scholarly journals Convective Heat Transfer of Spring Meltwater Accelerates Active Layer Phase Change in Tibetan Permafrost Areas

2021 ◽  
Author(s):  
Yi Zhao ◽  
Zhuotong Nan ◽  
Hailong Ji ◽  
Lin Zhao
Keyword(s):  
Heat Transfer ◽  
Soil Temperature ◽  
Convective Heat Transfer ◽  
Active Layer ◽  
Convective Heat ◽  
Thermal Regime ◽  
Transport Processes ◽  
Model Parameters ◽  
Soil Layers ◽  
The Impact

Abstract. Convective heat transfer (CHT) is one of the important processes that controls the near ground surface heat transfer in permafrost areas. However, this process has often not been considered in most permafrost simulation studies and its influence on the freeze-thaw processes of the active layer lacks quantitative investigation. The Simultaneous Heat and Water (SHAW) model is one of the few land surface models in which the CHT process is well incorporated in the soil heat-mass transport processes. We applied the SHAW model to investigate the impacts of CHT on active layer thermal dynamics on the Tanggula station, a typical permafrost site located at the eastern Qinghai-Tibetan Plateau with abundant meteorological and soil temperature/moisture observation data. The 2008–2009 observed hourly data were used to calibrate the model parameters and those of 2010 for validation. A control experiment was carried out to quantify the changes in active layer thermal regime affected by vertical advection of liquid water, consisting of three setups: using (1) the original SHAW model with full consideration of CHT; (2) a modified SHAW model ignoring the CHT due to infiltration from the surface, and (3) a modified SHAW model ignoring complete CHT processes in the system. The impacts of vapor convection are not considered in this experiment. The results show that the CHT events mainly happened during thawing periods when the active layer melted at shallow (0–0.2 m) and middle (0.4–1.3 m) soil depths, and its impact on soil thermal regime at shallow depths was significantly greater in spring melting periods than in summer. The impact was minimal in freezing periods and in deep soil layers. During melting periods, temperatures in the shallow and middle soil depths simulated under the scenario considering CHT were higher by up to 10.0 and 1.5 °C, respectively, than those under the scenarios ignoring CHT. The ending dates of zero-curtain effect were considerably advanced with CHT considered, due to the warming effect of CHT associated with infiltration. However, the opposite cooling effect also existed due to presence of upward liquid fluxes and thermal differences between the soil layers. In some certain period, the advection flow including partial return flow reduced the temperatures in the shallow and middle depths by as much as −5.0 and −1.0 °C, respectively. The overall annual effect of CHT by liquid flux is to increase soil temperature in the active layer and favors thawing of frozen ground at the study site.

Experimental Model of Temperature-Driven Nanofluid

2006 ◽  
Vol 129 (6) ◽  
pp. 697-704 ◽  
Author(s):  
A. G. Agwu Nnanna
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Small Volume ◽  
Volume Fraction ◽  
Convective Heat Transfer Coefficient ◽  
Isothermal Condition ◽  
Carrier Fluid ◽  
Small Volume Fraction ◽  
The Impact

This paper presents a systematic experimental method of studying the heat transfer behavior of buoyancy-driven nanofluids. The presence of nanoparticles in buoyancy-driven flows affects the thermophysical properties of the fluid and consequently alters the rate of heat transfer. The focus of this paper is to estimate the range of volume fractions that results in maximum thermal enhancement and the impact of volume fraction on Nusselt number. The test cell for the nanofluid is a two-dimensional rectangular enclosure with differentially heated vertical walls and adiabatic horizontal walls filled with 27 nm Al2O3–H2O nanofluid. Simulations were performed to measure the transient and steady-state thermal response of nanofluid to imposed isothermal condition. The volume fraction is varied between 0% and 8%. It is observed that the trend of the temporal and spatial evolution of temperature profile for the nanofluid mimics that of the carrier fluid. Hence, the behaviors of both fluids are similar. Results shows that for small volume fraction, 0.2⩽ϕ⩽2% the presence of the nanoparticles does not impede the free convective heat transfer, rather it augments the rate of heat transfer. However, for large volume fraction ϕ>2%, the convective heat transfer coefficient declines due to reduction in the Rayleigh number caused by increase in kinematic viscosity. Also, an empirical correlation for Nuϕ as a function of ϕ and Ra has been developed, and it is observed that the nanoparticle enhances heat transfer rate even at a small volume fraction.

scholarly journals Radiation heat transfer between human and cryocabin walls

2021 ◽  
Vol 2119 (1) ◽  
pp. 012146
Author(s):  
I A Burkov ◽  
S I Khutsieva ◽  
V A Voronov
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Human Body ◽  
Transfer Problem ◽  
Radiation Heat Transfer ◽  
Vertical Wall ◽  
Heat Transfer Problem ◽  
Radiation Heat ◽  
The Impact

