Yongpan Cheng

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.

Dynamic motion of red blood cells in simple shear flow

A three-dimensional numerical model is proposed to simulate the dynamic motion of red blood cells (RBCs) in simple shear flow. The RBCs are approximated by ghost cells consisting of Newtonian liquid drops enclosed by Skalak membranes which take into account the membrane shear elasticity and the membrane area incompressibility. The RBCs have an initially biconcave discoid resting shape, and the internal liquid is assumed to have the same physical properties as the matrix fluid. The simulation is based on a hybrid method, in which the immersed boundary concept is introduced into the framework of the lattice Boltzmann method, and a finite element model is incorporated to obtain the forces acting on the nodes of the cell membrane which is discretized into flat triangular elements. The dynamic motion of RBCs is investigated in simple shear flow under a broad range of shear rates. At large shear rates, the cells are found to carry out a swinging motion, in which periodic inclination oscillation and shape deform...

Microchannel flows with superhydrophobic surfaces: Effects of Reynolds number and pattern width to channel height ratio

Superhydrophobic surfaces are widely adopted for reducing the flow resistance in microfluidic channels. The structures on the superhydrophobic surfaces may consist of longitudinal grooves, transverse grooves, posts, holes, etc. In this paper their effective slip performances are systematically studied and compared in detail through numerical simulations. The numerical results show that channel wall confinement effects have a positive influence on the effective slip length for square posts and longitudinal grooves, and a negative influence for square holes and transverse grooves. Square posts, holes, and transverse grooves all exhibit deteriorating effective slip performances at higher Reynolds numbers, while the effective slip performance of longitudinal grooves remains independent of the Reynolds number. For small pattern width to channel height ratios and at low Reynolds numbers, for low shear-free fractions, the effective slip length of square posts is equivalent of that of transverse grooves, and both...

The Influence of Strip Location on the Pressure Drop and Heat Transfer Performance of a Slotted Fin

In this article, a numerical study is conducted to predict the air-side heat transfer and pressure drop characteristics of slit fin-and-tube heat transfer surfaces. A three-dimensional steady laminar model is applied, and the heat conduction in the fins is also considered. Five types of slit fins, named slit 1, slit 2, slit 3, slit 4, and slit 5, are investigated, which have the same global geometry dimensions and the same numbers of strips on the fin surfaces. The only difference among the five slit fins lies in the strip arrangement. Slit 1 has all the strips located in the front part of the fin surface, then, following the order from slit 1 to slit 5, the strip number in the front part decreases and, correspondingly, the strip number in the rear part increases, so that all the strips of slit 5 are located in the rear part. Furthermore, slit 1 and slit 5, slit 2 and slit 4, have a symmetrical strip arrangement along the flow direction. The numerical results show that, following the order from slit 1 and slit 5, the heat transfer rate increases at first, reaching a maximum value at slit 3, which has the strip arrangement of “front coarse and rear dense”; after that, it begins to decrease, as does the fin efficiency. Although they have the symmetrical strip arrangement along the flow direction, slit 5 has 7% more Nusselt number than slit 1, and slit 4 also has 7% more Nusselt number than slit 2, which shows that strip arrangement in the rear part is more effective than that in the front part. Then the difference of heat transfer performance among five slit fins is analyzed from the viewpoint of thermal resistance, which shows that when the thermal resistances in the front and rear parts are nearly identical, the optimum enhanced heat transfer fin can be obtained. This quantitative rule, in conjunction with the previously published qualitative principle of “front sparse and rear dense,” can give both quantitative and qualitative guides to the design of efficient slotted fin surfaces. Finally, the influence of fin material on the performance of enhanced-heat-transfer fins is discussed.

An Efficient and Robust Numerical Scheme for the SIMPLER Algorithm on Non-Orthogonal Curvilinear Coordinates: CLEARER

In this article, an Improved SIMPLER (CLEARER) algorithm is formulated to solve the incompressible fluid flow and heat transfer on the nonstaggered, nonorthogonal curvilinear grid system. By virtue of a modified momentum interpolation method in calculating the interface contravariant velocity in both the predictor step and the corrector step, the coupling between pressure and velocity is fully guaranteed, and the conservation law is also satisfied. A second relaxation factor is introduced in the corrector step, of which the convergent solution is independent. By setting the second relaxation factor less than the underrelaxation factor for the velocity to some extent, both the convergence rate and robustness can be greatly enhanced. Meanwhile, the CLEARER algorithm can also overcome the severe grid nonorthogonality. With the simplified pressure-correction equation, the convergent solution can still be obtained even when the intersection angle among grid lines is as low as 1°, which may provide valuable guidance in studying the fluid flow in complex geometries.

