Horizontal Air Research Articles

Characterization of horizontal air–water two-phase flow in a round pipe part II: Measurement of local two-phase parameters in bubbly flow

The current work seeks to develop an additional database in air–water horizontal bubbly flow through a 3.81cm inner diameter test section with a total development length of approximately 250 diameters. The experimental conditions are chosen to cover a wide range of the bubbly flow regime based upon flow visualization using a high-speed video camera. A database of local time-averaged void fraction, bubble velocity, interfacial area concentration, and bubble Sauter mean diameter are acquired throughout the entire pipe cross-section using a four-sensor conductivity probe for nine flow conditions. To investigate the evolution of the flow, measurements are made at axial locations of 44, 116, and 244 diameters downstream of the inlet. Using this database, the effects of gas flow rate, liquid flow rate, and development length on the local and area-averaged two-phase flow parameters are presented. From the local profiles, it is found that highly turbulent liquid flow conditions cause the void fraction profile to become more uniform with increasing development length through a turbulent mixing process which counters the effect of buoyancy. Bubble interactions are also observed to play a significant role in the evolution of the flow such that the area-averaged void fraction and interfacial area concentration can have different axial trends.

Experimental and numerical study of the airflow distribution in mixed-flow grain dryers

ABSTRACTThe aim of this study was to investigate the airflow distribution in a mixed-flow dryer (MFD) and to study the effect of different bed materials and air duct arrangements. The results were used to validate the numerical model developed in a previous work based on Computational Fluid Dynamics (CFD). A series of experiments have been conducted at a semi-technical MFD test dryer with horizontal and diagonal air duct arrangement. Wheat and rapeseed were used as bed materials. The experiments were performed under isothermal conditions. Two experimental methods were selected and adapted to the measuring problem—the measurement of the isobar distribution within the grain bed and the residence time analysis using the tracer gas pulse method. As could be shown, the isobar distributions measured for wheat and rapeseed agreed well with the model predictions. The numerical model could calculate the influence of the bed material with its different particle characteristics (e.g., particle shape, particle size, bed porosity). The results obtained from the residence time analysis confirmed the known quartering of the air stream flowing from one inlet air duct to the four surrounding outlet air ducts for the horizontal air duct arrangement; in the diagonal air duct arrangement, the air stream from one inlet air duct was nearly halved flowing to the two adjacent diagonal outlet air ducts. These results were confirmed by investigations of the air velocity distribution within the grain bulk. Further experiments are necessary to refine the model. The residence time and isobar measurements will be extended to study the influence of different air properties under real drying conditions, the effect of structural elements, and dryer designs.

Characterization of horizontal air–water two-phase flow in a round pipe part I: Flow visualization

As a part of characterizing the bubble interaction mechanisms and flow regime transition processes in horizontal gas–liquid two-phase flow, a flow visualization study is performed in an air–water test facility constructed from 3.81cm inner diameter clear acrylic round pipes. The test section is approximately 250 diameters in length to allow for development of the flow. Flow visualizations are performed using a high-speed video camera at 80 and 245 diameters downstream of the inlet to observe the development of the flow structures. A total of 27 flow conditions including bubbly, plug, slug, stratified, wavy, and annular flows are characterized in the present study.In highly turbulent bubbly flow conditions it is found that the distribution becomes more uniform with increasing development length through a turbulence penetration process that counters the effect of buoyancy. It is also found that plug bubbles form below a layer of small bubbles rather than at the upper pipe wall where the bubbles are most packed. In fact, it is consistently found that turbulence-based bubble interactions do not occur in the most densely packed regions as the eddies there are not large enough to effect the bubbles. Rather, bubble packing-induced coalescence occurs in these regions and contributes to the formation of plug bubbles. The newly formed plug bubbles move faster than, and ultimately pass, the smaller bubbles above due to the effect of the wall. These small bubbles are subsequently overtaken by the following plug bubble and coalesce with the nose region through a process denoted as drag-induced coalescence.

