Horng Wen Wu

Effect of an oblique plate on the heat transfer enhancement of mixed convection over heated blocks in a horizontal channel

Abstract This study presents a numerical investigation on heat transfer enhancement of mixed convective flow in a horizontal block-heated channel. The heat transfer enhancement in this study has been accomplished by the installation of an oblique plate for internal flow modification induced by vortex shedding. The oblique angle of the plate is changed (30–90°) under Reynolds numbers (260–530) and Grashof numbers (0–3 200 000) for the purpose of investigating the heat transfer performance. The results show that the installation of an oblique plate in cross-flow above an upstream block can effectively enhance the heat transfer performance of mixed convection in the horizontal channel flow.

LES analysis of turbulent flow and heat transfer in motored engines with various SGS models

Abstract This paper presents the application of three different subgrid-scale (SGS) models in a large eddy simulation (LES) for investigating the turbulent flow field and wall heat transfer during the compression–expansion strokes in two types of engine configuration under realistic engine conditions. Predictions were compared with experimental measurements (including the local heat flux and swirl velocity), and with those calculated from the conventional K – e model. The results of the Van Driest wall damping model for LES were found to be in the best agreement with experimental data. The variations of velocity vector plots, isothermal profiles with crank angle were realized in the “Pancake” chamber engine. The variation of squish strength in the cylinder was also investigated by illustrating the friction velocity variations at different radial locations in the “Deep Bowl Piston” engine.

HEAT TRANSFERPREDICTIONS AROUND THREE HEATED CYLINDERS BETWEEN TWO PARALLEL PLATES

The objective of this article is to investigate the heat transfer of convective flow across three heated cylinders arranged in an isosceles right-angled triangle between two parallel plates. The variations in drag coefficient and time-averaged Nusselt number around the surface of the three cylinders as well as the surface-averaged values of the time-averaged Nusselt number for each cylinder are investigated. This investigation is considered under the conditions where the gap-to-diameter ratio is changed (0.5 to 1.25) with forced convection (Re = 100 to 300) and mixed convection (Gr = 80,000 and 200,000). The maximum value of surface- and time-averaged Nusselt number for both forced convection and mixed convection is obtained at a gap-to-diameter ratio equal to 0.75 among the gap-to-diameter ratios considered in this article.The objective of this article is to investigate the heat transfer of convective flow across three heated cylinders arranged in an isosceles right-angled triangle between two parallel plates. The variations in drag coefficient and time-averaged Nusselt number around the surface of the three cylinders as well as the surface-averaged values of the time-averaged Nusselt number for each cylinder are investigated. This investigation is considered under the conditions where the gap-to-diameter ratio is changed (0.5 to 1.25) with forced convection (Re = 100 to 300) and mixed convection (Gr = 80,000 and 200,000). The maximum value of surface- and time-averaged Nusselt number for both forced convection and mixed convection is obtained at a gap-to-diameter ratio equal to 0.75 among the gap-to-diameter ratios considered in this article.

Effects of Temperature-Dependent Viscosity on Natural Convection in Porous Media

This analysis has studied natural convection for the temperature-dependent viscosity of fluids inside porous media between two concentric spheres by numerically solving the Brinkman–Darcy–Forchheimer model, vorticity transport, and energy equations. Parameters included Rayleigh numbers (5.0 × 103–8.0 × 104) at radius ratios of 1.5, 2.0, and 3.5 with porosities of 0.4 and 0.9 for variable-viscosity fluids with Prandtl numbers (158, 405, and 720) when the Darcy number was changed at 0.1 and 0.001. The results showed that the mean Nusselt number varied with Rayleigh number, porosity, radius ratio, and variable viscosity but did not change with the Darcy number.

Mixed Convective Heat Transfer Past a Heated Square Porous Cylinder in a Horizontal Channel With Varying Channel Height

The laminar mixed convection flow across the porous square cylinder with the heated cylinder bottom at the axis in the channel has been carried out numerically in this paper using a semi-implicit projection finite element method. The governing equations with the Brinkman-Forcheimer-extended Darcy model for the region of square porous cylinder were solved. The parameter studies including Grashof number, Darcy number, and channel-to-cylinder height ratio on heat transfer performance have been explored in detail. The results indicate that the heat transfer is augmented as the Darcy number and channel-to-cylinder height ratio increase. The buoyancy effect on the local Nusselt number is clearer for B/H=0.1 than for B/H=0.3 and B/H=0.5.

Transient mixed convective heat transfer predictions around three heated cylinders in a horizontal channel

Purpose – To investigate the effect of transient mixed convective flow interaction between circular cylinders and channel walls on heat transfer with three circular cylinders arranged in an isosceles right‐angled triangle within a horizontal channel.Design/methodology/approach – This paper uses a semi‐implicit finite element method to solve the incompressible Navier‐Stokes equation, energy equation and continuity equation in primitive‐variable form by assuming the flow to be two‐dimensional and laminar.Findings – Provides information indicating that the transient streamlines, isotherms, drag coefficient and time‐mean Nusselt number around the surfaces of three cylinders are affected by various gap‐to‐diameter ratio, Reynolds numbers and Grashof numbers. The results show that the maximum value of surface‐ and time‐mean Nusselt number along cylinders exists at S=0.75.Research limitations/implications – It is limited to two‐dimensional laminar flow for the transient mixed convective flow interaction between ...

