Existence Of Non-condensable Gas Research Articles

Numerical Simulation on Two-Phase Ejector with Non-Condensable Gas

The two-phase ejector is a simple and compact pressure boosting device and widely used in ejector steam-generator water feeding systems and core emergency cooling systems. The direct contact condensation of water and steam is the key process of a two-phase ejector. Usually, the high-temperature and high-pressure steam will inevitably induce non-condensable gases. The existence of non-condensable gases will reduce the condensation heat transfer rate between steam and water, and harm the equipment. This study carried out 3D numerical simulations of a two-phase ejector based on an inhomogeneous multiphase model. The steam inlet pressure and the non-condensable gas mass fraction rang in 0.6–2.9 MPa and 1–10%, respectively. The heat and mass transfer characteristics were analyzed under different conditions. The results show that the heat transfer coefficient and plume penetration length increased with the steam inlet pressure. Non-condensable gas prevents direct contact condensation between the steam and water. The non-condensable gas mass fraction rises from 1% to 10%, the heat transfer between steam and water deteriorates, and leads to a lower heat transfer coefficient.

Numerical study on the interfacial heat and mass transfer characteristics of bubble direct contact condensation with the effect of non-condensable gas

Bubble condensation is one of the fundamental issues in the direct contact condensation, which is also affected significantly by the existence of non-condensable gas. In this study, a further investigation is carried out on air-steam bubble condensation with the Proportion Integration Differentiation (PID) control model which adjusts the mass transfer coefficient of Lee model in each time step by comparing the parameters obtained in the simulation with the ones calculated based on the empirical correlations originated from the experiments. Furthermore, species transport equations are employed to obtain the local mass fractions of steam and air. The proposed simulation schema is verified by the existing experimental data.Results show that the air-steam bubble condensation can be divided into initial and stable stages. The mass transfer coefficient used in Lee model can be regarded as linear growth over time at each stage, but with different slopes. The non-condensable gas can weaken the heat and mass transfer of condensation, but stabilize the bubble deformation. The present study also reveals the influences of subcooling and initial bubble diameter on the air-steam bubble condensation, which is expected to be instructive for better understanding the effect of non-condensable gas on the interfacial characteristics of bubble condensation in subcooled water.

Experimental study of non-condensable gas effects on sonic limit of sodium heat pipe

Due to unexpected gas leakage and long-term operation, the non-condensable gas (NCG) inevitably exists in high-temperature alkali metal heat pipe. Previous studies have mainly focused on its influences on startup and steady state operation characteristics of heat pipes. However, few of them conducted experiments to study the effects of NCG on sonic limit, which usually occurs during heat pipe startup or during steady state operation when the heat transfer coefficient at condenser section is high. In present work, experimental and analytical studies are conducted to the explore the effects of NCG on sonic limit of sodium heat pipes. With the heating power ranging from 500 to 1400 W, the sonic limit of 1 m-long sodium heat pipes with artery wick and axial groove wick are obtained based on oil-cooling system, which can widely adjust the cooling capacity at condenser section. The amount of NCG in heat pipe is estimated by Heat Pipe Transient Analysis Code (HPTRAN) and verified by experimental data. It is found that the existence of NCG in heat pipe leads to the decrease of maximum heat transfer rate under sonic limit. The mechanism of NCG effects on sonic limit is discussed based on phenomenological analysis. By introducing the added back pressure, a modified model to predict sonic limit considering NCG effect is proposed and the comparison with experimental data shows good agreements.

Numerical evaluation of non-condensable gas influence on the heat transfer characteristics of high-temperature lithium heat pipe during reactor operation

The high-temperature lithium heat pipe utilized in the reactor will be irradiated by neutrons, causing non-condensable gas (NCG) to generate within and deteriorate thermal performance. This study develops a high-temperature heat pipe transient analysis code that takes into account the NCG effect. Heat pipe frozen start-up and key parameters sensitivity study are numerically evaluated. The NCG influence can be well described by the code. When compared to heat pipes without NCG, the start-up time of heat pipes with 25% NCG volume fraction increases by 54.2%. However, the existence of NCG can make the start-up process go more smoothly. The temperature drop in the NCG area changes from 40 K to 395 K as the NCG volume fraction increases from 5% to 25%. Increased input power can lower the volume fraction of NCG and hence diminish the negative thermal effects. This work provides foundation for transient analysis of high temperature heat pipe containing NCG.

