Air Channel Research Articles

Development of a three-dimensional numerical model of indirect evaporative cooler incorporating with air dehumidification

Indirect evaporative cooler (IEC) is treated as an energy-efficient and low carbon-emission air conditioning solution without utilizing the mechanical compressor and chemical refrigerants for cooling production. Currently, the widely-used one-dimensional (1-D) and two-dimensional (2-D) numerical models of IEC can expose the temperature and humidity variations of the airflow direction for predicting the cooling performance. However, the air profile along the channel width direction is usually simplified as constant in those models, neglecting the effect of non-uniformity distribution in the normal direction of heat exchanger plates. In this paper, a novel 3-D IEC numerical model of a cross-flow IEC was developed based on the computational fluid dynamics (CFD) method. The judgment on moist air condensation was incorporated in the model by a built-in generalized minimum residual iterative solver, which enables the performance prediction of IEC in hot and humid areas. Detailed air temperature and moisture content distributions along and perpendicular to the air channels were obtained with sufficient validations from published and experimental data. Parameter analysis was conducted on the primary air inlet properties and the channel gap distance. Results show that the prediction accuracies of temperature and humidity were improved by 5.8 and 6.7% compared with the 2-D model. The air dehumidification process started to appear from 27 ℃ given the 70% RHp,in. Under the circumstance of condensation state, the highest wet-bulb efficiency was 0.8 given the 0.5 m/s air velocity against 4 mm optimized gap distance, while the greatest dehumidification rate was 0.3 in the studied ranges. The maximum net energy saving was obtained as 4.7 kW under scorching and wet climate conditions. The proposed 3-D model can benefit future study on the heat and mass transfer mechanism of advanced plate materials by providing the non-uniformity of air profiles along the Y-axis, and thus contribute to the development of next-generation IEC technology.

Analytical impedance of two-layer oxygen transport media in a PEM fuel cell

A model for impedance of oxygen transport in two porous layers A and B (GDL and MPL) positioned between the air channel and cathode catalyst layer in a PEM fuel cell is developed. Analytical solution shows that the system transport impedance cannot be split up into a sum of separate layer A and B impedances. The two-layer system works as a unified oxygen transport media. This result suggests that in general, impedance of all oxygen transport medias in a fuel cell are interdependent.

Comparative study on the water uptake kinetics and dehumidification performance of silica gel and aluminophosphate zeolites coatings

Desiccant coated heat exchanger (DCHE) systems have significant potential in improving dehumidification performance. The moisture transfer occurring in one single air channel of DCHE can dominate the overall system performance. In this study, the adsorber with an approximate air channel of DCHE was designed to experimentally investigate the kinetics of silica gel and two aluminophosphate zeolites (FAM Z01 and FAM Z05). Their dehumidification performances also were simulated at regeneration temperature range of 50–80 °C to reveal the applicability in air-cooled cross-flow DCHE. Results showed that linear driving force (LDF) model could well describe the dynamic water uptake behaviors. Adsorption rate coefficient kads increased with increasing inlet air humidity ratio and velocity, but decreased with increasing coating thickness. It is worth noting that increasing regeneration temperature had a little effect on kads, while desorption rate coefficient kdes increased significantly. Simulation results showed that moisture removal capacity (MRC) and dehumidification coefficient of performance (DCOP) of FAM Z05 coated DCHE could reach 0.495 g/kg and 0.57 at regeneration temperature of 50 °C, which were 2.3 and 15.4 times higher than these of silica gel and FAM Z01 coated DCHEs. It demonstrated that FAM Z05 was more promising to utilize lower-grade heat energy.

Performance evaluation of a novel plate-type porous indirect evaporative cooling system: An experimental study

The traditional indirect evaporative cooler (IEC) needs the consistent working of the pump for water spraying to the secondary air channel wall, which consumes an amount of energy and causes the extra pressure drop. Improving the channel surface water retention capacity has drawn research attentions over the years, and using porous materials is expected to deal with this problem. However, there is still lack of quantitative experimental data on the thermodynamic performance of this environmental-friendly heat exchanger. In this study, a plate-type cross-flow indirect evaporative cooler that sintered the porous layer in each secondary air channel surface (PIEC) was proposed and manufactured. A series of experiments were carried out on an established test rig to assess the performance of the PIEC system. It was observed that the air cooling effect could still be observed over time without water supply owing to the water absorbed and retained in the porous layer, which indicated the feasibility of the intermittent spraying plans. Results showed that the PIEC maintained the supply air temperature up to 2105 s within 0.5 °C fluctuation under the test conditions without spraying water, lessening 94.6% of the operation time of the water system. Therefore, the power consumption was significantly reduced. The coefficient of performances (COPs) were enhanced by 117.5% on average compared with the conventional continuous spraying mode. In addition, the pressure drop of secondary channels was measured to be lower in the non-spraying mode than in the spraying mode. Eventually, the present experimental study recommended to combine the PIEC as a pre-cooling air-conditioning device with the air handling unit to guarantee the required outlet air temperature for the indoor environment.

