Effect Of Air Velocity Research Articles

Numerical Investigation of Cross-Flow Water Cooling Towers

Abstract In this study, thermal performance of the forced cross-flow water cooling tower is numerically investigated by using the commercial computational fluid dynamics software ansys-cfx. The temperature variation between inlet and outlet of the water, namely, process water temperature, is the main extracted result of simulations. Additionally, the cooling range (CR) that is the difference between inlet water temperature and outlet water temperature is the second representative result of the analysis. The effect of air velocity (Va), water droplet diameter (dw), and water temperature at the inlet of tower (Tw) are the variables that are considered to be the effective design parameters on the process temperature of the water. The process water temperature decreases, but the cooling range increases when the air velocity increases. When the inlet water temperature increases, the process water temperature and the cooling range also increase. On the other hand, the process water temperature decreases with the decreasing diameter of water droplets.

Experimental and Numerical Simulation Research on Combustion and NOx Emission of Low Calorific Value and High-Alkaline Coal Based on Jet Characteristics of Flow Field

A numerical model of a tower boiler burning low calorific value and high-alkali coal was established; based on the jet rigidity of the flow field in the furnace, the effects of air velocity and air volume flow on the combustion of high-alkali coal and NOx emissions were studied. The optimal parameters obtained by numerical simulation are applied to a typical experimental boiler. The experimental results show that when SOFA operates at about 65 m/s, the rigidity of the jet is obviously increased, the flow field and temperature field in the furnace are uniform, the temperature of the flue gas at the outlet is reduced by 53 K, the CO concentration at the furnace outlet was reduced from 3713 ppm to 57 ppm, the exhaust gas temperature was reduced by 6 K, and the concentration of NOx is reduced to 163 mg/m3. On the other hand, the combustion efficiency is increased by 0.86%, which translates into 2 g/kwh of standard coal, and the problem of easy coking has also been effectively solved. The collaborative optimization of high-alkali coal combustion and NOx emission is realized.

Evaporation coefficient determination during the capillary rise

Thermo-physiological comfort refers to the heat and moisture transport properties of clothing and how the clothing helps maintain the body's heat balance during various activities. To maintain the thermoregulation of the body, the resulting sweating should be absorbed by wicking fabrics close to the skin and evaporated to the ambient air. In this study, a mathematical model was developed considering evaporation during the capillary rise, based on the geometric configuration of a jersey-knitted fabric and taking evaporation into account. This model was used to calculate the evaporation coefficient. Based on the activities of the worker, sweat diffusion is controlled by capillary diffusion and moisture evaporation. The effect of air velocities of 0 m/s, 1 m/s, and 2 m/s, representing non-walking, walking, and running activities of a worker, respectively, was studied during capillary diffusion. The experiments were conducted at different relative humidities. The results show that the evaporation coefficient depends on the worker’s activities and the relative humidity ratio.

Influence of non-uniform distribution of fan-induced air on thermal comfort conditions in university classrooms in warm and humid climate, India

This study investigates how fan-induced non-uniform air movement affects thermal comfort conditions and productivity at different seats in different parts of a lecture hall. A four-month-long field study was conducted, involving 11 naturally ventilated lecture halls, 828 samples from the students, monitoring thermal environments, and measuring air movement at every seat. Thermal comfort conditions and productivity at every seat were obtained through questionnaire survey. During the study, outdoor temperature varied between 24.7 °C and 36.7 °C. From ANCOVA and cluster analysis, it was found that students seated farther from the fan experience less air velocity, resulting in a significant difference in comfort conditions and productivity. The thermal sensation of the students was found to be comparatively less in Zone-1 (Mean = 0.2, SE = 0.06) than Zone-2 (Mean = 0.53, SE = 0.05). In the zone near the fan, 8.1% of students voted hot sensation at 33°C–34 °C, whereas in the zone far from the fan, this number increased to 35.9%. The slope of the regression line plotted between thermal sensation and indoor operative temperature was found to be 0.368/°C. Regression analysis revealed that thermal comfort could be achieved up to 33.8 °C with an air velocity of 1.2 m/s. The acceptable temperature limit for 80% of students was between 23.4 °C and 29.7 °C. A narrower comfort band was observed compared to ASHRAE-55 and the Indian standard, indicating limited adaptive opportunities in lecture halls. An adaptive comfort model is proposed that includes the effect of air velocity.

