Chongam Kim
Department of Aerospace Engineering,
Seoul National Univ. Seoul 151-744, Korea
chongam@snu.ac.kr
ABSTRACT
This paper focuses on the role of CFD in developing rocket propulsion system by simulating major devices such as turbopump inducer, cryogenic storage tank and solid rocket propellant. These are closely related to operation reliability and fuel efficiency of rocket propulsions system.
The numerical computations on these devices have several issues owing to complex flow physics such as interaction between fluid, structure, and combustion domain or extreme flow conditions. This paper introduces these issues and corresponding numerical methods for more realistic simulation. Finally, several
numerical results are presented to show the contributable aspects of CFD in rocket development stage.
INTRODUCTION
The rocket systems for space exploration are composed of payload, guidance, propulsion system and structural assemblies. Among these elements, increasing the portion of payload has been on the rise as a recent major issue, because the mission of rocket system depends on payload itself. But most portion in entire weight of rocket is occupied by liquid propellant, and consequently, reducing structure weight of fuel tank or estimating optimum amount of fuel are demanded to save the redundant weight.
In this sense, liquid propulsion part of rocket system generally employs the turbopump inducer to omit unnecessary structure required for highly compressed cryogenics.
Furthermore, many research institutes are concentrating on precise prediction of remaining fuel in tank to achieve the high fuel efficiency.
At the same time, reliable launching should be secured to prevent enormous costs as well as tragic accidents by operation failure. It has been considered as a first fundamental issue to be assured for rocket propulsion system, but operation failures are occasionally occurred even in recent days. However, triggering reasons for failure are generally diverse and complex for examination. For example, failure of H-II rocket has come from vibration stress due to swirling cavitation in turbopump, while explosion of Titan IV rocket has come from bore choking resulting from excessive deformation of solid propellant.
Even though many other issues still exist to be resolved, this paper focuses on aforementioned topics, operation reliability and fuel efficiency. As stated above, turbopump inducer, which is employed to reduce the weight of fuel feed system for efficiency, is closely related to operation reliability because of cavitation.
The cavitation is commonly known as inevitable phenomena that phase is changed locally from liquid
to gas phase by pressure decrement around rapid rotating blade. This generated cavitation impedes not only suction performance of inducer, but also stability of propulsion system as H-II rocket or Ariane V failure.
Therefore, understanding the characteristics of cavitation and its effects on suction performance or operation stability are crucial in design process of propulsion system.
As the next problem, the behavior of cryogenic fluids in microgravity condition is handled correlating fuel efficiency. The remainder can be easily measured in normal gravity, but microgravity in space makes difficult to measure exactly because it obscures the interface between liquid and gas phase. Furthermore, various heat sources, which cryogenic storage tank may experience in space, make a loss in liquid phase by ebullition. In this sense, predicting optimum amounts of cryogenic fluids by considering ebullition loss and its change due to behavior of fluids in microgravity have important meaning in development stage of rocket propulsion system.
Finally, complex multi physical phenomena inside solid rocket is treated to avoid catastrophic accidents by rocket failure. The burning process of propellant grain generates high pressure and temperature gases, which gives a structure load and following deformation on to the grain. Additionally, it decreases the volume of propellant and changes the surface shape as well as fluid domain region. This change reversely
causes burning rate and pressure distribution on the propellant surface. Examining these problems can be conducted through experimental manner, but various limitations make difficult to accomplish the objectives.
For example, high pressure and temperature condition in solid rocket propellant hinder visualizing inside
rocket motor. Furthermore, reproducing various gravity or heat source condition similar to real mission through long period of time is also difficult. Recently, numerical approach is gathering the attention as an alternative to experimental research. It can help realizing local flow physics comparably in ease than experimental research, while it requires lower cost. The extreme flow conditions, which were difficult to reproduce in experimental manner, also can be simply implemented in numerical approach.
Furthermore, recent growth in computing power makes numerical research more attractive.
Based on this background, cavitating flows around turbopump inducer and behavior of cryogenic fluids in microgravity condition are numerically simulated in multiphase framework, while multidisciplinary physics inside solid rocket is computed by integrating fluid, structure, and combustion domain. The numerical issues and corresponding methods are introduced firstly. Then, simple validation and some examples, which can show the role of CFD in developing rocket propulsion system, are presented.