Phase-change lattice Boltzmann simulation of condensate falling and heat transfer on hybrid-wettability surface in the presence of non-condensable gas

• Condensate falling off hybrid-wettability surfaces is studied under non-condensable gas effect. • Pseudopotential MRT LB method is coupled with finite difference for phase-change multicomponent flow. • Droplet nucleation, growth and departure time prolong as non-condensable gas fraction increases. • Hydrophilic-superhydrophobic hybrid surface exhibits higher heat transfer performance. A hybrid phase-change lattice Boltzmann model is employed to investigate the condensate falling from subcooled surface in the presence of non-condensable gas. The effects of hydrophilic length ratio, the non-condensable gas fraction ( w NCG ) and the hybrid-wettability of the subcooled surface on condensate droplets dynamics and heat transfer characteristics are investigated systematically. It shows that the average heat flux at the subcooled surface first increases and then decreases with the increase of hydrophilic length ratio. Owing to the increased thermal resistance by the thicker non-condensable gas layer, the nucleation time, growth time as well as departure time of condensate droplets are prolonged as the non-condensable gas fraction increases. The nucleation time of w NCG = 0.4 is 1.5 times higher than that of w NCG = 0.1 , and the growth time and departure time of w NCG = 0.4 are 1.7 times higher than those of w NCG = 0.1 . As the surface wettability changes from hydrophilicity-superhydrophobicity ( θ y = 60 o - θ g = 150 o ) to hydrophilicity-more hydrophilicity ( θ y = 60 o - θ g = 30 o ), the condensation pattern transits from dropwise to filmwise mode. Moreover, the local heat flux at hydrophilic-superhydrophobic ( θ y = 60 o - θ g = 150 o ) and hydrophilic-hydrophobic ( θ y = 60 o - θ g = 105 o ) surface is obviously higher than that at hydrophilic-hydrophilic ( θ y = 60 o - θ g = 75 o ) and hydrophilic-more hydrophilic ( θ y = 60 o - θ g = 30 o ) surface. The present simulations clarify the mechanism of condensate droplets falling off subcooled surface with the existence of non-condensable gas and are conducive to the design of hybrid-wettability surfaces for condensation heat transfer enhancement, which is of great significance to energy resources conservation.

Experimental study of heat transfer characteristics on condensation in the presence of NCG through thermal resistance analysis

As an efficient approach of heat transfer, condensation is applied to the passive containment cooling system in the nuclear power plant for a fast export of heat. However, the existence of noncondensable gases in the containment will inhibit condensation heat transfer then increase the risks to the nuclear power reactors. In order to search effective methods to improve the condensation heat transfer, this study investigated condensation in the presence of NCG experimentally focusing on two aspects: (i) to search heat transfer characteristics of filmwise condensation; (ii) to evaluate possible heat transfer enhancement taken by dropwise condensation. For the first one, a new numerical model was developed to quantitatively analyze the thermal resistance distribution in the condensation process. The result indicated that the effect of various parameters on condensation heat transfer depended on the variation of both the thermal resistances of the liquid film and NCG accumulation layer. It provided a reliable explanation for the divergence of early perspectives around the effects on condensation heat transfer of subcooling or system pressure. For the second one, dropwise mode was proved to significantly enhance the condensation heat transfer when the noncondensable gas existed. However, this heat transfer enhancement is limited by the NCG accumulation in the gaseous mixture.

The effect of NCG on the characteristics of hydraulic cavitation

Hydraulic cavitation, as an important and complex hydrodynamic phenomenon, has long drawn attention. In this paper, the ZGB (Zwart-Gerber-Belamri) cavitation model is improved and the effect of NCG (noncondensable gas) on cavitation in water is studied by numerical simulation. The influence of NCG on the cavity length, the temperature of the cavities and the mixed viscosity of the cavities is investigated through the improved ZGB cavitation model. In addition, experiments on hydrodynamic cavitation produced by a Venturi tube are used to validate the improved ZGB cavitation model. The results show that NCG not only shortens the length of the cavity but also reduces the volume fraction of the vapor. The existence of NCG decreases the viscosity in the cavity of the Venturi tube but increases the viscosity at the sidewall of the tube. In addition, the temperature in the cavities increases with increasing NCG. Regardless of whether air is injected, the volume fraction of the vapor in the cavities increases first and then decreases with increasing temperature. However, the transition temperature decreases somewhat after injecting air. Therefore, the influence of NCG on hydraulic cavitation is significant, and the role of NCG should be considered in industry.

