Non-uniform Airflow Research Articles

Three-dimensional airflow distribution in a maize silo with peaked, levelled and cored grain mass configurations

Non-uniform airflow models for peaked, levelled and cored grain mass configurations in a maize silo were developed using the finite volume method. Airflow resistance due to porous media of grain material was implemented using Ergun’s equation. The linear porosity variation was implemented with low porosity at the centre and maximum at the side. Non-uniform airflow distribution for natural air drying with an airflow rate of 1.1 m3 min−1 t−1 was validated with experimental data using maize in grain bins with storage capacities of 180.0–450.0 Mt. Air velocity was predicted with reasonable accuracy for cored, levelled and peaked grain mass configurations having porosities ranging from 0.34 in the core to 0.38 towards the silo wall. Incorporation of variable porosities into the model improved the air velocity prediction. For the cored grain mass and variable porosity (0.34–0.38), the error was 4.4% at the centre and 23.1% at the periphery.

Effect of the Moments of Probability Density Function for Non-uniform Air Flow Distribution on the Hydraulic Performance of a Fin-tube Heat Exchanger

Effect of the Moments of Probability Density Function for Non-uniform Air Flow Distribution on the Hydraulic Performance of a Fin-tube Heat Exchanger

Compensation of flow maldistribution in fin-and-tube evaporators for residential air-conditioning

Compensation of flow maldistribution in multi-channel fin-and-tube evaporators for residential air-conditioning is investigated by numerical modeling. The considered sources of maldistribution are distribution of the liquid and vapor phases in the distributor and non-uniform airflow distribution. Fin-and-tube heat exchangers usually have a predefined circuitry, however, the evaporator model is simplified to have straight tubes, in order to perform a generic investigation. The compensation of flow maldistribution is performed by control of the superheat in the individual channels. Furthermore, the effect of combinations of individual maldistribution sources is investigated for different evaporator sizes and outdoor temperatures. It is shown that a decrease in cooling capacity and coefficient of performance by flow maldistribution can be compensated by the control of individual channel superheat. Alternatively, a larger evaporator may be used.

Electrical and Thermal Performance of a Solid Oxide Fuel Cell Unit With Nonuniform Inlet Flow and High Fuel Utilization

This study investigates the electrical performance of a planar solid oxide fuel cell unit with cross-flow configuration when the fuel utilization gets higher and the fuel inlet flows are nonuniform. A numerical code, solving the two-dimensional, simultaneous, partial differential equations of mass, energy, and electrochemistry and neglecting the stack direction variation effect, is developed. The results show that the fuel utilization increases with a decrease in the molar flow rate, and the average current density decreases when the molar flow rate drops. In addition, nonuniform pattern A induces more severe happening of nonreaction area in the corner of the fuel exit and the air inlet. This nonreaction area deteriorates the average current density and then reduces the electrical performance to 7%. This study suggests that the fuel inlet manifold should be located far from the inlet of air, which is able to decrease the deterioration to below 3% when using nonuniform profile of pattern B. On the other hand, employing a suitable air flow rate, we can easily control the operating temperature of a solid oxide fuel cell unit and the effect of nonuniform inlet air flow rate on the temperature distribution becomes negligible.

Performance of residential air-conditioning systems with flow maldistribution in fin-and-tube evaporators

Refrigerant and airflow maldistribution in fin-and-tube evaporators for residential air-conditioning was investigated with numerical modeling. Fin-and-tube heat exchangers usually have a pre-defined circuitry. However, the objective in this study was to perform a generic investigation of each individual maldistribution source in an independent manner. Therefore, the evaporator and the condenser were simplified to be straight tubes for the purposes of this study. The numerical model of the R410A system, its verification and an investigation of individual maldistribution sources are presented in this paper. The maldistribution sources of interest were: inlet liquid/vapor phase distribution, feeder tube bending and airflow distribution. The results show that maldistribution reduced the cooling capacity and the coefficient of performance of the system. In particular, different phase distribution and non-uniform airflow distribution reduced the performance significantly. Different feeder tube bendings only caused a minor decrease in performance.

