Combustion Dynamics

This guide is intended to help the new user understand the basics of Davidson’s fiber-optic based combustion dynamics monitoring system (CDMS).

1. Introduction

Fiber optic sensing technology offers a number of advantages for measurement of combustion dynamics in Low NOx gas turbines used for electrical power generation.

The Davidson fiber optic transducers are tolerant to high temperatures, intrinsically safe, and immune to electromagnetic interference.

Because the sensors can tolerate up to 1000°F, they can be placed in close proximity to the combustion zone where the highest fidelity measurements can be made.

2.  Safety

Davidson has designed its CDMS to be safe for use in industrial applications.  The Davidson CDMS is eye-safe and intrinsically-safe.

The Davidson CDMS uses 850nm LED light which operates at such a low optical power that the amount of light energy and the wavelengths of light transmitted into the optical fiber are not sufficient to damage the eye.

Furthermore, the power is not sufficient to ignite vapors in hazardous locations. The maximum energy transmitted in a fiber is below the standards set by ANSI/ISA-TR12.21.01-2004, Use of Fiber Optic Systems in Class I Hazardous (Classified) Locations.

3. How Does a Fiber Optic System Measure Combustion Dynamics?

See the illustration of the signal processing that occurs within the Davidson CDMS.

The illustration shows the light source and the characteristic of the light which is generally uniform across the spectrum.

Davidson’s fiber optic transducer contains an optical interferometer (sensor) at the tip of the transducer. Because the Davidson transducers are very small and designed to “take the heat”, the transducers can be inserted directly into the engine where the sensors are located very close to the combustion zone inside the engine.

In the illustration, the sensor is shown as two red parallel bars. The optical sensor is actually comprised of two parallel and partially reflective surfaces. The first partially reflective surface is a fixed optical window. The second partially reflective surface is an integral part of the pressure diaphragm.

The pressure diaphragm moves in response to pressure pulsations caused by combustion instabilities. The distance between the fixed optical reference and the surface of the diaphragm at any instant in time is directly related to the applied pressure at that instant in time.

The characteristics of the light that is transmitted to the sensor are changed at the sensor according to the distance between the fixed reference and the sensing diaphragm. In the illustration, this modulated light is illustrated by the sinusoidal waveform. This modulation of the spectral characteristics of the reflected light is the key to measuring the dynamic pressure.

The modulated light is reflected from the sensor and transmitted back to the electronics where the system demodulates the optical signal and precisely measures the length of the gap between a fixed optical reference and the sensing diaphragm.

This demodulation is performed instantaneously and optically using a second optical interferometer (wedge shaped optical component in the illustration). The cross-correlator converts the sinusoidal waveform into a measurement of the sensor gap at up to 80,000 times per second and outputs a voltage signal proportional to pressure as a function of time.

The time-series pressure output is run through a spectrum analyzer which converts the time series pressure signal into a spectral waveform that shows the pressure as a function of the acoustic frequency. That spectral waveform is updated many times each second and is displayed on the computer monitor as a spectral waveform.

Each engine and each separate combustor has a normal spectral waveform called an acoustic signature. When the combustion conditions change due to changes in fuel quality, air-to-fuel ratio, atmospheric pressure, or engine wear and tear, the acoustic signature will change, Alarm thresholds are set for certain pressures at certain frequency bands to alert the turbine operator of potential problems.

Schematic of the CDMS Signal Processing Steps

Typical Real Time CDMS Spectral Waveform

4. What are the Major Components of a CDMS?

There are three basic components in the Davidson’s CDMS:

  • Transducers (Sensors),
  • Cables, and
  • Signal Conditioning Electronics.

Transducers contain the sensors and are installed in the engine

Cables and junction boxes are used to transmit the light from the signal conditioner to the transducers and back to the signal conditioner.

Signal conditioners have a channel for each combustor (transducer) that contain light sources that transmit light through the cables to the transducer and back again where the modulated reflected light signal is detected and converted into an electronic signal that is transmitted to a spectrum analyzer for display and archiving. 

Davidson transducers, cables, and signal conditioners are generally not interchangeable with those manufactured by others.

Schematic of Fiber Optic CDMS

5. Which Gas Turbines Can Davidson CDMS Support?

Davidson has configured two CDMS that are specifically designed for the GE frame engines and the Siemens frame engines. Generally, the signal conditioners and cables are identical, and the only significant difference is the configuration of the transducers which need to fit within the mechanical constraints of the engine design.