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Modified serially–linked serpentine flow fields are proposed for proton exchange membrane fuel cells (PEMFCs) and their performances are numerically studied. The effects of the segment number and the channel path number on the cell performance are analyzed. Especially The effect of the secondary flow on the cell performance is particularly discussed. Numerical results indicate that the modified serpentine flow fields can obviously improve the cell performance in comparison with the parallel flow field. The cell performances can be improved by increasing the segment number or the channel path number in each segment. The reactant transport under the rib and the secondary flow in the channel are considered as the main causes for the high cell performance of PEMFC with modified serpentine flow fields. The secondary flow can be generated by the combination of the under–rib flow and the serpentine turn. In comparison with the parallel flow field, the modified serpentine flow fields have higher average oxygen concentration but lower uniformity of the oxygen concentration distribution.

The application of both the vortex generators and twisted belts were investigated in literature as they are effective in improving heat exchanger thermal performance. In this paper, a novel method to enhance the heat exchanger thermal performances obtained by combining the vortex generator and twisted belt is proposed. The circular tube thermo-hydraulic characteristics were numerically studied by combining the twisted belt and delta wing vortex generators (DWVGs). The effects of twist ratio, attack angles, pitch and vortex generator arrangement were investigated. The emphasis was put on obtaining the optimal parameters. The Reynolds number was based on the hydraulic diameter channel flow, which ranged from 5000 to 25,000. The results indicate that the most favorable vortex generator arrangement is staggered arrangement when the twisted belt twists counter-clockwise. The optimal twist ratio is 4, and PEC increases with the increase in the angle of attack and decrease of the vortex generator pitch. When considering the conditions studied in this paper, the PEC can be improved by 96% ~ 128% compared with the smooth tube. Said improvements are mostly obtained when using the novel combination structures with the staggered arrangement, optimal twist ratio of 4, attack angle of 75°, and pitch of 1.5D.

An inverse convection heat transfer in a two–dimensional channel mounted with square ribs was concerned in the present work. A simplified conjugate gradient method was adopted for optimizing the convection heat transfer. The optimal pitch ratio of the ribs was searched under the maximum heat transfer rate. The sensitivity and adjoint problems were not considered but a constant search step size was applied. The results showed that the simplified conjugate gradient method can be used to search the optimal pitch ratio of the ribs at various initial values. But the constant search step size may result in the oscillation of the numerical results. The searched optimal pitch ratio with the inverse method at one Reynolds number can be spread to a large range of Reynolds numbers.

Heatsink is a heat dissipation system which widely used in the present electronic devices. As the integration of electronic equipment increases, traditional axial fans are adopted in the heat dissipation for heatsink in electronic devices due to its size limitations may no longer ensure that its can work normally and efficiently. The electromagnetic driven oscillator (EDO) is mainly composed of a pair of oscillating sheets and an electromagnetic driving device. As an innovative way of heat dissipation, it has been gradually attracting attention in recent years. The work principle is that the electromagnetic force makes the oscillating fans vibrate and gives rise to surrounding airflow achieving the purpose of cooling. In this paper, the effect of the electromagnetic driven oscillator on heat dissipation is studied. And, there are two different fan sheets: the straight boundary fin (SBF) and the jagged top boundary fin (JTBF). The numerical simulation and experiment were combined to study the heat transfer and flow characteristics of the heatsink with EDO. The results indicate that the electromagnetic driven oscillator (EDO) can effectively decrease the temperature on the heatsink, and the airflow generated by the vibration of the oscillator can take away the heat; the heat dissipation effect of the jagged top boundary fan (JTBF) is better than the straight boundary fan (SBF). For the jagged top boundary fan (JTBF), the area of the high temperature of the heatsink is significantly smaller and 1.4K lower than the temperature of the straight boundary fan (SBF).

The buoyancy influenced heat transfer characteristics of S-CO2 have been investigated widely in previous studies, but limited to the conditions with constant wall temperature or constant heat flux. The present study numerically investigates the buoyancy effects on the convective heat transfer of S-CO2 and the thermal stress in parabolic trough receivers (PTRs) under the non-uniform solar flux distribution based on a coupled optical-thermal-fluid-mechanical numerical model. The results indicate that the drastic density variation associated with the large temperature gradient of the fluid occurs on the cross-section under the non-uniform solar flux distribution, which further induces the secondary flow. As the secondary flow improves the synergy between the velocity vector and the temperature gradient, the bulk average heat transfer is prominently enhanced due to the buoyancy effect. However, the buoyancy effects on the local heat transfer characteristics are more complex. The buoyancy effect enhances the heat transfer at the bottom but impairs the heat transfer at the top, and the heat transfer enhancement area is expanded along the flow direction. The temperature distribution in the absorber tube wall is greatly flattened due to the buoyancy effect, and the thermal stress is consequently reduced. The buoyancy effects on the thermal and mechanical performances become less significant with the increase in the operating pressure, the inlet velocity and the inlet temperature but with the decrease in DNI. The buoyancy effect in the PTR under non-uniform solar flux distribution can be ignored when Se/Re