Impact of European Black Carbon on East Asian Summer Climate

The remote response of the East Asian sum- mer monsoon (EASM) to European black carbon (EUBC) aerosols was studied by using an ensemble of sensitivity experiments with the Geophysical Fluid Dynamics Labo- ratory (GFDL) atmospheric general circulation model (AGCM) Atmospheric Model version 2.1 (AM2.1). The results show that EUBC causes an enhanced EASM. The resulted enhanced southwesterly brings more moisture supply from the Bay of Bengal, which causes an increase in precipitation over the Yangtze River valley, northeast- ern China, the eastern part of the Yellow River valley, and the Tibetan Plateau. Diagnostic examination suggests that EUBC induces enhanced tropospheric heating over most of the Eurasian Continent through a propagating wave train and horizontal air temperature advection. This phe- nomenon results in intensified thermal contrast between land and ocean, which accounts for the enhanced EASM. Moreover, reductions in EUBC emission in 1992 may have contributed to decadal weakening of the EASM in the early 1990s.  Keywords: European black carbon, East Asian summer monsoon, thermodynamic contrast, decadal shift Citation: Wan, J.-H., R. Mahmood, and S. Li, 2013: Im- pact of European black carbon on East Asian summer climate, Atmos. Oceanic Sci. Lett., 6, 375-380, doi: 10.3878/j.issn.1674-2834.13.0037.

Advanced modelling of the transport phenomena across horizontal clothing microclimates with natural convection.

The ability of clothing to provide protection against external environments is critical for wearer's safety and thermal comfort. It is a function of several factors, such as external environmental conditions, clothing properties and activity level. These factors determine the characteristics of the different microclimates existing inside the clothing which, ultimately, have a key role in the transport processes occurring across clothing. As an effort to understand the effect of transport phenomena in clothing microclimates on the overall heat transport across clothing structures, a numerical approach was used to study the buoyancy-driven heat transfer across horizontal air layers trapped inside air impermeable clothing. The study included both the internal flow occurring inside the microclimate and the external flow occurring outside the clothing layer, in order to analyze the interdependency of these flows in the way heat is transported to/from the body. Two-dimensional simulations were conducted considering different values of microclimate thickness (8, 25 and 52 mm), external air temperature (10, 20 and 30 °C), external air velocity (0.5, 1 and 3 m s(-1)) and emissivity of the clothing inner surface (0.05 and 0.95), which implied Rayleigh numbers in the microclimate spanning 4 orders of magnitude (9 × 10(2)-3 × 10(5)). The convective heat transfer coefficients obtained along the clothing were found to strongly depend on the transport phenomena in the microclimate, in particular when natural convection is the most important transport mechanism. In such scenario, convective coefficients were found to vary in wavy-like manner, depending on the position of the flow vortices in the microclimate. These observations clearly differ from data in the literature for the case of air flow over flat-heated surfaces with constant temperature (which shows monotonic variations of the convective heat transfer coefficients, along the length of the surface). The flow patterns and temperature fields in the microclimates were found to strongly depend on the characteristics of the external boundary layer forming along the clothing and on the distribution of temperature in the clothing. The local heat transfer rates obtained in the microclimate are in marked contrast with those found in the literature for enclosures with constant-temperature active walls. These results stress the importance of coupling the calculation of the internal and the external flows and of the heat transfer convective and radiative components, when analyzing the way heat is transported to/from the body.

Prediction system of erythemas for phototypes I and II, using deep-learning

Background: The sun is a natural source of electromagnetic radiation, upon which are found the ultraviolet (UV) rays, where only the types A and B are able to irradiate over the surface of the Earth in different proportions. Although the sun helps human skin in the formation of vitamin D, the mineralization of bones, and absorption of calcium and phosphorus in the organism, it can cause damage on the skin by prolonged exposure to UV radiation, generating adverse effects on human health like erythema formation, photo-toxicity, photo-allergy, idiopathic lesions, and photo-dermatitis, among others. This paper, shows the results of developing a prediction system of the exposure time of a person to UV rays coming from the sun, which can cause erythema on human skin, using the standards in UV index and the dose limits of radiation allowed for phototypes I and II, aiming to foresee the generation of these kind of lesions. This was made by the implementation of artificial intelligence algorithms like Deep Belief Networks and Backpropagation, based in the Deep Learning technique. These algorithms use as training parameters for the neural network, the meteorological data such as the sky clearness index, the radiation on the horizontal surface and average air temperature, supplied by the National Aeronautics and Space Administration (NASA). With the data, a neural network aiming to foresee the UV index for the following year of the data input was trained, in addition some mathematical regressions were applied allowing in this way, to obtain an approach to the behavior of the UV index along the day. Likewise, this information was used to estimate the maximum time of sun exposure, for the period of time contained between 6:00 a.m. and 6:00 p.m. This paper, also presents some conclusions based in the results found, which try to establish some important considerations in order to implement the neural networks.

