Ambient Crosswind Research Articles

Influence of different flow models on numerical simulation of solar updraft tower

Solar updraft tower is a new kind of solar heat utilization technology. In the numerical simulation of solar updraft towers, selecting different flow models will affect the accuracy of prediction results. To improve the reliability of the numerical simulation results of the solar updraft tower, this paper uses the solar load model is used to simulate the local conditions of Hohhot, Inner Mongolia Autonomous Region, China at different times, and adds the ambient crosswind air domain to simulate the operation of the device under the actual environmental conditions. Laminar flow, Re-Normalisation Group (RNG), transition, and k-omega shear stress transport (SST) models were selected according to the Reynolds number corresponding to the characteristic length of the system at different positions to simulate and predict the flow field in the solar updraft tower heat collection system, and the results were compared with the test data. The results show that in the simulation of Hohhot at 13:00 and 17:00 on July 19, compared with the RNG model, the relative errors of the calculated data and experimental data of the transition model are reduced by 1.04% and 0.1%, respectively. The surface temperature distribution of the absorber is more consistent with the actual physical law. The superiority of the transition model in the numerical simulation of solar updraft tower is verified.

Effects of Crosswinds on the Aerothermal Behaviour of a Double-Skin Façade

Abstract In double-skin facades, airflow is mainly induced by buoyancy forces due to temperature gradients and air compressibility. In practice, this passive system is also affected by ambient crosswind. The present work is focused on the analysis of the aerothermal behavior of an asymmetrically heated channel interacting with a transverse airflow. To achieve this, we utilized the computational fluid dynamics (CFD) technique based on the finite volume method to predict the system’s performance. After validating the adopted numerical model, we examined the flow structure for different channel widths, and we determined the corresponding ventilation flow rates. We integrated the effect of wind with the thermosiphon effect. The thermo-aerodynamic performance of the facade was presented as a function of wind velocities, which significantly influence the airflow through the cavity and its openings.

Air Equalizing Mechanism in Cooling Performance Improvement of Vertical Delta-Type Radiators

Based on the design and measured data of one actual tower, a three-dimensional numerical model for a natural draft dry cooling tower (NDDCT) was created and validated under constant heat load. This enabled the performance improvement mechanism of air equalizing on vertical delta-type radiators (VDRs) to be clarified by detailed analysis of key parameters, such as the exit water temperature, heat transfer coefficient, and mass airflow. Under the impact of typical ambient crosswind, all VDRs were retrofitted with air-side-equalizing devices. It was found that the exit water temperatures of the whole NDDCT decreased by 0.865 °C, 0.593 °C and 0.186 °C under the studied ambient crosswind speeds of 2.5 m/s, 4 m/s and 12 m/s, respectively. The performance improvement mechanism of air-side equalizing was investigated for three VDRs, which were located on the upwind, tower lateral, and downwind sides under crosswind impacts. Besides the studied VDRs, the performance of the neighboring VDRs behind them was also improved by the optimized aerodynamic field and the reduced hot wind recirculation around them. In addition, the average heat transfer coefficients of the VDRs were enhanced, which could lay the foundation for improving the cooling performance of thermodynamic devices with VDRs.

The mutual effect between dual thermal power units under the advanced configuration of two units sharing one dry cooling tower and the energy-efficient and low-emission operation strategy

For a thermal power plant configuration of two power units sharing one dry cooling tower, a comprehensive computational framework was developed to reveal the mutual effect between dual units via analyzing the competition between the air-cooled radiators which are owned by two units and coupled by the shared tower, under various environmental factors and unit power load schemes. Results showed that an increase in crosswind speed vcw aggravates the uneven performance distribution among radiators, but variations in ambient temperature ta have little impact. Each growth of 10 °C in ta results in a drop of ∼1.4% in power cycle efficiency ηe for both units operating at rated load, corresponding to increases of ∼9 g/(kW·h) and ∼26 g/(kW·h) in coal consumption and CO2 emission rates. ηe of the unit with its radiators facing towards ambient crosswind is 0.3% higher than ηe of the other unit as vcw exceeds 8 m/s. When both units operate with equal power load rate Np, a decline in Np aggravates the unevenness of air inflow among radiators and amplifies the predominance of air intake of windward radiators for the shared tower. An increase in Np causes a growth in ηe for both units, but this growth becomes slight as Np exceeds 75%. For both units operating with different Np, ηe declines more sharply with vcw for the unit with a higher Np, especially when radiators of the other unit face towards ambient crosswind. To maximize the overall operation economic efficiency, load difference between dual units should be minimized and radiators owned by the unit with a higher Np should be positioned against ambient crosswind. For dual units operating at a total Np = 150% on a strong-wind summer day with ta = 32.5 °C and vcw = 12 m/s, a maximum saving of 87.9 t standard-coal can be achieved by applying the optimal operating strategy, reducing daily CO2 emission by 250.3 t.