Supercritical CO2 (S-CO2) is an attractive heat transfer fluid (HTF) candidate for application in parabolic trough receivers (PTRs). The thermal and mechanical performances are of serious concern for the PTR using S-CO2 as HTF which suffers from the rigorous conditions of high operating pressure and non-uniform solar flux distribution. The present study develops an integrated numerical model for the optical-thermal-fluid-mechanical analysis of PTRs by coupling Monte Carlo ray-tracing method, finite volume method, and finite element method. It is found that the non-uniform solar flux distribution can induce significant secondary flow of S-CO2 which improves the synergy of the velocity vector and the temperature gradient in the fluid, benefiting the convective heat transfer. However, both of the high operating pressure and the non-uniform solar flux distribution can induce remarkable mechanical stress in PTR, and the superposition of the operating pressure and the solar flux distribution aggravates the mechanical stress. It should be noticed that the operating pressure has no contribution to the axial stress component, while the radial stress component induced by the non-uniform solar flux distribution can be neglected. Flattening the solar flux distribution by adding a secondary reflector is recommended as an effective approach to reducing the thermal stress, and the maximum thermal stress can be reduced from 50 MPa to 10 MPa under the conditions studied in this paper.

A geometry model for simulating the gas circulation, heat transfer and slab heating process in a reheating furnace was developed with numerical simulation. The slabs are heated in the furnace and the temperature of the slabs become above more than 1500 K, and then they are transported to the rolling mill. The slab heating in a reheating furnace is influenced by many factors, such as gas velocity, the movement speed of the slab, the height and spacing of the burners and others. The slab heating characteristics in the walking beam type reheating furnace were studied by considering the slab movement. The movement of the slabs was processed by transferring temperature data from the previous location to the next location. It was assumed that no time is spent for moving the slabs. Especially, the effects of different inlet boundaries of the burners were investigated. Results show that the entrainment effect from the inner air flow results in a larger coverage area of the flames, and there is an increase of more than 28.5% in the slab temperature over the whole furnace.

In this article, the influence of the flow field structure and temperature gradient of forefront interface on the shape of crystal-melt interface which may reflect the interface stability were analyzed through the method of numerical simulation by using CGSim software. In order to get a suitable interface shape and grow high-quality sapphire crystal, the heater arrangement should be adjusted during the KY process. The results indicate that the effect of Marangoni convection cannot be neglected at the last stage, the crystal-melt interface is governed by the flow field structure and the temperature gradient in melt at the crystal-melt interface. The phenomenon of shoulder concave appears at the stage of shoulder turning and interface inversion appears at the last stage during the crystal growth is discussed. Adjusting heater arrangement may effectively optimize the shape of crystal-melt interface.

In this paper, heat transfer characteristics of natural convection in an enclosure are investigated by considering variable thermal properties. The heated enclosure with an aspect ratio of unity is full of an alumina-water nanofluid. To analyze the effect of different thermal properties on the flow and temperature distributions, many comparisons are conducted for various cases at various Rayleigh numbers and volume fractions of nanoparticles (1–9.5%). The effect of low-gravity conditions on natural convection is investigated by using the gravity values of 0.25 g (2.45 m/s2), 0.5 g (4.9 m/s2), 0.75 g (7.35 m/s2) and 1.0 g (0.98 m/s2). It was found that larger temperature gradients near the heated and cold walls can be obtained by increasing the Rayleigh number. By increasing the value of gravity, the flow velocity increased along the y direction. This indicated that gravity has great influence on enhancement of heat transfer. The investigation of the volume fraction of nanoparticles shows that the variation of the Nusselt number for high volume fraction presented a more flat profile than for low volume fraction. The Nusselt number decreased with an increase of the volume fraction. Results for cases with connected and unconnected heated walls are presented. The results showed that heat transfer for the case with the nanoparticle volume fraction of 1% at 1.0 g increased by 89.9% using an unconnected heated wall compared to the connected heated wall.

