Study On Flow Boiling Heat Transfer Characteristics And Its Predication Model In Helically-Coiled Tubes

The research on flow boiling heat transfer characteristics of the new type and environmental protection working medium is important academic and engineering application value to reform and development new heat exchange equipment. We re-furnished and upgraded the original experimental system of two phase flow and heat transfer, then mainly undertook a series systematic study on boiling heat transfer properties of multi-component working medium R407c and single-component working medium R134a’s flow boiling in horizontal helically-coiled tubes, and model prediction. Pipe wall temperature’s features of flow boiling heat transfer had been obtained, furthermore, the heat transfer coefficient correlation has been developed, and criteria on heat transfer deterioration had been built up. The main work is as follows:Firstly, experimental study has been performed on flow boiling properties of R 134a and R407c in horizontal helically-coiled tubes as working medium. Two different working medium in helically-coiled tubes followed different rules, which were compared. The results show that averaged wall temperature for R134a shows a ripple along the pipe, and pipe wall temperature for R407c shows an obvious saw-tooth pattern along the pipe. The main reason is that the film thickness of boiling heat transfer is reduced with the increasing of the heated time; and the friction loss makes the working pressure reducing, which results in the decreasing of the evaporation temperature; the centrifugal and gravity force take effect on the working fluid in helically-coiled tubes together, the variety of the resultant force direction of which leads to the changing of the wall temperature in the different sections. Using R407c as working medium, circumferential variety of wall temperature is analyzed, and the whole section follows the similar rule, however, with mass flux increasing, temperature difference of outside and inside decreases gradually. The same experiment is performed with R134a instead, the circumferential temperature difference of inside and outside of wall is smaller. Experiment using R134a, shows flow boiling heat transfer coefficient increases along pipe generally, and wavy ripple occurs with this increasing; correspondingly, as to R407c, flow boiling heat transfer coefficient increases along the pipe slightly, and shows obvious saw-tooth pattern, and heat transfer coefficient of R134a in helically-coiled tubes is higher than R407c.For comprehensive consideration influence of nucleate boiling and convective heat transfer for helically-coiled tubes flow boiling heat transfer, we introduced Dn to correct a nuclear boiling number, and developed a new flow boiling heat transfer correlation in helically-coiled tubes. The result showed the new prediction and experiment result matched well. Therefore, this correlation can be used in prediction analysis of boiling heat transfer properties in helically-coiled tubes.The complexity of helically-coiled tubes geometric structure causes the complicated heat transfer phenomenon. In this paper, helically-coiled tube’s diameter, diameter of spiral, heating length, and helical pitch etc were combined together to an equivalent factor, and analyzed the influence of this factor to the CHF using the experimental data. CHF features along with the change of equivalent factor are obtained.In order to explore the heat exchange equipment "burnt" for wall temperature jump, the flow boiling heat transfer deterioration characteristics and prediction criterion in horizontal helically-coiled tubes with R314a was study. The experiment shows:at high quality region, low mass flux, generally, inside wall temperature ramps up first, and then outside wall temperature ramps up, but the difference of the startup point of ram pup is small, and start point is the point where the quality equals to 0.73 round; at high mass flux, inside and outside wall temperature ramp up simultaneously, and start point is the point where the quality equals to 0.78 round. This is due to the combined effects of gaseous bubbling and centrifugal force in low mass flux, and the liquid membrane outside becomes thinner for vaporization, on the contrary, the one inside keeps the certain thickness for the secondary fluid effect, that make the heat transfer inside exceeds the outside. However, the liquid membrane outside and inside are almost vaporized to dry in high mass flux simultaneously. Furthermore, the relationships between the critical heat flow density and the quality, the effect of heat flow density to averaged heat transfer, have been analyzed. According to experimental result, a prediction model applied to flow boiling heat transfer deterioration in horizontal helically-coiled tubes was proposed by fitting method. By experiment validation, the predicted value matched the experimental value well.Analyzed two major methods for judging heat transfer deterioration occurring on supercritical fluid:Look-up table method and empirical correlations method. Inspected heat transfer deterioration correlations on supercritical fluid by using exiting the experimental data, and provided the reference for the development and application of heat transfer deterioration correlations.