This paper is an experimental study on the combustion optimization of an Innovative Inverse Diffusion Flame (IDF) characterized by a central air jet surrounded by an array of fuel jets for impingement heating. An extensive investigation has been performed to explore the effects of the diameter ratio between air port and fuel port (d(air)/d(fuel)) on the IDF structure, particularly on its thermal and emission characteristics. Small, moderate and large d(air)/d(fuel) are investigated. It is found that under the same air flow rate ((Q) over dot(air)) and d(fuel), d(air) exerts a significant influence on the behavior of the IDF by changing air/fuel hydrodynamics including air/fuel mixing intensity and air entrainment intensity. The experimental results show that smaller d(air) produces a blue flame with better thermal characteristics, with higher maximum flame temperature (T-f,T-max), wider range of air jet Reynolds number (Re-air) for flame stability, and wider operation range of the overall equivalence ratio, Phi. On the emission side, smaller d(air) is found to produce more incomplete combustion products of CO and HC but less NOx, which is attributed to lower volume of high-temperature zone and shorter flame residence time. The current investigation provides a valuable input for combustion and design optimization of this innovative IDF burner for impingement heating. (C) 2011 Elsevier Ltd. All rights reserved.