Condensation Of Humid Air Research Articles

Experimental investigation of humidified air condensation in different rows of serpantine heat exchanger – Cooling water flow rate effect

Condensing economizers are used in the industry as they enable the acquisition of additional heat during vapor condensation. In condensing heat exchangers, water is the usual cooling agent due to its high specific heat capacity and thus efficient heat removal. Therefore, experiments of hot humid air condensation in a serpentine economizer being cooled by water were performed to reveal the effect the cooling water flow rate has on humidified air condensation in separate rows of the serpentine type economizer (with vertical tubes). The results have shown that the cooling ratio (mass flow ratio between the coolant and the humid air) had a minor effect on the distribution of the humidified air temperature along test section for inlet Reynolds number of 3000–10000. The effects on the condensation flux distribution in different rows were much stronger. The most optimal cooling ratio was determined to be 3. The results have shown that the biggest condensation efficiency is up to 35%. It was revealed that in some cases the convection was prevailing in the first rows of the economizer and this resulted in a decreased efficiency/performance of the exchanger.The obtained results from the practical point of view will provide an extended basis for the optimisation of the design of economizers for waste heat recovery in the cases of different cooling water flow rates. It could also be applied to validate computational models developed for condensation heat transfer and condensate flux numerical modelling along economizer.

Electrowetting assisted shedding of droplets by a shearing airflow in dropwise condensation

Condensation is a necessary part of most industrial processes dealing with phase change. Efficient condensate removal (shedding) increases the systems’ performance. This study investigated the influence of electrowetting on droplet shedding and heat transfer enhancement during humid air condensation on a horizontal substrate. A mini closed-looped wind tunnel was used to simulate the condensation environment and control condensation parameters. AC electric field with the frequencies of 1 and 10 kHz was applied and compared with the control case (no voltage). The transient inverse heat conduction method was used to determine the heat transfer coefficient, and heat transfer measurements were synchronized to a high-resolution camera to monitor the condensation dynamics. The results show that the area-weighted average droplet radius was increased by 61 % and 84 % for 1 kHz and 10 kHz, respectively, compared to the control case for 1 m/s airflow. At 10 kHz, the shedding time was reduced, particularly at higher airflow velocities. Additionally, the heat transfer coefficient (HTC) exhibited a significant increase at 10 kHz when shedding was present. Notably, the difference in HTC became pronounced at an airflow velocity of 15 m/s, where shedding was prominent. In this scenario, the HTC rose by an impressive 35 % compared to the control case. These findings provide a promising foundation for utilizing EW to enhance heat transfer and improve condensate shedding under shear flow conditions.

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Surface modification is a widely utilized technique for enhancing condensation heat transfer. Two main properties of surfaces in manipulation of condensation heat transfer are the contact angle and contact angle hysteresis. This study focuses on the influence of contact angle (CA) and contact angle hysteresis (CAH) on humid air condensation. For this, hydrophilic and hydrophobic surfaces with varying CA and CAH values were fabricated and tested in a humidity-controlled climate chamber at different relative humidity levels. Three hydrophilic surfaces samples with a contact angle of approximately 70° and CAH values of 10°, 20°, and 42° were tested. Two hydrophobic surfaces with a contact angle of approximately 110° and CAH values of 21° and 39°were also prepared as well as a hydrophobic surface with a contact angle of 96° and a CAH of 43°. The role of CA and CAH in different stages of condensation cycle was investigated. Our findings show that while CA plays the main role in droplet nucleation, CAH has a significant impact on droplet coalescence and departure. Increasing CAH while keeping CA constant has a negative effect on condensation heat transfer in all wettability levels. However, the relationship between changes in CA while keeping CAH constant does not have the same trend in the condensation heat transfer performance for every case. Changing CAH for lower CAH values led to a greater impact on enhancing condensation heat transfer than higher values. Moreover, increasing CAH on hydrophobic surfaces had a more significant effect than on hydrophilic surfaces. Additionally, decreasing CAH had a more pronounced effect on improving condensation heat transfer than increasing CA. The findings emphasize on the importance of considering both the contact angle (CA) and contact angle hysteresis (CAH) in the surface design to have the optimum condensation heat transfer performance.

Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface

To provide further insight into humid air condensation on hybrid surfaces, an experiment was conducted to visually investigate the condensation process on wettability-interval grooved surfaces, which had hydrophobic ridges and hydrophilic grooves. The droplet dynamic behavior and heat transfer performance of condensation on a wettability-interval grooved surface were explored and compared with four other functional surfaces, including the plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, and hydrophobic grooved surface. The presence of hydrophobic ridges perpendicular to the groove direction and hydrophilic grooves allowed for the exclusion and easy spreading of droplets, respectively. Compared with the other four functional surfaces, the coupling phenomena during condensation, i.e., the spontaneous suction and directional drainage via hydrophilic grooves, were only found on the wettability-interval grooved surface. These could not only remove condensate quickly but also suppress the formation of the flooded liquid film, which was beneficial to the enhancement of heat transfer performance. It was proven by the experimental results that at subcooling 12 K, the condensation heat flux on the wettability-interval grooved surface reached 1280 W/m2, which was 1.25 times that of the plain hydrophobic surface (1030 W/m2), and 15% higher than that of the hydrophobic grooved surface (1110 W/m2). This indicated that the wettability-interval microgrooves could effectively enhance humid air condensation heat transfer performance.

Performance evaluation of a solar humidification dehumidification desalination system employing a multistage bubble column dehumidifier

In humidification-dehumidification desalination (HDD) system, conventional dehumidifiers require a large condensing area due to the additional thermal resistance to the heat transfer caused by high concentrations (60–95% by mass) of non-condensable gases present in the air. In a bubble column dehumidifier (BCD), condensation of humid air via direct contact with cold water in a column can significantly decrease condensing area and cost. Hence, in this study, a solar-powered HDD using a multistage BCD is designed and tested in the climatic conditions of Kerala, India. Single-stage and two-stage configurations are investigated in BCD. A mathematical model based on the energy and mass balance of each component is also developed. The model results are observed to be in good agreement with experimental measurements. The results showed that the daily freshwater yield, day average gain output ratio (GOR), and average BCD effectiveness of HDD system with two-stage BCD are 4.571 kg/m2, 0.79, and 0.9, respectively. Furthermore, upgrading the single-stage configuration of BCD to a two-stage configuration increases freshwater yield, day average GOR, and BCD effectiveness by about 6.24%, 6.75%, and 17%, respectively. Detailed water quality analysis revealed that freshwater quality is well within drinking water standards. The cost of freshwater is about 0.0716 $/liter.

Exploring the water capture efficiency of covalently attached liquid-like surfaces.

The capture of moisture from the atmosphere through condensation has the potential to provide a sustainable source of water. Here, we investigate the condensation of humid air at low subcooling condition (11 °C), similar to conditions for natural dew capture, and explore how water contact angle and contact angle hysteresis affect the rates of water capture. We compare water collection on three families of surfaces: (i) hydrophilic (polyethylene oxide, MPEO) and hydrophobic (polydimethylsiloxane, PDMS) molecularly thin coatings grafted on smooth silicon wafers, which produce slippery covalently attached liquid surfaces (SCALSs), with low contact angle hysteresis (CAH = 6°); (ii) the same coatings grafted on rougher glass, with high CAH (20°-25°); (iii) hydrophilic polymer surfaces [poly(N-vinylpyrrolidone), PNVP] with high CAH (30°). Upon exposure to water, the MPEO SCALS swell, which likely further increases their droplet shedding ability. MPEO and PDMS coatings collect similar volume of water (around 5 l m-2day-1), both when they are SCALS and non-slippery. Both MPEO and PDMS layers collect about 20% more water than PNVP surfaces. We present a basic model showing that, under low heat flux conditions, on all MPEO and PDMS layers, the droplets are so small (600-2000 µm) that there is no/low heat conduction resistance across the droplets, irrespective of the exact value of contact angle and CAH. As the time to first droplet departure is much faster on MPEO SCALS (28min) than on PDMS SCALS (90min), slippery hydrophilic surfaces are preferable in dew collection applications where the collection time frame is limited.

