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ABSTRACT
Although microfin tubes are widely used in refrigeration and air-conditioning industries, relatively little data are available for small diameter tubes. In the present study, 7.0 and 5.0 mm outer diameter (O.D.) microfin tubes were tested for a range of mass flux (from 50 to 250 kg/m2s) and vapor quality (from 0.2 to 0.8). The saturation temperature was 8°C and the heat flux was 3.0 kW/m2. This heat flux is quite small, and existing investigations have not been gone to this low heat flux. Results showed that an optimum behavior was observed for the enhancement factor. It increased as mass flux increased up to 150 kg/m2s, and then decreased with a further increase of mass flux. The penalty factors were larger than 1.0 except at the lowest mass flux of 50 kg/m2s. Possible reasoning was provided based on flow patterns of the microfin and the smooth tube. The enhancement factor ranged from 1.24 to 1.93 for the 7.0 mm microfin tube, and 1.63 to 3.26 for the 5.0 mm microfin tube. The penalty factor ranged from 0.82 to 1.11 and from 0.48 to 1.39 for the 7.0 mm and the 5.0 mm microfin tube, respectively. Larger high fin height of the 5.0 mm microfin tube may be responsible for the larger enhancement and penalty factor of the 5.0 mm microfin tube. Comparison with predictions by existing correlations revealed that 7.0 mm microfin tube data were reasonably predicted. However, 5.0 mm microfin tube data were generally underpredicted, probably due to the large fin height of the 5.0 mm microfin tube.
Nomenclature
| A | = | heat transfer area, m2 |
| cp | = | specific heat, J/kg∙K |
| D | = | tube diameter, m |
| e | = | fin height, m |
| EF | = | enhancement factor |
| Fr | = | Froude number |
| G | = | mass flux, kg/m2s |
| h | = | heat transfer coefficient, W/m2∙K |
| ifg | = | latent heat of vaporization, J/kg |
| k | = | thermal conductivity, W/m∙K |
| L | = | length, m |
| = | mass flow rate, kg/s | |
| n | = | numbe of fins |
| Nu | = | Nusselt number |
| P | = | pressure, Pa |
| PF | = | penalty factor |
| Pw | = | wetted perimeter, m |
| Pr | = | Prandtl number |
| Q | = | heat transfer rate, W |
| q | = | heat flux, W/m2 |
| Re | = | Reynolds number |
| T | = | temperature, K |
| t | = | thickness, m |
| U | = | overall heat transfer coefficient, W/m2K |
| Xtt | = | Lockhart-Martinelli parameter |
| x | = | vapor quality |
| z | = | cordinate to flow direction |
Greek symbols
| = | void fraction | |
| = | helix angle, degree | |
| ΔTlm | = | logarithmic mean temperature difference, K |
| Δx | = | vapor quality difference |
| = | fin apex angle, degree | |
| ρ | = | density, kg/m3 |
Subscripts
| a | = | actual, acceleration |
| ave | = | average |
| exp | = | experimental |
| f | = | friction |
| fa | = | flow area |
| g | = | gas |
| h | = | hydraulic |
| i | = | inside |
| in | = | inlet |
| l | = | liquid |
| m | = | middle, melt-down |
| o | = | outside |
| out | = | outlet |
| p | = | preheater |
| pred | = | prediction |
| r | = | refrigerant, fin root |
| sat | = | saturation |
| t | = | tip |
| v | = | vapor |
| w | = | water |