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Parabolic trough collectors are essential components of solar thermal power plants, and the non-uniform heat flux on the walls may lead to low heat transfer coefficients and large wall temperature differences. A novel jet structure is proposed in this paper to explore the feasibility of adopting impinging jets to improve the heat transfer performance of the collector tube with supercritical carbon dioxide (S-CO2) as the working fluid. The physical model is built based on several assumptions, and numerical simulations are performed under the mass flow rate of 0.25–0.75 kg/s and the average surface heat flux of 14.7–33 kW/m2. Firstly, performance comparisons are made between the jet and tube-in-tube structures. The results show that the average heat transfer coefficient h of the jet structure is 46.5% higher than that of the tube-in-tube structure and the corresponding average temperature difference ΔT between the wall and S-CO2 is 31.7% lower. Secondly, the effects of the jet hole circumferential position and diameter on heat transfer are investigated. It's found that by adjusting the circumferential position of the jet hole from 60° to 120° and matching the impingement area of the jets and the high heat flux region, h can be increased by 14.2% and ΔT can be reduced by 12.4%. h is also found to be increased by 77.4% when the jet hole diameter is reduced from D=9.7 mm to D=5 mm. Furthermore, the jet structure is further modified by incorporating it with the eccentric configuration. The effect of eccentricity e is studied in detail, and results show that appropriate eccentricity results in higher h and smaller ΔT due to the decreased impingement distance and the increased heat transfer. This study can guide the design and optimization of parabolic trough collectors. [ABSTRACT FROM AUTHOR]

The present research delves into the intricate dynamics of Darcy-Forchheimer radiative magnetohydrodynamic flow of a Casson ternary hybrid nanofluid through a porous exponentially stretching cylinder, exploring the influence of several pivotal parameters on velocity, thermal, and mass profiles, as well as the rate of shear stress, heat, and mass transfer rates. Arrhenius activation energy and non-uniform heat flux are examined. By employing similarity conversions, the governing PDEs transformed into nonlinear ODEs. These are subsequently addressed computationally through the bvp4c technique within MATLAB for resolution. Physical factors, including the Casson fluid parameter, Darcy-Forchheimer parameter, magnetic field parameter, porosity parameter, radiation parameter, non-uniform heat sink/source parameters, chemical reaction parameter, and Arrhenius activation energy parameter, are analyzed. Significant findings from the investigation are plotted through graphs and tables. The study reveals that the introduction of the Darcy porous effect alters flow characteristics by decreasing velocity while increasing thermal and mass profiles. Additionally, the magnetic, Casson, and porosity parameters collectively reduce velocity but enhance the temperature and concentration curves, while radiation and non-uniform heat flux independently elevate temperature. The Casson fluid parameter decreases shear stress and heat/mass transfer rates, whereas Arrhenius activation energy boosts concentration, and the chemical reaction parameter diminishes it. It has been found that the Casson hybrid nanofluid exhibits an 11% increase in absolute skin friction compared to the Casson nanofluid. However, this increase sharply rises to 52.5% in the case of Casson ternary hybrid nanofluid when compared to Casson nanofluid. Similarly, the heat and mass transfer rates experience significant enhancements of approximately 15.5% and 8%, respectively, in Casson ternary hybrid nanofluid compared to Casson nanofluid. This model's practical utility lies in predicting impact flow behavior and transfer rates, aiding in optimizing designs for various industrial and engineering applications, like energy systems, or material processing. [ABSTRACT FROM AUTHOR]

The performance of the newly designed parabolic trough solar thermal collector system has been investigated experimentally in the current work. The receiver tube is placed below the focal axis, and a secondary reflector is placed over it, as specified in the design. The primary goal of this experiment is to investigate the effect of a secondary reflector intended for uniform heat flux distribution on the performance enhancement of a parabolic trough solar thermal collector under various flow rates and outdoor weather conditions. The experiments are carried out by using Therminol55 as the working fluid. Effects of installing a secondary reflector under actual outdoor weather conditions and for different flow rates of 2.0 L min−1, 5.0 L min−1, and 7.5 L min−1 have been evaluated and the amounts of outlet fluid temperature (To), useful heat gain (Q), and thermal efficiencies (ηth) have been reported. It is observed that the system performance with the secondary reflector is better than the conventional solar collector. It has been found that the efficiency of the solar collector is increased by 6.79% with the use of the secondary reflector. Using the secondary reflector, the average working fluid outlet temperature is observed as 461.35 K, which is 19.71% higher than the conventional PTC. The levelized Cost of Heat is 11.74 Cents/kWh according to the economic analysis, significantly lower than those found in literature. The experimental results and low cost of the system demonstrate that it can be used in the desired application, both technically and economically. [ABSTRACT FROM AUTHOR]

To enhance the performance of the parabolic trough solar collector (PTSC) and mitigate the circumferential temperature gradient within the absorber tube, a numerical simulation investigation was conducted involving the implementation of vortex generators. The vortex generator was composed of a ring belt and four ribs, and four types of vortex generators M1, M2, M3 and M4 were proposed. The flow and heat transfer of the absorber tube were analyzed at Reynolds numbers ranging from 10000 to 35000 and inlet temperatures between 373.15 K and 473.15 K, using the shear stress transport (SST) k-ω turbulence model. The results show that the four kinds of vortex generators—M1, M2, M3 and M4—can induce longitudinal vortices in the heat transfer fluid (HTF) within the absorber tube. These longitudinal vortices diminish the thermal boundary layer's thickness, expedite the mixing of cold and hot fluids, and enhance the overall heat transfer efficiency. Vortex generators M1 and M3 arranged in the reverse direction produce two pairs of reverse rotating longitudinal vortices, while vortex generators M2 and M4 arranged in the same direction form one pair of longitudinal vortices, so the flow resistance in M1 and M3 is slightly larger than M2 and M4. However, from the perspective of heat transfer enhancement effect and comprehensive performance, the vortex generator M3 has the best effect. In M3, the Nu/Nu0 reached the highest value of 1.238, and the performance evaluation criteria (PEC) for M3 exceeded 1, reaching a maximum of 1.11. [ABSTRACT FROM AUTHOR]

This study delves into the intricate dynamics of magnetohydrodynamic (MHD) flow, investigating the behaviour of a TiO2–Cu/H2O Jeffrey hybrid nanofluid cascading over a porous exponentially stretching surface. Employing MATLAB's bvp4c solver, we address a complex interplay of factors including aligned magnetic fields, radiation effects, non-uniform thermal fluxes, and the influence of Arrhenius activation energy. Our findings reveal a nuanced landscape wherein heightened Jeffrey fluid parameters amplify thermal and concentration gradients, yet diminish heat and mass transfer rates. Interestingly, variations in the Deborah number yield subtle effects, enhancing thermal and concentration profiles while moderating transfer rates. Augmented magnetic and porosity parameters deepen profiles but reduce velocity and transfer rates. Notably, magnetic field alignment reduces velocity while accentuating thermal and concentration gradients. Furthermore, non-uniform thermal fluxes elevate temperatures but restrain heat transfer rates. Activation energy augments concentration profiles distinctly from chemical reaction parameters. The comparison of Jeffrey hybrid nanofluid and its conventional counterpart reveals intriguing disparities: the former exhibits a notable 30 % increase in absolute skin friction while increasing heat transfer rate by 4.5 %. These insights underscore the pragmatic significance of our model for precise prognostication and evaluation across various applications.

