Airflow Profile Research Articles

Stochastic modelling of sandstorms affecting the optimal operation and cleaning scheduling of air coolers in concentrated solar power plants

The operation performance of air-coolers in concentrated solar power plants decays due to particulate deposition on heat transfer surfaces. The deposition process can be seen as a stochastic phenomenon. A modelling approach is proposed to capture the uncertainty and the effect of extreme events, such as sandstorms, affecting the performance of plants located in dry places through dust or sand deposition on the air coolers. A case study of a concentrated solar power plant located in Dubai is analysed. Sandstorms generate acute and drastic fouling of the air coolers, and this is modelled as a stochastic process using historical aerosol dispersion data. Ten scenarios are generated by sampling the probability distribution of sandstorms occurrence and intensity. The optimal operation (cleaning schedule and airflow profiles) of the air coolers is established using Benders decomposition to solve the resulting large-scale mixed integer non-linear programming problem. The results of the stochastic scenarios demonstrate that substantial savings of $ 0.6 M − $ 2.7 M per year are achieved by the optimal operation. Cost is minimized by a combined reactive and proactive cleaning policy which accounts for the frequency, intensity and seasonal variability of sandstorms, in addition to the variability on local radiation and weather conditions.

Analysis of the energy performance of an Opaque Ventilated Façade under winter and summer weather conditions

Nowadays, there is continuing worrying about energy efficiency and the reduction of GHG emissions in the building sector. It has been claimed that ventilated building envelopes help to reduce energy use in buildings and improve occupant comfort.This study proposes a comprehensive comparison of the thermal behaviour between an Opaque Ventilated Façade (OVF) and a conventional unventilated Façade (UF) considering two reference days for the winter and summer period. The analysis is developed investigating different façade orientations and two states of windiness, which are a state of calm wind and a state with wind velocity higher than zero (i.e. 5.0 m/s at 10 m of height) are taken into account.These analyses were developed utilizing fluid-dynamic calculation under dynamic conditions. Thus for the two façades were calculated: (I) the hourly surface temperatures of the most external, (II) the temperature profiles for all the facade’s layers; (III) the airflow profiles inside the cavity and near the façade; (IV) the hourly thermal fluxes that cross the façade.Finally, the daily energy fluxes and the energy-saving, achievable through the adoption of the OVF, is calculated for the different façade exposures and the conditions of windiness.The outcomes of this study highlight that the OVF guarantees an energy-saving ranging from 20 to 55%, with the highest rate during the summer day for the façade facing East/West.

Simulation of patient-specific bi-directional pulsating nasal aerosol dispersion and deposition with clockwise 45° and 90° nosepieces

Numerical simulations of the dispersion and deposition of poly-disperse particles in a patient-specific human nasal configuration are performed. Computed tomography (CT) images are used to create a realistic configuration of the nasal cavity and paranasal sinuses. The OpenFOAM software is used to perform unsteady Large Eddy Simulations (LES) with the dynamic sub-grid scale Smagorinsky model. For the numerical analysis of the particle motion, a Lagrangian particle tracking method is implemented. Two different nosepieces with clockwise inclinations of 45° and 90° with respect to the horizontal axis are connected to the nostrils. A sinusoidal pulsating airflow profile with a frequency of 45 Hz is imposed on the airflow which carries the particles. Flow partition analysis inside the sinuses show that ventilation of the sinuses is improved slightly when the 45° nosepiece is used instead of the 90° nosepiece. The flow partition into the right maxillary is improved from 0.22% to 0.25%. It is observed that a closed soft palate increases the aerosol deposition efficiency (DE) in the nasal cavity as compared to an open soft palate condition. The utilization of pulsating inflow leads to more uniform deposition pattern in the nasal airway and enhances the DE by 160% and 44.6%, respectively, for the cases with clockwise 45° and 90° nosepieces, respectively. The bi-directional pulsating drug delivery with the same particle size distribution and inflow rates as the PARI SINUS device results in higher total DEs with 45° nosepiece than with the 90°. Thus, the numerical simulation suggests that the 45° nosepiece is favorable in terms of the delivered dose.

