Concurrent evaporating spray jets in dilute gas-solid pipe flows
Department of Mechanical Engineering
Doctor of Philosophy
Dave, Rajesh N.
Ho, Teh C.
Evaporating spray jets
Evaporating spray jets into gas-solids suspensions are encountered in many industrial applications. The rapid evaporation of spray jets has significant effects on the gas-solids mixing near the jet nozzle regions, including non-uniform solids concentration dilution, gas/solids temperature reduction, and gas/solids velocity acceleration. Furthermore, the jet structure is altered by the change of solids loading, jet mass flow rate and initial droplet size distribution. These interactions of gas, solids and evaporating droplets dominate the process efficiency and product quality. There is an urgent need for exploring fundamental mechanisms of the multi-phase interactions with heat and mass transfer as there are few reported studies on this topic.
This study is aimed at better understanding the phase interactions and microstructures of concurrent evaporating spray jets in gas-solids pipe flows. A study that combined experiments, analytical modeling and numerical simulation has been carried out to explore the unique characteristics of phase mixing and spray evaporation. A continuously circulating gas-solids suspension flow apparatus has been constructed, and a concentric liquid nitrogen spray jet was concurrently introduced into the fully developed gas-solids flow. The spray structure was investigated based on both laser-assisted flow visualization and temperature measurements along the axis using a thermocouple array. In order to interpret the experimental findings and go beyond the experiment limits, modeling approaches were developed, including analytical models and full-field numerical simulations. The analytical model accounts for turbulent phase transfer and interactions between the spray and the ambient flow and successfully reveals basic characteristic phenomena such as the shortening of the spray penetration, evaporation induced temperature reduction and evaporation-induced gas acceleration. The numerical simulation, based on a hybrid EulerianLagrangian method, yields the local phase interactions, evaporation rate, and phase distributions that would otherwise be difficult to determine.
In this study, an innovative and practical method has been developed for demarcation of the dense spray region and the dilute spray region in gas-solids suspensions with strong evaporation. Based on the thermocouple measurements, the dense evaporation length can be clearly determined from a jump in the axial temperature distribution. The proposed thermocouple measurement method can quantitatively determine the dense spray region not only for the cases where optical visualization can be applied but also for sprays in dense gas-solids flows and in dilute gas-solids suspensions with indistinguishable droplets and solids particles. The proposed mechanistic model for thermocouple measurement has successfully interpreted the temperature profiles; the model suggests that the spray region demarcation is characterized by two time scales, namely the collision contact time between a droplet and the thermocouple tc and the time interval between two adjacent droplet collisions on the thermocouple tdd.
In this work, a systematic investigation is carried out for the shortening effect of solids loading on the spray evaporation length. The evaporation length is found to decrease monotonically and asymptotically when the solids concentration is increased. Within the range of this study, a solids volumetric loading of 1.0% leads to shortening of the total evaporation length by 50%, compared to that of the solids-free case, whereas the dense region length is shortened by nearly 70%. Due to the asymptotic nature, within the range of this investigation, the most sensitive range of shortening effect of solids loading occurs within 0.5%, whereas the shortening effect becomes relatively insensitive beyond this point. This indicates that the spray structure in a dense gas-solids suspension should be very similar to that in a dilute suspension flow.
The numerical simulation reveals a strong non-uniformity in phase distributions. It shows that a dense solids layer exists around the jet boundary, which affects the heat and mass transfer between the spray region and its surrounding flows. Another interesting finding is the existence of similarity in gas velocity profile in the main spray region. The dimensionless velocity distributions are not only similar but also match the Schlichting formula or "the Law of 3/2" of single-phase jets. It appears that the similarity model for a one-phase jet may be extended to three-phase jets with strong droplet evaporation.
njit-etd2002-057 (158 pages ~ 9,709 KB pdf)
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Created August 13, 2003