船用柴油机双工质并联涡轮复合新型联合循环的热力学研究

Waste heat recovering and slow steaming have become effective approaches to decrease CO2 emission from the marine transportation. To solve the problem of decreased efficiency under varied conditions of the typical marine engine combined cycle, this project proposes to increase the matching capacity regarding the flow characteristics of the turbocharged diesel engine in the topping cycle and eliminate effects of inefficient waste heat recovery components under off-design conditions with the mixture of exhaust gas and overheated steam flowing into the compounding turbines. A novel combined cycle based on the dual-fluid parallel turbo compounding system is established, which is expected to achieve a high efficient cooperative operation among the engine in-cylinder “Diesel cycle”, exhaust gas “Brayton cycle” and waste heat recovery “steam Rankine cycle”. Researches on key problems including the transformation and distribution of the exhaust energy in the dual-fluid bottoming cycle, mutually binding thermodynamic characteristics of the combined cycle and thermal and flow matching characteristics under varied conditions will be conducted to ascertain the exhaust energy transfer efficiency and irreversibility distributions in the bottoming cycle, highlight power and energy coupling mechanism of the combined cycle and further clarify high efficient operating mechanism under off-design conditions. This project can lay a solid theory foundation for the application of this new combined cycle and also provide a good guidance of exploiting new theories and approaches for higher efficient ship propulsion.

余热能回收利用及减速航行目前已成为降低船舶动力CO2排放的有效途径。本项目针对典型船机余热回收联合循环难以实现变工况下的高效配合运行，提出通过燃气、过热水蒸汽混流经复合涡轮做功来提高增压柴油机顶循环内部流通特性匹配的调节能力，同时避免余热回收底循环部件效率在非设计工况下的显著降低。进一步构建了船用柴油机双工质并联涡轮复合新型联合循环，有望实现变工况下柴油机缸内“狄塞尔循环”、增压换气“布雷顿循环”和余热回收“蒸汽朗肯循环”的高效配合运行。项目拟通过对双工质底循环内部能量传递及分配、联合循环之间互为约束的热力学特性、系统变工况下传热特性及流通特性等关键问题的研究，探明底循环中能量传递效率及㶲损分布规律，揭示联合循环内部耦合运行规律及变工况下的高效配合运行机制。本项目的研究可为新型联合循环的应用及推广奠定理论基础，同时对探索高效船舶动力的新理论及新技术提供思路。

余热能回收利用现已成为降低船舶内燃动力CO2排放、实现船运行业“碳达峰”的关键技术途径之一。考虑典型余热回收底循环难以实现同船舶内燃动力在变工况下的高效配合运行，本项目提出并研究了一种双工质并联涡轮复合余热回收系统，该系统主要通过燃气、过热水蒸汽混流经复合涡轮做功来提高增压柴油机顶循环内部流通特性匹配的调节能力，同时避免余热回收底循环部件效率在非设计工况下的显著降低。围绕该新型联合循环系统，本项目通过热力循环分析、一维数值模拟及试验研究的技术手段，主要开展了如下研究内容：双工质并联涡轮复合底循环中能量传递、分配及梯级回收热力过程研究；新型联合循环内部功、能传递及互为约束的循环特性研究；联合循环变工况下内部传热特性及流通特性的匹配研究。通过本项目的研究，已取得的重要结果及关键数据包括：对于二冲程柴油机，存在一最优涡轮当量流通面积使得扫气质量流量最大；动力涡轮旁通策略在负荷高于60%时显示出较大潜力，燃油消耗在100%负荷工况最高可改进5 g/kWh，采用动力涡轮旁通约10%的废气可实现排气温度升高约60 ˚C。蒸汽喷注热力循环分析结果表明，蒸汽喷注流量对底循环㶲损失分布的影响最大，蒸汽喷注温度和压力的影响较低，蒸汽喷注过程是㶲损失的重要环节；一维数值模拟结果表明采用双工质并联涡轮复合系统在指定机型额定转速下可有效降低比油耗5.1%，而常规复合涡轮系统比油耗改进约3.3%。试验研究验证了蒸汽喷注可有效改善中低负荷工况下柴油机的燃油效率，比油耗在中等转速最高可改进5.8g/kWh，对于高转速工况则由于泵气损失的加剧，蒸汽喷注难以改善柴油机的燃油效率。相较于耦合动力涡轮和蒸汽朗肯循环的余热回收系统，本项目所提出的新型余热回收系统具有更优的技术经济性。本项目的研究可为新型联合循环的应用及推广奠定理论基础，同时对探索高效船舶动力的新理论及新技术提供思路。

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