Low Air Velocity Research Articles

Experimental evaluation and optimization of the heat pipes integrated thermoelectric generator using response surface methodology

In this study, integration of heat pipes with thermoelectric generators is proposed for the recovery of waste heat. Heat energy is transferred from a heat source to thermoelectric elements via heat pipes, enabling energy conversion. To dissipate excess heat from the system after conversion, additional heat pipes are positioned on the cold sides of thermoelectric elements. Cooling of the condenser sections of these heat pipes is facilitated using air flow provided by a fan. A novel approach in this study involves the innovative integration of heat pipes with thermoelectric generators. This integration provides a new pathway for optimizing waste heat recovery systems by combining advanced thermal management techniques with thermoelectric technology. Experiments conducted at different power inputs (12–42 W) and air velocities (0.4, 0.8, 1.2 m/s) determine the performance of the thermoelectric generator. Furthermore, a power usage efficiency analysis was conducted to discuss the applicability of the proposed method in electronics. At low air velocities, values below the ideal value of 1 were achieved. Experimental data were analyzed using response surface methodology to assess the impact of varying input parameters on the power output and efficiency of the thermoelectric generator. Optimization studies highlighted the importance of effectively cooling the cold side of thermoelectric elements. The highest power output and efficiency are achieved under conditions of 42 W power input, 1.2 m/s air velocity, and use of 4 heat sinks, yielding approximately 0.8 W and 2 %, respectively. Furthermore, the response surface methodology analysis indicated that the most significant effect on system outputs was the power input. Moreover, the results of this study lay the groundwork for future strategic advancements in thermoelectric generators, enhancing their role in sustainable energy solutions.

Ventilation of large section blind headings under conditions of changing ore load size

Relevance. The need to ensure safe working conditions in the working areas of underground mines. Stagnant zones with low air velocities can be formed in large section blind heading behind the ore bulk when ore is shipped by mining machines. There is a possibility of the formation of local gas accumulations and their abrupt removal later in such stagnant zones due to the unsteadiness of accumulation and transfer of gas impurities in a blind heading. A sharp removal of gas will lead to contamination of the load-haul-dump operators’ workplace. Aim. To identify removal of exhaust gases in large section blind headings in conditions of changing ore load size. Objects. Large section blind headings with of complex geometry. Methods. Three-dimensional numerical simulation of unsteady turbulent flow of a gas-air mixture in the Ansys CFX, visualization and analysis of simulation data in the Wolfram Mathematica. Results. The paper presents the results of mathematical modeling of ventilation of large section blind headings of complex geometry under conditions of changing ore load size formed during mining operations by the load-haul-dump machines with an internal combustion engine. It is shown that either a single vortex is formed that ventilates the blind headings space, or a stagnant zone behind the bulk ore, which is characterized by a relatively low intensity of mass transfer. Vortex formation depends on the height of the ore bulk. The authors have obtained the dependences of the change in the average concentration of exhaust gases at the outlet of the blind headings space on the operating time of the machine. They determined the dependence of the correction volume coefficient on the geometric parameters of the blind headings. An analytical model of the removal of gases from the blind headings space under conditions of changing bulk ore volume is obtained. The expression of increment in gas concentration on an operator’s workplace makes it possible to derive formula to find maximum time of load-haul dump operation in a stope such that gas concentration is never higher than maximum allowable concentration.

Structural optimization and battery temperature prediction of battery thermal management system based on machine learning

Lithium-ion batteries significantly extend the driving range for electric motorcycles. The battery thermal management system (BTMS) is critical for achieving optimal battery performance. Moreover, precise battery temperature prediction is essential for efficient thermal management. Therefore, a battery thermal management system integrating air and phase change material (PCM) cooling is proposed. Initially, the impact of PCM height, PCM thickness, and air velocity on battery temperature is analyzed. Subsequently, with cost minimization as the objective and ensuring that the maximum battery temperature remains below a threshold, the Black Kite Algorithm (BKA) is employed to optimize the BTMS structure. Finally, a BKA-Convolutional Neural Network (CNN)-Self Attention (SA) model is introduced for battery temperature prediction. The results indicate that increasing the thickness of the PCM and air velocity facilitates battery heat dissipation but with diminishing marginal effects. An increase in PCM height enhances battery cooling at low air velocities but becomes detrimental at high air velocities. The optimized PCM height is 35 mm, resulting in a cost of 0.073 USD for the BTMS per battery. Additionally, the BKA-CNN-SA model achieved a maximum error of 0.45 °C on the validation set and accurately predicted battery temperature changes before and after PCM melting.

