Effective Parameters on Increasing Efficiency of Microscale Heat Sinks and Application of Liquid Cooling in Real Life

Abstract

Over the past two decades, electronic technology and miniaturization of electronic devices continue to grow exponentially, and heat dissipation becomes a critical issue for electronic devices due to larger heat generation. So, the need to cool down electronic components has led to the development of multiple cooling methods and microscale heat sinks. This chapter reviewed recent advances in developing an efficient heat sink, including (1) geometry parameters, (2) flow parameters that affect the hydraulic–thermal performance of the heat sink. Also, the main goal of this chapter is to address the current gap between academic research and industry. Furthermore, commercialized electronic cooling devices for various applications are highlighted, and their operating functions are discussed, which has not been presented before.

1. Introduction

Seminal advances in microelectronics technology have driven the Integrated Circuit Topographies (ICT) revolution over the last decades. Technologies of miniaturization, fabrication, and integrated circuit/system design are three vital parameters that have underpinned this revolution and allowed continuous and on-going breakthroughs. However, the heat generated by electronic devices is always a fundamental problem that forced researchers to improve cooling systems to increase thermal efficiency. Since 85 $°\mathit{c}$ is a critical temperature for electronic devices [1], exceed each 1 °c above critical temperature causes the reduction of 5% of devices life [2]. There are several methods to cool electronic devices as working fluid that are generally divided into (i) air cooling and (ii) liquid cooling. The efficiency of heat sinks increases due to the high thermal conductivity of liquids compared to air. Also, the increasing surface-to-volume ratio in heat sink leads to higher heat dissipation and extension of the electronic device’s lifetime. Tuckerman and Pease [3] studied liquid cooling microchannel in single and multi-phase for the first time. Several parameters have also been considered to improve microchannel heat sinks efficiency, such as changing the cross-sections, patterns, manifolds, and working fluids [4].

Some technical issues have been reported, like generating hotspots and pressure drop through the microchannel for different applications. For instance, Copeland et al. [5] illustrated the impact of pressure drop and temperature gradient on system functionality. Moreover, they reported high-pressure drops (2 bars) for reaching minimal thermal resistance due to the small size of channels. Although utilizing a pump could compensate, the generated pressure drop which is used in conventional applications, using these pumps on a micro-scale is almost impossible [6]. The thermal boundary layer in convectional channels is maintained in a fully-developed state; thus, the thermal resistance increases and caused non-uniform heat transfer performance, leading to an unreliable platform and system failure. A large number of researches have been carried out to address these limitations by changing geometrical parameters and fluid flow structures in a microchannel.

In the current study, all previously reported parameters relating to enhancing the heat sink efficiency are considered. The efficient parameters on the performance of micro heat sinks are divided into two main parts, i.e., (i) Geometrical and (ii) Flow parameters. Geometrical parameters include patterns, cross-sections, and manifolds of heat sinks that the prior studies in this area are sorted and are explained in detail, and a comprehensive table is presented for each section. Also, working fluids (nano-fluids, phase change materials (PCMs) slurries, and boiling flows) are investigated as subsections of flow parameters. Besides, almost all micro heat sink applications in real life are characterized and the most significant of them, such as PCs and laptops, PCRs, gaming consoles, and data servers, are explained in detail, and other applications are listed. Finally, the suggestions and future direction of heat sink research are presented.

2. Microchannel and micro pin-fin heat sinks

The difference between macro-, mini-, and microchannels remains a lack of complete definition. However, it is fair to say that these differences can be classified into two groups; phenomenology and dimensions. Forces and phenomena play an essential role in micro-scale rather than macro-, mini scales [7]. Channel classification is based on hydraulic diameter as a simple guide to examine the desired dimensional range. Kandlikar and Grande [8] presented a general scheme based on the channel dimensions shown in Table 1.

Macrochannels	$\mathbf{D}\mathbf{>}\mathbf{3}\mathbf{mm}$
Minichannels	$\mathbf{3}\mathbf{mm}\mathbf{\ge }\mathbf{D}\mathbf{\ge }\mathbf{200}\mathbf{\mu m}$
Microchannels	$\mathbf{200}\mathbf{\mu m}\mathbf{\ge }\mathbf{D}\mathbf{\ge }\mathbf{10}\mathbf{\mu m}$
Transitional microchannels	$\mathbf{10}\mathbf{\mu m}\mathbf{\ge }\mathbf{D}\mathbf{\ge }\mathbf{1}\mathbf{\mu m}$
Transitional nanochannels	$\mathbf{1}\mathbf{\mu m}\mathbf{\ge }\mathbf{D}\mathbf{\ge }\mathbf{0.1}\mathbf{\mu m}$
Nanochannels	$\mathbf{D}\mathbf{\ge }\mathbf{0.1}\mathbf{\mu m}$

Table 1.