Study of the influence of input parameters in an air channel on mass and heat transfer phenomena within a wall saturated with water: application to the renovation of old wet buildings

In order to control moisture in old buildings, a new technology consists in incorporating a ventilation system into the air space located between the thermal insulation and the wet wall. This technology is considered as one of the effective solutions to achieve the comfort of the occupants, the durability of the insulation. A mathematical approach based on the darcien model was developed, in order to analyse the influence of effective coupled parameters which are heat, moisture, and air on the evaporation performance of a porous layer. A hybrid coupling combining both the Boltzmann method and the finite volume method is proposed. The results show that blowing air in winter conditions affect markedly the heat and mass exchanges at the interface between the porous layer and the channel. However, blowing air in summer conditions provide drying the wall and decrease the risk of development of mould.

Experimental investigations on the performance of a rectangular thermal energy storage unit for poor solar thermal heating

A novel rectangular energy storage unit (RTESU) is proposed to effectively harvest low-radiant solar energy in poor solar areas for the application of preheating cold outdoor air. The RTESU consists of tube bundles to charge phase change material (PCM), paraffin wax PCM panels and air channels to discharge heat to cold outdoor air. Experiments on the charging and discharging processes with heat transfer fluid (HTF) temperature (varying from 28 to 36℃) and flow rate (varying from 0.96 to 2.04 m3/h) are performed to investigate the thermal performance of the device for poor solar areas. PCM temperature variations, heat transfer characteristics, heating capacity, and melting and solidification dynamics are analyzed. The results reveal that solar energy can be harvested and stored in PCM through melting process even under extremely poor solar radiation (about 138.68 W/m2) within 3.6 h. During the charging process, the natural convection plays a dominant role, while conduction is dominating throughout the discharging process. Further analysis of the thermal performance of discharging process shows that the heat extraction effectiveness can be improved as the air inlet temperature decreases. The highest heat extraction effectiveness can approach 100% in 10.2 h when the air inlet temperature ranges from 11.5 to 13 °C.

Thermal management evaluation of Li-ion battery employing multiple phase change materials integrated thin heat sinks for hybrid electric vehicles

The optimal performance of a Li-ion battery is directly impacted by temperature. In order to control the temperature rise and provide even temperature distributions in the battery pack, a thermal management scheme comprises thin heat sinks with multiple phase change materials (PCMs) and air channels is investigated in this paper. The cooling performance and temperature homogenization of the battery thermal management (BTM) system are carefully studied under various configurations of PCMs. The results show that increasing the air inlet velocity has less effect in suppressing the temperature rise at early discharge stage, but ameliorates as the discharge prolonged to 3600 s. The standard deviation of the temperature (STDV) and maximum temperature of the batteries can be decreased by arranging PCMs with a lower melting temperature at the midsection and a higher melting temperature at the air outlet region of the heat sinks. In addition, for volume fraction of PCMs, Case IV, having a PCM with a higher melting point adjacent to the air outlet region and occupying one-half the height of the heat sink, illustrates a lower temperature rise and decreases the maximum temperature in the battery module by 1.024 K, 2.186 K, and 2.553 K, compared to Case I, II, and III, respectively.

Experimental and Numerical Research on the Performance of a Seasonal Ice Storage Device in Summer Residential Rooms of Northeast China

• A 3D transient simulation of Seasonal Ice Storage Device is performed. • Experimental study is conducted to storage energy. • The impacts of various affecting factors on the performance of the ice making and storage device are analyzed. • The optimum size and energy saving potential of the device are given. Seasonal cold storage is an environment-friendly technique that utilizes natural cold source in winter for summer cooling. In this study, a seasonal ice storage device using cold ambient air in cold regions in winter to make and storage ice for cooling residential rooms in summer was developed. The mathematical model of a seasonal ice storage device was established and validated though designing and measuring the operation data of an experimental device under the winter condition in Shenyang, China. The impacts of various affecting factors, including the inlet wind speed, inlet size, and the air channel size on the performance of the ice making and storage device were analyzed. The results indicate that the time that is taken for ice-making decreases with the increase of the inlet size, decreases with the increase of the inlet wind speed and decreases with the increase of the air channel size. The optimal air channel size of the seasonal ice storage device was achieved. The proposed and optimized device can save cold energy for residential buildings, and provide some guidance for the understanding of ice making and ice storage characteristics in the seasonal ice storage device.

