Condensation Of Moist Air Research Articles

Optimization of impeller contour and diversion blade of a moist air turbo expander with spontaneous condensation

The turbo-expander is the crucial equipment for obtaining refrigeration capacity and power in the system, and the aerodynamic performance is vital for the system. Spontaneous condensation of moist air in a high-speed turbo expander has a significant effect on the efficiency and output power. Based on the model of non-equilibrium spontaneous condensation, the flow of moist air in a high-speed turbo expander was studied, which is verified by the experiments. The meridional surface shape affects the variation of the flow passage cross-sectional area. The impeller with a uniform variation in cross-sectional area has the highest efficiency. Based on the optimal meridional surface shape, the condensation flow of moist air with different impeller blade diversion curvature was compared. The optimal curvature can increase the efficiency of the moist air turbo-expander by 2.83 %, and the optimized impeller has 3.53 % higher output power. The largest curvature of the blade leads to an efficiency reduction by 3.06 %, but it has little droplet condensation in the impeller, which can help to avoid damage of high-speed blades.

Improved Heat Transfer Modeling of Moist Air Condensation on Hydrophobic Metallic Surface

For dropwise condensation of water over a textured surface, the condensation rate is limited by the conduction resistance of the droplet. Conduction models with varying levels of approximation have been reported in the literature. In the present study, the conduction resistance of a spherical-cap droplet is derived by regression analysis of data generated by 3D numerical simulation. Results show that conduction resistance is sensitive to the choice of contact angle and droplet radius. The proposed model has significant differences from reported models which are based on 1-D and 2-D approximations. An important source of discrepancy is traced to additional heat flux localized at the three-phase contact line. Equivalently, high local heat flux serves to lower thermal resistance between the vapor-liquid interface and the condensing wall. The prediction of the droplet growth rate using the proposed conduction model is in good agreement with the experiments reported in the literature. The improved conduction resistance correlation is incorporated in the model for a space- and time-resolved dropwise condensation cycle for vapor condensing from moist air. Simulations with the improved condensation model show close agreement with experimental data, justifying the utility of an improved correlation for conduction resistance.

Numerical and experimental investigation on condensation inside a turbine designed for an 100 kW polymer electrolyte membrane fuel cell system

This work presents mathematical modelling of moist air condensation process in a fuel cell turbine. A condensation model based on the classical nucleation theory and the Hertz–Knudsen droplet growth model is established. The numerical approach is validated through experimental results of the MS nozzle and a fuel cell turbine, and the simulation fits the experimental data well. Influences of inlet temperature and pressure on condensation process in the turbine is examined. Results show that increase of inlet temperature apparently weakens condensation and inlet heating can be realized by arranging a gas-to-gas heat exchanger in the air supply subsystem without consuming additional energy. Moreover, at the same mass flow rate increase of inlet pressure reduces the expansion ratio, thereby curbing the condensation. Compared with the matching scheme of a bypass valve, turbocharger matching with a back-pressure valve allows higher inlet pressure to be achieved and can effectively suppress the condensation.

Study on the spontaneous condensation of moist air in the high-speed turbo-expander

Study on the spontaneous condensation of moist air in the high-speed turbo-expander

Enhancing dropwise condensation of vapor from moist air over a copper substrate by temperature-controlled chemical etching

• Fabrication of superhydrophobic copper substrate via various chemical etching and deposition process. • Demonstrate proposed copper superhydrophobic substrate in atmospheric water generator device. • Optimization of atmospheric water production via moist air condensation at various environmental conditions. • Experimental trends on the proposed surface were supported by a mathematical model of dropwise condensation. Surface engineering of copper substrates for condensation of vapor in moist air in an atmospheric water generator (AWG) is an intensive area of research. The goal here is to enhance the vapor condensation rate, thus yielding a large quantity of potable water. In the present study, three scalable chemical processes such as deposition, etching, and etching with deposition have been explored for preparation of the copper substrate. In these processes, polished copper substrates are dipped in select chemical solutions for appropriate periods of time and temperature. The resulting surface morphology as well as surface chemistry has been characterized using SEM and EDS analysis. In addition, surfaces are characterized in terms of hydrophobicity with respect to water. Moist air condensation experiments have been carried out on the proposed substrates for estimating their performance in terms of vapor condensation from atmospheric air. Results show that the water collection rate at relative humidity of 90% and degree of sub-cooling at 6 °C achieved for the ‘ ea ’, substrate is as high as 1.2 l/m 2 -h; it is 1.4 l/kWh with respect to the power expended to maintain a low surface temperature. The surface efficacy is improved by 63% on the ‘ ea ’ substrate and 16% on the ‘ e 2 ’ substrate with respect to a clean polished copper surface. These results are validated against simulations from a time-dependent dropwise condensation model. It founded that the superhydrophobic copper substrate prepared as per the stated methodology serves as an improved surface for condensation of water vapor present in moist air.

