Air Velocity Profiles Research Articles

3D CFD analysis of the effect of inlet air flow maldistribution on the fluid flow and heat transfer performances of plate-fin-and-tube laminar heat exchangers

Plate fin-and-tube heat exchangers are widely used in several areas such as heating, ventilating, air conditioning and refrigeration systems. Nonuniformity of the inlet air flow in heat exchangers is first order importance and has decisive influence on their efficiency because it can intensify longitudinal wall heat conduction and the maldistribution of interior temperature.This study presents the results of three-dimensional (3D) Computational Fluid Dynamics (CFD) simulations aim to investigate the effect of inlet air flow maldistribution on the thermo-hydraulic performance of heat exchangers. Validation of the computation results with experimental data found in the literature has shown a very good agreement.Different computation test cases with a variety of inlet air flow distributions on in-line and staggered plate-fin-and-tube heat exchangers were run to systematically analyse their effects on system performance. The CFD results confirmed the importance of the influence of inlet fluid flow nonuniformity on heat exchanger efficiency. Results indicate that up to 50% improvement or deterioration in the Colburn j-factor are found compared to the baseline case of a heat exchanger with a uniform inlet air velocity profile. Moreover, the present investigation with respect to inlet flow maldistribution demonstrates that 3D CFD simulation is a useful tool for analysing, designing and optimising heat exchangers. Finally, the results of this study can be also of significant value for the optimum design of header and distributor configurations of heat exchangers to minimise maldistribution.

불균일한 풍속분포에 따른 응축기의 열전달 성능 변화

Heat transfer performance variation of a condenser caused by non-uniform distribution of air flow was investigated using a numerical simulation method. A heat exchanger used for a outdoor unit of a commercial heat pump system and represented by a numerical model was selected. Non-uniform profile of air-velocity was constructed by measuring the air velocity at various locations of the outdoor unit. Simulation was conducted for various refrigerant circuits and air flow conditions. Simulation results show that the heat transfer capacity was reduced depending on the air-flow rate and the refrigerant circuit configuration. It is also shown that the capacity reduction rate is increased as the average air velocity decreases.

Modeling and simulation of a solar flat plate collector as an air heater considering energy efficiency

The present research study includes thermal energy performance analysis of a flat plate solar collector, in order to cover a sensitivity analysis about effective variables contributed to performance increasing approach. To obtain that, energy and momentum governing equations on a flat plate thermal collector were developed to achieve air output temperature and velocity profiles both in model and experiment. The model theory is validated with experiments by a set of flat plate thermal collectors, those located in (35◦ 44′ 35′′ N, 50◦ 57′ 25′′ E) coordinates and then were applied to carry out experimental activities. Quantitative results depicted that the mean difference between predicted and measured output air temperature in natural and forced convection scenarii is 1.47 ◦ C (3.5%) and 0.9 ◦ C (1.5%), respectively; however earlier research works and studies mentioned in literature, include error percentages in the range of 4–10%. Another quantification about, the average error percentages of estimated amounts of output air velocity profile values in natural and forced convection scenarii is 9% and 4%, respectively. This issue realizes the developed model in forced convection scenario more accurate than natural convection scenario.

Experimental Study on Fan-Induced Airflow Evaluation by Comparing the Power Spectrum, Turbulence Intensity and Draught Rate Methods

In hot climates a comfortable indoor environment is important. Mechanical fans are often introduced to cool the indoor air. However, it has been found that the airflow from such fans is not comfortable, especially compared with natural wind. Artificial airflow in an enclosure has been known to disturb hair, irritate eyes, and distract occupants. This paper presents the result of an experimental study on the characteristics of household airflow inducing appliances. The details of experiments carried out in the laboratory are described. Four kinds of common appliances were tested and two operating conditions were chosen. The power spectrum density function was used to evaluate the airflow dynamic characteristics.The preliminary conclusion shows that the power spectrum exponent is determined by the mean air velocity and the intermittence of the air velocity profiles. However, even mechanical airflow with dynamic characteristics such as shift and intermittence cannot exactly replicate natural wind. A small ratio of height to width of the fan airflow outlets is required so that more surrounding air can be involved in the flow, and more comfortable airflow is available for occupants.

