Comparison of Unsteady Heat Release Rate by Measurements From Chemiluminescence and Two Microphone Techniques

In the reaction zone of flame, electronically excited species are formed such as CH*, OH* etc. During de-excitation these radicals emit electromagnetic radiation of certain wavelength. This process is called chemiluminescence. The intensity of chemiluminescence, is in general captured using a photo multiplier tube (PMT), which is used to measure unsteady heat release rate from premixed flames. This technique is well established and is now a standard for unsteady heat release rate measurements in the parlance of combustion instability, however has certain limitations. In fuel rich mixtures, unreacted heated carbon emits broad band black body radiation, which in some cases large enough to mask the chemiluminescence signal. Hence, this technique is not valid for fuel rich conditions. Moreover, it cannot be applied, when the heat source is diffusion/partially premixed flames or electrically heated wires. We propose an alternative in this regard: two microphone technique. In this technique, we relate the acoustic velocity jump across the heat source to measure the unsteady heat release rate. The up and downstream acoustic velocity, in turn is obtained by two microphone technique. Experiments are performed in a premixed multiple flame burner at fuel lean conditions. This burner is enclosed in a duct, which acts as an acoustic resonator. Results indicate that the magnitude of the unsteady heat release rate obtained from both the techniques is found to agree within 18 %. Experiments are conducted for various lengths of the duct, thereby changing the oscillating frequency. This method is valid as long as the heat source is compact in comparison to the duct, which is true in most of the combustors during combustion instability and is irrespective of its type.