A review of single flow, flow boiling, and coating microchannel studies

1 Introduction

The article has three main sections: single-flow, flow-boiling, and coating microchannel studies. In each section, the authors comprehensively review the relevant literature, summarizing the essential findings and highlighting areas where further research is needed. In the section on single-flow microchannel studies, the authors discuss studies investigating fluid flow and heat transfer characteristics for microchannels when a single fluid is flowing through them. The flow boiling microchannel studies section focuses on research investigating microchannels’ heat transfer and fluid flow characteristics when boiling occurs. Finally, the section on coating microchannel studies reviews research that has examined the effects of coatings on the heat transfer and fluid flow characteristics of microchannels. Overall, the article provides a valuable resource for researchers in microchannel heat transfer and fluid flow and engineers and designers working on developing microchannel-based devices and systems [1].

According to Kandlikar [2], when single-phase flow is employed in microchannels, heat dissipation, a crucial design factor for small heat exchangers, can increase to as much as 10 MW/m². Additionally, there is a significant pressure decrease and an uneven temperature distribution. On the other hand, flow boiling may achieve a significantly higher heat transfer rate with improved axial temperature regularity, low mass flux, and low-pressure drop or less pumping power. Furthermore, some concerns include flow instability, significant wall superheat at a start for nucleated boiling, early drying-out, and a lack of knowledge of the microchannel flow boiling features that hinder the microchannel heat sink from being widely used. Because of the aforementioned factors, this section critically analyzes the single-phase flow, flow boiling works, and microchannel augmentation methods.

2 Heat transfer of single-phase micro-channel

In the single phase, many investigators tested flow and heat transmission in microchannels. It is explained why some of the findings contradict the conventional theory. This could be due to various factors influencing the experimental results, including microchannel surface roughness, temperature-dependent physical characteristics, surface physical effects, measurements errors, and fabrication processes. It is unknown that elements influence microchannels’ flow and heat transfer conduct. Once employing traditional fabrication methods, surface roughness may be a significant issue affecting heat and flow transmission in microchannels, as shown in Table 1 [3]. They used deionized (DI) water as the working fluid in the circular micro pipes to study the effects of the substrate materials with varied roughness through the flow parameters. Stainless steel and fused silica micropipes with 1.36 and 1.7% relative roughness were utilized. The friction factor in the laminar area was discovered to be larger than the conventional theory expected. The findings of this experiment showed that laminar-to-turbulent flow happened between 300 and 1,500. According to the authors, surface roughness was thought to influence the change significantly. However, prior knowledge of the effect on thermo-hydraulic properties was required for the limitation in heat exchanger sizing [4]. A number of researchers have examined single-phase microchannel heat exchangers’ thermo-hydraulic performance like Qu and Mudawar [5], as shown in Figure 1.

Figure 1

Test module construction.

Investigated and numerical studies examined pressure drop and heat transfer in microchannel single-phase laminar flow. Traditional Navier–Stokes equations for microchannels were shown to have good agreement between observations and numerical predictions. Other researchers also researched the influence of hydraulic diameter on thermal performance [6]. This experiment investigated the heat transmission characteristics of single-phase forced convection of R134a through single circular microchannels with inner diameters of 1.7, 1.2, and 0.8 mm. The results were compared to correlations for heat transmission in macroscale channels and correlations indicated for microscale geometries. The results suggest that the classical correlations and experimentally measured data in the tumultuous region agree.

Rectangular microchannels have been used to predict the thermal behavior in single-phase flow by Lee et al. [7]. An experimental investigation was conducted to explore the validity of classical correlations based on conventional-sized channels. The investigated microchannels ranged in width from 194 to 534 m, with the channel depth being around five times the breadth in each instance. Each test section had ten parallel microchannels and was made of copper. DI water was used for the experiments, and the Reynolds (Re) numbers ranged from 300 to 3,500. The experimental data and the numerical predictions made using a classical continuum technique were in good agreement (with an average variance of 5%). Effect of inlet velocity on boiling curves is shown in Figure 2.

Figure 2

Effect of inlet velocity on boiling curves.

