Airflow Behavior Research Articles

Experimental and numerical heat transfer investigation in turbulent square-duct flow through oblique horseshoe baffles

An experimental and numerical work has been carried out to study the heat transfer enhancement in a heat exchanger square-duct fitted with 30° oblique horseshoe baffles (HB). In the current work, air is passed through the HB-inserted duct having a constant surface heat-flux. The air flow and heat transfer behaviors are presented for turbulent flow region, Reynolds number ranging from 4000 to 25,000. The pertinent parameters of the 30° HB elements include three relative baffle-pitches (PR=P/H=0.5, 1 and 2) and five relative baffle heights (BR=b/H=0.05, 0.1, 0.15, 0.2 and 0.25). Influences of those parameters on heat transfer and energy loss due to friction in terms of Nusselt number and friction factor, respectively are studied. The experimental result shows that at a given BR, the smallest pitch spacing (PR=0.5) provides the highest heat transfer and friction factor. The HB at BR=0.25 and PR=0.5 yields the highest heat transfer and friction factor but the one at BR=0.2 and PR=1 gives the maximum thermal performance. In addition, the thermal performance of using the HB is much higher than that of the wire coil insert, in comparison with other turbulators. To understand the heat transfer mechanism, a numerical inserted-duct flow simulation is also conducted and the obtained numerical results are in good agreement with measurements. Numerical flow and heat transfer behaviors such as streamlines, temperature and Nusselt number contours of the duct flow model are also reported.

Investigations of air flow behavior past a conical bluff body using particle imaging velocimetry

An annular jet flow past conical bluff body in an original configuration was investigated using particle imaging velocimetry for Reynolds (Re) numbers between 4900 and 58,800. The studied geometrical configuration finds its application in a novel low NO x burner; therefore, the flow conditions corresponding to burner operation were investigated in detail. It was found that the flow is laminar at Re = 4900 and the transition from a laminar to a turbulent flow regime starts at Re = 9800–14,700. The length of the recirculation zone increases abruptly when the flow becomes turbulent and is independent of the Re number within the turbulent flow regime. The fraction of the total flow rate representing flow circulating in the recirculation zone is relatively constant for the investigated flows. An increase in main stream velocity implies proportional decrease in the residence time in the recirculation zone observed within the turbulent flow regime. Two types of coherent structures shed from the trailing edge of the bluff body are identified using proper orthogonal decomposition technique. The first is vortices that cross the centerline of the bluff body and travel downstream along the centerline. The second is smaller vortex structures that are shed alternately from the trailing edge of the bluff body.

Numerical investigation on the airflow characteristics and thermal comfort in buoyancy-driven natural ventilation rooms

Numerical simulations are conducted to model the transient development of the buoyancy-driven natural ventilation. The simulations are compared with experimental data and theoretical predictions in the literature to validate the numerical model. The airflow behavior of three cases, i.e., the initial indoor temperature is equal to, higher and lower than the outdoor temperature, were analyzed by the numerical results, respectively. The results show that transient flow development of the last case is relatively complex in naturally ventilated rooms with inner heat sources due to the transformation from downward ventilation to upward ventilation. Vent area, heat source power and vertical position of the source are the main factors affecting the airflow rate, vertical temperature difference in the occupied zone and indoor thermal comfort, while the vent shape and the horizontal position of heat source on the floor seem to have marginal influence on the transient natural ventilation. In addition, initial indoor temperature is also a crucial factor influencing the establishment of steady state of the transient ventilation besides the vent and heat source characteristics.

Measurement of mass flow rate and evaluation of heat transfer coefficient for high-pressure pneumatic components during charge and discharge processes

Both the mass flow rate and heat transfer characteristics are significant factors to the flow behavior of the high-pressure air; however, they are not easy to be obtained by analytical model during discharge and charge processes. In this paper, the mass flow rate characteristics of high-pressure pneumatic components (HPPC) are measured by a compounding approach; two components under test with the same geometry and dimension are needed to be connected in series. Both the effective cross-section area and critical pressure ratio of HPPC are determined accurately, and only the pressure variation and the steady-state temperature of air in the chamber are utilized. The compared results between experimental and simulation data show that the accuracy of the measured effective cross-section area and critical pressure ratio of the HPPC is high when the sonic and adiabatic releasing time is less than 2s. And then, a new combined method of calculating the heat transfer coefficient during discharging and charging processes for the high-pressure air is proposed. The computational fluid dynamics (CFD) method is used to illustrate the intensity of heat exchange between the high-pressure air inside the chamber and outer atmosphere. The dynamic flow behavior is analyzed based on the tested flow rate characteristics of HPPC, mixed heat transfer theory and numerical results. The results show that the heat-transfer coefficient during charge process is much greater than discharge process, and the forced convection heat exchange happened owing to the strong “air agitation” during the charge process. The experimental results also validate that the proposed method of calculating the transient heat transfer coefficient is more reasonable to describe the heat transfer behavior. The findings may also have general implication in the development of the design and analysis of the high-pressure pneumatic system.

