Experimental Investigation Of Disruptive Burning Phenomena On Nanofuel Droplets
Authors: Ferrão I.A.S. , Mendes M.A.A. , Moita A.S.O.H. , Silva A.R.R. 
Published: July 2023

The use of sustainable and green fuels in the aeronautical industry has been implemented due to the environmental concerns and depletion of fossil fuels. The introduction of biofuels, a renewable energy source in the transportation sector, has shown advantages in terms of pollutant reduction. Recently, the addition of nanoparticles in the combustion of biofuel has been studied with the purpose of enhancing its combustion characteristics. Consequently, the present work evaluates nanofuel single droplet in a falling droplet method. In this way, the fiber suspension effect was neglected, and droplets with a diameter of 250 μm were evaluated. A comparison between pure biofuel and a nanofuel at two furnace temperatures (T = 800 ºC and T = 1000 ºC) was performed. Nanofuels can be defined as a novel class of fuel, where energetic or not energetic nanoparticles are stably suspended in liquid fuel. The preparation of nanofuels demands a methodic procedure to accomplish stable, long-term stability. Therefore, in the present work, the liquid fuel is a biofuel named NEXBTL (HVO) from NESTE. The nanofuel is composed of HVO and aluminum nanoparticles of 40 nm in a particle concentration of 1.0 wt.%. The combustion of pure biofuel and nanofuel was evaluated in a drop tube furnace. This droplet combustion facility consists of an electrically heated drop tube furnace (DTF), an illumination set, an image acquisition system, and an injector device. To follow the burning process from the injector tip until the micro-explosion, a CMOS high–speed camera (CR600×2, Optronics), coupled with a high magnification lens, was used. The image acquisition was pursued with 1000 fps with a resolution of 1200×500 pixels and an exposure time of 1/13000 s. Moreover, a Photron FASTCAM mini UX50, a high-speed camera with 1.3 Megapixel was used to acquire micro-explosions with more detail, and a high magnification lens was coupled to the camera. The results reveal that disruptive burning phenomena occur when aluminum nanoparticles are added to the biofuel. Consequently, a micro-explosion determines the end of the droplet lifetime, mainly affected by the furnace temperature. Disruptive burning phenomena identified in the experimental study are difficult to acquire since its duration is brief and occurs at the end of the droplet lifetime. Due to this, different camera operating ranges were considered. Several frame rates from 2000 to 5000 fps and exposure times of 1/16000s and 1/20000s were considered. The image data processing was performed in ImageJ and MATLAB software. First, the droplet size evolution was evaluated using ImageJ based on the droplet outline through the brightness gradient. Then, the bright spots provided from the micro-explosions were identified and counted by an algorithm developed in MATLAB. It was concluded that adding aluminum combustion in a proper particle concentration can potentially improve the biofuel combustion performance. Moreover, no puffing or micro-explosions were detected in the pure HVO. In contrast, nanofuels displayed puffing and micro-explosions at the end of the droplet lifetime. In addition, the furnace temperature affects the disruptive process, being more intense at T = 1000 ºC. It was observed that a higher number of fragments are ejected at the highest furnace temperature. In addition, the fragments after the explosion are mainly on the lower side, however more studies focusing on this topic should be performed since these disruptive events are unpredictable. The image data acquisition provided the visualization and description of micro-explosions in nanofuel droplets, as well as comprehension of its phenomenology.