Measurement of Turbulent Flames with high Temporal and Spacial Resolution

Investigation of Combustion Generated Noise with Optical Measurement Techniques

The main focus of the project is the development of measurement techniques for the analysis of the noise emission from turbulent premixed swirl flames. With these measuring techniques interrelations between the heat release in the reaction zone and the emitted noise spectrum can be investigated. Furthermore, the contribution of coherent structures to local volume fluctuations and thus, to the turbulent noise spectrum will be explored. The combustion process in swirl flames is also influenced by the type of confinement. Therefore, three configurations will be used in the project:

• Open premixed flame in an anechoic chamber
• Confined flame with a wide and short flametube
• Confined flame in a compact combustion chamber

Currently, only free field conditions without any confinement are considered.

The noise generation of turbulent flames is governed by temporal changes of the total flame volume due to local heat release fluctuations. On the basis of the wave equation an expression for the relationship between the acoustic power and the heat release fluctuations can be derived. Thus, thermal energy is transformed into acoustic energy.

For calculation of the acoustic power, first it has to be considered that only a small part of the overall heat release fluctuates and thus, contributes to noise emission. Secondly it has to be taken into account that the fluctuating heat release is influenced by turbulent motion and thus, statistically coherent to some extend. Including further non stochastic fluctuations, three coefficients can be calculated to describe the transformation of the energy.

Theory

The wave equation describes the propagation of noise without damping. It can be shown that for low Mach-numbers the dominant part of acoustics in turbulent reacting flows is arising from heat release fluctuations. Using Greens function the wave equation can then be solved yielding a dependency between pressure fluctuation p' at location x₀ and heat release fluctuation q' at location x_s.

From the pressure fluctuation the acoustic power P_ac can be calculated for free field conditions. Assuming a constant phase between acoustic velocity fluctuation and pressure fluctuation one may write

where r₀ describes the density in the far field and c₀ the sound speed. From literature it is further known that the acoustic power scales with the size of coherent volume V_coh, flame Volume V_flame and amplitude of the heat release fluctuation

The maximum possible acoustic power resulting from heat release can be estimated by calculating the maximum of the squared integral. Therefore the entire volume has to be coherent. Further the heat release is presumed to be harmonic with an amplitude of q_mean. Relating the result to the thermal Power gives an efficiency of h_the=10^-4…10^-3. For evaluation of the other two efficiencies two measurement techniques were developed as described below.

Burner Characteristics

A test facility with a vertical mounted burner was set up. The power is set to 30 kW, with l = 1.2 and an estimated swirl number between 0 and 0.7.

test rig

An open design was chosen to keep the constraints as small as possible.

Measurement Techniques

Since heat release is the most important parameter influencing the noise emission of turbulent premixed flames, two techniques were developed for its investigation.

To capture the size of the coherent volumes, one must know the spatial distribution of heat release. Thus, an optical system was developed, which images a well defined cylindrical volume onto the screen of a Photomultiplier. The measured voltage represents the chemiluminescence and thus, conclusions towards the local heat release can be drawn. Two apertures and a special set of lenses collect a parallel light beam and realize a homogenous exposure of the sensor as well as an easy alignment. The incoming signal is filtered by a L.O.T. Oriel band pass filter with a maximum transmittance at 307 ± 20 nm, matching exactly an energy transition within the OH-radical.

schematic sketch of the pmt setup

Two of these probes were used for estimating the correlation function in longitudinal, radial and tangential direction. While traversing one PMT over the flame volume, the second one was kept fixed. From the obtained data a length scale L could be calculated by fitting the correlation function

into the measured correlation coefficients.

For getting non-stochastic phenomena planar time resolved information is needed. Thus, a time resolved LIF-System was set up. OH-radicals were excited by a Nd:YAG pumped dye laser tuned to the Q₁(6) line (283 nm) using the A²S⁺ <- X²P(1,0) transition. Because of the poor intensity of high repetitive laser systems the light sheet was directed through the flame volume three times by two planconcave mirrors. Thus, a repetition rate of 1000 Hz with a good SNR becomes possible. During the OH LIF measurements the resulting light sheet was located at an axial position of 0 < z < 2D. Therefore, a section of the flame front is depicted with a high temporal and spatial resolution.

schematic sketch of the HS-LIF-setup

Results

Using the spatial resolving chemiluminescence probes the coherence length scale in radial, axial and tangential direction can be determined

correlation coefficients in axial, radial and tangential direction

With this information the size of the coherent volume can be calculated. The result is related to the overall flame volume and leads to an efficiency of h_coh»10^-3.

fluctuation of the local heat release

The time resolved LIF-System enables us to visualize the arrangement of positive negative heat release fluctuations.

The arrangement of the coherent volumes reduces the combustion generated noise to a factor of 20, which determines the third coefficient h_per=5·10^-2. Defining these three coefficients the transformation of thermal into acoustic energy can be understood.