Numerical simulation of dropwise condensation over hydrophobic surfaces using vapor-diffusion model

Dropwise condensation of humid air over hydrophilic and hydrophobic surfaces is numerically investigated using a phenomenological, Lagrangian model. Mass flux through droplets free surface is predicted via a vapor-diffusion model. Validation with literature experimental data is successfully conducted at different air humidities and air velocities. The accuracy of the implemented condensation model is compared with a standard analogy between convective heat and mass transfer, showing that the latter is not able to predict heat transfer performances in the investigated air velocity range.

Visualization study on the condensation heat transfer on vertical surfaces with a wettability gradient

The functional surface is a crucial approach to enhance the condensation heat transfer. This paper designs and fabricates a functional surface with wettability gradient by the use of etching and chemical vapor deposition methods. The dynamic phase change behaviors and heat transfer characteristics of humid air condensation on vertical surfaces with wettability gradient have been experimentally investigated and compared with the corresponding hydrophilic and hydrophobic surfaces by a visualization system. The results indicate that the heat transfer performance of humid air condensation on traditional gradient surface is superior to hydrophilic case but inferior to hydrophobic case in large-scale application due to small wettability gradient. Considering this, the periodic gradient surface with periodical layout of multi-layer wettability gradient is introduced to improve the condensation heat transfer. The presence of a larger wettability gradient for the periodic gradient surface induces the efficient droplet removal and the preferable inter-layer and intra-layer actions, leading to an improvement of condensation heat transfer.

Numerical simulation of dropwise condensation over hydrophobic surfaces using vapor-diffusion model

Dropwise condensation of humid air over hydrophilic and hydrophobic surfaces is numerically investigated using a phenomenological, Lagrangian model. Mass flux through droplets free surface is predicted via a vapor-diffusion model. Validation with literature experimental data is successfully conducted at different air humidities and air velocities. The accuracy of the implemented condensation model is compared with a standard analogy between convective heat and mass transfer, showing that the latter is not able to predict heat transfer performances in the investigated air velocity range.

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface.

Vapor condensation is a well-known phase-change phenomenon observed in nature as well as in different industrial applications. Superhydrophobic surfaces (SHSs) with low hysteresis can efficiently drain off the condensate and rejuvenate the nucleation sites further. In this work, three distinct SHSs were fabricated by nanocoating three hydrophobic agents, viz., perfluoro-octyl-triethoxy-silane (PFOTS), perfluoro-octanoic-acid (PFOA), and commercial Glaco solution on a hierarchical aluminum surface. The surface morphology of all surfaces was investigated, and its effects on the wetting, droplet departure, and overall heat-transfer coefficient (HTC) during condensation phenomena in the humid air (>95% noncondensable gases) were analyzed. The contact angle hysteresis of all three surfaces was very low (∼5°); however, different wetting behaviors were observed during the condensation, depending on the adhesion of the condensate drop with nanoscale textures in the microcavities. Dropwise condensation (DWC) was observed in silane and Glaco-coated surfaces. A gravity-assisted sweeping mechanism removed the condensate from the silane-coated surface. In contrast, the condensate was ejected out of the plane of the Glaco-coated surface by droplet jumping. The PFOA-coated surface has shown DWC initially and floods in the later stages due to highly pinned condensed droplets. This study reports an enhancement of ∼35 to ∼110% in the HTC for the SHS-exhibiting gravity-assisted sweeping mechanism compared to the droplet-jumping mechanism. The present work will provide substantial insights into the fabrication of efficient hierarchical interfaces for water-energy nexus applications.

