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A healthy and efficient ventilation system is essential for establishing a comfortable indoor environment and significantly reducing a building's energy demand simultaneously. This paper proposed a novel ventilation system and applied it to the IEA Annex 20 mixing ventilation enclosure to verify its feasibility through mathematical modeling and CFD simulations. First, two bionic ventilation systems, single-side and dual-side ventilations, were compared to a conventional constant-volume supply system using CFD simulations, with the results demonstrating that the bionic ventilation system could provide higher ventilation efficiency and more effective pollutant removal from stagnant regions. Furthermore, the present work exercised these two bionic ventilation systems with different temporal periods of sine and rectangular wave functions, identifying a turning point at a period of 0.06 τ n , which could contribute to further enhancement of these bionic ventilation systems. Finally, a methodology depending on the Bayesian inference algorithm was developed for identifying pollution sources in the bionic ventilation system with unstable flow fields, and factors influencing source identification accuracy were discussed. The results show that the peaks of the KDE distributions and the sampling average values of both the source location and intensity are all consistent with the actual source parameters. The potential of the proposed bionic ventilation systems has been well demonstrated by direct and inverse CFD models, paving the way for further engineering applications. [ABSTRACT FROM AUTHOR]

Solar photocatalysis has been an efficient option to eliminate indoor volatile organic pollutants (VOCs) due to its relatively low cost. Moreover, solar ventilation wall is a common heating and ventilating technology that supplies air circulation in a room and expels polluted air under the action of wind pressure. An efficient air purification type ventilation wall that meets indoor pollutant purification, natural ventilation and thermal fully driven by the combination of solar photocatalytic purification technology and natural ventilation wall has been developed by previous research. The aim of this updated review is to explore the development of the solar photocatalytic ventilation wall (SPVW) system and critically analyze the scientific literature on photocatalysts, models, mechanism, parameter analysis and evaluation of this multifunctional system. The review also included the effects of parameters such as initial concentrations of pollutants, ultraviolet (UV) light intensity, relative humidity, channel width, channel height on the solar photocatalytic characteristics and natural ventilation performance, and a brief performance evaluation on natural ventilation, indoor heating and solar photocatalytic degradation. The governing analysis reveals that the daily stability of the SPVW system could be achieved by enhancing the activity of the photocatalyst and decreasing fluctuation of light source in the external input. [ABSTRACT FROM AUTHOR]

Abstract The air flow and pollutant dispersion within a group of street canyons ventilated merely by thermal buoyancy force induced by heated building surfaces are examined by CFD model using SST k - ω turbulence model for different urban skyline configurations. Pollutants emitted from the bottom of street canyon roughly mimic the traffic exhaust releasing. Numerical results are validated well with former theoretical results on thermal boundary flow adjacent to a heated vertical wall. The air exchange rate per hour (ACH) and pollutant retention time are adopted to evaluate the canyon ventilation performance. An exponential relationship could be established between the pollutant retention time and the thermal boundary flow rate. A semi-empirical formula is proposed by using the theoretical results of thermal boundary layer and two empirical constants derived from the present simulation results, which could be used to evaluate the ventilation performance at the urban design. As the convergence flow at the street canyon roof decays from urban rim to urban center, the pollutant retention time differs from canyon to canyon. The " protuberant " skyline configuration is found more effectively in purifying the street canyons at urban edge, in contrast, the " concave " skyline configuration shows higher purification efficiency at urban center. Present research could benefit for design purpose and environmental impact assessment. Highlights • Canyon dispersion was modelled under the regime of pure buoyancy driven flows. • Theoretical evaluation on the air exchange rate at canyon roof was presented. • Air exchange rates were of linear relationships with thermal boundary flow rates. • Thermal plume merging was a key pattern for near-ground level pollutant dilution. • Pollutant retention time reduces exponentially with the increase of boundary flow rate. [ABSTRACT FROM AUTHOR]

