Effect of Nanofluids on the Enhancement of Boiling Heat Transfer: A Review

Boiling heat transfer can play a vital role in the two-phase flow applications. The analysis of the boiling hat transfer enhancement is of importance in such applications and the enhancement can be mostly conducted by using various active and passive techniques. One type of passive techniques is the enhancement of heat transfer by nanofluids. This article presents an extensive review on the effect of different nanofluids on the enhancement of heat transfer coefficient (HTC) and critical heat flux (CHF) for both pool as well as flow boiling. Nanoparticles addition to a working fluid is done arbitrarily to improve the thermophysical properties which in turn improves heat transfer rate. Numerous works have been done in the studies on nanofluid boiling. Among various nanoparticles, the most frequently used nanoparticles are Al2O3 and TiO2. In the case of binary nanoparticles, the most commonly used combination is Al2O3 and TiO2. After reviewing the relevant literatures, it is found that for pool boiling, the maximum HTC is increased to 138% for TiO2 nanoparticles and the maximum CHF is increased to 274.2% for MWCNTs. Conversely, in flow boiling the maximum HTC is increased to 126% for ZnO nanoparticles and the maximum CHF increased to as 100% for GO nanoparticles. In addition, when two or more nanoparticles in succession or binary nanofluids are used the CHF in pool boiling increased up to 100% for Al2O3 and TiO2 as well as the CHF in flow boiling increased up to 100% for Al2O3, ZnO, and Diamond. Though the information of the coefficient of heat transfer and the critical heat flux varied for different nanofluids and vary from experiment to experiment for each of the nanofluids. This variation happens because the coefficient of heat transfer and the critical heat flux in boiling is dependent upon several factors.


Current Situation of The Study of Nanofluids
Nanofluids can be measured as heat transfer fluids of the next generation because they suggest exciting options to increase the efficiency of heat transfer as compared to pure liquids. The growth of publications on nanoparticles research is shown in figure 1. Different researchers have shown that, compared to their base fluids, nanofluids provide improved heat transfer. It can be the motivating force for research to perform and eventually replace traditional functioning fluids. It was seen that arbitrary movement (Brownian movement) of nanoparticles and thermophoresis expand the energy transport performance. However, there are major problems such as erosion, clustering, sedimentation, and precipitation related to the use of nanofluids as the operating fluid. The influence on nanoparticle stability of changes in nanoparticle surface topology has also been well documented in the literature. In various experiments, nanofluids have been used to increase heat transfer in both single and two-phase systems. The findings of the earlier experiments indubitably indicate that with forced convection, nanofluids provide excellent heat transfer enhancement. In contrast, the normal convection and boiling of nanofluids in the literature has led to some contradictory findings [2]. Figure 1: Growth of publications on nanofluids [3] 2 BOILING HEAT TRANSFER A convection heat transfer process including a change in phase from liquid to vapour is referred to as boiling. It can also be defined as the change of phase from liquid to vapour at liquid vapour. Actually, boiling is a convective heat transfer process that includes phase change at a constant temperature from liquid to vapour [4]. During the boiling, the heat added to liquid for its phase change is the latent heat. Sometimes, the boiling process is also referred to as vaporization, when the latent heat of liquid is provided for its phase change such as the process of steam generation in the boiler. Boiling is also a mode of heat transfer that involves the creation of vapour bubbles at the solid-liquid boundary. The growth of vapour and its dynamics depend on temperature excess, nature of the surface and thermophysical properties of the liquid, such as its surface tension, latent heat, etc. Thus, the boiling may occur under various conditions. Based on the presence of bulk fluid motion [5], various modes of boiling are (i) pool boiling and (ii) flow boiling. Pool boiling refers to a situation of boiling in which the heated surface is immersed below the free surface of a stationary liquid and its motion adjacent the surface because of the natural convection only and the combination is made by bubble development and its separation [4]. Flow boiling refers to a situation of boiling, in which the fluid motion is induced artificially by an external means as well as by natural convection and the bubble induced mixing. This type of boiling takes place in high-pressure water tube boilers involving forced convection [5]. The schematic of the above-described types of boiling can be given by figure 2. Figure 3 shows the conventional pool boiling curve of water at atmospheric pressure and the schematic of each regime of it.  