CFD performance analysis of finned-tube CO2 gas coolers with various inlet air flow patterns

A detailed model of three-dimensional computational fluid dynamics (CFD) on a finned-tube CO2 gas cooler has been developed and validated. The model is then applied to investigate the effect of uniform and mal-distribution inlet airflow profiles on the coil performance. The airflow mal-distribution velocity profiles include linear-up, linear-down and parabolic while the effected coil performance parameters contain airside pressure drop, average airside heat transfer coefficient, approach temperature and coil heating capacity. The model also enables to predict the CO2 refrigerant temperature profile along the coil pipes from refrigerant inlet to outlet at different operation conditions. The simulation results reveal that different types of inlet airflow velocity profiles have significant effects on the gas cooler performance. The uniform airflow velocity profile case shows the best thermal performance of gas cooler. Compared with the cases of linear-up and parabolic air velocity profiles, the linear-down airflow profile can influence more on the coil heat transfer performance. Due to the thermal conduction between neighbour tubes through coil fins, reversed heat transfer phenomenon exists which can be detected and simulated by the CFD model. It is predicted that the linear-down airflow profile can increase greatly the reversed heat transfer phenomenon and thus lead to the highest approach temperature and the lowest heating capacity amongst these four types of airflow profiles. The research method and outcomes presented in this paper can have great potentials to optimize the performance of a CO2 gas cooler and its associated refrigeration system.

Effects of Airflow Profile and Condensation Pressure on Performance of Air-Cooled Condensers

The effect of airflow profile and condensation pressure on the performance of air-cooled condensers is investigated experimentally. Two large, flattened-tube air-cooled steam condensers are studied. The tube lengths are 10.7 and 5.7 m, with inner dimensions of 216 × 16 mm and aluminum fins on each side of the elongated-slot cross sections. Capacity and pressure drop are measured and discussed here. All tests are performed with a horizontal tube and co-current vapor and condensate flow. Four different profiles of cross-flowing air are tested: uniform air flowing upwards, uniform air flowing downwards, and two profiles of non-uniform air flowing upwards. For the 10.7 m tube, reversing airflow direction from upwards to downwards is found to significantly increase condenser capacity. Also for the 10.7 m tube, a favorable non-uniform air-velocity profile is shown to increase capacity in comparison to a uniform air-velocity profile. Both of these performance increases are shown to be the result of matching regions of maximum heat transfer coefficient on the air and steam sides. For the 5.7 m tube, a non-uniform airflow profile is shown to have no effect on capacity. Reducing condensation pressure is shown to decrease condenser capacity for both condenser tubes.

Impact of airflow on the heat transfer conditions inside an oven cavity, characterized using particle imaging velocimetry

The complex airflow in convection ovens directly influences the heat transferred to the product placed inside, thereby affecting product quality. Characterization of related airflow profiles can provide scientific understanding for improvement of oven designs as well as important parameters for simulation of involved thermal processes. In this study, the particle-imaging velocimetry (PIV) technique was applied to visualize airflow inside a household convection oven with samples placed at three different locations on a baking tray. The oven cavity was modified for optical access, and airflow was measured at room temperature. A 30 mW green laser was used for illuminating tracer particles in a laser sheet that were generated using incense sticks. The flow patterns were captured using a high-speed camera at 1000 fps. The vorticity and turbulent kinetic energy parameters derived from velocity fields reflected adequate mixing of air inside the cavity. The computed heat transfer coefficient distribution from the boundary layer flow fields to the sample surface ranged between 2.0 and 18.3 W m−2 K−1. The results showed separation of the laminar boundary layer from the object surface at angles of 85°–90°. The PIV-algorithms and boundary layer flow derived parameters developed in this study can be used for refined characterization of complex air or gas flows and related heat transfer characteristics in closed cavity convection ovens and the like arrangements.

