Natural Draft Research Articles

Prediction model of buoyancy-driven flow rate in inclined tunnels with a localized buoyancy source: Emphasis on stratification effects

In inclined narrow and long spaces such as tunnels, the buoyancy-driven flow pattern and the resulting ventilation flow rate are different from those in enclosure-type buildings, because of their large length-to-width aspect ratios. In this paper, the buoyancy-driven flow rate induced by a localized heat source in inclined tunnels was investigated. Theoretical analysis and three series of experiments including non-isothermal or isothermal buoyancy sources, i.e., large-scale fire tests, brine-water tests and helium-air tests were performed. Two typical buoyant flow patterns were identified in the experiments, which are stratified and well-mixed patterns, respectively. Both the stratification pattern and the longitudinal decay rate of temperature rise were proved to be correlated with a dimensionless factor, η=B01/3ΔLsina/(g0′1/2H3/2), which is composed of characteristic parameters referring to both buoyancy source and geometrical configuration. The flow stratification results in stack effect attenuation and consequently ventilation flow rate reduction. A model for predicting the buoyancy-driven flow rate in such spaces was proposed, with special emphasis on the effects of stratification pattern. The predictions were validated with the experimental data, and the necessity for accounting for the stack effect attenuation caused by stratification was proved. The prediction model possesses an explicit expression and can thus provide a convenient tool for design calculations.

Reassessing ISO 9972 constraints: A mathematical analysis of errors in building airtightness tests due to steady wind and stack effect

Building airtightness significantly impacts its energy use estimation. The fan pressurization test, outlined in ISO 9972, is the most commonly used assessment method. A crucial component is the zero-flow pressure difference, influenced only by wind and stack effects. The ISO 9972 mandates that the absolute value of this pressure difference be less than 5 Pa and the minimum pressure station be either 10 Pa or five times the zero-flow pressure difference. The rationale behind these constraints is not evident, often excluding high-rise and buildings in wind-prone areas from standardized testing.This research examines these ISO 9972 requirements, aiming to clarify their basis and propose alternatives. Equations were developed to connect airflow estimation error with the ratio of the measured pressure station to zero-flow pressure difference. The external pressure measurement location during the test was found pivotal in determining airflow errors due to ISO constraints. While existing constraints limit airflow errors in many scenarios, enhancements are possible. If ISO 9972 conditions are unfeasible, always maintaining a 10 Pa buffer across façades can contain maximum airflow errors. This study suggests an alternative constraint with a similar error range (about 15 %) to current ones, enabling standardized testing in environments where existing ISO conditions are unattainable.

Numerical study of cooling performance augmentation for panel-type radiator under the chimney effect

In order to augment cooling performance of transformer panel-type radiators under natural-convection, the fluid-flow and heat transfer for a novel panel-type radiator and its surrounding air are numerically studied in this paper. The novel radiator is equipped with wind deflectors on both lateral sides and a chimney cap on the top. Then the effects of the height and channel number of the chimney cap on the cooling performance of the radiator are simulated. The results show that wind deflectors can form several enclosed air channels with radiator fins. The air-flow can be accelerated under the chimney effect generated in these channels, and the cooling capacity of the radiator can be increased by 12.75%. The addition of a chimney cap can further extend the chimney channels and increase its cooling capacity by 15.74%. Furthermore, with the increase of the height and channel number, the total cooling capacity of the panel-type radiator increases first and then decreases. In this study, when the chimney cap has five channels and a height of 700 mm, the novel radiator can obtain the best cooling performance, where its cooling capacity and overall heat transfer coefficient can be increased by 26.54% and 28.21%, respectively, as compared with traditional panel-type radiators, and the temperature difference between the inlet and outlet insulating oil is 7.1?.

Simulation-based assessment of natural and mechanical ventilation solutions for renovated apartment buildings in Denmark

Many European buildings require renovation, often involving window replacements and improved airtightness, leading to increased mold risk and indoor air quality (IAQ) issues. Solutions include adjustable air inlets with mechanical exhaust or balanced heat recovery ventilation. Using building performance simulations, we studied control strategies for renovated apartment building with natural or mechanical ventilation. CO2 - controlled air inlets proved most effective for IAQ and energy reduction. In different apartment positions, air inlet performance varied based on available stack effect. Mechanical exhaust cases were less affected by airtightness changes. Altogether, the mechanical ventilation was more robust, but all strategies were sensitive to closed doors.