Figure 3: The a) pool boiling curve with b) schematic representation of each boiling regime [5,6] 3 MORE ABOUT NANOFLUIDS 3.1 Why Nanofluids in Boiling Heat Transfer? In recent times the experimental research has shown that nanofluids have more thermal conductivity than the base fluids. Nanofluids have many advantages of over the traditional solid-liquid suspensions. Some of them are as follows: 1. There are a greater specific surface area and greater stability of the colloidal suspension. 2. It lowered the pumping power to accomplish same heat transfer amplification compared to the base fluid. 3. It reduced particle obstruction in comparison with convention slurries, thus helping system reduction. 4. By changing particle concentrations to suit various applications, regulating properties, including thermal conductivity and surface wettability [7]. 5. The properties of the fluid will differ by adjusting the concentration of the nanoparticles in order to make it suitable for various applications. 6. Nanofluids have minimum pressure drop as the particles are nanometre in size. 7. Nanofluids are also fit for fast heating and cooling systems. 8. By using nanofluids heat exchanger systems can be made smaller and lighter that will result in cost as well as energy savings. It is notably observed that the heat transfer coefficient and critical heat flux can be increased by using nanofluids in the boiling heat transfer. This improved heat transfer coefficient and critical heat flux can be implemented in the design of heat exchangers and heat storage systems. For these reasons, nanofluids are important for boiling heat transfer research.

How Does Nanofluid Work?
Heat transfer relies on the stroll of the molecules within the substance. Temperature can be a range in which the movement energy of substance molecules is expressed in order to increase the temperature, energy from outside must be given to a substance. By altering the density and specific heat, nanofluid enhances the heat transfer characteristics of water. Despite the fact that water is generally utilized as a heat transfer medium, it is certainly not an ideal base liquid. The nanofluid heat transfer is more in contrast to the base fluid. As a result of this enhance in heat transfer nanofluid allows the heat to be transferred to the target faster and more efficiently. Nanofluid provides higher efficiency with lower energy [8].

PREPARATIONS OF NANOFLUID
The nanofluid generation to alter the rate of heat transfer of traditional fluids is the most significant step in the use of Nano phase particles. As a liquid-solid mixture, the nanofluid is not only stated. Nanofluids consist primarily of carbides, oxides, metals and carbon nanotubes which, with the addition of stabilizing agents, can simply be circulated into heat transfer fluids, like as hydrocarbons, water and ethylene glycol. Several processes, namely gas condensation, mechanical attribution, or chemical precipitation, may also create nanoparticles. Under cleaner conditions, these nanoparticles can also be produced and their surface can be saved from sudden coatings that can occur during the gas condensation phase [8,9]. The simple procedure of nanofluids preparation is shown in figure 4, Where nanoparticles are fixed up with base fluids in order to get the nanofluids [10]. Two key methods of preparing nanofluids are available. Single-step method and Two-step method. 4.1 Single-Step Method A single-step method has been developed to decrease the synthesis of nanoparticles. Using this process, there are many ways to prepare nanofluids, including direct condensation of evaporation, condensation of chemical vapour and chemical synthesis in one step [9]. The single-step process, also known as the direct evaporation condensation method, consists of following main stages. 1. The base fluid for example water or ethylene glycol is circulated in the inner part of a cylinder so that a thin film of the fluid is regularly expelled out of the upper of the chamber. 2. Small pieces of metal evaporate using heat to form nanoparticles. 3. Finally, the fluid is allowed to cool downwards to avoid any unwanted evaporation.

