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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

characteristic DSQC

characteristic DSQC infiltration curves, Figure 4.47. As a reference, data for a Saffil fibre preform with 0.24 of ceramic volume fraction (FA24) were taken from Mortensen et al. (105) . With the latter values, Dopler et al. (113) successfully validated the unsaturated flow model. Figure 5.5 shows the graphs of the calculated saturation function (Equation 29) between P0 and 100 MPa. By definition, P has to be larger than P0. Complete saturation (S=1) may never be reached in preform infiltration whereas, in the saturated flow assumed in Darcy´s model, it is by definition fully saturated behind and unsaturated in front of the infiltration front. Saturation S(S) 1.00 0.95 0.90 0.85 0.80 0.20 0.10 0.00 10 -1 10 0 209 10 1 Pressure P /MPa P (MPa) Saffil FA24 AOPC20 TOPC10 Figure 5.5 Saturation S as a function of the applied pressure P, plotted using Equation 29 with parameters of FA24 (105) , AOPC20 and TOPC10 presented in Table 5.1. FA24 showed a saturation of 0.99 at about 1 MPa whereas the same saturation in AOPC20 was reached at 10 MPa and in TOPC10 at 20 MPa. This was the result of the lower P0 and higher shape factor α of FA24. In terms of technical applications, the fibre preform may be infiltrated with relatively high saturation in the constant pressure gas pressure infiltration (GPI) mode, whereas residual porosity has to be expected for AOPC20 and TOPC10 since the maximum pressure of GPI is usually maintained below 15 MPa for safety reasons. Therefore, infiltration methods with higher final pressures had to be used for the particulate preforms in 10 2

the present work. These pressures were delivered by plunger driven melt pressurisation such as in squeeze (SQC) and high pressure die casting (HPDC) techniques which are constant flux methods to a first approximation. The unsaturated flow model of Dopler et al. (113) originated from water flow modelling in water reservoirs having a permeable soil base and were based on a constant pressure mode. An adaption of the model to the constant flux mode is discussed in the following section and validated with experimental preform infiltration results. 5.4. Modelling of fluid flow in preform infiltration As reviewed by van Dam (185) , Richards’ equation (Equation 30) has been predominantly used in hydrogeology to model unsaturated dynamic water flow in soils but was recently adapted to dynamic preform infiltration in the constant pressure (CP) mode by Dopler et al. (113) . In collaboration work with Pokora (186) and based on these two publications, a simple numerical modelling tool has been derived and additionally an impermeable wall and the constant flux mode have been introduced. In order to solve Equation 30, the model parameters have to be evaluated experimentally, which is the main contribution of the author. The parameters were: the saturation specific parameters P0 and α, the geometrical parameters such as preform thickness and ceramic volume fraction (1−Φtot), and the relative and specific permeabilities Kr and Ks. The dynamic fluid viscosity μ of the melt was set to 1.15·10 -3 Pa·s for all calculations, in accordance to Dopler et al. (113) . In the following section, the factors influencing the threshold pressure of infiltration P0, such as the wetting behaviour, and the specific permeability Ks are considered and the implication of their use in the calculations. Subsequently, the model is presented and its applicability is validated. 5.4.1. Reactivity of the metal-ceramic systems Several research groups (71,87,88,103) have proposed that reactivity between the fluid and the porous medium enhances wetting by reducing the wetting angle θ which, in accordance to 210

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