Condensation of Water Vapor from Humid Air Inside Vertical Channels Formed by Flat Plates

Condensation of humid air is a very important process in thermal and process engineering, as well as in several industrial applications. It is also a subject of many currently research-intensive scientific domains, such as atmospheric water harvesting and seawater desalination. The nature of (water) vapor condensation in the presence of non-condensable gas (NCG) such as air differs significantly from the case with the pure, quiescent vapor condensation. In the literature, simple models that describe the condensation of the forced flow of water vapor in the presence of air on a series of vertical flat plates forming vertical channels are hard to find. Here we present a simple and computationally efficient semi-empirical correlation that describes the condensation of the forced flow of humid air on a condenser geometry consisting of a series of vertical flat plates forming vertical channels. The correlation accounts air as a non-condensing gas, different heights of vertical plates and different thermal-hydraulic parameters. The correlation has been experimentally validated and shows excellent agreement, as 90% of theoretically predicted values are within ±12% of experimental data.

Effects of wetness and humidity on transonic compressor of gas turbine

Moist air flows with and without consideration of inlet wetness and assuming fogging were numerically investigated in a transonic compressor of an industrial gas turbine, and the causal relationship between the wetness and the deposition of water droplets on the blade surface was clarified. First, we simulated three-dimensional moist-air flows through the 1.5-stage rotor and stator blade rows in the transonic compressor of an industrial gas turbine operating at a Tokyo Electrical power plant, considering variations in the inlet wetness and the number of aerosol particles. The results indicated that the shock generated in the first-stage rotor passage was the primary factor in the evaporation of the moist air, decreasing its temperature, whereas the wetness was highly dependant on the deposition of water droplets on the blade surface. We further simulated two-dimensional humid-air flows through a transonic compressor cascade channel under a number of conditions, varying the aerosol particle size, number of aerosol particles, humidity, blade chord length and pressure ratio. The results indicated that the condensation of humid air was captured locally in the supersonic region, while the deposition of the condensed water droplets was mainly influenced by the first three factors. Overall, this study indicates that wetness and deposition are in a trade-off relationship with regard to the performance of gas turbines.

Direct numerical simulations of moisture transport in porous media by a multi-component/phase-change lattice Boltzmann method

In this paper, simultaneous heat and mass transport in porous media is studied numerically using a 2D multicomponent/multiphase phase-change lattice Boltzmann scheme. This newly developed LB model requires no correlation equations as inputs for physical properties of the porous medium (such as relative permeability, heat capacity, thermal conductivity and mass diffusivity) that is filled with a multi-component fluid of air/vapor mixture. Natural convection and condensation of humid air in an enclosure filled with and without a porous medium that is heated differentially by vertical walls are compared. The same numerical scheme is applied to simulate capillary rise of liquid in semi-infinite packed hydrophilic beds with different structures that are filled initially with a mixture of unsaturated vapor and air above a water reservoir. It is shown that evaporation in the packed hydrophilic bed plays an important role in capillary rise of liquid. Effects of contact angles as well as the influence of the directionality and heterogeneity of the packed bed are investigated. The directional and the heterogeneous packed beds are shown to be able to suck a larger amount of liquid in a shorter time period compared to homogeneous packed beds with the same porosity under the same condition.

Numerical Investigation of Influence of Nanoparticles Presence on Water Vapor Condensation Process inside a Vertical Channel

