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• Cylindrical fins in compact solar receivers are optimized using adjoint method. • 3-D fins have characteristics of varied cross-section along the height direction. • Thermal-hydraulic performance of 3-D fins is improved by 13 % and 20 %. • The highest entropy generation rate of 3-D fins is reduced by 8.5 % and 16.5 %. The compact solar receiver is a promising option for S-CO 2 solar receivers, designed to operate safely and efficiently under high-temperature, high-pressure, and high solar flux conditions. Adding fins with outstanding thermal-hydraulic performance in the mini-channel contributes to maximizing heat transfer performance and minimizing flow resistance for compact solar receivers. However, most of the research on high-performance fins is limited to two-dimensional optimization, ignoring the effect of shape changes in the height direction of the fins. The present study employs a combination of the finite volume method and the adjoint method to optimize single-row cylindrical fin shapes in three-dimensional for compact solar receivers, and the strengthening mechanism of high efficiency and low resistance is further revealed in the view of entropy generation. The results indicate: (1) three-dimensional fins exhibit significant variation in cross-section along the height direction, and each fin is unique; (2) three-dimensional fins exhibit more excellent thermal-hydraulic performance than cylindrical fins, showing an increase of 13 % and 20 % in the performance evaluation criterion (PEC) in case 1 (q w = 160 kW·m−2, v in = 0.487 m·s−1) and in case 2 (q w = 584 kW·m−2, v in = 1.778 m·s−1), respectively; (3) the entropy generation due to heat transfer is reduced by 4.9% and 3.9%; as well as the entropy generation due to pressure drop is decreased by 8.5 % and 16.5 %, in case 1 and case 2, respectively; (4) robustness results show that three-dimensional fins can maintain higher thermal-hydraulic performance than the initial cylindrical fin even under wide working conditions. [ABSTRACT FROM AUTHOR]

Highlights • The numerical simulations for thermal contact resistance are based on practical surface topography of 3D C/C-SiC needled composites. • Temperature distributions between the contact interface have been studied, and the proportion of the actual contact area has been calculated. • Thermal contact resistance are in the order of 6 × 10−4–1.2 × 10−3 K·m2·W−1 in this paper. Abstract The 3D C/C-SiC composite is a kind of widely used material in thermal protection system or other functional parts of the hypersonic vehicles. This paper conducts numerical simulation to predict the thermal contact resistance of the 3D C/C-SiC needled composite pairs. The practical surface topography is measured by a 3D optical microscope named Bruker Contour GT-K, and the rough surfaces for simulation are reconstructed in ANSA software with the help of Python code. The measured arithmetical mean roughness (Ra) of the two specimens are 12.04 μm and 11.75 μm respectively. The prediction is divided into two steps, static analysis for revealing the contact spot distribution of the contact interfaces, and thermal analysis for the temperature distribution of the contact interfaces to calculate the thermal contact resistance. Both two steps are finished with Abaqus. The prediction results show that actual contact area only occupies a small part of nominal contact area (when pressure is 5.52 MPa, the actual contact area just accounts for 7% of the nominal contact area), and the dependency curve approximately shows linearity between the proportion of actual contact area to nominal contact area and loading pressure. Besides, thermal contact resistance decreases with an increase in loading pressure and temperature. Thermal contact resistances of the studied composite materials are in the order of 6 × 10−4–1.2 × 10−3 K·m2·W−1. [ABSTRACT FROM AUTHOR]

An FEM (Finite Element Method) numerical approach of predicting the effective thermal conductivities of plain woven composites is presented in this paper. Three reducing-size unit cells are formulated by using different symmetries exhibited in the composite, including translational, reflectional and rotational symmetries. Corresponding thermal boundary conditions are derived and validated by the numerical results of the same problem of different unit cells. Thermal conductivities of the matrix with porosity and the woven yarns are calculated first, and then used as input data to numerically predict the effective thermal conductivities of plain woven composite. The influences of porosity and fiber volume fraction on effective thermal conductivities of studied composites are clarified. [ABSTRACT FROM AUTHOR]

