tailieunhanh - Heat Transfer Handbook part 78

Heat Transfer Handbook part 78. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | ENHANCED IN-TUBE CONDENSATION 765 typically have 30 to 80 trapezoidal fins that spiral down the tube axis with a helix angle a between 6 and 30 . The fin height e ranges between and mm which is approximately less than . The fin apex angle P varies from 20 to 60 for commercial microfin tubes where most are close to the middle of this range. Several factors are responsible for the passive enhancement of condensation in microfin tubes. First the fins provide additional heat transfer area over that of a smooth tube of the same cross-sectional flow area. The fins add in the range of 50 more surface area per tube length. Surface tension is another mechanism that can act to thin the film on the fin tips for very high vapor quaiities and tow mass velocities. Yang and Webb 1997 have modeled surface tension effects on the fin and vapor shear effects for small-diameter extruded aluminum tubes with microgrooves. Swirl effects due to the riffling of the fins along the tube axis may improve heat transfer for certain operating conditions. Finally the fins enhance heat transfer as a roughness would in the mixing of the flow at the wall. Currently the prediction of convective condensation heat transfer in microfin tubes is handicapped by the lack of flow pattern maps for microfin tubes. Consequently correlations and models that presently exist for the prediction of the condensation heat transfer and pressure drop in microfin tubes do not use them. Cavallini et al. 2000 provides an excellent review of predictive correlations models for flow condensation in microfin tubes. Cavallini et al. 2000 shows that the models of Cavallini et al. 1993 1995 Yu and Koyama 1998 and Kedzierski and Goncalves 1999 produce similar results for various tube geometries and mass fluxes. The Kedzierski and Goncalves 1999 model is presented here because it has the simplest form and is based on the largest number of data points. Although the Kedzierski and Goncalves 1999 model is based on 1489

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