An Investigation of Bread‐Baking Process in a Pilot‐Scale Electrical Heating Oven Using Computational Fluid Dynamics

A computational fluid dynamics (CFD) model was developed for bread-baking process in a pilot-scale baking oven to find out the effect of hot air distribution and placement of bread on temperature and starch gelatinization index of bread. In this study, product (bread) simulation was carried out with different placements of bread. Simulation results were validated with experimental measurements of bread temperature. This study showed that nonuniform air flow pattern inside the oven cavity leads to uneven temperature distribution. The study with respect to placement of bread showed that baking of bread in upper trays required shorter baking time and gelatinization index compared to those in the bottom tray. The upper tray bread center reached 100 °C at 1200 s, whereas starch gelatinization completed within 900 s, which was the minimum baking index. Moreover, the heat penetration and starch gelatinization were higher along the sides of the bread as compared to the top and bottom portions of the bread.

Modelling distribution of evaporating CO 2 in parallel minichannels

The effects of airflow non-uniformity and uneven inlet qualities on the performance of a minichannel evaporator with parallel channels, using CO 2 as refrigerant, are investigated numerically. For this purpose a one-dimensional discretised steady-state model was developed, applying well-known empirical correlations for calculating frictional pressure drop and heat transfer coefficients. An investigation of different correlations for boiling two-phase flow shows that the choice of correlation is insignificant regarding the overall results. It is shown that non-uniform airflow leads to maldistribution of the refrigerant and considerable capacity reduction of the evaporator. Uneven inlet qualities to the different channels show only minor effects on the refrigerant distribution and evaporator capacity as long as the channels are vertically oriented with CO 2 flowing upwards. For horizontal channels capacity reductions are found for both non-uniform airflow and uneven inlet qualities. For horizontal minichannels the results are very similar to those obtained using R134a as refrigerant.

CFD modeling of MPS coal mill with moisture evaporation

Coal pulverizers play an important role in the functioning and performance of a PC-fired boiler. The main functions of a pulverizer are crushing, drying and separating the fine coal particles toward combustion in the furnace. It is a common experience that mill outlet pipes have unequal coal flow in each pipe and contain some coarse particles. Unequal coal flow translates into unequal air-to-fuel ratio in the burner, deviating from the design value and thus increasing unburned carbon in fly ash, NO x and CO. Coarser particles at the mill outlet originate from poor separation and decrease the unit efficiency. In addition, coarser particles reduce burner stability at low load. Air flow distribution at the mill throat, as well as inside the mill, significantly influences the mill performance in terms of separation, drying, coal/air flow uniformity at the mill outlet, wear patterns and mill safety. In the present work, a three-dimensional computational fluid dynamics (CFD) model of the MPS Roll Wheel pulverizer at Alliant Energy's Edgewater Unit 5 has been developed. The Eulerian–Lagrangian simulation approach in conjunction with the coal drying model in Fluent, a commercial CFD software package, has been used to conduct the simulation. Coal drying not only changes the primary air temperature but it also increases the primary air flow rate due to mass transfer from coal. Results of the simulation showed that a non-uniform airflow distribution near the throat contributes significantly to non-uniform air–coal flow at the outlet. It was shown that uniform velocity at the throat improves the air and coal flow distribution at the outlet pipes. A newly developed coal mill model provides a valuable tool that can be used to improve the pulverizer design and optimize unit operation. For example, reject coal rate, which is controlled by the air flow near the mill throat, can be reduced. The model can also be used to further aid in identifying and reducing high temperature or coal-rich areas where mill fires are most likely to start.

Modelling refrigerant distribution in microchannel evaporators

The effects of refrigerant maldistribution in parallel evaporator channels on the heat exchanger performance are investigated numerically. For this purpose a 1D steady state model of refrigerant R134a evaporating in a microchannel tube is built and validated against other evaporator models. A study of the refrigerant distribution is carried out for two channels in parallel and for two different cases. In the first case maldistribution of the inlet quality into the channels is considered, and in the second case a non-uniform airflow on the secondary side is considered. In both cases the total mixed superheat out of the evaporator is kept constant. It is shown that the cooling capacity of the evaporator is reduced significantly, both in the case of unevenly distributed inlet quality and for the case of non-uniform airflow on the outside of the channels.

