Enhancing Radiator Cooling with CuO Nanofluid Microchannels

The study explores in employing copper oxide (CuO) nanofluid as a cooling medium in the vehicle radiators. To simulate the heat transfer process, the microchannel is construct-ed using electron discharge machining (EDM) and a computational fluid dynamics (CFD) modeling is employed. UV-visible spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS) are used to characterize the CuO nanofluid. CuO nanofluid surpasses water in the heat transfer capabilities, with a 40% improvement in thermal conductivity. The average size of CuO nanoparticles was determined via DLS to be 485.1 nm. The heat transfer coefficient of CuO nanofluid is 5366 W/m2K, which is 116% larger than that of water. The increased heat transfer capabilities of CuO nanofluid microchannel flow indicate to its potential as a viable replacement for conventional radiators in the automotive applications. Lower engine temperatures, increased fuel efficiency, and longer engine lifespan may result from improved cooling performance. Due of the small size of microchannels, more efficient and space-saving radiators for automobiles are conceivable. More research is needed to improve the microchannel design as well as to realize the practical benefits of CuO nanofluids in car cooling systems


Introduction
Computers, electronics, and communication have evolved considerably in recent decades.Power consumption and data storage in small chips have risen with these devices, making thermal management harder [1,2].These air-coolers are at capacity.Applications exceeding 100W/cm 2 need advanced cooling [3].Today, liquid cooling solves these challenges.These fluids' thermal conductivity makes energy-efficient heat transfer devices [4][5][6].Despite substantial heat transfer improvement research, heat transfer fluids have weak thermal conductivity, limiting cooling [7,8].At normal temperature, solid metals have far greater thermal conductivities than fluids [9,10].Heat flows better via metallic liquids.Thus, suspended solid metallic particle fluids should have better thermal conductivity than heat transfer fluids.Advanced applications need heat dissipation beyond water.Most solids transmit heat better than conventional heat transfer fluids [11,12].Thus, liquid-fluid microchannel heat sinks efficiently manage electrical and optical device temperatures.Micro channels are smaller, have less coolant inventory, and have a bigger heat transfer surface, making them useful in automotive, aerospace, refrigeration, air conditioning, gas turbine blade cooling, and processing [13][14][15][16].They have low channel pressure drop and strong convective heat transfer coefficient [17,18].Although pressure drop was high, Tuckerman and Pease [19] used direct water circulation in rectangular micro channels to reduce heat flux up to 790 W/cm 2 utilizing silicon microchannels for electronics cooling in 1981.Peng et al. [20] carried out studies on water flow fluctuations in rectangular micro channels.Kawano et al. [21] explored water-coolant pressure drop and heat transfer statistically and experimentally.Simulated pressure loss was lower than Reynolds number testing above 300.Channel thermal resistance raised to 0.1K/W.cm 2 with 100W/cm 2 heat flow.Yin et al. studied pressure loss in onephase microchannel heat exchangers with parallel circuits and intricate headers [22].These measurements revealed manufacturing issues.Moody chart friction factors were determined.Kim [23] suggested 3-D modeling of microchannel heat sink thermal resistance utilizing a fin model, porous medium model and optimization.In a microchannel heat sink, Min et al. [24] simulated the influence of tip clearance.Xu et al. [25] split the flow domain into longitudinal and transverse microchannel zones to form a microchannel sink.Lee and Garimella [26] studied heat transmission in copper micro channels of 194-534μm 202 diameters and 300-3500 Reynolds numbers.Wang et al. [27] developed a computer model to evaluate nanofluid cooling microchannel thermal and hydraulic performance.With fixed pumping power, pressure drop, and volume flow rate, channel number and aspect ratio were optimized.A silicon micro-nano pillar multilayer water-cooled heat sink was created by Dixit et al.The researchers found that silicon pillars increased heat dissipation.In ethylene glycol, de-ionized water, and oil, Choi and Eastman [29] distributed nanocrystalline particles to enhance heat transfer.Reiyu Chein and Janghwa Chen studied microchannel heat sink fluid flow and heat transfer using numerical FVM [30].P. Gunnasegaran et al. [31] calculated water flow pressure drop and friction factor in rectangular, trapezoidal, and triangular microchannel heat sinks for 100-1000 Reynolds numbers using finite volume technique.Wang et al. [32] provides an overview of nanofluid heat transfer characteristics, as well as enhancements and challenges in using nanofluids for heat transfer applications in microchannels.Timofeeva et al. [33] investigate the effect of particle shape on the thermophysical properties of alumina nanofluids, contributing to a better understanding of particle morphology's role in heat transfer enhancement.The investigation of the viscosity data for Al 2 O 3 -water nanofluid and the enhancement of heat transfer based on nanofluid viscosity behavior was conducted by Nguyen et al. [34].The temperature dependent thermal conductivity and viscosity of TiO 2 -water nanofluids are investigated by Duangthongsuk et al. [35] which is critical for understanding their heat transfer behavior at different temperatures.Putra et al. [36] explored the heat transfer trend in nanofluid microchannel flow under natural convection conditions.Ho,Wei and Li [37] studied the performance of microchannel heat sinks with Al 2 O 3 -water nanofluids, highlighting their potential for improved heat transfer over conventional coolants.Xie, Lee and Youn [38] investigated multi-walled carbon nanotube containing nanofluids and their improved thermal conductivities, contributing to a better understanding of nanofluid behavior with various nanoparticle materials.An experimental investigation of the forced convective heat transfer coefficient of nanofluids in a helically dimpled heat exchanger under turbulent flow conditions is carried out by M. Mehrali et al. [39], providing perspective for the heat exchanger design.Chon et al. [40] developed an empirical correlation relating temperature and particle size to the thermal conductivity enhancement of Al 2 O 3 nanofluids,allowing for more accurate predictions of nanofluid thermal conductivities.Ghadimi et al. [41] examined the properties of nanofluid stability and characterization methods in stationary conditions, which are critical for understanding nanofluid behavior in heat transfer applications.