The researchers also looked at how the surface quality of the microchannel affected the friction factor [8] and achieved research using water as the working fluid in a circular microchannel. The research was led on two surfaces, one of which was abrasive and the other hydrophobic (siliconated and Teflon), in comparison to a smooth glass duct. The friction factor exceeded the traditional theory’s estimate of the uneven surface’s minor diameter (126 m). Considering that these channels were ten times worse than untreated channels. Instead of a rough surface, the researchers ascribed the higher friction factor to the actual deformation of the channel’s circularity. This conclusion was reached after observing the analogous behavior of Teflon’s friction factor. At the same time, the investigation of the DI water flow properties in microtubes with diameters ranging from 2 to 30 m was studied by Xu et al. [9]. The experimental findings demonstrate that the flow properties in microtubes with a diameter greater than 16 m agree with the conventional theory. However, nonlinear flow characteristics predominate when the diameters are lowered to 5 and 2 m, and the findings show a considerable deviation in flow characteristics from the predictions of the traditional concept, with the magnitude of the departure increasing as the diameters decrease. As the Re number increases, the degree of nonlinear flow characteristics diminishes progressively, and the experimental results are roughly in accordance with the theoretical prediction. In this research, the minimal Re number is only 2.46 × 1 0 − 5 [10]. In a 0.85 mm stainless steel microtube with an average internal surface roughness of 1.7  μ m and relative surface roughness of 0.002, friction factor and heat transfer characteristics in single-phase fluid flow were studied. Laminar, transition, and turbulent flow regimes were tested with distilled water and R134a liquids at Re numbers from 100 to 10,000. The test segment had homogeneous heat fluxes, and the micro-tube was horizontal.

The Hagen–Poiseuille equation for laminar flow and the modified Miller correlation for turbulent flow and early transition from laminar to turbulent flow predicted the friction factor of water and R134a. Water and R134a, heat transfer findings, matched conventional theory in laminar flow and were lower than Adam’s correlation in a turbulent flow [11]. The bottom surface of a single-phase microchannel heat sink was a hybrid micro-sandblasting of elliptical patterns, and complete sandblasting was used to modify the material to increase heat transfer passively. The hydraulic diameter of the 26 mm × 5 mm × 0.35 mm microchannel is 654 μ m. The Re number range was 85–650, and DI water was the coolant. Fully sandblasting (FS) the microchannel bottom surface only minimally increased heat transfer.

Researchers investigate the utilization of water as a cooling medium by employing cylinders and parallelepipeds as fins on rectangular microchannels [12]. The Re numbers 67–153 and four heat inputs are measured with a plain copper microchannel with six parallel channels with dimensions of 350 (width) and 605 (depth) m. The numerical model is modified so that it may examine the impacts of cylindrical and parallelepiped micro-fin diameter, length, and breadth on the performance of the microchannel by making use of non-dimensional characteristics such as the Poiseuille number, the Nusselt number, and performance evaluation criteria. The experimental results validate the numerical model. The increase in heat transmission outweighs the decrease in pressure. Smaller cylindrical fins outperform conventional channels when heat transmission and pressure drop are considered.

In a microchannel made of copper, we investigated both the flow of a single-phase fluid and the transfer of heat [13]. The influence of artificial cavities on fluid flow and single-phase heat transfer in microchannel heat sinks was investigated by designing and manufacturing two straight microchannels (model 1 and model 2). Model 1 has a flat, smooth bottom surface, but model 2 contains 47 spaced artificial cavities throughout the microchannel. Both models have nominal dimensions of 300  μ m height and depth. However, their actual dimensions are 367  μ m width and 296  μ m depth. Working fluids were DI water. Laminar flow conditions were tested at 3 0 ° C input temperature by a Re number range (700–2,200). Horizontal microchannel studies were performed under adiabatic (friction factor calculation) and diabetic, experimental pressure drop changing with heat flux of three-inlet subcooled temperature, for mass fluxes 1,700 kg/m² s, as shown in Figure 3.

Figure 3

The experimental pressure drop changing with heat flux of three-inlet subcooled temperature, for mass fluxes 1,700 kg/m² s.

