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

The current study reviews the research on nanosuspension-enhanced heat pipe technologies. The reviewed studies are categorized based on the nanosuspension type incorporated in the heat pipe i.e., mono & hybrid. The study attempts to identify the heat transport modes in heat pipes and explore their dominance among each other. The dominance of the identified mechanisms was found to be a strong function of the heat pipe type investigated and get significantly influenced by the operating conditions. The current review paper will aid in properly understanding the thermal mechanisms prevalent in heat pipes filled with nanosuspensions and to further optimizing their thermal response.


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 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 TiO 2 and Al 2 O 3 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, Al 2 O 3 , 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 TiO 2 nanofluids were prepared using the spherical and wire-shaped TiO 2 nanoparticles respectively.The wire-shaped TiO 2 nanofluid offered an increment of 21.1% in efficiency while that offered by the spherical TiO 2 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].The authors carried out a search in the google scholar database for the articles reported in the period of 2017-2022 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.

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 Al 2 O 3 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 Fe 2 O 3 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 Al 2 O 3 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 Al 2 O 3 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-Al 2 O 3 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 TiO 2 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 TiO 2 nanofluid for battery cooling.They reported that uniform temperature distribution was achieved with an effective improvement rate of 60% employing TiO 2 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-SiO 2 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 0.5 m/s and 8 °C respectively.Jin et al. [42] reported that at an optimum fill ratio of 83%, using 3.0 wt.% MWCNT nanosuspension in an oscillating heat pipe the conversion efficiency of about 92% was attained attributed to the higher thermal conductivity of the prepared nano-suspensions.
Monroe et al. [43] examined the performance of an oscillating heat pipe filled with CoFe 2 O 4 nanosuspension.The surface modification of nanoparticles was carried out using citric acid to improve their suspensibility.It was observed that effective thermal conductivity declined by about 11% relative to that attained in the absence of the externally applied magnetic field attributed to the increased viscosity of the nanofluid owed to nanoparticle magnetization.Davari et al. [44] experimented on an oscillating heat pipe using Fe 3 O 4 nanofluid and observed enhanced performance with the corrugated horizontal condenser.
Zhou et al. [45] fabricated a heat pipe and used aqueous-ethanol-based CNT nanofluids (0.05 wt.% to 0.5 wt.%) and reported 0.2 wt.% nanosuspension to offer better heat transfer characteristics relative to the aqueous-ethanol solution.Aly et al. [46] suspended the γ-Al 2 O 3 nanoparticles of average size 20 nm in DI water to prepare the 3 vol.%alumina nanofluid and reported increased evaporation and condensation rate and decreased resistance at higher fluid fill ratio.Rui Zhou et al. [47] experimented on a microgrooved heat pipe filled with CuO nanosuspensions (0.5 wt.% to 1.5 wt.%).The start-up response of the heat pipe slowed while the heat transfer capability improved significantly by utilizing nanosuspensions.Veerasamy et al. [48] utilized graphene nanosuspension in a grooved heat pipe.
The improvement achieved in the response was reported to be a direct function of nanofluid concentration.
Bhullar et al. [49] evaluated the temporal performance of aqueous Al 2 O 3 nanofluids (0.005 vol.% to 1.0 vol.%.) in heat pipe and attained superior reliability at higher heat loads.The nano-sized coating of Al 2 O 3 nanoparticles on mesh caused performance augmentation.The researchers also fabricated a heat pipe with crimped edges to offer extended conduction length [50] and attained thermal resistance of about 22% employing the 1 vol.%Al 2 O 3 nanofluid.They reported that the thermal conductivity followed a non-linear relationship with the nanofluid concentration.
Channapattana et al. [51] investigated the influence of the number of screen mesh layers on heat pipe using 1.0wt.%CuO nanofluid.The capability of the heat pipe was reported to improve using a higher number of mesh layers.Gupta et al. [52] experimented on a heat pipe using nanosuspension and nanoparticle coating.The heat pipe was filled with TiO 2 nanofluids.Another heat pipe incorporated in the study was coated with TiO 2 nanoparticles and filled with DI water.They reported that the best thermal performance was attained using TiO 2 nanofluid followed by the nanoparticle-coated water heat pipe.The performance of CeO 2 nanofluid and nanoparticle-coated heat pipes was investigated experimentally.Both the heat pipes were tested under a similar set of conditions and it was found that both the heat pipes offered nearly similar thermal performance hence the nanoparticle-coated heat pipes were proposed as a substitute for conventional heat pipes [53].
Sharuk et al. [54] experimented on an aqueous TiO 2 nanosuspension (0.05 vol.% to 0.25 vol.%) filled mesh heat pipe.They reported that with advancement in nanosuspension concentration and applied heat load, the heat pipe conductivity increased.Anand et al. [55] experimented on a mesh & sintered wick heat pipe respectively.Both the heat pipes were filled with DI water-based Al 2 O 3 nanofluid.The maximum enhancement attained in the thermal efficiency in the mesh wick and sintered wick heat pipes using Al 2 O 3 nanofluid was found to be about 37.55% and 41.38% respectively.
Gupta et al. [56] experimented on a mesh heat pipe using CuO nanosuspension (0.5 vol.% to 2.5 vol.%) and achieved an efficiency of 66.5% at 150 W using 1.0 vol.%CuO nanofluid owed to its high thermal conductivity.Nizam et al. [57] prepared DI water-based Al 2 O 3 nanosuspension (0.5 vol.% to 1.5 vol.%) and employed them in a mesh heat pipe.The characterization of the prepared nano-suspensions was carried out through scanning electron microscopy and transmission electron microscopy.It was found that the nanoparticles were spherical in shape and formed a stable suspension within the base fluid.Sankar et al. [58] examined the employability of graphite nanosuspension (0.5 wt.%) in a mesh heat pipe.The nanosuspensions showcased no sedimentation till 9 weeks and improved the heat transfer coefficient attributed to the low density and high thermal conductivity of graphene nanosuspension.