To study the pressure drop characteristics of hypervapotron, which was designed as a water-cooling structure in the divertor dome of the fusion reactor, the pressure drop tests of subcooled water were carried out in a vertically upward hypervapotron. To simulate the one-side radiant heating condition in the engineering application, the non-uniform heat fluxes were obtained by using the off-center electrically heating method. The system parameters were as follows: mass flux G = 2000–5000 kg·m−2·s−1, inlet pressure p = 2–4 MPa, and equivalent one-side radiating heat flux qe = 0–5 MW·m−2. The effects of the parameters on the pressure drop were discussed in detail. It was observed that in the single-phase (SP) region, the pressure drop was little influenced by the inlet fluid temperature (Tb,in). However, in the subcooled boiling region, the pressure drop increased rapidly with the increasing Tb,in. A higher G leads to a high pressure drop. In the SP region, the influence of p on the pressure drop is not obvious, and the pressure drop decreased with the increasing qe. The test data are used to evaluate the typical pressure drop correlation, and the results show that none of these correlations can predict the pressure drop well under the test conditions. Therefore, a new pressure drop correlation is proposed for subcooled water in a hypervapotron under high and non-uniform heat fluxes. The new correlation has a high prediction accuracy for the test data, and the mean relative error (MRE) and root mean square error (RMSE) are 0.72% and 4.33%, respectively. The test results have a reference value for the design of the water-cooling structure of the diverter. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Refrigeration is the property of Journal of Refrigeration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract The persistence of the present numerical investigation is to evaluate the thermal and total entropy generation characteristics of flow within a partially filled porous wavy channel with finite wall thickness subjected to non-uniform wall heat flux. The average Nusselt number and the total entropy generation increase with the permeability value of the porous medium (Da) irrespective of asymmetric wall heat flux amplitude and wavelength as well as with the variation in thickness ratio ( δ s ) and thermal conductivity value of the solid substrate. However, the total irreversibility decreases for a high Darcy number value when the phase angle between the applied heat fluxes is varied. Moreover, the average Nusselt number decreases up to a critical value of phase angle ( θ H = 135 0 ) followed by a gradual increment for Da = 10−5 and 10−3, respectively. On the contrary, the reverse trend can be seen for Da = 10−1. The optimal choice of nonuniform heat flux amplitude, wavelength, and phase angle for a high heat transfer rate with minimum irreversibility generation is found to be 10 W/cm2, 5 mm, and 180°, respectively for a thickness ratio of 0.5 and thermal conductivity value of 646.07 W/m·K independent of Darcy number. [ABSTRACT FROM AUTHOR]

This study investigates numerically and experimentally the flow and heat transfer characteristics of ZnO/EG-H2O nanofluid flow in a parabolic trough solar collector at different flow rates (between 20 lit h−1 and 80 lit h−1) and nanoparticle volume fractions (φ = 1%, 2%, 3%, 4%). The effects of changes in parameters such as absorbed and heat loss parameters, collector efficiency, useful energy, and temperature differences between outlet and inlet have been investigated in the context of experimental results. To obtain meaningful results in the numerical study, a non-uniform heat flux distribution on the collector absorber has been generated by the Monte Carlo Ray Tracing method (MCRT) using the commercial code SOLTRACE. Friction factor, entropy generation, PEC number, Nusselt number, and Bejan number are the parameters studied. The ZnO/EG-H2O nanofluid significantly improves the efficiency of the collector, based on the findings obtained. The highest usable energy has been obtained at the flow rate of 80 lit h−1 with 4% ZnO/EG-H2O nanofluid. The results suggest that the temperature differential rises when ZnO/H2O has been used compared to EG-H2O. Moreover, when ZnO/EG-H2O is used with the flow rate of 80 lit h−1 and a volume fraction of 4% of nanoparticles, the Nusselt number increases by about 100% compared to EG-H2O as the working fluid. There is a negligible increase in the overall entropy production when ZnO/EG-H2O is utilized as opposed to the base fluid. Thus, the greatest possible nf may be suggested for parabolic trough solar collector. The goal of this study is to use the findings of ZnO/EG-H2O nanofluid research to parabolic trough solar collectors. The experimental data show that compared to traditional fluid, utilizing nanofluid results in significantly improved thermal performance. In this situation, it seems that nanofluid would be the best option. [ABSTRACT FROM AUTHOR]

In this paper, the effect of using four different types of inserts in cavity receiver tube of solar power tower to improve heat transfer and reduce non-uniform temperature distribution is investigated. Numerical simulation is performed in three dimensions and the effect of inserts configurations, position inside receiver tube, pitch and thickness of insert and the effect of non-uniform heat fluxon the Nusselt number, friction factor and temperature of outer surface of receiver tube are investigated. The flow is incompressible, steadystate, turbulent and the Reynolds number is in the range of 8000 to 20000. Numerical simulation results for four types of inserts: twisted-tape, wavy-tape, helical-tape and louvered-tape show that wavy-tape compared to the other three types, has the higher Nusselt number and the lower average temperature of receiver tubesurface. The Nusselt number of the wavy-tape is increased by 1.8%, 2% and 3.2% relative to the louvered-tape, twisted-tape and helical-tape, respectively and 10% relative to the receiver tube without insert. By increasing the insert thickness and decreasing the insert pitch, the Nusselt number increases and average temperature of receiver tubesurface decreases. Investigating the insert position in three cases: close to the non-uniform heat flux wall, the middle of the tube and close to the insulation wall show that by increasing the distance of insert from non-uniform heat flux wall, the mixing of hot and cold fluids flow is better and the temperature distribution is more uniform.Therefore, the best position to place insert is the farthest distance from the non-uniform heat flux wall. [ABSTRACT FROM AUTHOR]

