Increasing Air Velocity Research Articles

Preliminary investigation of thermal behaviour of PCM based latent heat thermal energy storage

Solid-liquid phase change is used to accumulate and release cold in latent heat thermal energy storage (LHTES) in order to reduce energy consumption of air cooling system in buildings. The storing capacity of the LHTES depends greatly on the exterior air temperatures during the summer nights. One approach in intensifying heat transfer is by increasing the air’s velocity. A LHTES was designed to be integrated in the air cooling system of a building located in Bucharest, during the month of July. This study presents a numerical investigation concerning the impact of air inlet temperatures and air velocity on the formation of solid PCM, on the cold storing capacity and energy consumption of the LHTES. The peak amount of accumulated cold is reached at different air velocities depending on air inlet temperature. For inlet temperatures of 14°C and 15°C, an increase of air velocity above 50% will not lead to higher amounts of cold being stored. For Bucharest during the hottest night of the year, a 100 % increase in air velocity will result in 5.02% more cold being stored, at an increase in electrical energy consumption of 25.30%, when compared to the reference values.

FROM HIGH-FIDELITY NUMERICAL SIMULATIONS OF A LIQUID-FILM ATOMIZATION TO A REGIME CLASSIFICATION

High-fidelity numerical simulations of spray formation were conducted with the aim of improving fundamental understanding of airblast liquid-film atomization. The gas/liquid interaction in the near-nozzle region is investigated for a multitude of operating conditions in order to extrapolate phenomenological and breakup predictions. To reach this goal, the robust conservative level-set (RCLS) method was used. For a fixed prefilmer geometry, we performed a parametric study on the impact of various liquid and gas velocities on the topological evolution of the liquid interface. The behavior and development of the liquid film is found to be influenced mainly by the relative inertia of the gas and the liquid, the liquid surface tension, and interfacial shear stresses. Preliminary regime maps predicting the prefilming liquid-sheet atomization behavior are constructed based on our numerical results. Three distinct types of “regime” are reported: accumulation, ligament-merging, and three-dimensional wave mode. In addition, these results also show the influence of vortex action and rim-driven dynamics on the breakup mechanism at the atomizer edge. An increase in liquid injection speed leads to the generation of smaller droplets; whereas, an increase in air velocity does not point to one simple conclusion.

Modeli i termodinamačke osobine tankih kriški paradajza sušenog vrelim vazduhom

The drying kinetics and determination of thermodynamic properties of sliced tomato samples during hot air drying in a hybrid solar-electric crop dryer were presented. One kilogram batch of freshly harvested tomato samples were sliced to 10 mm thickness and dried at varying air velocities (1.0, 1.5 and 2.0 ms-1) and temperatures (50, 55, 60, 65, and 70oC) using the hybrid mode. In order to choose the best drying model, eight thin-layer mathematical models were fitted to the experimental data. The high values of coefficient of determination, R2 and the low values of reduced sum of squares error (SSE) and root mean square error (RMSE) indicated that the Midilli et al. model adequately described the drying process of tomato slices, with its highest R2 (0.9999), lowest SSE (0.1136) and lowest value of RMSE (0.0212) at 70oC temperature and 2.0 ms-1 air velocity. Arrhenius model was used to represent the drying constant as a function of temperature. The effective moisture diffusivity increased with increase in temperature and air velocity; with the highest value obtained at air velocity of 2.0 ms-1 and temperature of 70oC, whereas it was lowest at air velocity of 1.0 ms-1 and temperature of 50oC. The mean activation energy required to dry 1kg batch of 10 mm sliced tomato samples was 39.34 kJmol-1. Enthalpy and Gibbs free energy values were found to decrease with increasing drying temperature as well as the entropy which also exothermically decreased with temperature. Recommendations for further studies were stated.