Channel classification [8].

In Table 1, D is the hydraulic diameter of the channel. In non-circular channels, it is recommended to use the smallest channel dimension in place of hydraulic diameter (e.g., the short side of a rectangular cross-section) [8]. Also, multiple microfluidic fabrication techniques have been developed, such as photolithography and soft lithography [9], laser cutting [10, 11], 3D printing [12, 13], microinjection molding [14] and glass etching for different applications like Point of Care (POC) and diagnosis [15, 16], microbiology [17, 18], drug delivery [19, 20, 21], oil and gas [22], micropump [23, 24], particle separation and enrichment [25, 26, 27], Organ on a chip [28, 29, 30], biosensor [31, 32, 33, 34].

Microchannel and micro pin-fin are two types of heat sinks that are used in electronic cooling systems. The microchannel heat sink consists of extended parallel channels in different cross-sections (such as rectangular, hexagonal, triangular, etc.) that coolant flow passes from channels and absorbs heat from the chip. With advances in nano/micro-manufacturing techniques, another type of heat sink used in cooling circuits is a micro pin-fin heat sink. This heat sink type consists of pin-fin arrays in different shapes (like rectangular, hexagonal, elliptic, circular, etc.) and due to the high flow mixing rate, thermal performance increases compared to the microchannel heat sink.

3. Effective parameters on the efficiency of heat sinks

The thermal/hydraulic performance of the heat sink is affected by geometrical parameters (such as the shape of the cross-section, pattern, inlet/outlet arrangement) and flow parameters (such as working fluids and boiling flow) [35]. In this section, the effective parameters on the heat sink’s thermal–hydraulic performance are presented.

3.1 Geometrical parameters

3.1.1 Patterns

Previous research indicates that changing pattern plays a fundamental role in enhancing the heat transfer rate [4]. The concept of periodic renewal of thermal boundary layers is a useful technique for enhancing heat transfer. Besides, secondary flows and fluid mixing are considered other factors for heat transfer enhancement that can be formed in pattern design.

Furthermore, research has shown that increasing heat transfer will reduce the pressure drop penalty [2, 3, 36]. Therefore, setting a balance between the heat transfer enhancement and the pressure drop penalty is required for discovering the optimum pattern design. Some relations, such as efficiency index (ɳ) and Performance Evaluation Criteria (PEC), could help to identify these crucial parameters [37].

Several works studied the impact of pattern designs on heat transfer including, periodic (wavy, zigzag, etc.) [38, 39, 40, 41, 42], serpentine [43, 44], pin-fin [45, 46], and oblique [47, 48, 49, 50] and most efficient pattern designs are summarized in Table 2.

Author	Type of heat sink	Pattern	Size of heat sink	Re/Flow rate	Heat flux/Power input
Y.Sui et al. [38]	MCHS	Wavy & Straight pattern	N.A.	100–800	${\mathrm{q}}^{″}=1.5\times {10}^6$; And $1.5\times {10}^6\mathrm{W}/{\mathrm{m}}^2$
Lin Lin et al. [51]	MCHS	Wavy	W = 10 mm; L = 14 mm	300–800	${\mathrm{q}}^{″}=100\mathrm{W}/{\mathrm{cm}}^2$
M. Khoshvaght-Aliabadi et al. [41]	MCHS	Zigzag pattern with rectangular, triangular, and circular nook	W = 20 mm L = 100 mm	10–900	N.A.
Ahmad F. Al-Neama et al. [43]	MCHS	Serpentine (SPSM, DPSM, TPSM) & Straight rectangular pattern	W = 45 mm; L = 41 mm	Flow Rate=; 0.1, 0.2 to; 1 L/min	Heat power = 100 W
Yogesh K. Prajapati [52]	MCHS	rectangular plate-fin	W = 3.7 mm; L = 15 mm	100–800	q″ = 100 to 500 (kW/m^2)
Mushtaq Ismael Hasan [53]	PFHS	Square, triangular and circular	W = 6 mm; L = 16 mm	100–900	N.A.
Zohreh Chamanroy and Morteza Khoshvaght-Aliabadi [54]	MCHS	Interrupted straight and wavy: SMHS, SMHS-SP, SMHS-WP 1, SMHS-WP 2, WMHS, WMHS-SP, WMHS-WP 1, and WMHS-WP 2	W = 1 mm; L = 100 mm	100–1000	N.A.
Lei Chai and Liang Wang [55]	MCHS	Five different configurations of ribs: rectangular, backward triangular, diamond, forward triangular and ellipsoidal	W = 0.25 mm; L = 10 mm	100–700	q″ = 106 (W/m^2)
Dawei Yang et al. [56]	PFHS	Triangle, square, pentagon, hexagon and circle geometries	W = 10 mm; L = 10 mm	Re = 2122	N.A.
Zekeriya Parlak [42]	MCHS	Zigzag, straight and wavy pattern	N.A.	Inlet velocity =; 0.5 to 5 m/s	q″ =2 $\times$106 (W/m^2)
Fatima Zohra Bakhti and Mohammad Si-Ameur [57]	PFHS	circular perforated pin fin	N.A.	100–400	Heat power; = 300 W
N. Pahlevaninejad et al. [58]	MCHS	wavy pattern with rectangular obstacles	N.A.	5, 50, 150 and 300	q″ =50000; (W/m^2)
Mir Waqas Alam et al. [59]	PFHS	triangular shape micro-pin-fin	W = 13 mm; L = 21 mm	500 to 10,000	q″ =2; (kW/m^2)

Table 2.