Two-Step Method
Most commonly, this process is used to make nanofluids. A two-step process where nanoparticles or nanotubes are formed as a dry powder chemically or physically for the first time. In the second level, the resultant nanoparticles are dispersed in the liquid with the aid of intense magnetic energy movement, ultrasonic movement, high shear mixing, homogenization, and ball milling. As the nano powder synthesis methods have already been measured at the industrial production level, the two-step approach is the most reasonably priced approach for manufacturing nanofluids on a large scale. The two-step method for nanofluids preparation is shown in figure 5. Due to a higher surface area and surface activity, nanoparticles tend to accumulate. Using surfactants is a vital method to improve the stability of nanoparticles in liquids. Several advanced nanofluid production technologies (including the one-step process) have been developed due to the complexity of preparing stable nanofluids using the two-step method [8].

NANOFLUIDS ENHANCEMENT ON BOILING
Boiling has an important character in several engineering application and technological regions, for example electricity generation, nuclear reactor cooling, cooling of electronic components, etc. There are a many research and review articles regarding nanofluids; few on their expected advantages on heat transfer applications and furthermore few on their heat conductivity improvement. The usage of nanofluids for boiling enhancement is a promising zone that is as of now being explored by various researchers for pool boiling and as of late for flow boiling applications. Figure 6 shows the quick advancement of yearly research publications on nanofluids boiling heat transfer. In latest years, a sharp increase in nanofluid boiling studies have found because of the higher thermal conductivity of nanofluids. In this review article, all prevalent pool boiling and flow boiling articles using nanofluids to date have been consolidated. A summary related to pool boiling and flow boiling of nanofluid studies has been provided in the later section. It is to be hoped that this article will give a brief and reasonable record of the benefits and restrictions of nanofluids in regard to their boiling performance and application.
In Figure 7, the first number means number of articles reviewed of a nanoparticle or a series of nanoparticles and the following percentage means the corresponding percentage of that nanoparticles within the total percentage.   Table 1 to Table  4. In this literature review, the impacts of nanofluids on the most significant parameters, like HTC and CHF, in pool boiling is studied. The recent enhancement pool boiling heat transfer with nanofluids are discussed below.
[10] experimentally and numerically scrutinized the heat transfer enhancement in pool boiling and condensation by changing the hydrophilicity or hydrophobicity properties of the working fluid, i.e. by using nanofluid solution. In order to determine the influence of nanoparticle concentration on heat transfer properties, two separate nanofluid solutions (h-BN/DCM and SiO2/DCM) were arranged and tested for three distinct volumetric concentrations as well as different heat flux conditions. The boiling curves for DCM, h-BN/DCM and SiO2/DCM nanofluids under saturation and after saturation conditions at various concentration is shown in figure 8. The enhancement rate of HTC for saturation boiling, boiling after saturation, and condensation methods was found to be 27.59%, 14.44%, and 15% respectively for the experimental outcomes of h-BN/DCM nanofluid and for SiO2/DCM nanofluid it was 20.69%, 16.67%, and 21.86%, respectively.
A theoretical analysis was carried out by J. Ham et al.
[11] on pool boiling features of Al2O3 nanofluids based on volume concentration and nanoparticle size by using the HFP (heat flux partitioning) model with variations in the contact angle. Their result showed that the CHF base of nanofluids is higher than that of the liquid. The most critical heat flux was found to be 1,515 kW/m 2 under a volume density of 0.05vol.% and was gradually reduced with the density of nanofluids and the size of L2 and L3-nanoparticles was 50 nm and decreased gradually.
Donga et al. [12] investigated an experiment on boiling heat transfer characteristics of Al2O3-water nanofluid in swirl micro channels subjected to an acceleration force. They applied three test sections with dissimilar geometric parameters. The concentration (vol.) of Al2O3 nanoparticles (average diameter of 13 nm) was changed from 0.07% to 0.1%. The mass flow rate and vapour qualities were between 3-6 kg/h and 0.4-1.0%, respectively. Their investigational outcomes showed that the direction and width of the acceleration had an important influence on the BHT and that HTC increased with the increasing of mass flow rate and volume concentration increased and reduced with the direction ratio.
