**3.3 Nanofluid as the working fluid**

Thermo-physical properties of the Nanofluids are superior among various working fluids, so which are suitable for high heat transfer applications.

**295**

**Figure 20.**

*kg/s [75].*

**Figure 19.**

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks…*

Significant research has been done on MCHS with Nanofluids using experimental and single and multi-phase CFD models. Multi-phase methods are noticed to be more precise compared to the single-phase numerical methods [68]. The inaccuracy of the 2-phase mixture method and the 1-phase methods for 1% Nanofluid compared to the experimental results were found to be 11.39% and

Ayoub Abdollahi et al. [70] proved that the water-based SiO2 Nanofluid has the best performance among the four water-based SiO2, CuO, Al2O3, and ZnO Nanofluids. In a similar study conducted on the hexagonal channeled MCHS using various water-based Nanofluids, it was observed that Al2O3-water Nanofluid has the highest heat transfer coefficient [71]. The analysis on CuO-Water Nanofluid flow in trapezoidal channeled MCHS proved that the thermal performance of heat sink

*The variation of average nu with Reynolds for various nanofluid volume fractions [72].*

*Distribution of nanofluid thermal conductivity at the outlet with various mass Flux (i) 0.0001 kg/s, (ii) 0.0003* 

*DOI: http://dx.doi.org/10.5772/intechopen.97087*

32.6% respectively [69].

**Figure 18.** *Schematic diagram of single nozzle with convex dimple arrangement [67].*

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks… DOI: http://dx.doi.org/10.5772/intechopen.97087*

Significant research has been done on MCHS with Nanofluids using experimental and single and multi-phase CFD models. Multi-phase methods are noticed to be more precise compared to the single-phase numerical methods [68]. The inaccuracy of the 2-phase mixture method and the 1-phase methods for 1% Nanofluid compared to the experimental results were found to be 11.39% and 32.6% respectively [69].

Ayoub Abdollahi et al. [70] proved that the water-based SiO2 Nanofluid has the best performance among the four water-based SiO2, CuO, Al2O3, and ZnO Nanofluids. In a similar study conducted on the hexagonal channeled MCHS using various water-based Nanofluids, it was observed that Al2O3-water Nanofluid has the highest heat transfer coefficient [71]. The analysis on CuO-Water Nanofluid flow in trapezoidal channeled MCHS proved that the thermal performance of heat sink

**Figure 19.** *The variation of average nu with Reynolds for various nanofluid volume fractions [72].*

**Figure 20.**

*Distribution of nanofluid thermal conductivity at the outlet with various mass Flux (i) 0.0001 kg/s, (ii) 0.0003 kg/s [75].*

*Heat Transfer - Design, Experimentation and Applications*

dimple arrangement is presented in **Figure 18**.

*Schematic model of* **(a)** *free-surface jet-arrays and* **(b)** *submerged jet-arrays [66].*

*Schematic diagram of single nozzle with convex dimple arrangement [67].*

**3.3 Nanofluid as the working fluid**

has superior overall performance among the three arrangements studied and it has a high heat transfer rate and lowest pressure loss. The single nozzle with a convex

Thermo-physical properties of the Nanofluids are superior among various working fluids, so which are suitable for high heat transfer applications.

**294**

**Figure 18.**

**Figure 17.**

was enhanced by increasing the Nanofluid volume fraction with a penalty of high pumping power [72]. The variation of Average Nu with the Reynolds number (Re) is shown in **Figure 19**.

The numerical investigation on the Nanofluid-based triangular and trapezoidal grooved MCHS revealed that the water-based Al2O3 Nanofluid was the best coolant for the trapezoidal grooved MCHS [73, 74]. In a numerical study conducted on Al2O3-water Nanofluid-based MCHS, the rise of Nanoparticles' concentration at the walls and non-uniform distribution of thermal conductivity was observed with increasing Discharge [75]. **Figure 20** shows the distribution of the Nanofluid thermal conductivity at the outlet.

Few studies used the advantages of both the geometries and the Nanofluids in the MCHS. Water-based Diamond Nanofluid has the best performance among the various Nanofluids used in the analysis of the wavy channeled MCHS [76]. M.M. Sarafraz et al. [77] witnessed a 76% improvement in the heat transfer performance of MCHS with a 20% increase in the pumping power at the Re higher than 1376. The variation of the Nu obtained from the experiments and existing correlations is shown in **Figure 21**.

The efficiency of an energy system can understand by finding its entropy generation. Some studies investigated the entropy generation of the MCHS to analyze its performance using various working fluids. In the first and second law thermodynamic analysis of offset strip-fin MCHS with CuO Nanofluid as coolant, it was found that the thermal characteristics of the MCHS improved with Re but the frictional entropy generation was also increased [78]**.** The rate of entropy generation of MCHS in the flow direction concerning the number of fins is presented in **Figure 22**.

