| With the deepening of the world economy and the deepening of global trade,shipping,as a cheap and convenient mode of transportation,has become an important part of driving the development of the world economy.However,the high energy consumption and high pollution brought by ship transportation have posed a major challenge to sustainable development.Only 40%-50% of the fuel consumed during the operation of ship main engines is converted into useful energy,and the remaining heat is discharged into the environment in the form of main engine exhaust and jacket water,resulting in a huge waste.Waste heat utilization system can efficiently utilize various forms of waste heat from ship main engine.However,as the research progresses,scholars have proposed more and more thermal cycle structures.Since the ship main engine has multiple waste heat sources,the traditional waste heat utilization system is only designed for a single heat source,without considering the multiple heat source characteristics of the ship main engine.Therefore,this thesis constructs the thermal cycle structure based on the superstructure method and conducts research on how to realize the selection of suitable thermal cycle structure for waste heat utilization under different heat source conditions,and the main work is summarized as follows:Firstly,the mathematical model of the waste heat utilization system of ship main engine is established and verified.Taking supercritical carbon dioxide Brayton cycle and organic Rankine cycle as examples,the mathematical model of the working process of the main components of the ship mainframe waste heat utilization system(including heat exchanger,compressor,working fluid pump and expander)is established,the calculation method of the thermodynamic performance index of the thermal system is given,and the correctness of the established model is verified by a typical waste heat utilization thermal cycle,so as to prepare for the subsequent superstructure model of the ship mainframe multiheat source waste heat utilization system.It will prepare for the subsequent superstructure modeling of the multi-source waste heat utilization system of the ship main engine.Secondly,the superstructure model of ship main engine multi-source waste heat utilization system is established and verified.Based on the principle of "temperature counterpart and gradient utilization" and the concept of splitting flow,the super structure model of ship main engine multi-source waste heat utilization system with top and bottom cycle gradient utilization is established for the scenario of ship main engine multi-source waste heat utilization.The coding method for the selection of circulating work material,circulating structure and design parameters is established to realize the synergistic optimization of the three.Seven organic work substances were selected for the superstructure bottom cycle considering organic work substance category,safety,thermal materiality and impact on the environment.The correctness of the superstructure model was verified by comparing the available literature data.Then,a multi-objective optimization study was conducted on the waste heat utilization system of ship main engine.NSGA-Ⅱ is selected as the optimization algorithm,and the net output work and cost per unit output work of the waste heat system are taken as the objective functions,and the maximum pressure of top cycle,minimum pressure,top cycle expander inlet temperature,cooler outlet temperature,precompressor boost ratio,discrete split ratio,continuous split ratio and work quality selection coefficient are taken as the optimization variables for the multi-objective optimization study.The calculation results show that the superstructure model established in this study realizes the construction of the cycle structure of the host waste heat utilization system under multiple heat sources and analyzes the influence of the host exhaust temperature on the optimization of the structure.When the exhaust temperature of the main engine is 350℃,the optimal structure of the top cycle is an intermediate cooling structure at the cold end,a partial heating structure with preheating at the hot end,and a simple organic Rankine cycle with R245 fa at the bottom cycle,with a net output power of 2.26 MW and a unit output power cost of 7.18$/GJ;when the exhaust temperature of the main engine is 400℃,a shunt structure is added at the hot end to change the When the exhaust temperature of the main engine is400℃,a shunt structure is added at the hot end to change the mass flow rate of the circulating mass into the gas heater and the return heater,thus improving the heat matching performance inside the gas heater and the return heater,and the optimal structure becomes an intermediate cooling structure at the cold end of the top cycle,a double shunt double heating double expansion structure with preheating at the hot end,and a simple organic Rankine cycle with R245 fa at the bottom cycle,with a net output power of 2.84 MW and a unit output power cost of 6.00$/GJ;when the exhaust temperature of the main engine is 450℃,a shunt structure and a pre-cooling structure are added at the cold end,and a part of the mass is compressed directly without cooling,thus increasing the temperature of the mass at the inlet of the cold end of the gas heater and improving the thermal matching performance of the gas heater.is a simple organic Rankine cycle with R123 as the working mass,the net output power is 3.31 MW,and the unit output power cost is 5.41$/GJ。... |
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