Square solar updraft tower coupled phase change material: An experiment

Solar updraft tower is a low-cost heat collection ventilation power generation technology in solar heat utilization technology, which has high research potential and application value. However, the traditional circular solar updraft tower has some problems, such as low thermal efficiency, short working time, and unstable heat collection performance, which are caused by ambient crosswind disturbance and low heat capacity of the absorber. Therefore, this study designed a new square one-sided inlet solar updraft tower to prevent most of the heat from being lost through the ambient crosswind. At the same time, a phase change paraffin thermal storage unit was coupled in the absorber, which could reduce the temperature field fluctuation of the device and extend the working time by releasing latent heat when the solar irradiance decreased. Control experiments were conducted on the two processes. The test results show that the working time of the coupled new device is at least one hour longer than that of the square solar updraft tower without phase change material. The average temperature difference between the inlet and the external environment after the solar radiation drops at 17:00 is 79.12% higher than that of the traditional circular solar updraft tower of the same size, and the average instantaneous heat collection efficiency is 99.98% higher. While the effectiveness of the improved solar updraft tower is proved, the economic analysis of the integrated application of the single-side inlet solar updraft tower on the agricultural solar greenhouse is made. The carbon emission reduction of at least 2.144 tons in the year of using this system, and the construction cost is saved about 1330USD. The use of the system will reduce carbon emissions by at least 2.144 tons per year, save at least 2.30 tons of standard coal, and save construction costs of about 1330 US dollars. The research can provide theoretical and technical support for the efficient commercial application of solar updraft towers.

CFD Analysis of a Small-Scale Solar Chimney Exposed to Ambient Crosswind

Solar chimneys are devices that use solar energy to generate a hot airflow that can be used for power production, the drying of agricultural products, and/or water desalination. The performance of a small-scale solar chimney is studied numerically. The computational domain includes the solar chimney, the ground, and the atmosphere. The turbulent airflow is simulated using the commercial CFD code Ansys Fluent. The only boundary conditions required for the simulation are the wind speed, the ambient temperature, and the absorbed energy from the ground, determined by an energy balance in the system. The system was simulated for one day in the summer in the city of Belo Horizonte, Brazil. The ambient crosswind plays an important role in the velocity and temperature. The velocity inside the solar chimney increased with the wind speed, increasing the heat transfer and decreasing the airflow temperature. When the wind speed increased from 0 to 10 m/s, the outlet velocity increased from 1 to 4 m/s, and the outlet temperature decreased from 313 to 304 K.

Numerical analysis of solar chimney power plant integrated with CH4 photocatalytic reactors for fighting global warming under ambient crosswind

Methane's global warming potential (GWP) is much larger than carbon dioxide and contributes significantly to global warming. Solar chimney power plant (SCPP) integrated with photocatalytic reactors can capture and remove atmospheric methane, and generate electrical power without fossil energy consumption simultaneously. In this paper, the performance of the flow characteristics, the CH4 removal, the CO2 emission reduction, and the power generation were analyzed for the SCPP integrated with different types of photocatalytic reactors under ambient crosswind (ACW). The results revealed that the SCPP integrated with a honeycomb reactor was more stable for the degradation of CH4 than that with a plate reactor. With an increase in ACW, the removal rate of atmospheric CH4 was reduced to a constant value of 0.41 g/s for the honeycomb reactor and 0.11 g/s for the plate reactor. The SCPP integrated with a honeycomb reactor achieved a maximum power generation of 88.31 kW, which was 1.63 times than that of the conventional SCPP when G = 857 W/m2 and ACW = 0 m/s. In addition, the improved SCPP could reduce CO2 emissions by 85.04 kg/h when G = 857 W/m2, ACW = 0 m/s, and △P = 320 Pa.

