Nanofluids are potential heat transfer fluids with enhanced thermophysical properties and heat transfer performance can be applied in many devices for better performances (i.e. energy, heat transfer and other performances). In this paper, a comprehensive literature on the applications and challenges of nanofluids have been compiled and reviewed. Latest up to date literatures on the applications and challenges in terms of PhD and Master thesis, journal articles, conference proceedings, reports and web materials have been reviewed and reported. Recent researches have indicated that substitution of conventional coolants by nanofluids appears promising. Specific application of nanofluids in engine cooling, solar water heating, cooling of electronics, cooling of transformer oil, improving diesel generator efficiency, cooling of heat exchanging devices, improving heat transfer efficiency of chillers, domestic refrigerator-freezers, cooling in machining, in nuclear reactor and defense and space have been reviewed and presented. Authors also critically analyzed some of the applications and identified research gaps for further research. Moreover, challenges and future directions of applications of nanofluids have been reviewed and presented in this paper. Based on results available in the literatures, it has been found nanofluids have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional fluids. This can be considered as one of the key parameters for enhanced performances for many of the applications of nanofluids. Because of its superior thermal performances, latest up to date literatures on this property have been summarized and presented in this paper as well. However, few barriers and challenges that have been identified in this review must be addressed carefully before it can be fully implemented in the industrial applications.

/pdf/a-review-on-applications-and-challenges-of-nanofluids-ibdekzmmqr.pdf

A review on applications and challenges of nanofluids

Science and technology of polymeric ablative materials for thermal protection systems and propulsion devices: A review

Nanofluids are described as a relatively new kind of colloidal solutions with a particle size smaller than one billionth of a meter (1–100 nm) suspended in the base fluid so as to enhance the thermophysical properties, which makes them an obvious choice for use in number of commercial applications including engineering, medical sciences, biotechnology, agriculture technology, transportation etc. With the advancement in nanotechnology during last few years, scientific community focuses on improvising combustion behavior, stability aspects, various engine performance parameters and emission characteristics of conventional diesel engine using nanoparticle laden diesel biodiesel fuel blends. Most recently few experimental works on above issues using nanosized metallic, non-metallic, organic and mixed particles in the base liquid fuel for diesel engine have appeared in the open literature. The obtained results are very encouraging due to multifold enhancement in thermo physical and chemical properties of modified fuel such as high surface to volume ratio, high reactive medium for combustion, enhanced heat and mass transport properties due to high thermal conductivity, improvement in flash point, fire point, pour point etc depending upon the type of nanoparticles used, their particle size and concentration with base fuel. Despite having all superiorities, somewhat unclear and contradictory results are found in the literature, further the experimental results of different researchers are not generalized so far as to reach at common consensus about this new approach of fuel modification. Keeping all these facts in mind, a serious attempt has been made to summarize the important published work on combustion and stability aspects of nanoparticle laden diesel, biodiesel fuels and their blends, and its effects on fuel and engine overall characteristics with the objective to provide a pathway to conduct further research in this area for utilizing maximum potential of nanoparticle fuel emulsion technology and to provide a promising future fuel for diesel engine.

A comprehensive review on combustion and stability aspects of metal nanoparticles and its additive effect on diesel and biodiesel fuelled C.I. engine.

The present study attempts to improve the ignition properties of diesel fuel by investigating the influence of adding aluminum and aluminum oxide nanoparticles to diesel. As part of this study, droplet ignition experiments were carried out atop a heated hot plate. Different types of fuel mixtures were used; both particle size (15 and 50 nm) as well as the volume fraction (0%, 0.1%, and 0.5%) of nanoparticles added to diesel were varied. For each type of fuel mixture, several droplets were dropped on the hot plate from a fixed height and under identical conditions, and the probability of ignition of that fuel was recorded based on the number of droplets that ignited. These experiments were repeated at several temperatures over the range of 688-768 degrees C. It was observed that the ignition probability for the fuel mixtures that contained nanoparticles was significantly higher than that of pure diesel.