Unsteady-state entropy generation analysis of the counter-flow dew-point evaporative coolers

The surging demand for air conditioning, especially in hot climates, underscores the urgency of strategies aimed at curbing fossil fuel reliance and fostering energy saving strategy. Despite the longstanding utility and advantages of the conventional mechanical vapor compression refrigeration system, sustainability hurdles persist owing to its pronounced electricity demand. While evaporative cooling, harnessing water evaporation, emerges as a more efficient alternative, its efficacy is contingent upon environmental conditions. Addressing this, dew-point evaporative cooling (DPEC) presents an innovative direction, showcasing superior efficiency by directing supply air into a wet channel. This study delves into the thermodynamic losses within a counter-flow configured DPEC system, employing a transient entropy generation model rooted in the second law of thermodynamics to provide a comprehensive analysis of DPEC’s performance. Key findings that emerged from this study reveal that (1) the transient entropy generation is intricately distributed across different layers of the dew point evaporative cooling (DPEC) system, including dry air, wet air, the channel plate, and the water film; (2) Parametric analyses highlight the significant impact of factors on entropy generation, inlet air temperature has a minimal effect on system efficiency, but higher temperatures increase thermal losses. Excessive humidity limits evaporative potential, while low humidity significantly increases entropy generation. The system performs optimally at a working ratio of 0.3 and lower air velocities (within the inlet air velocity range of 0.6 to 2.2 m/s). Channel length has little impact, while the system is more sensitive to channel height, with a height of 2.5–3.5 mm being conducive to better performance. Experimental validation demonstrates the model’s accuracy in predicting system performance. The research contributes to a deeper understanding of DPEC systems, offering insights for achieving optimal operating conditions and enhancing overall efficiency in the pursuit of sustainable development.

Energy-saving cooling strategies in tunnel-ventilated dairy buildings: Computational fluid dynamics (CFD) simulations and validation

Ventilation is a key strategy for addressing heat stress in dairy cattle. In this study, we developed computational fluid dynamics (CFD) models for a 252-head free stall tunnel-ventilated dairy building equipped with ridge openings and circulation fans. A detailed cow model was developed for a standard 625 kg Holstein cattle. Both tube and floor cooling were developed as supplemental cooling strategies using a ground source heat pump while the ventilation fans continued to operate at a reduced air speed (30 to 65 % reduction). The average air velocity within the stalls for the control (1.4 ± 0.32 m s-1) was significantly higher than the average velocity achieved with floor cooling (1.02±0.41 m s-1) and tube cooling (0.69±0.21 m s-1) (p = 0.043). Despite lower air velocities, CFD simulations showed that the average temperature at cow resting height for tube cooling (27.9 ± 0.21 °C) was significantly lower than that for the control (28.6 ± 0.36 °C) and floor cooling (28.3 ± 0.14 °C) (p = 0.021). For 252 cows, the total electricity consumption for the control was 40,590 kWh during summer, while for floor cooling, it varied between 30,683 and 36,181 kWh, and for tube cooling, it ranged from 15,879 to 21,378 kWh. Ventilation-related greenhouse gas (GHG) emissions from the tunnel-ventilated barn were 0.12 t of CO2 eq. per cow. Future studies could investigate the impact of reduced airflow rates on air quality inside the buildings.