Geometry Optimalization of the Ventilated Insulating Panel

Abstract An EPS ventilated panel, which may be applied as an external insulation to humid walls, is investigated. Dimensions of the air channel sections have been determined using the Ansys software. Afterwards, the drying rate of the walls externally insulated with EPS, mineral wool and EPS ventilated panel has been compared using the WUFI software. The ventilated panel increases the drying rate when compared with the standard polystyrene panel and increases the heat loss through the wall by less than 20%.

Experimental study on thermoelectric performance and power consumption characteristics of flat-plate thermoelectric generator

Thermoelectric generator can directly convert thermal energy into electrical energy, and applying it to recover waste heat from automobile exhaust is conducive to energy saving. In this paper, a thermoelectric generation experimental system is built, the power output performance and channel power consumption of the flat-plate thermoelectric power generation system under different exhaust gas flow rates are experimentally studied. The results show that when the exhaust flow rate is 40 Nm3/h and the exhaust inlet temperature is 400 °C, the maximum output power of the system is 26.8 W, and the maximum net output power is about 23.3 W. In addition, the exhaust flow rate is changed from 10 Nm3/h to 40 Nm3/h, and the proportion of power consumption in the hot air channel is changed from 1.91% to 12.85%, which has increased by more than six times.

Numerical analysis of transport phenomena in solid oxide fuel cell gas channels

The gas channel geometry in solid oxide fuel cells (SOFCs) influences the reacting thermo-fluid process and, thus, overall cell performance. This paper presents a dimensionless approach to the study of the transport phenomena in the gas channels of planar anode-supported proton-conducting SOFC. Out-of-scale modeling reduces the number of variables that should be investigated and offers generalized results, giving insight into similar fuel cells. A 2D numerical model for the multiphysics process in SOFC is developed. A dimensionless form of the governing equations is derived in order to identify the dimensionless quantities that characterize the transport phenomena in SOFC. Reynolds, Peclet, and Sherwood are the important parameter groupings of flow channels that influence mass and temperature distribution. The efficacy of the computational fluid dynamic model is confirmed by comparing simulated results with experimental data from the literature. The effect of fuel and air channels’ dimensionless parameters on cell performance is discussed. Similar changes in fuel and air channels exert various influences on SOFC electrical performance. It is found that reducing Pe in the fuel channel improves power generation. However, Sh and Re reduction effect neutralize the increase in power generation due to Pe reduction in the air channel.

Techno-Economic Assessment of an Air-Multiple PCM Active Storage Unit for Free Cooling Application

A latent energy storage (LES) unit is presented in this paper for free space cooling and ventilation application. The unit includes multiple phase change materials (PCM) to advance the thermal performance of common LES units. It is composed by metallic rectangular panels containing commercial paraffins with melting temperatures ranging among 20 °C and 25 °C and surrounded by air channels. The average cooling load of the unit corresponds to approximately 1 kW over 8 h. It fulfils the peak ventilation cooling load during summer of an office building in Portugal. The study provides a techno-economic analysis and the environmental benefits of the LES technology compared to a traditional air conditioning (AC) unit for the cooling and ventilation of an office building. During daytime, the air-multiple PCM unit allows reducing the energy consumption by nearly 200 kWh. The full charging of the PMs during nighttime, requires significant energy consumption due to the high air flowrate demand for full solidification. The competitiveness of such units can be achieved by introducing fins into the panels, allowing double the energy savings. In an overall perspective, the unit presents several benefits such as lower initial cost and reduced maintenance requirements (non-use of refrigerants and batteries) that also allows better personal health issues when related to traditional ACs.

Lossy mode resonances in photonic crystal fibers

The interaction effect of the fundamental mode in a special photonic crystal fiber (PCF) with a thin-film absorbing coating deposited on a surface of a fiber cladding on the optical transmission of the PCF is theoretically studied. It is shown that the transmission has a multi-peak spectrum that is determined by the resonance capture of the fundamental PCF mode energy by the coating. In some cases, this capture is explained by a resonance coupling between the fundamental core mode and leaky modes of the coating, or between the fundamental PCF mode and cladding modes located between PCF air channels and the coating. Examples are presented of using this effect to develop fiber-optic sensors of refractive index or pressure, and to sense a nanoscale adsorption layer of ammonia molecules deposited on a coating surface contacting air.

