Upward Airflow Research Articles

Microstructure of heat-treated PAN nanofibers

A simple and modified electrospinning technique was utilized to prepare aligned and heat treated Polyacrylonitrile nanofibers by using a rotating drum fixed on top of syringe needles and applying upward hot air flow which can facilitate to heat nanofibers in electrospinning zone. Polyacrylonitrile nanofibers were electrospun from its 14 wt% solution in dimethylformamide under practical conditions. Angular power spectrum analysis showed better fiber alignment with increasing take up speed, although SEM studies demonstrated wider diameters of nanofibers being produced by modified method. The glass transition temperature of all prepared samples were determined between 70 °C and 90 °C using DSC technique. The Quantitative analysis of WAXD patterns has revealed the positive effect of modified method on the degree of crystallization of nanofibers heat treated at higher take up speed. The maximum chain orientation factor of 0.27 was determined for nanofibers collected at linear velocity of 114.5 m/min in the modified set up using Raman Spectroscopy technique.

A simplified model of Passive Containment Cooling System in a CFD code

In this paper, we built up a simplified model of the Passive Containment Cooling System in a CFD code, including a steel plate, a condensing channel and an evaporating channel. In the inner side of the plate, the condensing channel is supposed to be the source of heat transfer into the steel plate. Along the outer side, an evaporating falling film is used to extract the heat from the steel plate. Upward flow of air is also considered along the evaporating film. In the condensing channel, a flow solver based on an asymptotic model of the Navier–Stokes equations at the low Mach number regime and two turbulence models (Buleev's model and Chien's k–ɛ model) are considered. The condensing channel model was used to model the COPAIN test, the computed heat flux and condensation rate were compared with the experimental data. In the evaporating channel, a simplified model developed by Aiello and Ciofalo (2009) was used, which considered the heat and mass balance between the falling film and the ascending air flow. The model was validated for two cases: a dry wall case and a completely wet wall case. In the former case, the results were compared with 2D predictions obtained by using the CFX-4 CFD code. In the latter case, the results were compared with experimental data obtained by Kang and Park. The comparison showed a satisfactory agreement on heat transfer rates, despite some overprediction depending on the air velocity. At the end, the condensing channel model and the evaporating channel model were coupled by the steel plate to establish the complete model for the PCCS. In the absence of relative tests, a case with general parameter values was implemented in the complete model.

Numerical analysis to determine the performance of combined variable ceiling and floor-based air distribution systems in an office room

This study analyses the performance characteristics of combined variable ceiling and floor-based air distribution systems in an office room using a numerical method to optimize the level of thermal comfort and indoor air quality. To validate the computational fluid dynamics simulations, temperature, air velocity and CO2 concentration were measured and compared. The simulation results of various combinations of ceiling and floor-based air distribution systems show that the exhaust should be located above the occupant and the window in summer; due to the large thermal plume as heat sources that could be generated by the window and the occupant. Whereas in winter, the supply should be located on the ceiling far from the window and the exhaust should be positioned on the floor near the occupant; as the downward airflow around the cold window surface would become more dominant when compared to the upward airflow associated with any other heat sources. The results have indicated that having both ceiling and floor-based air conditioning systems would allow the flexibility to change supply and exhaust air diffuser locations to optimize thermal and ventilation performance more efficiently in countries that are subject to distinct seasonal changes.

Variances in airflows during different ventilation modes in a dynamic U-shaped cave

Airflow dynamics were studied in Cisařska Cave (Moravian Karst, Czech Republic) under different seasonal conditions. The dependence of airflows on the difference between external and cave temperatures is nonlinear and roughly obeys the Darcy-Weisbach equation. The upward airflows were found to be systematically higher than the downward airflows under comparable driving forces. The principle reason is nonlinearity between air temperature and air density. U-shaped cave geometry magnifies this effect by feedback between external temperature and airflow driving forces. Whereas this feedback is positive during the upward airflow ventilation mode, it is negative during the downward airflow mode. To discuss the behavior in detail, a simplified model based on balancing the masses of two equivalent air columns of different temperatures and densities is proposed. The results contribute to a better understanding of cave microclimate evolution, cave CO2 dynamics, and speleothem growth.

Knowledge Discovery for Classification of Three-Phase Vertical Flow Patterns of Heavy Oil from Pressure Drop and Flow Rate Data

This paper focuses on the use of artificial intelligence (AI) techniques to identify flow patterns acquired and recorded from experimental data of vertical upward three-phase pipe flow of heavy oil, air, and water at several different combinations, in which water is injected to work as the continuous phase (water-assisted flow). We investigate the use of data mining algorithms with rule and tree methods for classifying real data generated by a laboratory scale apparatus. The data presented in this paper represent different heavy oil flow conditions in a real production pipe.

