An exploratory review on heat transfer mechanisms in nanofluid based heat pipes

Abstract

Keywords: Phase Change; Thermal Management; Thermosyphon; Hybrid Nanofluid; Heat Transfer Mechanism

Introduction

In the current era, a technological revolution has been observed to persist involving the development of new technological solutions and continuous advances in pre-existing technologies to proficiently meet societal requirements. A strong pursuit is prevailing to research and develop effective ways to carry out energy management efficiently. Nanotechnology is one of the most widely discussed areas of research attributed to its appreciable performance enhancement potential across various applications. Nanotechnology has been evident to be widely discussed across different thermal and heat transfer applications. Nanofluids are basically nano-sized particles suspended in the conventional thermofluidic [1]. The

potential of nanofluids based on metallic and carbon-based nanoparticles in base fluids like DI water, alcohols, refrigerants, lubricants, and other conventional working fluids [2]. Figures 1 (a) and (b) illustrate the pictorial representation of the process involved in the one-step and the two-step method respectively. The major goal in nanofluid synthesis is to achieve a stable nano-suspension avoiding nanoparticle agglomeration and sedimentation within the base fluid [3]. The nanofluids offer attractive thermophysical characteristics in contrast with that offered by conventional working fluids [4]. Two major classes of nanofluids are currently being widely discussed and investigated by researchers. The first is the mono while the other is the hybrid nanosuspension. The

*Corresponding author. *E-mail address: naveen.gupta@gla.ac.in This paper was recommended for publication in revised form by Regional Editor Hasan Köten Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

Figure 1. (a) One-step method, (b) Two-step method of nanofluid preparation. mono nanosuspensions are synthesized by adding a single nanoparticle type while the hybrid nanofluids involve suspending two or more nanoparticle types simultaneously in the base fluid [5]. Both the mono and hybrid nanofluids offer appreciable thermophysical properties in contrast with that offered by conventional working fluids. However, the hybrid nanofluids offer thermal and flow characteristics much superior to that offered by the mono-nanofluids attributed to the synergy achieved between the nanoparticles that further results in a potential combination offering competency to significantly enhance the heat transfer systems [6]. Several researchers have investigated the applicability of nanofluids across different applications. Kilic et al. [7] numerically studied heat transfer from a heated surface employing a swirling jet and nanofluids. They reported that by increasing the Reynolds number (12000 to 21000), an increment of about 51.3% was attained in the average Nusselt number. It was found that using the aqueous CuO nanofluid enhanced the average Nusselt number relative to the TiO2 and Al2O3 nanofluids. Abdulvahitoglu [8] investigated the effectiveness of using aqueous copper and nickel oxide nanosuspensions respectively for engine cooling. Out of all the considered nano-suspensions, the Cu nanofluid was reported to be the best coolant attributed to its superior thermophysical characteristics while the CuO nanofluid was reported as the least preferred choice out of all the considered nanofluids. The stability of nanofluids is a major issue since it deteriorates the performance of the nanofluids by declining their thermal and flow characteristics [9]. In the last few years, researchers working in the field of nanotechnology have tried to devise several methods to overcome the stability issue of nanofluids to increase their practical applicability. Several mechanical and chemical treatments have been introduced ranging from mechanical agitation to surface modification of nanoparticles to improve the nanofluid stability. However, the stability issue of nanofluids has been encountered to be relatively more in the mono-nanofluids as compared to the hybrid nanofluids. Heat pipes are special thermal devices used to carry out the transfer of heat energy from a source to a distantly

