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Keywords: spray cooling, parallel flow, vaporization, transient, modelling.

Summary

Spray cooling finds wide applications in various industrial processes. In the process of spray cooling in metallurgical systems all water droplets should completely be vaporized before they reach the filtering equipment and it is desirable for droplets not to hit the duct wall. The spray cooling may be controlled by the operational parameters such as nozzle configuration and nozzle capacity. Therefore, a good understanding of the spray cooling behaviour is important for designing and operating the spray cooling systems applied in metallurgical processes.

Little CFD modelling can be found related to the present work which considers the spray cooling of a hot gas. The present authors conducted the numerical simulation of the cooling of a hot gas by a water spray in a straight duct with the nozzle located at the duct wall [1]. The present paper aims to numerically investigate the cooling of the off-gas by a water spray in a straight duct in the condition of parallel flow with the nozzle located at the central axis and to examine the effect of the nozzle capacity on the flow. Details of transient flow and heat transfer performance are obtained for different nozzle capacities. Predicted results are verified against simplified analytical solutions of the mean gas temperature at the outlet.

The spray cooling case to be simulated is the cooling of the off-gas by a water spray in a straight duct. The duct is 8.0 m long and 2.1 m in diameter. The nozzle is located at the central axis of the duct. Parallel flow is formed between the spray and the off-gas. The gas cooling flow is modelled by using the Eulerian-Lagrangian approach. The droplet performance is computed by the discrete phase model (DPM). The coupled approach is used, i.e, the continuous phase flow pattern is impacted by the discrete phase or vice versa.

The gas phase is treated as a continuous phase and the droplets as a dispersed phase. The ideal gas law is applied to the off-gas phase. The turbulent dispersion of particles is modeled by a stochastic discrete particle approach. The collision and breakup of droplets during their motion is taken into account.

An air-blast atomizer is used. The nozzle spray angle is 55C. The temperature of water spray and the compressed air at the nozzle exit are set at 23C. The spray is modeled by fifty-six droplet streams. Every stream represents a number of droplets. Simulation are conducted for different nozzle capacities ranging from 0.2523 to 1.01 kg/s. The off-gas volume flow rate is 68000 Nm³/h and the inlet gas temperature is 800C.

The commercial code Fluent 6.1.22 is used. A refined mesh, with approxiamtely 300,000 elements, is utilized. The time step used is between 10s and s and 25 iterations within each time step are set, with which the scaled residuals of smaller than 10 for all variables can be reached.

Detailed results of the transient behavior of the droplet trace, volume fraction, mass source of H₂O on the duct wall, velocity and temperature are given. The change of outlet temperature with the nozzle capacity is obtained. Predicted mean temperature at the outlet is verified using a simplified analytical solution [1].

Results show that a quasi-steady state is reached at about t=0.35 s. The residence time of droplets is slightly smaller for parallel flow than that of the cross flow. Slight fluctuation in velocity and temperature fields persists in quasi-steady state due to different evaporation times of different droplet sizes. At the same nozzle capacity, the outlet temperature is higher for parallel flow cooling than for cross flow cooling, but this temperature difference becomes smaller with increasing the nozzle capacity.

The outlet temperature decreases with increasing nozzle capacity. At larger nozzle capacity, droplets are more likely to coalesce, more droplets hit the duct wall and more un-evaporated droplets may flow out of the outlet for the duct length considered. The reasonable nozzle capacity is about 0.25 kg/s. With nozzle located at the duct axis, the whole circumference of the duct wall about 3 m downstream from the nozzle location may be hit by the droplets.

References

1: J.L. Xia, J. Järvi, E. Nurminen, E. Peruniemi, and M. Gasik, "Cooling of gas by a water spray in a straight duct - cross flow", 10th International Conference on Liquid Atomization and Spray Systems, ICLASS-2006, 27 Aug.-1 Sept., Kyoto, Japan, 2006.