Local thermal non-equilibrium conjugate forced convection and entropy generation in an aircraft cabin with air channel partially filled porous insulation

PurposeThe purpose of this study is to numerically investigate heat transfer and entropy generation between airframe and cabin-cargo departments in an aircraft. The conjugate forced convection and entropy generation in a cylindrical cavity within air channel partly filled with porous insulation material as simplified geometry for airframe and cabin-cargo departments are considered under local thermal non-equilibrium condition.Design/methodology/approachThe non-dimensional governing equations for fluid and porous media discretized by finite volume method are solved using the SIMPLE algorithm with pressure and velocity correction.FindingsThe effects of the following parameters on the problem are investigated; Reynolds number, Darcy number, the size of inlet and exit cross-section, thermal conductivity ratio for solid and fluid phases, angle between the vertical symmetry axis and the end of channel wall exit and the gap between adiabatic channel wall and horizontal adiabatic wall separating cabin and cargo sections.Originality/valueThis paper can provide a basic perspective and framework for thermal design between the fuselage and cabin-cargo sections. The minimum total entropy generation number is calculated for various Reynolds numbers and thermal conductivity ratios. It is observed that the channel wall temperature increases for high Reynolds number, low Darcy number, narrower exit cross-section and wider the gap between channel wall and horizontal.

Application of Coal Gangue as a Coarse Aggregate in Green Concrete Production: A Review

Among the techniques for converting stacked coal gangue to reusable material, one of the most effective ways is to use coal gangue as a coarse aggregate in green concrete productions. The physical and chemical properties of rock and spontaneous-combustion coal gangue are generally suitable for being used as a coarse aggregate in green concrete. Coal gangue concrete is not recommended to be used in subsurface structures, as its water absorption law would be changed under a large replacement ratio. The mechanical performance of coal gangue concrete is degraded by raising the replacement ratio. Over-low and -high concrete grades are not suggested to be used as coal gangue aggregate, unless extra admixtures or specific methods are used. The influence of coal gangue on the durability of coal gangue concrete is remarkable, resulting from the porous structure of coal gangue that provides more transmission channels for air and liquid in concrete, but is beneficial for thermal insulation. The usage of coal gangue in structural concrete members is still limited. The mechanical behavior of some structural members using coal gangue concrete has been reported. Among them, concrete filled steel tubes are a preferable configuration for using coal gangue concrete, regarding both the mechanical and durability performance.

Experimental study on the performance of a novel hybrid indirect evaporative cooling/thermoelectric cooling system

A novel hybrid cooling system combining indirect evaporative cooling and thermoelectric cooling technologies was built and experimentally investigated. The system was mainly comprised of a cross flow regenerative indirect evaporative cooler and a water cooled thermoelectric cooler. The incoming air was firstly pre-cooled in the indirect evaporative cooler, and then further conditioned in the thermoelectric cooler to achieve desirable temperature and humidity. In particular, the thermoelectric cooler could operate under two modes of with or without spray water on the air channel surface. The performance of the proposed system under the above mentioned two operation modes was experimentally investigated and compared in detail under various operation parameters. The experimental results show that the product air temperature decreases more significantly with larger electric current and larger number of thermoelectric modules, smaller relative humidity, larger temperature and smaller velocity of inlet primary air. Under certain conditions, air condensation occurs leading to the increase of the product air temperature and the decrease of moisture content of the product air. Adding spray water in the air channel of the thermoelectric cooler would further decrease the temperature and increase the moisture content of the product air, which is more obvious under smaller electric current and number of thermoelectric modules, smaller relative humidity, smaller temperature and larger velocity of inlet primary air. The coefficient of performance of the system with spray water is smaller but the dew point effectiveness is higher compared to the system without spray water.

