Natural Draft Research Articles

Performance investigation of the natural draft wet cooling tower at different wet-bulb temperatures

This work experimentally investigated a natural draft wet cooling tower by varying the wet-bulb temperatures by shading. In this work, inlet air conditions were changed by shading of it using tripal and aluminium–zinc alloy-coated steel sheets at different water to air flow rate (L/G) ratios. It is observed that shading of cooling tower with tripal increases the tower characteristics up to 1.43 L/G ratio, whereas it increases up to 1.61 L/G ratio, when it is shaded with a steel sheet. Tripal shades increase the tower characteristics from 0.62% to 4.45% up to an L/G ratio of 1.43. Similarly, the steel sheet shading increases the tower characteristics from 1.46% to 14.48% up to an L/G ratio of 1.61. Finally, regression equations were developed for the coefficient of efficiency and water temperature difference with water flow rate at different wet-bulb temperatures.

Experimental investigations on two-sided upward flame spreading over thin flax fabric under the influence of restricted distance

Up to 2021, most previous work focused on upward flame spread over thin solid fuel completely attached to objects or with both sides freely exposed to the air, but did not take the restricted distance (distance between fuel and objects) effects into account. In this paper, the restricted distance effects on upward flame spread over thin solid fuels were investigated using 0.65 mm thick, 120 cm tall and 6.0 cm wide flax fabric sheets under various restricted distances of 1.0–3.5 cm. The essential parameters were monitored and analyzed simultaneously, including flame length, pyrolysis spread rate, surface temperature and ignition time. The main conclusions drawn are as follows: when the restricted distance is no more than 1.5 cm, the flame length on the unrestricted side is larger than that on the restricted side, whereas the variation exhibits the opposite trend when the restricted distance is beyond 1.5 cm. As the restricted distance increases from 1.0 to 3.5 cm, the flame length and flame spread rate first increase and then decrease, reaching a maximum value at 3.0 cm restricted distance, whereas the ignition time shows the opposite trend. The decrease rate of the surface temperature with the distance from the pyrolysis front first drops and then rises as the restricted distance increases, which qualitatively characterizes that the heat flux received by the virgin surface first increases and then decreases with restricted distance. The non-monotonic trends of heat flux received by the virgin surface and consequently the flame spread rate as a function of restricted distance are due to the combined restricted distance effects of the chimney effect, wall radiation and restricting oxygen supply. The results of this paper are not only helpful in better understanding the upward flame spread over a thin flax fabric under restricted distance, but also provide some basic data for fire prevention of thin solid fuels.

Influence of airflow movement on methane migration in coal mine goafs with spontaneous coal combustion

Spontaneous coal combustion was proved to cause methane accumulation and methane explosions in coal mine goafs. To avoid possible methane explosions, the empirical engineering measure, ventilation dilution, is proposed in coal mines though its disaster prevention mechanism has not been well understood. Through experimental and numerical simulations, the superposition effect of the air leakage and coal combustion-induced chimney effect was studied to reveal disaster prevention effect of ventilation dilution. Research results show that the high temperature area of coal combustion is steady and can provide continuous buoyancy force to form upward airflow even under ventilation dilution; the drifting methane accumulation is observed under the superposition effect of air leakage and upward airflow; ventilation dilution can weaken and even eliminate methane accumulation by overcoming chimney effect, but an increase in coal combustion temperature will enhance the upward airflow of chimney effect to cause methane accumulation again. The competitive relationship between the coal combustion-induced chimney effect and air leakage provides a new insight to study methane migration for the disaster formation and prevention mechanism.

Improvement of the Chimney Effect in Stack Ventilation

The article is focused on the airflow in a ventilation system in a building. The work examines the methods which enhance the chimney effect. In this paper, three cases with different chimneys were analyzed for the full-scale experiment. These cases were characterized by different geometrical and material parameters, leading to differences in the intensity of the ventilation airflow. The common denominator of the cases was the room with the air inlet and outlet to the ventilation system. The differences between the experimental cases concerned the chimney canal itself, and more precisely its part protruding above the roof slope. The first experimental case concerned a ventilation canal made in a traditional way, from solid ceramic brick. The second experimental case concerned the part that led out above the roof slope with a transparent barrier, called a solar chimney. In the third experimental case, a rotary type of chimney cap was installed on the chimney to improve the efficiency of stack ventilation. All these cases were used to determine the performance of natural ventilation—Air Change per Hour (CH). Additionally, the paper presents a technical and economic comparison of the solutions used.