Airflow patterns around obstacles with arched and pitched roofs: Wind tunnel measurements and direct simulation

Achieving accurate numerical predictions of airflow around buildings is challenging due to the dynamic characteristics of wind. Buildings are usually considered as obstacles to the wind. A time-dependent simulation model has been applied for the prediction of the turbulent airflow around obstacles with arched and pitched roof geometry, under wind tunnel conditions. The numerical model is based on the direct solution of transient Navier–Stokes and continuity equations using the Galerkin finite element method. To verify the reliability of the model an experiment was conducted inside a wind tunnel and the air velocity and turbulent kinetic energy profiles were measured around two small-scale obstacles with an arched-type and a pitched-type roof, respectively. The velocity components and the turbulent kinetic energy values were used to demonstrate a dynamic and statistical analysis of this complex flow. The wind tunnel tests presented good agreement with the numerical simulations with respect to airflow patterns. The different roof geometry of obstacles affected the instantaneous and time-mean averaged parameters of the flow. According to the instantaneous results of the numerical solution, airflow patterns presented fluctuating characteristics mainly downstream of the obstacles. Intense variations were shown in streamlines and velocity components, both at the arched-type and the pitched-type obstacle, starting from the upstream corner of the roof and the top of the roof respectively. The time-dependent simulation of the flow parameters can provide important information on instantaneous fluctuations of the complex flow phenomena around arched-type and pitched-type roof obstacles which cannot be obtained by the time-mean averaged approach.

Numerical Simulations of Two-Phase Flow in a Dorr-Oliver Flotation Cell Model

Two-phase (water and air) flow in the forced-air mechanically-stirred Dorr-Oliver machine has been investigated using computational fluid dynamics (CFD). A 6 m3 model is considered. The flow is modeled by the Euler-Euler approach, and transport equations are solved using software ANSYS-CFX5. Unsteady simulations are conducted in a 180-degree sector with periodic boundary conditions. Air is injected into the rotor at the rate of 2.63 m3/min, and a uniform bubble diameter is specified. The effects of bubble diameter on velocity field and air volume fraction are determined by conducting simulations for three diameters of 0.5, 1.0, and 2.0 mm. Air volume fraction contours, velocity profiles, and turbulent kinetic energy profiles in different parts of the machine are presented and discussed. Results have been compared to experimental data, and good agreement is obtained for the mean velocity and turbulent kinetic energy profiles in the rotor-stator gap and in the jet region outside stator blades.

Thermal analysis and design of a volumetric solar absorber depending on the porosity

The thermal evaluation of different absorber configurations for a volumetric solar receiver designed for a solar furnace has been carried out by means of commercial Computational Fluid Dynamics (CFD) software in a 2D numerical model. Simulation results for proposed configurations depending on the porosity are discussed and compared to find the optimum configuration for which flow instabilities and thermal stresses are minimized and higher efficiencies are reached. The results obtained from the comparison of air velocity and thermal profiles at the absorber outlet propose a gradual-porosity configuration as an alternative to a previous design of a porous silicon-carbide honeycomb structure in order to heat an air stream up to temperatures suited for several high-temperature industrial processes.

Effect of air-side flow maldistribution on thermal–hydraulic performance of the multi-louvered fin and tube heat exchanger

For the complex mechanical turbulence by micro-louvered fins coupled with the problems of thermal interaction between air side and refrigerant side, the airflow distribution in such a compact fin-tube air-to-refrigerant heat exchanger is perceived as a compound and non-homogeneous behavior, which is always quite difficult to calculate. In this research, based on the finite volume method (FVM), a verified model and methodology for the cross-flow condenser modeling with multi-louvered fin and tube structure is developed, for evaluating the coupling effects on the performance of multi-louvered fin and flat tube heat exchangers under airflow maldistribution. Firstly, a simplified control volume is selected as the calculating domain, and is validated under uniform airflow with 20 groups of experimental data, including root-mean-square (RMS) errors of condenser capacities, outlet refrigerant temperatures and outlet refrigerant pressures of ±5.99%, ±5.65% and ±5.66% respectively. Secondly, four representative airflow patterns in two-dimensional velocity distributions are adopted to characterize the air velocity profiles, and their effect of airflow maldistribution on the consequent heat transfer and pressure drop. The results indicate that airflow maldistribution affects the condensation capacity, refrigerant pressure drop, as well as the theoretical fan power consumption. The maximum capacity reduction and pressure drop increment are 6% and 34 % respectively for these types of airflow maldistribution. This discussion may be attractive for a great potential in designing and sizing an optimum air arrangement for making the flow distribution more uniform in a significant level.