located sink without undergoing any major energy losses [10]. Mesh wick [11], sintered wick [12], grooved [13], oscillating [14], pulsating [15], thermosyphon heat pipes [16], etc. have been employed for numerous applications owing to their high heat transfer capability and robust build. Some of the interesting applications of heat pipes range from the thermal management of electronic devices to solar, space, HVAC, refrigeration, and air conditioning applications, etc. The heat pipes are lined with a capillary material at its inner surface called the wick structure. The heat pipes are charged at sub-ambient pressures by thermofluids to undergo thermal energy transport without necessitating any external work input [17]. Figure 2 illustrates different working fluids used in heat pipe applications. Recently, attributed to their attractive thermophysical characteristics, nanosuspensions have been utilized as working fluids. Nanofluid-based heat pipe technologies are being widely investigated for solar thermal applications. Dehaj et al. [18] investigated a heat pipe solar collector using CuO nanosuspensions. The efficiency of the solar collectors got enhanced at higher nanofluid concentrations and flow rates respectively. The researchers carried out another study where they experimented using CuO, Al2O3, and MgO nanosuspensions on efficiency. The efficiency was found to enhance using the nanofluids with an improvement rate of up to 20% [19]. Hosseini et al. [20] investigated the effect of nanoparticle surface morphology on solar collector response. Two different samples of aqueous TiO2 nanofluids were prepared using the spherical and wire-shaped TiO2 nanoparticles respectively. The wire-shaped TiO2 nanofluid offered an increment of 21.1% in efficiency while that offered by the spherical TiO2 nanofluid was found to be 12.2% respectively. The response of the heat pipes augments appreciably by utilizing nanosuspensions. The thermal transport capacity of the heat pipes enhances considerably while offering augmented thermal conductivity and efficiency when filled with nanofluids. Different nanofluids have been investigated across different experimental and numerical studies. It has been widely proposed that the optimized operating and design parameters can augment the thermal response employing nanofluids [21, 22].

Figure 2. Heat pipe working fluids. The authors carried out a search in the google scholar database for the articles reported in the period of 20172022 using different keywords. The retrieved search results are illustrated in Figure 3. The trend number of studies on the nanofluid-filled heat pipes reported since 2017 can be observed to be increasing. However, it was noticed by the authors that the number of research studies on nanosuspensions & heat pipes investigating the heat transfer mechanisms is quite low. It is clear from the generated search results from the Google Scholar database that very few numbers of research studies on heat pipes discussed the reasons behind the exclusive performance. This could be attributed to the fact that the heat transfer mechanisms associated with nanofluid-filled heat pipes are quite complex since all three solid, liquid, and vapor states coexist during their operation. As per the available literature, the authors found that a limited number of studies have attempted to exhaustively identify and investigate mechanisms in different heat pipe types that rationalize the importance and novelty of the current work. The authors have considered nanofluid-filled thermosyphon, pulsating, oscillating, grooved, and mesh wick heat pipes respectively within the scope of this study. The current study will aid the researchers in the concurrent field in getting aware of the current research perspectives forming a firm understanding of the phenomena involved in the nanofluid-filled heat pipes.

Figure 3. Summary of search results generated from the Google Scholar database.

Applications Of MANO Nanofluids

Experimental Researches The heat pipe is a special type of heat exchanger that is widely being studied owing to its compact build and advanced response. Different heat pipes using various nanosuspensions have been reported in the open literature.

Moradgholi et al. [23] fabricated a thermosyphon system to generate electrical & thermal energy simultaneously. They employed methanol-based Al2O3 nanofluids of concentrations 1.0, 1.5, and 2 wt.%. The optimum nanofluid concentration and the filling ratio were reported to be 1.5 wt.% and 50% respectively. Das et al. [24] studied the properties of graphene nanosuspensions. They reported that the conductivity of the graphene nanosuspension was about 29% higher relative to DI water (at 45 °C) which augmented the thermosyphon characteristics. Sardarabadi et al. [25] utilized multi-wall carbon nanotube (MWCNT) nanosuspensions in a thermosyphon and attained enhanced heat transfer capacity. Sarafraz et al. [26] studied the response characteristics of a thermosyphon using zirconia-acetone nanosuspensions and reported geyser boiling phenomena. Kiseev et al. [27] studied the employability of thermosyphon for LED applications. They utilized Fe2O3 nanofluids (0.1 wt.% to 1.0 wt.%) and attained an improved heat transfer coefficient attributed to the nano-sized coating. Cacua et al. [28] studied the effects of nanosuspension stability on thermosyphon response. They prepared aqueous Al2O3 nanofluids of concentrations 0.1 wt.% using surfactants and attained a maximum decrement of about 24% in the heat pipe thermal resistance. Kaya [29] utilized 1.0 wt.% and 2.0 wt.% CuO nanosuspensions in a thermosyphon kept in the vertical position. The performance was found to enhance using CuO nanosuspensions relative to water. Anand et al. [30] prepared Al2O3 nanofluids using HFE 7000 & R134a and utilized them in a thermosyphon and reported augmented performance trends. Choi et al. [31] studied the boiling regime of a thermosyphon employing cellulose nanofiber nanofluid. They observed that at lower heat loads, geyser boiling occurred while at higher heat loads, churn boiling occurred. Shuoman et al. [32] experimented on a thermosyphon utilizing γ-Al2O3 nanoparticles of average size 40 nm suspended in distilled water (0.5- 2.0 vol. %) in the heat load range of 500-1250 W in terms of its overall conductivity. At 1000 W, the conductivity was reported to augment by three folds using 2.0% by vol. alumina nanofluid. Xing et al. [33] examined the response of a pulsating heat pipe employing hydroxylated MWCNT nanofluids and attained improved start-up performance. Nazari et al. [34] experimented on a pulsating heat pipe, filled with GO nanosuspensions. The GO nanoparticles were suspended in water to form stable nano-suspensions of concentrations ranging between 0.25 to 1.5 g/lit. At higher concentrations, the heat pipe performance degraded owed to poor flow characteristics of nano-suspension. Beydokhti et al. [35] experimented on a pulsating heat pipe filled with MWCNT nanosuspensions and reported enhanced convection currents. The surface modification of the MWCNT nanoparticles was carried out through chemical surface and polymer wrapping treatment. Akbari et al. [36] experimented on a pulsating heat pipe utilizing aqueous TiO2 nanofluid and graphene nanofluid (10 mg/ltr and

