Thermal Buoyancy Effects Research Articles

Effect of ventilation ducts on smoke spread between two adjacent cabins arranged along a corridor: An experimental and numerical investigation

When a fire occurs onboard a ship, ventilation ducts may become hidden pathways for smoke to spread, and this issue has long attracted attention in the industry. In this paper, an experimental and numerical investigation on the smoke movement and indoor temperature characteristics were carried out. The results show that under the effect of thermal buoyancy, ducts with the openings located on the ceiling can easily become pathways for smoke to spread across the cabins. Compared with the scenario where the ducts are removed, the smoke jets from the ducts make the smoke movement in the adjacent cabin more complicated, and the maximum temperature in the cabin is also significantly increased. Moreover, it is found that the mass flow rate (MFR) of the gas in the duct is mainly determined by the pressure difference Δp between the two ends of the duct, and the MFR increases linearly with the increase of Aflow(2Δp)1/2, but its maximum value is limited by the cross-sectional area of the duct. The research results will help to better understand the cross-compartment smoke spread under the effect of ventilation ducts and serve as a reference for the quantitative evaluation of the smoke flow in ducts.

Predicting heat transfer Performance in transient flow of CNT nanomaterials with thermal radiation past a heated spinning sphere using an artificial neural network: A machine learning approach

An efficient heat transfer phenomenon using nanofluid have greater challenges in various industries, engineering application the recent trend. Keeping this in present scenario, this study aims to optimize the heat transmission rate in the magnetized flow of nanomaterials through a rotating, spinning sphere. The heat transfer phenomena in the time-dependent fluid are enhanced by the incorporation of nonlinear radiation and a variable heat source. Additionally, the free convective flow is influenced by the effects of thermal buoyancy and a transverse magnetic field. The proposed model along with several factors is standardized through adequate transformation rules. Further, shooting-based Runge-Kutta technique is adopted with the help of built-in MATLAB function bvp4c for the solution of the transformed system. The prime focus of the proposed work is the optimizing heat transfer rate combined with regression analysis using artificial neural network and then it uses Levenberg Marquardt algorithm with well-posed training, testing, and validation data. The error analysis also presented briefly and the variation of characterizing parameters is depicted via graphs. Further, the important outcomes are; the particle concentration of carbon nanotubes contributes to decelerating the velocity profiles, leading to an increase in boundary layer thickness. In contrast, increasing magnetization has the opposite effect. Both nonlinear radiative heat and an additional heat source enhance the heat transfer phenomenon.

Theoretical investigation of the combined effects of solar energy and thermal buoyancy around a laminar jet placed in a porous medium

The current research examines the combined effects of solar energy and thermal buoyancy around a laminar jet placed in a porous medium. The governing boundary layer equations are dimensionalized by using appropriate dimensionless variables. The numerical solution of the dimensionless boundary layer equations is obtained using the finite difference method. The impact of physical parameters, which are Darcy number, dimensionless porous medium inertia coefficient, Prandtl number, radiation parameter, and dimensionless fluid's absorption parameter, on velocity and temperature profile, is shown graphically, while the influence of the above parameters on the heat transfer rate is presented in tabular form. It is keenly observed that for Darcy number velocity profile decreases while reverse behavior is noted for temperature distribution. For the dimensionless radiation parameter, both the velocity and temperature profile decrease. The main novelty of the current work is to improve the thermal performance of natural convection heat transfer system in the presence of thermal radiation placed in porous medium.

Melting heat transfer and irreversibility analysis in Darcy-Forchheimer flow of Casson fluid modulated by EMHD over cone and wedge surfaces

The present mathematical model analyses the melting heat transfer and irreversibility in Darcy-Forchheimer flow of Casson fluids modulated by the electroosmosis and magnetohydrodynamic mechanisms over the wedge and cone surfaces. The effects of thermal radiation, thermal buoyancy and heat generation/absorption are taken into consideration. Appropriate similarity transformations are utilized to establish a system of ordinary differential equations which are non-linear. The model has been numerically simulated using shooting approach in conjunction with the fourth order Runge-Kutta method under the bvp4c tool of MATLAB. Numerical results are computed for fluid velocity, fluid temperature, Nusselt number and Bejan number for analysing the fluid flow behaviour and heat transfer characteristics for the Casson fluid. A comparative analysis is performed to determine the irreversibility and heat transfer in the absence and presence of melting parameters, respectively. Variations in skin friction coefficient, local Nusselt number and Bejan number have also been examined under the effects of key parameters. Key findings of the present study indicate enhanced velocity distribution with increasing zeta potential and electric field parameters, while it diminishes with higher Prandtl number and Forchheimer terms. Moreover, fluid temperature reduces with rising zeta potential, while the Bejan number escalates with greater thermal radiation in the core region of geometries. The findings of the model may be useful in various applications of thermal systems.