Plasma nano-patterning for altering hydrophobicity of copper substrate for moist air condensation

Fabrication of durable hydrophobic metallic substrate is an emerging area of research for the condensation process in many industrial devices because its implementation enhances the efficacy of the devices. In this work, effective and durable hydrophobic copper (Cu) substrates were fabricated by plasma nano-pattering. Various discharge voltages were used to create various types of nano-patterns textured on copper substrates. For 30 min of irradiation of Argon gas plasma at various discharge voltages of 500, 650 and 750 V, morphologies of substrates were altered such that micro-litre water droplet has contact angle of 105 ± 3o, 97 ± 5o, and 95 ± 5o, respectively. However, pristine polished Cu substrate has water droplet contact angle of 56±5o. The micro-graph of scanning electron microscope (SEM) showed micro-order to nano-order roughness on etched substrates and the structure of nano-patterns was different for different discharge voltages for given period of argon ions irradiation. The etched copper substrate at 500 V was deployed as a condensing surface for moist air condensation of temperature 28 °C, relative humidity 70% and degree of sub-cooling 10°C. These results showed that the water condensation rate of plasma etching hydrophobic Cu substrate was 438 ml/m2-h, compared to 188 ml/m2-h for pristine Cu substrate.

Fabrication and analysis of hydrophobic mesh-metallic surface for moist air condensation

ABSTRACT In the advancement of materials and their processing technologies, the alteration in the morphology of metallic surfaces to enhance the condensation rate of moist air is an emerging area of research for improving the performance of atmospheric water generators. In this work, hydrophobic mesh-metallic surfaces were fabricated via a thermal diffusion process in which a micro-order copper mesh with a diameter of 6 µm thermally diffuses on a steel surface of grade 100. The prepared surface was chemical etched before hexadecyltrimethoxysilane coating via the immersion method to grow silica nanostructure in a hierarchical order. A custom-built goniometer was used for measuring the surface wettability in terms of contact angle. The proposed surface has a 140 ± 5° static contact angle of the water droplet. The coated thermally diffused copper mesh to the steel surface fosters low wettability with respect to water. Eventually, a moist air condensation experiment was carried out in controlled atmospheric conditions on both coated and uncoated surfaces to demonstrate water harvesting from moist air. The hydrophobic mesh-metallic surface has high efficacy for moist air condensation as compared to the uncoated thermally diffused mesh-metallic surface. It makes the proposed hydrophobic coating substrate applicable for water harvesting from moisture.

Development of a three-dimensional numerical model of indirect evaporative cooler incorporating with air dehumidification

Indirect evaporative cooler (IEC) is treated as an energy-efficient and low carbon-emission air conditioning solution without utilizing the mechanical compressor and chemical refrigerants for cooling production. Currently, the widely-used one-dimensional (1-D) and two-dimensional (2-D) numerical models of IEC can expose the temperature and humidity variations of the airflow direction for predicting the cooling performance. However, the air profile along the channel width direction is usually simplified as constant in those models, neglecting the effect of non-uniformity distribution in the normal direction of heat exchanger plates. In this paper, a novel 3-D IEC numerical model of a cross-flow IEC was developed based on the computational fluid dynamics (CFD) method. The judgment on moist air condensation was incorporated in the model by a built-in generalized minimum residual iterative solver, which enables the performance prediction of IEC in hot and humid areas. Detailed air temperature and moisture content distributions along and perpendicular to the air channels were obtained with sufficient validations from published and experimental data. Parameter analysis was conducted on the primary air inlet properties and the channel gap distance. Results show that the prediction accuracies of temperature and humidity were improved by 5.8 and 6.7% compared with the 2-D model. The air dehumidification process started to appear from 27 ℃ given the 70% RHp,in. Under the circumstance of condensation state, the highest wet-bulb efficiency was 0.8 given the 0.5 m/s air velocity against 4 mm optimized gap distance, while the greatest dehumidification rate was 0.3 in the studied ranges. The maximum net energy saving was obtained as 4.7 kW under scorching and wet climate conditions. The proposed 3-D model can benefit future study on the heat and mass transfer mechanism of advanced plate materials by providing the non-uniformity of air profiles along the Y-axis, and thus contribute to the development of next-generation IEC technology.

