Horizontal Airstream Research Articles

Using a coupled CFD – DPM approach to predict particle settling in a horizontal air stream

Preventing health and safety hazards such as dust explosions and respiratory exposure in the work force when handling and storing fine powders is a major challenge faced by plant operators [13]. Computational Fluid Dynamics (CFD) coupled with Discrete Phase Model (DPM) can be used as a tool to address this challenge by advancing the understanding of how particles deposit in a particular process. Particle settling, in air streams, is primarily dependent on the drag forces exerted on the individual particles through interactions with the suspension medium [2]. By improving the understanding of this interaction through repeatable experiments and simulations; more complex CFD – DPM simulations are possible, thus providing a significant step in reducing the risks associated with handling fine powders. To study the transport and settling of particles in air streams, an experiment was established where glass beads, alumina and iron ore dust were injected into a horizontal flow channel. The material was fed into the top of the test rig where it was then transported in a laminar air stream. Through this method particle settling, according to the particle size, can be observed by sampling different trays along the bottom of the test rig. Once the deposition of particles is analysed (using a particle size analyser) each diameter range can be tracked to determine the distance travelled. After evaluating these experiments a CFD coupled with DPM simulation was employed to predict particle deposition in the horizontal chamber. The results show a good agreement between experiments and CFD – DPM results.

The boundary condition for vertical velocity and its interdependence with surface gas exchange

Abstract. The law of conservation of linear momentum is applied to surface gas exchanges, employing scale analysis to diagnose the vertical velocity (w) in the boundary layer. Net upward momentum in the surface layer is forced by evaporation (E) and defines non-zero vertical motion, with a magnitude defined by the ratio of E to the air density, as w = E/ρ. This is true even right down at the surface where the boundary condition is w|0 = E/ρ|0 (where w|0 and ρ|0 represent the vertical velocity and density of air at the surface). This Stefan flow velocity implies upward transport of a non-diffusive nature that is a general feature of the troposphere but is of particular importance at the surface, where it assists molecular diffusion with upward gas migration (of H2O, for example) but opposes that of downward-diffusing species like CO2 during daytime. The definition of flux–gradient relationships (eddy diffusivities) requires rectification to exclude non-diffusive transport, which does not depend on scalar gradients. At the microscopic scale, the role of non-diffusive transport in the process of evaporation from inside a narrow tube – with vapour transport into an overlying, horizontal airstream – was described long ago in classical mechanics and is routinely accounted for by chemical engineers, but has been neglected by scientists studying stomatal conductance. Correctly accounting for non-diffusive transport through stomata, which can appreciably reduce net CO2 transport and marginally boost that of water vapour, should improve characterisations of ecosystem and plant functioning.

C&C efficiency of canopy hood exposed to horizontal air stream in commercial kitchen calculated by CFD analysis

ABSTRACTAs one of the commercial kitchen ventilation researches, the mechanism of capture phenomenon and containment one of a canopy hood has to be investigated. Several experiments intended to change the Capture and Containment (C&C) efficiency of combustion gas and cooking effluence were conducted under the conditions of different suction wind velocity at the bottom of a hood, sometimes called face velocity, which is caused by the different hood size. The overall C&C efficiency was analyzed by means of computational fluid dynamics (CFD). The C&C efficiency from the experiment and CFD analysis showed good agreement, and it was assumed that the disturbing ambient airflow around the hood can influence on the C&C efficiency. Therefore the kitchen space was assumed which includes the disturbing airflow over the counter between a dining hall and a kitchen. As a result, the C&C efficiency of the larger hood decreased more under the disturbing airflow.

Experimental Investigation of the Concept of a ‘Breathing Zone’ using a Mannequin Exposed to a Point Source of Inertial/Sedimenting Particles Emitted with Momentum