Cryogenic multiphase flows in liquid rockets
There are diverse numerical issues that must be resolved for the calculation of cryogenic multiphase flows. We categorize the issues into a group regarding multiphase flows and cryogenic flows. After addressing the numerical issues and the way we overcome each difficulty, several numerical results will be shown: flows inside the fuel tank and cavitation around turbo-pump inducer.
Figure 1 present comparison between numerical results and experimental data in various cavitation numbers. We can see that the locations of vortices or low density regions have good agreements with the shedding patterns from experimental data. So, we can conclude that the numerical model we proposed show similar behaviors of experiment and our multiphase model is validated. It can be certain when from the FFT analysis of both results as Figure 2, which presents the dominant frequencies of free shedding vortex.
Result 2: 2-D shock wave-water column interaction
Another famous validation case of multiphase flows is 2-D shock wave-water column interaction. It is a severe benchmark test for compressible multiphase flow because it need to capture the correct shock locations. This problem simulates that moving air shock whose speed is 1.47 Mach number strikes on a water-column. After striking, there are several complex shocks and expansions which can be seen as
Figure 3. And it also has good agreement with results of other research groups[8].
Cryogenic flow
Numerical issue 1: Reflecting the real fluid properties
Since most cryogens are working near their critical points, thermal effects play an important role. For example, when a cavitation occurs, the temperature drops more due to the low density ratio between liquid and gas phases near the critical temperature.
Consequently, additional cavitation is suppressed as the saturation pressure is decreased. These thermodynamic effects with phase change are all entangled with real fluid properties.
Therefore, exact equation of state such as MEOS or NIST database [9~11] for specific working fluid is essential for an accurate computation. Employing these equations directly, however, requires huge computational costs. We use tabular function with database hashing based on NIST as an equation of state for subcooled, superheated, and supercritical states for the computational efficiency. SRK equation of
state is also applied for non-condensable gases in cryogenic tank simulation.
t=0.0 𝛍𝐬(upper, left), t=3.0 𝛍𝐬(upper, left) , t=7.0 𝛍𝐬(upper, left) , t=100.0 𝛍𝐬(upper, left)
Result 1: Cryogenic cavitating flow around hydrofoil
As a validation case, numerical simulations of experiments by Hord [12] for liquid nitrogen is presented. This cryogenic cavitating flow is known to exhibit substantial temperature variation, hence the importance of accurate equation of state is emphasized.
Figure 4 clearly shows the mushy characteristics of cryogenic cavitation. Thermal effect is successfully reflected and the cavitation length is well predicted as in Figure 5.
From the results, we can find the use of hash function based on NIST database is appropriate for the computation of cryogenic multiphase flows.
Flows inside the propellant tank
Figure 6 represents pressurizing process of liquid nitrogen storage tank under normal gravity and microgravity circumstance. The injected hot helium gas approaches the liquid nitrogen surface triggering the boiling process in microgravity condition, while the helium is floating on the upper part of the tank due to its small density under normal gravity. The amount of loss in liquid fuel under microgravity situation can be considerable due to this active boiling. In preparing stage of vehicle launch, these fuel loss should be taken into account for the successful and efficient mission.
microgravity (lower), ASDL
Figure 7 shows another active pressurization in liquid oxygen tank without gravity.
The tank is located at NASA SSC E-1 test complex. We simulate the injection of supercritical oxygen. Pressure rises quickly due to the high mass flow rate as in figure 7 (f). Therefore, condensation is the major phase change phenomenon in this case. We can see vigorous mixing in ullage and consequent deformation of phase interface.
As seen in these example, numerical approach on cryogenic flow containing phase changing process can produce data for efficient management of fuel or oxidizer.
Cavitation around turbo-pump inducer
Cavitating flows around turbopump inducer are numerically reproduced. The target inducer is experimented by KARI. Figure 8 compares the experimentally measured pressure and suction performance of inducer with our computed results. It indicates that our numerical methods can predicts its suction performance reasonably, and hence it can be used as a substitution for experimental works. Furthermore, Figure 9 which represents surface pressure distribution and cavitation region implies that our numerical approach can help realizing characteristics of cryogenic cavitation. Further researches on unsteady physics are expected to be valuable for operation reliability.