A grid convergence index study of mesh style effect on the accuracy of the numerical results for an indoor airflow profile

The Grid Convergence Index () method requires at least three systematic mesh refinements, each requiring the numerical solution to be in the asymptotic range. When the observed order of accuracy differs from the formal order of accuracy, a lack of numerical precision exists which may result in erroneous conclusions. The aim of the work in this paper is to evaluate the effect the mesh resolution for structured hexahedral, unstructured hexahedral and tetrahedral mesh has on the observed order of accuracy and the accuracy of the numerical solution of the mean air flow velocity profile in indoor environments. The value was calculated based on the recommendation of the lower limit of the observed order of accuracy. For the structured hexahedral mesh, with successive grid refinements the observed order of accuracy converges close to the formal order of accuracy, while for the two other unstructured meshes it converges more gradually. The lowest value was obtained when the observed order of accuracy has reached its highest level of accuracy. In this study the turbulence model was adopted, and when the obtained numerical results were compared with another published numerical study, the comparison showed that the structured hexahedral mesh with turbulence model produced a similar result to that produced from unstructured hexahedral mesh using the turbulence model.

Experimental and numerical studies on an OWC axial-flow impulse turbine in reciprocating air flows

In this study, an experimental facility and a transient numerical model are established to predict the complicated unsteady behaviors of oscillating water column air turbines under reciprocating air-flow conditions, which are closer to the actual sea states. There are a total of 256 cases in the experimental test. Three starting modes for the axial impulse turbine in symmetrical reciprocating air flows are identified, and over 93% of the cases are found to be self-starting cases. The peak cycle-averaged turbine efficiency is 0.53, which is larger than that observed in the steady-state tests. Compared to the efficiencies derived from the steady-state tests, the turbine has the possibility to achieve a better performance in the reciprocating air flows even under the same pneumatic power input. The numerical model is compared and validated by the experimental results. Effects of constant load on turbine performance are investigated numerically. An irregular air-flow profile based on the local sea-state is generated. The turbine self-starting processes under various moments of inertia are simulated numerically, and the angular velocities, torque outputs and the pneumatic powers are predicted and compared.

A radial inflow air turbine design for a vented oscillating water column

The oscillating-water-column (OWC) is a ubiquitous style of wave-energy-converter for harnessing significant power from ocean waves. A new configuration of OWC described here as a vented-OWC provides opportunity to simplify air-turbine design by limiting the power extraction to inhalation mode. The asymmetric airflow profile of the vented-OWC requires a unidirectional-turbine configuration to be adopted.The focus of this study is to analyse a unidirectional radial air-inflow-turbine design using Computational-Fluid-Dynamics (CFD) suitable for application with a vented-OWC. It is found that downstream of the rotor can cause significant energy losses due to its narrow flow passage. Two configurations of an inward-flow radial turbine were also compared, the first by keeping the casing height constant throughout the turbine domain and the second by increasing the casing height in a way to keep a constant sectional area from inlet to the outlet of the turbine domain. The latter configuration obtained a ten percent gain in peak efficiency compared to the first. The difference is identified as fewer energy losses at the downstream section and comparably higher torque for the same flow-coefficient. Introduction of downstream-guide-vanes was found to retard the chocking phenomenon, which offers a wider operational range for the radial-inflow design.

Classification of Tidal Breathing Airflow Profiles Using Statistical Hierarchal Cluster Analysis in Idiopathic Pulmonary Fibrosis.

In idiopathic pulmonary fibrosis (IPF) breathing pattern changes with disease progress. This study aims to determine if unsupervised hierarchal cluster analysis (HCA) can be used to define airflow profile differences in people with and without IPF. This was tested using 31 patients with IPF and 17 matched healthy controls, all of whom had their lung function assessed using spirometry and carbon monoxide CO transfer. A resting tidal breathing (RTB) trace of two minutes duration was collected at the same time. A Euclidian distance technique was used to perform HCA on the airflow data. Four distinct clusters were found, with the majority (18 of 21, 86%) of the severest IPF participants (Stage 2 and 3) being in two clusters. The participants in these clusters exhibited a distinct minute ventilation (p < 0.05), compared to the other two clusters. The respiratory drive was greatest in Cluster 1, which contained many of the IPF participants. Unstructured HCA was successful in recognising different airflow profiles, clustering according to differences in flow rather than time. HCA showed that there is an overlap in tidal airflow profiles between healthy RTB and those with IPF. The further application of HCA in recognising other respiratory disease is discussed.

