ABSTRACT

Heat transfer enhancement in any heat exchanger duct is associated with an increase of the pressure drop required for pumping − particularly when the duct has an insert. Considering a whole duct as a thermodynamic system, the total generation of entropy in it has two irreversible “competing“ processes - heat transfer and fluid flow. The appropriate analysis method for this problem is named in literature as Thermodynamic Design (THD) or Entropy Generation Minimization (EGM). Three different tube geometries have been analysed: (i) smooth one; (ii) with a continuous spiral insert, and (iii) with a spaced spiral insert. In this analysis the dimensionless distance X (between the sections of spaced spiral inserts) varied in the range from 0 to ∞. These boundary values represent cases (ii) and (i), respectively, for which the heat transfer and friction coefficients are known. Apart from that, however, values of these coefficients, for the whole range of distance X, are not available. Their reliable values have been obtained from the computer simulation of fully developed 3D-flow with heat transfer and codes CFX and ANSYS-CFX have been used for this purpose. Special attention has been paid to simulations verification − using own results of different measurements on specially built experimental stands. They include velocity fields visualisation, in chosen parts of the experimental duct with inserts, using PIV system. The results are presented in a 3D-figure, in normalised and dimensionless co-ordinates − where the minimum of the entropy generated is clearly present as a function of the distance X and of the Reynolds number.