Air Flow Humidity Research Articles

Laminar mixed convection of humid air in a vertical channel with evaporation or condensation at the wall

The effects of simultaneous heat and mass transfer on downward laminar flow of humid air in a vertical channel with isothermal wet walls have been studied numerically using an elliptical formulation. The entering air is always warmer than the channel walls but its absolute humidity can be higher or lower than the one corresponding to the wall temperature. Thus, cooling of the air stream is accompanied by either condensation or evaporation. The axial evolutions of the interfacial transverse velocity, of the average air temperature and average vapour mass fraction, of the sensible and latent Nusselt numbers, of the friction factor as well as that of the Sherwood number are presented and analysed for different inlet air conditions. The effects of the buoyancy forces on the hydrodynamic field are very important while their influence on the average air temperature and average mass fraction is small. Even though sensible heat is always transferred from the air to the walls, the latter must be heated when evaporation is important. If evaporation is weak, or if condensation occurs, the walls must be cooled.

Sick building syndrome: A disease of modern age

Sick building syndrome (SBS) is a term used to describe situation in building when more than 30% of occupants suffer from various symptoms which tend to increase by severity during the time people spend in "sick" building and disappear when they leave the building. Typical cases of SBS report vague symptoms, which cannot be objectively measured, and sufferers usually show no clinical signs of illness. Symptom heterogeneity suggests that they do not represent a single disorder. The objective of our study was to establish if SBS is present in our town because new buildings have been built lately producing the artificial environment--exclusively artificial lightning and mechanical ventilation. A total of 812 subjects were included in our study. The investigation of SBS was performed by standardized questionnaires to determine the prevalence of symptoms and complaints. Questionnaires were used to collect data on perception of environment conditions and health during the period they work in this building. The subjects were divided in three groups according to sex, level of education and ownership. Our data suggested that the incidence of symptoms was higher in employers than in owners of the offices. Moreover, the prevalence of SBS was very high-up to 74.76%. It is obvious that certain physical, psychological and organizational factors are involved in the incidence of symptoms, but our investigation suggests that physical factor has a dominant role in development of symptoms, particularly low humidity and low air flow. In addition, our judgment is that SBS exists in our city, probably in the whole country and, therefore, it must be investigated properly.

A numerical study of boundary-layer heat and mass transfer in a forced flow of humid air with surface steam condensation

Data obtained by numerical solution of boundary-layer flow, energy and diffusion equations for laminar and turbulent flows of humid air are described. The effect of vapor concentration and vapor–gas mixture temperature on the intensity of and similarity between the heat- and mass-transfer processes in the presence of surface vapor condensation is considered. It is shown that the Reynolds analogy is valid in the range of mass vapor concentration in the flow core C 10<0.2. In the region of higher vapor concentrations (condensation of a vapor with incondensable species), the analogy between the transfer phenomena is violated and the assumption that Le=1 can no longer be used.

Intrinsic and relative permeability for flow of humid air in unsaturated apple tissues

Knowledge of intrinsic and relative permeabilities is important when studying pressure driven mass transport in unsaturated porous media. Little information is available on intrinsic and relative permeabilities of biomaterials. In this study, an experimental procedure was developed to determine intrinsic and relative permeabilities for the flow of humid air in porous apple tissues. Experiments were completed in two steps. In the first step, apple tissues were freeze-dried to remove moisture without changing the sample’s original pore structure. Dry air was passed through the sample and Darcy’s law was used to determine intrinsic permeability, K, as a function of porosity. In the second step, moist air was forced through partially saturated apple tissues to determine the air permeability, K g. The humid air was in thermodynamic equilibrium with the apple tissues to obtain an equivalent immiscible condition. The air relative permeability k rg was then calculated based on the value of the air permeability K g and the intrinsic permeability K. The intrinsic permeability for apple tissue varied from 8.89 × 10 −13 to 4.57 × 10 −11 m 2 for porosity ranging from 0.33 to 0.77. The porosity dependency of intrinsic permeability can be described by the Kozeny–Carman model. The gas relative permeability k rg was correlated to saturation by an empirical model.

