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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 airflow 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 °C. [ABSTRACT FROM AUTHOR]

Solar energy is one of the most feasible options to produce energy in countries where unexploited desert areas or solar radiation are abundant. An energy tower is an effective system for electrical power generation that can perform more efficiently along with solar radiation. As the primary aim of the present study, effects of different environmental parameters on total efficacy of energy tower were investigated. In this study, the efficiency of the energy tower system is investigated experimentally by an indoor fully adjustable apparatus. In this regard, a comprehensive set of influencing parameters like air velocity, humidity, and temperature and the effects of tower height on the performance of the energy tower are individually assessed. It is demonstrated that there is a direct relationship between an increase in humidity percentage of the surrounding and performance of energy tower, meaning that a 274% increase in humidification rate led to 43% elevation in airflow velocity. The kinetic energy increases in the direction of airflow from top to bottom, and as the height of the tower lengthens, the kinetic energy enhances and subsequently increases the overall efficiency of the tower. An elevation about 2.7% in airflow velocity was seen due to an increase from 180 to 250 cm in chimney height. Although the energy tower performs efficiently in the nighttime, airflow velocity increases averagely about 8% during the daytime and at the peak of the solar radiation, the airflow velocity enhances by 58% compared to nighttime. [ABSTRACT FROM AUTHOR]

Coal mine gangue hills have a significant impact on the ecological environment. To address this challenge, gravity heat pipes have been recognized as an effective means of controlling deep-seated temperature in coal mine gangue hills. Among various factors affecting heat transfer efficiency, the magnitude of external airflow velocity in the heat dissipation section of the heat pipe is critical. In this study, we develop a novel chimney effect model for gravity heat pipes in coal mine gangue hills. We employ the non-isothermal flow multiphysics transient solution method to analyze the single-factor effects of entrance radius, height of outlet section, and radius of outlet section on the average velocity at the chimney outlet. Furthermore, we utilize a comprehensive factorial response surface research method to fit the functional relationship between the average velocity at the chimney outlet and various chimney parameters. Notably, our study introduces the height and radius of the outlet section as constraint conditions based on field conditions. As a result, we determine the optimal chimney parameters for achieving the chimney effect in gravity heat pipes within coal gangue hills. Specifically, we set the chimney inlet radius at 1.05 m, the chimney outlet height at 0.03 m, and the chimney outlet radius at 0.145 m. Comparative analysis reveals that introducing the chimney improves the heat dissipation efficiency of the gravity heat pipe by 2.6%. Overall, our study offers innovative approaches to addressing the challenges associated with coal gangue hills and provides valuable insights into the optimization of gravity heat pipes for efficient heat dissipation. [ABSTRACT FROM AUTHOR]

Artificial CO2 was used as a tracer along ventilated karst conduits to infer airflow and investigate tracer dispersion. In the karst vadose zone, cave ventilation is an efficient mode of transport for heat, gases and aerosols and thus drives the spatial distribution of airborne particles. Modelling this airborne transport requires geometrical and physical parameters of the conduit system, including the cross-sectional areas, the airflow and average air speed, as well as the longitudinal dispersion coefficient which describes the spreading of a solute. Four gauging tests were carried out in one mine (artificial conduit) and two ventilated caves (natural conduits). In this paper, we demonstrate that it is possible to gain reliable airflow rates and geometric information of ventilated karst conduits using CO2 as a tracer. Airflow was gauged along two caves and one mine and compared with punctual measurements made with a hot-wire anemometer. Cross-sectional areas estimated with CO2 tests were compared with those measured in situ. Moreover, breakthrough curve (BTC) analysis displayed an accentuated tailing along the investigated natural conduits due to the presence of dispersive singularities which possibly enable aerosol deposition. The long tailing observed in Milandre and Longeaigue caves is probably due to cross-section variations. A 1-D advection-dispersion model tested for these sites was unable to fit BTC tailing in natural conduits. In Baulmes artificial conduit, where long tailing is not observed, the dispersion coefficient has been estimated using Chatwin’s method, and compared with the prediction of Taylor’s theory. Despite the regular geometry of Baulmes Mine, Taylor’s correlation significantly underestimates the dispersion coefficient deduced from field data, showing the need for more theoretical work on turbulent dispersion in mines. This paper gives a first insight into air motion and matter dispersion along ventilated karst conduits, preparing for proper aerosol dispersion modelling. [ABSTRACT FROM AUTHOR]

Shaft structures in high-rise buildings may increase fire coverage due to chimney effects. However, few previous studies have considered the motion of the flue gas under the combined effect of the chimney effect and thermal buoyancy. So,weset a continuous gradient differential pressure opening based on the characteristics of the chimney effect in winter. The CFD method is used to simulate 12 operating conditions with different fire source locations and rates of heat release from the ignition source. We compare and analyze the temperature rise, the flue gas rise law and the variation of the thermal pressure difference between the inner and outer shafts for different fire source powers. The results show that, in the case of low-floor fires, the range of temperature appreciation and fluctuation at the local measurement point increases with the fire power and the distribution of temperature appreciation decreases with the height. The relationship between the dimensionless time and the dimensionless height of the flue gas layer is exponential for different fire position conditions. The fire causes the neutral surface to shift, and below the original neutral surface, the higher the position of the fire source, the more pronounced the shift of the neutral surface. [ABSTRACT FROM AUTHOR]

The design and operation of a furnace for the incineration of solid industrial and household waste are described. A peculiarity of the furnace is the application of catalytic neutralization of high-temperature gaseous combustion products upon their filtering through a catalyst representing a fixed layer of granular solid particles. The results of experiments on the neutralization of combustion products are presented. A method for calculating the main thermohydraulic characteristics of a furnace operated under the conditions of chimney effect is proposed. [ABSTRACT FROM AUTHOR]