Effect of Air Velocity and Initial Conditioning on the Moisture Buffer Value of Four Different Building Materials.

Porous materials are able to exchange moisture with the surrounding air. The more hygroscopic they are, the more they contribute to regulate ambient humidity. This ability is characterized by the moisture buffer value (MBV) which is measured under dynamic solicitations according to different protocols. The NORDTEST protocol is the most commonly-used. It gives recommendations regarding the air velocity and the ambient conditions for initial stabilization. The purpose of this article is to measure the MBV according to the NORDTEST protocol and to study the effect of air velocity and of initial conditioning on the MBV results for different materials. Two mineral and two bio-based materials are considered: gypsum (GY), cellular concrete (CC), thermo-hemp (TH) and fine-hemp (FH). Following the NORDTEST classification, GY is a moderate hygric regulator, CC is good, TH and FH are excellent. When the air velocity ranges from 0.1 to 2.6 m/s, the MBV of GY and CC materials remains constant, but the MBV of TH and FH materials is highly affected. The initial conditioning has no effect on the MBV, but has an effect on the water content of the material, whatever the material.

Coupled CFD-DEM Simulation of Seed Flow in Horizontal-Vertical Tube Transition

A series of computational fluid dynamics–discrete element method (CFD-DEM) simulations were applied to seed flow in horizontal-vertical 90-degree elbows. The performance of one-way and two-way CFD-DEM coupling methods was compared. Additionally, simulated seed velocities were compared to the current pneumatic conveying theory for each coupling method. Simulated field peas (Pisum sativum) were pneumatically conveyed to study the effect of air velocity (20, 25, and 30 m/s), seed rate (0.07, 0.21, and 0.42 kg/s), elbow diameter, D, (48.3, 60.3, and 72.4 mm), and elbow bend radius (1.5D, 2.5D, 3.5D, and 4.5D) on seed attributes (trajectory, velocity, and force). Results showed that seed velocity was significantly different between one-way and two-way coupling. Both methods resulted in nearly identical seed trajectory and force. Overall, simulated seed velocities had a strong correlation to values calculated through the current pneumatic conveyance theory. Dimensional analysis revealed that seed contact force was proportional to the elbow diameter to the power of 0.26 and inversely proportional to the elbow bend radius to the power of 0.5. Simulation results indicated that one-way coupling could be suitable to describe seed flow when two-way coupling may not be possible or practical.

Three-dimensional simulation of the convective heat transfer coefficient of the human body under various air velocities and human body angles

This study aimed to investigate the effects of air velocity and human body angles on the convective heat transfer coefficient (hc) of the whole body and body segments. A 20-zone computational thermal manikin and climate chamber were constructed using a three-dimensional body scanning technique and ANSYS software. Subsequently, the hc values were calculated through computational fluid dynamics simulation and validated against the measurement and simulation results in the literature. Then, 50 cases (10 air velocities ranging from 0 to 20 m/s and 5 human body angles of 0°, 45°, 90°, 135°, and 180°) were considered to further analyze the combined effects of air velocity and human body angle on hc. The results indicated that the hc values of both the whole body and body segments increase exponentially with the air velocity. The whole body hc increases by approximately 23 times when the air velocity increases from 0.2 to 20 m/s. On the other hand, whole body hc is lower at a human body angle of 90° than at other angles; the human body angle has a significant effect on the hc of body segments. The difference in the hc of the whole body does not exceed 8.5 W/m2·K for the five human body angles, but the largest difference for the body parts reaches 52 W/m2·K. Finally, the combined effect of air velocity and human body angle on hc was expressed by a regression equation. This study can offer insights into the heat transfer process between the human body and its thermal environment, as well as human thermoregulation and skin burn injuries.

