| Subcooled boiling is especially attractive for the equipment with high heat flux such as nuclear reactors, EAST (Experimental Advanced Superconducting Tokamak), cooling system of integrated chip and internal combustion engines because of high heat transfer efficiency. Due to the compactness in volume and high efficiency in heat transfer, fluid flow and heat transfer in coiled tube has been widely used in the heat exchangers for food processing, air conditioning and cryogenic systems and nuclear reactors. When subcooled boiling occurs in the helically coiled-tubes, the heat transfer efficiency would be improved significantly. To date, a number of test channels are involved in the investigation of subcooled flow boiling such as the straight pipe with circle, rectangular, annular, square cross sections, bundle channel and micro-channel. The subcooled flow boiling in the helically coiled-tube is impacted by the gravity and centrifugal force. As a result, the flow and heat transfer characteristics of subcooled boiling are more complicated for the coiled tubes. Therefore, the research of flow and heat transfer characteristics of subcooled boiling in helical coiled-tubes have an important meaning in improving the theoretical analysis of two-phase flow and economy and safety of heat exchanger.By considering the experimental conditions of the subcooled flow boiling and the physical property of the R134a, the experimental system was improved by installing the subcooler for the present study. Followed by the debugging stage, the experimental investigation is carried out at mass flux ranging from 147.5 to 443.7 kgm-2s-1, inlet subcooled temperature from 4.7 to 15.1?, and pressure from 412.1 to 850.3 kPa. Moreover, the visual study is conducted in the coiled tube to understand the effects of bubble behaviors on the heat transfer characteristics in coiled tube. Curved flows are formed by pumping constant water flows through the coiled transparent PVC tube. To understand the departure and the trajectory of the bubbles, air bubbles are generated by injecting constant air flow at varied locations of the coiled tube.Based on the experimental data, the temperature distribution of the cross section and the heat transfer coefficient of the single-phase flow in helically coiled-tube are analyzed, which have been proved to be influential on the onset of nucleate boiling (ONB). As a result of the analysis of the wall temperature, the subcooled boiling starts at the point of the wall temperature deviate from the linear growth process due to the change in heat transfer mechanism. The parametric trends of the experimental parameters on the superheat and the heat flux at onset of subcooled flow boiling in the helical coils are presented, which include the angular position of the coil and cross section (?,?), mass flux, subcooling, pressure, and the construction parameters of the helical coil. The wall superheat and heat flux at ONB in the helical tubes increase with the increase in the mass flux and inlet subcooling but decreases with the increase in the system pressure and coil diameter. The pitch and the placement direction of the helical coil have a slight effect on the wall superheat and heat flux at ONB.The wall temperature distribution and heat transfer coefficient of subcooled flow boiling are investigated in the horizontal and vertical helically coiled-tubes. The experimental results indicate that the wall temperature distributions of the cross sections are non-uniform, which is attributed to the effects of secondary flow, velocity distribution of the main flow and the bubble behaviors. However, the wall temperature distribution of the vertical helically coiled-tube is different from that of horizontal coil. In the vertical coil, the temperatures of ?=90° and ?=270° are the highest and the lowest in the cross-section, respectively. However, the temperature of ?=0° is the highest temperature and the temperature of ?=180° is the lowest temperature in the cross-section for the horizontal coil. As a result of the forces and the geometric construction of the vertical coil, the generated bubbles on the side of ?=270° may always slide along the heated surface. It has been well known that the bubble sliding results in augmentation of microlayer evaporation and bulk turbulent enhancement, which contribute significantly to the heat transfer enhancement. Therefore, the heated surface of ?=270° can be further cooled because of the bubble sliding heat transfer mechanism. The standard deviation of the temperature of the cross section is introduced as the nonuniformity of the wall temperature. Meanwhile, the quantitative research of the nonuniformity of the cross section temperature is carried out for the subcooled boiling.Experimental results indicate that no much difference is observed on the variation trends in the heat transfer coefficients between vertical and horizontal helically-coiled tube. The heat transfer coefficients of the vertical and horizontal helically coiled tube increase with the system pressure, but decrease with increases in the subcooling and helical diameter. The pitch and mass flux have a negligible effect on the subcooled boiling heat transfer coefficient of the helical coil. The correlations of heat flux at ONB and heat transfer coefficient of subcooled flow boiling in helical coil are developed by using dimensional analysis and introducing the dimensionless numbers. The results indicate that almost all of the present experimental data can be predicted within ±20%, demonstrating a good agreement with the experimental data.The visual study of the bubble behavior in the horizontal coil is carried out at water velocity ranging from 0.13?1.13ms-1 and air velocity from 0.13?1.13ms-1. To explain the observed bubble behaviors in the curved flows, force analysis is conducted. Four forces on the bubble are taken into consideration:1) drag force due to the main flow; 2) drag force due to the centrifugally-induced secondary flow; 3) net gravitational force; 4) net centrifugal force. All the forces are combined to get the total forces in tangential and radial directions. Focus is put on the relationship of the bubble departure and traveling path with the water flow and injection location. The tangential force plays a major role in detaching the bubble from the injection nozzle, and that the effect of the radial detaching force embodies at the locations of ?=0° and ?=180°. The tangential and radial forces are influenced by the water and air velocity and the angular location, which will decide the departure rate of the bubbles.After departure, the bubbles travel with the main flow, and show radial displacement between the inner and outer sides of the helical coil. In the ranges of 0°<?<90° and 270°<?<360°, bubbles travel along the inner side (?=0°) of the helical coil without displacement toward the outer inside of the coil because the radial force is negative. In the ranges of 90°<?<180° and 180°<?<270°, the radial displacement is changed when the radial force transforms between the positive and negative. As a result, the dominant factor of the bubble trajectory is transformed from gravity to centrifugal force. |
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