In this paper, the stagnation line heat transfer characteristics and critical heat flux (CHF) of small slot jet impingement boiling were experimentally investigated. Water with the Reynolds number range 34,013–136,054, the size of slot jet nozzle of 12 mm × 1 mm, and the non-dimensional nozzle to target spacing of 0.2 were used. Besides, heat transfer surface sizes were 5 mm × 1 mm and 1.5 mm × 1 mm. It was found that the stagnation line heat transfer characteristics of the jet impingement nucleate boiling in the fully developed region are irrelevant to jet velocity, subcooling and heater size for the slot jet impingement in the present experimental range. The heat transfer correlation of nickel foil heater for the jet impingement nucleate boiling in the fully developed region was obtained. Meanwhile, the CHF of the jet impingement boiling on the stagnation line was also studied. Using the experimental data to modify the semi-empirical and semi-theoretical correlation proposed previously for predicting the CHF, its modified form with heater size effect was also obtained. Through increasing jet velocity and subcooling while decreasing heater size, extremely critical heat fluxes (>10 8 W/m 2 ) were achieved in the experiment, and its maximum value was 1.14 × 10 8 W/m 2 . In addition to decreasing heater size to a moderate value, it is also necessary to change heater’s surface features including CA to achieve a higher heat flux. [ABSTRACT FROM AUTHOR]

• Dual-stage blowers with different subunit structure are developed. • The evolution process and flow distribution of ionic wind are explored. • Working efficiency of dual-stage blower was evaluated. • The dual-stage blower performed better in cooling. Electronics have an increased thermal flow density due to the development of miniaturization and high integration. Effective heat management is a pressing technological issue that must be addressed. Ionic wind cooling technology is gaining popularity as a means of improving local heat dissipation. Ionic wind blowers with two non-uniform 'needles-to-ring' subunits in series were proposed in the present work. The dynamic evolution process and flow distribution characteristics of the ionic wind for different combined structures were investigated. The operation and maintenance characteristics were optimized to increase ionic wind strength. Experiments are performed to explore the variability of system temperature and mean heat transfer coefficient when the combined ionic wind blowers are employed to improve cooling performance. The results reveal that by employing an additional collecting electrode, charged particles may be accelerated twice, increasing the ionic wind intensity. The dual-stage ionic wind blower with D 1 =32 mm and D 2 =22 mm has a greater mean velocity and a wider flow dispersion than the others. There will be apparent backflow near the ring electrode during the early stages of ionic wind production, but the backflow steadily lessens until it evaporates. The operating performance of a dual-stage blower is better when cooling a high-power heat source. [ABSTRACT FROM AUTHOR]

Laminar and turbulent forced convection heat transfer and flow characteristics of nanofluids in small smooth tubes are numerically simulated using two kinds of multiphase-flow models. The simulated results are compared with the experimental results from the published papers and the traditional predicting correlations to investigate the applicability of these models for nanofluids. The multiphase-flow models including mixture model and Eulerian model, and both of them belong to the well-known Euler–Euler model. The effects of various parameters such as Reynolds number and nanoparticles concentration on the heat transfer and flow characteristics are investigated and discussed in each model. The study results show that little deviation exists between the simulated results and the traditional predicting correlations for low concentration nanofluid, which indicates that low concentration nanofluid has no meaningful nano-effect on forced convection heat transfer. While, non-traditional fluid characteristics occur and increase with increasing the nanoparticles concentration, and the simulated results using special models of multiphase flow are closer to the experimental data than that of the traditional correlations, which means the multiphase flow models are more accurate than traditional correlations for high concentration nanofluid. Moreover, the numerical results also indicate that the drag coefficients of simulated results have only little difference less than 0.4% with that of experimental results for nanofluids in the laminar flow region. However, the drag coefficients of simulated results have a increase by about 60–22% compared with the experimental results in the turbulent flow region. As conclusion, the present study indicates that the two-phase models, including mixture model and Eulerian model, can predict the forced convection heat transfer and flow characteristics of nanofluid well, and have important implications for the application of nanofluid. [ABSTRACT FROM AUTHOR]

