Study of the effect of entrance length on heat transfer to fibre suspensions in annular flow heat exchangers

In internal flow the boundary layer is unable to develop without eventually being constrained. There is no satisfactory general expression for the entry length in turbulent flow and it is approximately independent of Reynolds number and remains within 10–60Dh. Present experimental investigation has highlighted the effect of entry length (23 and 38Dh) on heat transfer to fibre suspensions in annular passage. It is observed that there is no significant variation of heat transfer coefficients with change in entrance length for water flow. Fibre suspensions flow at low consistency provides data with little variation whereas at higher consistency heat transfer coefficients data provides some variations but the trend remains similar for both short and long entrance lengths in the test section. Effects of fibre concentrations and flexibility in suspensions are providing similar trends in heat transfer irrespective of entrance length in annular flow, pipe flow and a larger annular gap.