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Analysis of the flow and heat transfer characteristics of supercritical water (SCW) become complicated by the extensive changes in the physical properties of SCW near its pseudo-critical line and by the secondary flow stemming from the buoyancy and centrifugal force in a helically coiled tube (HCT). To address the heat transfer deterioration (HTD) of SCW in HCTs, we investigated the distribution of circumferential wall temperatures and heat transfer coefficients of SCW in a vertical HCT under the HTD condition via numerical simulation. Wall temperature was highest on the inner side, and the local heat transfer coefficient was the lowest. With increasing heat flux, the wall temperature of each circumferential position around the tube rose, while the inner wall temperature rose the most. To explain the mechanism of HTD, the rapid decrease of the specific heat and thermal conductivity near the inner side played a certain role in deteriorating the heat transfer. Further, the buoyancy effect, and the thermal acceleration effect could be used to quantitatively assess the HTD condition. The threshold values of the dimensionless number Bu describing buoyancy effect and the dimensionless number Ac describing thermal acceleration effect were found to be 3.0 × 10−6 and 1.28 × 10−6, respectively. The concept of circumferential wall temperature inhomogeneity (CWTI) was used to quantify differences in local wall temperature around the HCT. CWTI significantly increased with increasing heat flux. At low heat flux levels (q w = 300–700 kW m−2), the CWTI decreased at first, and then increases. When the heat transfer deteriorated with increasing heat flux, the trend of CWTI reversed. These results indicated that differences in circumferential wall temperature became larger under HTD conditions. Increasing pressure reduced the physical property changes in the HCT, leading to more uniform wall temperatures and a smaller CWTI. Under a constant ratio of heat flux to mass velocity, increasing the mass velocity weakens the heat transfer and even triggers HTD. In contrast, increasing pressure suppresses HTD. • Heat transfer of supercritical water under HTD condition in a coil was numerically simulated. • Severe inhomogeneity of circumferential wall temperature existed when HTD occurred. • CWTI parameter δ t was adopted to quantitatively depict CWTI. • Influencing factors on CWTI & heat transfer coefficient were explored & analysed. [ABSTRACT FROM AUTHOR]

Film boiling of magnetic nanofluid (MNF) in non-uniform magnetic fields generated by current carrying wires is numerically studied in this paper. The calculation model for the flow and heat transfer of MNF film boiling is established by incorporating both Maxwell's equation for magnetic fields and transport equations for interfaces into the basic governing equations of two-phase flows. The interface of boiling vapor film is captured by the VOSET method. Influence of the non-uniform magnetic field generated by single and multiple current carrying wires on the MNF film boiling is studied by analyzing the coupling among the magnetic field, flow field and interface distribution based on the numerical results. It is shown that the locations of the current carrying wire have a great influence on the MNF film boiling. The non-uniform magnetic field generated by current carrying wires may promote or inhibit the growth and departure of the vapor bubbles, which is determined by the distance between the current carrying wire and the top of initial vapor film. Compared with the cases without magnetic field, when the current carrying wire is near the bottom of the initial vapor film (located at x = 0, λ 0 /6, 5 λ 0 /6 and λ 0), the average wall temperature of the heated wall decreases and the average Nu number increases, resulting in enhancing the heat transfer. While the wire is below the bottom of the initial vapor film and H 0 = 10 kA m−1, the averaged Nu number increases to 2.98, which is 4.2% higher than that in the case without magnetic fields. However, when the wire is near the top of the initial vapor film (located at x = λ 0 /3, λ 0 /2 and 2 λ 0 /3), the bubble departure time increases slightly, and the averaged Nu number decreases by 1.1%. Further, the influence of the coupled magnetic field of four current carrying wires on the MNF boiling heat transfer is explored. Compared with the single wire, the coexistence of multiple current carrying wires can further promote the heat transfer performance of MNF film boiling. When the H 0 = 20 kA m−1 generated by four current carrying wires, compared with the condition without magnetic field, the average Nu number increases from 2.87 to 3.80, increasing by 32.4%. • A calculation model for the flow and heat transfer of MNF film boiling was built. • Effect of single and multiple current carrying wires on MNF boiling was studied. • The current carrying wire location has a great influence on the MNF film boiling. • The average Nu number is increased by 32.4% under four current carrying wires. [ABSTRACT FROM AUTHOR]