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A chimney system is combined of two heated plates with adiabatic extension. The numerical analysis of the chimney system is carried out to clarify the performance of the free convection heat transfer and the augmentation of mass flow rate (m•) for the tested parameters. A Computational fluid dynamic (CFD) software, ANSYSFLUENT, is employed in this investigation. Itis found that there is good agreement between the numerical findings and the experimental data collected from the literature, as demonstrated by this validation. Two types of obstruction are used in the present investigation, which are rectangular and semi-circular. The two types are tested at different configurations. Results reveled that the obstructions reduce the velocity and form vortices, which led to the reduction of the mass flow rate. In addition, it is found that the highest (m• ) is achieved with the base case However, the semi-circular chimney found to achieve the highest among obstructions. [ABSTRACT FROM AUTHOR]

Waitomo Glowworm Cave is a highly visited cave where the highlight is viewing the bioluminescence display of a large colony of glowworms. Anthropogenic carbon dioxide build-up in the cave is prevented by management of chimney-effect ventilation aided by a network of microclimate sensors. A cave door prevents ventilationunder drying conditions and promotes it when necessary to clear CO2 and when inflowing air has high relative humidity. A COVID-19-related nationwide “lockdown” in New Zealand from March 2020 resulted in neither staff nor visitors being present in the cave for 60 days, and provided an opportunity to assess the natural microclimate of the cave, especially the natural variation in partial pressure of carbon dioxide (pCO2). In addition, comparison to the previous year showed that the presence of people in the cave increased the cave temperatures but the effect was short-lived due to cave ventilation. During the period of lockdown, the daily increase of carbon dioxide partial pressure (pCO2) due to visitors was absent. When the cave door remained sealed, pCO2 varied and tended to lie at levels above that of the external atmosphere (410 ppm). Notably, rain events raised pCO2 by up to 200 ppm (v/v), which appeared to be sourced from both stream water and drip water. These natural CO2 sources rarely reached the levels associated with cave visitation. The results support current management practices that use door control to enhance cave ventilation when people are in the cave or when natural conditions (high stream levels and high drip-water levels) promote CO2 outgassing into the cave. Suppressing ventilation outside of those times reduces the risk of introducing dry air that could desiccate the glowworms.

The concave building structure (CBS) featured with a partial sunken façade is prevalent in the high-rise residence and comprehensive buildings. The building is a 3/4 enclosed longitudinal shaft that creates a new channel for longitudinal fire propagation. The objective of the paper is to reveal the dynamic mechanism of fire propagation in CBS. A small-scale experiment and Fire Dynamics Simulator (FDS) were conducted to investigate the fire behaviour. The results show that CBS could enhance the longitudinal propagation velocity of the flame and smoke with a slender profile. It could reduce the heat loss and create a higher space temperature than no-concave building structure (NBS). The maximum temperature and smoke concentration extended from the external facade centre to two flanks with the building height increase. Raising the structure factor extended the flame length from 7.0 m to 8.6 m. It exhibited a typical three stages of entrainment velocity growth: rapid growth, medium growth and slow growth. The flame transferred from the external façade to the lateral façade when the heat release rate was 8–10 MW. The research is essential to distinguish the hazardous region and provide a basis for architectural design and personal safe evacuation in the CBS fire scenario. [ABSTRACT FROM AUTHOR]

In order to study the design parameters such as the spacing and width of transverse passage of super long highway tunnel at high altitude under the effect of chimney effect, taking Shengli Tunnel in Tianshan Mountains as an example, a three-dimensional fire burning model of highway tunnel at high altitude with different slopes was established by using the fire dynamics model(FDS), and the distribution law of fire temperature and smoke in high altitude tunnel was analyzed. The available safe evacuation time under fire mode in tunnel was given. Considering the factors of evacuation speed reduction caused by high altitude and smoke, human characteristics, vehicle type and passenger capacity, the three-dimensional personnel simulation model Pathfinder was used to establish the personnel evacuation model under different combinations of transverse channel spacing and width, and the necessary evacuation time was obtained. Based on the safety evacuation criterion, the recommended values of design spacing and width of cross passage of Tianshan Shengli Tunnel under different slopes were given. The results show that the larger the tunnel slope is, the more obvious the chimney effect is, the faster the temperature rises and the visibility decreases in the uphill direction of the fire source, and the less available safe evacuation time. When the tunnel slope is 0.5%, 1%, 1.367% and 1.8%, the available safe evacuation time at 250 m upstream from the fire source is 496, 456, 430, 415 s, respectively. The evacuation speed of children, adult male, adult female and elderly in Shengli Tunnel of Tianshan Mountains is 0.72, 1.07, 0.91, 0.65 m/s, respectively. When the tunnel slope is 0.5%, 1.0%, 1.367% and 1.8%, it is recommended that the spacing(width) of transverse passages be 250 m(2 m), 250 m(2.2 m), 220 m(2 m), and 220 m(2.2 m), respectively. [ABSTRACT FROM AUTHOR]

In the paper, experimental analyses of the free convection heating transfer in a flat wavy-fin heat exchanger with the dimensions of 500 × 500 mm were investigated. The experimental reserch mainly included determining the average heat flux and heat-transfer coefficient for two selected types of finned heat exchangers. First, tests were conducted for exchangers without considering the so-called 'chimney' effect; these tests will be treated as reference studies. Then, experiments for specially designed 'chimneys' over the exchanger with heights of 350, 850, and 1350 mm, respectively, were carried out again. The analyses were performed for an average temperature difference between the heat-exchange surface and the environment in the range of 18 to 55 K. The experimental results demonstrated that, compared to the exchanger without a chimney, the addition of a chimney significantly affects the improvement in the thermal performance of the heat exchanger under natural convection conditions. Regarding the variant without a chimney, when a chimney is used with the highest height of 1350 mm and a maximum temperature difference of 55 K, the average heat flux increases by approximately 450% and the average heat-transfer coefficient is approximately 10 times higher. The heat exchanger characterised by lower airflow resistance showed higher values of average heat flux of 5 to 45% in the Rayleigh number range of 25 to 180. Studies have indicated that in some cases, a simple modification of the geometry of the heat exchanger leads to significant improvements in thermal performance and, in extreme cases, to the elimination of supporting equipment such as fans. [ABSTRACT FROM AUTHOR]

