it is, the less content of the incondensable gas there will be in the experimental section, which can reduce the film resistance of the surface efficiently and enhance the condensation heat transfer.(4) The in-tube condensation experimental platform based on the surface of fusion welding technology was set up by using the mechanism of double tube type counter flow heat exchange. Through modifying inner-surface of tube by high temperature fusion welding craft to construct inner-surface fusion welding tube, the experimental study is carried out on inner condensation heat transfer characteristic of the steam, and comparing with that of the inner condensation steam in ordinary steel tubes, the regularity is obtained that how the Re of circulating cooling water in casing ring gap influences the overall heat transfer coefficient k of two kind of tubes under different inner steam pressure:at negative pressure (0.064MPa), at atmospheric pressure(0.105MPa) and at higher atmospheric pressure (0.185MPa). It is found that the k of the inner-surface fusion welding tube is26percent higher than that of ordinary tube when the inlet steam is of negative pressure, and the strengthening amplitude of overall heat exchange are30and34percent respectively when the pressure of inlet steam closes to and higher than the atmospheric pressure. At the same time, the contrast curve of inner condensation heat transfer coefficient.hi of an inner-surface fusion welding tube and that in an ordinary tube is obtained, showing that the inner condensation heat transfer coefficient hi of an inner-surface fusion welding tube is highest under negative pressure (0.064MPa), and also showing that the enhancing impact is weaken in some degree along with the increase of steam parameters. That is to say, when the heat exchange tube is in vacuum state, the inner-surface fusion welding tube is the best one in promoting the effect of in-tube steam condensation and heat transfer. The result indicates that in the heat transfer situation of in-tube steam condensation, modifying the inner-surface of the heat exchange tube by fusion welding craft can take effect in enhancing heat transfer.(5) Experimental correlative of condensation heat transfer coefficient in ordinary steel tube under the heat flux density qi range of160-430kW/m2and that of condensation heat transfer coefficient in inner-surface fusion welding tube under the heat flux density qi range of180-540kW/m2are derived from regression analysis based on experimental data of in-tube condensation heat transfer experiment, and Simultaneously error analysis was conducted.(6) Microstructure and specific elemental composition of fusion welding surface were studied by metallographic structure experiment and EDS experiment of tube surface. The results indicate that alloy solid solution combination layer (thickness between0.013mm and0.016mm) containing Ni, P, and few Cr element can be implanted to steel surface of main pipe by fusion welding technology. This combination layer is tight, smooth, and its micro hardness is high. These features make the surface fusion welding tube possess good mechanical strength and resistance to corrosion. In addition, because of the compact and smooth cheek of fusion welding surface, the combination layer can be persisted and not easily fall off, by which the high quality of condensate can be guaranteed. (7) The relation between the contact angle9and surface free energy was obtained through energy balance analysis of the steam condensation interface. And then, by combining the results of metallurgical and elemental analysis, it was found that implanted elements of fusion welding surface can reduce the free energy to enhance condensation heat transfer effect. Owing to the high degree of finish and disordered amorphous surface structure of fusion welding tube, the δc>δi,+Hz approximate dropwise condensation criterion of film-dropwise coexisting model which determined by critical film split thickness δc, surface uneven height H2, and average film thickness δi, is satisfied. As a result, further explantation is obtained for the reason why the fusion welding surface has better performance on condensation heat transfer in the experiment.(8) Numerical calculation model of inner surface condensation heat transfer was built based on relevant theoretical methods of numerical computation, whose accuracy is verified with the experiment conditions.. Reynolds averaged Navier-Stokes equation (RNS) is used to describe the flow of steam in the double tube, standard k-ε model to enclose the Reynolds stress term of the governing equation of steam motion, and standard wall functions to handle the near wall. Condensation model of steam in different tube was got based on the experiment data, meanwhile, the grid independence of the simulated results has been checked. The distribution of velocity, temperature and pressure of inner-surface fusion welding tube were analyzed. The heat transfer law and flow regularity of inner-surface condensation shows that:the parameters of steam is axially symmetrical distribution through the centerline of the tube, and that of the cross section become relaxer along the flow direction; the condensation quantity and pressure gradient of ordinary steel tube and inner-surface fusion welding tube are contrasted at the same time, which shows the superior condensation capacity of inner-surface fusion welding tube; the inner-surface condensation law is further researched with the changes of Re of circulating water and inlet pressure of steam. Overall, this dissertation revealed the condensation heat transfer enhancing mechanism of surface fusion welding technology by tube externally and internally research. Performance evaluation system of surface fusion welding tube is further consummated by study of heat transfer characteristic, and all the work have provided theoretical basis and practical guidance for comprehensive engineering application of surface fusion welding tube. |
PDF Full Text Request