### **3.4 Magneto-hydrodynamics (MHD) influence**

MHD is an interdisciplinary subject that has been used in various engineering problems like the design of MHD pumps and flows meters and cooling of nuclear reactors etc. In the field of microfluidics, MHD pimping is very favorable because of its uncomplicated design and less power consumption [79–82]. In the starting, conventional heat

**297**

**Figure 23.**

**Figure 22.**

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks…*

exchangers with magnetic fluid are investigated to know its performance by applying it. The proportionality of friction factor and Nusselts number with the applied magnetic field was noticed in the study of the MHD effect on circular tube flow of Fe3O4-Water Nanofluid [83]. Variation of Nusselt number with Magnetic field effects is presented in **Figure 23**. The working fluid velocity was observed to be reduced with Lorentz force generated because of magnetic flux applied. The increase of the local heat transfer coefficient was witnessed experimentally when an external magnetic field was applied to the W-40 (magnetic Nanofluid) flow in a rectangular duct [84]. Few research studies about the MHD effect on micro-channels noticed the improvement of their hydraulic and thermal behavior. The decrease of the gradient of velocity at the wall with a rise in the index of Power-law flow was observed in the flow of non-Newtonian fluid flow in a microchannel under a transfer magnetic field [85]. The increment of the gradient of velocity near the wall and reduction of maximum velocity was found with the rising Hartmann number. The increase in the Joule heating and the viscous dissipation was observed to decrease the Nusselt number. Chunhong Yang et al. [86] studied the thermal behavior of an incompressible MHD flow in a rectangular microchannel by taking the combined influence of the viscous dissipation

*The entropy generation of MCHS in the flow direction with the number of fins [78].*

*The nu variation with the magnetic field (a) in the range Re, (b) along the channel [83].*

*DOI: http://dx.doi.org/10.5772/intechopen.97087*

**Figure 21.** *The variation of the nu obtained from the experiments and existing correlations [77].*

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks… DOI: http://dx.doi.org/10.5772/intechopen.97087*

#### **Figure 22.**

*Heat Transfer - Design, Experimentation and Applications*

**3.4 Magneto-hydrodynamics (MHD) influence**

*The variation of the nu obtained from the experiments and existing correlations [77].*

is shown in **Figure 19**.

shown in **Figure 21**.

thermal conductivity at the outlet.

was enhanced by increasing the Nanofluid volume fraction with a penalty of high pumping power [72]. The variation of Average Nu with the Reynolds number (Re)

The numerical investigation on the Nanofluid-based triangular and trapezoidal grooved MCHS revealed that the water-based Al2O3 Nanofluid was the best coolant for the trapezoidal grooved MCHS [73, 74]. In a numerical study conducted on Al2O3-water Nanofluid-based MCHS, the rise of Nanoparticles' concentration at the walls and non-uniform distribution of thermal conductivity was observed with increasing Discharge [75]. **Figure 20** shows the distribution of the Nanofluid

Few studies used the advantages of both the geometries and the Nanofluids in the MCHS. Water-based Diamond Nanofluid has the best performance among the various Nanofluids used in the analysis of the wavy channeled MCHS [76]. M.M. Sarafraz et al. [77] witnessed a 76% improvement in the heat transfer performance of MCHS with a 20% increase in the pumping power at the Re higher than 1376. The variation of the Nu obtained from the experiments and existing correlations is

The efficiency of an energy system can understand by finding its entropy generation. Some studies investigated the entropy generation of the MCHS to analyze its performance using various working fluids. In the first and second law thermodynamic analysis of offset strip-fin MCHS with CuO Nanofluid as coolant, it was found that the thermal characteristics of the MCHS improved with Re but the frictional entropy generation was also increased [78]**.** The rate of entropy generation of MCHS in the flow direction concerning the number of fins is presented in **Figure 22**.

MHD is an interdisciplinary subject that has been used in various engineering problems like the design of MHD pumps and flows meters and cooling of nuclear reactors etc. In the field of microfluidics, MHD pimping is very favorable because of its uncomplicated design and less power consumption [79–82]. In the starting, conventional heat

**296**

**Figure 21.**

exchangers with magnetic fluid are investigated to know its performance by applying it. The proportionality of friction factor and Nusselts number with the applied magnetic field was noticed in the study of the MHD effect on circular tube flow of Fe3O4-Water Nanofluid [83]. Variation of Nusselt number with Magnetic field effects is presented in **Figure 23**. The working fluid velocity was observed to be reduced with Lorentz force generated because of magnetic flux applied. The increase of the local heat transfer coefficient was witnessed experimentally when an external magnetic field was applied to the W-40 (magnetic Nanofluid) flow in a rectangular duct [84].