Based on analyzing a semi-analytical heat transfer model with both contact angle and surface roughness effects in the pool boiling, the present paper concluded that the heat transfer coefficient (HTC) could be expressed as a uniform function of wall superheat, solid-liquid contact angle, surface roughness, influence parameter of heating surface material and the fluid thermophysical properties. The comprehensive effects of surface characteristics are expressed for the first time in one correlation, and they are thoroughly discussed. An improved correlation which is similar to Rohsenow's correlation are proposed by the induction using the experimental data of 7 kinds of fluids (water, ethanol, CCl4, acetone, n-hexane, R113, R141b). It is verified that the proposed correlation for predicting pool boiling heat transfer can be widely applied on the common plain surfaces without capillary structures under various pressures. These surfaces include ordinary metal, metal with coating and high thermal conductive nonmetal. It has a uniform correlation factor and fixed dimensionless parameter exponent, which will not vary with changing the surface-fluid combination. Its accuracy has satisfied the general engineering application requirements, which will offer a calculating basis for heat transfer engineering application. [ABSTRACT FROM AUTHOR]

Abstract: The jet impingement boiling is the most effective heat transfer method among all the steady-state heat transfer forms. However, various researches on the jet impingement boiling still lack experimental data for heat transfer characteristics under high impact velocity and special heated surface topography. Due to this reason, the present experimental study investigates the heat transfer characteristics of high-velocity circular jet impingement boiling on the nano-characteristic surface of the stagnation zone with different surface wettability. The wettability of the copper surface is varied to hydrophilic or hydrophobic nano-characteristic surface by modifying surface topography and chemistry. The effects of impact velocity, subcooling and solid–liquid contact angle (CA) on the onset of nucleate boiling (ONB), nucleate boiling heat transfer coefficient (HTC) and critical heat flux (CHF) are studied experimentally. It is found that the effects of impact velocity, subcooling and CA on the ONB are respectively independent. Comprehensive effects of impact velocity and CA on the nucleate boiling HTC are also obtained. The proportion of nucleate boiling HTC after the ONB point gets more significant with increasing wall superheat. The HTC will increase with increasing impact velocity and CA. Some empirical correlations are proposed for predicting the effects of impact velocity, subcooling and CA on the jet boiling heat transfer characteristics. What’s more, the method for predicting the wall superheat corresponding to the CHF is discussed. The comparison results indicate the predicted value agrees well with the experimental data. [Copyright &y& Elsevier]

Abstract: An experimental study was conducted to investigate the thermal performances of a new type of copper wire-bonded flat heat pipe using water, ethanol and nanofluids as working fluids. The influences of the working fluid, the operating temperature or operating pressure, the wire diameter and the space gap between two wires on the thermal performances of the heat pipe were investigated. Experimental results show that the wire diameter has an important effect on the heat transfer coefficient but has hardly impact on the maximum heat flux. The space gap between two wires not only has an important effect on the heat transfer coefficient but also has a significant impact on the maximum heat flux. The maximum heat power increases apparently with increasing the operating temperature. Using nanofluid as working fluid can improve the heat transfer performance of the heat pipe; the best heat transfer performance of heat pipe is achieved at the concentration of 1.0wt.% under different saturation temperature conditions with different nanofluid; nanofluid is a kind of new working fluid to the copper wire-bonded flat heat pipe. [Copyright &y& Elsevier]

