Thermal-hydraulic performance of zigzag channels with Bending number below unity for printed circuit heat exchanger

Abstract

• A dimensionless number is proposed for characterizing the bending of zigzag channel. • Zigzag channel with Bending number below unity is studied experimentally. • Empirical correlations are obtained for friction factor and Nusselt number. • Thermal-hydraulic performance of the tested zigzag channel is obtained. Zigzag-channel printed circuit heat exchanger is one of the leading candidates for the pre-cooler of supercritical carbon dioxide Brayton cycle. A dimensionless number, Bending number, is newly proposed to characterize the bending amplitude of zigzag channels in printed circuit heat exchanger. The existing studies are examined with the help of Bending number and it is found that the thermal-hydraulic performance of zigzag channels with Bending number below unity has not been investigated. To remedy this situation, the thermal-hydraulic performance of zigzag channels with Bending number 0.49 is tested. Experimental result shows an enhancement in convective heat transfer of supercritical carbon dioxide at its pseudo-critical zone. It is also found that there is no obvious transition from laminar flow to turbulent flow within Reynolds number 1560-45700. Flow and heat transfer correlations for the tested zigzag channels are proposed. The new developed Fanning friction factor correlation predicts almost 100 % of the experimental data within ±20 % error band, and the new developed heat transfer correlation predicts 78.6 % of the experimental data within ±25 % error band. Comparison with Ngo’s correlations, Kim’s correlations, and Saeed’s correlations demonstrates that both Fanning friction factor and Nusselt number are higher with larger Bending number. Analysis of the thermal-hydraulic performance, (Nu/Nu 0 )/(f/f 0 ) 1/3 , of the tested zigzag channels shows that its combined performance is improved obviously compared with straight channels when both Re and Pr are large.