**2. Microchannel heatsink (MCHS)**

*Heat Transfer - Design, Experimentation and Applications*

Conventional channels: Dh < 43 mm Mini-channels: 0.2 mm < Dh < 3 mm Micro-channels: 10 μm < Dh < 200 μm Transitional channels: 0.1 μm < Dh < 10

*The classification of the mini and microchannels.*

**S.G. Kandlikar and W.J. Grande [7] S.S. Mehendale et al. [8]**

Conventional channels: Dh < 46 mm Compact passages: 1 mm < Dh < 6 mm Meso-channels: 0.1 mm < Dh < 1 mm Micro-channels: 1 μm < Dh < 100 μm

*The increment of studies performed on the micro-channels from the year 1996 to 2019 [15].*

indicates the capacity of this technology.

[8], which is produced in **Table 1**.

and hydraulic behavior of the M [11].

by providing an efficient heat absorbing device like MCHS. The MCHS is also used in many other applications like LED cooling, fuel cells, refrigeration, combustors, chemical industry and food industry, etc. Huge literature availability on MCHS

The categorization of the microscale channels is different from the conventional flow channels, and it is done by considering the channel's hydraulic diameter. So many classifications are available from the literature. Many authors followed the classification given by S.G. Kandlikar and W.J. Grande [7] and S.S. Mehendale et al.

The microchannel heat sink was first developed in 1981 for electronic cooling applications, which has rectangular cross-sectional channels made of silicon. In this study, the maximum thermal resistance of 0.09 0C/W was observed at the heat flux of 790 W/cm2 over the 1 cm2 area [9]. Since then, noticeable work has been done to improve the micro-channels' fluid flow and heat transfer performance by improving the channel geometry, surface roughness of the channel, channel aspect ratio, working fluid and substrate materials, etc. The thermal resistance of 0.070 C/W was achieved for the MCHS developed for thermal management of the diode laser array manufactured by Indium phosphide (InP) [10]. The hydraulic diameter and aspect ratio of the channel was proved to be has a noticeable impact on the thermal

Initially, few studies claimed that the conventional correlations and theories are not applicable for the micro and mini channels. Eventually, researchers cleared about these ambiguities and concluded that the inaccuracies in the microchannel dimensional measurements are the main reason for the deviation of the results produced from the conventional correlations. The uncertainties in experiments

**284**

**Figure 1.**

**Table 1.**

The heat sink is a heat-absorbing device that takes heat from its surroundings by the various modes of heat transfer by using working fluids. Miniaturization makes the heat sinks as efficient and compact. MCHSs have fluid flow channels in the size of microns. MCHS application is found in the high-powered density energy system with less space availability. These applications include the computer components cooling (Storage devices, CPUs and GPUs, etc.) [16], thermal management of high power density electronic components (IGBTs) [17], cooling of fuel cells [18], diode laser arrays [19], and combustors [20], etc. Electronic cooling is the major application of the MCHS. **Figure 2** represents the schematic diagram of the transistor with a liquid-cooled heat sink.

Fabrication of MCHS is the biggest hurdle to perform the experimental investigations. Laser cutting [22], dry and wet etching [23–25], micro-cutting [26], and ultrasonic micro-machining [27] are very expensive fabrication methods for MCHS. Most of the researcher's attention is on developing a new low-cost manufacturing method with good surface characteristics. Kaikan Diao, Yuyuan Zhao [28] studied the performance of the sintered Copper microchannel manufactured by a low-cost fabrication method. This study proved that the pressure drop in the sintered copper microchannel was higher than the microchannel machined conventionally and noticeably lower than the porous Copper microchannel fabricated by the Lost carbonate sintering method (LCS). Ivel L. Collins et al. [29] performed the direct-metal-laser-sintering

**Figure 2.** *Schematic diagram of the transistor with liquid-cooled heatsink [21].*

#### *Heat Transfer - Design, Experimentation and Applications*

method (DMLS) for manufacturing of two MCHS models, PMM (permeable membrane MCHS) and MMC (manifold MCHS) heat-sinks shown in **Figure 3**.

