# Numerical Simulation of Hot Jet Detonation with Different Ignition Positions
## 1 Introduction  
Detonation combustion has attracted plenty of attention from researchers because of its high thermal efficiency, low entropy generation and self-pressurization characteristic [1; 2]. According to the formation process of detonation waves and operating characteristics in the engines, detonation engines can be divided into rotating detonation engine (RDE) [3; 4], pulse detonation engine (PDE) [5; 6] and standing detonation wave engine (SDWE) [7]. The detonation initiation technology is one of the bottlenecks and key technologies that restrict the engineering application of any detonation engines. The common detonation initiation techniques are mainly divided into two categories: one is direct detonation, and the other is indirect detonation initiation. Compared with direct detonation, indirect detonation initiation requires less ignition energy, thus becoming the main direction of the detonation domain. A weak energy ignition source triggers combustion and then leads to a transition to detonation through the accumulation of energy, which is a commonly used indirect detonation initiation method [8]. Deflagration-to-detonation transition (DDT) usually requires a transition distance. However, too long transition distance may cause oversize engines and performance loss. Therefore, it is necessary to explore suitable short-range detonation initiation technology [9]. In published literature, studies have been done on detonation initiation mechanism and enhancement approach, such as hot jet [10; 11], solid obstacle [12; 13], fluidic obstacle [14; 15], plasma [16] and shock focusing detonation initiation technology [17]. Especially, the hot jet detonation initiation firstly forms a high-energy flame in the jet tube, and then the jet flame rapidly forms a high-intensity turbulent flame in the detonation chamber, eventually forming a detonation wave in a short distance. Since the hot jet detonation initiation is an effective detonation technique with short distance and low flow loss, it is one focus of current research. [PERSON] et al. [18] firstly applied the hot jet tube on the detonation chamber to achieve a reliable detonation. Through visual experiments, it was found that a hot jet could quickly form a turbulent flame at the head of the detonation chamber and promote the formation of a detonation wave. [PERSON] et al. [19] used the numerical simulation method to study the hot jet detonation initiation, their results showed that the energy provided by the hot jet was 20 times that of the spark, and the hot jet technology could effectively reduce the initiation distance of DDT. Other literature got the same conclusion by experiments [20; 21]. Subsequently, plenty attention had been poured on effect of jet intensity on DDT. Lots of research had been investigated jet intensity by changing the structure of the hot jet tube, and the same conclusion was obtained that the initiation time and distance of DDT were short when the jet intensity is sufficient [22; 23; 24; 25]. Using ethylene/oxygen with nitrogen diluted, [PERSON] et al. [26] explored the effects of different jet velocity on DDT distance. From their experimental results, it was clearly seen that jet velocity played an important role in accelerating DDT, the faster jet velocity the shorter DDT distance. [PERSON] et al [27] numerically investigated propane/air hot jet detonation initiation process by changing the length of jet tube, it was found that the increasing length of jet tube resulted in faster jet velocity, thus decreasing the initiation time and distance of DDT. According to the review of previous work, it is concluded that the faster jet flame velocity was, the shorter DDT distance. Up to now, the common method to obtain fast jet flame was increasing the length of jet tube. However, too long length of the hot jet tube would increase the formation time of hot jet and increase the DDT time, so that the performance of the engines was seriously affected. In addition, oversize was inconvenient for practical applications.