Environmental Engineering Reference
In-Depth Information
It can also be proposed from the figure that a unit increase in percentage of unsaturation can
increase the ignition delay by 0.03° CA.
25.4.2.3 heat release rate
The details about combustion stages and events can often be determined by analyzing heat release
rates (HRRs) as determined from cylinder pressure history. Before further discussion, paying
attention to the following points may offer a successful understanding on investigation findings:
• Generally, a fuel that has a longer ignition delay should have a higher value of maximal
HRR as compared with fuels that have a shorter ignition delay. However, the maximal
HRR not only depends on ignition delay, but also upon heating value and the mass fraction
burnt for a given crank angle duration.
• Sauter mean diameter (SMD) has been shown to increase with increasing surface tension,
density, and viscosity.
• An increase in droplet size can reduce the fraction of fuel burned in the premixed combus-
tion phase.
• Density increases with increase in unsaturation.
From the aforesaid points it may be concluded that a fuel with more density may lead to an
increased droplet size, which in turn reduces the mass fraction burnt in the premixed combustion
phase as compared with a lower density fuel. Therefore, a higher density fuel may be expected
to have a lower value of maximal HRR. In addition, it was already found that the heating value
decreases with increase in unsaturation. Hence, for a given value of mass fraction burnt, fuel with
a lower heating value may release less heat energy as compared with the fuel with a higher heating
value. From the above discussions it may be concluded that the maximal HRR tended to decrease
with an increase in unsaturation. The correlation coefficient among peak HRR, density, unsatura-
tion percentage, and heating value is listed in Table 25.10.
From the table, it can be observed that the maximal HRR is highly negatively correlated with
percentage of unsaturation and density. Similarly, the peak HRR is positively correlated with heat-
ing value (however, the correlation coefficient is not so significant). From Table 25.7, it can be
observed that ROME has a lower value of maximal HRR and MOME has a higher value than that
of other biodiesel fuels. The table shows that the order of magnitude of peak HRR for the biodiesel
fuels is matched with the reverse order of percentage of unsaturation. This is because the density
increases whereas the heating value decreases with increase in percentage of unsaturation.
It can be said that the mass fraction burnt for a given angle decreases with an increase in the
percentage of unsaturation and the stoichiometric air-to-fuel ratio increases with an increase in
percentage of unsaturation. This can be explained as follows. For the same quantity of supplied air
(and hence oxygen), the burning volume for a less unsaturated biodiesel would be more. Because it
has a lesser air-to-fuel ratio, the less unsaturated biodiesel could find more oxygen at a given crank
angle than that of a more unsaturated biodiesel. From the above discussion, it can be concluded that
the peak HRR decreases with an increase in the percentage of unsaturation.
taBle 25.10
correlation coefficient among Peak hrr, density, Percentage
of unsaturation, and heating value
x
variable
y
variable
correlation coefficient
Density
Peak HRR
-0.951
Percentage of unsaturation
-0.995
Heating value
0.836
