Research On Detonation Initiation With Hot Jet And Its Application To Pulse Detonation Engine

It is important to enhance the process of deflagration to detonation transition(DDT), which can improve the performace of pulse detonation engine(PDE). As initial ignition condition is sensitive to the DDT process, A reasonable ignition method with high energy causes flame acceleration faster and detonation initiation earlier. The purpose of this paper is the engineering of PDE and it is focused on detonation initiation with hot jet, Series of experiments and numerical simulations were used to study on the mechanism related to detonation process and the design of multi-tube detonation engine with hot jet initiation. The main results are as follow:series of single-cycle experiments were performed where stoichiometric propane-air mixtures with nitrogen dilution at atmospheric initial pressure were used. It was found that the diameter of flame jet orifice slightly affected detonation initiation sensitivity, the critical nitrogen dilution was between 65% ~68% and the hot jet sub-chamber configurations exert a minor effect on DDT distance, but the DDT time decreased while the length of the hot jet sub-chamber increased. Furthermore, the lead shock wave was in front of the combustion wave at the stage of flame propagration in detonation tube. And the velocity of shock wave was faster than combustion wave when the nitrogen dilution was above the critical value. Both of the waves coupled together after the detonation initiated. A conventional spark ignition was investigated for a comparison experiment, the flame accelerated more rapidly with hot jet ignition, which suggested that turbulent combustion was an important reason that lead to form the shock wave and decreased the DDT time.Combined with the conditions and results of single-cycle experiments, series of 2D numerical simulations with a detailed propane mechanism including 34 reaction steps among 26 chemical species were performed, where 5 different length and orifice diameter of hot jet sub-chamber was used to simulation the details of the flame acceleration and detonation initiation. The results shown that turbulence played a key role in the step of flame propagation, flame / shock interaction dominated the step of detonation initiation. According to the definition of DDT time and distance, it was found that the minimum DDT time was obtained in the hot jet sub-chamber with 150 mm long and 8 mm in orifice diameter and the hot jet sub-chamber configuration exert a minor effect on the DDT distance. The calculated energies transferred to detonation tube by hot jet ignition was 9.96 J. It was about 20 times than the energies offered by spark ignition.Based on the Y-Jet nozzle, a internal mixing pneumatic atomizer with multi-unit was designed and improved that used in the head of hot jet sub-chamber, where the fuel was active controlled by a electronic injection.The ignition time and delayed time of filling fuel were also controlled. The experimental results shown that the electronic injection had optimal state at 0.4 duty cycle in different operation frequencies. The hot jet sub-chamber operated successfully at the frequency of 15 Hz and 20 Hz. In addition, the average propagration velocity of flame and shock wave increased as the fill fraction increased. The flame and shock wave propagrated rapidly and strongly while the ignition time shift earlier. However, the hot jet sub-chamber operated unstablitily when the ignition time shift later.In order to study on the characteristics of multi-cycle detonation initiation with hot jet, series of experiments were performed by an aero-valve pulse detonation engine. The characteristics of flame propagation and pressure histories were analyzed with different hot jet ignition position and length of jet sub-chamber, fill fraction and excess air coefficient of sensitive gas in hot jet sub-chamber. The experimental results shown that the hot jet is slightly affected the flame propagation velocity, which located in the recirculating zone. However, the initial flame velocity decreased as the distance between hot jet position and blind head increased; the initial average flame velocity increased from 380m/s to 420m/s as the fill fraction increased from 1.78 to 2.75 and it changed slightly for the fill fraction between 2.75 to 3.45. Moreover, the initial flame velocity increased as excess air coefficent incresased(from 350m/s to 446m/s). The length of sub-chamber had minor effect on initial flame velocity.Six tube PDE with cylindrical-rotary valve and central pilot-burner was designed. The hot jet ingiton and evaporated fuel were obtained by pilot-burner. Flow field was analysed by numerical simulation and combustion experiment was investigated. It was found that the average total pressure loss coefficient increased as inlet air pressure increased. However, the average flow resistance coefficient was between 18.8~21.5. It was indicated that the flow was in self-stimulated. Finally, the hot jet ignition operated successfully but the detonation initaiton was not observed in detonation tubes caused by low fuel evaporation efficiency and unable pilot-burner.