Heat transfer and pressure drop characteristics of suspensions of synthetic and wood pulp fibres in annular flow

Abstract

Pressure drop and heat transfer data were obtained from a specially designed flow loop with an annular flow test section housing a coaxially located heating rod with a calming extension. The heating element was embedded in the central rod with thermocouples located just below the surface at the heater in the flow direction. The pressure drop across the test section was obtained with a differential pressure transducer. Data were obtained for several suspensions of Kraft pine pulps, a spruce pulp, and an eucalypt pulp, as well as for five polymer model fibres. Both the heat transfer and pressure drop data for the wood pulp fibre suspensions in general validated the results obtained from the pipeline investigations. The effect of fibre concentration on heat transfer coefficient hc and pressure drop ∆P/L are reported, and results are presented for the change in these variables with flow velocity over the range 0.1–2.0 m/s. There are some anomalous pressure drop ∆P/L data due to insufficient calming length and the additional exit and entrance losses. Both hc and ∆P/L values at 1.5 m/s and 0.4 percent fibre concentration are plotted against fibre length, fibre length-to-perimeter ratio, fibre stiffness, coarseness, and population. The data for the pine and spruce pulps were generally well correlated. Synthetic fibres with specific lengths, diameters, and modulus of elasticity values provide some further insights into the effects of key variables on hc and ∆P/L. It appears that fibre stiffness and fibre population are key variables but further tests are required with more synthetic fibres of different dimensions to validate the findings.