Most practical burners are usually operated at high temperature, in which both limits of flammability and thermodynamic efficiency are improved enabling complete combustion and achieving more fuel saving. Furthermore, for the compact and efficient burner design, jet flames in a cross-flow are mostly desirable due to its enhanced ability of mixing fuel with air. Because of the practical importance for design of combustors, general characteristics such as lift-off and stability properties of these flames at normal temperatures have received much research attention. Regarding with such characteristics at high temperatures, however, little results have been reported so far. Considering that the assumption of normal temperature limits the performance of many practical burners, the investigation on the general characteristics of these flames at high temperatures is meaningful and challenging.
In this context, we present two main results which should be considered for any burners operated at high temperatures. First of all, this thesis describes the macroscopic characteristics of widely used jet flames in a cross-flow at high temperature, which are found to be fundamentally different from those at normal temperature. Secondly, new lift-off behaviors of the flames in a cross-flow at high temperature are discussed through the large-scale structures extending the effect of large-scale structures at normal temperature to the flame lift-off at high temperature.
In general, flames in a cross-flow at normal temperature can be categorized into liftable and never-lift flames, and then their characteristic modes are identified and finally stability are characterized by jet to wind momentum flux ratio R and a separation point.
An importantly novel observation in this thesis is that the flame behaviors and the stability characteristics observed in the cross-flow at high temperature (around 1080K) are different from those at normal temperature. Specifically, the flames cannot be categorized into either liftable or never-lift flames anymore at high temperature. And over the velocity range beyond the stability limits of normal temperature, flames exist with lifted form. Consequently, neither an unstable region nor a separation point could be found. The key contribution of this thesis is that the different characteristic flame modes from those at normal temperature are identified for the first time. In our knowledge, these lifted flames at high temperature have not yet been observed at normal temperature. And we believe that they will be a new evidence for understanding lift-off characteristics. Based on this new evidence, this thesis also illustrate that the large-structural mixing plays an important role in the lift-off at high temperature.
In conclusion, jet flames in a cross-flow of high temperature behave very differently from those at normal temperature, and the distinct characteristics established in this thesis should be considered for the design of the efficient burners.
Additionally, macroscopic and microscopic characteristics of the flames at high temperature are investigated through numerical simulation and also experiment, to confirm the results described above.
