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A mathematical model of the process of unsteady conjugate heat transfer of a thermionic thermal protection system during supersonic air flow around a spherically-blunted cone is investigated. Estimates of the effect of evaporation (emission) of electrons from the emitter surface on lowering the temperature of the composite shell of the thermoelectronic thermal protection are made. The influence of different angles of attack on heat transfer modes in the system of multielement thermionic thermal protection has been studied. Qualitative agreement of calculated results with the known data has been found. [ABSTRACT FROM AUTHOR]

Flat air solar collectors are used for heat transfer between the absorber and the heat transfer fluid, to improve this transfer there are several methods. Among these methods, the exchange surface lengthening and the creation of turbulence. In this work is done to give a comparison between two types of solar collectors, so we have made an improvement of Ben Slama Romdhane's solar collector by creating two air flow passages to increase heat transfer. We made a 3D simulation of a flat air solar collector with transverse baffles which causes turbulence and increases the exchange surface; we use the ANSYS calculation code to make the simulation and gives results with a brief time and minimal cost. [ABSTRACT FROM AUTHOR]

In this paper, a solution method for the temperature distribution of rectangular test specimen with a high-speed heat air-flow passing through is proposed based on the heat transfer theory and numerical calculation, and the feasibility of temperature prediction method is validated. Firstly, the partial differential equations to describe the average temperature in the section of the hot air-flow and the specimen are established and the solving method using MATLAB solver is proposed. Then, based on heat transfer conduction equation and the average temperature, the temperature distribution at different point in each section is calculated. The comparison between numerical computation and experiment shows that two results are in good agreement, which verifies the correctness of the presented prediction method of the temperature distribution of the specimen. [ABSTRACT FROM AUTHOR]

The paper presents a preliminarily study on an efficient, inexpensive and energy saving solar batch dryer for drying sludge. A concept of a mixed mode solar dryer was developed and designed. Air heated by the solar flat heater was forced through drying chamber by electric fans. A 3-D physical model was used to observe and predict the operation of the solar batch dryer at different time under designed conditions. The thermal performance and air movement of drying chamber in the designed dryer unit were evaluated through ANSYS-FLUENT software. By determining the external conditions, localization and the material properties, the model can predict temperature and humidity distributions in the designed drying chamber and sludge material layers, air-flow field according to the radiation and convection, as well as water quantity evaporated from the sludge. A special attention was paid to implementation of physical boundary conditions on the sludge surface, which is between air and dried sludge. The developed solar barn dryer can heat air at average temperature between 47 °C and 57 °C, which is optimum for dehydration of the sludge. The designed drying chamber can generate an adequate fow of hot air to increase the drying rate by above 30%. [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]

The importance of accounting for the temperature dependence of air properties in numerical simulations of air flows over pinned heat sinks is demonstrated by comparisons with recently published experiments. Numerical simulations, based on a conjugate heat transfer analysis, using the RANS-based modified k-ω turbulence model, with temperature-dependent air properties, are shown to be in significantly better agreement with experimental measurements of pressure drop, heat transfer coefficient, and heat sink base temperature, than those which employ constant air properties. [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]

Numerical simulations are carried out to study the unsteady air flow and heat transfer in a partitioned triangular cavity of differentially heated from inclined walls and heat source placed at the bottom wall. The finite volume numerical method has been employed to solve the governing equations. The grid sensitivity test has been carried out and obtained results have been validated against experimental results. The dependency of several parameters on fluid flow and heat transfer is studied including Rayleigh number from 10 3 to 10 6 , heater size from 0.2 to 0.6, heater position from 0.3 to 0.7 and aspect ratio from 0.2 to 1.0 with a fixed Prandtl number of 0.72 (air). A conductive partition of infinite conductivity is placed at the middle of the enclosure, which means that only heat can freely transfer between two fluid zones through the partition. Results are presented as a form of isotherms and streamlines. Also, heat transfer is presented as a form of Nusselt number. Both transient and steady states of solutions are presented in this study. [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]

An analysis of the convective heat exchange in the separation air and oil flows in banks of heated round tubes and their hydraulic losses at Reynolds numbers changing in a wide range from 100 to 400 for the laminar flow and from 10 to 8·10 for the turbulent fl ow has been performed. For solving the Navier-Stokes and energy equations, multiblock computational technologies realized in the VP2/3 package and original procedures for correction of the pressure gradient in a fl ow and its mean-mass temperature were used. The Reynolds-averaged Navier-Stokes equations were closed using the Menter shear-stress transfer model modified with account for the curvature of streamlines within the framework of the Leshtsiner-Rody approach with an Isaev-Kharchenko-Usachov constant equal to 0.2. The results of numerical simulation were compared with the corresponding experimental data of A. Zhukaukas. The dependence of the local and integral characteristics of a fl ow of a heat-transfer agent in a staggered bank of round tubes on the properties of this agent was determined. [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]

In this paper, a general numerical methodology is developed and validated for the simulation of steady as well as transient thermal and electrical behaviors of thermoelectric generator (TEG)-based air flow self-cooling systems. The present model provides a comprehensive framework to advance the study of self-cooling applications by combining fluid flow, heat transfer and electric circuit simulations. The methodology is implemented by equation-based coupled modeling capabilities from multidisciplinary fields to capture the dynamic thermos-electric interaction in TEG elements, enabling the simulation of overall heating/cooling/power characteristics as well as spatially distributed thermal and flow fields in the entire device. Experiments have been conducted on two types of self-cooling arrangements to measure the device temperature, voltage and power produced by TEG modules. It was found that the computational model was able to predict the experimental results within 5% error. A parametric study was carried out using the validated model to study the effect of heat sink geometry and TEG arrangements on device temperature and power produced by the device. It was found that the power for self-cooling could be maximized by proper matching of the TEG modules to the fluid mover. Although an increase in fin density results in a rise in fan power consumption, a marked increase in net power and decreases in thermal resistance are observed. [ABSTRACT FROM AUTHOR]