Khliyeva et al. [13] experimentally studied the impact of nanoparticle added substances to the refrigerant R141b on the pool boiling process. They observed that TiO2 nanoparticles additives to the R141b / Span-80 solution contributed to a 2-8-fold reduction in the amount of nucleation sites. They also observed that the increase in heat flux contributes to a rise in the disparity between the nucleation site density magnitudes in R141b and R141b/Span-80 boiling. At a heat flux of 7.5 kW/m 2 , compared to boiling on the stainless-steel surface, the amount of nucleation sites on the Teflon surface was 2 times lesser. They also found that the form of surfaces did not affect the number of nucleation sites and the frequency of vapor bubble take-off in nanofluid boiling in the whole explored the scope of heat fluxes and the frequency of vapor bubble departure in R141b boiling and R141b/Span-80 solution on the Teflon surface 1.5-2 times lower compared to boiling on the surface of stainless steel. An experimental study was directed by Sarafraz et al. [14] on the thermal behaviour of the ZnO nanoparticles dispersed in water-ethylene glycol (water/EG) as the base fluid under the pool boiling condition. They investigated the effects of various operational factors like as heat flux and mass concentration on HTC and thermal fouling resistance provided by heat ZnO nanofluid. Their findings showed that the HTC for ZnO/water/EG nanofluids increased with the rise in heat flux and mass concentration at a concentration of 0.5% by weight. With increasing in the nanoparticle mass concentration, the CHF point was increased because of the increase in the nanoparticle's deposition layer.
Kamel et al. [15] experimentally investigated the performance of pool boiling heat transfer by using tungsten oxide WO3 nanomaterial-based deionized water nanofluids. They made WO3-based water nanofluids by using twostep process. The most important steps they observed were to achieve a stable suspension during the creation of nanofluids. Figure 9 shows the different boiling phenomena of their experiment at different volume fractions. Their experimental findings showed that at concentrations of 0.005% vol., the PBHTC ratio increased by around 6.7% and 0.01 vol.% in the high heat flux area. While there was a decrease in this ratio by about 15% at a volume concentration of 0.05% vol. in the low heat flux area. Yanwei et al. [16] investigated the boiling heat-transfer properties of ethylene glycol (EG) and water (H2O) mixture based SiO2 nanofluid under realistic conditions. They used a 0.6 vol.% EG aqueous solution as a base liquid due to the perfect frost-proofing performance of CO2 nanofluids based on a mixture of E-glycol (EG) and water (H2O) in realistic situations. Their results showed that the nanoparticle diameter decreased from 120 nm to 64 nm, increasing the boiling heat-transfer coefficient (HTC). Also, with increasing in the concentration of nanoparticle, initially HTC was found to be increased quickly and then showed a decrease of 0.25-1.00% in the concentration range. Also, above 0.75 vol.%. nanoparticle, HTC decreased.
Shoghl et al. [17] conducted an experimental study regarding boiling performance of ZnO, α-Al2O3 and MWCNTs/water nanofluids. The ZnO and Al2O3 nanoparticles impairing heat transfer when add MWCNTs resulted in improving heat transfer. The essential effect of nanofluid which was attributed to the reference heat transfer coefficient improved by ZnO and Al2O3 while decreased by CNTs at the low concentrations (0.01 wt.%).
Aizzat et al. [18] conducted an experiment to identify the Heat Transfer Coefficient (HTC) in saturated pool boiling of single and hybrid water-based nanofluids. They chose Al2O3 and SiO2 nanoparticles and mixed into two different single nanofluids and mixed to achieve a ratio of 0:100, 25:75, 50:50, 75:25, and 100:0 % to 0.001 vol. percent final concentration. On a regular basis, they used these blends to achieve HTC standards through experimental work. They found that at the lowest concentration of Al2O3 nanofluids (0.00025 vol.%), HTC improved significantly but decreased for CO2 nanofluids. Distinctly, as with hybrid nanofluids, HTCs were dramatically improved in the early stages but gradually decreased over time, especially in the case of higher proportions of SiO2 nanofluids.