Abstract: A new water-based SiO2 functionalized nanofluid was used in a loop thermosyphon as the working fluid. Functionalized nanofluid has unique dispersing ability and has no deposition on heated surfaces. The flow evaporating heat transfer characteristics of functionalized nanofluid in a loop thermosyphon were investigated to understand the sole effect of thermal properties of nanofluid on the heat transfer characteristics of the loop thermosyphon, regardless of the effects of heated surface characteristics. Results show that: functionalized nanofluid deteriorates both the evaporating heat transfer coefficient and the maximum heat flux of the loop thermosyphon. The changes in the thermal properties of functionalized nanofluid mainly result in the heat transfer deterioration. No especial nano-scale effect is found in the present study. [Copyright &y& Elsevier]

• A compact jet receiver based on jet heat transfer is proposed. • The heat transfer and thermal stress properties of the jet channel were investigated. • This study examines the impact of channel structural parameters on performance. The compact solar receiver can withstand high temperature and high pressure, which provides promising solutions for direct supercritical CO 2 (S-CO 2) solar receiver applications. However, it still suffers from local high temperature and large thermal stress due to the non-uniform heating boundary condition. The impinging jet method can significantly enhance the local heat transfer coefficient, holding significant potential in reducing the local high temperature of the compact S-CO 2 receiver. Inspired by this, a novel design for a compact S-CO 2 receiver using the impinging jet was proposed in this work. The heat transfer performance and stress behavior of the jet channel within the receiver were numerically investigated. The results showed that the jet channel has a significantly higher heat transfer performance than the rectangular channel, and the shorter flow path of the jet channel also effectively reduces the pressure loss. Meanwhile, the jet pattern significantly enhances the temperature uniformity, thereby reducing the thermal stress. In addition, increasing the channel aspect ratio improves the thermo-hydraulic performance, but increase the stress at the same time. The increase in the number of jet holes and the jet hole diameters not only enhance the thermo-hydraulic performance, but also reduce the stress of the jet channel. [ABSTRACT FROM AUTHOR]

• A novel topology optimization approach based on the Lyapunov equation is proposed for transient heat conduction, which avoids transient analysis when evaluating transient temperature responses. • A new topology optimization formulation is introduced, including a modified governing equation for transient heat conduction and a new objective function. • The proper orthogonal decomposition (POD) method is adopted to reduce the size of the analysis model, further improving the efficiency of the optimization process. • Both 2D and 3D models are considered to validate the method's effectiveness. The results of the point-to-point heat conduction example indicate that the optimized topology design successfully reduce the heating time by approximately 35%. This study presents a novel topology optimization approach for transient heat conduction during the transition period based on the Lyapunov equation. For the problem under consideration, traditional topology optimization methods face computational challenges due to the time dependence of the transient responses. To address this issue, this article introduces a new optimization formulation that contains a modified governing equation for transient heat conduction and a new objective function. The new formulation aims to simplify the evaluation of transient response performance by utilizing the Lyapunov equation, thereby avoiding time-consuming transient analysis during the optimization iteration process. Additionally, this study employs model reduction techniques to reduce the size of the analysis model and further enhance the efficiency of the optimization process. After comparing two classic methods, the Proper Orthogonal Decomposition (POD) method is selected for its higher efficiency. Numerical examples consider both heat conduction and temperature control problems, employing 2D and 3D models to validate the proposed method's effectiveness. The results of the point-to-point heat conduction example indicate that, compared to the conventional design, the optimized topology design obtained using the proposed method successfully reduce the heating time by approximately 35%. Furthermore, the optimized designs display clear distinguishing features from conventional designs, further emphasizing the significance of implementing transient heat conduction topology optimization. [ABSTRACT FROM AUTHOR]

