Nan-Suey Liu

Investigation of Swirling Air Flows Generated by Axial Swirlers in a Flame Tube

An unstructured and massively parallel Reynolds-Averaged Navier-Stokes (RANS) code is used to simulate 3-D, turbulent, non-reacting, and confined swirling flow field associated with a single-element and a nine-element Lean Direct Injection (LDI) combustor. In addition, the computed results are compared with the Large Eddy Simulation (LES) results and are also validated against the experimental data. The LDI combustors are a new generation of liquid fuel combustors developed to reduce aircraft NOx emission to 70% below the 1996 International Civil Aviation Organization (ICAO) standards and to maintain carbon monoxide and unburned hydrocarbons at their current low levels at low power conditions. The concern in the stratosphere is that NOx would react with the ozone and deplete the ozone layer. This paper investigates the non-reacting aerodynamics characteristics of the flow associated with these new combustors using a RANS computational method. For the single-element LDI combustor, the experimental model consists of a cylindrical air passage with air swirlers and a converging-diverging venturi section, extending to a confined 50.8-mm square flame tube. The air swirlers have helical, axial vanes with vane angles of 60 degree. The air is highly swirled as it passes through the 60 degree swirlers and enters the flame tube. The nine-element LDI combustor is comprised of 9 elements that are designed to fit within a 76 mm 76 mm flametube combustor. In the experimental work, the jet-A liquid fuel is supplied through a small diameter fuel injector tube and is atomized as it exits the tip and enters the flame tube. The swirling and mixing of the fuel and air induces recirculation zone that anchors the combustion process, which is maintained as long as a flammable mixture of fuel and air is supplied. It should be noted that in the numerical simulation reported in this paper, only the non-reacting flow is considered. The numerical model encompasses the whole experimental flow passage, including the flow development sections for the air swirlers, and the flame tube. A low Reynolds number K-e turbulence model is used to model turbulence. Several RANS calculations are performed to determine the effects of the grid resolution on the flow field. The grid is refined several times until no noticeable change in the computed flow field occurred; the final refined grid is used for the detailed computations. The results presented are for the final refined grid. The final grids are all hexahedron grids containing approximately 861,823 cells for the single-element and 1,567,296 cells for the nine-element configuration. Fine details of the complex flow structure such as helical-ring vortices, re-circulation zones and vortex cores are well captured by the simulation. Consistent with the non-reacting experimental results, the computation model predicts a major re-circulation zone in the central region, immediately downstream of the fuel nozzle, and a second, recirculation zone in the upstream corner of the combustion chamber. Further, the computed results predict the experimental data with reasonable accuracy.Copyright

The Effect of Spray Initial Conditions on Heat Release and Emissions in LDI CFD Calculations

Abstract : The mass, velocity distribution, droplet size and distribution of liquid spray has a primary effect on the combustion heat release process. This heat release process then affects emissions like Nitrogen Oxides (NOx) and Carbon Monoxide (CO). Computational Fluid Dynamics gives the engineer insight into these processes, but various setup options exist (number of droplet groups, initial droplet temperature) for spray initial conditions. This paper studies these spray initial condition options using the National Combustion Code (NCC) on a single swirler Lean Direct Injection (LDI) flame tube. Using laminar finite rate chemistry, comparisons are made against experimental data for velocity measurements, temperature, and emissions (NOx, CO).

Computational Parametric Study of Fuel Distribution in an Experimental Trapped Vortex Combustor Sector Rig

Numerical simulations are performed to predict the flow properties in a liquid spray droplets fueled Trapped Vortex Combustor (TVC) sector rig. The quantities studied include aerodynamics, pressure drop, spray droplets trajectories, evaporation, mixing and combustion, and combustor exit temperature distributions. Previous numerical simulations of this TVC configuration have identified basic flow patterns and performance characteristics, and were generally in good agreement with experimental data. In the current effort, more detailed investigations were performed to understand the sensitivity of the TVC combustor to variations in the liquid fuel injection parameters. The computational model is described, including combustor geometry, boundary conditions for all combustion and cooling air injections, and spray droplets inlet conditions. A key finding is that liquid fuel injection boundary conditions for different types of downstream flows (cavity, high velocity cross flow) require different treatments, even though similar fuel injectors are used. This is evident in the large differences observed in the combustor exit plane pattern factor due to only minor differences in the fueling schemes. Combustor exit temperature profile strongly affects the design for turbine durability. With small changes in the temperature distribution, design modifications for the first turbine vane cooling schemes are required.Copyright

EMISSIONS PREDICTION AND MEASUREMENT FOR LIQUID FUELED TVC COMBUSTOR WITH AND WITHOUT WATER INJECTION

An investigation is performed to evaluate the performance of a computational fluid dynamics (CFD) tool for the prediction of the reacting flow in a liquid fueled combustor that uses water injection for control of pollutant emissions. The experiment consists of a multi-sector, liquid fueled combustor rig operated at different inlet pressures and temperatures, and over a range of fueVair and water/fuel ratios. Fuel can be injected directly into the main combustion air stream and into the cavities. Test rig performance is characterized by combustor exit quantities such as temperature and emissions measurements using rakes, and overall pressure drop fiom upstream plenum to combustor exit. Visualization of the flame is performed using grayscale and color still photographs, and high Me-rate videos. CFD simulations .re performed utilizing a methodology that includes CAD solid modeling of the geometry, parallel processing over networked computers, and graphical and quantitative post-processing. Physical models include liquid he1 droplet dynamics and evaporation, with combustion modeled using a hybrid finite-rate chemistry model developed for Jet-A fuel. CFD and experimental results are compared for cases with cavity-only fueling, while a numerical study of cavity and main fueling was also performed. Predicted and measured trends in combustor exit temperature, CO and NOx are in gcneral a~;n.~rment at the diffmrmt mtcr/fi~rI lmding rat-.. although quantitative differences exist between the predictions and measurements.