Multidisciplinary physics inside solid rocket
The numerical issues related to the solid rocket simulation can be categorized into two groups, domain deformation by the propellant burning process and methods for coupling fluid, structure and burning phenomena. Diverse methods are implemented to overcome each issue. With these methods, integrated Fluid-Structure-Burning Interaction (FSBI) simulations are performed to analyze multidisciplinary physics inside solid rocket.
Numerical issue 1: Propellant grain burn back simulation
Propellant grain burn back simulation requires a surface-tracking method for precise burning area prediction. Most popular surface-tracking method is Level Set Method (LSM). LSM traces surface in an implicit representation and treats topological changes easily. However, LSM suffers from high computational cost. Because efficiency is important in multidisciplinary physics simulation, it is almost impossible to implement LSM in our 3-D simulation.
Face Offsetting Method (FOM) [15] moves grid nodes and faces explicitly by assigned grid velocities. Although FOM cannot handle topological changes easily, this method can be used at low computational cost. FOM is adopted as our surface-tracking method for efficiency.
Numerical issue 2: Data transfer at non-matching interface
In general, grid element sizes for fluid are small compared to structure at the fluid-structure interface. Therefore, fluid and structure meshes are non-matched, and the ability to transfer data at this non-matching interfaces is required. Common-refinement method [16] using Sobolev minimization is one of the well-known data transfer method at non-matching interface. We use common-refinement method for 2-D simulation. In 3-D simulation, however, 3rd order interpolation method is adopted for efficiency.
Numerical issue 3: Time stepping scheme for integrated simulation
In multidisciplinary physics, a time integration method combining individual simulations is needed. Accuracy and stability of the integrated simulation depend on the time integration scheme. Conventional Serial Staggered (CSS) method is the most popular method in Fluid-Structure Interaction (FSI) simulation. This method is successfully implemented for Fluid-Structure-Burning Interaction (FSBI) simulation.
Integrated FSBI simulation
Fig. 7 shows ignition delay and secondary ignition phenomena during combustion.
[17] Initial cold gas and separation bubble make the ignition delay and secondary ignition occurs at propellant surface.
Fig. 8 represents an erosive burning effect in high L/D rocket model. Additional heat flux induced by high velocity core flow causes erosive burning effect on the propellant grain surface. This phenomenon is prominent as the flow approaches the nozzle exit.
Fig. 9 represents the failure mode similar to the explosion of Titan IV. Inward deformation of the aft propellant segment make the core gas flow choked at the aft segment.
Conclusion
The important physics related to operation reliability and fuel efficiency of rocket propulsion system are numerically studied in this paper. It includes cavitation around turbopump inducer, behavior of cryogenic fluids in microgravity condition, and multi-physical phenomena around solid rocket propellant. There are some numerical issues which may make difficult to provide reliable results. This paper introduces these
numerical issues and corresponding methods. Then, some numerical examples are successfully simulated.
The simulations on cavitation around turbopump inducer can help predicting cavitation breakdown point. Furthermore, it can provide valuable data to investigate unsteady cavitation instability characteristics. The capability predicting the remainder of fuel tank can be applied to maximize fuel efficiency. The CFD also can be used to understand complex unsteady physics inside solid rocket. It can predict unexpected
failure mode of solid rocket by examining fluid field around rocket propellant, which is complicatedly coupled with structure and combustion domain, and consequently improve the reliability in early design stage.
By summarizing these applicable aspects, it can be concluded that CFD can contributes to the development and performance enhancement for rocket propulsion system.
Acknowledgements
The author appreciate the financial supports provided by National Space Laboratory program(NRF-2014M1A3A3A02034856) and Development of Space Core Technology program (NRF-2015M1A3A3A05027630) through the National Research Foundation of Korea funded by the Ministry of Science, ICT \& Future Planning. The author also appreciates the help of Daeho Min, Hyeongjun Kim in preparation of this manuscript.