Detailed experimental investigation of air speed field induced by ceiling fans

Comfort cooling by ceiling fans is cost-effective and energy-efficient compared to compressor-based cooling and fans are commonly used in tropical and subtropical countries. There are however limited data and design tools supporting the design of fan systems, especially for situations where there are multiple fans. In this paper, we investigate airflow profiles induced by a single fan and multiple fans using high spatial resolution air speed measurements (5,760 and 20,160 measuring points for the two cases respectively) in a climatic chamber. To authors' knowledge, this is the first time that interaction between multiple fans has been reported. We developed typical airflow patterns from the measurements and further validated them via smoke visualization. The single-fan results are consistent with previous studies of this configuration, providing additional refinements. For the multiple-fan case, both the difference of fan speed levels and the distance between the fans affect the interacting airflow profiles of the fans in complex ways. When fans are close enough, the combined air speed profile cannot be extrapolated from the profile of a single fan. All the measurement results are open sourced: raw data is included in the supplementary materials and can be visualized via an online platform (single fan, multiple fans). The data and the tool can be used to validate CFD models and inform fan layout design.

Computational fluid dynamics (CFD) based analysis of the aerodynamic and thermodynamic performances of package designs during cooling of stacked pomegranates

The demand for pomegranate fruit is increasing due to the extensive knowledge acquired on the health benefits of pomegranate and the increased public awareness about functional food. Following this, there has been an increased interest in research to improve storability. A computational fluid dynamics (CFD) model was developed, validated and used to analyse the cooling characteristics of two different package designs used for the postharvest handling of pomegranate fruit. The model incorporated the geometries of the fruits, packaging box, tray and plastic liner. A thin layer of plastic material with conservative interface heat flux was used to model the liners. The accuracy of the model to predict the airflow and temperature distributions were validated against experimental data. The model predicted the airflow through the stacks and cooling rates within experimental error. The stack design markedly affected the airflow profile, rate and uniformity of cooling. The cooling rate of the two package designs differed by 30% and the plastic lining increased the average 7/8th cooling times significantly. The profile of the high and low temperature regions depended considerably on the packaging box design.

A CFD assisted segmented control volume based heat exchanger model for simulation of air-to-refrigerant heat exchanger with air flow mal-distribution

Air flow mal-distribution has been identified as one of the causes for performance degradation of air-to-refrigerant heat exchangers. The accurate prediction of such a non-uniform air flow profile requires full heat exchanger simulations using Computational Fluid Dynamics (CFD) models which are often much more computationally expensive than traditional lumped parameters or finite volume based heat exchanger models. The high computational cost of CFD makes it difficult to comprehensively study the effect of multiple parameters impacting the air flow distribution and the heat exchanger performance. In this study, an integrated CFD-segmented heat exchanger model based on a momentum resistance model is developed that is computationally efficient and accurate. The CFD simulation process is fully automated using script based open-source CFD code, OpenFOAM. The momentum resistance-based CFD model is validated against Particle Image Velocimetry (PIV) data and parametric studies are performed to investigate the influence of heat exchanger geometries (HX angle, HX depth, HX height and fin type) and air flow rate on the air flow mal-distribution under dry operating condition. In addition, an approach to scale the velocity profiles using multivariate linear interpolation is proposed. The interpolation model can further accelerate the simulation speed without compromising the accuracy compared to the CFD model.

Variable air assistance system for orchard sprayers; concept, design and preliminary testing

Conventional air-assisted sprayers with hydraulic nozzles generate large radial spray plumes which produce significant off-target losses and can consume high power (20–30 kW). Also, it is not uncommon to see these machines treating dwarf orchards, where the spray losses at the full leaf stage may be over 80% of applied spray volume. Significant reductions in spray losses can be obtained with targeted and wind-oriented airflow adjustment. The objectives of the presented studies were to develop an energy saving variable air assistance (VAA) system with continuous real-time adjustment of air volume and with spraying systems mounted on both sides of the sprayer. The system is based on a double axial fan system which allows for remote adjustment of air volume. The nominal air output was 20,000 m3 h−1, for use in typical dwarf and semi-dwarf orchards, and this was designed to be obtained with 10 kW power consumption. The system used variable speed impellers with fixed blades which showed greater suitability than a method with adjustable pitch blades working at constant speed because it provided a wider range of air volumes (±35%). The air volumes produced could be continuously adjusted to obtain airflow profiles, on both sides of the sprayer that were almost symmetrical. The results obtained in tests appear to meet the objectives and, therefore, the VAA system can be considered as a suitable prototype platform for variable rate technology and future intelligent orchard sprayers.

