On the Evaporation and Ignition of Kerosene Fuel Droplet
Qasim Sarwar Khan
†, Seung Wook Baek
*and Hojat Ghassemi
**케로신 연료 액적의 증발과 점화 현상에 관한 연구
카심 살월 칸
†, 백승욱
*, 호잿 가세미
**Key Words : Kerosene(케로신), evaporation(증발), ignition(점화) Abstract
In the present study, the evaporation rates and ignition delay times of kerosene were investigated at high temperatures (between 800 and 1100 K) and high pressures (between 0.1 and 2.0 MPa) under normal gravity.
Droplet with an initial diameter of 1.0 to 1.4 mm hanging at the tip of quartz fiber was subjected under high ambient conditions suddenly. The vaporizing/burning process was recorded through a CCD camera and evaporation history is thus evaluated through temporal variation of droplet diameter. The results show a reduction in ignition delay times upon increment of temperature and pressure. The evaporation follows d2-law and vaporization rates increases at high ambient temperature and pressure. The ignition delay time can be represented by the expression τ=APBexp(D/T).
Nomenclature Cv : Evaporation rate (mm2/s) P : Pressure (atm)
T : Temperature (K) τ : Ignition delay (ms)
do : Droplet initial diameter (mm)
1. Introduction
Combustion of liquid fuel droplet at high pressure and high temperature environments is one of the basic mechanisms in spray combustion for various applications such as industrial furnaces, gas turbines, diesel engines, and liquid propellant rocket engines. And for the
combustion of droplets, vaporization and ignition are the processes of fundamental importance with respect to efficient design of combustors.
There exists lots of study regarding single droplet ignition and combustion of various liquid fuels. Some important review papers present the state of the art in single droplet evaporation and combustion (1-2). The effects of temperature and pressure on vaporization of single droplet in normal and microgravity have been investigated experimentally (3-4). For single component fuel, the effects of several different parameters such as ambient pressure, temperature and oxygen concentration (5), fuel boiling point and fuel chemical type (6), droplet diameter and initial droplet temperature (7) and effects of natural convection (8) has been studied numerically and experimentally.
Kerosene is a common liquid fuel in applications.
However, there is a little useful data and information about evaporation and ignition of kerosene. The purpose
†
한국과학기술원E-mail : [email protected]
TEL : (042)869-5754 FAX : (042)869-3710
*
한국과학기술원**
한국과학기술원of this work is to observe the effects of ambient pressure and temperature on kerosene droplet vaporization and ignition delay. Kerosene is blend of relatively nonvolatile petroleum fractions. It typically consists of 60% of paraffins, 32% of naphthenes and 7.7% of aromatics by volume. The overall average properties of kerosene are very roughly equivalent to dodecane, C12H26 (9). The critical temperature and pressure of dodecane are 661 K and 1.81 Mpa respectively.
Kerosene that was used in the present study has 453-543 K boiling point range and 0.80 specific gravity at 288 K and is produced by Junsei Chemical Company from Japan. Experiments were performed with an individual suspended droplet. The initial diameter of droplets was 1.2±0.2 mm. Ignition delay times were measured for several ambient pressures and temperatures. Droplet evaporation histories were obtained from the measured temporal variations of droplet diameter at different ambient temperatures and pressures and hence the vaporization rates were calculated. In this study, temperature and pressure ranges were from 800 to 1100 K and 0.1 to 2.0 MPa respectively.
2. Experimental Setup
The detailed description of the experimental setup and procedure can be found in reference (10). A schematic of the experimental apparatus is shown in Fig. 1
Fig. 1 Experimental arrangement
A droplet hanging on a fine quartz fiber (0.125mm diameter) was subjected to the hot environment by a freely falling electric furnace, thereby resulting in evaporation/combustion. Air is used to fill the vessel in the ignition study while nitrogen was used for evaporation. This unit is enclosed within a pressure vessel installed with glass windows which enabled us to observe the vaporizing/burning process. The processes are observed using a CCD camera. Due to flexible feature of the furnace design the ambient gas temperature steps up from low to high stage in a short time. It means the heat leakage from furnace to outside is negligible.
Also, since a thin quartz fiber is used for droplet suspension, the heat transfer between droplet and fiber is minimized.
3. Evaporation
3.1 General behavior
Figure 2 depicts a sample of droplet evaporation history. It shows the square of droplet diameter versus time for kerosene droplet vaporization at 0.1 MPa and 700 oC. The initial diameter of droplet is 1.52 mm. This curve is composed of two completely different sequences.
Time (s) DiameterSquared,d2 (mm2 )
0 1 2 3 4 5 6 7 8
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50
Δd2)max (
t0
Fuel: Kerosene P= 0.1 MPa T= 700oC d0= 1.52 mm Cv= 0.39 mm2/s
d2-law evaporation lifetime tan-1Cv
Fig. 2 Temporal variation of the square of droplet diameter
The first sequence shows a non-linear behavior while the second one shows a linear regression in squared diameter. In the first or heat up period, the diameter of droplet increases and after some times decreases. This
3 behavior of droplet is due to its heat-up by thermal conduction from gas and subsequent thermal expansion.
As temperature of droplet surface increases and reaches its boiling temperature, evaporation starts. After that, a balance between thermal expansion and evaporation determines the diameter of droplet. When the temperature inside of droplet reaches to a quasi-steady state, only evaporation is controlling the droplet size.
From this stage, the d2-law is valid.
