NUMERICAL INVESTIGATION ON THE EFFECT OF COOLING WATER SPRAY ON HOT SUPERSONIC JET

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Volume 119 No. 12 2018, 59-63 ISSN: 1314-3395 (on-line version) url: http://www.ijpam.eu ijpam.

1 Volume 119 No , ISSN: (on-line version) url: ijpam.eu NUMERICAL INVESTIGATION ON THE EFFECT OF COOLING WATER SPRAY ON HOT SUPERSONIC JET Ramprasad T and Jayakumar J. S. Department of Mechanical Engineering, Amrita Vishwa Vidyapeetham, Amritapuri, India tv_ramprasad@hotmail.com Abstract Direct impingement of the high temperature supersonic jet causes the thermal erosion of materials on the launcher and the equipment surrounding it. Therefore, cooling water is generally injected in to the high temperature exhaust plume. Further, injection of water aids in the reduction of jet noise. The injection is modelled using the Discrete Phase Model available on the commercially available software, Fluent. The effect on temperature reduction of the exhaust plume due to the water addition is investigated by varying the parameters including the cone angle of injection and the droplet diameter size distribution. Keywords-supersonic jet, spray cooling, cone angle, discrete phase model. Nomenclature A area of droplet, m 2 C p h fg Ma r e u T x heat capacity of droplet, J/kg K latent heat of vapourization, J/kg Mach Number exit radius of nozzle, m velocity, m/s Greek symbols temperature, K length of the plume, m ρ density, kg/m 3 Subscripts c p Continuous phase Droplet particle I. INTRODUCTION Supersonic high temperature exhaust plume causes adverse damage to the launch facilities and equipment that are sensitive to high temperature. Hence, owing to the high heat capacity and high value of latent heat of vapourisation, water is used as a coolant for this high temperature exhaust plume. Water addition to the exhaust plume also aids in acoustic suppression. Further, addition of water also helps in reduction of nitrous oxide emissions [1]. The work carried out by Miller et al.[2] took into account the reduction in ablative erosion and the reduction in heat flux due to water injection. Further, a relationship between the measured heat flux and the water momentum was found which can be used for the scaling the water injection system to a full scale rocket. Li et al. [1] investigated the effects of injecting water into the exhaust plume and a dimensionless relation between momentum ratio and reduction in temperature has been calculated. The reduction in temperature by water spray injection has been studied in depth by Manikanda et al. [3] by incorporating the Eulerian-Langrangian framework. The pros of using the Eulerian-Lagrangian over Eulerian- Eulerian framework have been explained by Abdullah et al. [4] by investigating the pre-cooling of air into the natural draft dry cooling tower. Abdullah et al. [5] have further investigated on the representation of a real nozzle in a numerical work by including a size distribution of the droplets. In this work, the injection cone angle is varied and reduction in temperature is quantified. Further, modelling of a real nozzle is attempted by considering a distribution of droplet diameter instead of a constant size droplet. II. METHODOLOGY The simulations were carried out on the commercial finite volume CFD software, FLUENT. A. Governing Equations Eulerian-Lagrangian approach is typically used for the modelling of spray injection. In the discrete phase model, in addition to solving equations of the continuous phase, a discrete second phase is simulated in the Lagrangian framework. The second dispersed phase is taken to be consisting of spherical particles which are assumed as droplets. In the present work, the continuous phase is the host rocket exhaust plume and the injection of cooling water is modelled as the discrete phase. The trajectory prediction [6]of the discrete phase is done by integration of the force balance on the particle and is written as, du p gx ( p ) F ( u u p) F D c x (1) p Here, u c and u p is the velocity of the fluid phase and the droplet particle in m/s respectively, ρ and ρ p is the fluid and 59

In such a coupled approach, the discrete phase is impacted by the continuous phase and vice versa, and thus, the heat and mass transfer effects between the droplet and the rocket exhaust can be taken

