Numerical simulation of detonation inception in Hydrogen / air mixtures

Transcription

1 Numerical simulation of detonation inception in Hydrogen / air mixtures Ionut PORUMBEL COMOTI Non CO2 Technology Workshop, Berlin, Germany,

2 Introduction Objective: Development of detonation based pulsed propulsion system Increased thermodynamic efficiency => lower fuel consumption; No Carbon based emission (Hydrogen fuelled); Reduced NOx emissions (small residence time) Valveless Pulsed Detonation Chamber design to increase operating frequency Premixed air / fuel admission controlled by aerodynamic valves System of oscillating shock waves generated by two supersonic jets impinging on Hartmann oscillator

3 Geometry 1. Combustor casing; 2. High pressure air chambers; 3. Supersonic nozzle; 4. Lateral resonators; 5. Outer side walls of the lateral resonators; 6. Central detonation chamber; 7. End walls of the lateral resonators; 8. End wall of the central resonator; 9. Separation walls. Combustor (1) fed by high pressure air chambers (2) Fuel also fed and mixed into high pressure air chambers (2) Mixture fed into supersonic nozzles (3); Two lateral resonators (4), used to enhance amplitude and frequency of pressure waves Detonation occurs in central detonation chamber (6)

4 Computational domain 13 blocks, divided into approx 2.2 million cells; Structured, Cartesian grid; Computational cells clustered towards walls and the regions of increased interest (shock waves and detonation waves are expected) Uniform grid in the spanwise direction Resolution: η in cold flow

5 Numerical algorithm and modeling Solver: ANSYS CFX Numerical method: element based, finite volume, compressible Numerical scheme: bounded central differencing scheme, second order accurate Turbulence modeling: k ε model Combustion modeling: Finite rate chemistry using Eddy Dissipation Model Reaction mechanism: global reaction

6 Boundary conditions Inlet critical section Temperature 500 K; Mach = 1; Velocity normal to inlet surface; Pressure (total): 6 bar; Gas composition: stoechiometric Hydrogen / air mixture Outlet - supersonic Atmospheric pressure Walls Solid (zero normal velocity); No slip (viscous) (zero tangential velocity); Adiabatic (zero heat flux)

7 Results (t = ms) Flow enters PDC with higher than atmospheric temperature. Bow expansion waves appear at the supersonic nozzle exits. Air stream from supersonic jets first enter lateral resonators, creating pressure waves that propagate axially along flow direction both in lateral resonators, and into central detonation chamber. As high speed jets flow into resonators, flow is entrained from stagnant central detonation chamber into lateral resonators, increasing pressure levels there, and intensity of pressure waves propagating inside the lateral resonators. Entrained air interferes with incoming jets composed of premixed Hydrogen / air and creates vortices that push combustible mixture into the central detonation chamber. In central detonation chamber, pressure waves are propagating from central region towards closed end, driven radially by incoming supersonic jets. Waves travel slower in central detonation chamber than in lateral resonators, due to larger available volume.

8 Results (t = ms) Mixture in the central detonation chamber region is slowly convected in both axial and transversal direction by the low velocity flow existing there, and will play a critical role in transition from deflagration to detonation. Pressure waves mentioned earlier reach solid back wall of lateral resonators, and pressure in region close to closed ends of lateral resonators starts raising sharply. Initial pressure wave reaches back wall of central detonation region, and pressure in the pocket between reflected pressure wave and back wall of central detonation chamber is much lower than in the similar pockets created in lateral resonators. Combustible mixture does not have sufficient time to reach central detonation chamber, so no ignition occurs here.

9 Results (t = ms) Pressure waves are reflected back by back wall of lateral resonator and start moving towards incoming supersonic jets, which are still flowing towards the closed ends of the resonators. Flow behind the pressure waves reverses direction entrained by the pressure gradient. When pressure waves returning from back wall and meet the incoming jets, local pressure increases even higher and forces the flow in the supersonic jets to break suddenly, triggering formation of shock wave around middle of straight channel of lateral resonator. At this point, temperature rises sharply through shock wave due to the meeting of the two suddenly breaking air streams, and fuel air mixture ignites.

10 Results (t = ms) Combustion wave starts propagating both towards closed resonator edges and upstream, in the supersonic jets, as a deflagration wave. Secondary couple of ignition points appears on walls of lateral resonators, helping propagation of combustion wave into fresh incoming mixture. Possibly due to wall friction of high speed jets, but may be artefact of insufficient resolution of computational grid. The combustion region merge, so effect on numerical simulation results is negligible. Combustion process spreads through lateral resonators, which become almost completely filled with combustion products. A region near closed ends of lateral resonator remains untouched by combustion, possible optimization issue, but complete elimination may prove impossible, since a volume of increasingly higher pressure must exist at closed end of resonators to trigger shock wave that serves as ignition device. Shock wave propagates upstream through lateral resonators increasing local pressure.

11 Results (continued) Typical shock cell structures observed. Nozzle flow is under-expanded (jet pressure higher than surrounding air) and tends to further expand downstream of nozzle through expansion fan. Expansion waves reflect on both fluid boundary of the jet, and on solid surface of wall, but reflection effect is different. On fluid surface, constant pressure at boundary must be preserved; therefore reflection on fluid boundary creates shock waves returning into jet. To ensure no-slip wall condition at solid wall, waves reflected back into jet remain, in this case, expansion waves. Thus, alternating expansion and compression regions create a stationary structure of so-called shock cells. Similar behaviour of supersonic jets in close neighbourhood of solid walls is frequently reported in literature.

12 Results (t = ms) Increasing pressure starts flattening bow expansion wave at the nozzles exhaust, pushing it back. Local pressure in lateral resonator regions, close to lateral walls, increases, and direction of jets is deflected towards central region. Flow in lateral resonator reverses direction. Flame reaches central region, and starts propagating mainly transversally through fresh mixture brought by entrainment vortices. Sudden increase in temperature in central chamber inlet triggers sudden increase in pressure in central PDC region. Pressure gradient creates strong axial acceleration, up to supersonic values. Incoming supersonic jets suffer sharp turn immediately after exiting nozzles, and pattern becomes similar to non reactive case. Central supersonic jet oriented towards PDC outlet is formed, delimited by strong shock wave also propagating outwards. Combustion front is coupled and rides right behind this travelling shock wave, in typical detonation front pattern. Pressure in supersonic nozzle is higher than inlet pressure and flow of fresh mixture into PDC is blocked.

13 Results (continued) Once detonation wave is created in central PDC region, the strong pressure gradient it creates also propagates into central detonation chamber and overcomes reflected pressure wave, driving high speed flow into the central detonation chamber, towards back wall. Thus, central detonation chamber plays no active role in ignition phase.

14 Conclusions Simulations capture fuel self ignition and the ignition phase of the detonation process; Ignition triggered by shock waves created when reflected pressure waves meet incoming supersonic jets; Combustion wave propagates as deflagration into PDC central region; Here, combustion wave accelerates and turns into a detonation wave formed by coupling of combustion wave with the shock wave delimiting central jet core. Detonation wave subsequently propagates towards PDC outlet

15 Remaining Issues Numerical simulation fails to properly capture next phase of the detonation cycle due to limitations of combustion model and wall boundary conditions related issues; New combustion model needed, able to properly handle the very thin detonation front; Reaction mechanism needs to be extended to allow for capturing of quenching and re-ignition; New goal: Linear Eddy Mixing model with 3 steps chemistry; LEM implementation to be switched from constant pressure to constant density PDC design optimized to increase role of central detonation chamber during operation