The Effect of the Angle of Inclination on the Operation Limiting Heat Flux of Long R-134a Filled Thermosyphons

When traditional air-to-air cooling is too voluminous, long thermosyphons may offer a way out. For safe operation of heat exchangers equipped with thermosyphons, the limiting heat flux $qlim″$ is an important design parameter. Some literatures are found to deal with the operation limiting heat flux of closed two-phase thermosyphons. However, R-134a filled thermosyphons with large length-to-diameter (188) are hardly investigated up to now. Extrapolation of existing correlations to predict $qlim″$ in this case results in large scatter. The effect of the angle of inclination on $qlim″$ has not been considered until now. Dedicated experiments with a single thermosyphon with a large length-to-diameter ratio (188) and filled with R-134a are presented and analyzed. Effects of saturation temperature, filling ratio, and angle of inclination $β$ on the operational limiting heat flux have been investigated. The thermosyphon functions properly if $β<83deg$⁠, and $qlim″$ is found to increase with increasing $β$⁠. With decreasing saturation temperature, $qlim″$ increases. The filling ratio is found not to be crucial if it exceeds 25%. Correlations are presented to accurately predict the operation limiting heat flux for thermosyphons with a $L/d$ ratio up to 188. Because of the accounting for the above new aspects, these correlations are also relevant for filling refrigerants other than R-134a.