Experimental performance of mushroom fluidized-bed drying: Effect of osmotic pretreatment and air recirculation

Abstract The present work evaluates the effect of osmotic pretreatment (OP) and air recirculation (AR) on energy and exergy analyses of fluidized bed drying of sliced mushroom at different drying conditions. The drying was carried out at drying temperatures of 30, 40 and 50 °C, air superficial velocities of 3, 5, and 7 m/s for osmotic pre-treatment and untreated (control) samples under with/without air recirculation. Results showed that both air recirculation and osmotic pretreatment significantly effect on energy and exergy parameters. The energy consumption increased with increasing air velocity and drying temperature while it decreased with using osmotic pretreatment and air recirculation techniques. The energy utilization ratios vary between 0.10 and 0.269 for control/NAR and 0.890 to 0.964 for OP/100% AR, while exergy efficiency vary from 10.40 to 30.57% and 0.81–3.94% for OP/100% AR and control/NAR, respectively. Exergy loss significantly decreased with recycling the exhaust air.

The behavior of frost layer growth under conditions favorable for desublimation

The purpose of this study is to understand the behavior of frost layer growth under conditions favorable for desublimation. The frosting experiments were conducted on a horizontal cooling surface. Condensation did not occur at the initial stage of frosting, and feather-shaped frost crystals were formed on the cooling surface. These frost crystals grew one-dimensionally while maintaining their shapes. In addition, the effects of operating conditions (air temperature, air velocity, air absolute humidity, cooling surface temperature) on frost layer growth under the conditions favorable for desublimation were investigated. As the cooling surface temperature decreased, the increase in the amount of frost was insignificant. Additionally, an increase in air velocity increased the frost density but not the thickness of the frost layer.

Numerical study of mixed convection heat transfer for vertical annular finned tube heat exchanger with experimental data and different tube diameters

The commercial software along with the inverse method and the experimental data is used to obtain the mixed convection heat transfer and fluid flow characteristics of the single-tube vertical annular finned tube heat exchanger with different tube diameters. Various flow models along with near-wall treatments are introduced to investigate their effects on the obtained numerical results. The inverse method of the finite difference method along with the experimental data is first applied to determine the fin temperature and the heat transfer coefficient for a smaller tube. Afterwards, the air temperature and velocity profiles, the fin temperature distribution and the heat transfer coefficient on the fins are determined by the commercial software along with various flow models. More accurate results are obtained if the resulting heat transfer coefficient and the fin temperature at the measurement locations are respectively close to the inverse results and the experimental data. In addition, the comparison between the obtained air velocity pattern and the existing experimental pattern is desirable. An important finding is that the choice of appropriate flow models, near-wall treatments, friction factor, y+ value and the number of grid points varies with air velocity and tube diameter to obtain more accurate results. The heat transfer coefficient increases with increasing air velocity and fin spacing. However, the friction factor increases as the fin spacing increases and the air velocity decreases. The two vortices are symmetrical in the wake region behind the tube and rotate in the opposite direction. Increasing the number of grid points may not necessarily obtain more accurate results. The proposed correlation of the heat transfer coefficient is closer to the obtained inverse and numerical results than the existing correlation. To our knowledge, few researchers have used the similar method to investigate this problem in the open literature.

Parametric study on the outlet water temperature in a shower cooling tower and its application in different Iranian provincial capitals

This study investigates the performance of shower cooling towers under various geometric, physical, and environmental conditions. For this purpose, a shower cooling tower was thermodynamically modeled, and the governing equations were numerically solved using finite difference scheme. Contrary to the previous studies, the equation for droplet diameter variations during the evaporation process was also included among the governing equations, and the maximum error in the estimation of the outlet water temperature reduced to 1.2%. The results also showed that the decrease in the initial diameter of the sprayed water droplets lead to decrease of the process water temperature. While the process water temperature significantly drops because of the increase of air velocity, the changes in droplet spray speed have an insignificant effect on the outlet water temperature. The analysis of the effects of environmental conditions on the system's performance shows that as the air temperature decreases, the dependence of the outlet water temperature on the relative humidity of ambient air decreases. The comparison of countercurrent and countercurrent-concurrent towers indicates that using countercurrent-concurrent cooling tower cannot lead to significantly greater decreases in the outlet water temperature compared to countercurrent tower. The results obtained through the study of the performance of shower cooling tower utilized in Iranian provincial capitals also indicate that, the maximum and minimum temperature decrease occurs in cities located in cold and dry climates, and hot and humid climates, respectively.

