Air Mass Fraction Research Articles

Experimental investigation on thermal stratification induced by steam-air mixture vertical injection with shallow submergence depth

A series of experiments have been performed to investigate thermal stratification induced by steam-air mixture vertical injection, in the range of steam mass flux 3–30 kg/m2s, air mass fraction 0–5% and submergence depth 100–300 mm. Experimental results show that: thermal stratification occurs at the region where none of chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume is strong enough. For pure steam experiments, thermal stratification occurs when the steam mass flux above 15 kg/m2s. The pool is mixed well firstly, then thermally stratified, and then re-mixed in sequence, as the water temperature increases. A little air can make thermal stratification and re-mixing occur ahead of time and more air can completely remove thermal stratification. Gas bypass induced by shallow submergence depth makes thermal stratification occur earlier and more severe. The submergence depth has little effect on thermal stratification in chugging regime.

Effect of Cooling Water Flow Path on the Flow and Heat Transfer in a 660 MW Power Plant Condenser

The effect of the cooling water flow path on the flow and heat transfer in a double tube-pass condenser for a 660 MW power plant unit was numerically investigated based on a porous medium model. The results were used to analyze the streamline, velocity, air mass fraction and heat transfer coefficient distributions. The simulations indicate that the cooling water flow path is important in large condensers. For the original tube arrangement, the heat transfer with the lower-upper cooling water flow path is better than that with the upper-lower cooling water flow path. The reason is that the steam cannot flow into the internal of upper tube bundle and the air fractions are higher in the upper tube bundle with the upper-lower cooling water flow path. An improvement tube arrangement was developed for the upper-lower cooling water flow path which reduced the back pressure by 0.47 kPa compared to the original scheme. Thus, the results show that the tube arrangements should differ for different cooling water flow paths and the condenser heat transfer can be improved for the upper-lower cooling water flow path by modifying the tube arrangement.

Numerical Investigation of Natural Convective Condensation with Noncondensable Gases in the Reactor Containment after Severe Accidents

The heat and mass transfer processes of natural convective condensation with noncondensable gases are very important for the passive containment cooling system of water cooled reactors. Numerical simulation of natural convective condensation with noncondensable gases was realized in the Fluent software by adding condensation models. The scaled AP600 containment condensation experiment was simulated to verify the numerical method. It was shown that the developed method can predict natural convective condensation with noncondensable gases well. The velocity, species, and density fields in the scaled AP600 containment were presented. The heat transfer rate distribution and the influences of the mass fraction of air on heat transfer rate were also analyzed. It is found that the driving force of natural convective condensation with noncondensable gases is mainly caused by the mass fraction difference but not temperature difference. The natural convective condensation with noncondensable gases in AP1000 containment was then simulated. The temperature, species, velocity, and heat flux distributions were obtained and analyzed. The upper head of the containment contributes to 35.1% of the total heat transfer rate, while its area only takes 25.4% of the total condensation area of the containment. The influences of the mass fraction of low molecular weight noncondensable gas (hydrogen) on the natural convective condensation were also discussed based on the detailed species, density, and velocity fields. The results show that addition of hydrogen (production of zirconium-water reaction after severe accident) will weaken the intensity of natural convection and the heat and mass transfer processes significantly. When hydrogen contributes to 50% mole fraction of the noncondensable gases, the heat transfer coefficient will be reduced to 45%.

Numerical investigations on steam condensation in the presence of air on external surfaces of 3 × 3 tube bundles

Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure.

Experimental investigations on pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water

Present paper aims to experimentally investigate pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water. Dynamic pressure signals and jet images are recorded and analyzed. Experimental results show that: the pressure oscillation dominant frequency decreases with the rise of water temperature, inlet pressure and air mass fraction. Air mass fraction has a complex effect on pressure oscillation intensity. When water temperature is less than 55 °C, as the air mass fraction increases, the pressure oscillation intensity increases first, and then decreases slightly, and then increases quickly. When water temperature is more than 55 °C, as the air mass fraction increases, the pressure oscillation intensity decreases first, and then increases quickly. The interfacial fluctuation height and pressure oscillation intensity dependency to the air mass fraction is the same near the nozzle exit. In addition, the validity of previous analysis model for steam sonic jet pressure oscillation dominant frequency is evaluated. A modified function considering the effect of air mass fraction is developed. The predicted results of steam-air mixture gas sonic jet pressure oscillation dominant frequency agree well with the experimental results. More than 98% of the data points are inside the error band of ±20%.

