Humid Air Stream Research Articles

Investigating condensation from humid air under mixed convection regime

A wide range of industrial applications rely heavily on heat transfer during vapor condensation from a mixture of water vapor and noncondensable gases (NCG). For a vertically-mounted condenser plate, the vapor-diffusion boundary-layer thickness is influenced by the interplay of the thermogravitational and forced flow fields, eliciting classical mixed convection scenario. This thickness in turn dictates the condensation heat and mass transfer rates. While condensation in presence of NCG under free and forced convection scenarios are well-characterized in the literature, its counterpart in the mixed convection regime is relatively uncharted. Herein, condensation from an upward stream of humid air over a vertically mounted mild steel condenser surface is characterized under different flow velocities. The free-stream flow is thus directed opposite to the thermogravitational flow induced next to the plate. We observe that with increasing the magnitude of the upward flow velocity of the free stream, the condensation heat transfer coefficient (CHTC) initially decreases until it reaches a minimum at 0.4 m/s, beyond which the CHTC rises again with the flow velocity. Using the Nusselt analogy for mixed convection for the relevant flow regimes we substantiate our experimental findings and extend the observation for predicting condensation behavior under different experimental ambient conditions.

Design of a sheathed water condensation particle counter with variable saturation ratio

We consider a particle condensation device where a cold aerosol enters at one open end of a tube. The wall of the tube is porous beyond a certain section. A stream of cold air is injected radially and symmetrically through an initial focusing region of the porous wall, and a stream of warm humid air with a saturation ratio S is injected in a condensation region further downstream. The configuration offers several advantages over existing water condensation particle counters (WCPCs). First, S may be varied by mixing a dry gas stream with a humid stream prior to their passage through the porous tube wall. Second, radial entry of the humidified flow accelerates the penetration of the vapor into the core of the tube, such that the maximal saturation ratio is reached within axial distances comparable with the tube radius. Third, the aerosol is partially confined within the tube core by the radially entering dry and humid gases. The particles then sample the relatively uniform saturation field near the tube axis, resulting in sharp responses of the activation probability versus particle diameter. Calculations with various combinations of axial and radial Reynolds numbers are carried out in search of conditions resulting in the smallest possible variation of the maximal supersaturation reached along the various flow streamlines.

Transport and evaporation of exhaled respiratory droplets: An analytical model

An important vector for host-to-host infectious disease transmission is given by the transport of tiny pathogen-laden droplets. These are commonly exhaled by individuals while breathing, speaking, coughing, or sneezing. Depending on their size and ambient conditions, they may follow different paths, either settling on surfaces where the pathogen can be further transmitted by contact, or remaining airborne after evaporation where the pathogen can be inhaled. Our understanding of pathogen transmission from the fluid mechanics perspective is still somewhat limited, especially in quantitative terms. In the current work, starting from the fundamental laws of fluid mechanics and diffusion, a detailed analytical model of droplet transport and evaporation in humid air streams is presented and successfully validated against available data in the literature finding remarkable agreement. The model implements closed-form analytical solutions of the equations of transport, evaporation, and energy balance, and an algebraic model to account for the droplet chemical composition. It also features an analytical model of droplet transport within the buoyant exhaled breath cloud based on momentum conservation addressing both jet and puff phases and is able to handle periodic respiratory events. Turbulent dispersion is modeled with a discrete random walk approach. A simple inhalation model is also proposed. Such a model may help in better understanding droplets' fluid dynamic behavior and may be used to assess the risks associated with pathogen transmission under different scenarios for any type of respiratory event. Overall, the computational cost is relatively low, allowing extensive simulation campaigns to be performed easily.