A hybrid method for refrigerant flow balancing in multi-circuit evaporators: Upstream versus downstream flow control

A hybrid method for optimizing refrigerant distribution in evaporators is presented that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. The flow balancing valves could be located upstream or downstream of the evaporator. This paper presents the results of a study to investigate the benefits of this hybrid scheme for both upstream and downstream flow balancing for the case of air flow mal-distribution. In order to perform this investigation, a simulation model was developed to consider evaporator flow mal-distributions for a 10.55 kW residential R410A heat pump and then validated through comparisons of predicted results with measurements. Simulation results show that there are significant benefits in controlling the superheat of each circuit of evaporators through the hybrid–individual superheat control method. Furthermore, the upstream refrigerant flow control consistently outperforms the downstream refrigerant flow control, and recovers most of the loss in cooling capacity and COP due to non-uniform air flow distribution.

Numerical Simulation of the Performance Characteristics of the Hybrid Closed Circuit Cooling Tower

The performance characteristics of the Hybrid Closed Circuit Cooling Tower (HCCCT) have been investigated applying computational fluid dynamics (CFD). Widely reported CFD techniques are applied to simulate the air-water two phase flow inside the HCCCT. The pressure drop and the cooling capacity were investigated from several perspectives. Three different transverse pitches were tested and found that a pitch of 45 mm had lower pressure drop. The CFD simulation indicated that when air is supplied from the side wall of the HCCCT, the pressure drop can be over predicted and the cooling capacity can be under predicted mainly due to the non-uniform air flow distribution across the coil bank. The cooling capacity in wet mode have been calculated with respect to wet-bulb temperature (WBT) and cooling water to air mass flow rates for different spray water volume flow rates and the results were compared to the experimental measurement and found to conform well for the air supply from the bottom end. The differences of the cooling capacity and pressure drop in between the CFD simulation and experimental measurement in hybrid mode were less than 5 % and 7 % respectively for the uniform air flow distribution.

Impact of Occupant Modelling on the Prediction of Airflow around Occupants in a Ventilated Room

Localized ventilation systems typically create highly asymmetric or non-isothermal environments around occupants with significant vertical temperature gradient and highly non-uniform airflow regimes that could be directed toward a segment of the body. These effects may have pronounced impact on occupant’s thermal comfort. The airflow field and temperature distribution near the occupant can be determined either by performing full-scale measurements or by simulation methods. Usually, human subjects or manikins are used in field studies involving measurement techniques. However, as an alternative to full-scale measurement, Computational Fluid Dynamics (CFD) has been proven to be a practical and valuable tool for predicting the airflow field. At the same time, the accuracy of the predictions of the local airflow within the microclimate of the occupant is highly dependent on the proper modelling of the occupant itself. The human body not only has a complicated physical shape, but also has complex thermo-physiological properties. Modelling of all these aspects is a formidable challenge and an extremely time-consuming task. Therefore, various simplifications have been made in order to decrease the level of complexity so that the computation may be performed with the available computer resources.This paper reports the results of a detail numerical simulation to study the impact of occupant modelling on the airflow and temperature distribution and their influences on the occupant’s thermal comfort. First, the predictions made by the CFD model were compared with experimental data that were measured in a specially designed experimental chamber. Good agreement was observed. Four type of configuration were used to model the occupant: a conventional block form, three-node, six-node and finally eight-node configurations. Further simulations were carried out to investigate the assumption of uniform heat distribution. An assessment of uniform and non-uniform heat distribution scenarios for various occupant configurations and ventilation systems showed that the assumption of uniform heat distribution is valid for a wide range of operating conditions.

Evaluation of Thermal Comfort in Slightly Warm Ventilated Spaces in Nonuniform Environments

The present work analyzes and evaluates the global thermal comfort and local thermal discomfort levels of an occupant subjected to a symmetric nonuniform airflow, originated in common use ventilators. Several incident airflow directions are studied and their effects are described. The global thermal comfort level is evaluated through a multi-nodal numerical model that simulates human and clothing thermal responses, while the local thermal discomfort level is analyzed using an empirical model that predicts draft risks. The computational model of the human body and clothing thermal systems is based on the energy balance integral equations for human body tissue, blood, and clothing, as well as mass balance integral equations for the blood and transpired water in skin surface and the clothing. The human body is divided into 35 elements, each one in several layers of tissue, which could be protected through some clothing layers. A thermoregulatory system model was adapted to control the human body tissue temperature. The experimental tests were carried out in a test chamber in controlled environmental conditions; a thermal manikin was used to simulate the human posture, an indoor climate analyzer was used to measure the environmental variables around the occupant, and two ventilators were used to produce an airflow field around the occupant. The frontal and ascendant airflows from the ventilators placed in front of the occupant are characterized and their velocities around the occupant are measured for several incident angles. The global thermal comfort conditions of the occupant are evaluated both with and without ventilation, and the local thermal discomfort level is evaluated with ventilation for slightly warm, moderate environments.

