Effective Thermal Conductivity of Wall-Adjacent Layer in Gravity-Driven Vertical Dense Granular Flows

2018 ◽  
Author(s):  
Megan F. Watkins ◽  
Yesaswi N. Chilamkurti ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Granular Flows ◽  
Packing Fraction ◽  
System Configuration ◽  
Adjacent Layer ◽  
Potential System ◽  
Dense Granular Flows

Particle-based heat transfer fluids for concentrated solar power (CSP) tower applications offer a unique advantage over traditional fluids as they have the potential to reach very high operating temperatures. Our work studies the heat transfer behavior of dense granular flows through cylindrical tubes as a potential system configuration for CSP towers. Thus far, we have experimentally investigated the heat transfer to such flows. Our results corroborate the observations of other researchers; namely, that the discrete nature of the flow limits the heat transferred from the tube wall to the flow due to an increased thermal resistance in the wall-adjacent layer. The present study focuses on this near-wall phenomenon, examining how it varies with system configuration and flow rate. A correlation to predict the thermal resistance, in the form of an effective thermal conductivity, was developed based on the underlying physics controlling the heat transfer. The model developed focuses on heat transfer via conduction, considering the heat transfer to particles in contact with the wall, heat transfer to particles not in contact with the wall, and heat transfer through the void spaces. Discrete Element Method simulations were used to examine the flow parameters necessary to understand the heat transfer in the wall-adjacent layer, in particular the packing fraction in the wall-adjacent layer and the number of particle-wall contacts. Incorporation of the model into the single-resistance model developed by Sullivan & Sabersky [1] showed good agreement with their experimental results and those of Natarajan & Hunt [2].

Analytic Modeling of Heat Transfer to Vertical Dense Granular Flows

2019 ◽  
Vol 142 (2) ◽  
Author(s):  
Megan F. Watkins ◽  
Yesaswi N. Chilamkurti ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Heated Surface ◽  
Granular Flows ◽  
Packing Fraction ◽  
Heat Transfer Fluid ◽  
Limiting Factor ◽  
Adjacent Layer ◽  
Dense Flows ◽  
Dense Granular Flows ◽  
Near Wall

Abstract The high packing fractions of dense granular flows make them an attractive option as a heat transfer fluid or thermal energy storage medium for high temperature applications. Previous works studying the heat transfer to dense flows have identified an increased thermal resistance adjacent to the heated surface as a limiting factor in the heat transfer to a discrete particle flow. While models exist to estimate the heat transfer to dense flows, no physics-based model describing the heat transfer in the near-wall layer is found; this is the focus of the present study. Discrete element method (DEM) simulations were used to examine the near-wall flow characteristics, identifying how parameters such as the near-wall packing fraction and number of particle-wall contacts may affect the heat transfer from the wall. A correlation to describe the effective thermal conductivity (ETC) of the wall-adjacent layer (with thickness of a particle radius) was derived based on parallel thermal resistances representing the heat transfer to particles in contact with the wall, particles not in contact with the wall, and void spaces. Empirical correlations based on DEM results were developed to estimate the near-wall packing fraction and number of particle-wall contacts. The contribution from radiation was also incorporated using a simple enclosure analysis. The ETC correlation was validated by incorporating it into dense flow models for chute flows and cylindrical flows and comparing with the experimental data for each.

Effective Thermal Conductivity of Submicron Powders: A Numerical Study

2016 ◽  
Vol 846 ◽  
pp. 500-505
Author(s):  
Wei Jing Dai ◽  
Yi Xiang Gan ◽  
Dorian Hanaor
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Granular Materials ◽  
Gas Phase ◽  
Numerical Study ◽  
Transfer Model ◽  
Size Of Particles ◽  
Contact Unit

Effective thermal conductivity is an important property of granular materials in engineering applications and industrial processes, including the blending and mixing of powders, sintering of ceramics and refractory metals, and electrochemical interactions in fuel cells and Li-ion batteries. The thermo-mechanical properties of granular materials with macroscopic particle sizes (above 1 mm) have been investigated experimentally and theoretically, but knowledge remains limited for materials consisting of micro/nanosized grains. In this work we study the effective thermal conductivity of micro/nanopowders under varying conditions of mechanical stress and gas pressure via the discrete thermal resistance method. In this proposed method, a unit cell of contact structure is regarded as one thermal resistor. Thermal transport between two contacting particles and through the gas phase (including conduction in the gas phase and heat transfer of solid-gas interfaces) are the main mechanisms. Due to the small size of particles, the gas phase is limited to a small volume and a simplified gas heat transfer model is applied considering the Knudsen number. During loading, changes in the gas volume and the contact area between particles are simulated by the finite element method. The thermal resistance of one contact unit is calculated through the combination of the heat transfer mechanisms. A simplified relationship between effective thermal conductivity and loading pressure can be obtained by integrating the contact units of the compacted powders.