Comprehensive Method for Obtaining Multi-Fidelity Surrogate Models for Design Space Approximation: Application to Multi-Dimensional Simulations of Condensation Due to Mixing Streams

In engineering problems, design space approximation using accurate computational models may require conducting a simulation for each explored working point, which is often not feasible in computational terms. For problems with numerous parameters and computationally demanding simulations, the possibility of resorting to multi-fidelity surrogates arises as a means to alleviate the effort by employing a reduced number of high-fidelity and expensive simulations and predicting a much cheaper low-fidelity model. A multi-fidelity approach for design space approximation is therefore proposed, requiring two different designs of experiments to assess the best combination of surrogate models and an intermediate meta-modeled variable. The strategy is applied to the prediction of condensation that occurs when two humid air streams are mixed in a three-way junction, which occurs when using low-pressure exhaust gas recirculation to reduce piston engine emissions. In this particular case, most of the assessed combinations of surrogate and intermediate variables provide a good agreement between observed and predicted values, resulting in the lowest normalized mean absolute error (3.4%) by constructing a polynomial response surface using a multi-fidelity additive scaling variable that calculates the difference between the low-fidelity and high-fidelity predictions of the condensation mass flow rate.

Optimization of a rotary desiccant wheel for enthalpy recovery of air-conditioning in a humid hospitality environment

Excessive condensation on cooling coils can be costly energy-wise due to humid process air streams. An innovative desiccant cooling system is introduced into an existing hotel in Taoyuan, Taiwan, where the ambient is humid year-round. Rejected heat from the existing cooling system is used as the energy source to heat the regeneration airstream for dehumidification of the process airstream via a rotary desiccant wheel. The paper develops and validates a numerical model for the heat and mass (moisture) transfer of the rotary desiccant wheel. The validated numerical model is used to simulate the expected performance of the rotary wheel under various conditions and operating strategies. The desiccant wheel’s performance includes the steady-state moisture removal capacity (MRC) and the transient response time to reach steady-state. The examined factors include the ambient temperature and humidity, air flow rate, rotational speed of the wheel, wheel-split, and regeneration airstream temperature. From the simulation results, the paper offers the optimized control strategies for the operation of the rotary desiccant wheel system in the hotel.

Modeling the effect of humidity and temperature on VOC removal efficiency in a multistage fluidized bed adsorber

The competitive adsorption of volatile organic compounds (VOCs) and water vapor onto beaded activated carbon (BAC) in a fluidized bed adsorber is developed in the form of a two-phase model using the Manes method. The Manes method uses as input single component adsorption isotherms described by the Langmuir model for adsorption of VOCs and the Qi-Hay-Rood (QHR) model for adsorption of water vapor. The effect of temperature is accounted for using the linear van’t Hoff relationship for Langmuir affinity coefficient. The effects of relative humidity (RH) and temperature in the simulations are validated using experimental data, with very good agreement observed (e.g. R2 = 0.98 comparing overall removal efficiencies). Humidity begins to have an effect on the adsorption of 1,2,4-trimethylbenzene (TMB) on BAC starting at 70% RH in the form of a reduction in overall removal efficiency due to fewer available adsorption sites. The overall removal efficiency decreases from 93.0% at 70% RH to 85.4% at 90% RH, and then plateaus with further increase in RH up to 100%. In dry air, temperature variation has a small effect on removal efficiency, showing a reduction in modeled overall removal efficiency from 93.0% to 90.2% when the adsorption temperature increases from 22 to 50 ˚C. On the other hand, at high RH values, temperature has a larger and positive effect on removal efficiency due to RH change. Increasing the temperature of a stream of humid air (RH = 95%) from 22 to 27 °C increases the TMB removal efficiency by 6.9% from 85.3% to 92.2%. Taking into account the effect of humidity and temperature, the model can be used to help optimize fluidized bed adsorber operations in industrial applications.

Low Temperature Catalytic Oxidation of Ethanol Using Ozone over Manganese Oxide-Based Catalysts in Powdered and Monolithic Forms

Catalytic oxidation of low concentrations of ethanol was investigated in dry and humid air streams at low temperature (60 °C) over manganese oxide-based catalysts supported on a meso–macrostructured TiO2 using ozone as the oxidant. Ethanol was selected as a representative model VOC present in indoor air, and its concentration was fixed to 10 ppm. For that purpose, a series of Mn/TiO2 powder and monolithic catalysts was prepared, some doped with 0.5 wt% Pd. Whatever the catalyst, the presence of water vapor in the gas phase had a beneficial effect on the conversion of ethanol and ozone. The Pd–Mn/TiO2 catalyst containing 0.5 wt% Pd and 5 wt% Mn exhibited superior oxidation efficiency to the Mn/TiO2 counterparts by increasing ozone decomposition (77%) while simultaneously increasing the selectivity to CO2 (85%). The selectivity to CO2 approached nearly 100% by increasing the amount of catalyst from 20 to 80 mg. In a further step, alumina wash-coated cordierite honeycomb monoliths were coated with the 0.5Pd–5Mn/TiO2 catalyst. Full conversion of ethanol to CO2 without residual O3 emitted (less than 10 ppb) could be attained, thereby demonstrating that the proposed Pd–Mn/TiO2 monolithic catalyst fulfills the specifications required for onboard systems.

