Optimal Air Gap Research Articles

CFD analysis of the impact of air gap width on Trombe wall performance

AbstractIn recent years, there has been a lot of research and debate on how solar energy can be used instead of conventional sources of heating to power residential heating. In this study, the Trombe wall (TW) technique, based on natural convection and energy storage, was examined to predict mass flow rate, temperature field, and velocity field for the TW system under steady conditions. A numerical simulation model was investigated for further validation using k‐ε turbulence and discrete ordinates (DO) radiation models. Independent grid studies were conducted to ensure that there were no changes after varying the grid numbers. The effect of the air gap was carried out to enhance TW thermal performance. CFD simulation shows good agreement with published data in the literature, and the optimum air gap was set at 0.1 m. The results pave the way for future studies to improve passive solar heating systems, which will eventually help move towards more sustainable residential heating solutions.

Three-dimensional scanning measurement and characterization of air gap entrapped on air ventilation garments with different fabrics and clothing sizes

PurposeThe purpose of this paper is to determine the effect of clothing fabrics, sizes and air ventilation rate on the volume and thickness of the air gap under the air ventilation garments (AVGs).Design/methodology/approachThe geometric models of the human body and clothing were obtained by using a 3D body scanner. Then the distribution of the volume and thickness of the air gap for four clothing fabrics and three air ventilation rates (0L/S, 12L/S and 20L/S) were calculated by Geomagic software. Finally, a more suitable fabric was selected from the analysis to compare the distribution of the air gap entrapped for four clothing sizes (S, M, L and XL) and the three air ventilation rates.FindingsThe results show that the influence of air ventilation rate on the air gap volume and thickness is more obvious than that of the clothing fabrics and sizes. The higher is the air ventilation rate, the thicker is the air gap entrapped, and more evenly distributed is the air gap. It can be seen that the thickness of the air gap in the chest does not change significantly with the changes of the air ventilation rates, clothing fabrics and sizes, while the air gap in the waist is affected significantly.Originality/valueThis research provides a better understanding of the distribution of the air gap entrapped in ventilated garments, which can help in designing the optimal air gap dimensions and thus provide a basis and a reference for the design of the AVGs.

THE EFFECT OF AIRGAP THICKNESS ON THERMAL PROTECTIVE PERFORMANCE IN HEAT RESISTANT CLOTHING

The thermal protective performance is provided by thermal protective clothing worn by people who work in high temperature environments and is highly influenced by air gaps. In this paper, effect of air gap thickness on thermal protective performance as indicated by the time before degree burn were investigated. 3 layers of thermal protective clothing with 3 types of fabrics that have been selected will be tested using an experimental method. The research will be conducted through experimental tests using the fabric level method on a bench-scale test apparatus which is equipped with 4 thermocouples and using gas torch as a heat source. The air gap located between the layers of thermal protective clothing will be varied with different thickness in a vertical orientation. The size of the air gap used varies from 01 mm, 2.51 mm, 51 mm, and 7.51 mm. The results show that the temperature distribution in each layer of clothing from the outer shell to the thermal linear has decreased. In addition, time before degree burn which is an indicator on thermal protective performance shows a positive effect on the addition of air gap thickness where the optimum air gap thickness is shown at 7.5 mm variation. It is hoped that the results of this research can be a source or useful information in mechanical engineering, especially in the fields of thermal comfort and fire safety.

Numerical investigation of the effects of different influencing factors on thermal performance of naturally ventilated roof

Ventilated roofs can improve thermal performance and reduce the energy consumption of building roofs. In this study, the optimal design parameters and adaptability of naturally ventilated roofs were determined through numerical investigations of the effects of different influencing factors on their thermal performance. A three-field coupling numerical simulation method considering temperature, flow, and solar radiation fields was used to improve simulation accuracy. The research results show that the optimal air gap layer thickness of a naturally ventilated roof increases with increasing roof length, which has not been considered in previous studies. The peak interior surface temperature of the ventilated roof increases with increasing outdoor atmospheric pressure. Ventilated roofs with a 30°–40° slope have a stronger stack effect and can better discharge solar heat gains when compared to roofs with other slopes. Interior surface temperature is reduced when phase change materials is set in the upper rather than the lower roof during summer. Phase change materials should be set in the upper roof to save energy. Small damage holes in the upper roof don't apparently affect the ventilated roof thermal performance. The findings of this study can guide optimization design and help evaluate the energy-saving effects of naturally ventilated roofs.

