The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Thermal performance of non air-conditioned buildings with vaulted roofs in comparison with flat roofs

Vaulted or domed roofs have been frequently adopted by builders and architects throughout the Middle East and other hot dry areas. However, the thermal performance of such buildings under hot dry climatic conditions has rarely been quantitatively studied. In this paper, a detailed finite element model for the investigation of the thermal performance of non air-conditioned buildings with vaulted roofs (VR) is suggested based on two-dimensional unsteady heat transfer in such roofs and solar geometry. This model allows a comparison of the thermal performance of non air-conditioned buildings with a VR and a flat roof (FR) under different climatic conditions. Results obtained by numerical calculation show that, irrespective of building type the VRs are applied to, buildings with a VR have lower indoor temperatures as compared to those with a FR. The reason is that such roofs dissipate more heat than a FR does by convection and thermal radiation at night due to the enlarged curved surfaces. This implies that such roof forms are suitable for buildings located in hot dry regions but not for those located in hot humid areas, and reasonably explains why curved roofs have been extensively adopted by builders and architects in the hot dry areas in the past. However, with the decrease in the half rim angle of a VR, the difference of indoor thermal condition between a VR and a FR building becomes small and insignificant. Results also indicate that the indoor air temperature is slightly influenced by the half rim angle _θ0${\theta }_0$ and the orientation _φv${\phi }_v$ of the VR. To be effective to create a favorable thermal condition inside buildings with a VR under hot dry climatic conditions, the half rim angle of a VR should be θ₀>50°, instead of _θ0<50^°${\theta }_0<50^{°}$, which is the optimal half rim angle of a VR of air-conditioned buildings, as found by the present authors in a previous study.     

Thermal performance evaluation of domed roofs

Domed roofs have been used in Iran and many other countries to cover large buildings such as mosques, shrines, churches, schools. They have been also employed in other buildings like bazaars or market places in Iran due to their favorable thermal performance. The aim of this research is to study about domed roofs thermal performance in order to determine how they can be helpful in reducing the maximum air temperature of inside buildings during the warm seasons considering all parameters like air flow around them, solar radiation, radiation heat transfer with the sky and the ground as well as some openings on the building. The results of the study show that the thermal performance of the investigated domed roof is better than the building with flat roof, particularly when the dome is covered with glazed tiles. In addition to their aesthetic values, domes covered with glazed tiles have thermal benefits of keeping the inside air of these buildings relatively cool during the summer. Moreover, openings cause passive air flow inside building, which is helpful for human comfort.     

Evaluating the influence of the width of inlet slot on the prediction of indoor airflow: Comparison with experimental data

The aim of this work is to investigate the influence of two values of inlet slot width on the velocity characteristics and turbulent intensity of the airflow inside a rectangular room. The experimental data used to check the numerical results concerns a rectangular room where the air is supplied horizontally on the upper left and is exhausted through an opening on the lower right on the opposite side. The performance of three turbulence models, standard k-?, RNG k-?, and k-ω, in predicting the three-dimensional airflow in that room has also been investigated. The results for Reynolds number of 5000 are presented for dimensionless horizontal velocities and turbulent kinetic energy for two planes of the room and two inlet arrangements, one opening as large as the room and another with half of the width of the room. The results have indicated that the main features of the flow were captured by the three turbulence models investigated. On the whole, the performance of the standard k-? model was better than those of the other two turbulence models. In particular, the k-ω model performed better in the configuration with the largest air opening than in that with the smallest one, while the RNG k-? model presented the opposite behavior. The comparative study between both geometries demonstrated that for slots much smaller than the width of the room, three-dimensional effects become important.     

