An experimental study on critical heat flux (CHF) in a helically-coiled tube cooled with R-134a has been completed in order to assess present fluid-to-fluid modeling approaches. The investigated range of flow parameters for R-134a was: pressure from 0.2 to 0.5 MPa, mass flux values from 50 to 1500 kg m-2 s-1 and inlet quality from -0.2 to 0.1. The CHF data of R-134a have been compared with that of water by applying the Ahmad and the Katto modeling. The water equivalent CHF data translated from R-134a CHF data by using the two modeling approaches have shown a good agreement with the actual water CHF data from previous studies when mass flux exceeds 600 kg m-2 s-1. The results indicate that both the Ahmad and the Katto modeling can be applied only for the high mass flux conditions in helically-coiled tubes.
Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated horizontal straight tube and helically-coiled tube respectively with R-134a as the working fluid. The helically-coiled tube has the same heated length and inner diameter with the straight tube and experiments were performed under the following conditions: pressure from 0.4 to 2.5 MPa, mass flux values from 80 to 1500 kg m-2 s-1, inlet quality from -0.23 to 0.28 and critical quality from 0.65 to 0.86. The CHF data of the helically-coiled tube have been compared with that of the straight tube. The results show that the helically-coiled tube gets significant improvement in the CHF values vs. the straight tube under the same conditions and the degree of improvement depends on the mass flux, system pressure, inlet quality and critical quality.
Based on 2006 look-up tables to analysis the effect of pipe diameters on CHF, the correlations are established for horizontal helically coiled tube by introducing equivalent geometrical parameter. According to experimental data, it is found that the correlation is very suitable, and the regularity of horizontal helically coiled tube geometry parameters on CHF tends to be consistent with that of a vertical tube.
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