Single-phase flow research examined how artificial cavities in the microchannel affect the flow and heat transmission [27]. The trials used DI water at 3 0 ° C input temperature and a Re number range of 108.6–2,372. Three rectangular brass straight microchannel versions with a hydraulic diameter of 0.42 mm were made. Model 1 has a smooth surface, but model 2 features artificial cavities with 40 artificial nucleate cite (ANC) on the microchannel base along one of the sidewalls. Model 3’s artificial cavities are also at the microchannel’s base, closest to each sidewall. Each sidewall has 40 artificial nucleations (80 total). ANS increased heat transmission by 15.53 and 16.67% for model 2 and model 3, respectively, compared to model 1. Test section design and construction are shown in Figure 4.

Figure 4

Test section design and construction.

3 Flow boiling heat transfer in microchannel

Some studies reported nucleate boiling as the primary mechanism in microchannels, whereas convective boiling or convective-nucleate boiling was observed by others. The heat transfer coefficient in the nucleate boiling zone is primarily affected by heat flux, where the effects of mass flux and vapor quality are negligible. In contrast, the coefficient of heat transfer is larger in convective boiling. It depends on vapor quality and mass flux, as shown in Table 2.

Lee and Karayiannis [15] explored convective boiling mechanisms in copper microchannel heat sinks with subcooled input conditions at different heat and mass fluxes and reported their findings. The working fluid used was water. They found that mass flow and vapor quality are two factors that influence the heat transfer coefficient, but not heat flux, according to the authors. Both nucleate and convective boiling mechanisms stated in the study of Korniliou and Karayiannis [16] investigated flow boiling heat transfer into rectangular copper microscale channels with R134a. The nucleate boiling heat transfer mechanism dominated at the low vapor quality; however, when heat flux and vapor quality were high, annular film boiling predominated. The effect of inlet sub-cooling and mass flux on boiling and heat transfer onset was studied. It uses DI water as its medium of operation. Experiments were carried out on two microchannels, one of which had a hydraulic diameter that was different from the other. According to the observations, increased degree sub-cooled and mass flow not only delayed the commencement of boiling but also took a shallow effect on the boiling curve once onest nucleate boiling takes already occupied place. Nevertheless, nucleate boiling is the predominant mode of the boiling process. Qu and Mudawar [17] looked at how flow boiling characteristics were affected by microchannel size and operation factors. Microchannel widths, heat flux, and mass flux were studied in sub-cooled dielectric fluid experiments to determine the heat transfer coefficient and flow pattern. Seven microchannel widths were examined from 100 to 5,850 μ m with similar depths. Findings demonstrate that five flow regimes, bubble, slug, churn, wispy-annular, and annular, are found for widths less than 400  μ m. At the same time, narrower channels had convective boiling. Compared to smaller microchannels, the flow regimes change for these diameters, and bubble nucleation at the walls is controlled at a relatively modest heat flux. Lee and Mudawar [18] studied water flow boiling characteristics in two distinct single copper microchannels with the same length and aspect ratio but different hydraulic diameters and two distinct inlet subcooling. Experiments were conducted using various mass fluxes, heat fluxes, and vapor quality. It was reported that nucleate boiling was the predominant heat transmission mechanism for the smaller channel, as opposed to the more significant channel, where convective boiling may predominate [19]. The local flow boiling heat transfer coefficient was compared using pressure gradient measurement and linear pressure gradient. They employed DI water in the three microchannels with varied widths but the same length and depth to create three hydraulic diameters. Experiments used a constant mass flux and various heat fluxes. For hydraulic diameter 561 and 635  μ m channels, nucleate boiling dominated heat transmission, increasing the local heat transfer coefficient with heat flow. Because heat flow did not affect the local heat transfer coefficient, convective boiling dominated the 438  μ m channel. The two approaches yielded differing local heat transfer coefficients at large heat fluxes. Experimental friction factor at an inlet temperature of 3 0 ° C for the 0.438, 0.561, and 0.635 mm channels is shown in Figure 5.

Figure 5

Experimental friction factor at an inlet temperature of 3 0 ∘ C for the 0.438, 0.561, and 0.635 mm channels.

