The thermo-hydraulic performance of two shell and tube condensers was investigated with an experimental approach. The experiment is conducted in a water cooled centrifugal chiller test rig. The condensers are made of three-dimensional (3-D) and high fin density integral-finned (2-D) tubes. 2-D and 3-D tubes all have the diameter of 3/4 inch (19mm). The 2-D tube has external fin density of 56fpi (fins per inch), fin height 1.023mm and 48 internal ribs per circle. The 3-D enhanced tube has the external fin density of 45fpi, fin height of 0.981mm and 45 internal ribs per circle. The 3-D tube is widely used in the water cooled chillers. 2-D tube is a newly designed surface with enhanced external fin density. Condensing heat transfer coefficient of R134a outside single horizontal tube is firstly tested at saturate temperature of 40°C. At the internal water velocity of 2.2m/s, the overall heat transfer coefficients of 2-D tube is in the range of 10364.7 to 12420.9W/m 2 K, 4.2% ∼ 9.0% higher than 3-D tube. External condensing heat transfer coefficient is 16.3% ∼ 25.2% higher than 3-D tube. The condensers are manufactured with these two types of tubes. Both condensers have the same geometric parameters except the tubes and tube bundle space. The length of tube in the condenser is 4000mm. The tube bundles are arranged in a staggered mode. For the integral-fin tube condenser, the longitudinal tube pitch of tube arrays is 23mm in rows and the transverse is 20mm. At the same power input and cooling water inlet temperature of 32°C, the cooling power of 2-D tube condenser are respectively of 1755.4kW and 1769.4kW; 3-D tube condenser is 1727.5kW and 1770.5kW. The pressure drop increased about 11.2% ∼ 15.9% for the 2-D tube condenser compared with 3-D tube condenser. Generally, the two condensers have the same heat transfer performance, while the integral-fin tube condenser saves 15% of copper material consumption.

A non-isothermal two-dimensional two-phase numerical model is developed in this paper to investigate the heat generation and transport processes in a direct methanol fuel cell with anisotropic gas diffusion layers (GDLs). Thermal contact resistances at the GDL/CL (catalyst layer) and GDL/Rib interfaces, and the deformation of GDLs are considered together with the inherent anisotropy of the GDL. Latent heat effects due to condensation/evaporation of water and methanol between liquid and gas phases are also taken into account. Formulation of the two-phase mass transport across the membrane electrode assembly (MEA) is mainly based on the classical multiphase flow theory in the porous media. The numerical results show that the overall heat flux in MEA is mainly contributed to heat generation in anode and cathode CLs. And the three anisotropic factors of the GDLs, including the inherent anisotropy, the spatially varying contact resistances, and the deformation of GDLs, have a strong impact on the heat transport processes in the DMFC by altering the distribution of temperature across the MEA.

The effects of main geometrical parameters of the longitudinal vortex generator (LVG) called “common flow up” on heat transfer enhancement and pressure loss are numerically investigated. Taguchi method is used to guide the numerical simulations. Based on the results of the Taguchi method, a new fin with the combination of different vortex generators is proposed to substitute the original wavy fin-and-tube surface. The results show that the new fin with LVGs can save 9.08% of pumping power while reaching the same amount of heat transfer rate as the wavy fin at inlet velocity of 2m/s, and it can also save 33% of heat transfer area.

