A flow loop system for study of fibre flocculation insuspensions has been developed. The system is designed tosimulate the flow conditions in a paper machine headbox. It isequipped with a radial distributor feeding a step diffuser pipepackage, after which the flow is contracted in a 2-D nozzle.The flow system is also equipped with a secondary flowcontraction with an area reduction ratio of 2:1 after theheadbox nozzle, mimicking accelerations that may take placeduring forming. The flow system is equipped with heating andcooling devices for the study of temperature effects on fibresuspension flocculation. An online dosage device for the studyof chemical effects on fibre suspension flocculation is alsoincluded. The maximum flow velocity in the system is 16m/s. Flowing fibre suspensions were studied using a high speedCCD video camera and transmitted infra-red laser light pulseillumination. Images were taken either separately before andafter or along the secondary contraction. Images of fibreflocculation were evaluated by power spectrum analysis, and themean floc size and the flocculation index were calculated. Aconcept of mean floc area reduction, based on power spectrum,has been introduced to characterise the fibre network insuspensions. By comparingthe fibre flocculation before andafter the secondary contraction, or by following the fibreflocs along the secondary contraction, floc rheologyinformation can be obtained. The effects of chemical additivesand fibre surface modification can also be studied by comparingthe corresponding fibre flocculation. For a bleached softwood kraft pulp suspension at a fibreconcentration of 5 g/l, the fibre flocs along the secondarycontraction have been manually evaluated, and the resultsconfirm that the power spectrum analysis is applicable. Ca 1/5of the flocs were broken into two by the contraction. (...).

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