Yue-Tzu Yang

Influence of hydrogen functionalization on thermal conductivity of graphene: Nonequilibrium molecular dynamics simulations

The influence of hydrogen coverage on the thermal conductivity of typical armchair hydrogen functionalized graphene is investigated using a nonequilibrium molecular dynamics simulation. We also study the effects of easy-rotation of unsupported sp3 bonds. We find that the system exhibits a rapid drop in thermal conductivity with hydrogen coverage, where hydrogen coverage down to as little as 2.5% of the graphene carbon atoms reduces the thermal conductivity by about 40%. The simulation results indicate that the effect is due to a reduction in the phonon mean free path.

Calculation of turbulent flow and heat transfer in a porous-baffled channel

Abstract This study presents the numerical predictions on the turbulent fluid flow and heat transfer characteristics for rectangular channel with porous baffles which are arranged on the bottom and top channel walls in a periodically staggered way. The turbulent governing equations are solved by a control volume-based finite difference method with power-law scheme and the k–e turbulence model associated with wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi-implicit method for pressure-linked equation) method. The parameters studied include the entrance Reynolds number Re (1×104–5×104), the baffle height (h=10, 20 and 30 mm) and kind of baffles (solid and porous); whereas the baffle spacing S/H are fixed at 1.0 and the working medium is air. The numerical calculations of the flow field indicate that the flow patterns around the porous- and solid-type baffles are entirely different due to different transport phenomena and it significantly influences the local heat transfer coefficient distributions. Relative to the solid-type baffle channel, the porous-type baffle channel has a lower friction factor due to less channel blockage. Concerning the heat transfer effect, both the solid-type and porous-type baffles walls enhanced the heat transfer relative to the smooth channel. It is further found that at the higher baffle height, the level of heat transfer augmentation is nearly the same for the porous-type baffle, the only difference being the Reynolds number dependence. As expected, the centerline-averaged Nusselt number ratio increases with increasing the baffle height because of the flow acceleration.

Unsteady unidirectional flow of second grade fluid between the parallel plates with different given volume flow rate conditions

In this paper, the velocity profile and pressure gradient of the unsteady state unidirectional flow of second grade fluids between the parallel plates are considered. The flow motion in the plates is induced by a given but arbitrary inlet volume flow rate which varies with time. Based on the flow conditions described, two basic flow situations are solved, which are a sudden started and a constant acceleration flow, respectively. Then we apply these two results to a practical case that is a trapezoidal piston motion which contains three phases of piston motion, the constant acceleration from the rest to a fixed velocity, then keeping at this velocity, following with the constant deceleration to a stop. In addition, the oscillatory flow is also considered.

Investigation of planted pin fins for heat transfer enhancement in plate fin heat sink

The present study carries out numerical computations of the plate-circular pin-fin heat sink and provides physical insight into the flow and heat transfer characteristics. The governing equations are solved by adopting a control-volume-based finite-difference method with a power-law scheme on an orthogonal non-uniform staggered grid. The coupling of the velocity and the pressure terms of momentum equations are solved by the SIMPLEC algorithm. The plate-circular pin-fin heat sink is composed of a plate fin heat sink and some circular pins between plate fins. The purpose of this study is to examine the effects of the configurations of the pin-fins design. The results show that the plate-circular pin-fin heat sink has better synthetical performance than the plate fin heat sink.

Inverse Method for Estimating Thermal Conductivity in One-Dimensional Heat Conduction Problems

An inverse analysis is provided to determine the spatial- and temperature-dependent thermal conductivities in several one-dimensional heat conduction problems. A e nite difference method is used to discretize the governing equations, and then a linear inverse model is constructed to identify the undetermined thermal conductivities. The present approach is to rearrange the matrix forms of the differential governing equations so that the unknown thermal conductivity can be represented explicitly. Then, the linear least-squares-error method is adopted to e nd the solutions. The results show that only a few measuring points at discrete grid points are needed to estimate the unknown quantitiesofthethermalconductivities, evenwhenmeasurementerrorsareconsidered.Incontrasttothe traditional approach, the advantages of this method arethatno prior information is needed on thefunctional form oftheunknown quantities,no initialguessesarerequired,andno iterations in thecalculating processarenecessary and that the inverse problem can be solved in a linear domain. Furthermore, the existence and uniqueness of the solutions can be easily identie ed. Nomenclature A = coefe cient matrix of vector T B = coefe cient matrix of vector C C = vector constructed from the unknown thermal conductivities D = vector constructed from the functions of the unknown thermal conductivities E = productof A i1 and B F = error function g = heat generation, W/m 3 k = thermal conductivity, W/m ¢ ± C q = heat e ux, W/m 2 R = reverse matrix T = temperature, ± C T = temperature vector t = time, s x = spatial coordinate, m 1t = increment of time domain, s 1x = increment of spatial coordinate, m ae = standard deviation ! = random variable