Limitations of the previous study
Research on microchannel heat flow with and without nanofluids has improved heat transfer techniques.However, these studies have struggled to achieve true adiabatic conditions in experimental setups, address nanofluid stability, standardize experimental protocols, explore microchannel design parame-ters and consider nanofluid degradation.Since actual experiments generally include friction losses, pressure drops, and heat transmission to the surroundings, achieving real adiabatic conditions is difficult.The detailed knowledge of nanofluid concentration's nonlinear impact on heat transfer performance requires a greater emphasis on individual concentrations.Standardized procedures across research would improve repeatability and dependability.Further scaling the microchannel heat transfer to the industrial level at low cost while maintaining efficiency remains a challenge.The transient behavior of microchannel heat flow and its dynamic response needs further investigation for applications requiring rapid response.

Research gaps
Microchannel heat flow using nanofluids has advanced, yet there are certain research gaps.A thorough knowledge of the effect of nanofluid concentration on heat transfer performance is needed.While there are numerous microchannel designs, research is ongoing to determine the most efficient and effective geometries for specific applications.This entails experimenting with various shapes, aspect ratios and surface modifications to improve heat transfer.Researchers are investigating methods such as using nanofluids, additives or surface coatings to improve convective heat transfer coefficients, as well as investigating pulsating flows for improved performance.Understanding heat transfer phenomena at the nanoscale within microchannels is an area that is still largely unexplored.Investigating the effects of molecular dynamics, phonon transport and material interactions at this scale could greatly improve the understanding and efficiency in microscale heat transfer.Microchannel fabrication and manufacturing techniques require improvement.Techniques for creating complex microchannel structures that are cost-effective, scalable and precise are critical for practical implementation.The research to investigate heat transfer in microchannels under phase change conditions, especially for applications such as thermal management and refrigeration systems is still in nascent stage.

Objective of the experimentation
The aim of the current study is to prepare the nanofluid and conduct morphological analysis of the nanofluid to determine the size and dispersion of nano particles in the base fluid.CFD analysis is performed using COMSOL Multiphysics software to investigate the flow of nanofluid through rectangular microchannel.The theoretical results (COMSOL software) of the heat transfer coefficient for the flow of water and CuO nanofluid through rectangular microchannel are compared with the experimental results.The effect of velocity, Reynolds number and Nusselt number on heat transfer coefficient of water and CuO nanofluid and the dependence of Nusselt number on Reynolds number for water and CuO nanofluid is investigated.The research developed the microchannel heat flow system with high heat dissipation rate which can be employed effectively in the automobile radiator cooling resulting in cost effectiveness and eco-friendly environment.

