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To reduce the levelized cost of energy for concentrating solar power (CSP), the outlet temperature of the solar receiver needs to be higher than 700 °C in the next-generation CSP. Because of extensive engineering application experience, the liquid-based receiver is an attractive receiver technology for the next-generation CSP. This review is focused on four of the most promising liquid-based receivers, including chloride salts, sodium, lead-bismuth, and tin receivers. The challenges of these receivers and corresponding solutions are comprehensively reviewed and classified. It is concluded that combining salt purification and anti-corrosion receiver materials is promising to tackle the corrosion problems of chloride salts at high temperatures. In addition, reducing energy losses of the receiver from sources and during propagation is the most effective way to improve the receiver efficiency. Moreover, resolving the sodium fire risk and material compatibility issues could promote the potential application of liquid-metal receivers. Furthermore, using multiple heat transfer fluids in one system is also a promising way for the next-generation CSP. For example, the liquid sodium is used as the heat transfer fluid while the molten chloride salt is used as the storage medium. In the end, suggestions for future studies are proposed to bridge the research gaps for > 700 °C liquid-based receivers. [ABSTRACT FROM AUTHOR]

The geometrical dimensions of the rough structures as well as the non-condensable gases in the vapor mixture can have the great effect on the nucleation position and the wetting state of the droplet, which further influence the condensation heat flux. In this paper, the multispecies multiphase lattice Boltzmann method together with a thermal phase change model is used to investigate the dropwise condensation on a rough surface enhanced with pillars. The effect of the geometric dimensions including pillar height H, pillar width W and pillar space S is investigated. Then the effect of non-condensable gases on the contact angle of a droplet on textured surface is studied. The results show that the local heat flux and the wetting area are higher while the waiting time is shorter at larger S, W and smaller H on the rough surface. The nucleation position rises from the bottom of grooves to the top with the increase of pillar height and the decrease of pillar space. The contact angle is larger and it is easier to maintain the Cassie state for droplet. When there exists non-condensable gas, the non-condensable gas obviously enhances the hydrophobicity of the rough surfaces compared with pure vapor. [ABSTRACT FROM AUTHOR]

Thermal analysis of data centers is in urgent need to ensure that computer chips remain below the critical temperature while the energy consumption for cooling can be reduced. It is difficult to obtain detailed hotspot locations and temperatures of chips in large data centers containing hundreds of racks or more by direct measurement. In this paper, a multi-scale thermal analysis method is proposed that can predict the temperature distribution of chips and solder balls in data centers. The multi-scale model is divided into six scales: room, rack, server, Insulated-Gate Bipolar Transistor (IGBT), chip and solder ball. A concept of sub-model is proposed and the six levels are organized into four simulation sub-models. Sub-model 1 contains Room, Rack and Server (RRS); Sub-model 2 contains Server and IGBT (SI); Sub-model 3 contains IGBT and Chip (IC), and Sub-model 4 contains Chip and Solder-ball (CS). These four sub-models are one-way coupled by transmitting their results as boundary conditions between levels. The full-field simulation method is employed to verify the efficiency and accuracy of multi-scale simulation method for a single-rack data center. The two simulation results show that the highest temperature emerges in the same location. The Single-rack Full-field Model (SRFFM) costs 2.5 times more computational time than that with Single-rack Multi-scale Model (SRMSM). The deviation of the highest temperature of chips and solder balls are 1.57°C and 0.2°C between the two models which indicates that the multi-scale simulation method has good prospect in the data center thermal simulation. Finally, the multi-scale thermal analysis method is applied to a ship data center with 15 racks. [ABSTRACT FROM AUTHOR]

A reconstruction method is proposed for the polyurethane foam and then a complete numerical method is developed to predict the effective thermal conductivity of the polyurethane foam. The finite volume method is applied to solve the 2D heterogeneous pure conduction. The lattice Boltzmann method is adopted to solve the 1D homogenous radiative transfer equation rather than Rosseland approximation equation. The lattice Boltzmann method is then adopted to solve 1D homogeneous conduction-radiation energy transport equation considering the combined effect of conduction and radiation. To validate the accuracy of the present method, the hot disk method is adopted to measure the effective thermal conductivity of the polyurethane foams at different temperature. The numerical results agree well with the experimental data. Then, the influences of temperature, porosity and cell size on the effective thermal conductivity of the polyurethane foam are investigated. The results show that the effective thermal conductivity of the polyurethane foams increases with temperature; and the effective thermal conductivity of the polyurethane foams decreases with increasing porosity while increases with the cell size. [ABSTRACT FROM AUTHOR]