Weak geostrophic wind driven ventilation in street canyons with trees and green walls: Cooperating or opposing dispersions of airborne pollutants?

Urban high-rise buildings and dense street canyons significantly disrupt synchronised wind flows, potentially exacerbating urban heat island effect. Fortunately, urban heat island effect can be mitigated by the green infrastructure in cities, particularly through the presence of roadside trees and green walls. However, the combined mechanisms of tree resistance, shading effects, and cooling effect, along with sensible thermal buoyancy flows within canyon ventilation and pollution dispersion, remain undisclosed. This study employs refined computational fluid dynamics numerical simulations to analyse airflow and pollutant dispersion within urban street canyons. Air exchange rate and pollutant retention time are employed to evaluate ventilation and pollutant dispersion, respectively, inside the street canyons.In scenarios with leeward green wall layout, higher tree canopy spread and trunk height lead to a shift in high pollutant concentrations from the region near the windward side to that near the leeward side of the typical canyon (H/W = 1). Conversely, in deep canyons (H/W = 5), most pollutants are retained near the bottom leeward side. Moreover, differences in pollutant dispersion between typical and deep canyons increase with with rising tree canopy spread. Additionally, higher tree canopy spread and trunk height values result in longer pollutant retention time, which are unfavourable for pollutant removal, and particularly limiting pollutant dispersion as street canyon depth increases. Similarly, increasing tree canopy spread reduces the difference in air exchange rate between windward and leeward green wall layout patterns. Regarding Richardson number variations —indicating thermal buoyancy effects—lower Richardson number values lead to reduced differences in pollutant retention time between typical and deep canyons. Furthermore, ventilation performance of the windward green pattern surpasses that of the leeward pattern. This research provides valuable insights for implementing green infrastructure to locally mitigate urban air pollution.

Computational fluid dynamics numerical simulation of flue gas emission from stack and tower integration system of coal-fired power plant

ABSTRACT The method of Computational Fluid Dynamics (CFD) simulation has been extensively employed in the stack and tower integration technology (STIT) research; however, a significant number of studies overlooked the impact of thermal buoyancy on the external flow field of cooling tower. In light of this, an improved approach is proposed in this study. A model of a cooling tower and its surrounding flow field was established, and its reliability was confirmed too. Based on our model, the performance of three enhancement strategies under various crosswind speeds and ambient temperatures was simulated. The results indicate that all three enhancement strategies can effectively simulate the effects of thermal buoyancy. In the simulations, the elevation of the flue gases increased from 306 m to either 426 m or 476 m. In the present study, ambient conditions characterized by crosswind speeds below 4 m/s and temperatures below 297.15 K were deemed suitable for the dispersion of stack and tower integration (STI) flue gases. A power function distribution was identified between crosswind speed and the mean height of the plume, while a linear relationship was observed between ambient temperature and the mean height of the plume.

Combined Effects of Thermal Buoyancy, Wind Action, and State of the First-Floor Lobby Entrance on the Pressure Difference in a High-Rise Building

The stack effect in high-rise buildings, stemming from an inside/outside temperature difference, may produce a significant pressure difference on the elevator doors, potentially causing elevator malfunctions. This effect can also be influenced by wind action and human behaviors, e.g., opening/closing of building entrances. In this study, a wind tunnel test was conducted to determine the real wind pressure distribution on a high-rise building in northern China. A numerical simulation utilizing the Conjunction of Multizone Infiltration Specialists software (COMIS) was carried out to investigate the pressure difference of elevator doors under the effects of thermal buoyancy, wind action, and opening/closing of the first-floor lobby entrance. An alternative solution of a locally strengthened envelope is proposed and validated for the studied building zone. The study reveals that the opening of the first-floor lobby entrance increases the pressure difference regardless of the environmental conditions, and the increase of wind speed tends to increase the pressure difference in winter but decrease it in summer. The proposed countermeasure combination, involving using revolving doors instead of swing doors, increasing additional partitions, and strengthening the local building envelope, was found to be synergistic and effective in reducing the pressure difference inside the building. The research findings offer practical engineering solutions for mitigating elevator door pressure challenges in high-rise buildings.

