Hacia los nanofluidos de TiO2:Parte 2:Aplicaciones y desafíos

Absorción solar

Como fuente limpia de energía renovable, la energía solar tiene un impacto ambiental mínimo. Sin embargo, el desarrollo del colector solar térmico está restringido por las malas propiedades de absorción del fluido de trabajo convencional. Por lo tanto, en los últimos años, la tecnología de nanofluidos se ha utilizado gradualmente en los colectores solares para producir propiedades ópticas y térmicas superiores. Se espera que esta nueva generación de fluido de transferencia de calor y absorción solar pueda mejorar la eficiencia del uso de la energía solar.

Como se muestra en la Fig. 6, un diagrama esquemático típico de un sistema de calentamiento de agua solar de concentración basado en nanofluidos se puede observar en los informes de Khullar et al. [83]. Pensaron que se podría lograr una gran cantidad de reducciones de emisiones y ahorros de energía al implementar un colector solar de concentración basado en nanofluidos. Chaji y col. [84] investigó los efectos del contenido de partículas y la velocidad de flujo del líquido en la eficiencia de un colector de placa plana a pequeña escala con TiO ₂ nanofluidos. Encontraron el índice de eficiencia del colector usando TiO ₂ Los nanofluidos se incrementaron entre un 2,6 y un 7% en comparación con el fluido base según la norma europea EN12975-2. Said et al. [85] utilizó TiO ₂ –H ₂ O nanofluido como fluido de trabajo para mejorar el rendimiento de un colector solar de placa plana. Observaron que los nanofluidos preparados podrían mantenerse estables durante más de 1 mes. Los resultados mostraron que, en comparación con el fluido a base de agua, la eficiencia energética se puede aumentar en un 76,6% en volumen de carga y una tasa de flujo de 0,5 kg / min, y la eficiencia exergética más alta del 16,9% podría lograrse en estas condiciones de operación.

Esquema de un sistema de calentamiento de agua solar de concentración basado en nanofluidos. Rediseñado según la referencia [83]

La investigación teórica sobre el rendimiento de un colector solar utilizando nanofluidos también se ha desarrollado en los últimos años. Alim y col. [86] estudió teóricamente la generación de entropía, las características de transferencia de calor y la caída de presión de Al ₂ O ₃ , CuO, SiO ₂ y TiO ₂ nanofluidos en un colector solar de placa plana bajo flujo laminar. Descubrieron que todo tipo de nanofluidos pueden mejorar el rendimiento, mientras que el factor de fricción era casi similar al de los fluidos a base de agua. Faizal y col. [87] también llevó a cabo un estudio numérico sobre el rendimiento de esos cuatro tipos de nanofluidos en el colector solar. Observaron que los ahorros de energía de los cuatro tipos de nanofluidos pueden superar el 20%, lo que daría lugar a reducciones de emisiones de gases de efecto invernadero.

La mejora del rendimiento de los nanofluidos en los colectores solares se atribuye generalmente a dos factores principales:los caracteres mejorados de transferencia de calor y las propiedades ópticas. Por lo tanto, los investigadores también investigaron las propiedades ópticas de los nanofluidos en el sistema de absorción solar. Said et al. [88] realizó estudios experimentales y analíticos sobre el rendimiento de absorción solar del TiO ₂ y Al ₂ O ₃ nanofluidos. Utilizaron dos fracciones de volumen de 0,1 a 0,3% en volumen para la investigación de la propiedad fotosensible. Algunas teorías clásicas, incluidas las de Rayleigh, Maxwell-Garnett y Lambert-Beer, fueron adoptadas en su análisis analítico. Concluyeron que las propiedades ópticas del TiO ₂ los nanofluidos fueron más altos que los de Al ₂ O ₃ nanofluidos dentro del rango de luz visible para todas las cargas de partículas. He et al. [89] comparó la eficiencia de conversión de luz-calor del TiO ₂ –Agua y CNT – nanofluidos de agua en un colector solar de tubo de vacío tanto en condiciones soleadas como nubladas. Observaron que el incremento de temperatura del nanofluido CNT-agua es mayor que el del TiO ₂ –Nanofluidos de agua, lo que indicó que la característica de conversión de luz-calor del primero es mejor que del segundo.

