Experimental Study on Condensation Heat Transfer of Ethanol–Water Vapor Mixtures on Vertical Micro-tubes
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Abstract
The paper presents an experimental investigation of Marangoni condensation heat transfer of ethanol–water vapor mixtures on vertical micro-tubes with an outer diameter of 0.793 mm, 1.032 mm, and 1.221 mm. Experiments were performed over a wide range of ethanol mass fractions in vapor mixtures for different vapor velocities and pressures. Condensation heat transfer coefficients behaved nonlinear characteristics, increased, and then decreased with increasing vapor-to-surface temperature difference. Under the same experimental conditions, the condensation heat transfer coefficient at a 2 % ethanol mass fraction in vapor was the highest. At low ethanol mass fractions, the condensation heat transfer coefficient of the ethanol–water vapor mixture was 2 to 3 times greater than that for pure steam. The effect of vapor pressure and velocity on condensation heat transfer suggested a positive tendency on each micro-tube for all vapor mixtures with different ethanol mass fraction. Results showed that condensation heat transfer coefficients on micro-tubes with a diameter of 1.032 mm were higher than those on the other two micro-tubes, suggesting that there existed a critical diameter which gave the largest condensation heat transfer coefficient.
Keywords
Condensation heat transfer Ethanol–water vapor mixtures Marangoni condensation Micro-tubeNomenclature
- A
Outside heat transfer area of micro-tube \((\hbox {m}^{2})\)
- \(C\)
Vapor mass fraction of ethanol (%)
- \(C_{p}\)
Isobaric specific heat \((\hbox {J}{\cdot } \hbox {kg}^{-1}{\cdot } \hbox {K}^{-1})\)
- \(d_{\mathrm{i}}\)
Inner diameter (m)
- \(d_{\mathrm{o}}\)
Outer diameter (m)
- \(h\)
Condensation heat transfer coefficient \((\hbox {kW} {\cdot } \hbox {m}^{-2}{\cdot } \hbox {K}^{-1})\)
- \(h_{\mathrm{i}}\)
Convective heat transfer coefficient \((\hbox {kW} {\cdot } \hbox {m}^{-2}{\cdot } \hbox {K}^{-1})\)
- \(k\)
Overall heat transfer coefficient \((\hbox {kW}{\cdot } \hbox {m}^{-2}{\cdot } \hbox {K}^{-1})\)
- \(l\)
Micro-tube length (mm)
- \(P\)
Pressure (kPa)
- \(P{r}\)
Prandtl number (dimensionless)
- \(q\)
Heat transfer flux \((\hbox {kW}{\cdot } \hbox {m}^{-2})\)
- \(q_{\mathrm{v}}\)
Volumetric flow rate
- \(T\)
Temperature (K)
- \(\Delta T\)
Vapor-to-surface temperature difference (K)
- \(\Delta t_\mathrm{m}\)
Logarithmic mean temperature difference (K)
- \(Re\)
Reynolds number (dimensionless)
- \(R\)
Thermal resistance \((\hbox {m}^{2} {\cdot } \hbox {K}{\cdot } \hbox {W}^{-1}\))
- \(U\)
Vapor velocity \((\hbox {m}{\cdot } \hbox {s}^{-1})\)
Greek symbols
- \(\lambda \)
Thermal conductivity \((\hbox {W}{\cdot } \hbox {m}^{-1}{\cdot } \hbox {K}^{-1})\)
- \(\rho \)
Density of condensate \((\hbox {kg}{\cdot } \hbox {m}^{-3})\)
- \(\varPhi \)
Heat transfer rate (kW)
Subscripts
- i
Inlet
- o
Outlet
- sat
Saturated
Notes
Acknowledgments
The authors greatly appreciate Prof. Y. Utaka at Yokohama National University for his experimental assistance and academic discussion. The work was financially supported by the National Natural Science Foundation of China Nos. 51206133 and 51125027.
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