Convective Air Flow Research Articles

Plate versus mesh collecting electrode for electrohydrodynamic (EHD) drying

Electrohydrodynamic (EHD) drying is considered a highly efficient technology due to its ability to create powerful convective airflow with low energy consumption. EHD efficiency depends on multiple factors, such as voltage, current, environmental conditions, drying material, and electrodes geometry. This research is exploring the effect of collecting electrode geometry on the drying rate and energy efficiency of EHD drying. Although numerical studies predict a better performance of mesh collectors compared to solid plate collectors, experimental verification is still missing. This research addresses this gap by comparing the performance of plate versus mesh collecting electrodes in drying experiments with three discharge electrodes (nails, needles, pins) and two materials (wet paper and apple slices). Our experiments confirmed the benefits of mesh collector compared to the plate, namely up to 25% higher drying flux and a 20–35% smaller specific energy consumption. For example, EHD drying of apple slices with the mesh resulted in specific energy consumption of 719–731 kJ/kg compared to 935–968 kJ/kg with the plate collector. The better energy efficiency of the mesh collector provides a unique opportunity for the industrial upscaling of EHD drying technology.

The dynamics of water micro-particles in air

The dynamics of water micro-particles in air can be studied by introducing a viscous force in the equations of the motion, in addition to the weight of the particle and the buoyance force. In this work, by taking account of the buoyance force, the range of the micro-particle is calculated as a function of the angle at which it is released. An analytic-numerical procedure to find the angle giving the maximum range is indicated. The effect of a convective air flow is finally studied.

Controlled autoignition in stratified mixtures

The shockless explosion combustion (SEC) is a recently proposed concept aiming for pressure gain combustion through an unsteady process of multiple, distributed autoignitions occurring simultaneously. For this, a stratified fuel profile of dimethyl ether is injected into a continuous air flow. This profile is tailored such that the mixture residence time and ignition delay time are matched, allowing multiple ignition kernels to initiate simultaneously leading to an aerodynamic confinement during heat release. This work first presents an injection strategy for injecting a defined mixture profile into a convection air flow to control the local equivalence ratio throughout the combustor. Line-of-sight measurements are applied to visualize the concentration profile and subsequently used to develop a one-dimensional tool to predict the local equivalence ratio before ignition. Next, an extremum seeking control algorithm is applied to an existing SEC test rig to control the cycle averaged formation of different autoignition modes by optimizing the fuel supply. Pressure and ionization probe data indicate the successful initiation of specific modes of flame propagation by adjusting the fuel injection trajectory. The previously developed simulation tool is applied to the injection trajectories optimized by the controller. Correlating the fuel concentration distribution and the obtained autoignition modes reveal that the ignition process can be very well controlled by the fuel injection trajectory. Lastly, single representative ignition cycles are further investigated by applying optical measurement techniques to obtain OH* and CH* chemiluminescence. The results reveal a complex interaction between heat release and pressure waves influenced by temperature-dependent ignition behavior of the applied fuel. As a conclusion, four different flame propagation modes are identified, namely: turbulent deflagration, subsonic autoignition, supersonic autoignition and aerodynamic confinement by multiple simultaneous autoignition fronts.

Improved protocol for natural convection pumpkin drying

The most effective way to preserve agricultural product is drying. However, vegetable drying is an energy-consuming procedure. Convective drying is the mode considered in this work. The study intends to explore a new way of pumpkin drying, which reduces drying time and minimizes heat consumption. The study considers pumpkin thin slices and pumpkin samples with cubic shape. The samples were subjected to free convection airflow at different temperatures (40 °C, 46 °C, 52 °C, and 60 °C) for each run. A varying airflow temperature was also considered. Airflow velocity was generated by buoyancy forces for each temperature. Drying curves were plotted and fitted to the widely used thin-layer drying models. The modified Page model came out as the best-fitted model. The effective diffusivity coefficient was determined for each case using the slope moisture curve. It appeared that diffusivity was high and drying time was short, for high temperature. Drying processes for slice configuration and cube configuration showed that the latter was more efficient. When applying the regime of increasing temperatures to the cubic samples, data analysis showed that effective diffusivity was higher during the third step in comparison to all the other drying temperatures and the total drying time was similar to that obtained at drying regime on high temperature. With this procedure, the final consumed energy was much less and the time was shorter.

