Entry-region hydrodynamic and thermal conditions have been experimentally determined for laminar mixed-convection water flow through a horizontal rectangular duct with uniform bottom heating. Direct heating of 0.05 mm stainless steel foil was used to minimize wall conduction, and the foil was instrumented to yield spanwise and longitudinal distributions of the Nusselt number. Flow visualization revealed the existence of four regimes corresponding to laminar forced convection, laminar mixed convection, transitional mixed convection, and turbulent free convection. The laminar mixed-convection regime was dominated by ascending thermals which developed into mushroom-shaped longitudinal vortices. Hydrodynamic instability resulted in breakdown of the vortices and subsequent transition to turbulent flow. The longitudinal distribution of the Nusselt number was characterized by a minimum, which followed the onset of mixed convection, and subsequent oscillations due to development of the buoyancy-driven secondary flow.
The Potential for reducing emissions from gas turbines by injecting steam for Nox control and hydrogen for Co control is evaluated through laboratory-scale combustion experiments. Results showed that hydrogen addition into a steam-injected diffusion combustor at hydrogen/fuel molar ratios of approximately 20 percent was associated with somewhat increased NOx production and reduced CO emissions. Both effects are attributed to an increase in the local stoichiometric flame temperature. However, the decrease in CO was greater than the increase in NOx, resulting in a net emissions benefit, or a shifting of the NOx–CO curve toward the origin. Consequently, a greater range of NOx/CO emissions targets could be achieved when hydrogen was available. Additional experiments on premixed systems with hydrogen injection showed a significant increase in operability. Cost estimates for producing hydrogen with a conventional fired steam reformer suggested high capital costs unless ample steam, is already available. Hence, the technology is particularly well suited for turbines that use steam for power augmentation. Alternate reforming technology, such as catalytic partial oxidation, offers the potential for reduced capital costs.
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