Wen-Quan Tao

Field synergy principle for enhancing convective heat transfer--its extension and numerical verifications

Abstract The concept of enhancing parabolic convective heat transfer by reducing the intersection angle between velocity and temperature gradient is reviewed and extended to elliptic fluid flow and heat transfer situation. Five examples of elliptic flow are provided to show the validity of the new concept (field synergy principle). Two further examples are supplemented to demonstrate the importance of the concept in the design of the enhanced surfaces.

A unified analysis on enhancing single phase convective heat transfer with field synergy principle

Numerical simulations were conducted to reveal the inherent relation between the filed synergy principle and the three existing mechanisms for enhancing single phase convective heat transfer. It is found that the three mechanisms, i.e., the decreasing of thermal boundary layer, the increasing of flow interruption and the increasing of velocity gradient near a solid wall, all lead to the reduction of intersection angle between velocity and temperature gradient. It is also revealed that at low flow speed, the fin attached a tube not only increases heat transfer surface but also greatly improves the synergy between the velocity and the temperature gradient.

Nanoscale simulation of shale transport properties using the lattice Boltzmann method: permeability and diffusivity

Porous structures of shales are reconstructed using the markov chain monte carlo (MCMC) method based on scanning electron microscopy (SEM) images of shale samples from Sichuan Basin, China. Characterization analysis of the reconstructed shales is performed, including porosity, pore size distribution, specific surface area and pore connectivity. The lattice Boltzmann method (LBM) is adopted to simulate fluid flow and Knudsen diffusion within the reconstructed shales. Simulation results reveal that the tortuosity of the shales is much higher than that commonly employed in the Bruggeman equation, and such high tortuosity leads to extremely low intrinsic permeability. Correction of the intrinsic permeability is performed based on the dusty gas model (DGM) by considering the contribution of Knudsen diffusion to the total flow flux, resulting in apparent permeability. The correction factor over a range of Knudsen number and pressure is estimated and compared with empirical correlations in the literature. For the wide pressure range investigated, the correction factor is always greater than 1, indicating Knudsen diffusion always plays a role on shale gas transport mechanisms in the reconstructed shales. Specifically, we found that most of the values of correction factor fall in the slip and transition regime, with no Darcy flow regime observed.

NUMERICAL DESIGN OF EFFICIENT SLOTTED FIN SURFACE BASED ON THE FIELD SYNERGY PRINCIPLE

In this article, a numerical investigation of the flow and heat transfer in a three-row finned-tube heat exchanger is conducted with a three-dimensional laminar conjugated model. Four types of fin surfaces are studied; one is the whole plain plate fin, and the other three are of slotted type, called slit 1, slit 2, and slit 3. All four fin surfaces have the same global geometry dimensions. The three slotted fin surfaces have the same numbers of strips, which protrude upward and downward alternatively and are positioned along the flow direction according to the rule of “front coarse and rear dense.” The difference in the three slotted fins is in the degree of “coarse” and “dense” along the flow direction. Numerical results show that, compared to the plain plate fin, the three types of slotted fin all have very good heat transfer performance in that the percentage increase in heat transfer is higher than that in the friction factor. Among the three slotted fin surfaces, slit 1 behaves the best, followed by slit 2 and slit 3 in order. Within the Reynolds number range compared ( from 2,100 to 13,500), the Nusselt number of slit 1 is about 112–48% higher than that of the plain plate fin surface under the identical pumping constraint. An analysis of the essence of heat transfer enhancement is conducted from the field synergy principle, which says that the reduction of the intersection angle between the velocity and the temperature gradient is the basic mechanism for enhancing convective heat transfer. It is found that for the three comparison constraints the domain-average synergy angle of slit 1 is always the smallest, while that of the plain plate fin is the largest, with slit 2 and slit 3 being somewhat in between. The results of the present study once again show the feasibility of the field synergy principle and are helpful to the development of new types of enhanced heat transfer surfaces.

Three-Dimensional Numerical Simulation on Laminar Heat Transfer and Fluid Flow Characteristics of Strip Fin Surface With X-Arrangement of Strips

A numerical investigation of air side performance of strip fin surface is presented. Three-dimensional numerical computation was made for a model of a two-low. finned tube heat exchanger. The tube configuration is simulated with step-wise approximation, and the fin efficiency is also calculated with conjugated computation. Four types of fin surfaces were studied: A-the whole plain plate fin; B-the strip fin with strips located in the upstream part of the fin; C-the strip fin with strips located in the downstream part of the fin; and D-the strip fin with strips covering the whole fin surface

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.