Summary of studies in different types of patterns.

Note. MCHS: microchannel heat sink, PFHS: pin-fin heat sink, L: Length, W: Width, N.A: Not Applicable/Not Available, SPSM: Single path serpentine microchannel, DPSM: Double path serpentine microchannel, TPSM: Triple path serpentine microchannel, SMHS: Smooth straight mini-channel heat sink, SMHS-SP: SMHS with straight pin-fins, SMHS-WP: SMHS with wavy pin-fins, WMHS: Smooth wavy mini-channel heat sink, WMHS-SP: WMHS with straight pin-fins, WMHS-WP: WMHS with wavy pin-fins.

The impact of the microchannel heat sink’s pattern on thermal performance was investigated numerically by Lin et al. [51]. They reported that due to dean vortices formation in the channel’s cross-section, the fluid mixing enhanced, and the thermal boundary layers’ thickness decreased. Therefore, wavy heat sinks had better thermal performance compared to conventional straight heat sink due to higher Nusselt number and lower thermal resistance. After Lin et al. [51], another research group, Sui et al. [38] investigated the effect of wave amplitudes in wavy microchannel shown in Figure 1. Results illustrated that with increasing the amplitude to wavelength ratio (relative waviness), the thermal performance increased compared to the straight microchannel.

Figure 1.

Top view of wavy microchannels with (a) constant wavelength, (b) decreasing wavelength, and (c) shorter wavelength [38].

Mohammed et al. [60] numerically investigated the effect of different geometric patterns (zigzag, curvy, and step) on the heat transfer characteristics. The hydraulic- thermal characteristics (temperature profile, heat transfer coefficient, pressure drop, friction factor, and wall shear stress) were compared between considered patterns. They found that the highest heat transfer coefficient and the pressure drop belong to the zigzag, wavy, and curvy pattern, respectively. The step pattern obtains the lowest heat transfer coefficient and pressure drop; however, it was still higher than the conventional straight pattern. The main reason for the heat transfer enhancement is the periodic renewal of boundary layers. These boundary layers disturb by the formation of recirculation flow around the corners in the zigzag pattern and dean vortices’ formation in the wavy and curvy patterns. Thus, the zigzag and the step were the best patterns for achieving the optimum hydraulic- thermal performance, respectively [42, 60].

Junye and Hugh Wang [61] numerically investigated the effect of different layout configurations on the flow distribution and pressure drop. They simulated six configurations, including single serpentine, multiple serpentines with two channels, multiple serpentines with three channels, multiple serpentines with six channels, straight parallel and interdigitated configurations with U-type arrangement for inlet/outlet position. They reported that less pressure drop and higher flow maldistribution (MLD) belong to a straight parallel configuration. Single serpentine had the best uniform flow distribution with a higher pressure drop, while multiple serpentine configurations had the medium pressure drop and flow MLD. Moreover, the flow MLD had decreased with the decreasing channel number. Similarly, Al-Neama et al. [43] experimentally and numerically investigated the thermal performance of different serpentine patterns with straight rectangular microchannels (Figure 2). Results showed that the highest heat transfer belonged to Single path serpentine microchannel, followed by Double path serpentine microchannel and Triple path serpentine microchannel.

Figure 2.

Actual views of a serpentine microchannel with different layout configurations: (a) single, (b) double and (c) triple path multi-serpentine [43].

Many researchers in recent years studied the impact of different pin-fin patterns in heat sinks. A heat sink’s performance can be enhanced by using different pin-fin patterns in which secondary flows and fluid mixing can be formed. The effects of using oblique fin pattern on the thermo-hydraulic performance of microchannel heat sink were studied by Yong-Jiun Le [45]. They reported that the oblique fin pattern gained a higher heat transfer performance enhancement of about 47% compared to the conventional microchannel due to the secondary flow generation and redevelopment of boundary layers.

Evaluation of thermal performance in hexagonal pin-fin heat sink was studied by S. Subramanian et al. [45]. The results revealed that the hexagonal fins achieved a higher heat transfer rate compared to the conventional straight microchannel. The significant reason for heat transfer enhancement is the formation of recirculation flow zone around the pin-fins, increasing the fluid mixing and disturbs the thermal boundary layer.