Optimizing Air Contactor Design for Efficient Carbon Capture in Direct Air Capture Systems: A Multi-Disciplinary Approach

As carbon dioxide (CO2) emissions continue to rise, direct air capture (DAC) has emerged as an important technology for removing CO2 from ambient air to combat climate change. The air contactor design is a critical factor impacting DAC system performance and costs. This research describes a strategy for improving the design of a single air contactor unit to increase CO2 capture flux while minimizing costs using optimization techniques grounded in experimental data .The research focuses on a 100 m2 slab air contactor modeled utilizing cooling tower and scrubber principles. The system includes the chemical, mechanical, and structural engineering disciplines. The mass transfer coefficient, air velocity, packing depth, and capital cost per frontal area are four major design characteristics that were maximized.The model integrated requirements for low air velocity, pressure drop, and energy consumption, as well as adequate gas-liquid interaction via structured packing selection and depth.The design space was explored using a multi-objective optimization strategy based on gradient-based and heuristic methods. Initial sampling via a design of experiments methodology demonstrated response surface trends in the objective function across the design space. The response surfaces mapped the optimization objective as a function of the design variables. Using gradient information, sequential quadratic programming efficiently located an optimal design vector that met all constraints on the design variables and feasibility requirements. The constraints included bounds on mass transfer coefficient, air velocity, packing depth, and capital cost per frontal area. The optimized air contactor design achieved a CO2 capture flux of 10,440 kg/m2-yr at a cost of $ 0.068 per kg CO2 captured. The developed optimization framework maximizes contactor performance while decreasing operational costs. A multi-objective genetic algorithm approach was used to identify the trade-offs between the two competing objectives of maximized CO2 capture and minimized cost by expanding the formulation. The Pareto-optimal front of the optimization results offered numerous optimized solutions with specific DAC system setups. This methodology, as well as the optimum configurations identified, serve as the foundation for constructing larger-scale DAC systems, in future. Ongoing research aims to improve solution approaches by including advanced optimization software packages such as DAKOTA, which offers global optimization algorithms, as well as to boost model accuracy via integrating additional choice variables. Future optimization and modeling studies will include contactor interactions and the modeling of a full DAC facility to boost confidence that a globally optimal design has been established.Overall, this study illustrated an efficient optimization strategy for air contactor design and contributes to the improvement of sustainable carbon capture technology, which is crucial to controlling rising global CO2 levels. Contactor configuration optimization enables superior DAC system designs at lower costs, hence promoting the urgent deployment of negative emissions solutions.

Study on the Influence of Wind Fairing Parameters on the Aerodynamic Performance of Long-Span Double-Deck Steel Truss Suspension Bridge

A long-span double-deck steel truss suspension bridge is easy to produce vortex-induced vibration (VIV) at low air velocity, which affects bridge service life. Additional aerodynamic measures play a role in suppressing VIV by changing the aerodynamic shape, which is a common control method. As the main aerodynamic measure to suppress the VIV response, wind fairing is widely used in engineering practice. In order to obtain the optimal additional position and shape parameters of the fairing, Huangjuetuo Yangtze River Bridge is the research target. Through the combination of a wind tunnel test and numerical simulation, the VIV response of the original and fairing section is studied. Based on data analysis, it is revealed that these additional fairings to the upper chord can significantly reduce the VIV response. When the shape parameters of the fairing are h/D = 1/4 and l/D = 1, the VIV inhibition efficiency is the highest, which can reach 65.51%. By analyzing the flow distribution, it can be seen that VIV is caused mainly by vortex separation in the upper bridge board area. Although this wind fairing does not change the original vortex shedding forms, it changes the first separation point and movement direction of the airflow, making the vortex scale generated by the airflow smaller and the vorticity lower, thus effectively suppressing VIV.

Determination of Ambient Air Vaporizers’ Performance Based on a Study on Heat Transfer in Longitudinal Finned Tubes