Plenum-Aerated Biofilter for Local-Scale Nitrogen Removal

AbstractThe plenum-aerated biofilter (PA biofilter) is an unsaturated, single pass vertical flow biofilter with horizontal air plena within the media to enhance passive aeration. Plena enlarge the surface area of the air/media interface, increase oxygen ingress, and permit a reduction in the footprint of ammonium oxidizing biofilters. An experimental PA biofilter was operated on household wastewater pretreated in a primary settling tank. The prototype was a 7.3 cm (3 in.) inner diameter circular column, containing 61 cm (24 in.) of granular clinoptilolite media. Three 5.1 cm (2 in.) aeration plena were placed at successive 7.6 cm (3 in.) media thicknesses below the upper filter surface. The biofilter was operated for 28 weeks at hydraulic loading of 69.8 cm/day (17 gal./ft2-day) and a 60-min dosing cycle. Performance was characterized by monitoring from days 180 to 196. Influent NH4+−N was reduced by 99% (mean 51.6 mg/L to 0.61 mg/L, n=3), with substantial conversion to NO3−−N (mean 38.4 mg/L, n=3). ...

A CFD Study of Roadside Barrier Impact on the Dispersion of Road Air Pollution

This study evaluated road shape and roadside barrier impact on near-road air pollution dispersion using FLUENT computational fluid dynamics (CFD) model. Simulated road shapes are three types, namely atgrade, depressed, and filled road. The realizable k-e model in FLUENT CFD code was used to simulate the flow and dispersion around road. The selected concentration profile results were compared with the wind tunnel experiments. The overall concentration profile results show good agreement with the wind tunnel results. The results showed that noise barriers, which positioned around the at-grade road, decrease the horizontal impact distance (In this study, the impact distance was defined as the distance from road surface origin coordinate to the position whose mass fraction is 0.1.) lower 0.33~0.65 times and change the vertical air pollution impact distance larger 2.0~ 2.27 times than those of no barrier case. In case of filled road, noise barriers decrease the horizontal impact distance lower 0.24~0.65 times and change the vertical air pollution impact distance larger 3.33~3.55 times than those of no barrier case. The depressed road increase 1.53~1.68 times the vertical air pollution impact distance. It contributes the decrease of horizontal air pollution impact distance 0.32~0.60 times compare with no barrier case.

Convective heat transfer coefficients between horizontal plastic shading nets and air

Determining convective heat is essential for analyzing energy exchange with shading nets. Unlike solar and thermal radiation, the convection (Qn−a) and the convective coefficient (hn−a) have never been estimated or measured for plastic nets.In this study, an energy balance was applied to a horizontal net under outdoor conditions to predict Qn−a and hn−a. Therefore, the net was tacked onto a wooden frame; energy balance was applied below and above the net surfaces and above a black substrate underneath the frame. The wind speed (V), net-air temperature difference (ΔT), downward and upward solar and thermal radiation fluxes were measured, on sunny days, below and above the net to be used for solving the energy balance equations. Accordingly, the time dependence of Qn−a and hn−a were investigated. Nets with different porosities and colors were used. For each net tested, hn−a was expressed as a function of V and ΔT. The results showed that hn−a depends mainly on the texture structure of the net rather than its color or porosity. Therefore, a similar net texture would have nearly the same hn−a value. These expressions can be used to estimate hn−a for nets having similar texture structures, different colors, and different porosities.