1 mg/ltr respectively). They reported that the higher the stability of the nanofluid, the higher will be the enhancement attained in the response. Chen et al. [37] experimented with a pulsating heat pipe filled with TiO2 nanofluid for battery cooling. They reported that uniform temperature distribution was achieved with an effective improvement rate of 60% employing TiO2 nanofluid. Zhou et al. [38] fabricated a pulsating heat pipe using a copper tube bent into three turns. They reported that employing 0.05 wt.% GO nanofluid within the heat pipe improved its thermal performance by about 41%. Zhang et al. [39] experimentally studied a nanofluid-based pulsating heat pipe. A high-speed camera was used to visualize the flow dynamics. Aqueous-SiO2 nanofluids (0.5 wt.% to 2.0 wt.%) were used and they reported enhanced phase-transition in the working fluid and increased instantaneous velocity and driving force. Zhou et al. [40] experimented on an oscillating heat pipe using graphene nanoplatelet nanosuspensions (1.2 to 16.7 vol.%). They reported that the favorable nanofluid concentration varied between 2.0 vol.% to 13.8 vol.% at an appropriate fluid fill ratio of 55%, 62%, and 70% respectively. Meena et al. [41] used silver nanofluid in an oscillating heat pipe at a fluid fill ratio of 50%. They reported augmented performance at the air velocity and temperature of

Applications Of Hybrid Nanofluids In HEAT Pipes

Hybrid nanofluids have been reported to showcase a significant potential for heat transfer fluids applications owing to their advanced thermophysical properties. They offer noteworthy conductivity and stability making them suitable for practical employment. Recently, researchers have tried to explore various nanoparticle combinations to prepare exceptional thermo-fluidics. Xu et al. [71] experimented on a thermosyphon using mono and hybrid nanosuspensions of Al2O3 and TiO2 nanoparticles and attained the best response using the (25% Al2O3+75% TiO2) hybrid nanofluid. Çiftçi [72] experimented on a thermosyphon and attained a maximum enhancement of about 38.4% in the thermal efficiency and a maximum decrement of about 40.79% in the thermal resistance was attained utilizing (50% AlN+50% ZnO) hybrid nanosuspension. Zufar et al. [73] experimented on a pulsating heat pipe using 0.1 wt.% aqueous hybrid nanofluids of Al2O3, SiO2, and CuO nanoparticles and reported that the SiO2+CuO hybrid nanofluid performed most appreciable out of all the prepared nano-suspensions attributed to lower flow resistance due to lower dynamic viscosity. It was found that the Al2O3+CuO hybrid nanofluid had the highest conductivity and dynamic viscosity [74]. Pandya et al. [75] experimented on a grooved heat pipe utilizing (80% CeO2+20% MWCNT) nanosuspension (0.25 vol.% to 1.5 vol.%) and attained the best thermal response using the 1.5 vol.% nanosuspension. Veeramachaneni et al. [76] experimented on a miniature loop heat pipe using Cu+graphene hybrid nanofluid and attained a maximum decrement of about 24.42% and