Experimental Study of LiCl/LiBr-Zeolite Composite Adsorbent for Thermochemical Heat Storage

Adsorption-based thermochemical heat storage is a promising long-term energy storage technology that can be used for seasonal space heating, which has received significant amount of efforts on the research and development. In this paper, the heat storage capacity of composite adsorbents made by LiCl + LiBr salt and 3A zeolite was investigated. The basic characteristics of composite material groups were experimentally tested, and it was found that the adsorption composite with 15 wt% salt solution had excellent adsorption rate and adsorption capacity, which was considered as the optimal composite material. Furthermore, the heat storage density of the composite material could be as high as 585.3 J/g, which was 30.9% higher than that of pure zeolite. Using 3 kg of the composite material, the adsorption heat storage experiment was carried out using a lab-scale reactor. The effects of air velocity and relative humidity on the adsorption performance were investigated. It was found that a flow rate of 15 m3/h and a relative humidity of 70% led to the most released adsorption heat from the composite material, and 74.3% of energy discharge efficiency. Furthermore, an adsorption heat storage system and a residential model were built in the TRNSYS software to evaluate the building heating effect of such heat storage system. It is found that the ambient temperature will affect the heating effect of the adsorption heat storage system. The coefficient of performance (COP) of this model is as high as 6.67. Compared with the gas boiler heating system, the adsorption heat storage energy can replace part of the gas consumption to achieve energy savings.

An experimental study on the dynamic frosting characteristics on the edge zone of a horizontal copper plate under forced convection

To retard the negative effects of frosting, it is necessary to better understand the frosting mechanism on cold surfaces. In this study, a systematical study on the dynamic frosting characteristics on the edge zone of a horizontal copper plate under forced convection was conducted. The results showed that the air velocity effect and edge effect on droplet condensation, frozen and frost layer growth characteristics were significant. The duration of droplet condensation stage decreased from 365 to 113 s with a decrease of 69.0% when air velocity increased from 0.5 to 2.5 m/s. Besides, the average freezing wave propagation velocity for Region I (an area where a row of water droplets closest to plate edge) was significantly larger than that for Region II (the remained area). As a result, the duration of droplet frozen stage for Region I was much shorter than that for Region II. Accordingly, frost layer amount for Region I was notably larger than that for Region II. Besides, the frost surface roughness for Region I increased as air velocity increased at the early frosting stage, but decreased as air velocity increased at the later frosting stage. The general turning point for frost surface roughness at 1.5 m/s surpassed the value at 0.5 m/s was around 21.4 × 10−6 m at 390 s, and that between 2.5 m/s and 1.5 m/s was around 30.3 × 10−6 m at 450 s. This study can help to establish a better relationship among different frosting stages, and better understand the plate edge effect on frosting.

Ethylene permeance through perforated packaging film: Modelling and effect of air velocity, temperature, film thickness and perforation diameter

Perforation in packaging material is one of the efficient ways to remove ethylene from fresh produce packages for delaying the ripening of climacteric fruit and vegetables. The effect of storage conditions on ethylene permeance has not been studied previously, therefore, this study pursued to investigate and model the effect of cooling air velocity, temperature and perforation size on the ethylene permeance through oriented polypropylene packaging film. Ethylene permeance was estimated from 1.70 ± 0.02 to 4.06 ± 0.03 cm s−1 at different temperatures, air velocities and perforation sizes. The perforation diameter showed an inverse significant impact on ethylene permeance. The model based on the empirical approach and artificial neural network using validation data exhibited an R2 of 0.9515 and 0.9890, respectively. The presented study would be helpful in optimising the perforation size in order to avoid ethylene accumulation inside fruit packages, under different storage conditions.

