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This page calculates the head loss coefficient and head loss of a fluid in across angled lovers inside a pipe.

The head loss h_L can be calculated as:

h_L = k_loss x (u_m² / (2 x g) )

Where k_loss is the head loss coefficient, u_m is the flow velocity in the pipe, and g is the gravitational acceleration. k_loss is dependant on the Reynolds Number Re. If Re is smaller than or equal to 50, then (1 - 50 / Re) is taken to be zero:

k_loss = + (1 - 50 / Re) k_qu

Where k_qu is a function of the angle and the number of  louvers based on a table of values (Ref), and Re is the Reynolds Number and is calculated as:

Re = u_m x D_h / v

Where v is the kinematic viscosity of air and D_h is the hydraulic diameter of the tube. A tube of square cross section has a hydraulic diameter equal to the width of the tube.

References

Idel'chik, I E. and Fried, Erwin., Flow Resistance, a Design Guide for Engineers, Taylor & Francis, Washington D.C. 1989. p 306.