• Proposed a dual-iterative thermal-mechanical coupling method (DICM). • Thermal expansion effect on TCR prediction is analyzed. • DICM is recommended for simulation contact pairs with axisymmetric geometry. This study investigates the effect of thermal expansion on thermal contact resistance prediction, proposing a dual-iterative coupling method (DICM). The contact surfaces in the simulation model are reconstructed based on either the actual measured topography or the hypothetical topography, and a mathematical formulation for numerically predicting the thermal contact resistance (TCR) is established. The DICM includes four steps: first, mechanical analysis is conducted based on the ideal single point contact condition, according to the elastic–plastic constitutive equations. Second, heat transfer analysis is carried out based on the deformed geometry originating from the prior mechanical analysis. Third, another step of mechanical analysis is implemented to consider the effect of thermal expansion with the temperature distribution determined in the second step. Fourth, another step of heat transfer analysis is carried out based on the deformed geometry originating from the second-step mechanical analysis. The conventional prediction method only contains the first and second steps, and is known as the single sequential coupling method (SSCM). The TCRs of two engineering examples are predicted using both DICM and SSIM. The results show that the mechanical–thermal-mechanical–thermal dual-iterative coupling method, i.e., DICM, should be recommended for simulating contact pairs with axisymmetric geometries, while SSCM is suggested for contact pairs with non-axisymmetric geometry. [ABSTRACT FROM AUTHOR]

• The impacts of buoyancy and flow acceleration on S-CO 2 heat transfer are argued. • A threshold value was identified for buoyancy and thermal acceleration. • New empirical formulas for S-CO 2 heat transfer are proposed. The mini-channel in compact solar receiver deliver the high performance in both heat transfer (HT) and pressure resistance. However, the severe variation of thermophysical properties of supercritical CO 2 (S-CO 2) coupling with the complex nonuniform distribution of solar flux leads to the abnormal flow and heat transfer (FHT) performance of S-CO 2 in mini-channels. The present work numerically investigated the detailed FHT of S-CO 2 in the horizontal mini-channel with one-side heating. The impacts of buoyancy and flow acceleration on FHT of S-CO 2 are demonstrated in detail. The results show that the buoyancy effects can lead to the enhancement on circumferentially average heat transfer performance of S-CO 2 while the local deterioration at the top. In contrast, the thermal acceleration effects can worsen the S-CO 2 HT. The buoyancy and thermal acceleration effects can be ignored when Ri ≤ 2.7 × 10−3 and Ac ≤ 7.8 × 10−8, respectively. The empirical formula for S-CO 2 HT in the mini-channel under single-side heating conditions is developed, with a maximum error of 4.5%. [ABSTRACT FROM AUTHOR]

Highlights • The natural heat transfer of nanofluids was investigated under electric field. • The electric field forces can reduce a strong electric convection. • In the absence of gravity, the EHD effect on heat transfer is significant. • In the existence of gravity, the EHD effect on heat transfer is limited. Abstract Natural convective heat transfer of nanofluid consisted of transformer oil and Al 2 O 3 nanoparticles in a cavity under a non-uniform high electric field intensity was experimental investigated. The main objective is looking for a coupling enhanced method by nanofluid and electric field. It is found that when the effect of gravity is ignored, the electric field forces applied on the working fluids induce an electric convection and thence induce significant heat transfer enhancement. This enhancement effect increases rapidly with the increase of electric field intensity, the nanoparticles mass concentration of nanofluid and the heat flux. When the effect of gravity exists, a coupled convection is caused by both gravity and electric field force together. For pure transformer oil, the influences of electric field force and gravity on the heat transfer almost offset each other. However, for nanofluid, the impact of the electric filed force is dominant. The study results revealed the enhanced mechanism of natural convection heat transfer of nanofluid under electric field and provided some useful technical support for practical application. [ABSTRACT FROM AUTHOR]

Highlights • Interfacial wave and local heat transfer coefficient of steam jet condensation. • Empirical correlation for predicting the dimensionless wave amplitude. • Interfacial fluctuation promotes steam jet condensation heat transfer coefficients. Abstract An experimental study on the interfacial wave and local heat transfer coefficient of stable steam jet condensation in a rectangular mix chamber was carried out. In experiments, the interfacial wave amplitude was obtained and analyzed, and an empirical correlation predicting the dimensionless wave amplitude was established. To evaluate the local average heat transfer coefficient, a thermal equilibrium model was developed, and the local average heat transfer characteristics were obtained and discussed. The results indicate that the interfacial wave amplitude increases with water mass flux and the axial dimensionless position, decreases with the steam mass flux, and changes little with inlet water temperature. The local average heat transfer coefficients are within the range of 0.98–6.24 MW/m2 °C, and they increase with the interfacial wave amplitude. The interfacial fluctuation could promote the steam jet condensation and corresponding heat transfer coefficients. [ABSTRACT FROM AUTHOR]