Co-occupant's exposure to exhaled pollutants with two types of personalized ventilation strategies under mixing and displacement ventilation systems

Personalized ventilation (PV) system in conjunction with total ventilation system can provide cleaner inhaled air for the user. Concerns still exist about whether the normally protecting PV device, on the other hand, facilitates the dispersion of infectious agents generated by its user. In this article, two types of PV systems with upward supplied fresh air, namely a chair-based PV and one kind of desk-mounted PV systems, when combined with mixing ventilation (MV) and displacement ventilation (DV) systems, are investigated using simulation method with regard to their impacts on co-occupant's exposure to the exhaled droplet nuclei generated by the infected PV user. Simulation results of tracer gas and particles with aerodynamic diameter of 1, 5, and 10 μm from exhaled air show that, when only the infected person uses a PV, the different PV air supplying directions present very different impacts on the co-occupant's intake under DV, while no apparent differences can be observed under MV. The findings demonstrate that better inhaled air quality can always be achieved under DV when the adopted PV system can deliver conditioned fresh air in the same direction with the mainly upward airflow patterns of DV.

Impact of geometry of a sedentary occupant simulator on the generated thermal plume: Experimental investigation

The characteristics of the thermal plume generated by a sitting person were experimentally studied using four human body simulators with different complexities of geometry but equal surface area and heat generation: a vertical cylinder, a rectangular box, a dummy, and a thermal manikin. The experiments were performed in a climate chamber with an air temperature and mean radiant temperature of 23°C (73.4°F) and an upward airflow with a velocity of less than 0.05 m/s (0.164 ft/s). Distributions of air temperature excess and air speed were measured in the thermal plume 0.70 m (2.30 ft) above each of the simulators. The results show that the thermal plume generated by the dummy comprising head, torso, and legs has a similar shape of the cross-section and integral characteristics to the manikin plume; therefore, the dummy can successfully be used as a simulator of a sitting person. Simple shaped models of a sitting human without a clear indication of legs, such as a cylinder and a rectangular box, are not recommended for use. The plume of the cylinder is symmetrical and concentrated, while the plume above the rectangular box is sensitive to the surroundings and can have two maximums of air speed distribution.

Asymmetric combustion characteristics and NO x emissions of a down-fired 300 MW e utility boiler at different boiler loads

To investigate the aerodynamic field, cold airflow experiments were conducted under different boiler loads in a cold small-scale model of a down-fired pulverized-coal 300 MW e utility boiler. At 300 MW e and 250 MW e loads, a deflected flow field appeared in the lower furnace. In contrast, at a 150 MW e load, a U-shaped flow field appeared in regions near the left- and right-side walls in the lower furnace. Concurrently, the regions near the two wing walls adjacent to the front arch had received deflected upward airflow emanating from the region near the rear wall. Moreover, a symmetric W-shaped flow field appeared in the central regions below the front and rear arches. Industrial-sized experiments on the full-scale furnace were also performed at different loads with measurements taken of gas temperatures in the burner region and near the right-side wall, as well as heat fluxes and gas components in the near-wall region. Asymmetric combustion appeared at 300 MW e and 250 MW e loads, with large differences arising in gas temperatures, gas components, and heat fluxes between zones near the front and rear walls. At 150 MW e load, gas temperatures, gas components and heat fluxes are, in general, symmetrically distributed throughout the furnace. By decreasing the load, differences in gas temperatures, gas components, and heat fluxes near the front and rear walls decrease, as did NO x emissions. Meanwhile, the carbon content in fly ash essentially decreased, yielding an increase in boiler efficiency assisted by a drop in exhaust gas temperature.

Influence of staged-air flow on flow characteristics in a scale model of a down-fired utility boiler with swirl burners: An experimental study

Using an IFA300 constant temperature anemometer system, cold air experiments on a scale-model of a down-fired pulverized-coal 300MWe utility boiler with swirl burners were performed to investigate the influence of the staged-air flow ratio on flow characteristics in the furnace. This ratio was increased in incremental steps over the range of 7%–20%. The flame reach of the downward airflow was decreased with the ratio increased. A shallower reach is accompanied by a weaker mixing of vent and staged-air. At lower ratios, mixing is good and thus favors coal combustion but weak mixing at intermediate ratios produces a delay in the mixing point. This is compensated by an increasing recirculation zone under the arch that benefits ignition and the appearance of an upward airflow below the mixing point due to an increasing influence of vent air. At higher ratios still, this upward flowing disturbance is centered higher up in the furnace generated primarily by mixing with staged-air as vent-air mixing weakens.

