Different numerical methods are used, for the discretization of the axial domain, according to the flow direction. If the flow direction is in the positive x direction (from zero to the exit of the channel) then a secondorder backward finite diference method with 20 elements is used. For the reverse flows a second-order forward finite diference method is used. The PHE thermal model is approximated by a second-order centred finite diference method with 20 elements. These discretization schemes and orders where chosen so as to obtain accurate results.