Z.G. Qu

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.

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

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.

An acoustofluidic micromixer via bubble inception and cavitation from microchannel sidewalls.

During the deep reactive ion etching process, the sidewalls of a silicon mold feature rough wavy structures, which can be transferred onto a polydimethylsiloxane (PDMS) microchannel through the soft lithography technique. In this article, we utilized the wavy structures of PDMS microchannel sidewalls to initiate and cavitate bubbles in the presence of acoustic waves. Through bubble cavitation, this acoustofluidic approach demonstrates fast, effective mixing in microfluidics. We characterized its performance by using viscous fluids such as poly(ethylene glycol) (PEG). When two PEG solutions with a resultant viscosity 54.9 times higher than that of water were used, the mixing efficiency was found to be 0.92, indicating excellent, homogeneous mixing. The acoustofluidic micromixer presented here has the advantages of simple fabrication, easy integration, and capability to mix high-viscosity fluids (Reynolds number: ∼0.01) in less than 100 ms.

A NOVEL SEGREGATED ALGORITHM FOR INCOMPRESSIBLE FLUID FLOW AND HEAT TRANSFER PROBLEMS—CLEAR (COUPLED AND LINKED EQUATIONS ALGORITHM REVISED) PART II: APPLICATION EXAMPLES

In Part I of this article a novel algorithm, CLEAR, was introduced. In this article the relative performance of the CLEAR algorithm and the SIMPLER algorithm is evaluated for six incompressible fluid flow and heat transfer problems with constant property. The six examples cover three two-dimensional orthogonal coordinates. Comprehensive comparisons are made between the two algorithms on the subject of iteration number for obtaining a converged solution, and the consumed CPU time. It is found that CLEAR can appreciably enhance the convergence rate. For the six problems tested, the ratio of iteration numbers of CLEAR over that of SIMPLER ranges from 0.15 to 0.84, and the ratio of the CPU time from 0.19 to 0.92.

An Efficient Segregated Algorithm for Incompressible Fluid Flow and Heat Transfer Problems—IDEAL (Inner Doubly Iterative Efficient Algorithm for Linked Equations) Part II: Application Examples

In this article, comprehensive comparisons are made between the SIMPLER and IDEAL algorithms for four application examples. It is found that the IDEAL algorithm is efficient and stable not only for the simple, low-Re/Ra or coarse-mesh flow cases, but also for the complex, high-Re/Ra or fine-mesh flow cases. For the low-Re/Ra, coarse-mesh flow cases, the ratio of CPU time of IDEAL to that of SIMPLER ranges from 0.029 to 0.7. For the high-Re/Ra, fine-mesh flow cases, the IDEAL algorithm can obtain convergent results but the SIMPLER algorithm cannot, even though the underrelaxation factors are adjusted.

Polydimethylsiloxane-Paper Hybrid Lateral Flow Assay for Highly Sensitive Point-of-Care Nucleic Acid Testing

In nucleic acid testing (NAT), gold nanoparticle (AuNP)-based lateral flow assays (LFAs) have received significant attention due to their cost-effectiveness, rapidity, and the ability to produce a simple colorimetric readout. However, the poor sensitivity of AuNP-based LFAs limits its widespread applications. Even though various efforts have been made to improve the assay sensitivity, most methods are inappropriate for integration into LFA for sample-to-answer NAT at the point-of-care (POC), usually due to the complicated fabrication processes or incompatible chemicals used. To address this, we propose a novel strategy of integrating a simple fluidic control strategy into LFA. The strategy involves incorporating a piece of paper-based shunt and a polydimethylsiloxane (PDMS) barrier to the strip to achieve optimum fluidic delays for LFA signal enhancement, resulting in 10-fold signal enhancement over unmodified LFA. The phenomena of fluidic delay were also evaluated by mathematical simulation, through which we found the movement of fluid throughout the shunt and the tortuosity effects in the presence of PDMS barrier, which significantly affect the detection sensitivity. To demonstrate the potential of integrating this strategy into a LFA with sample-in-answer-out capability, we further applied this strategy into our prototype sample-to-answer LFA to sensitively detect the Hepatitis B virus (HBV) in clinical blood samples. The proposed strategy offers great potential for highly sensitive detection of various targets for wide application in the near future.

Advances in the understanding of nanomaterial–biomembrane interactions and their mathematical and numerical modeling

The widespread application of nanomaterials (NMs), which has accompanied advances in nanotechnology, has increased their chances of entering an organism, for example, via the respiratory system, skin absorption or intravenous injection. Although accumulating experimental evidence has indicated the important role of NM-biomembrane interaction in these processes, the underlying mechanisms remain unclear. Computational techniques, as an alternative to experimental efforts, are effective tools to simulate complicated biological behaviors. Computer simulations can investigate NM-biomembrane interactions at the nanoscale, providing fundamental insights into dynamic processes that are challenging to experimental observation. This paper reviews the current understanding of NM-biomembrane interactions, and existing mathematical and numerical modeling methods. We highlight the advantages and limitations of each method, and also discuss the future perspectives in this field. Better understanding of NM-biomembrane interactions can benefit various fields, including nanomedicine and diagnosis.