| Since it have the advantages of simple structure and convenient maintenance, organic Rankine cycles have been paid widely attention in the field of low and medium grade energy utilization in recent years. Due to the temperature of the most heat source is variable, and at the same time, the zeotropic mixture have the characteristics of temperature glide and composition shift. Therefore, compared with organic Rankine cycle of pure working fluids, organic Rankine cycles using zeotropic mixture as working fluids have significant advantages and characteristics. On the one hand, such as solar energy and waste heat, the temperature of the heat source is unstable; On the other hand, the temperature of the cooling fluid will seasonally change. Therefore, in most of the time an organic Rankine cycle operates in off-design working condition. Meanwhile, zeotropic mixtures in the thermodynamic cycle also have the characteristic of composition shift, which will be influenced by the variable temperature of heat source and heat sink. Under the condition that the changing working condition of heat source and heat sink, the cyclic mechanism of organic Rankine cycle with zeotropic mixture and heat and mass transfer characteristics in the evaporation process are the main research contents in this paper.At first, for the variable temperature heat source and heat sink with finite heat capacity, through thermodynamic derivation, the theoretical formula of predicting organic Rankine cycle of zeotropic mixture system performance(net thermal efficiency, output power and the available energy efficiency),including Jacob number(the ratio of the sensible heat and latent heat in the evaporation process) and REC(the ratio of the average thermodynamics evaporation temperature and condensation temperature) is obtained. On this basis, the influences of the heat source inlet temperature, cooling water inlet temperature and cooling water temperature increase on the best composition of organic Rankine cycle with zeotropic mixture are discussed. Results show that with the increase of the heat source inlet temperature, there is a heat source inlet temperature which makes the performance of organic Rankine cycle with pure working fluid better than that of organic Rankine cycle with zeotropic working fluid. When the temperature increase of the cooling water is less than the temperature glide, the local maximum value of output work exists when they are equal. While the temperature increase of the cooling water is greater than the temperature glide, the local maximum value of output work exists when the temperature glide is maximum.The experiment rig of flow boiling heat transfer of zeotropic mixture in a horizontal tube is independently established. For isobutane, pentane and their mixture, the effect of the evaporation pressure, mass velocity, and inlet vapor quality on the heat transfer performance are carried out and their heat transfer performance are compared. And at the same time, the heat transfer correlations are evaluated. Results show that under the same conditions, when the quality is less than 0.2, the heat transfer coefficient of pentane is higher than isobutane. And the quality is higher than 0.2, the heat transfer performance of isobutane is much higher than is pentane. For pure substance, Liu and Winterton correlations and Gunger and Winterton correlations can effectively predict the heat transfer coefficient. For zeotropic mixtures, Jung correlations can well predict the heat transfer coefficient.With the aforementioned heat transfer correlations, evaporator, condenser, expander and feed pump models are respectively developed, and then the circulating composition is calculated, based on which, the system parameters and performance of organic Rankine cycle with zeotropic mixtures are obtained in the case of composition shift. Meanwhile, it also discusses the local composition shift characteristic in the process of phase change and effects of charge and charge composition on composition shift. The results show that composition shift significantly influence the performance of organic Rankine cycle with zeotropic mixtures, which will result in a lower output work of expander, a higher power consumption of pump, a lower net output work and lower thermal efficiency.Transforming the aforementioned experiment rig, regarding isobutane and pentane as the research object, the impact of the key system parameters(heat source inlet temperature, heat source flow rate, cooling water temperature, cool water flow rate, evaporator heat exchanger length) on composition shift are experimentally carried out. The results show that when the heat source inlet temperature, flow rate and evaporator heat exchange length increase and cooling water temperature decreases, circulating composition will increase. For working fluid flow rate, when the evaporator outlet is in two phase region, and circulating composition will decreased with the increase of working fluid flow rate; while the evaporator outlet is in a state of overheating, circulating composition will increase as the working fluid flow rate rises. At the same time, the dependence of charge and charge composition on composition shift is experimentally verified.In consideration of the importance of local composition shift on the system composition shift, the simulation of composition shift during the evaporation process is developed. Based on the non-equilibrium film model, through the establishment of mass, energy and composition conservation equations, the local composition is obtained under the non-equilibrium condition, which is compared with the equilibrium conditions. Meanwhile, the effects of the thermodynamic parameters(evaporation pressure, heat flux, inlet composition and mass velocity) on the composition shift are discussed. The results show that under the non-equilibrium condition, the local low-boiling point composition is less than that under the equilibrium condition. The increase of mass velocity will reduce the gas phase heat transfer and mass transfer resistance and the increase of heat flux will increase the gas phase heat transfer and mass transfer resistance. For different inlet composition, the tendency of the concentration difference with the change of vapor quality is different. The rise of evaporation pressure will increase slightly the gas phase heat and mass transfer resistance, but on the whole, evaporating pressure on its impact is not big.Inspired by front research, a novel auto-cascade Rankine cycle is put forward, which is respectively applied in solar energy and waste heat recovery, is analyzed by the first law and second law of thermodynamics and is compared with single organic Rankine cycle and Kalina cycle to prove its advanced nature. At the same time the system parameter sensitivity analysis is also conducted. The results found that with the isopentane/R245 fa as working fluid, since the auto-cascade Rankine cycle is superior to single organic Rankine cycle. For solar auto-cascade Rankine cycle, the influence of the first solar collector outlet temperature is largest. The R245 fa mass fraction in the primary circuit has an optimum value with respect to the minimum total cycle irreversibility, for a set of other operating parameters. Separator temperature has a greater influence on the system performance than superheating temperature. At the fixed separator temperature and superheating temperature, with the evaporation pressure rises, the total cycle irreversibility decreases and work output increases. |
PDF Full Text Request