Investigation on heat exchanger arrangement in solar enhanced natural draft dry cooling towers under various crosswind conditions

Solar enhanced natural draft dry cooling tower (SENDDCT) is a novel heat rejection device for thermal power plant, which uses solar energy to reheat the air after the heat exchanger and therefore improves the ventilation of the tower for performance enhancement. This paper develops 3-D models to compare the effects of circular/sawtooth arrangements of heat exchanger on cooling performance of a 140 m high SENDDCT. The result shows that sawtooth arrangement can improve the ventilation and heat dissipation rate of SENDDCTs. Under no crosswind condition, the flow field distributions of sawtooth and circular arrangements are similar in the tower. The ventilation and heat dissipation rate of sawtooth arrangement increases by 34.0 % and 46.8 % respectively when compared with circular arrangement under no crosswind condition. Under crosswind condition, the flow field of sawtooth arrangement is less affected by crosswind than that of circular arrangement. The ventilation rate and heat dissipation rate of sawtooth arrangement increases by 38.5 % and 65.0 % respectively when compared with circular arrangement at crosswind speed of 3 m/s. This study verifies that the sawtooth arrangement can weaken the crosswind effect and improve the cooling performance of SENDDCT and provides guidance for the optimal arrangement of heat exchanger for the SENDDCT.

Effect of Parallel Curtain Walls on Upward Flame Spread Characteristics and Mass Loss Rate Over PMMA

The effects of parallel curtain walls on the characteristics and mass loss rate of the upward flame spread over polymethyl methacrylate (PMMA) have been experimentally studied. The experimental research variables were the sample size and separation distance of the curtain wall. In the experimental setup, a PMMA plate was attached to one of the curtain walls. The results were analyzed to assess the effect of the curtain wall separation distance on the flame height. The special condition of two curtain walls with only a small distance between them was also analyzed. Analysis of experiments with systematically varied distances between the curtain walls has provided insight into factors such as air entrainment and the chimney effect. The results show the flame height evolution trend with the separation distance, and a new correlation to predict the global mass loss rate of the PMMA plate under the influence of parallel curtain walls, which can potentially be used in curtain wall design through optimization of the separation distance given fire safety requirements and practical needs.Graphical Abstract

The effect of methane emission on air distribution in potash mine production units

Introduction. It has been found that due to light gas (including methane) emission in rooms under development, there develops an additional natural draught between the mine workings. The calculation has shown that methane emission from the rock mass conditions the low value of the additional natural draught. However, even minor additional natural draught interacting with a thermal drop of pressure caused by temperature rise in the conveyor shaft changes the direction of the delivery air stream. While in up the dip blocks and panels the resultant natural draught promotes ventilation, in the down the dip production units it prevents air circulation in the required direction. Research methods. The methods and results of calculating the value and direction of the natural draught for real panels of potash mines at the Upper Kama potash deposit under various conditions have been presented together with the simulation observations of methane distribution in the room with a point source of emission. Results. Model analysis has shown that even under the low amount of gas emission out of the point source in the blind room (of a hole drilled in the roof), the concentration of gas in the gas-air mixture entering the belt heading reaches 2%. Conclusions. To ensure the safety of mining and reduce the risk of emergency when calculating the volume of air required to ventilate the production units, the dynamics of methane emission out of the rock mass should be taken into account as well as its further distribution across the mine workings.