Numerical Analysis of Laminar Convective Condensation with the Presence of Noncondensable Gas Flowing Downward in a Vertical Channel

The purpose of this paper is to study and perform a numerical analysis of the simultaneous processes of mass and heat transfer during the condensation process of a steam in the existence of noncondensable gas (NCG) inside a descending vertical channel. In this study, the flow of the vapor-air mixture is laminar and the saturation conditions are prevailing at the inlet of the channel. The coupled control equations for liquid film, interfacial conditions, and mixture flow are solved together using the approach of finite volume. Detailed and valuable results are presented both in the liquid condensate film and in the mixing regions. These detailed results contain the dimensionless velocity and dimensionless temperature profiles in both phases, the dimensionless mass fraction of vapor, the axial variation of the dimensionless thickness of the film liquid δ⁎, and the accumulated condensate rate Mr as well the local Nusselt number Nuy. The relative humidity at the inlet varies from 60% to 100% and the inlet temperature from 40°C to 80°C. The results confirm that a decrease in the mass concentration of NCG by the increasing the inlet relative humidity has a direct influence on the liquid film layer, the local number of Nusselt, and the variation of condensation rate accumulated through the channel. The results also designate that an increase of the inlet relative humidity and the inlet temperature ameliorates the condensation process. The comparison made for the coefficient of heat transfer due to condensation process and the condensate liquid film thickness with the literature results is in good concordance which gives more credibility to our calculation model.

Numerical study of condensation flow regimes in presence of non-condensable gas in minichannels

A numerical research on flow regime of R134a in horizontal minichannels was conducted. The model was on the basis of the volume of fluid approach, and inserted user-defined routines which cover condensation mass and heat transfer. The observed annular flow, injection flow and intermittent flow of pure steam condensation, and annular (annular-wavy) flow, injection flow, annular-intermittent flow and intermittent flow of gas mixture condensation were qualitatively compared against experimental data. Flow patterns of lower non-condensable gas mole concentration show similar with that of pure steam, while the fluctuation of gas-liquid interface in annular flow and the noticeable annular-intermittent flow of higher non-condensable gas mole concentration were noted. Besides, injection flow characters with larger head volume and longer neck, and intermittent flow characters with larger slug sizes in existence of non-condensable gas were found. Furthermore, the influence of mole concentration of non-condensable gas and operating conditions on flow pattern transition lines were analyzed after validating the flow pattern transition. The results showed that the injection flow transition point has a relative fixed point at pure vapor condensation, while various in presence of NCG. Besides, lower non-condensable gas mole concentration, smaller inlet gas mass flux and larger wall heat flux could lead to earlier flow pattern regime.

Effect of evaporator tilt on a loop heat pipe with non-condensable gas

The coupling effect of non-condensable gas (NCG) and evaporator tilts on the steady state operation of a loop heat pipe (LHP) was investigated both experimentally and theoretically in this work. Nitrogen was injected quantitatively into an ammonia-stainless steel LHP to simulate NCG, and the steady state characteristics of the LHP were studied under three typical evaporator tilts. According to the experimental results, the main conclusions below can be drawn. (1) The temperature is the highest under adverse tilt and the lowest under favorable tilt no matter whether there is NCG in LHP. (2) The existence of NCG could cause the increase of temperature under all three typical evaporator tilts, but the temperature increment caused by NCG seems to be relatively small under adverse tilt. (3) The increments of the temperature caused by NCG display different patterns under different tilts. Theoretical analysis was conducted to explain the results: the temperature under the coupling effect of NCG and evaporator tilt was determined by the energy balance between the heat leak from evaporator to compensation chamber and the cooling capacity of returning subcooled liquid. With the increase of heat load, the augmentation of heat leak caused by NCG and the enhancement of subcooled liquid cooling effect were incongruent. The coupling effect of NCG and evaporator tilts should be considered in the terrestrial application of LHP.

