Nozzle and plate assembly

where χA is the mole fraction of species A andQN₂ is the volume flow rate of the excess nitrogen. The uncertainty in the measured values of the equivalence ratio, Φ, and the oxygen percentage, %O2:(O2+N2), are estimated to be 0.8% and 0.2%, respectively, when the mass flow meter uncer-

tainty is propagated.

The flow was seeded with particles using an in-house seeder before entering the jet-plenum, where screen (coarse to fine mesh) and honeycomb (1/8 in cell size, 1 in thick) sections were located for flow-uniformity and turbulence management. The flow is then accelerated through a high- contraction-ratio nozzle and impinges onto a stagnation plate. Flames are ignited in the stagnation flow using a custom spark igniter assembled from a commercially available “stun gun” (Panther 100,000 V). The apparatus is contained within an enclosing chamber to reduce drafts and prevent the small particles relied upon for velocimetry from entering the room. This chamber has openings to allow the laser beams to pass through the experimental assembly and allows for optical access for the imaging devices. The gas within this chamber is exhausted to the atmosphere after passing through a HEPA filter to remove particulates.

2.3

Nozzle and plate assembly

2.3.1

Mark I

The experimental assembly utilized in the study of cold impinging jets (see Chapter 4) is depicted in Figure 2.2. Room-temperature jets are generated in atmospheric pressure air from a contoured nozzle with an internal (nozzle-exit) diameter of d= 9.9 mm. The nozzle interior was designed by optimizing the inner radius profile, r(x), through the contraction-section, expressed in terms of a 7th-degree polynomial, to minimize the exit boundary-layer displacement thickness and avoid the formation of Taylor-G¨ortler vortices in the concave section (see Fig. 2.2, Appendix E, and Drazin & Reid 1981; Dowling 1988). The nozzle exterior was designed with attention to the upstream entrainment-induced flow, and to avoid flow separation and unsteadiness (see Fig. 2.2, Appendix E, and Landau & Lifshitz 1987). The nozzle-plenum system produced a uniform velocity profile in a free-jet configuration. The jet-exit velocity profile was measured with a flattened pitot probe (dpitot ≈0.4 mm in the radial direction) and an electronic-capacitance manometer (BOC Edwards

stagnation plate

nozzle

plenum

Figure 2.2: Schematic of nozzle and stagnation plate apparatus

W57401100) with a temperature-stabilized 1 torr differential-pressure transducer (BOC Edwards W57011419). Figure 2.3 compares the nozzle-exit velocity profile with the profile obtained from an axisymmetric-viscous simulation (performed by K. Sone), at a Reynolds number

Rej ≡ ρ d Uj

µ ∼= 1400 , (2.4)

whereUjis the centerline velocity at the jet exit,ρis the density, andµis the viscosity. The profile is uniform, with less than 1% variation outside the wall boundary layers (r/R ≤ 0.6, R = d/2). The slight disagreement between simulation and experiment in the wall boundary layer region is attributable to the finite pitot-probe extent, dpitot, in the radial direction, for which no corrections

were applied.

The jet axis was aligned normal to a solid wall (stagnation-plate assembly). The stagnation plate was a circular copper block, 7.62 cm (3 in) in diameter and 5.08 cm (2 in) thick, with a 2.03 cm (0.8 in) bottom-edge radius. A bottom-edge radius was introduced to mitigate upstream effects of flow-separation and edge-flow unsteadiness in the stagnation-flow region (see Fig. 2.2).

r [mm]

u

[c

m/s

]

-5

0

5

0

50

100

150

200

250

Figure 2.3: Nozzle-exit velocity profile (d = 9.9 mm, Rej = 1400). () experimental data. (dash

line) viscous-simulation results. Pitot-probe internal opening wasdpitot≈0.4 mm.

2.3.2

Mark II

Preliminary flame experiments were performed using the first apparatus. While steady flames could be stabilized over a wide range of conditions with this apparatus, for some combinations of the equivalence ratio, jet velocity, and separation distance, the flame would become unstable. This instability manifested itself as a “flapping” of the flame edges in the shear-layer region and appeared to be linked to a Helmholtz resonance of the nozzle-plenum-plate system and the flame. It was thought that the vortex roll-up in the annular jet-shear-layer could be responsible for exciting the resonance. In an effort to eliminate this instability, a new coflow nozzle was designed to stabilize the annular shear-layer at the edge of the flame. The coflow apparatus is depicted in Fig. 2.4. The inner profile of the inner nozzle was identical to that utilized in the impinging-jet study discussed above (see Fig. 2.2). The outer profile of the inner nozzle was designed to smoothly join the outer surface of the inner plenum and the tip of the nozzle and provide vertical outflow in the annular jet. The inner profile of the outer nozzle was identical to the outer profile of the inner nozzle, and acceleration was achieved through the reduction in area due to the radial contraction. The outer profile of the outer nozzle was designed to match with the entrained flow streamlines, as was done in the single-nozzle apparatus discussed above (see Fig. 2.2). Experiments were performed using either nitrogen or helium as the co-flow gas. Both inerts improved the stability of the flame. The density of helium is more closely matched to the hot-gas (post-flame) density, resulting in improved

Outer plenum Outer nozzle Inner nozzle

Inner plenum

Premixed fuel & air

Inert Inert

Cooling water out

(3) K-type thermocouples Stagnation plate

Water in

Figure 2.4: Coflow nozzle apparatus with water-cooled stagnation plate.

planarity in the “wing” regions. The flatter flames tended to produce superior velocimetry images and thus helium was utilized for the majority of the data presented here. The use of an inert co-flow also reduced the tendency of the flames to attach to the nozzle rim, as noted previously (Ishizuka

et al.1982).

One of the required boundary conditions for the simulation of premixed flames in stagnation flow is the wall temperature. It was desired to control and accurately measure the wall temperature for the archival flame experiments. Thus, a water-cooled stagnation plate was designed and fabricated (see Fig. 2.4). The stagnation wall was a circular copper plate, 10.16 cm (4 in) in diameter and 5.84 cm (2.3 in) thick, with a 1.91 cm (0.75 in) bottom-edge radius. The plate diameter was chosen to be large enough to ensure one-dimensional flow over the central region of the plate, but also small enough to ensure that the collection optics for the fluorescence measurements could be situated close to the experiment. This allows the maximum possible magnification for the fluorescence imaging. The plate is cooled by a flow of water that is introduced along the centerline of the plate, in a stagnation- type flow. The water flows radially outwards along the rear portion of the stagnation surface and out through (4) outlet ports radially distributed around the plate. The water flow is metered using a needle valve (Swagelok Nupro SS-4MG-MH), and allows the wall temperature to be (open-

loop) controlled to a reasonable accuracy. The plate has three embedded K-type thermocouples on the centerline, spaced vertically between the stagnation and cooled surface, to allow accurate measurement of the wall temperature and temperature gradient.