A Study of Heat Transfer in Microchannels using Aluminium oxide Nanofluids

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A Study of Heat Transfer in Microchannels using Aluminium oxide Nanofluids

A S tudy of Heat Transfer in Microchannels using Aluminium oxide Nanofluids

Arun Vir Singh Mechanical Engineering Department Thapar Institute of Engineering and

Technology

Patiala, Punjab, India Dr. D Gangacharyulu

Sumeet Sharma Mechanical Engineering Department Thapar Institute of Engineering and

Technology Patiala, Punjab, India

Chemical EngineeringDepartment Thapar Institute of engineering and technology

Patiala, Punjab, India

Abstract High performance heat exchanger devices with higher thermal conductivity of coolants are the need for micro industry, domestic and automobile industry. Thermal conductivity of coolants plays an important role in designing, selection, fabrication of high surface to volume ratio devices to extract higher heats from small spaces.Lower thermal conductivity of conventional fluids like water, ethylene glycol and oils has put a question on their credibility in small spaces high heat extraction devices.Nanofluids, suspensions have nano sized particles (upto 100nm) is seems to be promising solution of this problem. Furthermore, higher surface to volume ratio devices extract more heat than convention heat exchanging devices, microchannels stands by these constraints and proved a valuable asset for heat exchanger category.

Keywords Flow Rate; Heat Transfer; Nanofluid; Microchannel.

I.INTRODUCTION

A lot of advancement has been made by microelectronics industry,resulting in development of high heat generating microelectronic devices.The heat generated by these devices which is an important issue for consideration of their use in industry or everyday activities.High heat buildup in microelectronic devices can not only hamper its performance but can also damage the device.Hence finding solution to this heat buildup is an important but challenging task.Now this heat disscipation task can either be achieved by increasing surface to volume ratio of heat exchangers or by employing better coolants or by both the methods.The problem here with conventional coolants like water,oils,ethylene glycols is that that they have been proved futile due to their low thermal conductivity that leads to poor heat disscipation and slows down the device.

In 1873,it was put forward by J.C Maxwell [1] that to increase the thermal conductivity of base fluid,very small solid particles must be added to the base fluid which can lead to higher heat disscipation.This happens due to higher heat capacity of very small solid particles as compared to base fluid and gives a boost to the heat capacity as well as thermal conductivity of base fluids.However experiments have also shown that addition of micro particles and millimeter sized particles to base fluids also leads to problems like abrasive wearing of pipeline,channel clogging,sedimentation and pressure drop.The above problems has put a restriction on

their use in industry.To bypass these problems the use of nanoparticles was introduced.

Microsized flow passasges having hydraulic diameter range between [3]10 micrometer to 200 micrometers and that have other dimensions also in micro are called microchannels.Microchannels consists of high surface area to volume ratio enabling higher heat transfer rates.Microchannels can fit in very small spaces owing to their small size.It can fit into small spaces where heat generation is more and where conventional methods fail to disscipate the heat.Many researchers have done experimentation for studying the heat transfer through microchannels using nanofluidsThe experiments carried out by these researchers have shown increased thermal conductivity upto 790W/cm2 and the maximum temperature can rise upto 710C [2] higher than that of water.In the microchannels small channels which can be seen have hydraulic diameters ranging from 10mm 200mm.Microchannels are generally manufacteured on silicon wafers because of simple process of stereo lithography and ease of manufacturing.Although this method is easier but it has a drawback related to manucaturing accuracy and thus it leads to discrepancy between theoretical and experimental results.Hence due to this drawback a new method of cnc wire cutting on aluminium is generally utilized.The revolutionary work of tuckerman and Pease[2] in 1981 gave a boost to the microchannels research.After this the research focus shifted to design implementation from 1986- 1988.This was further followed by understanding the design fundamentals of flow of fluid through microchannels during the period of 1992-2002 and then the interest shifted towards practical application in 2002.

During the period after the focus shifted towards validating the research work which had already bee done.From the findings of Satish. G. Kandlikar [3] it has been found that for practical use more research needs to be done in microchannel area.