In Fig. 2, the lifetime of the droplet has been divided into two parts; t0 indicates the non-linear behavior and the remainder is related to d2-law evaporation lifetime.
For the purpose of evaporation study, t0 does not have importance, because the evaporation rate is determined by the second part of droplet lifetime. Base on mass flux of the evaporating liquid droplet (11), for a small droplet, of which heat-up period is negligible, d2-law is expressed as d2 =d02−Cvt. As indicated in Fig. 2, the evaporation rate can be expressed as the time derivative of droplet squared diameter, Cv =−d(d2) dt. This coefficient can be extracted from the linear part of the evaporation history curve. The slope of this line, which is passing through the second part, is the negative of the evaporation rate. The slope of the best straight line can be estimated using the least square regression.
Figure 3 shows the variations of normalized squared diameter with the normalized time for different environment temperatures at 1.0 MPa pressure.
t/d20(s/mm2) d2 /d2 0
0.0 0.5 1.0 1.5 2.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
T=500oC T=600oC T=700oC T=800oC T=900oC T=1000oC Fuel: Kreosene P= 1.0 MPa
Fig. 3 Normalized temporal histories of droplet evaporation
Each evaporation history follows the same general behavior. After a finite heat-up period, the variation of the square of droplet diameter becomes approximately linear with time while keeping d2-law.
3.2 Effect of temperature
The effects of ambient temperature on the evaporation rate have been investigated under four different environment temperatures. In Fig. 4, the effects of temperature on evaporation rate are shown for different environmental pressures.
Fig. 4 Effect of temperature on vaporization rate
As indicated in this figure, the pressure does not affect on the monotonic dependence of evaporation rate on the temperature.
3.3 Effect of pressure
Unlike the monotonic effect of temperature, the pressure shows a different effect on the evaporation rate of kerosene. Figure 5 shows the variation of evaporation rate versus ambient pressure for several environmental temperatures. Vaporization rate increases with an increase in pressure for all temperature cases. But as the pressure approaches critical value (2.0 Mpa approximately in this case), the increase in the rate of vaporization slows down and even diminishes in some cases. Some of the researchers reported a decrease in vaporization rate after passing critical pressure value.
Fig. 5 Effect of pressure on evaporation rate
4. Ignition delay
Ignition delay is defined as the time interval between the exposure of the droplet to high ambient temperature and the start of ignition. Chemical ignition delay and physical ignition delay are the two sub categories of ignition delay. As mentioned before kerosene is generally composed of 60% of paraffins and 30% of naphthenes by volume. Figure 5 shows the comparison of ignition delay of kerosene with its main constituent component’s group members i.e., paraffins and naphthenes. For paraffins and naphthenes, the data is taken from reference [6].
Fig. 6 Ignition delay comparison of Kerosene, Paraffins and Naphthenes
One important conclusion we can draw from the above comparison is that the kerosene, being a multicomponent fuel cannot be represented by only a single component of its constituents. Indeed its ignition delay times are effected by the boiling point (physical delay) and reactivity (chemical delay) of its constituent components at different ambient conditions.
4.1 Effect of temperature and pressure on ignition delay
Ignition delay data are presented in figure 6 with an Arrhenius type of expression where the logarithmic ignition delay on the ordinate is plotted versus the inverse of absolute ambient temperature on the abscissa for various ambient pressure conditions.
Fig. 7 Effect of temperature on ignition delay times at various pressure conditions
At all ambient pressure conditions, an increase in temperature reduces the ignition delay time. It seems that the effect of increasing ambient temperature on the reduction of ignition delay is more pronounced in the case of atmospheric pressure than in the case of higher ambient pressures. The experimental results in the figure are correlated with parallel linear lines, which imply that they are formulated with the following equation.
τ ∝ exp(D/T) (1)
The above equation is valid at full range of ambient pressures in the present study. It implies that equation (1) can be transformed to
τ ∝ f(P)exp(D/T) (2)
5 In order to determine the form of the function f(P), ignition delay is plotted versus pressure in the logarithmic coordinate as shown in figure 8.
Fig. 8 Effect of pressure on ignition delay times at various temperature conditions
It indicates that they can be correlated with linear lines which results in f(P) = PB. The above equation indicates that the ignition delay decreases with an increase in ambient gas pressure. Substitution of the function f(P) in equation (2) leads to the following equation.
τ = A PB exp(D/T) (3)
The values of the constants A, B and D for kerosene are calculated using averaged values of the ignition delay data obtained. The values are found to be A=2.6, B=-0.2 and D=6000.
5. Conclusions
The focus of this work was on the study of the evaporation and ignition delay of kerosene droplets to provide some useful data for high-pressure conditions and various ambient temperatures. The results are summarized as follows:
1. In spite of the multicomponetnt nature of kerosene, its evaporation follows the d2-law after an initial heating up period.
2. The evaporation rate of kerosene droplet increases monotonically with increase in ambient temperaure.
3. The evaporation rate increases as an increase in pressure but near critical pressure the increasing rate is diminished.
4. The ignition delay times of kerosene cannot be represented by any of its single constituent component (paraffins and naphthenes) solely, rather different constituent components have different effect on the physical and chemical ignition delay under different ambient conditions.
5. An increase in temperature and pressure reduces the ignition delay time of kerosene.
6. The ignition delay time of kerosene follows the following empirical relation:
τ = A PB exp(D/T)
Acknowledgement
The present work was supported by the Combustion Engineering Research Center at the Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, which is funded by the Korea Science and Engineering Foundation.
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