Emergence of elevated battery positioning in air cooled battery packs for temperature uniformity in ultra-fast dis/charging applications

Pure electric vehicles (EVs) are gradually becoming major interest of research in worldwide. Battery cells in EV battery packs must be kept in between the desired operational temperature range (∼30°C) and temperature should be homogeneous in packs to eliminate safety risks and prolong battery life. In this study, performance of a novel BTMS design was studied at various discharge conditions with fast and ultra-fast C-rate values. Cooling with natural convection exceeds desired operational temperature in the pack as well as forced air convection in Z-type manifold. Elevated battery positions yield flow resistance along the air channels in between battery cells to be uniform which yields flow rate sweeping the surface of each cell to be the same. Therefore, the maximum temperature in between cells decreases to less than 0.3K from the order of 12K. The temperature uniformity is essential for ageing and electrical resistance of cells to be homogeneous in a pack. In addition, heat transfer enhancement with various fin designs is documented as well as its effect on the temperature distribution. The accuracy of numerical studies is validated by experimental work. The results show that the peak temperature can be kept under the desired operational temperature with minimum deviation in the temperature difference for distinct operation conditions required for advanced electric vehicles (cars, airplanes, helicopters) with extreme charging and discharging capability.

Analytical Impedance of Oxygen Transport in the Channel and Gas Diffusion Layer of a PEM Fuel Cell

Analytical model for impedance of oxygen transport in the gas–diffusion layer (GDL) and cathode channel of a PEM fuel cell is developed. The model is based on transient oxygen mass conservation equations coupled to the proton current conservation equation in the catalyst layer. Analytical formula for the “GDL+channel” impedance is derived assuming fast oxygen and proton transport in the cathode catalyst layer (CCL) In the Nyquist plot, the transport impedance consists of two arcs describing oxygen transport in the air channel (low–frequency arc) and in the GDL. The characteristic frequency of GDL arc depends on the CCL thickness: large CCL thickness strongly lowers this frequency. At small CCL thickness, the high–frequency feature on the arc shape forms. This effect is important for identification of peaks in distribution of relaxation times spectra of low–Pt PEMFCs.

Thermal characteristics and operation efficiency of solid-state electro thermal storage

This work is dedicated to studying characteristics of proposed dynamic-type electro-thermal storages (ETS). Experimental (laboratory) studies of thermal status of magnesium oxide heat-storage cells of conventional design with two slot-shaped channels and chamotte heat-storage cells with two round-shaped channels, in the modes of heat charging and heat emission of ETS have been carried out. Results of experiments on temperature change of electro-thermal storage units and that of heated air in ETS, for a number of reference points, in operating modes of charging and heat emission were applied in calculations of thermal characteristics of ETS including average value of heat-transfer coefficient of ETS external surface, average value of heat-transfer coefficient, in air channels of heat-storage units of ETS, aggregate value of ETS heat emission, quantity of heat accumulated in ETS, in absolute and relative units, average rates of heating and cooling of heat-storage units, in charging and heat emission operating modes of ETS. The efficiency of operation of standard magnesium oxide ETS with heat-storage cells having slot-shaped channels and chamotte ETS with heat-storage cells having round-shaped channels has been estimated. The developed design of chamotte ETS with heat-storage cells having round-shaped channels makes it possible to increase heat emission in channels of heat-storage cells by 15 % and to reduce the final cost of ETS by, at least, 15 % compared to existing devices. Mass-scale implementation of electro-thermal storage systems for heat supply in rural areas and farmer enterprises will make it possible to reduce power loss in networks, to normalize the daily load curve in power network and to improve regional environmental situation. Application of ETS in combination with power installations based on renewable energy sources (such as wind and solar installations) is considered to be perspective solution, as well.

Highly Efficient Piezoelectrets through Ultra-Soft Elastomeric Spacers.

Piezoelectrets are artificial ferroelectrics that are produced from non-polar air-filled porous polymers by symmetry breaking through high-voltage-induced Paschen breakdown in air. A new strategy for three-layer polymer sandwiches is introduced by separating the electrical from the mechanical response. A 3D-printed grid of periodically spaced thermoplastic polyurethane (TPU) spacers and air channels was sandwiched between two thin fluoroethylene propylene (FEP) films. After corona charging, the air-filled sections acted as electroactive elements, while the ultra-soft TPU sections determined the mechanical stiffness. Due to the ultra-soft TPU sections, very high quasi-static (22,000 pC N−1) and dynamic (7500 pC N−1) coefficients were achieved. The isothermal stability of the coefficients showed a strong dependence on poling temperature. Furthermore, the thermally stimulated discharge currents revealed well-known instability of positive charge carriers in FEP, thereby offering the possibility of stabilization by high-temperature poling. The dependences of the dynamic coefficient on seismic mass and acceleration showed high coefficients, even at accelerations approaching that of gravity. An advanced analytical model rationalizes the magnitude of the obtained quasi-static coefficients of the suggested structure indicating a potential for further optimization.