A novel convection and radiation combined terminal device: Its impact on occupant thermal comfort and cognitive performance in winter indoor environments

Personal comfort systems usually use a single heat transfer mode to improve local thermal comfort by only stimulating one local body part. A novel hybrid convection and radiation combined terminal device, which could be used both in summer and winter, was proposed. A total of sixteen human subjects took part in a winter subjective experiment to test its performance on the improvement of occupants’ thermal comfort and cognitive performance in winter. Occupants’ thermal comfort, air movement perception, eye dryness and cognitive performance were investigated. The results showed that 88% of the subjects accepted 16℃ room temperature. 0.4 m/s air speed to face and abdomen parts is acceptable when convective airflow temperature is 4.5℃ higher than room temperature. No dry eye discomfort was reported while using a controllable device. Furthermore, subjects’ cognitive performance, including self-evaluate work performance, math (mental performance) and typing performance (dexterity), could be significantly improved by using the novel device. Energy efficient thermal comfort can be achieved by less intensifying ambient heating temperature to 16℃ in winter.

Experimental study on the effect of airflow conditions on the combustion characteristics of methane hydrate

Due to its large reserves and cleanliness, methane hydrate is widely regarded as a promising energy source. The study of the effect of airflow conditions on the combustion characteristics of methane hydrate can provide a theoretical basis for various engineering applications, such as realizing the method of in-situ combustion mining, improving the safety of mining and predicting the development of fire during the storage and transportation of methane hydrates. In this paper, the combustion process of methane hydrate under the conditions of natural convection, external horizontal airflow and internal vertical airflow was analyzed. This way, the effect of the form of airflow on combustion was studied, and the effect of airflow velocity on the characteristics of flame was documented. The results demonstrated that, under the conditions of natural convection and horizontal airflow, there was an “eruption phenomenon”, during which the ice layer broke and a large amount of methane gas instantly escaped, forming a high and bright flame. The vertical airflow reduced the mass transfer resistance of methane gas, whereas the combustion process was more stable, and the height of flame was more uniform. Under the condition of horizontal airflow, the tilted flame increased the heat transfer to the surface of hydrate. The flame tilt angles were different in different combustion stages. The flame tilt angle in the intensive combustion stage (61.26°) was smaller than that in the steady combustion stage (70.85°), whereas a higher combustion rate had a stronger capability to oppose the horizontal wind.

Mass Transfer Analysis of Air-Cooled Membrane Distillation Configuration for Desalination.

It has been proposed that the air-cooled configuration for air gap membrane distillation is an effective way to simplify the system design and energy source requirement. This offers potential for the practical applications of membrane distillation on an industrial scale. In this work, membrane distillation tests were performed using a typical water-cooled membrane distillation (WCMD) configuration and an air-cooled membrane distillation (ACMD) configuration with various condensing plates and operating conditions. To increase the permeate flux of an ACMD system, the condensing plate in the permeate side should transfer heat to the atmosphere more effectively, such as using a more thermally conductive plate, adding fins, or introducing forced convection air flow. Importantly, a practical mass transfer model was proposed to describe the ACMD performance in terms of permeate flux. This model can be simplified by introducing specific correction values to the mass transfer coefficient of a WCMD process under the same conditions. The two factors relate to the capacity (B) and the efficiency (σ), which can be considered as the characteristic factors of a membrane distillation (MD) system. The experimental results are consistent with the theoretical estimations based on this model, which can be used to describe the performance of an MD process.