9.8% was attained in the resistance and the wall temperature respectively. Ramachandran et al. [77] used (25% Al2O3+ 75% CuO) hybrid nanofluid to increase the heat transfer capacity of a mesh heat pipe by about 79.35%. Bumataria et al. [78] experimented on a mesh heat pipe and reported that the (75% CuO+25% ZnO) hybrid nanofluid offered appreciable performance. Vidhya et al. [79] experimented on a cylindrical heat pipe using (50% ZnO+50% MgO) hybrid nanosuspensions (0.0125% to 0.1% by vol.). They added CTAB to improve the nanosuspension stability and reported improved convection using the 0.1 vol.% (50% ZnO+50% MgO) hybrid nanofluid. Martin et al. [80] experimented on a conventional heat pipe using (50% Fe+50% CuO) hybrid nanosuspension (2% by mass) and reported the least wall temperatures. It is clear post-discussion that the hybrid nanofluids offer tremendous enhancement in the heat pipes since they offer superior thermophysical properties accompanied by appreciable stability which favors their applications in the heat pipes. Hybrid nanofluids perform much superior relative to mono-nanofluids. However, the full potential of hybrid nanofluids is not yet discovered completely since not many studies have been reported. Hence, it gets crucial to substantially focus on hybrid nanofluids across various applications like heat pipes, heat exchangers, etc. by carrying out experimental and numerical studies. Studies focused on the prospective combinations of different nanoparticles should be carried out to achieve hybrid nanofluids as efficient and competent heat transfer fluids. The research studies discussed on hybrid nanosuspensions are summarized in Table 2.

Table 2. Summary of the studies on the hybrid nanosuspensions Literature

The (25% Al2O3+75% TiO2) hybrid nanofluid performed the best. A maximum decrement of about 26.8% was attained in the thermal resistance. A maximum increment of about 26.8% and 10.6% were attained in the heat transfer coefficient and thermal efficiency.

An increment of about 34.8% and a decrement of about 40.79% were attained in the efficiency and resistance respectively using the 50:50 combination.

The SiO2+CuO hybrid nanofluid offered the best response attributed to its lower dynamic viscosity.

The performance deteriorated using the Al2O3+CuO hybrid nanofluid attributed to its high dynamic viscosity.

The operational capacity improved using the 1.5 vol.% nanosuspension. The capillary limit and the conductivity got improved using the 0.02 vol.% (30:70) hybrid nanofluid.

Studies on mesh heat pipe The hybrid nanofluids were found to increase the heat transfer capacity. The best performance was attained using (75:25) hybrid nanofluid at an inclination of 60°.

A maximum decrement of about 16.91% was attained in the resistance.

HEAT Transfer Modes In HEAT Pipes

A major share of the published studies on heat pipes is on the utilizing of conventional nanosuspensions in the heat pipes relative to hybrid nanosuspensions that report performance augmentation. The authors have noticed during the literature survey that the crucial factors for heat pipe improvement are an aspect of research that is still not very much discussed with a firm understanding and remains

hidden. The heat transfer modes found to be acting in such scenarios are summarized as follows: • The Mechanism-(A) of augmentation in the thermophysical characteristics employing nanoparticles acts in the nanofluid-filled heat pipes. Such augmentation is achieved mainly in conductivity owed to the high aspect ratio of the nanoparticles that improves the heat transfer across the resulting nano-suspension. • The Mechanism-(B) of increased capillary action also plays a crucial role owing to the reduced interface angle

that enhances the wettability & further improves the capillary pumping pressure. The Mechanism-(C) of Brownian motion of the nanoparticles leading to enhanced operation i.e., highly randomized movement by virtue of their high surface activity. Such randomized movement of the nanoparticles leads to inter-collisions and thermal energy exchange. The Mechanism-(D) of vapor bubble puncture due to nanoparticle activity also occurs in the heat pipes. During the heat pipe operation, vapor bubbles form at the inner wall which eventually hinders thermal exchange. In the case of nanofluid-filled heat pipes, the nanoparticles suspended in the base fluid under the action of externally supplied heat energy initiate chaotic motion. Such chaotic motion of nanoparticles bombards the aforementioned bubbles degrading the resistance. The Mechanism-(E) of nanoscale deposition on the inner surface of the container has been reported widely. The nanoparticles suspended in the base fluid lose their stability, start to sediment on the inner wall surface and form a nano-scale deposition. Such nanoparticle deposition results in numerous nano-sized nucleation sites which further results in vapor bubble formation of smaller size with high departure rates further reducing the resistance. The Mechanism-(F) of high mixing fluctuations within the nanofluid-filled heat pipe also had been observed in several research studies. The supplied heat energy progresses the mixing fluctuations within the nanofluids. Such increased mixing in the nanofluid leads to enhanced heat transfer when filled within the heat pipes. The Mechanism-(G) of nanoparticle thermophoresis across the base fluid has been evident to occur in the nanofluid-filled heat pipes such that a thermal gradient establishes. Such temperature gradient results in nanoparticle migration under the action of the discussed temperature gradient which further enhances the thermal exchange. The Mechanism-(H) of diffusiophoresis of the nanoparticles also has been observed to prevail in the heat pipes. The nanoparticles migrate across a density gradient resulting in heat exchange between them. However, this also results in nanoparticle agglomeration which further deteriorates the thermal exchange if the size of the agglomerates formed gets exceptionally large and blocks the wick pores. The Mechanism-(I) of ballistic transport of the nanoparticles also has been evident to act in the heat pipes owed to externally supplied heat energy carrying out phonon heat transfer which changes from scattered to ballistic transport resulting in increased conductivity. The Mechanism-(J) of the interfacial phenomena in nanofluids also prevails. The nanosuspensions are