Turbulent flow and heat transfer enhancement of mixed convection over heated blocks in a channel

Purpose – To investigate the heat transfer enhancement performed by installing a rectangular plate turbulator for internal flow modification induced by vortex shedding.Design/methodology/approach – The large eddy simulation (LES) and SIMPLE‐C method coupled with preconditioned conjugate gradient methods have been applied to the turbulent flow field and heat transfer enhancement of mixed convection in a block‐heated channel.Findings – Provides information about heat transfer performance indicating that heat transfer performance can be affected by various width‐to‐height ratio of turbulator and Grasehof numbers with a constant Reynolds number. The results show that the installation of turbulator in cross‐flow above an upstream block can effectively enhance the heat transfer performance by suitable width‐to‐height ratio of turbulator and Grasehof numbers.Research limitations/implications – It is limited to two‐dimensional mean flow for the turbulent vortex‐shedding flow past a long square cylinder.Practical ...

The Analysis of Three-Body Contact Temperature under the Different Third Particle Size, Density, and Value of Friction

Recently, many studies have investigated the friction, wear, and temperature characteristics of the interface between two relative movements. Such analyses often set the coefficient of friction as a fixed value and are analyzed in cases of two-body contact; however, the interface is often a three-body contact and the coefficient of friction varies depending on the operating conditions. This is a significant error in the analysis of contact characteristics, therefore, in this study, the actual interface and the change of the coefficient of friction were analyzed based on three-body micro-contact theory where the contact temperature was also analyzed and the difference between the generally assumed values were compared. The results showed that under three-body contact, the coefficient of total friction increased with an increase in particle size; and at a different particle size and area density of particles, the surface contact temperature increased with the plasticity index and load increases, and the particle contact temperature increased with the increasing particle size. The surface temperature rise was mainly affected by the ratio of the average temperature between surface 1 and surface 2 to the multiplication between the 100th root of the area density of particles and the square root of the equivalent surface roughness (Ts1s2_ave*/ηa0.01σ0.5) and the ratio of the 10th root of the mean particle diameter to the 100th root of the equivalent surface roughness (xa0.1/σ0.001). Particle temperature was mainly affected by the ratio of the 10th root of the mean particle diameter to the 100th root of the equivalent surface roughness (xa0.1/σ0.001) and the area density of particles ηa. Our study indicated that when the contact of surface with surface and the contact of the particles with the surface, the resulting heat balance was assigned to the particles and the surface in a three-body contact situation. Under this contact behavior, it could avoid a too high a rise in micro-contact temperature to achieve the material failure temperature.

Study of Contact Temperature in Polishing Surfaces

Based on a three-body micro-contact mechanism and contact temperature theory, a micro-contact temperature model was developed to investigate the effect of particle size, particle density, and rotational speed on temperature rise between particles and workpieces. The experiments with different particle sizes and rotational speeds verified the feasibility of the micro-contact temperature analysis. The results indicate that contact temperature between particles and workpieces linear increases as particle size and rotational speed increase. The particle density has a negligible effect on maximum contact temperature between particles and workpieces.

Effect of two-body and three-body microcontacts under dry friction on contact characteristics:

The wear debris generation is unavoidable between the contact interfaces of moving components. In three-body contact instances, friction and wear occur at these separate contact points. This paper discusses the characteristics of the three-body contact comprising the abrasive particle in the interface compared to the two-body contact. The results show that for the wear debris or foreign particles present in the interface of the three-body contact, as external load initially increases, the external load is fully borne by the contact characteristics of particle-to-surface. Until the external load rises to a particular critical external load, it enters the real three-body situation, and the critical external load thus increases with an increase in the ratio of particle diameter to surface roughness. For two contact surfaces, the summit deformation is the elastoplastic deformation in a wide range of external loads. As the external load is lower than the critical external load value of the three-body contact, the contact surface is under the particle-to-surface two-body contact, and the elastic deformation of surface peak has the largest proportion of contact area. When the external load is higher than the critical external load value, the elastoplastic deformation contact area quickly dominates, and the total contact area ratio approximates to the surface-to-surface two-body contact situation. In the range of engineering surface roughness (σu2009=u200950–400u2009nm), at each external load and surface roughness, the total friction coefficient decreases with the increase in the ratio of particle diameter to surface roughness under the three-body contact, and this shows that the friction coefficient of surface-to-surface contact is larger than that of the sphere wear debris between the contact interface. At the same surface roughness, the friction coefficient may increase or decrease with an increase in the external load because it is determined by particle diameter. At the same ratio of particle diameter to surface roughness and external load, the friction coefficient increases with the decreasing surface roughness.