Low Air Velocity Research Articles

Time-course estimation of evaporated water fluxes together with mean surface water activity in uncooked pressed cheeses during ripening using a purpose-built micro-bioreactor

During cheese ripening, surface water activity controls microbial flora activity and aroma development, essentially shaping the organoleptic properties of the cheese. An experimental micro-bioreactor was set up using air-product water balance to non-destructively estimate the time-course of mean water activity at the cheese surface under well-controlled airflow ripening conditions. Six trials were performed using this experimental device on uncooked pressed cheeses ripened until 30 days. These trials highlighted (1) the effects of a low air velocity and high relative humidity on the water exchanges occurring at the cheese surface, demonstrating strong surface sensitivity to external ripening air conditions, and (2) a close interaction between surface microbial flora development and the water transfers occurring both from cheese core to surface and at the surface itself, although further micro-bioreactor ripening trials are needed to fully clarify this interaction.

CFD Analysis of Two-Phase Flow Characteristics in a 90 Degree Elbow

Computational fluid dynamics (CFD) analysis was performed for a two-phase air-water flow through a horizontal to vertical 900 elbow with a 12.7 mm pipe diameter. Three different air velocities of 15.24, 30.48, and 45.72 m/sec along with three different water velocities of 0.1, 1.0, and 10.0 m/sec were used in this study. To analyze the flow behavior in the elbow, pressure and velocity profiles at six different upstream and downstream locations of the elbow were compared. Computational fluid dynamics (CFD) analysis was performed for 9 different cases using FLUENT commercial code. A mixture model was used to account for different gas and liquid velocities to solve continuity, momentum and energy equations. CFD analysis results showed a decrease in pressure as fluid leaves the elbow in addition to a larger pressure drop at higher air velocities. No significant change in pressure was observed when water velocity was increased from 0.1 to 1.0 m/sec compared to water velocity change from 1.0 to 10.0 m/sec. The normalized pressure drop was larger at lower air velocities compared to higher water velocities. CFD analysis results were compared with available experimental data showing a reasonably good agreement.

Evolution of bioaggregate strength during aerobic granular sludge formation

This work investigated the modification of aggregate properties during the formation of granular sludge in a sequencing batch airlift reactor (SBAR). The cohesion of biological aggregates was quantified by subjecting sludge samples to two different controlled shear stresses in a stirred reactor. For reference sludge (without granules), flocs broke and reformed easily, indicating that floc size was controlled by the turbulence micro-scale (Kolmogorov scale, here from 17 μm to 62 μm). In contrast, granules showed high strength which enabled them to resist turbulence and their size was no longer imposed by the Kolmogorov micro-scale. Different steps were observed during the granulation process: a first increase of aggregate cohesion associated with a decrease in sludge volume index (SVI), a growth of aggregates with detachment of fragile particles from the surface and, finally, an increase in the sizes of small and large granules to reach a pseudo-stable size distribution. Results suggest that small particles could have formed the seeds for new granules, as they were maintained in the bioreactor. Here, granular sludge was formed in an SBAR with a conventional settling time (30 min), i.e. without particle washout, and with a low superficial air velocity (SAV = 0.6 cm s −1): it is thus demonstrated that high SAV and low settling time are not necessary to produce granules, but probably only accelerate the accumulation of granules. It is shown that the increase of cohesion is the initial phenomenon explaining the granule formation concomitantly with bacterial aggregates densification. It seems important, in the future, to investigate the reasons for this cohesion increase, which is possibly explained either by bacterial bounding interactions or the excretion of extracellular polymeric substances (EPS).

Air‐flow field of the melt‐blowing slot die via numerical simulation and multiobjective genetic algorithms

AbstractThe air‐flow field in melt blowing plays a key role in fiber drawing and nonwoven web performance. A multiobjective optimization using genetic algorithms was proposed to obtain optimum air‐flow field with the lowest velocity decay and temperature decay of the air‐flow field of a melt‐blowing slot die. Four main geometry parameters, including slot width, head width, slot angle, and setback, were studied. The optimal results were achieved in the 50th generation with 20 individuals of each generation. The results also show that a smaller slot angle and larger slot width resulted in a lower air velocity decay and temperature decay. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Application on the Stratified Air Conditioning System with Low Velocity in the Main Hall of the Temple

