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

Chapter 6
Heat transfer
Wei et al., (2001) and Shudo and Suzuki, (2002) have measured instantaneous heat
transfer coefficients in hydrogen fuelled engines. Wei et al. (2001) found transient heat
transfer coefficients during hydrogen combustion to be twice as high as during gasoline
combustion. They evaluate heat transfer correlations and found Woschni’s equation to
underpredict the heat transfer coefficient by a factor of two.
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Shudo and Suzuki (2002) compared the heat transfer coefficients during stoichiometric
hydrogen and methane combustion, finding them to be larger in the case of hydrogen.
The shorter quenching distance of a hydrogen flame is put forward as the cause of this
increased heat transfer, leading to a thinner thermal boundary layer. Furthermore, for
near-stoichiometric combustion, flame speeds are high and cause intensified
convection. Hydrogen has also a higher thermal conductivity compared to
hydrocarbons. Shudo and Suzuki, (2002) construct an alternative heat transfer
correlation with an improved correspondence with their measurements. However, the
correlation contains two calibration parameters, dependent on ignition timing and
equivalence ratio. These dependencies are started to be the subject of further studies,
so the correlation is not useful for the present work.
The correlation by Woschni and Annand have cited to be inadequate [Borman and
Nishiwaki, (1987)], even for gasoline and diesel engines, although the correlations
have been based on measurements on such engines and use hydrocarbon mixture
properties. However, the development of heat transfer correlation for SI engines is not
within the scope of this work, it was decided to use the standard model of Woschni with
separate values during compression, combustion and expansion as reported by
Verhelst and Sierens, (2007). This calibration was made by matching a simulated
cylinder pressure trace to a measured pressure trace. The compression heat transfer
coefficient can be calibrated to a motored pressure trace. The other coefficients need
to be set more or less simultaneously.
Turbulent burning velocity
The turbulent burning velocity models need laminar burning velocity data of the
air/fuel/residuals mixture at the instantaneous pressure and temperature. As most
models use the laminar burning velocity as the local burning velocity, such as the
DamkÖhler model, the stretched burning velocity should be used.
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