Flame lift-off

Flame lift-off in oil-fired pressure jet burners occurs when the flame separates from the burner due to excessive combustion air. This separation often leads to the loss of flame detection by the photoelectric cell, causing a safety lockout of the control system. Other outcomes include delayed ignition if the oil spray is too far forward, resulting in explosive ignition; failure to ignite, leading to a safety lockout; and inefficient burning due to excessive cooling of oil particles.

The lift-off height of a non-premixed jet flame increases with fuel jet velocity until it reaches a critical point where the flame may be blown out. The stability of the lifted flame is crucial for combustor design. Various theories explain this phenomenon: Scholefield and Garside emphasized turbulence as essential for stabilization, while Gollahalli focused on flow balancing. Navarro-Martinez and Kronenburg linked excessive turbulence to lift-off, and Kiran and Mishra demonstrated a linear relationship between normalized lift-off height and jet velocity through visual experiments. Their equation is:

\[ \frac{H_L}{D_f} = 1.8 \times 10^{-3} \times \frac{U_f}{D_f} \]

where \( H_L \) is the lift-off height, \( D_f \) is the fuel tube diameter, and \( U_f \) is the fuel jet velocity.

Additionally, Broadwell et al. and Pitts studied the effects of air entrainment on flame extinction, noting that excessive dilution by cool air can lead to flame extinction.