Numerical Methods And Dynamic Charactertistic Analysis Of The Thermal And Hydraulic Transients In District Heating Network

As one of most important municipal infrastructures,district heating(DH)network has great significance to fit the demand of the end-user,balance the dynamic transmission-distribution and efficient operation of heating system.The thesis focuses on the numerical calculation method and dynamic characteristic analysis of the hydraulic and thermal transients in the DH network.Based on the analysis of the steady-state hydraulic characteristic of the network,the sensitivities of the resistance changes of the pipe sections and the heating substations to the flow and pressure distributions of the most unfavorable loop are studied.A novel resistance identification algorithm for DH network with multiple heat sources and ring topology based on the least square method is established,which can reduce the resistance identification error around ± 1% by using the pressure and flow data under multiple operational conditions.The distributed parameter and lumped parameter models of dynamic hydraulic operational condition of large heating network are established.The two models are applied to a case study of a long-distance DH system,and the numerical results of the two models are compared and analyzed.Based on the distributed parameter model,the dynamic hydraulic characteristics of a DH network with multiple sources ring topology are studied.Results indicate that the transient variations of the substation flow rates near the heat source is closely related to the change of the flow rate of the nearest heat source.The second-order implicit upwind scheme and the semi implicit QUICK scheme are proposed for dynamic thermal numerical calculation of DH network.The application of the two dynamic thermal calculation methods and the first-order implicit upwind scheme to Shijiazhuang DH network shows the validity and accuracy of the three numerical schemes with the measured data.The optimal time and spatial steps of the three numerical methods is determined by analyzing the calculation errors and the changes of the calculation time according under different time and spatial step.Results show that the performances of the second-order implicit upwind scheme is superior to these of the other two schemes.The coupling numerical calculation method of the dynamic hydraulic and thermal operational conditions is established by using the breadth first traversal algorithm,the steady-state hydraulic condition calculation method and the secondorder implicit upwind scheme.The dynamic thermal characteristics of the DH network under the hydraulic and thermal transcients are analyzed by the coupling numerical calculation method.Based on the algorithm,the influence of the substation flow rate changes on the dynamic thermal characteristics of the network is studied via a case study.Results show that the further the substation is from the heat source,the greater the influence of the flow change on the thermal dynamic characteristic of the network.The resistance identification algorithm,the numerical calculation method of dynamic hydraulic condition,the numerical scheme of dynamic thermal condition and the coupling numerical calculation method of dynamic hydraulic and thermal operational condition of the network established in the thesis are theoretically and practically significant for analyzing the dynamic characteristics and efficient operation strategies of DH network.