A study of flow mal-distribution in U-type micro-channel configuration is presented. Numerical simulations indicate that flow deceleration and associated pressure recovery in the inlet header lead to flow separation and recirculation which cause oscillations in channel-wise mass flow distribution. Increase in flow resistance by decrease in channel depth, width or number of channels or increase in channel length, results in a more uniform distribution. Mal-distribution increases at high flow rate or low viscosity due to the dominance of inertial phenomena. Experiments performed on a 25-channel setup illustrate that small manufacturing variations in channel dimensions introduce random fluctuations in flow distribution. © 2011 Elsevier Ltd.

T Sundararajan

Department of Mechanical Engineering

R. Manikanda Kumaran

Experimental investigations

Flow distribution

Flow maldistribution

Flow resistance

High-flow rate

In-channels

INLET HEADER

LOW VISCOSITY

MAL-DISTRIBUTION

MANUFACTURING VARIATION

Mass flow

Pressure recovery

RANDOM FLUCTUATION

RECIRCULATIONS

Uniform distribution

Flow separation

Inlet flow

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Communications in Heat and Mass Transfer

The present paper illustrates experimentally the impact of non-uniform heat generation in a microprocessor on the hot spot distribution and the method to cool the same, efficiently employing parallel microchannel cooling configurations. It is often assumed during design of microprocessor cooling systems that the heat load emitted by the device is uniform, however real time tracking of the device shows that such assumptions are far from the reality. An Intel® Core™ i7–4770 3.40 GHz quad core processor has been experimentally mimicked using heat load data retrieved from a real microprocessor with non-uniform core activity. Parallel microchannel based heat spreader configurations using U, I and Z type flow configurations have been employed to mitigate the thermal load from the mimicked device. The observations clearly show that the microchannel cooling system experiences two forms of hot spots, one due to the flow maldistribution within the system and the other due to the additional non-uniform heat generation by the device. The hot spots have been shown to exhibit drastically different shapes and core temperatures and this has been verified through simulations and infrared thermography. To efficiently cool hot spot core temperatures, nanofluids have been employed and ‘smart cooling’ has been observed and the same has been explained based on nanoparticle slip mechanisms. The present work shows that the notion that high flow maldistribution leads to high thermal maldistribution, is not always true and existing maldistribution can be effectively utilized to tackle specific hot spot location. The present work can be important to design cooling mechanisms for real microprocessors with high core activity leading to non-uniform hot spot formation probabilities. © 2017 Elsevier Masson SAS

International Journal of Thermal Sciences

Mitigating non‐uniform heat generation induced hot spot(s) in multicore processors using nanofluids in parallel microchannels

Design of effective microcooling systems to address the challenges of ever increasing heat flux from microdevices requires deep examination of real-time problems and has been tackled in depth. The most common (and apparently misleading) assumption while designing microcooling systems is that the heat flux generated by the device is uniform, but the reality is far from this. Detailed simulations have been performed by considering nonuniform heat load employing the configurations U, I, and Z for parallel microchannel systems with water and nanofluids as the coolants. An Intel® Core™ i7-4770 3.40 GHz quad core processor has been mimicked using heat load data retrieved from a real microprocessor with nonuniform core activity. This study clearly demonstrates that there is a nonuniform thermal load induced temperature maldistribution along with the already existent flow maldistribution induced temperature maldistribution. The suitable configuration(s) for maximum possible overall heat removal for a hot zone while maximizing the uniformity of cooling have been tabulated. An Eulerian-Lagrangian model of the nanofluids shows that such "smart" coolants not only reduce the hot spot core temperature but also the hot spot core region and thermal slip mechanisms of Brownian diffusion and thermophoresis are at the crux of this. The present work conclusively shows that high flow maldistribution leads to high thermal maldistribution, as the common prevalent notion is no longer valid and existing maldistribution can be effectively utilized to tackle specific hot spot location, making the present study important to the field. Copyright © 2017 by ASME.

Fulltext

Journal of Heat Transfer

Selecting Optimal Parallel Microchannel Configuration(s) for Active Hot Spot Mitigation of Multicore Microprocessors in Real Time

While parallel microchannel based cooling systems (PMCS) have been around for quite a period of time, employing the same and incorporating them for near–active cooling of microelectronic devices is yet to be implemented and the implications of the same on thermal mitigation to be understood. The present article focusses on a specific design of the PMCS such that it can be implemented at ease on the heat spreader of a modern microprocessor to obtain near-active cooling. Extensive experimental and numerical studies have been carried out to comprehend the same and three different flow configurations (U, I and Z) of PMCS have been adopted for the present investigations. Additional to focussing on the thermofluidics due to flow configuration, nanofluids (as superior heat transfer fluids) have also been employed to achieve the desired essentials of mitigation of overshoot temperatures and improving uniformity of cooling. Two modelling methods, Discrete Phase Modelling (DPM) and Effective Property Modelling (EPM) have been employed for numerical study to model nanofluids as working fluid in micro flow paths and the DPM predictions have been observed to match accurately with experiments. To quantify the thermal performance of PMCS, an appropriate Figure of Merit (FoM) has been proposed. From the FoM It has been perceived that the Z configuration employing nanofluid is the best suitable solutions for uniform thermal loads to achieve uniform cooling as well as reducing maximum temperature produced with in the device. The present results are very promising and viable approach for futuristic thermal mitigation of microprocessor systems. © 2017 Elsevier Ltd