In this study, the heat transfer characteristics of an improved absorber tube of parabolic trough solar collector LS-2 are investigated using ANSYS software. Oil syltherm 800 type is used as a heat transfer fluid. Three types of absorber tubes are tested; the first is a smooth tube and the others are finned tubes. One of the rough cases is rectangular cross-section fins with rounding at fin base and the other does not have rounding at fin base. Simulations are performed with fin thickness variations of 2, 4 and 6 mm. The fin lengths change at 5, 10, 15, 20 and 25 mm. The radii of rounding are 2, 3 and 4 mm and angles between fins are 45° and 90°. Wide range of operating parameters is considered, such as inlet fluid temperatures (300:600 K), flow rate (6:24 m3/h) and direct normal irradiance (500:1000 W/m2). The thermal efficiency, Nusselt number and thermal enhancement index are calculated under different operating conditions. The results show that thermal enhancement index of the fin with round edge radius of 4 mm is higher than that of the fin with the sharp edge (R = 0 mm) by about 10.74% under the considered conditions. The thermal enhancement index of the round edge fins with a length of 25 mm is nearly 25.6% higher than that of the round edge fins with a length of 5 mm. At a fin thickness of 6 mm, the thermal enhancement index of the round edge fin is nearly 7.8% higher than that of the sharp edge fin. At 45° angle and 25-mm fin length, the thermal enhancement index for round and sharp-edged fins is 1.644 and 1.532, respectively. When the inlet fluid temperature increased from 300 to 600 K, the heat enhancement index value increased by 14.57%; as the flow rate increased from 6 to 12 m3/h, the heat enhancement index value decreased by 11.63%. The thermal enhancement index increased from 1.265 to 1.359 as the direct normal irradiance varied from 500 to 700 W/m2. [ABSTRACT FROM AUTHOR]

High-power servo motor is widely employed as a necessary actuator in flight vehicles. The urgent problem to be solved restraining the working performance of servo motor is no longer the torque and power, but the heat dissipation capability under high-power working conditions, which may cause the overheat, even burn down of motor or other potential safety hazards. Therefore, a structure of mini cooling channels with appropriate channel density is designed in accordance with the non-uniform heat flux of servo motor in this paper. Combined with the regenerative cooling method, the cryogenic fuel supercritical methane is served as the coolant, which is easy to be obtained from the propulsion system, and the heat from the servo motor can be transported to the combustion for reusing. According to the actual working cases of servo motor, a numerical model is built to predict the thermal performance of cooling channels. In order to better represent the secondary flow of coolant in the cooling channels, especially the turbulent mixed flow in the manifold, the k-ε RNG model with enhanced wall treatment is employed resulting from its precise capacity to simulate the secondary and wall shear flow. On this basis, the heat transfer mechanism and thermal performance of cooling channels, as well as the influence of various heat flux ratios are investigated, which can offer an in-depth understanding of restraining excessive temperature rise and non-uniformity distribution of the servo motor. By the calculation results, it can be concluded that under the adjustment of the channel density according to the corresponding heat flux, the positive role of the appropriate channel density and the manifolds on heat transfer is manifested. Moreover, the maximum temperature difference of heating wall can be kept within an acceptable range of the servo motor. The heat transfer coefficient in the manifold is nearly 2–4 times higher compared with that in the straight cooling channels. The effect of buoyancy force cannot be neglected even in the manifold with turbulent mixed flow, and the pattern of heat transfer is mixed convection one in all the flow regions. The thermal resistance R and overall Nusselt number Nu are affected remarkably by all the operation parameters studied in the paper, except the pressure, while the overall thermal performance coefficient η demonstrates differently. The strong impact of heat flux ratio is implied on thermal performance of the cooling channels. Higher heat flux ratio results in the stronger non-uniform temperature distribution. Meanwhile, only tiny temperature differences of the fluid and inner wall in manifolds among various heat flux ratios are demonstrated, resulting from the positive effect of mixture flow on heat transfer. [ABSTRACT FROM AUTHOR]

• The application of sCO 2 effectively improves the temperature uniformity of MCHS. • Increase in heat transfer coefficient does not always improve temperature uniformity in MCHS. • Sharp drop in heat transfer efficiency post-pseudo-critical temperature. • Disruption of boundary layers intensifies heat transfer in microchannels. • The entropy production rate of sCO 2 under ideal conditions is only 46.6 % of that employing water. With the rapid growth of information technology and the evolution in the use of microelectronic chips, these components are facing major challenges of high heat dissipation and inhomogeneous heat flux. To address the hotspot issues, a combination of microchannel heat sinks (MCHS) with cavity-rib designs and supercritical fluids is a potential solution. This study compares the thermo-hydraulic properties of water and supercritical carbon dioxide (sCO 2) under various boundary conditions and hotspot locations during non-uniform heating. The results showed that sCO 2 can effectively improve temperature uniformity along the heat sink in the presence of hotspots as compared to water. Considering a mass flow rate of 600 kg/(m2·s), the inhomogeneity index of the basal temperature of sCO 2 is found to be 0.24, significantly lower than 4.22 for water. This indicates that sCO 2 eliminates hot spots by rapidly increasing specific heat capacity near the pseudo-critical temperature. Furthermore, it is noted that maintaining the operating temperature of sCO 2 near the pseudo-critical temperature can increase the specific heat capacity of the fluid in a short time, in addition to reducing the corresponding density and viscosity. Consequently, the entropy generation rate of sCO 2 under ideal conditions is 46.6 % of that of water. Therefore, the proposed combination of a microchannel of complex structure and sCO 2 in this study is demonstrated to effectively reduce the influence of hot spots and improve the overall thermal performance of heat sinks. [ABSTRACT FROM AUTHOR]

• Thermal boundary conditions of supercritical heat exchangers are introduced. • Effects of non-uniform heat flux on the heat transfer characteristics of supercritical fluids. • Optimization strategies for supercritical heat exchangers. • The spatiotemporal matching theory of supercritical substance flow and energy flow. The dual optimization of heat transfer and fluid flow constitutes subjects of paramount interest in heat exchange equipment. Non-uniform thermal boundaries are common in supercritical heat exchangers, interacting with drastic changes in properties to make the heat transfer process very complex. This paper first reviews the forms of boundary conditions in supercritical heat exchangers, including plate heat exchangers, shell-and-tube heat exchangers, printed circuit heat exchangers and some emerging heat exchangers. It attempts to translate structural impacts into non-uniform thermal boundary conditions. Then, based on single-tube analysis, the effects of axially and circumferentially non-uniform heat flux on the flow and heat transfer characteristics of supercritical fluids are examined. To reduce wall temperature and improve heat transfer performance, some methods for optimizing structures are introduced. Finally, referencing constructal theory, some optimization strategies and evaluation methods from laminar heat exchangers, which are similarly sensitive to thermal boundary conditions, are introduced into the supercritical domain. This establishes the spatiotemporal matching theory of supercritical substance flow and energy flow, providing reference and guidance for the intelligent design and safe, efficient operation of supercritical heat exchangers. Additionally, some reasonable suggestions for future research are given, including study and utilization of the supercritical thermal entrance effect and the exploration of optimal heating strategies. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The distributions of heat flux over the circumference of the receiver tubes have an immense influence on the performance and reliability of the parabolic trough solar thermal collectors. The location of the receiver tube and the secondary reflector configuration may largely influence the performance of the system. Therefore, in this study, the effect of receiver tube position and parabolic secondary reflector configuration has been analyzed, and the non-uniformity of solar flux distribution, heat gradient, and power output has been compared. The results of the Monte Carlo ray-tracing analysis to homogenize the receiver tube flux distribution and maximize the output power, making the use of the cutting edge solar optical simulation tool Tonatiuh, has been presented. A parabolic trough collector with a rim angle of 80 deg and aperture area of 40 m2 have been used for the analysis. It has been confirmed that the circumferential heat flux gradient and the local hot spot could be greatly diminished, while the power output tended to reduce slightly due to the shading effect of the secondary reflector. Under the conditions investigated in this work, although the output power decreased by 4.83%, flux gradient reduced significantly, and the non-uniformity of flux distribution has reduced from 0.9757 to 0.5176. A simple design procedure for receiver tube position and secondary reflector configurations to homogenize the receiver tube temperature distribution has also been proposed. [ABSTRACT FROM AUTHOR]

