Abstract
Thermal transport in nanofluids, i.e., suspensions of nanoparticles in a base fluid, is a subject of dispute in the literature with no conclusion as to the governing mechanisms describing it. The interaction between the suspended particles and the base fluid has been cited as the main contributor to the phenomenon. However, the exact cause of the enhancement of heat transfer in nanofluids is still not fully understood. In this study, a simplified computational approach for a nanofluid simulation is proposed. The fluid motions induced by the random Brownian motions of suspended particles are examined from a statistical perspective. The results confirm that the motions of the suspended particles induce a random flow field within the fluid and provide a statistical assessment of this field. As the Brownian time-step is reduced, the statistics of the fluid phase slowly converge. The statistics also indicate the existence of local convective eddies throughout the fluid phase as particles move about. Parameters related to the computational setup do not affect the statistics of the induced fluid velocity. However, implementation of the slip assumption affects the results of the simulation. The calculated local fluid disturbance varies with the particle size with the location of maximum depending on the slip assumption. Finally, it was observed that higher fluid temperature will have a stronger effect on the induced flow field.
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Notes
Using a one-pass algorithm for the standard deviation calculation eliminates the need to store all of the velocity data from the beginning of the simulation, thus reducing the computational cost of the simulations.
Excess kurtosis equals kurtosis minus 3, for which 3 is the kurtosis of a normal distribution.
Abbreviations
- C :
-
Arbitrary constant
- D :
-
Diffusivity of particles
- Fo :
-
Fourier number
- m :
-
Mass
- P :
-
Pressure
- r :
-
Radial distance
- R :
-
Particle radius
- Re :
-
Reynolds number
- ΔS :
-
Displacement of particles
- t :
-
Elapsed time since particle movement
- Δt :
-
Size of time-step
- U :
-
Velocity of discrete phase
- V :
-
Velocity of continuous phase
- δ :
-
Distance from particle surface
- Λ :
-
Sample size
- μ :
-
Dynamic viscosity
- ν :
-
Kinematic viscosity
- ρ :
-
Density
- σ :
-
Standard deviation
- τ :
-
Relaxation time
- Δτ :
-
Sampling frequency
- φ :
-
Polar angle
- ψ :
-
Stream function
- ω :
-
Random number (normally distributed)
- 0.05:
-
Location of 5 % of free-stream velocity
- D :
-
Diameter
- fluid:
-
Fluid
- max:
-
Maximum
- part:
-
Particle
- r :
-
Radial distance
- SS:
-
Steady state
- TS:
-
Transient
- θ :
-
Azimuthal angle
- φ :
-
Polar angle
- *:
-
Dimensionless quantity
- \(\bar{\alpha }\) :
-
Mean of quantity α
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Acknowledgments
This publication was made possible by the NPRP award 09-1183-2-461 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. The HPC (and/or scientific visualization) resources and services used in this work were provided by the IT research computing group at Texas A&M University at Qatar. IT research computing is funded by the Qatar Foundation for education, science, and community development. The authors would like to express their sincere appreciation for the valuable technical advice from Professor J. N. Reddy, Texas A&M University.
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Cheng, W.L., Sadr, R. Induced flow field of randomly moving nanoparticles: a statistical perspective. Microfluid Nanofluid 18, 1317–1328 (2015). https://doi.org/10.1007/s10404-014-1531-7
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DOI: https://doi.org/10.1007/s10404-014-1531-7