Ariane 5 Internal Cavities Beat the Heat
By Loïc Cheriaux and Jean-Marc Carrat, Thermal Aerodynamics & Hydrodynamics Analysis Department-EADS-Space Transportation, Les Mureaux, France
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The geometry of the launcher cavities, with the LOX tanks in blue and the LH2 tank in yellow |
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Ariane 5 on the launch pad in French Guiana |
The new 10-ton payload ARIANE 5 (the so-called A5ECA) is composed of two main cryogenic stages separated by internal ventilated cavities. These cavities are flanked by very different temperature extremes on the sides, so the development and validation of a thermal model for them, particularly including convective effects, is critical to the success of the project. Several studies have been performed throughout the life of this program using FLUENT to simulate convection and conjugate heat transfer. This work continues at EADS,with the goal of completing a global validation of both cavities using full scale test data and incorporating local weather conditions into the CFD model.
There are two main ventilated cavities, both with helium conditioning. The ESC/VEB cavity is between the third stage of the rocket (ESC-A) and the payload compartment, with a size of about 30 m3. The EPC/ESC-A cavity is between the two cryogenic stages, and is about 150 m3in size, with a height of nearly 6 m. There are many technical challenges associated with this thermal configuration. First, an acceptable thermal environment is needed for different types of equipment, such as the electronics and the engine. Ventilation of the cavities with helium gas can provide the proper environment, but it is expensive and there is an associated constraint on the tank pressure. Thus, the flow of helium in the cavities needs to be optimized. Second, a compromise must be found that provides efficient thermal protection to the tanks, but which is also lightweight and inexpensive.
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The geometry of the EPC/ESC-A cavity includes many details, most of which were included in the CFD model. The first two components above combine to form the geometry used for the simulation |
Consider the EPC/ESC-A cavity. This cavity has two very cold walls on its floor and ceiling, which are the upper and lower storage tanks for cryogenic fuels.The liquid hydrogen (LH2) is at a temperature of 20K, and the liquid oxygen(LOX) is at a temperature of 90K. The vertical side walls, on the other hand,are exposed to the hot temperatures of French Guyana, the site of the Ariane5 launch pad. In this environment, temperatures range from 273K (0°C) to313K (60°C). The walls of the cavity are made from different materials, such as aluminum and composites, which have different thermal conductivities. Using a helium gas flow rate through the cavity, the challenge is to maintain thermal equilibrium for several hours on the launch pad with the tanks filled!
Modeling the flow and heat transfer in the cavity poses a number of challenges as well. In addition to the complex geometry and physics, the numerical model must take into account several critical issues:
- The overall cavity is large (several meters across), yet the walls can be only a few millimeters thick, so this poses a meshing challenge.
- Very high thermal gradients are anticipated, so this also impacts meshing decisions. In addition to regions of refined mesh, a smooth, high quality mesh overall is desired for the combined calculation of conduction and convection.
- The conjugate heat transfer needs to account for boundary layers,turbulence, laminar/turbulent transitions, separation, a variety of thermal boundary conditions, and the effects of phase change of ambient water vapor.
- The mixed convection that will occur will include forced and natural components, and due to the height of the cavity, the latter cannot be neglected.
- The flow is compressible close to most of the ventilation inlets(where the injections are nearly sonic), and incompressible in the remainder of the cavity volume, especially in the many low Mach number recirculation zones. While this impacts the mesh refinement in the inlet regions, the choice of numerical solver(segregated vs. coupled) is still an issue in this situation.
- While steady-state flows provide much important information,transient analyses are also needed.
The forced convection in the cavity is the result of the helium gas flow into the volume. Natural convection results from the warm walls and cold floor and ceiling. It gives rise to a circulation loop, with upward flow at the hot lateral walls and downward flow, accelerated by the cold ceiling, through the center of cavity. For this cavity, both the Rayleigh number (Ra) and Reynolds number (Re) have the same order of magnitude: 109. The flow is therefore highly turbulent and buoyant. For these conditions, a full 3D Navier-Stokes simulation is required to understand and characterize the coupled convective and conductive heat transfer phenomena.
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Contours of the fluid velocity field on a slice through the EPC/ESCA cavity |
Construction of the 3D geometry for the EPC/ESC-A cavity required some simplifications in order to focus on the salient flow features. The model was not intended to reproduce the exact flow structure throughout the cavity,but instead, to provide the true global budget of mass and energy through-out the domain. It was created by a surface-based CAD-system, and included the convective flow domain as well as the material structures (for the conduction calculation). A conformal hybrid mesh of about 1.5 million cells was built using GAMBIT. This mesh size was chosen to meet several (often conflicting) requirements, including CPU and RAM resources, convergence time, and the need for solution precision. Both size functions and the automatic structured boundary layers option were used during the meshing process. The realizable k-ε turbulence model with standard wall-functions was used, since it is an industrial approach particularly well suited to this problem. Parallel computations were used to reduce the CPU time.
To date, validations have been made through full scale tests on the ground.The validation process involves
- computing the external environment of the launcher using meteorological data on the ground (solar and wind conditions)
- applying this specific environment to the global thermal models in FLUENT
- comparing the calculated and measured temperature, and
- identifying discrepancies, understanding them, and adapting the thermal models, as needed.
The validation work has proved to be very successful. Transient predictions of the average gas temperature inside the cavity and at a point on the wall are in very good agreement with experimental data.
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In a comparison between a transient simulation and measurements on the ground, a global discrepancy of less than 2°C was achieved for the average gas temperature (green) and wall temperature (blue) |
Based on the use of CFD software (FLUENT) with pre-flight validations using full scale tests, the thermal calculations and especially the inner convection field have been identified as critically important to the A5ECA program. CFD has demonstrated that it can help engineers to better understand complex phenomena, investigate local optimization, and/or define corrective action.In the future, there will be a shift from the current macro cavity model to amore detailed global computational model, by taking advantage of steady improvements in solver numerics, physical models, and computer capabilities. The end goal will be to reduce computation time yet still make progress on modeling the thermal coupling between the gas and wall.
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