MODELING OF A ONE-DIMENSIONAL ABLATION PROCESS WITH CHAR FORMATION

We model an ablation process in an semi-infinite solid. The solid rece ives an external heat flux, which is kept constant during ablation. Th e process involves vapourisation of the virgin material and degradatio n of this material to another one. The thermal properties of the two m aterials can be different. However, the properties of both materials a re independent of temperature. The fraction of material being vapouris ed per unit volume of the virgin solid is also kept constant throughou t the process. In the model, we also assume an infinite chemical react ion rate. In other words, the ablation process takes place (and thus t he secondary material is formed) instantaneously. In this paper, the s econd material will be referred to as char. Unlike pure ablation, in w hich regression rate always tends to a non-zero limit, the char formin g model predicts it reaches a maximum and eventually tends to zero. In the mathmatical model, a front fixing method is used to 'fix' the pos ition of the moving boundary. An iterative method is used to evaluate the rate of char formation. A brief discussion on the method of soluti on is included. In addition to the constant heat flux boundary conditi on, a convective heat loss and a radiative heat loss mechanism are als o incorporated. Heat losses are important in real ablation. It has a s ignificant effect on the mass loss rate (regression rate), and, in gen eral, determines whether the ablation takes place. It is also interest ing to note that the solutions for regression rate and for rate of cha r formation are qualitatively the same, with or without heat loss mech anisms present.