Design and fabrication of Microchannel
The microchannel in the shape of a rectangle was developed on Solid works 2014 as shown in Figure 1.Measurements such as length, height, breadth, hydraulic diameter, height of substrate, and width of substrate are among the different geometrical features that are associated with rectangular microchannels (Table 1).Polycarbonate was utilized for the cover plate, which worked as both a sealer and an insulator because of its dual purpose.On this cover plate, two general assemblies were machined, and they were connected to the microchannel in order to frame the test section.Both the three-dimensional rectangular shape and the manufactured model are shown in Figure 2.
Copper was chosen as the material for the substrate because of its high heat conductivity of 385W/mK.Additionally, channels were formed on the substrate by the use of micro precision wire electron discharge machining.There were several phases of planning that went into the design of the microchannel heat sink.The test piece also known as foundation plate was made of aluminum 6061 alloy.

Characterization of the nano fluid
UV spectroscopy was used in order to study the characteristics of the nanofluid that was generated using CuO nanoparticles.For the purpose of morphology and particle size analysis, further SEM and DLS were used.For the purpose of recording the spectra of CuO nanofluid, UV-visible spectroscopy was used.The ultraviolet spectrum has been recorded from 200 nm to 700 nm, and the absorbance peak occurs at 237 nm in CuO nanofluid.This peak is attributed to the transition from → * , as seen in Figure 3.It is possible to monitor the reaction process by UVvisible spectroscopy, which is made possible by the production of stable CuO dispersions.The scanning electron microscopy (SEM) technique has been used in order to investigate the surface morphology of the CuO nano particles.By placing a little quantity of liquid drop on a silicon wafer, as shown in Figure 4, and then drying the mixture for twenty-four hours in an airtight container, a very thin film of copper oxide was produced.This was done to prevent any moisture or dirt from getting into the film.After that, a scanning electron microscope (SEM) picture of a powdered thin film sample that had been dried was acquired, as shown in Figure 5.A picture obtained from a scanning electron microscope (SEM) may be used to clearly see the surface morphology, as shown in Figure 6, which demonstrates the homogeneity in the dispersion of CuO nano particles  In order to determine the average particle size distribution of copper oxide nanoparticles, the Dynamic Light Scattering technique was used.At the beginning of the nanofluid formation process, it was found that the sample of CuO nanofluid had an average particle size distribution of 485.1 nanometers.In Figure 7, the different characteristics of the system are as follows: temperature (25 degrees Celsius), measuring location (3 millimeters), count rate (7.2 kilocycles per second), duration (50 seconds), and attenuator (11).

CFD modeling of microchannel heat sink
The formulation of a computational fluid dynamics (CFD) model to study the thermal performance of a rectangular microchannel heat sink using water and the suggested nanofluid as the working fluid has been done in this section.The COMSOL Multiphysics program was used to model and simulate a rectangular microchannel in single phase flow.The dimensions of the microchannel were identical to those of the real experimental test setup.The finite element method known as COMSOL Multiphysics is used to simulate flow in all velocity regimes, provided the boundary conditions.This is accomplished by solving numerous variants of the Navier Stokes equations.Both the design and simulation of the microchannels were accomplished with the assistance of the structural mechanics and physics component of the MEMS Tool known as COMSOL.Figure 8 illustrates the meshing of the microchannel that is rectangular in shape.Structured mesh with triangular elements was used for meshing the geometry as shown in Figure 8. 100 X100 X10 nodes were created with element size of 0.0005 in all the three dimensional coordinates.The solution based on the governing equations and discretized mesh took 1000 iterations to converge.

Experimentation
In Figure 10, the experimental equipment for measuring heat flow via microchannels is shown.All of the components that make up the experimental apparatus seen in the picture are as follows: (a) Fluid Source (b) A peristaltic pump (c) The thermocouple of the K-type (d) meter of wattage (e) An exchanger for heat (f) System for the collecting of data (g) The multimeter, (h) heater for plates (i)D.A.C card.As working fluids, water and nanofluids that included nanoparticles of copper oxide suspended in a base fluid (such as water that had been diluted) were used.The accuracy of the sensors of the experimental apparatus is provided in Table 6.
M f = ρ (5) where the density is denoted by ρ, A represents the area and V represents the velocity.From 45 and 240 was the range of the Reynolds number.In order to determine the temperature of both the intake and the outflow, two thermocouples of the K-type were used.In order to determine the temperature of the substrate, a multimeter was being used.As shown in Figure 11, the KD2 Pro analyzer is used for the purpose of determining the thermal conductivity of the nano fluid.The specifications of the KD2 probe are provided in Table 5.The thermal conductivity of CuO nanofluid and water were found to be 0.842 W/mK and 0.6 W/mK respectively which is within the range provided in the literature (0.5 W/mK to 10 W/mK for CuO nanofluid depending upon size and concentration of the nanoparticles and 0.5 W/mK to 0.6 W/mK for water at room temperature depending upon the impurities in it.)Fig. 10.Experimental apparatus Fig. 11.KD2 probe to measure thermal conductivity