Building energy-saving potential of a dual-functional solar heating and radiative cooling system

Space heating and cooling devices that rely on renewable resources are in demand amid energy crises in parts of the world. However, common renewable space heating and cooling devices are mono-functional. For regions with heating and cooling seasons, using two mono-functional devices might double the installation and maintenance costs, and prolong the payback period. This study proposed a dual-functional renewable heating and cooling device by utilising solar power and nocturnal radiative cooling. The device is a modified solar heating (SH) collector that optimises the nocturnal radiative cooling (RC) to become an SHRC collector. Investigation of the SHRC collector’s performance and energy-saving potential of a building-integrated SHRC collector was conducted using CFD and EnergyPlus. Analysis of the SHRC collector’s performance in various environmental conditions shows that the SHRC collector can reach 42 % thermal efficiency at zero-reduced temperature and > 50 W/m2 of net cooling power. Also, studies on the optimal air duct and air gap height reveal that a 1 cm air duct and 4 cm air gap as the best options for the SHRC collector design. Simulations of the building-integrated SHRC with a collector area of 9.43 m2 for a typical 100 m2 house building demonstrate the multi-seasonal advantage of the SHRC collector, with at least 1.5 kWh more daily savings than the SH and RC collectors in typical winter and summer days. Furthermore, the simulations estimate the annual combined heating and cooling energy savings by the SHRC collector around 32.7 % in Madrid, 25.5 % in Tokyo, and 14.0 % in Isfahan.

Optimal Air Gap Length Design in Powder Core Inductors

Optimal Air Gap Length Design in Powder Core Inductors

Highly selective, fractal-configured UWB-FSS for sub-6 GHz 5G, GSM, WLAN, and C band electromagnetic stealth application

AbstractA miniaturized and flexible frequency-selective surface (FSS) has been presented in this article with a unit cell size of 0.049 λc × 0.049 λc where λc is the free space wavelength at the lower cut-off frequency. In order to achieve an ultra-wide (−3 dB) second order pass band of 151.3% with enhanced selectivity factor of 0.887, a cascaded triple layered hybrid resonating structure has been proposed with symmetrical Minkowski island-shaped fractal geometry pair and spiral-shaped middle layer in optimized air gap coupling. Furthermore, 149.8% ultra-wide pass band also ascertains the conformal feature of the proposed structure. In addition to this, the proposed FSS provides the stable angular response for both TE and TM polarization. An equivalent circuit model has been synthesized for accurate frequency response. Finally, a sample prototype has been fabricated to verify the experimental validation. Excellent angular stability under large oblique incident and significant conformal characteristics ensure the compatibility of the proposed structure for electromagnetic stealth in 0.9–1.8 GHz GSM band, 2.10–2.14 GHz wireless medical telemetry band, 2.4–2.5 and 4.9–5.8 GHz WLAN band, 3.4–3.7 and 4.4–4.9 GHz sub-6 GHz 5 G band, and 3.7–4.2 GHz C band.

Оптимизация величины неравномерного воздушного зазора синхронной машины с постоянными магнитами на роторе

The article proposes an approach to determining the uneven air gap of a synchronous machine with prismatic magnets on the rotor, in which the distribution of the normal component of magnetic induction along the inner circumference of the stator core has a shape close to sinusoidal. The calculation of the magnetic field of an electric machine model with the specified geometric dimensions and properties of the element materials is performed in the FEMM program. To solve the optimization problem, the Hook-Jeeves method is used, restrictions are taken into account by the method of penalty functions. As a result of modeling the magnetic field of an electric machine with a calculated optimal uneven air gap, a distribution curve of the normal component of magnetic induction along the inner circumference of the stator is obtained, the average deviation of which from the sinusoidal curve is 4,8 %.