  1. MICROCHANNELS

    1. Introduction

    2. Microlelectronics devices are very small in shape but they generate a lot of heat in a small area.To disscipate heat generated from microlectronic devices very small heat exchanging devices are required that can not only fit into small spaces but also are light weight.Microchannels are hand in glove with these requirements.Microchannels are small heat exchanging devices which comsists of very small passages through which coolants or heat exchanging fluid flows.These small microchannel passages have very high surface to volume ratio which allows very high heat transfer rates from small spaces.Microchannels can lent themselves useful in applications where there are restrictions of weight and space.These small passages have hydraulic diameters ranging from 10µm – 200µm [4] .[5] [6].

      .

    3. History and literature

    theory to the flow of liquid.

    Tuckerman and Pease [7] in 1981 gave a boost to the research in the area of microchannels and gave the direction in which the research is to be done.[8-15]Till 1988 the focus of the researchers was on design as well as implementation of microchannels technology.After thorough analysis of design and implementation the the focus of the researchers shifted towards studying the flow behavior through small microchannel passages. [15- 20]From 1990 2000 a lot of work was done by researchers using experiments but they could not find a solution or answer the problem of applying the continuum

    Fig.1 Microchannels

    Heat dissipation and work on design and implementation gets started, continues till 1988. After design and implementation the researchers researched upon the fundamental understanding of flow characteristics in microchannels. The experimental research work of many researchers in 1990s still cannot solve the question of credibility of continuum theory to the liquid flow. Later Xu et al [21] neglecting the entrance and exit effects, validated the applicability of conventional theory in microchannel. However later the study by Palm [22] concluded that the research of that time is still inconclusive regarding the applicability of continuum theory in microchannels. The researchers Qu and Mudawar [23] , Steinke and Kandlikar [24] presented the experimental data that confirms the validity of continuum theory in microchannels. Later Lee et al [25] validated the applicability of continuum theory in microchannels with careful consideration on boundary conditions during experiment. Hence the validation of continuum theory has been accepted in single phase flow in

    microchannels. Two phase flow is still in under active research and conclsion has to be made yet. Single phase flow is considered in this experimental work. However the research work by many researchers is taken as reference for designing of the microchannels. Gunnasegaran et al [19], in 2009, carried out the experiments to study pressure drop and flow friction measured in different shapes of microchannels, and found out that Poiseuille number decreases in series

    triangular, trapezoidal , rectangular channels. Zhang et al [26], in 2014, did computational work on Design optimization of microchannels and found that The geometry of the channels strongly influences the pressure needed for the flow. Manay et al [27], in 2012, numerically Investigated heat transfer characteristics and compared with experimental data and concluded that Mixture model theory can be applied to nanofluids flow and heat transfer enhancement was 2.87 and 3.21 times for Al2O 3 and CuO respectively. Farsad et al [28], in 2011, did the numerical simulation of microchannels and concluded that Heat transfer increases with increases with increase in concentration and metals have higher thermal conductivity than corresponding oxides. Mohammed et al [29], in 2011, did experiments on diamond , Al2O3, Ag, CuO, TiO2, SiO2 in triangular microchannels and found out that Diamond has highest heat transfer coefficient and alumina with lowest , SiO2-H2O has highest pressure drop, Ag-H2O shows no wall shear stress. Hamid et al, in 2011, investigated the performance index and efficiency of counterflow microchannel heat exchanger(CMHE) numerically and found out that performance index and effectiveness of CMHE decreases with increase in Reynolds number and pumping power and performance index are insensitive to volume fraction at all Reynolds number. Tannaz et al, in 2009, performed the experiments to check effect of channel geometry on performance of microchannels and concluded that Heat transfer coefficient is independent of channel width above 400m and cross sectional geometry of channels effects the heat transfer coefficients in microchannels. Manay et al, in 2016, investigated the effect of microchannel height on performance of nanofluids and found out that Increase in height of microchannels decreases the heat transfer coefficient and increases pressure drop.

  2. EXPERIMENTAL SETUP

    The directions for making the experimental setup was obtained from the literature review which described the effect of aluminium oxide water nanofluids on the heat disscipation capacity of microchannels.Experimental apparatus was designed at T.I.E.T Patiala and manufactured at the Global instrument company ambala Haryana.