An analytical study on heat transfer performance of radiators with non-uniform airflow distribution

Heat exchangers used in modern automobiles usually have a highly non-uniform air velocity distribution because of the complexity of the engine compartment and underhood flow fields; hence ineffective use of the core area has been noted. To adequately predict the heat transfer performance in typical car radiators, a generalized analytical model accounting for airflow maldistribution was developed using a finite element approach and applying appropriate heat transfer equations including the ε-NTU (effectiveness - number of heat transfer units) method with the Davenport correlation for the air-side heat transfer coefficient. The analytical results were verified against a set of experimental data from nine radiators tested in a wind tunnel and were found to be within +24 and −10 per cent of the experimental results. By applying the analytical model, several severe non-uniform velocity distributions were also studied. It was found that the loss of radiator performance caused by airflow maldistribution, compared with uniform airflow of the same total flowrate, was relatively minor except under extreme circumstances where the non-uniformity factor was larger than 0.5. The relatively simple set of equations presented in this paper can be used independently in spreadsheets or in conjunction with computational fluid dynamics (CFD) analysis, enabling a full numerical prediction of aerodynamic as well as thermodynamic performance of radiators to be conducted prior to a prototype being built.

A Numerical Analysis of the Local Anomalies in a Natural-Draft Cooling Tower

This paper presents experimental and numerical analyses of the aero-thermodynamic characteristics of a natural-draft cooling tower. The influence of local technical faults in the fill and rain region on the cooling tower's performance was estimated. Measurements of the air velocity and the temperature above the droplet eliminators showed a noticeable non-uniformity of both parameters. This is caused by a non-uniform airflow resistance and heat-transfer rate within the fill and rain region. Based on these measurements, a commercial CFD model was customized with additional relations describing the heat- and mass-transfer, as well as the airflow resistance in individual regions of the cooling tower. The results of a 3D numerical simulation of the cooling tower are the temperature and velocity distributions within the entire cooling tower. A comparison of nominal and actual cooling tower operation shows regions with unfavorable air temperatures or velocities. Thus, the inefficiently operating areas of the cooling tower's cross-section can be identified. These areas cause non-homogeneous aero-thermodynamic characteristics and have an influence on the integral characteristics of the cooling tower. A sample calculation of an actual cooling tower shows the usefulness of the method when it comes to improving the cooling tower's performance. The improvement can be achieved by modifying the fill resistance and water distribution.

Investigation of non-uniform airflow signal oscillation during high frequency chest compression

BackgroundHigh frequency chest compression (HFCC) is a useful and popular therapy for clearing bronchial airways of excessive or thicker mucus. Our observation of respiratory airflow of a subject during use of HFCC showed the airflow oscillation by HFCC was strongly influenced by the nonlinearity of the respiratory system. We used a computational model-based approach to analyse the respiratory airflow during use of HFCC.MethodsThe computational model, which is based on previous physiological studies and represented by an electrical circuit analogue, was used for simulation of in vivo protocol that shows the nonlinearity of the respiratory system. Besides, airflow was measured during use of HFCC. We compared the simulation results to either the measured data or the previous research, to understand and explain the observations.Results and discussionWe could observe two important phenomena during respiration pertaining to the airflow signal oscillation generated by HFCC. The amplitudes of HFCC airflow signals varied depending on spontaneous airflow signals. We used the simulation results to investigate how the nonlinearity of airway resistance, lung capacitance, and inertance of air characterized the respiratory airflow. The simulation results indicated that lung capacitance or the inertance of air is also not a factor in the non-uniformity of HFCC airflow signals. Although not perfect, our circuit analogue model allows us to effectively simulate the nonlinear characteristics of the respiratory system.ConclusionWe found that the amplitudes of HFCC airflow signals behave as a function of spontaneous airflow signals. This is due to the nonlinearity of the respiratory system, particularly variations in airway resistance.