A compact super wideband antenna with stable and improved radiation using super wideband frequency selective surface

A small bulb shaped planar super wideband (SWB) antenna with the dimension of 35 × 30 × 0.8 mm3 operating over 2.8–40 GHz (14.28:1) spectrum is proposed. The antenna offers a fractional bandwidth of 173.8%, bandwidth dimension ratio (BDR) of 1904, measured peak gain of 5.5 dBi and radiation efficiency of 80%. The proposed antenna is unified to an unique SWB frequency selective surface (FSS), having unit cell size equal to 0.114λL × 0.114λL × 0.008λL (λL is the wavelength at 3.1 GHz). The FSS operates in 3.05 to 35.92 GHz spectrum (168.7%) with transmission level S21 ≤ -10 dB and reflection S11 near 0 dB for TE and TM modes of polarization with adequate angular stability. The linearly decreasing S11 phase of FSS makes it appropriate for radiation improvement of the bulb shaped antenna in the broadside direction by placing the antenna at the top of FSS with an optimal air gap of 19 mm. The SWB ‘antenna-FSS’ exhibits wideband response over 3–36.2 GHz (169.4%) spectrum, including commercial ultra-wideband (3.1–10.6 GHz), ISM bands (5.725–5.875 GHz & 24–24.25 GHz), WiMAX (3.3–3.7 GHz), IEEE 802.11 WLAN bands, S-C-X-Ku-Ka bands, and 5G NR (n77, n78, n79, n257, n258, n261) bandwidth with stable and adequate gain of 5.2–7.16 dBi and an acceptable radiation efficiency.

Heat and mass transfer in a counter flow parallel plate membrane-based absorption heat pump (PMAHP)

Membrane-based absorption heat pumps (MAHP) operate based on the working principle of the absorption refrigeration system (ARS), which could be used to recover and transform low-temperature waste heat into useable heat sources with higher temperatures. This work focuses on the performance study of a counter-flow parallel-plate membrane-based absorption heat pump (PMAHP) to recover low-temperature waste heat from used cooling water at 40 °C. It consists of refrigerant (water) and absorbent (LiCl solution) streams flowing in neighboring channels formed by hydrophobic microporous membranes with air gaps sandwiched in between these channels. Air-gap design is incorporated to minimize the sensible heat loss through conduction between the two streams. Water vapor molecules travel from the water stream to the solution stream through the membrane and air gaps. As the water molecules are absorbed by the solution, they condense to release the latent heat of absorption and dilution. The recovered heat can be used subsequently for fluid heating or air-conditioning purposes. A three-dimensional, steady-state model based on the finite element method is used to study the conjugate heat and mass transfer mechanisms. Model validation results agree with experimental data with a general discrepancy of within 10%. Parametric studies on the performance of the PMAHP are carried out. Scaling analysis is applied to study the effects of geometrical parameters on the heat and mass transfer dimensionless parameters, fluid flow behavior, heat, and mass transport within the PMAHP. The optimal air gap width is determined to achieve maximum solution temperature lift, which shows an improvement of 99.6% compared to the base case. The findings of this study provide an insight regarding the potential aspects to be focused on for further enhancement in the PMAHP heat and mass transfer performance.

