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(33 problems)
Published online by Cambridge University Press: 18 December 2013
When blood is taken out of the body for processing into an extracorporeal device, a majorconcern is that the level of shear stress to which the blood is exposed should be less than acritical level (roughly 1000 dyne/cm2). For exposure to shear-stress levelshigher than this, lysis of the red blood cells can occur, together with platelet activation andinitiation of the clotting process.
Consider the flow of blood through a device that has a set of parallel tubes each with a diameterof 1 mm and a length of 10 cm. What is the maximum pressure drop that should be usedfor such a device if the highest shear levels in the device occur in these tubes? (Blood has aviscosity about five times that of water.) You may neglect entry effects and treat the flow as fullydeveloped.
A parallel-plate flow chamber is to be designed to study the effects of shear stress on adhesionof leukocytes to endothelia. However, endothelial cells can be damaged by shear stress greater than400 dyne/cm2. The width of the flow channel is to be 1 cm and its length 5 cm. The flowis to be driven by gravity, and a fluid column 1 m in height is available. The system must work bothfor saline and for blood. What should be the maximum separation (s ≪ 1 cm) between the two platessuch that the endothelial cells are not damaged? The schematic diagram below is not to scale. Youmay neglect entry effects and treat the flow as fully developed.
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