Numerical simulation of convective heat and mass transfer in banks of tubes

AbstractThis paper studies convective heat and mass transfer in laminar flows of humid air through banks of tubes. The flow structure on the airside is spatially periodic, with fully developed conditions prevailing after a short distance from the inlet section. In numerical simulations, periodicities in the velocity, temperature and mass concentration fields are taken into account to reduce the computational domain, both in the entrance region and in the fully developed region. The approach followed is completely general, even if the finite element method is used in the discretization process. In the application section, velocity, temperature and mass concentration fields are computed for a bank of tubes in a staggered arrangement. Quantitative results are also obtained for apparent friction factors, Nusselt numbers, Sherwood numbers and ratios between Colburn factors for heat and mass transfer. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Dynamics of ozone and OH radicals generated by pulsed corona discharge in humid-air flow reactor measured by laser spectroscopy

The dynamics of ozone and OH radicals are studied in pulsed corona discharge plasma in a humid-air environment. Ozone density is measured by the laser absorption method, and OH density is measured by the laser-induced fluorescence (LIF) method. A 100-ns pulsed corona discharge occurs between a series of 25 needle electrodes and a plate electrode. After the pulsed discharge, the time evolutions of ozone and OH densities are measured in humid air or a humid nitrogen-oxygen mixture. Results show that the addition of 2.4% water vapor to dry air reduces ozone production by a factor of about 6, and shortens the ozone formation time constant from 30 to 6 μs. Water vapor may reduce atomic oxygen levels leading to the decreased production of ozone by O+O2 reaction. The LIF measurement for OH radicals shows that OH density is approximately constant for 10 μs after the pulsed discharge, then decays by recombination reaction and reactions with the discharge products of oxygen, such as ozone or atomic oxygen. Absolute OH density is estimated; it is about 3×1015 cm−3 in streamers at 10 μs after discharge in the H2O(2.4%)/N2 mixture.

Experimental Study on Frost Structure on Surfaces With Different Hydrophilicity: Density and Thermal Conductivity

An experimental study has been conducted to investigate the effects of surface energy on frost formation. Test samples with three different surfaces of which dynamic contact angles (DCA) were 23, 55, and 88 deg were installed in a wind tunnel and exposed to a humid airflow. The airflow Reynolds number, humidity, the air and the cold plate temperatures were maintained at 9000, 0.0042 kg/kg′, +12 and −22°C, respectively. The thickness and the mass of frost layer were measured and used to calculate frost density while heat flux and temperature profile were measured to obtain thermal conductivity. Exact positions of thermocouple junctions were verified by means of visualization system in order to increase accuracy. Results showed that frost density and thermal conductivity increase with time. The surface with a lower DCA showed a higher frost density and thermal conductivity during a two-hour test, but minor differences have been found after two hours of frost generation. Empirical correlations for thickness, mass deposition, density and thermal conductivity were proposed as the functions of test time and surface energy.

Gas-Phase Photocatalytic Oxidation of Styrene in a Simple Tubular TiO2 Reactor

AbstractGas-phase photocatalytic oxidation (PCO) of styrene was studied. Styrene appeared to poison the photocatalyst easily degrading its PCO efficiency at concentrations above certain level. Below this level no poisoning of the photocatalyst was observed. The presence of humidity extended the photocatalyst’s lifetime. The yield of carbon dioxide also increased in humid air, although lower conversion degrees of styrene were observed. Carbon dioxide was the main gaseous PCO product; carbon monoxide was formed in trace amounts. The apparent styrene PCO rate was independent of temperature at the initial stage of oxidation. However, the PCO rate noticeably increased with temperature at stages close to complete photocatalyst poisoning. The photocatalyst’s activity was entirely restored by UV -irradiation in humid airflow: adsorbed by-products were successfully oxidised. The simultaneous PCO of styrene with oxygenated hydrocarbons-alcohols and ethers-resulted in the photocatalyst poisoning along the same pattern as for styrene alone.

Experimental study and modelling of the crystallization of a water droplet

Modelling the crystallisation of a water droplet into a cold humid airflow is the first step in modelling the behaviour of water droplets sprayed out of a snow gun. This modelling, which is based on an experimental study, deals with the behaviour of the droplet which is transformed successively from the supercooled liquid phase to the liquid–solid phase and then the solid phase. These three transformations are brought about by various exchanges: heat transfer with conduction and convection as well as mass transfer with evaporation and sublimation. The supercooling phenomenon is naturally observed during experiments and taken into account in the modelling.

Convective heat and mass transfer in wavy finned‐tube exchangers

The paper adopts a simplified two‐dimensional approach to deal with convective heat and mass transfer in laminar flows of humid air through wavy finned‐tube exchangers. The computational domain is spatially periodic, with fully developed conditions prevailing at a certain distance from the inlet section. Both the entrance and the fully developed flow region are investigated. In the fully developed region, periodicities in the flow, temperature and mass concentration fields are taken into account. The approach is completely general, even if the finite element method is used for the discretizations. In the application section, velocity, temperature, and mass concentration fields are computed first. Then apparent friction factors, Nusselt numbers, Colburn factors for heat and mass transfer, and goodness factors are evaluated both in the entrance and in the fully developed region.