Waitomo Glowworm Cave is a highly visited cave where the highlight is viewing the bioluminescence display of a large colony of glowworms. The visitation levels result in the build-up of anthropogenic CO2, to the extent that it could cause corrosion of speleothems. The cave experiences chimney-effect ventilation with air flowing either upward or downward through the main cave chambers depending on air density differences between the cave and the outside environment. Lack of airflow leads to CO2 build-up; however, unrestricted airflow can draw in cool, dry air which is harmful to the glowworms. Consequently, airflow is managed by controlling the opening and closing of a door that seals the upper-most entrance, preventing ventilation under drying conditions and promoting ventilation when it is necessary to clear CO2 and when inflowing air has high relative humidity. A network of microclimate sensors in the cave allows prediction and management of the ventilation pattern. Management leads to asymmetric airflow through the year, which has a flow-on effect on cave temperature. Microclimate monitoring supports the current management practices that use door control to enhance cave ventilation when people are in the cave. Suppressing airflow, especially in winter, reduces the introduction of dry air.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been demonstrate to be transmitted through fecal aerosols in building drainage systems (BDS), emphasizing the urgent need to understand its specific transmission pathways and mechanisms. This paper concludes the two periods of aerosols transmission in BDS: airflows caused by pressure fluctuations during toilet flushing and airflows induced by the chimney effect. It also compares the airflow velocity, air pressure, and transmission duration during these two periods. Subsequently, research methods (e.g., aerosol experiments, tracer gas experiments, and numerical simulations), influencing factors, and related conclusions regarding aerosols transmission in BDS were thoroughly concluded and analyzed. Finally, the advantages and limitations of the research methods are compared, offering valuable insights on essential future research directions and drainage system design. Although there is a qualitative understanding of the vertical transmission mechanism in BDS, which summarize the pathway of aerosols transmission as "respiratory/fecal aerosols-drainage pipes-faulty floor drains-bathrooms", it is imperative to establish a quantitative understanding of the relative importance and duration of aerosols transmission during these two periods. Further research urgently needs to assess the risk levels of aerosols transmission during both periods and establish pressure-coupling curves for these two periods. Overall, both designers and scholars can utilize this review as a reference to mitigate the potential risk of vertical aerosols transmission in BDS, so that BDS can be appropriately designed to minimize pressure fluctuations and the water seal in floor drains can be regularly checked. • A review of vertical transmission of virus-laden aerosols in drainage pipes. • The air pressure, airflow and duration in pipes during different periods are analyzed. • The influencing factors and results of research methods are discussed. • The advantage and limitations of research methods are discussed. • Several challenging topics are proposed to provide guidance for future studies. [ABSTRACT FROM AUTHOR]

The propagation characteristics of near‐inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves' vertical penetration are investigated using observations from a mooring array located in the northeast Atlantic. The year‐long observations show that near‐inertial motions are mainly generated by local wind forcing, and that they radiate equatorward and downward following several strong wind events (wind stress ≳0.5 N m−2). Observational estimates of horizontal group speed typically exceed those of vertical group speed by two orders of magnitude, consistent with predictions from the dispersion relation. Enhanced near‐inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales, shorter time scales, and confined vertical extent compared to mesoscale eddies. Plain Language Summary: Near‐inertial waves (NIWs) are excited mainly by variable winds at the ocean surface and can carry their energy into the ocean interior, thus playing an important role in mixing the deep ocean. However, the propagation behaviors of NIWs, and how such waves are affected by mesoscale and submesoscale processes, are still understudied, especially over periods of months to years. In this study, we examine an annual cycle of wind‐generated NIWs based on moored observations in a typical open‐ocean region of the northeast Atlantic. Our results show that NIWs propagate downward and equatorward following several strong wind events. Enhanced near‐inertial kinetic energy and vertical shear are found preferentially in regions of anticyclonic vorticity with Rossby number of O(0.1). By contrast, submesoscale anticyclones with Rossby number of O(1) are ineffective at trapping and accelerating near‐inertial motions into the ocean interior. This is due to the smaller horizontal scales, shorter time scales, and confined vertical extent of submesoscale motions compared to mesoscale eddies. Our findings highlight the major role of mesoscale anticyclones in draining NIWs from the upper ocean to the ocean interior, and have implications for detecting regions of active turbulent mixing driven by NIWs in the deep ocean. Key Points: We provide observational evidence of downward‐ and equatorward‐propagating near‐inertial waves over a full annual cycleEnhanced near‐inertial kinetic energy and vertical shear are found preferentially in regions of anticyclonic vorticityThe chimney effect for near‐inertial waves is very likely controlled by mesoscale, rather than submesoscale, anticyclones [ABSTRACT FROM AUTHOR]

An analysis of natural convection in horizontal slots has been carried out. It is demonstrated that a proper combination of heating and groove patterns can create a net horizontal fluid movement, which we refer to as the horizontal chimney effect. Groove shapes that can be easily manufactured as well as heating patterns that can be easily created using heating wires were considered. It has been shown that both patterns must be properly tuned. The direction of the net horizontal flow can be changed by changing the relative positions of the patterns. Changes of groove geometry can change the flow rate by up to 100%. Simultaneous use of grooves and heating at both plates can nearly double the system effectiveness. The strength of the flow increases with reduction of the Prandtl number. [ABSTRACT FROM AUTHOR]

Biodegradable plastic mulch is potentially a suitable alternative to conventional polyethylene mulch because of the limited disposal options of the latter. However, biodegradable plastic mulch must perform better or comparably to polyethylene mulch to be widely adopted. Gas exchange and soil microclimate are important factors impacted by the use of plastic mulch, which in turn have implications on crop productivity. A controlled-environment study was established in a greenhouse to assess gas exchange and soil microclimate dynamics under biodegradable plastic, polyethylene, and paper mulches with and without planting holes, as well as the impact of the mulches on the growth of sweet corn (Zea mays). A no-mulch condition was included as control. In addition, we monitored CO2 concentrations in the vicinity of planting holes (chimney effect) in a greenhouse and agricultural field conditions under sweet corn production. The plastic mulches (both biodegradable plastic and polyethylene mulches) decreased the soil O2 concentration to a minimum of 181-183 mmol mol–1, and when compared to the no-mulch, the plastic mulches reduced water loss within 50 days by 35-68 mm. The paper mulch inhibited light penetration more than did the plastic mulches. There was an increase in the CO2 concentration at 2.5 cm above the planting holes in the plastic mulches compared to that under the no-mulch. The plastic mulches (both biodegradable plastic and polyethylene mulches) decreased the soil growth of sweet corn, possibly, because the canopy height of sweet corn was more than 15 cm within a few days after planting. Overall, the plastic mulches did not reduce O2 concentration below 100 mmol mol–1, the minimum level at which plant growth becomes impaired. Also, the often reported improved growth of sweet corn from plastic mulching could be attributable to other factors, such as weed control, reduced water loss, and early season soil warming, rather than elevated CO2 concentrations and fluxes in the vicinity of planting holes. [ABSTRACT FROM AUTHOR]