Experiment of air flow behavior and heat transfer characteristics in microchannels with different aspect ratios

Experiments were conducted to investigate the flow behavior and heat transfer characteristics in minichannels with different aspect ratios. The widths of microchannels are constant of 0. 4 mm. The aspect ratios of the microchannels are 2 /5,1 /2,2 /3 and 1,respectively. All tests were performed with air. The experiments were completed with the Reynolds number in the range of 200 to 1 600. Results of experiments show that the aspect ratios of microchannels have a remarkable effect on the performance of flow behavior and heat transfer characteristics. The friction factors decrease with the increase of the Reynolds numbers. The Poiseuille numbers increase when the Reynolds numbers keep rising. Both of the friction factors and Poiseuille numbers drop as the aspect ratios increase. Nusselt numbers increase as Reynolds numbers increase,but decrease with the increase of aspect ratios. The corresponding empiric equations for the flow resistance and heat transfer characteristics in microchannels with different aspect ratios are fitted.

Large Eddy Simulation of Air Escape through a Hospital Isolation Room Single Hinged Doorway--Validation by Using Tracer Gases and Simulated Smoke Videos.

The use of hospital isolation rooms has increased considerably in recent years due to the worldwide outbreaks of various emerging infectious diseases. However, the passage of staff through isolation room doors is suspected to be a cause of containment failure, especially in case of hinged doors. It is therefore important to minimize inadvertent contaminant airflow leakage across the doorway during such movements. To this end, it is essential to investigate the behavior of such airflows, especially the overall volume of air that can potentially leak across the doorway during door-opening and human passage. Experimental measurements using full-scale mock-ups are expensive and labour intensive. A useful alternative approach is the application of Computational Fluid Dynamics (CFD) modelling using a time-resolved Large Eddy Simulation (LES) method. In this study simulated air flow patterns are qualitatively compared with experimental ones, and the simulated total volume of air that escapes is compared with the experimentally measured volume. It is shown that the LES method is able to reproduce, at room scale, the complex transient airflows generated during door-opening/closing motions and the passage of a human figure through the doorway between two rooms. This was a basic test case that was performed in an isothermal environment without ventilation. However, the advantage of the CFD approach is that the addition of ventilation airflows and a temperature difference between the rooms is, in principle, a relatively simple task. A standard method to observe flow structures is dosing smoke into the flow. In this paper we introduce graphical methods to simulate smoke experiments by LES, making it very easy to compare the CFD simulation to the experiments. The results demonstrate that the transient CFD simulation is a promising tool to compare different isolation room scenarios without the need to construct full-scale experimental models. The CFD model is able to reproduce the complex airflows and estimate the volume of air escaping as a function of time. In this test, the calculated migrated air volume in the CFD model differed by 20% from the experimental tracer gas measurements. In the case containing only a hinged door operation, without passage, the difference was only 10%.

Design and validation of a 2D CFD model of the airflow produced by an airblast sprayer during pesticide treatments of citrus

During plant protection treatments using airblast sprayers, part of the chemical is lost in the atmosphere (spray drift), ground, surface water, etc., causing risks to the environment. Although there is a growing interest in quantifying these losses, field measurements are extraordinarily complex and expensive. Computational Fluid Dynamics (CFD) generates mathematical models of this phenomenon that may help to understand and quantify it. The air flow produced by the fan is affected by the tree canopies, which modify the trajectories of spray droplets. Current state of the art in CFD considers canopies as porous bodies and uses the k–ε turbulence model. In a first step, this work proposes and validates a two dimensional CFD model to be applied in citrus tree applications from experimental data. This new CFD model considers canopies as solid bodies. Four different geometries for the first tree are compared using three different turbulence models: k–ε, SST k–ω and Reynolds Stress Model. Air velocities measured in front of a canopy in a previous field test are introduced as boundary conditions. We used the experimental data to adjust the model and select the geometry and the turbulence model. In order to test the validity of the model, air velocities obtained with the model are compared with the experimental data obtained in other experiment. The final CFD model was able to reproduce the airflow behaviour around the tree canopy, with the same turbulent structures. The solid body with the new turbulence model (SST k–ω) was considered as a good approximation to the real life.