INTERACTION EFFECTS OF HYDRODYNAMICALLY FULLY DEVELOPED PRIMARY FLOW AND SECONDARY FLOW IN THE THERMAL ENTRANCE REGION OF ANNULAR DUCT

Experiments have been conducted to study the local and average heat transfer by mixedconvection for hydrodynamically fully developed, thermally developing and fully developedlaminar upward air flow in an inclined annulus with adiabatic inner cast iron tube and uniformheated outer aluminum tube with an aspect ratio ( Ω = 0.72) and (L/Dh≈40) for both calming andtest sections). A wide range of Reynolds number from 859 to 2024 has been covered, and heatflux has been varied from 159 W/m2 to 812 W/m2 (these values of heat flux and Reynoldsnumber gave Richardson number range from 0.03 to 0.٣٨), with angles of annulus inclinationφ =0o (horizontal position), φ =60o (inclined position), and φ =90o (vertical position). The hydrodynamically fully developed condition has been achieved by using aluminum annulus(calming section) has the same dimensions as test section and has connected with it by Teflonpiece. The average Nusselt numbers have been correlated with the product of (Richardsonnumber and Reynolds number) and compared with available literature and showed satisfactoryagreement. The temperature and local Nusselt number profiles results have revealed that thesecondary flows created by natural convection have a significant effect on the heat transferprocess.

Effect of air humidity at the entrance on heat and mass transfers in a humidifier intended for a desalination system

Abstract The aim of the present study is to investigate numerically the effect of inlet air humidity on an upward airflow in a humidifier intended for a humidification–dehumidification desalination system. A vertical parallel-plate channel constitutes the humidifier. One of the plates is wetted by a liquid water film and maintained at a constant temperature, while the other is dry and thermally insulated. The airflow enters the channel with constant temperature, humidity and velocity. The results show that the increase of air humidity at the channel entrance affects seriously the performances of the humidifier as it induces condensation of the water vapour on the walls. On the other hand, it was stated that the humidifier works well for low inlet humidity.

Experimental and numerical study of mixed convection heat and mass transfer in a vertical channel with film evaporation

The objective of the present study is to investigate experimentally the effect of film evaporation on mixed convective heat and mass transfer in a vertical rectangular channel. Two parallel channel walls are wetted by a water film and heated by a constant heat flux, while the other walls are dry and thermally insulated. The liquid film temperature, the evaporated flow rate, the upward airflow temperature and humidity are measured. Wide ranges of inlet airflow velocity, heat flux and liquid film flow rate are considered. Cases of laminar and turbulent airflow are considered. The experimental results show that evaporation takes place on the majority of the surface of the two walls and, in some cases, evaporative cooling occurs especially for small heating flux and large air velocities. Moreover, a numerical study is carried out in the case of a laminar flow and its results are compared to experimental ones. These comparisons lead to a good agreement with respect to liquid film and airflow temperatures in addition to the evaporated flow rate.

Air circulation and its impact on microclimatic variables in the Císařská Cave (Moravian Karst, Czech Republic)

AbstractAirflows and their impact on microclimatic parameters were studied in the two‐entranced Císařská Cave (Moravian Karst, Czech Republic). Basing on the airflow direction with respect to the upper entrance (UE) and lower entrance (LE), three ventilation modes were distinguished: upward airflow one (UAF mode), downward airflow one (DAF mode), and transitional mode. A switching between the UAF and DAF happens in the transitional mode, in the range of external temperatures from 9.5 to 11.1 °C. Basing on the extent of cave ventilation, a period of active ventilation and period of stagnant ventilation were recognised. The cave air is intensively exchanged with external airs during the active ventilation period when the cave persists in either the DAF or UAF mode. During the stagnant ventilation period, the single airflow modes switch during diurnal cycle and only cave entering passages are vented (impacted). The airflows were found to depend (i) linearly on the differences in cave‐outdoor air densities, (ii) nonlinearly on the external temperature, and (iii) both linearly (LE) and non‐linearly (UE) on the temperature gradient. Majority of the airflows showed high‐frequency oscillations with the period from tens of seconds to minutes and with the amplitude up to 0.1 m3 s−1. Different airflow spectra were found at the individual entrances. The highest spectral densities were registered at low driving forces. The air circulation changed the pattern of temperature distribution depending on the ventilation mode and ventilation period. The cave relaxation length varied in the extreme range from 1 to 69.6 m. The asymptotic temperature and asymptotic specific humidity varied from 7.5 to 11.8 °C, and from 5.47 to 7.15 kg kg−1, respectively. The impact of airflows on the cave CO2 levels is discussed based on a dynamic model. Copyright © 2011 Royal Meteorological Society