International Journal of Heat and Mass Transfer

Heat spreader with parallel microchannel configurations employing nanofluids for near–active cooling of MEMS

Mitigation of 'hot spots' in microelectromechanical systems (MEMS) employing in situ microchannel systems requires a comprehensive picture of the maldistribution of the working fluid and uniformity of cooling within the same. In this paper, detailed simulations employing parallel microchannel systems with specialized manifold-channel configurations, i.e., U, I, and Z, have been performed. Eulerian-Lagrangian discrete phase model (DPM) and effective property model with water and alumina-water nanofluid as working fluids have been employed. The distributions of the dispersed particulate phase and continuous phase have been observed to be, in general, different from the flow distribution, and this has been found to be strongly dependent on the flow configuration. Accordingly, detailed discussions on the mechanisms governing such particle distribution patterns have been proposed. Particle maldistribution has been conclusively shown to be influenced by various migration and diffusive phenomena, such as Stokesian drag, Brownian motion, thermophoretic drift, and so on. To understand the uniformity of cooling within the device, which is of importance in real-time scenario, an appropriate figure of merit has been proposed. It has been observed that uniformity of cooling improved using nanofluid as working fluid as well as enhanced relative cooling in hot zones, providing evidence of the 'smart' nature of such dispersions. To further quantify this smart effect, real-time mimicking hot-spot scenarios have been computationally probed with nanofluid as the coolant. A silicon-based microchip emitting nonuniform heat flux (gathered from real-time monitoring of an Intel Core i7-4770 3.40-GHz quad-core processor) under various processor load conditions has been studied, and the evidence of enhanced cooling of hot spots has been obtained from DPM analysis. This paper sheds insight on-the behavior of nonhomogeneous dispersions in complex flow domains and the caliber of nanofluids in cooling MEMS more uniformly and 'smarter' than base fluids. © 2011-2012 IEEE.

IEEE Transactions on Components, Packaging and Manufacturing Technology

Thermally 'Smart' characteristics of nanofluids in parallel microchannel systems to mitigate hot spots in MEMS

This paper brings out the phenomenon of flow maldistribution in parallel microchannel systems, which is supposed to have an adverse effect on hot spot formation and temperature distribution in microelectronic devices. An extensive experimental study is carried out where in the parameters affecting the flow maldistribution such as number of channel, area of cross section of the manifold, channel hydraulic diameter, and Reynolds number are varied to study their effect on the pressure drop across the parallel channels designed for liquid cooling of a CPU. It is observed that the flow distribution among the channels improves significantly with a decrease in the channel hydraulic diameter due to higher pressure drop offered by each individual channels simultaneously. This results in a considerable reduction in both the peak temperature and the average temperature of the device with decreasing channel diameters. It is also inferred that the flow maldistribution is relatively invariant with Reynolds number for the microchannel system, which is not the case for the macrochannels. Flow maldistribution is found to increase with increase in number of channels and with a decrease in the manifold area relative to the channel area. The 'I' type flow configuration is found to have the least maldistribution while the 'U' type shows the maximum and Z type falls in between. A simple force analysis of the governing equation in the manifold of the parallel microchannel system reveals a strong dominance of the frictional force over the inertial force and both the forces contribute to the uniform flow distribution at smaller hydraulic diameters, where the 1-D theoretical models failed to achieve a concurrence with the present experimental results. Also the present 3-D numerical simulations give a satisfactory agreement with the experimental results projecting it as an effective tool in the design and analysis of microchannel cooling system. The potential zones of hot spots are identified as low fluid velocity zones or low pressure drop zones among the channels resulting from flow maldistribution. © 2011-2012 IEEE.

Investigation on flow maldistribution in parallel microchannel systems for integrated microelectronic device cooling

A detailed experimental study of flow through long microchannels of hydraulic diameter ranging from 60.5 to 211 μm has been carried out. The internal pressure distribution along the length of the channel has been measured to analyze the local flow behaviour. The effects of compressibility and transition to turbulence occurring in the microchannel flow were investigated in detail. In addition, the resulting flow has been analyzed numerically using a commercially available CFD code, FLUENT. It has been shown that there are no special micro-scale effects, including early transition to turbulence at least in the present range of hydraulic diameters after the significant effects of compressibility are accounted appropriately. © 2009 Elsevier Ltd. All rights reserved.

Effects of compressibility and transition to turbulence on flow through microchannels

Journal of Computers

CFD-based Study of Velocity Distribution among Multiple Parallel Microchannels

MEMS

Chemical Engineering Research and Design

Microchannel Plate Geometry Optimization for Even Flow Distribution at High Flow Rates

1st International Conference on Microchannels and Minichannels

Industrial Microchannel Devices: Where Are We Today?

A numerical and experimental investigation of flow maldistribution in a micro-channel heat sink

Journal	International Communications in Heat and Mass Transfer
ISSN	07351933
Open Access	No