Said et al. [90] pensó que la mayor parte de la investigación se centró en las propiedades termofísicas y ópticas fundamentales de los nanofluidos; Los estudios sobre algún factor importante para la dispersión y absorción, incluido el tamaño, la forma y el contenido de las partículas, así como el tipo de fluido base, rara vez se encontraron. Para examinar esos factores, llevaron a cabo una investigación relacionada y observaron que el tamaño de partícula tiene poco efecto cuando está por debajo de 20 nm, y el contenido de partículas era directamente proporcional al coeficiente de extinción. Para los nanofluidos que contienen TiO ₂ de 20 nm nanopartículas, la transmisividad fue casi nula para longitudes de onda que van desde 200 a 300 nm, pero 71% para 400 nm y 88% para 900 nm respectivamente. También sugirieron que la fracción de volumen de TiO ₂ las nanopartículas deben estar por debajo del 0,1%, con lo que se puede obtener un resultado mucho mejor.

Kim y col. [91] llevó a cabo una investigación teórica detallada utilizando MWCNT, Al ₂ O ₃ , CuO, SiO ₂ y TiO ₂ nanofluidos con PG (propilenglicol) –agua (20:80) fluido base en un colector solar de tubo en U de alta temperatura. Observaron que la eficiencia del colector de la eficiencia del colector solar tiene una correlación claramente positiva con la conductividad térmica de las nanopartículas agregadas, ya que está en la secuencia de mayor a menor:MWCNT, CuO, Al ₂ O ₃ , TiO ₂ y SiO ₂ nanofluidos. También analizaron la reducción de emisiones de CO ₂ y SO ₂ así como el consumo de electricidad y energía en todo el mundo. Sus resultados apoyan que los nanofluidos tienen un gran potencial para el ahorro de energía y la reducción de emisiones. Debido a que sus resultados teóricos no han considerado la situación de dispersión de diferentes nanofluidos, es necesario verificar experimentalmente el rendimiento real.

Coincidentemente, Verma et al. [92]. Los resultados experimentales indicaron que para solo un 0,75% de carga de volumen de partículas y a un caudal de 0,025 kg / s, la eficiencia exergética de los nanofluidos en comparación con el agua aumenta en 29,32, 21,46, 16,67, 10,86, 6,97 y 5,74%, respectivamente, para el grafeno. , CuO, Al ₂ O ₃ , TiO ₂ y SiO ₂ nanofluidos. Además, la caída en la generación de entropía siguió esta secuencia. Sus resultados también respaldaron que la eficiencia del colector solar tiene una correlación positiva con la conductividad térmica de las nanopartículas agregadas.

Sin embargo, también hay algunas investigaciones que arrojan resultados diferentes. Mahian y col. [93] analizaron el rendimiento de un colector solar basado en minicanal utilizando cuatro nanofluidos de agua diferentes con Cu, Al ₂ O ₃ , TiO ₂ y SiO ₂ nanopartículas respectivamente. Sus resultados mostraron que el Al ₂ O ₃ Los nanofluidos exhibieron el coeficiente de transferencia de calor más alto, mientras que el valor más bajo perteneció a SiO ₂ –Nanofluidos de agua, pero la temperatura de salida siguió esta secuencia:Cu> TiO ₂ > Al ₂ O ₃ > SiO ₂ nanofluidos. También observaron que la generación de entropía de TiO ₂ –El agua es más baja que la de Al ₂ O ₃ –Nanofluidos de agua a pesar de que la conductividad térmica del primero es menor que el del segundo.

Refrigeración

El nano-refrigerante es un tipo especial de nanofluido que consta de nanopartículas y refrigerante, así como el lubricante probable. Nano-refrigerant is a new generation of refrigerant for using compression or absorption refrigeration, air conditioning systems, heat pumps, etc. In recent years, many studies regarding nano-refrigerants have shown that adding nanoparticles into refrigerants or lubricant can achieve a better system performance and energy efficiency.