The analysis method implemented for the study of MCHS is also plays a key role in the accuracy of the study. Initially, researchers and scientists depended on expensive experimental methods only for their research but the development of numerical methods has upturned the studies on microfluidics. Novel computational fluid dynamic (CFD) techniques have been developed for accurate analysis of the MCHS. 3-dimensional simulation models give an accurate result than 2-dimensional simulation models but computational time is less for a 2-dimensional model. Similar outcomes were found in the 2D and 3D simulation model studies conducted on the

**Figure 3.**

*Images of the (a, c) manifold MCHS and (b, d) permeable membrane MCHS [29].*

**287**

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

microchannel fluid micro-mixing [30]. S. A. Si Salah et al. [31] implemented the control-volume FEM (CVFEM) to study microchannel flow, which has the advantages of both finite element method and the finite volume method. The slug flow in the microchannel of serpentine shape was studied using a Coupled-level-set and volume of fluid (CLSVOF) method, which accurately predicted heat transfer and fluid flow performance liquid–liquid 2-Phase flow [32]. J. Rostami and A. Abbassi [33] implemented the Eulerian–Lagrangian method to analyze the Al2O3-water fluid flow in the wavy channeled heat sink. Shuzhe Li et al. [34] and Zhibin Wang et al. [35] were also used the coupled level set and volume of fluid method for their study on coalescence between the moving liquid and the droplets in the microchannel. A flexible coupled-level-set and volume of fluid (flexCLV) method [36], Lattice Boltzmann method (LBM) [37, 38]. The coupled LBM [39] was also implemented for the accurate prediction of complex problems. **Figure 4** shows the schematic of

It is clear from the studies in the literature that MCHS has an eminent future in the field of thermal management of electronic equipment. The work performed on the micro-channel heat sink by Tuckerman and Pease [9] attracted the researchers towards the MCHS. Researchers and scientists have been working on MCHS to develop new ways to enhance heat transfer in micro-channels. The details of the few thermal performance-enhancing techniques developed for MCHS are produced in this section.

The microchannel geometry has a major impact on its heat transfer and fluid flow performance. The improvement of the microchannel geometry is a possible technique to decrease the pressure drop with a significant increase of the heat transfer. Various cross-sectional shapes of micro-channels used for the analysis are presented in **Figure 5**. Microchannel with a Trapezoidal-shaped cross-section has a good thermal performance than the rectangular channel [41]**.** The effect of the different parameters like aspect ratio (AR) [42], hydraulic diameter, channel spacing [31], channel width, channel height, etc., on the heat transfer behavior of

An experimental analysis performed on the rectangular microchannel with the working fluids FC770 and water proved that the critical Reynolds number (Re) increases to 2400 from 1700 with a reduction of aspect ratio (AR) to 0.25 From 1 [43]. The reduction of friction factor with increasing the AR was also noticed initially, and then it started increasing. The increase of both the Nusselt number (Nu) and the pressure loss with the channel height reduction was observed in the numerical study on MCHS with transfer channels [44]**.** The flow channel size shows a noticeable effect on the hydraulic performance as it was decreasing from the macro scale to the microscale. The effect on the hydraulic behavior of the microchannel was negligible as the space between the micro-channels decreases from 50 μm to 0.5 μm [31]**.** The Nu and Poiseuille number are found to be raised with rising the AR and side angle [45].

Some studies on MCHS have introduced the ribs, internal fins into the flow channels and changes the shape of the passage so that the area of heat transfer increased. A considerable decrease of pressure drop was noticed when the rectangular-shaped ribs and the sinusoidal cavities are provided to the MCHS [46]. In the various category of offset ribs on the channel sidewalls, the best performance was observed with the forward triangular ribs and the rectangular ribs showed the worst behavior at the Re

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

types of studies performed on the microchannel heat sinks.

**3. Thermal performance-enhancing techniques**

**3.1 Geometric improvements**

microchannel, were also studied.

#### **Figure 4.**

*Schematic of various studies on micro-channel heatsinks (MCHS) [15].*

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

microchannel fluid micro-mixing [30]. S. A. Si Salah et al. [31] implemented the control-volume FEM (CVFEM) to study microchannel flow, which has the advantages of both finite element method and the finite volume method. The slug flow in the microchannel of serpentine shape was studied using a Coupled-level-set and volume of fluid (CLSVOF) method, which accurately predicted heat transfer and fluid flow performance liquid–liquid 2-Phase flow [32]. J. Rostami and A. Abbassi [33] implemented the Eulerian–Lagrangian method to analyze the Al2O3-water fluid flow in the wavy channeled heat sink. Shuzhe Li et al. [34] and Zhibin Wang et al. [35] were also used the coupled level set and volume of fluid method for their study on coalescence between the moving liquid and the droplets in the microchannel. A flexible coupled-level-set and volume of fluid (flexCLV) method [36], Lattice Boltzmann method (LBM) [37, 38]. The coupled LBM [39] was also implemented for the accurate prediction of complex problems. **Figure 4** shows the schematic of types of studies performed on the microchannel heat sinks.