This paper presents a design scheme of the stratified air conditioning system, in accordance with the temple hall’s large space and function characteristics. The indoor stratified air conditioning environment is formed by the low velocity air supply of fan coil units embedded below the Buddha statue. The interface of stratified air is only 1.7m high, because the large amount of air unavoidably penetrates the room due to the prohibition of closing the door. From the tested results, the design not only meet the temple hall’s operational requirements for decoration and the open front door, but also greatly reduces the energy consumption of the air conditioning at the cost of the comfortable feeling of air conditioning. In this work, some suggestions are also given in order to further improve the comfort and lower the energy use.

Mechanically Agitated Fluidized Bed Drying of Cohesive Particles at Low Air Velocity

The drying of micrometer-sized, cohesive particles belonging to the group C type materials according to Geldart's classification is always challenging. An agitated fluid bed dryer (AFBD) of pilot-scale capacity was designed to study the effect of the type of agitator and its speed, gas velocity, and inlet temperature and feed loading on the hydrodynamic performance of the AFBD. Key hydrodynamic parameters such as pressure drop have a profound influence on determining the fluidization characteristics. The pressure drop across the AFBD system was expressed in terms of flow coefficient, ξ, which can be conveniently used as a design parameter. The choice of agitator has an even greater influence on the drying kinetics and the results have been summarized, paving the way for a more efficient spiral agitator of the helical ribbon type.

Characteristics of Liquid–Solid Mass Transfer in a Bubble Column Equipped with a Vertical Tube of Circular Fins

The solid-liquid mass transfer rate at a stack of circular fin surfaces in a bubble column was investigated. A diffusion-controlled dissolution technique of copper in an acidified chromate solution was employed. Variables studied included the number of actively exposed fins ranging from 5 to 20, pertinent physical properties of the solution, and air superficial velocity. Experimental data showed that the rate of the diffusion-controlled mass transfer increases with increasing superficial air velocity and decreases with increasing chromate solution acid concentration. Moreover, at relatively low superficial air velocity, increasing the number of actively exposed fins results into a continuous increase in the mass transfer coefficient. At relatively higher superficial air velocity, however, the mass transfer coefficient decreases in the 5 to 10 range of actively exposed fins and then reverts to increase in the 15 to 20 range.

Multiphysics modeling of building physical constructions

The paper presents an overview of Multiphysics applications using a Multiphysics modeling package for building physical constructions simulation. The overview includes three main basic transport phenomena for building physical constructions: (1) heat transfer, (2) heat and moisture transfer and (3) heat, air and moisture (HAM) transfer. It is concluded that full 3D transient coupled HAM models for building physical constructions can be build using a Multiphysics modeling package. Regarding the heat transport, neither difficulties nor limitations are expected. Concerning the combined heat and moisture transport the main difficulties are related with the material properties but this seems to be no limitation. Regarding the HAM modeling inside solid constructions, there is at least one limitation: the validation is almost impossible due to limitation of measuring ultra low air velocities of order μm/s.

Numerical probing of a low velocity concurrent pilot scale spray drying tower for mono-disperse particle production – Unusual characteristics and possible improvements

A pilot scale micro-fluidic-spray-dryer (MFJSD-II) utilizing a single-stream atomization to produce mono-disperse particles has been developed at Monash University. A unique feature of this unit is the use of relatively low air velocities in the range of 10 −1–10 −2 m s −1, which increases the residence time of the particles during the drying process, in comparison with conventional spray dryers. In this study, Computational Fluid Dynamics (CFD) simulation revealed that the effects of natural convection, caused by heat loss from the wall, were significant in deflecting the flows of air from the central jet-side recirculation pattern, causing them to spread towards the wall. The flow patterns formed a layer of relatively high velocity region adjacent to the wall. The understanding of these flow patterns would be crucial for future designs of low-velocity spray dryers. The spreading of the central jet towards the wall also created recirculation regions in the center of the tower, thus affecting the residence times particularly for smaller particles. More numerical simulations are required to optimize the design of the bottom bustle to be effectively used as a particle-separator.