Numerical investigation of the effects of heterogeneous air gaps during high heat exposure for application in firefighter clothing

For accurate prediction of thermal protective performance of firefighter clothing, a realistic assumption about the heterogeneous distribution of air gaps underneath the clothing is necessary. In this study, a numerical model of heat transfer through realistic heterogeneous air gaps under flash fire exposure was developed. First, the models of heat transfer and fluid motion were validated with data from scientific literature. The verified model was further developed and then used in a subsequent parametric study to quantify effects of heterogeneous air gap distributions. The results revealed that the difference in terms of heat transfer and skin burn injuries between heterogeneous air gaps with contact folds and equivalent homogeneous air gaps was greater than that between heterogeneous air gaps with non-contact folds and equivalent homogeneous air gaps. Fold aspect ratios showed a more prominent impact on heat transfer and skin burn times in the case of contact folds compared to non-contact folds. Exposure times to skin burn were continuously prolonged with increasing air gap thickness from 6.4 to 19.1 mm for homogeneous air gaps and heterogeneous air gaps with non-contact folds, while for heterogeneous air gaps with contact folds, there was an optimum air gap thickness around 12.7–15.9 mm.

Proximity coupled fed wideband printed magneto–electric dipole antenna

Results are presented fora wideband printed magneto-electric dipole antenna. The printed electric dipole and reflector are separated by an air-gap and is fed with a coaxial transmission line and a printed proximity coupled T-shaped feeding strip. The slot between the electric dipole halves forms the magnetic dipole radiating aperture. This feeding mechanism allows for good impedance matching at adjacent resonances of the electric and magnetic dipoles to achieve wideband operation. The two printed electric dipole halves are DC shorted to the reflector. The results show that this antenna can operate over a wide bandwidth with good gain, stable unidirectional radiation patterns and low cross polarization. The effect of the air-gap height and the relative permittivity of the substrate on which the dipole is printed are investigated. Results show that an optimum air-gap height for a specific substrate permittivity can be determined that will result in a gain with a small variation over the entire impedance bandwidth of the antenna. Simulated as well as measured results are presented for a prototype antenna with a small gain variation and overall dimensions of 1.33λ0 × 1.33λ0 × 0.19λ0. A measured impedance bandwidth of 54% with an average gain of 9.2 dBi is achieved.

Optimization of Air Gap Length and Capacitive Auxiliary Winding in Three-Phase Induction Motors Based on a Genetic Algorithm

There is a necessity to design a three-phase squirrel cage induction motor (SCIM) for high-speed applications with a larger air gap length in order to limit the distortion of air gap flux density, the thermal expansion of stator and rotor teeth, centrifugal forces, and the magnetic pull. To that effect, a larger air gap length lowers the power factor, efficiency, and torque density of a three-phase SCIM. This should inform motor design engineers to take special care during the design process of a three-phase SCIM by selecting an air gap length that will provide optimal performance. This paper presents an approach that would assist with the selection of an optimal air gap length (OAL) and optimal capacitive auxiliary stator winding (OCASW) configuration for a high torque per ampere (TPA) three-phase SCIM. A genetic algorithm (GA) assisted by finite element analysis (FEA) is used in the design process to determine the OAL and OCASW required to obtain a high torque per ampere without compromising the merit of achieving an excellent power factor and high efficiency for a three-phase SCIM. The performance of the optimized three-phase SCIM is compared to unoptimized machines. The results obtained from FEA are validated through experimental measurements. Owing to the penalty functions related to the value of objective and constraint functions introduced in the genetic algorithm model, both the FEA and experimental results provide evidence that an enhanced torque per ampere three-phase SCIM can be realized for a large OAL and OCASW with high efficiency and an excellent power factor in different working conditions.

Coupling optimization study of key influencing factors on PCM trombe wall for year thermal management

The proposed environment-interactive novel Trombe wall system was a passive building envelope integrated with phase change material (PCM) based on traditional Trombe wall. Compared with traditional Trombe wall system, this system can make full use of solar energy and nature ventilation, improving indoor thermal comfort. The dynamic heat transfer model of PCM Trombe room was established, and six key factors influencing thermal performance of PCM Trombe wall system were analyzed. Through the coupled operation of TRNSYS heat transfer model and GenOpt optimization software, the energy consumption characteristics of the system and the optimal value of the key influencing factors were analyzed and obtained. The optimal air gap thickness was 0.05 m, the optimal external sun-shading length was 0.78 m, the optimal thermal storage wall thickness was 0.68 m, the optimal vents area was 0.6 m2, the optimal melting temperature of lower temperature PCM layer was 16.5 °C, and the optimal melting temperature of higher temperature PCM layer was 27.75 °C. The annual total building load was reduced by 7.56% in optimized reference Trombe room compared with traditional Trombe wall, and the annual total building load was reduced by 13.52% in optimized PCM Trombe compared with reference Trombe wall.