3.1.2 Cross-sections

The shape of the cross-section plays a vital role in heat sink performance. The micro pin-fin/channel cross-section can affect the flow characteristics like flow distribution, thermal resistance, secondary flow generation, maximum wall temperature, and thermal resistance, which can influence the heat transfer and pressure drop [8].

Generally, the cross-section shape is divided into two parts, (i) shape of microchannel cross-section (ii) shape of micro pin-fin cross-section; Figure 3 shows different microchannel and micro pin-fins cross-sectional shape.

Figure 3.

(a) Shape of microchannel cross-section and (b) shape of micro pin-fin cross-section.

Gunnasegaran et al. [62] numerically investigated the effect of rectangular, triangular, and trapezoidal cross-section shapes on the microchannel heat transfer characteristics. The results showed that the rectangular cross-section gains the best maximum heat transfer coefficient of about 9.65 at the maximum Reynolds number (Re = 1000); while, the triangular shape showed the lowest heat transfer coefficient (9.38). In another study, Wang et al. [63] reported that the rectangular shape had the maximum, and the triangular shape had the minimum pressure drops. The effect of several aspect ratios on microchannel heat sinks’ performance was studied numerically by Alfaryjat et al. [64]. They found that the highest heat transfer coefficient and the lowest pressure drop belong to the hexagonal and circular cross-section, respectively.

Evaluation of thermal performance in the micro-pin heat sink with various cross-section shapes was studied by Hasan [53]. The results revealed that the circular fins present a higher heat transfer rate compared to other fins. The square has the highest pressure drop, while it is the lowest for circular cross-section. Besides, in another research, Tehmina Ambreen et al. [65] numerically investigated the effect of different cross-section shapes (circular, square, and hexagonal) on the micro pin-fin heat sink thermal performance. Their results showed that the upstream fins row considerably influences the heat sink flow distribution and thermal performance. Moreover, circular fins showed the highest thermal performance (Nu_ave = 10), followed by hexagon and square fins, whereas square fins showed the smallest thermal performance values (Nu_ave = 7).

Dawei Yang et al. [56] illustrated that pin-fins with triangular cross-sections create the maximum blocking region in the fins back-side area, reducing the heat transfer rate of the heat sink increases the pressure drop. The circular cross-section had the minimum blocking region and pressure drop. Furthermore, the hexagonal shape has a flow-guided effect, which conducts the coolant into the back area; consequently, it has the best heat transfer performance.

Another evaluation of the pin-fin thermal–hydraulic performance of heat sinks belongs to Yang et al. [66], and their results showed that the sine shape’s presented better heat transfer than hydrofoil and rhombus due to high fluid mixing. Besides, the rhombus has a maximum pressure drop because the flow path was smaller than other cases, while the pressure drop is minimum in the sine shape. Furthermore, due to the minimum stagnation area around the sine pin-fin heat sink, this one obtains the highest thermal performance. In a similar study, Ambreen et al. [67] investigated the effect of different micro-pin shapes on the heat sink’s thermal performance. Results showed that the highest heat transfer rate belonged to circular pin-fin, followed by square and triangular shape. They demonstrated that the largest separation area happens behind the square and triangular pin-fins, and the circular had the least separation region, which contributes to the optimized thermal performance of the circular pin-fins (Figure 4). Table 3 summarized the different types of cross-sections.

Figure 4.

Flow streamline of (a) square, (b) circular, and (c) triangular micro pin-fin [67].

Author	Type of heat sink	Size of a heat sink	Heat flux/Power input	Type of shapes
Yanjun Zhang et al. [68]	Microchannel	W = 2 mm; L = 20 mm	q″ = 35 × 105; (W/m^2)	Rectangular, Circular, Trapezoidal
A.A. Alfaryjat et al. [64]	Microchannel	W = 22 mm; L = 12 mm	q″ = 500; (kW/m^2)	Hexagonal, Circular, Rhombus
Hamdi E. Ahmed and Mirghani I. Ahmed [69]	Microchannel	N.A.	q″ = 1000; (kW/m^2)	Trapezoidal,; Triangular,; Rectangular
Gongnan Xie et al. [70]	Microchannel	W = 35 mm; L = 35 mm	Heat power = 300 W	Rectangular
Mushtaq Ismael Hasan [53]	Micro pin-fin	W = 6 mm; L = 16 mm	N.A.	Square, Circular, Triangular
S. Subramanian et al. [45]	Micro pin-fin; Plate pin-fin	W = 12.5 mm; L = 25 mm	q″ = 32 × 104; (W/m^2)	Hexagonal fin; Plate fin
Tehmina Ambreen and Man-Hoe Kim [65]	Micro pin-fin	W = 52.80 mm; L = 125 mm	q″ = 37.2; (kW/m2)	Square, Circular, Hexagonal
Yong Jiun Lee et al. [49]	Microchannel	W = 12.7 mm; L = 12.7 mm	Heat power = 273 W	Oblique fin
Dawei Yang et al. [56]	Micro pin-fin	W = 10.3 mm; L = 10.3 mm	q″ = 144; (W/cm^2)	Rhombus, Hydrofoil, Sine
Fatima Zohra Bakhti and Mohamed Si-Ameur [57]	Micro pin-fin	N.A.	Heat power = 300 W	cylindrical perforated fins
Gagan V. Kewalramaniet al. [71]	Micro pin-fin	W = 10 mm; L = 30.4 mm	q″ = 10, 50 and 100; (W/cm^2)	elliptical pin fin with different aspect ratio

