On the use of OH^* radiation as a marker for the heat release rate in high-pressure hydrogen-oxygen liquid rocket combustion

The identification of the volumetric heat release rate is of great importance in combustion research, e.g. for the investigation of combustion instabilities in liquid rocket motors. Since it is not feasible to measure this quantity directly with reasonable effort, the heat release is often assumed to be directly proportional to the radiation of the excited hydroxyl radical (OH^*). This paper investigates whether this assumption can be used for diagnostics of the turbulent combustion in liquid rocket motors. The study is based on counterflow flamelet simulations. Using this model, the physical processes leading to both the OH^* radiation and the heat release rate are examined. Within the calculated flamelets, only little spatial correlation exists between OH^* radiation and the local heat release rate. This is in agreement with earlier experimental results. The relevance of this fact is discussed for turbulent liquid rocket combustion. Since the flame thickness in highly strained high pressure flames is usually much smaller than the detection length scale, the spatial discrepancy is disregarded. Instead, if the turbulent flame is represented by an ensemble of strained laminar flamelets, the integrated quantities of counterflow flames have to be considered. The correlation factor c_f is defined as the proportionality of the flamelet-integrated heat release rate to the flamelet-integrated OH^* radiation. Batch simulations show that c_f increases linearly with the flamelet strain rate. A general proportionality between the heat release rate and the OH^* radiation is therefore not given for arbitrary flamelet ensembles. Due to the linear dependence, a normal distribution of the strain rate maps on to a similar normal distribution of the correction factor. If the strain rate distribution in the flamelet ensemble is constant, there is also a constant mean correlation factor ¯c_f. Under this strict condition, the heat release rate is indeed proportional to the OH^* radiation.