Discrete particle simulation of food grain drying in a fluidised bed

Drying is a common practice for post-harvest processing of food grains. Fluidised beds are often adopted for this purpose. It is of importance to understand the fluidised bed drying process for improving its energy efficiency. This work establishes a numerical drying model based on the combined approach of computational fluid dynamics and discrete element method for describing heat and mass transfer in the gas-solid flow system. Water evaporation is modelled in resemblance to a chemical reaction, thereby requiring fewer model parameters. The model is first described in detail. Then it is tested by comparing model predictions with those experimental data of corn kernel from the literature. General drying characteristics including grain and air moisture contents are reproduced qualitatively. The predicted drying rate curves are quantitatively comparable with those of experimental data. Finally, the effects of inlet air velocity and temperature are examined. The model predictions confirm that the drying rate increases with both the inlet air velocity and temperature. However, the drying product quality, here represented by the standard deviation of grain moisture distribution, increases with increasing air velocity or decreasing air temperature. This grain scale model would be useful to the design and control of the drying process.

PEMANFAATAN KOLEKTOR SURYA SEBAGAI ENERGI ALTERNATIF DESALINASI AIR LAUT UNTUK MENGATASI KRISIS AIR BERSIH

This research examined the effect of air velocity and the efficiency of double-pass flat plate solar air heater with two glass covers on the performance of solar desalination unit based on heat pump with using humidification and dehumidification processes. This unit consists of a heat pump, humidifier, dehumidifier, and double-pass flat plate solar air heater with two glass covers. The research was conducted in an indoor experiment. Solar energy generated from solar simulator using halogen lamps. In this research the air flow rate was varied at 3 metres per second, 4 metres per second, 5 metres per second, and 6 metres per second, while the intensity of solar radiation at 828 Watts per meter squared. At each variation in air velocity, sea water temperature was conditioned at a constant temperature of 45 degrees celcius, the compressor was operated at a constant rotation of 900 rpm, volumetric flow rate of sea water inlet 300 liters per hour into humidifier and sea water in this system was re-circulated. The research result showed that the volume of fresh water production increases with increasing air velocity inlet into humidifier. This solar desalination unit could produce fresh water maximum 2470 milliliters per hour in air velocity 6 metres per second.

Waste heat and water recovery characteristics of heat exchangers for dryer exhaust

ABSTRACTIn many industrial drying processes, water is evaporated in the drying section by a large volume of fresh air supply, which contains a large amount of sensible and latent heat. Recovering heat and water from the dryer exhaust is a potential way to realize energy and water conservation in drying systems. In this paper, the experimental and theoretical investigations of the waste heat and water recovery characteristics of heat exchangers for dryer exhaust are presented. A hot air and water vapor mixture was used to simulate dryer exhaust. Convection–condensation heat transfer and water recovery characteristics were first obtained under various water vapor mass fractions, flow velocities, and humid air inlet temperatures through experiments, which revealed that the convection–condensation heat transfer coefficient increased with the increase in water vapor mass fraction and humid air velocity, and decreased with the increase in inlet temperature. The condensed water rate increased with the increase in water vapor mass fraction and humid air velocity, while inlet temperature had almost no effect on it. The condensing ratio decreased with the increase in water vapor mass fraction and humid air velocity, while improving the inlet temperature slightly reduced it. Then, a theoretical calculation model was developed to predict the heat and water recovery characteristics of heat exchangers under a wide working condition. In this model, the pipe and suction effects caused by condensation on the tube wall were considered. The properties of humid air were changed according to the temperature and vapor content instead of regarding them as constant values. The calculation results were compared with the present experimental data and those of previous studies. The good prediction accuracy and the universality of the present model were verified. This study is of great significance to the design of heat exchangers.