Nanofluids have gained prominence due to their superior thermo-physical properties. The current paper deals with MHD nanofluid flow over a non-linear stretchable surface of varying thickness in the presence of an electric field. We investigated the effects of nanometer-sized copper (Cu) particles in water (base fluid) as a nanofluid, as well as non-linear thermal radiation, variable fluid viscosity, Joule heating, viscous dissipation, and non-uniform heat flux. The current study's aim is influenced by the immense applications in industry and machine building. It has been observed that linear stretching sheets have been extensively used in heat transfer research. Moreover, no effort has been made yet to model a non-linear stretching sheet with variable thickness. Furthermore, the effects of electromagnetohydrodynamics (EMHD) boundary-layer flow of a nanofluid with the cumulative impact of thermal radiation, variable viscosity, viscous dissipation, Joule heating, and variable heat flux have been investigated. Sheets with variable thicknesses are practically significant in real-life applications and are being used in metallurgical engineering, appliance structures and patterns, atomic reactor mechanization and paper production. To investigate the physical features of the problem, we first examined the model and identified all the physical properties of the problem. This problem has been formulated using basic laws and governing equations. The partial differential equations (PDEs) that govern the flow are converted into a system of non-dimensional ordinary differential equations (ODE's), using appropriate transformations. The Adam–Bashforth predictor-corrector technique and Mathematica software are utilized to numerically solve the resulting non-dimensionalized system. The interaction of various developing parameters with the flow is described graphically for temperature and velocity profiles. It is concluded that the velocity of nanoparticles declines as the intensity of the magnetic field increases. However, the temperature of the nanomaterials rises, as increasing the values of the electric field also increases the velocity distribution. The radiation parameter enhances the temperature field. The temperature of the fluid increases the occurrence of space- and time-dependent parameters for heat generation and absorption and radiation parameters. [ABSTRACT FROM AUTHOR]

The concentrated solar power generation system with supercritical CO 2 as working medium has high cycle efficiency, which can effectively reduce the volume of heat exchange equipment and reduce the investment cost. However, there is no systematic comparative study on the effect of tube structure on the performance of supercritical CO 2 , but the structure of tubes has a significant effect on the heat transfer performance of supercritical CO 2 , so it is necessary to study it. In this contribution, we firstly analyze the effects of different structures of tubes on the performance of supercritical CO 2. Then, we design a novel alternative elliptical twisted tube. Finally, we further analyze the effects of the novel tube on the heat transfer performance of supercritical CO 2 by using the field synergy principle. The results show that the novel alternative elliptical twisted tube is the best structure. When the mass flow is 0.08 kg s−1, the heat transfer coefficient of horizontal arrangement is 1625 W m−2 K−1, which is 107% higher than that of horizontal tube. And the heat transfer coefficient of vertical arrangement is 1620 W m−2 K−1, which is 116% higher than that of vertical tube. These results will provide a guidance for the structural optimization of practical receivers. •Analysis and comparison of enhanced heat transfer performance of corrugated structure tubes. •Design a novel alternative elliptical twisted tube based on comparison of different structures. •The field synergy principle is applied to analyze the difference of flow field in different tubes. •The novel alternative elliptical twisted tube has a better flow and heat transfer performance. [ABSTRACT FROM AUTHOR]

Summary: As one of the most promising advanced reactors, the lead‐based fast reactor has drawn great attention due to economic and safety advantages. Investigating thermal hydraulics is essential for the design of a lead‐based reactor. In this paper, a CFD simulation of a 19‐wire‐wrapped‐rod bundle with lead‐bismuth eutectic (LBE) as coolant is carried out using the Reynolds‐averaged Navier‐Stokes equation method, and four different types of axial non‐uniform heat flux are applied. Excellent validation of results for thermal and hydraulic aspects is obtained first by comparing simulation results using turbulent model k‐ω SST with experimental data and high‐fidelity large eddy simulation data. The mechanism of transverse flow variation in subchannels and faces resulting from changing locations of wires is studied. The strong transverse flow at the edge and corner subchannels lead to a more distinct oscillation in peripheral cladding temperature under non‐uniform heat flux. The hot spot issue for blockage conditions is studied, and it is found that the temperature increment at blockage is linear to the local heat flux. When the blockages locate at the peak normalized heat flux of 1.56, there is no evident difference in the maximum temperature when changing the heat flux pattern, with an averaged Tmax of 732 K. Even though the normalized heat flux is high at 1.89, the flatter and lower temperature distribution would not lead to a remarkable high hot spot temperature compared with that of 1.56. The distribution of hot spots, as well as the inhomogeneity of heat transfer in different heat flux patterns, provide a reference for lead‐based reactor design. [ABSTRACT FROM AUTHOR]

The solar flux distribution on the Parabolic Trough Collector (PTC) absorber tube is extremely non-uniform, which causes non-uniform temperature distribution outside the absorber tube. Therefore, it generates high thermal stress which causes creep and fatigue damage. This presents a challenge to the efficiency and reliability of parabolic trough receivers. To override this problem, we have to homogenize the heat flux distribution and enhance the heat transfer in the receiver's absorber tube to improve the performance of the PTC. In this work, 3D thermal and thermal stress analyses of PTC receiver performance were investigated with a combination of Monte Carlo Ray-Trace (MCRT), Computational Fluid Dynamics (CFD) analysis, and thermal stress analysis using the static structural module of ANSYS. At first, we studied the effect of the receiver tube material (aluminium, copper, and stainless steel) on heat transfer. The temperature gradients and the thermal stresses were compared. Second, we studied the effect of the addition of nanoparticles on the working Heat Transfer Fluid (HTF), employing an Al2O3-H2O based nanofluid at various volume concentrations. To improve the thermal performance of the PTC, a nanoparticle volume concentration ratio of 1%-6% is required. The results show that the temperature gradients and thermal stresses of stainless steel are significantly higher than those of aluminium and copper. From the standpoint of thermal stress, copper is recommended as the tube receiver material. Using Al2O3 in water as an HTF increases the average output temperature by 2%, 6%, and 10% under volume concentrations of 0%, 2%, and 6% respectively. The study concluded that the thermal efficiency increases from 3% to 14% for nanoparticle volume fractions ranging from 2% to 6%. [ABSTRACT FROM AUTHOR]

