Zeng-Yao Li

A NEW STABILITY-GUARANTEED SECOND-ORDER DIFFERENCE SCHEME

Based on the stability-controllable second-order difference (SCSD) scheme, a new stability-guaranteed second-order difference (SGSD) scheme is proposed whose merits are absolutely stable and adaptive. Its numerical accuracy is at least no less than that of the central difference (CD) and second-order upwind difference (SUD) schemes and sometimes higher than that of the QUICK scheme. The SGSD scheme can automatically choose a different difference scheme according to the available local field information in difference space or time. It automatically approaches the central difference scheme where or when diffusion is dominant, and approaches the second-order upwind difference scheme where or when convection is dominant. Computations for two benchmark problems using the SGSD and the other three schemes show its feasibility in engineering computations.

Phase Change Heat Transfer Simulation for Boiling Bubbles Arising from a Vapor Film by the VOSET Method

This article presents a numerical method directed towards the simulation of flows with changes of phase. The volume-of-fluid level set (VOSET) method, which is a new interface capturing method and combines the advantages of both volume-of-fluid (VOF) and level set methods, is used for interface tracking. A difficulty occurs for the problems studied here: the discontinuous velocity field due to the difference between mass-weighted velocity and volume weighted velocity caused by the phase change at the interface. In this article, some special treatment is made to overcome this difficulty. The VOSET method and the developed treatment for the difference between mass-weighted and volume-weighted velocities are adopted to simulate a one-dimensional Stefan problem, two-dimensional horizontal film boiling, and horizontal film boiling of water at near critical pressure. The predicted results in both Nusselt number and flow patterns are agreeable with experimental results available in the literature.

A Direct Numerical Simulation for Nucleate Boiling by the VOSET Method

This article presents a 2-D numerical simulation on nucleate boiling using the VOSET method. Heat transfer through a liquid microlayer around a three-phase contact point is incorporated by a multiscale system method. A temperature interpolation method is presented for solving temperature field in cells containing liquid-vapor interface. Apart from the single-bubble model, computations are carried out for two-bubble and bubble merger models. Heat fluxes predicted by simulation are compared with experimental correlations and good agreement is obtained. In addition, simulation results of bubbles behavior also verify some boiling mechanisms.

Analysis of Inconsistency of SIMPLE-like Algorithms and an Entirely Consistent Update Technique—The CUT Algorithm

The inconsistency of the second hypothesis of the SIMPLE-like algorithms is analyzed and a remedy—the Consistent Update Technique (CUT algorithm)—for velocity and pressure is proposed. In the CUT algorithm, at each iterative level the mass conservation condition is satisfied implicitly while the momentum equation is satisfied explicitly. In CUT algorithm, a modified equation for the explicit-velocity is suggested, and an adjustable coefficient is introduced. Four 2-D fluid flow and heat transfer problems are solved numerically by both the CUT and SIMPLER algorithms under the same other conditions. Comparisons of CPU time are made and it is found that for the four examples studied, the CUT can at least reduce CPU time by 15–63% with much better robustness.

A numerical study on the influence of insulating layer of the hot disk sensor on the thermal conductivity measuring accuracy

A numerical study on the influence of the insulation layer of the hot disk sensor on the thermal conductivity measuring accuracy has been conducted. It is found that the influences of the thermal contact resistance and the insulating layer could be excluded in the transient plane source method. Both the kapton5501 and the mica5082 sensor could measure stainless steel and ceramic with a deviation less than 3% while the deviation increases to 54.2% of silica aerogel because of the large heat loss proportion through the mica5082 sensor side. The simulation proved that the heat loss through sensor side and accuracy could be improved by increasing the radius of the sensor.

