Condensation Of Moist Air Research Articles

Condensation of Moist Air on Mesh-like Surfaces

Condensation of Moist Air on Mesh-like Surfaces

Experimental and modeling investigations of dropwise condensation out of convective humid air flow

In this work, experimental and modeling investigations of convective dropwise condensation from moist air are carried out for the mechanistic understanding of the influence of non-condensable gas. For modeling, different droplet growth models in the presence of non-condensable gas are firstly developed by treating the droplet as an isolated form. During dropwise condensation, the presence of non-condensable gas brings some specific condensation characteristics, including the interaction effect between droplets and convection mass transfer. However, those characteristics cannot be reasonably captured by the droplet growth model. In this work, the distributed point sink method and the droplet Sherwood number are used to evaluate the inter-droplet interaction and convection mass transfer, respectively. Combining those progresses, the droplet growth rate is predicted and the prediction is evaluated by the measurements. Based on the accurate prediction for the droplet condensation rate, a semi-analytical modeling method is released to predict convective dropwise condensation of moist air. It considers the latent heat by condensation and the sensible heat by forced convection. Finally, this work presents the direct comparisons of the overall heat flux measured in experiments and the prediction. Furthermore, mechanistic understanding is discussed simultaneously and conclusively. Throughout this work, we highlight the reduction (the diffusion resistance of water vapor and the inter-droplet interaction) and enhancement (convection mass transfer) factors for dropwise condensation in the presence of non-condensable gas. In addition, in the case of this work, large droplets are more beneficial for heat transfer of dropwise condensation.

Feature Extraction of Oscillating Flow With Vapor Condensation of Moist Air in a Sonic Nozzle

The sonic nozzle is commonly used in flow measurement. However, the nonequilibrium condensation phenomenon of moist air in the nozzle has a negative effect on the measuring accuracy. To investigate this complex phenomenon, the experiments on the oscillating condensation flow of moist air were conducted by an adjustable humidification apparatus with different relative humidities (0%-100%), temperatures (30-50°C), and carrier gas pressures (1-6 bar), where the microsize pressure measuring system was designed by Bergh-Tijdeman (B-T) model. The accurate mathematical model of nonequilibrium condensation was also built and validated by the experimental data of time-averaged pressure distribution. Then, the frequency and intensity of pressure fluctuation of oscillating flow at a wide range of operation condition were obtained combining experimental data and physical simulation model. Importantly, a new semiempirical relation of dimensionless frequency deduced from dimensionless analysis was identified accurately by experimental data. Finally, the signal nonstationarity was also observed using the continuous wavelet transform (CWT). The instantaneous frequency saltation and the energy attenuation of pressure signals were observed in the condensation flow.

A correlation of metallic surface roughness with its hydrophobicity for dropwise condensation

Abstract The surface morphology alike lotus leaf has high contact angle and low contact angle hysteresis with respect to water droplet because of nanoscale roughness superimposed on micro order roughness in well-organized manner. The major application of superhydrophobic metallic substrate is in the condensing unit of energy and water harvesting devices. In this work, nanoscale roughness is superimposed on micro-order roughness of Aluminum substrate. The chemical texturing method is used to create micro-scale roughness onto Al surface. However, 0.6 keV Ar ions are irradiated for various time span to fabricate nano order roughness on chemically etched substrate. The roughness of samples is measured by Atomic Force Microscopy and static contact angles and their hysteresis are measured by lab customize Goniometer. It is found that the static contact angle and contact angle hysteresis increases after irradiation on chemical etched surfaces. Multi-scale roughness is observed on irradiated substrate. Results also shows that contact angle increases with increasing time of irradiation. However, maximum change in static contact angle is observed for samples irradiated for 45 s. Eventually, the static contact angle is correlated with its surface roughness. This correlation is vital for optimizing parameter affecting condensation rate in dropwise condensation. Finally, the moist air condensation experiments are conducted to know the condensation efficacy of the irradiated substrate. Also, it is found that the drop sliding frequency on vertical irradiated Al substrate is higher than the chemical etched Al substrate.

Moist Air Condensation on Inclined Hydrophobic Metallic Surfaces: Simulation & Experiments

Atmospheric Water Generators (AWGs) are popularly used for harvesting portable water from atmospheric air of hot, humid and arid regions of the world. Hydrophobic metallic surfaces are preferred in AWGs systems because condensation of moist air on/underneath these surfaces have high efficacy. There are several issues to fabricate hydrophobicity on metallic surfaces. Although with the advent of Nanotechnology and thin film coating technologies, the fabrication of hydrophobicity on metallic surfaces has become easier and realizable in the recent era. In this manuscript, a comprehensive mathematical model is developed for simulating the moist air condensation in the form of droplets on various substrates and in different environmental conditions. The experiment is carried out for validation of the present model. Post validation, the effect of surface hydrophobicity, relative humidity and degree of sub-cooling on the condensation rate are addressed. The simulation results show that vertically orientated metallic surfaces having a high contact angle along with low contact angle hysteresis are efficient for condensing unit. Larger condensation rates are observed at higher relative humidity and a high degree of sub-cooling. This research is helpful for designing efficient and effective AWGs for the hot and humid region.

