Numerical Simulation Of Static Instability And Flow Distribution Of Double Parallel Heat Transfer Tubes

In heat exchangers such as boilers,condensers,and evaporators,the use of parallel pipes can effectively increase the heat transfer area and improve heat transfer efficiency.,The efficiency of heat exchangers with parallel pipes depends on the uniformity of the mass flow distribution in the parallel channels.Uneven distribution of flow will result in uneven wall temperature distribution and the quality of the outlet steam cannot be guaranteed.Therefore,it is necessary to study the flow heat transfer characteristics of parallel pipes and understand the principle of flow drift instability in parallel pipes.In this paper,based on the steam generator in the thermal power generation system,combined with the structural parameters of the steam-water two-phase experimental platform,a one-dimensional two-phase flow mathematical model of the parallel pipes is established.According to the export status of the working fluid,the experimental section is divided into a supercooling zone,a two-phase zone and a superheating zone.The MATLAB language was used to develop the steady-state simulation program.The dynamic simulation model was built by using MATLAB/SIMULINK module.The influence of operating conditions and geometric conditions on the flow characteristics and flow distribution in parallel tubes was studied.According to the steady-state simulation results,the flow rate and the resistance are not monotonous in a uniformly heated single tube,the two-phase region has a negative slope,and there is a case where the flow resistance decreases as the flow rate increases.When the single pipe system operated in the negative slope region,the flow drift causes the system to be unstable.In the double pipes system,the multi-value correspondence between flow and pressure is more complicated.When the outlets of the two pipes are two-phase,even the flow is in uniform distribution,the system is unstable.The flow drift causes the mass flow changing from uniform to very uneven.The split ratio increases from 0.5 to above 0.9.In addition,the influence of operating conditions and geometric parameters on the flow characteristic curve is also explored.As the fluid inlet temperature,operating pressure increases,or as the heat flux decreases,or as the pipe diameter increases and the pipe length shortens,the unstable region decreases and the stability of the system increases.The dynamic simulation results show that when the starting operating point is in the unstable range,even if the system is subject to small disturbances(such as reducing the flow rate of a tube by 0.01%),The flow in the parallel pipes will change from uniform distribution to be extremely uneven,the split ratio can reach 0.971.And there is a certain delay,and the response time to the disturbance is 0.3s.When the operating point changes linearly from the stable region to the unstable region,the flow drift instability phenomenon also occurs.And the change path is related to the speed of the linear change of the disturbance.When the change time is 0.5s,the range of pressure and flow is large;when the change time is 5s,the flow drift of the two pipes occurs earlier,but the range of pressure flow changes is smaller.And it fits more closely to the steady state curve.When the inlet temperature and the heat flux change linearly,if the change time is 0.5s,the operating point will jump to the new characteristic curve first,and then the flow drift occurs.If the change time is 5s,the disturbance speed becomes slower and the flow drift will occur earlier.When the geometric parameters of the parallel double tubes are different,the flow distribution after the disturbance is more uneven.The flow rate of the tube with a large mass flow rate continues to increase,and the flow rate of the tube with a small mass flow rate continues to decrease,resulting in a great difference in the split ratio of the two tubes.