A Numerical Assessment of the Air Flow Behaviour in a Conventional Compact Dry Kiln

Convective drying is the most common drying strategy used in timber manufacturing industries in the developing world. In convective drying, the reduction rate of the moisture content is directly affected by the flow topology in the inlet and exit plenums and the air flow velocity in the channels formed by timber layers. Turbulence, boundary layer separation, vortex formation and recirculation regions are flow features that are intrinsically associated with the kiln geometry, which in turn dictate the flow velocity across the timber stack and, ultimately, the drying rate. Within this framework, this work presents a numerical study of the effects of the plenum width and inlet flow velocity in a compact dry kiln aiming to establish design recommendations to ensure the highest possible level of flow uniformity across the lumber stack. The numerical solution of the mathematical model is obtained through the finite-volume based Ansys CFX R flow solver. Validation of the numerical approximation is performed by comparing numerical and experimental flow velocities for a scale model of a kiln available in the literature.

에어튜브의 직경비에 따른 건타입 버너의 출구 유동특성에 관한 연구

Swirl flow has an impact on the stabilization of the flame by the recirculation flow, improvement of the combustion efficiency. The swirl flow in the gun type burner is created by the spinner which is inside the airtube that guide the combustion air. Burner has generally the combustion device composed electronic spark plug, injection nozzle, combustion device adaptor, and spinner. These inner components change the air flow behavior passing through airtube. So, this study analyzed exhaust flow characteristics of the gun type burner according to the ratio of airtube diameter. Turbulence characteristics by the spinner was mean velocity, turbulence intensity, kinetic energy, shear stress and flattness factor of the air flow of axial direction and tangential direction from the exit of the airtube.

CFD simulation of airflow behavior and particle transport and deposition in different breathing conditions through the realistic model of human airways

In this work, the airflow behavior and particle transport and deposition in different breathing conditions such as light breathing condition (15L/min), normal breathing condition (30L/min) and heavy breathing condition (60L/min) are investigated. The realistic geometry data was reconstructed from CT-scan images of the human airways with 0.5mm thickness of slices. The CT-scan images (DICOM files) are imported in the 3D-DOCTOR software and all slices were segmented. Then, the output has been imported in CATIA-V5 software. Finally, face, volume, mesh and extension tubes at inlet and outlets were created and then imported into ANSYS FLUENT 15. The Lagrangian approach is used to evaluating the transport and deposition of inhaled micro-particles. The presented results showed that for dp=5μm and 10μm, when flow rate =30Lmin and for dp=1μm, when flow rate =15Lmin, the particle deposition fraction have maximum amount. For flow rate =15Lmin and =30Lmin, the maximum deposition occurs in the zone number 1 and for flow rate =60Lmin occurs in the zone number 4. Also, the maximum pressure distribution happens when flow rate =60Lmin which would be acceptable. According to the results, the particles tended to go to the right branch and the minimum number of particles crossed the zone numbers 6 and 11.

Verification of a CFD code use for air flow simulations of fractured and broken rock

In support of air flow resistance testing and conjugate heat transfer modelling of broken rock bodies a direct comparison has been undertaken between laboratory wind tunnel experimentation and computational fluid dynamics simulation of a 3D manufactured analogue of a body of fractured broken rock, created with CAD/CAM techniques. Detailed descriptions of the ‘design’ of the analogue (using a discrete element analysis multi-body simulator), its manufacture (using computer aided design and 3D printing technology) and the laboratory experimentation (inserting the physical analogue into a laboratory scale wind tunnel) are presented. The agreement of the results of direct experimental observations on the manufactured analogue with the values from CFD simulations of those experiments justifies the utilization of a CFD environment for the practical investigation of geometric factors controlling air flow and heat exchange in complex assemblages of broken rocks. The ultimate goal is described, that of developing functional relations between bulk properties of such assemblages (e.g. their porosity and bulk density) and the low-level geometric factors that dictate the air flow behaviour, such as the equivalent perimeter length of the cross-sectional area available for flow, or average flow path tortuosity.

