Ignition and combustion characteristics of boron-based nanofluid fuel

Abstract

With its high gravimetric and volumetric heat value, boron-based nanofluid fuel is potentially well suited for future high-energy propulsion systems. In this experimental study, we investigate the ignition and combustion of boron-based nanofluid fuel, with a focus on the effects of oleic acid (OA) concentration (0 to 10 wt.%), boron particle concentration (2.5 to 20 wt.%), and boron particle size (300 nm to 13 µm). Thermogravimetry−differential scanning calorimetry and laser ignition tests reveal that adding boron nanoparticles to a fuel can improve its combustion intensity, but without much affecting the oxidation and ignition characteristics, primarily because of agglomeration and thicker oxide layer properties relative to boron microparticles. Pure kerosene droplets are found to undergo pre-ignition heating and classic combustion, whereas OA or boron addition promotes puffing and micro-explosions, accelerating the classic combustion process. The co-existence of OA and boron is found to lead to more frequent and stronger disintegration of the nanofluid fuel droplet. The ignition delay reaches 156 ms with only 2.5 wt.% OA inclusion, compared to just 73 ms for pure kerosene. Both the ignition delay and combustion rate increase, while the solid residual decreases owing to enhanced micro-explosions as the OA concentration increases. Unlike OA, boron addition promotes ignition, with 2.5 wt.% boron yielding an ignition delay reduction of 24 ms, but this effect weakens with increasing boron concentration. The combustion rate is found to be relatively insensitive to boron addition. Although the particle size affects the ignition and combustion performance of boron powder, its effect on the combustion of the nanofluid fuel droplet is, under the present test conditions, relatively weak. Finally, a phenomenological model is presented to capture the ignition and combustion of a boron-based nanofluid fuel droplet. The predicted ignition delay and combustion rate are found to be within 7.15% of their experimental values. This study contributes to an improved understanding of boron ignition and combustion in high-energy liquid fuel systems.