The Optimum Nozzle Exit Position and the Behaviour of a Turbulent Flow in an Ejector Designed for Natural Draft Burner

The Optimum Nozzle Exit Position and the Behaviour of a Turbulent Flow in an Ejector Designed for Natural Draft Burner

Developing a Performance-Based Approach to the Effect of Roof Features on Fire Safety in Buildings with Atriums

Developing technology, changing social structures, and the threat of resource depletion have changed the design of buildings. Therefore, design approaches are developed to increase user comfort and reduce energy consumption by utilizing natural ventilation and lighting. The design of the distribution of outdoor air and light into the spaces through vertical and horizontal gaps reduces the energy demand of the mechanical systems and increases occupant comfort. The atrium is the preferred vertical gap in modern buildings for distributing natural air and light to interior spaces under appropriate conditions. However, in buildings with atrium, there is a risk of fire spreading in the event of a fire due to the uninterrupted gaps between the rooms. It is necessary to ensure the operability of the design by monitoring the measures to be taken in the early stages of design using performance-based fire safety methods. This study develops design strategies for fire analysis in atrium buildings using computational fluid dynamics (CFD) simulation technology. Atrium height, roof type, and slope characteristics are analyzed for the stack effect, which is the main factor in the movement of smoke and flames. As a result of the numerical analyses consisting of flat, unidirectional, and bidirectional sloping roof type, 10, 20, 30-degree roof slope, and one-meter rising atrium roof variables, the effect degrees for smoke dispersal and temperature control are investigated. Fire Dynamic Simulator, which uses CFD capabilities, and Smokeview software, which can visualize the results, were used for the numerical analysis. Correlation analysis was used to determine the effect of variables on temperature. The results showed that flat roofs and designs with increasing height were effective in delaying the spread of smoke and increasing the stack effect in the atrium, while the contribution of roof slope to fire safety was weak.

Study on flow characteristics of exhaust shaft with parallel fans in high-rise residential buildings

The exhaust shaft is often used to centrally extract cooking fumes captured by parallel fans on each floor of high-rise buildings. Inadequate airflow rate and excessive pressure in the exhaust shaft lead to poor ventilation and unsatisfactory air quality in kitchens. This issue is made worse by mixed installations of exhaust fans with different dynamic performances. In this study, the flow characteristics in the exhaust shaft with parallel fans are investigated by full-scale experiments and computational fluid dynamics (CFD). Based on the orthogonal experimental design, the cross-sectional area, concurrent operating rate, operating position distribution, and P-Q curves are key factors affecting the flow rate distribution significantly. Mixed installation of fans deteriorates the uniformity of the flow distribution along the shaft by even 62.6%. The competition for flow rate between different types of fans is obeying the Matthew effect. Concentration at the bottom and more simultaneous operations exacerbate this problem. An 28% enlargement in the shaft size increases the flow rate by 28.9% and improves the uniformity of the flow distribution by 50%. The stack effect is positive and unstable, with up to a 6.49% increase in system flow rate as a function of the cooking heat intensity. This work can provide guidance for the design of centralized exhaust systems in high-rise residential buildings.

Numerical investigation of the effects of different influencing factors on thermal performance of naturally ventilated roof

Ventilated roofs can improve thermal performance and reduce the energy consumption of building roofs. In this study, the optimal design parameters and adaptability of naturally ventilated roofs were determined through numerical investigations of the effects of different influencing factors on their thermal performance. A three-field coupling numerical simulation method considering temperature, flow, and solar radiation fields was used to improve simulation accuracy. The research results show that the optimal air gap layer thickness of a naturally ventilated roof increases with increasing roof length, which has not been considered in previous studies. The peak interior surface temperature of the ventilated roof increases with increasing outdoor atmospheric pressure. Ventilated roofs with a 30°–40° slope have a stronger stack effect and can better discharge solar heat gains when compared to roofs with other slopes. Interior surface temperature is reduced when phase change materials is set in the upper rather than the lower roof during summer. Phase change materials should be set in the upper roof to save energy. Small damage holes in the upper roof don't apparently affect the ventilated roof thermal performance. The findings of this study can guide optimization design and help evaluate the energy-saving effects of naturally ventilated roofs.