Numerical assessment of mixing of humid air streams in three-way junctions and impact on volume condensation

Flow mixing at three-way junctions is widely studied due to its usage in countless applications. Particularly, in air conditioning and internal combustion engines, the mixing of humid air streams can lead to volume condensation, provided that any local temperature achieves dew conditions. This work investigates and quantifies the correlation between mixing intensity and generated condensation. A method to define mixing indexes is developed, comparing its performance against literature references that employ temperature or passive scalar to evaluate mixing. A numerical campaign of 128 Reynolds-Averaged Navier Stokes three-dimensional (3D) simulations is designed to explore different junction operating conditions. A validated condensation submodel embedded in the 3D model enables the quantification of condensation at the junction outlet, which also can be estimated by means of a 0D model that provides the psychrometric state of a homogeneous mixture between the inlet streams. A strong linear correlation is found between condensation and the product of mixing indexes and 0D condensation mass flow rate, since the latter carries the psychrometric information of the working point. This connection allows to find junction design guidelines to reduce mixing and therefore condensation. Additional simulations confirm that removing junction valves, reducing mixing length, increasing the branch duct diameter and aligning the branch leg with the outlet duct significantly decrease condensation. In the framework of low pressure exhaust gas recirculation (EGR), this condensation damages the compressor wheel. If junctions are designed as suggested, the current constraints over EGR rates to limit condensation can be removed, which will result in internal combustion engines with lower emissions and better fuel consumption. • A new definition of mixing index is developed by calculating the unmixed area. • Flow mixing and condensation at T-junctions is assessed using 0D and 3D models. • Condensation is strongly correlated with psychometric conditions and mixing. • Mixing is reduced by decreasing length and momentum ratio and relocating valves. • Guidelines to design junctions with reduced volume condensation are provided.

Determining the radial distribution function of water using electron scattering: A key to solution phase chemistry.

High energy electron scattering of liquid water (H2O) at near-ambient temperature and pressure was performed in a transmission electron microscope (TEM) to determine the radial distribution of water, which provides information on intra- and intermolecular spatial correlations. A recently developed environmental liquid cell enables formation of a stable water layer, the thickness of which is readily controlled by pressure and flow rate adjustments of a humid air stream passing between two silicon nitride (Si3N4) membranes. The analysis of the scattering data is adapted from the x-ray methodology to account for multiple scattering in the H2O:Si3N4 sandwich layer. For the H2O layer, we obtain oxygen-oxygen (O-O) and oxygen-hydrogen (O-H) peaks at 2.84 Å and 1.83 Å, respectively, in good agreement with values in the literature. This demonstrates the potential of our approach toward future studies of water-based physics and chemistry in TEMs or electron probes of structural dynamics.

Development of an experimental test bench and a psychrometric model for assessing condensation on a low-pressure exhaust gas recirculation cooler

A test bench has been designed to assess condensation formation produced on the interior of a low-pressure exhaust gas recirculation cooler working with hot stream of humid air representing an engine warm-up stage, when its coolant starts from very cold conditions. An experimental campaign has been conducted with three different exhaust gas recirculation mass flow rates, four exhaust gas recirculation inlet temperatures and three different coolant initial temperatures, covering common conditions found in the low-pressure exhaust gas recirculation system of internal combustion engines under cold starts. The transient experimental results are analyzed and compared with a simple psychrometric condensation model, obtaining a good correlation and reproducing the trends of the condensation, even though an overprediction of the condensates of around 20%–40% exists due to the strong hypotheses assumed. The warm-up tests are most sensitive to the initial coolant temperature. For example, an engine starting at –10 °C ambient temperature could require 10 min to stop producing water in the low-pressure exhaust gas recirculation cooler, with an accumulated quantity during the warm-up of about 100 mL of condensates.

