Heat Transfer and Unsaturated Flow Phenomena in Rigid Dual-Scale Porous Media
Abstract
Composites are light weighted materials that can replace the metals in strength promisingly in future. The residual porosity of the composites alters the thermo-physical properties of the material to a maximum level. During the impregnation of fibre in matrix (injection), the presence of air voids changes the direction of the flow of resin. This also affects the material properties in terms of flexibility, durability but agitates the effective thermal conductivity (keff).The physics behind the effect of air void on the effective thermal conductivity cannot be captured in commercial software or experiments. The best way to solve this problem is by numerical codes using finite element approach, by dividing the whole macroscopic domain into numerous subdomains as possible. The divided subdomain should be periodic in nature with respect to the whole domain. If the heterogeneities are similar, dual scale approach is used and so on. If the heterogeneities are different and if they are of two types, triple scale approach is used. The contrast ratio and volume percentage of fiber is used as variables. The air void creates a giant leap for saturation in both, which in turn effect the effective thermal conductivity. Air void troubles the effective thermal conductivity mainly because of its insignificance in scalar values of thermal conductivity when it is compared to fiber or matrix mathematically.
Keywords
Download Options
Introduction
The Liquid Composite moulding (LCM) is one of the most predominant processes in the manufacturing of composites. The reinforcement of dry fiber called as perform is kept inside the moulding cavity. The moulding cavity is locked and the resin is injected to the moulding cavity. Initially, the resin that is injected to moulding cavity will rush the air entrapped between the fibrous networks outside the moulding cavity thus forming a composite on curing.
The major defect during injection is voids, These are due to entrapment of dissolved volatile gases in resin or due to improper balance of velocity and capillary force. There are voids formed within the tows and also in between the tows, naming micro porosity and macro porosity respectively. As described by R.J Bascom [1],the composite test rings having void contents of 0.2 Volume % or less had interlaminar shear strengths 40-100% greater than the conventional rings with high void contents(5%).The void formation is related to the liquid properties and fluid-liquid contact angle. The micro scale flow pattern and the void formation, movement and removal is related to fibre mat architecture [7].]
The laminates with the highest average content of voids had an transverse strain to failure as high as 2% whereas low void content laminates failed at 0.3%. Low void content laminates form only few large and well defined transverse cracks whereas the high void content laminates form multiple transverse cracks with irregular shapes and also small cracks. The irregularity of these cracks results in lower stress concentration as per the conclusion of J.Vama [2].
The LCM processes such as Resin transfer moulding is increasingly used to manufacture parts of industrial application and were cost efficient. The work by LS Lecrec [5] which was carried out on different type of fibrous reinforcements and the optimal condition for the impregnation of resin to avoid macro and micro voids relating the local capillary number. The contrasts in the thermal properties existing between the dry and the fully saturated reinforcement is used to determine the dynamic saturation curve. As explained by Maxime Viliere [6] the evolution of residual voids for several resin flow rates with respect to the capillary number.
Conclusion
By this research work the following studies had been done.
The contrast ratio influences effective thermal conductivity. The ratio of thermal conductivity between the fibre and matrix is known as contrast ratio. As this parameter increases the effective thermal conductivity increases drastically.
The volume percentage of fibre in macro scale can be determined directly by the fibre content in micro scale in dual homogenisation. In iterative homogenisation the volume percentage of fibre in macro scale is determined by fibre content in micro scale and the rate at which fibre is rooved.
The effective thermal conductivity depends on the position and shape of the fibre. As the volume percentage of fibre increases, the effective conductivity increases gradually in radial direction and increases linearly (most likely) in longitudinal direction.
The iterative homogenisation is most useful for realistic approach since the geometry of fibre in micro scale differs with the geometry of tow in the mesoscale. This method increases the accuracy in terms of volume percentage of fibre.
The micro and macro voids effects micro and meso saturation. The effective conductivity decreases when the saturation decreases or the volume occupied by the air void increases