Few research studies about the MHD effect on micro-channels noticed the improvement of their hydraulic and thermal behavior. The decrease of the gradient of velocity at the wall with a rise in the index of Power-law flow was observed in the flow of non-Newtonian fluid flow in a microchannel under a transfer magnetic field [85]. The increment of the gradient of velocity near the wall and reduction of maximum velocity was found with the rising Hartmann number. The increase in the Joule heating and the viscous dissipation was observed to decrease the Nusselt number. Chunhong Yang et al. [86] studied the thermal behavior of an incompressible MHD flow in a rectangular microchannel by taking the combined influence of the viscous dissipation

**Figure 23.** *The nu variation with the magnetic field (a) in the range Re, (b) along the channel [83].*

*The entropy generation of MCHS in the flow direction with the number of fins [78].*

and the Joule heating. This analysis shows the decrease of normal velocity with increment in the Hartmann number (Ha) without any applied lateral electric field. The increasing-decreasing trend of Velocity and temperature profiles was noticed with the rising the Hartmann number under the applied lateral electric field. The generation of entropy in the MCHS was diminished as the influence of the Lorentz force generated by the injected electric current and magnetic field applied transversely [87].

### **3.5 Flow boiling in MCHS**

Flow boiling in the MCHS can vanish the heat fluxes in the range of 30 to 100 W/cm2 with the acceptable channel surface temperatures [88]. The flow boiling implemented MCHSs were used for a variety of applications, like cooling of PEM fuel cells, thermal management of the IGBTs (insulated gate bipolar transistors), refrigeration systems, etc. [89]. Most of the researchers' attention is on estimating the impact of the mass flow rate, heat flux, vapor quality, and surface characteristics on the boiling heat transfer [90, 91]. Stable flow boiling is also one of the best way to enhance the heat transfer in the MCHS. The experimental analysis done by John Mathew et al. [92] on the copper hybrid MCHS with flow boiling revealed that the local heat transfer coefficient is consistent increases with the heat flux and becomes sensitive to the heat flux. The pressure loss was also found to increase in the 2-phase flow with heat flux under all mass flow rate conditions. **Figure 24** shows the variation of upstream heat transfer coefficient in the microchannel concerning the effective heat flux. A numerical study of Yang Luo et al. [93] on manifold MCHS with subcooled two-phase flow boiling proved that heat flux and the manifold ratio significantly influence the pressure drop and thermal resistance (Rth) of the microchannel. The authors suggested that the manifold ratios should be between 1 and 2 for low-pressure drop and the better thermal performance of the manifold heatsink.

The impact of the surface characteristics on the flow boiling of regasified and deionized water micro-channel was experimentally studied and found that the characteristics of the Cu microchannel surface are transient [88]. The heat transfer coefficients of the aged Cu microchannel surface were unchanged and even enhanced

#### **Figure 24.**

*Variation of upstream HT coefficient in the microchannel with respect to the heat flux with the images of flow visualization [92].*

**299**

**4. Conclusions**

**Figure 25.**

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks…*

after proper cleaning. Hongzhao Wang et al. [94] examined the wettability patterned microchannel and homogeneous hydrophilic microchannel and found a 22% higher heat transfer coefficient for wettability patterned microchannel. The heat transfer coefficient was noticed to be improved for the wettability patterned channel with the mass flux. The transient variation of Heaters' temperature variation is shown in **Figure 25**. The experimental and simulation analysis on microchannel with 2-phase continuous boiling revealed that heat transfer was improved at the fixed heat flux with increment in mass flux but it may tend to unstable boiling [95]. For the unstable boiling in a microchannel, the oscillation amplitude was observed to be influenced by the structural parameters of the microchannel and the thermal conductivity. In the experimental investigation on 2-phase flow regimes in a microchannel, the formation of the wave on the liquid film was observed in film flow regimes [96] and the wavelength of the waves on the liquid film is depending on flow rate of the gas and liquid. Along with the MCHS performance improvement methods discussed above, a few other inventive methods were noticed in the literature. C.J. Ho et al. [97] examined the microencapsulated PCM (MEPCM) based MCHS under the sudden pulsed heat flux and disclosed that the layer of MEPCM layer not effective in controlling the temperature rise in the MCHS. At the high amplitude of heat flux pulse, the MEPCM layer has the improved cooling performance. Soumya Bandyopadhyay, Suman Chakraborty [98] investigated the thermophoretic force effect and the interfacial tension by studying Newtonian fluid dynamics in a microchannel with the consideration of temperature dependency of viscosity. Linda Arsenjuk et al. [99] investigated the slug flow in parallelized microchannel and obtained static fluid distribution with high-pressure loss. Zan Wu et al. [100] analyzed the slug flow in a square microchannel and correlated the velocity of the slug in terms of the

*Transient variation of heaters' temperature (lines represents the average temperature) [94].*

Capillary number using bulk velocity and continuous phase viscosity.