Two-dimensional analytical model of dry air thermal convection

In the present work, the steady-state stationary dry air thermal convection in a lower atmosphere has been studied theoretically. The thermal convection was considered without accounting for the Coriolis force, and with only the vertical temperature gradient. The stream function has been analytically obtained within the framework of two-dimensional thermal convection model in the Boussinesq approximation with velocity divergence taken as zero. It has been shown that the stream function is symmetrical about the horizontal and vertical. The expressions for the horizontal and vertical air velocity components have been obtained. The maximal vertical velocities level is in the center of the convective cell where the horizontal air velocity component is equal to zero. It has been shown that the air parcel’s rotation period during the thermal convection is determined by the Brunt–Vaisala frequency. The expression for the maximal air velocity vertical component has been found. The dependence of the maximal air velocity vertical component on the overheat function at ground surface and on the atmosphere instability has been demonstrated. The expression for the pressure disturbance has been obtained. It has been demonstrated that at the points with maximal pressure disturbance the vertical velocity is equal to zero and the horizontal velocity is maximal. It has been found that the convection cell size depends on the atmosphere stability state.

Numerical Investigation of Different Airflow Schemes in a Real Operating Theatre

A study on performances of different ventilation schemes provided by vertical and horizontal uni-directional air flow was carried out in a standard orthopaedic operating theatre (OT). Starting from our previous studies of a real OT under operating use conditions, in this research different air flow configurations, considering some air curtain solutions on the ceiling and at the sliding door always assumed to be open as a basic boundary condition, were investigated by numerical simulations. Indoor air quality (IAQ) indexes and thermal comfort parameters, deduced from simulation results were calculated and discussed referring to the best performance and efficacy between the air flow schemes to contrast the incorrect use conditions of the OT. Referring to the studied schemes, the reciprocal comparison emphasizes that a successful outcome in preventing surgical site infection can depend as much on resolving human factors (i.e. operational use conditions, door opening/closing), as on overcoming physical and technical obstacles.

Experimental Investigation of Transient Heat Transfer on a Solid Surface, with Fire Retardant Fabric Under Hot Air Impinging Jet

This paper represents an experimental investigation of the transient heat transfer on a solid base plate (mimic to skin) covered by fire retardant fabric (Kevlar® 49), under hot air jet impingement. The study was carried out by a fabricated attachment with an axial flow wind tunnel for horizontal hot air jet impingement. The hot air jet was impinged on a vertical base plate at different velocities and temperatures. A set of experimental conditions was considered to understand the various heat transfer phenomenon. The hot air jet temperatures were 115 and 125°C respectively and jet velocities were 12, 15, and 19 ms−1 respectively at the exit point of the nozzle. The surface temperatures of the solid base plate are used to calculate the heat flux, local heat transfer coefficient, and Nusselt numbers. The maximum heat transfer is found on base plate, whilst the maximum decrease of heat flux is observed on fire retardant fabric. This experimental model can enhance the understanding and insights of the heat transfer process through permeable fabric.

Numerical investigation of low NOx combustion strategies in tangentially-fired coal boilers

A numerical model is developed to investigate the effects of horizontal bias combustion (HBC) and air staging combustion (over-fire air, OFA) technologies on the performance of a 200MWe tangentially-fired pulverized-coal boiler. The devolatilization rate and volatiles amount are determined by a kinetic devolatilization model, which predicts the coal devolatilization prior to the volatile matter and char combustion. The characteristics of the devolatilization, combustion, heat transfer and NOx emission are studied and compared to achieve a comprehensive understanding of the low NOx combustion. The prediction shows a good agreement with the on-site measurement results, which confirms that the model is capable of predicting the characteristics of the investigated boiler. The predicted results have shown that the OFA has a remarkable effect on the reduction of NOx emission. The HBC makes a significant NOx reduction in the primary combustion zone (PCZ) when there is no air staging. In terms of the NOx reduction, the air staging plays a dominant role in comparison with HBC burners. The application of OFA tends to lead to slagging in the PCZ, which can be avoided using HBC due to the higher stoichiometry close to the furnace wall. The details of this study improve the understanding of combustion and NOx emissions in tangentially-fired pulverized-coal boilers with low NOx combustion technologies, especially for boilers adopting HBC burners and OFA.