Three dimensional numerical simulations are performed to investigate the flow and heat transfer characteristics of the plain fin with multi-row delta winglets punched out from the fin. The Reynolds number based on the tube outside diameter varies from 360 to 1440. The effects of punched holes and their orientations on flow structure and heat transfer are numerically studied. Results show that a down-wash flow is formed through the hole punched at the windward side, which has little influence on the longitudinal vortices in the main flow, and a longitudinal main vortex is formed behind each delta winglet. An up-wash flow is formed through the hole punched at the leeward side, the up-wash flow impinges the longitudinal vortices generated by the delta winglet, and then a counter-rotating pair of main vortices is generated behind each delta winglet. The windward punched holes have little effect on the flow friction and heat transfer of the plain fin with delta winglets, while the leeward punched holes deteriorate the heat transfer and decrease the flow friction of the fin channel, the Nusselt number decreases by 3.5–5.0 % with a corresponding decrease of 3.9–4.8 % in the friction factor. The effect of the punched holes on the heat transfer of the fin can be well explained by the field synergy principle. The overall analysis of the thermal performance is performed for all fin configurations, including the slit fins and the wavy fins with one-row delta winglets, the plain fin with the windward punched delta winglets shows the better thermal performance than one with the leeward punched delta winglets.

The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger punched with combined rectangular winglet pairs (CRWPs) were numerically analyzed. The influences of several main parameters of the CRWP on heat transfer and friction were examined and compared to rectangular winglet pairs (RWP). The numerical results showed that the wavy fin-and-tube heat exchanger has a better heat transfer performance when the winglets punched on the suction side of the wavy fin. For the wavy fin punched with CRWPs, the Nusselt number and friction factor increase with increases of secondary attack angle, accessory winglet length or accessory winglet width. In comparison with RWP, the vortices generated by CRWP are more, larger and stronger. Hence, CRWP can more effectively improve heat transfer and increase friction.

A novel combined longitudinal vortex generator (LVG), comprising a rectangular wing mounted with an accessory rectangular wing, is developed and the turbulent flow and heat transfer characteristics are numerically analyzed. The influences of six main parameters of the combined rectangular winglet pair (CRWP) on heat transfer enhancement and fluid flow resistance characteristics in a rectangular channel are examined. The parameters include the location of accessory wing on the main wing and geometric sizes of the accessory wing. The Reynolds number range is from 2000 to 16,000. The numerical results show that in the range of the present study, the increase of the six parameters can result in the increase of heat transfer and pressure drop. Specially, the pressure drop decreases at large value of the distance of accessory wing from the channel bottom. In comparison with RWP, the CRWP generates vortices with larger area and lower core. Furthermore, the accessory wings generate vortices that swirl downward the channel bottom and disturb the boundary layer growth more effectively. Hence the heat transfer is enhanced. The numerical result for the Nusselt number of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.

A modified rectangular longitudinal vortex generator (LVG) obtained by cutting off the four corners of a rectangular wing is presented. Fluid flow and heat transfer characteristics of this LVG mounted in rectangular channel are experimentally investigated and compared with those of original rectangular LVG. Results show that the modified rectangular wing pairs (MRWPs) have better flow and heat transfer characteristics than those of rectangular wing pair (RWP). Near the positions of z = ±40 mm from the centerline of the heater plate, the local heat transfer is enhanced due to the strong longitudinal vortices generated by the presence of the LVGs. The down-sweep of the longitudinal vortices is beneficial to the heat transfer enhancement. The distance from the core of the main vortices of MRWP1 to the heater wall is slightly lower than those of RWP, and hence MRWP1 has a comparably better heat transfer characteristic.

Heat transfer in passage with pin-fin arrays for cooling blade trailing edge was studied numerically. Three-dimensional numerical simulations were carried out for steady laminar flow in passages with different wedge angles between pressure surface and suction surface of cooling blade trailing edge to study the effect of different wedge angles (from 0° to 30°) on heat transfer and pressure losses. Research was carried out for both in-line array and staggered array. From this investigation, wedge angle 10° gives the best heat transfer performance.

The heat dissipation performance of a panel type radiator attached with rectangular winglets on the air-side was numerically investigated. The natural convection heat transfer of the oil and air were concerned. The numerical results showed that the application of the rectangular winglets can obviously increase the heat dissipation of the panel type radiator. In the computational range of the present work, the total heat transfer coefficient is increased by 23.6–32.0% and the optimal condition is l = 2 mm, N = 10 and α = 15°. The rectangular winglet generates vortices that swirl down toward the radiator wall and disturb the boundary layer growth. Hence the heat transfer is enhanced.