Relationships between the properties of an urban street canyon and its radiant environment: Introduction of appropriate urban heat island mitigation technologies

The relationships between the properties of urban canopy components and the radiant environment in an urban street canyon are examined considering the introduction of appropriate urban heat island mitigation technologies. Radiant heat transfers between walls and roads are calculated according to Gebhart’s radiant absorption coefficients and using the Monte Carlo method. Roads are classified as either north–south or east–west; intersections are also considered. The key property of an urban street canyon is expressed by its aspect ratio W/H. A simple street canyon model and two actual urban street canyon areas are used as the objects of examination. Distributions of surface temperatures and solar radiation gains on street canyon roofs, roads, and walls are analyzed. The top priority for the implementation of urban heat island mitigation measures concerns the buildings with large roof areas. The other high-priority areas for implementing mitigation measures focus on smaller roofs and roads for which the street canyon aspect ratio W/H is greater than 1.5; the lowest-priority area is the walls.     

Modelling solar effects on the heat and mass transfer in a street canyon, a simplified approach

In urban areas, the climatic loads on buildings in summer conditions are largely affected by solar radiation. In this paper a modified simplified method for radiant interchange determination is used in a solar energy study. The good agreement with the radiosity method allows one to use this simplified method in the street canyon case. In a building pilot study, parametric analysis and building thermal behaviour can be assessed by simplified models which are useful for long-period simulation. Then this radiant interchange model is introduced in a zonal model of a canyon street and performed with a variable climatic conditions show case. The solar radiation is the only driving force in the street air movement. The interest of such approach for complex coupled phenomena studies is highlighted by obtained results and the assessment of variable climatic loads for different building zones can be considered with the model detailed herein. Future developments are planned in order to improve simulation accuracy by the addition of other local phenomena.     

Effect of gas radiation on radiative heat transfer in urban street canyon model

In this paper, we describe the results of numerical simulation of radiative heat transfer between the human body and an urban street canyon (building walls, pavement, and the sky) in the presence of participating non‐gray gas mixtures consisting of H₂O and CO₂. The ambient temperature in typical summer conditions and the concentration of gas mixtures during summer in Tokyo were assumed. Further, the parallel infinite plane model and simple urban street canyon model were used. The results show that the participating gas significantly affects the infrared radiation field in an urban street canyon. The radiation flux emitted by the participating gas is approximately 35% of the total radiation flux incident on the human body surface. This causes a homogenization of the infrared radiation field surrounding the human body. Gas radiation plays an important role in the heat transfer between the human body and the environment under hot and humid summer conditions. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20258     

Building cluster and shading in urban canyon for hot dry climate: Part 1: Air and surface temperature measurements

Under low latitude conditions, minimization of solar radiation within the urban environment may often be a desirable criterion in urban design. The dominance of the direct component of the global solar irradiance under clear high sun conditions requires that the street solar access must be small. It is well known that the size and proportion of open spaces has a great influence on the urban microclimateThis paper is directed towards finding the interaction between urban canyon geometry and incident solar radiation. The effect of building height and street width on the shading of the street surfaces and ground for different orientations have been examined and evaluated. It is aimed to explore the extent to which these parameters affect the temperature in the street. This work is based on air and surface temperature measurements taken in different urban street canyons in EL-Oued City (hot and arid climate), Algeria.In general, the results show that there are less air temperature variations compared to the surface temperature which really depends on the street geometry and sky view factor. In other words, there is a big correlation between the street geometry, sky view factor and surface temperatures.     

A process simulation model for a 2 ton h−1 incinerator (a combined bed combustion and furnace heat transfer model)

A process simulation model was constructed for a 2 ton h⁻¹ incinerator. The simulation model was designed to provide system performance parameters according to various operating conditions. In accommodating the wide variation of quality and composition of input wastes, the plant operating parameters such as amount of excess air, preheated air temperature, waste feed rate and primary air distribution over the stoker, etc. must be carefully controlled. The proposed model calculates operating variables of each submodule, by employing steady-state thermal and material balance equations. Combustion of waste bed, and its radiative heat transfer in the combusion chamber are considered. The calculated results of the combustion chamber performance are evaluated, in terms of temperature, locations and width of the flame band, and mean residence time in the secondary combustion chamber. These results are compared with a limited set of field test measurements for verification of the model. © 1998 John Wiley & Sons, Ltd.     

