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Detonation dynamics and engines

Detonation features self-sustained high-pressure wave propagation. The chemical energy release process is nearly constant-volume, resulting in a higher theoretical efficiency than those that can be achieved in traditional power cycles. As a means of energy conversion, detonation has a great potential in achieving improved performances and simplified mechanical design. Challenges when utilizing detonation include device size constraints, inadequate fuel-oxidizer mixing, poorly-controlled initiation, etc. An improved understanding towards chemical kinetics and shock dynamics of the detonation phenomenon is critical in stabilizing detonation wave propagation in a controlled manner.

Chemical kinetics

Addition of trace ozone reduces detonation cells

When the size of detonation cells (a typical detonation wave structure) approaches the characteristic length scale of the housing device (detonation tube, slot, etc.), detonation propagation often destabilizes or fails. Chemical kinetics in detonation controls post-shock ignition and heat release; thus is expected to affect the cellular behavior. A current investigation utilizes ignition promoters to reduce the detonation cell size and extend the detonation limit. The ignition promoter studied thus far includes ozone, which offers a unique advantage as it significantly reduces ignition delay time without affecting the thermodynamic properties of the mixture, allowing the effect of ignition delay to be studied in isolation. It has been shown that the addition of traces amount of ozone (up to 3000 ppmdv) reduces the detonation cell sizes of hydrogen/oxygen mixtures up to 70%. This advancement paves the way to the realization of future detonation engines in compact forms. Future studies aim to address the role of chemical kinetics in detonation stability as well as the non-equilibrium nature of the chemical process at detonation front.

Gasdynamics

A mathmatical interblast network to reproduce detonation cellular structure

The detonation cellular pattern reveals that detonation wave is likely to comprise a network of individual blasts. Understanding the blast dynamics and interactions remain the key to unravel the gasdynamics of wave propagation. The schematic on the left shows an example blast network of blasts with a prescribed frontal velocity decay function (a simple cosine function). A resulting cellular pattern (inset) is created by tracking the intersections between two adjacent blasts. The striking similarity between the cellular pattern and the detonation cells measured in experiments supports the stability mechanism stemming from blast dynamics and interactions. One implication from this understanding is that detonation cell size can therefore be predicted if the blast property is specified (or using one-dimensional cylindrical blast simulations). How the uncertainty and variation of individual blasts affect the final detonation cellular structure remains to be elucidated.

Geometric model

Geometric model demo

Available numerical tools to simulate detonation include (1) 2D or 3D simulations with fully resolved fluid physics, which are accompanied by significant computational costs, but also often inaccuracies, (2) 1D simulations, which are not able to resolve boundary features nor depict multidimensional detonation behaviors such as detonation cells, and (3) linear stability analysis and asymptotics, some of which can resolve multidimensional features, but again cannot account for boundary effects. Adding to the challenge is the reported need to consider detailed chemical kinetics in these simulations.

The improved understanding of detonation chemical kinetics and gasdynamics led to the idea of a detonation geometric model. Based on the knowledge of blast dynamics and interactions, the model incorporates the necessary pieces of physics governing detonation dynamics and simulates detonation in a computationally inexpensive and scalable manner. Current development efforts focus on the integration of chemical kinetics and various boundary treatments. The model along with its source code will be published on Github, allowing for continuous modification and improvement.

Detonation engines

A detonation generator

The overarching goal of this research project is to develop detonation engines for power and propulsion applications. A compressor-less, intermittently-operated, efficient, high-power-density, and miniaturized detonation engine offers a unique opportunity to operate as a propulsion device or an electric generator, enlisting a wide range of applications from range extenders for electric vehicles to household power generators and low-earth orbit satellite engines. Shown on the left is a prototype idea of a simple detonation generator. Leveraging modern simulation capability including the geometric model, a successful design, testing, and deployment of such a device will revolutionize the current landscape of power and propulsion solutions for emerging small-scale applications.