Numerical Investigation Of Formation Condition And Propagation Characteristics For Non-premixed Rotating Detonation Wave

Detonation is a type of combustion different from deflagration and it has low entropy production,self pressure gain and high heat release rate characteristics.Various detonation based combustors are realized by different propagation features of detonation waves.Rotating detonation wave（RDW）is confined within rotating detonation combustor（RDC）leading to stable thrust source production,wide range of inlet flow rate and low energy loss because of its inherent characteristics.Due to the high risk of flashback in premixed injection scheme,it is unreasonable to apply to rotating detonation combustor.Alternately,non-premixed injection scheme becomes the essential approach for practical implementation of rotating detonation combustor.Different fuel properties for pure gas and two-phase non-premixed rotating detonation combustors make these two kinds of combustors have different futures.Pure gas non-premixed rotating detonation wave is generally more stable than two-phase rotating detonation wave.Involving droplet atomization and evaporation in combustor,two-phase rotating detonation wave is more complex and basically has poor propagation characteristics compared with pure gas condition without pre-heating and pre-vaporization process.However,because of the high energy density of liquid fuel,two-phase non-premixed rotating detonation wave is still an important aspect to investigate.This paper numerically investigates the H₂/air pure gas and n-C₇H₁₆/air two-phase non-premixed rotating detonation wave,including detonation wave formation process,propagating mode and stability as well as propulsive performance of combustor.Based on the key points of current status,a reasonable approach is applied to investigate the rotating detonation of hydrogen and n-heptane.Firstly,the non-reacting mix process is analyzed focusing on the air injection direction.After obtaining the optimal injection scheme based on the non-reacting mix process,the effects of different factors on H₂/air non-premixed RDW characteristics are further investigated and figure out the physical mechnism of new detonation wave.For n-C₇H₁₆/air non-premixed RDW,the effects of pre-vaporized degree and droplet size on two-phase RDW are first analyzed then the geometric model which predictes contribution of droplet to detonation are obtained as well as the approach to improve detonation fraction,finally we gets the operation map of n-C₇H₁₆/air non-premixed RDC.According to the numerical results,the main findings as follows.（1）For H₂/air non-premixed rotating detonation combustor,air injection direction plays a significant role on mixing process.Air radical injection scheme performs good mixing efficiency compared to air axial injection scheme.In air radical injection structure,gas flow direction changes confined by the wall before getting into combustor leading to higher total pressure loss.Detonative area ratio generally similar for all injection structures,while it is higher near the inlet location for air axial injection structure.For air axial injection structure,hydrogen injection location makes the different total pressure distributions along inner and outer walls.Jointly considering the collection of pressure by the outer wall,detonative area ratio,mixing efficiency and ignition success,the injection structure of air axial injection and hydrogen injected from inner wall is selected as the following computation of full scale three-dimensional combustor.For two-phase n-C₇H₁₆/air non-premixed rotating detonation combustor,the mixing efficiency decreases with injection pressure but the total pressure loss increases.Total pressure loss increases with initial droplet diameter but has little effects on mixing efficiency.（2）Reactant mass flow rate,global equivalence ratio of combustor,hydrogen holes number have obvious influence on rotating detonation propagation mode,stability and propulsive performance of combustor.As the increase of mass flow rate,rotating detonation wave is bias to switching to multi-wave mode.Propagating mode switching process plays sight impact on propagation mode of rotating detonation wave and it is only determined by the current inlet condition.Single wave mode is obtained when global equivalence ratio is 1.2.Two wave mode is obtained when global equivalence ratio are 0.6,0.8,1.0 and 1.4.Stable and self-sustained rotating detonation wave is not observed when the global equivalence ratio is 0.4.Detonation wave formation time is the shortest on stoichiometric condition and the rotating detonation wave is the most stable amog all the cases.As the equivalence ratio varies from fuel-lean condition to fuel-rich condition,the working range of RDC enlarges then narrows.In current combustor geometric structure,single wave mode is obtained when hydrogen holes number is 40,60 and 120 while two-wave mode is obtained when hydrogen holes number is 90.Because of the homogeneous reactant ahead of rotating detonation wave when holes number is 90,rotating detonation wave is very stable.The formation process of rotating detonation wave is similar in different holes number and the final propagation direction has relationship with shock wave collision spot and the equivalence ratio around this spot.（3）Small n-heptane droplet can be fully consumed by rotating detonation wave.For large n-heptane droplet condition,unburned droplets can be observed behind the detonation wave and these survived droplets keep burning there.Detonation wave speed increases with pre-vaporized degree and detonation wave speed deficit decreases with initial droplet diameter decreases and pre-vaporized degree increases.The detonation speed deficit varies between 5%-30%in current investigation scope.Besides,detonation fraction increases with pre-vaporized degree.There is a critical initial droplet diameter（about 20μm）where detonation fraction reaches the lowest for the same pre-vaporized degree.The droplets is able to interact with deflagration surface when the initial injection location is far away from detonation front because they have long resident time in refill zone.However,when the droplets initial injection location is close to detonation front,they can be quickly consumed by detonation wave with short evaporation time in refill zone.Droplet evaporation height increases with pre-vaporized degree decreases and initial droplet diameter increases.Increasing gas pre-heating temperature can effectively improve detonation fraction and the deflagration surface promotes vaporization of droplets on the top layer.The blast wave stems from shock triple point is able to penetrate into refill zone from deflagration surface and further improve droplet vaporization.