Numerical Analysis of Periodically Developed Fluid Flow and Heat Transfer Characteristics in the Triangular Wavy Fin-and-Tube Heat Exchanger Based on Field Synergy Principle

In this article the three-dimensional periodically developed incompressible flow in the triangular wavy fin-and-tube heat exchanger is numerically simulated. A novel CLEARER algorithm is adopted to guarantee the coupling between the pressure and velocity, and it can also overcome the severe grid non-orthogonality in the complex wavy fin-and-tube heat exchanger. The influence of wavy angle, fin pitch, tube diameter, and wavy density on pressure drop and heat transfer characteristics is provided under different Reynolds numbers. The numerical results show that with the increase of wavy angle, tube diameter, or wavy density both the friction factor and Colburn factor will increase, while the increase rate of friction factor is higher than that of the Colburn factor. Opposite to the wavy fin-and-tube heat exchanger with uniform inlet velocity distribution, the wavy fin-and-tube heat exchanger with periodically developed flow owns a higher Colburn factor at larger fin pitch. The influence of wavy angle, fin pitch, tube diameter, and wavy density on the heat transfer performance can all be explained well from the viewpoint of the field synergy principle, and it means that the higher Colburn factor can be attributed to the better synergy between the velocity field and temperature field. Those results may provide an insight into the heat transfer enhancement in the wavy fin-and-tube heat exchanger.

Improvement of SIMPLER Algorithm for Incompressible Flow on Collocated Grid System

In this article an Improved SIMPLER (CLEARER) algorithm is proposed to solve incompressible fluid flow and heat transfer problems. Numerical study shows with the CLEARER algorithm on a collocated grid, in the correction stage the velocities on the main nodes are overcorrected with the pressure correction, which lowers the convergence rate; hence a second relaxation factor is introduced to overcome this disadvantage. By setting this factor less than the underrelaxation factor for velocities, the convergence performance can be significantly enhanced; meanwhile, the robustness can also be increased. Four numerical examples with reliable solutions are computed to validate the CLEARER algorithm, and the results show that this algorithm can predict the numerical results accurately. Compared with the SIMPLER algorithm, CLEARER can enhance the convergence rate greatly, and in some cases it only needs as little as 17% of the iterations required by SIMPLER to reach the same convergence criterion.

Numerical prediction of periodically developed fluid flow and heat transfer characteristics in the sinusoid wavy fin‐and‐tube heat exchanger

Purpose – In this paper three‐dimensional numerical simulations were conducted for the periodically developed laminar flow in the sinusoid wavy fin‐and‐tube heat exchanger.Design/methodology/approach – A novel CLEARER algorithm is adopted to guarantee the fully coupling between the pressure and velocity, and it can not only speed up the convergence rate, but also overcome the severe grid non‐orthogonality in the wavy fin‐and‐tube heat exchanger. The influence of wave amplitude, fin pitch, tube diameter and wave density on fluid flow and heat transfer characteristics is analyzed under different Reynolds numbers.Findings – The numerical results show that with the increase of wave amplitude, tube diameter or wave density, both the friction factor and Nusselt number will increase, and the increase rate of friction factor is higher than that of Nusselt number. It is interesting to note that, at low Reynolds numbers the Nusselt number increases with the decrease of fin pitch, while at high Reynolds numbers, the...

Implementation of CLEARER Algorithm on Three-Dimensional Nonorthogonal Curvilinear Coordinates and its Application

The newly proposed CLEARER algorithm is extended to three-dimensional nonorthogonal curvilinear coordinates. This algorithm can not only guarantee full coupling between the pressure and velocity terms, and solution independence of underrelaxation factors, it can also satisfy the physical and geometric conservation laws, hence it owns high efficiency and more robustness than its companion SIMPLE-like algorithms. To ease implementation of the CLEARER algorithm, the irregular physical domain is transformed to a regular computational domain, and correspondingly, the governing equations are also transformed. The mathematical formulations in both the predictor step and the corrector step are provided in detail, as well as the solution procedure. The CLEARER algorithm is adopted to simulate numerically the fluid flow and heat transfer in a triangular wavy fin-and-tube heat exchanger, and the influence of eccentricity and transverse tube pitch on the friction factor and Nusselt number is investigated in detail. It is found that the friction factor and Nusselt number increase with increasing eccentricity and decreasing transverse tube pitch.

Influence of electrostatics on instrumentation in fluidized bed measurements

In fluidized bed, electrostatics is a common phenomenon due to the frequent collisions among solid particles, as well as between solid particles and the wall. It can cause serious problems for fluidized beds, altering hydrodynamics, causing agglomeration, interfering with instrumentation, generating nuisance discharges and even creating the danger of explosions. Despite these negative effects of electrostatics, current understanding on this subject is far from completion. The present study will focus on electrostatics in fluidized bed. Through our experimental study, it is found that the strong electrostatics can greatly affect the instrumentation and can easily lead to the terminations of data acquisitions. With increasing fluidizing air velocity, the generated electrostatics is increased rapidly. In order to reduce the influence of electrostatics, the non-conductive tape is adhered to the inner walls of fluidized bed, and the generated electrostatics is greatly decreased, as such the instrumentation can...