Modulation of plume interaction and diameter prediction of boreholesinduced by novel flash boiling jet drilling

The formation of multilayer subboreholes using jet drilling is an efficient technology to improve coal seam permeability for CBM recovery,and its quality and efficiency are crucial in enhancing the CBM recovery rate and the life span of the wells. The flash boiling jet drilling was proposed to achieve an equivalent impaction and the subsequent formation of highquality and stable subboreholes. In this method,the multiphase jets are formed through the phase transition,accompanied by the thermal stress to efficiently break coal. To modulate plume interaction for further utilizing the merits of flashboiling phenomena,it is essential to obtain the criteria of plume interaction and their effects on jet characteristics. In this study,Diffused Back Illumination (DBI) imaging and Phase Doppler Anemometry (PDA) were performed to capture the macroscopic and microscopic characteristics of the superheated jet,respectively. The new nominal parameter calculated by the pressure enthalpy graph was introduced to analyze the jet morphologies and macroscopic characteristics. Based on the isentropeadiabatic model,the nominal geometrical CP and nominal operating condition parameter OP were obtained on the basis of flow condition at the nozzle throat. The new criteria considering fluid properties,nozzle geometries,and ambient pressure were obtained,and validated by various types of fluids,providing the basis for the adjustment of the impacting force and area. Based on the new criteria,the jet characteristics in various stages of plume interaction were investigated,and the smallest jet angles were achieved at the onset of the severe plume interaction. Values of Sauter mean diameter decreased with the intensified superheat degree and achieved a more uniform distribution when severe interaction occurred. The dominant factors accounting for the lateral expansion of the plume were examined,and thermal factors contributed to the plume expansion under high superheat degree conditions. High vapor pressure facilitates the bubble nucleation and burst and subsequent plume expansion,and the surface tension also affects the plume expansion due to the dependence of the energy barrier to nucleation. The flow conditions at the throat are crucial in determining the jet characteristics in the region of both flash boilingdominant and plume interaction dominant. The new nominal parameter calculated by the energy barrier and nucleation rate was introduced to predict the dimensionless diameter impacted by the flash boiling jet,showing good agreement under various operating conditions,with a confidence of 95%. The proposed criterion and prediction model supplements the guidance on the nozzle design and injection strategy,and are also suitable for the jet with considerable compressibility and supercritical jet,providing a basis for energy exploration.