DROPLET IMPACT ON SHEAR-DRIVEN LIQUID FILMS

This paper investigates the interaction between the impact of a liquid droplet and a moving liquid film in which a shearing airflow is present above the liquid film. Visualization of droplet trajectory, deformation, and impact outcome are provided. Droplet diameters used in this study range from 2.4 to 4.8 mm, water film thicknesses are between 0.2 and 1.7 mm, and the shearing air velocity within the rectangular duct is between 4.5 and 15 m/s. Two high-speed cameras are used to track the droplet and visualize the film surface. A laser focus displacement meter is used to obtain the instantaneous and average film height. By analyzing the acquired images, along with the film height, different regimes for both droplet deformations in shearing air and impact outcomes are observed. A regime map is produced relating the air, film, and droplet properties with the outcomes of the interaction between the droplet and the moving water film. The shearing air has an important effect on the interaction process before the impact by deforming the droplet and confining the film thickness; it also influences the impact process during and after the impact. Where the droplets retain their spherical shape before impact (low shearing air velocity), the transition between splash and deposition agree with published results obtained for droplet impact onto static films. The waviness of the film has no clear effect on the impact outcomes but does have an important effect on the way splash occurs with a big difference in the size of secondary droplets.

Thermal desorption and vapor transport characteristics in an explosive trace detector

Swipe-based explosive trace detectors rely on thermal desorption to vaporize explosive particles collected on a swipe. The vaporized material is carried by air flows from the desorption unit to the inlet of the chemical analyzer, typically an ion mobility spectrometer. We have observed that the amount of explosives detected from a swipe varies with the physical location of explosives collected on the swipe. There are two issues that may contribute to this effect: inhomogeneous or insufficient heating of the swipe during desorption and low velocity air flows that inefficiently transport desorbed vapor during the instruments analysis time. To better characterize this effect, we have simulated the air movements within a generic desorption unit using commercially available computational fluid dynamics software. Simulations are three dimensional, symmetric and solved under steady, laminar flow conditions. The calculated velocity field correlates directly with experimental detector response to the high explosive RDX. Results suggest that the limiting factor in this model thermal desorption unit is the flow-field around the swipe and flow rate into the detector, rather than heat transfer to the swipe itself. Buoyancy effects due to heating dominate the flow-field and produce a vertical bulk fluid motion within the domain that opposes much of the flow drawn into the analyzer.

Effects of Drying Parameters on the Drying Kinetics of Fermented Ground Cassava Using a Rotary Dryer

The effects of drying parameters on the drying kinetics of fermented ground cassava were studied. Fermented ground cassava (TMS 30572) was dried in a bench scale rotary dryer at different inlet air temperature (115-230°C), inlet air velocity (0.83-1.55 m/s), feed drive speed (12-100 rpm), drum drive speed (8-18 rpm), relative humidity of inlet air (50 to 80 percent), and mass of feed (50-500 g). It is shown that inlet air temperature and inlet air velocity have the most significant effects on the drying rate of fermented ground cassava. A model which predicts the drying rate of fermented ground cassava as function of inlet air temperature and inlet air velocity is presented. Predictions of the model are compared with experimental data, and good agreement is obtained. For proper gelatinization of fermented cassava mash, the inlet air temperature should be within the range 140-230°C, low inlet air velocity (< 1.55 m/s), low drum drive speed (8-12 rpm), moderate air humidity (50 to 65 percent), and low feed drive speed of about 12 rpm. For high mass of feed (> 500 g), inlet air temperature in the range 190-230°C is recommended for proper gelatinization of fermented cassava.

Experimental studies on the flow characteristics in an oscillating water column device

A fixed type oscillating water column (OWC) device was designed and tested in a 2-D wave channel. The air chamber was converged to its minimum area at the turbine section to obtain the maximum kinetic energy. The variations in the height of water in the water column and in the static pressure of the air caused by the oscillating waves were studied in detail. The airflow in the entire air chamber was documented with particle image velocimetry measurements. No turbine was installed in the device. The experiments were performed by varying the water depth and the wave frequency. It was found that the air velocities in the turbine chamber during the upward motion of water in the column are always larger than during the downward motion. While the airflow was strong most of the time, very low air velocities were recorded during the transition between the upward and downward flows indicating the need of an airflow regulator before the turbine to get a constant flow rate of air. The well-directed flow obtained at the turbine section can be used to drive a Savonius rotor, which is the most appropriate turbine for rectangular cross-section of the chamber.

