Browsing by Author "Majid, Abdul"

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Open Access
Two phase flow solver for solid particles in hypersonic Martian entry flows
(2011) Majid, Abdul; Röser, Hans-Peter (Prof. Dr. rer.nat.)
In the Martian atmosphere heavy storms occur, which transport dust particles even into the higher atmosphere, i.e. up to 40 km of altitude. These particles, with sizes of up to 20µm, consist of silicon oxides and iron oxides may affect the heat load on the heat shield during atmospheric entry. In this present study, these additional loads due to impingement of solid particles in hypersonic entry flows in Martian atmosphere are investigated. The Euler-Lagrangian approach is used for the modeling and simulation of solid particles in hypersonic Martian entry flows. For the simulation, the program SINA (Sequential Iterative Non-equilibrium Algorithm) previously developed at the Institut für Raumfahrtsysteme is used. SINA consists of different solvers that are loosely coupled. The main limitation of the code was that it could simulate only air flows consisting of eleven species. However, taking advantage of the loose coupling between the solvers, the capabilities of SINA are not only extended to simulate gases other than air, but also to two-phase flow applications. For the Martian atmospheric chemistry model, only carbon dioxide (CO2) is taken into account because Mars atmosphere consists of 95.3% CO2. Considering that the entry velocity in the Mars atmosphere is usually around 5-7 km/s, the dissociation of CO2 has to be taken into account. Therefore, a five species model (carbon dioxide (CO2), molecular oxygen (O2), carbon monoxide (CO), oxygen (O) and carbon (C)) is implemented in the code. CO2 has three modes of vibration, because it is a three atomic molecule. Previously in SINA, there was no provision to take into account vibrational energies for the three atomic molecule. Therefore, a vibrational model for the three atomic molecule is developed and implemented in the code. Both phases, the gaseous phase and the particle phase, interact with each other through one-way or two-way coupling. In one-way coupling, there is no influence of the particle phase on gas phase. However, two-way coupling takes into account particle phase impact on the gaseous flowfield. The model for the effect of the flowfield gas on a particle includes drag force and particle heating. An adequate model for the drag force computation is implemented in the model in order to take into account transitional and rarefied flows. The heat transfer model of the particle consists of convective heating and radiation cooling. The radiative heating from the gas to the particle is not taken into account because this model is not available in SINA. Due to relative velocity difference in the gas and particle, the development of the local shock may introduce extra heating to the particle. In order to take into account this effect, a normal stagnation point shock relation is also introduced for the computation of particle temperature. The phase change of the particle due to high temperature of the flowfield is also considered. A semi empirical model for the particle-wall interaction is presented. Depending on the input conditions, the erosion mass loss of a charring ablator using an engineering correlation is also discussed. Verification and validation are the primary means to assess accuracy and reliability in computational simulations. In order to obtain higher confidence and to have a code with as few errors as possible, the models and solvers being implemented in the code are verified and validated with external established resources. The TINA code of Fluid Gravity Engineering (FGE) is used for the verification of particle momentum and heat transfer models. The results of particle momentum agree closely but a notable difference in the values of temperature is found between both codes. In order to further assess the accuracy of thermal model of particle solver, the DSMC code DS2V is also employed. The chemical equilibrium constants of the Martian atmospheric model are validated using the CEA (Chemical Equilibrium with Applications) code of NASA. Parametric analysis is done regarding the impact of variation in the physical input conditions like position, velocity, size and material of the particle on particle-wall interaction. Particle movement is characterized by transitional and rarefied flow properties due to the low gas densities and small particle sizes. Convective heat fluxes onto the surface of the particle and its radiative cooling are discussed. Variation of particle temperatures under different conditions and for differently sized particle is presented. Mass loss or decrease in particle sizes due to higher temperature is explained. Heat fluxes onto the wall due to impingement of particles are also computed and compared with the heat fluxes from the gas.