Computational and experimental studies on solar chimney power plants for power generation in Pacific Island countries

Computational and experimental studies were performed on solar chimney power plants (SCPP). The first part of the work was optimization of the geometry of the major components of an SCPP of 10m height and 8m collector diameter using a computational fluid dynamics (CFD) code ANSYS-CFX to study and improve the flow characteristics inside the SCPP. The collector inlet opening, the collector outlet height, the collector outlet diameter, the chimney divergence angle, chimney inlet opening and the diameter of the chimney were varied and optimum values that give the highest power were obtained. Based on the best configuration achieved for the 10m high SCPP, a scaled down model of 1:2.5 was modeled and simulated. The 4m tall SCPP had a collector diameter of 3.2m. The collector outlet height was kept constant while the collector outlet diameter and the chimney throat diameter were varied in the second part. The collector inlet opening was also varied. The best configuration was then fabricated and extensive experiments were carried out on days of different solar insolations with and without water bags including the effect of atmospheric wind as the third and main part of this work. Detailed measurements of temperature variations along the collector and along the chimney height were performed. The air velocity at the location of turbine was measured and the power available to the turbine was estimated. It was found that, at higher wind speeds, the temperatures along the collector and along the chimney height drop a little; however, the air velocity and available power increase. Water bags were placed under the collector to obtain round-the-clock power. A 100m SCPP was later modeled and simulated to predict the power available for bigger sized towers at different solar insolations. Such SCPP plants will be very appropriate for Pacific Island Countries; most of these countries have islands with populations of only a few hundred people. Also, the solar insolation is very high in these countries.

Energy saving method for improving thermal comfort and air quality in warm humid climates using isothermal high velocity ventilation

Increased air velocity improves thermal sensation in warm humid climate under certain conditions. In halls and enclosed public zones, a great volume of room air needs to be exchanged with fresh air. This is crucial to keep the air quality up to a desired degree. Air conditioning requires relatively high amount of energy for cooling and dehumidification, especially by high moisture content. Although fresh air can be provided by exposing the space to the outdoor environment via windows or moving partitions, improving thermal sensation and air quality in such space will not take place in calm wind conditions due to the lack of air movement. This paper investigates the effect of inducing outdoor air via ceiling confluent high velocity air supply to mix it with and eventually replace existing air in the semi-outdoor space. The generated results from the conducted CFD simulations indicate high air change rate, low air velocity at human height and increased thermal comfort without the need for cooling or dehumidification. The presented method of room conditioning suggests a low energy solution for spaces in adjacent location to the outdoor context that can generate new and more urban spaces.

Effect of Drying Temperature and Air Velocity on the Essential Oil Content of Mentha aquatica L.

The various species of mint are used in different industries such as pharmaceutical, food, cosmetics and health industry. Mentha aquatica L. are important species of mint. Drying process, in order to maintain the quality and quantity of essential oil extraction of the plant, it has an important role in the processing medicinal plants. Dryer tests were conducted in June 2016. In this research, the effect of drying temperature and air velocity on the essential oil content of mint was studied. The drying test was performed in three air temperatures 40, 50 and 60°C, and two drying air velocity 1 and 2 m/s. The essential oils from each organ type were extracted by hydrodistillation and the chemical components were analyzed by GC-MS. The results showed that increasing the drying temperature of 40 to 60°C was caused the loss of compounds such as α-terpinene, piperitone and β-myrcene. In addition with increasing air velocity dryer of 1 to 2 m/s caused increasing the percentage of volatile compounds such as α-pinene, 1,8-cineole, β-pinene and menthol. According to results, menthol content decreased with an increase in the drying air temperature from 40 to 50°C and then increased as the drying air temperature increased to 60°C. Menthol content increased with an increase in the drying air velocity. The results of essential oil analysis showed that among all determined compounds: menthol (61.1%), menthyl acetate (15.1%), 1,8-cineole (9.8%) and menthofuran (8.6%).

Factors influencing the efficacy of insect-proof screens

The use of insect-proof screens as a method of control is widespread. These agrotextiles act as physical barriers preventing the contact between insects and plants. Their effectiveness as a method of crop protection has been sufficiently demonstrated. The use of insect-proof screens reduces pest populations, decreases the incidence of insect-transmitted diseases and as a result it reduces the need to apply pesticides. There is a wide variety of screens with different properties on the market. Some of these agrotextiles are known commercially like anti-aphids or anti-thrips screens although their efficacy is not proven. During the last decades different studies have evaluated the efficacy of insect exclusion screens both under laboratory and in field conditions against various pests. The results of these works offer constant values that represent the relationship between insects and fabrics. However, that relationship is not a constant and depends on other variables in addition to the properties of the textile and the characteristics of the insect. The velocity and air temperature also influence the relationship between screen and insect and modify the exclusion capacity of the protection screens. To take into account these variables, we have built a device to evaluate the effectiveness of the screens under laboratory conditions. With this device we can measure the capacity of exclusion of the screens at different air velocities and we can interpret the data according to the measured temperature values. The results show that the efficacy of the screens varies when the conditions change and, in general, the exclusion percentage of the screens decreases when air velocity increases. The temperature is also an influential variable on the efficacy of these textiles and it has been observed that if the temperature decreases the effectiveness of the screens increases due to the lower activity of insects.