• Assess typical Pr t models for supercritical fluids. • Fluid temperatures is measured to corroborate thermal stratification. • A new buoyancy-tuned Pr t model for improved predictions. • New model accurately predicts HTD and heat transfer enhancement. • Effective under axially non-uniform heat flux conditions. To improve the predictive accuracy of numerical simulations for supercritical water, the effect of turbulent Prandtl number (Pr t) on supercritical water heat transfer was extensively studied using the Shear-Stress Transport (SST) k − ω turbulence model. The efficacy of typical turbulent Prandtl number models for predicting heat transfer in horizontal tubes with supercritical water was evaluated based on wall temperatures measured experimentally and fluid temperatures obtained from multi-point temperature measurement devices. These models successfully demonstrated the phenomenon of thermal stratification due to physical property variations, but their predictive performance near the pseudocritical temperature, particularly in the top wall region, was not satisfying. Building upon this, the supercritical water buoyancy-tuned turbulent Prandtl number model (SWBT model) suitable for supercritical water in horizontal tubes was developed through coefficient calibration and the integration of buoyancy correction terms to ensure accuracy and applicability. This model effectively replicates severe heat transfer deterioration (HTD) and buoyancy-induced improvements in heat transfer, showing robust predictive capabilities with a wide array of experimental data from both the authors and other researchers. The model was tested across ranges of P = 24.5–26.5 MPa, q/G = 0.11–0.67, and tube diameters of 7.5 mm, 26 mm and 43 mm. Additionally, the model's performance was benchmarked against experimental results of axially non-uniform heat flux conducted by the authors, successfully reproducing the variations in wall temperature due to changes in heat flux. This study substantially enhances the accuracy of numerical simulations for supercritical water in horizontal tubes, offering valuable insights for the engineering applications of supercritical water and the design of heat exchangers. [ABSTRACT FROM AUTHOR]

This study presents exact solutions for the phenomenon of conjugate heat transfer occurring within a solar receiver tube. The investigation focuses on three distinct configurations: (a) tubes with negligible wall thickness, (b) thin-walled tubes, and (c) thick-walled tubes. The primary objective is to offer precise mathematical representations for the distribution of temperature, as well as numerical values of local and average Nusselt numbers. Furthermore, the study delves into the examination of limiting cases to enhance the understanding of the problem at hand and validating the outcomes. The derived solutions are also utilized to conduct a comprehensive analysis of the specific practical scenario involving the flow of molten salt in both Alloy 625 and stainless steel solar receiver tubes. Inspection of the configurations presented in this study demonstrates that the circumferential conduction heat transfer in the tube wall helps to distribute the heat more evenly along the circumference and mitigate the occurrence of localized hot spots. [ABSTRACT FROM AUTHOR]

Melting of phase change materials (PCMs) finds applications in engineering systems such as heat sinks and latent heat energy storage (LHTES) modules. In real-time devices and systems, the heat load distribution is generally spatially varying, and hence, accurate measurements and modeling of PCM melting subjected to spatially varying heat flux inputs are very important. In this context, velocity field measurements and numerical simulations of PCM melting subjected to non-uniform heat flux distribution are presented herein. A transparent experimental setup for PCM melting with spatially varying heat flux loads has been constructed. The non-invasive particle image velocimetry (PIV) technique is employed to visually capture the liquid region's flow and velocity field distribution. A finite-volume method (FVM) based numerical model is employed to simulate the conjugate heat transfer and melting of PCM under spatially varying heat flux loads. Five cases with different configurations of heat flux distribution are considered. The numerical predictions are in good agreement with the experimental measurements. The variations in melt morphology and flow features caused by the spatially varying heat flux are highlighted. The heat transfer characteristics in each case are quantified in terms of Nusselt number and melting rates. Increasing the heat flux in the gravity direction aids natural convection and promotes faster melting rates at the bottom end of the cavity. On the other hand, decreasing the heat flux in the gravity direction hindered the natural convective flow and melting in the bottom end of the cavity. 40% decrease in total melting time is achieved by increasing the heat flux in the gravity direction. Decreasing the heat flux in the gravity direction led to a 44% increase in total melting time. The results of this study provide deeper insights into the dynamics of the PCM melting process subjected to spatially varying heat loads. • PCM melting driven by non-uniform heat flux heating is studied • Combined experimental and numerical investigation is carried out. • Five cases with different heat flux distribution sequence are considered. • Increasing the heat flux in gravity direction led to 40% higher melting rate. • Decreasing the heat flux in gravity direction led to 44% slower melting. [ABSTRACT FROM AUTHOR]

Copyright of Experimental Technology & Management is the property of Experimental Technology & Management Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Regenerative cooling is considered promising in the thermal protection of hypersonic propulsion devices such as SCRamjet. However, the heat transfer deterioration (HTD) of hydrocarbon fuel is a severe threat to the thermal structure safety, especially under axially nonuniform heat flux caused by the thermal load difference in different components. In this work, the heat transfer of trans-critical n-decane in a mini-horizontal channel is numerically investigated. The influences of the axially nonuniform heat flux on the heat transfer is focused on. Two types of HTD are recognized and analyzed. The first type of HTD is induced by the near-wall flow acceleration and the local thickening of the viscous sublayer. The second type of HTD is closely related to the expansion of the low thermal conductivity λ and specific heat cp region, which is seriously worsened under axially nonuniform heat flux, especially when the heat flux peak locates where Tw ≥ Tpc. The minimum HTC deteriorates by 40.80% and the Tw_max increases from 857 K to 1071 K by 27.5%. The maximum fluctuation in pressure drop is 6.8% in the variation in heat flux distribution with Φ = 2. This work is expected to offer a reference to the proper match of fuel temperature distribution and the engine heat flux boundary in SCRamjet cooling system design. [ABSTRACT FROM AUTHOR]

The present article aims to investigate the Graetz-Nusselt problem for blood as a non-Newtonian fluid obeying the power-law constitutive equation and flowing inside the axisymmetric tube subjected to non-uniform surface heat flux. After the flow field is determined by solving the continuity and the momentum equations, the energy equation is handled by employing the separation of variables method. The resulting Eigen functions and Eigen values are numerically calculated using MATLAB built-in solver BVP4C. The analysis is first conducted for the situation of constant heat flux and subsequently generalized to apply to the case of sinusoidal variation of wall heat flux along the tube length, using Duhamel's Theorem. Furthermore, an approximate analytic solution is determined, employing an integral approach to solve the boundary layer equations. With respect to the comparison, the results of approximate solution display acceptable congruence with those of exact solution with an average error of 7.4%. Interestingly, with decreasing the power-law index, the discrepancy between the two presented methods significantly reduces. Eventually, the influences of the controlling parameters such as surface heat flux and power-law index on the non-Newtonian fluid flow's thermal characteristics and structure are elaborately discussed. It is found that switching from constant wall heat flux to non-uniform wall heat flux that sinusoidally varies along the tube length significantly improves the simulation's accuracy due to the better characterization of the heat transport phenomenon in non-Newtonian fluid flow through the tube. In the presence of sinusoidally varying wall heat flux with an amplitude of 200 W/m2and when the power-law index is 0.25, the maximum arterial wall temperature is found to be about 311.56 K.