This paper is aimed at investigating the nanofluid film condensation by mixed convection in the presence of water vapor, Cu nanoparticles, and air treated as a noncondensable gas (NCG) on the inner walls of a vertical channel. In this simulation, the flow is laminar, stationary, two dimensional, and axisymmetric. The coupled governing equations for the liquid film with the nanoparticles and the mixture air-humid-nanoparticles are solved together using the finite volume method. Since the application of humid air condensation is one of the most applicable methods of phase change processes that is observed in different industrial fields such as heating, ventilation, and air conditioning (HVAC) or cooling systems, for this purpose, the influence of injecting a uniform volume fraction of nanoparticles on improving heat and mass transfer is determined as a function of the variation in relative humidity, velocity, temperature, pressure, and volume fraction of Cu nanoparticles at the channel inlet. The numerical results indicate that under the best conditions in the range of variation studied RH in = 100 % , Re in = 2000 , T in = 50 ° C , P in = 0.5 atm , and φ in = 0.1 % , the use of nanoparticles has a greater impact, and the maximum improvement in the condensation film thickness, the local Nusselt number, and the accumulated condensation rate has an effective ratio strictly greater than one compared with the case of pure humid air.

Performance of the Condensation Process for Water Vapour in the Presence of a Non-Condensable Gas on Vertical Plates and Horizontal Tubes

The condensation of humid air is a crucial step in air conditioning and process engineering. However, the models that describe the condensation of vapour in the presence of a non-condensable gas require time-consuming numerical calculations that go beyond the Nusselt film theory. Only a small number of publications exist, where simple and computationally effective correlations for the condensation of water vapour in the presence of air are presented for specially designed condenser heat exchangers. Therefore, the objective of this paper is to extend the existing semi-empirical correlations for different geometries and process parameters. For the purpose of the study, an experimental setup with two different condenser heat exchangers based on vertical plates (height 74 mm) and horizontal tubes (3 tubes, diameter 40 mm and 7 tubes, diameter 15 mm) was built. Additionally, based on existing correlations, we developed two semi-empirical models that predict the condensation mass flux for the proposed geometries. Here, we report that the agreement between the experimental and theoretical values predicted by the new, semi-empirical correlations is excellent, with an average uncertainty of less than ±6%. Their usability was demonstrated by a possibly significant performance improvement of the condenser inside a condensation-type tumble dryer.

Determining transient heat transfer coefficient for dropwise condensation in the presence of an air flow

It is necessary to understand humid air condensation because of its various applications, such as water harvesting. In this study, the influence of airflow on the condensation heat transfer coefficient of humid air on a horizontal surface was investigated experimentally. Airflow velocities from 1 to 15 m/s were investigated, because the condensate's shedding usually happens in this range. A relative humidity of 10–80% and subcooling temperature (Tsc) of 0–10 °C were used to achieve both single-phase and condensation regimes. To study the relationship between condensate morphology and heat transfer coefficient (HTC), transient and instantaneous heat flux measurements were required. To facilitate this, a transient inverse heat conduction method was developed to characterize the time-varying surface heat flux and associated heat transfer coefficient. A 5-fold decrease in response time was found for the transient method compared to the steady-state method. The effect of condensation parameters on the heat transfer coefficient, as well as the relationship between condensate morphology and heat transfer coefficient, is discussed. The results show that HTC for the subcooling of T = 0 °C is smaller than for other temperatures. Also, the effect of RH on condensation was investigated and higher HTC was found for higher RHs. The results clearly show that the shedding of condensate kept the average droplet size low and doubled heat transfer performance improvement.

Improved humid air condensation heat transfer through promoting condensate drainage on vertically stripe patterned bi-philic surfaces

Even when dropwise condensation occurs on the super-hydrophobic surface, the condensed water droplets stick to the surface in the Wenzel state, resulting in lower drainage rate than the super-hydrophilic surface. Condensate that is not drained consequently degrades condensation heat transfer performance over time. In this study, it was considered that the striped patterned bi-philic surface could improve the condensation heat transfer in that droplet of the near super-hydrophobic area can be quickly removed and drained to a nearby super-hydrophilic region. Rapid drainage of condensate from the near super-hydrophobic surface can result in the renewal of new dropwise condensation, which can lead to increased heat transfer. Condensate discharges from bi-philic surfaces fabricated by material printing were measured and compared to condensate discharges from fully super-hydrophilic and near super-hydrophobic surfaces. As decreasing the stripe width of the bi-philic surface, the drainage rate is improved due to the rapid growth of the droplet nucleate on the near super-hydrophobic surface and the increment of the drainage path. The maximum condensate discharge from the bi-philic surface was measured to be 15% higher than that of the entirely super-hydrophilic surface. The amount of condensate recovery from the bi-philic surface was predicted through film and dropwise condensation combined modeling. Through this model, the amount of condensate recovered from the bi-philic surface was predicted within the 8% error range. Determination of the optimal stripe pattern to maximize the amount of condensate recovery can be made through developed modeling. The optimized stripe design of bi-philic surface resulted in a 25% increase in condensate discharge compared to the entirely super-hydrophilic surface.

