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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]

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]

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]

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]

This study investigates the heat transfer enhancement of a chimney system, both experimentally and numerically, by varying the height and diameter of the chimney, and the Prandtl number of the working fluid. Mass transfer experiments are carried out using a sulfuric acid and copper sulfate electroplating system based on analogy concepts. Numerical simulations are executed using FLUENT 6.3. Natural convection experiments and numerical calculations performed without a chimney showed good agreement with the Le Fevre correlation for natural convection on a vertical plate. As the chimney height is increased, the heat transfer rates are enhanced for all Prandtl numbers, but the enhancement rates decrease as the Prandtl number increases. An optimal chimney diameter is found that maximizes the heat transfer. An increase in heat input or heated length results in an additional enhancement of the heat transfer, increasing the buoyancy force. Numerical results provide visualizations of the temperature and velocity fields in the chimneys, showing their interactions and flow regimes. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of natural convection from a two dimensional “T” form cavity with rectangular heated blocks is conducted. The blocks are identical, and the domain presents a symmetry with respect to a vertical axis passing through the middle of the opening. The governing equations are solved using a control volume method, and the SIMPLER algorithm for the velocity–pressure coupling is employed. Special emphasis is given to detail the effect of Rayleigh number and block height on the heat transfer and the flow rate generated by the chimney effect. The results are given for the parameters of control as, 104⩽Ra⩽3×106, Pr=0.71, opening diameter (C=l′/H′=0.15), blocks gap (D=d′/H′=0.5) and blocks height (1/8⩽B=h′/H′⩽1/2). These results show that the heat transfer variation with Ra is in the same manner as those met in the case of the vertical smooth or ribbed channels. [Copyright &y& Elsevier]

[Display omitted] • Cooling of thermoelectric generators based on natural convection is very limited. • For performance enhancement the proposed cooling method utilizes the chimney effect. • For the presented heat exchanger, no auxiliary energy is necessary. • TEG performance is enhanced up to 46,2% by applying improved cooling method. • Further optimization potential for the cooling is investigated numerically by CFD. Due to the lack of sufficient auxiliary electrical energy to drive pumps or fans, the cooling of autarkic thermoelectric systems is confined to the natural convection mechanism. This is regarded as a key weakness and inhibits an adequate overall thermoelectric system performance. In order to enhance the performance of thermoelectric generators (TEG) for powering small consumer units like self-sufficient wireless sensor nodes (WSN), this paper presents a heat exchanger concept to improve the cooling of the TEG. The concept comes without any auxiliary energy and is based on utilizing the chimney effect to improve the cooling rate of a simple fin heat exchanger. With this concept it is experimentally proven, that the electrical output power of a TEG can be enhanced up to 46,2% compared to the conventional passive cooling method with a finned heat exchanger. Furthermore, this paper presents a CFD simulation to reveal further design optimization potential for the proposed heat exchanger apparatus. [ABSTRACT FROM AUTHOR]