Selected Research Topicss
1, Development of general theories on ignition, extinction and propagation of laminar spherical premixed flames
A general theory considering thermally sensitive intermediate kinetics was developed for describing the different flame regimes occurring during the flame ignition, like stationary flame balls, self-extinction flames and outwardly propagating spherical flames. Based on the general theory, the influences of preferential diffusivity of reactants and intermediates on the minimum ignition energy and flame bifurcations were investigated. The radical ignition mechanism was studied theoretically. These theories and results obtained are of great importance to understand the physics of near-limit combustion phenomena of premixed gaseous flames in internal combustion engines and gas turbine combustors.
2, Theoretical analysis of flame bifurcation and extinction of strained planar premixed flames
Flame bifurcation and extinction of strained planar premixed flames are theoretically investigated with asymptotic analysis , based on the large activation energy assumption. The correlation between flame stretch and flame front location is derived. Different flame regimes and their extinction characteristics can be predicted by the derived theory. It is found that fuel Lewis number affects the flame bifurcation qualitatively and quantitatively, whereas radical Lewis number only has the quantitative influence. Stretch rates at the stretch and radiation extinction limits respectively decreases and increases with fuel Lewis number before the flammability limit is reached, while radical Lewis number shows the opposite the tendency. In addition, the relation between the standard flammability limit and the limit derived from the strained near stagnation flame is affected by fuel Lewis number. However, it is not affected by radical Lewis number. Radical behaviors at flame front corresponding to flame bifurcation and extinction are also analyzed in this work. It is shown that the Lewis number and radiation heat loss have pronounced effects on radical concentration at the flame front.
3, Development of conditional moment closure model for turbulent non-premixed flames and multiphase flames
The governing equations for Conditional Moment Closure (CMC) (Klimenko and Bilger, Prog. Energy Combust. Sci.1999) which can be used for finite volume discretization were derived and implemented, which extends the capacity of the CMC model in predicting the combustion phenomena in complex configurations, such as gas turbine combustors. High-accuracy numerical flux calculation approaches for the discretization of the CMC governing equations were designed based on the data coupling between LES and CMC solvers. Two-phase CMC model was developed as well, towards modelling the pulverized fuel (e.g. coal and biomass) and spray combustion.
4, Development of parallel salable and computationally conservative CMC combustion solver with detailed chemistry for turbulent reacting flows
The in-house unstructured 3D-CMC solver is developed by our group and University of Cambridge. In this implementation, CMC cells are regenerated from the LES cells and can be arbitrarily polyhedral. The numerical flux conservation in physical space is beneficial in predicting the large variations of conditional mass fractions that may exist in such near-limit flame dynamics as extinction. The data transfer interface between the CMC and LES solvers is designed and the MPI based parallelization is employed in the 3D-CMC solver with the round-robin algorithm, ensuring the ideal load balancing among all the processors. The scalability of the solvers is assessed on the ARCHER (Cray XC30) clusters of the UK National Supercomputing Service and, particularly, the LES/3D-CMC solver can have a linear speedup until processor number is close to O(103). For more details, please refer to the Ph.D. Thesis of Huangwei ZHANG.
5, Localized extinction and re-ignition in turbulent non-premixed flames
Localized extinction and re-ignition are the major dynamic manifestations of the turbulence-chemistry interactions in gas turbine combustion. The investigated burner fueled by pure methane is a prototype of the gas turbine combustor. The local extinction and re-ignition characteristics were discussed through quantifying the variations of reactive scalars and the mixture fraction scalar dissipation. The effects of wall conductive heat loss due to bluff body on the local extinction and re-ignition were studied as well. The results from this work are conducive to understanding the mechanisms of these flame dynamic processes and also confirm the ability of the CMC combustion model as the predictive tool for gas turbine combustion with strong turbulent-chemistry interaction. Below are results from LES/CMC of Sydney swirl flames (Masri et al. Combust. Flame 2004) and Cambridge swirl flames (Cavaliere et al. Flow Turb. Combust. 2013). The left two burner figures are respectively from the above two references.
6, Flame blow-off in model gas turbine combustors
For developing and designing a new generation of gas turbine combustors, blow-off characteristics in various fuel types and operating conditions should be taken into consideration. In this work, the unsteady blow-off process was predicted and the measured blow-off curve covering broad operating conditions was aimed to be computationally reproduced. The results show that: (1) for individual blow-off transient, the computational results agree well with the experimental counterparts about extinction time, accompanied local extinction features during blow-off, dynamic evolutions of reactive scalar fields, etc. and (2) the blow-off range from simulation is close to that of the measured blow-off curve. This work demonstrates both the considerable theoretical (i.e. examining the performance of CMC sub-grid scale combustion model and the relevant sub-models therein) and engineering (i.e. quantitatively calculating the blow-off characteristics of gas turbine combustors from computational efforts) significance. Below are the modelling results about the global extinction of Cambridge swirl burner (Cavaliere et al. Flow Turb. Combust. 2013). The first vedio visualizes the blow-off transient with line-of-sight integrated heat release rate, while the second one the stoichiometric OH mass fraction at the middle plane of the burner.
7, Pulverized coal combustion modelling
Pulverized coal combustion is studied with Eulerian description for the dispersed coal particle phase and comprehensive modelling for devolatilization, char combustion, radiation and gas phase combustion. We aim to develop the advanced gas phase combustion model in order to predict the flame dynamic behaviors dominated by finite-rate chemistry effects, such as pollutant formation. Below are distributions of gas phase and particle phase properties from IFRF (International Flame Research Foundation) Furnace No. 1. The corresponding experimental measurements were performed by Michel and Payne (1980) in IFRF.
9, Turbulent fire modelling for buildings in urbanization areas
10, Pollutant dispersion and aerosol dynamics in atmospheric reacting flows
11, Turbulent premixed flame modelling