Study On Fluid-to-fluid Modeling Of Flow Boiling Critical Heat Flux

The critical heat flux (CHF) is the maximum heat flux beyond which the burnout of the heating wall may occur resulted from a steep jump of wall temperature. The research and exploration to CHF is an important subject in the field of thermal-hydraulic conditions for boilers, nuclear reactors, and many other thermal systems of higher heat fluxes. The CHF is divided into two basic types which are the pool boiling CHF and the flow boiling CHF. The present study belongs to the category of the flow boiling CHF. What differentiates the flow boiling CHF from the pool boiling CHF is the more complex mechanism of flow boiling CHF which is subject to the specific flowing conditions. For the deficiencies of current theoretical models and the less accurate description to the CHF with present mathematical equations, the experimental approach is the main ways to understand the CHF characteristics and mechanism. The distinct shortcomings of CHF experiment using water as the working fluid are the very stringent requirements to the experiment system and the great expense to run the system due to the high latent heat, high pressure and temperature of water. By applying the fluid-to-fluid modeling techniques and conducting CHF experiments using refrigerants with much lower pressure, temperature and latent heat of vaporization as the working fluid, it can not only overcome the above-mentioned difficulties, but also facilitate further and intensive research on the CHF phenomenon by employing the many advanced testing equipment. R134a is a kind of ideal modeling fluid for its environment friendly quality, low pressure, low temperature and low latent heat of vaporization. Thus, the main content of this thesis is to use R134a as the working fluid to carry out study on the fluid-to-fluid modeling of CHF for water flow boiling in vertical tubes, horizontal tubes and helically-coiled tubes which are common heat-exchange form in the cooled wall of boilers, steam generator of nuclear reactors and many other thermal systems of higher heat fluxes. The main objective of the present study is to provide a clear acknowledge to effect of the existing CHF fluid-to-fluid modeling methods for the different types of channel, and then develop the fluid-to-fluid modeling technique of R134a and water which applied to the different types of flow channels with higher accuracy.This paper firstly summarized the existing boiling flow CHF fluid-to-fluid modeling methods and then expounded the successful CHF fluid-to-fluid models by now which were Ahmad model, Katto model, Lu Zhongqi model and Stevens-Kirby model respectively. A detailed analysis of the solving ideas for these models are presented. The similarity conditions needed to satisfy including geometric similarity, hydraulic similarity, thermodynamic similarity and flow similarity are analyzed and compared for each model. Then the step of applying each models were described in details. The CHF fluid modeling properties for R134a and water was calculated, respectively, the flow scale factor and CHF scale factor for Ahmad model, Katto model and Lu Zhongqi model in different conditions were achieved, and a look-up table for the CHF fluid modeling properties was made. The look-up table provided a great convenience to the evaluation of the existing fluid-to-fluid models using present CHF data, as well as to the arrangement of CHF modeling experiments committed to verify the effect of present fluid-to-fluid models.In the inspection and evaluation for present flow boiling CHF fluid-to-fluid modeling methods, obtaining the accurate CHF data for R134a flow boiling in different channels plays a key role to the validity of the results undoubtedly. So it is very important to ensure the accuracy of the experimental CHF data for R134a. The experiments were carried out in the vapor-liquid two-phase flow and boiling heat transfer experiment platform in Institute of Refrigeration and Cryogenics affiliated to Shandong University. While ensuring the measurement errors for temperature, pressure, mass flow rate, current, voltage and so on as small as possible, the more precise thermal equilibrium experiments were designed. In the thermal equilibrium experiments, an accurate calibration of heat loss was made using the heating cycle of pure liquid water to avoid more complex two-phase parameter calculations which may produce big errors. The thermal equilibrium experiment guaranteed the flow boiling CHF data were more reliable. Another measure to ensure the validity of the CHF data was judging the occurrence of CHF with the control program which employed the Agilent3498data acquisition system aided by computer programming. A large number of CHF data for R134a flow boiling in the vertical tube, horizontal tube and helically-coiled tube were achieved from the present experiments, which provided a strong basis to analysis and evaluation of the existing flow boiling CHF fluid-to-fluid modeling techniques with more accurate and reliable data.Study on flow boiling CHF fluid-to-fluid modeling in a vertical tube was performed. Firstly, the parameter trends of CHF in a vertical tube were analyzed simply and the main factors that influenced the CHF in the vertical tube under the present experimental conditions were found. In the analysis and evaluation to the results of the present CHF fluid-to-fluid modeling methods, the water CHF data used for comparison with the R134a CHF data stem from the existing water CHF experiments directly and the classic Bowring CHF correlation indirectly. This study presented parameter trends for boiling number varied with flow modeling parameter of each fluid-to-fluid model when the similarity conditions were satisfied respectively. The results indicated that the CHF fluid-to-fluid modeling methods at present were basically applied to the vertical tube for R134a and water.Study on CHF fluid-to-fluid modeling in a horizontal tube was carried out. Firstly the CHF parameter trends were analyzed in the horizontal tube, and then the effects of flow boiling CHF fluid-to-fluid modeling methods, i.e., Ahmad model, Katto model, Lu Zhongqi model and Stevens-Kirby model applied to the horizontal tube were tested and evaluated. The influence of mass flux, liquid-gas density ratio and inlet subcooling on the accuracy of modeling were analyzed. Based on the analysis of the modeling results and investigation to influences of stratification on the CHF in horizontal tube, the thesis focused on the statement for development of new flow boiling CHF fluid-to-fluid modeling methods in the horizontal tube. A new flow similarity criterion number based on the Katto model and the corrected Fred number to reflect the effect of stratification on flow boiling CHF in the horizontal tube were proposed. The results of data analysis showed that the error band of the new method applied to the horizontal tube under the experimental conditions was about±25%.Compared to the straight tube, the helically-coiled tube which is the representative of the curved tube has the advantages of high heat transfer efficiency, compact structure, etc., and is a new type of heat transfer equipment developed in recent years. The CHF data of R134a and water in horizontal helically coiled tubes were analyzed by applying Ahmad model, Katto model, Lu Zhongqi model and Stevens-Kirby model. The modeling results of each models were presented in the coordinates of boiling number vs. CHF flow modeling parameters when the similarity conditions were matched. The deviations of the fluid-to-fluid modeling methods at present were given within the range of experimental parameters. It was found that the mass flux, pressures and inlet subcooling had a notable influence on the flow scale factor of horizontal helically-coiled tubes. This thesis presented an empirical correlations of flow scale factor of horizontal helically-coiled tubes through regression of the experimental data, and developed a new fluid-to-fluid modeling method for CHF in helically-coiled tubes. The results showed that the errors of the new fluid-to-fluid modeling method applied to the helically-coiled tubes under the present experimental conditions were within±20%.In summary, a comprehensive analysis and evaluation to the results of CHF fluid-to-fluid modeling methods at present applied to the vertical tube, the horizontal tube and the horizontal helically-coiled tube was presented based on the CHF experiments, and the new CHF fluid-to-fluid modeling methods suitable for the horizontal tube and the horizontal helically-coiled tube were developed which provided strong support for the carrying out of intensive CHF experimental research using working fluid with low latent heat.