**3. Thermal performance-enhancing techniques**

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.

#### **3.1 Geometric improvements**

*Heat Transfer - Design, Experimentation and Applications*

method (DMLS) for manufacturing of two MCHS models, PMM (permeable mem-

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

brane MCHS) and MMC (manifold MCHS) heat-sinks shown in **Figure 3**.

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**Figure 4.**

**Figure 3.**

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

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

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 microchannel, were also studied.

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

**Figure 5.** *Various cross-sections of micro/mini channels used for studies [40].*

less than 350 [47]**.** The increase of heat transfer was also observed by providing the internal fins with the increase of pressure drop, as a cumulative effect, the microchannel performance was increased. The proximity from the wall of the large row of pin fins showed the greatest effect on the velocity field, distribution of flow, temperature distribution, and streamline structure. As the gap between the pin-fins increases, the heat transfer is noticed to increase first and then decrease. The fin structured microchannel with equal gap and diameter shows better thermal performance [48]. A schematic model of finned MCHS is produced in **Figure 6**. The effect of proximity from the sidewall on the thermal performance is represented in **Figure 7**.

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**Figure 9.**

**Figure 7.**

**Figure 8.**

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

The large heat transfer (HT) enhancement was found in the periodically converg-

ing and the diverging and periodically expanded and constrained MCHS. 46.8– 160.2% improvement in HT was noticed in the converging and the diverging MCHS (**Figure 8**) [49]**.** 50–117% improvement of heat transfer was found in the periodically expanded and constrained MCHS (**Figure 9**) in the range of Re from 150 to 820 [50]**.** An experimental study on periodic jetting and throttling MCHS (**Figure 10**) has concluded that the mean temperature and maximum temperature in the throttling

*The influence of proximity from the sidewall on the thermal performance of MCHS [48].*

*Physical model of periodic converging–diverging microchannel [49].*

*Design of the periodic expanded and constrained MCHS [50].*

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

**Figure 6.** *Schematic model of finned MCHS,* **(a)** *physical model and* **(b)** *boundary conditions [48]***.**

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

#### **Figure 7.**

*Heat Transfer - Design, Experimentation and Applications*

*Various cross-sections of micro/mini channels used for studies [40].*

less than 350 [47]**.** The increase of heat transfer was also observed by providing the internal fins with the increase of pressure drop, as a cumulative effect, the microchannel performance was increased. The proximity from the wall of the large row of pin fins showed the greatest effect on the velocity field, distribution of flow, temperature distribution, and streamline structure. As the gap between the pin-fins increases, the heat transfer is noticed to increase first and then decrease. The fin structured microchannel with equal gap and diameter shows better thermal performance [48]. A schematic model of finned MCHS is produced in **Figure 6**. The effect of proximity

from the sidewall on the thermal performance is represented in **Figure 7**.

*Schematic model of finned MCHS,* **(a)** *physical model and* **(b)** *boundary conditions [48]***.**

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**Figure 6.**

**Figure 5.**

*The influence of proximity from the sidewall on the thermal performance of MCHS [48].*

The large heat transfer (HT) enhancement was found in the periodically converging and the diverging and periodically expanded and constrained MCHS. 46.8– 160.2% improvement in HT was noticed in the converging and the diverging MCHS (**Figure 8**) [49]**.** 50–117% improvement of heat transfer was found in the periodically expanded and constrained MCHS (**Figure 9**) in the range of Re from 150 to 820 [50]**.**

An experimental study on periodic jetting and throttling MCHS (**Figure 10**) has concluded that the mean temperature and maximum temperature in the throttling

**Figure 8.**

*Physical model of periodic converging–diverging microchannel [49].*

**Figure 9.** *Design of the periodic expanded and constrained MCHS [50].*

#### **Figure 10.**

*Physical model of* **(a)** *jetting MCHS* **(b)** *throttling MCHS [51].*

MCHS are less than the jetting MCHS with huge pressure loss [51]. The heat transfer rate in the trapezoid cross-sectional MCHS with grooved structure was noticed to be improved by 28% because of the breaking and regeneration of the thermal and hydrodynamic boundary layer [41]**.**

Huan-ling Liu et al. [52] developed new annular MCHS designs, MRNH and MRSH presented in **Figure 11,** and concluded that the consistency of the substrate temperature of the interleaved structure was better than the sequential structure. The increase of the total thermal resistance was noticed with rising the slant angle in the MRSH design. The variation of the average Nu with the pumping power is represented in **Figure 12**.