On the numerical study of contaminant particle concentration in indoor airflow

The application of three turbulence models—standard k–ε, re-normalization group (RNG) k–ε and RNG-based large eddy simulation (LES) model—to simulate indoor contaminant particle dispersion and concentration distribution in a model room has been investigated. The measured air phase velocity data obtained by Posner et al. [Energy and Buildings 2003;35:515–26], are used to validate the simulation results. All the three turbulence model predictions have shown to be in good agreement with the experimental data. The RNG-based LES model has shown to yield the best agreement. The flow of contaminant particles (with diameters of 1 and 10 μm) is simulated within the indoor airflow environment of the model room. Comparing the three turbulence models for particle flow predictions, the RNG-based LES model through better accommodating unsteady low-Reynolds-number (LRN) turbulent flow structure has shown to provide more realistic particle dispersion and concentration distribution than the other two conventional turbulence models. As the experimental approach to access indoor contaminant particle concentration can be rather expensive and unable to provide the required detailed information, the LES prediction can be effectively employed to validate the widely used k–ε models that are commonly applied in many building simulation investigations.     

Effects of plume buoyancy and momentum on the near-wake flow structure and dispersion behind an idealized building

Dispersion simulations of buoyant and neutral plume releases within the recirculation cavity behind a cubical building were performed using a commercially available CFD code and the RNG k-ε turbulence model. Plume buoyancy was observed to affect the size and shape of the cavity region and the flow structure and concentration profiles within. Source momentum of a neutral plume release had similar effects on the flow structure and the cavity region to that caused by plume buoyancy. However, the effects of momentum on the concentration profiles were noticeably different from that caused by plume buoyancy. Plumes released immediately downwind of a cubical building appear to alter the flow field and dispersion characteristics of the cavity recirculation region due to their inherent momentum and buoyancy. A greater fraction of a plume was captured inside the wake as the plume became increasingly buoyant. Contrarily, greater plume momentum resulted in smaller plume fractions captured inside the wake. Inclusion of these effects in the downwash algorithms would improve the accuracy of modeling results for far-field concentration distributions and would be mandatory in accident assessments where accurate predictions of short-term, near-field concentration fluctuations near source releases are required.     

The effect of source motion on contaminant distribution in the cleanrooms

In the recent decades, cleanrooms have found growing applications in broad range of industries such as pharmacy and microelectronics. Concerns about negative effects of the contaminant exposure on the human health and product quality motivate many researchers towards understanding of the airflow and contaminant distribution though these environments. With an improvement in computational capacity of the computers, computational fluid dynamics (CFD) technique has become a powerful tool to study the engineering problems including indoor air quality (IAQ). In this research, indoor airflow in a full-scale cleanroom is investigated numerically using Eulerian-Eulerian approach. To evaluate the ventilation system effectiveness, a new index, called final efficiency, is introduced which takes all aspects of the problem into account. The results show that the contaminant source motion and its path have a great influence on the contaminant dispersion through the room. Based on the results, the contaminant distribution indexes, e.g. final efficiency and spreading radius, are improved when the source motion path is in the dominant direction of the ventilation airflow. Consequently, the efficiency of an air distribution system which provides a directional airflow pattern shows the least source path dependency. This study and its results may be useful to gain better understanding of the source motion effects on the indoor air quality (IAQ) and to design more effective ventilation systems.     

Experimental and numerical study of a full scale ventilated enclosure: Comparison of four two equations closure turbulence models

Full-scale experimental and computational fluid dynamics (CFD) methods are used to investigate the velocity and temperature fields in a mechanically ventilated enclosure. Detailed airflow fields are measured in three cases of ventilation air temperature: an isothermal case, a hot case and a cold case. The ventilation system creates an axisymmetric jet which is developing near the ceiling. The experimental data are used to test four two equations turbulence models: a k–ε realizable model, a k–ε RNG model, a k–ω model and a k–ω SST model. It is found that, even if the models can predict reasonably the hot and isothermal cases global values of temperature and velocity, none of the models is reliable concerning the cold case. Moreover, a detailed analysis of the jet shows that none of the models is able to predict the exact experimental velocity and temperature fields.     