A Comprehensive Review of Ferromagnetic Liquid Dynamics and Nonlinear Thermal Buoyancy Effects

Ferromagnetic liquid is a unique class of fluids that exhibits strong magnetic properties when subjected to a magnetic field. This liquid has been extensively studied in various industrial and scientific applications due to its unique properties such as high thermal conductivity, heat capacity, and electrical conductivity. One of the key phenomena associated with this type of liquid is its nonlinear thermal buoyancy effect, which refers to the motion of the liquid due to variation in temperature caused by a non-uniform magnetic field. In recent years, there has been a growing interest in studying the nonlinear thermal buoyancy effect in ferromagnetic liquids, specifically in the context of heat transport processes.
 Non-Fourier heat flux is another crucial aspect of ferromagnetic liquids that has gained considerable attention among researchers. Non-Fourier heat flux is characterized by the deviation from the linear Fourier's law of heat conduction, where the rate of heat transfer is not proportional to the temperature gradient. In ferromagnetic liquids, this phenomenon is mainly attributed to the presence of magnetic fields, which influence the heat transfer processes within the liquid. This nonlinear heat flux behavior has significant implications in a wide range of applications, including heat dissipation, cooling, and thermal insulation.
 Radiated elastic surfaces are another key feature of ferromagnetic liquids that has recently emerged as a promising area of research. These surfaces are formed when a magnetic field is applied to a ferromagnetic liquid, resulting in the formation of elastic waves on the surface. These waves are created due to the interaction between the magnetic field and the elastic properties of the liquid. The formation of these surfaces has been observed to have a considerable impact on the heat transfer behavior of the liquid, where they act as efficient heat dissipators and significantly enhance the rate of heat exchange between the liquid and its surroundings.
 In summary, the interplay of nonlinear thermal buoyancy, non-Fourier heat flux, and radiated elastic surfaces in ferromagnetic liquids has attracted significant attention in recent years due to its potential for a wide range of practical applications. The understanding of these phenomena has the potential to significantly improve the design and efficiency of various heat transfer processes, making ferromagnetic liquids an important area of study in materials science and engineering. Future research in this field is expected to focus on developing more comprehensive models and experimental techniques to further explore the complex thermal and magnetic interactions in ferromagnetic liquids.

Simulation Study on Natural Ventilation Performance in a Low-Carbon Large-Space Public Building in Hot-Summer and Cold-Winter Region of China

Recently, climate governance has entered a new phase of accelerating decarbonization. In order to achieve low-carbon buildings, natural ventilation has been widely used as it requires no fan power. However, there are great challenges for achieving effective natural ventilation in large-space public buildings especially in areas characterized by hot-summer and cold-winter climatic regions, due to empirically unsuitable ambient temperatures and theoretically complex joint effect of wind pressure and thermal buoyancy. Therefore, this numerical study was conducted on the performance of a natural ventilation strategy in a large-space public building in a hot-summer and cold-winter region by using computational fluid dynamics (CFD) methods. Simulations were performed by applying FLUENT software for obtaining airflow distributions within and around a typical low-carbon public building. The temperature distribution in the atrium of the building was simulated particularly for analyzing the natural ventilation performance in a large-space area. Results demonstrated that thermal pressure was dominant for the large-space building in the case study. The average indoor airflow velocities on different floors ranged from 0.43 m/s to 0.47 m/s on the windward side which met indoor ventilation requirements. Most areas of wind velocities could meet ventilation requirements. The natural ventilation performance could be improved by increasing the relative height difference between the air inlets and air outlets. These findings could help provide references and solutions for realizing natural ventilation in low-carbon large-space public buildings in hot-summer and cold-winter regions.

