Flow regimes in the emptying of pipes filled with a Herschel-Bulkley fluid

The emptying of pipework from fluids of high viscosity is a significant multiphase flow problem in many food and personal care industries. Maximising product recovery whilst minimising cleaning time and effluent volume is important in minimising the environmental footprint of the plant. The cleaning of pipework fully filled by toothpaste by water under different process conditions has been studied and monitored by weighing pipes at intervals. Three flow regimes have been identified; a short core removal stage of product recovery, before water breaks through the filled pipe, and two in subsequent cleaning, film removal when there is a continuous wavy annular film on the wall, and patch removal in which the material is present as patches on the wall. The amount of product recovered in core removal is here not a function of flow conditions; however, conditions during core removal significantly affect the overall cleaning time. Overall cleaning time can be reduced by at least 25% by selecting the best removal conditions in the different stages. It is hypothesised that this is due to changes in the wall layer induced during core removal, with a very wavy wall layer leading to rapid subsequent removal. If this effect could be understood and scaled up it may be possible to improve commercial cleaning processes.