Numerical Prediction of Non-Reacting and Reacting Flow in a Model Gas Turbine Combustor

The three-dimensional, viscous, turbulent, reacting and non-reacting flow characteristics of a model gas turbine combustor operating on air/methane are simulated via an unstructured and massively parallel Reynolds-Averaged Navier-Stokes (RANS) code. This serves to demonstrate the capabilities of the code for design and analysis of real combustor engines. The effects of some design features of combustors are examined. In addition, the computed results are validated against experimental data. The numerical model encompasses the whole experimental flow passage, including the flow development sections for the air annulus and the fuel pipe, twelve channel air and fuel swirlers, the combustion chamber, and the tail pipe. A cubic non-linear low-Reynolds number K-e turbulence model is used to model turbulence, whereas the eddy-breakup model of Magnussen and Hjertager is used to account for the turbulence combustion interaction. Several RANS calculations are performed to determine the effects of the geometrical features of the combustor, and of the grid resolution on the flow field. The final grid is an all-hexahedron grid containing approximately two and one half million elements. To provide an inlet condition to the main combustion chamber, consistent with the experimental data, flow swirlers are adjusted along the flow delivery inlet passage. Fine details of the complex flow structure such as helicalring vortices, recirculation zones and vortex cores are well captured by the simulation. Consistent with the experimental results, the computational model predicts a major recirculation zone in the central region immediately downstream of the fuel nozzle, a second recirculation zone in the upstream corner of the combustion chamber, and a lifted flame. Further, the computed results predict the experimental data with reasonable accuracy for both the cold flow and for the reacting flow. It is also shown that small changes to the geometry can have noticeable effects on the combustor flowfield.Copyright

HIGH FIDELITY 3D TURBOFAN ENGINE SIMULATION WITH EMPHASIS ON TURBOMACHINERY-COMBUSTOR COUPLING

Introduction The 3-dimensional flow in the primary flow path of the GE90-94B high bypass ratio turbofan engine has been achieved. The simulation of the compressor components, the cooled high pressure turbine and the low pressure turbine was performed using the APNASA turbomachinery flow code. The combustor flow and chemistry were simulated using the National Combustor Code, NCC. The engine simulation matches the engine thermodynamic cycle for a sea-level takeoff condition. The fan, booster and OGV are corrected to the cycle condition from component simulations, whereas the high pressure compressor and turbines have been simulated at the cycle condition and coupled to the NCC code by passing profiles. Details of this coupling are presented. Significant gains in parallel computing are demonstrated which allow simulations to take place that can impact design. One of the goals of the Numerical Propulsion System Simulation (NPSS) Program at NASA Glenn Research Center has been to demonstrate a high-fidelity 3D Turbofan Engine Simulation. This simulation will support the multi-dimensional, multi-fidelity, multidiscipline concept of the design and analysis of propulsion systems for the future. This paper describes the current status of one major part of that goal: the complete turbofan engine simulation using an advanced 3-D Navier-Stokes turbomachinery solver, APNASA, coupled with the National Combustion Code, NCC. A production engine has been chosen for this demonstration: the GE90 turbofan engine shown in Fig. 1. A sea level, Mach 0.25, takeoff condition has been chosen for the simulation. The main reason is that detailed cooling flows for the turbine are well known at takeoff since this represents the cooled turbine design condition. Since the cooling flow represents a significant amount of the boundary condition information required for the simulation, it was felt this was a good point for the simulation. It also represents a condition where there are the highest temperatures and most stress in the engine, and is therefore a practical point to gain further understanding. _ C A S b A The GE90 development program included component testing of all the turbomachinery as well as the combustor. The full engine simulation effort has taken advantage of this. All the turbomachinery components 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 7-10 July 2002, Indianapolis, Indiana AIAA 2002-3769 Copyright

Numerical Studies of a Single Hydrogen/Air Gas Turbine Fuel Nozzle

A numerical study of a Hydrogen/Air fuel nozzle was conducted using the National Combustion Code (NCC). The numerical results are for a single -fuel nozzle injection element extracted from a full -scale fuel nozzle design. The prototype full -scale fuel nozzle consists of 25 Hydrogen/Air injection elements, each with two cross injection hydrogen ori fices. Detailed results about the mixing of fuel -air, flame structures and combustion species are presented. As part of the investigation, the effects related to the selection of wall function for turbulent flow near the wall were investigated. Two types of wall functions, a standard wall function and a generalized wall function, were evaluated. This numerical information will assist an ongoing experimental study of Hydrogen/Air fuel nozzle concepts.

Shock Waves and Plume Prediction of a Rocket Thruster With an Attached Panel

Reynolds-Averaged Navier-stokes (RANS) numerical simulations are performed to predict the supersonic flow field induced by a H2-O2 rocket thruster with an attached panel, under a variety of operating conditions. The simulations have captured physical details of the flow field, such as the plume formation and expansion, formation of a system of shock waves and their effects on the temperature and pressure distributions on the walls. Comparison between the computed results for 2-D and adiabatic walls and the related experimental measurements for 3-D and cooled walls shows that the results of the simulations are consistent with those obtained from the related rig tests.Copyright