Biodegradable plastic mulch is potentially a suitable alternative to conventional polyethylene mulch because of the limited disposal options of the latter. However, biodegradable plastic mulch must perform better or comparably to polyethylene mulch to be widely adopted. Gas exchange and soil microclimate are important factors impacted by the use of plastic mulch, which in turn have implications on crop productivity. A controlled-environment study was established in a greenhouse to assess gas exchange and soil microclimate dynamics under biodegradable plastic, polyethylene, and paper mulches with and without planting holes, as well as the impact of the mulches on the growth of sweet corn (Zea mays). A no-mulch condition was included as control. In addition, we monitored CO2 concentrations in the vicinity of planting holes (chimney effect) in a greenhouse and agricultural field conditions under sweet corn production. The plastic mulches (both biodegradable plastic and polyethylene mulches) decreased the soil O2concentration to a minimum of 181–183 mmol mol-1, and when compared to the no-mulch, the plastic mulches reduced water loss within 50 days by 35–68 mm. The paper mulch inhibited light penetration more than did the plastic mulches. There was an increase in the CO2 concentration at 2.5 cm above the planting holes in the plastic mulches compared to that under the no-mulch. However, the differences were not discernible at 15 cm above the ground. Consequently, we did not observe significant impacts on the growth of sweet corn, possibly, because the canopy height of sweet corn was more than 15 cm within a few days after planting. Overall, the plastic mulches did not reduce O2 concentration below 100 mmol mol-1, the minimum level in which plant growth becomes impaired. Also, the often reported improved growth of sweet corn from plastic mulching could be attributable to other factors, such as weed control, reduced water loss, and early season soil warming, rather than elevated CO2 concentrations and fluxes in the vicinity of planting holes. Highlights - Gas exchange and soil microclimate dynamics under biodegradable plastic, polyethylene, and paper mulches were assessed - Elevated CO2 levels were observed near planting holes of plastic mulches (both biodegradable and polyethylene) - The plastic mulches inhibited O2 exchange, but not to a level that could impair plant growth - Polyethylene mulch conserved soil water better than biodegradable plastic and paper mulches - Paper mulch inhibited light penetration better than plastic mulches

Waitomo Glowworm Cave is a highly visited cave where the highlight is viewing the bioluminescence display of a large colony of glowworms. Anthropogenic carbon dioxide buildup in the cave is prevented by management of chimney-effect ventilation aided by a network of microclimate sensors. A cave door prevents ventilation under drying conditions and promotes it when necessary to clear CO2 and when inflowing air has high relative humidity. A COVID-19-related nationwide "lockdown" in New Zealand from March 2020 resulted in neither staff nor visitors being present in the cave for 60 days, and provided an opportunity to assess the natural microclimate of the cave, especially the natural variation in partial pressure of carbon dioxide (pCO2). In addition, comparison to the previous year showed that the presence of people in the cave increased the cave temperatures but the effect was short-lived due to cave ventilation. During the period of lockdown, the daily increase of carbon dioxide partial pressure (pCO2) due to visitors was absent. When the cave door remained sealed, pCO2 varied and tended to lie at levels above that of the external atmosphere (410 ppm). Notably, rain events raised pCO2 by up to 200 ppm (v/v), which appeared to be sourced from both stream water and drip water. These natural CO2 sources rarely reached the levels associated with cave visitation. The results support current management practices that use door control to enhance cave ventilation when people are in the cave or when natural conditions (high stream levels and high drip-water levels) promote CO2 outgassing into the cave. Suppressing ventilation outside of those times reduces the risk of introducing dry air that could desiccate the glowworms. [ABSTRACT FROM AUTHOR]

The first China-Russia crude oil pipeline (CRCOP-I) and the second China-Russia crude oil pipeline (CRCOP-II) went into operation in January 2011 and January 2018, respectively. On-site monitoring data have indicated that the seasonal thawing depth (STD) of a borehole 2 m away from CRCOP-I reached 10.8 m by the end of 2021. To improve mitigation measures of thaw settlement, an inverted T-shaped crushed-rock pipeline embankment (ITCPE) is proposed in this study. The long-term thermal activity of three types of pipeline embankments is predicted and compared with the underground structure. This study finds that none of the current pipeline structures has solved the thaw settlement issue for the CRCOP, whether with insulation layers, in an unprotected pipeline embankment (UPE), or in a traditional horizontal crushed-rock pipeline embankment (THCPE). However, the cross-ventilation attained in the proposed approach did mitigate thawing. It is estimated that after 50 years of operation, the natural permafrost table will be 2.9 m deep, whereas the proposed ITCPE central pipeline embankment could maintain the artificial permafrost table at a depth of 0.7 m. This study provides technical support for the CRCOP and provides a reference for the design and maintenance of other pipeline projects in permafrost areas worldwide. • In-site warning system and ERT testing reveal that the permafrost surrounding the CRCOP is thawing. • Insulation cannot change the thermal evolution trend of the permafrost surrounding the CRCOP. • A newly designed ITCPE system for thaw settlement mitigation is employed. • ITCPE is effective at mitigating thaw settlement and non-uniform deformation using ambient cool energy. [ABSTRACT FROM AUTHOR]