A theoretical model on MHD hybrid nanofluid flow in accordance with non-uniform heat flux and solar energy radiation has been studied in our work. Also, the impact of multiple slip conditions is presumed at the boundary. Comparative flow analyses for hybrid nanofluid (Al2O3/Cu–H2O) and single nanoparticle-based nanofluid (Cu–H2O) are addressed here with graphs and charts. The leading partial differential equations with boundary conditions have been converted into ordinary differential equations with the aid of similarity transformation. The final system is tackled via the fifth-order Runge–Kutta–Felberg method with shooting procedure and the computation is done using Maple 17. One of the interesting results shows that with the growth of thermal radiation, the Nusselt number for Cu–H2O is reduced by 26.16%, whereas for the same, Nusselt number for Al2O3/Cu–H2O is lessened by 27.38%. Fallout shows that with the growing values of velocity slip parameter, the thermal boundary layer thickness enlarges faster for Al2O3/Cu–H2O in comparison to Cu–H2O. [ABSTRACT FROM AUTHOR]

This study proposes a method for calculating the fatigue–creep of a supercritical carbon dioxide (sCO 2) solar receiver based on the linear damage accumulation (LDA) theory. The effects of temperature and stress on creep and fatigue were considered through the Manson–Coffin formula and Mendelson–Roberts–Manson (M–R–M) correlation, and the interaction between creep and fatigue was reflected by adopting the damage allowable region (DAR). Based on the DAR, a comprehensive damage coefficient K was proposed to assess the damage and safety margin of the receiver. Furthermore, this study used this method to analyze the impact of critical design parameters, namely the flow rate, tube wall thickness, and tube radius on the fatigue–creep damage of a single tube of an sCO 2 solar receiver. The results demonstrated that increasing the design flow rate or decreasing the tube radius could reduce the fatigue–creep damage of the receiver, and the effect of wall thickness on creep was related to the heat flux at the location of the receiver. For the same design parameters, the creep damage was evidently greater than the fatigue damage and thus, the influence of creep on the receiver should be given priority in the design process. • Comprehensive fatigue-creep damage evaluation of tubular sCO2 tower solar receivers. • The fatigue-creep damage of receiver is analyzed by LDA theory. • Increasing flow rate or decreasing radius resulted in a fatigue-creep damage decrease. • The effect of wall thickness on creep damage was related to the heat flux level. [ABSTRACT FROM AUTHOR]

This study assesses effects of the non-uniform circumferential heat flux on horizontal flow boiling of R141b in a stainless-steel tube at ambient system pressure with an inner diameter of 10.92 mm. The main motivation for this are application in Concentrated Solar Power using the Direct Steam Generation process inside horizontal absorber tubes. Experiments were conducted for two mass velocities (50 and 150 kg m−2s−1) and two mean heat fluxes (3.4 and 11.0 kW m−2), with three different heating conditions (uniform, top-concentrated, and bottom-concentrated). The differences in the Onset of Nucleate Boiling between uniform and bottom-concentrated heating are assessed, and the transient behavior of the wall temperatures is investigated. It is found that the heating condition significantly affects the mean heat transfer coefficient in the case of non-annular flow regimes but practically not in annular flow. The circumferential wall temperature gradient is with uniform heating highest for non-annular flow regimes. But with non-uniform heating the circumferential wall temperature gradient is highest for annular flow. The use of empirical correlations for the heat transfer coefficients are evaluated. • Transient behavior of single to two-phase flow under circumferential non-uniform heating. • Annular flow yields uniform temperature even for non-uniform circumferential heating. • The HTC is not affected by the circumferential heat flux distribution for annular flow. • HTC increases for stratified-wavy flow when heating from the bottom. • Comparison of several correlations from literature with non-uniform heating data. [ABSTRACT FROM AUTHOR]

The city scale buoyancy-driven flow is crucial for the wind and thermal environment in urban areas, exerting a significant impact on the city ventilation, pollutants dispersion and heat removal. The intra-urban variations of hot spots and heat flux are drawing more attention on beating the urban heat. This study aims at analyzing the influences of non-uniform heat flux and background wind on the city-scale heat removal ability and underlying mechanisms. The large eddy simulation model was used and verified with water tank experiments. Results show that positive heat flux and negative heat flux in the rural areas will enhance and depress the strength of the urban heat dome flow respectively compared to the adiabatic condition. The high-speed regions are wider and stronger when the rural area has positive heat flux condition. Under positive rural heat flux condition, the time-average mixed layer height in urban areas across the city center plane is nearly twice that under negative rural heat flux condition. Besides, the city-scale heat transfer coefficients in positive rural heat flux condition are higher than those under negative rural heat flux and adiabatic conditions. For cases of non-uniform heat flux in urban areas, the differences of time-average mixed layer height and heat transfer coefficients are not obvious. The heat transfer coefficients decrease initially and then increase with increasing background wind speed indicating the non-linear interaction between approaching wind and urban heat dome flow. Moreover, the growth rate of the heat transfer coefficient decreases when the non-dimensional wind speed exceeds 1.19. • The non-uniform heat flux does change the overall urban heat dome flow structure. • The urban heat dome flow interacts with background wind non-linearly. • The bulk heat transfer coefficients are larger under positive rural heat flux condition. • The overall 'S' shape curve of heat transfer coefficients versus wind speed was found. [ABSTRACT FROM AUTHOR]

The alternating-laminated-microchannel-tube (ALMT) heat exchanger has emerged in the refrigeration and air conditioning industry. However, existing studies on evaporation heat transfer coefficient are limited to a single microchannel tube, while research on multi-path ALMT heat exchanger is lacking. The evaporation heat transfer in actual multi-path heat exchangers is influenced by secondary fluid non-uniform heating and refrigerant flow maldistribution, making direct measurement more difficult. Therefore, the infrared thermography-based method is proposed to predict evaporation heat transfer coefficient distribution in actual multi-path heat exchangers. Firstly, the secondary fluid-side temperature distribution based on infrared thermography derives non-uniform heat flux distribution by finite volume partition, equal to that on the refrigerant side. Then, the refrigerant-side evaporation heat transfer coefficient distribution along length is obtained. Further, the refrigerant flow distribution is quantified according to the pressure drop balance and evaporation coefficient distribution with vapor quality is also obtained. The prediction method is experimentally validated within a 10% deviation. Results show that the evaporation heat transfer coefficient distribution curve exhibits diverse peak values as influenced by non-uniform heat flux and refrigerant flow maldistribution. These peaks are at a vapor quality of around the 0.44–0.79 range. Moreover, the heat exchanger is optimized based on the predicted results, with micro-finned tubes placed in the appropriate region to bring a maximum increase of 8.9% in heat transfer capacity. • Evaporation coefficient distribution in multi-path heat exchanger is studied. • Infrared thermography-based method is proposed to predict this distribution. • Refrigerant maldistribution and non-uniform heat flux are quantified. • Diverse evaporation coefficient peaks at a quality of around 0.44–0.79 range. • Heat exchanger is optimized with a maximum increase of 8.9% in heat capacity. [ABSTRACT FROM AUTHOR]