Ambient air vaporizers (AVVs) are the most commonly used type of heat exchanger for cryogenic regasification stations. The transfer of heat from the environment for heating the liquefied gas and its vaporization is a cost-free and efficient method. Designing ambient air vaporizers for regasification or fueling stations requires accepting the size and related thermal power of the AVV considering the operating conditions and the type of liquefied gases to be vaporized. The nominal capacity of the ambient air vaporizer depends on its design, the frosting of longitudinal finned tubes, and the airflow through the vaporizer structure. This paper presents the results of experimental studies and computational fluid dynamics (CFD) analysis on determining the heat output of AVV longitudinal finned tubes depending on their design. This experiment was conducted in order to establish a numerical model. The relation between the longitudinal finned tubes thermal power and the air flow velocity is demonstrated and the beneficial effect of forced convection is proved. The obtained results are used for verification calculations of ambient air vaporizers’ performance depending on the size of the AVV, the profile cross-section, and the airflow velocity for different liquefied gases. Under conditions of forced convection, profiles with 12 equal-height fins were discovered to be the most efficient for higher airflow velocity providing up to 7% higher heat rate than profiles with 8 equal-height fins. However, at low air velocity, profiles with 8 equal-length fins showed a comparable heat output to profiles with 12 equal-length fins. Profiles with 8 and 12 unequal high fins differ in average heat output by about 28%. The profile with 12 unequal high fins turned out to be the least effective when 2D airflow was considered in this analysis.

Investigation on fixed pitch Darrieus vertical axis wind turbine

Small-scale Darrieus wind turbines have a wide scope in areas that are isolated from the power grid for such small-scale household applications. Applications of wind turbines on house roofs are one potential way to generate electricity from wind energy harvesting in low-wind urban locations. This work studies the aerodynamic behavior of a vertical axis wind turbine based on a MATLAB programming mathematical model. The NACA0021 airfoil profile blade was used in this present research investigation. The turbine was fabricated with dimensions such as chord length, c = 95 mm, blade height, h = 600 mm, and turbine diameter, D = 600 mm. The experimental results of the turbine for air velocity from 1 to 12 ms−1 were used in this paper and compared with analytical results. It has been observed that the fixed-pitch turbine does not start by itself at a low air velocity of 1 to 5 m/s due to a minimum and negative torque.

Numerical investigation of multiple affecting parameters on mixed convective heat transfer for arrays of buildings

This study aims to explore relationships between wind speed, wind direction, exterior surface roughness, temperature difference, plan area density and convective heat transfer coefficients (CHTCs) for external surfaces of arrays of buildings. The present investigation provides correlations for CHTCs at the windward, leeward, top, left, and right lateral surfaces. Three different plan area densities of 0.25, 0.111 and 0.027 are considered and 249 numerical simulations are conducted to investigate the influential parameters including density (λp), wind angle (θ), wind speed (U10), temperature difference (ΔT), and surface roughness (ε). Findings reveal that higher plan area density results in lower CHTC due to heightened airflow obstruction. Increase of the wind angle amplifies CHTC on leeward and right lateral facades while reduces it on the left lateral side. With increased wind speed, CHTC experiences significant increase. The results also shows that the temperature difference's impact is pronounced at lower air velocities, leading to heightened CHTC. Besides, increased surface roughness enhances heat transfer coefficients. Finally, the present study employs extensive simulations to present correlations for different facades of the buildings in the studied arrays offering insights into CHTC variations in terms of plan area density, wind angle, speed, temperature difference, and roughness height.

Filtering Efficiency and Design Properties of Medical- and Non-Medical-Grade Face Masks: A Multiscale Modeling Approach

Approved medical face masks have been shown to prevent the spread of respiratory droplets associated with coronavirus transmission in specific settings. The primary goal of this study was to develop a new strategy to assess the filtering and transmissibility properties of medical- and non-medical-grade face masks. In this study, we designed and assessed the filtering efficiency of particles through six different masks with a diverse set of fabrics, textures (woven and non-woven), fiber diameters, and porosity. The filtering and transmissibility properties of face mask layers individually and in combination have been assessed using mathematical analyses and new experimental data. The latter provided velocity profiles and filtration efficiencies for which the data were shown to be predictable. The filtration efficacy and pressure drop across each fabric have been tested using an aerosol particle spray and scanning electron microscopy. To assess clinical significance, the temperature and humidity of the masks were tested on a group of healthy volunteers spanning various age ranges (9–79 years old), utilizing an embedded temperature sensor disc. Also, a mask filter model was developed using fluid dynamic simulations (Solidworks Flow) to evaluate the aerodynamic dispersion of respiratory droplets. Overall, the FFP2 and FFP3 masks demonstrated the highest filtration efficiencies, each exceeding 90%, a feature of multi-layered masks that is consistent with simulations demonstrating higher filtering efficiencies for small particles (<5 µm). The velocity and temperature simulations of all six masks revealed a low air velocity (~1 m/s) inside the mask and a temperature variation of approximately 3 °C during the breathing cycle.

