The existed flow pattern map of flow boiling are mainly developed for horizontal smooth tubes, whichare not suitable for predicting the flow patterns in micro-finned tubes. This paper presents animproved flow pattern map for flow boiling in horizontal micro-finned tubes considering the effect ofhelix angle. Visualization of R410A flow boiling was carried out through the transparent smooth andmicro-finned tubes with 0°, 10° and 18° helix angle. The result shows that the flow pattern map formicro-finned tube with 0° helix angle is generally similar to that in the smooth tube. For the tube with10° and 18° helix angle, the annular flow occurs at lower vapor quality and lower mass flux. Amodified model accounting for the early transition to annular flow in the micro-finned tubes is built.
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