Waitomo Glowworm Cave is a highly visited cave where the highlight is viewing the bioluminescence display of a large colony of glowworms. The visitation levels result in the build-up of anthropogenic CO2, to the extent that it could cause corrosion of speleothems. The cave experiences chimney-effect ventilation with air flowing either upward or downward through the main cave chambers depending on air density differences between the cave and the outside environment. Lack of airflow leads to CO2 build-up; however, unrestricted airflow can draw in cool, dry air which is harmful to the glowworms. Consequently, airflow is managed by controlling the opening and closing of a door that seals the upper-most entrance, preventing ventilation under drying conditions and promoting ventilation when it is necessary to clear CO2 and when inflowing air has high relative humidity. A network of microclimate sensors in the cave allows prediction and management of the ventilation pattern. Management leads to asymmetric airflow through the year, which has a flow-on effect on cave temperature. Microclimate monitoring supports the current management practices that use door control to enhance cave ventilation when people are in the cave. Suppressing airflow, especially in winter, reduces the introduction of dry air. [ABSTRACT FROM AUTHOR]

In the paper, experimental analyses of the free convection heating transfer in a flat wavy-fin heat exchanger with the dimensions of 500 × 500 mm were investigated. The experimental reserch mainly included determining the average heat flux and heat-transfer coefficient for two selected types of finned heat exchangers. First, tests were conducted for exchangers without considering the so-called ’chimney’ effect; these tests will be treated as reference studies. Then, experiments for specially designed ’chimneys’ over the exchanger with heights of 350, 850, and 1350 mm, respectively, were carried out again. The analyses were performed for an average temperature difference between the heat-exchange surface and the environment in the range of 18 to 55 K. The experimental results demonstrated that, compared to the exchanger without a chimney, the addition of a chimney significantly affects the improvement in the thermal performance of the heat exchanger under natural convection conditions. Regarding the variant without a chimney, when a chimney is used with the highest height of 1350 mm and a maximum temperature difference of 55 K, the average heat flux increases by approximately 450% and the average heat-transfer coefficient is approximately 10 times higher. The heat exchanger characterised by lower airflow resistance showed higher values of average heat flux of 5 to 45% in the Rayleigh number range of 25 to 180. Studies have indicated that in some cases, a simple modification of the geometry of the heat exchanger leads to significant improvements in thermal performance and, in extreme cases, to the elimination of supporting equipment such as fans.

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. • Studies have shown that both in the mine adits and in the grottoes of the cave, strong seasonal radon variations are observed. • The pattern of radon variations in caves depends on the position of the cave relative to the neutral pressure plane. In the caves located below NPP, the highest radon concentrations are observed in summer, and in the caves located above NPP, high concentrations are recorded in winter. • The obtained values of the EEC of radon progeny periodically by several times exceed the International and Russian permissible levels for the work spaces (310 Bq m−3 in Russia). [ABSTRACT FROM AUTHOR]

The electrocatalytic hydrodehalogenation (EHDH) process mediated by atomic hydrogen (H*) is recognized as an efficient method for degrading halogenated organic pollutants (HOPs). However, a significant challenge is the excessive energy consumption resulting from the recombination of H* to H 2 production in the EHDH process. In this study, a promising strategy was proposed to generate piezo-induced atomic H*, without external energy input or chemical consumption, for the degradation and dehalogenation of HOPs. Specifically, sub-5 nm Ni nanoparticles were subtly dotted on an N-doped carbon layer coating on BaTiO 3 cube, and the resulted hybrid nanocomposite (Ni-NC@BTO) can effectively break C-X (X = Cl and F) bonds under ultrasonic vibration or mechanical stirring, demonstrating high piezoelectric driven dehalogenation efficiencies toward various HOPs. Mechanistic studies revealed that the dotted Ni nanoparticles can efficiently capture H* to form Ni-H* (H abs ) and drive the dehalogenation process to lower the toxicity of intermediates. COMSOL simulations confirmed a "chimney effect" on the interface of Ni nanoparticle, which facilitated the accumulation of H + and enhanced electron transfer for H* formation by improving the surface charge of the piezocatalyst and strengthening the interfacial electric field. Our work introduces an environmentally friendly dehalogenation method for HOPs using the piezoelectric process independent of the external energy input and chemical consumption.

In hot climate areas, the proper understanding of thermal comfort parameters leads to help building designers in providing a suitable and refreshing environment for occupants. The rational model such as ASHRAE 55/2013, ISO: 7730 defines comfort boundaries based on the experimental results, which are conducted in climatic chambers. Research works have been rarely conducted in buildings with high density of occupants like mosques. In order to study the thermal comfort and the effect of air velocity in mosques in hot dry climate, a rational and adaptive comfort study by applying ASHRAE 55, ISO: 7730/10551 is carried out in Eloued city, Algeria. The results show that the applicability of ISO7730 is suitable for a natural air velocity v_air∈[0.35,0.8]m⁄s, where the comfortable operative temperature is 30.7°C. However this outcome doesn’t exceed 26.8°C for air velocity vair< 0.35m/s.

Abstract Cave 222Rn has been a major health issue and subject of scientific debate for decades. While the basics of natural ventilation physics are well understood, it is difficult to make blind predictions of 222Rn concentrations in a given cave due to the complexity of cave systems. In-situ continuous observation is necessary to improve our ability to quantify radiation dose exposure and reduce radiation hazard to cave users, and trace the air exchange patterns occurring in caves. In this study, continuous monitoring using a RAD7 radon detector revealed high 222Rn concentrations and large fluctuations in 222Rn concentration in a small karst cave in southwest China, Shawan Cave. From August 2016 to July 2017, the average annual concentration was 47,419 Bqm−3 and ranged between 3720 and 123,000 Bqm−3, with lower values during summer than other seasons. Taking Shawan Cave as a case study, we suggest a framework to evaluate the potential dose exposure, allowing cave users to minimize risk of exposure to hazardous levels of 222Rn. Furthermore, we comparing results from this study with other studies in 35 caves worldwide, and conclude that there are three patterns of seasonal 222Rn variation. They were classified into five types of ventilation mode based on diversity of cave locations, geometry and connectivity of bed rock fracture networks, together with temperature differences between outside atmosphere and cave air. Highlights • Monitored cave 222Rn concentrations as high as 123000 Bqm−3. • Dose exposure and advices were given according to ICRP recommendations. • Seasonal variations of 222Rn concentration of 35 caves were summarized. [ABSTRACT FROM AUTHOR]

