code: def Nu_L(self, L, theta, Ts, **statef):
          
  
          return self.Nu_x(L, theta, Ts, **statef) / 0.75
docs: 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