Heat Transfer and Pressure Drop Under Dry and Humid Conditions in Flat-Tube Heat Exchangers With Plain Fins

Flat-tube heat exchangers could be an interesting alternative to make indirect cooling of display cabinets more energy-efficient. This application involves low air velocities in combination with condensation of water vapor on the air side, so plain fins could be suitable. Two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally. One of the heat exchangers had continuous plate fins, and the other had serpentine fins. The performances during dry and wet test conditions were compared and related to theoretical predictions for different assumptions. The influence of air velocity, air humidity, and inclination angle was investigated. The results show that, in most cases, the heat transfer performance is somewhat reduced under wet conditions in comparison with dry test conditions, and that wet heat transfer surfaces lead to an increased pressure drop. At the lower air velocity range that was investigated, the heat exchanger having continuous plate fins drained better than the one with serpentine fins.

Development of a device and method for the time-course estimation of low water fluxes and mean surface water activity of food products during ripening and storage

Abstract Accurate measurement of water activity ( a w ) is an important goal for the food industry because a w is a key parameter in microbial growth, biological reaction rates and physical properties. An experimental device was setup using air-product water balance to non-destructively estimate the time-course of mean a w at the food product surface under well-controlled airflow conditions. The device is especially suited for studying the ripening of cheeses and fermented meat products, where water fluxes exchanged between products and air are very low. The validation tests performed with a w -known model products showed that water fluxes of 10 −7 kg s −1 can be estimated with an accuracy better than 2% over very short periods of time, and that surface a w can be estimated with an absolute uncertainty of less than 0.01 a w units. A handful of cheese ripening trials illustrate the potential of the method, highlighting the effects of a low air velocity and high air RH on the water exchanges occurring at a cheese surface, thus demonstrating the strong surface sensitivity to external air conditions.

A new integrated CFD modelling approach towards air-assisted orchard spraying. Part I. Model development and effect of wind speed and direction on sprayer airflow

The current trend in modelling flow phenomena within trees such as in orchards follows the assumption of the space occupied by the trees as a porous and horizontally homogeneous medium to avoid the flow details associated with the individual plants. This being sufficient at a larger field or regional scale much has to be done at a plant scale to analyse the flow details within the plant and its elements especially for sensitive agricultural operations such as spraying. This article presents an integrated 3D computational fluid dynamics (CFD) model of airflow from a two-fan air-assisted cross-flow orchard sprayer through non-leafed orchard pear trees of 3 m average height. In this model the effect of the solid part of the canopy on airflow was modelled by directly introducing the actual 3D architecture of the canopy into the CFD model. The effect of small canopy parts, such as very short and thin branches and flowers that were not incorporated in the geometrical model, on airflow was simulated by introducing source-sink terms in the Reynolds averaged Navier–Stokes (RANS) momentum and k– ɛ turbulence equations in a sub-domain created around the branches. This model was implemented in a CFD code of ANSYS-CFX-11.0 (ANSYS, Inc., Canonsburg, PA, USA). In this work it was possible to link the real 3D architecture of pear canopy into a CFD code of CFX. The model was able to capture the local effects of the canopy and its parts on wind and sprayer airflow directly by inserting the tree structure into the model which gave realistic results. The model showed that within the injection region of the sprayer there was an average reduction of the jet velocity by 1 m s −1 for a distance of 2.3 m from the sprayer outlet due to the presence of leafless pear canopy. This reduction was variable at different vertical positions due to the difference in the canopy density. Maximal effect of the canopy was observed in the middle height of the trees between 0.25 m and 2.5 m which is the denser region with a bunch of several branches. The maximum velocity difference observed between these two positions was 1.35 m s −1 at 1.75 m height. Thus, regions of high and low air velocity zones of the sprayer due to the variable branch density of the pear tree were predicted. The effects of wind speed and direction on the air jet from the sprayer were investigated using the model. For a cross- (direction of 90°) wind speed of 5 m s −1 there was about 2 m s −1 reduction in the sprayer jet velocity at the jet centre and 0.5 m horizontal shift of the jet centre towards the wind direction. Generally there was a decrease in the jet velocity with increasing cross-wind and decreasing wind direction with respect to the jet direction.