STUDY OF DRUG FORCE AND THE FLOW FIELD ON ROAD VEHICLES Khalid M. Sawoud

Improving road vehicles performance needs to deep understanding the science of aerodynamics, in order to control flow field by means of active and passive control techniques. The airflow behavior when passes over the road vehicles surfaces will be changes in patterns and resulting in different pressure regions. This pressure region causes drag force and thereby increases in fuel consumption of the road vehicles. This paper include experimental study to investigate the effect of road vehicles for three most common rear end configurations design such as (square, notch and fastback) on drag force, drag coefficient and pressure distribution. The experiments were carried out an open, low speed, and three dimensional wind-tunnel, on geometrical similarity to the prototype (Audi 80 1987) in scale down (1:18) wooden models for four different velocities (11.31, 13.86, 17.89 and 22.98) m/sec. The results obtained from the wind-tunnel investigations showed that the drag force increases with the increasing of free-stream velocities. The minimum drag coefficient can be achieved with fastback configuration, comparing with the other tested models and result in approximately 16% and 48% lower than that for notch and square back configurations, respectively. These results are demonstrated by the pressure distribution curves which provide a deep understanding of the flow behavior above the tested models.

Computational Fluid Dynamics Analysis of Empty Railway Freight Wagons

Increase in the speed of train leads to aerodynamic problems. These problems occur due to turbulent behaviour of air flow around the train. So it is of great concern to study the dynamics of fluid flow. This paper deals with the dynamic analysis of empty freight wagons due to air turbulence using computational fluid dynamics. The effect of important aerodynamic coefficients such as aerodynamic drag, pressure distribution and velocity stream line has been studied on realistic and simplified freight train geometries. The geometry is a scaled model of freight open wagon type‘BOXN25’ from Indian Railways. The aerodynamic analysis for empty wagons shows that the critical stress zone and the air drag effect are comparable between the two considered geometries.

Influence of fibre cross-sectional shape on air permeability of nonwovens

In this work, a series of needle-punched nonwoven fabrics was prepared by mixing polyester fibres of circular and deep-grooved cross-sections at different mass proportions and evaluated for their mean fibre shape, pore diameter and air flow behaviour. A simple and easy-to-use expression was developed for prediction of mean fibre shape in nonwoven fabrics prepared by mixing of fibres of different cross-sections. The mean fibre shape factor was found to be a weighted arithmetic mean of the shape factors of individual fibre components, where the weightage was determined by the product of volume and fineness fractions of individual fibre components. The nonwoven fabric prepared by circular fibres alone displayed the highest pore diameter, while the nonwoven fabric prepared by deep-grooved fibres alone exhibited the lowest pore diameter. The pore diameter was found to increase non-linearly with an increase in mass proportion of circular fibres in the nonwoven fabrics. The relationship between fibre shape and pore diameter was theoretically derived and experimentally verified. The air flow behaviour of nonwoven fabrics was found to be explained by Darcy’s law reasonably well. The nonwoven fabric prepared by circular fibres alone exhibited the highest air permeability, while the nonwoven fabric prepared by deep-grooved fibres alone displayed the lowest air permeability. The air permeability was found to decrease with a higher proportion of non-circular fibres in the nonwoven fabrics. This behaviour could not be explained reasonably well by Hagen-Poiseuille’s equation, however. But when the Reynolds number and the friction factor were taken into account, the predicted results were found in close agreement with the experimental ones.

Premixed CH4-Air Flame Structure Characteristic and Flow Behavior Induced by Obstacle in an Open Duct

To study the fuel gas combustion hazards, the methane/air flame structure and flow characteristic in an open duct influenced by a rectangular obstacle were explored by experiment and realizable k-∊ model (RKE). In the test, the high-speed schlieren photography technology and dynamic detection technology were applied to record the flame propagation behavior. Meanwhile, the interaction between flame front and flame flow field induced by the obstacle was disclosed. In addition, the laminar-turbulence transition was also taken into consideration. The RKE and eddy dissipation concept (EDC) premixed combustion model were applied to obtain an insight into the phenomenon of flow change and wrinkle appearing, which potently explained the experimental observations. As a result, the obstacle blocked the laminar flame propagation velocity and increased pressure a little in an open duct. Some small-scale vortices began to appear near the obstacle, mainly due to Kelvin-Helmholtz instability (KHI), and gradually grew into large-scale vortices, which led to laminar-turbulent transition directly. The vortices thickened the reaction area and hastened the reaction rate; reversely, the higher reaction rate induced larger vortices. The RKE model result fitted the test data well and explained the wrinkle forming mechanism of two special vortices in the case.