Girish Sapre et al. [19] conducted an experiment on Pool Boiling Heat Transfer using Titanium Oxide (TiO2) and Magnesium Oxide (MgO) nanofluids. Their experimental setup consisted with a Borosil transparent glass vessel having size 130 mm diameter and 205 mm height. Due to increased thermal conductivity of nanofluid and other reasons such as presence of Brownian motion and diffusion of nanoparticles in distilled water the Critical Heat Flux of TiO2 and MgO nanofluid are superior to the distilled water and keep on increasing with the volume fraction, both fluids show significant increment in the CHF. Their result also showed that CHF improves in the range of 10.51% to 52.42% in case of TiO2 and 23% to 60.88% in case of MgO nanofluid with increasing in volume fraction. Figure 9: Different boiling phenomena for WO3-based water nanofluids at different volume fractions [15] 5.2 Literature Review on Flow Boiling Heat Transfer of Nanofluids A boiling heat transfer process in which an external means, as well as natural convection and bubble-induced mixing artificially causes the fluid motion is known as flow boiling. It can be used in a range of industrial applications, for example air conditioning, cooling of nuclear reactors, components of power plants (boilers), refrigeration, petroleum, and high-tech electronic cooling of components. In order to improve the efficacy of such applications in terms of heat removal or cooling systems, it is important to boost the heat transfer method of flow boiling to find a substantial decrement from the consumption of energy. The use of solid nanoscale particles with traditional cooling fluids is one of the most successful methods to enhance the flow boiling heat transfer. This solution will give new and imaginable energizing results compared to host fluids to increase thermal exchange efficiency. For certain fluids, nanofluids build a fresh fluid category and a promising next-generation. Since 2003 a lot of research paper have been published on nanofluids flow boiling, that are listed in Table 5 to Table 8. In these studies, similar to pool boiling, the influence of nanofluids on the significant factors, like as HTC and CHF, in flow boiling have been studied. The recent flow boiling heat transfer enhancement on nanofluids are discussed below. Sudheer et al. [20] investigated an experimental studies of heat transfer and flow regimes during flow boiling of water and alumina nanofluids at different heat and mass fluxes. They investigated the flow boiling heat transfer in a vertical pipe of internal diameter 7.5mm with pure water and Al2O3/water nanofluid as working fluids. For their study, they considered the particle concentrations of 0.001%, 0.005% and 0.01%. From the results, it was seen that flow boiling HTC was increased with mass flux for both water and nanofluids. It was also found that the HTC increased with particle concentration due to the altered characteristics of the heater surface and changes in the process of bubble formation. The average rise in heat transfer coefficient, in comparison with the base fluid, for the nanoparticle concentrations of 0.001%, 0.005% and 0.01% at a mass flux of 905.42 kg/s-m 2 was found to be 12.11%, 21.75% and 27.97%, respectively. The flow boiling HTC was seen to be more for the nanofluids compared to water.
Wang et al. [21] investigated experimentally the study on Al2O3/H2O nanofluid flow boiling heat transfer under different pressures. By ultrasonic oscillation, the Al2O3/H2O nanofluid was prepared. The nanoparticle concentration was 0.1 and 0.5 vol. percent with diameter 20 nm. Their results indicated that the heat transfer rate of Al2O3/H2O nanofluid was increased in flow boiling by roughly 86% as compared to pure water. In their range of work, the rise in the average number of Nusselt nanofluids compared to deionized water was 35 percent.
