• Energy Research

Abstract The detailed flow structure and heat transfer characteristics in a newly designed heat transfer surface geometry were investigated. The surface geometry proposed is the combination of a conventional dimple cavity with a protrusion structure mounted within it. The underlying design concept of this surface geometry aims to enhance the flow mixing and the corresponding heat transfer in the flow-recirculating region that is generated by a conventional dimple cavity. Four different protrusion heights were considered as the main design parameter of the present study. The numerical simulations were carried out with a Reynolds number of 2800 and Prandtl number of 0.71 (air) corresponding to the mean velocity and channel height. The calculated pressure drop and heat-transfer capacity were assessed in terms of the Fanning friction factor and Colburn j factor. The overall performances, estimated in terms of area goodness factor for several protrusion-in-dimple cases, were higher than that found by a conventional dimple. Compared to the conventional dimple case, the pressure drop and heat-transfer capacity were slightly augmented in the case of a protrusion height of 0.05 since this leads to an improvement in the mixing of the turbulent flow in the dimple cavity.

Abstract Two-dimensional numerical simulations were conducted for the natural convection phenomena in a cold square enclosure containing four hot circular cylinders with Rayleigh number in the range of 103 ≤ Ra ≤ 106 and a Prandtl number of Pr = 0.7. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the cylinders based on the finite volume method (FVM). In the present study, the variation in the vertical position of the cylinders was compared with those in the diagonal and horizontal positions. The change in the flow and thermal structures and corresponding heat transfer characteristics were investigated in regard to the transition of the flow regime from the steady state to the unsteady state at a relatively high Rayleigh number. In addition, the effects of the cylinder positions of a rectangular array in the square enclosure on the heat transfer characteristics are highlighted.

This paper presents the change of non-dimensional characteristics and thermal behavior of different sized tilting pad journal bearings (TPJBs) with the same Sommerfeld number. A three-dimensional (3D) TPJB numerical model is provided considering the thermo-elastic hydro-dynamic (TEHD) lubrication model with pad thermal-elastic deformation. The pivot stiffness is assumed to be the combination of linear and cubic stiffness based on the Hertzian contact theory. The TPJBs in a configuration of load between pad (LBP) with the same Sommerfeld number having seven different sizes are simulated, and their non-dimensional dynamic and static characteristics and thermal behavior are compared. Pad thermal and elastic deformation are both taken into account. If the changes in lubricant viscosity, thermal deformation, and elastic deformation of journal/pads due to viscous shearing are ignored, the bearings with identical Sommerfeld numbers should show the same performance characteristics. However, the heat generation at the bearing clearance during operation (a) induces a decrease in viscosity and heat transfer to journal/pads and (b) results in a thermal deformation. Furthermore, the elastic deformation of the tilting pads and pivots also affects the bearing dynamic performance. For the same Sommerfeld number, the numerical analyses provide how the viscous shearing and elastic deformation lead to a change in bearing performance. For the small bearings with the same Sommerfeld number, the non-dimensional characteristics did not change significantly, where the heat generation was small being compared to the large sized bearing. The largest change in non-dimensional characteristics occurred when the maximum temperature of the oil film increased by 30 °C or more compared to the lubricant supply temperature. The root cause of the change in the non-dimensional characteristics is the viscous shearing in the oil film, and the thermal deformation of the structures surrounding the oil film acts in combination. These results provide insight into the Sommerfeld number, which can be used for the early stage of bearing design.

The present study established two different models based on the convolutional neural network (CNN) and the encoder–decoder (ED) to predict the characteristics of the flow and heat transfer around the NACA sections. The established CNN predicts the aerodynamic coefficients and the Nusselt number. The established ED model predicts the velocity, pressure and thermal fields to explain the performances of the aerodynamics and heat transfer. These two models were trained and tested by the dataset extracted from the computational fluid dynamics (CFD) simulations. The predictions mostly matched well with the true data. The contours of the velocity components and the pressure coefficients reasonably explained the variation of the aerodynamic coefficients according to the geometric parameter of the NACA section. In order to physically interpret the heat transfer performance, more quantitative and qualitative information are needed owing to the lack of the correlation and the resolution of the thermal fields. Consequently, the present approaches will be useful to design the NACA section-based shape giving higher aerodynamic and heat transfer performances by quickly predicting the force and heat transfer coefficients. In addition, the predicted flow and thermal fields will provide the physical interpretation of the aerodynamic and heat transfer performances.

The present study numerically investigates laminar fluid flow past a rectangular cylinder with different width to height ratios. The mass and momentum conservation equations to govern the fluid flow around the rectangular cylinder are solved using the finite volume method which has the second order accuracy and the immersed boundary method is used to handle effectively the rectangular cylinder. The Reynolds number Re and the width to height ratio W/H are varied in the range of 50 Re 150 and 0.1 W/H 1, respectively. The distribution of the Strouhal number and the drag and lift coefficients strongly depend on the fluid flow and vortex structure around the rectangular cylinder varying as a function of the width to height ratio and the Reynolds number. The Strouhal number decreases monotonically with increasing width to height ratio in the Reynolds number varying in the range of 50 to 80. However, as the width to height ratio increases in the Reynolds number varying in the range of 90 to 150, the Strouhal number increases until it has a maximum value, followed by the decrease in its value.