Turbulent mixed convection in a uniformly heated vertical channel with an assisting moving surface

A numerical investigation of mixed convection in air due to the interaction between a buoyancy flow and the flow induced by a moving plate in a vertical channel is carried out. An unheated plate moves at a constant velocity in the direction of the buoyancy force and the principal walls of the channel are heated at uniform heat flux. The effects of the channel spacing, heat flux and moving plate velocity are investigated and results in terms of the channel walls and moving plate temperatures, air velocity profiles inside the channel, wall stress along the channel wall and the moving plate and Nusselt numbers are given. A composite correlation between Nusselt number and Reynolds and Richardson numbers is proposed in the range from natural convection to forced convection up to Re = 1.32 × 104 and 9.86 × 10−6 ≤ Ri ≤ 1.15 × 103, for the channel aspect ratio in the [20.3–81.2] range.

Experimental evaluation of air distribution in mechanically ventilated residential rooms: Thermal comfort and ventilation effectiveness

The effect of low ventilation rates (1 or 0.5 air change per hour) on thermal comfort and ventilation effectiveness was experimentally studied in a simulated residential room equipped with radiant floor heating/cooling and mixing ventilation systems. The tests were performed for various positions of supply and extract air terminals and different winter and summer boundary conditions. Vertical air temperature, operative temperature and air velocity profiles were measured in different positions in the room, and equivalent temperatures were derived, in order to characterize thermal comfort.Contaminant removal effectiveness (CRE) and local air change index was measured in order to characterize ventilation effectiveness in the occupied zone. Acceptable thermal comfort was found in most experiments; however, air temperature differences higher than 3°C occurred when floor cooling was combined with unconditioned outdoor air supply, i.e. at the supply air temperatures higher than the room air temperature. Moreover, low floor temperatures were needed to maintain the desired reference temperature in the stratified thermal environment. Mainly in cooling conditions the ventilation effectiveness depended on the positions of supply and extract air terminals and on the difference between the supply and the room air temperature, and it could be as low as 0.5, where 1 is complete mixing.

Simulation of gas–solid two-phase flow in the annulus of drilling well

This paper investigates the hydrodynamic behavior of gas–solid two-phase flow in the annular space of an air drilling well under different arrangements by using three-dimensional approach. Two-fluid model is used to solve the governing equations in the Eulerian–Eulerian framework. Effect of eccentricity and drill pipe rotation on the pressure drop, volume fraction and velocity profile are examined. The results are compared with available data in the literature and good agreement is found. The results show that the presence of solid particles in the annulus change the air velocity profile significantly and create two off-center peaks velocity close to the walls instead of one peak velocity in the middle. Eccentricity of drill pipe makes more accumulation of the cuttings in the smaller space of the annulus. Increasing the eccentricity increases pressure drop due to impact of particles with annulus wall and also particles collision with each other. Rotation of the drill pipe shifts maximum air velocity location toward smaller space of the annulus which results more uniform cutting distributions in the annulus and improvement in their transportations. Pressure drop in the annulus increases as eccentricity and rotation of drill pipe increase.

Fully Automated System for Air Velocity Profile Measurement

The paper presents the idea of a system for controlling the movement of a flowmeter for air velocity profile measurement. In such a system, due to massive amount of data and limitations of the Data Acquisition Equipment, it is necessary to use moveable sensors. The flowmeter sensor is moved with the use of a linear module with a stepper motor and a tooth-belt drive. The location and speed of the sensor are controlled by a program based on the idea of virtual instrument. The proposed structure allows the user to control operation of the stand and provides automatic measurement. A wide range of velocity and step increments of the stepper motor drive, and flexibility of the virtual instrument software, allow one to create effective measurement systems ensuring sufficiently precise location with optimal time duration of measurement. It is shown that the linear module with tooth-belt is an effective alternative for similar modules with micro-screw drives.

Air distribution and ventilation effectiveness in an occupied room heated by warm air

Air distribution, ventilation effectiveness and thermal environment were experimentally studied in a simulated room in a low-energy building heated and ventilated by warm air supplied by a mixing ventilation system. Measurements were performed for various positions of the air terminal devices and at different simulated outside conditions, internal heat gains and air change rates. Floor heating was also simulated and compared with the warm air heating system. Vertical air temperature profiles, air velocity profiles and equivalent temperatures were derived in order to describe the thermal environment. Contaminant removal effectiveness and air change efficiency were used to evaluate ventilation effectiveness. No significant risk of thermal discomfort due to vertical air temperature differences or draught was found. When the room was heated by warm air, buoyancy forces were important for ventilation effectiveness at low air change rates. The effect of increasing air change on the ventilation effectiveness depended on the position of air terminal devices. Depending on the position of air terminal devices, the ventilation effectiveness varied between 0.4 and 1.2, where 1 is complete mixing. When a radiant floor heating system was simulated, the cooler ventilation air introduced to the room mixed well and created uniform conditions with a ventilation effectiveness of about 1.