The interaction effects between droplets condensing from moist air using a distributed point sink method

In modelling dropwise condensation, growth of the droplet is frequently assumed in an isolated form. However, for dropwise condensation of moist air the vapor diffusion from the surrounding to the droplet surface will be tremendously influenced by the blocking effect of the neighboring droplets. The influenced spatial distribution of water vapor totally determines a different condensation rate comparing with that by the isolated droplet model. In this work, a distributed point method (DPSM) as the methodof Green function is developed to capture the interaction effects of droplets without requiring solution of the diffusion equation and the numerical discretization. Due to its nature, the automaticallysatisfied boundary conditions make sure the solution accuracy based on the uniqueness theorem. The significant characteristics for the interaction effects between droplets are investigated by handling a series of droplet arrays. During dropwise condensation, a typical droplet array containing up to 1000 droplets is considered. The results indicate that the interaction effect between droplets is critical in accurately predicting the condensation behavior for dropwise condensation in the presence of non-condensable gas.

Modeling of heat and mass transfer for dropwise condensation of moist air and the experimental validation

A single droplet model is developed to describe the droplet growth during dropwise condensation of moist air on a cold substrate. The condensation process is divided by the droplet surface into two parts. The first step, i.e. the processes of mass transfer from the surroundings to the droplet surface, is modeled by the Kinetic theory and the laws of continuum fluid dynamics formulated using the two-region concept (Knudsen layer and continuum region) at any droplet size and at any concentration of non-condensable gas (NCG). The second step, i.e. the heat transfer across the droplet, is governed by Fourier’s law of heat conduction. These three regions (the continuum region, the Knudsen layer and the region inside the droplet) are incorporated by the matching both the mass flow rates and the energy flow rates. From these, the droplet growth rate, the nucleation size of droplet, the temperature at the droplet surface and Knudsen layer interface can be evaluated depending on different conditions. For this, a numerical algorithm is developed to reflect the droplet dynamics sufficiently detailed, including nucleation, growth/coalescence, slide-off/fall-off, re-nucleation. This is applied to the simulation of the entire condensation process by putting the growth rate and minimum radius from single droplet model into the growth algorithm. Additionally, dropwise condensation experiments of moist air in different relative humidity (RH) are carried out for validation of the simulation results. Good agreement is obtained which demonstrates that the present droplet growth model for dropwise condensation of moist air is credible. The current model and experiments also indicate that the diffusion resistance of water vapor in air from the free stream toward the droplet surface has a significant influence for the heat transfer performance of dropwise condensation of moist air.

二次元直線フィンにおける結露を含む伝熱簡易計算法 : 第1報-結露を伴う二次元フィン効率モデルの提案

二次元直線フィンにおける結露を含む伝熱簡易計算法 : 第1報-結露を伴う二次元フィン効率モデルの提案

二次元直線フィンにおける結露を含む伝熱簡易計算法 : 第2報-フィン温度近似関数と潜熱バイパスファクターモデルの提案

二次元直線フィンにおける結露を含む伝熱簡易計算法 : 第2報-フィン温度近似関数と潜熱バイパスファクターモデルの提案

Experimental investigation of moist air condensation on hydrophilic, hydrophobic, superhydrophilic, and hybrid hydrophobic-hydrophilic surfaces

This study experimentally investigates the condensation performance amid hydrophobic, hydrophilic, superhydrophilic and hydrophobic-hydrophilic hybrid patterned surfaces with air velocity ranging from 0.5m/s to 4.0m/s and relative humidity of 85%, 60% and 40%. The hybrid novel surface employs inverted V shape channels design with alternate hydrophilic and hydrophobic channel to direct condensate, and the accumulated condensate is gathered at vertical hydrophilic channel for further effective condensate removal. It is found that the heat transfer coefficient for hydrophobic surface is higher than that of hydrophilic surface irrespective of the operational velocity and relative humidity. Dropwise condensation prevails for the hydrophobic surface and a twig-like structure of condensate is seen for the hydrophilic surface, and this phenomenon becomes more pronounced when the relative humidity is increased. The superhydrophilic surface shows the worst heat transfer performance due to filmwise condensation. The hybrid surface shows superior heat transfer performance over other surfaces. The heat transfer coefficient obtained is around 3–9% higher than that of hydrophobic surface and is about 6–16% higher than hydrophilic one. The proposed novel design offers a shorter cyclic condensate removal time and better condensate drainage. It is found that the maximum diameter for the hybrid surface is about 80–90% smaller than the hydrophobic surface and the droplet size before falling off is relatively independent of operational velocity.