Evacuated tubes are extensively utilized for converting solar energy into thermal energy owing to their exceptional thermal properties and affordability. The circumferential temperature gradient of the evacuated tube is significant to the heat and mass transfer inside, but it is often neglected. This research proposed an improved model incorporating non-uniform heat flux and circumferential conduction to obtain the temperature distribution. In the experiment, A thermal insulation material was inserted inside the inner tube to create an adiabatic boundary. Various conditions (non-uniform, rectangular, and uniform heat fluxes, circumferential conduction of the coating) were numerically compared. The enhanced combined thermal transmission model was validated, demonstrating a relative error of 6.45 % in temperature increase calculation and increasing the prediction accuracy by 42.15 % at least. The distribution of the heat flux and the thermal conduction of the coating were identified as the primary factors affecting the heat transfer properties of the evacuated tube. An imbalanced distribution of heat flux resulted in non-uniform heat transfer, whereas the circumferential conduction within the coating effectively modulated the temperature distribution to achieve uniformity. The local heat transfer coefficient exhibited non-uniformity, with calculated values ranging from 0.23 to 3.25 W/(m2∙K), and an error range of −16 to +3 %. • A theoretical model incorporating circumferential conduction was developed. • The non-uniform heat transfer model exhibits the minimum RE avg (6.45 %) in dT. • An imbalanced distribution of heat flux results in non-uniform heat transfer. • The circumferential conduction modulates the temperature distribution to achieve uniformity. • The local heat transfer coefficient exhibits non-uniformity but the value is small. [ABSTRACT FROM AUTHOR]

The objective of present study is to investigate the effect of a novel type of insertion, i.e. semi-annular and fin shape metal foam (SAFM) hybrid structure on thermohydraulic and thermodynamic performance of absorber tube in parabolic trough collector (PTC). The influences of SAFM fin shapes (rectangle, triangle and trapezoid) on turbulent forced convection heat transfer and flow under non-uniform heat flux are numerically analyzed. The local thermal non-equilibrium method is adopted to reflect the energy imbalance between fluid and metal foam structure, and Monte Carlo Ray-Trace Method is used to determine the non-uniform heat flux on absorber tube wall. The results show that the insertion of SAFM improves thermohydraulic and thermodynamic performance of absorber tube; compared with smooth tube, Nusselt number is increased by 256.3%–838.7%, friction factor is increased by 440.3%–788.8%, and performance evaluation criteria increment, total entropy generation reduction as well as exergetic efficiency increment maximally reach 360%, 93.3% and 10.2% respectively. It is also found that SAFM achieves higher performance than individual semi-annular metal foam or fin shape metal foam, and SAFM with triangular fin achieves higher performance than that with rectangular or trapezoidal fin.

Regenerative cooling is considered promising in the thermal protection of hypersonic propulsion devices such as SCRamjet. However, the heat transfer deterioration (HTD) of hydrocarbon fuel is a severe threat to the thermal structure safety, especially under axially nonuniform heat flux caused by the thermal load difference in different components. In this work, the heat transfer of trans-critical n-decane in a mini-horizontal channel is numerically investigated. The influences of the axially nonuniform heat flux on the heat transfer is focused on. Two types of HTD are recognized and analyzed. The first type of HTD is induced by the near-wall flow acceleration and the local thickening of the viscous sublayer. The second type of HTD is closely related to the expansion of the low thermal conductivity λ and specific heat cp region, which is seriously worsened under axially nonuniform heat flux, especially when the heat flux peak locates where Tw ≥ Tpc. The minimum HTC deteriorates by 40.80% and the Tw_max increases from 857 K to 1071 K by 27.5%. The maximum fluctuation in pressure drop is 6.8% in the variation in heat flux distribution with Φ = 2. This work is expected to offer a reference to the proper match of fuel temperature distribution and the engine heat flux boundary in SCRamjet cooling system design.

In this study, the thermal and flow characteristics of a parabolic-trough solar collector have been numerically investigated. The turbulent flow inside the receiver tube was modeled via the finite volume method, while a non-uniform concentrated heat flux was imposed on the absorber tube. A Cu–water nanofluid was specified as the heat transfer fluid. The results showed that increasing the Cu nanoparticle concentration led to an increase in the Nusselt number (Nu). Furthermore, the effect of Cu nanoparticle addition on the heat transfer enhancement became more significant as the Reynolds number decreased. This was because nanoparticle addition mainly improved the heat transfer via conduction. As the Reynolds number increased, the role of forced convection overcame that of conduction. Furthermore, it was shown that although Cu nanoparticle addition increased the thermal efficiency, it also increased the pressure drop slightly. The effect of direct normal irradiance changes on the performance of the solar collector was assessed. At Reynolds numbers of 104, 105 and 106, as direct normal irradiance increased from 900 to 1100 W m−2, Nu increased by up to 8.6%, 9.78% and 11.43%, respectively, leading to increases in thermal efficiency of 3.87%, 3.82% and 2.04%. This study provides new insight into the effect of Cu nanoparticle addition on the thermal and flow characteristics of parabolic-trough solar collectors. [ABSTRACT FROM AUTHOR]

This paper explores the influence of entropy production on MHD hybrid nanofluid (Al2O3–Cu/H2O) flow due to permeable stretching sheet with variable heat flux in the existence of electric field. The partial differential equations are converted into ordinary differential equations by using appropriate transformations. The obtained dimensionless model is then numerically solved with the help of Adams–Bashforth method. The velocity, temperature and entropy generation are obtained under the influence of various emerging quantities examined. It is concluded that existence of magnetic field and suction decelerates the motion of fluid, whereas it magnifies for greater values of an electric field which tends to resolve sticky effects. Furthermore, friction factor and heat transfer rate have been evaluated through tables and bar charts. Under some specific conditions, the present results are matched with the available results to check the accuracy and validity of the present study and we have observed a good agreement with it. [ABSTRACT FROM AUTHOR]