Using Observed Signals from the Arctic Stratosphere and Indian Ocean to Predict April–May Precipitation in Central China

AbstractA linear regression model is constructed to predict the April–May precipitation in central China (PCC) with a lead time of 1–2 months. This model not only reproduces the historical April–May PCC from 1985 to 2006 but also demonstrates strong robustness and reliability during the independent test period of 2007–16. Two preceding factors are selected to build the model, the February–March Arctic stratospheric ozone (ASO) and Indian Ocean sea surface temperature (IOSST), indicating a synergistic impact of Arctic and tropical signals on the midlatitude climate. A possible mechanism of ASO changes affecting Chinese precipitation is that the stratospheric circulation anomalies related to ASO changes may downward influence circulation over North Pacific, and then extend westward to influence East Asia, leading to changes in Chinese precipitation. Anomalies of the other predictor, IOSST, are associated with a baroclinic structure over central China. For example, warm IOSST causes anomalous convection over central China and affects the warm and humid airstream flowing from the Pacific to China. These processes related to the two predictors result in the April–May PCC anomalies. Sensitivity experiments and a transient experiment covering a longer period than the observations/reanalysis support the results from our statistical analysis based on observations. It implies that this statistical model could be applied to the output of seasonal forecasts from observations and improve the forecasting ability of April–May PCC in the future.

Comparison of the Efficiencies of Carbon Sorbents for the Preconcentration of Highly Volatile Organic Substances from Wet Gas Atmospheres for the Subsequent Gas-Chromatographic Determination

The efficiencies of various carbon sorbents in the preconcentration of volatile organic substances from a stream of humid air for their subsequent gas-chromatographic determination are compared. A hybrid procedure for rapid analysis including the sorption preconcentration of analytes on a selected carbon–fluoroplastic composite sorbent and their one-stage thermal desorption in the injector of a chromatograph with a capillary column and a flame-ionization detector is proposed. This procedure makes it possible to determine lower alcohols and ketones in air saturated with water vapor for 5–10 min with determination limits at a level of several μg/m3.

Adsorbent Regeneration by Non‐thermal Plasma for Elimination of Odorous Compounds from Indoor Air

AbstractThe non‐thermal plasma (NTP) technique was shown to be a method to improve indoor air quality. In particular in kitchens, odorous emissions can be removed by NTP. A combined concept of adsorption of volatile organic compounds (VOCs) and plasma regeneration of the adsorber was tested in adsorption‐regeneration‐adsorption cycles. As reference VOCs, 2‐methylthiophene, 2‐methylpyrazine, 2‐acetylthiazole, nonanal, and trans‐2‐nonenal were selected in humid air streams. These odorous compounds are emitted during cooking and frying processes. The adsorption‐regeneration concept was also tested during a simulated frying process with garlic in rape oil. A hydrophobic zeolite was chosen as adsorber material and placed directly into the discharge zone of a plasma reactor.

Energy consumption in humidification process

The purpose of the air-conditioning system is to provide people in an air-conditioned room or facility with thermal-humidity comfort. For this purpose, the external air before entering into the room is processed for its treatment. These processes include, among others, humidification of the air. Too high a humidity level also does not benefit the building and its people. Humidification of the air can be realized as directly, then the steam or water mist injection takes place in the room, and indirectly, when the supply air to the room is humidified in the ventilation system. The energy consumption in the humidification process depends primarily on stream of humid air, methods of humidifying the air (water or steam), the type of energy carrier used to produce steam or secondary heating of humid air, range of relative humidity maintained, external humidity parameters. In the case where both the steam and the secondary heating use the same energy sources, the costs in both cases will be comparable. In this paper the calculation of energy demand in hourly and annual period as well as for water and steam humidifier unit have been calculated and presented in many tables and figures. The semidetached house were taken under consideration to analyzed the energy consumption for humidification process. The amount of energy needed to humid the air during the annual season varies from 127 to 545 kWh/ar, depending on the humidifier used and the pump power, while for steam humidification the amount of energy for humidification is less than 9421 kWh/a. As a result, water humidification should be considered more economical. The basic difference between water and steam humidification, as evidenced by the hx graphs is decrease in the air temperature during water humidification and need to reheat it. Taking into account the energy requirements in both cases is similar. The humidifying costs of the steam humidifiers are much higher than those of water humidification.