Effect of Air Velocity, Type and Size of Biomass on Performance of Downdraft Gasifier

Investigation on thermal gasification of different biomass materials viz., Pigeon pea stalk (Cajanus cajan),Cotton stalk (Gossypium hirsutum) and Vilaytee babool (Prosopis juliflora) in a downdraft gasifier (1 Kw capacity) was conducted to study the effect of air velocity, type and particle size of biomass. The influenceof three different particle sizes viz. 25-50, 50-75, 75-100 mm, at three different air velocities viz., 2.4, 7.4 and12.4 m s-1 on biomass consumption rate, gas flow rate and gasification yield was investigated. With theincrease in particle size, the reduction in biomass consumption rate was observed, whereas the increase inair velocity increased the biomass consumption rate. Similar trend was observed in gas flow rate andgasification yield respectively. Under the experiment conditions, the biomass consumption rate, gas flowrate and gasification yield of selected biomass material varied in the range of 3.80 to 7.76 kg h-1, 6.13 to 14.40Nm3 h-1 and 1.40 to 2.30 Nm3 kg-1, respectively. The experimental results showed that the gasificationperformance was better for Vilaytee babool (Prosopis juliflora) biomass with 25-50 mm size at 7.4 m s-1 airvelocity.

Energy loss caused by the elbow of stiff shotcrete pneumatic conveying based on response surface method and CFD-DEM

To explore the energy loss caused by the elbow in the pneumatic transportation of stiff shotcrete, the numerical simulation of the gas-solid flow of stiff shotcrete was carried out based on CFD-DEM. The accuracy of the simulation results was verified by on-site test and parameter calibration. The relationship between air velocity, feed rate, R/D ratio, energy loss caused by elbow and maximum wear rate was studied by response surface methodology. A second-order response surface model between the energy loss caused by elbow and each factor was established. The relationship between the maximum wear rate and each factor was obtained. The effects of air velocity, feeding rate, and R/D ratio on the spatial position and velocity distribution of particles were revealed. This paper can provide a reference for the pneumatic conveying of shotcrete. • The effects of the solid-gas ratio on the energy loss caused by the elbow are studied. • Energy loss caused by the elbow shows a quadratic perturbation with the change of factors. • The maximum wear has a linear relationship with the factors. • The spatial and velocity distributions of particles at different positions are revealed.

Fluid dynamics and thermal characteristics of a conical bubbling fluidized bed riser

Conical fluidized beds show a unique inherent dynamic feature due to a lower pressure drop than conventional fluidized beds. Although the literature deliberating on the hydrodynamics of this type of riser is rich, the thermal characteristics of this type of riser are yet to be investigated. Moreover, the interphase heat transfer has not been deeply understood in previous literatures by considering the effect of air velocity. As in real fluidized bed industries, hot air is supplied through the inlet of a riser to heat the particles in the drying process and other phenomena. As a result, the interphase heat transfer characteristics are more important than the core-to-wall thermal characteristics. The thermal phenomena are ineluctably directed by the two-phase flow hydrodynamics. The present investigation is conducted to research fluid dynamics and interphase heat transfer phenomena of the air-sand mixture in a conical bubbling fluidized bed riser of cone angle 10° with varying inlet air velocity. Air velocity is the main parameter which leads to the fluidization in fluidized beds. It is this parameter that materializes the elutriation of particles and determines the nature of the bed, whether bubbling or recirculating. This, in turn, affects the heat transfer and overall efficiency of the bed. Experiments along with corresponding three-dimensional (3-D) numerical simulations are performed at five superficial air velocity conditions (1, 1.25, 1.5, 1.75 and 2 m/s) and a 30 cm height of bed materials. The results are then compared between experimental and simulation conditions. Both the experimental and 3-D numerical results indicate that a velocity of 2 m/s results in the lowest pressure drop of 2830 Pa. Similarly, the average bed temperature in this velocity is 444 K which is the highest, whereas the interphase heat transfer coefficient is 320 W/m2K. The bed-to-wall heat transfer coefficient is found to increase by 47.2% and 45.8% at the central axial locations of 10 and 20 cm, respectively. Hence, it can be concluded that air velocity plays an important role in the operation of a fluidized bed in terms of pressure drop, heat transfer and thermal efficiency. Air velocity of 2 m/s is found to be the best value for the present study because the bed pressure drop decreases and heat transfer increases.