Numerical Researches
The majority of the numerical studies reported in open literature have attributed the advance thermal and flow characteristics of nanosuspensions responsible for the response improvement of the heat pipes.Some of the numerical studies recently reported by the researchers on the same are discussed in the current study.
Alagappan et al. [59] experimented on a nanofluid-filled thermosyphon following the Box-Behnken Design method.They reported that Fe 3 O 4 nanofluid offered better thermal efficiency relative to CeO 2 nanofluid attributed to enhanced convection between the container and nanosuspension.Sarafraz et al. [60] prepared a model of a thermosyphon-assisted solar collector to optimize the operating parameters and maximize efficiency.They reported that utilizing CNT nanofluids promoted nucleate boiling within the working fluid and resulted in enhanced efficiency.Wang et al. [61] prepared a steady-state model of thermosyphon.The nanofluids were reported to reduce the evaporator temperature and further decrease the overall entropy generation.
Xu et al.
[62] experimented on a pulsating heat pipe utilizing silver nanofluids.The simulation was carried out on the FLUENT 15.0 software.The 1 vol.%silver nanofluid was found to offer stable thermal performance in the system.Malekan et al. [63] investigated an oscillating heat pipe with artificial intelligence methods using Fe 3 O 4 nanofluids considering heat load, the conductivity of nanofluids, and the aspect ratio.They reported that the resistance decrement attained was more utilizing Fe 3 O 4 nanofluids relative to γ-Fe 2 O 3 nanofluids.
Gupta et al. [64] carried out heat pipe simulation on Ansys FLUENT (14.0) software at heat loads of 10, 15, and 20 kW/m 2 .The 1.0 vol.%CeO 2 nanofluid offered the least heat transfer resistance and maximum efficiency out of all the considered nano-suspensions.Maddah et al. [65] studied the characteristics of a heat pipe utilizing an aqueous copper oxide nanofluid.They reported that the residuals of the considered parameters scatter around the zero axis and attainment of response augmentation.
Poplaski et al. [66] carried out a numerical simulation on nanofluid-filled mesh heat pipes to study the influence of nanosuspension concentration.They reported that optimal concentration is different for different nanofluids employed within the heat pipe and was found to be 35% for CuO nanofluid and 25 vol.% for Al 2 O 3 and TiO 2 nanofluids considering capillary limit.Herrera et al. [67] numerically studied heat pipe response considering nanoparticle agglomeration and deposition.The capillary limit was reported to augment by about 30 to 40% using nanofluids relative to the base fluid.At high nanosuspension concentrations, the advancement in the capillarity limit was found to be variable and elevated resistance was observed.
Gupta [68] carried out the optimization of a mesh heat pipe using the Taguchi technique.It was found that the heat load has the maximum impact on the efficiency of the heat pipe relative to heat pipe inclination, nanofluid concentration, and fluid fill ratio respectively.Herrera et al. [69] investigated the response of a mesh heat pipe filled utilizing aqueous-Al 2 O 3 nanosuspension using a model taking into account the nanoparticle agglomeration.The prepared model predicted that the capillarity augments by about 32% using nanofluid.Reddy et al. [70] prepared a model for a mesh heat pipe using the response surface methodology along with MINITAB-17 software.They reported that the minimum resistance and maximum convection were attained in the heat pipe when kept at an inclination angle of 57.2° under a heat load of 200 W filled with 0.159 vol.% concentration TiO 2 nanofluid.
It is clear from the discussion that the heat pipe response depends on various parameters.The heat pipe type, heat load, inclination angle, and fluid fill ratio significantly control its heat transfer characteristics.The nanofluids have an appreciable ability to enhance the heat-pipe performance owing to their advance and favorable thermophysical properties.All the studies discussed till now (both experimental and numerical studies) are summarized in Table 1.