Effect of non-condensable gas on the startup of a loop heat pipe

It is essential to address the startup issues prior to the wide application of loop heat pipes (LHPs) in both space and terrestrial surroundings. As non-condensable gas (NCG) is an important factor affecting the startup behavior, its effects on the startup performance of an ammonia-stainless steel LHP with and without preconditioning were experimentally investigated in this work. Nitrogen with controlled amounts was used to simulate the NCG, and the temperature overshoot, liquid superheat and startup time were employed as the evaluation criteria. Four situations relating to initial liquid/vapor distribution in the evaporator were examined: (1) both evaporator core and vapor grooves are filled with liquid, (2) vapor exists in vapor grooves and the evaporator core is filled by liquid, (3) vapor grooves are filled by liquid and vapor exists in the evaporator core, and (4) vapor exists in both evaporator core and vapor grooves. Experimental results showed that with NCG presence in the LHP, the startup could only proceed in situation 1 with preconditioning, while it could proceed in situations 1, 3 or 4 without preconditioning. For the startup in situation 1, a larger NCG inventory led to much degraded startup performance, and a higher startup heat load could benefit the startup. For the startup in situation 3, the most difficult startup situation, NCG resulted in a very high temperature overshoot, which may even exceed the maximum allowable value. For the startup in situation 4, the existence of NCG in the vapor grooves could facilitate the evaporation there, leading to a very desirable startup.

Accurate Prediction of Preheat Temperature in Solar Flash Desalination Systems Using Kernel Ridge Regression

AbstractThermal desalination systems consist of phase-change operations to separate freshwater from bulk seawater. Solar desalination systems involve partial vaporization of seawater, known as flashing, using solar heat then condensation of flashed steam to produce fresh water. The latent heat of the condensing steam is usually utilized to preheat seawater, thus increasing the energy efficiency of the desalination systems. Evaluating the feasibility of a solar desalination system requires accurate determination of seawater preheat temperature exiting the condenser to enter the evaporator. Determining this temperature is very challenging due to the complicated phase-change dynamics and the existence of noncondensable gases in the condenser that were dissolved in seawater. The preheat temperature depends on several factors such as seawater flow rate, system vacuum, and flashed vapor temperature. This study utilizes the kernel ridge regression (KRR) method to predict the preheat temperature as a function of ...

Dynamic process modeling on depressurization by cooling-controlled condensation in a closed chamber

It has long been realized that condensation in a chamber prefilled with condensable vapor leads to chamber depressurization, and the condensation rate can be cooling controlled. While the final state can be reasonably estimated based on the thermodynamic equilibrium, the dynamic process of depressurization is not only transient but also in thermal non-equilibrium. This transient and non-equilibrium characteristics, mainly governed by the rate of condensation, has not been satisfactorily modeled, which is due to the complicated coupling mechanisms of heat and mass transfer during the condensation, the presence of non-condensable gas (NCG) within vapor, as well as the complex geometry and properties of chamber and cooling device involved. In this paper, we have conducted an experimental study on depressurization by steam condensation onto an internal cooling coil in a steam-prefilled closed chamber. To reveal various parametric effects on the depressurization process, a parametric model consisting of a set of coupled ordinary differential equations has been established, with some simplified assumptions including lumped heat capacity sub-models for chamber walls, cooling coils and the gas phase. To further explore the thermal non-equilibrium characteristics during the process, a full-field and transient simulation of computational fluid dynamics (CFD) is also conducted using FLUENT with user-defined function (UDF) on boundary of condensation. Both parametric and CFD models consider the existence of NCG that is pre-mixed with the vapor as impurity. By comparison with the experimental measurements, both models reasonably predict the dynamic and asymptotic characteristics of depressurization with time. The CFD simulation indicates an almost instant equilibrium in pressure within the chamber and yet non-equilibrium in temperature with noticeable temperature gradients over the gas phase. The simplified parametric model gives quick predictions of some major parametric effects (e.g., vapor purity, coolant flow rate, and vessel volume) on the rate of depressurization. The detailed mechanistic understanding, gained from proposed models, provides insights essential to the optimized design and operation of the depressurization by cooling-controlled condensation.

Performance characteristics of barometric-type open-cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996

Performance characteristics of barometric‐type open‐cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996

バロメトリック型オープンサイクル海洋温度差発電システムの動作特性

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996