Air sampling methods for VOCs related to field-scale biosecure swine mortality composting

Monitoring specific volatile organic compounds (VOCs) as markers of biosecure carcass degradation is a promising method to test progress and completion of the composting process. The objective of this study was to test the feasibility of using existing aeration ducts in composting units as practical sampling locations. The secondary objective was to test the feasibility of using marker VOC concentrations in aeration ducts to elucidate information about airflow patterns inside composting units. Marker VOC concentrations were significantly higher in the upper aeration duct and this duct can typically be used to collect air samples instead of placing special air sampling probes inside the composting units. Occasionally, the airflow direction inside composting units can change. Marker VOC concentrations can be used to decide the airflow direction inside the composting units. In this study, higher VOC concentrations were measured from the upper aeration duct, and this duct was shown to be an outlet.

Investigation of the action of hygroscopic particles on a warm convective cloud from the results of numerical simulation

The results of a numerical simulation of the action of hygroscopic particles on a warm convective cloud with the purpose of obtaining additional precipitation are presented. The one-dimensional numerical model considered in this work describes the evolution of the cloud medium in the central part of an axisymmetric convective cloud at the specified height-variable velocity of the upward air flow which forms the cloud. Our model comprehensively describes microphysical processes in the cloud medium with the use of the kinetic equation for the size distribution of cloud droplets. This model makes it possible to obtain the spatiotemporal pattern of the cloud formation and development and to analyze regular features in the cloud evolution under the action of hygroscopic reagents. The cloud characteristics calculated with the use of this model correspond to the cloud parameters observed in natural conditions of the atmosphere. The process of precipitation stimulation by hygroscopic particles in convective clouds with vertical thicknesses of 2.5–4.5 km was analyzed on the basis of the results of numerical calculations. The results of calculations of the dependence of the intensity and total amount of precipitation on the vertical cloud thickness and parameters of particles introduced into the cloud are presented. It is shown that hygroscopic particles with root-mean-cube radii of 1–1.5 μm are the most effective for acting on warm convective clouds with the purpose of obtaining additional precipitation. In this case, the required reagent expenditure is 100–200 kg/km2. The conditions necessary for obtaining the maximal positive effect of the action are elucidated.

Determination of the integral characteristics of an asymmetrical thermal plume from air speed/velocity and temperature measurements

A method, named the Approximate Distributions Integration Method (ADI-method), is proposed for calculation of parameters of the asymmetrical thermal plume above a heat source, such as maximum air temperature excess and velocity, their position in the plume cross-section, the widths of the temperature and velocity profiles, asymmetry parameters of the plume cross-section, and the integral characteristics. The method is based on an approximation of the measured profiles of air velocity and air temperature excess in the plume cross-section. A procedure for conversion of the air speed measured by omnidirectional sensors into air velocity is incorporated with the ADI-method. Experiments were performed in a climate chamber with air temperature of 23 °C, radiant temperature equal to the air temperature and upward airflow with velocity of less than 0.05 m/s. Air speed and temperature in a thermal plume, generated by a thermal manikin resembling the complex body shape and heat generated by a sitting person, were measured. Using the measured data, the integral characteristics of the generated asymmetrical thermal plume were calculated by the ADI-method, and the uncertainty in determination of the characteristics was identified. At a height of 0.7 m above the manikin head, the mean integral characteristics of the plume and their 95% certainty range (in parentheses) were: volume flux 258 m 3/h (1.8%), momentum flux 0.0087 N (2.0%), buoyancy force density 0.0038 kg/s 2 (3.8%) and enthalpy flux 16 W (2.8%).