Polymer Based Triboelectric Nanogenerator for Cost‐Effective Green Energy Generation and Implementation of Surface‐Charge Engineering

Performance of triboelectric nanogenerators for harvesting mechanical energy from the ambient environment has been limited by structural complexity, cost‐effectiveness, and mechanical weakness of materials. Herein, a cost‐effective vertical contact separation mode triboelectric nanogenerator using polyethylene (PE) and polycarbonate (PC) in a regular digital versatile disc is reported. This cost‐effective nanogenerator with simplified structures is able to generate an open‐circuit voltage of 215.3 V and short‐circuit current of 80 μA. The effects of the distance of impact and the air gap between the triboelectric layers have also been tested from 3 to 9 cm, and 0.25 to 1 cm, respectively. It is determined that 0.5 cm is the optimal air gap. The nanogenerator is also tested in different real‐life scenarios including stresses produced by a moving car, walking, and a rolling skateboard over the nanogenerator. The surfaces of the triboelectric layers are further modified by surface‐charge engineering which induced a 460% increase in the output power. These tests reveal a significant electrical response and mechanical stability under stress. In summary, this study demonstrates that the relatively inexpensive PE and PC triboelectric pair has the potential to be used for highly efficient, mechanically robust triboelectric nanogenerators.

Experimental study of heat and moisture transfer in vertical air gap under protective clothing against dry and wet heat exposures

PurposeThe purpose of this paper is to study heat and steam transfer in a vertical air gap and improve thermal protective performance of protective clothing under thermal radiation and hot steam.Design/methodology/approachAn experiment-based model was introduced to analyze heat and moisture transfer in the vertical air gap between the protective clothing and human body. A developed test apparatus was used to simulate different air gap sizes (3, 6, 9, 12, 15, 18, 21 and 24 mm). The protective clothing with different air gap sizes was subjected to dry and wet heat exposures.FindingsThe increase of the air gap size reduced the heat and moisture transfer from the protective clothing to the skin surface under both heat exposures. The minimum air gap size for the initiation of natural convection in the dry heat exposure was between 6 and 9 mm, while the air gap size for the occurrence of natural convection was increased in the wet heat exposure. In addition, the steam mass flux presented a sharp decrease with the rising of the air gap size, followed by a stable state, mainly depending on the molecular diffusion and the convection mass transfer.Originality/valueThis research provides a better understanding of the optimum air gap under the protective clothing, which contributes to the design of optimum air gap size that provided higher thermal protection against dry and wet heat exposures.

Improvement of Transverse-Flux Machine Characteristics by Finding an Optimal Air-Gap Diameter and Coil Cross-Section at the Given Magneto-Motive Force of the PMs

This paper presents analysis and study of the single-phase transverse-flux machine. The finite element method results of the machine are compared with the laboratory measurements to confirm the accuracy of the computer model. This computer model is then used to investigate the effect of the machine’s geometry on its output characteristics. Parametric analysis of the machine is carried out to find the optimal air-gap diameter at which the cogging torque of the machine is minimal. In addition, the influence of the coil cross-section on the torque and output power characteristics of the machine is investigated and discussed.

