Premix flames are favored in modern combustion systems to reduce emissions, because they have low NOx emissions due to their low flame temperatures. Unfortunately, with this type of combustion, and in conjunction with combustion-chamber acoustics, so-called self-excited combustion dynamics emerge very quickly. These are often also referred to as combustion instabilities, thermoacoustic instabilities, or, in accordance with the audible frequencies, also known as rumbling, humming, screeching or pulsation.
Strong pressure fluctuations at one or more frequencies are the underlying cause of such vibrations. Pressure fluctuations can achieve such high amplitudes that the combustion system itself or components connected upstream and downstream are destroyed. These vibrations can be observed in a range of situations from household heating systems to large-scale firings and from stationary gas turbines to rocket engines.
As an engineering office for thermoacoustics, IfTA has been concerned with this problem for several decades. In addition to planning services for prevention strategies, we have also developed an extensive portfolio of products to diagnose the causes of combustion dynamics, to monitor their potential occurrence or to avoid them.
Diagnosis with IfTA DynaMaster
IfTA DynaMaster provides an optimal setup, ready to perform the basic tests for diagnosis. Our Flexense technology is of particular note. It allows the high-temperature sensors for pressure and acceleration and strain gauges often required for these measurements to be connected directly to the measuring equipment. If thermoacoustic instabilities occur only sporadically over longer periods, e.g. under certain environmental conditions, the measurement device can record this autonomously via the integrated trigger function. Relevant operating conditions such as equivalent ratio, thermal power, etc. can be recorded in the process. Vibration and operating data can be evaluated with IfTA Trend, the extremely powerful yet easy-to-use analysis software.
Monitoring and protection with IfTA OMDS
IfTA OMDS (Oscillation Monitoring and Diagnostic System) is used when a combustion system (e.g. in gas turbines) is to be monitored and protected from any undesirable vibrations that should occur. It has a CPU that analyzes the combustion dynamics detected via suitable sensors in the combustion chamber – in real time. This means that, with the appropriate algorithms, the combustion system can be protected before damaging high amplitudes occur. In this protection mode, IfTA OMDS works directly with the combustion system's control system. The relevant data is stored in the system, ready for analysis via IfTA Trend software. IfTA OMDS has been used very successfully in large stationary gas turbines in combined cycle power plants for over a decade.
Active suppression with IfTA AIC
If combustion dynamics cannot be avoided through, e.g. modified combustion control, active suppression may provide the appropriate solution. This is achieved by actively controlling a physical quantity involved in a vibration by means of an actuator, and thereby creating a damping effect. Fuel supply, for example, can be modulated by a rapid valve to stabilize a flame. The IfTA AIC has been specially developed for this task and has been used very successfully in gas turbine power plants around the world. The modular design allows the system to be retrofitted easily to any type of gas turbine.