Experimental and Theoretical Study of Ventilation and Heat Loss From Isothermally Heated Clothed Vertical Cylinder in Uniform Flow Field

The flow characteristics and heat transfer are studied in a vertical annulus of a heated cylinder surrounded by a permeable cylinder, subject to cross uniform wind with open end to the environment and in the presence of natural convection. The objective here is to develop a computationally efficient model capable of capturing the physics of the flow and heat transport to predict air renewal rates in the vertical annulus. The small quantities of air infiltrating/exfiltrating through the porous cylinder over its upstream/downstream regions do not substantially affect the external flow pattern around the clothed cylinder. The air annulus flow and heat transport model predicted the radial and vertical mass fluxes and the mass flow rate at the opening as a function of environment conditions, porous cylinder thermal properties, wind speed, and annulus geometry. Experiments were performed in a low speed wind tunnel (0.5–5 m/s), in which an isothermally heated vertical cylinder surrounded by a clothed outer cylinder was placed in uniform cross wind. The tracer gas method is used to predict total ventilation flow rates through the fabric and the opening. Good agreement was found between the model and experimental measurements of air renewal rate and predicted heat loss from the inner cylinder at steady conditions. A parametric study is performed to study the effect of wind speed and temperature difference between the wind and skin temperature on induced ventilation through the clothing and the opening. It is found that natural convection enhances ventilation of the annulus air at wind speed, less than 3 m/s, while at higher speeds, natural convection effect is negligible. As the temperature difference between external wind and inner cylinder surface increases, the vertical air temperature gradient and total upward airflow through the opening increase.

MATHEMATICAL MODELING Of THE DRYING KINETICS OF SUGARCANE SLICES

Sugarcane crop is most important economically, socially and environmentally. Brazil is the largest sugarcane producer in the world. With the objective of contributing towards precision agriculture, the air drying characteristics of sliced sugarcane ( Saccharum spp ) were investigated and made to fit into semi-theoretical models used to describe drying behavior. The drying tests were performed in an experimental fixed-bed dryer with upward air flow. The drying was carried out at two air temperatures: 50 and 60oc with air relative humidity of 17.9 and 11.1, respectively. The time required for sugarcane drying, from an initial moisture content of 70% w.b. to the final moisture content of 6% w.b., was 7.5 and 3.5 h for drying temperatures of 50 and 60oc, respectively. Experimental data were adjusted to 4 traditional mathematical models in order to represent the drying process of agricultural products. The Midilli model was the one that best described the sliced sugarcane drying process.

Determination of laminar and turbulent flow ranges through vertical packed beds in terms of particle friction factors

Despite the presence of a variety of studies dealing with the magnitude of particle Reynolds number, Re p defining transition from laminar to turbulent regime for flow through packed beds, the manner is still one of the unknowns. An approach based on the experimental data concerning upward airflow through fixed cylindrical packed beds is presented in this paper. The utilized packed beds had the following ranges of; sphericity, Φ, 0.55 ⩽ Φ ⩽ 1.00, packing material diameter to bed length ratio, D p/ L, 0.04 ⩽ D p/ L ⩽ 0.72, and bed porosity, ε, 0.36 ⩽ ε ⩽ 0.56. The test cases covered the ranges of particle Reynolds number, Re p 708 ⩽ Re p ⩽ 7772 and particle Froude number; Fr p 2.86 ⩽ Fr p ⩽ 10.39. The measurements of pressure drop through packed bed; Δ P Bed and superficial mean exit velocity; U are used to determine bed frictional effects in reference to the available literature on particle friction factors, f p. The magnitude of Re p defining transition is assumed to be 2000 with particular emphasis to the flow dynamics. The definitions of Bird et al. [R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena , John Wiley and Sons, NY, 1960] are used to calculate f p. The calculated f p for the covered test cases are given as a function of pressure coefficient, Δ P* and Re p, Fr p, Φ, ε, D p/ L in the approximate ranges of laminar and turbulent flow for Re p < 2000 and Re p > 2000, respectively. The proposed separate equations of f p = f p(Δ P*, Re p, Fr p, Φ, ε, D p/ L) are satisfied for laminar and turbulent flows with corresponding average error margins of ±7.6% and ±18%. Furthermore ranges of transitional and fully rough flow through packed beds are estimated as 2000 ⩽ Re p ⩽ 4000 and Re p > 5000 with an analogy to the well-known Moody Chart in pipe flows.

Motion of a single hailstone

A vertical motion (up and down) of a single hailstone in an upward flow of supercooled air is studied with consideration of hailstone mass changes. A mathematical model is proposed on the basis of theoretical and experimental data on the process of hailstone growth. This model is used to estimate the lift height of the hailstone, its maximum size, the time of its upward motion, and the time of its return to the initial height. Simple formulas are derived to approximately calculate these characteristics.