Current Research Projects
Detonation Fundamentals
The study of detonation waves has been ongoing for over, however industrial explosions still pose a significant risk, and cause billions of dollars of damage to the economy every few years. This highlights difficulties in accident prediction, prevention, and mitigation. While it's known that a slow moving flame (deflagration) can develop into an explosion, or detonation, during an incident, its prediction remains extremely challenging. A significant thrust of our research is to better understand the detonation phenomenon, using the latest CFD tools and models available. For example, the following videos demonstrate situations where the CLEM-LES turbulent combustion model was applied to learn more about the turbulent nature of detonation waves in hydrocarbon mixtures.
Combustion Model Reduction
Modeling of the chemistry and thermodynamics is a crucial component in numerical simulations that attempt to accurately simulate reactive flows generated by shocks, such as flame acceleration and detonation phenomena. Our group has recently improved a global 4-species, 4-step premixed combustion mechanism that captures detailed chemistry reaction behavior and properties for a wide range of initial densities, temperatures, and inert gas dilution. We also expanded the performance of this model to encompass a wide range of hydrocarbon fuels, beyond its original implementation for stochiometric acetylene-oxygen. The current model only requires 3 transport equations, which has a significantly lower memory overhead compared to detailed chemistry mechanisms. It also provides a much better description of the combustion stiffness and response to conditions compared to simplified one, two, and three-step models.
Turbulent Mixing Layers
Turbulence is everywhere. Some of our work has focused on exploring the nature of turbulence in high speed flows. The following link is a simulation of a supersonic shear layer. We do these types of simulations to validate our work.
Hydrogen Leaks
With the emergence of new hydrogen technologies, there has been much focus on advancing
research to understand ignition behavior of hydrogen leaks in order to assess explosion and safety hazards. A number of experiments have shown, to date, that hydrogen jets are easily ignitable, and have a wide range of ignition limits (between 4% to 75% by volume). It is therefore of paramount interest to understand the dispersive nature of hydrogen, a highly compressible gas, in order to adequately develop codes and standards. Our group has used three-dimensional large eddy simulation to model the dispersion of compressible hydrogen leaks, from a realistic piping configuration, in order to determine the extent of ignition limits associated with the respective release conditions considered.
Rotating Detonation Engines (RDEs)
The RDE is a unique type of pressure-gain combustion engine in which a highly reactive wave, the detonation, continuously propagates against the reactive mixture within a combustion chamber. Typical axially-oriented RDEs supply air to an annulus, while the detonation wave continuously propagates in the azimuthal direction. Higher efficiencies can be potentially realized compared to cycles relying on heat addition through constant pressure deflagrations such as the well-known Brayton cycle. Moreover, the conceptually simple geometry and configuration allows for compact devices to be constructed, leading to a wide range of scales at which RDEs can be designed for. The detonation wave is a key feature of RDEs, which not only provides compression of the reactive fuel-air mixture through incident shock compression, but also provides additional compression through a rapid and nearly constant volume combustion process. This results in elevated temperatures and pressures at the combustor outlet, which may permit increased thrust or mechanical work extraction compared to deflagration-based cycles. The detonation thermodynamic cycle has the potential increase of available compression ratios by about an order of magnitude compared to the deflagration-based Brayton cycle. In the video featured, a model natural-gas powered RDE has been simulated in an unwrapped configuration.
Pulse Detonation Systems
Our work is also focussed on advancing fundamental understanding of pulse detonation systems. Like the RDE, a detonation wave may be pulsed inside a tube to deliver thrust, potentially much more efficiently compared to the conventional Brayton cycle. Also, pulse detonation systems may be used for additive manufacturing, to deliver protective coatings to a manufactured part. Such methods are known to provide the densest and strongest protective coatings.
Supernovae
In a type Ia supernova (SN1A), which occurs in a binary star system, a white dwarf (WD) consisting of mostly carbon-oxygen nearing its life cycle becomes so dense that it accrues stellar matter from a nearby companion star. This causes the WD's core to exceed the fusion temperature of carbon, triggering an intensely luminous thermal runaway nuclear fusion reaction that unbinds the star, ejecting stellar matter and the companion star into space. The brightness of an SN1A, now referred to as a universal standard candle, make it particularly interesting to study. It permits us to measure cosmological distances and study expansion of the universe. This consequently permits us to better understand the origins of our universe. Our work aims to investigate and model the nuclear physics involved in the potential unconfined deflagration to detonation transition (DDT) as a progenitor for the SN1A phenomena. This mechanism is also referred to as the Delayed Detonation Model. We are attempting to isolate and advance a fundamental understanding of the roles of turbulence in promoting the DDT process, and attempting to confirm whether or not DDT is a possible or likely mechanism to explain such types of stellar events.
Facilities and Resources
Explore the Computational Fluid Dynamics and Supersonic Combustion (CFDSC) Research Lab’s facilities, publications, and ongoing research projects.
CWRU High Performance Computing Cluster
Our lab is a member of, and has access to, CWRU's high performance computing (HPC) cluster. Currently, the HPC has more than 280 compute nodes, more than 6,800 CPU cores, and over 400,000 GPU cores. CWRU also has a NIST 800-171 controlled Secure Research Environment for handling research data subject to HIPAA and other regulatory requirements requiring high levels of security.
Computational Fluid Dynamics (CFD) and Turbulent Combustion Modelling Capabilities
Our in-house CFD codes, written in C++, use a variety of Godunov-type compressible flow solvers (Exact, Roe, HLLE), and equipped with Adaptive Mesh Refinement (AMR) for increased efficiency. This reduces computational overhead significantly by computing high resolution solutions only in regions of interest, see Fig. 1. The code is also equipped with the Compressible Linear Eddy Model for Large Eddy Simulation (CLEM-LES) turbulent combustion model, originally developed to model detonation waves. This approach is a grid-within-a-grid approach, as illustrated in Fig. 2, that resolves mixing and reaction on the subgrid scales, while providing significant savings compared to direct numerical simulation (DNS) of the governing Navier-Stokes equations. The advantage of CLEM-LES is its ability to handle a wide range of combustion regimes. High fidelity detail of the reaction zone structures are preserved on one-dimensional subgrid domains, which are supplemented by re-mapping procedures, or `stirring events', to account for the effect of turbulence on the subgrid. Application of the CLEM-LES to capture experimental observations of detonation waves is demonstrated in Fig. 3.
Figure 1: Example AMR grid topology (right) and corresponding temperature field (left) for a detonation wave in methane--oxygen at 4.5 kPa. The x and y scales go from 0 to 10 units and 615 to 635 units, respectively.
Figure 2: Computational cells required for DNS (left) compared to CLEM-LES (right). The coupling of energy between subgrid scale grids and large scale grids is through pressure changes, obtained on the large scales, and energy released due to combustion, obtained on the subgrid.
Figure 3: Comparison of experimental and simulated (CLEM-LES) detonation structures using a) and b) soot-foils, and c) and d) Schlieren (exp) and density (sim) flow fields. The scale on the left images indicates 10cm, and the scale in the right images indicates 20cm. The right images (c and d) indicate multiple cells on the detonation front, pockets of dense unburned gas, shockwaves, and turbulent flame surfaces.
Publications
For a full list of Professor Maxwell’s publications, please see his Google Scholar and Researchgate pages.
Simulation videos and online presentations of Professor Maxwell’s work can be found on YouTube.