Spout geometry of fine particle conical spouted beds equipped with internal devices

The size and geometry of the spout are crucial parameters for the design and scaling-up of spouted beds because they condition heat and mass transfer rates. It is well-established in the literature that spouted beds without draft tube show axisymmetric spout shape, but this does not occur with open-sided draft tubes. Particle velocity has been measured at several bed heights and angular coordinates, under different geometric parameters and operating conditions, to determine the location of the spout-annulus interface in beds provided with open-sided draft tubes. Narrow draft tube wall slits and low air velocities lead to spouts of small size and triangular shape. The greatest spout expansion (high air velocities and wide draft tube wall slits) leads to a Reuleaux triangle shape, in which the spout swallows the tube. The results allow a better understanding of fine particle hydrodynamics in conical spouted beds, which is essential information for their scaling-up.

Effects of the deformation and size of the upper airway on the deposition of aerosols

Aerosol drug delivery in the human airway is significantly affected by the morphology and size of the airway. This work developed a CFD-DEM model to simulate and analyze air flow and powder dynamics in combined inhaler-airway systems with different degrees of airway deformation (non-deformed, 50%, and 75% deformed) and sizes (adult, 0.80, and 0.62 scaled). The airways were generated based on a regular airway constructed from the MRI images through finite element method (for deformed airways) or scaling-down (for smaller airways). The airways were connected to Turbuhaler® through a connector. The results showed that under the same flow rate, the variation in the airway geometry and size had a minimum impact on the flow field and powder deposition in the device and the connector. However, deformation caused more particle deposition in the deformed region. Notably, the airway with 50% deformation had the most particles passing through the airway with the largest particle sizes due to its lower air velocity in the deformed area. Reducing airway size resulted in more powder deposition on the airway, particularly at the pharynx and mouth regions. This was because, with the same flow rate, the flow velocity in the smaller airway was higher, causing more particle–wall collisions in the mouth and pharynx regions. More importantly, the deposition efficiency in the 0.62-scaled airway was significantly higher than the other two airways, highlighting the importance of the different administration of aerosol drugs for young children.

Numerical study on thermal-hydraulic characteristics of streamwise-interrupted flying-wing finned tubes

To improve the thermal-hydraulic performance, this study conducted numerical studies on streamwise-interrupted flying-wing finned tubes utilising both single- and double-channel interruption modes. The interruptions varied in width from 0.5 to 10 mm and were strategically positioned in nine distinct locations along the flow direction. The RNG k-ε turbulence model was applied for viscous flow simulation. The findings reveal that fin interruptions create a “breakpoint thermal expansion effect” at specific locations, enhancing heat transfer downstream while diminishing it upstream. For the single-channel interruption mode with a centrally positioned interruption channel, a 5–6 mm width is optimal at a low air velocity (150 Pa total pressure drop and a Reynolds number of 1282.31 for the continuous flying-wing finned tubes). However, at high air velocities (550 Pa total pressure drop and a Reynolds number of 2944.91), interruption is unnecessary. In both modes, standardising the interruption channel width to 2 mm is more effective when the interruptions are placed closer to the downstream end, as opposed to an even distribution. This study provides valuable insights for enhancing the thermal-hydraulic performance of flying-wing finned tubes and similar continuous-finned designs through strategic flow-channel interruptions.