The opening location plays an important role of buildings’ environment and natural ventilation effectiveness. Present research investigated three categories of ventilation configurations: single-sided ventilation with an opening on the windward wall; single-sided ventilation with an opening on the leeward wall and cross ventilation with two openings. The velocity distribution and pressure distribution as well as the pressure coefficient are presented to analyze the representative fluid dynamic characteristics of these three configurations. Numerical results indicate that cross ventilation could achieve the best ventilation performance, and the situation for opening on the leeward wall is better than that on the windward wall. According to these results, the study gives some guidance for building design on natural ventilation and provides some useful ideas as well as approach and methodology for the development of energy-efficient buildings. [ABSTRACT FROM AUTHOR]

In the present work, an open-type thermoelectric heating system with multiple channels was developed. A mathematical model of heat transfer, based on one-dimensional treatment of thermal and electric power, is conducted. The heating coefficient and production are both correlated in terms of temperature difference, thermal conductivity, electric resistance and electric current. The looped air circulation was designed to simultaneously recycle heat and enhance heater system performance. Experimental investigations were conducted to identify thermal performance of the thermoelectric heater. Effects of airflow rates through the heating side and cooling side, temperatures of heating side and cooling side on the performance were investigated. The heating coefficient was calculated upon that surface temperatures of hot and cold sides were recorded. The results show that the average heating coefficient of the thermoelectric heating system could reach to 1.3, which is greater than that of a typical electric heater with heating coefficient of less than one. The optimal isolation thickness for this thermoelectric heater, i.e., 14 mm, is confirmed by the experimental rig system. Generally, heating coefficient was found to increase first and decrease afterwards when the current was continuously increasing. Analytical and experimental results demonstrate that the optimum performance of the thermoelectric heat recovery heater strongly depends on the intensities of these operating parameters. [ABSTRACT FROM AUTHOR]

Periodic vortex shedding from the building can induce pumping ventilation (PV) and promote the ventilation rate of single-sided ventilation with well-arranged open windows. The present study aims to investigate the influence of sheltering conditions (presence of upwind and/or downwind buildings) on pumping ventilation using wind tunnel experiments. Different building gap aspect ratios and sheltering conditions were considered as main influencing factors. The mean streamwise velocity and velocity fluctuation around the target building were measured. The oscillation frequency of pumping ventilation was obtained from the time history of window center velocity. The effective ventilation rate was utilized to quantify the ventilation effectiveness of pumping ventilation through constant-injection tracer gas method. The experimental results show that the influence of upwind building on the mean velocity and velocity fluctuation are much more significant than that of downwind building. The presence of upwind or downwind building can reduce the pumping ventilation frequency compared with the isolated case. The downwind building has larger reduction effect on PV frequency of the target building compared with the upwind building. Increase of gap aspect ratio could linearly promote the PV frequency. Sheltering could promote the effective ventilation rate of pumping ventilation compared with the isolated case and the ventilation rate could be overall the highest when sheltered by both upwind and downwind building. Upwind building has much larger effect on the concentration fluctuation inside the target building than downwind building. The findings of this study may help to better understand pumping ventilation mechanism in urban areas. • Wind tunnel experiments on pumping ventilation were conducted with reduced scale models. • The influence of upwind building on the velocity was more significant than downwind building. • Downwind building had larger reduction effect on PV frequency than upwind building. • Sheltering promoted the ventilation rate of pumping ventilation compared with isolated one. • This study may help to understand pumping ventilation mechanism in urban areas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Classroom thermal comfort and indoor air quality under natural ventilation. [•] Detailed correlations of heat loss and ventilation effectiveness ratio. [•] Displacement natural ventilation energy conservation and performance analysis. [•] Full CFD modeling and on-site measurements on the environment. [•] Effect of thermal buoyancy on air flow pattern, thermal comfort and air quality. [ABSTRACT FROM AUTHOR]