Ceiling fan air speeds around desks and office partitions

Ceiling fans may cool room occupants very efficiently, but the air speeds experienced in the occupied zone are inherently non-uniform. Designers should be aware of several generic flow patterns when positioning ceiling fans in a room. Key to these are the fan jet itself and lateral spreading near the floor. Adding workstation furniture redirects the jet's airflow laterally in a deeper spreading zone, making room air flows more complex but potentially increasing the cooling experienced by the occupants.This paper presents the first evaluation of the effects of tables and workstation partitions on a room's generic air flow and comfort profiles. In a test room with a ceiling fan, we moved five anemometers mounted in a “tree” at heights of 0.1, 0.6, 0.75, 1.1, and 1.7 m to sample a dense measurement grid of 7 rows and 6 columns. We tested five different table and partition configurations and compared them to the empty room base case. From the results we propose a simplified model of room airflow under ceiling fans, useful for positioning fans and workstation furniture. We also present comfort contours measured in two ways that have comfort standards implications. The measured data are publicly available on the internet.

Two-way coupled Eulerian-Eulerian simulations of drifting snow with viscous treatment of the snow phase

A novel two-way coupled Eulerian-Eulerian CFD formulation is presented, to simulate drifting snow based on a new viscous model of the snow phase. This approach allows explicit resolution of the saltation layer without resorting to empiricism, unlike other Eulerian-Eulerian approaches based on mixture formulations and one-way coupling. Initial validations carried out against detailed snowflux, airflow, and turbulence measurements in a controlled drifting snow wind tunnel experiment show the present approach is capable of accurately predicting snowflux, airflow profiles, as well as snow transport under developing and nearly developed flow conditions, when polydispersity is taken into account.

A Physical Interpretation of the Wind-Wave Instability as Interacting Waves

AbstractOne mechanism for the growth of ocean surface waves by wind is through a shear instability that was first described by Miles in 1957. A physical interpretation of this wind-wave instability is provided in terms of the interaction of the surface gravity wave with perturbations of vorticity within the critical layer—a near-singularity in the airflow where the background flow speed matches that of the surface gravity wave. This physical interpretation relies on the fact that the vertical velocity field is slowly varying across the critical layer, whereas both the displacement and vorticity fields vary rapidly. Realizing this allows for the construction of a physically intuitive description of the critical layer vorticity perturbations that may be approximated by a simple vortex sheet model, the essence of the wind-wave instability can then be captured through the interaction of the critical layer vorticity with the surface gravity wave. This simple model is then extended to account for vorticity perturbations in the airflow profile outside of the critical layer and is found to lead to an exact description of the linear stability problem that is also computationally efficient. The interpretation allows, in general, for the incorporation of sheared critical layers into the “wave interaction theory” that is commonly used to provide a physical description and rationalization of results in the stability of stratified shear flows.

Experimental and numerical study on air flow uniformity in the isobaric windbox of a 600 MW supercritical CFB boiler

