OJM  Vol.8 No.1 , February 2018
A Comprehensive Review on Heat Transfer Design Parameters in MEMS through Experimentations and Numerical Modeling
Author(s) Yasir M. Shariff
ABSTRACT
Micro electro mechanical systems are studied in this paper along to comparing the design, fabrication, and operation of such devices especially in the electronic industry. The advancements of such systems are important as scale reduction is becoming more economical and also in heat dissipation efficiencies. Furthermore, the fabrication techniques are becoming more of a challenge in today’s industrial market. In this paper we will introduce a comparative study of experimental research in the micro scale heat exchanger with varieties of industrial application primarily cooling integrated circuits. Significant effect in the process of cooling electronic devices has showed experimentally that scale variations impact into this improvement. Numerical studies have been a leading technique in the development of thermal systems especially at the micro or small scales which helps the industry to cut cost and provide innovative products at high efficiencies. There are so many approaches to numerical analysis and research, where the availability of ready to use software and data acquisition modules is variant. Material selection is also a variable in the efficiency and operations of these micro channel thermal system and heat exchangers.

1. Introduction

There are developments in the industry of MEMS such as fabrication, design parameters in today’s engineering systems whether as micro devices and/or the materials used in making these complicated devices [1] . There is a need and demand for better understanding micro scale devices at high power and pulse lasers in both the processing and diagnoses of these micro scaled structures where heat and energy become of demand during long operation which introduced studies as to compare from a macro to micro scale which requires experimental data base for the industry [2] . There are different areas of understanding in these systems such as micro length scale, micro structure materials, and the micro timing [3] . There are many variations of scaled micro channel heat exchangers which are an array of small tiny channels in the range of 10 to 100 micro meters, where this does show enhancement in reducing temperatures in electronic devices that does exert a large amount of heat during its operation [4] . Three dimensional numerical and experimental investigations were done to prove the heat transfer and pressure dropdone [5] . The results were done on a silicon edge microchannel as a very small thermal resistant, in the range of 0.1 K∙cm2/W @ Reynolds number equal to 300. In the design of the micro heat exchanger, the pressure drops made significant change to the heat dissipation part. Most of these studies measured the pressure drop with agreements to the published results done in analytical studies under fully developed laminar flow [6] .

These developments enhanced the removal of heat from electrical devices were possible using micro heat exchanger channels in arrays. It was proven by these studies through the numerical and experimental results that heat removal from silicon films was possible where these systems in long operations exert large amounts of heat which raise the temperature for these devices and in the long run will reduce the life expectancy of these systems. These systems introduce a new scale size which fits most electronic micro scales and in its operation, they would increase the life expectancy and durability [7] .

Material selection through numerical research made it easy for finding the perfect thermal conductivity appropriate and serves the purpose of these devices, where numerical simulations provided the drop of the thermal conductivity values at the boundaries of these systems which has improved the HTC efficiency of the micro channel apparatus and the reason for that is the linear heat conduction within the flow parameters of the channels as diameter was variant [8] . The HT gradients were affected through the increase of the value of the thermal conductivity as it is directly proportional to the heat transfer gradient specially at the edges of the micro channels within the heat exchanger and that does directly affects the HTC’s efficiency specially within a parallel flow micro channel heat exchanger [9] . The methods explained by most researchers comes along the same line of investigation which is to improve on the HTC’s efficiency as this has been presented and compared in this manuscript.

2. Experimental Analysis

There are high current systems in the market today which increases the heat dissipation values which needs to be managed through heat sinks which requires to be joint with the surfaces of these electronic systems during the fabrication and also taking considerations of the material that produce thermal conductivities or could also be used as an insulator to exchange heat transfer. The use of fluid flow through micro channels in arrays as heat exchanger could be effective in cooling these devices and to study the efficiencies in these systems is becoming more important as to the manufacturers of these devices. The introduction of micro channels as a heat exchanger in throughout device was studied to insulate the silicon surfaces [10] .

In the fabrication models used for heat transfer applications are rapidly used in the small scale channel heat exchangers particularly in the micro range. The mathematical models used in these systems are different than that of regular size evaporator channels. The initial variations under fully developed flow boiling can be observed as compared to regular sizes to that of micro structures. Single and two phase flow boiling is introduced and data collected will assist the industry in the design of such operative systems. Measuring the HTC in a continuous flow during nucleate boiling in micro channels are done in an experimental study which shows tremendous developments in the bubble generation especially if the surface of these channels are also varied in terms of surface roughness [11] .

Flow boiling of carbon dioxide in micro scale channels shows that the HTC for reduced boiling effect for micro channels as compared to regular large-scale channels which are used in today’s industry. It was shown that through experimental application during the flow boiling of carbon dioxide in micro channels under mass flux in the range of 300 to 800 kg/m2∙s with heat fluxes in the range of10 to 30 kW/m2 this was measured through a flow meter which gave variations of flow velocities. Through the experimental results it was indicated that HTC of the carbon dioxide in these arrays of micro channels are to the most independent of both heat and mass flux which indicates that the heat dissipation was dependent on the size of these channels as well as the design geometry. However, in addition to the exceptional improvement in its thermal properties carbon dioxide provides better heat dissipation management as compared to other refrigerants such as R-407C or other refrigerant mixtures [12] . Flow boiling in micro channels particularly in porous media was experimented. The effect of this scale on the micro channels in the heat dissipation were done at the beginning numerically. The results showed also comparison to experimental data collected. The COP of the micro channels as well as the porous media in that small-scale channels were done in comparison to other research work in its field of study. The results showed better COP for using porous media then that of micro channels with a larger presser loss; however, over a variation in experimental conditions the highest value of the HT coefficient under porous media gave us 85 MW/m3∙K, a wmf rate equal to 0.07 kg/s, & pressure loss equal to 4.3 bars. Over a variation in experimental conditions the highest value of the HT coefficient under micro channels array gave us 40 MW/m3∙K, a wmf rate equal to 0.29 kg/s, & pressure loss equal to 0.6 bars [13] .