basically composites where the nanoparticles form the core structure while the base fluid acts as a matrix. This leads to a formation of a region where the intermediate of their properties is attained called the interfacial layer. The interfacial layer resembles a multi-phase system that offers high thermophysical characteristics. Such availability of high thermophysical properties results in enhanced heat transfer and efficient performance. • The Mechanism-(K) of enhanced heat transfer area availability owed to the nanoparticle deposition has been reported across several experimental studies on heat pipes. The nanoparticle deposition reasoned as in the aforementioned mechanism (E) leads to a porous nanoparticle coating on the wick and inner wall resulting in an increment in thermal transport while allowing the smooth condensate return. • The Mechanism-(L) of favorable increment in the viscosity using nanoparticles has been reported to prevail. Nanoparticles added to the base fluid cause advanced conductivity (favorable) and viscosity (opposing). So, it gets essential that a trade-off is attained between them. It can also be declared as a reason that researchers have reported the optimum nanofluid concentrations for best performance. • The Mechanism-(M) of improvement in the critical heat flux due to nanofluids has been reported to act. The nanofluids filled offer high convection further improving the critical heat flux. An increase in the critical heat flux eventually results in improved heat pipe capacity. • The Mechanism-(N) of gravitational influence on heat pipe performance has been reported in several research studies on heat pipes. The gravity force interacts with the capillary force altering the fluid flow. In the gravity-assisted positions, the gravity favors the condensate flow towards the evaporator while in the gravity-opposed positions, the gravity opposes the same. • The Mechanism-(O) of synergy achieved between the suspended nanoparticles has been reported to act in the heat pipes. The synergetic combination of dissimilar nanoparticles is attained such that superior thermophysical characteristics are attained as compared to that attained using mono-nanofluids of the counterparts. This results in improved stability, flow, and performance. Figures 4 (a)-(e) illustrate the percentage of studies in which the identified mechanisms A to N were reported based on the type of heat pipe incorporated. Figure 4 (a) shows the variation of the reported thermal exchange modes in thermosyphons. Mechanism A of thermophysical augmentation of base fluid has been reported in the majority followed by mechanisms E, K, and C with the dominance of 97%, 67%, 61%, and 59% respectively in the thermosyphon. The rest of the reported mechanisms were reported in less than 50% of the reviewed studies such that the mechanisms D, L, M, J, N, G, F, I and H had dominance of 43%, 38%, 29%, 22%, 17%, 12%, 8%, 7%, and 5% respectively in the

nanofluid-filled thermosyphon heat pipes. Out of all the identified mechanisms, mechanism B was not reported in thermosyphon attributed to the absence of capillary phenomena. Figure 4 (b) illustrates the domination of different mechanisms in the case of pulsating heat pipes. The action of mechanism A dominated followed by mechanisms F, K, and E with the dominance of 93%, 71%, 65%, and 59% respectively. The rest of the discussed mechanisms were reported in less than 50% of the reviewed studies such that the mechanisms C, L, J, G, M, N, H and I had dominance of 49%, 41%, 28%, 15%, 14%, 13%, 9% and 4% respectively. Similar to thermosyphon, mechanism B was also reported to be absent in pulsating heat pipes owed to the absence of

capillarity. Mechanism D was also not reported owing to the fluid flow dynamics of the pulsating heat pipes. Figure 4 (c) shows the domination of different mechanisms in oscillating heat pipes. Mechanism A of thermophysical augmentation of base fluid has been reported in the majority of the studies followed by mechanisms F, K, E, and C with the dominance of 89%, 84%, 72%, 69%, and 62% respectively. The rest of the reported mechanisms were reported in less than 50% of the studies such that the mechanisms L, J, G, N, M, I, and H had dominance of 44%, 34%, 24%, 17%, 12%, 9% and 4% respectively. Mechanism B was not reported in oscillating heat pipes attributed to the absence of capillary phenomena in them.