Results and Discussion
With the help of the COMSOL Multiphysics software, flow through the microchannel sink is simulated for both water and CuO nano fluid at the same flow rate (Table 2).Since it is not possible to create an adiabatic setup that is ideal, the discrepancy between the theoretical and practical findings may be attributed to the numerous losses that occur in the experimental settings which include friction loss or pressure drop and heat transfer to the surroundings.
h=Q/A ∆Tn (6) where Q is the amount of heat that is applied to the substrate, n is the number of channels, which is equal to 21, and ∆T is the difference between Twall and Tmean.The fluctuation of the heat transfer coefficient with velocity is seen in Figure 12 for both water and nano fluids based on CuO.Table 3 presents the results of fifteen tests that were carried out by altering the heat flux and flow rate that was introduced into the system for both water and CuO nanofluid.The fluctuation in the heat transfer coefficient of water and CuO nanofluid when subjected to varying heat fluxes and flow rates while maintaining the same Reynolds number is seen in Figure 13.When both the heat flow and the Reynolds number are increased, there is a corresponding rise in the heat transfer coefficient for the flow.It was observed that the Nusselt number rises with an increase in the Reynolds number which concludes that the Nusselt number is dependent upon the Reynolds number.Nusselt number and heat transfer coefficient of both fluids are plotted against the Reynolds number at 75 W (Figure 14 and 15).As can be seen in Figure 14, the Nusselt number value for water falls somewhere in the range of 1.1 to 3.7, but the value for the suggested nanofluid falls somewhere in the range of 0.9 to 5.6.Nusselt number variation for water and nanofluid are shown in Figure 16 and Figure 17 respectively.Both of these figures demonstrate the variance of Nusselt number with respect to heat transfer coefficient.The presence of CuO nanoparticles enhanced the thermal conductivity of the nanofluid compared to water.This alteration in thermal properties could impact the Nusselt number differently across various Reynolds numbers.Generally, nanofluids tend to exhibit higher Nusselt numbers compared to base fluids due to improved thermal properties however it is observed that Nusselt number of water is higher than the nanofluid at lower Re values.

Conclusions and Future Scope
In the present research microchannel having rectangular dimensions was manufactured and the flow of water and CuO based nanofluid through the microchannel was analyzed experimentally and simulated using COMSOL Multiphysics software.The software is used to conduct the investigation, which is focused on the heat transfer and fluid flow analysis of water and nano fluids traveling via rectangular micro channels.CuO nanofluid microchannel flow was discovered to have a thermal conductiv-ity that is forty percent greater than that of water-based microchannel flow via the experimental study.When CuO nanoparticles were initially manufactured, their average size was found to be 485.1 nm using DLS technique.With the use of CuO nano fluid rectangular microchannels, it is possible to attain a high heat transmission coefficient of 5366 W/m 2 K, which is 116% more than the water flow through the microchannel.When the Reynolds number remains the same, the heat transfer coefficient for both the water and CuO nanofluid rises as the heat flux increases whereas at the same heat flux the Nusselt number shows 209 an upward trend with the Reynolds number.On the other hand, the value of the Nusselt number for CuO nanofluid is lower than water when the Reynolds number is low because of high thermal conductivity of nanofluid and it is larger than water when the Reynolds number is elevated.Because of the high thermal conductivity, the Nusselt number of CuO nanofluid is low, despite the fact that it has a high heat transfer coefficient.It is possible that the Nusselt number of CuO nanofluids might be up to 200 percent higher compared to the conventional fluid (water) at higher heat transfer coefficient.The scope of future research is to widen the study in order to examine the wear characteristics of radiator material when nanoparticles are added to the base fluid.Extensive study can be done to increase the heat transfer coefficient by considering the appropriate suspension, the usage of microchannels with varied aspect ratios and the impact of nanofluid on double-layer microchannels can be investigated further.

Table 2 .
Heat transfer coefficient for water and nanofliud (simulated and experimental results)

Table 3 .
Heat Transfer Coefficient at different heat flux and flow rate

Table 5 .
Specifications of KD2 probe

Table 6 .
Accuracy of the sensors of the experimental setup (Figure10)The author declares that there is no conflict of interest in the study.