Natural ventilation nowadays has been paid great concerns due to its zero carbon emission and good performance on the human health. In engineering applications, cross ventilation driven by winds has been frequently restricted in building clustered cities. Instead, single-sided natural ventilation becomes an alternative mode in wind driven natural ventilation strategies for clustered urban buildings. This research has reviewed the former published researches on single-sided natural ventilation in terms of the classification, features, influence factors, investigation methodologies and evaluation indices/parameters. Existing researches on a novel ventilation mechanism of single-sided natural ventilation—"pumping ventilation" have been comprehensively reviewed, which could be a promising ventilation strategy of single-sided natural ventilation. This critical review demonstrates that single-sided ventilation has raised increasing concerns of researchers. In current and future investigations, different methodologies and other advanced technologies should be coupled together to promote the predicting capability of single-sided ventilation. This review could facilitate the fundamental researches and engineering applications of natural ventilation in modern urban buildings. [ABSTRACT FROM AUTHOR]

• Modern cities could be vulnerable to radionuclides and other airborne pollutants;. • Urban ventilation capacity could be reduced or enhanced by the urban skylines;. • Increase of inhomogeneity of adjacent buildings reduced the age of air in the canyons;. • Retention became longer in well-ventilated regions, and it shrank in poorly-ventilated regions;. • Built array morphology essentially affects the dispersion of radionuclides in urban blocks. In the present work, a parametric CFD research of an urban block model was conducted to investigate the effects of street aspect ratio and urban skyline on their ventilation and radionuclide dispersion within such urban blocks. Representative ventilation indicators, including velocity ratio (V R), age of air (τ P), retention time (τ c) and atmospheric dispersion factor (A D F), were introduced here to evaluate urban blocks ventilation and radionuclide dispersion. Our numerical results indicated that the street aspect ratio has a dominant impact on the air age of the main street canyons. The age of air in the canyon increased with the expansion of its street aspect ratio. When remarkable downwash and weak wind regions were observed around the higher buildings, the age of air on the windward side of a higher building became small, whereas it prolonged on the leeward side. Similarly, canyon retention time increased positively with the street aspect ratio. There was also a longer canyon retention time on the windward side of higher buildings. Reversely, short canyon retention time was found on the leeward side. Additionally, mechanical disturbance of buildings and the rugged underlying surface primarily influence the radionuclides dispersion in the urban region, whereas wind wake flows heavily affect the radionuclides dispersion in the downstream of urban blocks. This research has provided a theoretical reference for the ventilation and far-field pollutant dispersion in urban blocks located near mining or industrial areas. [ABSTRACT FROM AUTHOR]

Abstract: The backward time modeling of diffusion–convection pollutant dispersions has been developed with quasi-reversibility method in this work. The procedure is applied to the backward time identification of the contaminant release history and source location in a three-dimensional slot ventilated building enclosure. Spatial distributions of pollutant concentrations are known in priori. The effects of supplying air velocity, pollutant source location, pollutant diffusivity property, and pollutant release time on the accuracy of the pollutant dispersion history recovery have been investigated. Numerical results demonstrate that the accuracy of the pollutant dispersion history recovery can be enhanced with different approaches and measures, including the promotion of room ventilation rate, the shrinkage of distance between the pollutant source and supplying air port, and the reduction of pollutant diffusivity. The facilitated implementations of boundary conditions and the improved generality of quasi-reversibility methods make the pollutant source history identifications of less computational efforts. Particularly, the good agreement of the backward time identified source location with the true situation fully shows that quasi-reversibility method is more competitive in the engineering applications involving with convective fluid flows. [Copyright &y& Elsevier]

Abstract: Slot ventilated enclosure flows have been simulated, respectively in displacement ventilation and mixed ventilation covering from the forced convection dominated flow to the natural convection dominated flow. Direct convection simulation together with the turbulent streamlines and turbulent heatlines demonstrate that the enclosure flow pattern, indoor thermal level and heat transfer potential will depend on the interactions of external forced flow and thermal buoyancy driven flows, i.e., Reynolds number and Grashof number. In subsequent inverse convection modeling, the inverse determination of enclosure wall heat flux profiles was conducted by the use of adjoint methodology, in which the direct, sensitivity and adjoint problems are formulated and solved by finite volume method. The effects of the supplying air flow rate, thermal source strength, ventilation mode, flux functional forms, and the measurement errors on the accuracy of inverse turbulent convection estimation have been investigated. The inverse solutions of turbulent convections are of low level accuracy as the flow becomes thermal-driven turbulent flows, and they deteriorate as the noise levels increase. This work is of fundamental importance for the room air flow design and measurements involving the turbulent thermal convections. [Copyright &y& Elsevier]

