Enhancement of heat transfer by a combination of a single-start spirally corrugated tubes with a twisted tape

Heat transfer and isothermal friction pressure drop results have been obtained experimentally for two single-start spirally corrugated tubes combined with five twisted-tape inserts with different relative pitches in the range of Reynolds number 4 x 10(3) - 6 x 10(4). The characteristic parameters of the tubes are: height-to-diameter ratio, e/D-i = 0.0371 and 0.0441; and relative pitch, H/D-i - 7.7, 6.25, 3.95, 2.95, 2.4. Significantly, higher friction factor and inside heat transfer coefficients than those of the smooth tube under the same operating conditions have been observed. Extended performance evaluation criteria (PEC) equations for enhanced heat transfer surfaces have been used to assess the multiplicative effect. Thermodynamic optimum can be defined by minimizing the entropy generation number compared with the relative increase of heat transfer rate or relative reduction of heat transfer area. (C) 2002 Elsevier Science Inc. All rights reserved.