| In recent years,the demand for high energy density,high energy conversion rate and miniaturized propulsion systems is continually increasing.However,thrust devices that operate with common hydrocarbon fuels have approximately reached the thermal efficiency limit under the traditional combustion mode.Thus,a more efficient combustion regime is exceedingly required since it is difficult to find an alternative kind of energy source for thrust devices,whereas detonation can be a potential solution to this problem because the detonable mixture can be quickly self-pressurized and release its chemical energy,offering a high thermal efficiency nearly the same as that of Humphrey cycle.Therefore,the present dissertation is established upon the new concept of microscale detonation and focuses on the detonation phenomenon which is substantially influenced by the scale factor A series of experiments were conducted to solve three major problems towards the engineering application,which are also the main research topics of the present study:1)Mechanisms of the deflagration to detonation transition(DDT)process under multiple scales;2)Evaluation of the detonation impulse(or specific impulse)affected by the scales;3)Detonation initiation within shortest distance and time utilizing microscale detonation.The first topic is related to the phenomenon and mechanism of flame acceleration.In a smooth tube,flame initiated from the closed end will typically undergo three distinct acceleration stages(S1?S3)before detonation occurs.First is the exponential acceleration after ignition(S1),then is the flame propagation at a quasi-steady speed(S2),and finally is the secondary acceleration until detonation takes place(S3).However,hardly any experiments have systematically provided the flame acceleration characteristics under multi-scales.Meanwhile,the theoretical analyses and numerical simulations have mostly differed from the few existing experimental results.Hence,in this paper,the DDT process was qualitatively and quantitatively studied in detail under a wide range from microscale to mesoscale.Parametric analysis shows that,when the mixture is fuel rich(equivalence ratio 0=1?1.5),the laminar flame speed(SL)and the gas expansion ratio((?))are rather large,then the maximal flame speed vtip,max in S1 and the quasi-steady flame propagation speed(saturation speed cs)in S2 are probably supersonic,but when 0 is other than 1?1.5,SL and(?)are relatively small,the above two speeds may be subsonic.By defining a characteristic Reynolds number Re*the effects of tube diameter d,along with SL and(?),on flame acceleration are evaluated globally.It is found that when Re*is within the range of 1000?1500,d just equals 8 mm.At the same time,vtip,max and cs also reach their maximal values under all experimental conditions,and the non-dimensional flame propagation distance is much shorter accordingly.This means that the acceleration is most effective at this moment.Furthermore,it is also demonstrated that the supersonic cs is conducive to shortening the DDT distance.Optical observations on the DDT process have shown that,in S1,the flame shape is finger-like and it will generate a leading shock during acceleration.In S2,when cs is subsonic,the flame skirt of the finger flame will catch up with the flame front,leading to the flame deceleration and transformation into tulip flame.While cs is supersonic,the flame skirt and flame front will always keep some distance from each other,and the flame will never change into tulip shape.Inspired by the multi-stage acceleration phenomenon,local obstacles were added to the different flame propagation stages to test the acceleration effects in a thin gap.Using shadowgraph technology,the flame-shock interaction affected by the local obstacles was observed and it is discovered that the best location of the obstacles is within S2 in a gap with cross section of 2×8 mm.The second topic is to measure the detonation impulse(or specific impulse)using multi-scale smooth tubes under different detonation initiation conditions.The results obtained by the method of thrust surface pressure integration and the method of ballistic pendulum were compared.It is found that their results show a greater discrepancy as the tube diameter decreases.This is because they have different definitions of the time for single cycle detonation,and the small pressure perturbations during the late stage of one detonation cycle cannot be measured by the pressure transducer due to its pressure charge effect when using the pressure integration method,which leads to the underestimate of the specific impulse.Though previous studies have demonstrated that the specific impulses generated by the direct detonation initiation and the DDT induced detonation initiation are the same,the present experiments further discover that under DDT conditions,the retonation wave or expansion wave contributes most of the produced impulse,and the contribution grows as the DDT distance occupies a smaller portion of the whole tube length.The effects of tube diameter and tube exit conditions on the characteristics of the thrust surface pressures were also tested.With the method of ballistic pendulum,it is found that the impulse is proportional to the tube' s length to diameter ratio(L/d),but the specific impulse will first increase with it and then decrease.When L/d is 40 to 50,the specific impulse of the stoichiometric ethylene-oxygen mixture reaches its maximum value(about 160 s?180 s).At the same time,the friction losses caused by the tube wall can be neglected.While under other length to diameter ratios,the effect of energy loss will emerge,especially for long and thin tubes.Such result can provide a criterion for the design of detonation engines.In addition,high initial pressure experiments were conducted to evaluate the detonation performance.It shows that the specific impulse will rise from 180 s under initial pressure of 0.1 MPa to 250 s under initial pressure of 1.0 MPa.The growth rate is 38.8%.The third topic is to initiate the mixture in the main detonation chamber utilizing overdriven detonation in the pre-detonator.Different detonation re-initiation modes were found under various lengths of the pre-detonator and mixture components.Two major re-initiation categories,the fast detonation re-initiation(FDR)mode and the slow detonation re-initiation(SDR)mode,were determined by the DDT time and distance in the main chamber It is found that the non-dimensional parameter ?,defined as the ratio of the distance between the overdriven detonation position and the pre-detonator outlet to the pre-detonator length,is crucial to the onset of FDR mode.When Cis smaller than 0.2?0.3,FDR mode may be achieved.When ?is larger than 0.3,SDR mode will take place,and the non-dimensional DDT distance in the main chamber has a linear relation with ?.By evaluating the hot jet energy,it is found that the energy generated by the overdriven detonation cannot directly initiate the mixture in the main chamber when the nitrogen dilution ratio ? is above 0.4.Thus,the initiation mechanism under the FDR mode(?>0.4)should be related to the wall reflection effects on the diffracted detonation wave.The critical tube diameter(dc)which is needed for successful detonation diffraction is also evaluated by measuring the detonation cell width.In comparison,the actual diameter of the pre-detonator can be as small as one seventh of dc under the same condition,if FDR mode is acquired with the help of overdriven detonation and wall reflection.Furthermore,the initiation effects using hot jet and obstacles were also tested respectively.The results show that a combination of the two may lead to the shortest DDT distance in the main chamber. |
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