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 84 PROCEEDINGS OF THE FIFTH INTERNATIONAL CONFERENCE ON ENGINEERING COMPUTATIONAL TECHNOLOGY Edited by: B.H.V. Topping, G. Montero and R. Montenegro Paper 174 Modelling of the Cooling of a Hot Gas Using a Water Spray in a Duct J.L. Xia¹, J. Järvi², E. Nurminen¹, E. Peuraniemi³ and M. Gasik¹ ¹Laboratory of Materials Processing & Powder Metallurgy, Helsinki University of Technology, Espoo, Finland ²Outokumpu Technology, Research Center, Pori, Finland ³Outokumpu Technology, Research Center, Espoo, Finland doi:10.4203/ccp.84.174 purchase the full-text of this paper Full Bibliographic Reference for this paper , "Modelling of the Cooling of a Hot Gas Using a Water Spray in a Duct", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Fifth International Conference on Engineering Computational Technology", Civil-Comp Press, Stirlingshire, UK, Paper 174, 2006. doi:10.4203/ccp.84.174 Keywords: spray cooling, parallel flow, vaporization, transient, modelling. Summary Spray cooling finds wide applications in various industrial processes. In the process of spray cooling in metallurgical systems all water droplets should completely be vaporized before they reach the filtering equipment and it is desirable for droplets not to hit the duct wall. The spray cooling may be controlled by the operational parameters such as nozzle configuration and nozzle capacity. Therefore, a good understanding of the spray cooling behaviour is important for designing and operating the spray cooling systems applied in metallurgical processes. Little CFD modelling can be found related to the present work which considers the spray cooling of a hot gas. The present authors conducted the numerical simulation of the cooling of a hot gas by a water spray in a straight duct with the nozzle located at the duct wall [1]. The present paper aims to numerically investigate the cooling of the off-gas by a water spray in a straight duct in the condition of parallel flow with the nozzle located at the central axis and to examine the effect of the nozzle capacity on the flow. Details of transient flow and heat transfer performance are obtained for different nozzle capacities. Predicted results are verified against simplified analytical solutions of the mean gas temperature at the outlet. The spray cooling case to be simulated is the cooling of the off-gas by a water spray in a straight duct. The duct is 8.0 m long and 2.1 m in diameter. The nozzle is located at the central axis of the duct. Parallel flow is formed between the spray and the off-gas. The gas cooling flow is modelled by using the Eulerian-Lagrangian approach. The droplet performance is computed by the discrete phase model (DPM). The coupled approach is used, i.e, the continuous phase flow pattern is impacted by the discrete phase or vice versa. The gas phase is treated as a continuous phase and the droplets as a dispersed phase. The ideal gas law is applied to the off-gas phase. The turbulent dispersion of particles is modeled by a stochastic discrete particle approach. The collision and breakup of droplets during their motion is taken into account. An air-blast atomizer is used. The nozzle spray angle is 55C. The temperature of water spray and the compressed air at the nozzle exit are set at 23C. The spray is modeled by fifty-six droplet streams. Every stream represents a number of droplets. Simulation are conducted for different nozzle capacities ranging from 0.2523 to 1.01 kg/s. The off-gas volume flow rate is 68000 Nm³/h and the inlet gas temperature is 800C. The commercial code Fluent 6.1.22 is used. A refined mesh, with approxiamtely 300,000 elements, is utilized. The time step used is between 10s and s and 25 iterations within each time step are set, with which the scaled residuals of smaller than 10 for all variables can be reached. Detailed results of the transient behavior of the droplet trace, volume fraction, mass source of H₂O on the duct wall, velocity and temperature are given. The change of outlet temperature with the nozzle capacity is obtained. Predicted mean temperature at the outlet is verified using a simplified analytical solution [1]. Results show that a quasi-steady state is reached at about t=0.35 s. The residence time of droplets is slightly smaller for parallel flow than that of the cross flow. Slight fluctuation in velocity and temperature fields persists in quasi-steady state due to different evaporation times of different droplet sizes. At the same nozzle capacity, the outlet temperature is higher for parallel flow cooling than for cross flow cooling, but this temperature difference becomes smaller with increasing the nozzle capacity. The outlet temperature decreases with increasing nozzle capacity. At larger nozzle capacity, droplets are more likely to coalesce, more droplets hit the duct wall and more un-evaporated droplets may flow out of the outlet for the duct length considered. The reasonable nozzle capacity is about 0.25 kg/s. With nozzle located at the duct axis, the whole circumference of the duct wall about 3 m downstream from the nozzle location may be hit by the droplets. References 1 J.L. Xia, J. Järvi, E. Nurminen, E. Peruniemi, and M. Gasik, "Cooling of gas by a water spray in a straight duct - cross flow", 10th International Conference on Liquid Atomization and Spray Systems, ICLASS-2006, 27 Aug.-1 Sept., Kyoto, Japan, 2006. purchase the full-text of this paper (price £20) go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £105 +P&P)
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