| OCR Text |
Show not easy. IL can be measured directly during the experiment with a power meter. Generally, the A and B coefficients are detennined by independent measurements of radiative lifetimes, and experimental and theoretical determination of the molecule's spectroscopic characteristics. The value of Q is dependent on pressure, temperature, and flame composition. It can be measured directly (see below) in low pressure flames but at atmospheric pressure its value must usually be inferred from independent measurements. Temperature Measurements with LIF The distribution of molecules over rotational and vibrational levels is governed by a competition between minimization of energy and maximization of entropy, resulting in a Boltzmann distribution of population. We consider the distribution over rotational levels, labeled by the quantum number J, which can be used to determine the temperature: (3) Here, N J is the number of molecules in the J quantum level. gJ is the quantum level degeneracy (given by the value 21+ 1), EJ is the energy of the level, and k is the Boltzmann constant We thus plot the logarithm of NygJ, where NJ is obtained from the LIF signal via Eq. (2), vs. EJ, for a series of rotational levels ]; the resulting straight line has the slope I/kT and permits us to determine the temperature of the flame at the point of the LIP measurement Fig. 3 shows three spectra used for temperan.rre determinations. These are made at different heights in a 30 Torr methane/air flame. Each shows the same series of absorption lines for different rotational levels of the OH molecule in the flame. Note how the pattern changes as we proceed from low in the burner, where the gases are not yet burnt, through the flame front, and into the burnt gases. The temperatures given in the scans are obtained from the analysis using Eqs. (2) and (3). LOW PRESSURE FLAME EXPERIMENTS Real combustors operate at atmospheric pressure and above, and are certainly the subject of many LIF studies. However, a detailed understanding of flame chemistry is best conducted on low pressure burners. The primary reason is that the flame front, that region where most of the "important flame chemistry occurs, spreads out as the pressure is reduced. At atmospheric 8 |
