Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 6
For all the above expressions, it is assumed that the unburned mixture is frozen in
composition and the burned mixture is always in equilibrium. Finally, the well-known
ideal gas relation is given by:
pV
= mR T
(6.22)
s
6.1.3 Heat Transfer
The heat transfer from gas to the walls is formulated as:
( )
Q
= h A T −T (6.23)
g g w
tel-00623090, version 1 - 13 Sep 2011
where h g is the heat transfer coefficient, A is the area in contact with the gas, T g is the
gas temperature and T w is the wall temperature. In the single zone analysis, the heat
transfer coefficient is the same for all surfaces in the cylinder. In general, a classic
global heat transfer model is applied to calculate the heat transfer coefficient and an
area-averaged heat transfer rate.
Several correlations for calculating the heat transfer coefficient in SI and CI engines
have been published in the literature. These studies have generally relied on
dimensional analysis for turbulent flow that correlates the Nusselt, Reynolds, and
Prandtl numbers. Using experiments in spherical vessels or engines and applying the
assumption of quasi-steady conditions has led to empirical correlations for both SI and
CI engine heat transfer. These correlations provide a heat transfer coefficient
representing a spatially-averaged value for the cylinder. Hence, they are commonly
referred to as global heat transfer models, e.g. Woschni (1967), Annand (1963), or
Hohenberg (1979). In this code one applies the classical Woschni’s correlation.
The Woschni heat transfer correlation is given as:
h () t a B P() t T() t v()
t
g
−02 . 08 . −055 . 08 .
=
s
(6.24)
where a s is a scaling factor used for tuning of the coefficient to match a specific engine
geometry calculated and used by Hohenberg, (1979) as 130, B is the bore (m), P and T
are the instantaneous cylinder pressure (bar) and gas temperature (K), respectively.
The instantaneous characteristic velocity, v is defined as:
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