• Green roof influenced on the outdoor thermal buoyancy flow distributions in a wide range of H/W. • Aiding and opposing effects were observed as wind and thermal buoyancy simultaneously acted. • A "free air flow region" arose when the leeward green roof deployment was adopted. • Lateral deployment of green roof system performed better than that of rooftop deployment. • Rational design of the green roof system could enhance its cooling capacity. The "thermal effect" of the green roof system on the pollutant dilution process would be analyzed in this research. Different arrangement schemes (on the rooftop, windward side and leeward side) of the green roof system would alter the thermal buoyancy distribution in the street canyon, thus change the overall flow field and pollutant dilution condition Computational Fluid Dynamics (CFD) technique along with the Renormalization Group (RNG) k-ε turbulence model has been adopted for the simulation work. Cases without the green roof system are assumed to be uniformly heated and are prepared for benchmark comparisons. Six aspect ratios (H/W) and ten ranks of unstable thermal stratifications (Rb) are selected to consider the influence of street canyon morphology and thermal buoyancy. Results demonstrate that the lateral deployment of green roof system performs better than the rooftop deployment and the situations without green roof system. In the windward cooling cases, a single vortex is formed in the canyon center and dramatically reduces the pollutant retention time τ when H/W > 1, while in the leeward cooling cases, a "free airflow" region is formed in all the tested aspect ratios expect for H/W = 0.5. The rooftop deployment scheme has negative effects, not just decaying ventilation performance, but also delaying pollutant dispersion period. Present research could benefit the rational design of the green roof system in the urban building clusters, and further enhance its cooling capacity for the built energy conservation. [ABSTRACT FROM AUTHOR]

Pumping ventilation is a special single-sided wind driven building ventilation induced by periodic vortex shedding at the building wake. This work aimed to investigate the oscillating frequency at the building façade opening and ventilation rate of pumping ventilation. The effects of opening separations on different floors of a three-story reduce-scaled building with two openings were considered using boundary-layer wind tunnel experiments. The wind velocity of the surrounding flow field and the center of the opening were measured. The ventilation rate was obtained by tracer gas method with constant and continuous injection rate. The results showed that the oscillating frequency was independent of the opening separation except on the third floor probably because of the disturbance of the rooftop shedding frequency. The oscillating frequency on the first floor was the lowest due to the resistance of the ground to the vortex shedding. Pumping ventilation indicated that its ventilation rate is greater than that of general ventilation across the single opening with the same total opening area. The promotion of ventilation rate was up to about 123% on the first floor, about 65% on the second floor and about 44% on the third floor. Expansion of opening separations could boost the ventilation rate. Meanwhile, the fluctuation of opening center wind velocity and indoor species concentration were not positively correlated with the ventilation rate of pumping ventilation. Conclusions of this research could provide some useful reference to the design of natural ventilation for buildings. • Wind tunnel experiments on pumping ventilation were conducted with reduced scale models. • Influence of opening separations and ventilating floors were experimentally investigated. • The promotion of ventilation rate could be up to about 123% from SS1 to SS2. • Pumping ventilation could be applied for future low carbon built environment. [ABSTRACT FROM AUTHOR]

