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 Engineering Amrita School of Engineering Clappana P.O., Kollam 690525, Kerala INDIA kpriya.kamesh@gmail.com ** Scientist/Engineer SF, ISRO, Trivandrum Abstract - Computational predictions were done for a typical subsonic thrust chamber plume and a parametric study was carried out for both impinging plate orientation and configuration. The CFD predictions were validated by conducting table top experiments in a heat sink chamber using a heat sink chamber of air-acetylene combustor. The exhaust plume from the chamber was directed to a plate and temperature variation across the system was measured. An Inverse conduction analysis was carried out to get heat fluxes. A parametric study was done for the deflector plate orientation and thickness to arrive at an optimized configuration so that this design tool could be used for the design of actual ground level testing of liquid propulsion rocket engines. Key-Words: - Exhaust plume, Deflector Plate, CFD, Subsonic, Inverse Conduction 1 Introduction Liquid propulsion rocket engines that have applications in space exploration require thorough design check, testing and qualification before usage in flight. Thrust chamber assembly with injectors and sea level extension nozzle is tested in ground level. For conducting the ground level test, the high temperature exhaust gases from the nozzle exit are to be guided through a properly designed deflector pit/plate. The high temperature plume gases will impinge on the pit/plates and heat them extensively. The plate thicknesses used in the pit, orientation of them etc are to be designed by taking into account the heat transfer from the plume in such a way that the plate temperature is within the operating limit of the temperature of plate material. The final arrangement of plate is arrived at after a series of CFD analysis and evaluation of them before implementation. The system configuration to be finalized for a typical semi cryogenic cluster of four engines having around 700 KN thrust giving a total of 2800 KN, the inlet pressure is around 70 bar, inlet temperature around 3300 K, fuel Flow rate 25kg/s, mixture ratio (oxygen/fuel) 5, outlet temperature is 2000 K and Mach number at the exit of the nozzle is 3.2. In the present study, the heat transfer from the exhaust plume to the deflector plates that carry the plume out of the system. In order to define the plume characteristics coming out of the exhaust nozzle, we have conducted experiments to replicate the plume coming out of a nozzle. In combustion chamber acetylene fuel is mixed with air and ignited by a spark plug. The mass flow rate in the acetylene combustion chamber is 0.4 kg/s, mixture ratio is 150, inlet temperature is 600 K, inlet pressure is 1.2 bar and Mach number is 0.0744. 2 Nomenclature T g Temperature of the plume striking the ISSN: 1790-5095 252 ISBN: 978-960-474-158-8
plate. T ip Inner wall temperature of plate T op Outer wall temperature of the plate h Heat transfer coefficient k Thermal Conductivity of the deflector plate. t Thickness of the deflector plate Q Heat transferred to the plate A s Area of the plate (520x520) mm F Force exerted by the plume gases on the deflector plate. ρ Density of the plume V θ Velocity of the Plume Angle of the deflector plate with respect to the central axis of the combustion chamber. computer analysis is not always reliable or useful for making predictions of many of the plume characteristics. However the models help in understanding the plumes and in extrapolating test results to different conditions of operation. 4 Problem Formulation 3 Plume Studies The source of exhaust plume heating is the rocket engines including the main propulsion, control, ullage and retro engines. Radiation and convection from plume gases splashing from the launch pad can be significant at lift off. The heating is transmitted to the structure by a combination of convection, particle impingement and radiation. Portions of the vehicle structure, which lie within the exhaust plume, are heated by gaseous convection, particle impingement and due to a lesser degree by radiation. In most liquid propellant engine plumes, only the convection mode of heat transfer may require consideration.the influence of the launch pad on base heating depends upon flame deflector geometry and whether the flame deflector operates dry or water-cooled. Simulations are aimed at predicting different plume parameters such as temperature, velocity or pressure profiles, and impact forces. The analysis has many different assumptions about the dynamics or steadiness of the flow. The mathematical models are complex and use one, two or three dimensional mesh models. The analysis of a plume often requires more than one model to solve for different predictions. Many solutions are based in part on extrapolating measured data from actual plumes to guide the analysis. The result of Fig. 2a Fig 1: Schematic of Test setup I X A C B D J Y ISSN: 1790-5095 253 ISBN: 978-960-474-158-8
A B I C D J X Y 5 Problem Solution To present the design and orientation of the deflector plate, contours of temperature, velocity and velocity vectors were examined for the plume from the combustion chamber and the deflector plate. From the temperature contour we computed the heat transfer coefficient of the plume. The amount of heat transferred by convection from the plume to the plate is equated with the heat conducted in the plate. Fig. 2b Figure-1 gives the schematic of the test set up and Figure-2a &2b illustrate the geometry and mesh used in the present simulation. The cell system involves four regions. The wall of the combustion chamber is above AC and below BD. AB and CD are the inlet and outlet of the combustion gases in the chamber respectively. After CD the exhaust gases expand in the region IJ. The deflector plate XY is kept at a distance of 400 mm. The dimension of the deflector plate used is (520x520x2.5) mm. In the first case, the deflector plate is kept at an angle of 0 degree and in the second case at an angle of 30 degree. The flow is in the sub sonic region. The experiment was performed in the lab under same