In hot climate areas, the proper understanding of thermal comfort parameters leads to help building designers in providing a suitable and refreshing environment for occupants. The rational model such as ASHRAE 55/2013, ISO: 7730 defines comfort boundaries based on the experimental results, which are conducted in climatic chambers. Research works have been rarely conducted in buildings with high density of occupants like mosques. In order to study the thermal comfort and the effect of air velocity in mosques in hot dry climate, a rational and adaptive comfort study by applying ASHRAE 55, ISO: 7730/10551 is carried out in El-Oued city, Algeria. The results show that the applicability of ISO7730 is suitable for a natural air velocity vair ∈ [0.35,0.8]m/s, where the comfortable operative temperature is 30.7 °C. However, this outcome doesn't exceed 26.8®C for air velocity vair< 0.35 m/s. [ABSTRACT FROM AUTHOR]

This work presents a numerical study of the dynamic and thermal behavior of the air flow circulating in the vertical rectangular cavity of height H and width W. The geometry consists of a vertical wall of low thermal conductivity and an opposite wall which acts as a ventilated facade with five openings subjected to a heat flow. The methods of analyzing the flow input and output behavior through the openings throughout the ventilated facade, make the use of CFD tools "Fluent 14.0" mandatory for a detailed description. The flow is considered to be turbulent, steady, incompressible and bi-dimensional and computations are performed using the standard k-ε and RNG k-ε models for Rayleigh number ≈ 1011. The results included mean velocity profiles; flow structure and thermal field which were presented and discussed. A comparative study with conventional cavity (closed) and classical cavity with two openings (input-output) under the same thermal conditions was conducted to quantify energy savings by the use of such configuration. The increase of the number of the openings enhances the wall cooling. Moreover, above a certain heat flux absorbed by the ventilated wall, natural cooling is obsolete, it is necessary to use forced devices. [ABSTRACT FROM AUTHOR]

Summary: This paper investigates a novel hybrid system combining thermal electrical generators (TEGs) and a wind turbine. The mathematical model of the system is derived and solved to investigate the performance of the proposed system. In the proposed system, solar energy is converted to heat by an absorber plate. Some portion of this heat is converted to electricity using TEG, while another portion of the heat is used to heat up air flowing in an inclined duct placed underneath the absorber plate. Heating the air inside the system enhances the current speed because of the effect of buoyancy. A wind turbine is placed inside the duct parallel to air flow before it exits to the atmosphere. The wind current is accelerated before passing through the turbine by using venture effect. The TEGs are exposed to the concentrated solar radiation. This can be obtained using a compound parabolic concentrator. The proposed configuration has multiadvantages: (1) the wind is used to drive a wind turbine; (2) air cools the rear surfaces of TEGs to increase the temperature difference between the opposite surfaces, thus generates more electrical power; and (3) it uses buoyancy effect to increase the wind stream speed, thus enhancing the power generated from turbine. It is found that the solar concentration ratio is the most important factor contributing to enhancing the TEG efficiency. The buoyancy effect leads to turbine power boost at high wind speeds more than at low wind speeds. [ABSTRACT FROM AUTHOR]

The three-dimensional characteristics of natural convection from isothermal vertical plate arrays with finite lengths are numerically analyzed. Studied are single- and multi-channel plate arrays with different plate length/height ( L / H ) ratios. To account for the extra plume buoyancy and the entrance flow resistance, extended computation domains encompassing the whole plate arrays are arranged. Highly non-uniform distributions of heat transfer coefficient exhibit along the plate, with the highest value at the plate end due to the effect of side flow. Inward, the heat transfer coefficient decreases drastically with boundary layer development but re-rises over the inner region as a result of higher plume buoyancy therein. As L / H increases, the stronger plume buoyancy promotes the overall heat transfer coefficient to approach the 2-D value with a difference of less than 4% when L / H  ≥ 2. The extra plume buoyancy varies in different channels, weakest in the edge channel, leading to complex flow above the outermost channels. The plume-buoyancy enhancement on the overall heat transfer coefficient is less pronounced for arrays with finite L / H ratios than for a 2-D array due to the plume shrinkage associated with the side flow. [ABSTRACT FROM AUTHOR]

Summary: A hybrid solar photovoltaic/wind system is proposed and investigated theoretically. The hybrid system is based on attaching a converging inclined duct beneath the photovoltaic (PV) panels and directed upward after the end of the panels. A wind turbine is attached at the exit of the converging duct. The converging duct will capture wind currents that at its inlet and enhances these current by buoyancy effect created by the rejected heat from the panels. The mixed convection air flow is used in cooling the PV panels and in generating electricity by driving the wind turbine at the duct exit. A mathematical model is proposed to describe the system hydrodynamic and thermal behavior. In addition to the mixed convection case, the pure free convection case, when there is no wind speed, has been tested. The design of the wind duct capturing system is not included in this study, which should be carefully manufactured to eliminate the reversed flow. The simulation results show that the integration of both systems not only enhances the performance of PV cell due to the effective cooling but also generates more electric power from the inserted turbine. At low wind speeds, it is found that the ducting system helps more in cooling the panels rather than driving the wind turbine. At these low wind speeds, the buoyancy effect may have a significant effect. However, at high wind speeds, the ducting system acts in both cooling the panels and driving the turbine, and at these high speeds, the buoyancy effect is insignificant. [ABSTRACT FROM AUTHOR]

In this paper, hot smoke test was carried out in the atrium of 60m high in Shanghai Tower, the law of fire smoke movement and smoke control effect has been studied under different fire release rate and different ventilation conditions. In this paper, the Influence of different air supply volume on mechanical smoke exhaust in the atrium of Shanghai Tower has also been studied by numerical simulation. The results showed that: in the tall atrium, smoke rise rapidly with the speed of 2-3 m/s because of “chimney effect”; Because of the “thermal barrier effect”, when the fire power is small, the smoke tends to subside and cannot be effectively discharged; When air supply volume is 50% of smoke exhaust, it is beneficial to exhaust smoke. [ABSTRACT FROM AUTHOR]

