Michael Tran
September-December 2009
Experimental Work

Motivation
Unfavorable environmental consequences as the result of burning fossil fuel and its limited resources have spurred the research for alternative transportation renewable fuels.
Among the renewable resources, bio-fuel is the most attractive alternative energy since it has been shown that combustion of bio-fuel produces significantly lower levels of emission such as particular matter, unburned hydrocarbons, and carbon monoxide compare to combustion of petroleum based fuel. Current research of bio-fuel includes the production, combustion, and emission properties. For combustion and emission studies, many researches use practical systems such as compression-ignited engine to characterize engine performance and emission control. However, few have investigated the combustion and emission of bio-fuel using laboratory diffusion flame. Therefore, the purpose of this project is to investigate the emission properties, particularly soot formation, of different blends of bio-fuel and ultra low sulfur diesel
(ULSD) in a laboratory diffusion flame.
Theoretical Background
Some common studies of soot formation in a laminar diffusion flame include smoke point test, light-extinction measurement, and laser-induced incandescence (LII) technique. The smoke point test is described in the American Society for Testing and Materials standard ASTM
D1322-08. It has been shown that the sooting propensity of the fuel is correlated with its smoke point, the maximum flame height in millimeter at which a liquid fuel can burn without producing
smoke. ASTM D1322-08 standard utilizes the smoke lamp, shown in Figure 1, to determine the
smoke point of a liquid fuel. The higher the flame height measured at smoke point, the less is its sooting propensity.
Alternative technique to characterize soot formation includes laser-extinction measurement across a flame. Extinction consists of two phenomenon, scattering and absorption.
Figure 2 shows as the laser beam passes through the flame, the beam’s intensity is attenuated due
to the scattering and absorption of soot particles presence in the flame. Scattering is determined by the scatter factor Q.
Q


4
4

4 d
6 f
 m , N

(eq.1)

Where

is the wavelength of the light, d is the diameter of the scattering particle, f is a function depending on m and N , the complex refractive index of soot particle and particle number concentration, respectively. The absorption is determined by the absorption coefficient K abs with
K abs
 


2
E
 
Nd
3
E
 
IM

 m m
2
2 

1
2


(eq.2)
(eq.3)
Where E ( m ) is the soot refractive index function. Comparing the two equations, the scattering factor is related to d
6

4 while absorption factor is related to d
3
 . Therefore, when d
 is small, (K abs
>> Q), the influence of scattering can be neglected. The domain where scattering can be neglected is called the Rayleigh regime (eq.3).
 d


0 .
3 (eq.3)
Soot particles in the early stage of soot formation is on average 30-50 nm in diameter, small enough to use the Rayleigh assumption. In order to calculate the extinction coefficient
(eq.4), the incident light intensity (I
0
), transmitted light intensity (I
T
), and absorption length (L) are measured. Since light-extinction method generates line-of-sight measurements, the common
Inverse Abel transformation is applied to convert the line-of-sight integral extinction data of an assumed axisymmetric flame into a spatially resolved field distribution. Once the spatially resolved field distribution of extinction measurement, A
  ext is obtained, soot volume fraction
(f v
) can be calculated using (eq.5)
K ext

1
L ln


I
0
I
T

 f v

6

E

  A ( K ext
)
(eq.4)
(eq.5)
A third method to study soot formation in a diffusion flame is laser-induced incandescence (LII). For LII measurement, an external heating source, such as a pulsed laser, can be used to heat soot particles to 3500-4000K. At this temperature range, the increased
Planck radiation or incandescence can be detected using shot gate CCD Camera. Both

theoretical and experimental investigations have shown that the LII signal detected during the laser pulse is approximately proportional to soot volume fraction. Unlike light-extinction measurement, the LII signal yields relative soot concentration measurements that require calibration to extract quantitative soot concentrations. Light-extinction measurement described previously can be used as calibration method to quantify the LII signal.
Experimental Set-Up
Blending ratios of 0% , 5%, 10%, 15%, 20%, 25% by volume of soybean bio-fuel in ultra low sulfur diesel fuel mixture are burned in ASTM smoke lamp and laboratory diffusion flame.
The smoke point measurements results, using the ASTM D1322-08 method, are plotted in Figure
4. As the concentration of soybean bio-fuel is increased in the fuel mixture, so is the flame
height measurement at smoke point which indicates less soot is produced. The result is consistent with combustion studies of bio-fuel in compression-ignited engines where increasing bio-fuel content in the fuel mixture reduces emission of soot particles.
The experimental apparatus for LII measurement is illustrated in Figure 3. There are
three main components in LII set-up: the heating source, the optics set-up, and the electronic trigger and image capture mechanism. A Pulse Nd:YAG laser operating at 532nm is used as a heating source for soot particles. The optics set-up is composed of steering mirrors and telescopes arrangement to collimate and provide a 22mm x 0.60mm laser sheet across the flame.
A function generator, photo-detector, and oscilloscope electronics are used to time the laser pulse and control the CCD camera gate duration. By placing a bandpass filter centered at 450nm in front of the CCD camera, LII signal is recorded at a fixed 20mm flame heigh. The LII signal for
0%, 5%, 10%, 15%, 20%, and 25% blends of bio-fuel are shown in Figure 5. For visualization purposes, the contour plots of the LII signal are illustrated in
Figure 6. As indicated previously, soot volume fraction is proportional to the LII intensity signal.
Figure 6 shows that as blend of soybean content increases in the fuel mixture, the LII
contour plot shows less concentrated soot particle zones. In addition, further analysis of LII intensity signal indicates that B15% yields the weakest LII signal; hence, the fewest soot particles produced among the various blending ratios tested. Future work includes extinction calibration in order to quantify the LII signal to obtain soot volume fraction.

In summary, as the blending ratios of soybean increase, smoke point test and LII results indicate fewer soot particles produced in the diffusion wick flame. Unfortunately, bio-fuel cannot be stabilized in a wick lamp for blending ratios above 25%. In order to test higher blending ratios, bio-fuel and diesel mixtures need to be burned in gaseous form. An alternative approach is to vaporize the fuel mixture and burn the gaseous fuel in a counterflow flame burner.
The advantage of utilizing the counterflow flame is for uniform, 1D, and stabilized flame at higher blends of bio-fuell. The counterflow flame is designed and the complete assembly is
shown in Figure 8.

Appendix
Figure 1. ASTM D1322-08 standard smoke lamp
Figure 2. Light-extinction measurement of initial and transmitted intensity of laser across a flame

Figure 3. Laser, optics, and electronic equipments set-up for laser induced incandescence measurement
Figure 4. Flame height measurement at smoke point of soybean results following ASTM
D1322-8 standard

Figure 5. Laser-induced incandescence signals of soybean and diesel mixtures for 20 mm flame height
Figure 6. Contour plots of LII signal for B0-B25 soybean bio-fuel

Figure 7. Comparison of LII signals with different blending ratios of soybean bio-fuel
Figure 8. Complete assembly of counterflow burner.