Heat transfer and pressure drop characteristics of ammonia during flow boiling inside a horizontal small diameter tube

In this article, a series of experiments was conducted to investigate the flow boiling heat transfer and pressure drop characteristics of ammonia in a 4 mm horizontal plain tube. The experiments were performed for heat fluxes at 9 and 21 kW m−2 K−1, mass flux from 50 to 100 kg m−2 s−1, and saturation temperature from −15.8 to 5 °C. The experimental results show that with the increase of heat flux, the heat transfer coefficient increases. Meanwhile, it also increases with a rise in mass flux in the low vapor quality region, whereas reversed situation can take place in the high vapor quality region when the saturation temperature is decreased. The saturation temperature has little effect on the heat transfer coefficient in the low vapor quality region, however at higher mass fluxes, the heat transfer coefficient can decrease with decreasing saturation temperature in the high vapor quality region. The comparisons of the experimental data with existing correlations for flow boiling heat transfer coefficient show that Gungor and Winterton correlation can give good agreement with mean absolute deviation of 19.6% even though it is developed for turbulent flow. The adiabatic two-phase frictional pressure gradient increases with the increase of vapor quality and mass flux, while decreases with the increase of saturation temperature. The comparisons with existing correlations for two-phase frictional pressure gradient indicate that Müller-Steinhagen and Heck correlation can predict the experimental data well with mean absolute deviation of 16.1% and 93.4% of the data is within the ±30% error band.