Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
tive diameter is d2. This is the analytical case irrespective of whether the flow is acting in a
diffusing manner as in Fig. 2.15(b) or in a nozzle fashion as in Fig. 2.15(c). The logical
diameter for each of the sections is that area which represents the mean area between the start
and the conclusion of each section. This is shown below:
Al = a + b and A2 = b * c (2.15.1)
The diameters for each section are related to the above areas by:
,2 ,2
and d2 = .M c - (2.15.2)
The analysis of the flow commences by determining the direction of the particle flow at
the interface between section 1 and section 2 and the area change which is occurring at that
position. If the flow is behaving as in a diffuser then the ensuing unsteady gas-dynamic analysis
is conducted using the theory precisely as presented in Sec. 2.10 for sudden expansions. If
the flow is behaving as in a nozzle then the ensuing unsteady gas-dynamic analysis is conducted
using the theory precisely as presented in Sec. 2.11 for sudden contractions.
2.15.1 Separation of the flow from the walls of a diffuser
One of the issues always debated in the literature is flow separation from the walls of a
diffuser, the physical situation being as in Fig. 2.15(b). In such circumstances the flow detaches
from the walls in a central highly turbulent core. As a consequence the entropy gain is
much greater in the thermodynamic situation shown in Fig. 2.9(a), for the pressure drop is not
as large and the temperature drop is also reduced due to energy dissipation in turbulence. It is
postulated in such circumstances of flow separation that the flow process becomes almost
isobaric and can be represented as such in the analysis set forth in Sec. 2.10. Therefore, if
flow separation in a diffuser is estimated to be possible, the analytical process set forth in Sec.
2.9 should be amended to replace the equation that tracks the non-isentropic flow in the
normal attached mode, namely the momentum equation, with another equation that simulates
the greater entropy gain of separated flow, namely a constant pressure equation.
Hence, in Sec. 2.9, the set of equations to be analyzed should delete Eq. 2.10.4 (or as Eq.
2.10.9 in its final format) and replace it with Eq. 2.15.3 (or the equivalent Eq. 2.15.4) below.
The assumption is that the particle flow is moving, and diffusing, from section 1 to section
2 as in Fig. 2.15(b) and that separation has been detected. Constant superposition pressure
at the interface between sections 1 and 2 produces the following function, using the same
variable nomenclature as in Sec. 2.9.
Psl-Ps2 = 0 (2.15.3)
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