Natural Draft Research Articles

Optimizing stack ventilation in low and medium-rise residential buildings in hot and semi-humid climate

Nowadays, optimization of methods that can reduce energy consumption is efficient for various reasons, such as the increasing mean temperature, the changes in climate, and the shortage of non-renewable energies like fossil fuels. Hence, the stack effect is one of the passive cooling strategies promoting using renewable energy. In this context, Stack ventilation is studied in a hot and semi-humid climate to introduce AEC industry parties. A notable contribution of this study to the existing knowledge landscape involves the integration of stack ventilation (vertical ventilation) with roof channels (horizontal ventilation). Direct and indirect ventilation are scrutinized and compared. Then stack position in the building is investigated for optimizing it in one-, two-, and four-story buildings in Dezful city. Considering being the most frequent residential building type, the four-story archetype has been the main focus of the present study. The models’ behaviors are studied by Design Builder (Energy Plus engine). The CFD (fluid dynamics calculations) module has been used to explore the behavior of air in terms of velocity and temperature quantities by numerical simulation. Findings suggest that indirect ventilation is better than direct ventilation in this climate if individuals do not want to use evaporating cooling strategies inside natural ventilation. Roof channels are particularly effective for collecting and discharging the hot air gathered on the roofs and air circulation within the buildings. The result is appropriate for an architect design in hot regions.

Supercapacitors Based on Spider Nest–Shaped Nickel Foam Electrodes Operating in Seawater

Abstract An environmental-friendly supercapacitor based on aqueous electrolyte was fabricated. Electrodes with conductive spider nest–shaped three-dimensional (3D) porous structures were prepared for the assembly of symmetric supercapacitors. The nickel foam was modified by multiwalled carbon nanotubes and β-cyclodextrin. The construction of the spider nest was stabilized via the chemical bond inside carbon nanotubes, π–π stack effects among carbon nanotubes, and physical adsorption between nickel foam and carbon nanotubes substrate. The role of β-cyclodextrin is a dispersant to prevent agglomeration of carbon nanotubes, thereby enhancing electroactive surface area of nickel foam and improving the specific capacitance of the electrodes. Furthermore, the electrodes exhibited excellent rate capability. The obtained symmetrical supercapacitors exhibited an excellent power density of 17,561.3 W kg−1, a good specific capacitance of 398.8 F g−1, and an energy density of 154.8 Wh kg−1 for 4000 cycles with outstanding cycling stability. In addition, the specific capacitance, energy density, and power density of the supercapacitor operating in seawater were found to be 100.2 F g−1, 17.8 Wh kg−1, and 2568 Wh kg−1, respectively, for 3000 cycles. Overall, our findings indicate that the supercapacitor could stably operate in seawater and shows potential for use as an ecofriendly power supply to marine engineering equipment.

Numerical-simulation study on the influence of wind speed and segregation effect on spontaneous combustion of coal bunker

Coal bunker facilities are important places for coal storage and supply in coal-fired power generation, coal mines, ports, etc. Coal bunker spontaneous combustion(CBSC) disasters are mostly caused by the interaction of multiple field effects. In order to prevent and control coal bunker fires, it is necessary to study the impact of “chimney effect” wind speed and “segregation effect” on CBSC. This article establishes a two-dimensional geometric model based on the No.4 warehouse of Huanghua Port and the model is a two-dimensional representation of the longitudinal section along the warehouse axis. The research results indicate that at low temperatures in winter (273K), coal bunkers are less prone to spontaneous combustion; Under normal temperature (298K) and high temperature (308K) conditions, an increase in the initial wind speed in the vertical direction of the coal bunker inlet has a significant impact on the shortening of the CBSC period. The enhanced “segregation effect” of coal particles can shorten the spontaneous combustion period of coal bunkers and significantly affect the location of high-temperature zones. The research in this article has guiding significance for preventing and controlling coal bunker fires.