conditions and cases. The flow conditions for the cases are listed below. Table-1. Flow Conditions/Cases h[t g - T ip ] = k/t [T ip - T op ] (1) The heat transferred to the plate is calculated by Q = ha s [T g T ip ] (2) The temperature and velocity vector contours show the deflection of plume after they hot the deflector plate. The backflow of plume to the system can be detrimental so we observe plume behavior after they hit the deflector plate. From the velocity contour and velocity vector contour we calculate the force exerted by the plume gases on the deflector plate. From this we can design an appropriate cooling mechanism for the deflector plate. F = ρa s V 2 sinθ (3) Case 1: Plate on vertical position Chamber Temperature Chamber Pressure Jet Mach Number Ambient Temperature Ambient Pressure T0 601 K P0 1.2 bar M 0.0744 Ta 300 K Pa 1 bar Fig. 3 Temperature Contour of exhaust plume for the plate kept at 0º. (i.e. Vertical position) ISSN: 1790-5095 254 ISBN: 978-960-474-158-8
Fig. 4 Contours of velocity magnitude for the exhaust plume for the plate kept at 0º Fig. 7 Contours of Velocity Magnitude of exhaust plume for plate kept at 30º inclination Fig. 5 Contours of velocity vectors for exhaust plume for the plate kept at 0º Fig. 8 Contours of velocity vector of the exhaust plume for plate kept at 30 º inclination Case-2. Plate at 30º inclination Fig. 6 Contours of Temperature of exhaust plume for plate kept at 30º inclination For both the cases we found the variables T g (Temperature of the plume striking the plate), T ip (Inner wall temperature of plate), T op (Outer wall temperature of the plate) are 619 K, 515 K, and 507 K respectively from the temperature contours. Therefore, from equation 1 we got the value h of the exhaust plume gas as 170.6 W/m2K. This value of h is used to calculate the heat transfer into the plate using equation 2. We got the value of Q as 4428.5 J. The maximum velocity of the plume when it strikes the deflector plate is 5.46 m/s. Therefore, the force acting on the plate due to the impingement of the plume is calculated by equation 3 and was found to be 1.15 N. ISSN: 1790-5095 255 ISBN: 978-960-474-158-8
For Case 1, from the temperature/velocity profiles we see that the flow field of the plume is restricted to midway of the distance between the plate and the exit of the chamber. This gives us an idea of the minimum distance that the plate must be placed from the exhaust to protect the components from backflow of the plume. However, for Case 2 we see that the flow field of the plume extends beyond the exhaust. This could damage the system components particularly for plume coming from rocket nozzles which are at temperatures around 1500 K. 5.1 Case 1 Plate in vertical position Considering the section XYZ in Figure 3, temperature of the plume varies from 587 K at Point Y to ambient temperature at Point X. Along this temperature variation line the temperature is 533 K at a point which is at a distance of 70mm from the plate, 475 K at a point which is at a distance of 110mm from the plate and then it reaches ambient temperature at a distance of 200mm (which is at the midway of the exit of the combustion chamber and the plate) from the plate to Point X. The exit velocity from the combustion chamber is 6.66m/s. Analyzing the velocity contour along the line XY in Figure 4, at a distance of 25mm from the plate the velocity is 5.4m/s and this reduces to zero at a distance 175mm from plate. Due to collision of the plume from the combustion chamber and the backflow of plume from the plate the velocity is zero at the point O (Mid point of the plate). Figure 5 shows the velocity vector diagram for this case. The same temperature and velocity profiles are observed in section ABC also due to axissymmetry. 5.2 Case 2 Plate at 30º from vertical Along the section XYZ in Figure 6 just above the plate the temperature was found to be 587K and this reaches 523K at about 10mm from plate and it reaches ambient temperature at about 200mm from plate. The system components lie in this region. Analyzing the velocity contours along XY in Figure 7, impinging velocity on the plate is found to be 4.7m/s and this reduces to zero at midpoint of XY. Now analyzing the temperature contour along the section ABC the temperature at the point where the plume strikes the plate is 619K, it reduces to 491 K at 185mm from plate and finally to ambient temperature at 280mm from plate. Analyzing the velocity contour along AB in Figure in 7 the impinging velocity of the plume is 5.13 m/s. At the point O, the velocity is zero. Figure 8 shows the velocity vector diagram for this case. 5.3 Computation/convergence history The execution speed of the code for the flow considered in the study, measured by CPU time/particle/time step, is about 6 microseconds on a pentium WX processor. The flow domain consists of about 1,49,000 cells in four sub regions. The flow is sampled for convergence every one time step for 20,000 time steps. The CPU time for the computation was about 1.5 and 2 days for Cases 1 and 2 respectively. 6 Conclusion The heat transfer coefficient of the plume that has been calculated from the CFD simulations helps in designing the required thickness of the deflector plate. This value of h is verified by performing table top experiments. The force calculated can be used to determine the required holding forces for the deflector plate. The flow fields clearly indicate that using a straight deflector plate is much safer than one kept at an angle of 30 degrees because the plume gas after backflow from the deflector plate can affect the components of the system. The straight plate when used along with another connecting plate kept at 90 degrees with respect to it will be able to divert the plume gases entirely from the system. For high temperature plumes emerging from typical nozzle divergent may require adequate additional water cooling. ISSN: 1790-5095 256 ISBN: 978-960-474-158-8
References: [1] George P. Sutton, An Introduction to the Engineering of Rockets. [2] John D. Anderson Jr., Computational Fluid Dynamics The Basics with Approach,McGrawHill. ISSN: 1790-5095 257 ISBN: 978-960-474-158-8