To reduce the energy consumed by ventilation in a building is a critical task for the related researchers. Using the solar chimney effect to obtain the nature ventilation at day time is possible. Here a hybrid wall with PCM was provided, and the effect of the position of PCM in the hybrid wall on the solar chimney effect was studied. The thickness of PCM was 1cm, the air gap was 30cm and the power of the simulation solar light was 780W. The results show that when PCM was in front of absorber, the temperature of the air in the gap was higher than that of the air with a PCM behind the absorber. As for the air velocity in the gap, when the simulation solar light was turn on, the air velocity with PCM in front of the absorb was higher than that of the air with PCM behind the absorb. While when the simulation solar light was turn off, the air velocity with PCM in front of the absorb was lower than that of the air with PCM behind the absorber It is interesting to guide the construction of the hybrid wall. [ABSTRACT FROM AUTHOR]

In a high temperature gas-cooled reactor (HTGR), one can postulate simultaneous ruptures of the pressure boundary at points near the top and bottom of the reactor vessel. In this extremely low probability event, depressurization and heat-up of the core can lead to ingress of air into the vessel driven by the buoyancy of the hot helium relative to the colder, cavity air (‘chimney effect’). Although this is considered a ‘beyond design basis accidents’ (BDBA), it receives high attention because it may lead to graphite oxidation of reflectors and fuel elements so as to weaken the structural strength and lessen the ability of the coated particles to retain fission products. A study of the rate and severity of the air ingress and potential for oxidation in the 200 MWe Pebble-bed Modular High Temperature gas-cooled Reactor (HTR-PM) is described in this paper. The TINTE code was used to simulate the air ingress accident caused by the simultaneous rupture of the fuel charging tube and fuel discharging tube. A mitigation measure, the injection of nitrogen into the vessel, has also been proposed and evaluated with the TINTE model. The results indicate that, due to the high flow resistance of the pebble-bed core, the air ingress flow rate is limited. The oxygen content in the reactor cavity is also limited due to the design of the Ventilated Low Pressure Containment (VLPC) that prevents the corrosion of the fuel elements and reflectors by limiting the amount of air that can enter the vessel. The results also indicate that the injection of the nitrogen in a suitable location and with a suitable mass flow rate can effectively prevent significant corrosion of the graphite. [ABSTRACT FROM AUTHOR]

Purpose: CO2 leakage along the trocar (chimney effect) has been proposed to be an important factor underlying port-site metastasis after laparoscopic surgery. This study aimed to test this hypothesis by comparing the incidence of port-site metastasis between B-ultrasound-guided and laparoscopically-assisted hyperthermic intraperitoneal perfusion chemotherapy (HIPPC). Materials and Methods: Sixty-two patients with malignant ascites induced by gastrointestinal or ovarian cancer were divided into two groups to receive either B-ultrasound-guided or laparoscopically-assisted HIPPC. Clinical efficacy was assessed from the objective remission rate (ORR), the Karnofsky Performance Status (KPS) score, and overall survival. The incidence of port-site metastasis was compared between the two groups. Results: Patients in the B-ultrasound (n=32) and laparoscopy (n=30) groups were comparable in terms of age, sex, primary disease type, volume of ascites, and free cancer cell (FCC)-positive ascites. After HIPPC, there were no significant differences between the B-ultrasound and laparoscopy groups in the KPS score change, ORR, and median survival time. The incidence of port-site metastasis after HIPPC was not significantly different between the B-ultrasound (3 of 32, 9.36%) and laparoscopy (3 of 30, 10%) groups, but significantly different among pancreatic, gastric, ovarian, and colorectal cancer (33.33, 15.79, 10.00, and 0.00%, p<0.001). Conclusion: The chimney effect may not be the key reason for port-site metastasis after laparoscopy. Other factors may play a role, including the local microenvironment at the trocar site and the delivery of viable FCCs (from the tumor or malignant ascites) to the trauma site during laparoscopic surgery. [ABSTRACT FROM AUTHOR]

• The chimney effect is induced by attaching aluminum tape to a pin fin heat sink. • The width and attachment position of the aluminum tape are optimized. • The heat dissipation performance is improved without changing geometry. Conventional methods of improving the heat dissipation performance of heat sinks require either modifying existing heat sink geometries or installing additional structures to induce chimney effect. These methods can be problematic to apply if there are limitations of space and cost. In this study, we address the aforementioned problems by developing a novel method of inducing the chimney effect by attaching aluminum tape to a pin fin heat sink. Geometric parameters for optimization were the width and attachment position of the aluminum tape, and changes in the thermal resistance of the pin fin heat sink with a rectangular base were observed according to its orientation angle. First, the widths and attachment positions of the outer and inner aluminum tape strips that minimized the thermal resistance of the heat sink were derived through simulation. The thermal resistance of the heat sink was estimated to be reduced by 10.8% or 7.7%, respectively, only by attaching the outer or the inner aluminum tape strip with optimal width to the top of the pin fin, and it was found to be further reduced by attaching both the outer and inner aluminum tape strips simultaneously. As the next step, the simulation results were verified by experimentally measuring the thermal resistance of actual pin fin heat sinks with and without aluminum tape. It was found that the thermal resistance of the pin fin heat sink was reduced by up to 17.1% by inducing the chimney effect simply through the attachment of both the outer and inner aluminum tape strips with optimal widths to the top of the pin fin. For all heat sink orientation angles tested in the range 0°−180°, the thermal resistance was found to be lower when the aluminum tape was attached than when it was not attached. As a result, the heat dissipation performance could be improved without changing heat sink geometry and by requiring no additional space to install a chimney structure. Our study results show that the safety of heat-generating devices that require heat dissipation can be significantly improved by enhancing the heat dissipation performance of heat sinks if the width and attachment position of the aluminum tape are optimized. [ABSTRACT FROM AUTHOR]