Numerical Study of Convective Condensation Heat Transfers for Moist Air on a 3-D Finned Tube

The condensation of moist air is a common process in industry and increase its heat transfer coefficient is of great importance for improving energy efficiency. This study used 3-D finned tube to enhance moist air condensation, and the condensation process was calculated through numerical method aligned with Diffusion Layer model and Eulerian Wall Film model. The total heat transfer, sensible heat transfer and latent heat transfer during the condensation process were discussed. The results indicated that 3-D finned tube increased heat transfer amount and the condensation was mainly on fin surface. Besides, the influence of 3-D fin height was also obtained. The results indicated that the increase of fin height in circumferential direction can increase heat transfer in the tail of tube, and sensible heat transfer process was more sensitive to the area increment.

Modelling and Analysis of Factors Affecting Moist Air Condensation on Mesh Like Surface for Sustainable Water Harvesting

Moist air condensation is a complex process because it relies on numerous significant parameters such as physio-chemical properties of a surface, thermal-physical properties of the moist air, and environmental conditions. Thus, the manuscript intent to structure the model-framework of sustainable water harvesting based on moist air condensation on mesh-like surfaces to increase the availability of potable water. Hence, the significant key factors related to sustainable water harvesting via condensation are identified from extant literature, consultation from researchers, and practitioners and subsequently confirmed by fabricating the experimental setup. Further than, an attempt by applying Total Interpretive Structural Modelling (TISM) is attained to provide a model framework. Thereafter, the procedure for analysis using MICMAC is implemented which performed to categorize the identified factors into the distinct clusters’ dependent on their driving and dependence powers to justify their interrelationship with one another. The research reveals that surface coating, mesh geometry, and material of condensing surface are the most dominant factor in the required hierarchy which helps in improving the condensation rate. Thus, the prioritization of the factors with improvisation intends to fill the gap for scalable, economic, sustainable, and environment-friendly condensation in various fields. This research work can be benefited to the industry to the area of water harvesting and energy conservation. Also, the researchers can consider the most influential factors identified in this research while fabricating experimental setups.

Super-Hydrophobic Nanostructured Silica Coating on Aluminum Substrate for Moist Air Condensation

Super-Hydrophobic Nanostructured Silica Coating on Aluminum Substrate for Moist Air Condensation

Moist air condensation heat transfer enhancement via superhydrophobicity

The main objective of the present study is to experimentally investigate the moist air condensation behavior subject to hydrophilic and superhydrophobic surface with a much broader operating conditions such as different relative humidities (RH = 40 – 85%) and degree of subcooling (ΔTsub= 1 – 16 K). The flow visualization of condensation behavior showed that the main condensate departure mechanism of the hydrophilic surfaces is gravity-driven sliding. In contrast, the primary mechanisms for the superhydrophobic surfaces are direct jumping and bounced-jumping caused by the coalescence induced phenomena. The superhydrophobic surfaces yielded a maximum of 36% improvement in heat transfer coefficient at 85% relative humidity over the hydrophilic surfaces, while the enhancement is about 16% for 60% and 40% relative humidities. Similarly, the maximum heat transfer coefficient of 37 W m-2K−1 is attained at the subcooling temperature of 16 K, and it is reduced to 31 W m-2K−1 and 20 W m-2K−1 respectively when the subcooling is reduced to 6 K and 1 K.

Modeling analysis of moist air condensation on a plate under forced convection conditions

A modeling analysis of the filmwise condensation for water steam outside a vertical plate under forced convective conditions was carried out in this study. The latent heat and sensible heat transfer under different moist air temperature, relative humidity, velocity, and wall temperature were theoretically analyzed. The model was developed based on the conservation equations of mass, momentum, energy, diffusion, and was solved numerically by finite difference methods. Meanwhile, the presence of condensate was neglected due to the low condensation load. Firstly, the results were compared with classical correlations and showed good agreement. Then the variation of latent and sensible heat transfer changing with operation conditions, such as moist air temperature, water steam mole fraction, wall temperature, and moist air velocity, were obtained and analyzed in detail. The results indicated that the latent heat transfer coefficient increased with the increase of relative humidity, moist air temperature, velocity, and cooling wall temperature. At the same time, the sensible heat transfer coefficient remains almost constant except under different moist air velocities, and latent heat transfer coefficient accounts for more than 70.3% in all the tested conditions of this study. The results obtained in this study can give a reference for the design of heat exchangers used in moist air condensation conditions.