Table 2 shows summary of related studies on TiO₂ nanoparticle-based nano-refrigerants. It can be seen that the TiO₂ nanoparticles can work normally and safely with many kinds of refrigerants, including R134a, R600a, R436a, R436b, R141b, R123, R12, R22, and R410a. It can be seen that most results showed that adding TiO₂ nanoparticles could bring benefits to the refrigeration system and the lubricating oil system, such as improving the performance [94], reducing the energy consumption [95,96,97], and the irreversibility [98]. Also, some research focused on the heat transfer [99, 100] and pressure drop [101] of the nano-refrigerant system to investigate the effect mechanism of the nanoparticles. Li y col. [102] investigated the coefficient of performance (COP) of the refrigeration system for both the cooling cycle and heating cycle, and the results showed that adding TiO₂ nanoparticle would lead to a slight decrease in COP of the cooling cycle but a significant increase in COP of the heating cycle based on the power consumption of the compression. Bi et al. [96] experimentally investigated the reliability and performance of a domestic refrigerator with HFC134a as refrigerant and Mineral oil with TiO₂ nanoparticles mixtures as lubricant. As illustrated in Fig. 7, the system main consists of fresh food storage room and frozen food storage room as well as refrigeration system pipelines. The results showed that the system TiO₂ nanoparticles works normally and safely and adding 0.1 wt.% TiO₂ nanoparticles can reduce 26.1% energy consumption while particle type has little effect on performance.

Schematic diagram of a domestic refrigerator with HFC134a, mineral oil and TiO₂ nanoparticles [96]. Reproduced with permission from Elsevier

In addition, there is likewise a forward-looking study on the effect on the environment. Javadi and Saidur [103] observed that adding 0.1% of TiO₂ nanoparticles to mineral oil-R134a could result in the maximum energy savings of 25% and reduce the CO₂ emission by 7 million tons by year of 2030 in Malaysia.

It can be seen from Table 2 that the amounts of nanoparticles used in refrigerants were very low as below 0.1% [94,95,96,97,98,99,100,101,102,103], which can prevent clogging by the possible sedimentation of nanoparticles. However, although all results seemed positive, the long-term performance of the refrigeration system using nano-refrigerants is a great challenge.

Lin et al. [15] investigated the suspending ratio of (0.1 to 1%) nanolubricant–refrigerant after continuous alternation processes of condensation and evaporation. The schematic diagram and photographic view of their experimental system is shown in Fig. 8. They found that the degradation ratio was 28 to 73% after 20 times’ alternate operations. Also, they found lower particle loading can reduce the degradation speed. It can be concluded that the longtime performance of nano-refrigerant system is the essential step for further application in nano-refrigeration system.

un , b Experimental setup for condensation–evaporation alternation [15]. Reproduced with permission from Elsevier

Energy Storage

The storage of latent heat is through the most efficient mean of storing thermal energy. The conventional PCMs have a shortcoming of inadequate heat transfer performance which can reduce the rate of storing and releasing thermal energy. Therefore, some researchers have studied the method of improving the heat transfer performance by adding nanoparticles into PCMs.

Usages of PCMs mainly include energy storage of heating or cooling capacity. Research on cool storage application of TiO₂ nano-PCMs is relatively rare. Liu y col. [104] find that thermal conductivity of saturated BaCl₂ aqueous solution increases remarkably when adding a small amount of TiO₂ nanopartículas. They found the thermal conductivity was increased by 15.65% as the TiO₂ nanoparticle volume fraction was 1.13% at temperature of 15 °C. They thought this nanofluid is a good phase change materials (PCMs) with higher cool storage/supply capacity and rate compared with its base fluids, which exhibited good potential for being applied to cool storage as a substitute for conventional PCMs.

Another similar study was conducted by He et al. [105]. They also found the thermal conductivity of saturated BaCl₂ aqueous solution can be distinctly enhanced by 12.76% when adding a small amount of TiO₂ nanoparticles at −5 °C. Although decreases in the latent heat and specific heat and an increase in viscosity were found, those varieties have little effect on the cool storage system since the supercooling degree is reduced by 84.92%. They also thought that TiO₂ -saturated BaCl₂ aqueous solution is suitable for low-temperature energy storage industries.

Studies on cool storage of TiO₂ nano-PCMs are in the minority, while most PCM applications focus on the heat storage. Table 3 shows a brief summary on the thermal conductivities and the latent heat of TiO₂ nano-PCMs for thermal storage applications in existing literatures. Sharma et al. [106] prepared a composite of palmitic acid (PA) and TiO₂ nanoparticles with SDBS as dispersant for thermal energy storage application. The preparation steps of PA–TiO₂ composites are shown in Fig. 9. It can be observed that the dispersion methods including adding surfactant, stirring, and ultrasonic vibration were implemented under the condition that the temperature of the base PA is above the melting temperature to confirm its liquid state.

