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 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].

Heat Transfer to Vertical Dense Granular Flows at High Operating Temperatures

2017 ◽  
Author(s):  
Megan F. Watkins ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Granular Flows ◽  
Concentrated Solar Power ◽  
Ceramic Particles ◽  
Dense Flows ◽  
Operating Temperatures ◽  
The Heat Transfer Coefficient ◽  
Dense Granular Flows

Ceramic particles as a heat transfer fluid for concentrated solar power towers offers a variety of advantages over traditional heat transfer fluids. Ceramic particles permit the use of very high operating temperatures, being limited only by the working temperatures of the receiver components, as well as demonstrate the potential to be used for thermal energy storage. A variety of system configurations utilizing ceramic particles are currently being studied, including upward circulating beds of particles, falling particle curtains, and flows of particles over an array of absorber tubes. The present work investigates the use of gravity-driven dense granular flows through cylindrical tubes, which demonstrate solid packing fractions of approximately 60%. Previous work demonstrated encouraging results for the use of dense flows for heat transfer applications and examined the effect of various parameters on the overall heat transfer for low temperatures. The present work examined the heat transfer to dense flows at high operating temperatures more characteristic of concentrated solar power tower applications. For a given flow rate, the heat transfer coefficient was examined as a function of the mean flow temperature by steadily increasing the input heat flux over a series of trials. The heat transfer coefficient increased almost linearly with temperature below approximately 600°C. Above 600°C, the heat transfer coefficient increased at a faster rate, suggesting an increased radiation heat transfer contribution.

Effect of Flow Rate and Particle Size on Heat Transfer to Dense Granular Flows

2016 ◽  
Author(s):  
Megan F. Watkins ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Particle Size ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Rate ◽  
Granular Flows ◽  
Heat Transfer Fluid ◽  
Storage Medium ◽  
Flow Rates ◽  
Dense Granular Flows

The increasing demand for renewable energy sources necessitates the development of more efficient technologies. Concentrated solar power (CSP) towers exhibit promising qualities, as temperatures greater than 1000°C are possible. The heat transfer fluid implemented to capture the sun’s energy significantly impacts the overall performance of a CSP system. Current fluids, such as molten nitrate salts and steam, have limitations; molten salts are limited by their small operational temperature range while steam requires high pressures and is unable to act as an effective storage medium. As a result, a new heat transfer fluid composed of ceramic particles is being investigated, as ceramic particles demonstrate no practical limit on operation temperature and have the ability to act as a storage medium. This study sought to further investigate the use of dense granular flows as a new heat transfer fluid. Previous work validated the use of such flows as a heat transfer fluid; the present work examined the effect of flow rate, as well as the particle size and type on the heat transfer to the particle fluid. Three different types of particles were tested, along with two different diameter particles. Of the three materials tested, the particle type did not appear to effect the heat transfer. Particle diameter, however, did effect the heat transfer, as a smaller diameter particle yielded slightly higher heat transfer to the fluid. Flow rates ranging from 30 to 200 kg/m2-s were tested. Initially, the heat transfer to the flow, characterized by the convective heat transfer coefficient, decreased with increasing flow rate. However, at approximately 80 kg/m2-s, the heat transfer coefficient began to increase with increasing flow rate. These results indicate that a dense granular flow consisting of small diameter particles and traveling at very slow or fast flow rates yields the best wall to “fluid” heat transfer.

Experimental Characterization of Heat Transfer to Vertical Dense Granular Flows Across Wide Temperature Range

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Megan F. Watkins ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Rate ◽  
Convective Heat Transfer Coefficient ◽  
Granular Flows ◽  
Effect Of Temperature ◽  
Cylindrical Tubes ◽  
Operating Temperatures ◽  
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. Gravity-driven dense granular flows through cylindrical tubes demonstrate potential for CSP applications and are the focus of the present study. The heat transfer capabilities of such a flow system were experimentally studied using a bench-scale apparatus. The effect of the flow rate and other system parameters on the heat transfer to the flow was studied at low operating temperatures (<200 °C), using the convective heat transfer coefficient and Nusselt number to quantify the behavior. For flows ranging from 0.015 to 0.09 m/s, the flow rate appeared to have negligible effect on the heat transfer. The effect of temperature on the flow's heat transfer capabilities was also studied, examining the flows at temperatures up to 1000 °C. As expected, the heat transfer coefficient increased with the increasing temperature due to enhanced thermal properties. Radiation did not appear to be a key contributor for the small particle diameters tested (approximately 300 μm in diameter) but may play a bigger role for larger particle diameters. The experimental results from all trials corroborate the observations of other researchers; namely, that particulate flows demonstrate inferior heat transfer as compared with a continuum flow due to an increased thermal resistance adjacent to the tube wall resulting from the discrete nature of the flow.

scholarly journals Experimental and Computational Studies of Gravity-Driven Dense Granular Flows

2015 ◽  
Author(s):  
Yesaswi N. Chilamkurti ◽  
Richard D. Gould
Keyword(s):  
Three Dimensional ◽  
Granular Flows ◽  
Packing Fraction ◽  
Velocity Shear ◽  
Spherical Particles ◽  
Tube Length ◽  
Soft Particle ◽  
Mean Velocity ◽  
Gravity Driven ◽  
Dense Granular Flows