Table 3.

Summary of some studies in a different type of cross-sections.

Note. L: Length, W: Width, and N.A: Not Applicable/Not Available.

According to results, a microchannel with a rectangular cross-section presented maximum performance compared to other microchannel cross-sections. The shape with no sharp corners obtains higher performance in micro pin-fins. It is hard to conclude precisely the best cross-section because the applied conditions play a significant role in micro pin-fin/channels heat sink performance.

3.1.3 Manifolds

Designing the manifolds is another primary geometrical parameter that researchers focus on to achieve a high-performance heat sink. Studying manifolds can be classified into three categories, including (a) location of inlet and outlet, (b) fluid inlet and outlet configuration (horizontal or vertical), and (c) header shape types. Some differences between experimental and theoretical results have been reported due to the maldistribution (MLD) in microchannel’s branches and forming non-uniform temperature distribution in the edges of multiple microchannels. On the other hand, no difference was observed in single microchannels’ results [72, 73, 74, 75]. So, it can be concluded that an essential goal of studying manifolds is to achieve uniform flow and temperature distribution and remove hot spots for obtaining optimal performance.

Anbumeenakshi and Thansekhar [76] experimentally examined the effect of header shapes and inlet configurations in flow MLD in a rectangular microchannel heat sink (Figure 5). Results illustrated that trapezoidal and triangular types showed better flow uniformity at low flow rates. Also, the rectangular header improved flow MLD at high flow rates.

Figure 5.

Different header shape and inlet configurations. (a) Trapezoidal-inline. (b) Rectangular-inline. (c) Triangular-inline. (d) Trapezoidal-vertical. (e) Rectangular-vertical. (f) Triangular-vertical [76].

Xia et al. [77] analyzed the effects of three inlet and outlet flow arrangements (I, C, and Z-type), as well as header shapes (triangular, trapezoidal, and rectangular). The results illustrate that the I-type arrangement generated a uniform flow distribution compared to other configurations. Similarly, the rectangular header shape produced better flow uniformity than other headers. Critical parameters for flow distribution in the manifold are summarized in Table 4.

Author	Parameters	Inlet type	Outlet type	Max MLD	Min MLD
A.B. Datta et al. [78]	U/Z/Mixed-type	H/H/H	H/H/H	N.A.	Mixed
S.S. Sehgal et al. [79]	P/U/S-type	V/H/H	V/H/H	N.A.	S-type
Chi-Chuan wang et al. [80]	U/Z-type	H/H	H/H	Z-type	U-type
Junye Wang and Hualin Wang [81]	U/Z-type	H/H	H/H	Z-type	U-type
Guodong Xia et al. [77]	Tri./Trp./Rec. header shape & C/I/Z-type	V/V/V & V/V/V	V/V/V & V/V/V	Tri. & Z-type	Rec. & I-type
C.Anbumeenakshi et al. [76]	Rec./Trp./Tri. header shape	H/H/H & V/V/V	H/H/H & V/V/V	Rec. in low flow rate & Trp./Tri. In high flow rate	Rec. in high flow rate & Trp./Tri. in low flow rate
Chun-Kai Liu et al. [82]	Type of entrance flow	A: Side & B: front	A: Side & B: front	A	B
Kevin P. Drummond et al. [83]	hierarchical manifold	V	V	N.A.	N.A.
Wang Yabo et al. [84]	location of the inlet and outlet	V	V	N.A.	N.A.

Table 4.

Review literature about manifolds influence on flow distribution.

Note. H: Horizontal, V: Vertical, Min: Minimum, Max: Maximum, Rec: Rectangular, Trp: Trapezoidal, Tri: Triangular, MLD: Maldistribution, and N.A: Not Applicable/Not Available.

In order to reach the optimal flow distribution, maldistribution should be reduced along the microchannel. In most cases, Rectangular header shapes with vertical configurations cause low MLD compared to horizontal ones, and I-type has a symmetrical flow distribution that attracts most of the researchers for different applications.