Impact of street design on urban microclimate for semi arid climate (Constantine)

Urban Planning has an immense impact on local microclimate which in turn affects the comfort and space quality within a city. The urban open spaces play an important role in creating the urban climate. The urban streets vary in geometry as defined by height/width ratio, sky view factor (SVF) and the orientation that is defined by its long axis. This directly influences the absorption and emission of incoming solar and outgoing long wave radiation which has a significant impact on the temperature variations within the street as well as the surrounding environment (Urban Heat Island).The objective of this research is to evaluate and to compare how the microclimate variation of urban street canyon can affect the built environment. Therefore the main aim of this paper is to discuss and assess the impact of the geometry on the street climate, in down town of Constantine-Algeria (semi arid climate).In order to achieve this goal, a series of site measurements, are utilised. The preliminary results strongly indicate an air temperature difference of about 3–6 ^°C between the urban street and its surrounding rural environment.     

燃烧参数影响高温空气燃烧特性的数值分析

高温空气燃烧技术具有高效节能和低NOx排放等多重优越性，是一种新型燃烧技术。为了深入研究高温空气燃烧机理和低氮氧化物排放特性，将湍流N—S方程与扩散燃烧模型和热力型NO生成模型相结合，研究了低氧浓度条件下，燃烧参数，如燃气供应量，过量空气系数，进口空气预热温度以及进口空气氧含量对燃烧的影响，为发展高温空气燃烧技术提供了理论依据。     

Effects of asymmetry, galleries, overhanging façades and vegetation on thermal comfort in urban street canyons

The present paper deals with the dependence of outdoor thermal comfort on the design of an urban street. The effects of the street vertical profile, including asymmetrical canyon shapes, the use of galleries and further shading devices on the façades, arranged in various orientations are assessed. The study is conducted by means of numerical modelling by using the three-dimensional microclimate model ENVI-met 3.0 which prognosticates the microclimatic changes within urban environments. Thermal comfort is evaluated for the daytime hours across the canyon in high spatial resolution and by means of the physiologically equivalent temperature PET.The results revealed that all design aspects investigated have a moderate impact on the air temperature and a strong effect on the heat gained by a human body and hence on the resulting thermal sensation. The larger the openness to the sky of the canyon, the higher the heat stress. For canyons with a smaller sky view, the orientation is also decisive: E–W canyons are the most stressful and deviating from this orientation ameliorates the thermal conditions. Basically, galleries and further shading through overhanging façades or vegetation enable a sensitive decrease of the period of time and of the area of thermal discomfort. Yet, this efficiency varies with the orientation and the vertical proportions of the canyon. Therefore, if appropriately combined, all investigated design elements can effectively mitigate heat stress in the summer and promote thermal comfort.     

Evaluating the impact of canyon geometry and orientation on cooling loads in a high-mass building in a hot dry environment

In the present paper, thermal simulation software (IDA Indoor Climate and Energy, developed by EQUA, Sweden) was employed in order to evaluate the effect of different street canyon geometries and orientations on building cooling loads in a dry environment – Sede-Boqer, located in the Negev desert, Israel. A residential building was monitored from January to August 2006 to calibrate the thermal and energy simulations in summer, using diverse solar geometries and axis orientations as input data. Simulations were carried out using the local TMY, taking into account different aspect ratios (H/W): 0.33, 0.66, 1 and 2 and street axis orientations: east–west, north–south, northwest–southeast (parallel to prevailing wind) and northeast–southwest (perpendicular to prevailing wind). Results are shown in terms of energy consumption for cooling for each canyon configuration. From the obtained results, general recommendations can be traced for urban planning in arid regions, which could help promoting sustainable and energy-responsive buildings.     

A new dynamic clothing model. Part 2: Parameters of the underclothing microclimate

Based on a new modeling, described in the first part of this paper, which takes into account the pumping effect under garments, the various parameters characterising the confined air, and managing its dry and latent losses, are determined. The mean temperature, calculated from heat exchanges with skin (or underwear) and with the garment, progresses exponentially as a function of the trapped time, until a limit. The mean humidity amount, determined from the energy of total evaporation, from the air layer renewal rate and from the water vapour diffusion through the fabric, increases linearly. Using a movable thermal manikin, walking at various speeds, and with a combined effect with wind, the intrinsic air speed and convection coefficient are defined. The intrinsic air speed combines the effects of external air and body motions. The intrinsic convection coefficient is a linear function of the square root of the inner air speed. The relations expressing the trapped time are obtained, for thin and thick garments, by comparison between this new dynamic model and the model built by Lotens and Havenith (1991) which predicts the effect of posture, motion and wind on the clothing insulation. The evaluation of the amount of heat and mass transferred by pumping effect requires the knowledge of all these parameters.     

