An exhaustive experimental study of the critical heat flux (CHF) in R-134a-cooled tubes has been recently completed. The objective of this study was to provide a consistent set of CHF data to be used as a reference for future separate effect studies. The investigated range of flow parameters in R-134a was: outlet pressure 0.96–2.39 MPa (equivalent to 6–14 MPa in water), mass flux 500–3000 kg m−2 s−1 (equivalent to 700–4300 kg m−2 s−1 in water), and critical quality −0.1 to +0.9. To extend the range of critical qualities, different heated lengths (0.45–2 m) and two-phase flow at inlet to the test section were used. Scaling laws were applied to convert the standard CHF look-up table from water to R-134a equivalent conditions. The comparison of the data with the water-based look-up table showed that for high mass fluxes (G = 1400–4300 kg m−2 s−1 in water equivalent), the look-up table provides an excellent CHF prediction for R-134a-cooled tubes. The differences were mainly noticeable for the limiting critical quality range and for low mass flux. A discussion of the observed limiting critical quality phenomenon is also included.

This paper examines the critical heat flux (CHF) behaviour for annular flow in bilaterally heated annuli and compares it with that in tubes and unilaterally heated annuli. It was found that the differences in CHF between bilaterally and unilaterally heated annuli or tubes strongly depend on pressure and quality. The CHF in bilaterally heated annuli can be predicted by tube CHF prediction methods for the simultaneous CHF occurrence at both surfaces, and the following flow conditions: pressure 7–10 MPa, mass flux 0.5–4.0 mg m−2 s−1 and critical quality 0.23–0.9. The effect on CHF of the outer-to-inner surface heat flux ratio, was also examined. The prediction of CHF for bilaterally heated annuli was based on the droplet-diffusion model proposed by Kirillov and Smogalev, 1969, Kirillov and Smogalev, 1972. While their model refers only to CHF occurrence at the inner surface, we extended it to cases where CHF occurs at the outer surface, and simultaneously at both surfaces, thus covering all cases of CHF occurrence in bilaterally heated annuli. From the annuli CHF data of Becker and Letzter (1975), we derived empirical functions required by the model. The proposed equations provide good accuracy for the CHF data used in this study. Moreover, the equations can predict conditions at which CHF occurs simultaneously at both surfaces. Also, this method can be used for cases with only one heated surface.

This paper examines the differences in critical heat flux (CHF) between internally heated annuli and tubes, cooled by water, based on an 8 mm tube CHF look-up table as the reference. The quality, gap size and pressure were identified as the most important parameters to account for differences in CHF between concentric annuli and tubes. Eccentricity, void migration parameter γ and quality caused the main differences in CHF between eccentric and concentric annuli. These parameters were used to develop CHF correlations for both types of annuli. The proposed correlations predict the CHF with an r.m.s. error of 9.26% for concentric annuli (based on 1547 CHF data points), and 11.6% for eccentric annuli (based on 331 CHF data points). These correlations predict the correct parametric and asymptotic trends, and are more accurate than other CHF prediction methods for annuli.

The influence of low-frequency harmonic oscillations on the natural convection occuring at vertical walls of a ship tank has been investigated. The velocity field outside the boundary layer of the liquid in the tank has been determined by the use of potential flow theory. Two analytical models of heat transfer have been solved. The first of them is based on the method of small perturbations; while, in the second model, averaging of conservation equations is adopted. The model are valid for a laminar boundary layer. The results obtained agree well with experimental results obtained elsewhere1. Comparison has also been made, on the assumption that the laminar sublayer determines the heat transfer, with results of experimental investigations carried out by the authors for the turbulent flow range.