Preparation steps of PA–TiO₂ composites [106]. Reproduced with permission from Elsevier

Their results showed that the thermal conductivity increased by 12.7, 20.6, 46.6, and 80% when the mass fractions of TiO₂ nanoparticle were 0.5, 1, 3, and 5%, respectively. And they considered this PCM could be a good candidate as potential solar thermal energy storage materials due to its high latent heat and thermal reliability of palmitic acid. Harikrishnan et al. [107] dispersed TiO₂ nanoparticles into PCM stearic acid and found this composite can accelerate the melting and solidification rates due to the enhanced heat transfer performance. They also observed that the addition of 0.3% nano-TiO₂ nanoparticles can increase the thermal conductivity of stearic acid by approximately 63%. In their another research [108], they used stearic acid and lauric as base PCM and found an increment of 42% in thermal conductivity and a reduction of only 2% of latent heat of fusion. Motahar et al. [109] dispersed the TiO₂ nanoparticles into organic PCM n-octadecane and found that the maximum enhancements of thermal conductivity in solid and liquid phases occurred at 3 and 4 wt.%, respectively. Moreover, the maximum average thermal conductivity enhancement for both phases was 26.6% when loading 5 wt.% nanoparticles.

Another experimental research focusing on the solidification process of PCM containing TiO₂ nanoparticles was also performed by Motahar et al. [110]. They observed that the rheological behavior of liquid PCM–TiO₂ at higher loading tends to Bingham fluids so that their solidification experiments were performed within 0–2.17 Bingham numbers. The results showed that the addition of TiO₂ nanoparticles can enhance the thermal conduction process and hence increase the solidified volume. For particle mass loading of 1, 2, and 4%, the solidified volume fraction was increased by 7, 9, and 18%, respectively. At last, they proposed a universal correlation to predict the solidified volume fraction as a function of Fourier number, Rayleigh number, solid Stefan number, Bingham number, and particle loading.

Most of the results showed that when adding TiO₂ nanoparticles, the thermal conductivity of PCMs can be greatly increased, while the latent heat will be decreased slightly, which is probably as a result of the thermal conductivity of nanoparticles which is much larger than the base composite, while the nanoparticles will not take part in the phase changing process as the base composite. However, in some case, both of the thermal conductivity and latent heat capacity of PCMs were considered to be elevated. Wang y col. [111] prepared nano-PCMs by adding TiO₂ nanoparticles into paraffin. They found the addition of TiO₂ nanoparticles can change the phase transition temperature and latent heat capacity of paraffin. The phase transition temperature dropped with <1% loading, while increased with>2% loading of particles. The latent heat increased firstly and then decreased as the loading of particles increase. And the turning concentration is 0.7 wt.%, at which a maximum latent heat capacity can be achieved. While the thermal conductivity of the nano-PCMs increased monotonously with the loading of TiO₂ nanopartículas. When the loading of TiO₂ nanoparticles reached 7 wt.%, the thermal conductivity was increased by 13% but the latent heat was reduced by 9%.

Heat Pipes

The characteristics of boiling heat transfer and critical heat flux enhancement of nanofluids can be utilized in the heat pipe to improve its performance and broaden the application range. And some numerical results [112, 113] have shown that for thermosyphon heat pipe, using the nanofluid could achieve a better heat transfer characteristics. Also, some researchers have carried out related research using TiO₂ nanofluids.

Zhou et al. [114] tested gravity heat pipes filled with DI water and TiO₂ nanofluids, where the concentration and filling ratio of nanoparticles were varied and the initial temperature distribution was given. The result indicated that the heat pipes filled with nanofluids had a lower start-up temperature and a shorter start-up time in evaporation section under the condition of a water bath. And the biggest temperature drop between the evaporation section and the condensation section for heat pipes filled with TiO₂ nanofluids was lower than those filled with DI water. The start-up time of heat pipes with filling ratios ranged between 50 and 70% in the evaporation section increased with the increase of the filling ratio and heating temperature, but the small inclination angle had a negative effect on the start-up performance.

Saleh et al. [65] collected data from different nanofluid experiments, where particle volume loading was up to 1.0% and the temperature of measurements ranged from 10 to 60 °C. They discovered that these data agreed with the classical Brownian motion theoretical model. They also investigated experimentally the effect of nanofluids on the thermal performance of heat pipes by measuring the wall temperature and thermal resistance distributions between the evaporation and condensation section. They found that distilled water and nanofluids achieved the best heat transfer performance when the inclination was set to 45° and the charge volume ratio of working fluid was 60%.