Experimental study of pure steam and steam–air condensation over a vertical corrugated tube

The natural convection condensation heat transfer outside the vertical smooth tube and corrugated tube was experimentally investigated in pure steam and steam–air conditions, respectively. The aim of the research was to evaluate the heat removal capacity of the corrugated tube through comparing with the test results of the smooth tube. In the experiment, a corrugated tube was tested, the bulk temperature and the wall temperature of crest and trough were measured in detail. The result of the temperature measurement demonstrated that the difference of heat transfer characteristics at the crest and trough is not significant. In the pure steam condition, the influence of subcooling on condensation heat transfer coefficient of the corrugated tube is reduced. Comparing with the smooth tube, the condensation heat transfer coefficient of the corrugated tube has been increased for more than 10%. In the steam–air test condition, the condensation heat transfer coefficients have been obtained for the air mass fraction from 0.1 to 0.95 and the total pressure from 0.2 MPa to 0.5 MPa. The corrugated tube shows a slight enhancement with an average increase of condensing heat transfer coefficient about 10%.

Numerical simulation of air–steam mixture condensation flows in a vertical tube

In this study, a fully compressible three-phase numerical model for steam condensation flow in the presence of air is presented. The model solves the Reynolds-averaged Navier–Stokes equation coupled with two interface advection equations. The Lee condensation model was applied to simulate the mass and energy transfer processes via interfaces. Several test cases were simulated for the air–steam mixture condensation flow in a vertical tube with different air mass fractions to verify the numerical model. The validity was demonstrated through comparisons of the centerline temperatures along the tube and experimental data; the results were in good agreement with experimental data. A detailed radial profile of the temperature, species mass fraction, and velocity was analyzed. A developing thin condensate water film along the tube and the thickest film layer at the outlet tube wall of approximately 4.0 mm were observed. Air accumulation at the steam–water interface was also represented. In addition, the effects of air mass fraction on the characteristics of the condensation rate were investigated.

Experimental Study on the Condensation of Steam With Air Out of the Vertical Tube Bundles

In this paper, experimental study was carried out in an open natural circulation loop,to figure out the condensation heat transfer characteristic on the outside of the vertical pipe tube bundle and single- tube. Condensation heat transfer coefficients have been obtained under the wall subcooling ranging from 5 to 22◦C, total pressure ranging from 0.3 to 0.6MPa and air mass fraction ranging from 0.05 to 0.65. The influence of mixed gas pressure, condensate depression of walls, and content of non-condensable gas on the condensation heat transfer performance is analyzed. Under the condition of the same air mass fraction, the condensation heat transfer coefficient increases with the increase of pressure. Also, as the air mass fraction is more than 30%, the effect of pressure will be weakened. The heat transfer features of pipe bundle and single tube is compared and studied, and an empirical correlation of the pipe bundle for the heattransfer coefficient is developed, covered all data points within 10%. According to the research results, heat transfer coefficient for pipe bundle decreases with the increase in non-condensable gas quality and wall subcooling, but increases with the increase in pressure.

Effect of non-condensable gas on pressure oscillation of submerged steam jet condensation in condensation oscillation regime

The pressure oscillation is an important characteristic of direct contact condensation of steam in subcooled water. Many experimental works have been performed on pure steam submerged jet condensation. However, the effect of non-condensable gas content on the characteristics of the pressure oscillation is not yet fully understood. So, present paper aims to investigate the effect of air mass fraction on the characteristics of the pressure oscillation. Experimental results show that: for pure steam jets, the pressure oscillation dominant frequency decreases with the rise of water temperature and nozzle diameter. While it increases with the rise of steam mass velocity, which is consistent with the most of previous research results. The pressure oscillation intensity increases with the rise of water temperature, steam mass velocity and nozzle diameter. For air-steam mixture gas jets, the effect of water temperature and steam mass velocity on pressure oscillation characteristics is the same as pure steam jets. The pressure oscillation dominant frequency rapidly decreases with the rise of air mass fraction. However, air mass fraction has a complex effect on pressure oscillation intensity. As the air mass fraction increases, the pressure oscillation intensity rapidly increases at first, then slowly decreases and then slowly increases. In addition, new correlations for pressure oscillation dominant frequency and intensity are developed. The predicted results agree well with the experimental results.