An influence of roughness through thermal performance and hydrodynamic in micro channel evaporator was examined [20]. Three microchannels with similar dimensions were produced, but they had different levels of roughness. The investigations utilized R134a as the experimental substance. The findings showed that increasing the surface roughness improved the low to moderate heat flow values of up to 45%. Mirmanto [21] experimentally examined the flow boiling properties of DI water in a single micro channel made of rectangular copper. Experiments were carried out with a constant intake temperature and pressure and a variety of heat and mass fluxes. For all mass flux levels, unstable flow boiling incipience was discovered. Additionally, it was noted that the major heat transport mechanism is unclear. Jafari et al. [22] carried out an investigation of copper multi micro-channel heat to evaluate the flow boiling heat transfer of R134a over a wide range of mass and heat fluxes. Camera was used in order to capture flow pattern while also measuring heat transfer, as shown in Figure 6. When the heat flux was steadily increased, three flow patterns were observed: bubbly, slug, and wavy-annular flow. Because there was no mass flux effect and the heat transfer coefficient rose with heat flow, nucleate boiling was the primary heat transfer process.

Figure 6

Nucleation in the liquid film in annular flow for G = 100 kg/m 2  s and q = 152.1 kW/m 2 .

Studies with microchannel heat sinks were carried out to investigate the two-phase boiling heat transfer in DI water [13]. The heat sink comprised a single microchannel with nominal dimensions of 300 m width and 300 m height (hydraulic diameter of 300 m). The heat sink is made of oxygen-free copper and is 72 mm in length and 12 mm in breadth. At 31 K subcooled intake temperature, experimental operating settings included heat flux (78–800 kW/m²) and mass flux (1,700 and 2,100 kg/m² s), as shown in Figure 7. The boiling heat transfer coefficient is calculated and compared to previous correlations. The results reveal that increased mass flux leads to a more significant boiling heat transfer coefficient, with convective boiling being the major process. Furthermore, it was discovered that an existing connection offers a reasonable forecast of the heat.

Figure 7

Mass flux effectiveness upon the local heat transfer coefficient at subcooled inlet temperature of 31 K.

Some researchers [14] reported that the effect of subcooling on heat transfer coefficients and flow boiling instability was significant. They conducted an experimental investigation into the impact of sub-cooling on pressure drop and heat transfer in a single microchannel (Figure 8). DI water was used in the trials as the working fluid and various heat and mass fluxes for three inlet sub-cooled degrees. It was shown that the sub-cooled heat transfer coefficient is generally independent of mass flux and heat flux dependent, indicating that nucleate flow boiling is the primary means of heat transmission. With increasing fluid temperature and decreasing mass flux, the possibility for unstable boiling behavior increased along with the average heat transfer coefficient.

Figure 8

Schematic of the microchannel test section: (a) isometric view and (b) sectional view.

However, Lee and Karayiannis [15] made contradictory statements on how subcooling affects the average heat transfer coefficient. Using HFE-7200 as the working fluid, they examined how inlet sub-cooling affected the flow boiling properties in a multi-microchannel system. The trials were conducted with a system pressure of one bar and a mass and heat flux range of three degrees of inlet sub-cooling. The experimental findings show that increasing the sub-cooled degree lowers the pressure drop. Furthermore, the local heat transfer coefficient was less affected by the greater intake sub-cooled degree, which led to a lower average heat transfer coefficient. Additionally, it was shown that flow instability correlated with input sub-cooling. Instants of alternating flow patterns are shown in Figure 9.

Figure 9

Instants of alternating flow patterns at 10 mm from the channel entry, for a constant heat flux of q ″ = 255 kW/m 2 , lowest mass flux of of G = 200 kg/m 2  s at high inlet sub-cooling of Δ T sub = 50 K .

Other researchers [16] explored the effect of inlet sub-cooling on flow boiling heat transfer for a heat sink of square microchannels utilizing DI water as the working fluid. The authors discovered that increasing the degree of sub-cooling reduces both the local heat transfer coefficient and the pressure drop, as shown in Figure 10. Flow reversal was also seen at large heat fluxes with low mass fluxes. This caused flow instabilities and changes in pressure drop, working fluid temperature, and wall temperature.