Numerical study on upstream smoke propagation and induced airflow velocity in a tilted channel under natural ventilation

A series of fire simulations were executed to explore the upstream smoke propagation characteristics and induced airflow velocity in tilted channels under natural ventilation. The channel slope ranging from horizontal to 58% (30°) and five heat release rates were used. Simulative results indicate that the airflow introduced by stack effect can enable the upstream smoke spread to reach sub-critical, critical and super-critical states without forced longitudinal ventilation. Meanwhile, taking the effect of fire heat release rate and height difference between the fire source and the upper portal into account, a piecewise function is developed to estimate the induced airflow velocity. It is found that when Ld∗·Q∗0.2 is less than or equal to 1.05, the airflow velocity increases exponentially with the variety of dimensionless elevation difference, but slowly rises logarithmically when Ld∗·Q∗0.2 is greater than 1.05. Besides, for scenarios in the sub-critical state, the relationship between the induced airflow velocity and the upstream smoke backflow length is revealed, which demonstrates that the dimensionless reverse flow length is in inverse proportion to the 1.5 power of dimensionless airflow velocity. For scenarios in the critical and super-critical states, there is still smoke backflow upstream of the fire source at the early period of a fire. And the disappearing time of smoke backflow reduces exponentially with the increase of heat release rate and elevation difference. Based on dimensional analysis and simulative data, a novel predictive model for the backflow disappearing time is proposed.

Fire Effect and Performance of Bridge Pylon Columns under Construction

The fire effect and performance of bridge pylons under construction were investigated via an analysis conducted on two types of pylons with different wall thicknesses. Three fire scenarios, namely internal fire, external ring fire, and external side fire, were established for a 40 m high section of the bridge pylon under construction. The distribution of fire smoke and temperature was obtained using fire dynamics simulation software for different fire scenarios. In addition, a finite element simulation was performed using the thermal–mechanical coupling method to obtain the temperature, stress, and deformation of the columns. The simulation results demonstrate that the average temperature of the internal fire is higher. The chimney effect extends the height range of temperature influence. In the vertical direction, the temperature decrease curve for the internal fire follows a single negative exponential function, while the external fire adheres to a double negative exponential function. The thickness of the temperature influence in the bridge pylon is extended by heating to approximately 200 mm. The stress value considering the thermal expansion coefficient is nearly 27.5 times that without the expansion coefficient, while the deformation value increases by 1 to 8 times. In conclusion, the calculations of the coupled expansion coefficient are helpful in improving the fire safety of bridge pylons.

Effect mechanism of cooling delta aerodynamic field equalizing on the cooling characteristics of dry cooling tower

As the key heat transfer components for large-scale natural draft dry cooling tower (NDDCT), the cooling deltas perform poorly when subjected to ambient crosswinds, thus leading to the cooling performance degradation of NDDCT. To enhance the cooling delta's heat transfer capabilities, the effect mechanisms of aerodynamic field equalizing, i.e. air equalizing, on the deltas were studied by the validated three-dimensional (3D) numerical model. For the deltas arranged around NDDCT, the aerodynamic fields, the air flow rates, and the water temperature were all analyzed for cooling columns composing deltas. By comparing the two situations with air equalizing or without in the midst of cooling delta, it could find that the air equalizing diminished the air flow rate differences between the two columns in one delta, which improved the cooling characteristics of the poorer-performing columns mostly at the delta's leeward but deteriorated those of the better-performing columns mostly at the delta's windward. With the air equalizing, the cooling characteristics of the two columns in one delta were balanced. And then the cooling characteristics of most deltas and the whole NDDCT were improved to some extent at varying crosswind speeds, which was significant for the design optimization of conventional delta-type radiators.

Water spray effects on fire smoke temperature distribution beneath a symmetrical V-shaped tunnel ceiling

The coupling effects of the double-slope stack effect, the thermal buoyancy induced by the V-shaped tunnel fire and the drag force generated by the water spray system alter fire smoke flow regulation compared to fire smoke flow in a horizontal tunnel. By numerical simulation, this work investigates the effect of water spray flow rate (0 L/min – 600 L/min), atomization angle (0–150°) and the length within fire source and the grade change point (0–120 m) on smoke temperature distribution beneath a symmetrical V-shaped ceiling. The findings indicate that the cooling effect on smoke temperature can be significantly improved by increasing the rate of water spray flow, while water spray increasing angle and tunnel slope have little contribution to the cooling smoke temperature. Thus an improved ceiling smoke temperature distribution model is proposed by importing the flow rate, angle coefficients and tunnel slope. As the fire source distance from the grade change point increases, stack effect on fire smoke flow could be categorized into three zones, which are the double-slope control zone, the transition zone and the single-slope control zone, which causes that downstream smoke temperature become higher gradually than the upstream one, while the smoke temperature difference tends to stabilize at the single-slope control stage. Finally, a global ceiling smoke temperature distribution in the symmetrical V-shaped tunnel installed a water spray system is established by introducing fire source distance coefficients.

