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Abstract
A numerical analysis of heat transfer and pressure drop for turbulent flow in a series of 15.54-mm inside diameter helically ribbed tubes has been performed. The ranges of geometric parameters were number of rib starts (10 to 40), helix angle (25 to 55 degrees), and rib height (0.3 to 0.6 mm). The effect of grid independence was extensively examined. The computational results match well with the experimental data to validate the accuracy of the numerical model. The effect of each main parameter, rib starts, helix angle, and rib height, on heat transfer and pressure drop is investigated. Considering fouling in practical situations, the ratio of pitch over rib height is an important parameter to select the tubes. It is advisable to select tubes with pitch over rib height ratio greater than 3.5, which have better heat transfer and lower fouling potential.
NOMENCLATURE
| A | = | heat transfer area, m2 |
| C1, C2 | = | constants |
| Cp | = | specific heat, J/(kg-K) |
| ct | = | coefficient for temperature correction, dimensionless |
| Di | = | internal tube diameter, or diameter to root of ribs, m |
| e | = | internal rib height (average value), mm |
| f | = | Fanning friction factor, dimensionless |
| Gk, Gb | = | generation rates of turbulence kinetic energy due to the mean velocity gradients and to buoyancy, respectively, s−1 |
| I | = | turbulence intensity, dimensionless |
| j | = | Colburn j factor (= StPr2/3), dimensionless |
| l | = | length of the tube, m |
| Ns | = | number of ribs |
| Nu | = | Nusselt number, dimensionless |
| p | = | axial element pitch, m |
| P | = | pressure, Pa |
| Pr | = | Prandtl number, dimensionless |
| Re | = | Reynolds number, dimensionless |
| Rf* | = | asymptotic fouling factor, m2-K/W |
| St | = | Stanton number, dimensionless |
| Sk | = | constant |
| T | = | temperature, K |
| ui, | = | i-axis velocity component and time-averaged value, m/s |
| YM | = | fluctuation in compressible flow, dimensionless |
Greek Symbols
| α | = | helix angle, degree |
| β | = | included angle of section shape, degree |
| ϵ | = | turbulence kinetic energy dissipation rate, J/s |
| κ | = | turbulence kinetic energy, J |
| η | = | = (j/jp) /(f/fp), dimensionless |
| η′ | = | = (j/jp) /(f/fp)1/3, dimensionless |
| σk, σϵ | = | constants |
| ν, νt | = | kinematic viscosity, turbulent kinematic viscosity, m2/s |
| μ, μt | = | dynamicviscosity, turbulent dynamic viscosity, Pa-s |
Subscripts
| ex | = | Webb's experimental data |
| i, j | = | refers to an axis (X, Y, or Z) |
| p | = | plain surface |
| t | = | turbulent |
Additional information
Notes on contributors
Wei Li
Wei Li received his B.S. from Xi’an Jiao Tong University, and after 5 years of engineering practice in Gree Appliances in China, continued his education at Penn State, where he obtained his M.S. and Ph.D. degrees with Prof. Ralph Webb in 1998. After 8 years of engineering practice serving as a director of engineering in the United States, he joined Zhejiang University as a full professor in 2006. His group has been focused on two-phase heat transfer, falling film evaporation, fouling, nanofluids, compact heat exchangers, supercritical fuels, and so on. He is a fellow of the ASME.
Ping Fu
Ping Fu is a master's student in Zhejiang University, Hangzhou, China. She received her B.S. from North China Electric Power University in 2012 and continued her study at Zhejiang University for the master's degree with Prof. Wei Li. Her study is focused on mechanisms of enhanced heat transfer and fouling characteristics. She is working on enhanced heat transfer and fouling in heat exchangers under numerical simulation and experimental aspects.
Hongxia Li
Hongxia Li received her B.S. from Southeast University, majoring in thermal and power engineering, and continued her study at Zhejiang University for the M.S. degree. Her study focuses on experimental and numerical investigation of heat transfer enhancement and fouling performance in compact heat exchangers. She received her M.S. degree with Prof. Wei Li in 2013. Now she is working in Philips as a thermal engineer.
Guanqiu Li
Guanqiu Li received his B.S. in mechanical and energy engineering from Zhejiang University in 2006. He obtained his M.S. and Ph.D. degrees with Prof. Wei Li in thermal engineering from Zhejiang University in 2012. His research interests include compact heat exchangers and fouling, phase-change heat transfer, combined cooling and power generation cycle, and dropwise condensation on nanostructured surfaces. He passed away on December 25, 2014 when he was serving as a post doctor in the Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, in Abu Dhabi in UAE at age of 27.
Petur Thors
Petur Thors received his M.S. degree in mechanical engineering from Penn State University in 1984 under the supervision of Prof. Ralph Webb. He held the position of vice-president of research and development and chief technical officer at Wolverine Tube, Inc., developing highly enhanced heat transfer surfaces for the heating, ventilation, and air conditioning (HVAC) industry for 30 years, until April 2013. He has more than 150 worldwide patents in the field of heat transfer and specialty tools, of which 13 are U.S. patents. He has authored and co-authored several research papers in the Journal of Heat Transfer. He currently holds the position of Director of Research & Development with Wieland Copper Products, LLC.