Figure 4. Heat transfer mechanisms for (a) thermosyphon, (b), pulsating, (c) oscillating, (d) grooved, and (e) mesh wick heat pipes.

Figure 4 (d) shows the variation of domination of different mechanisms in grooved heat pipes. Mechanism A of thermophysical augmentation of base fluid has been reported in the majority of the studies followed by mechanisms K, E, C, B, L, and D with the dominance of 86%, 81%, 78%, 72%, 67%, 63%, and 53% respectively. The rest of the reported mechanisms were reported in less than 50% of the studies such that the mechanisms N, M, J, G, F, H and I had dominance of 48%, 47%, 31%, 27%, 21%, 13% and 10% respectively. Figure 4 (e) shows the variation of domination of different mechanisms in mesh heat pipes. Likewise, for other heat pipes, mechanism A has been reported in the majority of the studies followed by mechanisms K, E, C, B, L, D, N, and M with the dominance of 98%, 85%, 85%, 81%, 72%, 70%, 67%, 66% and 53% respectively. The rest of the reported mechanisms were reported to act in less than 50% of the studies such that the mechanisms J, G, F, H and I had dominance of 38%, 19%, 15%, 14%, and 7% respectively in the nanofluid-filled mesh wick heat pipes. Figure 5 illustrates the percentage of studies attributing the mechanisms A to N to be acting during heat pipe operation irrespective of the type of heat pipe investigated. It is clear from the plot shown in Figure 5 that mechanism A of thermophysical augmentation due to nanoparticle addition has been reported significantly while mechanism I of nanoparticle ballistic phonon-based heat transfer has been reported in the least of the reviewed studies. The mechanism O of the synergy effect between the nanoparticles has been reported for hybrid nanosuspensions but it has not been reported widely owing to the scarcity of studies. Mechanisms A, K, E, C, and L have been reported to act in about 92%, 73%, 71%, 65%, and 51% of the total reviewed studies respectively. Contrary to them, the mechanisms F, D, N, M, and J were reported to act in about 40%, 33%, 32%, 31%, and 30% respectively. Mechanisms B, G, H, and I were reported to act in the least of the reviewed studies. All the discussed thermal exchange modes act simultaneously.

The identified thermal exchange modes have their respective contributions that vary with the operating conditions & their dominance depends on several other parameters which are yet to be identified.

Conclusion

A review of the studies reported on the heat pipe technologies was carried out and categorized on the basis of the type of nanofluid incorporated. The thermal exchange modes were identified and their dominance over each other was presented in different heat pipes. The key conclusions are as follows: • The performance of the heat pipe-based technologies augments appreciably when filled with nanosuspensions where the hybrid nanosuspension offers better heat transfer characteristics relative to mono nanosuspensions. • The thermal exchange modes are highly controlled by fluid-flow dynamics within the container. • The thermal exchange mode attributed with the highest dominance is the property augmentation of the base fluid when added with the nanoparticles. • The heat transfer mechanisms pertaining to the nanoparticle Brownian motion and their deposition are widely accepted modes attributed to heat transfer augmentation. • The synergy between the different nanoparticles enhances the thermal exchange across the hybrid nanosuspensions. • The thermal exchange mode of nanoparticle thermophoresis & diffusiophoresis, and ballistic transport have not been much discussed in the open literature and are required to be further investigated to properly understand their dominance over other heat transfer mechanisms.

Future Research Scope

The dominance of the heat transfer mechanisms in the pipes depends on various heat pipe parameters which are required to be investigated in terms of their quantitative influence by carrying out experimental and theoretical studies. Further research work can be carried out to optimize the heat pipes using artificial intelligence and machine learning.

Nomenclature

Photovoltaic Thermal system Light Emitting Diode Sodium Dodecyl Benzene Sulfonate Cationic Cetyl Trimethyl Ammonium Bromide Graphene Nanoplatelet Carbon Nanotube

Volume of Fluid method Computational Fluid Dynamics Mono Nanofluids Not Available Hybrid Nanofluids

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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