Experimental investigations on the influence of ‘chimney-effect’ on fire response of rainscreen façades in high-rise buildings

In recent years, external wall fires (or façade fires) are occurring due to mixed contribution of exterior cladding and continuous insulation in the chimney like construction of rainscreen façades. In the present work, a new experimental setup is developed and demonstrated for the initial screening of façade assembly at a lab-scale. It incorporates the real physics for investigating the fire hazards of flammable polymeric materials and the chimney effect together. Different combinations of exterior cladding (Al-45, Al-45 Class 0, Al-45 Class B) and continuous insulation (Expanded polystyrene, Polyisocyanurate and Mineral wool) with and without chimney are selected for studying their fire behaviour. The pressure differential combining with re-radiation inside the cavity led 3–6 times higher mass burning rate, flame height and temperature in tests with chimney effect to that of without for the same combination of products. With chimney, secondary fire sources are generated due to the dripping of products which further enhanced the burning and spread rate. A critical width of the chimney (13–50 mm) is established at which maximum vertical fire spread was noticed. Furthermore, the visual observations indicated that the products having thin outer Al sheets are prone to 30% early structural failure. Comparison of results with large-scale tests validated the usefulness of the current setup as an intermediate test between small and large scale for studying fire behaviour of façade materials.

The effect of fire location and the reverse stack on fire smoke transport in high-rise buildings

In this paper, smoke transport in high-rise buildings through elevator shafts and stairwells is investigated for various fire location and stack effect conditions. For this purpose, a transient network model, Fire-STORM, is upgraded and used. The results are benchmarked by using a computational fluid dynamics (CFD) model. Six scenarios are tested, which are 1st floor, mid-floor, and top-most floor fires under normal stack (cold environment) and reverse stack (hot environment) conditions. For each scenario, the time history of pressures, temperatures, and soot mass fractions in the fire floors, elevator shafts, and stairwells and the average soot mass fraction in all stories of the building are presented. Overall, Fire-STORM has reasonably good accuracy compared to CFD with significantly faster computation times (90 s on a single core vs. 4 days on 32 cores in parallel). One of the intended uses of this fast low-order model is a data generation engine for neural network modeling of high-rise building fires. As such, one of the unique features of this work is the development of a realistic random heat release rate (HRR) modeling approach created using a Gaussian process.

Influence of Operation Schemes on the Performance of the Natural Draft Hybrid Cooling System for Thermal Power Generation

For thermal power generation, the natural draft hybrid cooling system (NDHCs) with airflows in parallel design gives a multi-objective solution for water saving, performance enhancement and maintenance issues, like corrosion, by switching the loads of wet and dry sections. Performances of dry and wet sections interact with each other in the highly integrated system, increasing the complexity of operation strategies. In this context the present paper examines eight different operation schemes to reveal the relationships of ambient conditions and operation schemes. Comprehensive comparisons in the view of cooling efficiency with a same water inlet temperature are conducted firstly. Results show that there exists energy-saving potentials of the water evaporated rate, cooling performances and the pump power for different schemes. Based on the practical boundary conditions, including those of weather data, operation hours and market factors, optimal operation strategies of hybrid cooling are designed to minimize the operation costs of the energy system. For the 660 MW power generating unit integrated with a natural draft dry cooling system (NDDCs), operation costs based on NDHC after optimization decreased about 0.8% in 2010 and 0.35% in 2018 compared with that of the basic system. When comparing with the designed operation modes of hybrid cooling, 0.07 million dollars is saved after optimization.

Numerical investigation on novel water distribution for natural draft wet cooling tower

Natural draft wet cooling tower is widely used in cold end system of thermal power plants, however, the water droplets in the central area cannot be sufficiently cooled due to small air flow rate. In this paper, a novel water distribution mode is proposed for the wet cooling tower, in which the water distribution area is divided into four subdomains equally in the radius direction, with the diameters of sprayed water droplets from the inside to the outside of 4 mm, 3 mm, 1 mm and 0.5 mm, respectively. The three-dimensional numerical model of natural draft wet cooling tower is developed and validated by the published data, and the cooling performance with the novel water distribution mode is then obtained. The results show that the air velocity in the central area is significantly increased thus the velocity gets more uniformly distributed with the novel water distribution mode. Besides, the thermal performance in the external area is also improved thanks to the larger contact area between the cooling air and water droplets. By comparing the proposed water distribution with the conventional one, it can be found that the outlet water temperature decreases by up to 0.69 °C, and the tower with the proposed water distribution can achieve better heat and mass transfer performances with a lower air mass flow rate, which can be recommended in engineering applications for an ideal energy efficiency of cold end system.