An Optimized Design of Finned-Tube Evaporators Using the Learnable Evolution Model

Optimizing the refrigerant circuitry for a finned-tube evaporator is a daunting task for traditional exhaustive search techniques due to the extremely large number of circuitry possibilities. For this reason, more intelligent search techniques are needed. This paper presents and evaluates a novel optimization system called ISHED1 (intelligent system for heat exchanger design). This system uses a recently developed non-Darwinian evolutionary computation method to seek evaporator circuit designs that maximize the capacity of the evaporator under given technical and environmental constraints. Circuitries were developed for an evaporator with three depth rows of 12 tubes each, based on optimizing the performance with uniform and nonuniform airflow profiles. ISHED1 demonstrated the capability to generate designs with capacity equal or superior to that of best human designs, particularly in cases with non-uniform airflow.

AIRFLOW DISTRIBUTIONS IN THE FILLET SPACES OF A TIMBER STACK

An important requirement for producing evenly dried timber (lumber) in a kiln is that the airflow is uniformly distributed throughout the timber stack. The airflow distribution within the timber stack of a semi-scale kiln, which has multiple fans, was investigated to determine the possible causes of any airflow non-uniformity. Airflow measurements along the length of the timber stack illustrated the importance of ensuring that each fan generates the same flowrate, and that end-baffles are placed on either side of the timber stack to reduce bypass around these edges. Airflow measurements down the height of the timber stack demonstrated the effects of a separation zone near the air-inlet face of the stack, which partially reduced the airflow through the upper portion of the stack, and of non-aligned boards, which generated a saw-toothed velocity profile. Once the data had been smoothed to account for these effects, the down-stack velocity profile took on the expected form, with the maximum and minimum velocities at the top and bottom of the stack, respectively. Comparison of the velocity profiles at the air-inlet and outlet faces of the stack indicated a significant redistribution of the flow through the stack. The prediction of a one-dimensional model of airflow in a kiln was shown to be in good agreement with the smoothed experimental data at the air outlet of the stack.

A 3-dimensional nonhydrostatic dispersion modeling system for modeling of atmospheric transport and diffusion over coastal complex terrain in the Hongkong-Shenzhen area

A dispersion modeling system consisting of a three-dimensional PBL model NHECM (non-hydrostaticE-e closure model) and SLPTDM (seven-level puff transport and diffusion model) is developed to simulate the transport and dispersion of pollutant over coastal complex terrain. As an application of the system, the transport and dispersion of SO2 released from an elevated point source are simulated during typical sea-land breeze circulation in the Hongkong-Shenzhen area of China. The NHECM provides time-varying, three-dimensional distributions of wind and turbulence fields to the SLPTDM. The NHECM predictions compare well with observational data. Reflection of both the ground and the mixing layer top and penetration of the mixing layer top are improved in the SLPTDM. Results obtained from the system indicate that temporal variation and nonuniformity of airflow and turbulence obviously affect the concentration distributions, especially during the sea-land breeze transition period. A diurnal cycle of the GLC (ground-level concentration) is discussed. The results are compared with those estimated using a Gaussian model. The study's results illustrate the complexity of the dispersion patterns due to diurnal effects and mesoscale circulations, and demonstrate the potential of the mesoscale atmospheric dispersion modeling system for studies of air quality in complex terrain.

Sensitivity analysis of a mathematical model to simulate aeration of wheat stored in Brazil

Abstract The research objective was to determine the relative importance of several variables in a non-equilibrium mathematical model for simulating aeration of stored wheat with uniform and non-uniform airflow distributions. The relative importance of each variable was determined by adding or subtracting fixed uncertainties from that variable and by calculating the effect of these changes on the predicted grain deterioration, after 1 year of storage. Results of simulations using weather data from Curitiba, Brazil, ventilation time of 06:00–12:00, fan temperature rises of 1 and 3 °C, and linear airflows of 0.0056 and 0.0278 m3/s per m2 indicated that the most important variables, in decreasing order, were the fan temperature rise, thin-layer wetting equation, and thin-layer drying equation. Wheat bulk density can be a constant and the net heat of sorption can be neglected. The ratio of bin diameter to bin height and the accuracy of the equations to describe wheat specific heat, air resistance to airflow, and equilibrium moisture content for adsorption and desorption were not important in the mathematical model. The deterioration model must be improved because the uncertainty in the calculation of wheat deterioration was much higher than the uncertainty generated by many of the other variables for the Brazilian climate.