scholarly journals The Effect of Hydraulic Diameter on Flow Boiling within Single Rectangular Microchannels and Comparison of Heat Sink Configuration of a Single and Multiple Microchannels

Energies ◽  
2021 ◽  
Vol 14 (20) ◽  
pp. 6641
Author(s):  
Konstantinos Vontas ◽  
Manolia Andredaki ◽  
Anastasios Georgoulas ◽  
Nicolas Miché ◽  
Marco Marengo
Keyword(s):  
Heat Transfer ◽  
Thermal Resistance ◽  
Single Phase ◽  
Transient Flow ◽  
Numerical Study ◽  
Flow Boiling ◽  
Hydraulic Diameter ◽  
Rectangular Microchannel ◽  
Rectangular Microchannels ◽  
Parallel Microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh, whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob  was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.

A Numerical Study of Flow Boiling Heat Transfer of Nanoemulsion in Mini-Channel Heat Exchanger

2018 ◽  
Author(s):  
Jiajun Xu ◽  
Musa Acar ◽  
Naresh Poudel ◽  
Jaime Rios ◽  
Thanh N. Tran
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Boiling Heat Transfer ◽  
Single Phase ◽  
Numerical Study ◽  
Flow Boiling ◽  
Heat Transfer Fluid ◽  
Working Fluid ◽  
Vof Model ◽  
Flow Boiling Heat Transfer

In this study, a numerical study has been performed on the two-phase heat transfer of a new nanostructured heat transfer fluid: Water-in-Polyalphaolefin (PAO) Nanoemulsion Fluid inside a mini-channel heat exchanger using ANSYS FLUENT. Nanoemulsion fluids are liquid suspensions of nanosized droplets, which are part of a broad class of colloidal dispersions. The nanoemulsion fluid can be formed spontaneously by self-assembly, in which these nanodroplets are in fact swollen micelles. To simplify the complexity of the numerical model, the nanoemulsion fluid was then treated as a homogenous fluid during single-phase and only the water vaporizes during the phase change. The volume of fraction (VOF) model with Pressure-Velocity coupling based Semi Implicit Method for Pressure Linked Equations (SIMPLE) iterative algorithm is employed to solve the continuity, momentum, energy equations in two dimensional domains. The thermophysical properties of the nanoemulsion fluid were measured and used for the current simulation. The results were verified using the experimental results and has shown good agreement. This study has demonstrated the feasibility of simplyig the simulation of flow boiling heat transfer of this new heat transfer fluid through treating it as a homogenous fluid during single-phase convective heat transfer and separating the vapor phase of the nano-micelles during flow boiling. This study has also shown that this Water-in-PAO nanoemulsion could function as a good and alternative conventional working fluid in heat transfer applications.

scholarly journals Effect of hydraulic diameter and aspect ratio on single phase flow and heat transfer in a rectangular microchannel

2017 ◽  
Vol 115 ◽  
pp. 793-814 ◽  
Author(s):  
Amirah M. Sahar ◽  
Jan Wissink ◽  
Mohamed M. Mahmoud ◽  
Tassos G. Karayiannis ◽  
Mohamad S. Ashrul Ishak
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Single Phase ◽  
Hydraulic Diameter ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Rectangular Microchannel ◽  
Flow And Heat Transfer

A Numerical Study on Flow Boiling in Parallel Microchannels

2008 ◽  
Author(s):  
Jinho Jeon ◽  
Woorim Lee ◽  
Youngho Suh ◽  
Gihun Son
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Bubble Dynamics ◽  
Numerical Study ◽  
Reverse Flow ◽  
Flow Boiling ◽  
Experimental Studies ◽  
Two Phase ◽  
Flow And Heat Transfer ◽  
Parallel Microchannels

Flow boiling in parallel microchannels has received attention as an effective cooling method for high-power-density microprocessor. Despite a number of experimental studies, the bubble dynamics coupled with boiling heat transfer in microchannels is still not well understood due to the technological difficulties in obtaining detailed measurements of microscale two-phase flows. In this study, complete numerical simulation is performed to further clarify the physics of flow boiling in microchannels. The level set method for tracking the liquid-vapor interface is modified to include the effects of phase change and contact angle. The method is further extended to treat the no-slip and contact angle conditions on the immersed solid. Also, the reverse flow observed during flow boiling in parallel microchannels has been investigated. Based on the numerical results, the effects of channel shape and inlet area restriction on the bubble growth, reverse flow and heat transfer are quantified.

