def h_L(self, L, theta, Ts, **statef):
Nu_L = self.Nu_L(L, theta, Ts, **statef)<EOL>k = self._fluid.k(T=self.Tr)<EOL>return Nu_L * k / L<EOL>
Calculate the average heat transfer coefficient.

: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 Tf: [K] bulk fluid temperature

:returns: [W/m2/K] float