An inhaling mannequin, CALTOOL, was used in a specially ventilated room to compare the concentrations inhaled with those sampled by samplers mounted across the breathing zone. The CALTOOL is made from metal sheets and consists of a cylindrical torso (42 x 24 x 54 cm) with a circular cylinder as head. A circular nozzle simulates the mouth. This nozzle is part of a cassette that holds a filter. The inhalation rate is not periodic but kept constant at nominally 20 l min(-1). The CALTOOL was placed in a horizontal air stream ( approximately 10 cm s(-1)) either facing or back to the wind. In front of the lower chest of the CALTOOL, a particle source was mounted which emitted particles with a momentum directed upwards at an angle of 45 degrees towards the CALTOOL. Five monodisperse aluminium oxide powders were used as test aerosols. The mass median aerodynamic diameters of the test aerosols ranged approximately 10 to 95 mum. Six conically shaped aerosol samplers were mounted horizontally and over the breathing zone of the CALTOOL, one on each shoulder, three across the upper torso, and one at the lower torso centre. Four to six runs per test aerosol and CALTOOL orientation in the airflow were conducted. The samples were analysed gravimetrically. The concentration ratio aerosol sampler to the CALTOOL cassette was determined for the investigated mounting positions. The results showed that when the CALTOOL was exposed to particles emitted with momentum from a point source in front of the lower chest, the variation in concentration over the breathing zone was large. The ratio of the concentration sampled by an aerosol sampler mounted somewhere within the breathing zone to the CALTOOL cassette concentration, would, for specific particle sizes, easily differ by a factor of 3, but may extend up to 10-100, depending on the particular conditions. The basic concept of a breathing zone consisting of a hemisphere of radius 25-30 cm is therefore not well suited for workers handling a point source emitting large particles. For such sampling situations, it is suggested that the radius of the breathing zone is reduced to 10 cm, which may be achieved by a head-mounted sampler.

Pilot Flame Effects on Gas Jet Flames in Crossflow

Flame structure, which includes the radiation power and temperature profiles, soot concentration profiles, and pollutants emissions (NO, NO x , CO, and CO 2 ) of propane and propylene gas diffusion flames with and without ahydrogen pilot flame in crossflow is presented. All of the profiles were obtained at a distance of 50% of the total flame length from the burner. The purpose was to investigate the effects of a pilot flame on the structure of flames in crossflow. The fuel was injected vertically into a horizontal airstream through a burner. The flames at nonpiloted conditions were partially or completely detached form the burner. For the piloted flames, hydrogen was added to completely attach the flames to the burner. Experiments were conducted at two different crossflow speeds, 2.68 m/s and 3.58 m/s. The piloted flames, in general, had lower emissions of NO, NO x , and CO 2 . The temperature profiles in all flames showed a peak near the flame center. The pilot flame did not significantly influence the flame temperature profiles at the crossflow speed of 2.68 m/s. However, at 3.58 m/s, the overall flame temperature for both fuels was lower in the piloted flames. The nonpiloted detached flame showed higher concentrations of NO x , CO 2 , and O 2 , but had a lower soot concentration. All of the piloted flames appeared to be longer and narrower compared to detached flames. Piloted flames also produced higher radiation, produced more smoke, and appeared more luminous than detached flames.

Dynamic control of temperature and humidity in a chamber to test moisture-adsorbing insulations

ABSTRACT Theoretical studies have previously suggested that insulation materials with fibres which adsorb and desorb moisture have useful thermal properties under time-varying conditions. These properties are observable with outdoor exposure, which provides diurnal swings in temperature and humidity, but may be studied more closely under controlled dynamic conditions. A test chamber of 3.2 m square floor plan has been constructed to generate these conditions. Within it, panels of sample materials were located in a uniform horizontal airstream, recirculated over pans of glycerol/water solution which were controlled at the temperature required to achieve the desired chamber humidity. This and the control of air temperature (via a fan-coil unit) were implemented through a dual-loop PID controller, receiving updated set-point instructions from a computer at 1-minute intervals. Chamber response matched sinusoidal and ramp target waveforms acceptably well under all but extreme conditions over the design temperature range of 15–40°C and humidity range of 10–90%. There was some distortion of large amplitude sinusoids when temperature and humidity were out of phase, but sufficient waveform fidelity was achieved to allow a detailed intercomparison of insulation samples. The temperature response at the centre-plane of test panels was used as the primary indicator of thermal performance and is discussed in terms of some indicative results.

Influence of Fruit Size and Weight on Wind Dispersal in Betula lenta, a Gap-colonizing Tree Species

-The prevailing model of wind-borne seed dispersal assumes that variation in seed size and weight controls patterns of seed distribution by limiting exposure to horizontal airstreams. To test the applicability of this model in a natural dispersal situation, seeds of Betula lenta were collected downwind of a population along transects which trapped seeds (a) after primary dispersal through the air, and (b) after primary aerial dispersal plus secondary dispersal across a snow cover. In order to examine mechanisms of seed flight, weights of 296 seeds were experimentally modified and duration and distance of seed flight measured in laboratory trials. Rate of descent of modified seeds depended strongly on individual seed weight and wing loading, and lateral movement in a horizontal airstream was determined by rate of descent, as predicted. In the field, however, distance traveled by aerial movement was only weakly linked to seed size and weight, and the relationship did not hold during secondary dispersal near the ground. Thus, seed size and weight are not the principal factors controlling seed movement under all conditions. It may be necessary to invoke models based on airstream turbulence.

