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Turbulent swirling combustion of premixed natural gas and air Zhang, Dehong


Constant-volume combustion of a stoichiometric homogenous mixture of natural gas and air with global rotational motion (swirl) in a short cylindrical chamber has been studied experimentally and simulated numerically. Swirl was generated by a rotating disc in the combustion chamber with variable intensity. Turbulence intensity was varied by changing the swirl level as well as changing the size of roughness on the rotating disc. Combustion was initiated at the centre of the cylindrical combustion chamber. Combustion pressure signals were used to determine the combustion rate at different swirl levels. High-speed laser schlieren photography was used to obtain schlieren images of flame kernel development at different swirling levels. Combustion pressure measurements showed that: (i) at given turbulence intensity, there is a swirl level limit, below which swirl enhances the burning rate; above which swirl reduces the burning rate; (ii) the turbulence intensity has greater effect on combustion duration at high swirl than at low swirl; (iii) increased swirl leads to increased heat transfer rate; at the intermediate swirl, the total heat loss during combustion was the minimum. High-speed laser schlieren pictures showed some evidence of small flame kernel elongation along the rotating axis of swirl. Multi-dimensional numerical modeling, which was based on the KIVA II code, was applied to simulate combustion. A combustion model with a two-step chemical reaction scheme, in which the fuel was treated as a mixture of a number of simple chemical components, was developed to evaluate the burning rate with, and without, swirl. The numerical results show that (i) at the zero, low, and intermediate swirl levels, the predicted combustion rates were closely consistent with the measured combustion rates; at the high swirl level, the combustion rates were over predicted; (ii) the numerical simulation is consistent with the observed effect of swirl on the flame kernel development.

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