Discussion on Numerical Treatment of Periodic Boundary Condition for Temperature

For periodic fully developed fluid flow and heat transfer problems, it is sufficient to confine attention to a single cycle. The temperature periodic conditions at the computational domain inlet/exit for fluid temperature can be implemented by two methods: either by extending the computational domain by several control volumes and replacing the field data at the domain inlet and outlet by each other, or by using linear interpolation while restricting the computation within one module. By carefully examining the numerical solutions of the fluid streamwise bulk temperature obtained from the two methods, it is found that at the fluid bulk temperature varies abruptly at the domain exit. A comprehensive analysis reveals that this abnormal phenomenon is caused by the fact that the domain exit fluid bulk temperature is actually not updated with iteration. A remedy is proposed: The domain exit bulk temperature is updated by an upwind-based interpolation method. Numerical examples show that the second-order interpolation method is feasible and reliable.

Numerical Study on Some Improvements in the Passive Cooling System of a Radio Base Station Base on Multiscale Thermal Modeling Methodology–Part I: Confirmation of Simplified Models

Passive cooling schemes, such as natural convection, are the most reliable heat dissipation apprpaches for electronic equipment. Some times, the highest temperature of the printed circuit board (PCB), rather than the highest of chips, is very much concern because of some technological reasons. In order to reduce the maximum PCB temperature, this article analyze the PCB temperature distribution by multiscale simulation. A top-to-down approach is adopted in order to reveal the details of temperature distribution of some interested local position. In the top-to-down approach, the system level simulation by a not-too-fine grid system is the key to obtain a reliable solution. In order to reach such a goal each component of the PCB should be reasonably simplified. The thermal analysis method is proposed to simplify the components, and the implementation details are provided for three types of components. In the companion article, the settlement of boundary conditions from the data of system level simulation and several improvements in heat transfer by numerical simulation will be presented.

Nonlocal Effects and Slip Heat Flow in Nanolayers

Guyer-Krumhansl (G-K) equation is a promising macroscopic model to explore heat transport in nanoscale. In the present work, a new nonlocal characteristic length is proposed by considering the effects of heat carriers-boundaries interactions to modify the nonlocal term in G-K equation, and a slip heat flux boundary condition is developed based on the local mean free path of heat carriers. Then an analytical solution for heat flux across 2-D nanolayers and an in-plane thermal conductivity model are obtained based on the modified G-K equation and the slip heat flux boundary. The predictions of the present work are in good agreement with our numerical results of direct simulation Monte Carlo (DSMC) for argon gas nanolayer and the available experimental data for silicon thin layers. The results of this work may provide theoretical support for actual applications of G-K equation in predicting the thermal transport properties of nanolayers.

Numerical simulation of heat transfer at an array of co-planar slat-like surfaces oriented normal to a forced convection flow

Numerical study of the per-slat heat transfer coefficient was made for an array of co-planar slat-like surfaces which face upstream into an oncoming airflow. The flow washes over the slats as it passes into slot-like gaps between the slats. The computations were performed for several different slat-slot configurations characterised by the ratio of the slot-slat width to the slat width (P/W), the Reynolds number varied from 500 to 7000, and the range of Prandtl number was 0.7-10. Based on the SIMPLER algorithm and the central difference scheme (CDS), parametric studies on the effects of the Reynolds number, P/W and the Prandtl number were performed. Numerical results agree well with the test results for mass transfer (Sca2.5). From the numerical results, a general heat transfer correlation was obtained which covers the following range of parameters 500 Re 7000, 1.25 P/W 5, 0.7 Pr 10.

The influences of microstructural parameters on the gaseous thermal conductivity in nanoporous material

A three dimensional direct simulation Monte Carlo method which is suitable for the porous medium is developed for gaseous thermal conductivity within silica aerogel. The structure of random arranged nanoporous material is generated by three dimension diffusion-limited cluster-cluster aggregation method. The effects of the microstructure parameters such as particle size, porosity, specific surface area and mean pore size on the gaseous thermal conductivity are investigated quantitatively. It is found that the thermal conductivity of gas within nanoporous material is much smaller than that in free space at the same atmosphere. The effect of microstructure can be reflected from two aspects. One is that the existence of solid skeleton will reduce the collision frequency between molecules; the other is noncontinuum effect. Finally, the effects of microstructure parameters are summarized into a formula.