Figure 4. Fluid centerline temperature profiles in the streamwise
direction of the pre-mixed homogeneous-heterogeneous hybrid system.
The velocity contour plots in the pre-mixed homogeneous-heterogeneous
hybrid system are illustrated in Figure 5 with combustion of small
alkanes on noble metal surfaces. The fuel-air admixture is maintained at
about a constant theoretical adiabatic flame temperature by measurement
of various parameters and delivery of interdependent amounts of fuel and
air which amounts depend on these parameters by employing any convenient
means. For instance, air flow metering and temperature sensing means,
coupled with a valve or separating means to regulate the air flow to the
combustion zone in relation to the fuel flow to the combustion zone, for
example, a venturi meter or like device in combination with a
thermocouple and an air flow control valve can be used to determine the
amount of air to be admixed with the fuel for a given inlet air
temperature. A compressor speed indicator together with a valve position
indicator on the air flow valve can alternatively be used in place of
the venturi meter. The fuel flow control valve is regulated by a fuel
flow controller, based on power requirements of the turbine. Adiabatic
combustion systems, from a practical standpoint, have relatively low
heat losses, thus substantially all of the heat released from the
combustion zone of such systems appears in the effluent gases as thermal
energy for producing power. In general, conventional adiabatic thermal
combustion systems operate by contacting fuel and air in inflammable
proportions with an ignition source to ignite the mixture which then
will continue to burn. Frequently the fuel and air are present in
stoichiometric proportions. These conventional systems usually operate
at such high temperatures in the combustion zone as to form nitrogen
oxides [65, 66]. Many thermal combustors employed in turbine systems
utilize separate injection of air and fuel into the combustion zone
without premixing [67, 68]. Such combustors frequently have a fixed
combustor air inlet geometry, so that a predetermined fraction of the
inlet air enters the primary or combustion zone and the balance enters
the secondary or dilution zone. This allows for variation of the turbine
power by adjusting the fuel rate to the combustor and thus varying the
temperature of the effluent to the turbine inlet and consequently the
turbine power. Conventional combustors frequently produce high amounts
of pollutants because of inefficient combustion. In the type of
combustor previously described, the fuel delivery system can normally be
designed for optimum fuel delivery over only a small portion of the
operating range of the combustor. Such narrow limits of most efficient
operation tend to produce high levels of carbon monoxide, unburned
hydrocarbons, soot and the like in certain operating modes. For example,
at idle conditions the fuel flow may be so low as to result in improper
atomization because of low fuel pressure at the fuel nozzle. Further,
the global air to fuel ratio tends to be relatively high in the
combustion zone whenever a decrease in power level occurs because only
the fuel flow is decreased while the air flow remains constant for at
least a short period of time thereafter. This excess air results in
premature quenching which produces carbon monoxide, unburned
hydrocarbons, and the like. For operation above the optimum design
limits, the combustion zone tends to operate excessively fuel rich at
least in certain random regions, with the result that unburned fuel
droplets are coked to make soot and quenched in the dilution zone with
high emissions of carbon monoxide, soot, and unburned hydrocarbons, for
example, many commercial aircraft on takeoff operate at such conditions.