Parallel Air Flow Research Articles

CFD simulation of the interaction between water droplets on the GDL of a low temperature PEM FC in conditions of air cross-flow

Abstract Droplet dynamics in low temperature proton exchange membrane fuel cells (LT-PEMFCs) are significantly important with respect to water management and removal. Three main phenomena are linked to water generation/transport in a LT-PEMFC: H2O production on the cathode side, electro-osmotic drag (anode to cathode) and back diffusion (cathode to anode). The excess/lack of water, depending on membrane humidity, strongly affects LT-PEMFC performance, involving its instability. In this work CFD simulation of droplets was performed when they were deposited in tandem on the gas diffusion layer (GDL) and subjected to 15 m/s parallel air flow. The volume of fluid (VOF) method was adopted and droplets with diameters that range from 0.3 mm to 0.8 mm were considered. Simulations demonstrate that deformations, oscillations and sliding of droplets were affected by their relative size and position. Results can contribute to define predictive models of water droplet dynamics on the GDL surface and their detachment towards the FC gas channel.

Study on the influence of spray characteristics and air flow direction on heat transfer of spray evaporative condenser

Spray cooling is effectively utilized in evaporative condensers to achieve efficient heat and mass transfer transmission, while also effectively preventing performance degradation caused by packing blockage. The heat transfer efficiency is strongly affected by the relative flow direction between the spray and air flow. This work employs computational fluid dynamics to evaluate and examine the heat transfer effects of parallel and countercurrent air and spray flow in evaporative condensers. The findings indicate that increasing spray density and wall temperature enhances heat transfer efficiency. Smaller droplets with lower initial velocity exhibit superior heat transfer capabilities in parallel flow, while smaller droplets also perform well in countercurrent flow as long as the initial velocity is not too high. Additionally, a slight increase in air velocity improves heat transfer efficiency in both parallel and countercurrent flow conditions. It is important to note that the countercurrent condition has a larger heat transfer effect than the parallel flow.

Heat and moisture transfer through skin-clothing microclimate

The microclimate between clothing and human skin plays an important role in the heat and moisture exchange between body and ambient environment, which is essential to the thermo-physiological comfort. Despite extensive research on microclimate heat transfer, the sweating process accompanying heat transfer has often been neglected, and the investigation of transport mechanisms is limited. In this study, the heat and moisture transfer from skin to environment was experimentally and numerically simulated with focus on factors such as microclimate thickness (0 mm ∼ 40 mm), tilt angle (0° ∼ 90°), and airflow direction (parallel and normal to the fabric surface). The skin was represented by a surface with uniform temperature in experiments and numerical simulations, and the fabric was modeled as an air-permeable porous medium in the numerical simulations. The experimentally validated numerical model provided detailed distributions of temperature, vapor concentration, and airflow patterns in the microclimate for both dry and wet skins. The airflow of natural convection featured multiple circulation cells, and their circulation times were calculated and then related to the microclimate transport performance. It was found that natural convection dominates the heat and moisture transfer in thicker microclimates. It was also found that normal airflow was more effective in facilitating the heat and moisture transfer as compared to parallel airflow. The results are useful for developing high-performance clothing optimized for thermal regulation and moisture management, improving comfort and safety in different conditions.

Weakly nonlinear stability analysis of non-isothermal parallel flow in a vertical porous annulus

