| Since hot and cold fluid contact each other directly,the direct contact condensation heat transfer between water and steam produces very high condensation heat transfer rate,and thus is widely used in the mixing type heater of electricity plant,steam ejector and passive safety system of nuclear power plant.Recently,the study of utilizing steam ejector to recycle exhausted low-pressure steam is emerging,due to a further requirement of improving energy utilization efficiency.Influence of non-condensable gas on the direct contact condensation has been investigated in this dissertation,in addition to its influence on the performance of steam ejector.Numerical method and model are proposed to calculate the phenomenon of direct contact condensation and predict the performance of steam ejector.Firstly,steam ejection experimental facilities were constructed and air was taken as non-condensable gas.The gas volume fraction distribution was studied under different fraction of air(with a concentration 10%)in the ejected steam.The condensing gas jet plume was recorded by a high-speed camera,and eventually the volume fraction was obtained by a MATLAB procedure to process the instant images of condensation.Meanwhile,difference of the gas volume fraction obtained by such way with the traditional one was depicted,for the sake of making corrections to the numerical gas volume fraction afterwards.The results show the jet plume can be distinguished into jet region and plume region,each of which are composed of countless tiny bubbles.When the temperature of water in the pool or the air content in the ejected steam increase,the gas volume fraction decreases in a slower manner at both axial and radial direction.This indicates an enlarged jet plume due to the deteriorated condensation heat transfer by the non-condensable air.Self-similarity of the volume fraction distribution was investigated and the shape factor and diffusion rate was presented for all experimental cases.Based on a one-dimensional condensation model for pure steam scenario and a few reasonable assumptions,a direct contact condensation model for mixing gas was proposed,in addition to a semi empirical correlation to predict length of jet plume and a semi empirical correlation to predict the averaged condensation heat transfer coefficient,both of which contain a few unknown constants awaiting to be determined from experiments.The temperature field was hence measured in a mass flux range of 100-330 kg/m2/s and air content below 15%.The results show addition of air into the steam would raise the temperature distribution both in axial and radial directions,due to deteriorated heat transfer and enlarged jet plume.No distinct ending of the plume exists owing to the non-condensable,and thus the length of the jet plume was taken as the location where the temperature has decreased 90%of the temperature difference between ejected steam and pool water.This method was proved reasonable by comparison with the correlations for pure steam jet plume from literatures.The constants in above two correlations were then obtained from experiment,and the dimensionless length of jet plume and condensation heat transfer coefficient in this dissertation range from 3-17and 0.7-2MW/m2/K,respectively.Order of magnitude of latent heat and sensible heat was analyzed,in addition to the influence of gas superheat.The analyzation shows,when the air content in steam is not very high,the transfer of sensible heat can be neglected in comparison with latent heat,even with a mild gas superheat.Commercial CFD software ANSYS CFX was used to establish a numerical method,based on the Euler-Euler two-fluid-model,with water and gas taken as continuous and dispersed phase respectively.Particle model was used when considering the interphase mass,momentum and heat transfer,and the usage of thermal phase change model when non-condensable gas exist was justified.Numerical results show the temperature distribution can be well predicted,as the experimental temperature distribution located between the calculated two sets of temperatures.In order to make a comparison of the gas volume fraction,the value from simulation was corrected and found to match well with experimental results.The simulation also shows the addition of air into steam enlarges the jet plume and makes the velocity field and pressure distribution less fluctuated.The performance of steam ejector with the existence of non-condensable was studied by constructing a small steam ejector and responding test loop.The experiments show the water flow rate increases firstly and decreases after the air content in steam increases to a point.This can be explained as when air is added,the size of jet plume increases,leading an enlarged shear effect between phases and reduce the cross-section area where water flows through.With a low air content,the effect of the former is stronger and water flow rate thus increases,while air content is high,the effect of the latter is stronger,resulting in a decreased water flow rate.With different air content,the water flow rate would increase as steam flow rate increases,nozzle diameter decreases or water temperature increases.When air content is low,an ejector with a smaller throat nozzle distance would produce higher water flow rate,and when air content is high,an ejector with a larger throat nozzle distance would produce higher water flow rate.As a constant water level and total flow friction condition are applied at the water side,the pressure at the water inlet shows the opposite trend with water flow rate,and ideal suction lift shows the same trend with water flow rate because of a small inlet water velocity.Meanwhile,the entrainment ratio decreases with the increase of steam flow rate,and influences of other factor require further investigation.With the above direct contact condensation model,a three-dimensional ejector model was firstly established using the same dimensions of experiment,and the mass flow rate boundary conditions are applied to the model based on experiment data.The simulations show the variation trends of water inflow pressure along air content variation are the same with that from experiment,indicating the simulation model can well predict the performance of steam ejector.By this simulation method,it can be found the water inflow pressure decreases with an increase water temperature,same trend with experiment.The increase of water inflow rate leads to an increased water inflow pressure and makes the best air content lower.The increase of dimeter of ejector throat would increase the water inflow pressure while make the best air content higher.From simulation,the reason of the existence of best air content proved to be the simultaneously increased shear area between phases and the(?)duced flow cross section due to added non-condensable air.The pressure distribution,condensation rate and gas volume fraction in the ejector were also shown under different conditions.To further simplify the simulation model,a two-dimensional ejector was then established with the same dimensions of experiment and more common pressure boundary conditions were applied.It was found,the entrainment ratio of ejector increases firstly and decreases afterwards as the air content in the steam increases,but this is only true within certain range of water inflow pressure.If the inflow pressure is high than this range,the entrainment ratio would decrease monotonously,and if the inflow pressure is too low,there would be reversed flow at water inlet.Finally,a large steam ejector working under large pressure was investigated numerically,and same trend of performance with air content variation was found. |
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