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ABSTRACT
In the present research work, heat transfer and friction characteristics of in-line elliptical tubes are studied experimentally and numerically with the aiming to enhanced the thermal performance. The elliptical tubes with an aspect ratio (AR) of 0.35 and 0.50 and with circular tubes of the aspect ratio of 1 are selected for Reynolds number (Re) ranging from 5000 to 21000. The results indicate that the elliptical tube shows higher heat transfer rate with a significant reduction in the friction factor for the entire range of Reynolds number. The superior performance of the elliptical tube is often due to the aerodynamic cross-section profile of the tubes which promotes better heat transfer with a reduction in the frontal area of the tubes. The study is performed for the two different pitch to major axis ratios (PR) of 1.25 and 1.5, respectively. The results show that at a pitch ratio of 1.25 the elliptical tube with an aspect ratio of 0.50 leads to the superior heat transfer rate, while at pitch ratio of 1.50 elliptic tubes with an aspect ratio of 0.35 indicate better thermo-hydraulic efficiency relative to 0.50.
Nomenclature
| a | = | Major axis of the elliptical tube (mm) |
| b | = | Minor axis of the elliptical tube (mm) |
| Deq | = | Equivalent diameter of the circular and elliptic tubes (mm) |
| NL | = | Number of tubes in the longitudinal direction |
| m | = | Mass flow rate of hot water (kg/s) |
| n | = | The number of tubes in tube bank array |
| k | = | Thermal conductivity of the air (W/mK) |
| ∆P | = | Pressure drop with tube bank (Pa) |
| Vmax | = | Maximum velocity of the air in the tube bank (m/s) |
| Nu, Nuo | = | Nusselt number for test tubes and circular tubes |
| SL | = | Longitudinal pitch of tubes |
| ST | = | Transverse pitch of tubes |
| ρ | = | The density of air (kg/m3) |
| μ | = | Viscosity of air (Pa-s) |
| η | = | Thermo-hydraulic efficiency |
| To | = | Free stream temperature of the air (K) |
| Tw | = | Tube wall surface temperature (K) |
| U | = | Free stream velocity of the air (m/s) |
| Re | = | Reynolds number |
| Cp | = | Specific heat of the hot water (J/kgK) |
| h | = | Heat transfer coefficient (W/m2K) |
| Q | = | Heat transfer rate (W) |
| As | = | Surface area of the tubes (m2) |
| Ta | = | Temperature of air past test tubes (K) |
| f, fo | = | Friction factor for test tubes and circular tubes |
| ui,uj | = | Velocity components in the i th or j th directions (m/s) |
| AR | = | Aspect ratio of tube (b/a) |
| PR | = | Tubes pitch ratio (SL/a) |
| ε | = | Dissipation rate of turbulent kinetic energy |
| k | = | Turbulent kinetic energy (m2/s2) |
Acknowledgments
The authors highly acknowledge the efforts of Shri. Akula Srinivasa Rao, Nagpur for fabricating the experimental test facility at VNIT, Nagpur, India.
Conflict of interest
There is no conflict of interest.