The relative importance of wind-driven and chimney effect cave ventilation: Observations in Postojna Cave (Slovenia)

Density-driven chimney effect airflow is the most common form of cave ventilation, allowing gas exchange between the outside and the karst subsurface. However, cave ventilation can also be driven by other mechanisms, such as barometric changes or pressure differences induced by the outside winds. We discuss the mechanism and dynamics of wind-driven ventilation using observations in Postojna Cave, Slovenia. We show how seasonal airflow patterns driven by the chimney effect are substantially modified by outside winds. Wind flow over irregular topography forms near-surface air pressure variations and thus pressure differences between cave entrances at different locations. These pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect and can either enhance, diminish or even reverse the direction of the density-driven airflows. To examine the possibility of wind-driven flow, we used a computational fluid dynamics model to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. We compared these values with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the cave system, the comparisons showed satisfactory agreement. This allowed a more general assessment of the relative importance of wind pressure for subsurface ventilation. We are certain that this example is not unique and that the wind-driven effect needs to be considered elsewhere to provide better insights into the dynamics of cave climate, air composition or dripwater geochemistry.

Numerical simulation on influence of noise barrier on thermal performance for natural draft wet cooling towers

The noise barrier installed around the natural draft wet cooling tower can reduce the operating noise, but it will lead to an adverse effect on the thermal performance of the tower. In order to study the influence of noise barrier on the thermal performance, a three-dimensional numerical model of the tower with noise barrier was established in this paper, and the influence of ventilation shutters on the thermal performance are also investigated. In this study, the outlet water temperature t2 and the ventilation rate G are selected to evaluate the thermal performance of the cooling tower. The simulation results showed that noise barrier destroys the uniformity of the circumferential air intake of cooling tower, reduces the G of the tower, and weakens the cooling capacity. The overall thermal performance deteriorates with the increase of the height H, and the central angle θ of the noise barrier, but improves with the increase of its distance from the tower L. The maximum t2 is 28.25 °C, which is about 0.36 °C higher than that condition without noise barrier, the minimum G reduces to 10235.50 kg/s, which is about 6.8% less than that without noise barrier. Additionally, the ventilation shutters installed on the noise barrier can weaken the adverse effects of noise barrier on the thermal performance. When the ventilation shutters are installed on the middle of the noise barrier and the louvers angle β is 90°, the improvement to the adverse effect reaches to the optimum value. Compared with that condition without ventilation shutters, the G reaches to 10501.95 kg/s, increases by 2.6%, and the t2 is 28.08 °C, reduces by 0.17 °C.

Effect of the blockage ratio on the smoke extraction efficiency in tunnel fires with natural ventilation

Natural ventilation by shafts is widely applied in urban shallow tunnels due to its simple structure, steady smoke extraction efficiency and environment-friendly means. In order to investigate the influence of vehicle blockage on the smoke extraction efficiency of the shaft in tunnel fires, a series of n-heptane fire experiments were conducted in a 1:15 small-scale tunnel model with seven blockage ratios (0, 0.16, 0.24, 0.32, 0.40, 0.48, 0.56). The smoke extraction efficiency is calculated by the measured smoke temperature and CO volume fraction in the shaft and in the tunnel near the exhaust vent. Results show that the smoke extraction efficiency of the tunnel with shafts decreases firstly, then increases and drops lastly with the increasing blockage ratio. The smoke temperature in the shaft and at the downstream tunnel change similarly, which indicates that the vehicle blockage can affect the thermal energy discharged by a shaft. The main influence factors are the blocking effect, the necking effect caused by the blockage and the stack effect of the shaft, resulting in the variability of the smoke extraction efficiency. The smoke extraction efficiency reaches the maximum around the blockage 0.48 in the present tunnel, which can be referred to design the tunnel cross section in engineering applications.