Nanofluid has attracted the attention of many scholars for the excellent heat transfer enhancement capacity. In addition, electric field can further strengthen the nanofluid’s ability of heat transfer. The thermal conductivity enhancement rate of nanofluid under electric field was studied using transient hot wire method under different electric field intensities, temperatures and particle concentrations. The influences of those parameters have been discussed. Besides, based on the static thermal conductivity and the dynamic heat conduction coefficient, a novel prediction model was established, which had a good agreement with experiment data. The mechanism of thermal conductivity enhancement under electric field was analyzed according to the prediction model. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations of the convection heat transfer of n-decane at supercritical pressures in vertical tubes with inner diameters of 0.95 and 2.00 mm for various inlet pressures, heat fluxes and flow directions. The effects of the heat fluxes, property variations, buoyancy and flow acceleration on the convection heat transfer were studied. The results show that for high inlet Reynolds numbers neither buoyancy nor flow acceleration affects heat transfer significantly, while for low inlet Reynolds numbers buoyancy strongly reduces heat transfer coefficient in upward flow cases. The threshold for Bo ∗ was found to be about 2.0 × 10 −7 . Two local Nusselt number correlations for the convection heat transfer of supercritical pressures n-decane with and without buoyancy effect were developed based on the experimental data. [ABSTRACT FROM AUTHOR]

• The influence of hydrodynamics on biomass growth in PBR is theoretical analyzed. • The particle-averaged and volume-averaged growth rates are equal when the SDF is included. • Fluid kinematics shows that the average SDF should be uniform for single-phase flows. • The effect of hydrodynamics on the growth is very limited when the PBR is well-mixed. • CFD with Lagrangian tracking can have artificial particle accumulation near walls. This work presents a theoretical analysis for understanding the influence of hydrodynamics on the growth of microalgae in the photobioreactor (PBR). In the analysis, it is assumed that the growth rate is determined by the instantaneous light intensity and the light distribution is not influenced by the hydrodynamics. It is proved that the average growth rate over the tracked particles and the volume-averaged growth rate are the same if the average spatial distribution function (SDF) of the particles is included. In addition, the fluid kinematics shows that the SDF is related to the Jacobian or volumetric change ratio of the particle trajectories. For single-phase flows, the SDF should be uniform. The analysis justifies the conclusion that if the PBR is well-mixed and the dynamic growth kinetics are not included, the growth rate is only influenced by the light condition and the geometry of the PBR, but is not influenced by the hydrodynamics. The theoretical analysis is validated by the CFD simulations of the average growth rates of Taylor vortex PBR with single-phase flows, bubble column PBR and airlift PBRs with two-phase flows. It is also found that the particles tend to accumulate near the wall or boundary regions in the Lagrangian particle tracking method. This artificial effect is contrary to the theoretical analysis and can generate errors when predicting the growth rate. Therefore, further validations and examinations are needed for the numerical models. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of transformer oil natural convection in a cavity with a linear electrode were investigated by experiment and numerical simulation. The coupled physical and mathematical models of electric field, flow field and temperature field were established by the lattice Boltzmann method. The experimental results are in good agreement with the numerical simulation. In the absence of gravity, the enhancement effect of electric field on heat transfer is significant, and increases with the increase of voltage and heat flux. As to coupled convection caused by gravity and electric field force together, the electric field force destructs the natural convection caused by gravity, so applying an electric field at a low voltage will degrade heat transfer performance; as the voltage increases, heat transfer gradually becomes intensive, but the effect is limited. The study results revealed the enhancing mechanism of natural convection heat transfer under electric field and provided useful technical support for practical application. [ABSTRACT FROM AUTHOR]

