Turbulent mixing enhancement of compact swirl puffs

The context of the research
Pulse 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 objectives
This 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 benefits
The 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.

The proposed research integrates the strengths of continuous swirling jet combustion into the pulse combustion technique, to further enhance its mixing efficiency, hence to improve its overall thermal efficiency and to reduce pollutant emissions resulting from insufficient combustion of fuels. Ultimately it leads to a wider applicability of the pulse combustion technique, which may benefit more than one type of industries. Additionally, a number of broader impacts in terms of People and Knowledge have been identified. Overall, the outcomes of the project have a broad range of beneficiaries.

1. General public: The impact of the improved pulse combustion mixing efficiency leads to an improved overall thermal efficiency of such combustion technique and a lower emission of air pollutants. Each year in the UK, over 3000Kt pollutants are produced from combustion of fossil fuels. Together with greenhouse emissions, these pollutants lead to extra expenditure of £16 billion p.a. from NHS on health care service and produces, cause an effect equivalent to 29,000 deaths and shorten everyone's life by 6 months on average. Even a small percentage reduction of these pollutants will have a huge impact to the UK's society and economy.
2. Industries involving pulse combustion: In addition to engine and electric power generation industries, who directly benefit from higher combustion efficiency and lower emissions, its application has increased significantly in industrial drying. For example, pulse combustion's excellent ability for handling heat sensitive biomaterials and high-viscosity suspensions makes it an important drying method for pharmacology (e.g. antibiotics production) and food industry (e.g. milk powder production). Its advantage of burning diversified fuels allows it to contribute significantly to the development and utilization of renewable energy resources in fields such as sewage sludge treatment and hazard solids incineration.
3. Academic community: In a recent review article in renewable energy, it mentions that the importance of improving pulse combustion efficiency necessitates a substantial community of researchers underpinning expertise from a range of different disciplines. Focusing on a fundamental aspect in fluid mechanics, this project will both springboard the applicant's early career in this area and impact on the PDRA and project students working with / in collaboration with the applicant. Making available the knowledge and understanding required to apply the flow control to enhance mixing in pulsation flows will also enable other academic researchers, in both reacting and non-reacting flow communities, to draw on this approach to get otherwise unobtainable information using the results of the proposed experimental study.
4. Outreach: The project outcomes will be disseminated to the parties mentioned above through the active outreach programmes organized by the Durham University Science Outreach Department and the Durham Energy Institute.