Mie-scattering and Particle Image Velocimetry

Mie-scattering is an elastic scattering process on particles which are large compared to the wavelength of the scattered light. Elastic means that there is no permanent exchange of energy between light and particle. Consequently, the wavelength of the scattered light remains unchanged [126].

In combustion research, Mie-scattering on particles which are added to the fuel-oxidizer mixture can be used to visualize the position and shape of the flame front. As the gas density abruptly decreases at the flame front due to the combustion-induced gas expansion, the density of seeding particles in the post flame gases is reduced. This corresponds to a reduction of Mie-scattering intensity. Consequently, bright regions in Mie-scattering images represent un-burned mixture whereas dark regions correspond to combustion products.

The boundary between bright and dark regions can be detected and defined as flame front.

In this work, a pulsed Nd:YLF laser at 527 nm and a repetition rate of 3 kHz

3.3 Measurement Techniques

(a) Instantaneous (b) Time-averaged

Figure 3.4:Examples for Mie-scattering images of a premixed hydrogen-air flame atφ=0.71. White line represents detected flame front.

is used to illuminate TiO₂ seeding particles in the burner center plane. The laser beam is extended to a light sheet as indicated in Fig. 3.2 using a con-vex spherical lens and a plano-concave cylindrical lens. The scattered light is captured with a Photron Fastcam SA5 high speed camera (HSC2) with a re-solution of 1024×1024 pixels. A 527±1 nm band pass filter is combined with a 180 mm optics to reduce background noise. An example for the obtained Mie-scattering images with detected flame front is presented in Fig. 3.4(a).

The flame front is identified by edge detection in the binarized image and is included in Fig. 3.4(a) as a solid white line. If the captured instantaneous Mie-scattering images are averaged, the time-averaged flame shape is obtained (Fig. 3.4(b)). From the time-averaged flame front the averaged flame angle can be defined as introduced in [35]. The obtained flame angles are used for model validation in Sec. 6.2.5.

If two Mie-scattering images are captured with a short time difference∆t, the displacements of the particles∆x and∆y in x- and y-direction can be identi-fied to calculate the particle velocity field. Under the assumption that the par-ticles are small enough to perfectly follow the fluid flow, the particle velocity is equal to the fluid velocity and is determined via

~v(x,y,t)= 1

∆t

µ ∆x(x,y,t)

∆y(x,y,t)

¶

. (3.4)

This method is called Particle Image Velocimetry (PIV) and is widely used to measure the velocity fields in liquid or gaseous fluid flows. For a detailed de-scription of this method and the post processing algorithm the reader is re-ferred to Merzkirch’s Chap. 16 of [126]. In order to calculate the velocity field, the Mie-scattering image pairs are subdivided into small interrogation areas.

At the position of each area one velocity value is determined. As the interro-gation areas have a minimum size, some overlap can be designed to increase the resolution of the obtained velocity field. The averaged displacement of the particles is calculated using the autocorrelation of the gray values of all pixels forming the interrogation area.

Here, PIV measurements are performed with the same setup as in the Mie-scattering imaging. Each cavity of the pulsed two-cavity Nd:YLF laser is ated at 3 kHz. This means that the high speed camera (HSC2) has to be oper-ated at 6 kHz to capture each laser pulse on a separate frame. A synchroniza-tion scheme for the camera and the two laser cavities is presented in [120]. A time difference ∆t =20µs between the laser pulses is chosen for the velocity range investigated in this work. The open-source tool PIVlab 1.4 [127, 128] is used for post processing. The spatial resolution is set to 0.5 mm in x- and y-direction. The computed velocity fields ~v(x,y,t) can be converted into time averaged velocity or turbulence intensity as described in Sec. 2.2. The turbu-lence intensity information is used for model validation in Sec. 6.2.5.

4 Influence of Acoustic Oscillations on Boundary Layer Flashback

In this chapter, the CTA results are used to determine the excitation configu-rations for the flashback experiments. Afterwards, the flashback limits of the current setup are compared to literature data to ensure that the acoustic forc-ing section does not influence the burner’s stability. Finally, flashback limits are presented for different excitation amplitudes and frequencies in combi-nation with OH* images of the flashback process. In the discussion of the in-fluence of both parameters, two flashback regimes are identified.

4.1 Excitation Configurations

Figure 4.1 shows two examples for normalized velocity oscillation amplitude curves resulting from the CTA measurements. The two curves originate from the configuration with 2 and 6 installed speakers of type 2 (2T2, 6T2). At each frequency 190 excitation periods are averaged to obtain the oscillation ampli-tude. Maximum velocity oscillation amplitudes are found around 115 Hz for configuration 2T2 and around 135 Hz for configuration 6T2. It can be seen that installing and activating four additional speakers only slightly increases the achieved maximum oscillation amplitude but changes the natural frequency due to the change in geometry. Additional local maxima with sufficient oscil-lation amplitudes are observed at 330 Hz for 2T2 and 350 Hz for 6T2. Above 400 Hz, further local maxima are found, but the oscillation amplitude with speaker type 2 is not high enough to perform meaningful flashback tests. With speaker type 1 only the maximum at 135 Hz delivers sufficient velocity am-plitudes to perform flashback experiments. By inserting an additional chan-nel segment downstream of the flame arrestor, the natural frequency can be

50 150 250 350 450 550 650 750 0

10 20 30 40 50 60 70

f [Hz]

Ω[%]

6T2 2T2

Figure 4.1:Normalized velocity oscillation amplitudes at the burner exit aver-aged over 190 excitation periods for varying excitation frequencies.

2T2 and 6T2 represent configurations with 2 and 6 speakers of type 2. [119]

changed from 135 to 120 Hz. The natural frequencies depend only slightly on flow velocity. Consequently, all flashback tests for one configuration can be performed at a constant excitation frequency.

The resulting configurations analyzed in the flashback experiments are sum-marized in Tab. 4.1. First, a reference case (RC) is studied to ensure that the speaker section and an inserted channel segment upstream of the burner do not influence the flashback limits. The obtained flashback data are compared to existing data in the following section. Apart from this reference case, eight different excitation configurations are investigated. The configuration names consist of the number of active speakers (2 or 6) followed by the speaker type (T1 or T2) and the excitation frequency in Hz. The eight configurations are analyzed in flashback tests at varying excitation amplitudes. The maximum analyzed excitation amplitude isΩ=36 %. Although, higher excitation ampli-tudes up to A=68 % can be achieved with the current design, no meaningful flashback limits can be determined because flashback always occurs in the channel corners due to limitations in the efficiency of the corner injection.