2 particle density in kg/m 3 respectively. The term F ( u u p ) D represents the drag force per unit mass of particle. In order to account for the interaction of the dispersed phase with the continuous phase, a two- way coupling approach is used. In such a coupled approach, the discrete phase is impacted by the continuous phase and vice versa, and thus, the heat and mass transfer effects between the droplet and the rocket exhaust can be taken into account. The droplet particle type available in the Discrete Phase Model takes into account the heating and also the phase change of the droplet on interaction with the continuous phase. The rate of energy transfer by the droplet is given by the equation dtp dmp mpcp hap( Tc Tp) h f (2) g Here, C p is the droplet heat capacity in J/kg-K, T c and T p is the continuous phase and droplet temperature respectively. dm p The term denotes the rate of evaporation of the particle. B. Validation of the model (DPM) The work by Abdullah et al. [4] has been taken up for validation. This work involves the precooling of air before entering a natural draft cooling tower for enhancing the cooling tower performance. This is done by spraying of water onto the incoming air which is modelled using the discrete phase model. The domain is a rectangular duct of 1m by 1 m by 10m. Water is sprayed from a distance of 0.5m from inlet and at a height of 0.7m. The area weighted average of temperature is taken at various planes downstream of the injection point and is compared with the work done in the paper. III. RESULTS AND DISCUSSION A. Computational Domain The nozzle used for the simulation of the situation is a standard NASA TP-1953, configuration 2 nozzle which is taken from literature [7].The chamber pressure is given as bar and the temperature as 3400K. The 3-D cylindrical computational domain has been designed based on the length of the exhaust plume which is given by the relation [1] x 2 6.9( Ma) r e (3) Figure 2a: The C-D Nozzle The diameter of the cylindrical domain is taken as 10 times the nozzle exit diameter to allow for the complete expansion through the nozzle and to provide adequate spacing for injection. Figure 2b: The computational domain Fig. 1.Temperature (expressed as a dimensionless parameter) along the length of the duct The values at individual locations agree with that given in the literature and the error obtained is of the order of 1.08%. The mesh constructed is of 23,59,152 cells with an average orthogonality of The turbulence is modelled by RNG k-ɛ, pressure inlet is given for the nozzle inlet, the walls are considered to be adiabatic wall condition. Two injection points are taken at 180 degrees apart along the Y-axis. The mass flow rate per injection is taken as 7 kg/s, therefore the total injection flow rate is 14 kg/s. The injector is modelled as cone type and the temperature of the injection is 300K. The interaction between the continuous phase and discrete phase is activated to allow for the heat and mass transfer characteristics to be monitored. 60

B. Free jet expansion without water injection The hot gas mixture is expanded freely without water injection and the parameters including mass flow rate, temperature and Mach number at exit is

