Research On The Design Of Wet Steam Calibration Facility

Wet steam is a complex two-phase flow coexisting with liquid and steam.Influenced by the mechanism of wet steam flow,wet steam flow meter must rely on experimental calibration and correction.The wet steam calibration facility should provide stable wet steam and standard information for the wet steam flow meter.In this study,the quality design of the wet steam calibration facility is evaluated on two points of view,whether the steam at the experimental section becomes the saturation state or not and whether the tested wet steam flow patterns is almost identical to those in the industrial field or not.Meanwhile,in order to achieve a better design of the wet steam calibration facility,the comparative study of the regular patterns of liquid phase distribution between by use of the condensing heat transfer and the spray cooling has been deeply carried out here.According to the wet gas test data of vortex flowmeter,the droplet diameter,entrainment ratio and cross-section void fraction have been calculated on the changing in liquid volume fraction from 0.1% up to 0.2% and 0.3%.From the results,it can be deduced that the increase of liquid film thickness is the main cause of vortex instability.As we know,most of the vortex flowmeter can still output stable measurement signal in wet steam environment and few of vortex instability happens,so the following conclusions can be drawn: the flow pattern of wet steam in industrial field is most probably mist or annular mist,and the main body of liquid phase in wet steam exists is in the form of droplets.The regular patterns of the liquid phase distribution in the condensing heat transfer and the spray cooling wet steam calibration facility has been comparatively studied here and the mechanism of droplet generation in two kinds of facilities has been discussed.The particle size control method is obtained on the basis of analyzing the influencing factors of droplet size.The design of the condensation heat transfer wet steam calibration facility shall adopt the method of dropwise condensation.When the steam velocity is more than 10m/s,the drag force of steam plays a leading role in the droplet dropping process and the droplet dropping diameter is less than 2mm.The spray cooling wet steam calibration facility can make the droplet diameter less than 300 micron through reasonable selection of nozzle and operating pressure.According to the comparison of two types of calibration facility,the quality design of wet steam calibration facility has been realized successfully.In order to ensure the steam and water in the experimental section are saturated,the research of how to balance the momentum & heat of the super-cooled droplets' running with the super-heated steam has been studied.Applying the Cheng resistance coefficient model and Pasamehmetoglu & Nelson heat conduction model,the conditions of droplet reaching velocity stability and temperature stability were determined as 10% and 1% difference between the droplet and steam,and three key parameters can be achieved,like the time for droplets to reach a steady velocity and temperature,the length of straight pipe segment necessary for wet steam developing enough and the ideal installation position for tested flowmeter.On the basis of the above theoretical model,the spray cooling system includes the nozzle and the gear pump has been realized.The spray cooling system is installed in the main steam pipe of the superheated steam calibration facility,and the vortex flowmeter is installed downstream at a distance of 3m from the nozzle.The pressure and temperature information of each part of the pipeline was obtained and the vortex flowmeter status has been analyzed.Compared with the test results of the condensing heat transfer calibration facility,the wet steam at the experimental section of the spray cooling calibration facility has been assured to be saturated.