In the present paper, the entropy generation in a double-sine duet, which is frequently used in plate heat exchangers, is investigated by numerical methods. The Reynolds number (Re) at inlet covers ranges from 86 to 2000, and wall heat flux (q '') varies as 160, 320 and 640 W/m(2) . Three aspect ratio (Lambda/a) cases, including Lambda/a = 2, 4 and 8, are investigated. The results indicate that the entropy generation, S-gen(*), in cases with larger Re and smaller q '' is dominated by the entropy generation due to frictional irreversibility (S-p(*)), whereas the entropy generation is dominated by the entropy generation due to heat transfer irreversibility (S-p(*),) in cases with smaller Re and larger q ''. There is no monotonic relationship between Ala and S or S. For all cases with different Re and q '', S is found to be minimal in Lambda/a = 2 case, whereas S-T(+), is minimal in Lambda/a = 4 case. The optimal aspect ratio, (Lambda/a)(opt), with which the minimal S-geb(*) can be obtained, is 2 found to be dependent on Re and q ''. In q '' = 160 W/m(2) case, (Lambda/a)(opt) = 2 when Re > 540; (Lambda/a)(opt) = 4 when Re < 540. In q '' = 320 W/m_ case, (Lambda/a)(opt), = 2 when Re > 662; (Ala),p, = 4 when Re < 662. In q '' = 640 W/m(2) case, (Ala),p, = 2 when Re > 1008; (Lambda/a)(opt) = 4 when Re < 1008. An optimal Re, which is found to increase with q '', exists for every case with specific Ala and q" to induce the minimal S-gen(*) These results provide important information for the heat exchanger design since the thermal system could have the least irreversibility and best exergy utilization if the optimal Re and optimal aspect ratio is used according to the practical design conditions. (c) 2005 Elsevier Ltd. All rights reserved.