CFD simulation of anti-fogging coatings performance in refrigerated display cabinets

A numerical procedure for the simulation of both fogging and the defogging process on the glass door of a vertical refrigeration display cabinet for chilled food is presented. The large scale problem within the cabinet (convective air flow and heat transfer, moisture mass transport, unsteady conduction through a multi-glazed door) is modelled via a commercial CFD code, whereas the water layer is modelled, via in-house user defined routines, as a collection of tiny droplets in order to take into account the surface wetting properties, the latent heat contributions and the area effective wet fraction. Local water layer equivalent thickness and time needed for the complete defogging are evaluated for different indoor conditions. The influence of anti-fog coatings is investigated comparing different droplets contact angle, from 30° to 90°.

Study of Abnormal Seasonal Variations in the Radon Exhalation Rate in a Fault Zone

Strong seasonal variations in the radon exhalation rate from the soil surface were revealed during a two-year monitoring cycle in a fault zone on a slope of Mount Beshtau in the Caucasus Mineral Water Resort area, Russia. The variations are characterized by very high radon levels in summer and low levels in winter. In the warm season (from late April to early October), the radon exhalation rate values ranged from 4.5 to 23.8 Bq m–2 s–1. During the cold period (from November to March), the radon levels decreased to 0.04–0.17 Bq m–2 s–1. A strong correlation was found between the radon exhalation and atmospheric air temperature (the Pearson correlation coefficient is 0.8). The simulation results show that seasonal fluctuations in the radon exhalation rate are caused by an inversion of the convective radon transport direction in the fractured zones of the rock massif. In summer, the convective flow of radon is directed from the rock massif to atmosphere, whereas this flow in winter comes from the atmosphere to rock massif. The rate of convective transport varies from 5 to 28 m day–1 during the warm season and from –2 to –4 m day–1 during the cold one. Seasonal radon anomalies in fault zones are most probably caused by thermally induced convective air flows in the highly permeable rock zone due to the temperature difference between the rock air and atmospheric air. Seasonal radon anomalies should be taken into account both when mapping radon risk and when using radon as a tracer of gas transport in geochemical and geophysical research.

Cave radon exposure, dose, dynamics and mitigation

Many caves around the world have very high concentrations of naturally occurring 222Rn that may vary dramatically with seasonal and diurnal patterns. For most caves with a variable seasonal or diurnal pattern, 222Rn concentration is driven by bi-directional convective ventilation, which responds to external temperature contrast with cave temperature. Cavers and cave workers exposed to high 222Rn have an increased risk of contracting lung cancer. The International Commission on Radiological Protection (ICRP) has re-evaluated its estimates of lung cancer risk from inhalation of radon progeny (ICRP 115) and for cave workers the risk may now (ICRP 137) be 4–6 times higher than previously recognized. Cave Guides working underground in caves with annual average 222Rn activity > 1,000 Bq/m3 and default ICRP assumptions (2,000 workplace hours per year, equilibrium factor F = 0.4, dose conversion factor DCF = 14 µSv/(kBq h m-3) could now receive a dose of > 20 mSv/y. Using multiple gas tracers (d13 C--CO2, Rn and N2O), linked weather, source gas flux chambers, and convective air flow measurements a previous study unequivocally identified the external soil above Chifley Cave as the source of cave 222Rn. If the source of 222Rn is external to the cave, a strategy to lower cave 222Rn by passively decreasing summer pattern convective ventilation, which draws 222Rn into caves, is possible without harming the cave environment. A small net annual average temperature difference (warmer cave air) due to geothermal heat flux produces a large net annual volumetric air flow bias (2–5:1) favoring a winter ventilation pattern that flushes Rn from caves with ambient air. Rapid anthropogenic climate change over decades may heat the average annual external temperature relative to the cave temperature that is stabilized by the thermal inertia of the large rock mass. Relative external temperature increases due to climate change (Jenolan Caves, 2008–2018, 0.17°C) reduces the winter pattern air flow bias and increases Rn concentration in caves.