In 2015, Monirimanesh et al. [115] designed a thermosyphon-type heat pipe heat exchanger (HPHX) using TiO₂ nanofluids as the working fluid to save energy in an air conditioning system. Their experimental apparatus was constructed as shown in Fig. 10. They establish a pre-cooling and pre-heating device to produce altered conditions of the inlet air for investigating the performance of HPHX. The evaporator and condenser section of the HPHX functioned as a pre-cooler and reheating coil for the air conditioning system respectively. They also employed an electric heater and electric boiler to supply heat and steam into the entered fresh air by a fan for the purpose of simulating the hot and humid climate. Their results showed that using TiO₂ nanofluids and increasing the HPHX number of rows could make a part of air condensed on the evaporator fin, which could enhance the energy in the pre-cooling section. The use of 3 wt.% TiO₂ –methanol nanofluids in a four-row HPHX could achieve the highest energy savings ranging from 30.6 to 32.8% when the inlet air under the properties of 45 °C and 50–74% relative humidity. Based on a comprehensive consideration of the main purpose of supplying the energy required for reheating, they suggested that 2 wt.% TiO₂ –methanol nanofluid for the four-row HPHX would have been adequate and more economical.

Schematic of the experimental apparatus [115]. Reproduced with permission from Springer

Mass Transfer

The mass transfer of nanofluids is another important application aspect of TiO₂ nanofluids. Current research has shown that TiO₂ nanofluids can be used to enhance the absorption process of CO₂ and NH3 as well as the mass transfer coefficient of electrolyte fluids.

Li y col. [116] prepared stable N -methyldiethanolamine (MDEA)-based nanofluids to strengthen the absorption performance of CO₂ in the MDEA solution. The CO₂ absorption characteristics in the gas/liquid interface of nanofluids were investigated by measuring the absolute pressure drop of gas. The mass fraction of MDEA was 50%. And they used two particle mass fractions of 0.1 and 0.4%. The results showed that the CO₂ absorption rate increases with increasing temperature and it is enhanced by the added nanoparticles. However, at 20 and 30 °C, the enhancement caused by the mass fraction of nanoparticle (0.1 and 0.4%) reduced gradually. The effective absorption ratio varied from 1.03 to 1.14. Also, CO₂ bubble absorption ratio increased with the increase of nanoparticle mass fraction.

Yang [117] prepared stable TiO₂ nanofluids without adding dispersant and then carried out a comparative experiment on the falling film performance of absorption of ammonia gas by nanofluids and pure water. The schematic diagram of the experimental system for NH₃ –H₂ O nanofluid falling film absorption is shown in Fig. 11. They found that the absorption rate of ammonia gas can be increased by 10% when adding anatase TiO₂ nanofluids. Wu [118] used the similar experimental device but changed the falling film tube of Fig. 11 into a fin tube. He investigated the effect of rutile TiO₂ nanofluids on the ammonia absorption performance of falling film outside a fin tube. The result showed that the combined use of zigzag tubule and TiO₂ nanofluids can strengthen the ammonia–water falling film absorption and the maximum increment can reach 60.8%.

Schematic diagram of the experimental system for NH₃ –H₂ O nanofluid falling film absorption [117]. 1 NH3 vessel, 2 decompression valve, 3 constant pressure controller, 4 , 11 container of solution, 5 inlet of cooling water, 6 , 10 constant flow controller, 7 falling film tube, 8 visible absorber body, 9 solution distributor, 12 tubes for balancing pressure, 13 outlet of cooling water, 14 HP data acquisition instrument, 15 computer

Beiki et al. [119] investigated experimentally the turbulent mass transfer characteristics of TiO₂ and γ-Al₂ O ₃ electrolyte nanofluids in a circular tube. The results showed that adding 0.015 vol.% TiO₂ and 0.01 vol.% γ-Al₂ O ₃ could bring an increase in mass transfer coefficient of the electrolyte solution by 18 and 10%, respectively. They found that the enhancement ratio was independent of Reynolds number. The mass transfer coefficients increased firstly and then decreased as the nanoparticle loading increase. They attributed the cause of the existing of optimal particles’ loading to the clustering of nanoparticles and forming bigger agglomerates with smaller Brownian velocity when exceeding the optimum loading.

Coolant of Milling

As a coolant, nanofluids’ heat transfer enhancement characteristic can improve the cooling performance [120]. Moreover, when nanofluids are used for milling, another characteristic of nanofluids viz. enhancement in wear resistance can also play an important role in extending the lifetime of the milling tool.