Passive cooling of cement-based roofs in tropical climates

In tropical climates, dwellings are made of cement-based materials like concrete to be able to withstand tropical storms and severe weather conditions. However, cement-based materials exhibit undesirable thermal properties including low thermal conductivity and thermal diffusivity which make living conditions almost unbearable. The purpose of this research project was to investigate the impact of a newly designed passive cooling system which can minimize heat transfer through concrete roofs. The passive cooling system consists of a corrugated aluminum sheet with a unique orientation to promote heat dissipation. A layer polyurethane is also used to minimize heat transfer. Experimental results based on lab-scale prototypes show that the well-designed roof insulation system can reduce the typical thermal load by over 70%. The passive cooling system also shows a desirable slow response time to irradiation, which is a desirable characteristic necessary to effectively control thermal fluctuations and reduce thermal loads simultaneously. The results also indicate that the cement-based roof midpoint temperature can be modeled accurately using an appropriate empirical model.     

Performance and influence of numerical sub-models on the CFD simulation of free and forced convection in double-glazed ventilated façades

Double-glazed façades (DGF) are an attractive option in contemporary architecture and are increasingly used in commercial buildings. They offer some advantages compared with single façade systems but require careful design. The solar-collector-like construction leads to high temperatures in the façade cavities and the possibility of the building overheating. This is undesirable effect, especially in Mediterranean climates. A possible solution for reducing thermal overheating is to use the air channel between the two layers of glass to evacuate the solar radiation absorbed by the façade. A suitable simulation procedure for modeling these façades would be very useful for designing buildings of this type.     

Effects of roof tile permeability on the thermal performance of ventilated roofs: Analysis of annual performance

This paper shows the results of the second part of an experimental study aimed at analysing the effects of roof tile permeability on the thermal performances of ventilation ducts. Ventilation ducts under the layer of tiles are typically used in south European countries to limit the energy load during the summer period. The results of the first part of the study, carried out by analysing 14 different types of roof, proved that the air permeability of the layer of tiles determines a certain amount of heat to be released, in addition to the release connected with the stack effect, in ventilation ducts which have the same characteristics but are perfectly airtight. However, the study did not completely resolve some issues since it was carried out on a model roof (6 m × 1.5 m) with devices to raise the layer of tiles and to create the ventilation duct but without those building elements which are present in real roofs and are used to stop insects and small animals from entering the ventilation duct. These elements narrow the inlet and outlet and consequently cause important reductions in pressure. Moreover, the measurements were based on data collected for limited periods of time during the summer season.     

CFD modelling of natural displacement ventilation in an enclosure connected to an atrium

Computational fluid dynamics (CFD) is used to investigate buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and is warmed by heat gains inside the single-storey space which produce a column of warm air in the atrium and drive a ventilation flow. CFD simulations were carried out with and without ventilation openings at the bottom of the atrium, and results were compared with predictions of analytical models and small-scale experiments. The influence of key CFD modelling issues, such as boundary conditions, solution controls, and mesh dependency were investigated. The airflow patterns, temperature distribution and ventilation flow rates predicted by the CFD model agreed favourably with the analytical models and the experiments. The work demonstrates the capability of CFD for predicting buoyancy-driven displacement natural ventilation flows in simple connected spaces.     

Airflow analysis in an air conditioning room

The aim of superior air conditioning system is no longer constrained to advancing the efficiency of cooling machine, but includes the study of airflow with the assistance of the distribution of several significant parameters. A simple numerical study of the turbulent flow over an enclosed air conditioning system was not practicable a few decades ago since the computer facilities were not sufficient. In this paper, a standard office room was taken up for simulation. Temperature and velocity distribution over various virtual planes for different locations of the air conditioner blower were analyzed to achieve the maximum comfort for the occupant. With Fluent, as solution tool, k–epsilon and Reynolds stress models for turbulence flow were used for the analysis. The different locations of blower placement are analyzed for better comfort of occupant in the room and it is found that the occupant will experience most comfort if the air conditioner blower is placed on location II compared to the other two locations. This work can also be extended to a more complex air conditioning system like in the industries, hospitals as well as the gigantic shopping malls.     