Investigating the impact of stack effect on ventilation performance of residential kitchen exhaust shaft system

Investigating the impact of stack effect on ventilation performance of residential kitchen exhaust shaft system

PV PANEL COOLING USING STACK EFFECT

Unsatisfactory feat of the photo-voltaic cell is solitary in every of the foremost issues among the promotion of PV technology. A vital strand changing cell performance is operative temperature of the cell. The voltage of the cell, declines near directly with a rise in the operative temperature of the photo-voltaic cell. The temperature condition on the in-operative surface of a photo-voltaic panel remains usually 22–30°C beyond the normal temperature conditions. Upon the premise concerning those issues, the potency of the photo-voltaic cell will be refined by sustaining their operative temperatures as low as possible. This paper portrays a technique of PV/solar panel cooling using convection generated by the chimney effect. This paper considers the diminution of warmth from the Photo-voltaic panel for both active and inactive conditions.

The Effect of Wind Direction on the Cooling Capacity of Short Dry Natural Draft Cooling Towers Operating in Close Proximity

Abstract The layout of multiple natural draft dry cooling towers could have a significant influence on the performance of the cooling system in concentrated solar power (CSP) plants; however, this has never been quantified. Hence, this work used computational fluid dynamics (CFD) modeling to analyze the cooling capacity of two short natural draft dry cooling towers (NDDCTs) on a common site for a range of tower spacings, wind-speeds, and wind incidence angles. The results show that the cooling performance of the towers is a strong function of tower spacing and their orientation with respect to the wind direction. It was found that when the wind came at a 90-deg wind incidence angle (i.e., normal to a line drawn between the two towers), their cooling capacity was improved at tower spacings of less than 1.8 tower diameters (1.8D), though for the other tower spacings, there was no interaction between the towers. However, with the wind at 45 deg to the towers, the flow around the towers resulted in a decrease in their cooling capacity at tower spacings of 1.8D and 2.6D. Most interestingly, it was found that orienting the towers in line with the prevailing wind direction delivered improvements in the cooling capacity of up to 30%. This is due to the windward tower acting as a passive windbreak. Hence, as CSP plant capacity is increased, and additional cooling towers are required, these should be placed close to any existing tower and oriented along the line of the prevailing winds.

A Study on the Effect of Stack Effect on Performance Change of Underground Parking Area Smoke Control System Using Simulation

Recently, constructed large-scale high-rise buildings, multiple cores are often interconnected with an underground parking area. Performance variations of the smoke control system were observed when operating in this interconnected structure. Particularly, the pressure difference varied as the underground parking area, connected to several cores, underwent modifications. A correlation analysis was performed to determine the relationship between the fluctuating pressure differences. The findings indicated that the pressure difference was inversely proportional to the underground parking area. Moreover, the analysis revealed no correlation between evaluating a single core and simultaneously analyzing two cores. However, the variations in pressure difference were minimal and did not impact the performance of the smoke control system.

An Investigation of the Thermal Performance of a Tropical Greenhouse Constructed With an Earth Air Heat Exchanger

Abstract To maintain optimum plant growth temperature (i.e., 25–35 °C), the thermal performance of an earth air tunnel heat exchanger (EATHE)-integrated single-span saw-tooth greenhouse (GH) was assessed in peak summer. With the side and roof vents opened, natural ventilation due to wind and stack effect controlled the air movement and temperature inside the GH. In this configuration, the average temperature inside the GH remained higher than the ambient temperature by 1.5 °C for the entire period of the experiment. For EATHE (installed at a depth of 3.2 m) assisted GH with polyethylene (PE) cover, the air from the EATHE outlet entered inside the GH at 33 °C, and the average temperature within the GH was maintained at 4 °C lower than the ambient temperature. When the shading net was installed over the PE cover with the EATHE, the transmitted radiations into the GH were reduced from the roof, and the inside temperature was maintained 7 °C below the average ambient temperature (i.e., 45 °C). The measured temperatures along the length of the EATHE were compared with the indigenously developed code named PEAT (performance analysis of earth–air tunnel) and found to be in good agreement within ±4.5% deviation. The temperature distribution inside the GH was predicted using a computational fluid dynamics (CFD) model in ansys fluent with ±5% deviation from experimental results. With parametric analysis from the PEAT code and CFD model, the desired depth of the EATHE and the mass flowrate of air required to bring down the GH indoor temperatures to the optimum plant growth range were determined.