Study of the Diffusion Law of Harmful Gases in Tunnel Construction on Plateaus and Optimization of Ventilation Parameters.

To explore the diffusion law of harmful gases near the excavation face in tunnel construction on plateaus, select the optimal ventilation parameters, and improve the ventilation efficiency, Ansys Fluent was used to set the environmental parameters according to the highland, where the tunnel is located, and simulate and curve fit the diffusion phenomenon of CO and NO2 based on the fluid control equations and species transport model. The effects of air velocity, ventilation time, and duct position on the diffusion law of harmful gases were analyzed, from which the optimal ventilation conditions were selected, and the ventilation effects under the optimal conditions were compared with those under the original conditions. The study shows that after the fresh airflow passes through the outlet, part of it flows out along the tunnel wall toward the cave entrance, and the other part interacts with the return air to form a vortex; when the air supply speed is 10 m/s and the distance from the duct outlet to the excavation face is 25 m, the maximum concentration of harmful gases decreases by 92–99% after 20 min of ventilation compared with that before optimization and the smoke exhaust efficiency increases by 2.5% per minute.

Studies on thermal energy storage system with ceramic honeycomb channels

In this study, a ceramic-based sensible thermal energy storage system is analysed using analytical and numerical models, and the results subsequently validated with laboratory experiments. Corundum mullite monoliths are used as the storage material which is thermally cycled using compressed air as the heat transfer fluid (HTF). Here, hexagonal channels in the monolith allow direct contact heat exchange between the air and the storage material. The storage system is composed of a packed bed of ceramic blocks having honeycomb flow passages. However, unlike the case of packed bed of unconsolidated storage materials, the arrangement of these blocks plays a crucial role in the performance of the storage system. In this study, we try to bring out this crucial aspect of block arrangement and the effect it has on the flow characteristics and hence on the charging and discharging dynamics of the system. To understand the effect of ceramic block configuration in a storage system, two separate block arrangements are explored: parallel flow arrangement and series flow arrangement subjected to a single charging and discharging cycle. The storage systems are subjected to a constant temperature front of 450 K during charging cycle and 300 K during discharging cycle. Firstly, an analytical model is developed for airflow through a ceramic channel that predicts the temperature-time history of the storage system and the HTF at the system outlet. Subsequently, a full CFD analysis for coupled flow and conjugate heat transfer is performed on the two block arrangements, considering the actual three-dimensional ceramic block geometry with honeycomb channels. To validate the results of the analytical and numerical models, experimental testing is performed on a laboratory scale test setup. Finally, the effect of air velocity on the charging-discharging dynamics for the two arrangements of the ceramic blocks is also discussed. It is observed that the series flow arrangement charges and discharges the storage system 1.5 times faster than the parallel flow arrangement of the ceramic blocks but with four times higher pressure drop.

The effect of air velocity on the performance of the direct evaporative cooling system

This experimental study aimed to determine the effect of airflow velocity on the performance of a direct evaporative cooling system. Rectangular-shaped honeycomb cooling pads with a length of 34 cm, a width of 25 cm, and a thickness of 3.5 cm are used as cooling media. The main parameters of the study are low air velocity (2.3 ms−1), medium (3.2 ms−1), and high velocity (3.7 ms−1). The data collected include dry bulb temperature, wet bulb temperature, output air temperature, input and output air velocity, input and output humidity, and solar radiation. These data are used to determine saturation efficiency, cooling capacity, temperature decreases, and feasibility index. The experimental results are presented in the form of tables and graphs and analysed based on existing theories. The results showed that the evaporative cooling system could produce output temperatures up to 27.5°C with input 31.4°C at low airspeed, 27.97°C with input 31.47oC at medium speed, and 27.7°C with input 31.30°C at high air speed. It was concluded that a low airflow rate would add to the cooling efficiency, and the higher the airflow rate, the lower the cooling efficiency. The results showed that evaporative cooling is achievable with a feasibility index of 19.89 ≤ F*≤ 20.67. The results also affirmed that cooling capability is higher where the feasibility indexes are comparatively low.