The study examines the stability of parallel airflow within a tall vertical coaxial circular annulus filled with a highly permeable porous medium under stable stratification. The linear stability theory predicts that the least unstable mode can be either axisymmetric or non-axisymmetric, depending on the values of the governing parameters. The curvature parameter, defined as the gap between coaxial circular cylinders, contributes to the stabilization of the flow. Depending on various controlling parameter values, the stably stratified non-isothermal annular parallel flow (SNAPF) can manifest three instability modes: thermal-shear, thermal-buoyant, and mixed or interactive. The instability type can change abruptly with Reynolds number, curvature parameter, and media permeability. A finite-amplitude instability analysis is examined using weakly nonlinear stability theory. The analysis demonstrates both subcritical and supercritical bifurcations near the instability boundary. The range of Reynolds numbers for subcritical bifurcation expands when switching from axisymmetric to non-axisymmetric disturbance. Bifurcation transitions between supercritical and subcritical occur due to changes in the least stable azimuthal mode. The study also investigates the equilibrium/threshold amplitude within the supercritical/subcritical instability regime, and it verifies the existence of subcritical/supercritical bifurcation through nonlinear kinetic energy balance. Furthermore, the finite-amplitude analysis anticipates that the lower critical Rayleigh number increases as the Reynolds number, curvature parameter, or media permeability decreases. An increase in the Reynolds number shifts the point of inflection in the velocity profile toward the inner cylinder, which subsequently leads to a reduction in the size of the core vortex region. In the subcritical instability regime, the radius of the primary vortex cell decreased significantly as the Rayleigh number increased. When the curvature is altered, a structural transition occurs in the isotherms from multicellular to unicellular.

The effect of PCM layer thickness and coolant mass flow rate on the efficiency of a double-channel photovoltaic/thermal-PCM system: Numerical and artificial neural network analysis

A photovoltaic/thermal (PV/T) system containing a phase change material (PCM) layer located between two parallel air flow channels was numerically studied in the present research. With numerous numerical simulations, the effect of PCM layer thickness (ϕ=5–30 mm) and air mass flow rate (m˙a=0.0123–0.0368 kg/s) on system efficiency were determined. It was found that from the thermal point of view, it is better to have a smaller thickness of the PCM layer, while from the electrical point of view, the average thickness is more effective. The highest thermal efficiency (22.95–59.42 %) and the highest electrical efficiency (16.05–16.79 %) occurred in ϕ=5 mm and ϕ=20 m, respectively. Also, it was revealed that the increase in m˙a is associated with an increase in the overall efficiency. Among the examined cases, the highest overall efficiency (39.64–77.66 %) belonged to case of ϕ=5 mm and m˙a=0.0368 kg/s. After identifying this point, the artificial neural network method was used to determine the relationship that can predict the optimal overall efficiency at different hours and at different m˙a with an acceptable accuracy.

The realization of parallel airflow after flow rectification by the perforated plate under complex inflows

The parallel push-pull ventilation system uses uniform, low turbulence, and wide parallel airflow to achieve low diffusion transport of pollutants and reduce the system's energy consumption. Complex inflows caused by local connectors (fans, elbows, and reducers) in front of the air supply device seriously affect the air supply distribution. Currently, most existing studies focus on the performance of parallel push-pull ventilation systems, while lacking theoretical support and parameter guidance to rectify these complex inflows into parallel airflow. This study investigates parallel airflow after rectifying various complex inflows using perforated plates. Within the study context, three typical complex inflows models (fan, elbow, and reducer inflows) are established using dominant numerical and supporting experimental methods. In addition, the rectification effects of inflow velocity, porosity, and hole size of the perforated plate on various complex inflows are compared. The results show that the perforated plate's porosity and hole size should be reasonably matched under different complex inflows to achieve a uniform and low turbulence parallel flow. The perforated plate should have a porosity between 40% and 55% and a dimensionless hole size between 0.014 and 0.031 for elbow and reducer inflows. Swirling flow causes a reduction in the perforated plate's rectification capacity for fan inflow, requiring lower porosity and hole size, while also meaning that the resistance increases significantly. Overall, the study results are anticipated to aid in achieving supply airflow with parallel airflow.

