Summary and evaluation on the heat transfer enhancement techniques of gas laminar and turbulent pipe flow

A systematic survey and evaluation on the thermal-hydraulic performance of gas inside internally finned, twisted tape or swirl generator inserted, corrugated, and dimpled, totally 436 pipes is conducted in this work. The gases in the investigations involve air, nitrogen, exhaust gases, and helium. Prandtl number is around 0.6-0.7. It is found that in the Reynolds number of 2 × 103 to 100 × 103, the ratios of Nusselt number over Dittus-Boelter equation for internal finned tubes are typically in the range of 1-6, tubes with twisted tape and other inserts are 1.5-6, corrugated tubes are 1-3 and dimpled tubes are 1-4, including compound enhancement techniques. The ratios of friction factor over Blasius equation is normally in the range of 1.5-14 for internally finned tubes, 2-200 for inserted twisted tapes and swirl generators, corrugated tubes is 1.5-10 and dimpled tubes is 1-8. The heat transfer enhancement ratios for gases are generally similar with liquid, while the friction factor increased ratios for gases are higher than that for liquids. The number of investigations on the tubes fitted with twisted tapes inserts, coil loops, and swirl generators are more than other three enhancement methods. The increment of pressure drop for twisted tape inserts are also the largest. By using performance evaluation plot, different enhancement techniques with the same reference are compared for their effectiveness. It indicates that the efficiency of pipes with different types of inserts for gases are mostly lower than internal finned, corrugated and dimpled tubes in this survey.