| Due to the efficient heat and mass transfer,direct contact condensation of sub-merged steam jet in subcooled water attracted more and more attention and widely used in district-heating systems,steam accumulators,steam-water heat exchangers for power plant and passive safety system of nuclear power plant.In this paper,both experimental and numerical method were used to investigate the direct contact condensation charac-teristics of submerged steam jet in water flow in a pipe.The main research work is as follows:A visualization study on submerged steam jet in water flow in a pipe was carried out,in which a high-speed camera system was used to record the condensation phe-nomenon in the transparent test section and MATLAB digital image processing method was employed to obtain a clear vapor-liquid interface to identify the shape of the steam plume and extract the characteristic parameters of the steam plume,including the pen-etration length and the interface area.Steam plume shape,jet penetration characteristics and direct contact condensation heat transfer characteristics on the submerged steam jet were analazed in a wide range of operating conditions(steam mass flux 226-750 kg/(m2·s),water inlet temperature 7.5-66.8 ℃,Re number of water flow 7000-32215).A total of five typical steam plume shapes were observed under 150 test conditions:hemispherical shape,conical shape,contraction-expansion-contraction shape,ellipsoi-dal shape and divergent shape,which were plotted in a three-dimensional distribution map with coordinate axes of steam mass flux,subcooled water temperature,and Re number of water flow.As the steam mass flux and the temperature of the subcooled water increase,the dimensionless penetration length of the steam plume gets signifi-cantly longer.However,as the Re number of water flow increases,the dimensionless penetration length of the steam plume will be greatly shortened.The values of measured dimensionless penetration length of steam plume are 0.8-4.5.Based on the one-dimen-sional model of the dimensionless penetration length for the steam jet in a water pool,the Re number of water flow is considered and the one-dimensional model of dimen-sionless penetration length for steam jet in water flow is proposed.The steam mass flux,water inlet temperature and water flow rate were dimensionless into dimensionless steam mass flow density,condensation driving potential and Re number of water flow,respectively.Based on the three dimensionless parameters.The correlation for the di-mensionless steam plume penetration was proposed by fitting obtained experimental data under 140 conditions.The deviation between the predicted value and the measured value is within±25%.The average heat transfer coefficient of the steam jet in water flow is significantly higher than that of the steam jet in the stagnant water.As the steam mass flux and the temperature of subrcooled water increase,the average heat transfer coefficient decreases.However,as the Re number of water flow increases,the average heat transfer coefficient increases.The measured average heat transfer coefficient ranges from 1.6 to 5.5 MW/(m2 K).The correlations for the average heat transfer coef-ficient and the average Nu number were proposed by fitting the experimental data of 140 operating conditions.The deviation between the experimental value and the pre-dicted value is within ±20%.Based on the Euler-Eulerian two-fluid model,particle model has been employed to investigate the interfacial characteristics of the steam jet condensation in subcooled water flow,in which a thermal phase change model was inserted into ANSYS CFX as CFX Expression Language(CEL)to simulate the interphase heat and mass transfer process.A condensation model based on the interface area and interphase heat and mass transfer was developed.Combined with the theory of expansion and compression waves,the formation mechanism of three typical steam plumes was revealed,and the mechanism of steam plume expansion and contraction characteristics is analyzed.A comprehensive analysis of the axial total pressure,dynamic pressure and static pressure distribution reveals the internal mechanism of the pressure-lift phenomenon at the tail of the steam plume.By analyzing the distribution of thermodynamic parameters along the axial direction of the nozzle,it is found that at the low steam mass flux,weak ex-pansion occurs when the steam is ejected from the nozzle,and then the steam plume flows inwardly under the effect of the compression wave,thus forming a conical steam plume.At high subcooled water temperature,after ejecting from the nozzle,the steam first undergoes a weak compression process,which causes the flow direction inflexed inward.Then it undergoes a weak expansion process,making the flow direction in-flexed outward,and finally it undergoes a compression wave that restricts expanding outward.Therefore,the steam plume forms a contraction-expansion-contraction shape under the effect of two compression waves and one expansion wave.At relative high steam mass flux and subcooled water temperature,when the steam is ejected from the nozzle,the pressure is higher than the environment pressure,which causes further expansion.The steam flow bends outward under the reaction of the expansion wave.When the pressure is less than the environment pressure,a compression wave occurs,which makes steam flow direction inflexed inward.In the case of an expansion wave and a compression wave,the steam plume exhibits ellipsoidal shape.According to the simulation,the steam can continue to expand and accelerate after being ejected from the nozzle,and the supersonic flow is reached at a distance outside the nozzle.The numerical results are consistent with the experimental results.Based on the overall ther-mal equilibrium of the interface,i.e.the dynamic balance of steam inflow rate and con-densation rate,the expansion and contraction characteristics of the steam plume are analyzed.The simulation results show that as the steam mass flow rate and water tem-perature increase,the steam plume penetrates longer distance and the vapor-liquid in-terface expands.However,as the Re number of water flow increases,the gas-liquid interface contracts and the steam plume length shortens.As the steam inlet mass flux increases,the local heat transfer coefficient at the interface increases.As the water inlet temperature increases,the local condensation rate decreases.The Re number of sub-cooled water flow has a great influence on the condensation characteristics.As the Re number of water flow increases,the local heat transfer coefficient increases and the local condensation rate increase.The size of the interface depends on the rate of con-densation and it also affects this rate.The steam plume will contract when the conden-sation rate increases.When the condensation rate is reduced,the steam plume will ex-pand.By analyzing the total pressure distribution of the axis and the contours of gas volume fraction,it is found that there exists a pressure-lift phenomenon at the tail of the steam plume.The dynamic pressure of the water also shows a pressure-lift at this position,so the dynamic pressure of the water is the main factor of the total pressure-lift.The effect of non-condensable gas on the four typical steam plumes of pure steam jet was investigated by visualization method.The dimensionless penetration length and average heat transfer coefficient were calculated by digital image processing.The non-condensable gas content of the study was 0-1%.It was found that at 1%non-condensa-ble gas content,compared with the submerged pure steam jet,the dimensionless pene-tration length was extended by 16-70%,and the average heat transfer coefficient de-creased by 12-50%.A numerical model for the condensation of submerged steam con-taining non-condensable gas jet in water flow was developed.The effect of non-con-densable gas on submerged steam jet is considered by the species transport equation of the gas phase.The effects of non-condensable gas on steam plume shape,air mass frac-tion distribution,axial thermodynamic parameter distribution,local heat transfer coef-ficient and local condensation rate of steam jet in water flow were analyzed.It was found that when the steam with non-condensing gas jet occurs,the steam plume pene-tration length is prolonged.Non-condensable gas is mainly distributed near the vapor-liquid interface,forming resistance from both heat transfer and mass transfer.Based on the analysis of the axial thermodynamic parameters,as the non-condensable gas content increases,the expansion in the nozzle becomes weaker and the condensation rate de-creases,resulting in a decrease of pressure fluctuation in the three-phase mixing region.The momentum decay is slow due to the non-condensable characteristics of the air,so the higher the non-condensable gas content corresponds to higher flow rate in the mix-ing region.More importantly,the non-condensable gas reduces the local heat transfer coefficient and local condensation rate at the interface,which explains the main reason for the prolongation of steam plume.In this paper,the experimental and numerical results on submerged steam jet con-densation in water flow in a pipe will enrich the theory of steam-water direct contact condensation and provide an important basis for the design and optimization of related industrial equipment. |
PDF Full Text Request