Water Harvesting by Molecular Sieves Using Self-sustained Continuous Flow

A way of harvesting water from the air that avoids the discontinuity of the adsorption/desorption cycles is theoretically analyzed. A rectangular prism-shaped adsorbent bed is immersed in low-humidity air, at an angle to the horizontal and subject to a temperature gradient between two opposite and open faces. The other four faces of the prism remain isolated. Water is adsorbed on the adsorbent colder face, causing a density gradient in the surrounding air, parallel to the surface, that results in a self-sustained continuous air flow. On the opposite face, a self-sustained continuous air flow parallel to the surface also arises, but this time due to a temperature gradient in the air surrounding the hot bed face. In addition, its higher temperature causes the desorption of water from the adsorbent. This overall water exchange produces the enrichment of water content in one of the air streams that is crucial to produce water harvesting. The performance of Al-Fumarate, MOF-303, SAPO-34 and Zeolite 13X is tested, unveiling the key factors that increase flow rate and water concentration at the enriched phase. It has been found that the diffusive mass transport at the air-solid interphase is the bottleneck of water harvesting in continuous flow conditions. Therefore, if high concentration of water is desired, it is necessary to use porous materials with very high diffusitivities. These findings provide the foundations for the design of continuous water harvesting devices.

Experimental and numerical investigation of flat plate fins and inline strip fins heat sinks

The flow and heat transfer characteristics of two analogous heat sinks were obtained from laboratory experiments and compared to each other and to numerical simulations. One contained continuousstraight fins, and the other inline strip fins, both cooled by forced airflow parallel to their base. The averageairflow velocity in the interfin channels ranged from 4 to 20 m/s, corresponding to Reynolds numbers from810 to 3,800. The measurements indicated that despite its smaller heat exchange area, the strip fins heat sinkconvective coefficient was larger enough to obtain a thermal resistance smaller than that of the continuousfins. Numerical simulations were performed to compare their results with the experiments. Two distinct fintreatments were used: one considered fins with no thickness, isothermal with the fins base temperature. The other considered the fins thickness and perfect thermal contact with the heat sink base. The Nusselt number simulation results for the continuous fins agreed within 3% with the measurements, but larger deviations were observed for the strip fins heat sink.

Study on oxygen transport and titanium oxidation in coating cracks under parallel gas flow based on LBM modelling

The oxygen transportation from surrounding air to coating cracks is an important factor in the oxidation and ignition of titanium alloy. In this work, the oxygen transport and surface oxidation of titanium in inclined cracks of coating under parallel airflow are studied with the lattice Boltzmann method. A boundary scheme of LBM about surface reaction is developed. The conversion factors are utilized to build the relationship between the physical scale and the lattice scale. The reliability of the LBM model is validated by the FEM method. The results show that the convective mass transport driven by the surrounding airflow and the vortex structure formed inside the crack are the two significant factors that influence the oxygen transport in cracks. The convective mass transfer plays a major role in oxygen transport when the inclination angle of the crack is small. For the cases with a large inclination angle, the oxygen transfer from the top to the bottom of the crack is mainly controlled by mass diffusion mechanism. The oxygen concentration in inclined cracks is generally less than that in vertical cracks, and oxidation and ignition of the substrate titanium might be more likely to occur in relatively vertical cracks.

Experimental investigation of heat transfer by forced convection from three dimensional suspended bodies subjected to free air stream

ABSTRACT The present research work aims at providing data on heat transfer by forced convection from three-dimensional bodies suspended in a free stream of air. The shapes investigated covered cubic and rectangular bodies with air flow parallel and normal to the body’s axis. The experiments were carried out inside an air duct in which air flows over a model for the three-dimensional body. The results obtained were represented by power function relations between the Nusselt and Reynolds number. The results were divided into the main body axis was parallel to the direction of the air flow. The heat transfer from the whole cube could be broken to the face components. This showed that the sides and rear face were responsible for more heat transfer. Increasing the length of the rectangular body increased the heat transfer. The equation representing the increase of heat transfer with length is suggested. The present results are essential in designing air-conditioned railway coaches, heated liquid wagons, refrigerated cars, and food and fluid carriers.