Analytical Determination of the Equilibrium Temperature of an Adiabatically Evaporating Liquid

A simple analytical expression is derived to determine the equilibrium temperature of an adiabatically evaporating liquid (true temperature of a wet-bulb thermometer). Comparison of the calculated data with well-known experimental results confirms the rather high degree of their coincidence in the entire investigated range of parameters of the process of water evaporation into a humid air flow.

Heat Rate Predictions in Humid Air-Water Heat Exchangers Using Correlations and Neural Networks

We consider the flow of humid air over fin-tube multi-row multi-column compact heat exchangers with possible condensation. Previously published experimental data are used to show that a regression analysis for the best-fit correlation of a prescribed form does not provide an unique answer, and that there are small but significant differences between the predictions of the different correlations thus obtained. It is also shown that it is more accurate to predict the heat rate directly rather than through intermediate quantities like the j-factors. The artificial neural network technique is offered as an alternative technique. It is trained with experimental values of the humid-air flow rates, dry-bulb and wet-bulb inlet temperatures, fin spacing, and heat transfer rates. The trained network is then used to make predictions of the heat transfer. Comparison of the results demonstrates that the neural network is more accurate than conventional correlations.

Membrane porosity and hydrophilic membrane-based dehumidification performance

An experimental study examined the relevance of membrane surface properties to the overall performance of hydrophilic membrane-based dehumidifiers (membrane condensers). Specifically, this study examined a model for the overall mass transfer coefficient derived from surface-science sticking-coefficient arguments, which states that performance is a function of a porosity-weighted average of the surface properties of the bulk membrane material and the surface of the fluid filling the membrane pores. Membranes composed of hydrophilic polyvinylidene fluoride, hydrophilic polyethersulfone, mixed cellulose ester, and 316 stainless steel with porosities between 6 and 80% were used in a flat format for mass transfer measurements in an insulated humid airflow chamber with controlled gas-phase fluid dynamics. While the experiments did not disprove the model, they did not find any meaningful performance differences among the various membrane materials tested, due to membrane surface properties, compared with the gas-side fluid dynamics.

Étude numérique de l'évaporation dans un courant d'air humide laminaire d'un film d'eau ruisselant sur une plaque inclinée

Numerical study of the evaporation in laminar humid air flow of a liquid film flowing over an inclined plate. By using an implicit centered finite differences method with a non-uniform grid, the authors study numerically the evaporation of a thin liquid film flowing over an inclined plate in a forced humid-air flow. They consider the existence of two-dimensional laminar boundary-layers with variable physical properties and show that the term of enthalpy diffusion is always negligible, whether the plate is adiabatic, isothermal or heated by a constant heat flux density. By using in the liquid film transfer equations which are one-dimensional, partially two-dimensional and two-dimensional, the authors additionally show the following features. If the plate is adiabatic, the liquid mass flow rate is without influence on the transfers and the gas–liquid interface behaves like an isotherm surface at rest. In this case, one may use a one-dimensional model in the film whatever liquid mass flow rate is. If the wall is isotherm or heated by a constant heat flux and when the liquid mass flow rate is less than 10 −3 kg·m −1·s −1, the one-dimensional model is sufficient; if it is included in the interval [10 −3 kg·m −1·s −1, 10 −2 kg·m −1·s −1[, the partially two-dimensional model is useful; if it is superior to 10 −2 kg·m −1·s −1, it is necessary to use the two-dimensional model. Generally, whatever the thermal conditions on the plate are, heat transfer is dominated by the liquid-vapor transition.

Modeling of frost formation over parallel cold plates

This paper numerically evaluates some of the parameters involved in modeling the process of frost formation on flat cold surfaces subject to the flow of humid air. The model employs one-dimensional transient formulation based upon the local volume averaging technique. The modeling process was validated by comparison with available experimental data. Numerical experiments were realized to determine the best initial values of the diffusivity, initial radius and geometry of the ice crystals. This model was applied to the known case of flow of humid air over a single flat cold plate to predict the frost temperature, density and thickness distribution along the flow direction and also the void fraction. The results were compared with available results in the literature. The model was then extended to solve the case of flow of humid air between two parallel cold plates for which there are no available results.