The advancements in the thermal performance enhancement methods for microchannel heat sinks are discussed so far. Each method is selected based on

*DOI: http://dx.doi.org/10.5772/intechopen.97087*

*Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks… DOI: http://dx.doi.org/10.5772/intechopen.97087*

#### **Figure 25.**

*Heat Transfer - Design, Experimentation and Applications*

**3.5 Flow boiling in MCHS**

100 W/cm2

and the Joule heating. This analysis shows the decrease of normal velocity with increment in the Hartmann number (Ha) without any applied lateral electric field. The increasing-decreasing trend of Velocity and temperature profiles was noticed with the rising the Hartmann number under the applied lateral electric field. The generation of entropy in the MCHS was diminished as the influence of the Lorentz force generated

by the injected electric current and magnetic field applied transversely [87].

Flow boiling in the MCHS can vanish the heat fluxes in the range of 30 to

implemented MCHSs were used for a variety of applications, like cooling of PEM fuel cells, thermal management of the IGBTs (insulated gate bipolar transistors), refrigeration systems, etc. [89]. Most of the researchers' attention is on estimating the impact of the mass flow rate, heat flux, vapor quality, and surface characteristics on the boiling heat transfer [90, 91]. Stable flow boiling is also one of the best way to enhance the heat transfer in the MCHS. The experimental analysis done by John Mathew et al. [92] on the copper hybrid MCHS with flow boiling revealed that the local heat transfer coefficient is consistent increases with the heat flux and becomes sensitive to the heat flux. The pressure loss was also found to increase in the 2-phase flow with heat flux under all mass flow rate conditions. **Figure 24** shows the variation of upstream heat transfer coefficient in the microchannel concerning the effective heat flux. A numerical study of Yang Luo et al. [93] on manifold MCHS with subcooled two-phase flow boiling proved that heat flux and the manifold ratio significantly influence the pressure drop and thermal resistance (Rth) of the microchannel. The authors suggested that the manifold ratios should be between 1 and 2 for low-pressure drop and the better thermal performance of the manifold heatsink. The impact of the surface characteristics on the flow boiling of regasified and deionized water micro-channel was experimentally studied and found that the characteristics of the Cu microchannel surface are transient [88]. The heat transfer coefficients of the aged Cu microchannel surface were unchanged and even enhanced

*Variation of upstream HT coefficient in the microchannel with respect to the heat flux with the images of flow* 

with the acceptable channel surface temperatures [88]. The flow boiling

**298**

**Figure 24.**

*visualization [92].*

*Transient variation of heaters' temperature (lines represents the average temperature) [94].*

after proper cleaning. Hongzhao Wang et al. [94] examined the wettability patterned microchannel and homogeneous hydrophilic microchannel and found a 22% higher heat transfer coefficient for wettability patterned microchannel. The heat transfer coefficient was noticed to be improved for the wettability patterned channel with the mass flux. The transient variation of Heaters' temperature variation is shown in **Figure 25**.

The experimental and simulation analysis on microchannel with 2-phase continuous boiling revealed that heat transfer was improved at the fixed heat flux with increment in mass flux but it may tend to unstable boiling [95]. For the unstable boiling in a microchannel, the oscillation amplitude was observed to be influenced by the structural parameters of the microchannel and the thermal conductivity. In the experimental investigation on 2-phase flow regimes in a microchannel, the formation of the wave on the liquid film was observed in film flow regimes [96] and the wavelength of the waves on the liquid film is depending on flow rate of the gas and liquid.

Along with the MCHS performance improvement methods discussed above, a few other inventive methods were noticed in the literature. C.J. Ho et al. [97] examined the microencapsulated PCM (MEPCM) based MCHS under the sudden pulsed heat flux and disclosed that the layer of MEPCM layer not effective in controlling the temperature rise in the MCHS. At the high amplitude of heat flux pulse, the MEPCM layer has the improved cooling performance. Soumya Bandyopadhyay, Suman Chakraborty [98] investigated the thermophoretic force effect and the interfacial tension by studying Newtonian fluid dynamics in a microchannel with the consideration of temperature dependency of viscosity. Linda Arsenjuk et al. [99] investigated the slug flow in parallelized microchannel and obtained static fluid distribution with high-pressure loss. Zan Wu et al. [100] analyzed the slug flow in a square microchannel and correlated the velocity of the slug in terms of the Capillary number using bulk velocity and continuous phase viscosity.

#### **4. Conclusions**

The advancements in the thermal performance enhancement methods for microchannel heat sinks are discussed so far. Each method is selected based on heatsink application, the heat flux needs to be dissipated, space availability, etc. The primary conclusions drawn from this chapter are,