Improved ventilation in an urban residential neighborhood is a determinant for better energy efficiency and air quality of the neighborhood. This is particularly important for the real estimates recently developed in many cities in China, as these neighborhoods are built with high density. In this paper, we consider in an idealized neighborhood with two categories of packaging building densities ( λ p ) of 0.25 and 0.44, which respectively represent medium and compact urban development. For each packaging density, the frontal area density of arrays is varied from 0.125 to 0.5 through adding building blocks and reducing building size simultaneously. Computational fluid dynamics simulations are carried out to predict the wind flow through the building arrays by the use of renormalization group (RNG) k - ε turbulence model. Our predictions are validated by comparing with experimental data. Numerical results reveal that ventilation rate could be greatly enhanced when a low level of packaging building density is maintained. The momentum exchange rate at the canopy roof level is effectively strengthened when the frontal area density decreases, together with a reduction of ventilation in the urban canopy layer (UCL). Additionally, a linear relationship is found between the UCL ventilation rate and the momentum exchange rate at the urban canopy roof level. For a constant packaging density, reducing frontal area density improves the city block ventilation by enhancing the air exchange rate at the roof level rather than increasing horizontal drag. This paper also illustrates a practical strategy in optimum neighborhood building layout design to minimize the pollutant level. [ABSTRACT FROM AUTHOR]

For energy efficient ventilation and low carbon design, solar chimney has been extensively applied on buildings. In the present work, three representative solar chimneys were introduced, i.e., Model 1 with top-horizontal outlet, Model 2 with upper-vertical outlet and Model 3 with middle-vertical outlet. Updraft ventilation from the built enclosure to the outlet and heat transfer from the absorber to the fluid by heat convection were numerically and theoretically investigated, and their limitations and correlations with governing parameters were also discussed. Streamlines and heatlines were used to visualize transport paths of air and heat respectively. Reverse flow could not be observed in the solar chimney channels of Model 2 and Model 3; Therefore, total volume flow rate induced by the solar chimneys Model 2 and Model 3 and convection heat transfer rate of the absorber increases positively with thermal Rayleigh numbers (Ra). Model 2 chimney produces greater thermal buoyancy than that by Model 3, and its air flow rate is higher than that of Model 3. Generally, volume flow rate induced in Model 1 is higher than those of Model 2 and Model 3 as Ra is no more than 2.5 × 106, and simultaneously, convection heat transfer rate Nusselt number (Nu) is also greater than those of Model 2 and Model 3. When Ra exceeded 2.5 × 106, reverse air flow was intensified further, Nu of Model 1 started to decline. The three models with discrete heat sources could induce more air from indoor than that respective model without discrete heat sources. The maximum increment rate is obtained by the Model 1 solar chimney with discrete heat sources, then Model 2 and the least done by Model 3. Their power relationships between Nu and Ra , V*and Ra are regressed respectively. The dependence of volume flow rate on broad thermal Rayleigh numbers covering all the three flow regimes showed that there were linear correlations between discrete numerical volume flow rates and the scaling predictions, except in Model 1 for high Ra (exceeding critical value 9.0 × 105), where volume flow rate was remarkably reduced due to reverse air flow. This research could provide useful theoretical information for applying small sized vertical solar chimneys attached to the building sidewall to effectively reduce built energy consumption. [ABSTRACT FROM AUTHOR]

Abstract An active thermoelectric ventilated (ATEV) system coupling with multiple thermoelectric coolers and heat sinks was proposed in the present work. Depending on global energy balance and finite time thermodynamics, performance parameters were firstly presented, including cooling capacity, entropy generation and coefficient of performance (COP). Subsequently, two analytical sub-models respectively for parallel flow heat exchangers and counter flow ones were developed. Input current of thermoelectric coolers, quantitative numbers of thermoelectric coolers and two heat exchanger types (parallel and counter flows) were sensitively varied to optimize overall performance of this system. In a representative residential space, demo case comparisons between traditional thermoelectric cooling units and present ATEV system have been conducted. When the number of thermoelectric coolers exceeded eight, cooling capacity of this new system decreased remarkably. For parallel flow heat exchangers, the entropy generation rate (S g) firstly dropped and then started to increase; for counter flow heat exchangers, whereas, it continuously decreased with increasing unit cell numbers. Overall, present ATEV system integrating built envelope wall and thermoelectric modules simultaneously could be a promising technology to enhance space cooling, whatever for built residents or electronic facilities. Highlights • ATEV system aimed to enhance efficiency and reduce entropy generation simultaneously. • A network model for the ATEV system was analytically proposed and experimentally validated. • On site experimental rig system and environmental chamber were set up to validate theoretical results. • Parallel flow and counter flow heat exchangers were taken into accounts for ATEV system. • Active thermoelectric ventilation system has been operated in a residential space for demo comparisons. [ABSTRACT FROM AUTHOR]