Om Shankar et al. [22] studied an experiment on the flow boiling HTC by using ZnO-water nanofluids. Their 780 mm long annular test section was consisted with an electrically heated rod and a 21.8 mm internal diameter outer borosilicate glass tube. The heater was made with hollow stainless-steel rods of 12.7 mm diameter welded at both ends to solid copper rods. By using an ultrasonic vibration mixer, they prepared ZnO-water nanofluids with volume concentrations of ZnO particles ranging from 0.0001 to 0.1%. They found that, at a higher concentration of nanoparticles (0.1%), the HTC increased by 126% than the base fluid and increased surface roughness of the heater rod by 1367%, because of the deposition of ZnO/water nanofluid. Figure 10 shows the effect nanoparticle concentration on heat transfer coefficient at various heat flux. Sarafraz et al. [23] conducted an experiment on flow boiling heat transfer coefficients of DI-H2O and CuO-H2O based nanofluids at various working environments. They found that the flow boiling HTC enhanced for DI-water and CuO-water nanofluid in forced convective and nucleate boiling areas by enhancing the applied heat flux. The HTC also increased significantly in both regions by rising the flow rate of fluids. Their findings also revealed that the inlet temperature of fluids played an important character in HTC in nucleate boiling area.
Sarafraz et al. [24] performed an experimental investigation on the flow boiling heat transfer characteristics of MgO/therminol 66 nanofluid as a potential coolant on a copper-made disc. They prepared nanofluids with the help of two step method at wt.%=0.1, and wt.%=0.3. Results of their experiment showed that the existence of MgO/therminol 66 growths the flow boiling HTC as compared to the conventional base fluid. However, the coefficient of heat transfer of the nanoparticles decreased with increasing nanoparticle concentration. The HTC was also found to decrease with operating time because of the existence of nanoparticles on the boiling surface, resulting in thermal resistance on the surface being formed. The highest rise in the coefficient of heat transfer was 23.7% at wt.% of 0.1. For wt.% of 0.2 and 0.3, the greatest enhancement was found 16.2% and 13.3% respectively. Figure  12 shows the several boiling phenomena of their experiment. Figure 11: Some of the experimental findings [24] Nakhjavani et al. [25] conducted flow boiling heat transfer characteristics of titanium oxide/water nanofluid (TiO2/water) in an annular heat exchanger. They had seen that increasing the volume flow and the heat flux applied would increase the HTC while increasing the nanofluid weight concentration, initially improving the HTC so that the maximum increase in the HTC was 35.7% at wt. % of 0.15 and Re = 13500 as shown in figure 12, but over time, the nanofluid HTC decreased. Due to the rise of the IT from 333 K to 363 K, the maximum change in HTC was 4.2 percent at wt. percent = 0.15 and Re = 13500. The formation of the bubble was also found to be a strong function of the heat flux applied, so that the rate of bubble formation increased with the increase in the heat flux, which was also the reason behind the improvement in the HTC at larger heat fluxes applied. A persistent fouling layer of nanoparticles (NPs) was also found to be formed on the boiling surface, which induced thermal resistance against the transfer of boiling heat. The production of fouling reduced the HTC of the NF so that after 1000 minutes of the heater operation, the extreme reduction in the HTC was 21.6%.
Wang et al. [26] conducted a mechanism of heat transfer enhancement and decrease of nanofluid flow boiling heat transfer. By considering the pressure, mass flux, and heat flux they experimentally studied the heat transfer characteristics of SiC/H2O and Graphite/H2O nanofluids. They found both enhancement and decrease in boiling heat transfer of nanofluids compared to based fluid. In both cases, they used 0.1vol.% the concentration of the nanofluids. The flow BHT capacities of SiC/H2O nanofluid were varied from -24.7% to 30.6% and from -29.4% to 13.3% for Graphite/H2O nanofluid as compared with deionized water several heat and mass flux condition. Figure 13 shows effects of mass flux and wall heat flux on Nusselt number of their experiment.