Numerical simulation of TBM construction ventilation in a long diversion tunnel

A three-dimensional Renormalization-group (RNG) k-ε model has been performed for forced ventilation to the working face of a long diversion tunnel, taking into account the effects of air leakage and the frictional resistance along the tunnel. The case study involves the working face during TBM construction of the Xinjiang 81 Daban long diversion tunnel, China. Analysis of the flow-field distribution and the pressure distribution near the working face and tunnel outlet revealed the relationships among the air leakage rate per 100 m, pressure difference at the air leakage port, and quantity of air in the air duct. The simulation results show that the air flux, velocity, and leakage rate gradually decrease along the tunnel. The air leakage rate per 100 m increases logarithmically along with pressure growth when the latter is limited within a certain pressure difference range. A low-pressure area can be found on the duct wall near the air leakage port, and the pressure inside the tunnel gradually decreases from the working face to the tunnel outlet; the velocity is relatively high near the leakage port, and is low in the tunnel. The simulated results were in good agreement with the experimental work by Cigdem Aydin and Hui-min Wang, and the simulated axial velocities of the tunnel were validated with the empirical value.     

考虑特征湍流影响的体育场悬挑屋盖脉动风压谱模型

基于风洞试验对体育场悬挑屋盖的脉动风压谱进行系统研究,旨在得到适用于此类结构的脉动风压谱模型,为风振响应分析提供必要的信息。通过对屋盖表面脉动风压进行谱分析,可知在屋盖前缘处的风压谱与来流风速谱较接近,但屋盖后缘处则差异很大,表现出明显的漩涡脱落特征。因此脉动风压自谱采用来流谱与漩涡脱落谱相结合的形式来描述,并通过权数因子体现屋盖表面不同位置处流场作用的特点。对于脉动风压互谱则用指数衰减函数来表示,并确定了适用于悬挑屋盖的衰减系数。为验证所提出风压谱模型的有效性及特征湍流对风致效应的影响,对系列悬挑屋盖结构进行风振响应分析,风荷载时程分别采用风洞试验测得的风压时程、基于建议风压谱模型模拟生成的风压时程、按拟定常假设生成的风压时程。基于建议模型得到的响应结果与试验结果基本一致,基于拟定常假设的风振响应极值偏小10%～15%,均方根值偏小30%～40%,脉动风压谱建模中不可忽略特征湍流的影响。     

On the comparison of the ventilation performance of street canyons of different aspect ratios and Richardson number

In this paper, the ventilation performances of (1) isothermal street canyons of different building-height-to-street-width (aspect) ratios (h/b) and (2) a ground-heated street canyon of h/b=1 at different Richardson numbers (Ri) are examined numerically by solving the Reynolds-averaged Navier-Stokes (RANS) equations with the use of the Renormalization Group (RNG) k-ɛ turbulence model. The mean () and turbulent (ACH’) air exchange rates (ACH) are calculated by the eddy-viscosity model instead of the turbulence kinetic energy (TKE) used elsewhere. For the isothermal street canyons, the ACH’ is found to account for 90% of the total ACH for 0.5 ≤ h/b ≤ 2. Similar to the previous large-eddy simulation (LES) and k-ɛ turbulence model, the magnitudes and shapes of the roof-level profiles of mean and fluctuating vertical winds are close to each other for different h/b. This suggests that turbulent mixing is important for the ventilation of isothermal street canyons. For the ground-heated street canyon, both the mean wind and turbulence are strengthened as illustrated by the increasing and ACH’ with decreasing Ri. A secondary recirculation is developed at the ground-level windward corner that pushes the primary recirculation upward and enhances and ACH as well.     