Yogeswaran et al. [121] investigated experimentally the effects of coolant of TiO₂ –EG nanofluid on the tool wear and workpiece temperature at the various milling conditions when used for milling a stainless steel AISI 304. The milling tool was made of a TiN-coated carbide insert. The results showed that comparing to pure base fluid, the workpiece temperature was reduced by 30% when using the nanofluid as coolant. The tool wear from milling using the EG-based TiO₂ nano-coolant is much less than using the normal commercial coolant because the nanofluids can reduce the heat penetrating into the inserts. And the tool life is increased as a result of the nanoparticles reduces the damage on the edge of the tool.

Muthusamy et al. [122] also compared the efficiency of nanoparticle-based coolant (TiO₂ –EG) and conventional water-soluble coolant on the tool life and wear performance of a TiN-coated carbide insert in the end-milling process of AISI304 stainless steel. The results showed that using TiO₂ –EG nanofluid as coolant could increase the tool life from 32.67 to 54.9 min (increased by 40.55%) comparing to that using TiO₂ –EG nanofluid as coolant instead of water-soluble coolant. They attributed the cause to a Ti nanoparticle layer on the edge of the insert formed during the milling process when using TiO₂ –EG nanofluid, which can be proved from the SEM and EDX of cutting edge as shown in Fig. 12. When using nanofluids as coolant, the oxidation still occurred despite the cutting temperature was reduced at the interface of the tool and workpiece since it can be found from Fig. 12 the O peak on the EDX spectrum. The hard oxidation layer was formed due to the entering of oxygen from TiO₂ –EG nanofluid into the tool–workpiece interface. Then, the hard oxidation layer can protect the tool from micro-cracking and chipping wear because it could not be easily detached despite under the severe impact of the milling force and took parts of the tool surface from the workpiece.

SEM and EDX of cutting edge [122]. At a cutting speed of 1500 rpm, feed rate of 0.02-mm tooth, and axial depth of 0.1 mm using nanoparticle-based coolant at a cutting distance of 180 mm (×60 magnification). Reproduced with permission from Springer

Challenges and Future Works

Challenges

The above energy-related examples have exhibited the extensive application prospect and excellent properties of TiO₂ nanofluids. Although in some cases, especially in heat transfer applications, the heat transfer performance of TiO₂ nanofluids are not better than that of Ag, Cu, and CNT nanofluids, TiO₂ nanofluid is also a good choice due to their comprehensive properties for instance better dispersion and chemical stability, security, and economy.

Although TiO₂ nanofluids have showed great enhancement in heat transfer of solar collectors, refrigeration, energy storage, heat pipes, and coolant of milling, the investigations on the performances including dispersion stability and heat transfer performance after running operations are in great lack. Most dispersion stability studies are in static conditions, but it is important that the nanofluids prepared should be treated in the practical application conditions to examine the dynamic cycle stability and the sustainability of both system performance and components of nanofluids.

Generally, the biggest downside in application of nanofluids is the sedimentation and degeneration of nanoparticles after long running which makes the long-term performances of nanofluid system challenged. Some researchers have proposed a new idea and a novel method to re-disperse the aggregates in real time of the running system [123]. However, the concrete effect of the device has to be verified experimentally, and then, the design and location of the re-dispersion device needs to be improved. The surfactant is expected to have positive effect on the re-dispersion characteristic of aggregates. However, one of the biggest flaws in using surfactants is the occurrence of foaming when the fluids are under flowing or heating conditions which would have adverse impacts on the heat or mass transfer application of nanofluids. This defect suggests the amount of surfactant employed in the nanofluids should be limited.

Another great limitation in application of nanofluid is the increase in pumping power and pressure drop of nanofluids, which is essential for the high-quality application of solar collectors, refrigeration, and heat pipes. For instance, Sajadi and Kazemi [124] found the proportional increase in pressure drop of TiO₂ nanofluids is higher than that of heat transfer coefficient. While Teng et al. [125] found the pressure drop proportion of TiO₂ nanofluids for turbulent flow is lower than that for laminar flow. Therefore, if the extra energy consumption by the increased viscosity of nanofluids exceeds the benefit from the heat or mass transfer enhancement, there will be no application prospect. The most extreme case is when a large amount of agglomerations emerge, the pumping power and pressure drop of nanofluids will be greatly increased, which might lead to serious impact on system performance. Moreover, based on the similarity principle in heat transfer study, for instance in forced convection process, Nusselt number is determined by Reynolds number and Prandtl number, different thermal conductivity and viscosity will induce different Nu even though for the same experimental heat transfer coefficient. Therefore, the properties of nanofluids are essential for quantitative study in those application fields.

