Two Modified Correlations for Flow Boiling in Enhanced Tubes of Three Different Materials

Based on literature data and previous empirical formulas, this study investigates the accuracy of ten classic correlations for flow boiling heat transfer coefficients (HTCs) using experimental data points from eight heat exchange tubes made of three different materials under varying mass flux conditions. The materials of the heat transfer tubes are copper, stainless steel, and aluminum. The copper tubes include one smooth tube (ST) and one enhanced tube (ET), while both the stainless steel and aluminum tubes consist of one ST and two ETs. The enhanced structures for the copper tube (Cu-E1) are rectangular fins, while the stainless steel tubes (SS-E1 and SS-E2) are treated with laser etching and sandblasting on surfaces with two different staggered screw patterns (a, b). The enhanced structures for the aluminum tubes (Al-E1 and Al-E2) are chevron and ripple patterns, respectively. The experimental conditions of this study include a saturation temperature range from 279 K to 283 K, a mass flux range from 100 to 350 kg/m2s, and inlet and outlet qualities of 0.2 and 0.8. The refrigerant used is R32, with an inner diameter of 8.32 mm and a tube length of 2 meters. Ten empirical flow boiling correlations (Gungor and Winterton, Liu and Winterton, Lazarek and Black, Kandlikar, Wu, and Tran et al.) were used to evaluate the accuracy of the HTC predictions for STs of three different materials. Preliminary predictions indicated that four correlations exhibited significant errors, prompting further verification of the remaining correlations for STs. Comparative analysis revealed that the models proposed by Gungor and Winterton and Liu and Winterton had relatively high accuracy for the three STs, predicting over 85% of the data points within a ±20% error margin. These two correlations were then used to predict the flow of boiling HTCs for five ETs. The results showed that these correlations could not accurately predict the experimental data for the ETs, possibly due to discrepancies between the experimental conditions and the original research assumptions, such as refrigerant choice, mass flow rate, and quality range. To improve the accuracy, this study modified the Gungor and Winterton and Liu and Winterton correlations by introducing the Reynolds number (Re) to better capture the impact of mass flux on the heat transfer performance of ETs. Additionally, an exponent n and a coefficient K were introduced to the area ratio to maintain the integrity of the correlations. After modification, both correlations provided accurate predictions for the flow boiling HTCs of copper, stainless steel, and aluminum ETs, with the Liu-Winterton correlation showing that the same exponent and coefficient could be used for different materials. The error margin for the modified correlations remained within ±20%.