This document summarizes a study that examined convective boiling heat transfer of refrigerants R-22, R-134a, and CO2 in horizontal minichannels. The researchers obtained local heat transfer coefficients for varying heat fluxes, mass fluxes, and vapor qualities up to 1.0 in test sections made of stainless steel tubes with diameters of 1.5mm and 3.0mm. Nucleate boiling was the main heat transfer mechanism, and heat transfer coefficients increased and decreased with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO2 was approximately 1.0:0.8:2.0. A new boiling heat transfer coefficient correlation

1. Thermal Energy Conversion Control Lab. Chonbuk Nat’I Univ.
Boiling heat transfer of R-22, R-134a, and CO2
in horizontal smooth minichannels*
Kwang-Il Choia, A.S. Pamitran a, Chun-Young Oh b, Jong-Taek Oh c,*
A Graduate School, Chonnam National University, San 96-1, Dunduk-Dong, Yeosu, Chonnam 550-749, Republic of Korea
B Refrigeration Research Institute, Chonnam National University, San 96-1, Dunduk-Dong, Yeosu, Chonnam 550-749, Republic of Korea
C Department of Refrigeration and Air Conditioning Engineering, Chonnam National University, San 96-1,
Dunduk-Dong, Yeosu, Chonnam 550-749, Republic of Korea
Sudheer Nandi
(Ph.D.),M.Tech,MBA.
Sustainable Energy . S.korea

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• http://www.sciencedirect.com/science/article/pii/S0140700702000403#
http://www.youtube.com/watch?v=s-YmfZNKnlU

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Qualitative classification flow regimes
.
MIT Department of Nuclear Science and Engineering

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Heat transfer and flow regimes in a vertical heated channel. (Thermal non‐equilibrium effec
ts have been neglected in sketching the bulk temperature)

6. Thermal Energy Conversion Control Lab. Chonbuk Nat’I Univ.
Abstract
• This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO2.
• The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW 𝑚 −2
, mass fluxes
ranging from 200 to 600 kg 𝑚−2
𝑠 −1
, a saturation temperature of 10 °C, and quality up to 1.0.
• The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of
2000 mm. The section was heated uniformly by applying an electric current to the tubes directly.
• Nucleate boiling heat transfer was the main contribution, particularly at the low quality region.
• An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux
and mass flux.
• The mean heat transfer coefficient ratio of R-22:R-134a:CO2 was approximately 1.0:0.8:2.0. Laminar flow was
observed in the minichannels.
• A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in
minichannels was developed with a mean deviation of 11.21%.
Keywords: Refrigerant; R-22; R-134a; R-744; Carbon dioxide; Experiment; Heat transfer; Boiling; Micro channel;
smooth tube; Horizontal tube
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Experimental apparatus
The experimental test facility and test section.

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Experimental conditions
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The effect of mass flux on heat transfer coefficient: (a)R-22, (b) R-134a, and
(c) CO2

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The experimental data on Wojtan et al. [20] flow pattern map for R-22.
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The effect of heat flux on heat transfer coefficient: (a) R-22,(b) R-134a, and (c) CO2.

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The comparison of the heat transfer coefficient
The effect of inner tube diameter on heat transfer
coefficient for R-22.

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Development of a new correlation

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Two-phase heat transfer multiplier as a function of ∅2
𝑓
Diagram of the experimental heat transfer coefficient, hexp.,
vs prediction heat transfer coefficient, hpred.
Heat transfer coefficient comparisonNucleate boiling contribution

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Thank you for listening