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A system with optimum thermal control and noise control is proposed to allow placement of servers at workspace for low operation office cost. A computational fluid dynamics (CFD) model is developed to predict the air flow and temperature distribution of the system. To improve the accuracy of the simulation, the geometry of the axial-flow fan is reproduced in detail and the rotor region is treated with the multiple reference frame model. A porous model is applied to describe the hydraulic and thermal behavior of the evaporator. The integrated system simulation model is validated by measurements. Four layouts of fans are proposed to analyze the airflow organization and temperature distribution. The inhomogeneity of the temperature field is quantitatively evaluated via information entropy and variance. The results reveal that the airflow organization scheme avoids the occurrence of an evident stagnant zone or a hot spot. There are low-velocity zones with insufficient heat transfer in the evaporator. Increasing the number of fans can improve the heat transfer and result in a more uniform temperature distribution. Setting up ventilation fans to increase the ventilation rate of the server is an effective way to meet the challenges of higher heat density. [ABSTRACT FROM AUTHOR]

For natural draft direct dry cooling system, the triangularly arranged air-cooled condenser may lead to the unexpected airflow deviations and unbalanced flow distributions although with an additional buoyancy force from the dry-cooling tower. In this work, an annularly arranged air-cooled condenser is proposed for natural draft direct dry cooling system. By numerical simulations, the flow and heat transfer performances of these two dry cooling systems are analyzed and compared at various wind speeds. The results show that for the annularly arranged air-cooled condenser, the air flow interactions between the neighboring cooling columns are totally avoided at small wind speeds, and moreover, the vortices at the inlets of the cooling columns may vanish for the middle sector at high wind speeds. The air inflow and outflow deviations through the cooling columns are clearly restrained, which improve the local thermo-flow performances, thus increase the heat rejection conspicuously and recover the cooling efficiency of natural draft direct dry cooling system compared with the triangularly arranged air-cooled condenser, especially at small wind speeds. The annular configuration of air-cooled condenser could be recommended for the potential engineering applications thanks to its more energy efficient performance. [ABSTRACT FROM AUTHOR]

In this work, natural convection heat transfer from two horizontally attached cylinders in air has been studied numerically over the range of Rayleigh number 10 ⩽ Ra ⩽ 10 5 . A new model proposed by Bejan et al. (1995) is applied here, and it has been proved to be a more accurate and effective method for the numerical simulation of the natural convection in free space than other models used previously. Furthermore, the representative results for streamlines, isothermal contours, local Nusselt number and local drag coefficients have been presented with different Rayleigh numbers. It can be observed that there form two recirculation vortexes in the wake region when the two plumes begin to merge, and their sizes grow with the increasing Rayleigh number due to the downstream movement of the front stagnation point and the upstream movement of the separation point. Owing to the interactions of their plumes, the location of the maximum value of local Nusselt number moves downstream along the cylinder surface, i.e., it displaces to 133–150° depending on the Rayleigh number whereas it always occurs at the front stagnation point corresponding to 90° for a single cylinder. However, because the thinnest boundary layer in this work still hardly penetrates the small clearance between them and then influences their heat transfer, their interactions are independent of the Rayleigh number. Finally, a new correlating equation of the average Nusselt number with the Rayleigh number for the present configuration, has been proposed. [ABSTRACT FROM AUTHOR]

The lubrication and cooling design is important for high-speed rolling bearings. A multiphase numerical simulation and the corresponding tests were conducted to investigate the stratified air-oil flow inside jet cooling ball bearings. The simulated results of the volume fraction distribution has the same trend with measured observations. The heat transfer was also considered. It was found that the temperature distribution of the bearing was affected by the volume fraction distribution. The higher temperature always appears in the lower oil volume fraction region. The conventional oil-jet lubrication mechanism cannot achieve an efficient cooling on the inner ring of the high-speed ball bearing. The oil volume fraction increases with an increasing nozzle number. The number should not be larger than six for increasing the oil volume fraction. The jet velocity and the oil flow rate have great effects on the average oil volume fraction inside the bearing rather than the oil volume fraction distribution around the circumference. The flow pattern inside the jet-cooling ball bearing is significant for its temperature forecast and control. [ABSTRACT FROM AUTHOR]

Well-resolved direct numerical simulation (DNS) is applied to investigate strongly heated air flow in a vertical pipe ( L = 30 D ) at inlet Reynolds numbers of Re 0 = 4240 and 6020. The DNS is based on the experimental cases of Shehata and McEligot (1995) and shows excellent agreement with heat-transfer and flow statistical results. Flow relaminarization is observed in the strongly heated cases. We apply a new semi-local wall coordinate to replace the conventional one. With the semi-local wall coordinates, which considers the local property variation, both the velocity and temperature fields show the process of relaminarization. This relaminarization is also indicated by the significant decrease of turbulence intensity and Reynolds shear stress. In the quasi-laminar flow, the viscous sublayer becomes thicker. Turbulence in this layer shows a growing anisotropic character. And turbulence in the pipe center becomes approximately isotropic. This two-layer character is clearly displayed by flow visualization. [ABSTRACT FROM AUTHOR]

