Improvement of heat transfer in heat exchangers with spiral springs with the square cross-section area
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ORIGINAL
Improvement of heat transfer in heat exchangers with spiral springs with the square cross-section area Alireza Ghasemi Jolaghani 1 & Arash Mirabdolah Lavasani 2 Received: 2 July 2019 / Accepted: 13 June 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The purpose of this study is to investigate the effect of spiral springs with the square cross-section area on heat transfer and pressure drop in heat exchangers. The springs were fitted inside a circular cross-section copper tube. The diameter and length of the copper tube were 25.4 mm and 1820 mm, respectively. The experiments were performed with 9 spiral springs of 12 and 15 mm diameter in four pitches of 2, 5,10,20 mm and 21 mm diameter in three pitches 10, 20 and 30. The fluid used in all experiments was water and the Reynolds number for the hot water flow into the copper tube was in the range of 22,462 to 36,456. Cold water was used to cool around the copper tube and rate of cold water flow was also 0.1547 kg / s. The heat transfer coefficient increased by 45% after the springs were placed inside the copper tube in comparison to the no-spring condition. However, the pressure drop in the worst case increased by 77%. It can also be seen that the heat transfer coefficient increased to 20% by increasing the diameter of the springs by 75%. Meanwhile, the springs with a diameter of 21 mm and 12 mm has the most and the least effect on increasing the heat transfer coefficient, respectively. Keywords Heat exchanger . Heat transfer . Pressure drop . Spiral spring . Thermal performance factor
Nomenclature Ai Area of the heat transfer surface (mm) Cp Specific heat of fluid [J /kg K] D Diameter of the copper tube (mm) Dh Hydraulic diameter (mm) Do Inner diameter of the outer tube (plastic tube) (mm) d Diameter of the spiral spring (mm) f Friction factor fp Friction factor of plain tube fs Friction factor for the tube with the spiral spring h Heat transfer coefficient [W/m2 .K] k Fluid thermal conductivity [W/m.K] L Test tube length (mm)
˙ m Nu Nup Nus ΔP P Pr Qave Q Rec Re ΔTlm Tin.c Tout.c Tin.h
* Arash Mirabdolah Lavasani [email protected] Alireza Ghasemi Jolaghani [email protected] 1
Master of Mechanical Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
2
Department of Mechanical Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
Tout.h U
Mass flow rate [Kg/ s] Nusselt number Nusselt number for the plain tube Nusselt number for the tube with the spiral spring Pressure drop of copper tube [pa] Pitches of the spiral spring (mm) Prandtl number Average heat transfer rate (W) Heat transfer (W) Reynolds number of cold water Reynolds number of hot water Logarithmic temperature difference Cold water temperature at the entrance of the copper tube [°C] Cold water temperature at copper tube outlet [°C] Hot water temperature at the entrance of the copper tube [°C] Hot water temperature at copper tube outlet [°C] Overall heat transfer coefficient [W/m2. K]
Greek symbols η T
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