Investigation of the through-plane effective oxygen diffusivity in the porous media of PEM fuel cells: Effects of the pore size distribution and water saturation distribution

A thorough understanding of the oxygen transport properties in the gas diffusion layers (GDLs) of polymer electrolyte membrane (PEM) fuel cells is crucial for improving cell performance. However, because of the complex pore structure of the GDLs and the presence of water in them, the effective oxygen diffusivity (EOD) in porous medium of a PEM fuel cell is still not precisely understood. In this paper, the EODs were measured in two typical porous materials (TGP-H-120 carbon paper and carbon cloth, commonly used as GDLs) by using an oxygen sensor based on a galvanic cell under both dry and partially saturated conditions. The measurements were performed for various pore size distributions (PSDs) and water saturation distributions (WSDs); different PSDs were obtained by increasing the PTFE loading and different WSDs were realized via two impregnation methods using liquid water, namely vacuum impregnation and moist air condensation impregnation. The EOD was found to strongly depend on the PSD and WSD in the GDL, not simply on the average porosity and average water saturation, as reported in previous studies. Finally, this paper presents the experimentally verified equations that account for PSDs and WSDs under both dry and partially saturated conditions for the prediction of the EODs in PEM fuel cells.

Characteristics of transonic moist air flows around butterfly valves with spontaneous condensation

Effects of spontaneous condensation of moist air on the shock wave dynamics around butterfly valves in transonic flows are investigated by experimental and numerical simulations. Two symmetric valve disk shapes namely- a flat rectangular plate and a mid-plane cross-section of a prototype butterfly valve have been studied in the present research. Results showed that in case with spontaneous condensation, the root mean square of pressure oscillation (induced by shock dynamics) is reduced significantly with those without condensation for both shapes of the valves. Moreover, local aerodynamic moments were reduced in case with condensation which is considered to be beneficial in torque requirement in case of on/off applications of valves as flow control devices. However, total pressure loss was increased with spontaneous condensation in both the valves. Furthermore, the disk shape of a prototype butterfly valve showed better aerodynamic performances compared to flat rectangular plate profile in respect of total pressure loss and vortex shedding frequency in the wake region.

SPONTANEOUS NONEQUILIBRIUM CONDENSATION IN SUPERSONIC JETS IMPINGING ON CAVITY

Spontaneous condensation of moist air in supersonic jets is of considerable interest in a variety of natural and industrial processes. During impingement of supersonic moist air jets, the nonequilibrium homogeneous condensation can be experienced at the region between downstream of nozzle exit and an obstacle. The subsequent release of latent heat thus results in a deceleration of the flow and a rise in pressure, known traditionally as the condensation shock; likely have strong effect on the flow features. The present paper reported of the effect of spontaneous nonequilibrium homogeneous condensation of moist air on the aerodynamic and oscillatory flow features of supersonic jets impinging on cavity. A total variation diminishing (TVD) scheme was used to solve the time dependent Favre averaged Navier–Stokes equations, and the droplet growth equation of liquid phase production for simulating the condensing jets. Both qualitative and quantitative validations of the numerical model were accomplished, and the results showed a good agreement between the computed results and experimental data. Predicted flow and oscillatory features of jets were presented.

Condensation of Moist Air in Heat Exchanger Using CFD

Condensation of Moist Air in Heat Exchanger Using CFD

Transonic moist air flows with release of latent heat by non-equilibrium condensation

This study proposes a numerical method for solving transonic moist air flows with non-equilibrium condensation by taking FLUENT as the secondary development platform. The governing equations consist of moist air motion equations with an additional source term in the energy equation, which considers the effects of heat addition, and a set of four ordinary differential equations that are related to the generation of condensate mass. The moist air motion equations are solved using the standard features of the code in FLUENT, whereas the four ordinary differential equations are modelled using the user-defined-scalar transport modelling provided by FLUENT. This numerical method is validated both under internal and external flow conditions in a turbine cascade and over the NACA 0012 airfoil. Further investigations include testing the moist air flows in a compressor cascade channel and over the asymmetric RAE 2822 airfoil. The results show that the non-equilibrium condensation of moist air under transonic flow condition has a significant influence on the flow field structure and the aerodynamic performance of the turbine and compressor cascade.

