Numerical Simulation Of Duct Flow Heating With Rotating Gas

Improving the thermal efficiency is critical for gas turbines.According to Carnot's theorem,the fundamental method is to improve the total heat absorption in a thermodynamic cycle,which means to increase the maximum temperature.The increase of gas temperature is limited by the temperature limit of turbine blades.Temperature represents the average translational kinetic energy of thermal motions of a large number of molecules.If the thermal energy of the working fluid can be converted into other forms,the total heat absorption,and then the thermal efficiency,can be improved without increasing the temperature.Here,the path of converting the thermal energy of the working fluid to macroscopic rotational kinetic energy is determined.In this thesis,the duct flow heating with rotating gas is predicted by means of CFD simulation.Firstly,the flow field of spinning premixed methane/air gas under non-reacting and reacting conditions in the annular combustor was calculated,and the swirling characteristics of the flow field were analyzed.Based on this,the existence of energy storage mechanism and its physical mechanism were verified.The velocity distributions show that the rotational velocity of the gas in combustion state is significantly higher than that in cold state;as the equivalence ratio increases,then the heat increases,and the rotational velocity increases.Therefore,heating can improve the rotational velocity of the spinning gas.The increased rotational kinetic energy comes from the thermal energy of the gas.That is,the thermal energy of the spinning gas can be converted into the rotational kinetic energy.This result proves the existence of the energy storage mechanism.The pressure and density distributions indicate that the radial gradients of the pressure and density in combustion state are lower than that in cold state.This phenomenon indicates that when the spinning gas is heated,the gas molecules,which are distributed near the outer wall due to the centrifugal force,act against the centrifugal force and move towards the axis.Since the total angular momentum of the gas molecules is conserved and the average radius of rotation of the spinning gas becomes smaller,then the rotational velocity of the gas must be increased.This result explicates the physical mechanism for the improvement of the rotational velocity.Secondly,the flow field of spinning premixed methane/air gas under non-reacting and reacting conditions in the convergent variable cross-section combustor was calculated,and the swirling characteristics and energy storage characteristics were analyzed.It is found that the heating effect in the variable cross section combustor increases the rotational velocity much more than that in the constant cross-section combustor.Therefore,the use of variable-section combustors allows more energy to be stored in the form of rotational kinetic energy.Besides,increasing the axial velocity and reducing the swirling strength can effectively improve rotational kinetic energy of the gas.