Figure 10

Subcooled length as a function of wall heat flux for inlet subcooling Δ T sub = 5 , 10, and 20 K at P = 1 bar and G = 200 kg/m 2  s.

On the contrary, Mohammed and Fayyadh [28] found that subcooling had a noticeable effect on the local heat transfer coefficient, which got worse as subcooling got worse, when they looked at how subcooling, heat, and mass movement affected flow boiling in a single microchannel heat sink with a square cross-section. With DI water as the working fluid, the tests were done at high mass flux rates of 1,700 and 2,100 kg/m s. Also, it was found that the heat transfer coefficient went up as the mass flow went up. However, the convective flow cooking process is in charge [26]. A single microchannel was used to study the effect of the microchannel aspect ratio on the flow boiling heat transfer properties of DI water. Heat and mass flows were also examined to see their impact. Three rectangular microchannels made of copper and oxygen were used. Each had the same hydraulic width and length. The results of the tests showed that heat transfer was better when the aspect ratio was smaller and the heat flux was between 400 and 500 kW/m. On the other hand, as the heat flows went up, the effect of the aspect ratio became insufficient. Also, the researchers saw four main different regimes: bubbly, slug, churn, and circular flow. Also, it was said that two microchannels with a small aspect ratio became unstable when the flow kept going back and forth. Finally, they found that the relationships between microchannels could not predict the results of the experiments.

Flow boiling heat transfer in a single rectangular micro-channel was tested with an investigation [27]. The micro-channel was made by cutting a groove 300 m wide and 700 m deep (with a hydraulic diameter of 420 m) into the top of a 60 mm long brass block. Ionized water was used as the solution. Experiments were done with a mass flux range of 300–600 kg/m² s, a heat flux range of 5.4–376.5 kW/m² (based on the wall), an input subcooling of 20 K, and a working pressure of 1 atm. The findings of this investigation indicated that the local coefficient of heat transfer increased with increasing heat fluxes, even if the heat fluxes were relatively low. When heat fluxes surpass 64.2, 77.9, 93.8, and 116.7 kW/m², respectively, for mass changes of 300, 400, 500, and 600 kg/m² s, this trend is reversed. At these heat fluxes, the heat transfer coefficient decreases as the heat flux increases until it reaches its maximum value, which remains unchanged. These heat transfer correlations of mini channel and microscale perform a decent job of describing the findings of the tests compared to the results of the experiments, which are compared to the correlations.

4 Flow boiling heat transfer in microchannel coating

Heat transfer and pressure drop were investigated by Ammerman and You [29] about flow velocity, inlet sub-cooling, and surface coating. They concluded that coating enhanced critical heat flux (CHF) by 14–36%. The vapor condensed quickly due to the sub-cooling, resulting in little pressure loss. Khanikar et al. [30] investigated how the coating of carbon nanotubes on the bottom wall of a rectangular microchannel would affect the rate at which heat would be transferred. The nanotubes, which were initially almost vertical, bent onto the heated surface and created a repetitive “fish-scale” pattern at high mass fluxes. Heat transfer was improved in the nucleate boiling zone, thanks to the near-zero-angle cavities supplied by the voids between the fish scales. This improvement in CHF was also due to the fish scales’ ability to provide these cavities. It is important to note that CHF was reproducible at low velocities, but the quality of the results suffered in subsequent repeated experiments conducted at high mass velocities.

Phan et al. [31] conducted studies to investigate the effects of surface wettability on atmospheric pressure flow bubbling of water. The test channel is a solitary rectangular channel measuring 180 mm long and 0.5 mm in height, as shown in Figure 11. The mass flux was set at 100 kg/m² s, while the basal heat flux varied between 30 and 80 kW/m². The water entering the test channel is subcooled. Surfaces of silicon oxide (SiOx), titanium (Ti), diamond-like carbon (DLC), and carbon-doped silicon oxide (SiOC), with static contact angles of 26, 49, 63, and 10 3 ° , respectively, comprise the samples. The results demonstrate that surface wettability has significant effects on heat transfer coefficient. Observations indicate that micro-structured surfaces exhibit up to 85% greater heat transfer than flat surfaces.

