Deposition of magnetite particles from flowing suspensions under flow‐boiling and single‐phase forced‐convective heat transfer

TLDR: In this article, the authors measured the deposition rate of colloidal magnetite particles under both single-phase forced-convective and flow-boiling conditions, and found that particle removal rates were significantly lower than deposition rates; analysis using the theory of turbulent bursts suggests a removal efficiency of only 10−9% for each turbulent burst.

Abstract: The deposition rate of colloidal magnetite particles was measured under both single-phase forced-convective and flow-boiling conditions. All measurements were made at alkaline pH where both the heat transfer surface and the surface of the magnetite particles appear to be negatively charged. For single-phase forced convection, the deposition rate constant is lower than the mass transfer coefficient for colloidal particles, and the difference is attributed to the force of repulsion between the negatively charged surfaces of the particle and substrate. The deposition rate measured under flow-boiling conditions is lower than that reported for the deposition of colloidal particles at neutral pH. The difference is, again, attributed to the force of repulsion between the particle and substrate. Particle removal rates were significantly lower than deposition rates; analysis using the theory of turbulent bursts suggests a removal efficiency of only 10−9% for each turbulent burst. The low removal efficiency is consistent with the particle diameter being significantly smaller than the thickness of the laminar sublayer in these tests. On a mesure la vitesse de deposition de particules de magnetite colloidales dans des conditions de convection forcee et d'ebullition monophasique. Toutes les rnesures ont ete prises a un pH alcalin ou la surface de transfert de chaleur et la surface des particules de magnetite semblent chargees negativement. Pour la convection forcee monophasique, la constante de vitesse de deposition est inferieure au coefficient de transfert de matiere pour les particules colloidales, et la difference est attribuee a la force de repulsion entre les surfaces chargees negativement de la particule et du substrat. La vitesse de deaposition mesuree dans des conditions d'ebullition est inferieure a celle publiee pour la deposition des particules colloidales a pH neutre. La difference est, de nouveau, attribuee a la force de repulsion entre la particule et le substrat. Les vitesses de retrait des particules sont considerable-ment plus petites que les vitesses de deposition; l'analyse par la theorie des elatements turbulents suggere une efficacite de retrait de seutement 10−9% pour chaque elatement turbulent. La faible efficacite de retrait est consistante avec le diametre des particules qui est nettement plus petit que l'epaisseur de la sous-couche laminaire dans ces essais.