Increased Hot-Plate Ignition Probability for Nanoparticle-Laden Diesel Fuel

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35−14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

/pdf/functionalized-graphene-sheet-colloids-for-enhanced-fuel-28gnzsi50s.pdf

Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion

Performance comparison of HTPB-based solid fuels containing nano-sized energetic powder in a cylindrical hybrid rocket motor

Nano-sized aluminum- and boron-based solid-fuel characterization in a hybrid rocket engine

Nitromethane was investigated in this study due to the push for higher performance and reduced toxicity monopropellant. A comprehensive detailed model for its flame structure and linear regression rate was developed and validated with experimental data. The model considered one-dimensional behavior with surface vaporization and detailed gas-phase kinetics based on the RDX mechanism of Yetter et al. combined with the nitromethane decomposition mechanism of Glarborg, Bendtsen, and Miller, resulting in a mechanism consisting of 47 species and 250 elementary reactions. The predictive model was implemented using a custom FORTRAN code wrapping the CHEMKIN 4 PREMIX gas-phase solver coupled with the condensed-phase solution. Predicted burning rates using the model showed good agreement with measured rates up to 15 MPa. Calculated species and temperature profiles showed three distinct regions based upon the appearance and consumption of certain species. The first region was marked by decomposition of nitromethane, the second region by consumption of all intermediate species except CH4 and NO, and the third region by the rise to final temperature and species concentrations near the equilibrium values. Among the intermediate species, CH4 and NO had higher concentrations than those of CH2O, N2O, HNO, and HONO. CH4 served as fuel species and NO provided a portion of the oxidizer for the third-region reactions to reach equilibrium composition. Sensitivity analysis identified the importance of two elementary reactions involving HNO to the temperature profile, and therefore the burning rate. Although the absolute level of NH and HCO were low, they served as an important intermediate species transporting nitrogen and carbon, respectively, between other higher-concentration species. The calculated flame zone thickness is consistent with that measured by microthermocouples.

Modeling of nitromethane flame structure and burning behavior

Conventional polymeric fuels for hybrid rocket propulsion have relatively low regression rates. Two possible solutions were examined: utilization of energetic nano-sized particles, and adoption of non-polymeric paraffin fuel. Addition of 13 wt.% of nano-sized tungsten powder to HTPB-based fuel resulted in an increase of 38% in fuel regression rate compared to the pure HTPB fuel. The use of nano-sized tungsten powders in solid fuels for volume limited propulsion systems is greatly beneficial due to its high density, high heat of oxidation, and low oxidation temperature. SEM/EDS micrographs of the newly processed energetic paraffin-based solid fuels have shown that the nano-sized Silberline aluminum flakes are homogenously mixed in the fuel matrix. Paraffin-based solid fuels containing aluminum flakes showed a significant increase in regression rate over the non-aluminized paraffin fuel. A real-time X-ray radiography system enables the measurement of the instantaneous radius of the solid fuel grain. The radial increment of the regressing fuel surface can be correlated with time in a power-law form. An implicit relationship showing the dependency of instantaneous fuel regression rate on the total mass flux was obtained. The functional relationships for aluminized HTPB and paraffin fuels were obtained in graphical forms. Results show that the conventional power-law relationship between the average regression rate and average oxidizer mass flux cannot be applied to the instantaneous regression rates of solid fuel burning in hybrid rocket motor conditions.

Characterization of Nano-Sized Energetic Particle Enhancement of Solid-Fuel Burning Rates in an X-Ray Transparent Hybrid Rocket Engine

R low mass and linear regression rates of solid fuels have been among the major drawbacks of classical hybrid rocket engine technology due to low density, inertness of conventional solid fuels, and the diffusion-controlled combustion process. The thermal degradation in the pyrolysis process of inert polymeric fuels has been considered to be one of the key processes occurring in hybrid rocket engines and solid-fuel ramjet engines [1]. The fuel-surface regression rate generated by this process is a very important design and performance parameter and is strongly affected by the operating conditions and the composition and thermophysical properties of the solid fuel. In addition, fluid dynamic, heat-transfer, and combustion processes in these solid-fuel systems are characterized by complex interactions involving numerous physical phenomena, simultaneously taking place in the combustion chamber and the fuel grain. These complex interactions include solid-fuel pyrolysis; metal vaporization for metallized solid fuels; oxidizer atomization and vaporization; gas-phase species mass diffusion; mixing and combustion between the fuel-rich and oxidizer-rich species; turbulent flow with mass addition; conductive, convective, and radiative energy transfer; and time-varying

Metals, Energetic Additives, and Special Binders Used in Solid Fuels for Hybrid Rockets

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