Literature
Working fluid Key results
Increment of about 1.42 W in the power generated by the module.
Increment of 27.3% in the total exergy efficiency of the module.
Reduction of about 13.9% and 72% in the evaporator temperature and resistance.
The MWCNT nanoparticles functionalized with sodium persulfate offered better stability in the base fluid relative to that functionalized by potassium persulfate.
The optimum inclination angle and charging ratio were found to be 65° and 60% respectively.The nanofluid added with SDBS offered better stability relative to that added with CTAB.
High bubble formation rate achieved using surfactant-aided nanofluids.
The 2.0 vol.% nanofluid offered high thermal performance at 1000 W.
The 1.5 g/lit GO nanofluid offered deteriorated performance attributed to its high viscosity.
The maximum temperature of the battery was limited to 42.22 °C.

Meena et al. [41] [Ag MNF]
(N.A.) The best effectiveness was attained at a hot air temperature of 80 °C.

Monroe et al. [43] [CoFe 2 O 4 MNF]
(15 mg/mL) The effective thermal conductivity decreased by about 11% in the presence of bias magnets due to the agglomeration of nanoparticles.
Graphene nanofluids improved the capillary action.The response improved with an increase in mesh layers.

Gupta et al. [52]
[TiO 2 MMNFs] (0.5%-1.5% by vol.) A maximum decrement of about 17.3% and a maximum increment of 13.4% were attained in the thermal resistance and thermal efficiency respectively.

Sankar et al. [58] [Graphite MNF]
(0.5% by wt.) A decrement of about 32.50% was observed in the resistance.The overall entropy generation was reduced using Cu nanofluids.

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 Al 2 O 3 and TiO 2 nanoparticles and attained the best response using the (25% Al 2 O 3 +75% TiO 2 ) 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 Al 2 O 3 , SiO 2 , and CuO nanoparticles and reported that the SiO 2 +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 Al 2 O 3 +CuO hybrid nanofluid had the highest conductivity and dynamic viscosity [74].Pandya et al. [75] experimented on a grooved heat pipe utilizing (80% CeO 2 +20% MWCNT) nanosuspension (0.25 vol.% to 1.5 vol.%) and attained the best thermal response using the 1.5 vol.% nanosuspension.
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.

Literature
Working fluid Key results

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  The (25% Al 2 O 3 +75% TiO 2 ) 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.
Çiftçi [72] [AlN+ZnO] 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 operational capacity improved using the 1.5 vol.% nanosuspension.

Studies on mesh heat pipe
Ramachandran et al.The best performance was attained using (75:25) hybrid nanofluid at an inclination of 60°.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.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.

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.

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

Figure
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.Figure4(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.Figure5illustrates 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 Figure5that 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.

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.

NOMENCLATUREFigure 5 .
PV/T system Photovoltaic Thermal system LED Light Emitting Diode SDBS Sodium Dodecyl Benzene Sulfonate CTAB Cationic Cetyl Trimethyl Ammonium Bromide GNP Graphene Nanoplatelet CNT Carbon Nanotube Thermal exchange modes in heat pipes.

Table 1 .
Summary of the studies on the mono nanosuspensions (continue)

Table 1 .
Summary of the studies on the mono nanosuspensions (continue)

Table 2 .
Summary of the studies on the hybrid nanosuspensions