For combined cycle operations, especially during startup and shutdown, safety concerns have always been the top priority. Residual fuels can escape to and accumulate in the downstream heat recovery steam generator (HRSG) if appropriate purge is not taken, which may cause deflagration, an explosion at subsonic condition, during startup when heat source exists. Residual fuel deflagration will lead to pressure rise and therefore, structural and or tube damages depending on the level of pressure increase. Therefore, a careful study of the residual fuel distribution in HRSG and the pressure rise due to possible deflagration is wanted.Historically, thermodynamics relationships and chemical equilibrium calculations are used to estimate the pressure rise. These approaches either assume that all fuel energy is converted into heat, or make assumptions on the amount of fuel in chemical equilibrium that contributes to the pressure rise. These assumptions tend to overestimate the pressure rise. With the development of computational fluid dynamics (CFD) simulation, a more accurate prediction of the fuel concentration in HRSG during transient startup and/or shutdown process is possible. The local fuel concentrations can now be calculated and therefore, how much fuel that are within the explosion limits and contribute to the pressure rise can be detected readily.This article presents the transient CFD modeling results of residual fuel concentration in a typical HRSG configuration during a gas turbine/combined cycle startup failure process. The article then refines the adiabatic mixing model with the predicted local fuel concentrations to provide a better estimation of the pressure rise due to deflagration.

The primary reaction zone of a modern propulsion engine combustion system has a profound effect on overall combustor performance, including efficiency, lean stability, exit temperature pattern factor, and emissions production rates. A unique full-annular, throughflow combustion system with interchangeable fuel injectors, dome mixing devices, and liners was used to evaluate the effects of several primary zone design variables on efficiency and exit temperature distribution. The combustor was tested in a rig designed for extreme exit temperatures, at heat release rates up to 30 M Btu/hr-ft3-atm (0.31 kW/M3-atm) and exit temperatures in excess of 3500F (1927C), through a range of overall fuel/air ratios up to stochiometric. An automated, six-port, rotating exit gas sampling system was used to map the exit gas concentrations for determining gas temperatures and combustion efficiency. Both liquid (JP-4) and natural gas fuels were used to examine the impact of evaporation and fuel momentum effects. Gaseous fuel was injected through nozzles designed to mimic the initial fuel spatial distribution and spray cone angle of the liquid nozzles. Thick thermal barrier coatings on the liners eliminated the need for and interference of film cooling. The high reference velocity, short residence time, and absence of any dilution zone further highlighted the primary zone design impact on exit conditions. Test results show that combustor efficiency becomes volume-limited at a point lean of stochiometric, with both the shape of the efficiency curve and the fuel/air ratio for maximum efficiency dependent on the primary zone design and fuel type. The average exit temperature raidal profile was outside diameter (OD)-peaked at low fuel/air ratios, rotating toward a flat profile at stochiometric for all of the configurations.

No abstract

No abstract