Study On Characteristics Of Critical Heat Flux In Horizontal Helically-Coiled Tubes And Fluid To Fluid Modeling Methods

Critical heat flux (CHF) is one of the most important monitoring parameters in boiling heat transfer. It will result in heat transfer deterioration that is directly related to the safety and economy of the heat transfer equipment when CHF occurs. Therefore, CHF phenomenon has attracted researchers'much attention in the field of two-phase flow boiling heat transfer investigation. Among the heat transfer enhancement tubes, horizontal helically-coiled tube has been widely used for heat exchanger design, which has the advantages of compact structure and high efficiency over the traditional straight tube. However, CHF characteristics in horizontal helically-coiled tubes are more complicated and the mechanism for CHF occurrence is still unknow. In general, CHF characteristics experiments are usually carried out with vapor-water system or low latent heat substance system. The vapor-water system with high temperature and high pressure has high initial investment and severe conditions for operation, while the low latent heat substance system such as R134a system runs with lower system conditions which ensure the safety and the stability of the experiment performance. Moreover, results from the low latent heat substance system can be converted and applied to the vapor-water system by fluid to fluid modeling method derived from the dimensional analysis and similarity theory. The purpose of this paper is to investigate the R134a CHF characteristics in coils and develop new fluid to fluid modeling methods for R134a-water CHF in horizontal helically-coiled tubes according to theoretical analysis and experimental data.Based on the similarity theory and dimensional analysis methos, a generalized factor Dn is introduced to the fluid-to-fluid modeling to take into account the effect of flow channel geometrical parameters on CHF. Thirteen similarity dimensionless products were derived based on the dimensional analysis and similarity theory and were compared with those derived by Ahmad. Ahmad's results are considered to be the special situation of current result when n=1, and for different flow channels different n can be calculated according to experimental data to establish the specific relationship between Dn and the geometrical parameters. On the other hand, a new equivalent characteristic size De was developed, which could reflect the effects of complex flow channels on the occurrence of CHF.Experimental platform for CHF characteristics investigation using alternative refrigerants was designed and built, considering the requirements for thermal properties, environment protection and operating stipulation. A parallel test section was designed for boiling flow experiments in both horizontal helically-coiled tube and horizontal straight tube, and a reservoir and two visual sections made of electric melting-quartz glass were designed to observe flow patterns. Test results show that copper tubes are fit for the construction of the system main pipeline; electric melting-quartz glass is hard enough and utilizable for visual study; the heat loss of the system is no more than 5%; the leak ratio is less than 6 kPa/24h under 2.0 MPa; the respond time for adjusting system pressure and mass flux are 0.2 s/kPa and 1.5 s/L h-1, respectively. Five kinds of typical flow patterns, bubble flow, slug flow, stratified flow, wave-stratified flow and annular flow, has been observed and analyzed. Besides, a new flow pattern named wave-annular flow was found, which appears between wave-stratified flow and annular flow.A simple but accurate method for manufacturing horizontal helically-coiled tubes was proposed according to the theorem "three given points determine a circle" and the definition of the helix angle of helically-coiled tubes, and the manufacturing equipment and its control system were designed. In this design, three die wheels A, B and C made of wearable steel are used to adjust the positions of the raw materials in order to determine the product geometric parameters expected in advance. Subsequently, a method based on event-driven with Agilent BenchLink Data Logger Pro. software was developed to determine the occurrence of CHF. CHF phenomena is considered to occur once any wall temperature detected by thermocouples satisfies the function relationship given in this paper and then the Agilent software sends signal to cut off power supply within 0.1 second, which greatly improved the experimental accuracy.Fifteen horizontal helically-coiled test sections with different geometrical parameters were used for the R134a CHF experimental studies under the system parameters of P=0.20-1.15 MPa, G=50-500 kg m-2s-1, x=-0.32-0.36, q=6-90 kW. The wall temperatures distribution when CHF occurs, the effect of system parameters on CHF and the effect of the geometrical parameters were investigated and discussed in detail. The main results are as follows:The wall temperature will increase abruptly near the sections where CHF occurs and the average temperatures among other sections differ not much from each other, which is obviously different from the situation in regular heat transfer state. Along the circumference of the same section, the temperature at front side (90°) usually rises firstly when CHF occurs, while the temperature at offside (270°) usually fall down firstly when the power is cut off. In general, the temperatures at front side and offiside (90°and 270°) frequently have higher values than those at inner side and outer side (180°and 0°). CHF values increase approximately linearly with increasing mass fluxes, but decrease approximately linearly with increasing inlet vapor qualities. They also decrease with increasing critical vapor qualities, and this trand becomes more obvious when xcr<0.5 than xcr<0.5, especially for high mass flux conditions. Among all the system parameters, mass flux has the most significant effect on CHF, but pressure has the least in present experimental conditions.CHF values have a decline trend with increasing heated tube length L, inner diameter di and helical diameter, and this trend can also be influenced by mass fluxes and inlet vapor qualities. The helical pitch has little effect on CHF. For the combined geometrical parameters, helical diameter-to-inner diameter ratio Dc/di is more important than heated tube length-to-inner diameter ratio L/di for CHF analysis in horizontal helically-coiled tubes. The experimental data report that L/di nearly has no effect on DNB type CHF when L/di>200. Besides, Dc/Pt has little influence on CHF.Based on the theoretical analysis and experimental data, new correlations were developed for current experimental conditions; a fluid to fluid modeling method based on classical models was proposed; the specific expression of Dn for horizontal helically-coiled tubes was determined and new fluid to fluid modeling method based on this generalized influence factor was developed to apply for R134a-water CHF data conversion. All the correlations and modeling methods mentioned above were compared to verify their validity. The details are shown as follows:Experimental CHF data were compared with the calculated results from Bowring correlation and Shah correlation under the same conditions. It shows that the calculated values from the two correlations are dispersive much from the experimental results. Actually, both of them can give accurate CHF prediction in straight tube for water or R12, but are not suitable for R134a CHF in horizontal helically-coiled tubes under present experimental conditions. Considering the difference for DNB type CHF and dry-out type CHF based on experimental analysis, correlations are developed by inlet conditions and outlet conditions, respectively. Since the DNB type CHF is influenced mainly by the local conditions, boiling number Bo, Reynolds number Re, Dean number Dn, liquid-to-gas density ratio Nd and outlet quality xo were used to develop the new correlations, while boiling number Bo, Reynolds number Re, Dean number Dn, liquid-to-gas density ratio Nd and inlet quality xi were selected as similarity criterion for dry-out type CHF. They work well with an error of±20% according to the comparison between calculated data and experimental data.General way to develop fluid to fluid modeling method was suggested in this paper, and two fluid to fluid modeling methods were developed for R134a-water CHF in horizontal helically-coiled tubes. An average fluid modeling method was proposed based on Ahmad model and Katto model, and all the scaling factors for R134a-water were calculated. Subsequently, the average mass flux scaling factor and the average heat flux scaling factor were presented with a satisfied agreement with experimental data. On the other side, the specific expression of Dn for horizontal helically-coiled tubes was determined according to experimental data, and a new fluid to fluid modeling method derived from the analysis of the generalized influence factor Dn. The prediction error of this method is within±20% according to the comparison between calculated data and experimental data.