Abstract The paper considers the particular case of intensive radiation heat transfer in the system consisting of a human body and cryocabin walls of cryosauna. Calculations for three models have been made, namely, human-vertical wall, which is arranged parallel to a human, human-vertical wall, which is positioned at a certain angle, and a human-cryosauna. Analytical calculations are compared with Ansys-bassed numerical calculations. The impact of radiation heat transfer in this radiation-convective heat transfer problem is estimated. Conclusions are drawn about taking into account the radiation heat transfer and a rational method for calculating this heat transfer problem.

scholarly journals A Simplified Technique for Thermal Analysis of a Fenestration system with a Venetian Blind

2021 ◽  
Author(s):  
Hidayat Ullah Shahid
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Performance ◽  
Approximate Model ◽  
Vertical Surface ◽  
Shading Devices ◽  
The One ◽  
The Impact ◽  
Venetian Blind

A simplified 1-D numerical model of a window and horizontal Venetian blind assembly has been developed. This model provides a realistic estimate of the advantage of using blinds to control the window heat gain or loss. The free convective heat transfer rate from an isothermal vertical surface adjacent to a set of horizontal louvres has been studied numerically. This configuration is an approximate model of an indoor window glazing with a Venetian-type blind. Knowledge of the effect of blinds on the free convection at the indoor window surface is important for understanding and predicting the impact of shading devices on the overall thermal performance of a window. The convective heat transfer results are used in the one-dimensional model of the complete fenestration system to study the effect on key performance parameters. The results show that louvred blinds can have a significant beneficial effect on window thermal performance.

scholarly journals Review of Convective Heat Transfer Modelling in CFD Simulations of Fire-Driven Flows

2021 ◽  
Vol 11 (11) ◽  
pp. 5240
Author(s):  
Georgios Maragkos ◽  
Tarek Beji
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Flame Spread ◽  
Wall Region ◽  
Advantages And Disadvantages ◽  
Wide Range ◽  
Liquid Evaporation ◽  
Transfer Modelling ◽  
The Impact

Progress in fire safety science strongly relies on the use of Computational Fluid Dynamics (CFD) to simulate a wide range of scenarios, involving complex geometries, multiple length/time scales and multi-physics (e.g., turbulence, combustion, heat transfer, soot generation, solid pyrolysis, flame spread and liquid evaporation), that could not be studied easily with analytical solutions and zone models. It has been recently well recognised in the fire community that there is need for better modelling of the physics in the near-wall region of boundary layer combustion. Within this context, heat transfer modelling is an important aspect since the fuel gasification rate for solid pyrolysis and liquid evaporation is determined by a heat feedback mechanism that depends on both convection and radiation. The paper focuses on convection and reviews the most commonly used approaches for modelling convective heat transfer with CFD using Large Eddy Simulations (LES) in the context of fire-driven flows. The considered test cases include pool fires and turbulent wall fires. The main assumptions, advantages and disadvantages of each modelling approach are outlined. Finally, a selection of numerical results from the application of the different approaches in pool fire and flame spread cases, is presented in order to demonstrate the impact that convective heat transfer modelling can have in such scenarios.

CFD Approach Analysis of Chemical Reactions Coupled Convective Heat Transfer in Reformer Ducts

2008 ◽  
Author(s):  
Jinliang Yuan ◽  
Guogang Yang ◽  
Bengt Sunde´n
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Chemical Reactions ◽  
Convective Heat ◽  
Porous Layer ◽  
Gas Flow ◽  
Transport Processes ◽  
Fuel Gas ◽  
Steam Reformer ◽  
Flow Duct

Thermo-mechanical failure of components in a compact steam reformer is a major obstacle to bring this technology to real-life applications. The probability of material degradation and failure depends strongly on the convective heat transfer in the fuel gas flow duct and local temperature distribution in multifunctional materials. It is of significant importance to accurately predict the convective heat transfer coupled with catalytic reactions within the reformer components. In this paper, the simulation and analysis of combined chemical reactions and transport processes are conducted for a duct relevant for compact design steam reformer, which consists of a porous layer for the catalytic reforming reactions of methane, the fuel gas flow duct and solid plates. A fully three-dimensional computational fluid dynamics (CFD) approach is applied to calculate transport processes and effects of thermal conductivities of the involved multi-functional materials on convective heat transfer/temperature distributions, in terms of interface temperature gradients/heat fluxes and Nusselt numbers. The steam reformer conditions such as mass balances associated with the reactions and gas permeation to/from the porous anode are implemented in the calculation. The results show that the classic thermal boundary conditions (either constant heat flux or temperature, or combined one) may not be applicable for the interfaces between the fuel flow duct and solid plate/porous layer.

scholarly journals Numerical modelling of convective heat transport by air flow in permafrost-affected talus slopes

2016 ◽  
Author(s):  
Jonas Wicky ◽  
Christian Hauck
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Regime ◽  
Air Flow ◽  
Cooling Capacity ◽  
High Porosity ◽  
Frozen Ground ◽  
Talus Slope ◽  
Talus Slopes

