Rheological Behavior of Molten Polyimide Matrix Resins
Polyimide melt rheology governs composite void content and impregnation by defining exact viscosity windows and pressure timing before gelation.
Viscous Dissipation Heating
Meaning
Thermodynamic processes convert kinetic energy into thermal energy through the internal friction of a moving fluid. In high speed industrial mixing or lubrication systems, viscous dissipation heating can cause a measurable rise in the temperature of the medium. This effect is most pronounced in highly viscous fluids like polymers or heavy oils where the resistance to flow is great.
Energy Transfer
Work done by shear forces in the fluid is not recovered as motion but is instead released as heat. While viscous dissipation heating is often a source of inefficiency, it can be used intentionally in some manufacturing processes to melt materials during extrusion. The amount of heat generated is proportional to the square of the velocity gradient and the viscosity of the fluid.
High pressure hydraulic systems often require external cooling to counteract this effect and prevent the breakdown of the working fluid.
Fluid Temperature
Managing the heat balance in a recirculating system requires an understanding of how much energy is being added by the pump and the piping. Uncontrolled viscous dissipation heating can degrade sensitive chemical compounds or reduce the effectiveness of a lubricant. Cooling systems must be sized to remove this internal heat in addition to any heat absorbed from the environment.
Mechanical Effect
Minimizing sharp bends and narrow passages in a fluid circuit reduces the intensity of the shear forces. Designers use calculations of viscous dissipation heating to predict the temperature profile of the fluid along the flow path. This prediction ensures that the materials used in the system can withstand the maximum possible temperature without failing.