A New Design Method For Organic Rankine Cycles Coupling Heat Source And Numerical Simulation Of Modulating Flow Pattern

Low grade thermal energy (heat) such as waste heat, geothermal, and heat from low to moderate temperature solar collectors, accounts for more than one half of the total heat generated worldwide. It is significantly to utilize the energy from low grade thermal energy for human sustainable development. Organic Rankine Cycle (ORC) is applied efficiently to recover the low grade thermal energy as a kind of the thermal power conversion technology. At present, however, the development and application of the ORC are restricted due to the unitary thermodynamic analysis mode and the requirement of the large size for heat transfer equipment. In this paper, the research on ORC is carried on from both system and component to improve the ORC performance.A new design method for Organic Rankine Cycles coupling heat source and solution strategy are proposed firstly based on the ORC system analysis. The new design method is not only building the connection between expansion power and thermal efficiency, but also considering influence of the heat source and pinch temperature difference on the ORC performance. The results indicate that With constraint of the given heat source and pinch temperature difference, the system thermal efficiency, expander inlet pressure and mass flow rate of the organic fluid are decreased with increases in the expander inlet temperatures. The optimal condition appears at the saturated or slightly-superheated vapor state at the expander inlet. The increase in the pinch temperature differences yields the decreased expander inlet pressure to reduce the system thermal efficiency. Furthermore, the three organic working fluid, toluene, benzene and cyclohexane, are selected for medium temperature ORC system due to the higher thermal efficiency and larger operation range applying the new design method.As the main heat transfer equipment, the evaporator and condenser are influence on the ORC system performance strongly. It is point that the exergetic efficiency of the condenser is lowest in the ORC system via analyzing based on the new design method. Besides that, the low temperature difference heat transfer process in the evaporator and condenser leads to the requirement of the larger size and higher manufacture cost. Phase separation condensation tube is applied to solve this scientific problem. In this paper, the numerical simulation is carried on to quantify the relationship among the different parameters. The simulation results reveal that the mechanism of the phase condensation tube enhance heat transfer is contributed by the extra liquid film and three-levels of liquid circulation. The tube behaves the upward mixture flow in the annular region and downward liquid flow in the core region. Void fractions are exact zero in the core region and larger in the annular region, indicating the gas phase flowing in the annular region and inside of the mesh cylinder is liquid. Liquid film thicknesses are significantly decreased by the modulated flow. The three-levels of liquid circulation promote the liquid mixing over the whole tube length and within the radial direction. These circulations were performed through mesh pores. Besides that, the critical criterion is proposed to prevent the bubble leakage deteriorating heat transfer. The research will further developing into the small gravity and microgravity environment. The phase distribution still keeps "gas near the tube wall and liquid in the tube core". The liquid film thickness and condensation heat transfer quantity decrease significantly as the gravity decreasing. Especially for the microgravity, the annular region of the tube is occupied by gas totally.During the numerical simulation process, some kinds of the numerical technology, including the multiscale grid system, dynamic mesh adaption and the frame of reference coordinate, are developed and employed to reduce the computing period.The conclusions of this paper will promote the ORC system and modulating flow pattern theory research, and provide support for the ORC system and phase separation condensation tube design and application.