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.