Diagnóstico y/o determinación de necesidades 2.1 Caracterización de la muestra.
SESION 4. Título: ¡ sexo!
2.4 Pre-experimento Pedagógico
An isolated fuel drop in hot oxidizing atmosphere is a source for fuel vapour surrounding the droplet surface within a limited distance called the “flame reaction zone” [Krier & Wronkiewicz (1972)] or the flame radius. The reaction zone forms a flame envelope around each droplet where the fuel vapour defuses into the oxidant and reacts with it. In other words, the reaction zone is the “sink” where both the fuel substance and oxygen are consumed and turned into thermal energy [Annamalai & Puri (2007), Ch.16]. When a flame reaction zone is smaller than the distance between droplets, each droplet can be considered to be burning individually generating its own flame front [Brennen (2005), Ch.12]. Rapid reaction rates are connected with the large surface area between the micro-‐droplets and
the hot oxidizer [Lu, et al. (1978)]. In the experiments of Lu, et al. the exploding “sensitivity”
of the air-‐fuel mixture was found to increase as the fuel drop size decreases. Recent
publications show strong relationship between the droplet size and the soot formation1.
Large droplets do not completely burn, but turn into particulate matters (soot) to be later exhausted into the air. According to [ Faiz et al.(1996), p.93], in order to have a “quick”
1 See [Van Basshuysen (2009),pp. 67-‐70] for particle size and soot emissions at high fuel pressure values (200-‐
vaporisation (in gasoline engines), the SMD of the fuel droplets should be between 10 and 20 microns for direct injection, and around 100 microns for indirect injection.
The reduction in a fuel drop diameter with time is represented in the classic D2-‐
theory by the mass loss (evaporation) rate parameter (k) [Lawes M. (2007); Glassman &
Yetter (2008), Ch.6; Annamalai & Puri (2007), Ch.10; Turns (2000), Ch.3]:
2
D
dt
d
k
=−
(3-‐3)The mass loss rate is dependent on the fluid properties, and it increases as the temperature and pressure increase. It defines the speed in which a liquid fuel is turning into its gaseous phase. The temperature profile at the surface of a fuel drop, in the case of non-‐ flamed evaporation, is different from the temperature profile of a burning droplet. In the first case, the temperature decreases by the heat absorption of the fluid drop; in the second case, the temperature gradually increases due to the chemical reactions during the combustion process. The evaporation rate during the combustion incident is sometimes referred to as “mass burning rate”, while the evaporation rate in the absence of combustion is called “mass transfer rate”.
Smaller droplets evaporate faster and mix better with the oxidant than larger drops due to their lower weight. The rapid mixture formation is essential in the IC engines especially those with a direct injection (DI) strategy since the time between the injection incident and the ignition spark is extremely short. Increasing the fuel velocity within the combustion chamber by applying a higher pressure behind the injector can improve the evaporation rate and the atomisation quality. This helps in performing a better heat transfer through the chamber walls and a better mixing with air in the induced vortexes [van
Basshuysen, 2009, Ch.3]. Additional velocity components can be gained when injecting the mixture during the compression stroke (either in piston or rotary engines). Beside the piston/rotor guidance of the flow, the high pressure reduces the vortexes diameter in the turbulent air, and therefore increasing the angular velocity.
In his experiments, Glassman [Glassman (1997), Ch6] has investigated the soot formation process during the fuel combustion (Figure 3.1-‐A). It is clear that the burning rate of a fuel drop decreases as the drop diameter increases for the same ambient temperature. The effect of increasing the ambient temperature is always positive on the evaporation rate (see: Appendix 3.3). (A) (B)
Figure 3.1 (A) Fuel drops (heptane) burning rate against Initial droplet diameter, source [Glassman (1997), Ch6]; (B) Changes in kerosene drop diameter squared against time. Initial diameter is 1.52mm at temperature of 700°C and pressure of 0.1 MPa (1 Bar). The calculated evaporation rate was 0.39 mm2/s,
source [Ghassemi et al. (2006)].
Related experiments were made by Ghassemi et al. [Ghassemi et al. (2006)] for
stages during its lifetime, as shown in Figure 3.1 (B). In the first, changes in the squared
diameter are nonlinear during a time interval (t0) related to the initial diameter of the drop,
the temperature of the surrounding gas and the chemical composition of the fuel. This time delay can be explained as the period needed for a drop to reach the boiling point where the evaporation during this period is unimportant. The diameter changes become linear
afterward to follow the D2-‐theory where the evaporation process becomes rapid. The
evaporation of the JP8 jet fuel starts from the temperature of 165°C [Kutz (2006)].
The presented general behaviour of evaporation explains why increasing the ambient temperature has a very small effect on the spray area close to the injector nozzle but much more effect downstream from the nozzle. This agrees with the results reported in different resources with differences in time and rate values depending on the combination of the fuel itself. The effect on the pressure was found fluctuating between positive and
negative in Ghassemi et al. study [Ghassemi et al. (2006)]. Chin and Lefebvre [Chin and
Lefebvre (1931), in Lefebvre (1989), Ch8] defined the pressure influence more precisely: the evaporation rate for kerosene (JB-‐5) was found to increase as the pressure increases only
for an ambient temperature higher than 800 K (around 527°C). For temperatures lower than
600 K (around 327°C), increasing the pressure was found to have a negative impact to the
evaporation rate. Evaporation rate was independent of pressure between the two temperatures.