Melting of N-eicosane-based composite phase change materials for thermal energy storage application: A computational analysis in a square cavity

The present numerical study examines the thermal performance of composite n-eicosane phase change material (CPCM) with the addition of different nanoparticles such as Al2O3, Cu, Cuo, and GnP, identifying them as CPCM 1, CPCM 2, CPCM 3, and CPCM 4. This study focuses on the melting behavior of CPCMs with concentration ranges of 2, 5, 8, and 10 wt% in a square encapsulated (SE) geometry aimed at designing an energy-efficient CPCM-based thermal energy storage (TES) system for solar photovoltaic (SPV) applications. Mass fraction contours showed that heat flux at higher concentrations (8 and 10 wt%) contributed to a strong solid-liquid interface, which makes the melt front too wide at all CPCMs. Altogether, the addition of GnP in CPCM (CPCM 4) caused enhancement in thermal conductivity and buoyancy effect, leading to earlier completion of the melting than other CPCMs, irrespective of the concentrations. In addition, the 20–30 % reduction in total time with CPCM 4 over the other three CPCMs at high concentrations of 10 wt% With the influence of nanoparticles, the energy storage capacity (ESC) of CPCMs increased by about 10 %, with concentrations ranging from 2 to 10 wt%. In addition, the variation in total energy release rate between CPCM 2 and CPCM 3 was found to be less significant. However, the ESC of CPCM 4 is 8.5 %, 11 %, and 13 % higher than that of CPCM 1, CPCM 2, and CPCM 3, respectively. With the influence of the inherent thermal properties of GnP, the better thermal performance achieved by CPCM 4 confirms its suitability for SPV system designs. Study provides insights into the design of energy-efficient CPCM-based TES systems for SPV applications.

A qualitative study of mixing a fluid inside a mechanical mixer with the effect of thermal buoyancy

A qualitative study of mixing a fluid inside a mechanical mixer with the effect of thermal buoyancy

Influence of GI configurations and wall thermal effects on flow structure and pollutant dispersion within urban street canyons

Numerous evidence has shown that within the semi-enclosed street canyon, traffic emissions and the secondary pollutants are prone to accumulating in poorly ventilated areas, which is a serious threat to people's health. Among many factors affecting the flow field and pollution diffusion characteristics in street canyon, the effects of wall thermal buoyancy caused by solar radiation coupled with the aerodynamics and shading effect of different forms of green infrastructures (GIs) have not been paid enough attention. This study conducted the numerical simulations validated by wind-tunnel experiments to investigate the influence of different GIs & wall heating (WH) conditions on flow and dispersion within urban street canyons. Eight GIs and four WH configurations are considered. The validation results indicate that SKE-SWFs have better prediction accuracy. Numerical results show that the green belt has little effect on flow and dispersion within street canyon, while other GIs will cause increased pollution on the leeward side to varying degrees, depending on the planting location and volume of tree crown. Under the WWH, the RSTs will lead to the reverse flow at the canyon bottom, so that the pollutants will completely turn to the windward side. Conversely, the LSTs can promote the migration of pollutants to the canyon center, and has little impact on two side walls and pedestrian respiration planes, therefore, the LSTs is suggested to be the optimal tree planting configuration. This study can provide technical guidance for the optimization design of urban green facilities to achieve well-targeted control of local air quality.

Stagnation Point Flow of Bioconvective MHD Nanofluids over Darcy Forchheimer Porous Medium with Thermal Radiation and Buoyancy Effect

Stagnation Point Flow of Bioconvective MHD Nanofluids over Darcy Forchheimer Porous Medium with Thermal Radiation and Buoyancy Effect

An investigation of the MHD Cu-Al2O3/H2O hybrid-nanofluid in a porous medium across a vertically stretching cylinder incorporating thermal stratification impact