Experimental analysis of single loop solar heat collector with jet impingement over indented dimples

An experimental examination was conducted to consider the impact of convective heat transfer and airflow characteristics through an impingement jet solar heat collector (SHC) with indented dimple roughness geometry on the absorber plate. The experimental results included the arc angle (αd) from 30° to 75°, the relative indented roughness pitch (p/Dh) from 0.269 to 0.810, the relative indented roughness height (e/Dh) from 0.016 to 0.0267. The comprehensive performance of SHC was adequately assessed by fluctuating the Reynolds number (Re) from 3000 to 21000. The Nusselt number (Nuct) increases with the increase in Re, as predicted. It is observed that the value of Nuct is a vigorous function of αd. However, the values of Nuct for the indented dimples on the absorber plate are significantly higher than the values for the smooth absorber plate. The maximum value of the combined friction factor (ffct) takes place at an e/Dh of 0.0267, while the minimum value is at 0.016. The experimental outcomes confirm that the maximum thermohydraulic performance (ηct) enhancement value of 2.15 was obtained at αd = 60°, p/Dh = 0.269, and e/Dh = 0.0267 compared to conventional SHC. These experimental values were used to generate the correlations for the Nuct and friction factor ffct as a function of the indented dimpled roughness parameters.

Bat guano minerals and mineralization processes in Chameau Cave, Eastern Morocco

The decay of bat guano deposits in caves produces mineral accumulations, mainly phosphates and secondary sulfates. Chameau Cave, Eastern Morocco, is located in the semi-arid Bni Snassen Mountains. It is composed of semi-active and dry passages, and is featured by strong condensation-corrosion on the walls, presence of fluvial sediments, and old corroded flowstones. Due to forced and convective airflow, the cave is generally very dry, with some damp sites related to condensation. Samples collected on the surface of different passages and along two sediment profiles yielded minerals related to bat guano decay. On recent or fresh guano, precursor minerals correspond to sulfate (gypsum), phosphate-sulfate (ardealite) and phosphate (brushite). Phosphates (hydroxylapatite, fluorapatite) occur at the interface with host rock or carbonate speleothems. At the contact of phyllosilicates contained in allogenic fluvial deposits or shale partings, or with pyrite-rich sediments, various phosphates occur (Al-rich strengite, Fe-rich variscite, phosphosiderite, leucophosphite, spheniscidite, crandallite, minyulite, variscite, and strengite), the latter two minerals being the stable end-members. Black seams of oxyhydroxides (goethite, hematite, birnessite) line the contact between carbonate host rock and weathered fluvial deposits. After “digestion” by acidic guano leachates, fluvial deposits only display the most resistant minerals (quartz, muscovite, K-feldspars and Na-plagioclases) and weathering byproducts (kaolinite). We discuss the origin of a pure gypsum particle cone, possibly related to evaporation at the edge of a wet cupola and subsequent detachment of sulfate particles. Among environmental conditions, humidity is required for decay. In this dry cave, most of the damp originates from either permanent or seasonal condensation. Dust particle advection seems to be essential in providing compounds that are not present on fresh guano (quartz, clay minerals). Bat guano phosphatization has probably occurred since >100 ka. The Chameau Cave appears as an outstanding site for bat guano-related minerals (n = 12), including rare phosphates (spheniscidite and minyulite).

Influence of a Surfactant on Evaporation Intensity of Suspended Water Droplets

Experimental data have been presented on the evaporation rate of suspended droplets of both pure water and water containing ≈1.5 wt % surfactant, sodium dodecyl sulfate. Evaporation occurs in a convective flow of dry air, with its velocity and temperature being varied within ranges of U0 = 1–5 m/s and T0 = 20–80°C, respectively. The density of the mass flux from the surface of the surfactant-containing droplets is 2–3 times lower than that in the case of pure water. For droplets in a dry air flow, the change in their diameter can be described by a set of similarity criteria, and a generalized linear empirical dependence of the evaporation rate on thermodynamic parameters may be constructed for these droplets.