Figure 11

Schematic view of the experimental apparatus.

The equipment for experiments and method of flow boiling studies are designated [32]. Test results at R407, R22, R134a that are pure and when they are mixed with polyester oil. The tests took place in a tube that had as a guide, a covering that is porous and a tube made of smooth stainless steel. It was either 1% or 5% of the mass that was oil. During the trials, the level of the vapor quality at the input was set to 0, while the level at the outflow was set to 0.7. The mass velocity ranged from 250 to 500 kg/m² s at various points. Experiments were carried out at a temperature of 0 ° C , which is the typical saturation temperature. Refrigerants flow boiling in a duct with a porous covering has a better average heat transfer coefficient and smaller pressure drop than flow boiling in a smooth tube with the same mass velocity. [33] In this investigation, the accelerated boiling of anhydrous ethanol in porous-coated microchannels with a hydraulic width of 540 mm was studied, organized, and compared to boiling in microchannels with no coating. A solid-state sintering method made three heat sinks with porous coatings and parallel microchannels. In samples 1, 2, and 3, the copper pieces were about 30, 55, and 90 mm in diameter, respectively. The effects of adding porous surfaces on pressure drop, flow boiling instability, and heat transfer during flow boiling were discussed. The results of the experiments show that adding porous surfaces can make the pressure drop a little bit higher and reduce the amount that the pressure drop changes in the microchannels by a lot. Changes in how bubbles move can explain why flow boiling stability has gotten better, and mass flux and the quality of the exit gas have a lot to do with it. The best particle width was thought to be 55 mm, which was the middle size.

Experiments have been conducted to examine the effect that a nanoporous layer has on a microchannel’s convective heat transfer capability [34]. Experiments were carried out with DI water serving as the coolant in a microchannel with a hydraulic diameter of 672 m. Electrophoretic deposition of Al 2 O 3 nanoparticles was used to generate a nanoporous layer on the heat transfer surface of the microchannel. Experiments involving single-phase and two-phase convective heat transfer were carried out at various mass fluxes. The microchannel data with no coating on its surface were utilized as the baseline for comparison. CHF increases for the coated surface; an increase of up to 45% in CHF was recorded. In contrast, HTC decreased for covered surfaces compared to the naked surface. Flow boiling experiments were conducted to determine the influence of spray-pyrolyzed Fe-doped Al 2 O 3 – TiO 2 composite coatings applied to copper heater blocks on the CHF and boil heat transfer coefficient [35]. Studies on heat transmission were carried out with de-mineralized water serving as the working fluid in a mini-channel with dimensions of 30 mm × 20 mm × 0.4 mm. To investigate the effectiveness of the heat transmission, each coated sample was subjected to two different mass flux tests. The atomic force microscope and static contact angle were used to examine the effect of adding Fe on the wettability and porosity of the coated surfaces, and their influence on flow boiling heat transfer was also explored. Compared to a sandblasted copper surface, all coated samples exhibited a considerable improvement in CHF and boiling heat transfer coefficient. This was the case across the board. For a mass flow of 88 kg/m² s, a CHF enhancement of 52.39% and a heat transfer coefficient enhancement of 44.11% were measured to be the highest for 7.2% Fe-doped TiO 2 – Al 2 O 3 , respectively. Yang et al. investigated flow boiling in nanostructured microchannels using HFE-7000, a low-surface-tension dielectric coolant [36]. This is the first time such an investigation has been conducted. When the heat flow is less than 120 W/cm², the heat transfer coefficient may be significantly improved by using Si NWs due to the nanowire-enhanced nucleate boiling and evaporation. This is possible because of the increased surface area of the Si NWs. Because of the increased liquid film evaporation over the whole channel, the enhancement is more visible when the mass flux is at a low value of 1,018 kg/m² s. Two-phase separation with capillary flow reduced pumping power by up to 40% and improved CHF by up to 14% under low mass flux. The capillarity-induced annular flow is the primary enhancement mechanism when mass flux and vapor quality are low. These data demonstrate patterns consistent with our earlier research on DI water.

