| 1. |
- Akkerman, V'yacheslav, et al.
(författare)
-
Fast flame acceleration and deflagration-to-detonation transition in smooth and obstructed tubes, channels and slits
- 2013
-
Ingår i: 8th US National Combustion Meeting 2013. - : Western States Section/Combustion Institute. - 9781627488426 ; , s. 970-978
-
Konferensbidrag (refereegranskat)abstract
- This work is devoted to the comprehensive analytical, computational and experimental investigation of various stages of flame acceleration in narrow chambers. We consider mesoscale two-dimensional channels and cylindrical tubes, smooth and obstructed, and sub-millimeter gaps between two parallel plates. The evolution of the flame shape, propagation speed, acceleration rate, and velocity profiles nearby the flamefront are determined for each configuration, with the theories substantiated by the numerical simulations of the hydrodynamics and combustion equations with an Arrhenius reaction, and by the experiments on premixed hydrogen-oxygen and ethylene-oxygen flames. The detailed analyses demonstrate three different mechanisms of flame acceleration: 1) At the early stages of burning at the closed tube end, the flamefront acquires a finger-shape and demonstrates strong acceleration during a short time interval. While this precursor acceleration mechanism is terminated as soon as the flamefornt touches the side wall of the tube, having a little relation to the deflagration-to-detonation transition (DDT) for relatively slow, hydrocarbon flames; for fast (e.g. hydrogen-oxygen) flames, even a short finger-flame acceleration may amplify the flame propagation speed up to sonic values, with an important effect on the subsequent DDT process. 2) On the other hand, the classical mechanism of flame acceleration due to wall friction in smooth tubes is basically unlimited in time, but it depends noticeably on the tube width such that the acceleration rate decreases strongly with the Reynolds number. The entire DDT scenario includes four distinctive stages: (i) initial exponential acceleration at the quasi-incompressible state; (ii) moderation of the process because of gas compression; (iii) eventual saturation to a quasisteady, high-speed flames correlated with the Chapman-Jouguet deflagration; (iv) finally, the heating of the fuel mixture leads to the explosion ahead of the flame front, which develops into a self-supporting detonation. 3) In addition, we have revealed a physical mechanism of extremely fast flame acceleration in channels/tubes with obstacles. Combining the "benefits" of 1) and 2), this new mechanism is based on delayed burning between the obstacles, creating a powerful jet-flow and thereby driving the acceleration, which is extremely strong and independent of the Reynolds number, so the effect can be fruitfully utilized at industrial scales. Understanding of this mechanism provides the guide for optimization of the obstacle shape, while this task required tantalizing cut-and-try methods previously. On the other hand, our formulation opens new technological possibilities of DDT in micro-combustion.
|
|
| 2. |
- Linne, Mark, 1952, et al.
(författare)
-
Correlation of Internal Flow and Spray Breakup for a Fuel Injector Used in Ship Engines
- 2013
-
Ingår i: 8th US National Combustion Meeting. - 9781627488426 ; 2, s. 1150-1157
-
Konferensbidrag (refereegranskat)abstract
- A full-scale fuel injector for a large marine engine has been studied inside an optically accessible, high pressure spray research chamber. The injector tip was made of quartz and it had two holes oriented nearly normal to the injector centerline. Realistic nozzle internal flow passages were used, but a Scania XPI injector body delivered the fuel. The injector body was mounted in the side of the high pressure and temperature spray chamber at Chalmers (one of the windows was replaced), and the jets it produced pointed downward into a spray catch facility. Commercially available Diesel fuel was provided by an accumulator at 110 bar delivery-line pressure. The spray was ejected into flowing air at room temperature and pressures of 10 bar (to achieve relevant cavitation numbers), with injection durations on the order of hundreds of ms. The steady flow portion of the injection process was studied. Internal flow was observed using white light imaging, while spray breakup dynamics were observed using a Ti:sapphire laser-based, time gated ballistic imaging system. Spray dynamics were also studied using a spray impingement measurement inside the chamber. The internal and external (near field breakup) flows seemed to be correlated. Under high levels of cavitation the spray appears to break up similar to an aerated spray, producing a dense field of large primary drops at a fairly large spray angle. Under non-cavitating conditions the spray seems to break up similar to a more classical Diesel injector; including such features as an intact core, surface waves, some bag breakup, and what appears to be air entrainment.
|
|
| 3. |
- Valiev, D.M., et al.
(författare)
-
Stage of quasi-steady propagation in premixed flame acceleration in narrow channels
- 2013
-
Ingår i: 8th US National Combustion Meeting 2013. - : Western States Section/Combustion Institute. - 9781627488426 ; , s. 947-956
-
Konferensbidrag (refereegranskat)abstract
- The present work investigates the spontaneous acceleration of premixed flames in micro-channels in the process of deflagration-to-detonation transition. It has recently been shown experimentally [Wu et al., Proc. Combust. Inst. 31 (2007) 2429], computationally [Valiev et al., Phys. Rev. E 80 (2009) 036317] and analytically [Bychkov et al., Phys. Rev. E 81 (2010) 026309] that the flame acceleration undergoes a number of stages from an initial exponential regime to quasi-steady fast deflagration. The present work focuses on the final saturation stages in the process of flame acceleration, during which the flame propagates with supersonic velocity with respect to the tube wall. It is shown that an intermediate stage with quasi-steady velocity noticeably below the Chapman-Jouguet deflagration speed may be observed during the acceleration process. The intermediate stage is followed by additional flame acceleration and subsequent saturation to the Chapman-Jouguet deflagration regime. We explain the intermediate stage by the combined effects of gas pre-compression ahead of the flame front and the hydraulic resistance. We estimate the first quasi-steady saturation velocity theoretically and compare it with the numerical results. Numerical simulation shows that, in agreement with the theoretical prediction, heating due to viscous stress at the wall is minor before the flame reaches the first quasi-steady stage and is prevailing afterwards. The additional acceleration is related to viscous heating at the channel walls, being of key importance at the final stages. The possibility of explosion triggering is also demonstrated.
|
|
