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5 Flame Lift-Off and Blow-Out Stability Limits and Their Application in Gas Burners

Part 3. Gaseous and Liquid Fuels

  1. Yajue Wu

Published Online: 15 JUL 2010

DOI: 10.1002/9783527628148.hoc044

Handbook of Combustion

Handbook of Combustion

How to Cite

Wu, Y. 2010. Flame Lift-Off and Blow-Out Stability Limits and Their Application in Gas Burners. Handbook of Combustion. 3:5:121–140.

Author Information

  1. Sheffield University, Department of Chemical and Process Engineering, Sheffield, UK

Publication History

  1. Published Online: 15 JUL 2010

Abstract

Flame stability is usually characterized by lift-off velocity, lift-off height, and blow-out velocity. The lift-off velocity is defined as the mean jet velocity at which the flame becomes lifted above the jet exit rim. If the jet velocity is further increased, the flame moves downstream to a position where it stabilizes. Lift-off height is the distance between the lifted flame base and the jet exit. When the jet velocity reaches the blow-out velocity, the reaction cannot be sustained and the flame is extinguished. For positions between the jet exit and the flame base, a turbulent mixing region exists which is very similar to that for the unignited jet. The stability limits of turbulent jet diffusion flames are important for operation of combustion systems and have safety implication for handling combustible fuels. The lift-off and blow-out behaviors of the turbulent jet diffusion flames have been the subject of numerous research efforts. The focus point was the physical mechanisms responsible for flame stabilization, which has become a subject of considerable confusion and controversy. This chapter provides a comprehensive review of three main theories of the lift-off of a turbulent jet flame which have been proposed over the years. Following the review, the chapter provides the most practical theories on the prediction of the stability limits of flames for hydrocarbon fuels, including hydrogen fuel. It also provides a discussion on the uncertainties over the stabilization theories through a comparison of measured and predicted stability data. Energy-efficient burners require the flame to operate within the stability limits yet with smallest possible fuel consumption. Industrial burners usually use air entrainment to provide partially premixed flames; this chapter reports a recent study carried out on a series of burners which examined the role of burner diameter, burner head geometry, and the flame conditions at the flame base. The practical corrections for the stability limits are extended to the partial premixed flames from the gas burner and the operating conditions for industrial gas burners are discussed.

Keywords:

  • flame stability;
  • stability limits;
  • lift-off velocity;
  • blow-out velocity;
  • jet velocity;
  • turbulent jet flame;
  • gas burners