Combined infrared-convective drying of banana: Energy and quality considerations

This work evaluated the infrared (IR) drying combined with forced convection for processing banana using response surface methodology and desirability function. The effect of IR power (118–238 W) and unheated air velocity (1.5–4.5 m/s) on drying time, drying rate, effective moisture diffusivity (Deff), specific energy consumption (SEC), CO2 emission and product colour parameters were investigated at a significance level of 0.05. The experimental drying curves were predicted by the diffusive model solved with Robin boundary condition and a good agreement was obtained (R2 ≥ 0.9651). The values of Deff ranged from 1 × 10−8 to 4 × 10−8 m2/s increasing at higher IR powers and lower air velocities. The optimal condition (P = 238 W and v = 1.5 m/s) reached a global desirability value of 0.96 under which the final moisture content was 15.6 % with SEC of 12.51kWh/kgH2O and 0.050 gCO2/kWh. Therefore combined infrared-convective drying holds promise for the dried banana production.

Particle Flow Distribution in a Fluidized-Particles Multitube Solar Receiver

A fluidized-particles two-tube solar receiver was tested at ambient temperature at PROMES Laboratory to investigate the influence of an inhomogeneous solar flux density on the particle mass flow rate between the tubes. The principle of this 3rd generation solar receiver is to fluidize the particles in a container, called dispenser, in which the tubes’ bottom are immersed. The fluidized particles are flowing upward the tubes by applying an overpressure in the dispenser. Air velocities are changed inside the tubes, thanks to air mass flow controllers, to represent temperature heterogeneity between the tubes. Air velocities from 0.05 up to 0.52 m/s were tested, both in homogeneous and heterogeneous conditions. In the heterogeneous ones, the differences in air velocity between the tubes aim to mimic a difference in temperature from 20 to 100 % with homogeneous air flow rates injected. The following conclusions were drawn. First, the particle mass flux in the tubes are the same with homogeneous air velocities, each one following a calibration map previously obtained. Second, different air velocities lead to different particle mass flux. Third, the rise of the total particle mass flux diminishes the pressure in the dispenser. Four, this diminution of pressure leads to a decrease of the particle mass flow rate of the tube with the lower air velocity. This influence can lead in some cases to the stop of the fluidized bed circulation in the affected tube.

Overheating and Air Velocities in Modern Office Buildings During Heating Season

Proper design and operation of buildings are expected to result in optimal thermal comfort and energy performance at the same time. If occupants are not satisfied with thermal conditions, corrective actions by building managers and maintenance staff may lead to elevated room temperatures with evident energy penalties. Because of complicated technical systems and control logic, it is worth studying how well the design intent has been realised in new office buildings. In this study, thermal comfort was analysed by measurements of draught, room, and supply air temperature as well as with occupant questionnaire surveys in five modern office buildings. Both short‐ and long‐term measurements were conducted to demonstrate problems in the operation and to find potential solutions for improvement. The results revealed an issue of excessive overheating during the heating season despite generally low air velocities. Radiant ceiling panels had the lowest velocities in both summer and winter, while buildings with active chilled beams showed the potential to meet Category II air velocity and temperature requirements. The building with thermally activated building systems experienced the most overheating during the heating season. Occupants were satisfied with the heating season temperatures of 23°C–25°C that can be attributed to lighter clothing (0.7 clo) instead of the standard 1.0 clo. Ventilation supply air and indoor temperature analyses indicate that elevated setpoints have been used to compensate for draught, resulting in overheating. As a measure of improvement to avoid overheating, we propose control curves for room temperature based on the outdoor running mean temperature and for supply air temperature based on the extract air temperature.

Joint OH-PLIF and Mie scattering imaging of enhanced water mist suppression of buoyant fires

