Influences of street aspect ratios and realistic solar heating on convective heat transfer and ventilation in full-scale 2D street canyons

Yang, Hongyu; Chen, Guanwen; Wang, Dongyang; Hang, Jian; Li, Qi; Wang, Qun

doi:10.1016/j.buildenv.2021.108125

Cited by 28 publications

(10 citation statements)

References 84 publications

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“…32 The idea NEV has been applied for evaluating outdoor urban ventilation performances as well. [35][36][37][38] Although studies related to NEV have advanced in recent years, the research results of Lim et al 30,31,33,34 and Chung et al 32 Therefore, this study comprehensively reviews the theoretical background of NEV as discussed in previous papers. [31][32][33][34] Additionally, we derived mathematically rigorous definitions for NEV and NEP.…”

Section: Practical Implicationmentioning

confidence: 98%

“…Note that Equations (11) and (12) are consistent with the original NEV and NEV* definitions in Lim et al, 31 although the formulations are generalized into an arbitrary CV. In some previous studies applying the NEV concept for urban breathability, the area to define the NEV in Equation (11) is assumed as the area geometrically defined by building morphologies to convert the PFR to the velocity scale 35–38 . However, the area,

S_{o}

, must be defined as the area where the outflux is observed to interpret the NEV as the escaping parts of NEV*.…”

Section: Theorymentioning

confidence: 99%

“…In some previous studies applying the NEV concept for urban breathability, the area to define the NEV in Equation ( 11) is assumed as the area geometrically defined by building morphologies to convert the PFR to the velocity scale. [35][36][37][38] However, the area, S o , must be defined as the area where the outflux is observed to interpret the NEV as the escaping parts of NEV*.…”

Section: Here C O Indicates the Volume-averaged Concentration With Re...mentioning

confidence: 99%

“…In addition to such case studies, Chung et al 32 further developed the NEV concept by defining the net escape probability (NEP), which is the probability that a pollutant generated at a local point is directly discharged from the exhaust outlet of a ventilation system so that the NEV can decompose into the returning and escaping components of pollutant transport 32 . The idea NEV has been applied for evaluating outdoor urban ventilation performances as well 35–38 …”

Section: Introductionmentioning

confidence: 99%

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Rigorous mathematical formulation of net escape velocity and net escape probability determining a macroscopic concentration

2022

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Net escape velocity (NEV) and net escape probability (NEP) are concepts that describe that scalar quantity discharged from a source in an indoor air environment is expressed by the unique velocity scales of the returning and escaping populations. Despite the conceptual description and applications of several numerical simulations, the definitions were not precisely explained using a mathematical formula. Here, we derive rigorous mathematical formulations of the NEV and NEP. These formulations provide us with the physical interpretation of NEV, clarify the sufficient condition of perfect escape, and lead to a further formulation of the transfer probability of the scalar. To justify and apply the derived relationships, two simple problems were numerically solved: One was a diffusion equation, and the other was an advection–diffusion equation. The results of the diffusion problem clearly demonstrate that only the outgoing scalar flux exists on the surface of the control volume, covering the source at any location. In contrast, the advection–diffusion problem reveals that there is a returning population of the scalar in most locations, despite both diffusion and turbulent parts working to remove the scalar. This rigorous formulation contributes to apply NEV as an appropriate air quality index with the clear physical interpretation to determine the local scalar concentration.

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Section: Practical Implicationmentioning

confidence: 98%

S_{o}

, must be defined as the area where the outflux is observed to interpret the NEV as the escaping parts of NEV*.…”

Section: Theorymentioning

confidence: 99%

Section: Here C O Indicates the Volume-averaged Concentration With Re...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Rigorous mathematical formulation of net escape velocity and net escape probability determining a macroscopic concentration

2022

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“…Babakhani et al [18] investigated the forced convective heat exchange at the façade with different building geometries and wind directions, in which a coefficient F was developed to modify the correlation between Nu and Re. In addition to the geometry of the building itself, the surroundings may also change the h c /Nu at the façade, such as adjacent buildings in the street canyon [19] and exterior components attached to the façade [20,21]. For the latter one, Jiang et al [21] analyzed the convective heat exchange of the windward building façade with louver blinds.…”

Section: Introductionmentioning

confidence: 99%

Investigation on Forced Convective Heat Transfer at the Building Façade with Louver Blinds under Different Wind Directions

Tao

et al. 2022

Buildings

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Exterior louver blinds are one of the most common shading devices attached to the building façade, which will not only screen solar radiation but also affect the flow around the building and change the convective heat exchange at the façade. To better understand how different wind directions affect the convective heat properties of louvered building façades, this paper conducted a CFD simulation with the validation of previous wind-tunnel experiments. A total of 63 shading cases and 7 unshaded cases were both investigated by simulations. It was found that with the increase in wind attack angle θ, the surface-averaged convective heat transfer coefficient (CHTC) at the louvered surface will first increase by about 40~50% (θ from 0° to 60°) and then be cut down by 60% (θ from 60° to 90°). In addition, the convective heat exchange rate at the louver slats will gradually decrease with the growth of θ. Finally, modifications for previous correlations were proposed to quantify the effect of wind directions on the CHTCavg at the blinds and façade, of which the maximum deviation for convective heat exchange rate is less than 18%.

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Possible high COVID-19 airborne infection risk in deep and poorly ventilated 2D street canyons

et al. 2023

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Despite the widespread assumption that outdoor environments provide sufficient ventilation and dilution capacity to mitigate the risk of COVID-19 infection, there is little understanding of airborne infection risk in outdoor urban areas with poor ventilation. To address this gap, we propose a modified Wells-Riley model based on the purging flow rate (QPFR), by using computational fluid dynamics (CFD) simulations. The model quantifies the outdoor risk in 2D street canyons with different approaching wind speeds, urban heating patterns and aspect ratios (building height to street width). We show that urban morphology plays a critical role in controlling airborne infectious disease transmission in outdoor environments, especially under calm winds; with deep street canyons (aspect ratio > 3) having a similar infection risk as typical indoor environments. While ground and leeward wall heating could reduce the risk, windward heating (e.g., windward wall ~10 K warmer than the ambient air) can increase the infection risk by up to 75%. Our research highlights the importance of considering outdoor infection risk and the critical role of urban morphology in mitigating airborne infection risk. By identifying and addressing these risks, we can inform measures that may enhance public health and safety, particularly in densely populated urban environments.

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Influences of street aspect ratios and realistic solar heating on convective heat transfer and ventilation in full-scale 2D street canyons

Cited by 28 publications

References 84 publications

Rigorous mathematical formulation of net escape velocity and net escape probability determining a macroscopic concentration

Rigorous mathematical formulation of net escape velocity and net escape probability determining a macroscopic concentration

Investigation on Forced Convective Heat Transfer at the Building Façade with Louver Blinds under Different Wind Directions

Possible high COVID-19 airborne infection risk in deep and poorly ventilated 2D street canyons

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