A Smart Embedded System for Humid Air Condensation and Water Harvesting

The people in the south of Iraq in the coastal region of the Arabian Gulf, especially in the Basra government, are suffering from waves of high humidity throughout the year, average annual humidity of 60.6 %. The increment of salinity of the Shatt-Al-Arab river, the main source of the drinking water in this city, effected by the gulf water caused a lack of drinking water. What made matters worse is the frequent power outages caused by many reasons, one of these reasons that Basra considers one of the highest temperature cities in the world. For that, it is required to design an efficient device to harvest the water from the air to reduce the humidity and consider as a source for drinking water, taking into consideration it’s adopting a renewable source of energy. This article is presenting a low cost, portable, power-efficient, multi-purpose device that able to harvest water from the air efficiently. The results show a promising prototype that able to reduce significantly the relative humidity (RH), produce water, and consumed low power, with overall efficiency 40%-60%.

Dropwise condensation of humid air - Experimental investigation and modelling of the convective heat transfer

In this investigation dropwise-condensation heat transfer of out of a turbulent flow of humid air is experimentally investigated. The test section consists of a horizontal channel, sized 12 × 32 × 750 mm. The condensation takes placed on a 9 × 6 mm sized section of the vertical channel wall. A model based on the analogy of heat and mass transfer - previously used in the context of filmwise condensation - is adopted in order to describe the data obtained for varying bulk Reynolds numbers (3000–22000), temperatures (36∘C–55∘C) and relative humidities (36%–55%). Modifications are made in order to capture the effect of fog formation within the mixture and the enhancement of the area available for heat transfer induced by the droplets on the substrate. The formulation of the model is intended to be easily adoptable to other geometries, boundary conditions, types and orientations of the substrates. Previous investigations of this group were focused on the thermal resistance of the condensate. Based thereon, the present modelling completes the former approach, resulting in a model capable of predicting the total heat transfer coefficients of dropwise condensation from humid air. The predictions are validated using own experimental data as well as external data.

Electrowetting-based enhancement of droplet growth dynamics and heat transfer during humid air condensation

Condensation heat transfer can be significantly enhanced by condensing vapor as droplets (instead of a film), which rapidly roll-off. This work studies the use of electrowetting to enhance coalescence, growth and roll-off dynamics of condensed droplets, thereby enhancing the condensation rate and associated heat transfer. This enhancement depends on the nature of fluid motion (translation of droplets, oscillations of the three phase line), which in turn depends on the magnitude and frequency of the applied electrical actuation waveform. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation under the influence of an electric field. It is seen that droplet growth is enhanced as the voltage and frequency of AC electric fields is increased, with AC electric fields seen to be more effective than DC electric fields. Roll-off dynamics also depends on the frequency of the AC field. Overall, electric fields alter the droplet size distribution and move the condensate to more favorable states for removal from the surface. The condensation rate depends on the roll-off diameter of the droplet, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. An analytical heat transfer model is utilized to relate the measured condensation rate with condensation heat transfer. It is noted that this study deals with condensation of humid air, and not pure steam. Overall, this study reports more than 30% enhancement in condensation rate resulting from the applied electric field, which highlights the attractiveness of electrowetting for condensation heat transfer enhancement.