Abstract. Talus slopes are a widespread geomorphic phenomenon in the Alps. Due to their high porosity a gravity-driven internal air circulation can be established which is forced by the gradient between outside (air) and internal (talus) temperature. The thermal regime is different from the surrounding environment often leading to cold microclimates and permafrost occurrences. So far this phenomenon has mainly been analysed by field studies and only few explicit modelling studies of this phenomenon exist. Numerical simulations of permafrost sometimes use parameterizations for the effects of convection, but mostly neglect the influence of convective heat transfer in air on the thermal regime. On the contrary, in civil engineering many studies were carried out to investigate the thermal behaviour of blocky layers and to improve their passive cooling capacity. The present study further develops and applies these concepts to model heat transfer in air flow in a natural scale talus slope. Modelling results show that convective heat transfer has the potential to develop a temperature difference between the lower and the upper part from about 0.7 °C (boundary closed to the atmosphere) to 2.5 °C (boundary open to the atmosphere). A seasonally alternating chimney-effect type circulation develops. Modelling results also show that this convective heat transfer leads to a cold reservoir in the lower part of the talus slope which can be crucial for maintaining the frozen ground conditions under climate change.

scholarly journals Auxiliary Draught Base Performance Improvement of Air Distribution System of Cold Store

2019 ◽  
Vol 8 (4) ◽  
pp. 4737-4748
Keyword(s):  
Heat Transfer ◽  
Flow Velocity ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Cold Storage ◽  
Distribution System ◽  
Storage System ◽  
Air Transport ◽  
Air Distribution ◽  
The Impact

Air distribution enable convective heat transfer in cold storage operation. Thermal behaviors of the cold storage system are based on air transport arrangements. Transport characteristics can handle with auxiliary arrangements such as induce draught system. Experimental investigation for the impact of auxiliary draught system (ADS) on air transportation is carried out. Air transport velocity was measured in the cold chamber with a hot wire anemometer. Experimental results show significant enhancement by three times mid-section air flow velocity and overall one- and half-time greater flow velocity observed, while return air velocity measured almost tow time of general condition during the experiment. COP of plant improve by 21% with 25% less time required to achieve desired temperature. 26% saving in power consumption observed during experiments. Auxiliary draught ensures homogeneous environment inside the plant through proper mixing of air and support convective heat transfer. Designing and analysis of airflow patterns with temperature distribution in large entity like cold storage is a difficult task thus Computational Fluid Dynamics (CFD) can address the issue with high degree of precision. It has been observed that SST K-ℇ model has average 26% error with experimental values

Influence of Various Nanofluid Types on Wavy Microchannels Heat Sink Cooling Performance

2013 ◽  
Vol 420 ◽  
pp. 118-122 ◽  
Author(s):  
Prem Gunnasegaran ◽  
Noel Narindra ◽  
Norshah Hafeez Shuaib
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Heat Sink ◽  
Convective Heat Transfer Coefficient ◽  
Volume Fractions ◽  
The Impact

This paper discusses the impact of using various types of nanofluids and nanoparticle volume fractions on heat transfer and fluid flow characteristics in a wavy microchannel heat sink (WMCHS) with rectangular cross-section. Numerical investigations using three different types of nanofluids including Al2O3-H2O, CuO-H2O, and diamond-H2O with a fixed nanoparticle volume fraction of 3% and using a diamond-H2O with nanoparticle volume fractions ranging from 0.5% to 5% are examined. This investigation covers Reynolds numbers in the range of 100 to 1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite-volume method (FVM). The computational model is used to study the variations of convective heat transfer coefficient, pressure drop and wall shear stress. It is inferred that the convective heat transfer coefficient of a WMCHS cooled with the nanofluid flow showed marked improvement over the pure water with a smaller pressure drop penalty.

scholarly journals Numerical modelling of convective heat transport by air flow in permafrost talus slopes

2017 ◽  
Vol 11 (3) ◽  
pp. 1311-1325 ◽  
Author(s):  
Jonas Wicky ◽  
Christian Hauck
Keyword(s):  
Heat Transfer ◽  
Numerical Modelling ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Regime ◽  
Field Studies ◽  
High Porosity ◽  
Frozen Ground ◽  
Talus Slope ◽  
Talus Slopes

Abstract. Talus slopes are a widespread geomorphic feature in the Alps. Due to their high porosity a gravity-driven internal air circulation can be established which is forced by the gradient between external (air) and internal (talus) temperature. The thermal regime is different from the surrounding environment, leading to the occurrence of permafrost below the typical permafrost zone. This phenomenon has mainly been analysed by field studies and only few explicit numerical modelling studies exist. Numerical simulations of permafrost sometimes use parameterisations for the effects of convection but mostly neglect the influence of convective heat transfer in air on the thermal regime. In contrast, in civil engineering many studies have been carried out to investigate the thermal behaviour of blocky layers and to improve their passive cooling effect. The present study further develops and applies these concepts to model heat transfer in air flows in a natural-scale talus slope. Modelling results show that convective heat transfer has the potential to develop a significant temperature difference between the lower and the upper parts of the talus slope. A seasonally alternating chimney-effect type of circulation develops. Modelling results also show that this convective heat transfer leads to the formation of a cold reservoir in the lower part of the talus slope, which can be crucial for maintaining the frozen ground conditions despite increasing air temperatures caused by climate change.

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