Enhanced Heat Transfer Performance of Multiple Triangular Air Flow Passages in Parallel With Inclined Fins for Flat Plate Solar Air Heater

Abstract This paper presents results of a study of heat transfer performance of multiple equilateral triangular parallel air flow passages below a uniformly heated flat plate having inclined fins of different thickness, length, and material covering a part of the side walls of the duct for transition to early turbulent regimes using appropriate experimental heat transfer coefficient and friction factor correlations from the literature. The heat transfer performance enhancement at equal pumping power has been measured in terms of a performance parameter (hA/hsAs), where hA is the product of heat transfer coefficient and total heat transfer area for the finned triangular duct, and hsAs is the product for the triangular duct with heated flat plate without fins and adiabatic side walls. The values of the performance parameter for steel fins integral with the heated plate are found to be 1.71–1.78 for 1.0 mm thick fins of 15 mm length and 1.94–2.04 for 1.5 mm thick fins of 20 mm length. For aluminum fins, the performance parameter is 2.36–2.54 for 30 mm long fins of 1–1.5 mm thickness. The results of the presented novel scheme of the finned heated plate can be utilized for the development of enhanced performance solar air heater with the finned absorber plate. Since the presented scheme enhances heat transfer without increased pumping power penalty, the existing smooth rectangular duct solar collectors can be modified for enhanced performance.

Experimental evaluation of used engine oil based thermal energy storage coupled with novel evacuated tube solar air collector (NETAC)

The significant challenge in vacuum tube solar air collector is worse performance after sunset which prompts the thermal energy storage. In present manuscript, the used engine oil based thermal energy storage coupled with novel evacuated tube solar air collector (NETAC) is developed. The NETAC investigation is evaluated during winter season for hot air production for 13 h at diverse air flow rates and different directions of air flow (parallel air flow & counter air flow). The maximal temperature difference (24.8 oC) of air is obtained at low air flow rate (79.5 Kg/h) and maximal mean efficiency (28.8%) of NETAC at high air flow rate (159 Kg/h) with circular fin and counter flow arrangement. The maximal mean energy efficiency (27.15%) and exergy efficiency (24.8%) of the oil thermal storage tank is attained at high air flow rate (159 Kg/h) with circular fin and parallel flow arrangement. The maximal mean energy gain (538.7 W) by air is obtained at high air flow rate (159 Kg/h) with circular fin and parallel flow arrangement.

Finite element model to investigate the dynamic instability of rectangular plates subjected to supersonic airflow

In this paper, a numerical method is presented to study the dynamic behaviour of an isotropic rectangular plate subjected to aerodynamic loads induced by parallel supersonic airflow. A finite element model, based on bi-dimensional polynomial displacement functions and linear Piston theory, is used to study the dynamic behaviour of the solid plate coupled with aerodynamic loads. The developed approach is able to model flat plates and shallow shells in which the fluid–structure coupling at the interface is applied synchronously using a monolithic method. The solid model is based on Sanders’ shell theory. The stiffness and damping matrices that result from application of the aerodynamic load are coupled with those obtained from a structural model and are calculated using analytical integration. By assembling the matrices, the global mass, damping and stiffness matrices for the plate are obtained and then dynamic equations of the governing problem are derived. The eigenvalues of the system are calculated using the equation reduction technique. The critical non-dimensional aerodynamic pressure of the airflow that induces flutter of the structure is determined for various boundary conditions and geometries. The obtained results are compared with other published research works and very good agreement is observed.