Analysis of convective condensation heat transfer for moist air on a three-dimensional finned tube

The condensation of moist air plays an important role in daily life and industry. During the condensation process, single-phase and phase-change heat transfers occur simultaneously. A three-dimensional finned tube is an effective element for enhancing heat transfer. However, the latent and sensible heat transfer during moist air condensation outside a three-dimensional finned tube has not been experimentally studied. In this study, experiments were performed using a three-dimensional finned tube to enhance the moist air condensation process. For a better understanding of the enhancement mechanism, latent and sensible heat transfer variations with relative humidity ranging from 0.75 to 0.95 and Reynolds numbers ranging from 838 to 3444 were obtained and analysed. A smooth tube was used for the comparison. The experimental results indicated that the three-dimensional finned tube was favourable for the enhancement of moist air condensation, especially for latent heat transfer. When the Reynolds number was 2504, the latent and sensible heat transfers of the three-dimensional finned tube were 43.6%–81.8% and 15.6%–22.9% higher than those of the smooth tube, respectively. In addition, the latent and sensible heat transfers of the three-dimensional finned tube and the smooth tube were obtained under different velocities of moist air, and empirical correlations were developed to predict the sensible and latent heat transfers with deviations of less than ± 20%. The results obtained can provide a reference for the design of three-dimensional finned tubes used in moist air condensation.

A Study of Moist Air Condensation Characteristics in a Transonic Flow System

When water vapor in moist air reaches supersaturation in a transonic flow system, non-equilibrium condensation forms a large number of droplets which may adversely affect the operation of some thermal-hydraulic equipment. For a better understanding of this non-equilibrium condensing phenomenon, a numerical model is applied to analyze moist air condensation in a transonic flow system by using the theory of nucleation and droplet growth. The Benson model is adopted to correct the liquid-plane surface tension equation for realistic results. The results show that the distributions of pressure, temperature and Mach number in moist air are significantly different from those in dry air. The dry air model exaggerates the Mach number by 19% and reduces both the pressure and the temperature by 34% at the nozzle exit as compared with the moist air model. At a Laval nozzle, for example, the nucleation rate, droplet number and condensation rate increase significantly with increasing relative humidity. The results also reveal the fact that the number of condensate droplets increases rapidly when moist air reaches 60% relative humidity. These findings provide a fundamental approach to account for the effect of condensate droplet formation on moist gas in a transonic flow system.

Experimental study of moist air condensation outside three-dimensional finned tubes with different fin parameters

The condensation of moist air is a common phenomenon in daily life and industry, and increasing its heat transfer coefficient has always been a hot spot in research. Three-dimensional (3-D) finned tube is an effective heat transfer enhancement element and its fin parameters have an important influence on the enhancement result. This study carried out experiments using orthogonal design and Taguchi method to determine the influence of 3-D fin parameters on moist air condensation. The contribution ratios of axial fin spacing, fin height, fin width, and circumferential fin spacing on the heat transfer coefficient were obtained and analyzed in detail. Meanwhile, the photos of condensation processes were obtained. Experimental results show that all the heat transfer coefficient increased with the increase of relative humidity (RH) or the decrease of cooling water inlet temperature. Besides, all the 3-D finned tubes had higher heat transfer coefficient compared with smooth tube. The contribution ratio of fin height on the heat transfer coefficient was the highest, which can be 75.7% at most. Besides, the contribution ratio of axial fin spacing increased from 10.1% to 24.0% with the increase of RH. The optimal parameters combination of 3-D fins within the range of this experiment was also recommended. The results obtained in this study can provide guidance for the design and optimization of 3-D finned tubes used in moist air condensation.

Moist air condensation on teflon coated copper helical coil

The condensation transport coefficients are higher on the metallic surface because of the high thermal conductivity of metallic surfaces. However, the metallic surfaces have a higher affinity to water in the condensation process which leads to corrosion and results in damage to the condensing metallic surface. Low surface energy and low wettability surfaces promote efficient condensation in drop mode. Therefore, several surface modification techniques have been adopted for modifying the surface wettability of the metallic surfaces. Teflon coating on the metallic surface has been an economically viable and simple method for fabricating low surface energy coating on metallic surfaces. In this work, Cu coil is coated with PTFE film for fabricating hydrophobicity on Cu helical coil. Moist air condensation experiments are carried out on Teflon coated and bare Cu coil to investigate the effect of Teflon coating on the coil. In addition, the effect of moist ambient conditions on the Teflon coated coil is addressed to examine the aging of the Teflon coating on the Cu coil. The results show that the characteristics of hydrophobic Teflon coating deplete with increases in exposure time so the dropwise condensation transits into mixed condensation and ultimately in film mode as the coil is exposed to repeated condensation cycles. The results of the present study are useful to appropriately investigating in which all applications the Teflon coating can be implemented such that all its properties are retained in that time duration.

