Research On Hydraulic Analysis Method Of Long-distance District Heating Pipeline With Variable Flow

With the rapid development of economy in China,the pace of urbanization is accelerating,and the demand for heating in cities is greatly increased,China’s district heating system is experiencing a rapid development stage,with the heating scale getting larger and larger,and the transmission distance of heat source to the user getting longer and longer.As of 2017,there were six long-distance heating supply pipelines with a length between 20 kilometers and 40 kilometers in China.In these heating pipelines,supply temperature control is adopted in primary side to meet the demand of users,but supply temperature control method has the disadvantages of temperature delay and high energy consumption of transmission.Variable flow regulation method can improve the above problems,but when the motor frequency of the circulating pump changes,it will cause the phenomenon of hydraulic fluctuation,and even affect the operation of the system.Therefore,in order to ensure the safe operation of the long-distance heating supply pipeline,the establishment of hydraulic model and the analysis of the hydraulic characteristics during the operation of variable flow are urgent problems to be solved,which are the main contents of this paper.In this paper,based on the basic theory of water hammer,combining with the characteristics of long-distance heating pipe,several kinds of boundary conditions of district heating system were proposed,and the transient hydraulic model of longdistance heating pipeline was established based on the motion equation and continuity equation,its numerical solution was derived.At the same time,through a large number of numerical experiments,the optimum space step length as well as the time step length of the numerical solution were determined,in order to implement the dynamic hydraulic calculation of long-distance heating pipe more efficiently.By comparing the lumped parameter model with the transient hydraulic model established in this paper,the applicability of the model to the dynamic hydraulic analysis of long-distance heating pipeline is verified,which provides a reference for the operation regulation of variable flow.Compared with lumped parameter model,this model can better reflect the transient fluctuation characteristics of flow and pressure in pipeline.Based on the transient hydraulic model established in this paper,the influence of different regulation methods of pump frequency on hydraulic characteristics of longdistance heating pipeline is analyzed.The results show that,compared with the upfrequency of the circulating pump,the down-frequency of the circulating pump has a greater influence on the fluctuation of the flow rate and pressure in the pipeline.The shorter the frequency modulation time and the larger the frequency modulation amplitude,the more drastic the fluctuation of flow and pressure.The lower the initial frequency of the circulating pump motor,the stronger the flow fluctuation and the longer the pressure fluctuation time.