The turbine hall houses a number of components with intense heat release, resulting in a hot and humid air environment and making it very unpleasant for plant staffs to conduct maintenance work there, particularly at a hot outdoor environment. The traditional pure buoyancy driven natural ventilated could fulfil the overall heat exhausting requirement, however, creating hot environment at several positions due to the vortex airflow structure. As only part of the building need to be cooled, a localized cooling system with assisting mechanical fans is used. Buoyancy driven natural ventilated flows primarily occupying this turbine hall have been investigated by a computational fluid dynamics (CFD) model and an on-site measurement. The ventilation airflow is solved by 3D steady state Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with the SST k - ω turbulence model. The numerical codes and procedures were validated by comparing with on-site measurements. The concept of mean age of air is used to evaluate the natural ventilated efficacy. Following that, the potential of using mechanical vents and slabs to cool equipment work area at this buoyancy driven natural ventilated hall is analysed. The simulation results show that the mechanical vents could decrease air temperature and increase air exchange efficiency at work area effectively on condition that the local airflow pattern is well organized, which is strongly related to the ejection direction and velocity. Present research could benefit future development of the ventilation design in the power plant buildings. [ABSTRACT FROM AUTHOR]

A thermoelectric heat recovery unit could actively use thermoelectric refrigeration process to recover waste heat of the air exhausted from buildings. Theoretical analysis of a thermoelectric heat recovery unit is firstly conducted in the present work, and a corresponding mathematical model is proposed to incorporate with the thermoelectric cooling effect and the method of effectiveness-number of transfer units. Subsequently, the cost-performance model considering the relationship between system performances and operating cost is set up, aiming to critically evaluate the global performance of thermoelectric heat recovery system. Overall thermal conductance is determined analytically from steady-state model, where the optimal operating parameters, including the quantity, filling factor, length of P-N legs and couple numbers of thermoelectric coolers are identified. Finally, this prediction method is used to analyze system performance and operating cost in a practical thermoelectric heat recovery system operated in a representative residential building area. The simulation results demonstrated that the lower operating cost of a thermoelectric heat recovery system could be lower than 0.02 $/kWh under the optimal parametric conditions. In addition, present demo investigations indicated that the use of multiple thermoelectric cooling units in the heat recovery ventilation system could further reduce operating cost and promote the cooling capacity simultaneously. Present work illuminated that the thermoelectric heat recovery unit has a great potential for being applied in future energy-efficient buildings. [ABSTRACT FROM AUTHOR]

The pedestrian level pollutant transport in street canyons with multiple aspect ratios ( H / W ) is numerically investigated in the present work, regarding of various unstable thermal stratification scenarios and plain surrounding. Non-isothermal turbulent wind flow, temperature field and pollutant spread within and above the street canyons are solved by the realizable k - ε turbulence model along with the enhanced wall treatment. One-vortex flow regime is observed for shallow canyons with H / W = 0.5, whereas multi-vortex flow regime is observed for deep canyons with H / W = 2.0. Both one-vortex and multi-vortex regimes could be observed for the street canyons with H / W = 1.0, where the secondary vortex could be initiated by the flow separation and intensified by unstable thermal stratification. Air exchange rate (AER) and pollutant retention time are adopted to respectively evaluate the street canyon ventilation and pollutant removal performance. A second-order polynomial functional relationship is established between AER and Richardson number ( Ri ). Similar functional relationship could be established between retention time and Ri , and it is only valid for canyons with one-vortex flow regime. In addition, retention time could be prolonged abruptly for canyons with multi-vortex flow regime. Very weak secondary vortex is presented at the ground level of deep canyons with mild stratification, where pollutants are highly accumulated. However, with the decrease of Ri , pollutant concentration adjacent to the ground reduces accordingly. Present research could be applied to guide the urban design and city planning for enhancing pedestrian environment. [ABSTRACT FROM AUTHOR]