Axial Ventilation and Blade Row Effects on Transient Natural Convective Shutdown Cooling in a Gas Turbine

Abstract As a large civil gas turbine is cooling down, natural convective flows cause components to cool asymmetrically – the bottom sector cools faster than the top. This can lead to a number of issues that have the potential to damage engine components and affect operability. The ability to predict this cooling cycle of a gas turbine has proven to be extremely difficult, owing to the complex nature of natural convective flow and its dependency on a considerable number of design parameters. An experimental and numerical investigation into the impact of axial ventilation (interaction between the annulus and the external air) and blade rows on the natural convective flow in a large civil gas turbine high-pressure compressor has led to some key discoveries. Axial ventilation caused a 70% increase in the peak top-to-bottom temperature difference in the cooling cycle, when compared to the baseline sealed case. The combinations of four blade rows and axial ventilation caused a 130% increase in peak temperature difference over the baseline case. Numerical simulations illuminated that the root cause of this was the cold air drawn into the lower section of the annulus led to a relatively high heat flux, coupled with a blockage effect on the natural draft in the upper section of the annulus. This study has highlighted the importance and inter-dependency of these effects in defining the level of rotor bow that is observed. Therefore, it is imperative that they are included and sufficiently captured in order for a shutdown.

Numerical analysis on the cooling and electrical performances of photovoltaic noise barrier

A photovoltaic noise barrier (PVNB) is a useful device for installing PV systems on a noise barrier, which can save the space and cost associated with the installation of PV systems. Since a noise barrier usually consists of porous materials with low thermal conductivities, investigation on the cooling performance of the PVNBs is essential because the temperature of the PV module has a significant impact on its electrical performance. In this study, the cooling and electrical performances of a PVNB were numerically investigated using four different models: PV module-only, PVNB model without a heat sink, PVNB model with a conventional heat sink, and PVNB model with the proposed heat sink configuration. The simulation was validated by comparing the predicted and measured cell temperatures of the PV module at different heat fluxes, and the results showed good agreement. The average cell temperature in the PVNB model without a heat sink significantly increased by 42.2% from 86.98 ℃ to 123.66 ℃ because of the low thermal conductivity of the noise absorber, compared to the PV module only case. In the case of proposed heat sink configuration, the average cell temperature decreased by 42.7% from 123.66 ℃ to 70.90 ℃, compared with the PVNB model without a heat sink, owing to the additional injection of the cold ambient temperature air to the heat sink channel as well as natural convection and stack effect. The electrical performance of the PVs in the tested PVNBs was analyzed, and the results indicated that the maximum output power and voltage in the PVNB with the proposed heat sink configuration increased by 26.6% and 19.0%, respectively, compared to the PVNB model without a heat sink. The results of this study provide useful fundamentals regarding natural convection and the stack effect for optimizing heat sink designs for PVNB.

Energy performance enhancement of PV system through proposed back cooling techniques and natural air draft: an experimental study

ABSTRACT One main cause of concern about solar system performance is how well PV modules work in high-temperature areas with high yearly average temperatures. Typically, solar panels are tested for power output at a temperature of 25°C. For every degree above 25°C in temperature, the panel’s output power will decrease. This article represents a novel development in the domain of solar panel cooling techniques. In this work, a new cooling system using the backside water cooling of the solar panels with an absorbent fabric yet with natural air draft. The suggested alternative is simple to implement and suitable for projects with modest budgets. The current research presents an experimental technique carried out on large-scale test hardware in order to demonstrate its applicability in a real-life model application. Results obtained indicate that panels fitted with high-density foam and natural air draft have an efficiency improvement of 41%. Such a finding is entirely novel and has not been seen before in any other independent investigation. The commercial impact of using water cooling technology (with natural air draft) on a large-scale solar facility is shown in detail in this paper’s life cycle cost research. In comparison to a conventional system’s payback duration of 13 years and total income of EGP 1,282,509, the study found that the system using the innovative cooling approach had a payback period of 12 years and generated total revenues of EGP 1,514,587.

The Predicted Performance of Natural Ventilation Strategies in Islamic Boarding School Design in Magelang

The importance issue in providing comfortable indoor air quality conflicts with the use of energy for mechanical cooling in tropics region such as Indonesia. Natural ventilation is seen as a potential to provide comfort through the change of air within the inside and outside of the space. Natural ventilation becomes a strategy to achieve building resilience towards climate change because it requires no energy consumption. Therefore, this research aims to justify the design of natural ventilation of low-income Islamic Boarding School in Magelang using steady state simulation. The results indicated that among three ventilation strategies simulated in this study, it can be concluded that stack effect is predicted to produce higher air flow rate which results to comfortable predicted observation on adaptive comfort.

