Experimental study on critical heat flux characteristics of R134a flow boiling in horizontal helically-coiled tubes
- Authors: Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen
- Date: 2011
- Subjects: Critical heat flux , Helically coiled tubes , Heat transfer
- Type: Article
- Identifier: uj:5284 , http://hdl.handle.net/10210/14953
- Description: Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8e11 mm, coil diameters of 135e370 mm, helical pitches of 40e105 mm and heated lengths of 0.85e7.54 m. The experimental conditions are pressures of 0.30e1.10 MPa, mass fluxes of 60e480 kg m 2 s 1, inlet qualities of 0.32e0.36 and heat fluxes of 6.0 103e9.0 104Wm 2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/di> 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at xcr< 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.
- Full Text:
Fluid-to-fluid modelling of two-phase flow critical heat flux in horizontal helically coiled tubes
- Authors: Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen
- Date: 2011
- Subjects: Critical heat flux , Fluid flow , Helically coiled tubes
- Type: Article
- Identifier: uj:5287 , http://hdl.handle.net/10210/14956
- Description: The new similarity laws for fluid-to-fluid modeling of two-phase flow critical heat flux (CHF) in horizontal helically coiled tubes were derived based on the dimensional analysis and similarity theory considering the effect of the geometrical parameters on CHF. A generalized factor Dn was introduced to the new similarity laws, and all the new dimensionless numbers were derived from the classical theorem of Buckingham for dimensional analysis. The obtained dimensionless parameter sets were a reasonable extension to Ahmad’s compensated distortion model, which may be considered as a special case of the new dimensionless parameter sets when the variable n is equal to unity. Based on the experimental data, the specific similarity numbers were determined for CHF phenomena in horizontal helically coiled tubes. A new equivalent characteristic parameter De-helix was developed, which could reflect the influence of complex flow channels on the occurrence of CHF. The equivalent characteristic parameter consists of the essential geometrical parameters of tubes and the fluid thermophysical properties. The new fluid-tofluid modeling methods were proposed for CHF of R134a-water in horizontal helically coiled tubes, which could be used readily to derive the CHF data of water through the CHF data of R134a at the corresponding experimental conditions.
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