(a) (b) Figure 12: Effect of the Reynolds number on a) HTC b) Pressure drop [25] (a) (b) Figure 13: Effects of a) mass flux and b) wall heat flux on Nusselt number [26] 6 SUMMARY The following tables show the summary of the pool and flow boiling since 2003. Before making the summary table the nanoparticles are classified into four broad classes namely metal, metal oxide, non-metal, and nanoparticle mixture. The available metals that used for the enhancement of boiling heat transfer are Au and Copper. H2O is the only base fluid that used with metal to form the nanofluids. The several metal oxides that used for the enhancement of boiling heat transfer are Al2O3, TiO2, Bi2O3, GO, CuO, CuO2, ZnO, Fe3O4, ZrO2, CuO2, WO3, and MgO. The several base fluids that are used with the metal oxide to produce nanofluids are H2O, DI H2O, ethanol, ethylene glycol, pentane, CMC solution, different refrigerants, and polyester oil. Different non-metals like ceramic, CNs, MWCNTs, and non-metallic oxide like GO, SiO2 are used for the enhancement of boiling heat transfer. Many researchers use binary nanoparticles or a series of individual nanoparticles that are termed as nanoparticles mixture in the summary table. Table 4 give some brief information regarding the maximum enhancement percentage of pool boiling heat transfer with different nanofluids. Some of the results are related to HTC enhancement percentage, and others are related to CHF enhancement percentage. Many of the research papers didn't mention the amount of enhancement percentage. Few results show that HTC and CHF can also be decreased even by using nanofluids. Based on the availability of the data figures 14 and 15 show the maximum enhancement percentage for different nanoparticles and binary nanoparticles. From the summary Table 1 to Table 4, the maximum increased pool boiling HTC is found as 138% for TiO2 nanoparticles and the maximum increased critical heat flux is found as 274.2% for MWCNTs. When two or more nanoparticles in succession or binary nanofluids are used the CHF increased up to 100% for Al2O3 and TiO2. Tables 5-6 give some brief information regarding the maximum enhancement percentage of flow boiling heat transfer with different nanofluids. Some of the results are related to the HTC enhancement percentage, and others are related to the CHF enhancement percentage. Many of the research papers didn't mention the amount of enhancement percentage. Few results also show that HTC and CHF can also be decreased even by using nanofluids. This depends on some operating conditions. Based on the availability of the data, figure 16 shows the maximum enhancement percentage for different nanoparticles and binary nanoparticles. From the summary table, the maximum increased heat transfer coefficient is found as 126% for ZnO nanoparticles and the maximum increased critical heat flux is found as 100% for GO nanoparticles. When two or more nanoparticles in succession or binary nanofluids have used the flow, boiling increased up to 53% for Al2O3, ZnO, and Diamond. BHT enhanced between 11 and 21%. Rising particle concentration, BHT increased. HTC ratio was increased by about 6.7%.    Copper H2O [54] 50 nm parallel minichannels of dh = 800 μm.
With increasing nanoparticle concentration, BHT enhanced.   BHT decreased by 55% as compared to uncontaminated R-134a.
(a) (b) Nanoparticles/Binary Nanoparticles 4. Nanofluids boiling heat transfer activity depends on different variables as shown in figure 17 which includes the thermophysical characteristics of nanofluids, mutual interactions, type of the heating surfaces, type of coatings in the plate, type of nanoparticles, the concentration of the nanoparticle (by volume and mass), size of the nanoparticle, heat and mass fluxes, pressure system, vapor quality and additives, etc. 5. In the case of pool boiling the maximum increased heat transfer coefficient is found as 138% for TiO2 nanoparticles and the maximum increased critical heat flux is found as 274.2% for MWCNTs. 6. In the case of flow boiling the maximum increased HTC is found as 126% for ZnO nanoparticles and the maximum increased critical heat flux is found as 100% for GO nanoparticles. 7. When two or more nanoparticles in succession or binary nanofluids are used the CHF in pool boiling increased up to 100% for Al2O3 and TiO2. Also, the CHF in flow boiling increased up to 53% for Al2O3, ZnO, and Diamond. 8. Binary nanofluids are superior CHF for both pool and flow boiling rather than HTC. 9. Further work will be needed in order to find more suitable nanofluids for BHT as the findings of various researchers are not constant. So, testing more and more nanoparticles are essential. 10. To make applicable nanofluids, stability is the most important thing. It should thus be examined and developed for both steady and flow conditions in nanofluids boiling.