Bubble characterization in horizontal air–water intermittent flow

Elongated bubbles were characterized experimentally for air–water flow in a horizontal pipe at nearly atmospheric conditions. The range of flow rates employed covered regimes at the transition from elongated bubble to slug. Ensemble-averaged digital image processing techniques were applied for detection of the liquid–gas interface with aid of a set of photo gates to synchronize bubble passage with image acquisition. Quantitative data of front and tail parts of the bubbles were analysed for different mixture velocities and the results confirmed visual observations frequently reported in the literature. Close to transition, a linear tendency of the bubble nose to move towards the pipe centerline position, for increasingly higher values of the Froude numbers, was observed and quantified. Bubble tail shapes were quantified and the hydraulic jumps were shown to be dependent of Froude number, while the liquid film thicknesses were governed by the liquid volume fraction. Changes on the bubbles characteristics are apparently linked to variations in the bubble velocities and seem to reflect a competition between viscous and inertia effects.

The effect of screen type on crop micro-climate, reference evapotranspiration and yield of a screenhouse banana plantation

Most banana screenhouses are currently equipped with transparent screens, with a nominal shading of 8–15%. Two types of screens are widely used, woven or knitted, which differ in their texture and hence in their radiative and aerodynamic properties. These differences may induce different micro-climatic conditions and hence different atmospheric water demand, plant response and yield.We have investigated four types of screens based on different shading levels (8%, 10% or 13%) and different screen textures (woven or knitted). A large commercial banana plantation in Northern Israel was covered with patches of the four different screen types, each in 4–5 replicates, randomly located. Air velocity, temperature, humidity and net radiation were measured simultaneously below two out of the four screen types: the 10% woven and 10% knitted. The results showed that both the net radiation and air temperature were similar under these two screens. Nevertheless, under the knitted screen the horizontal mean air velocity was 18% higher and the specific humidity 8% lower than under the woven screen. Leaf lamina tearing (typical wind damage) and estimated reference evapotranspiration were higher under the knitted screen; the latter mainly during the fall–winter season. However, the horticultural measures of flowering and fruit yield characteristics were the same under all four screen types, with results typical of screenhouse banana plantations in this region. Hence, the results suggest a potential increase in water use efficiency under the woven as compared to the knitted screen.

Effect of Measurement Density on Characterizing Air Velocity Distribution in Commercial Broiler Houses

Abstract. Increasing air velocity of tunnel ventilation systems in commercial broiler facilities improves production efficiency. As a consequence, many housing design specifications require a minimum air velocity in the house. Air velocities are typically assessed with a hand-held anemometer at random locations, rather than systematic traverses. Simultaneous measurement of air velocity at multiple locations in the facility would provide a more accurate estimation of air velocity distribution. The objective of this study was to assess the effect of measurement density on accuracy of estimating air velocity distribution in a tunnel-ventilated broiler production facility. An array of 40 anemometers was placed on a series of transverse cross-sections in a commercial broiler production facility with curtain sidewalls (no birds present) measuring 12.8 A— 121.9 m. The house was equipped with ten 121.9 cm exhaust fans. Cross-sectional air velocity measurements were taken along the length of the house in increments of 3.05 m axially. Data were sampled at 1 Hz for 2 min; three 2 min subsamples were obtained at each cross-section. Horizontal plane air velocity distribution maps were generated using 12.19, 6.10, and 3.05 m axial measurement distances between cross-sections at 0.46 m above the litter. Vertical plane air velocity distribution maps were created using 10, 20, and 40 symmetrical sampling points from the original data set. Cross-validation analysis revealed that higher spatial measurement density in the axial direction yielded a higher correlation between observed and predicted values (79%) and lower mean squared prediction error (MSPE; 0.10 m s -1 ) when compared to decreased sampling densities. Vertical cross-section measurement density comparisons showed a reduction in MSPE and an increase in correlation between observed and predicted values at higher sampling densities in all cases tested excluding one. In the case of improved interpolation results with fewer measurement points, the cross-section demonstrated high variation in air velocity and velocity values being very low. Axial cross-sectional measurement distances of ≤3.05 m and vertical plane measurement densities of ≥40 measurement points should be used to accurately characterize air velocity distribution in a 12.8 A— 121.9 m broiler production facility. Although more sensors and time are required to collect 40-point cross-sections at 3.05 m, the improved visualization allows better identification of distribution effects caused by equipment placement in the facility.

