A MINIMUM ENTROPY GENERATION PROCEDURE FOR THE DISCRETE PSEUDO-OPTIMIZATION OF FINNED-TUBE HEAT-EXCHANGERS

This paper presents a novel method which can be helpful in assessing t he optimal configuration of finned-tube heat exchangers. The method is an extension of the local irreversibilities method [17], and it is ba sed on the determination on a local basis of the two components of the entropy generation rate: the one caused by viscous dissipations and t he one due to thermal irreversibilities. Depending on the engineering purpose for which a technical device was designed, it can be argued th at the optimal configuration will be that in which either one (or both ) of these two entropy generation rates is minimized. For a heat excha nging device, it is important to minimize thermal irreversibilities, b ut more important is to minimize the mechanical power lost in achievin g a prescribed heat-exchange performance: to this purpose, one can for m a relative irreversibility index (named Bejan number here and in [17 ] because the original seed of this procedure can be found in [1]), an d use it to assess the merit of a given configuration. In the procedur e presented here, a circular, single-tube, finned heat exchanger confi guration is considered: the velocity and temperature fields are comput ed (via a standard finite-element package, FIDAP) for a realistic valu e of the Reynolds number and for a variety of geometric configurations (various fin external diameters and fin spacing); then, the entropy g eneration rate is calculated from the flowfield, and is examined both at a local level, to detect possible bad design spots tie, locations w hich correspond to abnormally high entropy generation rates, which cou ld be cured by design improvements), and at an overall (integral) leve l, to assess the entropic performance of the heat exchanger. Optimal c urves are given, and the optimal spacing of fins is determined using a lternatively the entropy generation rate and the total heat transfer r ate as objective functions: different optima arise, and the difference s as well as the similarities are discussed in detail.