Investigation Of Internal And External Flow Characteristics Of Internal-Mixing Twin-Fluid Atomization

Liquid atomization is an essential process in many applications.There has been a tremendous growth of interest in the physics and techniques of atomization to overcome the numerous challenges that face the atomization processes,resulting in designing many types of atomizers.One of the most efficient of these types is the internal-mixing twin-fluid（IMTF）atomizer as it is advantageous over conventional atomizers,and as a result,they are used in many critical industrial applications.However,so far,the IMTF atomization faces challenges,including the difficulty in the accurate prediction of the discharge coefficient of the IMTF atomizers,the produced spray by the IMTF atomizers suffers a major unsteadiness problem,and the current IMTF atomizers cannot fully overcome the challenges of spraying the high-viscosity liquids.Since the IMTF atomization is a two-phase flow-based technique,the mentioned problems are linked to internal flow character.This study has addressed those problems through conducting comprehensive experimental investigations on different IMTF atomizers,including conventional and newly designed types.Firstly,the accurate prediction of the discharge coefficient for IMTF atomizers is still difficult since there are complex relations between dependent and independent factors due to the nature of the internal two-phase flow that affects the nozzle’s discharge.The effect of control factors remains inadequately understood,and the comparative data on C_D of IMTF atomizers are unavailable.Despite its importance in the nozzles’design process,the full and accurate calculation model for C_D still is unavailable.Therefore,in this study,a comprehensive and systematic experimental investigation is conducted on the discharge coefficients of five different types of IMTF atomizers.The effects of important factors on C_D are considered,including the operation conditions,the liquid types,the geometry of the exit orifice,and the internal mixing mechanism of the two phases.The ranges of these factors covered in this study are much wider in comparison with the previous studies.The results show that although the flow visualization inside the exit orifice of all the tested atomizers showed a diverse internal flow character for each atomizer at the same operation condition,the differences in the values of C_D between all the atomizers are very small and may approach zero.This result indicates that C_D of the IMTF atomizers is independent of the form of the internal flow regime at any given condition,and hence,is independent of the internal mixing mechanism.The ANOVA test shows that the parameters of the gas-to-liquid ratio（GLR）,the relative mixing pressure（RΔ_p）,the length-to-diameter ratio of the exit orifice（L_o/d_o）,and the relative viscosity ratio（μ_L/μ_G）have a significant effect on C_D of IMTF atomizers.The results demonstrate that C_D is governed mainly by GLR,and reduces if GLR,L_o/d_o,orμ_L/μ_G increase.While C_D shows a non-monotonic trend when varying RΔ_p;C_D decrease with increasing RΔ_p up to a value of0.98,thereafter C_D starts to increase with further increase in RΔ_p.Finally,a general model for the C_D prediction with an R²≥0.99 is proposed,which is valid for any type of IMTF atomizer and with wider validity ranges for the included factors.Secondly,the IMTF atomization suffers a major problem of spray unsteadiness.So far,the inherent instability problem in the produced spray has not been solved.The spray unsteadiness is directly linked with the internal flow character,which has not been fully understood because of its complexity.Therefore,in this study,a comprehensive experimental investigation is conducted on one of the newly designed IMTF atomizers called the“outside-in-liquid”（OIL）atomizers.By high-speed visualization and digital image processing,the two-phase flow characteristics inside the OIL atomizers are explored and linked with the stability and quality of the produced spray.The results show that the inhomogeneous bubbly and slug flow regimes cause an unwanted intermittent flow inside the exit orifice and an unstable coarse spray,while the stable-annular or the wavy-annular chamber flows provide a favorable continuous annular flow inside the exit orifice and a stable fine spray.By proposing a new parameter considering liquid/gas momentum ratio per liquid injection hole(Ф_ih),a flow regime map is constructed in the We _LS–Ф_ih space,and the transition criterion between the favorable annular flow and the unfavorable intermittent flow regimes inside the exit orifice is obtained.A direct relation between the Sauter mean diameter of the spray droplets and the internal flow transition parameter is found.The results demonstrate that the internal geometry of the OIL atomizer dominantly affects the internal flow and hence the spray characteristics.Under specific internal geometries,the OIL atomizer can produce a fine spray without any unsteadiness at any operating conditions.Finally,the stable spraying of high-viscosity liquids is challenging,and previous studies show that even the current IMTF atomizers cannot fully accomplish this task.Also,since there are many types of IMTF atomizers,the internal-mixing mechanism of the two phases is believed to affect the internal flow and the spray characteristics;however,no study has investigated this point.Therefore,in this study,the ability of different internal-mixing mechanisms on the efficient spraying of high-viscosity liquids is investigated.The internal flow inside two different IMTF atomizers,i.e.,the OIL and the OIG atomizers,is explored by visualization when spraying a high-viscosity liquid.Thereafter,the produced spray stability is investigated and linked with the internal flow.For each atomizer,two configurations with different perforated chamber sizes are used.The results show that the mixing mechanism of the two phases inside IMTF atomizers has a remarkable effect on the two-phase flow pattern and the spray stability,while the effect of the liquid’s viscosity on the flow regime varies with varying the diameter of the perorated chamber.In general,introducing the gas phase from the outside into a flowing liquid stream inside the OIG atomizer results in forming unstable two-phase flow regimes（i.e.,inhomogeneous bubbly,bubbly-slug,or slug flow）.In contrast,introducing the liquid phase from the outside into a flowing gas stream inside the OIL atomizer results in forming a stable and fully developed annular flow regime at all GLRs.Increasing the liquid’s viscosity promotes the formation of unstable and unfavorable flow regimes for both the OIG and OIL atomizers.However,the effect of the elevated viscosity on the flow character inside the OIL atomizer becomes negligible with increasing the diameter of the perforated chamber.Namely,modifying and increasing the size of the perorated chamber of the OIL atomizer results in generating a stable annular flow inside the whole atomizer regardless of the liquid viscosity.Also,the flow character inside IMTF atomizers directly governs the stability of the primary atomization near the exit orifice.The stable annular flow produces the most stable spray,while the abovementioned unfavorable flow regimes generate highly unstable sprays.Overall,the outcomes from this study amend the present knowledge about the discharge characteristics of IMTF atomizers,allow design optimization,and provide flow rate prediction for a variety of IMTF atomizers.Additionally,this study has evidenced that the performance of the outside-in-liquid（OIL）atomizers is superior to the other IMTF atomizers considered in this study since it can overcome the spray unsteadiness problem as well as can atomize liquids with low and high viscosities（μ≤0.2202 Pa·s）steadily and efficiently at any operating conditions.Therefore,the optimized OIL atomizer is suitable for applications requiring stable and efficient spraying of low and high viscosity liquids,such as combustion,surface coating,and power generation.