Darcy friction factor in annular pipes

[MassimoCimmino]

There might also be a mishandling of the hydraulic diameter in pipes.convective_heat_transfer_coefficient_concentric_annulus:

pygfunction/pygfunction/pipes.py

Lines 3257 to 3274 in 443d866

	# Hydraulic diameter and radius for concentric tube annulus region
	D_h = 2 * (r_a_out - r_a_in)
	r_h = D_h / 2
	# Cross-sectional area of the annulus region
	A_c = pi * ((r_a_out ** 2) - (r_a_in ** 2))
	# Volume flow rate
	V_dot = np.abs(m_flow_pipe) / rho_f
	# Average velocity
	V = V_dot / A_c
	# Reynolds number
	Re = rho_f * V * D_h / mu_f
	# Prandtl number
	Pr = cp_f * mu_f / k_f
	# Ratio of radii (Grundmann, 2016)
	r_star = r_a_in / r_a_out
	# Darcy-Wiesbach friction factor
	fDarcy = fluid_friction_factor_circular_pipe(
	m_flow_pipe, r_h, mu_f, rho_f, epsilon)

The code above calls fluid_friction_factor_circular_pipe using r_h instead of r_in as an attempt to avoid the implementation of a dedicated function for annular pipes. However, this may lead to a mistake due to the use of r_h to evaluate the fluid velocity.