In the present paper, the forced convective heat transfer was experimentally studied in packed beds with uniform and non-uniform spheres. The pore structures of different packings were reconstructed with discrete element method, and the lateral porosity distributions were analyzed. Furthermore, the pressure drop, heat transfer and overall heat transfer performances of different packings were investigated and compared with each other. Firstly, it is found that, using laterally layered composite packing (LLM), smaller pores would be formed close to the tube wall and big flow channels would be formed in the inner-tube region, which would reduce the tube-wall effect in the near tube wall region of packed bed. When compared with mono-sized packing with lower tube to particle diameter ratio, both the heat transfer and overall heat transfer capacities would be improved with LLM packing method. Secondly, it is found that, using randomly composite packing (RCM) or axially layered composite packing (ALM), both the lateral porosity magnitude and amplitude of packed bed are lower than those of mono-sized packing with higher tube to particle diameter ratio in the near tube-wall region. The tube-wall effect would be further reduced, and both the heat transfer and overall heat transfer capacities would also be improved with RCM and ALM packing methods.

Pipe cable is considered as an important form for underground transmission line. The maximum electrical current (ampacity) of power cable system mostly depends on the cable conductor temperature. Therefore, accurate calculation of temperature distribution in the power cable system is quite important to extract the cable ampacity. In the present paper, the fluid flow and heat transfer characteristics in the pipe cable with alternating current were numerically studied by using commercial code COMSOL MULTIPHYSICS based on finite element method (FEM). The cable core loss and eddy current loss in the cable were coupled for the heat transfer simulation, and the difference of heat transfer performances with pure natural convection model and radiation-convection model was compared and analysed in detail. Meanwhile, for the radiation-convection model, the effects caused by radiant emissivity of cable surface and pipe inner surface, as well as the cable location in the pipe, were also discussed. Firstly, it is revealed that the radiation and natural convection heat flux on the cable surface would be of the same order of magnitude, and the radiation heat transfer on the cable surface should not be ignored. Otherwise, the cable ampacity would be underestimated. Secondly, it is found that the overall heat transfer rate on the cable surface increases as the cable surface emissivity increases, and this is more remarkable to the upper cable. While the effect caused by the radiant emissivity on the pipe inner surface would be relatively small. Furthermore, it is demonstrated that, as cable location in the pipe falls, the natural convection heat transfer would be enhanced. These results would be meaningful for the ampacity prediction and optimum design for the pipe cable.

In the present paper, the heat transfer of gravity-driven dense particle flow around five different shapes of tubes is numerically studied using discrete element method (DEM). The velocity vector, particle contact number, particle contact time and heat transfer coefficient of particle flow at different particle zones around the tube are carefully analyzed. The results show that the effect of tube shape on the particle flow at both upstream and downstream regions of different tubes are remarkable. A particle stagnation zone and particle cavity zone are formed at the upstream and downstream regions of all the tubes. Both the stagnation and cavity zones for the circular tube are the largest, and they are the smallest for the elliptical tube. As the particle outlet velocity (v_out) changes from 0.5 mm/s to 8 mm/s at d_p = 1.72 mm/s, when compared with the circular tube, the heat transfer coefficient of particle flow for the elliptical tube and flat elliptical tube can increase by 20.3% and 15.0% on average, respectively. The proper design of the downstream shape of the tube can improve the overall heat transfer performance more efficiently. The heat transfer coefficient will increase as particle diameter decreases.

Transformer is one of the most important pieces of equipment in power system. The insulation aging and lifespan of transformer are significantly affected by hot spot distributions of internal components inside. In the present paper, the electromagnetic losses of different components and heat transfer process in a three-phase forced oil circulation transformer (400 kVA-15 kV/400 V) are numerically studied with finite element method. The leakage magnetic flux and eddy current loss density for metal components and oil tank are carefully analyzed, and the effect of metal components’ electromagnetic loss on hot spot temperature of different components and oil flow in transformer is also studied. It is found that the surface current of metal components is generated by leakage magnetic flux, and surface current density is large when leakage magnetic flux concentrates. The effect caused by relative magnetic permeability of metal components is remarkable on electromagnetic loss of metal components and oil tank, while the effect caused by relative magnetic permeability of transformer tank is relatively small. Due to the mixing of metal components on oil flow, the heat transfer of core is enhanced, its hot spot temperature is lowered, and the hot spot locations of coil and core also change. These results are meaningful for further understanding of heat transfer process in transformer and important for the optimal design of transformer.

Packed beds are widely used in catalytic reactors or nuclear reactors. Reducing the pressure drop and improving the heat transfer performance of a packed bed is a common research aim. The dimpled structure has a complex influence on the flow and heat transfer characteristics. In the present study, the flow and heat transfer characteristics in structured packed beds with smooth or dimpled spheres are numerically investigated, where two different low channel to particle diameter ratios (N = 1.00 and N = 1.15) are considered. The pressure drop and the Nusselt number are obtained. The results show that, for N = 1.00, compared with the structured packed bed with smooth spheres, the structured packed bed with dimpled spheres has a lower pressure drop and little higher Nusselt number at 1500 < Re_H < 14,000, exhibiting an improved overall heat transfer performance. However, for N = 1.15, the structured packed bed with dimpled spheres shows a much higher pressure drop, which dominantly affects the overall heat transfer performance, causing it to be weaker. Comparing the different channel to particle diameter ratios, we find that different configurations can result in: (i) completely different drag reduction effect; and (ii) relatively less influence on heat transfer enhancement.

In this paper, the heat transfer of pin-fin plate unit (PFPU) under static and oscillating conditions are numerically studied using the discrete element method (DEM). The flow and heat transfer characteristics of the PFPU with sinusoidal oscillation are investigated under the conditions of oscillating frequency of 0–10 Hz, amplitude of 0–5 mm and oscillating direction of Y and Z. The contact number, contact time, porosity and heat transfer coefficient under the above conditions are analyzed and compared with the smooth plate. The results show that the particle far away from the plate can transfer heat with the pin-fin of PFPU, and the oscillating PFPU can significantly increase the contact number and enhance the temperature diffusion and heat transfer. The heat transfer coefficient of PFPU increases with the increase of oscillating frequency and amplitude. When the PFPU oscillates along the Y direction with the amplitude of 1 mm and the frequency of 10 Hz, the heat transfer coefficient of PFPU is increased by 28% compared with that of the smooth plate. Compared with the oscillation along the Z direction, the oscillation along the Y direction has a significant enhancement on the heat transfer of PFPU.