INCA Wall interactions and radiation

Wall interactions and radiation


Industry requirements The combustion chambers in both aircraft and rocket engines experience very high thermal stress due to the extreme temperatures and other harsh conditions. Designers must incorporate appropriate cooling or protection systems to ensure chamber integrity either over a very long lifespan (aircraft engines), or for the duration of the mission (rocket engines on expendable launch vehicles). There is absolutely no material today that can stand up to the temperatures achieved in these engines without a cooling system. Aircraft engine combustors are traditionally cooled by diverting part of the airflow from the high-pressure compressor and reinjecting it in the chamber via small holes. Industry requires a very precise prediction of the thermal flux transmitted to the chamber walls to design cooling systems that avoid hot points and use cooling air efficiently. Increasing compression ratios to boost performance exacerbates the problem. One interesting research goal would be the use of high-temperature materials to decrease cooling airflow rates. For example, we are now considering the manufacture of combustors out of thermostructural composites rather than metal . Combustion chambers in liquid rocket engines are generally cooled by having one of the propellants circulate in tubes making up the chamber and nozzle. Called regenerative cooling, this method transfers heat from combustion to the propellant via the chamber wall, and optimizes performance. A successful system design depends on fully understanding the heat transfer of combustion products to the inside wall, as well as the heat transfer from the chamber wall to the cooling propellant – whose characteristics can change as it is heated (for a cryogenic fuel such as liquid hydrogen). Combustion chambers on the solid rocket motors used in ballistic missiles or launch vehicles are cooled by a thermal protection system using ablative or thermodegradable materials. The heat transferred from combustion products to the wall causes its thermal decomposition, which limits the thermal flux transmitted; the non-degraded part of the wall provides additional insulation. Nozzles and other heavily loaded parts of the engine may be made of highly refractory materials such as a carbon-carbon composite, which is also consumed very slightly by thermochemical ablation. Industry’s requirements can be summarized as follows: heat transfer predictions that are as reliable and accurate as possible, not only in terms of mean values, but more importantly in terms of spatial-temporal variations; and the design of optimized thermal protection and wall cooling systems that can deal with these transfers. Scientific challenges Conventional heat transfer modes – conduction, convection, radiation – are now well identified. However, the conditions prevailing in the air and space propulsion environments are relatively complex, and involve a series of scientific challenges: Heat transfer arises from hot, complex flows: turbulent, reacting and often multiphase flows, which are very heterogeneous, or even recirculating in some cases. One widespread problem which is still poorly understood is convective transfer from a reacting flow (flame-wall interaction); another major problem is the impact of droplets from a mechanical and energetic standpoint, and the possible formation of a runoff film. An inert wall’s behavior may be relatively simple; in the case of pyrolysis or ablation, it becomes more complex. Phase changes in the cooling systems of liquid rocket engines create complex multiphase flows, that may also generate instabilities. Given the temperatures reached in aerospace engines, heat transfer via radiation is not negligible; in fact, it is even predominant in certain areas of the engine. Simulation of this transfer is complex for multi-species multiphase flows with high gradients at the walls. Today we must meet an ambitious goal: develop numerical forecasting tools providing the accuracy needed for the design of effective thermal protection and cooling systems. Only this type of tool could significantly reduce development costs and times. State of the art As in all cutting-edge sectors, a certain degree of understanding has been achieved and applied at the industrial level. However, there is still a clear lack of understanding in the following areas: Flux measurement: some techniques applied behind an inert wall provide measurements using inverse methods, but there is practically no reliable, accurate method to measure overall flux and radiation flux in the presence of a very hot multiphase flow (e.g., for the internal thermal protection in solid rocket motors). Flux predictions: flux predictions using established formulas under simplified conditions are generally rather inaccurate, because of the environments in question and strong temperature variations. An effort is being made to improve the numerical prediction of fluxes via Navier-Stokes equations, but this method is limited by a lack of validating data, due to the difficulties inherent in modeling mechanical and thermal turbulence. Furthermore, heat transfers from a multiphase flow are relatively poorly understood. Radiation: there are now numerical tools that allow the calculation of heat transfer from a gaseous environment which is more or less charged with droplets or particles. The weak point is in data acquisition for the particles in multiphase flows (size, shape, optical characteristics), and once again in validating models. Unsteady aspects of heat transfer: these may be due to the behavior of the thermal protection elements, or to fast transients or operational instabilities in the case of cooling systems for liquid rocket engines. How INCA can help meet industry requirements Aerothermodynamics, still a relatively young science, now plays a critical role in the design of aircraft and rocket engines. Achieving higher performance and lower emissions demands increasingly harsh operating conditions in combustion chambers. This means that the scientific challenges of aerothermodynamics and radiation become even more complex, and mastering the technology involved is a key to improving component reliability. The INCA initiative pools the complementary skills and expertise of CNRS, ONERA and Snecma, to rethink basic thermal problems in aircraft and rocket engines. Cutting across traditionally divided sectors, it aims to list objectives in order of priority and develop research paths that will take us further along the way to understanding and resolving these problems. PRESS KIT SNECMA, ONERA and the CNRS launch the INCA project INCA Advanced Combustion Initiative Combustion dynamics and control Injection and mixing Emissions and the Environment Wall interactions and radiation Snecma group ONERA, the French aerospace research agency The CNRS, French national center for scientific research Video, photographic report, inauguration speeches