Investigations on the penetration length of steam-air mixture jets injected horizontally and vertically in quiescent water

Gas submerged jet penetration length is a key parameter for many industrial applications. Many experimental and theoretical works have been performed on pure steam and non-condensable gas submerged jets. However, the steam-air mixture gas submerged jets are not yet fully understood. So, a series of visualization experiments are performed to investigate the effect of air mass fraction, jetting direction, water temperature and inlet pressure on mixture gas submerged jet penetration length. Two flow regimes are distinguished based on the evolution of the interface between water and mixture gas, which are referred to as stable regime and oscillation regime. A flow regime map is developed based on inlet pressure and air mass fraction. A momentum balance model for predicting jet penetration length is proposed. Results show that the mixture gas jet penetration length increases with the increasing inlet pressure and decreases with the increasing air mass fraction. The water temperature has little effect on the jet penetration length. A new correlation for dimensionless vertical jet penetration length is developed based on the form of model analysis. The discrepancy between predicted results and present experimental data is within ±5%. A new correlation for dimensionless horizontal jet penetration length is developed based on experimental results. The discrepancy between predicted dimensionless horizontal jet penetration length and experimental value is within ±18%.

Development of a new empirical correlation for steam condensation rates in the presence of air outside vertical smooth tube

In the past several decades, experimentalists have proposed a large number of correlations to estimate steam condensation rates in the presence of noncondensable gases within free convection regime. However, these correlations are in different forms, and even express conflicting dependencies of the condensation coefficient on some parameters. This study is to aim at developing a new correlation which can faithfully reflect the complex dependencies of condensation coefficient on pressure, air mass fraction and wall subcooling. Considering that more measured data can certainly help to analyze the relationship between condensation coefficient and these parameters, total 374 sets of experimental data have been obtained. It is found that there is a negative power function relation between condensation coefficient and wall subcooling, and the exponent varies with wall subcooling and total pressure. The new correlation, covering two orders of magnitude in the condensation heat transfer coefficient, is valid for pressure ranging from 0.2 to 0.5 MPa, air mass fraction ranging from 0.1 to 0.8 and wall subcooling ranging from 10 to 70 °C. Finally, we have made comparison with experimental data obtained by other investigators which shows that the new correlation is applicative for not only steam-air condensation but also steam-nitrogen and steam-argon condensation, and it can be extended to larger scope of engineering applications.

Enhancement of Condensation Heat Transfer Rate of the Air-Steam Mixture on a Passive Condenser System Using Annular Fins

This paper presents an experimental investigation on the enhancement of the heat transfer rate of steam condensation on the external surfaces of a vertical tube with annular fins. A cylindrical condenser tube, which is 40 mm in outer diameter and 1000 mm in length, with annular disks of uniform cross-sectional area is fabricated in the manner of ensuring perfect contact between the base surface and fins. A total of 13 annular fins of 80 mm diameter were installed along the tube height in order to increase the effective heat transfer area by 85%. Through a series of condensation tests for the air-steam mixture under natural convection conditions, the heat transfer data was measured in the pressure range of between 2 and 5 bar, and the air mass fraction from 0.3 to 0.7. The rates of heat transfer of the finned tube are compared to those that are measured on a bare tube to demonstrate the enhanced performance by extended surfaces. In addition, based on the experimental results and the characteristics of steam condensation, the applicability of finned tubes to a large condenser system with a bundle layout is evaluated.

Local phenomena analysis of steam condensation in the presence of air

In severe accident scenarios (e.g. LOCA), the steam condensation inside the containment area is of vital importance. This is because the containment pressure peak and long-term residual heat removal are closely related to the steam condensation efficiency. Note that the gas inside the containment area is a mixture of steam and air/hydrogen. Steam condensation consisting of non-condensable gases has its own characteristics. Compared to pure steam condensation, it has a much lower heat transfer coefficient and the local phenomena are more complex. Considering that steam condensation is closely related to other phenomena in containment thermal-hydraulic analysis, further studies should be carried out. In the present work, the STAR-CCM+ software is used with a diffusion boundary layer condensation model. Various cases from the COPAIN and Su experiments are simulated and discussed. In the simulation of the COPAIN experiment, the effect of the mesh density (main-stream mesh size and the Y+ value) on the boundary heat flux is compared with the experimental data. Additionally, the local velocity, temperature, and air mass fraction are compared and discussed. In the simulations of the Su experiment, calculated results for the main-stream temperature, near-wall region temperature, and heat transfer coefficient are compared with experimental results and discussed. The COPAIN numerical results indicate that (i) the boundary heat flux decreases linearly with any increase in the air layer thickness of up to 5 mm, and the effect of the air layer weakens with further increases in thickness; (ii) the convective heat flux is only responsible for up to 9.1% of the total boundary heat flux; and (iii) the automatic wall treatment gives a reasonable prediction of steam condensation. The simulations of the Su experiment show that (i) there is no obvious temperature stratification in the main-stream along the vertical direction and (ii) the thickness of the air layer close to the condensation tube is around 6 mm. These results provide basic guidance for the future design of condensation heat transfer enhancement structures.

Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

Abstract. Pollution transport from the surface to the stratosphere within the Asian monsoon circulation may cause harmful effects on stratospheric chemistry and climate. Here, we investigate air mass transport from the monsoon anticyclone into the stratosphere using a Lagrangian chemistry transport model. We show how two main transport pathways from the anticyclone emerge: (i) into the tropical stratosphere (tropical pipe), and (ii) into the Northern Hemisphere (NH) extratropical lower stratosphere. Maximum anticyclone air mass fractions reach around 5 % in the tropical pipe and 15 % in the extratropical lowermost stratosphere over the course of a year. The anticyclone air mass fraction correlates well with satellite hydrogen cyanide (HCN) and carbon monoxide (CO) observations, confirming that pollution is transported deep into the tropical stratosphere from the Asian monsoon anticyclone. Cross-tropopause transport occurs in a vertical chimney, but with the pollutants transported quasi-horizontally along isentropes above the tropopause into the tropics and NH.

Interface dynamics and pressure oscillation of stable steam jet condensation in water flow in a confined channel with the presence of non-condensable gas

Though direct contact condensation (DCC) has been widely used in many industrial fields, the noise and vibration caused by pressure oscillation becomes an unignorable problem during the operating of related equipments. So there is an urgent need to find methods to reduce noise and vibration. In the present work, experimental study was conducted on the dynamic of steam-air mixture condensation in water flow in a confined channel, expecting to reduce the pressure oscillation intensity by means of adding a proper amount of air into steam. The experiments were tested in stable flow pattern region with steam mass flux, water Reynolds number, water temperature and air mass fraction in the range of 350–600kg/m2s, 78,000–164,000, 30°C and 0–5%, respectively. The observation and image processing of interface behavior and flow pattern transitions provides insight into the pressure oscillation mechanism with non-condensable gas involved. Dynamic pressures on the wall of channel were obtained. The results showed that the DCC of steam-air mixture was determined by two important factors, e.g. gas velocity at nozzle outlet and air content. When gas had a higher velocity, the air layer around the interface would be destroyed so that the effect of air on condensation would be weakened, but the interface was relatively stable. With air content increased to a critical value, stable flow patterns would transform into unstable due to the increase of air quantity. Moreover, the flow pattern transitions could be reflected also by the variation of pressure oscillation intensity. When air content was lower than the critical value, the gathering of air around the interface decreased the fluctuation of interface penetration length, which resulted in the dramatic decrease of pressure oscillation intensity. However, with air content increased to be higher than the critical value, stable jet would transform into unstable, leading to a rapid increase of pressure oscillation intensity. In addition, the results of various test conditions showed that the critical air mass fraction increased with the increase of steam mass flux but decreased with the increase of water Reynolds number. Furthermore, an optimal air mass fraction which could minimize the pressure oscillation intensity under various test conditions was suggested.

Experimental study on heat transfer characteristics of a condenser in the presence of air

In this study, the heat transfer characteristics of a condenser in the presence of air were investigated experimentally. A condenser system with a 450mm×447mm×630mm chamber was designed and constructed. The condensing tube was copper coil tube each with a total length of 4650mm, and the number of tube rows was 17. The experiments were conducted with different air mass fractions, inlet cooling water temperatures, cooling water flow velocities, and heat loads. With regard to the overall performance of the condenser, experimental results showed that the overall heat transfer coefficient decreased, whereas the condenser pressure and terminal temperature difference of the condenser both increased with an increase in the air mass fraction. With regard to the local heat transfer characteristics, the experimental results showed that the condensation heat transfer coefficients initially decreased slightly, then rapidly, and finally slightly again as the number of the tube row became higher in the flow direction of the steam when air leakage occurred in the condenser. The variation rate of the condensation heat transfer coefficient with the tube row significantly changed twice in the condenser. As the air mass fraction and the cooling water velocity increased, the critical rows, where the variation rate of the condensation heat transfer coefficient with the tube row significantly changed, generally moved upward. By contrast, the critical rows generally moved downward with increases in the inlet cooling water temperature and heat load.

