| The shortage of oil resources created an unprecedented energy crisis around the world. Internal-combustion engines(ICEs) are main oil-consuming equipment. When it runs, only one third of the fuel energy transformed into useful work, while the remaining energy is emitted into the environment in the form of waste heat, with the heat contained in exhaust gas accounting for the major part. If such waste heat is reused, engine efficiency will be greatly enhanced, bringing tremendous economic and environmental benefit. Organic Rankine Cycle(ORC) can convert heat energy into electricity energy and increase energy efficiency, and has received wide attention in ICE industry. For high temperature ORC used in engine waste heat recovery, it’s very critical to select a high temperature working fluid. Hydrocarbons(HCs) and siloxanes usually have excellent cycle performance, but the flammability limits their practical application. Considering the flame-retardant property of retardants, the paper proposes an application of mixtures based on HCs and siloxanes blending with retardants, to partly suppress the flammability and take advantage of the cycle feature of the mixtures. Besides, for zeotropic mixtures, there exists a certain degree of temperature glide during the phase change process, providing a better thermal match with the heat source and sink profiles, reducing the exergy loss and promising a better cycle performance.Based on the experimental data of a certain type of Yuchai diesel engine and the first and second law of the thermodynamic analysis, the paper built a theoretical model for the combined ICE-ORC waste heat recovery system in MATLAB software. The model was validated by previous work. Depending on the degree of the temperature glide created by the mixtures, different cycle modes were designed for matching. Moreover, combined with energy and exergy optimization theory, cycle performances of the selected mixtures were calculated and analyzed. Net output power, thermal efficiency, exergy efficiency and exergy destruction are evaluated under different mass fractions and operating parameters, in order to find the relatively optimal mixture under different cycle modes.Through theoretical analysis, using subcritical saturated cycle, zeotropic mixtures have a higher efficiency and lower exergy loss than the relative pure working fluids at a certain mixture ratio. There exist the optimal mixture ratios(OMR) for different zeotropic mixtures, and the OMR gradually approaches the side of fewer retardants. Adding internal heat exchanger(IHE) to the system, the performance of mixtures has a significant increase. And the higher the IHE’s efficiency, the better the performance. There is a turning point. Before it, the performance curve slope is very small. After it, the slope becomes very large. In a dual loop ORC, there exists optimal evaporation pressure in the high-temperature cycle and mass fraction of retardants to obtain maximum net power and utilization of heat source(UHS) for mixtures. Siloxanes mixtures performance better than HCs mixtures. D4/R123(0.4/0.6) has the highest thermal efficiency of 21.19%, and its exergy efficiency can reach up to 47.56%., while its exergy destruction is only 20.43 kW. Based on D4/R123(0.4/0.6) system, the irreversibility in the IHE and turbine(Tur1) contribute most to the total exergy destruction, which can serve as the optimized parameters for further study. In transcritical cycle, under a given pressure, the smaller the temperature glide, the more the net output power. The larger temperature glide doesn’t take on its advantage.Research shows that it’s of great significance to study the mixtures in ORC used in engine waste heat recovery, which can realize the energy saving and emission reduction of engines. |
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