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ABSTRACT
This paper experimentally investigated the heat transfer performance of two internal helically finned tubes covering low Reynolds number in turbulent flow. A water-ethylene glycol mixture flowed inside and R134a boiled outside. The numbers of fin, the helix angles, and the ratios of fin height to diameter for the two test tubes were 38 and 60, 60° and 45°, and 0.0534 and 0.0222, respectively. The experimental Reynolds number was lower to 5400, covering the critical Reynolds number Recr for turbulent flow. The data was divided into good and poor linearity regions by the modified Wilson plot method, which was much suitable for this case than that the data was treated as one region. The j factors showed ascending trend for Re below Recr and descending trend for Re above Recr. Enhancement factors of friction factor or j factor of the test tubes increased with increasing Reynolds number and eventually approached at the maximum for the Reynolds number above Recr. The test tubes exhibited maximum efficiency indexes of 2.1 and 1.88, indicating superior overall performance compared to the majority of existing tubes. By utilizing the experimental data, a correlation was established basing on the critical Reynolds number, resulting in over 96% of the predicted errors falling within the range of ± 8%. This study has the potential to provide valuable insights for industrial implementation and the advancement of general correlation development.
Nomenclature
| A | = | heat transfer area(m2) |
| Ci | = | coefficient for internal heat transfer correlation (-) |
| Co | = | constant for internal heat transfer correlation (W/m2·°C) |
| cp | = | specific heat capacity(J/kg·°C) |
| D | = | diameter(mm) |
| e | = | fin height (m) |
| F | = | enhancement factor (-) |
| f | = | friction factor (-) |
| hi | = | internal heat transfer coefficient (W/m2·°C) |
| hi,Gni | = | internal heat transfer coefficient calculated by Gnielinski equation (W/m2·°C) |
| ho | = | outside heat transfer coefficient (W/m2·°C) |
| j | = | Colburn j factor, Nu/Re/Pr1/3 (-) |
| k | = | thermal conductivity (W/m·°C) |
| L | = | test section length (m) |
| Ns | = | number of fins (-) |
| P | = | pressure (Pa) |
| p | = | pitch (m) |
| Pr | = | Prandtl number (-) |
| Q | = | heat transfer rate (W) |
| q | = | heat flux(W/m2) |
| Re | = | Reynolds number (-) |
| Rwal | = | thermal resistance of tube wall (m2·°C/W) |
| Rfoul | = | thermal resistance of fouling (m2·°C/W) |
| s | = | fin base width(m) |
| T | = | temperature (K or °C) |
| U | = | overall heat transfer coefficient (W/m2·°C) |
| u | = | flow velocity(m/s) |
| V | = | volumetric flow rate(m3/s) |
| Greek symbols | = | |
| α | = | helix angle (deg) |
| θ | = | fin apex angle (deg) |
| η | = | efficiency index (-) |
| ρ | = | fluid density(kg/m3) |
| ΔTm | = | logarithmic mean temperature difference(K) |
| Subscripts | = | |
| ave | = | average |
| exp | = | experiment data |
| h | = | hot medium |
| i | = | inside |
| in | = | inlet |
| o | = | boiling outside |
| out | = | outlet |
| p | = | plain tube |
| pre | = | predicted by equations |
| sat | = | saturation refrigerant |
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51606029), the Fundamental Research Program of Shanxi Province (202203021212200), the Research Project Supported by Shanxi Scholarship Council of China (2023-073), and the Special Fund for Science and Technology Innovation Teams of Shanxi Province (No. 202304051001011).
Disclosure statement
No potential conflict of interest was reported by the author(s).