• Energy Research

The conversion of renewable feedstocks into new added-value products is a current hot topic that includes the biodiesel industry. When converting vegetable oils into biodiesel, approximately 10% of glycerol byproduct is produced. Glycerol can be envisaged as a chemical platform due to its chemical versatility, as a scaffold or building block, in producing a wide range of added-value chemicals. Thus, the development of sustainable routes to obtain glycerol-based products is crucial and urgent. This certainly encompasses the use of raw carbonaceous materials from biomass as heterogeneous acid catalysts. Moreover, the integration of surface functional groups, such as sulfonic acid, in carbon-based solid materials, makes them low cost, exhibiting high catalytic activity with concomitant stability. This review summarizes the work developed by the scientific community, during the last 10 years, on the use of biochar catalysts for glycerol transformation.

Spray impingement on smooth and heated surfaces is a highly complex thermofluid phenomenon present in several engineering applications. The combination of phase Doppler interferometry, high-speed visualization, and time-resolved infrared thermography allows characterizing the heat transfer and fluid dynamics involved. Particular emphasis is given to the use of nanofluids in sprays due to their potential to enhance the heat transfer mechanisms. The results for low nanoparticle concentrations (up to 1 wt.%) show that the surfactant added to water, required to stabilize the nanofluids and minimize particle clustering, affects the spray’s main characteristics. Namely, the surfactant decreases the liquid surface tension leading to a larger wetted area and wettability, promoting heat transfer between the surface and the liquid film. However, since lower surface tension also tends to enhance splash near the edges of the wetted area, the gold nanospheres act to lessen such disturbances due to an increase of the solutions’ viscosity, thus increasing the heat flux removed from the spray slightly. The experimental results obtained from this work demonstrate that the maximum heat convection coefficients evaluated for the nanofluids can be 9.8% to 21.9% higher than those obtained with the base fluid and 11.5% to 38.8% higher when compared with those obtained with DI water.

This study addresses the effect of nanofluid synthesis on the rheological properties of the resulting fluid and their consequent effect on the characteristics (size and velocity distribution of droplets, spray cone angle, etc.) of the sprayed nanofluids. The results are discussed in the light of how the spray characteristics affect the use of the resulting nanofluid spray for cooling purposes. Nanoparticles of alumina (Al2O3) and zinc oxide (ZnO) are mixed in water-based solutions, for concentrations varying between 0.5% and 2 mass% for alumina and between 0.01% and 0.1 mass% for the zinc oxide particles. FeCl2·4H2O (0.1 mass%) was also used to infer on the effect of the nature (material) of the particles in the physicochemical properties of the resulting solutions. Among the various surfactants tested, citric acid (0.15%) was chosen for the final working mixtures, as it assured a stable behaviour of the solutions prepared during the entire study. The nanoparticles were characterized in detail, and the physicochemical properties of the fluid were measured before and after atomization, to evaluate any possible particle loss in the liquid feeding system or retention in the atomizer. The nanofluids were sprayed using a pressure-swirl atomizer at 0.5 MPa injection pressure. Droplet size and velocity in the spray were probed using phase Doppler anemometry. For the range of experimental conditions covered here, the results show that liquid viscosity is an important parameter in predetermining the spray characteristics of nanofluids, as it affects the primary liquid breakup. Despite this, only a mild increase is observed in the nanofluids viscosity, mainly for higher concentrations of alumina, which was not sufficient to significantly affect the spray characteristics, except for a small decrease in the spray cone angle and the size of the atomized droplets. Hence, for cooling purposes, the atomization mechanisms are not compromised by the addition of the nanoparticles and their using is beneficial, as they enhance the thermal properties without a significant deterioration of other fluid properties such as viscosity and spray characteristics. Present spray characteristics promote liquid adhesion to the cooling surfaces and droplet size and velocity are kept within a range that is appropriate for spray cooling, following the literature recommendations and our analysis.

This work aims to study the impact of nanofluids with alumina particles on pool boiling performance. Unlike most studies, which use a trial-and-error approach to improve boiling performance parameters, this study details the possible effects of nanoparticles on the effective mechanisms of boiling and heat transfer. For this purpose, biphilic surfaces (hydrophilic surfaces with superhydrophobic spots) were used, which allow the individual analysis of bubbles. Surfaces with different configurations of superhydrophobic regions were used. The thermophysical properties of fluids only vary slightly with increasing nanoparticle concentration. The evolution of the dissipated heat flux and temperature profiles for a nucleation time frame is independent of the fluid and imposed heat flux. It can be concluded that the optimal concentration of nanoparticles is 3 wt%. Using this nanoparticle concentration leads to lower surface temperature values than those obtained with water, the reference fluid. This is due to the changes in the balance of forces in the triple line, induced by increased wettability as a consequence of the deposited particles. Wherefore, smaller and more frequent bubbles are formed, resulting in higher heat transfer coefficients. This effect, although relevant, is still of minor importance when compared to that of the use of biphilic surfaces.