Heat Release and OH* Radiation in Laminar Non-Premixed Hydrogen-Oxygen Flames

Abstract

The local volumetric heat release rate is an important property in combustion systems. For combustion instabilities, it is the coupling parameter between the flame and the acoustic field. However, this theoretically defined quantity cannot be measured directly. Especially in industrial devices, access is limited, and complicated measurement techniques often cannot be applied. A common approach is to approximate the heat release rate through the radiation of radical species, mostly hydroxyl (OH*). In many applications like liquid rocket combustion chambers, the turbulent flame can be described as an ensemble of laminar flamelets. To explore the relationship between heat release rate and OH* radiation on this fundamental level, a laminar, non-premixed hydrogen-oxygen flame is investigated in this study. The configuration consists of a coaxial hydrogen jet in oxygen co-flow. The experimental setup was operated at atmospheric pressure. Flame radiation was measured for different mass flow rates. A monotonically increasing and slightly nonlinear dependence between the overall band-pass filtered OH* radiation and the fuel mass flow rate was observed. To obtain the local heat release rate, the configuration was computed in a 2D CFD simulation. The flame was modeled using detailed chemistry and differential diffusion. A reaction mechanism for OH* was included in order to compare the simulation with the experiment. The calculated OH* concentration is in good agreement with the experimentally measured radial and axial profiles. While OH* appears in the hottest region of the flame, the heat release rate appears in the zones of temperature increase. OH* radiation can therefore not be used as a marker for the local heat release rate within laminar nonpremixed hydrogen-oxygen flamelets, although on the global level OH ⇤ correlates to the heat release.