Numerical Investigation on the Urban Heat Island Effect by Using a Porous Media Model

The urban heat island (UHI) effect resulted from urbanization as well as industrialization has become a major environmental problem. UHI effect aggravates global warming and endangers human health. Thus, mitigating the UHI effect has become a primary task to address these challenges. This paper verifies the feasibility of a three-dimensional turbulent porous media model. Using this model, the authors simulate the urban canopy wind-heat environment. The temperature and flow field over a city with a concentric circular structure are presented. The impact of three factors (i.e., anthropogenic heat, ambient crosswind speed, and porosity in the central area) on turbulent flow and heat transfer in the central business district of a simplified city model with a concentric circular structure were analyzed. It is found that the three-dimensional turbulent porous media model is suitable for estimating the UHI effect. The UHI effect could be mitigated by reducing the artificial heat and improving the porosity of the central city area.

Ambient crosswind effect on the first integrated pilot of a floating solar chimney power plant: experimental and numerical approach

Floating Solar Chimney Power Plant (FSC) proposed by Papageorgiou is regarded as a novel type of Solar Aero-Electric Power Plants with fundamental characteristics of low cost and unaffected seismic chimney. The main disadvantage of the proposed system is the tilting of the floating chimney in windy conditions compared with a conventional reinforced concrete one. In the present research, the ambient crosswind (ACW) effect is studied on an FSC with experimental and numerical approaches. An experimental floating chimney integrated with an appropriate solar collector was designed, built, and tested under real-world conditions, and the experimental results were used for validations of the numerical analysis. Three-dimensional numerical analysis was performed to study the performance of the tilted floating solar chimney exposed to the ACW and then compared with a conventional type. Numerical results show that a tilting FSC with lower apparent height can operate more efficiently than a vertical concrete chimney in windy conditions due to eliminating adverse effects of tip vortices on updraft flow in the system. The results also show that an optimum tilting angle (OTA) exists for the specified plant and critical or prevailing wind speed, making ACW adverse effects minimum. Floating solar chimney design can be enhanced significantly based on achieving optimum tilting angle in windy conditions.

Performance analysis of a laboratory-scale tilted solar chimney system exposed to ambient crosswind

The tilted solar chimney (TSC) system has been introduced as one of the novel solar thermal approaches, whose performance in the presence of ambient crosswinds (ACW) is still somewhat unclear. The present research offers an in-depth experimental and numerical investigation of a TSC operating at a range of ambient wind velocities. Besides ACW investigations, numerical simulations examine the influence of solar radiation and chimney tilt angle (measured from the vertical line) on the fluid flow and thermal performance of the system. Results suggest that the ACW has an adverse effect on power performance at relatively low crosswind speeds. It is found that tilted solar chimney in windy conditions at a range of tilt angles between 10o - 20o has better flow performance compared with the fixed chimney arrangement. Dimensional analysis is carried out to establish the dimensionless parameters for predicting the system power output in a large-scale prototype with 200m chimney height. The results indicated that at 15o chimney tilt angle, the power output increases between 5% to 20% depending on the ACW velocities compared to the vertical chimney at the same crosswind speeds. The present research would be valuable for evaluating the power capacity of similar full-scale facilities.

Effect of cooling water salinity on the cooling performance of natural draft wet cooling tower

To clarify the effect of the cooling water salinity on the cooling performance of natural draft wet cooling tower (NDWCT) and relevant impact mechanisms, a three-dimensional (3D) numerical model of NDWCT was established with full consideration of the cooling water salinity, which was validated by comparing with the theoretical code method and the field test data of NDWCT with fresh water. The influences of the cooling water salinity and ambient crosswind on the key parameters, such as cooling tower outlet water temperature, ventilation rate and evaporation of circulating water, were all analyzed. The results show that the cooling performance of NDWCT with salinity circulating water is weaker than that with fresh water. The ambient crosswind had a great influence on the heat and mass transfer of the cooling tower. When the wind speed exceeded 6.6 m/s, the performance of the cooling tower is improved, this trend is less affected by salinity. The established 3D numerical model comprehensively considered the influence of salinity and ambient crosswind, the analyzed results are more valuable for practical guidance.

Evaluation of the Hot Air Recirculation Effect and Relevant Empirical Formulae Applicability for Mechanical Draft Wet Cooling Towers

Due to the hot air recirculation, the inlet air enthalpy h1 of mechanical draft wet cooling towers (MCTs) was usually greater than the ambient air enthalpy ha. To realize the cooling performance and accurate design of MCTs, this paper clarified the feasibility of the inlet air enthalpy empirical formula presented by the Cooling Technology Institute (CTI) of the USA. A three-dimensional (3D) numerical model was established for a representative power plant, with full consideration of MCTs and adjacent main workshops, which were validated by design data and published test results. By numerical simulation, the influence of different wind directions and wind speeds on hot air recirculation (HAR) and the influence of HAR on the cooling performance of the MCTs were qualitatively studied based on the concept of hot air recirculation rate (HRR), and the correction value of HRR was compared with the calculated value of the CTI standard. The evaluation coefficient ηh, representing the ratio of the corrected value to the calculated value was introduced to evaluate the applicability of the CTI formula. It was found that HAR was more sensitive to ambient crosswind, and an increase in HRR would deteriorate the tower cooling performance. When the crosswind speed increased from 0 to 15 m/s, ηh, changed from 2.42 to 80.18, and the calculation error increased accordingly. It can be concluded that the CTI empirical HRR formula should be corrected when there are large buildings around the MCTs, especially under high-speed ambient crosswind conditions.

