Abstract
Heat transfer and friction factor characteristics in a circular tube fitted with core-rod and multi-tube inserts at high temperature have been investigated experimentally. In the experiments, ambient air with Reynolds numbers in a range of 6000–20,000 is passed through a circular tube with uniform wall temperature and convection and radiation heat transfer phenomena are studied. Experiments have been performed at four constant wall temperatures in tubes with core-rod and multi-tube inserts. For each wall temperature considered, convection and radiation heat transfer coefficients have been determined. The experimental results show that at uniform wall temperature of 373, 473, 553 and 633 K, the average share of the radiation heat transfer coefficient to the total heat transfer coefficient is 13.9, 18.3, 24.7 and 28.7 % for core rod and 16.7, 19.1, 20.2 and 22.3 % for multi-tube insert, respectively. In addition it was noted that for the mentioned temperatures, the heat transfer coefficient increased by 227, 299, 327 and 369 % for core rod and 289, 407, 505 and 572 % for multi-tube insert, respectively in comparison to the plain tube.
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Abbreviations
- A :
-
Heat transfer surface area, m2
- C p,a :
-
Specific heat capacity of air, J kg−1 K−1
- \(D_{\text{h}}\) :
-
Hydraulic diameter, m
- f :
-
Friction factor
- \(F_{jk}\) :
-
Shape factor of surface j with respect to surface k
- \(\bar{h}\) :
-
Average heat transfer coefficient, W m−2 K−1
- K :
-
Thermal conductivity, W m−1 K−1
- L :
-
Length of test section, m
- \(\dot{m}\) :
-
Mass flow rate, kg s−1
- \(\bar{N}u\) :
-
Average Nusselt number
- Pr:
-
Prandtl number
- Q :
-
Heat transfer rate, W
- R :
-
Electrical resistance of the heater element, Ω
- Re :
-
Reynolds number
- T :
-
Temperature, K
- \(\tilde{T}\) :
-
Average temperature, K
- U :
-
Mean axial velocity, m s−1
- V :
-
Voltage output from the auto-transformer, V
- \(V^{\prime}\) :
-
Volumetric flow rate, m3 s−1
- X :
-
Distance between the two areas
- \(\eta\) :
-
Enhancement efficiency
- \(\upsilon\) :
-
Kinematic viscosity, m 2 s−1
- \(\varepsilon\) :
-
Wall emissivity
- \(\theta\) :
-
Angle between the unit normals and the area
- \(\varDelta T_{\ln }\) :
-
Logarithmic mean temperature difference, K
- ΔP :
-
Pressure drop, Pa
- A :
-
Air
- B:
-
Bulk
- Cor:
-
Core rod
- Conv:
-
Convection
- I :
-
Inlet
- J :
-
Inner
- k :
-
Outer
- loss:
-
Losses
- Mul:
-
Multi-tube
- O:
-
Outlet
- P:
-
Plain tube
- Pac:
-
Packing
- Pp:
-
Pumping power
- rad:
-
Radiation
- rod:
-
Rod
- s :
-
Side long area of heater
- t:
-
Turbulator
- tot:
-
Total
- vol:
-
Voltage
- w:
-
Wall
- \(\infty\) :
-
Atmospheric air
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Thereby due to financial support of this research is to be thanked and appreciate from Islamic Azad University Mahshahr Branch.
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Technical Editor: Fernando Alves Rochinha.
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Alijani, M.R., Hamidi, A.A. Experimental investigation of the convection and radiation heat transfer in the tube with core-rod and multi-tube inserts. J Braz. Soc. Mech. Sci. Eng. 38, 2173–2180 (2016). https://doi.org/10.1007/s40430-015-0358-9
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DOI: https://doi.org/10.1007/s40430-015-0358-9