The thermal aspects of 𝐶𝑢 − 𝐴𝑙2𝑂3/𝑤𝑎𝑡𝑒𝑟 hybrid nanofluid in a porous medium across a ver-tically stretched cylinder with the incorporation of heat sink/source impact are investigated in this numerical study. A magnetic field along the transverse direction of the stretching cylinder and the thermal buoyancy effect is considered in the flow problem. A pertinent similarity vari-able has been employed to simplify the boundary layer equations which govern the flow and convert the coupled nonlinear partial differential equations into a set of non-linear ordinary differential equations. The numerical results are computed using the 3-stage Lobatto IIIa tech-nique, Bvp4c. The impacts of non- dimensional parameters, including Prandtl number, heat source/sink parameter, magnetic parameter, porosity parameter, curvature parameter, ther-mal stratification parameter, and thermal buoyancy parameter on the velocity curve, thermal curve, skin-friction coefficient, and Nusselt number, are illustrated graphically and numeri-cally portrayed in tables. The important results demonstrate that hybrid nanofluids are more thermally conductive than nanofluids. Therefore, the hybrid nanofluid has a considerable im-pact on improving thermal developments. It has been found that the absolute skin friction of the hybrid nanofluid is up to 31% higher compared to the nanofluid. The heat transport rate of the hybrid nanofluid is 7.5% enhanced in comparison to the nanofluid. The influence of heat stratification of the hybrid nanofluid flow is appreciably significant.

Model of multiphase flow, superheat transport, and particle capture in a steel continuous caster

A model of turbulent multiphase flow, heat transfer, and particle entrapment during continuous casting of steel is presented. The model includes the top 7m of the vertical and curved strand, and considers the effects of argon gas injection and thermal buoyancy. RANS model flow results are compared with LES. Lagrangian particle transport through the Eulerian-Eulerian multiphase flow field from the k-ɛ RANS model is based on random walk and features advanced particle capture criteria, improved from previous work by including anisotropic turbulent velocity fluctuations near the walls. Particle capture via 3 mechanisms is included: capture by solidified hooks at the meniscus, entrapment between dendrites, and engulfment by surrounding large particles. The fluid flow and bubble/inclusion capture results are validated with plant measurements, including nail board dipping tests and ultrasonic tests of particle locations, and good agreement is seen. The superheat has negligible effect on flow in the mold region but causes complex flow in the lower strand by creating multiple recirculation zones due to the thermal buoyancy. With high (30 K) superheat, this leads to less penetration, and slightly fewer and shallower capture of particles. Lower (10 K) superheat may enable significant top surface freezing, leading to very large internal defect clusters. Lower superheat also leads to deeper meniscus hooks, and more surface capture. Capture bands occur near the transition from vertical to curved, where downward liquid flow velocity balances the particle terminal velocity.

Falling film evaporation in a partially heated parallel plate channel: Elliptic numerical analysis with an excess moisture condensation model

A fully coupled elliptic numerical method is used to conduct a detailed numerical analysis of laminar water falling film evaporation with a co-current laminar gas vapour mixture flow inside a partially heated vertical parallel plate channel. In order to obtain a fully coupled implicit solution, a complete set of elliptic two-phase two-dimensional governing equations is solved. The liquid-gas interface is obtained over a non-orthogonal structured moving mesh (grid) during the solution process. An excess moisture condensation model is proposed as a novel numerical method to address supersaturation in the system. Comparisons with the previous studies are made to verify the accuracy and capability of the present numerical model. The effects of the liquid and mixture Reynolds numbers variation on the hydrodynamic and thermal characteristics of the evaporation process inside the partially heated channel are investigated. Thermal and solutal buoyancy effects are seen in the form of recirculating flow in the gas phase.

Scaling method between sub-scale helium and full-scale smoke tests of smoke spread during solar roof fires

Increasing numbers of photovoltaic or solar panels have been installed on building roofs worldwide. Meanwhile, fires from solar roofs were also reported to increase when they are overheated. During a solar roof fire driven by wind and thermal buoyancy effects, the toxic smoke can spread around and infiltrate back into the building and creates major concerns over occupants’ health and safety. To understand this complex heat and mass transfer process, it is possible to conduct full-scale fire studies, which are often constrained by the cost, so the associated studies have been found inadequate in the literature. In comparison, a sub-scale experiment in a wind tunnel can be a cost-effective method for understanding the mechanism of the smoke spread from solar roof fires. However, this may require a fireproof wind tunnel, and the use of actual fires in a lab context often creates safety and cost concerns and is thus impractical. Therefore, this study proposes a scaling method for conducting sub-scale wind tunnel tests with helium to generate a similar thermal plume and smoke spread behavior as an actual fire. The method was demonstrated by comparing the similarity of a 1/15 scale helium experiment in a wind tunnel and a full-scale smoke test by computational fluid dynamics (CFD) simulations using a fire dynamics simulator (FDS). A good agreement was found with an average difference of 7.02% for dimensionless velocity and 7.87% for dimensionless temperature. The difference among all cases was less than the maximum of 21.66%, which occurred only in the transient 45° roof scenario near the free stream region close to the indoor space. Therefore, the proposed scaling method of the wind tunnel helium test is justified in this paper when studying the smoke spread during solar roof fires.