Stainless steel hypodermic microtubes with gradient crosslinked polyhydroxy ethyl methacrylate (pHEMA)/polyperfluorodecylacrylate (pPFDA) coatings had high porosity and varied wettability for flow boiling heat transfer [37]. DI water was utilized to coat the inner walls of 502  μ m microtubes using initiated chemical vapor deposition (iCVD). The coated microtubes had one pHEMA (hydrophilic) end and one pPFDA (hydrophobic) end. Thus, wettability varied along the microtube. At 9,500 kg/m²s, hydrophobic and hydrophilic inlet instances were compared to plain surface cases. Coatings significantly increased boiling heat transfer in experiments. The pHEMA-coated outlet case (hydrophobic intake and hydrophilic exit) had the highest heat transfer coefficients with a maximum heat transfer enhancement ratio of 64%. The effect of pHEMA (polyhydroxyethylmethacrylate) nanostructure-coated surfaces on flow boiling was examined by Khalili Sadaghiani et al. [38].

In a rectangular microchannel of 14 cm length, 1.5 cm width, and 500  μ m depth, DI water was utilized to study flow boiling. High-speed cameras investigated the influence of pHEMA coatings on 1.5 × 1.5 cm² silicon plates on heat transfer coefficients and flow patterns. Nano-coated surfaces have a lower contact angle, yet surface porosity increases the boiling heat transfer coefficient. Visualization showed a reduced nucleate boiling zone on uncoated surfaces. Coated surfaces dry out with more significant heat fluxes. pHEMA-coated characters exhibit lower wall temperatures at nucleate boiling than uncoated surfaces at low wall superheats. Coated surfaces transport heat better at low-wall superheats. At the same heat flux, plain surface bubbles are more giant.

Shustov et al. [39] observed water boiling in a single microchannel measuring 0.2 mm in height, 3 mm in width, and 13.7 mm in length with a uniform heating surface or coating of aluminum oxide nanoparticles. For various mass flows, heat flux depended on microchannel wall temperature. A correlation based on empirical data estimates the vacancy fraction for boiling in a microchannel without coating. The microchannel with a nanoparticle covering has a higher boiling vacancy fraction than the one without. The behavior of the void fraction in the microchannel without coating differs from that of the adiabatic flow. The application of nanoparticles increases the microchannel vacancy fraction [40]. In this study, flow boiling heat transfer properties of copper surfaces covered with Cu– TiO 2 nanocomposite materials have been experimentally investigated for prospective usage in heat transfer applications. A DI water-cooled bottom-surface-heated mini channel experiments are conducted. Copper surfaces covered with thin Cu-TiO₂ nanocomposite films are created using electrodeposition. The CHF increases by up to 92% for characters covered with nanocomposite materials. Cu– TiO 2 -coated surfaces have grown CHF and heat transfer coefficient due to improved wettability, surface roughness, and high-density active nucleate sites. On the bottom and the sidewalls of the silicon microchannel, nanowires made of silicon were produced [41]. The performance of the thermal and hydraulic systems was demonstrated via a closed-loop experimental setup. Results from experiments with mass fluxes between 250 and 1,250 kg/m² s and subcooled inlet temperatures between 15 and 65 K were given. Results for the SiNW device under the same wall superheat circumstances indicated a 20% increase in heat flux rejection over the plain-wall device. It has been demonstrated that a subcooled intake temperature of 65 K combined with a mass flux of 1,250 kg/m² s may dissipate an effective heat flux of 431.3 W/cm² at a wall superheat of around 85 K. Overall, the SiNW coatings showed promise for improving flow boiling heat transfer with a slower rise in pressure drop. Schematic of experimental setup is shown in Figure 12.

Figure 12

Schematic of experimental setup.