Discolouration during kiln drying of South AfricanPinuselliottii

AbstractThe effect of different parameters on the surface discolouration (yellow stain and kiln brown stain) and thermal discolouration of Pinus elliottii during kiln drying was investigated. Boards were dried with different kiln schedules and discolouration was assessed at different depth levels from the surface of each board. The discolouration data were analysed using notched boxplots, multivariate analysis of variance and canonical variate analysis biplots. Thermal and surface discolouration were distinguishable by the distribution of colour data point plots in the CIE-L*a*b* colour system. Thermal discolouration was less intense and less varied than surface discolouration. The greatest degree of thermal discolouration occurred in one of the higher temperature (Tdb≥90°C during first drying phase) schedules that was run for an excessively long period due to low air velocity. Discolouration progressively decreased as the boards were planed to a greater depth. Higher temperature schedules yielded 77.5-82...

An experimental study of energy-saving pneumatic conveying system in a horizontal pipeline with dune model

Abstract In order to reduce power consumption and conveying velocity, a pneumatic conveying system where a dune model is mounted in a pipeline is proposed in this paper. The experimental study focuses on the effect of the mounted dune model in the horizontal pneumatic conveying system in terms of pressure drop, power consumption and conveying velocity. The test pipeline consisted of a horizontal smooth acrylic tube with an inside diameter of 80 mm and a length of about 5 m. Polyethylene spherical particles with a density of 952 kg/m3 and diameters of 2.3 and 3.3 mm are used as conveying materials. The mean air velocity is varied from 9 to 16 m/s, and the solid mass flow rate is from 0.25 to 0.45 kg/s. Firstly, the effect of the dune model location on pneumatic conveying is experimentally studied. It is found that in the lower air velocity range, the pressure drop of the pneumatic conveying with a mounted dune model is lower than that of a conventional pneumatic conveying system. A lower conveying velocity and energy-saving conveying can be realized by installing a dune model in the conveying pipe. Especially the case of fixing the dune model on the bottom of the pipe at the inlet of particle feed is more effective. The particle flow patterns also show that the dune model reduces the deposition of particles. Then, the effect of different surface materials of the dune model is examined. By using a surface material of the dune model with a large coefficient of restitution, the pressure drop of conveying large particles is the lowest. When conveying relatively small particles, however, the pressure drop becomes the lowest by a small coefficient of restitution. The maximum reduction rates of the minimum velocity and power consumption by the dune model are about 19% and 34%, respectively.

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

To study the influence of back feeding particles on gas–solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds (CFB). The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u 0 = 4 m/s and G s = 21 kg/(m 2 s), the back feeding particles were found to penetrate the lean gas–solid flow near the entrance (rear) wall before reaching the opposite (front) wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate ( u 0 = 4 m/s, G s = 33 kg/(m 2 s)) resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity ( u 0 = 3.2 m/s) and G s = 21 kg/(m 2 s), beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the fluidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of u 0 = 4 m/s and G s = 21 kg/(m 2 s), addition of coarse particles ( d p = 1145 μm) into the bed made the radial distribution of solids more symmetrical.

Canopy Penetration and Deposition of Barrier Sprays from Electrostatic and Conventional Sprayers1

An experimental study was conducted to investigate the usefulness of electrostatic and conventional sprayers for barrier applications. Two conventional and three electrostatic sprayers were used in the study. Usefulness of the sprayers was rated based on penetration of spray into and deposition onto 2 sides of leaves on natural vegetation. Bifenthrin (Talstar adulticide) was applied at labeled rate, fluorescent dye was added to the tank mix as tracer, and all sprayers applied the dye and insecticide at the same rate. The results indicated that sprayers producing larger droplets produced significantly higher deposition on vegetation in barrier applications than the sprayers producing smaller droplets. Sprayers with higher air velocity at the nozzle discharge proved significantly better for barrier sprays than the sprayers with lower air velocity. Electrostatic sprayers did not show any improvement in deposition on vegetation or in penetration into vegetation over the conventional sprayers. There was no difference in deposition between truck-mounted and backpack sprayers.