Aerosol transmission of SARS-CoV-2 due to the chimney effect in two high-rise housing drainage stacks

Stack aerosols are generated within vertical building drainage stacks during the discharge of wastewater containing feces and exhaled mucus from toilets and washbasins. Fifteen stack aerosol-related outbreaks of coronavirus disease 2019 (COVID-19) in high-rise buildings have been observed in Hong Kong and Guangzhou. Currently, we investigated two such outbreaks of COVID-19 in Hong Kong, identified the probable role of chimney effect-induced airflow in a building drainage system in the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We injected tracer gas (SF6) into the drainage stacks via the water closet of the index case and monitored tracer gas concentrations in the bathrooms and along the facades of infected and non-infected flats and in roof vents. The air temperature, humidity, and pressure in vertical stacks were also monitored. The measured tracer gas distribution agreed with the observed distribution of the infected cases. Phylogenetic analysis of the SARS-CoV-2 genome sequences demonstrated clonal spread from a point source in cases along the same vertical column. The stack air pressure and temperature distributions suggested that stack aerosols can spread to indoors through pipe leaks which provide direct evidence for the long-range aerosol transmission of SARS-CoV-2 through drainage pipes via the chimney effect.

CO2 dynamics and heterogeneity in a cave atmosphere: role of ventilation patterns and airflow pathways

Understanding the dynamics and distribution of CO2 in the subsurface atmosphere of carbonate karst massifs provides important insights into dissolution and precipitation processes, the role of karst systems in the global carbon cycle, and the use of speleothems for paleoclimate reconstructions. We discuss long-term microclimatic observations in a passage of Postojna Cave, Slovenia, focusing on high spatial and temporal variations of pCO2. We show (1) that the airflow through the massif is determined by the combined action of the chimney effect and external winds and (2) that the relationship between the direction of the airflow, the geometry of the airflow pathways, and the position of the observation point explains the observed variations of pCO2. Namely, in the terminal chamber of the passage, the pCO2 is low and uniform during updraft, when outside air flows to the site through a system of large open galleries. When the airflow reverses direction to downdraft, the chamber is fed by inlets with diverse flow rates and pCO2, which enter via small conduits and fractures embedded in a CO2-rich vadose zone. If the spatial distribution of inlets and outlets produces minimal mixing between low and high pCO2 inflows, high and persistent gradients in pCO2 are formed. Such is the case in the chamber, where vertical gradients of up to 1000 ppm/m are observed during downdraft. The results presented in this work provide new insights into the dynamics and composition of the subsurface atmosphere and demonstrate the importance of long-term and spatially distributed observations.

Experimental and numerical studies on the smoke extraction strategies by longitudinal ventilation with shafts during tunnel fire

With the surge of extra-long tunnels, the vertical shafts are widely used to shorten the construction period and become a part of the ventilation system during the operation period. Because of the towering mountains, the depth of vertical shafts is usually about 100 m or more, which is quite different from those of urban road tunnels. As a result, it is indispensable to set up perfect smoke extraction strategies, including exhaust velocity and supply velocity, featuring longitudinal ventilation system with vertical shafts to reduce the great loss during tunnel fire. To control smoke better, 1: 4.8 reduced-scale tests were carried out. In addition, a series of full-scale numerical simulations were established, using FDS, to refine the supply velocity of three HRRs of three fire source locations. From this study, some conclusions were drawn: (1) Without ventilation system, the temperature at the junction of the main tunnel and shaft is higher, because of more smoke vents and stronger stack effect provided by vertical shafts. (2) Due to the stack effect of vertical shafts, the exhaust velocity is smaller than the supply velocity under a bigger HRR. (3) The HRRs and the fire locations made no difference to the exhaust velocity which is 3 m/s. When the fire source is located at the exhaust region and middle region, the respective supply velocity is in the range of 3 ~ 4 m/s and 4 ~ 6 m/s. When the fire source is situated in the supply region, the supply velocity of 6 m/s is suitable for all three HRRs. (4) The fitting formulas of the exhaust region and middle region are expressed. The results in this work will provide references for tunnel designers and engineers to devise smoke extraction strategies during tunnel fires.