    Rate of flow through the microchannels was controlled by computer operated syringe pumps.Flow meters were needed in this study since syringe pumps already provide highly precise flow rates.Thus highly accurate rate of flow was achievable by syringe pumps.The heat flux for heating of the microchannels was provided by the heater installed in the setup.This heat flux was controlled by carrying the current and voltage.Further the current and voltage were controlled by a dimmersta provided in the setup.The nanofluids after pasing through the microchannels go into the reservoir.In this

    study single pass flow is considered.However the study could also be conducted using continuous flow.The main issue of leakage in microchannels is solved by using grease paper.

    1. Fabrication of test section and setup.

      Syringe pump: They form the main part of the experimental apparatus.Syringe pumps are devices that cause the nanofluids to flow through the microchannels.They provide the required pressure to flow the nanofluids through the microchannels.Syringe pumps can provide constant flow rates through the microchannels ofr set duration of time with very high degree of accuracy.Since syringe pumps are highly accurate,there is no requirement of flow meter in the passage.The syringe pumps used in this study have been procured from E-spin nanotech,Kanpur,UP.

      Heaters: The heater used in the study is a low watt heater with a adjustable variate.These low watt heater have a wattage rating of only 35 W.These heater are highly effective in adjusting the heat fluc to the microchannels.The heat flux can be controlled with help of a adjustable variate.Using these heaters the temperature can be kept at a constant 400 C

      .The temperature can be increased further by adjusting the variate.These heater allows us to maintain the temperature in required range successfully.

      1. Temperature sensors: The temperature sensors employed in this study are PT 100.These temperature sensors are highly accurate temperature sensors for the measurements of temperature.These temperature sensors encapsulate the concept of resistance thermometers.The concept is that the resistivity of sensor changes with change in temperature.These temperature sensors are also more preferred as compared to thermo couple sensors due to high precision.

      2. The calibration was done at NIIRT Lab situated at Industrial area,Panchkula,Haryana.The calibration is valid for one year from the date on which calibration was done.The calibration was done and the report was provided.

        CNC wire cutting technique was found to be the most cost effective technique for manufacturing of microchannelsand was also available at location from where transportation was also viable.

        Fig. 2. A) Experimental setup layout

        Fig.2. B) Pictorial view of setup

        The calibration was done at NIIRT Lab situated at Industrial area,Panchkula,Haryana.The calibration is valid for one year from the date on which calibration was done.The calibration was done and the report was provided CNC wire cutting technique was found to be the most cost effective

        Table 1.

        Cross section

        Material

        Type of flow

        Type of channe ls

        Process of manufa

        ct uring

        Rectangul ar

        Aluminum

        Single pass

        Split

        flow type

        CNC

        wire cutting

        The cross sectional view with dimensions of channels is shown as

        Fig. 3.Cross sectional view of microchannel

        The dimensions of the microchannels are given in the Table 2.

        Dch

        Wch

        Lch (each side)

        2mm

        0.25mm

        20mm

        Table-2: Dimensions of micochannels

        • The width of the microchannel was chosen as .25 mm since it is the limiting width which can be produced from CNC wire cutting.

        • Only rectangular geometry of microchannel can be manufactured by CNC wire cutting this fixes the geometry of microchannel.

        • The dia for inlet and outlet manifold pipes is fixed to

          2.5 millimetre due to availability constraint.

          This is the little glimpse of designing procedure has been given here.

    2. Flow loop and working

      The syringe pumps are computer controlled and are operated through software interface which is installed on the computer from CD provided with the syringe pumps.The name of the software installed on the computer for operating the syringe pumps is SP 102.Various inputs are feeded into the software and the syringe pumps work according to these inputs.The various input parameters required to be feeded into the software before operation are :- a) Dia of syringe b) rate of volume flow c) Volume of the syringe d) Time duration for which the experiment is to run.According to the feeded inputs the computer computes the revolutions per minute of the motor.This information is then fed to the syringe pump controller.The controller then provides input to the syringe pump.

    3. Preparation and properties of nanofluids of nanofluids

      Nanofluids can be prepared by the following two methods

      :-

      Table 3. Specification of nanoparticles

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