The current paper focusses on the characterization of gravity-driven dry granular flows in cylindrical tubes. With a motive of using dense particulate media as heat transfer fluids (HTF), the study was primarily focused to address the characteristics of flow regimes with a packing fraction of ∼60%. Experiments were conducted to understand the effects of different flow parameters, including: tube radius, tube inclination, tube length and exit diameter. These studies were conducted on two types of spherical particles — glass and ceramic — with mean diameters of 150 μm and 300 μm respectively. The experimental data was correlated with the semi-empirical equation based on Beverloo’s law. In addition, the same flow configuration was studied through three-dimensional computer simulations by implementing the Discrete Element Method for the Lagrangian modelling of particles. A soft-particle formulation was used with Hertz-Mindilin contact models to resolve the interaction forces between particles. The simulation results were used to examine the velocity, shear rate and packing fraction profiles to study the detailed flow dynamics. Curve-fits were developed for the mean velocity profiles which could be used in developing hydrodynamic analogies for granular flows. The current work thus identifies the basic features of gravity driven dense granular flows that could form a basis for defining their rheology.

scholarly journals SIMULATION OF WATER DROPLET IMPINGEMENT ON A HEATED SURFACE- SINGLE AND DOUBLE DROPLETS

2021 ◽  
Vol 25 (4) ◽  
pp. 95-113
Author(s):  
Ruaa B. Namaa ◽  
◽  
Adnan A. Rasool ◽  
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Water Drop ◽  
Heated Surface ◽  
Heat Removal ◽  
Heat Transfer Fluid ◽  
Heated Plate ◽  
Single Drop ◽  
Transfer Coefficients ◽  
Numerical Work

The present numerical work is concerned with the single drop and double drops impingement on a heated surface. Fluid flow and heat transfer coefficients were modeled using a volume of fluid (VOF) code. The stainless –steel thin plate surface is uniformly heated to reach a constant temperature at (50C°), this was done by using relatively thicker plate underneath the heated plate. The thick plate is made of high conductivity aluminum alloy 2mm thickness. Relatively a lower temperature water drop is used for cooling to ensure that drop temperature remains below the boiling point of water. The drop –plate initial impingement distances were varied in the range (10-60) cm which represent an impact velocity in the range (1.4-3.4) m/s. The single drop fluid flow simulation results are compared with that in the literature ,while the heat transfer fluid flow results are represented as instantuous heat transfer coefficient variation as alternative to values of heat flux on the surface. Double drops impingement results are then presented and its features are compared to the single drop. Results show that the flow characteristics for the double drops are similar to the single drop at small distances with smaller coverage areas during impingement with lower heat removal rates. As distances increase rebound and splash occurs leading to bigger coverage areas during impingement with relatively smaller heat coefficients compared to the single drop one. The present results shows the same behavior for drop deformation when compared with M.pasandideh-Fard et al. [1] numerical results with an agreement of 90 % and 95 % in calculations the spread factor and impact velocities respectively. The calculated average heat coefficients show acceptable values with that given in litreture

Eddies, mixing and heat transfer in dense granular flows

2013 ◽  
Author(s):  
P. Rognon ◽  
T. Miller ◽  
I. Einav
Keyword(s):  
Heat Transfer ◽  
Granular Flows ◽  
Dense Granular Flows

Dense Granular Flows as a New Heat Transfer Fluid for Concentrated Solar Power

2015 ◽  
Author(s):  
Megan F. Watkins ◽  
Richard D. Gould
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Solar Power ◽  
Granular Flows ◽  
Heat Transfer Fluid ◽  
Concentrated Solar Power ◽  
Transfer Coefficients ◽  
Ceramic Particles ◽  
The Heat Transfer Coefficient

The increasing interest in concentrated solar power as a new form of renewable energy necessitates an improvement in overall system efficiency. Current heat transfer fluids employed to capture the concentrated heat demonstrate limited working temperature ranges. This study sought to investigate the use of dense granular flows as a possible new heat transfer fluid, as ceramic particles present virtually no restriction on working temperature. A bench-scale system simulating a single tube of a concentrated solar power central receiver was constructed and used to evaluate the heat transfer properties of the flow at low temperatures. Ceramic particles, 270μm in diameter, were gravity-fed through a vertical tube, resulting in granular flows with particle packing fractions of approximately 60%. Radial temperature profiles were measured and used to calculate the mean temperature of the fluid at different axial tube locations. The heat transfer coefficient was then calculated based on the input heat flux and measured wall and mean temperatures. The effect of the mass flow rate on the heat transfer coefficient was examined by using different orifices at the tube exit. As expected, the heat transfer coefficient increased with increasing flow rate. Heat transfer coefficients ranging from 330 to 380 W/m2-K were obtained for bulk temperatures ranging from 40 to 70°C. Previous works demonstrated comparable heat transfer coefficients at higher bulk temperatures. Thus, our preliminary heat transfer coefficient results demonstrate the potential of dense flows of ceramic particles for obtaining beneficial heat transfer properties at extremely high operating temperatures.

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