This study presents the first near-field measurements of interactions between reaction layers and water mist droplets suppressing a buoyant turbulent diffusion flame that is representative of a fire. Joint Mie scattering and OH-PLIF are employed at a high repetition rate of 400 Hz to image the droplet field and OH structure, which are deemed here to approximate the high-temperature reaction zone. Compressed natural gas (CNG) buoyant diffusion flames are stabilised on the Sydney Turbulent Buoyant Fire Burner, which is surrounded by a low-velocity air co-flow that is seeded with fine water mist droplets (d32 ∼ 50 µm). The water mist is doped with varying concentrations of metal alkali salts (NaHCO3, KHCO3) that release radical scavenging molecules, providing enhanced suppression capabilities through chemical inhibition in addition to the quenching effect from the water droplets. The spray boundary condition across the wind tunnel exit plane is quantified using Phase Doppler Interferometry. Flame extinction limits are measured by defining the critical condition at which the flame destabilises and lifts off the burner exit plane. A stability map is developed to quantify suppression efficacy in terms of the water mist flow rate and inhibitor concentration. Joint Mie-OH images of selected cases at varying extinction levels show that the addition of inhibitor introduces frequent breakages in the OH layers, marking regions of local extinction and reignition. Large mass fractions of inhibitor in the water, which, once evaporated, produces excess salt crystals in the local extinction zones that causes streaks of salt clouds that line both the lean and rich side of the flame. The chemical scavenging effects of the OH radical by NaHCO3 and KHCO3 is quantified by evaluating the reduction of the mean OH fluorescence signal in the flame at varying locations. Probability density functions for the OH area characterise local extinction in the flame due to the radical scavengers. The order of OH radical depletion and localised extinction in the flame due to the water mist follows: KHCO3 > NaHCO3 > H2O.

Evaluation of Phase Change Materials for Pre-Cooling of Supply Air into Air Conditioning Systems in Extremely Hot Climates

This research investigates the use of phase change materials (PCMs) in thermal energy storage (TES) unit-based cooling systems to increase the efficiency of air conditioners (ACs) by reducing the air inlet temperature. This study aims to evaluate different configurations of PCM enclosures, and different PCMs (paraffin and salt hydrate), by changing the speed of inlet air to achieve heat reduction of inlet air. The study includes experimental and simulation investigations. Every configuration simulates the hot-season atmospheric conditions of the UAE. A duct containing enclosures of paraffin RT-31 and salt hydrate (calcium chloride hexahydrate) was used for the simulation study using ANSYS/Fluent. A conjugate heat transfer model employing an enthalpy-based formulation is developed to predict the optimized PCM number of series and optimum airflow rate. Four designs of the AC duct were modelled and evaluated that contained one to four series of PCM containers subjected to different levels of supplied air velocities ranging from 1 m/s–4 m/s. The simulation study revealed that employing four series (Design 4) of PCM enclosures at a low air velocity of 1 m/s enhanced the pre-cooling performance and reduced the outlet air temperature to 33 °C, yielding a temperature drop up to 13 °C. The performance of salt hydrate (calcium chloride hexahydrate) was observed to be better than paraffin (RT-31) in terms of the cooling effect. Characterization of paraffin wax (RT-31) and salt hydrate was performed to establish the thermophysical properties. The experimental setup based on a duct with integrated PCM enclosures was studied. The experiment was repeated for three days as the repeatability test incorporating RT-31 as the PCM and a 3 °C maximum temperature drop was observed. The drop in the outlet air temperature of the duct system quantifies the cooling effect. Net heat reduction was around 16%.

Numerical modelling of two-phase flow hydrodynamics of column flotation - validation against ERT data

ABSTRACT In mineral processing, column flotation is being used extensively for fine particle beneficiation. Although a few attempts were made to develop computational fluid dynamics (CFD) models for column flotation in the past, validation against comprehensive experimental data is yet to be demonstrated. This work employs an Eulerian-Eulerian model coupled with the standard k-ε turbulence model for modeling the hydrodynamics in the column flotation. The two-phase simulations were performed on both laboratory and industrial-scale column flotations. The two-fluid model was further modified with the appropriate interphase forces (Ishii-Zuber drag force and Tomiyama lift force) and population balance method (PBM) to accurately simulate and predict the bubble dynamics. The axial and radial gas holdup variations along with the axial liquid velocity are predicted at different air and liquid superficial velocities. The numerical predictions were then validated against the Electrical Resistance Tomography (ERT) data in a 4-inch laboratory column flotation and conductivity probe data in an industrial column flotation. The modified two-fluid model was able to predict accurate hydrodynamics close to ERT data. The gas holdup was found to increase with the superficial air velocity, liquid height, and liquid velocity. The gas holdup found uniformly dispersed radially at low superficial air velocities and heterogenous dispersed bubble flow at higher superficial velocities. A parabolic profile of liquid flow observed at higher superficial air velocities.