Self Assembly of Super-hydrophobic Nanotextured Methyl Functionalized Silica on Copper and Aluminium Surfaces for Moist Air Condensation

The super-hydrophobic surfaces enhance the transport coefficients of the condensation process and therefore it is very useful in water harvesting devices. In the present study, super-hydrophobic substrates are fabricated by growing self-assembled silica nanostructures on copper (Cu) and aluminum (Al) surfaces by a sol-gel based solution immersion method at room temperature. The contact angles measured were 155 ± 2° and 156 ± 2° on coated Cu and Al surfaces respectively. The water condensation rate on the coated Cu surface significantly increased from 188 ml/m2-hr to 630 ml/m2-hr, while that of the coated Al surface increased from 190 ml/m2-hr to 650 ml/m2-hr. The present work shows that the metallic substrates with super-hydrophobic coatings have excellent potential for water harvesting from moist air, and they were effectively used for more than 120 days.

Dropwise condensation from moist air over a hydrophobic metallic substrate

Moist air condensation is of significant scientific and societal interest especially for harvesting potable water from atmospheric air. Dropwise condensation of water vapor present in moist air is preferred when compared to filmwise condensation because of high transport coefficients that augment condensation rate of the distillate. The efficacy and sustainability of dropwise condensation depends on various operating parameters, for instance, temperature of moist air, relative humidity, subcooling of the substrate, thermophysical properties of moist air and physico-chemical properties of the condensing substrate. In this context, a mathematical model of dropwise condensation of moist air over a textured surface has been developed in the present work. Jointly, an experimental study of moist air condensation over a specially prepared superhydrophobic surface has been carried out for model validation. By suitably deriving the correction factor for the interfacial resistance to account for the presence of noncondensable gases, good agreement has been obtained between the model and experiments. Thereafter, simulations have been performed over a range of conditions from the viewpoint of maximizing the condensate output. Results of the study show that a vertical substrate with a high equilibrium contact angle and low contact angle hysteresis increases drainage of water drops. Hence, there is an increase in the water productivity of the surface. In addition, the condensation rate increases with increase in relative humidity, degree of subcooling and saturation temperature of moist air. The results of the study can be utilized towards designing devices for water harvesting from the atmosphere.

Condensation heat transfer characteristics of moist air outside a three-dimensional finned tube

The condensation heat transfer of moist air is a common phenomenon in industry processes, and improving the efficiency of condensation heat transfer is a common focus of research. Many studies have shown that extending the surface of the heat exchanger can effectively enhance the condensation heat transfer efficiency. Therefore, this study focused on a three-dimensional (3-D) finned tube heat exchange element with an extra-large extended surface, and experiments were conducted to study the condensation heat transfer characteristics of moist air outside the 3-D finned tube. The effects of different steam mole fractions, moist air velocities, cooling water inlet temperatures, and moist air temperatures on the condensation heat transfer characteristics of the moist air outside the 3-D finned tube were studied under atmospheric pressure. A smooth tube was also tested under the same conditions for comparison. The experimental results showed that the 3-D finned tube can achieve a higher heat transfer coefficient compared with the smooth tube under all tested conditions. Moreover, for both the 3-D finned tube and the smooth tube, the heat transfer coefficient increased with an increase in the steam mole fraction and moist air velocity, and decreased with an increase of moist air temperature or cooling water inlet temperature. Furthermore, in a single-phase convective heat transfer case, the 3-D finned tube could achieve 3.1 times the heat transfer coefficient of the smooth tube. In a moist air condensation heat transfer case, the 3-D finned tube could achieve 1.7 times the heat transfer coefficient of the smooth tube when the steam mole fraction was 0.05. Although the 3-D finned tube had a more serious condensate retention phenomenon compared with that of the smooth tube, the 3-D fins could still increase the heat transfer area and effectively break the flowing boundary layer outside the tube wall. In addition, the 3-D finned tube had a higher heat transfer coefficient compared with the smooth tube.