Effect of rotation and cross thermal buoyancy on the nanofluidic transport around a circular cylinder

We numerically explore the coupled effect of a primary free stream nanofluid flow and secondary induced flows due to rotation and thermal buoyancy around a rotating and heated circular cylinder. The free stream flow of the Cu–H2O nanofluid is considered for a Reynolds number range 10≤Re≤30. The solid fraction (Cu-nanoparticles) varies in the base fluid (H2O) in the range 0%≤φ≤10%. The rotation and thermal buoyancy induced flows are considered for the range of dimensionless rotational speed, 0≤Ω≤3, and Richardson number, 0≤Ri<5. We estimate the first and the second critical rotational speeds characterizing the complete suppression of the steady and unsteady wakes. We also demonstrate a second vortex shedding mode originated at high rotational speeds. Furthermore, this study determines the critical thermal buoyancy to initiate the vortex shedding. The critical buoyancy parameter is found to increase with the increasing rotation rate and decrease with the increasing Reynolds number. However, it drops, rises, or remains constant depending on the solid fraction present in the base fluid.

Study on the formation mechanism of a radon source during coal spontaneous combustion in a goaf

Surface isotope radon detection technology (SIRDT) is an effective physical exploration technology that provides a solution for detecting hidden underground fire sources. However, because of the inaccessibility of deep underground spaces, current research on the formation mechanism of radon sources during coal spontaneous combustion in a goaf is insufficient, which severely restricts the use of SIRDT to detect such fires. In this study, the quantitative relationship between the coal temperature and radon exhalation rate was revealed through radon exhalation experiments during the low-temperature oxidation of coal, which provides measurement data for the radon source term of coal spontaneous combustion in a goaf. A multi-physical, coupled numerical model of coal spontaneous combustion in a goaf was constructed, and the intrinsic dynamics of radon migration and the formation mechanism of the radon source in the process of coal oxidation were thoroughly investigated. The results show that the radon distribution in a goaf is because of the competition between air leakage and the “porous chimney effect.” When the temperature of the coal was low, air leakage dominated the radon transportation in the goaf, causing radon to accumulate in the area close to the air-return side. When the temperature of the coal increased, the thermal buoyancy and vortex phenomena appeared in the high-temperature area of the goaf, at this point, the “porous chimney effect” took the leading role. The updraft generated by the thermal buoyancy effect rapidly carried the radon to the top of the goaf. The vortex promoted airflow from the surrounding area of coal spontaneous combustion into the interior of the “porous chimney,” which accelerated the process of coal spontaneous combustion and increased the amount of radon transported upward; thus, further intensifying the accumulation of radon at the top of the goaf. This accumulation becomes a source of radon that continues to migrate toward the surface.

Thermal fluid–structural interaction of three cylinders undergoing flow-induced vibration with cross thermal buoyancy

The effect of cross thermal buoyancy on the characteristic of flow-induced vibration and mixed convection of three circular cylinders is numerically studied. Two-dimensional simulations were conducted for a Reynolds number (Re) of 100 and five Richardson numbers (Ri) of 0–1.00. The range of the reduced velocity is 3 ≤ U* ≤ 15. Three circular cylinders C1, C2, and C3 are arranged in an equilateral triangle with C1 in upstream. The results show that the maximum amplitude of the C1 increases by up to 21% with considering cross thermal buoyancy in comparison to the case of Ri = 0. The galloping-like response is observed on the C1 at Ri = 1.00. The lift coefficient of three circular cylinders increases with the increase in Ri at U* ≥ 6. When U* exceeds a critical value, the vibrations of the C2 and C3 are in-phase, and the “2S” pattern is observed in the near-wake of three circular cylinders. The near-wake becomes wider, and the vortex shedding frequency increases at U* = 6 and Ri = 0.25 and 0.50. The higher the reduced velocity, the more significant is the effect of cross thermal buoyancy in enhancing heat transfer. The maximum space time-averaged Nusselt number increases by 10.42% in comparison to the case of fixed cylinders.