A numerical approach is developed to simulate the formation of pseudomorphs with presence of chemical fluids at the mesoscopic scale. This approach consists of the lattice Boltzmann method (LBM) for transport of chemical species in the pore space, a chemical reaction model including basic kinetics of the coupled dissolution and precipitation reactions, and a mesoscopic model for nucleation and crystal growth. Our study confirms the mechanism of the solution chemistry-driven interface-coupled dissolution-precipitation for the formation of pseudomorphs and identifies several sources for the generation of porosity in the pseudomorphs. We demonstrate that epitaxial precipitation is not necessary and random crystal growth may be more favorable for pseudomorphs. We show that the difference of precipitation barrier on the surface of the primary and secondary minerals should not be too large. Otherwise only the rim of the primary phase is roughly preserved. [ABSTRACT FROM AUTHOR]

Fully developed flow and heat transfer in metal-foam filled tube with uniform wall temperature (UWT) is semi-analytically investigated based on the Brinkman-Darcy model and the two-equation model, in which the inertia term, axial conduction, and thermal dispersion are ignored. A two-dimensional numerical simulation that adopts the full governing equations is also conducted to analyze the effects of neglected terms on flow and thermal transport performance by comparing with the semi-analytical solution. The effects of the relevant parameters and thermal boundary conditions including UWT and uniform heat flux (UHF) on the heat transfer characteristics are discussed based on the semi-analytical solution. The results show that the inertia term has a significant effect on the prediction of pressure drop, but has a relatively mild effect on Nusselt number. The axial conduction has significant effect on the Nusselt number at lower Reynolds number, and the effects of thermal dispersion can be neglected when the thermal conductivity ratio between fluid and solid is remarkably smaller for air/metal foam as example ( k/ k<3×10). The predicted Nusselt number of the semi-analytical solution is about 8% to 15% lower than that of the numerical solution with full model in the range of 4×10< k/ k<3×10. Moreover, the temperature profile of solid is more sensitive to pore density and porosity than that of fluid under UWT condition. The Nusselt number under UWT is about 7% to 25% lower than that under UHF, and the difference is mainly determined by interfacial convection rather than solid conduction. [ABSTRACT FROM AUTHOR]

A new air-water dual source heat pump water heater with heat recovery is proposed. The heat pump system can heat water by using a single air source, a single water source, or air-water dual sources. The water is first pre-heated by waste hot water, then heated by the heat pump. Waste heat is recovered by first preheating the cold water and as water source of the heat pump. According to the correlated formulas of the coefficient of performance of air-source heat pump and water-source heat pump, and the gain coefficient of heat recovery-preheater, the formulas for the coefficient of performance of heat pump in six operating modes are obtained by using the dimensionless correspondence analysis method. The system characteristics of heat absorption and release associated with the heat recovery-preheater are analyzed at different working conditions. The developed approaches can provide reference for the optimization of the operating modes and parameters. The results of analysis and experiments show that the coefficient of performance of the device can reach 4-5.5 in winter, twice as much as air source heat pump water heater. The utilization of waste heat in the proposed system is higher than that in the system which only uses waste water to preheating or as heat source. Thus, the effect of energy saving of the new system is obvious. On the other hand, the dimensionless correspondence analysis method is introduced to performance analysis of the heat pump, which also has theoretical significance and practical value. [ABSTRACT FROM AUTHOR]

In the present paper, the classical pressure correction method was extended into low Mach number compressible flow regime by integrating equation of state into SIMPLE algorithm. The self-developed code based on this algorithm was applied to predicting the lid-driven cavity flow and shock tube problems, and the results showed good agreement with benchmark solutions and the Mach number can reach the magnitude of as low as 10−5. The attenuation of sound waves in viscous medium was then simulated. The results agree well with the analytical solutions given by theoretical acoustics. This demonstrated that the present method could also be implemented in acoustics field simulation, which is crucial for thermoacoustic simulation. [ABSTRACT FROM AUTHOR]

The slotted fin concept was employed to improve the air cooling performance of plate-fin in heat sinks. Numerical simulations of laminar heat transfer and flow pressure drop were conducted for the integral plate fin, discrete plate fin and discrete slotted fin heat sinks. It is found that the performance of the discrete plate fin is better than that of the integral continuum plate fin and the performance of slotted fin is better than that of the discrete plate fin at the same pumping power of the fan. A new type of heat sink characterized by discrete and slotted fin surfaces with thinner fins and smaller spaces between fins is then proposed. Preliminary computation shows that this type of heat sink may be useful for the next generation of higher thermal load CPUs. The limit of cooling capacity for air-cooling techniques was also addressed. [ABSTRACT FROM AUTHOR]

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. [ABSTRACT FROM AUTHOR]

A three-dimensional physical and numerical model of the basic pulse tube refrigerator (PTR) was developed. The compressible and oscillating fluid flow and heat transfer phenomenon in the pulse tube were numerically investigated using a self-developed code. Some cross-section average parameter variations such as velocity, temperature and pressure wave during one cycle were revealed. The variations of velocity and temperature distributions in the pulse tube were also analyzed in detail for further understanding of the working process and refrigeration mechanism of PTRs. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. [ABSTRACT FROM AUTHOR]