Simulation of natural ventilation of different configurations of a room equipped with a solar chimney under the city of Ouarzazate (Morocco) climatic conditions

Natural ventilation is considered as an effective way to improve ventilation and thermal comfort and reduce energy consumption in residential buildings. In this sense, natural ventilation is done by a solar chimney. It is used to increase the air flow through the room by converting thermal energy into kinetic energy. In this work, we intend to present the results of a three dimensional numerical simulation by ANSYS software of natural ventilation through a room equipped with a solar chimney in semiarid climatic conditions of Ouarzazate (Morocco). The chimney is modelled by a rectangular channel with two South-East oriented glass faces. The main objective is to investigate effects of room dimensions for a solar chimney with of fixed dimensions. We also investigate effects of air velocity, turbulence of kinetic energy and air-flow rate in each area of room and solar chimney and determine optimal dimensions of solar chimney for each room size.

Numerical simulation of flow characteristics in vertical elbows of pneumatic conveying

In this paper, the numerical simulation method of computational fluid dynamics (CFD) is used to establish the flow field model of elbow, including the determination of the applicable gas-solid two-phase flow model and the establishment of the actual basic fluid dynamics equations. After meshing the elbow with ICEM software, the effects of air velocity, bend diameter ratio and other factors on the flow characteristics of the elbow pneumatic conveying are simulated based on FLUENT software. The results show that in the pneumatic conveying with vertical elbows, the flow characteristics of gas-solid two-phase flow have obvious differences with the changes of many factors.

Experimental and numerical investigation of spherical food product during forced convection cooling

Purpose of this study is to make improvements in the heat transfer behaviour during cooling of food products in forced convection environment. In the present study the effect of air velocity during pre-cooling of spherical food product i.e. orange is seen. The experiments are performed on an air blast apparatus and the results are compared with a 3D CFD model. This model is studied at (0.5, 1.0, 2.0, 2.5 and 3.0 m/s) velocities for orange. It is observed that the cooling is improved significantly up to a particular speed and increasing the speed above its critical speeds not enhance cooling significantly i.e. 2.0 m/s, hence is not advisable to increase the speed above its critical speed as it leads to loss of energy which in turn increases the overall operational cost. . The half cooling time(HCT) and the seven-eight cooling time(SECT) consequently decreases by 44.27% and 43.31% respectively for orange by a rise in air-inlet velocity from0.5 - 3.0 m/s.

THE EFFECT OF AIR VELOCITY ON THE CHARACTERISTICS PRODUCED BY CONVENTIONAL DRYING OF Tectona grandis L.f. WOOD

The effect of air velocity of 2.5 and 3.5 ms -1 was evaluated in the artificial drying of eight loads of 60 boards each, at two height positions of the shaft: the base and the top, with initial humidity in green and in the PSF by air drying, using twenty Tectona grandis Lf trees from a 33-year-old plantation in Chanchamayo, Peru. The drying time, rate, and quality of the wood were determined. In air drying, the Pearson correlation was positive linear between the free water drying rate with temperature (0.997) and air velocity (0.987), and negative linear with relative air humidity (-0.996), precipitation (-0.97), and drying time (-0.976). The artificial drying time of the wood showed a significant direct relationship with the initial humidity level, and an inverse relationship with the air velocity. The drying rate of free water showed a significant direct relationship with the air velocity, while it was significantly higher than that of hygroscopic water. The position of the wood at the height of the shaft did not significantly influence the time and rate of both air and artificial drying. The quality of the wood drying did not show significant dependence on the air velocity, the position of the wood at the height of the shaft, and on the initial drying humidity. The drying defects observed do not compromise the production of quality products with a uniform final moisture content of 10%.