Performance Augmentation of Solar Air Heater for Space Heating Using a Flexible Flapping Guide Winglet

A new idea is presented in this paper for improving the performance of solar air heater (SAH) designed for space heating by employing a thin flexible guide winglet. In addition to the role of winglet in pushing the convective airflow toward the heated surface, it behaves as a vortex generator (VG) due to its vibration by fluid-solid interaction (FSI) that causes flow mixing and breaking thermal boundary layer. In flow simulation, the finite element method (FEM) is employed with considering a two-way strongly-coupled FSI approach at transient condition. Numerical solution of the governing equations, including the continuity, momentum and energy for convective flow and the equation of motion for VG are obtained by COMSOL multi-physics. The well-known model is employed for computation of turbulent stress and heat flux. The present numerical results are validated against the most recent relevant literature. To provide a clear and deep understanding of the proposed concept, extensive comparisons are made between different test cases. Results reveal considerable performance enhancement of SAH with elastic guide winglet compared with clean solar air heater (CSAH), such that 56% increase in the natural airflow rate and 9% decrease in the average absorber temperature is seen because of the flapping winglet. But, the air outlet temperature decreases about 14% due to flapping VG. This study aims to make the proposed SAH as an essential renewable thermal-solar system more efficient and attractive so that this improvement pushes the industrial society toward more sustainable infrastructure.

Poboljšani protokol za prirodno konvektivno sušenje bundeve

The most effective way to preserve agricultural product is drying. However, vegetable drying is an energy-consuming procedure. Convective drying is the mode considered in this work. The study intends to explore a new way of pumpkin drying, which reduces drying time and minimizes heat consumption. The study considers pumpkin thin slices and pumpkin samples with cubic shape. The samples were subjected to free convection airflow at different temperatures (40 °C, 46 °C, 52 °C, and 60 °C) for each run. A varying airflow temperature was also considered. Airflow velocity was generated by buoyancy forces for each temperature. Drying curves were plotted and fitted to the widely used thin-layer drying models. The modified Page model came out as the best-fitted model. The effective diffusivity coefficient was determined for each case using the slope moisture curve. It appeared that diffusivity was high and drying time was short, for high temperature. Drying processes for slice configuration and cube configuration showed that the latter was more efficient. When applying the regime of increasing temperatures to the cubic samples, data analysis showed that effective diffusivity was higher during the third step in comparison to all the other drying temperatures and the total drying time was similar to that obtained at drying regime on high temperature. With this procedure, the final consumed energy was much less and the time was shorter.

Защита персонала от аэрозольных выбросов при проведении дезактивационных работ при выводе из эксплуатации радиационно опасных объектов

This article discusses the experience of using low-pressure fog generation systems to ensure the safety of work in contaminated and emergency rooms for fixing suspended particles of dust fractions containing alpha-emitting nuclides mobilized by convective air flows, and provides data on the required size of droplets and the method of their further deposition.

Limiting condition for auto-ignition of finite thick PMMA in forced convective airflow