3 B. Free jet expansion without water injection The hot gas mixture is expanded freely without water injection and the parameters including mass flow rate, temperature and Mach number at exit is determined. The exit Mach number is 2.53, the mass flow rate at the outlet is 6.795kg/s and the temperature at the outlet of nozzle is 1618K. D. Variation in droplet diameter Further a study comparing the droplet size as a constant value and as a Rosin-Rammler size distribution has been taken up. The constant size diameter was taken as 50µm. As for the Rosin-Rammler distribution, the diameter of the droplet at the outlet of the nozzle is a variable and varies according to the jet breakup. This water jet breakup takes place at a certain distance away from the nozzle. The input parameters for the Rosin-Rammler have been taken from literature[8] such that the range of diameters includes 50μm. The parameters are: minimum diameter 10µm, mean diameter is 69μm and the maximum diameter is 174μm, the spread parameter is taken as 3.41 and the number of diameter classes is taken as 40. Figure 3: Radial temperature contour of free jet expansion C. Variation of cone angle The injection cone angle is an important parameter deciding the spread of the spray onto the plume. Figure 4: Temperature distribution for different cone angle From the figure, for the cone angle of 60, the temperature of the core is decreased as compared to that of cone angle 75. Further, it is seen that the temperature at the outer profile of the plume is nearly equal for both cases that is cone angle 60 and 75. It can be therefore understood that, in case of a higher cone angle, the spread of the particles is higher and thereby is unable to penetrate the hot rocket plume to the degree that is penetrated by the injection done at cone angle of 60. Figure 5: Temperature distribution for different droplet size distribution From figure 5, it is seen that there is a decrease in the core temperature of the plume in both cases. In case of the Rosin- Rammler distribution, the smaller size particles, though possess a higher evaporation rate by virtue of their high surface area to volume ratio, have a low momentum as compared to the exhaust plume hence are carried away as they lose their momentum. The larger size droplets, possess a higher momentum, which when participate in momentum transfer with the exhaust lose their momentum more slowly. This causes the larger diameter droplets to penetrate into the plume and come in intimate contact with the plume causing a larger decrease in temperature towards the core region owing to the higher evaporation. IV. CONCLUSION Numerical simulation of the cooling of hot supersonic jet by injecting water is carried out and investigations have been done to understand the effect of parameters of injection including cone angle and droplet size distribution. It is seen that the temperature reduction is non-uniform due less injection locations and hence a larger number of injection points at uniform spacing around the plume are required for adequate cooling. It is seen that the lower cone angle allows for a smaller and more focused spread of droplets resulting in a higher temperature reduction as compared to a bigger cone angle. 61

4 Also, it is seen that the initial characterization of the spray is essential for the accurate modeling of the injection system, as the breakup of the jet into droplets depends on various factors including injection rate, jet velocity at nozzle exit, injection pressure and the ambient conditions, in this case, the exhaust plume. REFERENCES [1] Jing Li, Yi Jiang, fan Zhou, Cooling Effect of Water Injection on a high temperature Supersonic Jet, Energies, Vol.8, (2015), Ppp [2] Miller, M.J.; Koo, J.H. Effect of water to ablative performance under solid rocket exhaust environment. In proceedings of the 29th Joint Propulsion Conference and Exhibit, Monterey, CA, USA, November [3] Manikanda R., D. Rajamanohar, 2014, Modeling of an Exhaust gas cooler in a high-altitude test facility of large-area ratio rocket engines, American Society of Civil Engineers, ISSN / [4] Abdullah Alkhedhair, Hal Gurgenci, Ingo Jahn, Zhiqiang Guan, Suoying He, Numerical simulation of water spray for pre-cooling of inlet air in natural draft dry cooling towers, Applied Thermal Engineering, Vol.61,(2013),Pp [5] Abdullah Alkhedhair, Ingo Jahn, Hal Gurgenci, Zhiqiang Guan, Suoying He, Yuanshen Lu, Numerical simulation of water spray in natural draft dry cooling towers with a new nozzle representation approach, Applied Thermal Engineering, Volume 98, 2016, Pages [6] Discrete Phase Models, Chapter-19, Ansys User Guide,2001. [7] Carson, G. T., Jr. and Lee, E. E., Jr., Experimental and Analytical Investigation of Axisymmetric Supersonic Cruise Nozzle Geometry at Mach Numbers from 0.60 to 1.30, NASA TP-1953, [8] Fergusson, Ashley Ann Marie, "Numerical simulation of a fuel nozzle's spray" (2011). Theses and dissertations. Paper 881. [9] Suresh, S., Venkatesh, G., Ignatius, J., Jayakumar J S& S, Sankaran.. Numerical simulation on the interaction of water inejction with the under-expanded hot supersonic jet related to scaled solid rocket motor, In proceeding of the 44th National Conference on Fluid Mechanics and Fluid Power, Kollam, India, December

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Prediction of Transient Deflector Plate Temperature During Rocket Plume Impingment and its Validation through Experiments

Prediction of Transient Deflector Plate Temperature During Rocket Plume Impingment and its Validation through Experiments PRIYA KAMESH KAMATCHI*, VISHNU POOVIAH*, PISHARADY.J.C ** *Department of Mechanical