3.2 Flow parameters

Flow parameter is another crucial parameter in the heat sinks thermal performance and according to the latest research, flow parameters can be classified into two categories: (1) working fluids and (2) Boiling flow.

3.2.1 Working fluids

The right selection of coolant fluid is a critical factor in removing the generated heat and according to literature, the air-cooling systems have received more attention in the past. However, with advances in electronic technology and the miniaturization of devices, conventional (air) cooling systems cannot remove more than 100 W/cm² generated heat by electronic devices. Therefore, liquid cooling such as water due to high specific heat capacity, high thermal conductivity, and more availability is considered more than ever [85]. Nowadays, with advances in nanotechnology, researchers classified working fluids into two categories, (1) nanofluids and (2) phase change material (PCM) slurries. They reported that improved thermophysical properties and higher thermal conductivity could be the advantages of these liquid coolants [86].

3.2.1.1 Nanofluids

Since fluids conductivity plays a vital role in the heat transfer from a solid surface to a fluid domain, conventional heat transfer coolant such as water, ethylene glycol, and paraffin have low thermal properties compared to metals and even metal oxides [87]. The use of nanoparticles is an effective method for modifying the heat transfer properties of fluids. Masuda et al. [88] were the first to study changes in the thermal conductivity and dynamic viscosity of base liquids with the additional fine metallic and non-metallic oxide particles.

The nanofluid and PCM’s thermodynamical properties are defined based on the base fluid (i.e., DI water) [89].

Due to nanotechnology nanofluids advancement, different types of nanoparticles, nanotubes, and various distributed sizes were developed to investigate the stability and of nanofluidic during the cooling process [90].

3.2.1.2 Phase change material (PCM)

Advanced liquid coolants such as PCM are reported as effective substitutions for conventional coolants to enhance the heat transfer rate of microchannel heat sinks [91]. Furthermore, using phase change material (PCM) improves the coolants’ thermophysical properties using the latent heat of melting.

PCM slurries are created by adding micro/nano encapsulated PCM particles to the base fluid (water, ethylene glycol, and paraffin). The PCM reveals a higher heat transfer rate when the PCM particles undergo a phase change transition [92, 93, 94, 95]. One of the disadvantages of using nanofluids and PCM slurries is the higher viscosity than the base fluid, which imposes high pumping power on the system. Therefore, establishing a balance between heat transfer enhancement and pressure drop penalty is essential to distinguish the optimum advanced coolant [89]. Some of the significant nanofluids and PCM slurries used as working fluids are listed in Table 5.

Author	Phase one (base fluid)	Phase two (particles)
K.S. Suganthi et al. [96]	Ethylene glycol and water	ZnO as the nanoparticles
Zhou. Nianyong et al. [97]	Water	—
A.M. Bayomy et al. [98]	Water	—
Mingoo Choi and Keumnam Cho [99]	Water	5% Paraffin slurry as PCM
Bahram Rajabifar [86]	DI water	n-Octadecane as PCM and; Alumina as nano particles
Jasim M. Mahdi and Emmanuel C. Nsoforet [100]	Water	RT82 as a PCM and; Al2O3 as the nanoparticles
Lisi Jia et al. [101]	Water	TiO2 as the nanoparticles
Thaklaew Yiamsawas et al. [102]	Ethylene glycol and water	TiO2 and; Al2O3 as the nanoparticles
Min Li [103]	Paraffin	Nano-graphite
Hamideh Sardarabadi et al. [104]	Water	Multi-walled carbon nanotubes
O. Pourmehran et al. [95]	Water	CuO as the nanoparticles
Arash Karimipour et al. [105]	Water	Al2O3 and AgO as the nanoparticles
Vivek Kumar and Jahar Sarkar [106]	Water	Al2O3–MWCNT

Table 5.

Different types of working fluids used in previous investigations.

3.2.2 Boiling flow

From the cooling performance perspective, two-phase flow boiling in microchannel heat sinks is more efficient than its single-phase equivalent. A temperature lower than the critical temperature and the coolant’s boiling points provide sufficient factors for increasing heat transfer rate. Due to the nature of boiling and turbulent flow in the microchannel, while requiring a low rate of coolant flow and maintaining the wall temperatures relatively uniform, the boiling heat transfer coefficient is much higher than conventional systems [107]. The main problems in this type of system are higher pressure drop and instability of the system. Due to the biphasic flow inside the channels and the creation of larger bubbles, the higher pressure drop in the system may occur because of the channels’ poor design.