Effect of condensing moist air on unsteady shock waves around a symmetric disk in transonic internal flows

Shock waves are often observed around a symmetric disk in transonic or supersonic internal flow fields. In these cases, shock wave-boundary layer interaction becomes complex and induces the unsteady self-excited shock oscillations in the flow field. However, when water vapor is contained in the main flow, rapid expansion of the flow may give rise to non-equilibrium condensation of moist air. In the present study, the effect of condensing moist air flows on unsteady behavior of shock waves around a symmetric disk is studied numerically. The flow fields were investigated at two pressure ratios (total back pressure to total reservoir pressure) of 0.739 and 0.500. Results showed that in the absence of non-equilibrium condensation, the shock wave oscillates alternatively between the upper and lower disk surfaces at the pressure ratio of 0.739 and in the wake region at 0.500. However, in the presence of non-equilibrium condensation, the flow field aerodynamic instabilities such as root mean square of pressure oscillation and shock induced oscillations frequencies were reduced significantly compared with those in the absence of non-equilibrium condensation. In addition, the total pressure loss increased and wake frequency was reduced in case with condensing moist air flows.

Numerical study of shock tube flows with homogeneous and heterogeneous condensations in rarefaction wave

Shock tubes are devices in which the state of a gas is changed suddenly from one uniform state to another by the passage of shock and expansion waves. In the theory of ideal shock tube flow, it is customarily assumed that the unsteady expansion and shock waves generated by diaphragm rupture are a perfectly centered plane wave. However, such waves are generally not centered, or may not even by plane in practice. In the present research, the time-dependent behavior of homogeneous and heterogeneous condensation of moist air in the shock tube is investigated by using a computational fluid dynamics work. Further, the numerical and experimental studies were carried out in order to investigate the effect of the diaphragm rupture process on the flow characteristics of expansion and shock waves generated near the diaphragm.

Transonic Moist Air Flow around a Symmetric Disc Butterfly Valve with Non-Equilibrium Condensation

Transonic flow around a symmetric disc butterfly valve is associated with the appearance of shock waves on valve surfaces. In this case, the interaction between shock waves and boundary layers becomes complex and thus generates the flow-induced aerodynamic instability. In the transonic or supersonic flow where vapour is contained in the main flow, the rapid expansion of the flow may give rise to non-equilibrium condensation. In the present study, the effect of non-equilibrium condensation of moist air on the shock-induced flow field oscillation around a butterfly valve was investigated numerically. The results showed that in cases with non-equilibrium condensation, the flow field aerodynamic instabilities such as root mean square of pressure oscillation and shock-induced oscillation frequency are reduced significantly compared with those without the non-equilibrium condensation. Moreover, the total pressure loss increases and the vortex shedding frequency is reduced with non-equilibrium condensation.

Supersonic moist air jet impingements on flat surface

Pronounced aeroacoustic resonances are exhibited in the flowfield where a jet emerges from an orifice or a nozzle and impinges on a solid surface. One instance where such resonances are produced is in a high speed jet impingement, such as in the space launch vehicle systems, jet-engine exhaust impingement, and in the short take-off and vertical landing (STOVL) aircraft, etc. A highly unsteady flowfield leading to a drastic increase of noise level with very high dynamic pressure and thermal loads are noticed on nearby surfaces results dramatic lift loss, severe ground erosion and hot gas ingestion to the inlet in the jet engines. This highly unsteady behavior of the impinging jets is due to a feedback loop between the fluid and acoustic fields. In actual jet flow, the working gas may contain condensable gas such as steam or moist air. In these cases, the non-equilibrium condensation may occur at the region between nozzle exit and an object. The jet flow with non-equilibrium condensation may be quite different from that without condensation. Therefore, in this study, the effect of the non-equilibrium condensation of moist air on the axisymmetric under-expanded supersonic impinging jet on a vertical flat plate was investigated numerically.

Transonic moist air flow around a circular arc blade with bump

The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan, compressor blades and butterfly valves, and this causes often serious problems such as aeroacoustic noise and the vibration. In recent years, the effect of bump wall on the flow field around an airfoil has been investigated experimentally and as a result, it was observed that the bump wall is effective for the control of shock wave on the airfoil. In the transonic or supersonic flow field, a rapid expansion of moist air or steam gives rise to non-equilibrium condensation. In the present study, the effect of non-equilibrium condensation of moist air on the self-excited shock wave oscillation around a circular arc blade with or without a bump on the blade was investigated numerically. The results showed that the non-equilibrium condensation significantly reduced the flow field unsteadiness such as root mean of pressure oscillation and frequency compared to the case without the non-equilibrium condensation.