Transient Measurement Of Gas Concentration Field And Temperature Field Based On Background Oriented Schlieren Method

In the research field of heat and mass transfer,the transient and quantitative measurement of flow fields,especially the concentration field,the temperature field,and other important parameters of gas,is of great significance.In recent years,the development of high-speed imaging and digital image processing algorithms has promoted the development of background oriented Schlieren.As an optical method with easy implementation and good portability,the background oriented Schlieren method can not only show the details of the flow field,but also has a good ability to quantitatively measure the flow field density,which has become a unique experimental method for some flow problems.In this study,based on the background oriented Schlieren method,combined with the principle of inverse Radon transform,high-speed photography,and image processing technology,the transient concentration field during the impact of volatile droplets on solid surfaces and the temperature field of hot air jets were measured.The background oriented Schlieren system was improved in order to overcome the problem of simultaneously focusing on both the subject and the background.Firstly,the transient concentration field during the impact of volatile droplets on solid surfaces was measured using the background oriented Schlieren system.The direct visualization of vapor distribution and quantitative measurement of vapor concentration plays an important role in studying droplet evaporation.However,there is still a lack of experimental techniques to measure the vapor concentration quantitatively.The background oriented Schlieren method can be used to measure the transient concentration field during the impact of volatile droplets on solid surfaces.Compared with the other methods for concentration measurement,this method can measure fast-evolving vapor concentration fields at a high sampling rate.The results of the vapor distribution reveal that the complex structure of the vapor cloud is the result of the interplay among fluid inertia,fluid evaporation,vapor diffusion,vapor convection,and shape evolution during the impact and the spreading of the droplet.Moreover,the distribution of vapor is remarkably different from the diffusion-limited model because of the strong convection.Even in the late stage of the droplet spreading as the droplet achieves a static shape,the vapor cloud is not hemispherical but is flattened because the heavier-than-air vapor flows downwards under the influence of gravity.Besides,based on the background oriented Schlieren method,the temperature field of a hot air jet was measured.By comparing the hot air jet temperature field results measured by the background oriented Schlieren method with the single point temperature measured by a thermocouple,it was found that the temperature measurement accuracy of this method is high.Compared with intrusive temperature measurement methods like thermocouples,this method is for the whole flow field instead of a point,and it can accurately measure the temperature of the hot air jet at a higher sampling rate.