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The high temperature receivers in CSP plants should startup frequently, so the thermal stress and the fatigue failure are critical important for its safety and stability. In our present study, a computation model is proposed for a water/steam cavity receiver in CSP plant. The model couples the light propagation, the light-heat conversion, the thermo-elasticity and the thermal fatigue analysis together. Monte Carlo ray tracing method (MCRT), finite volume method (FVM) and boiling heat transfer are coupled to get the thermal boundary of the boiling panel. Finite element method (FEM) is adopted to obtain the temperature and stress–strain distribution of the boiling panel. Elastic stress analysis and equivalent stress fatigue assessment method is used to investigate the fatigue failure of the boiling panel. Two different structures of the boiling panel are researched, following results are obtained: The heat flux on the boiling panel is highly non-uniform and it results in the similar non-uniform distributions of temperature and stress-strain as well as the displacement of the boiling panel. The non-uniform stress-strain leads to the warping of the boiling panel and the largest displacement is in the direction normal to the heat absorbing surface toward the cavity internal, the largest displacement range is 17 cm. Fins of the boiling panel at the elbow region (case-1 in the present study) can lead to hot spots of which the temperature is extremely higher than that of boiling tubes, cutting off the fins at the elbow region (case-2 in the present study) can eliminate the hot spots and has no impact on temperature distribution of the boiling panel’s main part. The temperature difference on the cross section of the boiling tube is very high (about 130 °C while the heat flux is about 350 kW/m 2 ) both in radial and circumferential direction, and this high temperature difference results in the high thermal stress on the boiling panel and the maximum thermal stress occurs at the welding region. The welding quality has a significant impact on the fatigue failure of the boiling panel. For a receiver design life of 20 years, membrane wall with cutting off the fins at the elbow region (case-2 in the present study) is a good design and the welding quality level 3 should be ensured at least and only one cold startup for a day is permitted. [ABSTRACT FROM AUTHOR]

• Bed-to-tube heat transfer in fluidized bed with mixed-size particles are simulated. • Average HTC increases nonlinearly with the proportion of small-size component. • Local HTC is improved by enlarging the size difference between components. • A modified heat transfer correlation for complex particle systems is proposed. Mixed-size particles with low processing cost have great potential to act as the absorbing media in the next-generation concentrated solar power plants. However, the research on the heat transfer from the particles to heated surfaces in gas–solid fluidized beds is limited. In this paper, the bed-to-tube heat transfer coefficient (HTC) in a shallow fluidized bed with immersed tubes is determined numerically. The Eulerian–Eulerian approach that incorporated the kinetic theory of granular flow has been implemented in a two-dimensional model. The distribution of the local HTC around the tube is investigated for various particle systems. The results show that the HTC reaches a maximum at the upper right (45°) and upper left (135°) of the tube, respectively, indicating a strong relation between the local HTC and solid volume fraction (SVF). The average HTC increases as the mean particle size gets smaller. For monosized particles, the HTC increases from 925 W/(m2·K) to 3033 W/(m2·K) as the particle size decreases from 300 to 100 µm. For binary particles, the growth in the HTC is nonlinear, with an increase in growth rate being observed as the small-size component becomes dominant. For mixed-size particles with the same mean size, a higher HTC can be achieved by enlarging the size difference between the components. A modified correlation for predicting the bed-to-tube HTC is established. Besides, the impacts of gas velocity and temperature, and heat radiation on the bed-to-tube HTC are discussed. It is found that an optimal gas velocity exists to maximize the average HTC and SVF simultaneously, while the gas temperature has a minor contribution. Due to the small heat transfer temperature difference, the effect of thermal radiation on the HTC can be neglected even though the bed temperature reaches 800 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microfluidic strategy for continuous and high-throughput synthesis of nanofluids. • Flexible preparation of boehmite nanofluids with controllable morphology and sizes. • Effect of morphology on heat transfer performance was systematically explored. • CHF and HTC of sphere nanofluids were increased by 24% and 14.4%, respectively. • Enhanced heat transfer mechanism was analyzed via bubble image processing. Enhanced boiling heat transfer through nanofluids represents a significant approach to improving the cooling performance of electronic devices. However, the preparation of nanofluids poses challenges regarding production efficiency, morphology control, and structural stability. In addition, the underlying mechanism of flow boiling heat transfer enhancement of nanofluids, particularly changes in vapor behavior, remains a limited understanding. This study presents a facile and straightforward microfluidic strategy for the controllable synthesis of nanofluids toward efficient heat transfer enhancement. The microfluidic flow synthesis approach effectively enhances mixing efficiency at various flow rates, enabling continuous and high-throughput synthesis of boehmite nanofluids with different morphologies. The synthesized nanofluids exhibit long-term stability (>50 days) at room temperature. Both rod- and sphere-shaped nanofluids under equivalent concentrations can significantly improve critical heat flux (CHF) and maximum heat transfer coefficient (HTC) across various operating parameters, but with similar enhancement characteristics. Analysis of the period of bubble growth and burst, bubble size distributions, number of bubbles, and vapor phase fractions reveals that rod- and sphere-shaped nanofluids could shorten the period, enhance surface wetting, increase nucleation sites, and reduce bubble size efficiently and similarly. Furthermore, the effect of the concentration of nanofluid on phase-change heat transfer performance is systematically explored. 0.01 wt% nanofluid behaves the most remarkable enhancement, CHF and maximum HTC are improved by 24 % and 14.4 %, respectively. These findings not only shed new light on the controllable synthesis of functional nanomaterials but also provide important guidelines for the rational design of nanofluids for efficient two-phase cooling of electronic devices. [ABSTRACT FROM AUTHOR]

