Interfacial Momentum Transfer During The Relaxation Process Of High Speed Gas-droplet Two-phase Flow

Interfacial momentum transfer and velocity slip during the relaxation process of high speed gas-droplet two-phase flow are studied by numerical simulation and experimental investigation. In the light of characteristics of the relaxation process, mathematical models are proposed to predict the flow inside and outside a Laval jet and the interactions between the two phases in the transient section of the two-phase flow. The flows are supersonic gas-droplet two-phase flow inside the Laval jet and two-phase turbulent jets outside the Laval jet (in a reactor).Considered the effects of dilution, compressibility and non-isothermal between the two phases on the interfacial momentum transfer, a mathematical model of supersonic two-phase flows in the Laval jet is built. Simulations are made to the momentum and heat transfer between supersonic gas stream and spray droplets during the relaxation process. The relaxation time is introduced to describe the characteristics of the hydrodynamic equilibrium two-phase system. The initial velocity slip between gas and droplets causes an interfacial momentum transfer flux as high as (2.0-5.0) x 104 Pa. The relaxation time corresponding to this transient process is in the range of 0.015?.030 ms for the two-phase flow formed inside the Laval jet. Due to compression by the shock wave at the end of the Laval jet, the flow pattern becomes two dimensional and viscous outside the Laval jet. The two-phasemturbulent jets are simulated with k-e model for the gas phase and deterministic trajectory model for droplets. The equations of droplet motion are described in Euler coordinate, which simplifies the numerical simulation and makes it possible to obtain continuous velocity distribution of droplets. The relaxation time and the momentum flux in the two-phase turbulent jets are in the same order of magnitude with the values of the flow inside the Laval jet. It demonstrates the unique performance of this system to match the fast chemical reactions using electrically active media with a lifetime in the order of 1 ms.The droplet velocity field is investigated with the aid of Laser Doppler Velocimeter. The distributions of mean velocity and r.m.s. velocity of droplets are obtained. LDV measurements of droplet velocities outside the Laval jet are in reasonably good agreement with the results of the simulation. It shows that the two-phase flows are satisfactorily well represented by the mathematical models described in the paper.The results suggest a unique system suitable for fast chemical reactions with electrically activated media. The study in this paper can be served as a basic study for the supersonic plasma-liquid two-phase reaction system. It is possible to study the mass transfer in high-speed gas-droplet two-phase flow with a similar method.