def Nu_L(self, L, theta, Ts, **statef):
return self.Nu_x(L, theta, Ts, **statef) / <NUM_LIT><EOL>
Calculate the average Nusselt number.

:param L: [m] characteristic length of the heat transfer surface
:param theta: [°] angle of the surface with the vertical
:param Ts: [K] heat transfer surface temperature
:param **statef: [K] bulk fluid temperature

:returns: float