Experimentally, the increase of flow boiling heat transfer utilizing a porous nanocoating in copper microchannels has been studied by Lee et al. [42]. Same operational conditions, including mass flux of 200 kg/m² s, inlet subcooling of 10 K, input pressure at 1 bar, and wall heat fluxes ranging from 24.5 to 160.7 kW/m², the coating of a microchannel heat sink has been compared to the conventional, CNC-machined copper heat sink. Tiny, well-defined cavities as little as 0.6 m were abundant on the coated surface, but shallow, bigger holes measuring 6 m were discovered on the end-milled copper channels. Because coated surfaces have better nucleation sites, bubble formation frequency in the coating channels is much greater than in the conventional channels. Similar flow pattern progression, from bubbly to slug, churn, and annular flow with increased heat flux, happened in both test sections. When the nucleate boiling mechanism is prominent and modest heat fluxes are present, the heat transfer microchannel in the flow boiling can be increased by up to 43.5%. As heat flux increases further, heat transfer enhancement decreases, reaching 13.2% at the highest wall heat flux (Table 3).

The present study provides a comprehensive literature review of microchannel heat sinks, with a particular focus on single-phase flow, flow boiling, and coating microchannels. The review highlights the potential and challenges of microchannel heat sinks in increasing heat transfer rates in a compact size. It identifies the impact of various parameters, such as channel size, fluid properties, and flow conditions, on the heat transfer rate in single-phase flow. The study also explores the advantages and challenges associated with flow boiling and strategies developed to mitigate these issues. Additionally, it analyzes the effects of surface coatings on flow behavior and heat transfer performance.

The present study builds upon previous research on microchannel heat transfer by providing a comprehensive review of research on single-phase flow, flow boiling, and coating microchannels. This review identifies gaps in the literature and highlights areas that require further research to advance our understanding of microchannel heat transfer and improve the performance of microchannel heat sinks. The review also compares and contrasts the findings of different studies in these areas, providing insights into the similarities and differences in research outcomes.

The review’s findings have several implications for the design and optimization of microchannel heat sinks. For instance, the review highlights the potential of flow boiling to significantly increase the heat transfer rate compared to single-phase flow. However, it also identifies the challenges associated with flow boiling, such as flow instability and wall superheat, which can be mitigated by optimizing channel geometry, nucleation enhancement, and surface coatings. The review also explains the effects of surface coatings on flow behavior and heat transfer performance, providing insights into the impact of coating materials and thickness on microchannel performance.

The main strength of this study is the comprehensive review of the literature on microchannel heat transfer, providing insights into the potential and challenges of microchannel heat sinks. However, the study’s limitations include the inherent limitations of a review study, such as the potential for bias in selecting and interpreting studies and the inability to provide empirical data.

5 Conclusion

The literature review on single flow in a microchannel, flow boiling microchannel, and coating microchannel has highlighted the potential and challenges of the microchannel’s heat sink of increasing heat transfer rates in a compact size. Single-phase flow has been extensively studied, and researchers have investigated how the heat transfer rate is affected by various parameters such as channel size, fluid properties, and flow conditions. Flow boiling has also been the subject of numerous studies, with researchers investigating strategies to mitigate flow instability, wall superheating, and premature drying. Coatings have been explored to modify the surface properties of microchannels, affecting the flow behavior and heat transfer performance. Coatings made from materials such as metals, polymers, and self-assembled monolayers have been investigated, and their effects on flow boiling heat transfer have been analyzed.

The literature review highlights that single-phase flow and flow boiling can achieve high heat transfer rates, but flow boiling has the potential for significantly higher rates. However, flow boiling also presents challenges, such as flow instability and wall superheat, which can be mitigated by optimizing channel geometry, nucleation enhancement, and surface coatings. Coatings have been shown to impact flow behavior and heat transfer performance significantly, but further research is required to determine the optimal coating material and thickness for different applications. Despite the significant progress made in understanding microchannel heat transfer, challenges such as flow instability, premature drying, and a limited understanding of microchannel flow boiling properties still need to be addressed for the widespread adoption of microchannel heat sinks. Further research is required to develop innovative solutions to these challenges and fully realize microchannel heat sinks’ potential for various applications, including electronics cooling, microreactors, and biomedical devices.

By addressing these research areas, we can better understand the behavior of microchannel heat sinks and develop innovative solutions to improve their performance and reliability for various applications.