Numerical study of multiple impinging slot jets with an inclined confinement surface

This study presents numerical predictions on the fluid flow and heat transfer characteristics of multiple impinging slot jets with an inclined confinement surface. A nonorthogonal body-fitted coordinate system was used to handle the complexity of the geometry, and a control volume based finite difference method was employed to solve the governing equations. Two turbulence models are used to describe the turbulent structure: the standard k-e turbulent model associated with wall function and the Lam-Bremhorst version of the low-Re k- e model. The parameters studied include the angle of inclined confinement surface θ(0° ≤ θ ≤15°) and entrance Reynolds number (11,000 ≤ Re ≤ 21,000). The numerical results show that the maximum local Nusselt number and maximum pressure on the impinging surface move downstream while the inclination angle θ is increased. The maximum local Nusselt number decreases while the value of the local Nusselt number downstream increases with increasing inclination angle θ. The calculated s...

Free convection heat transfer of non-Newtonian fluids over axisymmetric and two-dimensional bodies of arbitrary shape embedded in a fluid-saturated porous medium

Abstract The problem of natural convection of a non-Newtonian power-law fluid with or without yield stress about a two-dimensional or axisymmetric body of arbitrary shape in a fluid-saturated porous medium is analyzed on the basis of boundary layer approximation. For a high modified Rayleigh number, similarity solutions are obtained by using the fourth-order Runge—Kutta scheme and shooting method for two-dimensional bodies without yield stress and a cone with yield stress. The effects of the surface heat transfer rate qW(x), the local Nusselt number Nux, the overall heat transfer rate Q* and the power indices n of fluids with the yield stresses on the free convection heat transfer characteristics are discussed. It is found that the results depend strongly on the high values of the yield stress parameter Ω/a at the boundary.

The inverse estimation of the thermal boundary behavior of a heated cylinder normal to a laminar air stream

Abstract This study provides an inverse analysis to determine the thermal boundary behavior of a heated cylinder normal to a laminar air stream. A finite-difference method is used to discretize the governing equations and then a linear inverse model is constructed. The present approach is to rearrange the matrix forms of the governing differential equations and to estimate the unknown conditions of the cylinder from the linear inverse model. The linear least-squares error method is then adopted to find the solutions of the unknown thermal boundary conditions such as surface temperatures, local heat flux, local Nusselt numbers, and even the unknown temperature of the hot wire imbedded in the center of the cylinder. The results show that only few measuring points inside the cylinder are needed to estimate the unknown quantities of the thermal boundary behavior even when measurement errors are considered. From this study it is confirmed that the proposed method is effective and applicable for the two-dimensional inverse heat conduction problems.

A three-dimensional inverse heat conduction problem approach for estimating the heat flux and surface temperature of a hollow cylinder

This study is intended to provide a different perspective for solving the three-dimensional, inverse, steady heat conduction problem for a hollow cylinder. At the beginning of the study, finite-difference methods are employed to discretize the problem domain and then a linear inverse model is constructed to identify the boundary conditions. The present approach is to rearrange the matrix forms of the differential governing equations and estimate unknown conditions. Then, the linear least-squares method is adopted to find the solution. The results show that one needs only a few measuring points in order to estimate the boundary temperature and heat flux when the measurement errors are negligible. When the measurement errors are considerable, more measuring points are needed in order to increase the congruence of the estimated results to exact solutions.

Natural convection of non-Newtonian fluids along a wavy vertical plate including the magnetic field effect

Abstract A Prandtl transformation method is applied to study the natural convection of non-Newtonian fluids along a wavy vertical plate in the presence of a magnetic field. A simple transformation is proposed, to transform the governing equations into the boundary layer equations, and solved numerically by the cubic spline approximation. A simple coordinate transformation is employed to transform the complex wavy surface to a vertical flat plate for a constant wall temperature by the numerical method. The effects of the magnetic field parameter, the wavy geometry and the non-Newtonian nature of the fluids on the flow characteristics and heat transfer are discussed in detail. It is found that the action of the magnetic field is to decelerate the flow, thus decreasing the Nusselt number.