Optimization of mooseer (A. hirtifolium Boiss.) dehydration under infrared conditions.

In recent years, infrared drying has gained popularity as an alternative drying method for a va- riety of agricultural products. The use of infrared radiation technology in drying agricultural products has several advantages. These may include decreased drying time, high energy efficiency, high-quality finished products and uniform temperature in the product. With intermittent infrared and convection heating of a thick porous material, the drying time can be reduced compared to convection alone, while keeping good food quality and high energy efficiency. Response surface methodology (RSM) was employed to optimize the drying con- ditions of mooseer under infrared-convective drying. Experiments were performed at air temperatures of  40, 55 and 70°C, infrared powers of 500, 1000 and 1500 W, air velocities of 0.5, 1.5 and 2.5 m/s and slice thicknesses of 2, 4, and 6 mm. In this study, effective moisture diffusivity (Deff), shrinkage, color changes and specific energy consumption (SEC) were investigated. The central composite design (CCD) was selected for the design and optimization of the process. Deff was obtained between 1.4×10 and 3.57×10    m /s. With increasing air temperature and slice –10                                  –9       2 thickness, Deff also increased. The level of shrinkage rose as slice thickness increased. The highest and lowest values of color changes were calculated at air temperatures of 70°C (52.3%) and 40°C (5.65%), respectively. Increasing air velocity led to an increase in SEC. Optimum conditions for mooseer drying were achieved at air temperature of 70°C, infrared power of 867.46, air velocity of 0.59 m/s and slice thickness of 2 mm. At this point, Deff, shrinkage, color changes and SEC was obtained as 1.32×10–9 m2/s, 29.58%, 17.62% and 4.64 MJ/kg, respectively. The desir- ability value of 0.689 was achieved for the drying process. &nbsp.

Investigation of efficient air pollutant removal using active flow control

Efficiency in maintaining indoor air quality is central to the operation of high-performance buildings. The purpose of this work was to investigate the effect of airflow velocities generated by a low-power annular, multi-orifice synthetic jet actuator (SJA) on the degradation rate of a model air pollutant, nitrogen dioxide (NO2), by a photocatalytic surface. In this study, the active flow fields generated by the SJA were first characterized, and the effect of SJA-to-wall distance was analyzed as the airflow impinged on a wall of varying surface textures. Second, the impact of flow characteristics, namely surface velocity and velocity distribution, on the removal rates of NO2 by the photocatalytic surface was investigated in a closed chamber. Results showed that a SJA-to-wall (L) distance of 315 mm had the greatest reduction (dampening) on peak airflow velocities. Also, the surface with the highest roughness used in this study resulted in increased turbulence at the wall surface. The use of the SJA enhanced the removal rate of NO2 compared to passive (control) conditions. Increases in air velocity, however, did not monotonically enhance the removal rate of NO2. The highest removal rate (k = 0.0013 min−1) was measured at L = 315 mm, where the highest velocity dampening was observed. It also corresponded to an average surface velocity of approximately 0.1 m/s across the photocatalytic surface.

Experimental study on gas-solid flow characteristics in an internally circulating fluidized bed cold test apparatus

A self-designed internally circulating fluidized bed cold test apparatus was built to investigate the gas-solid flow characteristics in a new kind of internally circulating fluidized bed furnace. The test material was circulating ash from a power plant CFB boiler. Optical fiber probes and differential pressure transmitters were employed for the measurements. The particle internal circulation rate and the solids holdup in the upper space of the furnace were studied by changing the fluidization air velocity in the main chamber, the height of partition wall and the initial static bed height. The results showed that with the increase of fluidization air velocity in the main chamber, the particle internal circulation rate increased at first then decreased. Meanwhile, the particle internal circulation rate decreased with the increase of the partition wall height and increased with the increase of initial static bed height in the main chamber. The solids holdup in the upper space of the internally CFB cold test apparatus was 1–4% of that in the normal CFB cold test apparatus. The proportion of particle external circulation rate was relatively low in the circulating system.