Improved k-ε model and wall function formulation for the RANS simulation of ABL flows

The simulation of Atmospheric Boundary Layer (ABL) flows is usually performed using the commercial CFD codes with RANS turbulence modelling and standard sand-grain rough wall functions. Such approach generally results in the undesired decay of the velocity and turbulent profiles specified at the domain inlet, before they reach the section of interest within the computational domain. This behaviour is a direct consequence of the inconsistency between the fully developed ABL inlet profiles and the wall function formulation.The present paper addresses the aforementioned issue and proposes a solution to it. A modified formulation of the Richards and Hoxey wall function for turbulence production is presented to avoid the well-documented over-prediction of the turbulent kinetic energy at the wall. Moreover, a modification of the standard k-ε turbulence model is proposed to allow specific arbitrary sets of fully developed profiles at the inlet section of the computational domain.The methodology is implemented and tested in the commercial code FLUENT v6.3 by means of the User Defined Functions (UDF). Results are presented for two neutral boundary layers over flat terrain, at wind tunnel and full scale, and for the flow around a bluff-body immersed into a wind-tunnel ABL. The potential of the proposed methodology in ensuring the homogeneity of velocity and turbulence quantities throughout the computational domain is demonstrated.     

Modeling and analysis of a fan–pad ventilated floricultural greenhouse

The application of greenhouse technology in the field of floriculture is rapidly expanding worldwide. In India solar radiation is abundant and the climate in the plains are rather hot and dry in summer months while the coastal parts witness a hot and humid climate. For greenhouses in such climates, cooling and ventilation are major factors influencing the production of quality flowers. In the present paper a thermal model of a greenhouse is presented based on fan–pad evaporative cooling. Thermal performance of the greenhouse, as predicted by the model under different climatic conditions is analyzed and compared with a reference study available in the literature. The analysis reveals that a suitable combination of evaporative cooling, shading and ventilation arrangements can effectively maintain the inside microclimate of the greenhouse within permissible limits throughout the year.     

Modeling the heat diffusion process in the abiotic layers of green roofs

Green roofs have been increasingly installed to alleviate some common environmental problems. The thermal benefit of living vegetation on rooftop has been extensively studied. The individual and joint contribution of the non-living green roof layers, namely soil, rockwool (water storage) and plastic drainage layers, to thermal performance of green roof has seldom been assessed. This study evaluates the insulating and cooling effects of these abiotic materials. A one-dimensional theoretical model was developed to assess the heat diffusion process in the layers. The model was validated with empirical results from three experimental plots. A calibration procedure was successfully applied to determine key model parameters. The model can capture the most critical features of temperature variations and thermal performance of common abiotic green roof materials. The appreciable water-retention capacity of rockwool plays the dual role of supplying water to the soil to enhance evaporative cooling, and increasing the specific heat capacity of the green roof. The plastic drainage sheet with ample air spaces serves as an excellent thermal insulator. The model remains robust despite seasonal and weather variabilities. Our research findings contradict with some researches in the temperate region that the thermal dissipation in green roofs with dense vegetation is lower than thermally insulated bare roofs. The theoretical model could be used to simulate the micro-environmental conditions and predict the thermal performance of different materials to improve green roof design.     

Lethal synergy of solar UV-radiation and H2O2 on wild Fusarium solani spores in distilled and natural well water

Environmentally-friendly disinfection methods are needed in many industrial applications. As a natural metabolite of many organisms, hydrogen peroxide (H₂O₂)-based disinfection may be such a method as long as H₂O₂ is used in non-toxic concentrations. Nevertheless, when applied alone as a disinfectant, H₂O₂ concentrations need to be high enough to achieve significant pathogen reduction, and this may lead to phytotoxicity. This paper shows how H₂O₂ disinfection concentrations could be significantly reduced by using the synergic lethality of H₂O₂ and sunlight the first time for fungi and disinfection. Experiments were performed on spores of Fusarium solani, the ubiquitous, pytho- and human pathogenic fungus. Laboratory (250-mL bottles) and pilot plant solar reactors (2 × 14 L compound parabolic collectors, CPCs) were employed with distilled water and real well water under natural sunlight. This opens the way to applications for agricultural water resources, seed disinfection, curing of fungal skin infections, etc.