• Heat transfer of supercritical water with high mass velocity in a coil was experimentally and numerically studied. • Severe inhomogeneity of circumferential wall temperature existed and was dominated by the secondary flow. • A new parameter was defined to quantitatively depict the circumferential wall temperature inhomogeneity. • The circumferential wall temperature inhomogeneity reached its minimum value at pseudo-critical temperature. To explore the flow and heat transfer characteristics of supercritical fluid with high mass velocity in a helically coiled tube, experiments and numerical simulations were conducted to evaluate the wall temperature distribution and heat transfer coefficient in a vertical helically coiled tube under a pressure of 24–28 MPa, with a mass velocity of 2500–4000 kg·m−2·s−1 and with a heat flux of 210–420 kW·m−2. Results show that the coil's inner wall temperature is the highest and the outer wall temperature is the lowest because the centrifugal effect is much stronger than the buoyancy effect at a high mass velocity in a helically coiled tube and thus the fluid with lower temperature and consequently higher density shifts to the outer side of the coil. To describe the wall temperature difference in the different circumferential points, a metric called the circumferential wall temperature inhomogeneity (CWTI) is proposed, and the influence of the parameters on the CWTI is then analysed. As bulk fluid temperature increases, the CWTI first decreases to a minimum at the pseudo-critical temperature and then increases thereafter. The increase of mass velocity reduces the CWTI, and the influence of mass velocity on the CWTI is the most significant in the high enthalpy region. In the high enthalpy region with the bulk fluid temperature of 410.1 °C, when the mass velocity increases from 2500 to 3250 kg·m−2·s−1, the CWTI decreases by 34.2%. The CWTI also increases almost linearly with the increase in heat flux. Finally, the factors influencing the average heat transfer coefficient in the helical tube are discussed. The coefficient first increases and then decreases with increase in the bulk fluid temperature, and the peak value of the coefficient appears at the pseudo-critical point corresponding to the supercritical pressure. Increasing the mass velocity improves the average heat transfer coefficient, and the improvement effect is more obvious after the pseudo-critical point. Increasing the pressure causes the bulk fluid temperature at which the peak average heat transfer coefficient is achieved to increase, and the peak value gradually decreases. When the pressure increases from 24.0 MPa to 26.0 MPa and to 28.0 MPa, the peak value of the average heat transfer coefficient decreases by 34.2% and 46.4%, respectively. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer of water in a coil was experimentally investigated. • Severe inhomogeneity of circumferential wall temperature existed. • The flow pattern of two-phase boiling flow in the coil was divided into four regions. • Influencing factors on CWTI and heat transfer coefficient were explored and analysed. • A new FBHT correlation in the HCT with better accuracy was proposed. Helically coiled tubes (HCTs) have been widely used in chemical industry and nuclear engineering because of its compact structure and excellent heat transfer performance. This paper designed and built an experimental platform to study the flow boiling heat transfer (FBHT) of subcritical water with high temperature and high pressure in an HCT. The results showed that the wall temperature on the inner side is higher and it gradually decreases when moving to the outer side along the circumferential direction. The variation law of circumferential wall temperature inhomogeneity (CWTI) under different pressures, mass velocities, heat fluxes and vapour qualities was obtained. The distribution of circumferential wall temperature varies greatly in the single-phase region while it varies slightly and is relatively uniform in the two-phase boiling region. As the vapour quality increases, the CWTI decreases first, and maintains at a low value when the vapour quality is about 0.0 ∼ 0.9. When the vapour quality reaches about 0.9, the CWTI increases rapidly. When the vapour quality is less than 0.5, the heat flux and mass velocity have little effect on the CWTI. When the vapour quality is larger than 0.5, reducing the heat flux or increasing the mass velocity can effectively reduce the CWTI. In addition, the flow pattern of two-phase boiling flow in the HCT is divided into four regions, namely bubble flow, slug flow, annular flow and mist flow, and the transition vapour quality x t1 = 0.2, x t2 = 0.5 and x t3 = 0.93 are respectively selected according to the sudden abrupt change of the wall temperature or the differential pressure. Finally, the existing FBHT correlations are collected and evaluated and the calculation accuracy needs to be improved for more accurate calculation. Therefore, a new FBHT correlation with better accuracy is proposed for calculating the heat transfer coefficient of two-phase flow boiling in the HCT. [ABSTRACT FROM AUTHOR]

• A multi-view 3D measurement based on geometric constraints is employed to acquire highaccuracy depth information from different perspectives. • A cycle-positioning-based registration scheme with the establishment of the pose-informationmatched 3D standard digital model is proposed to realize rapid alignment of the measured point cloud and the standard model. • A minimum 3D distance search is driven by a dual-thread processing mechanism for simultaneous scanning and detection to quantify and locate 3D surface defects in real time. • Our technology can compare the 3D shape of the measured workpiece versus the corresponding standard 3D model in real time without any auxiliary equipment for position control, allowing for highefficiency high-accuracy surface defect detection in an extremely simple (hand-held) manner and saving lots of operational expenses on precision alignment and position-orientation adjustment. Since a slight variance in production processes can make the entire production run defective, defect inspections are indispensable procedures in manufacturing processes to ensure high quality of each item before entering the next manufacturing step. Three-dimensional (3D) optical shape measurement technologies are widely applied for surface defect inspection of complex workpieces because of its high-accuracy and digitization. However, the complex surface structure and position of the test object can pose serious challenges, making inspections still relatively slow, expensive, and complicated in implementation and maintenance. In this work, we propose a real-time 360∘ 3D surface defect inspection approach based on fringe projection profilometry without any auxiliary equipment for position control. Firstly, a multi-view 3D measurement based on geometric constraints is employed to acquire high-accuracy depth information from different perspectives. Then, a cycle-positioning-based registration scheme with the establishment of the pose-information-matched 3D standard digital model is proposed to realize rapid alignment of the measured point cloud and the standard model. Finally, a minimum 3D distance search is driven by a dual-thread processing mechanism for simultaneous scanning and detecting to quantify and locate 3D surface defects in real time. Experimental results show that our method can accurately identify the surface defects of complicated objects in real time in an extremely simple (hand-held) manner, saving a lot of operational expenses on precision alignment and position-orientation adjustment. The proposed method holds tremendous potential for quality control in many facets of industry, such as product development, testing, and manufacturing. [ABSTRACT FROM AUTHOR]