Turbulent mixing enhancement of compact swirl puffs

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The context of the researchPulse combustion is an intermittent combustion technique, which is characterized by the oscillatory mass flow rate accompanied by a periodic variation in temperature, pressure and velocity field, but without a reciprocating mechanism as in IC engines. It is considered to be a promising combustion technique, which promotes renewable energy sources since it can burn fuels of different quality ranging from high-grade natural gas and propane to low-grade fuels like biogas. It also has higher combustion efficiency and lower pollutant emissions, compared to conventional steady combustions like it in modern jet engine combustors. These characteristics render it an ideal candidate for sustainable development (for natural resource and environment), which is a major global challenge. In a recent review article, it mentions that although with 80 years of R&D history, pulse combustion remains a relatively obscure technique, which requires fundamental research across a wide range of disciplines ranging from fluid mechanics to chemistry. While the research so far has mainly focused on the overall system performance, systematic studies on each of the many subsystems are still in high demand. The fluid mechanics associated with the fuel injection strategy is one of them.The flow field associated with pulse combustion is usually highly energetic and compact, which is in the form of a turbulent puff. It propagates at a considerable self-induced velocity. In non-premix combustion applications, this results in insufficient mixing of fuel and the surrounding oxidizer and hence undesirable combustion conditions. To tackle this problem, a method to enhance such mixing efficiency is sought in the project.Its aim and objectivesThis project, from a fundamental non-reacting fluid mechanics' point of view, aims to find an optimal scalar mixing enhancement by superposing a swirl component of a variety of strengths on to the pulsed puffs. Such a hybrid injection method will introduce complex coherent turbulence structures and hence promote high mixing efficiency. It combines the advantage of swirling jet combustion, which is commonly adopted in current jet engines, and pulse combustion. A feasible static flow control strategy will also be explored in order to further enhance turbulent mixing. In order to achieve these aims, systematic experiments will be conducted using advanced laser diagnostic techniques for simultaneous measurements of velocity and scalar fields in a non-invasive way. Its potential applications and benefitsThe main application of the research is pulse combustion, whose key advantage is to reduce emission and promote renewable energy sources. These two elements are crucial for global sustainable development, especially for developing countries. In the UK, over 75% of the energy demand is provided by the combustion of fossil fuels, at the cost of emitting over 3000Kt of air pollutant each year. These pollutant plus greenhouse emissions make extra £16 billion p.a. from NHS on health care service and products. A small reduction of these emissions will bring a huge impact to the UK economy.