Heat transfer augmentation using turbulators in PTC is well known. However, based on the methodology its thermo-hydraulic rating can vary. In the present experimental study, variations of wire matrix and twisted tape have been used to analyse the thermo-hydraulic behavior of the PTC by referring two approaches viz. thermo-hydraulic efficiency and performance evaluation criterion. An innovative method comprising, analytical model-SOLTRACE®- differential resistance heating is incorporated to simulate the highly non-uniform solar irradiance over the receiver in an indoor test rig. Under the same operating condition, thermo-hydraulic efficiency of medium dense wire matrix is maximum whereas twisted tape of twist ratio 3.37 yields better result as per performance evaluation criterion. However, thermo-hydraulic efficiency based analysis is recommended due to the role of realistic pumping power. Further, variation in energy parameters viz. thermal efficiency, pumping power, Nusselt number and their effect on receiver length have been analyzed. The trade-off between two inserts is also discussed with respect to operating condition. Image 1 • The realistic non-uniform solar flux is simulated in the indoor setup. • Variation in PTC performance rating by referring two thermo-hydraulic approaches have been discussed. • Effect of thermal performance enhancement on the receiver length is analyzed. [ABSTRACT FROM AUTHOR]

In deep geothermal exploitations, the U-shaped borehole heat exchanger (UBHE) has a longer fluid retention time and a larger contact area with the formation. In order to improve the utilization efficiency of geothermal energy, it is becoming increasingly important to accurately evaluate the performance of the deep UBHEs. In this study, the authors develop a new semi-analytical model for characterizing the heat transfer behavior of the deep UBHE geothermal system. In this developed model, the temperature change within the wellbore is numerically simulated by the finite difference method, whereas the temperature change between the formation and the wellbore is analytically simulated by the Green's-function. Besides, the non-uniform heat flux distribution along the horizontal section is considered to characterize three-dimensional heat transfer behaviors. By means of this model, the influences of developing strategies and well configurations on the outlet temperature and total thermal power are investigated. The calculation results indicate that the traditional assumption of uniform temperature along the horizontal section can induce significant errors in evaluating the performance of the UBHE geothermal systems; the horizontal section length is more conducive to improving the extraction performance in a shallower geothermal formation, and a longer horizontal section leads to a higher outlet fluid temperature, together with a larger total thermal power; a higher injection rate, as well as a lower injection fluid temperature, is more favorable for improving the total thermal power; an insulation casing with a length equaling the depth of injection point can lead to the maximum thermal power of the UBHE geothermal system. [ABSTRACT FROM AUTHOR]

Copyright of China Sciencepaper is the property of China Sciencepaper and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Appraisal of till date reviews on parabolic trough collector has been presented. • Most recent innovations in all aspects of PTC are deliberated. • Importance and limitations of passive heat transport mechanism for collector are discussed. This study gives a comprehensive overview about the recent advances related with the heat transfer augmentation techniques, absorber and cover treatment, direct absorption receiver, trough geometry and natural circulation loop applicable to parabolic trough collector. The papers reviewed including theoretical, numerical and experimental up to date works with special focus on passive heat transport systems. It was found that the use of inserts is justified in laminar flow. All nanofluids viz. conventional, eco-friendly, hybrid and magnetic type have the basic drawback of agglomeration effect. Both absorber and cover modification resulted in efficiency enhancement at the cost of pumping power/system complexity. If absorption tendency of flowing particle in direct absorption receiver is resolved, the technology is worth to imply. The concept of non-uniform heat flux application in indoor setups is also dealt with. Further, implementation of natural circulation loop in parabolic trough is the notable advancement as it is self-powered and self-sustained mechanism. [ABSTRACT FROM AUTHOR]

Abstract The underlying physics behind the effect of non-uniform heating on the heat transfer characteristics for flow of Al 2 O 3 -water nanofluid through a duct having finite wall thickness subjected to sinusoidal heat flux has been numerically investigated in this study. The nanofluid is modelled as a single fluid with two phases using the Eulerian-Lagrangian formulation. The concomitant implications of thermal conductivity ratio of solid to fluid, outer tube diameter to inner tube diameter ratio, amplitude of sinusoidal heat flux and volume fraction of nanoparticles on the thermal attributes have been explored and quantified by local and average Nusselt number. The results elucidate a close resemblance between the nature of the variation of local Nusselt number and the applied heat flux, particularly at the intermediate region of the channel. The results also reveal a damping effect on the conjugate heat transfer due to the increase in conducting wall thickness, irrespective of the volume fraction of nanofluid. The confluence of various key parameters on the variation of the magnitudes of the crests and troughs of local Nusselt number are analyzed and this variation proves to be a prime factor in determining the variation of average Nusselt number. Furthermore, the average Nusselt number decreases with amplitude of heat flux. The rate of heat transfer for flow of base fluid through a duct having finite wall thickness is always lower in case of non-uniform heating. However, the use of nanofluid as the heat transfer medium can compensate the decrement in heat transfer due to non-uniform heating. Graphical abstract Unlabelled Image Highlights • Investigation on conjugate heat transfer in a duct subjected to sinusoidal heat flux • Nanofluid modelled as single fluid with two phases using Eulerian-Lagrangian approach • Effect of wall thickness, solid to fluid conductivity ratio, heat flux amplitude and volume fraction of nanoparticles • Wall thickness compensates the decrement of Nusselt number at the troughs of heat flux • Decrement in average Nusselt number with amplitude is compensated by nanoparticles [ABSTRACT FROM AUTHOR]

Highlights • Supercritical heat transfer coefficient of carbon dioxide in microchannel (D = 0.75 mm) measured. • Effects of mass flux, heat flux, reduced pressure and bulk temperature investigated. • Data compared with subcritical and supercritical correlations and effects of buoyancy and flow acceleration in non-uniform boundary conditions explored. Abstract Supercritical carbon dioxide (sCO 2) experiences a drastic change in its thermophysical properties near the thermodynamic critical point. A non-linear thermophysical property variation can influence the heat transfer behavior of sCO 2 which is not predicted well by conventional single phase heat transfer theory. This can become a major hindrance in the effective design of heat exchangers using sCO 2 as a heat transfer fluid and operating in the vicinity of the critical point. Previous investigations of sCO 2 heating have been primarily focused on macroscale, circular and uniformly heated channels at relatively low heat fluxes. It is unclear if models and correlations developed from large circular tube data can be scaled down to the microscale, non-circular channels subject to non-uniform heating. The present study experimentally investigates the turbulent heat transfer performance of sCO 2 in a microchannel heat exchanger operating in a horizontal configuration with a single wall non-uniform heat flux boundary condition. The test section has five parallel channels with a 0.75 mm hydraulic diameter and an aspect ratio of 1. The channels are fabricated using computer numerical control machining and the test section sealed using a diffusion bonding approach. Data analysis techniques which employ 2-D and 3-D heat transfer models of the experimental test section are developed to calculate the average heat transfer coefficients for a given set of experimental conditions. Data are obtained over a wide range of experimental parameters including test section applied heat flux (20 ⩽ q ″ ⩽ 40 W cm−2), mass flux (500 ⩽ G ⩽ 1000 kg m−2 s−1), reduced pressure (1.03 ⩽ P R ⩽ 1.1), and inlet temperatures (16 ⩽ T in ⩽ 50 °C). The heat transfer data were screened for the presence of buoyancy and flow acceleration effects and then compared against correlations developed for turbulent subcritical and supercritical fluid flows. [ABSTRACT FROM AUTHOR]