Multiscale simulation is a rapidly evolving area of research that will have a great impact on computational mathematics and numerical modeling in engineering. In this keynote lecture following parts are included. First, what is multiscale problem. In the thermal and fluid science multiscale problems may be classified into two categories: multiscale process and multiscale system. By multiscale process we mean that the overall behavior is governed by processes occur at different length scales. By multiscale system we refer to a system that is characterized by a large variation in length scales. The cooling of an electronic system is such a typical multiscale system. Existing numerical methods for three geometric scales (macro, meso and micro) are briefly mentioned. In the second part the necessity of multiscale simulation is discussed. Examples are provided for multiscale process and multiscale system. In this lecture focus is put on the simulation of multiscale process. In the third section numerical approaches developed for the simulation of multiscale processes are presented. There are two types of simulation approaches. One is the usage of a general governing equation and solving the entire flow field involving a variation of several orders in characteristic geometric scale. The other is the so-called “solving regionally and coupling at the interfaces”. In this approach the processes at different length level is simulated by different numerical methods and then information is exchanged at the interfaces between different regions. The exchange of information should be conducted in a way that is physically meaningful, mathematically stable, and computationally efficient. The key point is the establishment of the reconstruction operator, which transforms the data of few variables of macroscopic computation to large amount of variables of microscale or mesoscale simulation. For different coupling cases the existing methods for such operators are briefly reviewed. In the fourth part, four numerical examples of multiscale simulation are presented: liquid flow in nanochannels with roughness by using MDS and FVM, flow and heat transfer in a micro nozzle by using DSMC in fluid and FVM in solid, flow past a cylinder and natural convection heat transfer in a square cavity by using coupled FVM and LBM. Finally, it is pointed out that we have a long way to go in order to have a successful full multiscale simulation for the complicated engineering problems as transport process in PEMFC and refrigerant condensation process on a enhanced surface. Further researches are highly required to establish robust and quick-convergent numerical solution approaches. Some further research needs are proposed.

In this paper, heat transfer enhancement and pressure loss penalty for fin-and-tube heat exchangers with rectangular winglet pairs (RWPs) were numerically investigated in a relatively low Reynolds number flow. The purpose of this study was to explore the fundamental mechanism between the local flow structure and the heat transfer augmentation. The RWPs were placed with a special orientation for the purpose of enhancement of heat transfer. The numerical study involved three-dimensional flow and conjugate heat transfer in the computational domain, which was set up to model the entire flow channel in the air flow direction. The effects of attack-angle of RWPs, row-number of RWPs and placement of RWPs on the heat transfer characteristics and flow structure were examined in detail. It was observed that the longitudinal vortices caused by RWPs and the impingement of RWPs-directed flow on the downstream tube were important reasons of heat transfer enhancement for fin-and-tube heat exchangers with RWPs. It was interesting to find that the pressure loss penalty of the fin-and-tube heat exchangers with RWPs could be reduced by altering the placement of the same number of RWPs from inline array to staggered array and simultaneously maintain the heat transfer enhancement level. The results showed that the rectangular winglet pairs (RWPs) can significantly improve the heat transfer performance of the fin-and-tube heat exchangers with a moderate pressure loss penalty.

First the flow friction characteristics of nitrogen and helium in stainless steel microtubes, glass microtubes, square glass microchannels, and rectangular silicon microchannels are tested. The data in glass microtubes with diameters from 50 to 201 microns and in square glass channels with characteristic diameters from 52 to 100 microns show that the friction factors are in good agreement with the conventional predictions. The friction factors in stainless-steel tubes with diameters from 119 to 300 microns are much higher than the conventional ones. The results for two of the four silicon microchannels with characteristic diameters from 26–60 microns are in good agreement while those of the other two channels are larger. This discrepancy is resulted from the large relative surface roughness. Smaller friction factors in glass microtubes with diameters from 10 to 20 microns are obtained due to the rarefaction effect. Second the flow friction experimental data for deionized water flow in glass microtubes with diameters from 50 to 530 microns show that friction factors and transition Reynolds numbers are in good agreement with the conventional predictions. However, the friction factors in stainless steel microtubes with diameters from 50–1570 microns are much higher than the conventional predictions. This discrepancy is attributed to the large surface relative roughness or denser roughness distribution. Numerical simulations considering electroviscous effect are carried out. The simulation results show that the electroviscous effect does not play a significant role in the friction factor for channel dimensions of the order of microns though it does affect the velocity profile and hence it could be neglected in engineering applications for channel dimensions of the order of microns. Third the measured local Nusselt number distribution of deionized water along the axial direction of the stainless steel tubes of 373–1570 microns with uniform heat flux do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. Numerical study reveals that the large ratio of wall thickness to tube diameter at low Reynolds number causes significant axial heat conduction in the tube wall, leading to a non-linear streamwise distribution of the fluid temperature. The axial wall heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness. In conclusion, the conventional fluid flow and heat transfer theories should still be applied for single-phase flow in smooth microchannels. Nevertheless, micro-channels do raise some issues to be paid special attention to when being applied.