Horizontal two-phase jet behavior with an annular nozzle ejector in the water tank

The behavior of gas–liquid two-phase jets has been extensively studied over many years, due to their fundamental importance in many multiphase systems. Understanding jet behaviors is essential for predicting many processes of practical importance. The objective of this study is to investigate horizontal air–water jet behavior with an annular nozzle ejector in a shallow water tank and to present the effect of overall jet behavior on oxygen transfer characteristics from air bubbles to water. The experimental apparatus consisted of an electric motor-pump, an annular nozzle ejector assembly, a high-speed camera unit, a circulation water tank, and measuring or controlling accessories. The images of the air–water mixed jet issuing from the ejector are analyzed qualitatively, and the dissolved oxygen concentration is measured using the clean water unsteady state technique. The horizontal mixed jet behaves like a buoyancy jet, a quasi-horizontal bubble jet, or a horizontal momentum jet owing to the momentum of the primary flow and air bubble size. It can be concluded that the higher the momentum of primary water flow and the lower the entrainment ratio, the smaller the air bubble size. The buoyancy force of the air bubble and the penetration of the mixed flow are found to be important parameters for the oxygen transfer rate owing to the contact area and time of the two phases.

Multi-range sensors for the measurement of liquid film thickness distributions based on electrical conductance

The paper presents an approach toward an enhancement of the measuring range of high-speed sensors for the measurement of liquid film thickness distributions based on electrical conductance. This type of sensors consists of electrodes mounted flush to the wall. The sampling of the current generated between a pair of neighboring electrode is used as a measure of the film thickness. Such sensors have a limited measuring range, which is proportional to the lateral distance between the electrodes. The range is therefore coupled to the spatial resolution. The proposed new design allows an extension of the film thickness range by combining electrode matrices of different resolution in one and the same sensor. In this way, a high spatial resolution is reached with a small thickness range, whereas a film thickness that exceeds the range of the high resolution measurement can still be acquired even though on the costs of a lower spatial resolution. A simultaneous signal acquisition with a sampling frequency of 3.2kHz combines three measuring ranges for the characterization of a two-dimensional film thickness distribution: (1) thickness range 0–600µm, lateral resolution 2×2mm2, (2) thickness range 400–1300µm, lateral resolution 4×4mm2, and (3) thickness range 1000–3500µm, lateral resolution 12×12mm2. The functionality of this concept sensor is demonstrated by tests in a horizontal wavy stratified air–water flow at ambient conditions. Using flexible printed circuit board technology to manufacture the sensor makes it possible to place the sensor at the inner surface of a circular pipe.

Application of Swirling Flows in Aspiration Systems

Horizontal air ducts of aspiration systems are subject to the formation of dust deposits. The organization of swirling flows is suggested for the prevention of this effect. The equations of dust particle motion in horizontal swirling flow running inside cylindrical air duct are presented. The paper gives the results of the determination of dust particle entrainment velocity with axial and swirling gas flows. The description of a device aimed at organizing the flow swirling inside aspiration systems air ducts is provided

Evolution of heat feedback in medium pool fires with cross air flow and scaling of mass burning flux by a stagnant layer theory solution

This paper quantifies experimentally the evolution of heat feedbacks through conduction, convection and radiation in medium square pool fires (10–25cm) with horizontal cross air flows ranged in 0–3.0m/s. Ethanol and heptane are used as representative fuels producing typically less-sooty and sooty flames. Results show that the overall conduction heat feedback flux through the four sides increases with cross air flow speed, being more prominent for smaller pools than for larger ones. And the rate of overall conduction heat feedback increment is more prominent for heptane, of which the rate of increase with flow speed nearly 2 times that of ethanol. Meanwhile, the radiation heat feedback declines with cross air flow speed, with its contribution fraction nearly negligible at larger flow speeds for both fuels. The convection feedback fraction increases with cross air flow speed. Its increment is more prominent (a) for heptane (increasing fast with flow speed to be even the dominate one among the three heat feedbacks) than ethanol (increasing slowly with flow speed; being nearly constant and maintaining to be the dominant one); and (b) for larger pools than for smaller ones. The changing of dominant heat feedback mechanism with cross air flows, results in the change of the scaling behavior of the mass burning flux with pool size. A stagnant layer solution theory is then proposed, by including fuel mass transfer Spalding number B, to describe the change of mass burning flux of different size pool fires with cross air flow speed for different fuels in relative strong cross air flows [as indicated by Froude number (Fr=u/[gd]1/2) larger than about unity]. Experimental data are shown to be well correlated by the proposed theory.