Micro channels are fabricated in the range of 20 to 1000 micro meters within two dimensions are to be tested at high temperature gradient devices. Furthermore, there are so many parameters that could also affect the HTC in a two phase flow boiling condition such as the pressure drop, the heat and mass flux which effects highly the thermal transportation. Experimental studies were done in an apparatus that tested these micro channel array systems with ranges of power input from 15 to 250 Watts and vfm equal to 30 to 250 ml/m and fluid x in the range of 0 to 0.6, the range of the internal sub-cooling ranged from 5 to 20˚C. The study showed that the HTC, delta P are functions of the fluids x, the heat and mass fluxes which related to the super heat effects. The HTC dropped from 12 to 9 kW/m2∙K @ 80˚C superheat at the wall jumped from 15˚C to 70˚C. The HTC dropped by 25% where the xv at the exit was increased from 0.02 to 0.7. Other experimental studies were provided of nucleate pool boiling using R- 123 as a working fluid @ 30˚C Tsat in dual channels that were designed for enhancing the surface to improve its performance and the tests were compared to that of pool boiling for the same working fluid in different surface and scale variations. The goal of such studies was to provide live data for manufacturers to enhance their design models and also a better performance to the heat dissipation of these systems and could be fabricated in a cold working process.

The aim for such a design to the heat exchangers was to reduce the budgets for O & M during its operations through improvements in both the surface and channels fabrication. The surface effect on the micro channels has shown improvement in the heat dissipation on electronic devices in ranges of 15 to 185 micro meters two dimension [14] .

The two phase flow HTC are effected by different variables that creates bubbles during the boiling operation such as the diameter where the small scale also is a big effect in the geometry and the surface of these channels provide heat transfer.

Furthermore, delta P on the heat flux and mass flux affects it significantly. Experiments were done on an apparatus that was made for different micro channel heat exchanger designs. Different variations were made in this experiment such as the power input in the range between 30 to 380 W, mfr between 40 to 320 ml/min, and x in the range between 0 to 1.3, and Tsub at 7˚C. Finally, it is necessary to point out the variables which show an effect of the flow which are the diameter of the channel, pressure drops and finally the working fluids thermal physical properties.

3. Numerical Analysis

Catalytic purifiers were studied and with the use of micro channel systems enhanced the efficiency where the hot air was flown through the air purifier system to extract the toxic substances and where the size of the particles within the toxic waste as well the weight and power used for these thermal catalytic systems. The study provided the use of a CFD code which directly worked on the design and modeling and later fabrication of these thermal systems at the micro scale level and a counter flow HX was used to bring the efficiencies higher. Furthermore, the CFD model was used in providing the characteristics of the heat transfer and flow patterns to improve the counter flow heat exchanger and make it more effective for which the availability of experimental data made it feasible to compare it to that of the numerical values. Both T and e agreed with that of the CFD model from the experimental data available such that it was possible to evaluate these thermal systems COP’s as a function of the diameter and scale for the operation schemes. Results showed limitation for the COP for a linear conduction surface boundary conditions and hence the scale and parameters were obvious to enhance these conditions.

4. Summary

It has proven through experiments that micro channel heat exchanger devices had an exceptional heat dissipation management in the removal of access heat from electronic MEMS devices. So many experiments were done to study the in-channel enhancements such as surface roughness, micro fined channels which show the heat transfer and pressure loss within these special conditions for a single or two phase fully developed to turbulent flow effects. It is found that the HTC as a function of x and mass/heat flux and the surface parameters superheat, where the HTC reduced at values of 10 kW/m2∙K @ 12˚C to 7 kW/m2∙K @ 95˚C. It was found also that HTC reduced 22% as x is increased within the ranges of 0.02 to 0.58. Finally, the pressure loss increased as heat transfer flux also was increased. The use of forced convection with the selecting a working fluid can affect the size parameters for a dimensional analysis for the heat transfer correlations where the use of micro channels gave a better approach in reaching the fully developed flow laminar and turbulent respectively. It also was observed that Re was varied during the study and for laminar and turbulent flows the values were 0.5 and 0.86 respectively and this gave important design parameters with diameter variations as scale was a factor in enhancing efficiencies of these thermal micro channel heat exchanger systems.

Acknowledgements

This work was supported by the National Science, Technology and Innovation Plan (NSTIP) through the Science and Technology Unit (STU) at Taibah University, Al Madinah Al Munawwarah, KSA, with the grant number 08-NAN20-5.

Cite this paper
Shariff, Y. (2018) A Comprehensive Review on Heat Transfer Design Parameters in MEMS through Experimentations and Numerical Modeling. Open Journal of Microphysics, 8, 1-6. doi: 10.4236/ojm.2018.81001.
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