Some of the researchers [53–55] investigated the effect of the channel surfaceroughness on the thermal performance of MCHS. Their work disclosed that the HT in MCHS was augmented for rough-surfaced channels and its effect is very significant at high Re. Yan Ji et al. [56] analyzed the low Knudsen number (Kn) gas

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**Figure 13.**

*Variation of nu with the relative roughness [56].*

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

flow in rough channels and observed a decrease of local heat flux with increasing the relative roughness for rarefied and compressible flow. The variation Nu with the

Secondary flow in MCHS is an effective method to reduce the pressure drop, which is a major limitation in the above-discussed models. One of the secondary flow MCHS models is shown in **Figure 14**. The maximum enhancement in heat transfer in secondary flow MCHS was obtained by optimizing the ratio of the parameters of secondary channel width to the main channel width (), the ratio of secondary channel half-pitch to the main channel width (), and tangent value of

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

roughness is shown in **Figure 13**.

**Figure 12.**

the angle of the secondary channel () [57].

*The variation average nu with pumping power at three slant angles [52].*

**Figure 11.** *Schematic design of MCHS, (a) MRNH (b) MRSH configuration [52].*

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

**Figure 12.** *The variation average nu with pumping power at three slant angles [52].*

flow in rough channels and observed a decrease of local heat flux with increasing the relative roughness for rarefied and compressible flow. The variation Nu with the roughness is shown in **Figure 13**.

Secondary flow in MCHS is an effective method to reduce the pressure drop, which is a major limitation in the above-discussed models. One of the secondary flow MCHS models is shown in **Figure 14**. The maximum enhancement in heat transfer in secondary flow MCHS was obtained by optimizing the ratio of the parameters of secondary channel width to the main channel width (), the ratio of secondary channel half-pitch to the main channel width (), and tangent value of the angle of the secondary channel () [57].

**Figure 13.** *Variation of nu with the relative roughness [56].*

*Heat Transfer - Design, Experimentation and Applications*

hydrodynamic boundary layer [41]**.**

*Physical model of* **(a)** *jetting MCHS* **(b)** *throttling MCHS [51].*

**Figure 10.**

MCHS are less than the jetting MCHS with huge pressure loss [51]. The heat transfer rate in the trapezoid cross-sectional MCHS with grooved structure was noticed to be improved by 28% because of the breaking and regeneration of the thermal and

Huan-ling Liu et al. [52] developed new annular MCHS designs, MRNH and MRSH presented in **Figure 11,** and concluded that the consistency of the substrate temperature of the interleaved structure was better than the sequential structure. The increase of the total thermal resistance was noticed with rising the slant angle in the MRSH design. The

Some of the researchers [53–55] investigated the effect of the channel surfaceroughness on the thermal performance of MCHS. Their work disclosed that the HT in MCHS was augmented for rough-surfaced channels and its effect is very significant at high Re. Yan Ji et al. [56] analyzed the low Knudsen number (Kn) gas

variation of the average Nu with the pumping power is represented in **Figure 12**.

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**Figure 11.**

*Schematic design of MCHS, (a) MRNH (b) MRSH configuration [52].*

**Figure 14.** *Design of* **(a)** *secondary flow MCHS* **(b)** *computational domain [57].*

A novel MCHS model namely, permeable membrane MCHS (PMM) and manifold microchannel heatsink (MMC) manufactured by Direct metal laser sintering method using an aluminum alloy was studied experimentally and noticed the better performance of the PMM heat sink [29]. **Figure 3** shows the images of PMM and MMC heat sinks.

#### **3.2 Jet impingement arrangement**

A microchannel heat sink with a jet impingement arrangement is an active heat sink with high heat transfer coefficients. Jet impingement in the flow of a fluid using liquid coolants produces very high heat transfer coefficients and it is more significant in the stagnation region [58–60]. Substantial work has been performed on the jet impingement, with various working fluids, at various nozzle configurations, standoff distances, and lengths. Jet impingement in heat sinks shows uniform temperature distribution for both micro-scale and macroscale applications. Microscale jet impingement is most suitable for dissipating the heat from high-powered electronic systems [61]. Hybrid MCHS with micro jet impinging developed for photovoltaic solar cell cooling was enhanced the solar cell electrical efficiency by 39.7% [62]**.** The numerical study conducted on the hybrid MCHS with a slot-jet module and various channel cross-sections revealed that the trapezoidal channel shows the better cooling effect [63]**.** The decreasing order of the pressure drop in the various channels at the fixed flow rate was circular cross-section, a trapezoidal cross-section, and rectangular cross-section. The rectangular channel was not favorable for impingement jet to produce vorticities, so it has a low-pressure loss. Afzal Husain et al. [64] modeled a new hybrid MCHS with impingement and pillars (**Figure 15**). It was noticed that the MCHS model with the low jet pitch to diameter ratio produces a greater heat transfer coefficient. The hybrid MCHS design with the ratio of standoff (distance between jet impingement surface and nozzle exit) to the diameter of the jet close to 2 and 3 results in low thermal resistance and pumping power.