PARAMETRIC NUMERICAL STUDY OF FLOW AND HEAT TRANSFER IN MICROCHANNELS WITH WAVY WALLS

Wavy channels were investigated in this paper as a passive scheme to improve the heat transfer performance of laminar fluid flow as applied to microchannel heat sinks. Parametric study of three-dimensional laminar fluid flow and heat transfer characteristics in microsized wavy channels was performed by varying the wavy feature amplitude, wavelength, and aspect ratio for different Reynolds numbers between 50 and 150. Two different types of wavy channels were considered and their thermal performance for a constant heat flux of 47 W/cm 2 was compared. Based on the comparison with straight channels, it was found that wavy channels can provide improved overall thermal performance. In addition, it was observed that wavy channels with a configuration in which crests and troughs face each other alternately (serpentine channels) were found to show an edge in thermal performance over the configuration where crests and troughs directly face each other. The best configuration considered in this paper was found to provide an improvement of up to 55% in the overall performance compared to microchannels with straight walls and hence are attractive candidates for cooling of future high heat flux electronics.

DISCUSSION ON MOMENTUM INTERPOLATION METHOD FOR COLLOCATED GRIDS OF INCOMPRESSIBLE FLOW

Discussions are given of the different momentum interpolation methods to evaluate the interface velocity in the collocated grid system. It is pointed out that the interface velocity is used in three cases in the overall numerical procedure of the solution of Navier-Stokes equations by utilizing a collocated grid: in the continuity equation; in the interface flow rate computation for the determination of the coefficients in discretization equation; and in the mass residual in the coefficient Ap. Analysis shows that it is better to adopt the momentum interpolation method in the three cases. Two new momentum interpolation methods, called MMIM1 and MMIM2, are proposed. Analysis shows that the two new methods can achieve numerical solutions that are independent of both the underrelaxation factor and the time step size. Taking lid-driven cavity flow as an example, numerical computations are conducted for several Reynolds numbers and different mesh sizes using the SIMPLE algorithm, and the results are compared with benchmark solutions. Numerical tests demonstrate that both MMIM1 and MMIM2 can give unique solutions for different underrelaxation factors and time step sizes, solutions from MMIM1 are slightly better than that of the momentum interpolation of Majumdar, and solutions from MMIM2 have an appreciably better accuracy when the mesh is not fine.

An Efficient Segregated Algorithm for Incompressible Fluid Flow and Heat Transfer Problems - IDEAL (Inner Doubly Iterative Efficient Algorithm for Linked Equations) Part I: Mathematical Formulation and Solution Procedure

An efficient segregated solution procedure for incompressible fluid flow and heat transfer problems is proposed. The new algorithm is called IDEAL (Inner Doubly Iterative Efficient Algorithm for Linked Equations). In the new algorithm there exist inner doubly iterative processes for the pressure equation, which almost completely overcome two approximations in the SIMPLE algorithm. Thus the coupling between velocity and pressure is fully guaranteed, greatly enhancing the convergence rate and stability of the iteration process. The mathematical formulation and solution procedure of the IDEAL algorithm are described in this article. In Part II, application examples are provided to show the features and feasibility of the new algorithm.

A NOVEL SEGREGATED ALGORITHM FOR INCOMPRESSIBLE FLUID FLOW AND HEAT TRANSFER PROBLEMS—CLEAR (COUPLED AND LINKED EQUATIONS ALGORITHM REVISED) PART I: MATHEMATICAL FORMULATION AND SOLUTION PROCEDURE

A novel segregated solution procedure for incompressible fluid flow and heat transfer problems is proposed. The new algorithm is called CLEAR (Coupled and Linked Equations Algorithm Revised). It differs from all SIMPLE-like algorithms in that it solves the improved pressure directly, rather than by adding a correction term, and no term is dropped in the derivation of the pressure equation. Thus the effects of the neighboring velocity values are fully taken into account, and the coupling between velocity and pressure is fully guaranteed, greatly enhancing the convergence rate of the iteration process. Its robustness is improved by introducing a second relaxation factor. The mathematical formulation and the solution procedure of the CLEAR algorithm are described in detail in this article. Comprehensive discussion is conducted to describe the difference between the CLEAR algorithm and all other existing algorithms of the SIMPLE family. In Part II, six numerical application examples with available numerical solutions are provided to show the feasibility of the new algorithm.