An experimental study on the interfacial behavior of stable steam jet condensation in a rectangular mix chamber was conducted. By high-speed imaging and image processing procedure, the interfacial wave was observed and discussed. From correlation analysis, interfacial wave velocity was obtained, and an empirical correlation for predicting the interfacial wave velocity was also established. Besides, interfacial fluctuation characteristics and corresponding mechanism were also analyzed. The results indicated that, interfacial wave observed in present work was proved to be a manifestation of K-H instability, and the interfacial wave velocities, which were in the range of 25.1–130.4 m/s, were proportional to steam plume penetration length, while inversely proportional to axial position. High frequency component of the interfacial height fluctuation was induced by the interfacial wave, while low frequency component was caused by waving of the whole interface. The results in present work would be significant to understanding the interfacial behavior of steam jet condensation and would be useful for a better design and safe operation on the TFSI. [ABSTRACT FROM AUTHOR]

Though direct contact condensation (DCC) has been widely used in many industrial fields, the noise and vibration caused by pressure oscillation becomes an unignorable problem during the operating of related equipments. So there is an urgent need to find methods to reduce noise and vibration. In the present work, experimental study was conducted on the dynamic of steam-air mixture condensation in water flow in a confined channel, expecting to reduce the pressure oscillation intensity by means of adding a proper amount of air into steam. The experiments were tested in stable flow pattern region with steam mass flux, water Reynolds number, water temperature and air mass fraction in the range of 350–600 kg/m 2 s, 78,000–164,000, 30 °C and 0–5%, respectively. The observation and image processing of interface behavior and flow pattern transitions provides insight into the pressure oscillation mechanism with non-condensable gas involved. Dynamic pressures on the wall of channel were obtained. The results showed that the DCC of steam-air mixture was determined by two important factors, e.g. gas velocity at nozzle outlet and air content. When gas had a higher velocity, the air layer around the interface would be destroyed so that the effect of air on condensation would be weakened, but the interface was relatively stable. With air content increased to a critical value, stable flow patterns would transform into unstable due to the increase of air quantity. Moreover, the flow pattern transitions could be reflected also by the variation of pressure oscillation intensity. When air content was lower than the critical value, the gathering of air around the interface decreased the fluctuation of interface penetration length, which resulted in the dramatic decrease of pressure oscillation intensity. However, with air content increased to be higher than the critical value, stable jet would transform into unstable, leading to a rapid increase of pressure oscillation intensity. In addition, the results of various test conditions showed that the critical air mass fraction increased with the increase of steam mass flux but decreased with the increase of water Reynolds number. Furthermore, an optimal air mass fraction which could minimize the pressure oscillation intensity under various test conditions was suggested. [ABSTRACT FROM AUTHOR]

· Shield is a feasible way to improve the bypass effect and enhance the heat transfer of heat sink. · Experimental and numerical studies of fin array with wedge shield installed in the top bypass are proposed. · Shield inclination angle has a significant effect on the thermal and hydraulic performance. · The maximum enhancement of heat transfer is about 12% and 16% under identical pressure drop and pumping power. In this study, the thermal and hydraulic performances of fin array with a wedge shield installed in the bypass to alter the flow were experimentally and numerically investigated. The effect of the wedge shield inclination angle was explored at Reynolds numbers ranging from 2753 to 20,825. The mean Nusselt number and friction factor were measured in a wind tunnel with the varying volume rate of inlet air flow. The comprehensive performance evaluation criteria under identical pressure drop and identical pumping power were adopted to evaluate the effect of the wedge shield. The velocity, temperature and pressure fields, and the local Nusselt number were characterized using ANSYS Fluent. The results indicated that the heat transfer enhanced and the flow resistance increased by the wedge shield effect. At a same Reynolds number, the mean Nusselt number and friction factor reach their maximum values at θ S = 10 ∘ and θ S = 60 ∘ , respectively. According to the comprehensive performance evaluation, the applicable condition of wedge shield to enhance fin array performance was the inclination angle θ S = 135 ∘ at Reynolds number Re =3286, in which the enhancement was approximately 9% under identical pressure drop and 14% under identical pumping power. The numerical results showed that the velocity increase in fin channel and recirculation zone, caused by the blockage of the wedge shield, was the fundamental mechanism of wedge shield to enhance the heat transfer and increase the flow resistance. The velocity increase and recirculation zone, along with the low temperature region and high pressure in the entrance region of the fin channel caused by them, were all affected by the wedge shield inclination angle. [ABSTRACT FROM AUTHOR]