• An experimental investigation of flow boiling on micro-pin-fined surfaces in a horizontal narrow microchannel was performed. • The CHF and HTC were enhanced up to 51% and 240%, respectively. • The bubble behaviors were captured to explain the heat transfer enhancement mechanism. • A new correlation was proposed to predict flow boiling HTC in the microchannel by considering the micro-pin-fins. In this paper, an experimental investigation of flow boiling in a horizontal narrow microchannel with hydraulic diameter of D h = 952 μm was conducted. Flow boiling heat transfer characteristics of micro-pin-finned surfaces were evaluated over the mass fluxes range from 200 - 500 kg/m2s (corresponding flow velocity: 0.2 - 0.5 m/s), using deionized water as the working fluid with different inlet temperatures T in = 30 - 50 °C. The outlet pressure of the microchannel was about 1 atm. The boiling curves, heat transfer coefficients, bubble behaviors and heat transfer enhancement mechanism were discussed with the variation of heat flux, mass flux and inlet temperature. The results indicated that the micro-pin-finned surfaces could improve the critical heat flux (CHF) and heat transfer coefficient (HTC) greatly due to numerous nucleate sites and large heat transfer area enhancement ratio. The CHF up to 360 W/cm2 has been demonstrated at a mass flux of 500 kg/m2s. Compared with a smooth surface, a 240% higher heat transfer coefficient (18.4 W/cm2K) on the S30-120 surface has been achieved at a mass flux of 500 kg/m2s with an inlet temperature of 30 °C. The boiling curves of the micro-pin-finned surfaces presented an obvious "hook back" phenomenon after reaching the onset of the nucleate boiling (ONB) and then the wall temperature had a slight rise with the increase of the heat flux. Moreover, the intensive micro-pin-finned arrangements showed a significant wicking effect and promoted liquid replenishment, which could destroy the liquid boundary and enhance turbulence. The bubble behaviors were also captured to explain the heat transfer enhancement mechanism in a horizontal narrow microchannel. [ABSTRACT FROM AUTHOR]

Abstract: Experiments were conducted to study the subcooled flow boiling heat transfer performance of FC-72 over silicon chips. For boiling heat transfer enhancement, two kinds of micro-pin-fins having fin thickness of 50 μm and fin heights of 60 and 120 μm, respectively, were fabricated on the silicon chip surface with the dry etching technique. The fin pitch was twice the fin thickness. The experiments were conducted at the fluid velocities of 0.5, 1 and 2 m/s and the liquid subcoolings of 15, 25 and 35 K. The micro-pin-finned surfaces showed a sharp increase in heat flux with increasing wall superheat and a large heat transfer enhancement compared to a smooth surface. The nucleate flow boiling curves for the two micro-pin-finned surfaces collapsed to one line showing insensitivity to fluid velocity and subcooling, while the critical heat flux values increased with fluid velocity and subcooling. The micro-pin-finned surface with a larger fin height of 120 μm provided a better flow boiling heat transfer performance and a maximum critical heat flux of 145 W/cm2. The wall temperature at the critical heat flux for the micro-pin-finned surfaces was less than 85 °C for the reliable operation of LSI chips. [Copyright &y& Elsevier]

Abstract: For the purpose of cooling electronic components with high heat flux efficiently, some experiments were conducted to study the flow boiling heat transfer performance of FC-72 on silicon chips. Micro-pin-fins were fabricated on the chip surface using a dry etching technique to enhance boiling heat transfer. Three different fluid velocities (0.5, 1 and 2m/s) and three different liquid subcoolings (15, 25 and 35K) were performed, respectively. A smooth chip (chip S) and four micro-pin-finned chips with the same fin thickness of 30μm and different fin heights of 60μm (chip PF30–60) and 120μm (chip PF30–120), respectively, were tested. All the micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth one, and the critical heat flux increases in the order of chip S, PF30–60 and PF30–120. For a lower ratio of fin height to fin pitch and/or higher fluid velocity, the fluid velocity has a positive effect on the nucleate boiling curves for the micro-pin-finned surfaces. At the velocities lower than 1m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat, and the wall temperature at the critical heat flux (CHF) is less than the upper limit, 85°C, for the reliable operation of LSI chips. The CHF values for all surfaces increase with fluid velocity and subcooling. The maximum CHF can reach nearly 150W/cm2 for chip PF30–120 at the fluid velocity of 2m/s and the liquid subcooling of 35K. [Copyright &y& Elsevier]