Figure 1. Multiple aluminum particle combustion. Aluminum (Al), with its exceptional attributes of high energy content, and carbon-free combustion products, has been recognized as a promising avenue for future energy systems. However, despite the awareness that the Al-steam reaction yields hydrogen as a valuable byproduct, the potential of harnessing this reaction for sustainable energy remains insufficiently explored. Consequently, the primary objective of this study is to comprehensively investigate the underlying mechanisms of the Al-steam reaction, develop numerical models for rigorous analysis, and illuminate the characteristics of aluminum combustion in steam. One significant aspect to consider is the heterogeneous nature of Al particle combustion, where the fuel comprises solid Al particles characterized by chemical and transport timescales significantly different from those observed in the gaseous phase. Additionally, the transformation of an Al particle from solid to liquid and, ultimately, to gas phase as it reacts with the gas-phase oxidizer introduces a substantial pre-heating delay. The modeling of such a complex process is challenging. This study aims to develop numerical models of aluminum particle combustion.
Project Details
Project term
October 27, 2023–October 25, 2024
Affiliations
RWTH Aachen University
Institute
Institute for Combustion Technology (ITV)
Principal Investigator
Methods
Simulations are carried out using the in-house code CIAO. The aluminum particles are treated as Lagrangian particles. The energy, momentum, and mass transfer between the Lagrangian particles and the Eulerian gas phase are modeled. The gas-phase reacting Navier-Stokes equations are solved in the low-Mach limit, and the solver implements a finite-difference method on a spatially and temporally staggered grid with the semi-implicit fractional-step method. Further, velocity and scalar spatial derivatives are discretized with a second-order finite-differences centered scheme. A pressure-correction step involving the solution of a Poisson equation is incorporated to ensure mass conservation. The code decomposes the computational domain over a number of processors and implements a distributed memory parallelization strategy using the Message Passing Interface (MPI). Combustion in the gas phase is described with a finite-rate chemistry approach, and a standard Strang splitting algorithm is employed to decouple the integration of the convective and diffusive parts of the partially differential equation (PDE) system from the integration of the source terms that
describe chemical reactions. The stiff ordinary differential equation (ODE) system for the chemical reaction is solved with the library CVODE.
Results
Simulations of single and multiple particle combustion have been carried out. Visualization of the multiple particle combustion is given in section 6. The most essential parameter in the study revolves around the prediction of burn time of single particles and the flame speed of multiple particle combustion. In this study, two Lagrangian particle models have been developed for the combustion of aluminum particle: one based on reactive boundary layer, and another on inert boundary layer. Besides, two different oxide cap coverage models have been implemented, one based on 90o contact angle between molten alumina cap and aluminum sphere, and another based on equilibrium of surface tension forces at the contact point. A comparative study was performed to assess the model predicted burn-times against those reported in literature for different oxidizers, particle sizes, and thermodynamic conditions. The burn time predictions from the proposed models align well with experimental data and literature. However, improvements are needed to better capture the burn time response to increasing pressure. Flame speed of multiple particle combustion is also compared with experiments. Relatively good agreements are archived.
Additional Project Information
DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: CIAO
Cluster: CLAIX