A new dynamic clothing model. Part 2: Parameters of the underclothing microclimate

Based on a new modeling, described in the first part of this paper, which takes into account the pumping effect under garments, the various parameters characterising the confined air, and managing its dry and latent losses, are determined. The mean temperature, calculated from heat exchanges with skin (or underwear) and with the garment, progresses exponentially as a function of the trapped time, until a limit. The mean humidity amount, determined from the energy of total evaporation, from the air layer renewal rate and from the water vapour diffusion through the fabric, increases linearly. Using a movable thermal manikin, walking at various speeds, and with a combined effect with wind, the intrinsic air speed and convection coefficient are defined. The intrinsic air speed combines the effects of external air and body motions. The intrinsic convection coefficient is a linear function of the square root of the inner air speed. The relations expressing the trapped time are obtained, for thin and thick garments, by comparison between this new dynamic model and the model built by Lotens and Havenith (1991) which predicts the effect of posture, motion and wind on the clothing insulation. The evaluation of the amount of heat and mass transferred by pumping effect requires the knowledge of all these parameters.     

An air filter pressure loss model for fan energy calculation in air handling units

Air filters consume a significant part of the fan power in air handling systems. Due to lack of suitable models, the fan energy associated with the filter pressure drop is often estimated based on average airflow and average pressure drop across the filter. Since the pressure drop varies nonlinearly with airflow and the filter resistance varies with dirt build‐up, current methods often produce erroneous results. This paper presents a new air filter pressure loss model that has been developed and verified using experimental data. The model projects the pressure losses across the filter for both constant and variable airflows. The inputs to the model are the airflow rate, the time of use, the initial design and final pressure losses at the design flow rate, and the coefficient of a power law regression of pressure loss as a function of airflow rate. The air filter pressure loss model may be implemented in hourly building energy simulation programs that perform hourly simulation at the air handling unit level. Copyright © 2003 John Wiley & Sons, Ltd.     

Building cluster and shading in urban canyon for hot dry climate: Part 2: Shading simulations

Under low latitude conditions, minimisation of solar irradiance within the urban environment may often be an important criterion in urban design. This can be achieved when the obstruction angle is large (high H/W ratio, H=height, W=width). Solar access to streets can always be decreased by increasing H/W to larger values.It is shown in this paper that the street canyon orientation (and not only the H/W ratio) has a considerable effect on solar shading and urban microclimate. The paper demonstrates through a series of shading simulation and temperature measurements that a number of useful relationships can be developed between the geometry and the microclimate of urban street canyons. These relationships are potentially helpful to assist in the formulation of urban design guidelines governing street dimensions and orientations for use by urban designers.     

The effect of urban layout,street geometry and orientation on shading conditions in urban canyons in the Mediterranean

The paper presents the results of shading analysis which was carried out as part of a wider comparative analysis of two sites with different characteristics in terms of street geometry and urban density. The first experiment site was a traditional settlement in the island of Tinos, Greece, and the second was a relatively newly built part of the capital city of the island. Also a parametric shading analysis was carried out in order to examine a number of parameters that influence shading conditions in urban canyons.The paper aims in analyzing the effect of parameters such as urban layout, street geometry and orientation on solar access and shading conditions, which strongly affect urban canyon microclimate. The results of shading simulations are compared to the results of experimental measurements of air and surface temperatures and to parametric thermal analysis results. The conclusions can contribute in the formulation of urban design guidelines.     

Direct ethanol fuel cell anode simulation model

The electrochemical behavior of a direct ethanol feed proton exchange membrane fuel cell (DEFC) operating under steady-state isothermal conditions at 1 atm at both anode and cathode sides is considered. A mathematical model that describes in one phase and one dimension the ethanol mass transport throughout the anode compartment and proton exchange membrane is developed. The influence of the operation parameters such as current density, temperature, catalyst layer thickness and ethanol feed concentration on both anode overpotential and ethanol crossover rate has been examined. According to the simulation results, it was found that the anode overpotential is more sensitive to the protonic conductivity than to the diffusion coefficient of ethanol in the catalyst layer. It was concluded that in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover is a serious problem for a DEFC performance. Finally, it was found a good agreement between simulation and experimental results.     