Results of experimental studies of the gas-dynamic behavior of airflow in the circulation line of the air condenser of steam-turbine plants

The results of experimental studies and a physical model of the three-dimensional flow of cooling air in the circulation line (CL) of a dummy air condenser (AC) incorporating a fan, heat-exchange modules, a shell, and other auxiliary components are analyzed. The local air velocity fields determined experimentally at the AC CL inlet and at the fan diffuser outlet are presented. The guidelines for determining the head-capacity characteristics of the airflow through the AC CL are proposed.

연소효율 개선을 위한 스월제트의 난류유동 특성에 관한 연구

Swirl flow in the gun type burner has a decisive effect on the stabilization of the flame, improvement of the combustion efficiency, and also a reduction of NOx. This swirl flow is created by the spinner which is inside the airtube that guide the combustion air. Gun type burner has generally the inner devices composed nozzle adapter, spark gap ignitor, and spinner. These inner components change the air flow behavior passing through air tube. Meanwhile, turbulent characteristics of this air flow are important to understand the combustion phenomena in the gun type burner, because the mixture of fuel and air are depended on. However, nearly all of the studies have been analyzed the turbulent flow of simplified combustion formation without the inner devices. So, this study conducted the measurement using by hot-wire anemometer and analyzed turbulent flow characteristics of the swirl flow discharged from the air tube with inner devices. Turbulence characteristics come up in this study were turbulence intensity, kinetic energy and shear stress of the air flow with the change of the distance of axial direction from the exit of the air tube.

Augmenting simulations of airflow around buildings using field measurements

Computational fluid-dynamics (CFD) simulations have become an important tool for the assessment of airflow in urban areas. However, large discrepancies may appear when simulated predictions are compared with field measurements because of the complexity of airflow behaviour around buildings and difficulties in defining correct sets of parameter values, including those for inlet conditions. Inlet conditions of the CFD model are difficult to estimate and often the values employed do not represent real conditions. In this paper, a model-based data-interpretation framework is proposed in order to integrate knowledge obtained through CFD simulations with those obtained from field measurements carried out in the urban canopy layer (UCL). In this framework, probability-based inlet conditions of the CFD simulation are identified with measurements taken in the UCL. The framework is built on the error-domain model falsification approach that has been developed for the identification of other complex systems. System identification of physics-based models is a challenging task because of the presence of errors in models as well as measurements. This paper presents a methodology to estimate modelling errors. Furthermore, error-domain model falsification has been adapted for the application of airflow modelling around buildings in order to accommodate the time variability of atmospheric conditions. As a case study, the framework is tested and validated for the predictions of airflow around an experimental facility of the Future Cities Laboratory, called “BubbleZERO”. Results show that the framework is capable of narrowing down parameter-value sets from over five hundred to a few having possible inlet conditions for the selected case-study. Thus the case-study illustrates an approach to identifying time-varying inlet conditions and predicting wind characteristics at locations where there are no sensors.

Influence of geometric parameters on the hydrodynamics control of solar chimney

Hydrogen generating station (HGS) by water electrolysis is based on three principal compartments: hydraulic pumping system, water electrolysis process and energy supply whose can be free source like solar chimney power plans (SCPP). The HGS can be controlled if governed one of these three compartments especially the energy supply SCPP. The aim of this study is to investigate numerically the airflow behavior through a Solar Chimney. The transport equations that describe the flow, for different geometric tower parameters were modeled and solved numerically using the finite volume method. This methodology allows us a detailed visualization of the effects of some geometrical parameters such as the tower height and tower radius. It has been shown in this work that the dimensions of the tower play an important role in designing of such systems by increasing or decreasing the mass flow rate. Consequently, the hydrodynamic field is directly controlled by varying the tower dimensions. However the thermal field is indirectly controlled by decreasing the system mean temperature.

Design and Development the Pressure Chamber of the Continuous Belt Air-Blast Freezer in Freezing Shrimp Process

The air-blast freezer is common freezing technique for shrimps in seafood industry. In many companies encounter with developing this technology for food industry. In this study, Development in effects and behavior of airflow over in pressure chamber of continuous belt air-blast freezer, were investigated by simulating the airflow field by using computational fluid dynamics (CFD). Pattern holes in plenum plate and Modifying shape tunnel were cases study in redesign of development. The average velocity at outlet holes of airflow were determined and the streamline and vector of airflow were analyzed. The effects of redesign could increase the average relative velocity at outlet up 22- 32% compared with the original design.