Theoretical Investigation on Effective Thermal Conductivity of a Disk-Shaped Composite System

2010 ◽  
Author(s):  
Yasushi Koito ◽  
Toshio Tomimura ◽  
Shuichi Torii
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Composite System ◽  
Microsoft Excel ◽  
Resistance Network ◽  
Estimation Equations ◽  
The Difference ◽  
Wiring Board

This paper addresses the methodology to estimate the effective thermal conductivity of the wiring board, where the metal wiring network is very complicated and then the thermal conductivity of the metal wiring is more than 1000 times higher than that of the resign board. Based on the concept of analogy between the electric and the thermal resistance network, two types of estimation equations are derived by dividing the composite system parallel or perpendicular to the heated/cooled surface. When the ratio of higher to lower thermal conductivities is less than 10, the estimated values by these equations agree with each other. However, the difference is clearly found between them when the ratio is larger than 100. The estimated values are moreover compared with the exact solutions, which are obtained by numerical simulation of heat transfer using Microsoft Excel, and then the applicability of the present estimation methodology is discussed. It is found that the thermal resistance network obtained by dividing the composite system perpendicular to the heated/cooled surface is effective to estimate the effective thermal conductivity of the composite system.

scholarly journals Investigation of Effective Thermal Conductivity for Ordered and Randomly Packed Bed with Thermal Resistance Network Method

Energies ◽  
2019 ◽  
Vol 12 (9) ◽  
pp. 1666 ◽  
Author(s):  
Jian Yang ◽  
Yingxue Hu ◽  
Qiuwang Wang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Packed Bed ◽  
Random Packing ◽  
Packing Factor ◽  
Resistance Network ◽  
Network Method ◽  
Thermal Conductivities

In the present paper, the effective thermal conductivities of Li4SiO4-packed beds with both ordered and random packing structures were investigated using thermal resistance network methods based on both an Ohm’s law model and a Kirchhoff’s law model. The calculation results were also validated and compared with the numerical and experimental results. Firstly, it is proved that the thermal resistance network method based on the Kirchhoff’s law model proposed in the present study is reliable and accurate for prediction of effective thermal conductivities in a Li4SiO4-packed bed, while the results calculated with the Ohm’s law model underestimate both ordered and random packings. Therefore, when establishing a thermal resistance network, the thermal resistances should be connected along the main heat transfer direction and other heat transfer directions as well in the packing unit. Otherwise, both the total heat flux and effective thermal conductivity in the packing unit will be underestimated. Secondly, it is found that the effect of the packing factor is remarkable. The effective thermal conductivity of a packed bed would increase as the packing factor increases. Compared with random packing at similar packing factor, the effective thermal conductivity of packed bed would be further improved with an ordered packing method.

scholarly journals FUNDAMENTALLY NEW APPROACHES TO SOLVING THERMOPHYSICAL PROBLEMS IN THE FIELD OF NANOELECTRONICS

2021 ◽  
Author(s):  
Vladimir Khvesyuk ◽  
Aleksandr Barinov ◽  
B. Liu ◽  
W. Qiao
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Solid State ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Contact Thermal Resistance ◽  
New Approaches ◽  
Rough Boundaries

The paper discusses current problems related to the heat transfer in solid-state nanostructures: the influence of real rough boundaries on the effective thermal conductivity and contact thermal resistance

Practical analytical steady-state temperature solution for annealed pyrolytic graphite heat spreader

2017 ◽  
Vol 27 (1) ◽  
pp. 174-188 ◽  
Author(s):  
Nhat Minh Nguyen ◽  
Eric Monier-Vinard ◽  
Najib Laraqi ◽  
Valentin Bissuel ◽  
Olivier Daniel
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Steady State ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Pyrolytic Graphite ◽  
Heat Spreader ◽  
Content Type ◽  
Heat Spreaders ◽  
Operating Limits

Purpose The purpose of this paper is to supply an analytical steady-state solution to the heat transfer equation permitting to fast design investigation. The capability to efficiently transfer the heat away from high-powered electronic devices is a ceaseless challenge. More than ever, the aluminium or copper heat spreaders seem less suitable for maintaining the component sensitive temperature below manufacturer operating limits. Emerging materials, such as annealed pyrolytic graphite (APG), have proposed a new alternative to conventional solid conduction without the gravity dependence of a heat-pipe solution. Design/methodology/approach An APG material is typically sandwiched between a pair of aluminium sheets to compose a robust graphite-based structure. The thermal behaviour of that stacked structure and the effect of the sensitivity of the design parameters on the effective thermal performances is not well known. The ultrahigh thermal conductivity of the APG core is restricted to in-plane conduction and can be 200 times higher than its through-the-thickness conductivity. So, a lower-than-anticipated cross-plane thermal conductivity or a higher-than-anticipated interlayer thermal resistance will compromise the component heat transfer to a cold structure. To analyse the sensitivity of these parameters, an analytical model for a multi-layered structure based on the Fourier series and the superposition principle was developed, which allows predicting the temperature distribution over an APG flat-plate depending on two interlayer thermal resistances. Findings The current work confirms that the in-plane thermal conductivity of APG is among the highest of any conduction material commonly used in electronic cooling. The analysed case reveals that an effective thermal conductivity twice as higher than copper can be expected for a thick APG sheet. The relevance of the developed analytical approach was compared to numerical simulations and experiments for a set of boundary conditions. The comparison shows a high agreement between both calculations to predict the centroid and average temperatures of the heating sources. Further, a method dedicated to the practical characterization of the effective thermal conductivity of an APG heat-spreader is promoted. Research limitations/implications The interlayer thermal resistances act as dissipation bottlenecks which magnify the performance discrepancy. The quantification of a realistic value is more than ever mandatory to assess the APG heat-spreader technology. Practical implications Conventional heat spreaders seem less suitable for maintaining the component-sensitive temperature below the manufacturer operating limits. Having an in-plane thermal conductivity of 1,600 W.m−1.K−1, the APG material seems to be the next paradigm for solving endless needs of a thermal designer. Originality/value This approach is a practical tool to tailor sensitive parameters early to select the right design concept by taking into account potential thermal issues, such as the critical interlayer thermal resistance.