Investigation of the fire hazard of underground space fire scenarios in urban metro tunnels under natural ventilation: Analysis of the impact of tunnel slope on smoke back-layering length

The smoke back-layering length is a crucial parameter for evacuating people in both road and subway tunnel fires. This study investigates the fire hazard induced by carriage fire in inclined metro tunnels under natural ventilation. The parameter ‘transition slope’ is defined to measure the smoke flow from the carriage head in the upstream direction to the tunnel or not due to the stack effect of the tunnel slope. The aim of this paper is to analyse the effects of changes in cross-section, downstream length, tunnel slope, and carriage side-door coupling on smoke behaviour characteristics by experiment and simulation methods. A piecewise function expression between dimensionless smoke back-layering length, downstream length, and tunnel slope for carriage fires in an inclined tunnel under natural ventilation is proposed by theoretical analysis. At the same time, a 1:15 scale model experiment was conducted to initially analyse the characteristics of smoke movement. Following this, full-scale numerical simulations were employed to complement the model experiment and quantify the principles governing smoke movement. The experimental results show that the tunnel slope has a significant effect on the smoke back-layering length. In contrast, the influence of the heat release rate was found to be relatively minor. In addition, simulation results show that the tunnel slope has no significant effect on the smoke back-layering length when the fire location is approximately 20 m from the train head, and the tunnel slope is in the range of 2.29° ∼ 3.43° (4% ∼ 6%). For small tunnel slopes, smoke spreads in the tunnel, and the smoke back-layering length produced by the virtual fire source shows a different law from the previous study model. Finally, the correlation coefficient of the piecewise function in theoretical analysis is fitted by combining the experimental and numerical simulation results. Practical application This study provides valuable insights into the practical implications of controlling and mitigating the impact of fires in inclined metro tunnels. By understanding the critical role of tunnel slope and providing a quantitative tool for smoke spread law assessment, this study contributes to the enhancement of safety measures and the protection of lives in tunnel environments during fire incidents.

Detection of Spontaneous Combustion Areas of Coal Gangue Dumps and Comprehensive Governance Technologies: A Case Study.

Spontaneous combustion of coal gangue dumps not only releases toxic and harmful gases, polluting the environment, but also leads to explosion accidents and casualties due to improper handling. This paper focuses on delineating the fire area, constructing a comprehensive fire prevention and extinguishing system, and restoring the ecological environment. Infrared thermal imaging was used to detect the shallow fire area, while intensive drilling was conducted to detect the deep fire area. The stability of the coal gangue dump was enhanced by perfusing three-phase foam for cooling and using a curing material to fill the cracks. Land reclamation was then performed to restore the ecological environment. The results indicate that spontaneous combustion of coal gangue dumps can trigger the spread of the fire area from the outside to the inside, gradually expanding due to the 'stack effect'. The sources of spontaneous combustion in gangue fire areas are mainly located 3-5 m below the flat surface, and the shallow and deep fire areas are interconnected, posing a significant danger. These research findings can serve as a reference for detecting fire areas in coal gangue dumps and controlling environmental pollution.

Field Study on Impact of Mechanical Pressurization on Pressure Distribution in High-Rise Buildings

In high-rise buildings, the excessive pressure differences cause various problems, and architectural and mechanical measures are always applied. In this study, pressure measurements on a 67-story high-rise building were conducted to evaluate the effect of mechanical pressurization on the pressure distribution. Absolute pressure measurement devices were installed at 28 points on 10 floors, a full-scale pressure profile of the test building was derived, and the pressure distributions on the main floors were reviewed. Four pressurization modes for the test building were considered, and the variation in the pressure distribution for each mode was analyzed. The results showed that mechanical pressurization reduced the pressure difference on the lobby floor by approximately 18%. Although it did not exert an apparent impact on the pressure difference due to the stack effect, pressurizing the entire floor serves as the most effective way of reducing the excessive pressure difference.

Comparative studies on radon seasonal variations in various undeground environments: Cases of abandoned Beshtaugorskiy uranium mine and Kungur Ice Cave

It is well known that one of the most important risk factors in underground environment is the harmful effects of radon. The reasons for strong seasonal fluctuations in radon content in underground environments remain not fully understood. The purpose of this article is to improve existing ideas about this phenomenon. The article presents the results of a study of radon transport in two different underground spaces – the Beshtaugorskiy uranium mine (North Caucasus) and the Kungur Ice Cave (Middle Ural). We have used the direct measurements of the equilibrium equivalent concentration (EEC) of radon progeny in air, as well as the air flow velocity. A very wide range and strong seasonal variations in the radon levels have been recorded in both cases. The EEC has a range of 11–6653 by Bq m−3 and 10–89,020 Bq m−3 in the Kungur cave and the Beshtaugorskiy mine, respectively. It has been established that seasonal fluctuations in radon levels both in the mine and in the cave are caused by the same process – convective air circulation in the underground space due to the temperature difference between the mountain massif and the atmosphere (so called chimney effect). Overall, these results indicate that due to convective air circulation, underground spaces are periodically intensively ventilated with atmospheric air, and then, on the contrary, they are filled with radon-enriched air that seeps into caves or adits from rocks and ores. In both cases, the EEC of radon progeny exceeds the permissible level for the population and workers. The results of this study highlight the need for the development of measures to limit the presence of people in the surveyed underground spaces.

