Physical Modelling of Turbulent Jets for Nuclear Sludge Mobilisation

The impingement of a fluid jet onto a surface has broad applications across many industries. Within the UK nuclear industry, during the final stages of fuel reprocessing, impinging fluid jets are utilised to mobilise settled sludge material within storage tanks in preparation for transfer and ultimate immobilisation through vitrification. Despite the extensive applications of impinging jets within the nuclear and other industries, the study of two-phase, particle -laden, impinging jets is limited, and generally restricted to computational modelling. Surprisingly, very little fundamental understanding of the turbulence structure within such fluid flows through experimental investigation is found within the literature. The physical modelling of impinging jet systems could successfully serve to aid computer model validation, determine operating requirements, evaluate plant throughput requirements, optimise process operations and support design. Within this work a method is considered, capable of exploring the effects of process and material variables on the flow phenomena of impinging jets. This is achieved on a number of experimental test rigs of varying scale employing both intrusive and non-intrusive measurement techniques Particle image velocimetry (PIV), ultrasonic Doppler velocity profiling (UDVP) and high speed imaging, through to visual observations and direct measurements, are all techniques that can be deployed. The influence of a number of parameters on the erosion characteristics of sediment beds following application of an axisymmetric impinging jet is presented in detail. Bed erosion is found to be enhanced as the jet height above the sediment bed is increased, due to greater turbulence development. Different erosion characteristics, as jet outlet velocity increased, were found for the particulates tested; sand, fine Mg(OH)₂ (test simulant representative of waste sludge, has similar particle size to sand, 200–1000μm) and coarse Mg(OH)₂ (1000–2000μm). The crater diameter increased with increasing velocity as expected. However, the effect of the increase in velocity on the crater depth was very different, particularly for the coarse material which was found to re-deposit in the crater when the velocity increased above 1.3 ms⁻¹, most likely due to enhanced re-circulation at the higher velocities.