Comparison on cooling performance of pre-cooled natural draft dry cooling towers using nozzles spray and wet medium

Evaporative pre-cooling technology improves the cooling performance of natural draft dry cooling towers (NDDCTs) by introducing a certain amount of water into the air entrance. This paper compares the effects of different evaporative pre-cooling systems (i.e., normal wet-medium arrangement, A-frame wet-medium arrangement and nozzle spray) on the cooling performance of a 167 m high NDDCT. Based on the meteorological data of Naomao lake in Xinjiang in 2015, two MATLAB programs were developed for case studies. This study finds all the evaporative pre-cooling systems can improve the NDDCT's cooling performance in high temperature period. In non-high temperature period, the NDDCT's performance improvement is unremarkable, and the NDDCT's cooling performance even deteriorates in the case of evaporative pre-cooling using wet medium. In the hottest month of July, the water outlet temperatures of the NDDCT pre-cooled with normal wet-medium arrangement, A-frame wet-medium arrangement and nozzle spray drop 3.2 K, 3.5 K and 3.3 K, respectively when compared with no pre-cooling counterpart. The annual average water consumption values are 39.0 kg/s, 56.4 kg/s and 124.1 kg/s, respectively. Considering all the year around performance improvement, it is suggested to use nozzle spray pre-cooling system while intermittent operation or certain-time period operation should be considered to save water.

Crosswind influence on heat and mass transfer performance for wet cooling tower equipped with an axial fan

To enhance the thermal performance and relieve the impact of crosswind on the super large-scale natural draft wet cooling towers (SLWCTs), based on our previous research, through the three-dimensional numerical simulation of the SLWCT equipped with an axial fan, the impact of the fan positions on the thermal performance was investigated at various crosswind velocities (1–9 m/s) when the fan power was fixed to 200 kW. It is obtained that the optimized fan diameter and the installation height are 20 m and 18 m. Additionally, the characteristics of the aerodynamic and temperature field inside the tower with different fan positions were discussed based on the above results, subsequently, the change rules of water temperature drop, evaporation loss, and heat transfer coefficient are analyzed. The results manifested that the optimization scheme efficiently relieves the adverse effects of the crosswind, and the air velocity near the fan is significantly increased. By comparison with the case of no fan, the heat transfer coefficient and water temperature drop increase by 4.79% and 0.226 °C, respectively, and the evaporation loss increases by 2.86% at the utmost.

Comprehensive Analysis of Solar Dryer with a Natural Draught

ABSTRACT A solar dryer has been examined critically by using a constant drying model. The effect of the dynamic losses, intrinsic properties such as thermodynamic, hydrodynamic and psychometric factors, and natural draught during the practical application of the solar drying unit has been investigated. Carson and Moses, and the dynamic and static pressure correlations with the flow field have been used to determine the flow pattern inside the collector, the drying chamber, and the chimney. The energy allocation throughout the drying process has been evaluated by a Sankey diagram. The psychrometric calculation has been made by applying the steady-state-steady flow conditions (SSSF) on the carrier fluid (air). The average drying rate for the given design is 0.31 kg·h−1. The overall drying system efficiency is found to be 19%. The net static-regain (S.R) in the ducting system due to the sudden expansion is 2.22 N·m−2. The plume height for an efficient dilution of outgoing hot air is 1.17 m in addition to the given design. The flat plate collector efficiency for the given design was 26%. The surface temperature of the specimen should not be more than 321 K during a constant rate period. The pressure loss due to sudden contraction was found to be 73.48% lower than that of sudden expansion. Drying consumed 0.3% of the global irradiance for the drying purpose, whereas 57% of net received energy was absorbed by carrier fluid. The study encompassed the variability in the input that influence the performance of the solar collector since quantitative workout might not be effective unless the rheology is considered.