A numerical study of the hydro-thermal behaviour of nanofluids in rectangular microchannels using a mixture model

2011 ◽  
Vol 225 (4) ◽  
pp. 791-798 ◽  
Author(s):  
H Aminfar ◽  
R Maroofiazar
Keyword(s):  
Heat Transfer ◽  
Mixture Model ◽  
Single Phase ◽  
Numerical Study ◽  
Pure Water ◽  
Convection Heat Transfer ◽  
Phase Model ◽  
Convection Heat ◽  
Rectangular Microchannel ◽  
The Difference

In this article, laminar flow and convection heat transfer of wateralumina nanofluid in a rectangular microchannel have been investigated numerically. Because of the existence of slip velocity between nanoparticles and base fluid, the mixture model is used and results are compared with the single-phase model. The results indicate that using nanofluids can enhance convective heat transfer and pressure drop in a microchannel in comparison with pure water. Also, the enhancement of convection heat transfer is higher in the developing region and the difference of the mixture model and single-phase model is slightly great in this region, but in fully developed region the differences are very low.

Experimental and Numerical Study of Methanol-Water Mixture Single-Phase Heat Transfer in a Micro-Channel Heat Sink

2012 ◽  
Author(s):  
Chun K. Kwok ◽  
Matthew M. Asada ◽  
Jonathan R. Mita ◽  
Weilin Qu
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Heat Sink ◽  
Single Phase ◽  
Numerical Study ◽  
Pure Water ◽  
Molar Fraction ◽  
Water Mixture ◽  
Micro Channel ◽  
Numerical Predictions

This paper presents an experimental study of single-phase heat transfer characteristics of binary methanol-water mixtures in a micro-channel heat sink containing an array of 22 microchannels with 240μm × 630μm cross-section. Pure water, pure methanol, and five methanol-water mixtures with methanol molar fraction of 16%, 36%, 50%, 63% and 82% were tested. Key parametric trends were identified and discussed. The experimental study was complemented by a three-dimensional numerical simulation. Numerical predictions and experimental data are in good agreement with a mean absolute error (MAE) of 0.87%.

Numerical Study of the Effect of Inlet Constriction on Bubble Growth During Flow Boiling in Microchannels

2005 ◽  
Author(s):  
Abhijit Mukherjee ◽  
Satish G. Kandlikar
Keyword(s):  
Stokes Equations ◽  
Numerical Study ◽  
Flow Boiling ◽  
Navier Stokes ◽  
Channel Cross Section ◽  
Vapor Bubbles ◽  
Reversed Flow ◽  
Cross Sectional ◽  
Parallel Microchannels ◽  
Energy Equations

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls and their rapid growth as they fill the entire channel cross-section. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow maldistribution that leads to reversed flow in certain channels. The reversed flow is detrimental to the heat transfer and leads to early CHF condition. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present numerical study, a nucleating vapor bubble placed near the restricted end of a microchannel is numerically simulated. The complete Navier-Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid-vapor interface is captured using the level set technique. The results show that with no restriction the bubble moves towards the nearest channel outlet, whereas in the presence of a restriction, the bubble moves towards the distant but unrestricted end. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.

scholarly journals Heat transfer and entropy generation in a microchannel with longitudinal vortex generators using Nanofluids

2020 ◽  
Author(s):  
Amin Ebrahimi ◽  
Farhad Rikhtegar Nezami ◽  
Amin Sabaghan ◽  
Ehsan Roohi
Keyword(s):  
Heat Transfer ◽  
Entropy Generation ◽  
Single Phase ◽  
Pure Water ◽  
Vortex Generators ◽  
Longitudinal Vortex ◽  
Working Fluid ◽  
Two Phase ◽  
Rectangular Microchannel ◽  
Important Conclusion

Conjugated heat transfer and hydraulic performance for nanofluid flow in a rectangular microchannel heat sink with LVGs (longitudinal vortex generators) are numerically investigated using at different ranges of Reynolds numbers. Three-dimensional simulations are performed on a microchannel heated by a constant heat flux with a hydraulic diameter of 160 μm and six pairs of LVGs using a single-phase model. Coolants are selected to be nanofluids containing low volume-fractions (0.5%–3.0%) of Al2O3 or CuO nanoparticles with different particle sizes dispersed in pure water. The employed model is validated and compared by published experimental, and single-phase and two-phase numerical data for various geometries and nanoparticle sizes. The results demonstrate that heat transfer is enhanced by 2.29–30.63% and 9.44%–53.06% for water-Al2O3 and water-CuO nanofluids, respectively, in expense of increasing the pressure drop with respect to pure-water by 3.49%–16.85% and 6.5%–17.70%, respectively. We have also observed that the overall efficiency is improved by 2.55%–29.05% and 9.78%–50.64% for water-Al2O3 and water-CuO nanofluids, respectively. The results are also analyzed in terms of entropy generation, leading to the important conclusion that using nanofluids as the working fluid could reduce the irreversibility level in the rectangular microchannel heat sinks with LVGs. No exterma (minimums) is found for total entropy generation for the ranges of parameters studied.

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