• The guide vane is added to the air inlet chamber of the intercooler. • An end-to-end model of the intercooler guide vane based on PSO-SVR algorithm. • Structure parameters of guide vane are optimized using genetic algorithm. In order to fundamentally solve the problem of poor air flow uniformity in the cooling pipe of the heavy-duty vehicle intercooler, which leads to poor heat exchange effect, this paper innovatively proposes to add a guide vane in the air chamber of the intercooler. The air flow uniformity in the cooling pipe of the heavy-duty vehicle intercooler is investigated under different structural parameters of guide vane based on the established end-to-end model. In fact, the end-to-end prediction model is innovatively proposed to characterize the relationship between the structural parameters of guide vane and performance parameters without conducting the high-cost and long-term experiments or numerical simulation. Besides, structure parameters of guide vane with the optimal airflow uniformity coefficient are obtained using genetic algorithm (GA), that is, the diversion angle (α) is 18°, the width of rectangular holes in the guide vane (L) is 6 mm and the number of rectangular holes in the guide vane (N) is 3. To demonstrate the effectiveness of the proposed method, the optimized intercooler is validated and it is found that the airflow uniformity coefficient of the optimized intercooler is 0.983, which is 5.25% higher than the original structure, the outlet temperature is reduced by 8.2 °C. [ABSTRACT FROM AUTHOR]

Numerical study is made of heat transfer enhancement in the narrow channel with insulated walls in air steady flow around a heated spherical dimple, when its relative depth is varied from 0 to 0.26 (in terms of spot diameter) at a defined Reynolds number 4 × 10 4 based on bulk velocity and dimple spot diameter. The applicability of multiblock computational technologies for solution of Reynolds and energy equations with the implication of an implicit factorized finite-volume algorithm and overlapping different-scale structured grids of different topology, as well as the verification of the shear stress transfer model (SST model) modified with regard to the streamline curvature within the framework of Leschziner–Rodi–Isaev’s approach is assessed from the comparison of numerical predictions obtained by different SSTM versions. Flow regimes in a spherical dimple, as its depth is increased, are classified on the basis of the analysis of change in the jet–vortex flow structure in the dimple and its wake in the channel. In what follows, special attention is paid to asymmetric flow around a dimple with the greatest vortex heat transfer enhancement. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the transient 3D numerical simulations of natural convection of air around a circular finned tube (24.9 mm OD ) kept in a small chimney. The annular plain fins are considered in this study. The effects of fin spacing to fin diameter ratio (0.057 mm ⩽ S / D f ⩽ 0.24 mm), chimney height (400–1000 mm) and ambient to surface temperature difference (10 K ⩽ T s ⩽ 65 K) on the heat transfer and the driving force have been investigated. The results are presented in terms of the temperature contours, velocity vectors, heat transfer and the driving force. It has been found that the heat transfer coefficient increases with an increase in the fin spacing upto an optimum value ( S = 8 mm) for all the fin geometries, and beyond S = 8 mm, the heat transfer coefficient decreases. The separation of the thermal boundary layer with a variation in the fin spacing and its effects on the heat transfer and driving force has been shown. For a fixed fin spacing, the heat transfer rate, heat transfer coefficient and the driving force increases with an increase in the fin diameter, however, for D f > 41 mm, the rate of increase in the heat transfer coefficient reduces. The heat transfer coefficient increases with an increase in the chimney height and it has been found that the effect of chimney height on the heat transfer coefficient is a resultant effect of the air outlet temperature and the driving force generated by the chimney. The base to ambient temperature difference has been varied to observe the temperature sensitivity on the heat transfer coefficient and flow patterns. In the last section, various circular and elliptical tube designs have been investigated, and it is found that, the elliptical tube with minimum ellipticity ( b / a = 0.33) and circular tube with smallest diameter (7 mm) provides better heat transfer coefficient than the other circular and elliptical designs. [ABSTRACT FROM AUTHOR]

Abstract: Punched longitudinal vortex generators (LVGs) were employed to enhance air-side heat transfer on the wavy fin surface of flat tube used in direct air-cooled condenser. The heat transfer enhancement of four types of the longitudinal vortex generators with different attack angles were compared by numerical simulations. It was found that the delta winglet pair with attack angle 25° could reach the greatest performance evaluation criteria (PEC) under the conditions of the inlet air flow velocity varied from 1m/s to 5m/s. The influences of locations on the wavy fin surface and the row number of the longitudinal vortex generators were also discussed. One delta winglet pairs at the middle of the wavy fin surface and the minimum row number, n =1, with the average PEC is 1.23, has the best heat transfer performance of all conditions, which can be recommended for practical applications. Experimental study in wind tunnel with flow field visualization by Particle Image Velocimetry (PIV), as well as the numerical simulations verified that the delta winglet pairs can generate obvious longitudinal vortex pairs at the down-sweep zone, which can enhance the heat transfer between the cooling air flow and heated wall surface with acceptable pressure loss. [Copyright &y& Elsevier]

• Direct numerical simulation of droplet evaporation without any empirical parameter. • Identification of a sharp layer of interface cells for occurrence of phase change. • Algebraic compressive differencing scheme to maintain high resolution of interface. • Satisfactory prediction of droplet evaporation for a wide range of cases. • Sessile droplets gain averagely 29 percent of heat for evaporation from ambient gas. This study presents a sharp-interface algebraic volume-of-fluid (SA-VOF) approach towards direct numerical simulation of evaporating droplets in a finite-volume framework. An interface identification scheme is introduced to determine a sharp layer of interface cells where liquid-vapor phase change occurs. The rate of vaporization is directly computed from the local gradient of the vapor mass fraction at the interface without any tuning parameter in the evaporation model. In addition, a modified compressive differencing scheme is employed to maintain high resolution of the interface. A solver based on the SA-VOF approach is developed and validated by several case studies including the Stefan problem for evaporation in one dimension, evaporation of sessile droplets in the constant contact angle mode, and evaporation of droplets suspended in a flowing air stream. Comparisons between numerical predictions and either analytical solutions or experimental data show that the SA-VOF approach not only computes the evaporation rate with high accuracy but also ensures highly resolved liquid-vapor interface. The solver demonstrates the capability in revealing minute details of the complex characteristics associated with droplet evaporation. [ABSTRACT FROM AUTHOR]

Abstract: Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers. [ABSTRACT FROM AUTHOR]