Numerical investigation of transient natural convection in a vertical channel-chimney system symmetrically heated at uniform heat flux

In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.

Direct Two–Phase Numerical Simulation of Snowdrift Remediation using Three–Dimensional Deflection Fins

We present a versatile three – dimensional two – phase model for simulating snow drift relocation around buildings utilizing deflection fins of various shapes and sizes. The first phase involves numerically obtaining the air velocity profile around the building and fin using a velocity – pressure Navier – Stokes algorithm, while the second phase involves direct classical simulation of snowfall with particle – particle, particle – surface and one – way particle – gusting wind interactions introduced to control accumulation, erosion, clumping and drifting. Because the simulation technique is direct, it is potentially useful for storms and surfaces with widely varying conditions. We are also able to consider the effect of crosswinds.

Airflow measurements in and around scale model cattle barns in a wind tunnel: Effect of ventilation opening height

Animal houses require an adequate ventilation system to allow for efficient production and, indirectly, high product quality. Indoor air quality is also important in relation to a healthy work and housing environment for both farmer and animal. Furthermore, ventilation is directly linked with emissions of gases with environmental impact, such as ammonia, methane and nitrous oxide. To acquire a better understanding of the complex natural ventilation process in and around animal houses, air velocity measurements were carried out in 1:60 scale models of a dairy cattle house placed in a wind tunnel, using a reference air velocity of 3.5 m s−1. Six different ventilation opening configurations were compared, in order to quantify their effect on internal and leeward air velocity profiles. The different scale model designs clearly gave rise to variable indoor and outdoor air velocities. Enlarging the inlet opening height led to lower velocities near the inlet, while higher velocities were measured at the outlet. The air velocities at the centre of the house were hardly affected by the inlet opening height, even with the front wall completely removed. Removing the outlet wall at the same time, however led to much higher velocities at the centre of the scale model (3–4 times higher). Higher airflow rates were found in scale models with larger ventilation opening surface areas. The presented results additionally provide a useful experimental basis towards validation of Computational Fluid Dynamics (CFD) simulations.

Influence of Laval nozzles on the air flow field in melt blowing apparatus

Melt blowing combines extrusion of molten polymer through small orifices with stretching of the hot extrudate by hot air jets to create long, small diameter, fibers. Simulations and experiments were performed to examine: (1) the influence of increasing air pressure inside the melt blowing die (Pinlet) on the air jet in a typical melt blowing process and (2) the influence of a Laval nozzle (a converging–diverging nozzle) on the air jet. The baseline case without a nozzle was simulated and examined based on the y-component of the air velocity profile, vy(y), at the centerline as a function of Pinlet. As Pinlet increases: (1) the air flow goes from subsonic to supersonic and (2) the maximum value of vy(y) increases with increasing Pinlet, then starts to oscillate with the formation of compression waves in the supersonic region, Pinlet≥15psig. Simulation also showed that a Laval nozzle influences the air flow field by increasing the maximum value of vy(y) and eliminating the compression wave at a predictable value of Pinlet. Actual density oscillations in the supersonic flow field exiting a melt blowing die, with and without a Laval nozzle, were captured using a Schlieren visualization technique. In both limits the experimental results are in good agreement with the density oscillation in the supersonic flow field of the air jet anticipated by the simulations.

EFFECT OF AIRFLOW PROFILE ON REDUCING HEAT STRESS, ENHANCING AIR DISTRIBUTION AND DILUTING GASEOUS CONCENTRATIONS IN DAIRY BARNS