Limiting condition for auto-ignition of finite thick PMMA in forced convective airflow

This contribution experimentally addresses the auto-ignition of finite thick PMMA (polymethyl methacrylate) subjected to parallel forced convective airflow. 1–15 mm thick samples, 30–60 kW/m2 heat fluxes (HF) and 0–1.2 m/s airflow velocities (u) were utilized to examine the auto-ignition mechanism and the limiting condition of non-ignition cases. Critical temperature, surface temperature evolution and ignition time were recorded. A modified numerical model, considering both PMMA and thermal insulation layers, and theoretical correlations developed for thermally intermediate solid were employed to analyze the experimental data. Results show that three regimes corresponding to different controlling mechanisms are identified: (1) solid-dominant regime, u ≤ 0.4 m/s, where auto-ignition is primarily determined by the heat transfer and pyrolysis in solid; (2) transitional regime, 0.4 m/s < u < 1.0 m/s, where both solid and gas phases have nonnegligible contributions; (3) gas-dominant regime, u ≥ 1.0 m/s, where gas phase chemical kinetics is responsible for the observed non-ignition phenomenon. Critical temperature is reliable only in the solid-dominant regime. In all the ignition scenarios, critical temperature increases with increasing sample thickness and u, but it barely changes with varying HFs. In the gas-dominant regime, surface temperature measurement suffers greatly from the generated large bubbles, worsening the agreement with numerical predictions. Based on the simulated in-depth temperature, estimated Biot number and thermal penetration depth, 1 mm PMMA is not thermally thin, whereas 15 mm sample is approximately thermally thick. Furthermore, thermal diffusivity of PMMA and the critical HF are estimated by fitting the measured ignition times.

Sensitivity analysis of the precooling process of strawberry: Effect of package designing parameters and the moisture loss.

Strawberry is one of the most perishable fruits, and precooling of strawberry increases the percentage of marketable fruits. To assess the sensitivity of strawberry cooling uniformity with respect to package vents and tray design, the previously proposed modified parallel airflow system (MPAS) was modified and a sensitivity analysis was conducted in this paper. Some improvements in homogeneous strawberry precooling process were made to give improved parallel airflow system (IPAS). To evaluate its performance, the cooling process of strawberries was simulated using the mathematical models of heat, momentum, and mass transfer, which were validated experimentally. Results showed that the IPAS was able to distribute cold air uniformly throughout the packages. A difference of 0.1°C was observed between the average fruit temperatures of the packages after 3 hr of cooling. Therefore, the cooling process of strawberry could be done at lower airflow rate, cooling time, and heterogeneity. In addition, the precooling process of strawberry was studied considering the moisture loss during the cooling process and comparing the data with the models without this term. The results indicated that the moisture loss of strawberries during the cooling process is not negligible and the cooling rate increased and the cooling time decreased (31%) by considering this term in the modeling. However, the moisture loss did not affect the heterogeneity of cooling process.

Effect of heat exchanger plates geometry on performance of an indirect evaporative cooling system

At present, there is great interest in indirect evaporative coolers (IECs), which can be effectively used to increase energy efficiency of HVAC systems, with particular attention to data center applications. In such devices, performance significantly depends on the formation of an adequate water layer on heat exchanger plates. Therefore, efforts should be made to promote water spreading through an appropriate design of the system.In this study, an experimental analysis of IECs manufactured with heat exchangers having different geometry is carried out. The scope of the research is to investigate the effect of different plates protrusion and pitch on IEC wet bulb effectiveness and pressure drop. Tests are carried out with counter and parallel air and water flow arrangement.Results highlight that plates geometry influences both surface wettability and the heat transfer rate. A reticular plates protrusion appears to be a good compromise to enhance system effectiveness. Tests also highlight that the parallel water and secondary air flows arrangement leads to higher wet bulb effectiveness and lower pressure drop compared to the counter flow configuration.

