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The ribbed surface is a widely used convection heat transfer enhancement technology. The geometric parameters of the ribbed surface have great influences on its heat transfer performance. It is crucial to determine the optimum geometric parameters. In the present work, the optimal arrangement of ribs mounted in a rectangular channel is provided by solving the inverse optimization problem with the entransy dissipation as the objective function. The parameter analysis based on the direct solution is firstly carried out to get an approximate range near the optimal pitch ratio which can provide the initial value for the inverse optimization problem; afterwards, the inverse optimization problem is solved by a modified simplified conjugate gradient method, and the accurate optimal pitch ratio is obtained; finally the optimization results are discussed in detail. The results indicate that, (1) the inverse optimization analysis presented in this paper can provide an accurate optimal value of the pitch ratio; (2) taking the entransy dissipation as the objective function could lead to a faster converge process and give a more stable result than taking the temperature data as the objective function; (3) the optimal value under the fixed heat flux boundary condition is slightly larger than that under the fixed temperature boundary condition.

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.

For district heating systems, prediction of the heat load is a very important topic for energy storage and optimized operation. For large and complex heating systems, most prediction models in previous publications only considered the influence of outdoor temperature, whereas the indoor temperature and thermal inertia of buildings were not included. For an energy-efficient residential building in Shijiazhuang (China), the heat load prediction is investigated using various prediction models, including a wavelet neural network (WNN), extreme learning machine (ELM), support vector machine (SVM) and back propagation neural network optimized by a genetic algorithm (GA-BP). In these models, the indoor temperature and historical loads are considered as influencing factors. It is found that the prediction accuracies of the ELM and GA-BP are slightly higher than that of WNN, so the ELM and GA-BP models provide feasible methods for the heat load prediction. The SVM shows smaller relative errors in the model prediction compared with three neural network algorithms.

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.

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.

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.

This study is focused on the preparation and performance of a building energy storage panel (BESP). The BESP was fabricated through a mold pressing method based on phase change material particle (PCMP), which was prepared in two steps: vacuum absorption and surface film coating. Firstly, phase change material (PCM) was incorporated into expanded perlite (EP) through a vacuum absorption method to obtain composite PCM; secondly, the composite PCM was immersed into the mixture of colloidal silica and organic acrylate, and then it was taken out and dried naturally. A series of experiments, including differential scanning calorimeter (DSC), scanning electron microscope (SEM), best matching test, and durability test, have been conducted to characterize and analyze the thermophysical property and reliability of PCMP. Additionally, the thermal performance of BESP was studied through a dynamic thermal property test. The results have showed that: (1) the surface film coating procedure can effectively solve the leakage problem of composite phase change material prepared by vacuum impregnation; (2) the optimum adsorption ratio for paraffin and EP was 52.5:47.5 in mass fraction, and the PCMP has good thermal properties, stability, and durability; and (3) in the process of dynamic thermal performance test, BESP have low temperature variation, significant temperature lagging, and large heat storage ability, which indicated the potential of BESP in the application of building energy efficiency.

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.