Validation and sensitivity analysis of an in-flow water condensation model for 3D-CFD simulations of humid air streams mixing

The mixing of gaseous streams at different humidity and temperature conditions may generate bulk condensation, which is a particular thermodynamic process that usually implies a great computational effort. A model that provides in-flow condensation predictions without heavily impacting on the simulation time becomes a potential tool for studying situations where the presence of water droplets is a problem. For instance, in a Long-Route Exhaust Gas Recirculation (LR-EGR) system, intake fresh air is mixed with humid and hot exhaust gases, generating water droplets that may impact on the turbo-compressor impeller and produce harmful erosion. Serrano et al. [1] developed a condensation submodel for such purpose. In this work, the 3D CFD methodology is analyzed by means of sensitivity studies on the numerical setup and intake throttle valve angle. Validation of the model is addressed by means of measurements in a fully instrumented continuous flow turbocharger test bench, obtaining quantitative and qualitative agreement. Particularly, a consistent correlation was found between predicted condensation rate and the impeller damage for seven LR-EGR configurations. Moreover, the T-joint geometry is noticed to have a great impact on condensation generation, thus showing the potential of such a 3D model for improving the LR-EGR design.

Optonumerical technique for mapping freezing droplet interactions on substrates

A visualization technique was developed to accurately resolve the locations and phases of liquid and frozen water droplets naturally evolving on a subfreezing surface exposed to a humid air stream. The technique utilized the lens-like properties of unfrozen condensate droplets to focus reflected light in a manner that made it possible to reliably distinguish the unfrozen from frozen droplets. By inducing this lensing effect in all visible droplets on the surface prior to freezing, a small, high contrast region within each liquid droplet was produced. When freezing occurred, the higher opacity of the ice phase caused the frozen droplets to suddenly take on a darker appearance than the unfrozen droplets. A reflected light microscope and digital camera apparatus were used to feed data into an image analysis algorithm capable of mapping the time-evolution of the freezing water droplets on the surface. The algorithm processed raw image sequences into binary image stacks from which the light foci associated with the droplets could be reliably isolated into high contrast white pixel clusters on an otherwise black background. An edge detection method was employed within each binary image to locate the coordinates of the pixels residing along the periphery of each droplet’s focal cone. Those data were subsequently used to compute the centroidal coordinates and state of each droplet over time. Compared with more laborious, frame-by-frame analyses with various commercially available software tools, this technique correctly identified the locations and states of the droplets through the entire growth and freezing processes with accuracies in excess of 99%. This technical approach also allowed for simultaneous tracking of all droplets and freezing events present within the region of interest—typically numbering in the hundreds or thousands—over time, paving the way for interdroplet coalescence and freezing interactions to be studied in detail.

An advanced impact of Arctic stratospheric ozone changes on spring precipitation in China

The effect of spring Arctic Stratospheric Ozone (ASO) changes on spring precipitation in China is analyzed using observations, reanalysis data, and the Whole Atmosphere Community Climate Model version 4 (WACCM4). We find that February–March mean ASO changes have a significant impact on April–May mean precipitation over Loess Plateau and middle–lower reaches of the Yangtze River—two important grain-producing regions with large populations. Changes in the polar vortex link the ASO to precipitation in China. Stratospheric circulation anomalies caused by ASO changes can propagate to the North Pacific. An increase in ASO leads to enhanced westerlies in the high and low latitudes of the North Pacific but weakened westerly in the mid-latitudes of the North Pacific. The circulation anomalies over the North Pacific, forced by the increase of ASO, can extend westwards to East Asia, leading to an abnormal anticyclone in the East Asian upper and middle troposphere, and an abnormal cyclone in the lower troposphere. This enhances the warm and humid airstream from Western Pacific to Chinese mainland and strengthens upwelling over Loess Plateau and middle–lower reaches of the Yangtze River. These conditions enhance precipitation in central China during positive ASO anomaly events and reduce precipitation during negative events. The WACCM4 simulations support the results from statistical analysis of observations and reanalysis data. Our results suggest that ASO variation can serve as a predictor of spring precipitation variation over Loess Plateau and middle–lower reaches of the Yangtze River.