Separation of particles in a horizontal air stream

Separation of particles in a horizontal air stream is examined and the effect of air velocity and feed rate is discussed. Cleaning and grading of grains are reviewed and a method is proposed for examining the feasibility of achieving a separation based on the suspension velocity of the particles. A method for increasing the effectiveness of discriminating between particles, based on the paths of grain and threshed crop material in an air stream is studied. Theoretical and experimental results for the horizontal drift of particles were compared.

Separation of Alfalfa Silage Into Stem and Leaf Fractions in a Horizontal Airstream

ABSTRACT Aerodynamic drag coefficient was used as the differentiating characteristic in the separation of alfalfa haylage into leaf and stem fractions in a horizontal wind tunnel. The effects of air velocity, moisture content, feed rate, and air stream height on particle distribution and separation efficiency were investigated. To evaluate the separator a protein improvement index and separation efficiency were calculated. Results indicated that both increased airstream height and air velocities (up to 8 m/s) improved separation efficiency. An increase in either moisture content or feed rate resulted in a decrease in separation efficiency. Regression equations were developed to estimate protein improvement and separation efficiency as functions of airstream velocity, moisture content, and feed rate.

FREE AND FORCED CONVECTION HEAT TRANSFER FROM SHORT INCLINED CIRCULAR CYLINDERS

Free and forced convective heat transfer rates from circular cylinders whose length to diameter ratio is small have been experimentally measured. The cylinders were arranged with their axes in a vertical plane and tests were conducted with the axes set at angles of between 0° and 90° to the horizontal. In the forced convective tests the cylinders were exposed to a horizontal air stream. The majority of the tests were undertaken with models having diameters of 19 mm and 25 mm with length to diameter ratios of between 1.5 and 16. The heat transfer rates were determined by the transient method i.e. by using solid aluminum models, heating them and measuring their temperature-time variation while they cooled. Significant differences between the present results and those observed in previous studies with models having large length to diameter ratio have been found. Methods of correlating both the free convective results and the forced convective results have been derived.

Combined Convective Heat Transfer from Vertical Cylinders in a Horizontal Air-Stream

Mean heat transfer rates from a series of circular cylinders mounted vertically in a horizontal air-stream have been measured. Five cylinders were used, having diameters of approximately 19 to 25 mm and heights of approximately 150 to 300 mm. Heat transfer rates from these cylinders were measured at a number of different air velocities ranging from zero to about 1.6 m/s. The results indicate that the heat transfer rate in the combined convection range is higher than it would be under the same conditions with purely free convection or purely forced convection. An attempt has been made to correlate the combined convective heat transfer rates in terms of the heat transfer rates that would exist under the same conditions in these two limiting cases of purely forced and purely free convection.

The dependence of hot-wire calibration on gas temperature at low reynolds numbers

Heat-transfer measurements from a hot-wire placed perpendicular to a horizontal air stream, have been made with a constant-temperature anemometer for 0.03 < Re < 0.80. Gas temperatures were varied between 283 and 353 K at a constant wire temperature of 473 K. After correction for heat conduction to the wire supports the experimental results could be represented by the relation Nu m = a m + b m Re m While a m and b m are approximately constant, m is found to vary with gas temperature in the same manner as the thermal conductivity of the gas. Gas thermal conductivity and kinematic viscosity are taken at the gas temperature and not at the film temperature, as is common practice.

Two-dimensional convection from heated wires at low Reynolds numbers

Measurements of heat transfer from circular wires placed normal to a horizontal airstream have been made in the Reynolds number range 0·01 to 140. The Nusselt number can be related to the Reynolds number and temperature loading by an expression of the form where the values of n, A and B (see table 3) depend on whether the Reynolds number is above or below the value for which a vortex street exists in the wake of the wire. This value of the Reynolds number (R [eDot ] 44) is independent of the intensity and scale of the stream turbulence. The theoretical heat transfer relation based on the Oseen approximation is approached asymptotically as R → 0, provided free convection is negligible.Free convection effects diminish rapidly with increasing Reynolds number so that the orientation of the wire with respect to the vertical has a negligible influence on heat transfer except at very low velocities. For horizontal wires at very low Reynolds numbers, free convection is significant, when, roughly speaking, the Reynolds number is less than the cube root of the Grashof number.

Societies and Academies

Societies and Academies