code: def Nu(L: float, h: float, k: float) -> float:
      
  
      return h * L / k
docs: Calculate the Nusselt number.
  
      :param L: [m] heat transfer surface characteristic length.
      :param h: [W/K/m2] convective heat transfer coefficient.
      :param k: [W/K/m] fluid thermal conductivity.
  
      :returns: float