Numerous studies have been conducted to explore the convection heat transfer characteristics of two-phase flow boiling in micro pin-fin/channel heat sinks in recent years. Wei Wan et al. [108] experimentally examined the effect of cross-section shape on flow boiling characteristics of micro pin-fin heat sinks. Four types of staggered micro pin-fins with different cross-section shapes, i.e., square, circular, diamond, and streamline, were tested in this study. Results showed that the square shape presents the higher boiling heat transfer, followed by circular and streamlined ones. The diamond micro pin-fins presented the smallest pressure drop, while their main problem is instability at moderate to high heat fluxes. The streamline micro pin-fins presented the largest two-phase pressure drop. Besides, the square and circular micro pin-fins showed their superiority in reducing two-phase flow instabilities. Matthew Law and Poh-Seng Lee [49] conducted an experimental study of flow boiling heat transfer and pressure characteristics in straight-finned and oblique-finned microchannels. They reported that the oblique-finned microchannels’ thermal performance is higher than straight-finned ones due to the increase in the density of bubbles generated in the convective boiling regime. The high pressure-drop in oblique-finned microchannels causes a sudden change in the flow orientation, where the fluid is being forced to flow through secondary channels. The pressure drop fluctuations in the oblique-finned pattern are much lower compared to the straight-finned; consequently, the pressure instabilities in the oblique-finned microchannels are relatively smaller than the straight-finned microchannels.

4. Applications of heat sinks in real life

Today, we can observe all levels of technological devices in everyday life, such as smartphones, gaming consoles, PCs, and laptops, etc. Companies changed many parameters to achieve the best performance of devices, including decreasing thickness, enhancing GPU and CPU numbers, and changing the appearance [109].

Based on the last researches, with the increasing number of microprocessors, the heat generation is intensified; thus, heat dissipation becomes a critical issue for thermal researchers. The manufacturer’s primary purpose is to achieve the best performance of devices by minimizing device and heat sink size and increasing the heat transfer rate utilizing the different items such as changing the geometrical parameters and working fluids.

In this section, some commercial electronic cooling systems and their operating principles are discussed. The purpose of this section is to establish a link between the scientific and commercial platforms. All effective parameters discussed in previous sections are required to fabricate optimum microchannel/micro pin-fin heat sink.

4.1 Computers and laptops

High-performance computing is the ability to process data and perform complex calculations at high speeds. One of the most common questions any manufacturer receives from customers is, “Why does my laptop or computer generate so much heat?” Heat is a normal byproduct of computer operation; for instance, a high-performance computer generates considerable heat than a lower performance computer. So, if the computer cannot disburse its heat, it may overheat, and device life will reduce. Therefore, this is a significant challenge for the manufacturer to solve. A heat sink is a thermally conductive device placed over a CPU or GPU to absorb some generated heat. Faster and multi-core processors require more high-efficiency heat sinks to keep their temperatures within acceptable levels. Nowadays, some companies professionally work in cooling system designing fields.

Asus [110], Alphacool [111], CoolIT Systems [112], Cool Innovations Inc. [113], Xtreme Performance Gear (XPG) [114], Coolermaster [115], Antec [116], Swiftech [117] and Thermaltake [118] are popular companies that work in cooling field. According to their operating principles, all companies used water as a cooling fluid. Figure 6 shows some of the micro pin-fin/channel heat sinks manufactured by cooling companies.

Figure 6.

(a) Alphacool, (b) Swiftech, (c) Thermaltake, (d) and (e) cool innovations Inc.

As shown in Figure 6a–c, some companies use a straight pattern microchannel, and others use a pin-fin pattern (presented in Figure 6d and e). Although complex patterns increase the heat transfer rate, they cause a significant pressure drop. Some companies use different patterns without considering the pressure drop penalty, such as a micro pin-fin pattern to enhance heat sink performance.

According to Section 3.1.3, fluid and temperature MLD are decreased by using I-type flow arrangements and vertical inlet/outlet type. It concludes that the vertical inlet/outlet and I-type arrangements are implemented in commercialized products. Some of the heat sinks are made by several manufacturers and use vertical inlet/outlet type. Asus ROG GX700 is the first liquid-cooled gaming laptop in the world and the liquid coolant is circulated around the heat-generating components like GPU by pumps. The coolant then heads back to the cooling module, where two heat sinks (radiators) help dissipate heat, that is shown in Figure 7a.

Figure 7.

(a) Asus ROG GX700 cooling system [110], (b)PlayStation 5 (PS5) heat sink type [119], (c) CPU RX3 [112].

4.2 Gaming consoles

Many consoles were designed with cooling fans in the past, but due to advances in thermal engineering, novel consoles are designed with liquid cooling fins and heat sinks to keep the internal components safe from high generated heat. PlayStation 5 (PS5) is the new video game console developed by Sony [119] Interactive Entertainment that utilized a heat sink with a hard copper plate with aluminum dissipation fins, shown in Figure 7b. The generated heat from the chips was conducted into the heat sink, which used a heat pipe to move the heat across the fins.

4.3 Data server

High-performance servers are specially designed to handle large computational loads with fast data processing however, to reach higher speeds, the power dissipation of high-performance CPUs was challenging [112].

