Studies on Heat Transfer and Pressure Drop Characteristics of Nanofluids in Microchannels

Abstract

High heat fluxes devices in an electronic industry pose a thermal challenge to the researchers of this area. Over the past decade, the use of microchannel heat sink to remove the high heat load from the active heat source has been studied. But as the amount of heat generated is quite high, so, an efficient fluid is required to enhance the cooling performance of microchannel heat sink. The suspensions of nanoparticles in the base fluids term as “nanofluids” proves an efficient fluid for the microchannels.

This present work focuses to compare the different types of nanofluids flowing through the microchannels and find the nanofluids with high heat removal capacity. In the present work, nanoparticles such as alumina, copper oxide (CuO) and multiwalled carbon nanotubes (MWCNT) are extensively used with different types of base fluids such as water (W) or water/ethylene glycol (W/EG) mixtures (90:10, 80:20, 70:30, 60:40 and 50:50). Nanoparticles are dispersed at different concentration ranges from 0.1 vol % to 5 vol % in the base fluids by using two step method. Alumina nanofluids are stable without the use of any kind of surfactant while CuO nanofluids get stabilized by using 0.2 wt % sodium dodecyl sulphate and MWCNT are stabilized with 0.25 wt % of Gum arabic. Further, sonication can be done by using ultrasonicator water bath at a fixed frequency to make more stable nanofluids. As there is no fixed time for sonication has been reported, so optimization of sonication can be done on the basis of thermal conductivity measurements at 40 min, 60 min, 80 min and 100 min. For 80 min of sonication, maximum enhancement in thermal conductivity is observed.

The stability of nanofluids can be determined by measuring the absorbance, thermal conductivity and zeta potential of nanofluids in terms of days without disturbing or shaking the samples. The results showed that CuO nanofluids are least stable nanofluids with stability of 2-4 days while MWCNT nanofluids shows a maximum stability of 24-36 days and alumina nanofluids remain stable for 19-28 days. Stability of nanofluids is affected by changing the base fluids and it increases with increase the ethylene glycol ratio in water. Maximum stability of nanofluids is achieved with W/EG (50:50) base fluids either any type of nanoparticles used. So, ethylene glycol itself acts as a stabilizer in water to enhance the nanofluids stability.

Thermophysical properties of nanofluids are important to determine to know the exact performance of nanofluids in microchannels. The objective of thesis is to determine the thermal conductivity, specific heat, viscosity and density of nanofluids at various temperatures ranges from 20 C to 80 C. The thermal conductivity is measured by KD2 Pro, viscosity is measured by Brookfield viscometer and density is measured by Pycnometer. Specific heat measurements of few samples of nanofluids can be done by Differential scanning calorimeter and compare with the theoretical mixture model. The experimental results show a ± 1 % deviation with the model data so further all the predictions of specific heat are done with mixture model.

The thermophysical properties of nanofluids are influenced by various factors such as type of nanoparticles and its concentration, type of base fluids and temperature. An appreciable amount of enhancement is observed in thermal conductivity of nanofluids and it increases with increase in temperature and nanoparticle concentration. MWCNT nanofluids show maximum thermal conductivity in comparison with alumina and CuO nanofluids. Thermal conductivity enhancement increases with increase in the ethylene glycol ratio in water and highest increasement observed with W/EG (50:50) base fluid. Viscosity and density are also an important property which decided the thermal performance of nanofluids while using in thermal systems. There is an insignificant rise in viscosity and density observed after the addition of nanoparticles in the base fluids. This result makes no adverse effect on thermal systems as the thermal conductivity rise is more in comparison with viscosity and density enhancement which gives overall better thermal performance.

Thermal performance of nanofluids are further determined for aluminium microchannel heat sink (MCHS). Rectangular shaped microchannels are fabricated with width, depth and length of 250×2000×40000 μm. There are 21 number of parallel microchannels fabricated on a single unit of aluminium block with a fin spacing of 200 μm. Nanofluids are used to extract the extra amount of heat generated by the heat sink. Overall convective heat transfer is improved by using nanofluids in microchannels with an insignificant rise in pressure drop.

Experiments are performed at various flow rates such 0.2 ml/min to 2 ml/min and at different heat inputs of 2 W, 4 W and 6 W. At very low flow rates, no significant rise in convective heat transfer is observed with nanofluids. Above 0.8 ml/min, flow is fully developed in microchannels with a maximum heat transfer enhancement of 80.49 % at 2 ml/min with 1 vol % nanoparticle concentration of MWCNT-W/EG (50:50) nanofluids. Friction factor and thermal resistance are quite low at high flow rates which show the improved thermal and hydraulic performance of nanofluids. An analytical model is developed by using Turbo C++ to analyze the heat transfer and fluid flow behavior of nanofluids in microchannels. The predicted model data have a good agreement with experimental data with a deviation of ± 10 % with Reynolds number and friction factor and ± 15 % with Nusselt number. Nanofluids extract upto 70 % heat from the heat sink which makes it a promising heat transfer fluids for thermal systems.

Further, a new MADM-TOPSIS approach is introduced for the evaluation, comparison and selection of best suitable nanofluids for any thermal systems. Actual performance of nanofluids is affected by the various parameters and it’s important to identify parametric quantities which concurrently improve the performance of nanofluids. A three stage evaluation scheme is proposed for evaluation, comparison, and optimum selection of a nanofluid known as MADM-TOPSIS approach. The method ensures that the selected nanofluid is closest to hypothetical best nanofluid and farthest from hypothetical worst nanofluids. Selection of nanofluids is done by considering the government rules as well as cost factor requires for a particular application. An example is considered by selecting few nanofluids used in the present work. All the performance attributes are considered to evaluate the best suitable nanofluids for MCHS. This scheme proves very user friendly as simple mathematical procedure is followed for the evaluation. This will be very helpful for the different researchers or people working in the different organizations or industries so that they will know the exactly performance and stability of nanofluids before use it any application.