Robust thin film composite PDMS/PAN hollow fiber membranes for water vapor removal from humid air and gases

Abstract Water vapor in humid air and industrial gases needs to be removed for various applications such as air conditioning, foodstuffs storage, aviation and space flights. Membranes can remove water vapor from gas streams without involving phase change and regeneration. A robust and productive membrane is vital for water vapor removal but it is challenging to produce it. Here we present a defect-free thin film composite (TFC) polydimethylsiloxane (PDMS)/polyacrylonitrile (PAN) hollow fiber membrane with an ultrathin PDMS selective layer of about 260 nm. Different from previous works, the PAN substrate was spun at a high take-up speed of 60 m/min. It has an average of surface pore size of 5.6 nm and superior mechanical properties. After the optimal PDMS coating, the composite membrane shows a N2 permeance of about 280 GPU, an O2/N2 selectivity of 2.2 and a water vapor permeance ranging from about 800 to 3700 GPU depending on operation conditions. In addition, the PDMS/PAN composite membrane has been tested for 48-hour outer door dehumidification and shown very stable and robust performance with a water vapor removal rate of 65% from the humid air. Besides, the PDMS/PAN composite membrane is not only able to efficiently dry the humid biogas but also upgrade its methane content simultaneously. Therefore, the newly developed TFC composite membrane has great potential for water vapor removal from humid air and various gas streams.

Esterification for biodiesel production with a phantom catalyst: Bubble mediated reactive distillation

The initial aim of the paper is to dramatically improve the pretreatment stage of biodiesel production, which converts problematic free fatty acids to fatty acid methyl esters, by introduction of a microbubble mediated reactive distillation stage instead of acid pretreatment. This will shift the conventional esterification process towards completion with a yield higher than 80%, even without high excess methanol. Application of ozone microbubbles has the advantage over acid gas catalysis in that it gives higher conversion and leaves no catalyst residue and requires no further catalyst recovery separation steps (a “phantom” catalyst). Unreacted ozone breaks down into oxygen, so the off-gases are just a humid air stream that can be vented. Importantly, ozonolysis breaks carbon–carbon double bonds into aldehydes and carboxylic acids. Many ester species were found after contacting the feedstock with ozone-rich microbubbles, depending on the molecular structure of the alcohols for the ozonolysis of oleic acid with alcohols, i.e., methanol, ethanol, n-propanol, iso-propanol, and n-butanol. In the case of ozonolysis of used cooking oil mixed with methanol, the results from the GC–MS show that all saturated free fatty acids (including palmitic acid, stearic acid, and myristic acid) are converted to methyl esters within 20 h of 60 °C ozonolysis, whereas trace amounts of these chemicals remain at lower temperatures. The results also show that the conversion of oleic acid to form oleic acid methyl ester is 91.16% after 32 h of ozonolysis at 60 °C. Therefore, the free fatty acid content in used cooking oil is less than 1.33%, which makes it suitable as a reactant for biodiesel production via transesterification. However, this result is different from the result provided by ASTM D974 in that the acid numbers decrease dramatically by 25% at the beginning of ozonolysis followed by a plateau. Moreover, if the fluidic oscillator is used to generate bubbles in ozonolysis of oleic acid mixed with methanol, the results show that the yields of ozonolysis product 1-nonanal increase by 30%. This observation means that ozonolysis of oleic acid is relative to the specific interfacial area, and favoured at low liquid temperatures.

Development and verification of an in-flow water condensation model for 3D-CFD simulations of humid air streams mixing

Bulk flow condensation caused by the mixing of air streams at different temperatures and humidities is a thermodynamic process that requires strong assumptions to be calculated with low computational effort. The applicability of a model that correctly predicts this phenomenon has grown recently due in part to the deployment of the Long Route Exhaust Gas Recirculation emission reduction technique in combustion engines and the damage to the turbocharger caused by the condensation produced when the intake air is mixed with the combustion gases. This work is addressed to expose a condensation model that is implemented in a commercial 3D-CFD code and is then verified, checking whether the implemented physical equations are behaving as intended. Finally, a practical application is made, showing the potential of model to predict water condensation in a LR-EGR T-joint.