Due to the increase in generated heat by high-performance servers, air cooling systems could not dissipate this generated heat because of their lower thermal efficiency. Therefore, new methods of cooling (i.e., liquid cooling) should be tested. Coolit system [112] is one of the companies working in the data server cooling field that utilized Coolant Distribution Unit (CDU) instead of high failure rate components such as internal pumps to circulate coolant, see in Figure 7c.

4.4 Other applications

Wherever cooling is required, liquid cooling capabilities can be used to improve the system’s thermal performance instead of traditional cooling methods. Liquid cooling systems are widely used to dissipate the heat generated by process operations in many applications, such as Aerospace systems, All-in-One devices, Food Industry instruments, and Biology fields.

In aerospace fields, thermal engineers have a major concern for keeping the plane at a steady temperature while avoiding ice buildup problems and protecting sensitive electronic components from extreme heat and cold. For that reason, companies should manufacture a range of heat sinks to keep planes and electronics components at optimal performance.

As shown in Figure 7a, an external port is required to cool down the All-in-one devices such as laptops and tablets, which may cause problems in the transportation and thermal performance of these devices. New liquid cooling methods can be used in these devices to optimize performance and facilitate transportation. It uses a small micro pump to recirculate a coolant in the integrated closed-circuit heat sink to dissipate heat away. Finally, heated liquid flows through the peltiers (Thermoelectric modules) to cool down and return to the circuit.

In traditional Food Industry fields, the lineup of food processing chillers works with air-cooled systems. Air-cooled food processing chillers use ambient air to dissipate heat from food cooling processes, which have low efficiency. New liquid cooling systems chillers use water from an external cooling tower to remove heat from food processes. These systems are long-lasting, quiet, and feature energy-efficiency properties. These cooling devices are best suited for medium temperature food processes, such as cheese, meat, and sauce production; potable liquid cooling systems tend to be economical solutions that deliver excellent cooling efficiency.

In biological fields, cooling devices are a common tool used in research labs. A thermal cycler (also known as a PCR device) is a laboratory instrument that facilitates DNA amplification through the polymerase chain reaction. PCR typically requires 20 to 35 cycles comprising two to three temperature steps. In the past, air-coolers used to control the heating and cooling process; by developing in the thermal engineering field, liquid cooling methods can be involved in thermal cycling to optimize the thermal efficiency and increase the number of tests per unit time [120].

5. Future research directions

Finally, according to the reviewed papers and commercialized devices, some research gaps recognized and the following directions are suggested:

1. Combination of micro pin-fin and oblique patterns to obtain novel patterns that may increase thermo-hydraulic performances

2. Investigation of hybrid nanofluids and PCMs in commercialized heat sinks can be an exciting field

3. Study of flow containing phase change particles and hybrid nanofluid within microchannels

4. Using the hybrid pattern (micro pin-fin + microchannel) in commercialized heat sinks for increased thermal performance.

5. Using jet impingement heat sink that can remove high heat fluxes

6. Using liquid cooling in small digital devices such as tablets and cellphones

6. Conclusion

Liquid cooling techniques in heat sinks have been using to increase the heat transfer rate in electronic devices. The performance of the heat sink is affected by some parameters. Geometry parameters (shape of the cross-section, pattern, manifolds) and flow parameters (working fluids and boiling) are the most common methods for enhancing the thermal–hydraulic performance of micro pin-fin/microchannel heat sink. Effective parameters are divided into two parts Geometry parameters and Flow parameters.

1. Flow parameters:

   1. pattern: It was found that changing the channel flow path leads to generating the phenomena such as secondary flows in the channel cross-section that result in improving thermal performance. The previous data demonstrated, the geometric parameters play a significant role in heat sinks’ performance

   2. Cross-section: rectangular cross-section presents maximum performance compared to other microchannels heat sinks. While in micro pin-fins, it is impossible to conclude precisely the best cross-section due to the different operating conditions.

   3. Manifold: it concludes that flow and temperature maldistribution should be reduced along the microchannel to reach the ideal flow distribution for obtaining optimal performance. Also, other parameters are crucial as well, such as header shape, inlet configurations (vertical and horizontal flow inlet/outlet), inlet/outlet arrangements (I, Z, C, S, V, and U-types), location of inlet/outlet.

2. Flow parameters:

   1. Working fluid: since fluids conductivity plays a role in the convective heat transfer, conventional heat transfer coolant such as water, ethylene glycol, and paraffin have low thermal properties compared to metals and even metal oxides. Many studies reported that the heat transfer capability could be further enhanced by using nanofluids (ZnO, Al2O3, TiO2, etc.) and PCM (Paraffin slurry, n-Octadecane, etc.) due to their better heat transfer characteristics.

   2. Boiling: flow boiling heat transfer in microchannels has been a subject of wide interest due to its ability to dissipate high heat fluxes; however, using this method still has critical challenges.