This study focuses on the improvement of the natural convection heat dissipation performance of a fully closed cabinet. Both experiments and simulations are performed for eight different cases to examine the effect of the heat sink with chimney effect, baffle, ceiling and ceiling angle with different perforation. A heat sink is introduced to form dual chimneys to separate/facilitate the airflow circulation in the interior and exterior of the cabinet. The heat generation is 90 Watts and the exterior ambiance is around 25 °C. The chimney effect of the heat sink with baffle improved the airflow circulation by reducing 11–14% of the thermal resistance (TR) than the reference case. TR was found to increase with the increase in baffle height and decrease with the increase of perforation and ceiling angle, respectively. The overall analysis recommends the optimized parameters namely, baffle length 175 mm, 20 mm opening height under the baffle, length of ceiling plate to be 75 mm with 20% perforation, ceiling height 75 mm with 20% perforation and 27-degree ceiling angle. The optimized design reduced the TR by 17.9% than the base case standardizing the natural convection potential that can enhance the airflow and heat transfer. [ABSTRACT FROM AUTHOR]

In this study, an indirect natural convection updraft solar dryer was designed and fabricated with objectives to; suit mid-latitude applications, determine its performance efficiency, analyze air properties by use of psychrometric chart and assess the quality of dried product. The dryer was fabricated using low cost and locally available materials. For mid-latitude regions, zenith angle is dependent on latitude, solar declination angle, time of the year and of the day. Based on these changing parameters, the solar dryer was designed to suit the conditions. The main parts of the dryer are; hinged flexible angled solar collector, drying chamber consisting of three drying trays, updraft chimney with metallic absorber plate. The experiments were carried out to dry sliced apples of 2.5 mm thickness spread over the drying trays. Temperature and relative humidity of airflow were taken at the collector inlet, drying chamber inlet and drying chamber outlet to chimney using data loggers. Airflow inlet to solar collector and from drying chamber was determined using hot wire anemometers, irradiation in W/m 2 was measured by radiation meter. Drying characteristics such as the collected solar energy to evaporated moisture, average drying rate, moisture content, etc., were determined. Natural convectional flow was adopted with an advantage of utilizing updraft air chimney to induce thermo-siphoning effect hence improving efficiency. In our current study, fresh apples of 886.64 grams with moisture content of 86% were dried to moisture content of 8.12% (wet basis) within 9 hours 20 minutes at an average irradiance of 534.45W/m 2 . The overall dryer efficiency was estimated to be 17.89%. The possibility of formation of water droplets within the drying chamber, which is a challenge to most direct solar dryers was considered to be minimal. This was based on analysis of the dew-point of drying chamber inlet air in comparison to outlet air properties. This design is simple; it can easily be replicated elsewhere in the world, products can be dried in one day and has capability to dry products at commercial scale while retaining the color, flavor and nutrients. [ABSTRACT FROM AUTHOR]

The effects of design parameters on natural convection heat transfer from a horizontal heated surface with a heated cylindrical pipe were estimated experimentally. The parameters considered were the heat flux of the horizontal heater and the aspect ratio of the heated cylinder. The flow fields around the horizontal heated surface and the heated cylinder were visualized. The results showed that flow characteristics have three distinctive patterns: non flow reversal, flow reversal occurred only in the heated cylinder and reached to the horizontal heater. The necessary conditions to determine the flow fields were discussed. With decreasing the distance of between the heaters and the aspect ratio of the heated cylinder, the ascending flow velocity in the pipe becomes low and flow reversal occurs. In all flow fields, an increasing in the heat flux of the horizontal heater enhances the heat transfer. In the case of no flow reversal and flow reversal occurred only in the heated cylinder, the heat transfer shows the same tendency and increases with increasing the aspect ratio of the heated cylinder. On the other hand, when flow reversal reaches to the horizontal heater, the heat transfer has a little influence on the heated cylinder conditions. Nusselt number is expressed as a function of the modified Rayleigh number of heaters and the nondimensional height.

In this study, the natural convection heat transfer from chimney-based radial heat sinks was investigated numerically and experimentally. The thermal performance of a radial heat sink with chimney was estimated and compared with that of a radial heat sink without a chimney. The effects on the heat sinks’ thermal performance were investigated in regard to the orientation angle with respect to gravity (0° ⩽ θ ⩽ 180°), the fin number (15 ⩽ N ⩽ 30), and the distance between the base plate and chimney (5 mm ⩽ H c ⩽ 25 mm). The results show that there exist optimal values for H c and N . The orientation angles 0° and 45° result in slightly better thermal performance than 135° and 180°, while 90° results in the worst thermal performance. The chimney-based heat sink with optimal structure enhances the thermal performance by up to 20% compared with the radial heat sink without a chimney. [ABSTRACT FROM AUTHOR]

• Natural convection heat transfer processes suffer from low heat transfer coefficients. • Heat transfer enhancement in internal natural convection is considered. • Enhancement by discrete rings and the chimney effect are evaluated individually. • Compound enhancement by combining above two processes is proposed and investigated. Natural convection is an indispensable heat transfer process considered in the design of advanced autonomous nuclear reactors. The renaissance of inherently safe micro nuclear reactors calls for increased post shut down heat rejection rates by passive means. This study numerically investigated heat transfer enhancement in a heated cylinder by discrete rings and the chimney effect. Five equally spaced rings of varying thicknesses and heights were fitted in the heated channel to effect enhancement by flow interruption. The chimney effect was studied by appending adiabatic chimney extensions of different heights, giving rise to extension ratios of 1.5, 1.6, 1.65, and 1.75. Compound enhancement was achieved by attaching chimney extensions to a ringed channel. Nusselt number ratios, mass flow rates, temperature, and velocity profiles were presented. The results showed that compound heat transfer enhancement could increase Nusselt numbers by between 44 and 54% and lower temperatures by over 30% depending on the chimney height. Furthermore, very short chimney heights would be required in the compound setup in comparison to the tall chimneys in the chimney-only cases, for the same target heat transfer rates. The resulting compactness could greatly influence the design of very small reactors and contribute to improvements of other air-cooled passive systems. [ABSTRACT FROM AUTHOR]