Airflow velocity profile of the primary air in the isobaric windbox of the first 600MW supercritical CFB boiler worldwide under cold state and various boiler loads was experimentally measured and numerically simulated to investigate uniformity of the primary air. Experimental measurements were conducted in the furnace under different boiler loads (100%, 85% and 70% BMCR for the right windbox; 100% BMCR for the left windbox) by a hot wire anemometer. Meanwhile, the effect of key structural parameters (including the pressure-stabilization section height W1, inlet area ratio AR, inclined angle θ and windbox depth) on the airflow uniformity of the isobaric windbox were analyzed based on two-dimension simulations of a series of isobaric windboxes with 43 different configurations, using the CFD software package FLUENT. The measured results showed that while the airflow velocity was basically uniform along the width direction of the windbox, it exhibited an obvious non-uniform profile along the depth direction. The airflow velocity was relatively higher in the front wall and obvious lower in the back wall. As the boiler loads increased, the airflow uniformity became worse because of intensified flow turbulence. Moreover, due to the jet flow at the entrance of the isobaric windbox, the airflow velocity in the initial a few simplified tubes was always lower than that of other parts regardless of the windbox depth. In addition, the airflow profile of the isobaric windbox within 12m was significantly different from those beyond 15m. The structural parameters of the isobaric windbox had obvious influence on the airflow uniformity, and this influence decreased with the increase of windbox dimension. The backflow phenomenon resulting from the interaction between W1 and AR was observed in some simulation cases. Finally, the optimal windbox structural parameters for primary air uniformity were determined by an optimization study of the isobaric windbox structural parameters.

Monitoring, modeling, and characterizing single-speed ventilation fans for an animal building

Ventilation rate is equally important as pollutant concentration in aerial emission quantification, but is technically challenging to determine for animal buildings. A simple and reliable method using 240-VAC to 6-VDC unregulated adapters to monitor 24 single-speed wall-mount ventilation fans was applied in a 12-room swine environmental research building. A portable Wall Fan Tester (WFT), consisting of nine equally-distanced impeller anemometers, was designed and developed to measure airflow rates of the wall fans of 508-mm and 356-mm diameters. Fan ventilation rates were tested using the WFT at 5 or 6 static pressures from −1.8 to −133.9 Pa and power voltages from 185 to 245 VAC at 15-VAC increments in five fan test sessions. Fan ventilation models were developed using a single independent variable (static pressure) and two independent variables (static pressure and fan power voltage). The innovative system and methodology considerably improved ventilation rate quantification for the 12 rooms. The study also found that the wall fans in the building delivered 16.9–40.1% less ventilation under different static pressures compared with the fan specifications. There were 17–29% ventilation rate variations among identical fans in different rooms. Daily power supply voltages to the fans often fluctuated for 3–6%, which could introduce errors in ventilation calculations if not taking them into account. Cross-sectional airflow profiles were not always symmetric and there were backflows in some sub-areas for the 356-mm fan inlet at conditions of low power voltages and high negative static pressures.

Design of a Pressurized Smokeproof Enclosure: CFD Analysis and Experimental Tests

Pressure differential systems have the purpose of maintaining tenable conditions in protected spaces for different types of building safe places, like escape routes, firefighting access routes, lobbies, stairwells and refuge areas. The aim of pressure differential systems is to establish airflow paths from protected spaces at high pressure to spaces at lower or ambient pressure, preventing the spread of toxic gas released during a fire. This strategy ought to be supported by a detailed design of the necessary air supply, considering also the cycle of opening and closing doors during the egress phase. The paper deals with the design of a simple pressure differential system intended to be used in a building as a pressurized smokeproof enclosure. Specifically, experimental tests and numerical modelling are conducted with the objective of characterizing the pressure evolution in a small compartment under different conditions and through a cycle of door opening. Experimental tests are conducted in a simple 3-m side cubic enclosure with two doors and no vent openings. While a centrifugal fan blows constant airflow inside the structure, the pressure trend in time is recorded during steady state and transient conditions; additionally, the velocity of the airflow across the doors has been measured by means of an anemometer. Numerical CFD (computational fluid dynamics) simulations are carried out to reproduce the same smokeproof enclosure configuration (both geometrical and boundary conditions) using the fire dynamics simulator (FDS). Furthermore, specific attention is paid to the modelling of the leakage across the doors, directly inserted in the model through a localized HVAC (heating and venting air conditioning) advanced leakage function. Comparisons between experimental tests and numerical simulations are provided. Once the model was correctly calibrated, other geometrical and mechanical configurations have been studied, looking for convenient and efficient positions of the fan in order to fulfill the requirements of the pressure differential, airflow velocity and door handle force. The paper highlights some fundamental aspects on the pressurization and depressurization during steady state and transient phases, trying to identify if there are airflow profiles typical of some geometrical configurations.