Influence mechanism of the three-zone synergistic efficiency-enhancing pattern on the 3D flow field and thermal resistance characteristics for wet cooling towers

For a super-large natural draft wet cooling tower (S-NDWCT) equipped in a 1000 MW unit, a 3D numerical model of the three-zone synergistic efficiency-enhancing pattern (TSEP) was established and validated. This study focused on the influence mechanism of the three-zone parameters on the 3D flow field and thermal resistance characteristics, including the installation height h, installation angle θ, total coverage area S, length L and number n of the split-flow plate, water distribution partition radius R1, the proportion of inner zone water amount P, and fillings partition radius R2. The results showed that the TSEP reconfigures and comprehensively optimizes the airflow field, and presents the more uniform air-water ratio and water temperature fields. The volumetric heat transfer coefficient hv increases and then reduces as h, θ, S, n, R1, P and R2 increase, and increases continuously with increasing L in the range studied. Additionally, the ventilation G increases and then reduces with the increase of h, θ, R1 and P, increases with rising S, n and R2, and reduces with L increases. Finally, the relatively optimized values of the three-zone parameters are obtained. In this case, compared with a conventional tower, the hv and G increase by 11.3% and 10.6%, respectively.

Effect Mechanism of Ambient Air Parameters on the Thermal Performance for Cooling Towers

Abstract As the volume of natural draft wet cooling towers (NDWCTs) continues to increase, the influence of ambient air on the thermal performance of the NDWCT is not clear. Therefore, the main parameters such as gas–water ratio, circulating water temperature difference, and heat transfer in each zone were calculated, and the temperature field and humidity field were also investigated. The results showed that the ambient temperature had the greatest influence on the cooling capacity of the NDWCT and the increase of ambient temperature led the circulating water temperature difference decreases the most, which was 7.63 °C (74.15%). The increase of relative humidity and atmospheric pressure led to an increase in convective heat transfer and the decrease in evaporative mass transfer, while both the convective heat transfer and evaporative mass transfer reduced with the decreasing temperature. This study establishes a theoretical foundation for optimizing of the thermal performance and energy-saving design of NDWCTs.

A Comprehensive Study of Different Types of Cooling Towers

Abstract: Cooling towers are essential components in a wide range of industries and processes, designed to dissipate excess heat generated during various operations. The efficient cooling of water or other heat transfer fluids is crucial for maintaining the optimal performance of machinery and equipment. This project presents a comprehensive study of different types of cooling towers, aiming to provide a thorough understanding of their functions, applications, and advantages. The study begins with an introduction to the fundamental principles of cooling towers, emphasizing the importance of heat exchange in industrial processes. It then digs into the various types of cooling towers, which include natural draft cooling towers, mechanical draft cooling towers, and hybrid cooling towers. Additionally, this study addresses showcasing successful applications of different cooling tower types in industries such as power generation, chemical manufacturing, and HVAC systems. Ultimately, the comprehensive study of different cooling tower types presented in this paper aims to provide engineers, industrial professionals, and researchers with valuable insights into selecting the most appropriate cooling tower design according to their needs

Upward flame spread along flammable hollow cylindrical structure

Hollow structure combustible, e.g. vertical drainpipe, is common in buildings, and can produce chimney effect once on fire. However, investigation on flame spread over the hollow structure solids has not been addressed yet. In this study, the hollow corrugated paper tube was chosen as the sample to experimentally study the flame spread characteristics under single-sided restriction. Here, 48 experiments were carried out by varying eight kinds of thicknesses (denoted by δ, 1–8 mm) and six kinds of inner diameters (d, 30–80 mm). The experimental results show that with the increasing inner diameter, the burning inside the sample changes from non-flame (regime I) to jet flame regimes (regime II). The effect of sample thickness on mass loss rate is less evident than that of sample inner diameter. Based on the laminar boundary layer theory and chimney effect, a calculation model of the mass loss rate inside the sample is proposed. Moreover, a minimum mass fraction of pyrolysis gas to produce regime II is obtained. In regime II, a dimensionless relationship between the jet flame height and the heat release rate is proposed. In addition, for flame spread over the outer surface of sample, dimensionless average flame spread rate has an exponential dependency on the dimensionless diameter. The results of this study can provide an important reference for fire prevention and fire protection design of high-rise buildings.

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.