docs: def Nu_plate_Martin(Re, Pr, plate_enlargement_factor, variant='1999'):
      r'''Calculates Nusselt number for single-phase flow in a 
      Chevron-style plate heat exchanger according to [1]_, also shown in [2]_
      and [3]_. 
      
      .. math::
          Nu = 0.122 Pr^{1/3} \left[f_d Re^2 \sin (2\phi)\right]^{0.374}
          
      The Darcy friction factor should be calculated with the Martin (1999) 
      friction factor correlation, as that is what the power of 0.374 was 
      regressed with. It can be altered to a later formulation by Martin in the
      VDI Heat Atlas 2E, which increases the calculated heat transfer friction 
      slightly.
  
      Parameters
      ----------
      Re : float
          Reynolds number with respect to the hydraulic diameter of the channels,
          [-]
      Pr : float
          Prandtl number calculated with bulk fluid properties, [-]
      plate_enlargement_factor : float
          The extra surface area multiplier as compared to a flat plate
          caused the corrugations, [-]
      variant : str
          One of '1999' or 'VDI'; chooses between the two Martin friction
          factor correlations, [-]
  
      Returns
      -------
      Nu : float
          Nusselt number with respect to `Dh`, [-]
  
      Notes
      -----
      Based on experimental data from Re from 200 - 10000 and enhancement 
      factors calculated with chevron angles of 0 to 80 degrees. See 
      `PlateExchanger` for further clarification on the definitions.
          
      Note there is a discontinuity at Re = 2000 for the transition from
      laminar to turbulent flow, arising from the friction factor correlation's
      transition ONLY, although the literature suggests the transition
      is actually smooth.
      
      A viscosity correction power for liquid flows of (1/6) is suggested, and
      for gases, no correction factor.
  
      Examples
      --------
      >>> Nu_plate_Martin(Re=2000, Pr=.7, plate_enlargement_factor=1.18)
      43.5794551998615
      
      References
      ----------
      .. [1] Martin, Holger. "A Theoretical Approach to Predict the Performance 
         of Chevron-Type Plate Heat Exchangers." Chemical Engineering and 
         Processing: Process Intensification 35, no. 4 (January 1, 1996): 301-10. 
         https://doi.org/10.1016/0255-2701(95)04129-X.
      .. [2] Martin, Holger. "Economic optimization of compact heat exchangers."
         EF-Conference on Compact Heat Exchangers and Enhancement Technology for 
         the Process Industries, Banff, Canada, July 18-23, 1999, 1999. 
         https://publikationen.bibliothek.kit.edu/1000034866.
      .. [3] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      '''
      try:
          fd_correlation = _Nu_plate_Martin_correlations[variant]
      except KeyError:
          raise Exception("Supported friction factor correlations are Martin's"
                          " '1999' correlation or his 'VDI' correlation only")
      fd = fd_correlation(Re, plate_enlargement_factor)
      # VDI, original, and Björn Palm and Joachim Claesson recommend 0.122 leading coeff
      # The 0.205 in some publications is what happens when the friction factor
      # is in a fanning basis; = 4^0.374*1.22 = 2.048944
      Nu = 0.122*Pr**(1/3.)*(fd*Re*Re*sin(2.0*plate_enlargement_factor))**0.374
      return Nu