Heat stress in dairy cows is one of the leading causes of decreased production and fertility. Increasing air velocity, using ceiling fans, to enhance convective heat transfer and accordingly body heat dissipation is highly required. However, this might have negative effects such as increasing emission mass flux of the harmful gases. Therefore, this study aims at investigating the effect of ceiling fans on the dairy cows, air velocity profiles, and the distribution of gaseous concentrations throughout a naturally ventilated dairy barn. Three air velocity measurements campaigns, with two repetitions each, were carried out during summer season 2010. The air velocity was measured inside the barn using ultrasonic anemometers. The air temperature and humidity were measured using temperature-humidity sensors. The climatic data were recorded by weather station. The concentrations of CO2, NH3, CH4, and N2O were measured using a multi-gas monitor. The heat stress was estimated by determining the Temperature-Humidity Index (THI). A thermal infrared imaging camera was used to investigate the heat relief from the cows and freestalls under 2 conditions, which were: ceiling fans on and ceiling fans off. The results showed that the implementation of ceiling fans reduced the THI from 79 (significant stress) to 68 (no stress). The average air velocities were 0.98 and 0.59 m s-1 as the ceiling fans were “on” and “off”, respectively. It was concluded that the ceiling fans have cooling effect, alleviate the heat stress, and enhance the air movement and distribution throughout the barn. However, the ceiling fans indirectly increase the ventilation rates in higher potential then the disguised decrease of the gaseous concentrations, which ultimately results in increasing the gaseous emissions. 1Assistant Professor, Depart. of Agric. Eng., Faculty of Agriculture, Cairo University, Egypt 2Research Scientist, Leibniz Institute for Agric. Eng. Potsdam-Bornim (ATB), Potsdam, Germany Therefore, a balance must be achieved among the different contradictions: air velocity optimization, heat stress alleviation, air distribution enhancement, and gaseous emissions reduction.

Unimpeded Air Velocity Profiles of an Air-Assisted Five-Port Sprayer

A capability that relies on tree structure information to control the flow rates of liquid and air is the preferential design in the development of variable-rate orchard and nursery sprayers. Unimpeded air jet velocities from an air-assisted, five-port sprayer in an open field were measured at four heights above ground, seven distances up to 3 m from the sprayer outlets, and five sprayer travel speeds from 0 to 8.0 km h-1. Air jet velocities were adjusted by changing the sprayer fan inlet diameter. Calculations of the air jet initial region length, transition length, and expansion angle from the five-port nozzles were computed with an air jet distribution model. The intersection between adjacent air jets from the five-port nozzles was determined from the air jet expansion angle. Air velocities were measured with a constant-temperature anemometer system coupled with hot-wire sensors. The air jets expanded at a 50 angle and intersected with adjacent air jets at 0.027 m from the five-port nozzles. When the sprayer was stationary (0 km h-1), axial air velocities from the nozzle outlets increased as fan inlet diameter increased and decreased as a hyperbolic function as the distance increased. Variations in the peak air velocities and airflow pressures with travel speeds of 3.2 to 8.0 km h-1 and heights of 0.2 to 2.0 m were statistically insignificant. When the sprayer was in motion and due to air entrainment and air jet disturbance, the peak air velocities decreased and airflow pressures increased as distance from the nozzle outlets increased. For all parameters tested, the peak air velocities and airflow pressure increased as fan inlet diameter increased, demonstrating that changing the fan inlet diameter achieved variable airflow rates with uniform air profiles.

Assessing effect of wind tunnel sizes on air velocity and concentration boundary layers and on ammonia emission estimation using computational fluid dynamics (CFD)

The objective of this study was to investigate the effect of different geometric sizes of wind tunnels on aerial boundary layers above the emission surface and therefore their effect on ammonia emission using CFD tool. Five wind tunnels of different sizes were used for the CFD simulation. Detail experimental measurements on air velocity and concentration profiles above the emission surface were realised using average inlet velocities of 0.1, 0.2, 0.3, and 0.4 m s −1 in two wind tunnels under isothermal condition. The TAN (Total Ammoniacal Nitrogen) and pH of ammonia aqueous solution were kept constant during the experiments. The boundary conditions necessary for the CFD study were obtained from the measured experimental data. The CFD model used for simulations was first validated using the data from the two wind tunnel experiments. For assessing the effects of wind tunnel size, the air velocity range of 0.1–0.6 m s −1 was used in CFD investigations. It was found that the velocity and concentration boundary layer thickness decreased with the increase of inlet air velocity where the concentration boundary was thinner than the corresponding velocity boundary layer. Wind tunnel sizes affected both velocity and concentration boundary layer thicknesses ( P < 0.001). The velocity and concentration boundary layer was thicker in a wind tunnel with larger heights than in small height. The over estimation of ammonia emission of smaller wind tunnels were observed comparing to the largest wind tunnel or open field situation ( P < 0.001). Non-linear regression equations were developed using four wind tunnels data for velocity boundary layer thickness ( δ u ), concentration boundary layer thickness ( δ c ), and ammonia mass transfer coefficient ( k G ) as a function of wind tunnel height ( H) and average inlet velocity ( u m ), which given clear quantitative indication of effects of H, and u m on δ u , δ c , and k G , respectively.