Numerical Studies of Ventilation Effect on Methane Layering Behaviour in Underground Coal Mines

Layering of methane in underground coal mines owing to poor ventilation leads to methane explosion hazard. We study the methane layering phenomenon and the effect of ventilation on dispersion of methane in underground coal mines at air velocities varying from 0.5 to 4.0 m/s. Three-dimensional simulations using CFD code ANSYS Fluent 12.0 were performed assuming the flow to be unsteady, turbulent and incompressible. The study demonstrated that ventilation significantly affects the behaviour of methane layering and dispersion of methane in underground coal mines. The layering length decreased with increase in air velocity. At air velocity of 4.0 m/s, the methane layering length considerably reduced to a safer level of about 1 m. Moreover, the simulation results showed a good agreement with the experimental results.

Numerical and experimental study of mixed convection heat transfer and fluid flow characteristics of plate-fin heat sinks

This study applies three-dimensional computational fluid dynamics (CFD) commercial software along with the inverse method and experimental data to determine the mixed convection heat transfer and fluid flow characteristics of a plate-fin heat sink in a wind tunnel. The inverse method of the finite difference method along with the experimental temperature data is applied to determine the unknown heat transfer coefficient on the fin. Commercial software combined with various flow models is used to obtain air temperature and velocity profiles, heat transfer coefficient on fins, fin surface temperature and pressure drop. More accurate heat transfer and fluid flow characteristics can be obtained by the appropriate flow model and the number of grid points, if the resulting heat transfer coefficient and the fin temperature at each measurement location are close to the inverse results of the heat transfer coefficient and the experimental temperature data, respectively. The interesting finding is that the results obtained by the RNG k-ε turbulence model are more accurate than those by the laminar flow model. FLUENT 4 has better accuracy than FLUENT 15 along with standard wall functions and enhanced wall treatment. In addition, the total number of grid points needs to be increased with increasing air velocity and fin spacing. The dimensionless wall distance can vary with air velocity. The pressure drop has a large variation in the specific range of the fin spacing, and the secondary vortices can be found at both corners of the wind tunnel. It is worth mentioning that the strength of the secondary flow decreases with decreasing fin spacing. The effect of the flow model, near-wall treatment, FLUENT version and grid points on the results obtained cannot be ignored. To our knowledge, few researchers have used similar methods to investigate this problem in the open literature. The two proposed correlations are closer to the obtained inverse and numerical results than the existing results.

Sweating-boosted air cooling using nanoscale CuO wick structures

Low heat transfer coefficient (HTC) in air/fin-side is the bottleneck of dry cooling strategies for thermal power plants. Inspired by the phase change heat transfer during the perspiration of mammals, a sweating-boosted air cooling strategy with on-demand water dripping is proposed. The testing samples are featured with macroscale grooves for global liquid delivery, and with nanoscale hydrophilic copper oxide (CuO) wick structures for local liquid spreading. The experiments of sweating-boosted air cooling are conducted in a wind tunnel system. There are three wetting conditions with increasing dripping rates: dry, partially wetted, and flooded conditions. In the partially wetted conditions, the surface temperatures reduce and HTCs increase with increasing dripping rates. For a given dripping rate of water, HTCs are enhanced and surface temperatures are reduced with increasing air velocities. High air velocity and low surface temperature have a trade-off effect on the evaporation process. This effect results in almost constant saturated dripping rates for a given thermal load. A linear relationship between the saturated dripping rates and the thermal loads confirms that the evaporation dominates the heat transfer process of sweating-boosted air cooling. Complete surface wetting is obtained on the designed surfaces, but no obvious effect of groove width on HTCs is observed. Sweating-boosted air cooling can significantly increase air-fin side HTC in air cooled condenser (ACC), and dramatically reduce the water consumption compared to current water evaporative condenser (WEC). This research provides a fundamental understanding on the sweating-boosted effects on the air cooling.