Low Reynolds Numbers Convective Heat Transfer in Single/Two-Phase Roughened Microchannels

2021 ◽  
pp. 117468
Author(s):  
Shou-Shing Hsieh ◽  
Ching-Feng Huang ◽  
Jian-Siang Pan
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Reynolds Numbers ◽  
Two Phase ◽  
Low Reynolds Numbers ◽  
Low Reynolds

Numerical investigation of nanofluid particle migration and convective heat transfer in microchannels using an Eulerian–Lagrangian approach

2019 ◽  
Vol 878 ◽  
pp. 62-97 ◽  
Author(s):  
Omar Z. Sharaf ◽  
Ashraf N. Al-Khateeb ◽  
Dimitrios C. Kyritsis ◽  
Eiyad Abu-Nada
Keyword(s):  
Heat Transfer ◽  
Brownian Motion ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Particle Distribution ◽  
Reynolds Numbers ◽  
Streamwise Direction ◽  
Nanofluid Flow ◽  
Low Reynolds Numbers ◽  
Low Reynolds

An Eulerian–Lagrangian modelling approach was employed in order to investigate the flow field, heat transfer and particle distribution in nanofluid flow in a parallel-plate microchannel, with a focus on relatively low Reynolds numbers ($Re\leqslant 100$). Momentum and thermal interactions between fluid and particle phases were accounted for using a transient two-way coupling algorithm implemented within an in-house code that tracked the simultaneous evolution of the carrier and particulate phases while considering timescale differences between the two phases. The inaccuracy of assuming a homogeneous particle distribution in modelling nanofluid flow in microchannels was established. In particular, shear rate and thermophoresis were found to play a key role in the lateral migration of nanoparticles and in the formation of particle depletion and accumulation regions in the vicinity of the channel walls. At low Reynolds numbers, nanoparticle distribution near the walls was observed to gradually flatten in the streamwise direction. On the other hand, for relatively higher Reynolds numbers, higher particle non-uniformities were observed in the vicinity of the channel walls. Furthermore, it was established that convective heat transfer between channel walls and the bulk fluid can either improve or deteriorate with the addition of nanoparticles, depending on whether the flow exceeded a critical Reynolds number of enhancement. It was also established that Brownian motion and thermophoresis had a major role in nanoparticle deposition on the channel walls. In particular, Brownian motion was the main deposition mechanism for nano-sized particles, whereas due to thermophoresis, nanoparticles were repelled away from channel walls. The result of the competition between the two is that deposition gradually increased along the streamwise direction.

Heat Transfer Mechanisms During Flow Boiling in Microchannels

2004 ◽  
Vol 126 (1) ◽  
pp. 8-16 ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Contact Line ◽  
Channel Wall ◽  
Flow Boiling ◽  
Reynolds Numbers ◽  
Nucleate Boiling ◽  
Two Phase ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Transfer Mechanisms

The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K1 and K2, relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100–1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper.

Enhanced Heat Transfer Through the Use of Nanofluids in Forced Convection

2004 ◽  
Author(s):  
Daniel J. Faulkner ◽  
David R. Rector ◽  
Justin J. Davidson ◽  
Reza Shekarriz
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Convective Heat Transfer ◽  
Flow Regime ◽  
Forced Convection ◽  
Convective Heat ◽  
Reynolds Numbers ◽  
Enhanced Heat Transfer ◽  
Low Reynolds Numbers ◽  
Turbulent Flow Regime

Much attention has been paid in recent years to the use of nanoparticle suspensions for enhanced heat transfer. The majority of this work has focused on the thermal conductivity of these nanofluids, which can be as much as 2.5 times higher than that of the plain base fluid. The present work moves beyond measurements of non-flowing liquids, to explore the role that nanofluids can play in enhancing convective heat transfer within microscale channels. A unique pseudo-turbulent flow regime is postulated, which simulates turbulent behavior at very low Reynolds numbers, in what are nominally laminar flows. The resulting fluid mixing has the potential to raise the average convective heat transfer coefficient within the channel. Numerical modeling, using the lattice Boltzmann method, confirms the existence of the pseudo-turbulent flow regime. Finally, experimental results are presented which demonstrate a significant heat transfer enhancement when using nanofluids in forced convection. The current results are especially relevant to microchannel heatsinks, where the low Reynolds numbers impose limitations on the maximum Nusselt number achievable.

Combined Natural and Forced Convective Heat Transfer In Spherical Annuli

1984 ◽  
Vol 106 (4) ◽  
pp. 811-816 ◽  
Author(s):  
S. Ramadhyani ◽  
M. Zenouzi ◽  
K. N. Astill
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Outer Sphere ◽  
Reynolds Numbers ◽  
Numerical Work ◽  
Low Reynolds Numbers ◽  
Forced Convective Heat Transfer ◽  
Nusselt Numbers ◽  
Different Temperatures

This paper presents numerical finite difference solutions of combined natural and forced convective heat transfer in spherical annuli. The flow is assumed to enter the annulus through a port in the bottom of the outer sphere and exit through a diametrically opposite port. The spheres are isothermal and at different temperatures. The governing conservation equations are reduced to dimensionless form and the nondimensional parameters of the problem are identified. The influence of these parameters of the problem are identified. The influence of these parameters on the solution is studied. Details of the flow field and temperature field are presented by means of velocity vector and isotherm maps. Circumferential average and local Nusselt numbers are presented and compared with earlier numerical work in which the effects of natural convection were ignored. It is shown that the buoyancy effects can have a very significant impact on the heat transfer and fluid flow, particularly at low Reynolds numbers.

scholarly journals Effect of Thermal Creep on Heat Transfer for a Two-Dimensional Microchannel Flow: An Analytical Approach

2013 ◽  
Vol 135 (10) ◽  
Author(s):  
Barbaros Çetin
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Slip Flow ◽  
Reynolds Numbers ◽  
Thermal Creep ◽  
Thermal Systems ◽  
Low Reynolds Numbers ◽  
Nusselt Numbers ◽  
Fundamental Understanding

In this paper, velocity profile, temperature profile, and the corresponding Poiseuille and Nusselt numbers for a flow in a microtube and in a slit-channel are derived analytically with an isoflux thermal boundary condition. The flow is assumed to be hydrodynamically and thermally fully developed. The effects of rarefaction, viscous dissipation, axial conduction are included in the analysis. For the implementation of the rarefaction effect, two different second-order slip models (Karniadakis and Deissler model) are used for the slip-flow and temperature-jump boundary conditions together with the thermal creep at the wall. The effect of the thermal creep on the Poiseuille and Nusselt numbers are discussed. The results of the present study are important (i) to gain the fundamental understanding of the effect of thermal creep on convective heat transfer characteristics of a microchannel fluid flow and (ii) for the optimum design of thermal systems which includes convective heat transfer in a microchannel especially operating at low Reynolds numbers.

Sign in / Sign up

Export Citation Format

Share Document