Effect mechanism of exit-water temperature distribution characteristics on the anti-freezing of natural draft dry cooling tower

For thermal power plants with natural draft dry cooling towers (NDDCTs) in arid and severely cold regions, the anti-freezing of finned-tube bundles is critical for the safe and economic running of a thermodynamic system. Using volume source term models representing the water flow in each pass of a cooling column, a 3D numerical model for NDDCT was established and validated. The exit water temperature and its relevant distribution characteristics could then be analyzed from cooling columns, deltas to the whole tower. Based on the analyses, the non-equilibrium temperature difference and relevant anti-freezing parameters were defined. It could be observed that the ambient crosswind, ambient temperature, heat load, circulating water flow rate, and convection pattern have different impact mechanisms on the exit water temperature distribution characteristics, among which, the ambient crosswind plays a major role but the ambient temperature has a negligible effect. Finally, the critical anti-freezing average exit-water temperatures were presented based on real work conditions, which would facilitate the safe and economic operation of NDDCTs in severely cold weather.

Critical impact factors on the cooling performance design of natural draft dry cooling tower and relevant optimization strategies

Because of the inherent water saving merit, natural draft dry cooling towers (NDDCTs) were widely used by power units in arid regions. The cooling performance of NDDCT plays an important role in the economic and safe running of power units. Then, the design of NDDCTs should be optimized from the design stage. Considering the following factors, such as the tower shape, the radiator arrangement, the ambient crosswind, and the mass flow ratio of circulating water to exhausting steam (m), an optimization model was put forward based on theoretical analyses. Through orthogonality analysis, two critical tower shape factors were selected, which were the ratio of the tower height to its bottom diameter and the ratio of the tower throat area to its bottom shell area. Combining the number of cooling column and m, the cooling performance of NDDCTs was optimized and relevant strategies were proposed. Meanwhile, the height of tower was optimized and the minimum construction cost of NDDCT was obtained with optimized cooling performance.

New retrofit method to improve the thermal performance of natural draft wet cooling towers based on the reconstruction of the aerodynamic field

Abstract The heat transfer deterioration that occurs in the inner rain zone weakens the thermal performance of natural draft wet cooling towers (NDWCTs). Existing NDWCT retrofit methods including the air deflectors and the cross wall have limited effects on this deterioration. In this paper, we propose a new retrofit method in which air ducts are installed in the rain zone and air deflectors are installed around the air inlet to improve the total tower thermal performance. To clarify the effect and mechanism of our retrofit method, a hot test for a NDWCT model is performed under various crosswind velocities, and a 3D numerical model for a NDWCT with air deflectors and air ducts is established and validated. Using the proposed method, the thermal performance of a NDWCT is substantially improved with less crosswind sensitivity. It is found that the flow diversion efficiency of the air deflectors weakens the adverse impact of the ambient crosswind on air inflow of the tower, and the additional ambient air introduced through the air ducts enhances the heat transfer in the central rain zone. Compared with the single effect of the air deflectors or the cross wall, the combined effect of the air ducts and air deflectors is more efficient in improving the thermal performance of NDWCTs.

Numerical investigation of the crosswind effects on the performance of a hybrid cooling-tower-solar-chimney system

The hybrid cooling tower solar chimney is a hybrid system capable of simultaneously dumping waste heat from a coupled thermal power plant and generating electricity from the self-induced updraft. To investigate the influences of crosswind and ambient pressure on the system’s thermal performance, a three dimensional model that considered the ambiance was developed. Numerical analysis results indicated that the self-power-generation capacity of the HCTSC system decreased with the increasing crosswind velocity, reaching its minimum when the crosswind velocity reached 8m/s. On the other hand, the turnabout for the heat dissipation capacity was 4m/s, below which the heat dissipation rate decreased with the increasing crosswind speed and above which it did the reverse. A further comparative study showed that, unlike the case of a solar chimney power plant, a circular blockage a few meters in front of the solar collector entrance barely improved the heat dissipation performance of the hybrid system when exposed to a strong ambient crosswind, however, it slightly boosted its self-power-generation capacity.