Lattice Boltzmann simulation of condensation in the presence of noncondensable gas

In this paper we use the multiphase multispecies Lattice Boltzmann method to investigate the influence of non-condensable gas on condensation. Condensation on a horizontal cold wall as well as that on a vertical wall with droplet movement is investigaged. The presence of non-condensable gas obviously reduces the condensation mass rate as well as the heat flux compared to condensation from pure vapor. The waiting time before nucleation is increased with non-condensable gas, and the wetting characteristics are also changed (the contact angles are increased), which further influences the heat transfer. Correlations of the relationship between droplet diameter and condensing time for different surface wettability, or contact angles, as well as different air mass fraction are obtained. As for condensation on a surface parallel to the gravitational force, it’s demonstrated that the presence of non-condensable gas reduces the droplet departure diameter and increases the period between subsequent droplet formation and departure.

Effect of the non condensable gas type during condensation of water vapor

In this paper, a numerical study is performed to investigate the influence of the non?condensable gas type in a vapor mixture of water gas (water vapor?krypton, water vapor?argon, water vapor?air, and water vapor?neon) during the condensation along a vertical pipe with a wall cooled by air-flow. The applied numerical method solves the coupled parabolic governing equations in both gas and liquid phases with the appropriate boundary and interfacial conditions. The equations systems, obtained by using an implicit finite difference method are solved by Thomas algorithm. The numerical results obtained show that the heat and mass transfer is influenced by increasing the molar mass of non?condensable gases. The comparisons of air mass fraction, bulk temperature, local condensate heat transfer coefficient, and average Nusselt number of sensible heat with the literature results and the available experimental data are in good agreement.

An experimental study of air–steam condensation on the exterior surface of a vertical tube under natural convection conditions

To investigate the heat transfer characteristics of a condenser tube in a Passive Containment Cooling System (PCCS) of an advanced nuclear power plant, an experimental investigation has been conducted on the air–steam condensation over external surfaces of a vertical tube under natural convection conditions. Local temperature data and average heat transfer coefficients were measured on a test condenser of 40mm in O.D. and 1.0m in length while axial air concentrations and wall temperatures were kept almost uniform. The experiments covered the pressure ranging from 2 to 5 bar, the air mass fraction from 0.10 to 0.88, the wall subcooling from 19 to 70K, and the Grashof number of an air-steam mixture varied from 1.36·1010 up to 1.42·1011 accordingly. In particular, the effect of the wall subcooling on the condensation heat transfer coefficient and the total heat removal rate was evaluated. From the comparison to Dehbi’s tube data, it was found that the condensation heat transfer coefficients on a vertical tube were significantly influenced by the strength of natural convection of the air-steam mixture, which can be described by the Grashof number. To complement the inadequacy of physical bases in deriving previous correlations, a new empirical correlation for the condensation heat transfer coefficient was proposed by using a consolidated data of this study and Dehbi’s test; the Nusselt number is correlated in terms of the Grashof number, Jakob number, and the air mass fraction. The suggested correlation was assessed by experimental data and its deviation was turned out to be less than 26%.

Lumped Parameter Modelling of Cavitating Orifice Flow in Hydraulic Systems

Gaseous cavitation is an important issue that introduces negative effects on the performance of hydraulic systems. The lumped parameter modelling approach is widely adopted in the research of hydraulic applications due to its quickness and convenience to apply at the system level. For this reason, a novel lumped parameter model of cavitating orifice flow based on a control volume concept is presented in this paper. In particular, a procedure of calibrating the unknown model coefficients is provided by using the computational fluid dynamic (CFD) method as well as test data. A test rig for studying cavitation in an external gear pump is set up with a variable orifice at the inlet port. The four coefficients in the CFD cavitation model is first identified by the measured mass flow rate through the test orifice; then the CFD simulated fluid field offers required information including the average air mass fraction in downstream region to determine the two coefficients of lumped parameter (LP) model. To further verify the calibrated models, another test at a different pump speed is carried out to compare it to the model prediction. Good matching with experiments justifies the proposed approach and the calibration procedure. Suggestions for future work include extending the approach to the study of hydraulic valves and high-speed pumps.