A Study On Air Humidification Process Of Humid Air Turbine Cycle

Humid air turbine cycle is very promising among the future advanced power cycles. Its excellent thermodynamic performance benefits from the recovery and utilization of low-grade heat in the turbine system via the humidification process of compressed air. In past two decades, the related studies focused on cycle configuration analysis and key technology development, such as pressurized humidification, combustion with humid air and water recovery from the exhaust. Humidifier is a key component of HAT cycle, and its performance has significant effect on the general performance of the system. However, the experimental data and design theory for humidification process under elevated pressure have been deficient even until recent years. Therefore, the main purpose of the present work is aimed towards understanding of the internal flow and heat and mass transfer mechanism of humidification process by using both experimental and numerical methods, and also, to find the possibility of the comprehensively optimum designing of a humidifier by combining CFD, 1-D simulation and system optimization methods.The dissertation was organized as follows:Firstly, as a basis of thermodynamical analysis of pressurized humidification processes, an analytical method was developed for calculating the pinch points and minimal enthalpy difference driving force in humidification towers. The indices for evaluating the performance of humidifiers were provided, which will support the optimization of HAT cycle.Secondly, a test rig of pressurized humidification tower was built. The internal flow and overall performance of the tower were studied experimentally. The mechanism of heat and mass transfer inside the tower was investigated. The influence of spray characteristicsof the nozzle on the performance of heating and humidifying during the humidification process was analyzed. The law of pressure loss of the tower as a function of air velocity and water/air ratio was acquired. Also, the influence of operating pressure on the internal flow pattern was studied.Thirdly, the complicated processes of water droplet-air two-phase flow and heat and mass transfer in a humidification tower were simulated by an improved 1-D theoretical model as well as a 3-D CFD model. The numerical results were compared to experimental data. On the basis of the 1-D model, the influence of droplet diameter on the steady-state heat and mass transfer characteristics of the humidification tower was analyzed. Also, the dynamical response of the parameters of outlet humid air to the disturbance of inlet water temperature and flow rate was studied. In the 3-D CFD model, the more complicated mechanism of water atomizing including droplets collision and merging was considered. The Lagrangian method together with the stochastic tracking approach, which is related to the influence of turbulent dispersion on droplet movement, was used to predict droplet trajectories. By using the 3-D model, the influence of the parameters of water droplet-air two-phase flow on the performance of the humidification tower was analyzed.Finally, on the basis of the previous evaluating work of the models, a frame of the method of comprehensively optimized designing of humidification towers was proposed. Also proposed was the design principle of tower diameter and height. At the same time, the problem of water consumption of HAT cycle was studied preliminarily. The principle for the cycle operating at self-balance of water and the reasonableness of selecting the techniques for water condensing in flue gas were discussed.