Cooling performance of natural draft hybrid system with parallel air path

The flexibility of the dry cooling system in thermal power plant needs to be improved to suit the load undulation caused by the increased renewable energy power. For overcoming the weakness of conventional cooling system, a natural draft hybrid (dry/wet) cooling system (NDHCS) is proposed to fully take advantage of dry and wet cooling technologies, which comprises the wet and dry cooling sections with parallel air flow path. The three-dimensional numerical models are developed to investigate the thermal-flow performances of the NDHCS under various environment and operation conditions, by which the flow fields near the NDHCS as well as the heat and mass transfer between circulating water and cooling air are obtained. The results show that both the dry and wet cooling sections dominate the cooling capacity of the hybrid system but the performance of the former is inferior to the latter in most of the cases. Low ambient humidity, large spraying water flow ratio in the wet cooling section and small wind speed are of benefit to the thermal performance of hybrid cooling system. For the dry and wet cooling sections themselves, the cooling performance can be improved thanks to the ambient wind under specific conditions. Besides, NDHCS can effectively avoid visible plume especially under windy conditions.

Energy modeling and analysis of variable airflow parallel fan-powered terminal units using Energy Management System (EMS) in EnergyPlus

The prevalence of variable airflow (VA) parallel fan-powered terminal units (FPTUs) points to the need of developing a performance model for building energy simulation. However, current version of EnergyPlus does not allow modelling variable speed fans in FPTUs using existing standard modules. This paper provided an alternative approach to modelling VA parallel FPTUs in EnergyPlus using Energy Management System (EMS) and user-defined modules to demonstrate the feasibility of integrating the model of VA parallel FPTUs with the rest of HVAC system. Also, the effect of constant airflow (CA) and VA parallel FPTUs on the component and system energy consumption was investigated based on the energy simulation results conducted on a single-story, five-zone small office prototype building model. The results showed that although VA parallel FPTUs have great potential in saving terminal unit fan energy, the selection of VA units over CA units alone does not guarantee net energy savings.

Thermohydraulic performance enhancement of a new hybrid duct solar air heater with inclined rib roughness

In this work, a new hybrid duct configuration of solar air heater is analytically investigated to improve the thermal performance of the system. The proposed design consists of parallel pass air flow paths with rectangular and triangular cross sections at upper and lower sides of the absorber plate. Both the sides of the absorber plates are roughened with inclined shape ribs. The roughness parameters are configured with relative roughness pitch at the bottom of the plate (Pb/eb) of 4–16, relative roughness height of e/Dh ranges from 0.021 to 0.050 and relative angle of arc of (α/60) 0.5–1. Based on the analytical results it is concluded that the maximum effective efficiency is 80.1% for roughness pitch of 8, relative roughness height of 0.021 and relative angle of arc of (α/60) 0.5, at the mass flow rate of 0.045 kg/s at the upper and lower duct. The effect of the mass flow fraction (i) on the thermal performance of the solar air heater is also analyzed. The system improves the thermal and effective efficiency by 22.4% and 18.1% when compared to conventional rectangular duct parallel pass SAH. The design plots are developed to find the optimum values of roughness parameters with respect to ambient conditions.

A naturally optimized mass transfer process: The stomatal transpiration of plant leaves

Stomatal transpiration of leaves is a dominant pathway of plant physiological water loss. The leaf transpiration rate when stomata are fully open is commonly at the same level as the evaporation rate of a wet surface of the same area as that of the leaf area, although the cumulative area of the stomatal pores is typically less than 3% of the leaf area. To elucidate the highly efficient diffusion of the stomatal array from the perspective of mass transfer theory, stomatal distribution characteristics of various kinds of leaves were obtained with optical microscope, and steady diffusions of water vapor from isolated zero-depth circular stomata, elliptical stomata, and distributed stomatal arrays without airflow parallel to the surface were simulated with the finite element method. It was found that the long perimeter of the elliptical stomata and the specific distribution characteristics of the stomatal array are the dominant reasons for the highly efficient diffusion of the stomatal array on the leaves. Furthermore, the simulation results reveal that extremal transpiration rates exist for the stomatal arrays with different distribution characteristics. It was found that the transpiration rates of the vegetation tend to approach the extremal values for flourishing development in the process of natural optimization.