Characteristics Of Sewage Flow And Heat Transfer In Intaking Water And Heat Transfer Process Of Sewage Heat Pump

Sewage heat pump technology with a significant role of energy conservation and emissions reduction has been widely applied in the heating area of northern China. However, up to now, there has an enormous of difficulties in the fetching and receding water process for the design of sewage heat pump system (SHPS), such as uncertainty in the physical property data of sewage water and randomness of fluid flow and heat transfer in the heat exchanger, because it is the key parameter models (sewage blocking coefficient, friction coefficient, convective heat transfer coefficient and dirtiness resistance) which are necessary to design systematically the sewage heat pump system that are absent. Based on the above reasons, the systematical design methods of the sewage heat pump system are still deficient, with the result that the sewage heat pump technology can not be further popularized by engineering design institutes, but be only applied by the few professional corporation of sewage heat pump. Therefore, to reveal the fluid flow and heat transfer characteristics of sewage water in the heat exchange tube and improve the design methods of the sewage heat pump system, the blocking characteristics of the fetching facility of sewage and the fluid flow and heat transfer characteristics of sewage water in the heat exchange tube for the sewage heat pump system are researched by theoretical and experimental methods in this thesis.The continuous regeneration of filtration surface for the filtering orifice plate in the opening anti-blocking device is a key technology to obtain continuous process of fetching water. So the structural style and size of orifice plate, the rotation velocity of the orifice plate, the distribution regularity of filtration holes in the filtering orifice plate and the size of filtration hole are key parameters which will be confirmed to develop the opening anti-blocking device. Firstly, to resolve optimization design of the semi-submerged rotation filtering orifice plate in the opening anti-blocking device, the influence factors of blocking coefficient of filtering orifice plate are studied by theoretical method. Based on the dynamic hydraulic mathematics model, the blocking coefficient model was obtained. Then the influence of some factors, such as concentration of the suspended solids in the sewage, the orifice plate submerged height and the rotation velocity of the orifice plate, on the blocking coefficient are studied numerically by the discretization of blocking coefficient model. And the engineering design method of the opening anti-blocking device is proposed according to these results.Building experimental test system for the fluid flow and heat transfer characteristics based on the real sewage water has great significance to research the fluid flow and heat transfer characteristics of sewage water in tube. So, at the scene of the sewage source heat pump engineering in Dalian, an experimental test system of flow friction coefficient and convective heat transfer coefficient for the flowing sewage water in the heat exchange tube is built owing to their experimental test requirement. And the test method and accuracy of the experimental system are analyzed for the fluid flow and heat transfer characteristics. The building of experimental test system provides the experimental conditions for the research of following chapters.The friction resistance coefficient in the heat exchange tube is a fundamental parameter to calculate for total friction resistance for sewage heat exchanger. Firstly, constitutive properties of sewage water flow in the tube, rheological properties without dirty sublayer in the tube and the resistance coefficient properties with dirty sublayer are researched by theoretical method. Then that the friction resistance of sewage water flow in the tube is similar to power law properties of non-newtonian fluid is proved by theoretical analysis and experimental test results. The calculation formula of turbulent shear stress for the sewage water flow in the rube is obtained by the application of mixing length theory for the suspended solids in the sewage based on the regularity of turbulent shear stress and the additional shear stress in the pipe. And the velocity distribution formula of turbulent flow for the sewage in the pipe is further obtained. The expression of friction resistance coefficient is proposed for the sewage flow in the pipe at the real condition (with dirty sublayer). It provides the theoretical model to calculate the friction resistance for the sewage heat pump engineering design.The convective heat transfer coefficient is another key parameter to develop the heat exchanger. The general formula of temperature distribution for the turbulent flow in the pipe is also attained by theoretical derivation based on e velocity distribution formula of turbulent flow for the sewage in the pipe. The general expression formula of sewage heat transfer heat resistance is formed at the conditions without considering the dirt thickness in the pipe. And the rationality, which the induction period during the progress of forming dirt for the sewage near the tube wall can be ignored, is proved based on the concept of critical fouling and rheological characteristic of sewage flow. The sewage side fouling deposition model is built based on the Kern-Seaton differential equation. The results show that the sewage side fouling deposition amount is satisfied with asymptotic distribution. Incremental value of the fouling deposition amount is proportional to the dirt thickness in the sewage and inversely proportional to mean flow velocity of sewage. And the asymptotic time is inversely proportional to the square of velocity. The fouling resistance model is derived by the fitting of experimental data at the condition with dirty sublayer. The design method of sewage intake and heat exchange process for sewage heat pump system is summarized based on the above results. This method provides the design basis for the engineering design of sewage heat pump system.Finally, to verify the accuracy of the above expressions in this thesis, the flow resistance and heat transfer coefficient of the sewage heat exchanger based on the expression of flow friction coefficient, convective heat transfer coefficient and heat resistance are compared with experimental results from a SHPS project in Dalian. The results that sewage intake and heat transfer process has been redesigned by the empirical formula show that sewage pump power consumption can reduce by50.7%by using the frequency control of motor speed of the sewage pump based on the variable properties of the friction coefficient, the heat exchanger area can be reduced by25%at the same heat transfer temperature difference, the heat transfer temperature difference can be reduced by25%at the same heat transfer area, and the inlet water temperature of the heat pump unit can increase1℃. So the performance of heat pump unit can be improved.