4. Conclusions
The effect of wall material thermal conductivity on butane flame
stability is investigated for microscale gas fired burners in terms of
wall outer edge temperature, flame location, critical external heat loss
coefficient, and critical flow velocity. The present study aims to
provide a fundamental understanding of the butane flame stability of
microscale gas fired burners at different wall material thermal
conductivities. Particular emphasis is placed upon the stability limits
over a range of equivalence ratios and the effect of wall material
thermal conductivity on the butane flame stability. The major
conclusions are summarized as follows:
- The most effective way of increasing the lean stability limits of the
burner is an increase in primary stream inlet temperature.
- Completing the combustion process near homogeneous stoichiometric
conditions, by intensifying the mixing process, may increase nitrogen
oxides emissions.
- To ensure that the combustion process of the furnace is not adversely
affected by the presence of the device, the device should not
adversely interfere with the flow of products of combustion away from
the combustion zone for each burner.
- Low wall thermal conductivities result in large axial wall-temperature
gradients and high maximum temperatures. High wall thermal
conductivity leads to uniform temperature profiles without hotspots.
- Low wall thermal conductivities cause the flame to shift downstream.
Increasing wall thermal conductivity has little effect on flame
location unless there are significant external heat losses.
- Typical ceramics allow maximum external heat loss coefficients.
Materials with lower wall thermal conductivities limit the upstream
heat transfer. Materials with higher wall thermal conductivities
result in enhanced heat transfer to the surroundings.
- The inlet flow velocity plays a competing role in flame stability.
There is only a relatively narrow envelope of flow rates within which
combustion can be stabilized.
- The maximum fluid temperature exceeds the adiabatic flame temperature
of butane-air mixture computed for room temperature.