Heat transfer to flowing model fibre suspensions to elucidate a new method of characterising of wood pulp fibres

摘要

Heat transfer data were obtained for a wide range of synthetic fibre suspensions flowing in a specially designed flow loop with an annular test section The heating rod located coaxially in the test section comprised an embedded heating element,four thermocouples set immediately below the surface and downstream of the heater,and an upstream calming extension The fibre suspension concentration was maintained at 0 4%(by mass)at a flow velocity of 1 5m/s for direct comparison with previous research using wood pulp fibres Various fibre properties such as fibre length L,length-to-perimeter(UP)ratio,fibre stiffness,fibre moment of inertia,relative fibre number,and coarseness(mass per unit length),were plotted against the heat transfer coefficient h_c values The results for all synthetic and wood pulp fibres show that many variables are interactive In general the results validated previous findings obtained from pipeline flow loop studies when the fibre physical dimensions corresponded The second aspect of this work was to compare heat transfer data for synthetic fibres with the results for wood pulp fibre suspensions at the same experimental conditions For valid comparison only synthetic fibres with similar physical dimensions and density were selected The results are disparate,but fibre stiffness,fibre population,fibre size,and fibre surface characteristics appear to be key parameters affecting heat and momentum transfer In general the stiffer fibres are associated with a higher heat transfer coefficient h_c values These are most likely to transmit momentum and hence thermal energy between neighbouring eddies and eddies adjacent to the boundary layer.Further experiments with synthetic fibres over a greater range of physical characteristics and closer to those for wood pulp fibres are necessary to isolate the dominant factors and validate these findings.