Annular flow of high-viscosity epoxy in circular pipes

Abstract

Paper presented at the 7th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Turkey, 19-21 July, 2010.

A combined experimental and numerical investigation was done to study the flow of viscous epoxy propelled through circular tubes by compressed air. Epoxy moved through a pipe in annular flow and spread uniformly on the inner surface of the pipe, forming a thin, uniform coating. The objectives of the study were to determine the effect of varying process parameters such as air flow rate, temperature, and pressure, on the movement of epoxy within a pipe; to visualize epoxy flows through straight pipe sections and around elbows; and to detemine how pipe geometry length and orientation affect epoxy flow. High pressure air from a compressor was used to drive a slug of epoxy through clear poly-vinyl chloride (PVC) pipes. The epoxy was mixed from two parts, resin and hardener, and hardened in an irreversible exothermic reaction. A video camera was used to record the movement of the epoxy inside the pipe. Once the epoxy had hardened sections were taken through the pipe and the thickness of the coating measured Tests were done varying a variety of experimental parameters including air pressure, airflow rate, piping configuration and epoxy temperature. A one dimensional numerical algorithm was developed to model the fluid flow of epoxy and air within the pipe, the heat transfer between air, epoxy and walls, as well as the curing rate of the epoxy as it is moving alongside the pipe. Results from the model were used to predict the epoxy front velocity and coating thickness and were compared to the experimental observations. Heating the epoxy was found to slow its motion, since the epoxy sets faster at a higher temperature and its viscosity becomes greater. Before curing occurs, the viscosity decreases as the temperature is increased. The viscosity then increases when hardening takes place. The coating was significantly thicker at the bottom of a horizontal pipe than at the top due to sagging of the epoxy coating after it had been applied, resulting in flow from the top to the bottom of the pipe. Sagging could be reduced by maintaining airflow until curing was almost complete and the epoxy had hardened enough to prevent it from moving easily. The most important parameters controlling the speed of the epoxy and coating thickness were the air flow rate and temperature, since they determine the shear forces on the epoxy layer and the rate at which the epoxy cures. Raising air temperature increases the reaction rate and therefore decreases the time required for the epoxy to cure inside the pipe.