This contribution experimentally addresses the auto-ignition of finite thick PMMA (polymethyl methacrylate) subjected to parallel forced convective airflow. 1–15 mm thick samples, 30–60 kW/m2 heat fluxes (HF) and 0–1.2 m/s airflow velocities (u) were utilized to examine the auto-ignition mechanism and the limiting condition of non-ignition cases. Critical temperature, surface temperature evolution and ignition time were recorded. A modified numerical model, considering both PMMA and thermal insulation layers, and theoretical correlations developed for thermally intermediate solid were employed to analyze the experimental data. Results show that three regimes corresponding to different controlling mechanisms are identified: (1) solid-dominant regime, u ≤ 0.4 m/s, where auto-ignition is primarily determined by the heat transfer and pyrolysis in solid; (2) transitional regime, 0.4 m/s < u < 1.0 m/s, where both solid and gas phases have nonnegligible contributions; (3) gas-dominant regime, u ≥ 1.0 m/s, where gas phase chemical kinetics is responsible for the observed non-ignition phenomenon. Critical temperature is reliable only in the solid-dominant regime. In all the ignition scenarios, critical temperature increases with increasing sample thickness and u, but it barely changes with varying HFs. In the gas-dominant regime, surface temperature measurement suffers greatly from the generated large bubbles, worsening the agreement with numerical predictions. Based on the simulated in-depth temperature, estimated Biot number and thermal penetration depth, 1 mm PMMA is not thermally thin, whereas 15 mm sample is approximately thermally thick. Furthermore, thermal diffusivity of PMMA and the critical HF are estimated by fitting the measured ignition times.

Desalination of seawater by spray freezing in a natural draft tower

The freeze-melting process can be a viable method for the purposes of desalination because of its low energy consumption, ignorable corrosion issues, and without huge pressure or membrane replacement work. Large contact area for heat and mass transfer per unit mass of water between the water and air and low heat resistance results in higher energy efficiency during spray freezing desalination process compared to other freezing desalination methods. A 200 m high desalination tower was proposed in this paper that could generate 27.7 kg/s fresh water in the form of water droplets with 2 mm diameter at an atmospheric temperature of −26°C. This research has founded that the natural convective airflow induced by the heat released by the warm water in the freezing process could generate through the wind turbine mounted in this system approximately one-third of the energy consumed by the water pump of the system. This free energy has never been studied in previous research. The power consumption required to produce 1 m3 fresh water in this system is approximately 1.07 kWh. Compared to traditional desalination methods, the power consumption of our new spray freezing desalination system is much lower than previous systems with the same mass flow rate of fresh water. Only 375.4 kJ cold energy to produce one-kilogram fresh water. Thus, this spray freezing desalination system could be employed in desalination industry if free cold energy (e.g. from the cold atmosphere or the regasification process of LNG) and seawater resources are available.

Performance analysis of solar updraft tower using Ansys fluent

The solar updraft tower (SUT) is a renewable energy power plant design concept for generating electricity by utilizing convective air flow due to low temperature heat absorption from incoming solar radiation. A parametric study on the performance power output of the SUT at various canopy angles and different insolation levels were carried out to investigate the relationship between them with respect to key parameters i.e. in terms of updraft tower velocity, temperature, pressure and height to choose turbine position for maximum air velocity absorption and energy conversion using Ansys-fluent software version 15 for analysis. The results obtained were analyzed comparatively to draw conclusions. The results identified using simulation software mentioned above were validated with earlier results and identified significant improvement in the power generated at various collector angles, when compared with the earlier works on SUT.

A Physicist View of COVID-19 Airborne Infection through Convective Airflow in Indoor Spaces

General Idea: Naturally produced droplets from humans (such as those produced by breathing, talking, sneezing, and coughing) include several types of cells (e.g., epithelial cells and cells of the immune system), physiological electrolytes contained in mucous and saliva (e.g. Na+, K+, Cl-), as well as, potentially, several infectious agents (e.g. bacteria, fungi, and viruses). In response to the novel coronavirus SARS-CoV-2 epidemic, which has become a major public health issue worldwide, we provide a concise overview of airborne germ transmission as seen from a physics perspective. We also study whether coronavirus aerosols can travel far from the immediate neighbourhood and get airborne with the convective currents developed within confined spaces. Methodology: Methods of fluid dynamics are utilized to analyse the behavior of various-size airborne droplets containing the virus. Study Findings: We show that existing vortices in the air can make a location far away from the source of the virus be more dangerous than a nearby (e.g., 6 feet away) location. Practical Implications: Our study reveals that it seems reasonable to adopt additional infection-control measures to the recommended 6 feet social distancing. We provide a recommendation that could help to slow down the spread of the virus.