Obstructed flow around single and two paratactic circular cylinders were investigated with two-dimensional molecular dynamics simulation (MDS) methods in the view of discrete particles. The transient and time-averaged profiles of streamline and density were obtained in order to analyze the characteristics of the wake flow. For single cylinder case, different flow patterns, i.e. Stokes flow, steady vortices flow, periodic vortices-shedding flow with the Ka´rma´n vortex street and supersonic flow, were divided based on Reynolds number ( Re ), 4< Re <12, 12< Re <20, 20< Re <62 and Re >62 respectively. For two paratactic cylinders case, different flow patterns, namely periodic vortices-shedding flow, periodic vortices-shedding flow with gap-flow, bistable flow and synchronized vortex-shedding flow, were observed with different center-to-center pitch ratios ( D * /d * ), D * /d * = 1.0, 1.0< D * /d * <1.1, 1.1< D * /d * <1.8 and D * /d * >1.8 respectively. The results show qualitative or quantitative agreement with those obtained from experiments and other MDS and indicate that most macroscopic obstructed flow patterns still exist even in nanoscale.

Three dimensional numerical simulations of the laminar fluid flow and heat transfer of water in silicon microchannels with non-circular cross-sections were performed. Two kinds of non-circular microchannel were investigated: trapezoidal and triangular. The continuum medium assumption was adopted and the corresponding governing equations and boundary conditions were used. The finite volume method was used to discretize the differential equations. The QUICK scheme was used for the discretization of convective term and the CLEAR algorithm was adopted to deal with the coupling between velocity and pressure. The water thermal physical properties were assumed to be constants except the viscosity. For both microchannels the grid system of 82 × 42 × 142 was used. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results reveal the effects of the geometric parameters of the microchannels, and indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around Re = 100. In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increase trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, the fully developed Nusselt number for the microchannels simulated increases with the increasing of on Reynolds number, rather than a constant. The present study shows that for liquid such as water the heat transfer and fluid flow in microchannels with geometric dimension in the order of 10–100 micrometer without the effect of external electro-field, the continuum assumption is still valid and numerical simulation can be conducted with conventional model and approach.

Recently, a large number of experiments have been conducted to examine the applicability of Navier-Stokes equations to predict the friction factor for the laminar flow of polar fluids in microchannels. However, significant discrepancies still exist between various results. In order to investigate the effect of electric double layer on the pressure-driven flow of polar fluid in smooth rectangular microchannels and to reveal whether continuum model can still be applied, numerical investigations are conducted in this paper. The simulated microchannels are made of silicon engraved substrate with Pyrex cover, and the measured relative roughness of channels is less than 0.5%. Deionized water and tap water with different ion concentration and electrical conductivity are used as the working fluid. The governing equations include the two-dimensional, non-linear Poisson-Boltzmann equation, the modified N-S equation and the electric field E z equation. The steady state electric field E z equation is coupled with the momentum equation. The FVM (finite volume method) is adopted to discretize the governing equations. The non-uniform grid systems 152×102 is applied and the Reynolds number ranges from 0.1 to 300. Good agreements are achieved between the numerical results and the experimental data available in the literature. The effects of electrical potential, fluid ion concentration, electrical conductivity and the channel dimensions on the EDL profile, and the electroviscous effect and the friction coefficient are presented in detail. The simulation results reveal that the Debye thickness depends on ion concentration greatly. Only when the ratio of D h /δ (the ratio of channel hydraulic diameter to the Debye thickness) is low, the electroviscous effect should be considered. According to the experimental conditions, the numerical results show that if the ratio of D h /δ is greater than 15, the predicted friction factors agree well with the macroscale classical law, and the disparities are less than 4%.