Fig.9 Relationship between smoke reduction rate and the mixing parameter for the different CCD injection timings and CCD chamber volumes.
Thus smoke reduction indexes clue to the turbulent jet generated during the main combustion process can be correlated to the mixing parameter defined by characteristic times of soot particle oxidation and the time for mixing. From this relationship, it can be said that the goal of combustion-system design should be to achieve the largest mixing parameter and also the largest smoke reduction index for this mixing parameter. The former item is to generate the strongest jet at the most effective period, and the later one is to achieve the maximum smoke reduction with the same jet energy and period, which is more related to microscopic mixing of fuel and air as will be discussed in the next chapter.
6. OBSERVATION OF JET IMPINGEMENT ON FLAME
In order to know the condition to give the maximum mixing with the same energy of CCD jets, combustion process was observed with a constant volume combustion vessel. A constant volume combustion vessel was made as outlined in Figs. 10 and 11. This vessel has a 100 mm diameter 30 mm thick test section. A small amount of fuel is injected in a turbulence generating cell corresponding to the CCD (volume 2.5 cc and path diameter 3.4 mm). The combustion in the cell creates a jet aimed at the main flame and generating strong turbulence. Main spray nozzles are located on the cylinder wall and on the end plate, so that different settings of the main spray are possible.
Initially the chamber is filled by a pre-mixed charge of ethylene (5 vol%), nitrogen (60 vol%), and oxygen (35 vol%) at a pressure of 0.8 MPa. Then the charge is ignited by a spark, creating a hot, high pressure atmosphere for the following spray injection. At an appropriate pressure, 900 K and 3.0 MPa in the present experiments, the main spray is injected into the chamber filled with combustion gas with sufficient oxygen, and this is followed by the secondary injection into the CCD.
Three different distances and four different directions of main fuel spray relative to the CCD jet were examined to investigate the effect on soot reduction. The distances between the nozzle and CCD orifice was 100, 75 and 50 mm for Cases 1, 2, and 3 in Fig.11. The directions of the main spray relative to the CCD jet were varied in cases 4 to 7 as shown in Fig.11. The θ angles in the figure are 180°, 135°, 90°, and 45°, and the distance between the main spray and the center of the chamber was 32.5 min. The injection tuning of the CCD jet was coordinated to collide when the main spray reached the same length in Cases 1 to 3. When the directions were varied, the timing was coordinated so that the CCD jet impinges on the main spray at the center of the chamber, Cases 4 to 7.
When the main nozzle is at the cylinder wall as shown in Fig.10 for Case 1, there are quartz glass windows at the both sides of the chamber for shadowgraph pictures. An Ar-ion laser was used for the shadowgragh. A band pass filter with center wavelength 488 nm was used to remove the luminous flame image. In cases other than Case 1 only direct photographs were taken, because the nozzles were on a steel side plate as shown in Figs. 10 and 11.
Figures 12 and 13 show photographs of the direct flame images and soot shadowgaraphs with and without CCD jet at the Case 1 nozzle location. The time in figures is the period after the start of the main injection. The pictures in Figs. 12 and 13 were photographed under very similar conditions. In the figures the main fuel is injected horizontally from left to right, and the CCD jet from the fight to left.