A fuzzy control method was used to accelerate iteration convergence in numerical fluid dynamic simulation using SIMPLER algorithm. The residual ratio of momentum or energy equation between two successive iterations was used as the input variable. A fuzzy logic algorithm was developed in order to obtain the relative increment of the under-relaxation factor and its new value was then used for the next iteration. The algorithm was tested by four benchmark problems. In all cases considered, when the fuzzy control logic was used, convergence was achieved with nearly the minimum number of iterations, showing the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Performance comparisons are composed of two parts: the first part contains the systematically investigation of six difference schemes including CDS, FUDS, HDS, PLDS, SUDS and QUICK for convection terms in numerical fluid flow and heat transfer based on the finite volume method using staggered and Rhie-Chow’s momentum interpolation collocated grids, the second part contains the comparative computations being conducted on Rhie-Chow’s momentum interpolation collocated grid and Thiart’s finite difference scheme based nonstaggered grid. Three 3-D cases that have analytical or benchmark solutions are adopted. For the first part, the results of computations indicate that, all the six schemes have the same numerical accuracy when the diffusion term is predominant. With the increase of convection, the FUDS, HDS and PLDS almost have the same accuracy in two of those grid systems, while the SUDS and QUICK have higher accuracy than the former. The accuracy of CDS is something in between. For the same under-relaxation factors and convergence criterion, the convergence rate of each scheme on those two grid systems are nearly equal with that on the staggered grid being a little bit faster. For QUICK and CDS, smooth, non-oscillating solutions can be obtained even when local Peclet number may be as large as 31.2–31.3. For the second part, it is concluded that simplified collocated grid system is preferable from numerical accuracy, grid Peclet number limit, sensitivity to the under-relaxation factor and the freedom in choosing finite difference scheme for convection term. [ABSTRACT FROM AUTHOR]

A method is proposed to apply specified pressures at the entrance and exit of a nanochannel/nanotube. The pressure at the entrance is controlled by imposing an external force on fluid atoms within the pump region, while the pressure at the exit is adjusted by applying another external force on the piston which is an extension of the back reservoir. The method is validated by simulating argon flowing through a nanochannel/nanotube with different pressures at the ends, and the simulation results show that the pressures can be well controlled by this method. To further test the method in an unsteady case, the external force in the pump region is linearly increased while that on the piston keeps unchanged. The results show that the pressure in the back reservoir keeps constant while the pressure in the front reservoir increases as expected. The end effect on pressure drop for argon flowing through a nanotube is also investigated. Finally, the method has been proven to work effectively for water transport through carbon nanotubes with different diameters and different wall models. The advantage of the method is that it controls the absolute pressures in both the front and back reservoirs continuously and it is more applicable to long nanochannels/nanotubes compared to previous methods. [ABSTRACT FROM AUTHOR]

Porous structures of shales are reconstructed using the markov chain monte carlo (MCMC) method based on scanning electron microscopy (SEM) images of shale samples from Sichuan Basin, China. Characterization analysis of the reconstructed shales is performed, including porosity, pore size distribution, specific surface area and pore connectivity. The lattice Boltzmann method (LBM) is adopted to simulate fluid flow and Knudsen diffusion within the reconstructed shales. Simulation results reveal that the tortuosity of the shales is much higher than that commonly employed in the Bruggeman equation, and such high tortuosity leads to extremely low intrinsic permeability. Correction of the intrinsic permeability is performed based on the dusty gas model (DGM) by considering the contribution of Knudsen diffusion to the total flow flux, resulting in apparent permeability. The correction factor over a range of Knudsen number and pressure is estimated and compared with empirical correlations in the literature. For the wide pressure range investigated, the correction factor is always greater than 1, indicating Knudsen diffusion always plays a role on shale gas transport mechanisms in the reconstructed shales. Specifically, we found that most of the values of correction factor fall in the slip and transition regime, with no Darcy flow regime observed. [ABSTRACT FROM AUTHOR]

The inner doubly iterative efficient algorithm (IDEAL), an advanced fully implicit algorithm for solving the pressure-velocity coupling problem, lacks a comprehensive implementation procedure on commonly used collocated unstructured grids. Therefore, this study provides a thorough demonstration of the fundamental principles, solving procedures, discretizations of momentum and pressure equations, Rhie-Chow interpolation implementation, mass continuity processing, and boundary condition treatment associated with the IDEAL algorithm on a collocated unstructured grid. Additionally, a comparative analysis between the semi-implicit method for pressure-linked equations (SIMPLE) and the IDEAL algorithm is conducted concerning computational efficiency under different grid types and flow conditions. The results indicate a significantly higher computational efficiency achieved by the IDEAL algorithm across all tested cases, attributed to its ability to overcome two fundamental assumptions inherent in the SIMPLE algorithm. The average acceleration ratio for these cases is approximately 3.13 (tSIMPLE/tIDEAL). This study serves as a valuable reference for the further development and application of the IDEAL algorithm in solving flow problems with complex irregular domains discretized by unstructured grids. [ABSTRACT FROM AUTHOR]