| The velocity measurement technology is essential in investigation of complex combustion flow of scramjet.Among the various velocity measurement techniques,hydorxyl tagging velocimetry(HTV)is a un-seeded,no-intrusive technique based on laser.HTV relies on the photodissociation of water vapor into OH radicals and their subsequent tracking using planar laser-induced fluorescence.HTV can be considered as a direct measurement of flow field due to using molecular tracer.Comparing with particle tracer,there is no tracer following problem in high speed flow.Addition,OH is easy to generate and survive in a high-temperature environment.Thus,HTV is suitable for the velocity measurement of complex high-speed and high-temperature flow field of scramjet.However,because of the complexity of scramjet flow and experiement environment,there are still some issues about HTV to be solved or clarified for the velocity measurement of scramjet flow field.First,the characteristics of photodissociation OH are not described in detail,the effective guidance for practical application is lack.Second,although there have been a lot of works on the methods of tagging line position recognition,the accuracy of the commonly used tagging line still needs to be improved.Third,the problems of applicability in harsh application environment,especially how to restrain the interference of strong vibration to measurement,needs to be solved.Moreover,there is a lack of suitable uncertainty evaluation method for actual application.In this work,intensive studies were conducted on the HTV technology to meet the application requirement of scramjet model experiments.The yield,lifetime,diffusion and spectrum characteristics of photodissociation OH were studied.The relationship between the OH yield and temperature was obtained by the calculation model of the OH concentration produced by photolysis in flame.And the required ratio of water molecules to OH for HTV application in the reaction zone was evaluated.According to the experiments of lifetime,diffusion and spectra of photodissociation OH,the applicable range of delay time between tagging and displaying was obtained,and the minimum measurable velocity of HTV in common environment was estimated to be about2m/s.The position recognition method of single tagging line was studied.A total image correlation calculation was used to recoginize tag baseline.Compared with the traditional row-by-row correlation method,the accuracy of baseline position was improved by about 5times(SNR = 2).For moved tag line,the position recognition accuracy was improved by one time by smoothing the data after the row-by-row correlation operation.The velocity measurements of non-reactive and reactive jet flow fields were carried out.According to the actual tagging images,a method for evaluating the uncertainty of velocity measurement was proposed by using the actual image background noise and combining with simulated tagging images.Experimental results show that in the non-reactive flow field,the uncertainty of velocity measurement of HTV technology is less than 1m/s.In the reaction flow field,the uncertainty of velocity measurement is larger than that of non-reaction flow field(less than 2m/s)due to OH background interference caused by combustion.A novel crossed tagging and displaying optical setup was designed to carry out two dimensional velocity vector measurement.The optical setup is simplier than the usual used grid tagging setup,and the refined optical adjustment is needless,and accurate recognition of a tag spot is more convenient.In a jet flow,the velocity vector distribution based on 8×10 tag spots was measured.A vibration suppression method was developed by recording real-time baseline images.In this method,the baseline image is generated by the tag laser induced Rayleigh scattering or fluorescence,and the baseline image and moved tag line image are recorded by a dual frames dual exposed ICCD camera at almost the same time.The experimental results of flow field measurement in scramjet model show that the velocity uncertainty is below 10m/s when the signal-to-noise ratio is greater than 2.An HTV measurement system for the flow field measurement of scramjet model test was established by means of device integration,vibration isolation design and remote control design.The flow field velocity measurements of scramjet models with incoming flow Mach2.0,2.5,3.5 were carried out by HTV.And the velocity results were obtained of the isolated section,combustor and outlet of combustor.By the measured velocity data of the upstream flow field of the combustion zone,the relationship between the relative turbulence intensity and the local Mach number shows that,during the ram-to-scram process,the turbulence intensity decreases exponentially with an increase in the local Mach number of the flow field upstream of the combustion zone. |