Determining the thermal resistance of a highly insulated wall containing vacuum insulation panels through experimental, calculation and numerical simulation methods

2020 ◽  
pp. 174425912098003
Author(s):  
Travis V Moore ◽  
Cynthia A. Cruickshank ◽  
Ian Beausoleil-Morrison ◽  
Michael Lacasse
Keyword(s):  
Thermal Conductivity ◽  
Thermal Resistance ◽  
Effective Thermal Conductivity ◽  
Edge Effect ◽  
3D Simulation ◽  
Simulation Methods ◽  
Zone Method ◽  
Vacuum Insulation ◽  
Thermal Bridges ◽  
Wall Assembly

The purpose of this paper is to investigate the potential for calculation methods to determine the thermal resistance of a wall system containing vacuum insulation panels (VIPs) that has been experimentally characterised using a guarded hot box (GHB) apparatus. The VIPs used in the wall assembly have not been characterised separately to the wall assembly, and therefore exact knowledge of the thermal performance of the VIP including edge effect is not known. The calculations and simulations are completed using methods found in literature as well as manufacturer published values for the VIPs to determine the potential for calculation and simulation methods to predict the thermal resistance of the wall assembly without the exact characterisation of the VIP edge effect. The results demonstrate that disregarding the effect of VIP thermal bridges results in overestimating the thermal resistance of the wall assembly in all calculation and simulation methods, ranging from overestimates of 21% to 58%. Accounting for the VIP thermal bridges using the manufacturer advertised effective thermal conductivity of the VIPs resulted in three methods predicting the thermal resistance of the wall assembly within the uncertainty of the GHB results: the isothermal planes method, modified zone method and the 3D simulation. Of these methods only the 3D simulation can be considered a potential valid method for energy code compliance, as the isothermal planes method requires too drastic an assumption to be valid and the modified zone method requires extrapolating the zone factor beyond values which have been validated. The results of this work demonstrate that 3D simulations do show potential for use in lieu of guarded hot box testing for predicting the thermal resistance of wall assemblies containing both VIPs and steel studs. However, knowledge of the VIP effective thermal conductivity is imperative to achieve reasonable results.

Figure of Merit-Based Optimization Approach of Phase Change Material-based Composites for Portable Electronics Using Simplified Model

2021 ◽  
Author(s):  
Ayushman Singh ◽  
Srikanth Rangarajan ◽  
Leila Choobineh ◽  
Bahgat Sammakia
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Phase Change ◽  
Effective Thermal Conductivity ◽  
Phase Change Material ◽  
Figure Of Merit ◽  
Volume Fraction ◽  
Simplified Model ◽  
Transient Temperature ◽  
Change Material

Abstract This work presents an approach to optimally designing a composite with thermal conductivity enhancers (TCEs) infiltrated with phase change material (PCM) based on figure of merit (FOM) for thermal management of portable electronic devices. The FOM defines the balance between effective thermal conductivity and energy storage capacity. In present study, TCEs are in the form of a honeycomb structure. TCEs are often used in conjunction with PCM to enhance the conductivity of the composite medium. Under constrained composite volume, the higher volume fraction of TCEs improves the effective thermal conductivity of the composite, while it reduces the amount of latent heat storage simultaneously. The present work arrives at the optimal design of composite for electronic cooling by maximizing the FOM to resolve the stated trade-off. In this study, the total volume of the composite and the interfacial heat transfer area between the PCM and TCE are constrained for all design points. A benchmarked two-dimensional direct CFD model was employed to investigate the thermal performance of the PCM and TCE composite. Furthermore, assuming conduction-dominated heat transfer in the composite, a simplified effective numerical model that solves the single energy equation with the effective properties of the PCM and TCE has been developed. The effective thermal conductivity of the composite is obtained by minimizing the error between the transient temperature gradient of direct and simplified model by iteratively varying the effective thermal conductivity. The FOM is maximized to find the optimal volume fraction for the present design.

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