On the Use of OH* Radiation as a Marker for the Heat Release Rate in High-Pressure Hydrogen Liquid Rocket Combustion

Abstract

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 factorc f . Under this strict condition, the heat release rate is indeed proportional to the OH * radiation. Nomenclatureδ flame thickness [m] ω production rate [kmol/s] q volumetric heat release rate [W/m 3 ] a strain rate [1/s] C molar concentration [kmol/m 3 ] c f correlation factor [W/kmol] g 0 mf Gibbs enthalpy of formation [J/kg] h specific enthalpy [J/kg] M molar mass [kg/kmol] p pressure [bar] R m universal gas constant [J/(kmol K)] T temperature [K] X mole fraction

Measuring the phase between fluctuating pressure and flame radiation intensity in a cylindrical combustion chamber

“…Usually, the measurement of ' q ′ (t) is approximated using the §ame radiation which is easy to measure. In hydrogen/oxygen combustion, the radiation of OH* [25] has been used as a marker for ' q ′ (t) due to its good detectability [31,32]. This is also the approach chosen for the presented work.…”

“…This is also the approach chosen for the presented work. As discussed in detail in other work by the authors [33], the selected measurement setup allows the measured OH* radiation intensity signals to be interpreted as an approximation for the oscillation of ' q ′ [32]. For the described analysis method, only the frequency and phase information of the ' q ′ (t) signal and not the amplitudes are required.…”

Chemiluminescence as a diagnostic in studying combustion instability in a practical combustor

“…However, studies that observed OH* chemiluminescence in turbulent flames and flames with high strain rates reported poor agreement with heat release in that the emission varies strongly with turbulence levels and even disappears at high strain rates 14;20-22 . At elevated pressures, it has been shown computationally that thermal excitation of OH molecules is the primary source for OH* radicals rather than chemical excitation that would be associated with the heat generating reaction path 23 .…”

Flame response to high-frequency oscillations in a cryogenic oxygen/hydrogen rocket combustor

“…Areas of high combustion rate and density gradients can be detected by OH* emission and visible imaging, respectively. Recent work has highlighted the limitations of OH* imaging in high-pressure oxygenhydrogen combustion [11]. There are rising doubts as to its often assumed analogy with local heat release rate.…”