Volatility of propoxur from different surface materials commonly found in homes

The purpose of this study was to determine the volatilization rates of propoxur from different surface materials commonly found in homes, and to conduct field measurements under ventilated and no-nventilated conditions. Since it is known that temperature, humidity and constant air flow most significantly affect volatility, various surface materials were sprayed using a constant amount of propoxur under the controlled conditions of an exposure chamber. Acetonitrile was used to desorb both XAD-2 resin that collected airborne propoxur and surface materials containing propoxur residue. HPLC was used to analyze propoxur concentrations. Based on multiple regression models, temperature most significantly affected volatility, followed by humidity. Volatilization rates of propoxur were highest from quartz surfaces and lowest from glass. Interaction was most readily found on glass surfaces based on humidity-air flow and humidity-temperature factors. In field applications, propoxur was sprayed in a room under two conditions with ventilation and without in order to measure the concentrations of propoxur in the air and on a quartz surface. Findings showed both airborne and settled concentrations peaked after a half hour then decreased under both conditions, both more sharply in the ventilated room. Under both conditions, no propoxur was detected on the quartz surface after three and a half hours but airborne concentrations remained detectable after thirty-three and one half hours. We conclude that to maintain good air quality, ventilation is important and special care must be taken when spraying insecticides on different surfaces.

Thermal Stability of Complex Oxide Combustion Catalyst Supports

As part of a search for more thermostable supports for combustion catalysts for gas turbine applications, a series of complex oxide materials were prepared by sol−gel synthesis and two different coprecipitation routes. The materials were chosen on the basis of literature studies; two hexaaluminates (LaAl11O18 and BaMnAl11O19-α), a perovskite (SrZrO3), a spinel (MgAl2O4), and a pyrochlore (La2Zr2O7). After synthesis, the materials were aged for 16 h in a flow of humid air at temperatures between 1100 and 1400 °C, simulating the actual conditions in a catalytic combustor. The aged samples were characterized by X-ray diffraction, nitrogen adsorption for determination of BET surface area, and scanning electron microscopy. The sol−gel materials generally exhibited higher thermal stability than their coprecipitated counterparts. One exception was LaAl11O18, which had the highest surface area of all materials, 8 m2/g, after aging at 1400 °C, both for the sol−gel synthesized and one coprecipitated material. The r...

Characterization of chargeability of biological particulates by triboelectrification

Triboelectric chargeability of pecan pollen and lycopodium spores, using commercial Teflon and nylon chargers, has been investigated as reported in this paper. The charge-to-mass ratio of lycopodium, measured at different powder feed rates, i.e., 50-500 mg/s, indicated that both Teflon and nylon chargers increased particle charge by approximately elevenfold over background with Teflon charging particles positively to +11.06 mC/kg and nylon negatively to -11.52 mC/kg. Pecan pollen charge increased by approximately 11 times for Teflon charging positively to +7.59 mC/kg, and five times for nylon charging negatively to a value of -3.72 mC/kg. Pollen charging was not affected by carrier-air humidity in the range of 18.1%-47.6% relative humidity (RH) and at a temperature of 22.5/spl deg/C. Pollen chargeability by Teflon measured as a function of the duration of air flow over the charger surface indicated that triboelectric charging was not degraded, even by 3.5 h continuous flow of humid air (44.2% RH at 19.2/spl deg/C). Results indicate that tribocharging is a feasible method for imparting charge to pollen in electrostatic pollination technology currently under development.

Parametric study of a reverse flat plate absorber cabinet dryer: A new concept

In this communication, the concept of reverse flat plate collector has been used as a heating medium of air for the drying of agricultural products in a cabinet dryer. The reverse flat plate absorber is a non-concentrating collector which can collect solar heat at high temperature unlike conventional nonconcentrating collectors. The thermal performance of the proposed dryer is analyzed by solving the various energy balance equations. An attempt has been made to optimize the vent area of the dryer for speedy flow of humid air from the drying chamber to the atmosphere. In order to have parametric studies, numerical computations have been carried out for a typical day in June for Delhi climatic conditions. The performance of this system is compared with that of conventional cabinet dryers. It is found that the reverse flat plate absorber dryer gives the better performance.

Effect of humid air flow rate on the filmwise condensation inside a vertical cooled pipe: numerical and experimental study

This study consists of evaluating the effect of the saturated humid air flow rate on the heat and mass transfer in a vertical exchanger. One of the affected parameters is the shear stress along the liquid vapor interface. The second studied point is the effect of flow rate on the diffusion of the vapor in the mixture. A simple mathematical model is built from one-dimensional equations for the mass conservation, enthalpy and momentum. In order to validate the model, an experimental apparatus allows to have access to the local heat transfer passing through the wall. The experimental results are in good agreement with the theoretical predictions. They show that the cocurrent gas/liquid flow allows a better recovery of energy.