Study on temperature distribution and smoke diffusion characteristics of cable shafts in high-rise buildings

The smoke and poisonous gas produced during combustion is one of the direct hazards of cable fire. For power cables laid vertically in the cable shaft of high-rise buildings, the chimney effect will speed up the spread and expand the harm of cable fire. The paper proposes a simulation method based on fire dynamics theory to analyze the diffusion and spread characteristics of cable fire. First, a three-dimensional model of the double-floor cable shaft with multiple cables laid vertically inside was built in the paper. Second, with the abnormal heating caused by overload or insulation breakdown as the excitation condition, the development process of cable fire is simulated. The diffusion of smoke and gas during the whole combustion process is studied and discussed. Then, the paper compares the simulation results of the model with different fire blocking materials. The simulation results show that the application of blocking materials will cause differences between the spread of fire and flue gases, and the difference can be used to evaluate the quality and performance of the fire blocking material. Finally, A test platform is constructed to verify the validity of the simulation analysis.

Computational investigation of NOx emission in biogas combustion.

Landfills and anaerobic digesters in the waste treatment processes generate biogas. Biogas can be used as a fuel and excess biogas is typically burned in a flare to reduce the greenhouse effect. However, burning biogas produces several pollutants, including CO2, NOx, and SO2. To minimize these emissions, the amount of excess air used in the combustion process needs to be considered, which has a significant impact on NOx emissions. This study developed a Computational Fluid Dynamics (CFD) model to simulate a small-scale biogas combustion system and analyses the effect of excess air on heat output and NOx emissions during biogas combustion. The GRI-Mech reaction mechanism was used to simulate reactions, and the model was validated by comparing it to experimental data from the DLR-Stuttgart CH4/H2/N2 Jet Flame. To reduce computational costs, a Tabulation of Dynamic Adaptive Chemistry (TDAC) algorithm was used to dynamically adapt the reaction mechanism in real time. Turbulence in the DLR flame was simulated using Reynolds-Averaged Navier-Stokes (RANS). The CFD model used a co-flow of a natural draft to provide additional air, while the air was premixed with fuel. The CFD model was used to simulate various premixed equivalent ratios, and the resulting emissions and heat outputs were compared. The study found that the optimal premixed equivalent ratio for the studied system was between 0.85 and 1.1, as this range produced the highest temperature and lowest NOx emissions. This model facilitates emission analysis of gas-phase combustion systems.

SACTI model in prediction and assessment of large scale natural draft cooling tower environmental impact of nuclear power plant

Large Scale Natural Draft Cooling Tower has become a hot topic in China because it is an important part of the nuclear power plant, and its environmental impacts include shading, solar energy loss, water deposition and salt deposition. In China, there is no built large-scale natural draft cooling tower of nuclear power plant. Therefore, model prediction becomes an effective way to solve this problem. This paper introduces the basic principles and structure of SACTI (Seasonal and Annual Cooling Tower Impact) model. SACTI is a cooling tower assessment model developed by Argonne National Laboratory, USA. A comparative case study between China's Pengze Nuclear Power Plant and the US Amos Power Plant is also presented. Calculations were carried out for the Pengze and Amos power plants, and the results showed that the maximum value of salt deposition at the Pengze plant was about 166.5 kg/(km2-month) at a distance of 800 m from the cooling tower. The maximum value of salt deposition at the Amos plant was about 92.85 kg/(km2-month) at a distance of 600 m from the cooling tower. Conclusions show that the research work can provide a useful solution in future work, the simulation results of the SACTI model have a potential mean in the absence of monitoring data. This research provides a way to generate simulation data through SACTI program in the design process of nuclear power plant cooling tower, and designers can use these data to determine how the cooling tower will affect the natural environment and manage within an appropriate range to reduce the impact on the environment.

ADDITIONAL ROOF VENTILATION TO PRODUCE A STACK EFFECT IN UME KBUBU, THE TRADITIONAL HOUSE OF THE ATONI TRIBE

Indoor air quality affects the health of building users so that every building needs ventilation. The ume kbubu building was built without using ventilation to get warm temperatures in the building but the air quality inside is highly contaminated with carbon dioxide. The addition of roof ventilation is expected to produce a stack effect phenomenon in ume kbubu. The purpose of this research is to measure the level of carbon dioxide in ume kbubu and the speed at which smoke moves out of the building. The method used in this research is computer simulation using CFD software to determine the effectiveness of adding roof ventilation in reducing carbon dioxide levels and simulation without a computer using mock-ups to see the movement of smoke in ume kbubu. The results of this study indicate that the effectiveness of roof ventilation reaches 60% so that the air quality in the ume kbubu room is safer for its users.