Numerical Simulation on the Effect of Vehicle Movement on Pollutant Dispersion in Urban Street

Combined processes of ambient crosswind and vehicle-induced turbulence have a very significant effect on the distribution of pollutants emitted by vehicles in street canyon. So the purpose of this study is to analyze the process of pollutants dispersion when moving vehicles pass the urban streets based on the basic theory of hydromechanics. The air flow and pollutants dispersion in a street canyon in the presence of a moving vehicle by a dynamic mesh approach using computational fluid dynamics (CFD) is investigated. The standard k-epsilon model and enhanced wall functions are employed to establish airflow field, and species transport model and the tracer gas technique are combined to simulate pollutants dispersion. The simulation results are compared with the published experimental data of mean velocity and turbulence for the validation. Then on the base of this validation, we further explore the effect of vehicle speed and the vehicle driving lanes. This study is a good understanding of the mechanism of vehicle-induced pollutants dispersion within the urban street canyon. Thanks to the different piston effect formed by moving vehicle in different lanes, the concentration on the leeward side in the descending order are L1, No vehicle, L4, L3, L2. Meanwhile, the concentration of pollutant behind the vehicle on the leeward side decreases with increasing vehicle moving speed. Higher moving speed could lead to a reduction of concentration on the windward side.

Improvement of Air Quality and Thermal Environment in an Old City District by Constructing Wind Passages

A case study in an old city district with hot-humid climatic conditions in Wuhan, China was conducted to explore the potential renewal strategies favorable to the local residents and pedestrians. For this purpose, a comprehensive mathematical model considering the parameters such as ambient crosswind, solar radiation, natural convection, and a previously established heat transfer mechanism was employed to analyze the fluid flow and heat transfer characteristics of the study area. In addition, in the urban renewal process, five alternative renewal strategies, namely, Central Demolition (CD) Plan, Edge Demolition (ED) Plan, Wedge Shape Demolition (WSD) Plan, “L” Shape Demolition (LSD) Plan, and Cross Shape Demolition (CSD) Plan, were adopted to improve the thermal and ventilation environment of Wuhan old city district. Through simulation analysis, the temperature and velocity distributions of the original urban layout and five alternative renewal strategies were compared. It is found that the construction of an air passage within the old city district can improve the local air quality, air ventilation, and thermal environment to some extent. Among the five alternative strategies to construct air passages, CSD Plan is much better than the others. Accordingly, corresponding suggestions and strategies for urban renewal were presented.

Modeling Thermal Comfort and Optimizing Local Renewal Strategies—A Case Study of Dazhimen Neighborhood in Wuhan City

Modeling thermal comfort provides quantitative evidence and parameters for effective and efficient urban planning, design, and building construction particularly in a dense and narrow inner city, which has become one of many concerns for sustainable urban development. This paper aims to develop geometric and mathematical models of wind and thermal comfort and use them to examine the impacts of six small-scale renewal strategies on the wind and thermal environment at pedestrian level in Dazhimen neighborhood, Wuhan, which is a typical case study of urban renewal project in a mega-city. The key parameters such as the solar radiation, natural convection, relative humidity, ambient crosswind have been incorporated into the mathematical models by using user-defined-function (UDF) method. Detailed temperature and velocity distributions under different strategies have been compared for the optimization of local renewal strategies. It is concluded that five rules generated from the simulation results can provide guidance for building demolition and reconstruction in a neighborhood and there is no need of large-scale demolition. Particularly, combining the local demolition and city virescence can both improve the air ventilation and decrease the temperature level in the study area.

Numerical analysis on the thermal environment of an old city district during urban renewal

Urban renewal has become a key issue all through China's Transitional Period. The social, economic, political factors, and their correlations have been seriously considered. However, the effects of outdoor pedestrian thermal comfort and fluid ventilation effects as well as the residence living conditions in old city districts under urban reconstruction should, to some extent, be paid closer attention to. As an example, take an old city district in Wuhan where a comprehensive mathematical model describing the fluid flow and heat transfer characteristics has been presented. Considering the influences of ambient crosswind, solar radiation, and natural convection and radiation heat transfer, numerical analysis based on the original layout has been executed. By analyzing the temperature and velocity distributions of the old city district, a new planned layout has been presented in this research. By comparison, some basic rules have been advanced to achieve favorable thermal comfort and flow ventilation effects.