街道峡谷内污染物对流扩散模拟分析

采用数值模拟和现场实验观测相结合的方法,对街道峡谷内汽车污染物的扩散规律进行研究。在进行数值模拟时,依据污染物对流扩散的基本理论,对不同风速下污染物的对流扩散特征进行模拟分析。将结果与现场观测结果进行比较,从而验证了数学模型的正确性,同时也获得了城市街道峡谷内污染物对流扩散规律。研究结果可为城市街道规划建设、建筑高度和密度控制、机动车尾气的排放控制和交通流量控制提供科学依据。     

Modeling of inhomogeneous compression effects of porous GDL on transport phenomena and performance in PEM fuel cells

A comprehensive, three‐dimensional model of a proton exchange membrane (PEM) fuel cell based on a steady state code has been developed. The model is validated and further be applied to investigate the effects of various porosity of the gas diffusion layer (GDL) below channel land areas, on thermal diffusivity, temperature distribution, oxygen diffusion coefficient, oxygen concentration, activation loss and local current density. The porosity variation of the GDL is caused by the clamping force during assembling, in terms of various compression ratios, that is, 0%, 10%, 20%, 30% and 40%. The simulation results show that the higher compression ratio on the GDL leads to lower porosity, and this is helpful for the heat removal from the cell. The compression effects of the GDL below the land areas have a contrary impact on the oxygen diffusion coefficient, oxygen concentration, cathode activation loss, local current density and cell performance. Generally, a lower porosity leads to a smaller oxygen diffusion coefficient, a less uniform oxygen concentration, a higher activation loss, a smaller local current density and worse cell performance. In order to have a better cell performance, the clamping force on the cell should be as low as possible but ensure gas sealing. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of ambient vapor concentration on droplet evaporation in a perspective of comparison between diffusion controlled model and kinetic model

A comparative study of purely diffusion controlled, kinetic, and empirical models has been executed on the predictions of evaporation characteristics of water droplet in a quiescent ambience of air with varying vapor concentrations at different pressures and temperatures. A significant increase in droplet lifetime with free stream vapor concentration has been observed at lower values of the free stream temperature, as predicted by all the models. An increase in ambient pressure increases the droplet lifetime at low temperature, while it does the opposite at higher ambient temperature. The droplet lifetime predicted by the kinetic model is always greater than that predicted by the purely diffusion controlled model. For a droplet of initial radius 20 μm, the purely diffusion controlled and kinetic models underpredict the droplet lifetime as compared to that obtained from the empirical model. For a droplet of initial radius of 5 μm, the purely diffusion controlled model underpredicts the droplet lifetime while the kinetic model overpredicts the same as compared to empirical values. The predictions of lifetime by kinetic models are closer to those obtained from empirical ones in case of evaporation in vapor rich ambience.     

Reducing spatial dimensionality in a model of moisture diffusion in a solid material

A model of the moisture movement in solid wood under isothermal conditions taking into consideration coating (insulation) of the surface of the material, is presented in a 2-D-in-space formulation. The validity of using a corresponding 1-D model on a 2-D problem is investigated. A measure of reliability of the 1-D model is introduced. This paper presents a new technique, which adjusts the width of a material as well as the degree of coating of the edges, to ensure the relative error of the problem solution is less than the required, resulting from reducing the model from 2-D to 1-D-in-space. This technique is based on the computer simulation of 2-D diffusion to estimate the reliability of the corresponding 1-D diffusion model.     

Determination of moisture diffusion coefficient in transformer paper using thermogravimetric analysis

This paper presents the methodology used and the results obtained in the determination of moisture diffusion coefficient of non-impregnated transformer insulating paper. In order to establish the diffusion coefficient, drying curves of paper samples were obtained by means of thermogravimetric experiments. The diffusion coefficient parameters were found by applying an optimization process based on genetic algorithms. The error function between measured and simulated curves was determined, and the parameters achieving the best correspondence between measured and estimated values were obtained. As a result, a new equation for the diffusion coefficient of non-impregnated insulating paper is proposed, depending on average moisture concentration, temperature and insulation thickness. The proposed coefficient was validated through experimental cases finding a good agreement between the experimental drying curves and those obtained by simulation using the diffusion coefficient. The proposed diffusion coefficient can be used for the determination of the time required to dry power transformers in factory.