Future Works

As a widely used material in considerable fields, TiO₂ has been explored several hundred years, and its nanofluidic form is also firmly worth studying and expected to make greater contributions owing to the outstanding physical and chemical properties. This paper provides a summary of the research outcomes of TiO₂ nanofluids up to now, including the preparation and stability of TiO₂ as well as three vital properties of TiO₂ nanofluids. It can be concluded that TiO₂ nanofluids show very comprehensive applications in heat transfer or other energy fields due to their good dispersion stability in both hydrophilic and lipophilic liquids, nontoxic and non-corrosive natures, chemical stability, lower price, and good appearance. Therefore, TiO₂ nanofluid is thought as one of the closest kinds to practical industrial application environment because of their better dispersion and chemical stability, security, and economy.

However, although TiO₂ nanofluids have shown enormously exciting potential applications, before commercialization of nanofluids, some urgent problems are summarized as follows:

Firstly, acquiring high-quantity nanofluid with outstanding long-term and high-temperature stability is the fundamental of the entire research since in any practical application, it is essential to have a stable suspension.

Secondly, the way to enhance and keep the stability of nanofluids in real time is a key issue in the actual use since the sedimentations of nanoparticles seem inevitable after a long-term running. The method to re-disperse the aggregation of nanoparticles in real time by adding some dispersion device in the system with functions of ultrasound or agitation might be a useful option [123].

Thirdly, although the surfactants were used to improve the dispersion and adhesion performance of nanoparticles in liquid, the effect of surfactants on the physical properties and system performance needs to be investigated. The amount of surfactants should be investigated experimentally owing to the positive and negative effects of surfactants.

Fourth, the pumping power or pressure drop of nanofluids is another challenge for the engineering application. Using nanofluids with higher viscosity than base fluids will induce a higher pressure drop and hence needs more pumping power [125]. The method to achieve higher heat transfer coefficient and lower pressure drop needs to be further studied.

Fifth, the waste management of the invalid nanofluids should also be considered when applying them to industrial systems. The impact on the environment by the nanofluids restricts many kinds of nanofluids containing heavy metal, toxic substance, or other hazardous substances. The super whiteness dyeing behavior of TiO₂ nanofluids should also be noticed to prevent the environment getting contaminated.

Sixth, although some studies have analyzed the entropy generation in tubes [126], microchannels [127], sheet, and other types of flow [128, 129], the entropy generation characteristic of nanofluid in the full system is actually the most important parameter for the full-system application or designing.

Last but not least, there is lack of evaluation index on the performance of nanofluids, especially on the stability, adhesion, and property sustainability of nanofluids. There is no unified indicator to evaluate the stability and adhesion of nanofluids. The uniform evaluation indexes on the different properties of nanofluids are needed [130].

The above problems are urgently needed to solve for the further application of TiO₂ nanofluids, which point out the directions of the future works in this field. It is believed that these problems and challenges will be solved or reduced with the development of nanofluid technology in the future.

Conclusions

This second part of the review summarizes recent research on application of TiO₂ nanofluids and identifies the challenges and opportunities for the further exploration of TiO₂ nanofluids. It can be concluded that although particle loading has exhibited a positive correlation with thermal conductivity of nanofluids, the effects of other factors including particle shape, size, base fluid type, temperature, surfactant, and sonication are unified. Even for particle loading effect, the intensities of growth in thermal conductivity differ widely for different samples. TiO ₂ nanofluids have shown good applications in many energy-related filed. However, the indeterminacy of long-term performances for both nanofluid and system and the increment in pressure drop are needed to investigate for further application. The forecast research hotspots are regarded as the long-term and high-temperature stability and re-disperse the aggregation of nanoparticles in real-time system, the required amount of surfactants, the heat transfer and pumping power characteristics, and the evaluation index on the stability, adhesion, and property sustainability of nanofluids.

Nanodots de carbono como nanosensores de modo dual para la detección selectiva de peróxido de hidrógeno Investigaciones de estructuras de vacantes relacionadas con su crecimiento en la hoja h-BN