P. Naphon et al. [65] applied the ANN model (**Figure 16**) of the Levenberg– Marquardt Backward propagation (LMB) training algorithm and Computational

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**Figure 16.**

**Figure 15.**

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

fluid dynamics to study the jet impingement of Nanofluids in the MCHS. The maximum deviation of the predicted results from the measured data was found to be 1.25%. With increasing the nozzle level, the heat transfer from the heat sink module to Nanofluid was tended to decrease, which causes the high fins tip temperature. Generally, there are two different jet impingement arrangements: the free-surface and submerged jet-arrays [66]**.** The schematic model of the free surface

The heat transfer was also enhanced effectively by introducing various shapes of dimples on the HT surface with impinging jets [67]**.** Convex dimple arrangement

and submerged jet arrays are produced in **Figure 17**.

*Optimal ANN model proposed by P. Naphon et al. [65].*

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

*Schematic model of the hybrid MCHS with pillars and jet impingement [64].*

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

#### **Figure 15.**

*Heat Transfer - Design, Experimentation and Applications*

*Design of* **(a)** *secondary flow MCHS* **(b)** *computational domain [57].*

A novel MCHS model namely, permeable membrane MCHS (PMM) and manifold microchannel heatsink (MMC) manufactured by Direct metal laser sintering method using an aluminum alloy was studied experimentally and noticed the better performance of the PMM heat sink [29]. **Figure 3** shows the images of PMM and

A microchannel heat sink with a jet impingement arrangement is an active heat sink with high heat transfer coefficients. Jet impingement in the flow of a fluid using liquid coolants produces very high heat transfer coefficients and it is more significant in the stagnation region [58–60]. Substantial work has been performed on the jet impingement, with various working fluids, at various nozzle configurations, standoff distances, and lengths. Jet impingement in heat sinks shows uniform temperature distribution for both micro-scale and macroscale applications. Microscale jet impingement is most suitable for dissipating the heat from high-powered electronic systems [61]. Hybrid MCHS with micro jet impinging developed for photovoltaic solar cell cooling was enhanced the solar cell electrical efficiency by 39.7% [62]**.** The numerical study conducted on the hybrid MCHS with a slot-jet module and various channel cross-sections revealed that the trapezoidal channel shows the better cooling effect [63]**.** The decreasing order of the pressure drop in the various channels at the fixed flow rate was circular cross-section, a trapezoidal cross-section, and rectangular cross-section. The rectangular channel was not favorable for impingement jet to produce vorticities, so it has a low-pressure loss. Afzal Husain et al. [64] modeled a new hybrid MCHS with impingement and pillars (**Figure 15**). It was noticed that the MCHS model with the low jet pitch to diameter ratio produces a greater heat transfer coefficient. The hybrid MCHS design with the ratio of standoff (distance between jet impingement surface and nozzle exit) to the diameter of the jet close to 2 and 3 results in low thermal resistance and

P. Naphon et al. [65] applied the ANN model (**Figure 16**) of the Levenberg– Marquardt Backward propagation (LMB) training algorithm and Computational

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pumping power.

MMC heat sinks.

**Figure 14.**

**3.2 Jet impingement arrangement**

*Schematic model of the hybrid MCHS with pillars and jet impingement [64].*

fluid dynamics to study the jet impingement of Nanofluids in the MCHS. The maximum deviation of the predicted results from the measured data was found to be 1.25%. With increasing the nozzle level, the heat transfer from the heat sink module to Nanofluid was tended to decrease, which causes the high fins tip temperature. Generally, there are two different jet impingement arrangements: the free-surface and submerged jet-arrays [66]**.** The schematic model of the free surface and submerged jet arrays are produced in **Figure 17**.

The heat transfer was also enhanced effectively by introducing various shapes of dimples on the HT surface with impinging jets [67]**.** Convex dimple arrangement

**Figure 17.** *Schematic model of* **(a)** *free-surface jet-arrays and* **(b)** *submerged jet-arrays [66].*

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