Reflective thermal insulation in non-ventilated air-gaps: experimental and theoretical evaluations on the global heat transfer coefficient

The refurbishment of existing buildings represents a priority target to reduce global energy consumption. An interesting solution is represented by multilayer systems made by insulating panels mounted on a suitable frame to form non-ventilated air-gaps with the existing envelopes. This solution is attractive because it allows for renewal and energy requalification of building façades by interventions addressed exclusively to the external surfaces, without interfering with occupants. However, it produces an increase of the building volume that could be in contrast with local construction regulations. In this context, the application of reflective (low-ε) thermal insulation panels inside the air–gap seems appropriate to reduce the system thickness without penalizing the thermal transmittance of the renovated envelope. However, addressed investigations conducted on the combined convective and radiative heat transfer coefficients inside enclosures equipped with low-ε materials, are lacking. In this paper, an experimental campaign conducted in a climatic chamber by the heat flux meter method on three different samples of commercial reflective panels inside a non-ventilated air–gap has shown, for the same sample thickness, an increase of the air–gap thermal resistance up to seven times. In order to evaluate the performances on a real scale, the experimental results were employed to tune a model developed in the COMSOL environment to determine the attainable global heat transfer coefficient in real scale air cavities. The results showed that thermo-reflective panels can produce the same effect of at least 6 cm of traditional insulating materials by avoiding additional space. Moreover, useful design information concerning the attainable thermal resistance growth related to the air–gap thickness and the thermo-reflective panel emissivity, were introduced. Results showed that by setting the vertical wall height, an optimal air–gap thickness that allows for minimizing the global heat transfer coefficient, can be identified, however, this is mainly when the emissivity coefficient of the panel surfaces is lower than 0.5. Furthermore, by setting the optimal thickness, the increase of the transferred thermal flux by quadruplicating the wall height is moderate being slightly greater than 16%. By monetizing the saved volume growth, the multilayer system equipped with thermo-reflective panels is economically profitable, especially with the insulating panel cost increase and in zones where the average building value is high.

Experimentally validated model of a self-switched solar motor for pumping application

This paper presents an original and autonomous solar pumping system, based on a Switched Reluctance Motor commutated by photovoltaic cells (SRMPV) and powered by a photovoltaic generator. In this work, experimental tests characterizing the SRMPV are presented. Based on these experiments, the first mathematical model of this complex system is developed and implemented under the Matlab-Simulink platform. In order to optimize the design of the SRMPV, the simulation platform is used to perform a sensitivity study. This research will focus on the determination of the optimal number of coils, the optimal air gap, and the optimal winding cross area of the switched reluctance motor.

Air gap technique is recommended in axiolateral hip radiographs.

PurposeTo investigate the replacement of conventional grid by air gap in axiolateral hip radiographs. The optimal air gap distance was studied with respect to radiation dose and image quality using phantom images, as well as 26 patient axiolateral hip radiographs.MethodsThe CDRAD phantom, along with polymethylmethacrylate slabs with thicknesses of 10.0, 14.6, and 20.0 cm was employed. The inverse image quality index and dose area product (DAP), as well as their combination, so called figure‐of‐merit (FOM) parameter, were evaluated for these images, with air gaps from 20 to 50 cm in increments of 10 cm. Images were compared to those acquired using a conventional grid utilized in hip radiography. Radiation dose was measured and kept constant at the surface of the detector by using a reference dosimeter. Verbal consent was asked from 26 patients to participate to the study. Air gap distances from 20 to 50 cm and tube current‐time products from 8 to 50 mAs were employed. Exposure index, DAP, as well as patient height and weight were recorded. Two radiologists evaluated the image quality of 26 hip axiolateral projection images on a 3‐point nondiagnostic — good/sufficiently good — too good scale. Source‐to‐image distance of 200 cm and peak tube voltage of 90 kVp were used in both studies.Results and conclusionBased on the phantom study, it is possible to reduce radiation dose by replacing conventional grid with air gap without compromising image quality. The optimal air gap distance appears to be 30 cm, based on the FOM analysis. Patient study corroborates this observation, as sufficiently good image quality was found in 24 of 26 patient radiographs, with 7 of 26 images obtained with 30 cm air gap. Thus, air gap method, with an air gap distance of 30 cm, is recommended in axiolateral hip radiography.