An experimental study on the direct contact condensation (DCC) of steam jet in subcooled water flow in a rectangular mix chamber was conducted. A high-speed camera was employed to record the flow field of DCC in the visualized experimental rig, in which, the steam nozzle and water nozzle were both rectangular. The flow field was filmed when the steam mass flux, water mass flux and water temperature were in the range of 200–600 kg/m 2 s, 6000–18000 kg/m 2 s and 293–333 K, respectively. The results indicated that, the flow field of DCC in the rectangular mix chamber consisted of four regions, and they were steam region, interface, mixture layer and ambient water region. The interface between steam region and mixture layer was clearly observed. The flow pattern of DCC was performed as bubble flow, oscillatory jet, stable jet and divergent jet at various test conditions. The steam region dimensionless penetration length and average heat transfer coefficient of the stable jet were found to be in the range of 1.73–4.40 and 2.89–7.89 MW/m 2 K, respectively. Furthermore, an analytical model was established and a correctional correlation was developed to predict the dimensionless penetration length. An empirical correlation was proposed to predict the average heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Abstract: The heat transfer performances of the fin array with shield in the bypass had been experimentally investigated. The variations of the mean Nusselt number and friction factor were obtained under varied parameters, such as the heated power input, the Reynolds number, the position and height of the shield. The optimum position and height of shield were analyzed under the identical pressure drop and the identical pumping power criteria. Experimental results indicated that the heat transfer and flow performances of fin array were marginally influenced by the power input. As the presence of the shield, the heat transfer of fin array was enhanced and the flow resistance increased. The heat transfer coefficient increased as the dimensionless shield position decreased and the shield height increased. The friction factor increased with the dimensionless position and height of shield. Evaluated under the identical pressure drop and the identical pumping power criteria, the optimum position of shield was the position where the dimensionless shield position equaled 0, and the optimum shield height was half of the fin height. Finally, the correlations of mean Nusselt number and friction factor were fitted considering the influences of Reynolds number, the shield position and the shield height. The predicted values fitted the experimental data with the deviations less than ±15.0%. [Copyright &y& Elsevier]

Abstract: In this paper, a three-dimensional numerical heat transfer analysis has been performed in order to obtain the temperature distribution and the fin efficiency using the experimentally determined local heat transfer coefficients from the naphthalene sublimation technique and heat and mass transfer analogy. The influences of the fin material, fin thickness, and transversal tube pitch on the fin efficiency are studied for flat tube bank fin heat exchangers. The fin efficiency, obtained by a numerical method using the averaged heat transfer coefficient, is compared with that using the local heat transfer coefficient. The reliability of the generally used formula for fin efficiency is tested also, and then a modified fin efficiency formula with a new equivalent fin height is provided. The results show that the difference between the fin efficiency obtained by the numerical method using the local heat transfer coefficient and the fin efficiency using the averaged heat transfer coefficient is small, but the fin efficiency obtained by the generally used formula is lower than that obtained by the numerical method using the local heat transfer coefficient; the fin efficiency obtained by the modified formula matches very well with the fin efficiency obtained by the numerical method using the local heat transfer coefficient. The modified formula for the fin efficiency calculation is more reliable, and it can be applied directly to the design of a flat tube bank fin heat exchanger and also will be useful in engineering applications. [Copyright &y& Elsevier]