Win-Jet Luo

Transient analysis of electro-osmotic secondary flow induced by dc or ac electric field in a curved rectangular microchannel

This study investigates transient secondary flow in a rectangular curved microchannel in which the fluid is driven by the application of an external dc or ac electric field. The resultant flow field evolutions within the microchannel are simulated using the backwards Euler time stepping numerical method in order to clarify the relationship between the changes in the transverse flow field conditions and the intensity of the applied electric field. The transient secondary flow evolutions provide evidence of the growth and decay of vortices in the transverse section. As the applied dc or ac electric field intensity is activated, a small vortex appears in each corner of the microchannel. Both upper and lower corner vortices gradually grow in size and strength and finally merge to form a single vortex, which compresses the original recirculation in the upper and lower half of the transverse section. In this study, the formation of these vortices is investigated through total applied force per unit area existing in the flow. The velocity magnitude of the vortices can be as high as 15% of the core axial speed.

Electroosmotic Flow Driven by DC and AC Electric Fields in Curved Microchannels

The purpose of this study is to investigate electroosmotic flows driven by externally applied DC and AC electric fields in curved microchannels. For the DC electric driving field, the velocity distribution and secondary flow patterns are investigated in microchannels with various curvature ratios. We use the Dean number to describe the curvature effect of the flow field in DC electric field. The result implies that the effect of curvatures and the strength of the secondary flows become get stronger when the curvature ratio of C/A (where C is the radius of curvature of the microchannel and A is the half-height of rectangular curved tube.) is smaller. For the AC electric field, the velocity distribution and secondary flow patterns are investigated for driving frequencies in the range of 2.0 kHz (Wo=0.71) to 11 kHz (Wo=1.66). The numerical results reveal that the velocity at the center of the microchannel becomes lower at higher frequencies of the AC electric field and the strength of the secondary flow decreases. When the applied frequency exceeds 3.0 kHz (Wo=0.87), vortices are no longer observed at the corners of the microchannel. Therefore, it can be concluded that the secondary flow induced at higher AC electric field frequencies has virtually no effect on the axial flow field in the microchannel.

Analysis of Electrokinetic Mixing Using AC Electric Field and Patchwise Surface Heterogeneities

In this paper, the authors investigate the use of an applied AC electric field and microchannel surface heterogeneities to carry out the microfluidic mixing of two-dimensional, time-dependent electroosmotic flows. The time-dependent flow fields within the microchannel are simulated using the backwards-Euler time-stepping numerical method. The mixing efficiencies obtained in microchannels with two different patchwise surface heterogeneity patterns are investigated. In general, the results show that the application of an AC electric field significantly reduces the required mixing length compared with the use of a DC electric field. Furthermore, the presence of oppositely charged surface heterogeneities on the microchannel walls results in the formation of localized flow circulation regions within the bulk flow. These circulation regions grow and decay periodically in accordance with the periodic variation of the AC electric field intensity and provide an effective means of enhancing species mixing in the microchannel. Consequently, the use of an AC electric field together with patchwise surface heterogeneities permits a significant reduction in both the mixing channel length and the retention time required to attain a homogeneous solution.

A User-Friendly Web Content Management System

This study applies two open source projects, namely PHP and MySQL, to create a powerful, user-friendly Web-based system for creating, maintaining and publishing a database comprising news information in the form of text and graphical images. The proposed system is specifically designed to enable non-technical users with no knowledge of computer programming, graphic imaging tools, or markup languages such as HTML to add new material to a Website or to modify the existing contents as and when required in an intuitive and real-time fashion. As a result, a technically competent Website management team is not required, and thus the cost and flexibility of the Website management process is significantly improved.

Effects of process air conditions and switching cycle period on dehumidification performance of desiccant-coated heat exchangers

This study investigated the dehumidification effect of silica gel and sodium polyacrylate desiccants coated on fin and tube heat exchangers under various humidity and temperature conditions. In the system, the two desiccant-coated heat exchangers are periodically operated in mass equilibrium states between dehumidification and regeneration processes. The corresponding cyclic switching times between the two processes are investigated to determine suitable switching times that will further improve the operational efficiency of the system. The experimental results show that while water at a temperature of 50°C was supplied during the regeneration process, the sorption ability was greater and the vapor was discharged in a shorter period of the regeneration process for the sodium polyacrylate desiccant than for silica gel. Thus, the moisture sorption ability of sodium polyacrylate in the system is superior to that of silica gel under the same operational conditions. Suitable cyclic switching times for both desiccants in different operational conditions were recommended in this study. By adapting the corresponding recommended switching times in the dehumidification and regeneration processes, a 10–14% increase in thermal performance can be attained for the two desiccants under the operating conditions.

EVAPORATION OF NON-NEWTONIAN FLUID IN POROUS MEDIUM UNDER MIXED CONVECTION

This study investigates the problem of non-Newtonian liquid evaporation from an impermeable vertical plate in a porous medium under mixed convection conditions. The analysis examines the effects of non- Darcian flow phenomena on the evaporation process and investigates the heat and mass transfer character- istics under a range of flow conditions in the aiding and opposing flow types. The parameters of interest include Rak,non, Pex,non, Pr and Sc. The data required for the calculations include a fixed wall temperature of Tw = 45°C, a free flow temperature of T∞ = 20°C and an air free flow rate of u∞ = 4m/sec and the corre- sponding humidity of 50% flows over fixed temperature and moist wall. The results indicate that an increasing buoyancy force enhances the mixing effect and therefore im- proves the rate of heat transfer, the latent heat flux is the primary mode of heat transfer in evaporation processes and the velocity gradient at the wall increases with an increasing buoyancy effect. Further- more, the evaporation rate increases with a reducing flow index and an increasing buoyancy effect.

FLOW BIFURCATION AND TRANSITION IN A GAP BETWEEN TWO-ROTATING SPHERES

ABSTRACT This study investigates the flow bifurcation and transition between stable states in the gap between two concentric rotating spheres. A continuation method is used together with linear stability analysis to investigate the bifurcation structure of the discretized governing equations and to determine the stability of the calculated states. The constructed bifurcation diagram is used to illustrate the restricted range of Reynolds number within which each equilibrium state exists. The diagram also identifies the permissible transitions between these states and indicates their terminative states. In the present study, it is shown how appropriate control of the angular velocity of the outer sphere results in the evolution of the flow state through a series of permitted stable states. The time-dependent transitions between these states are investigated by means of a backwards-Euler time stepping formulation. The terminate state of transition process can also be used to confirm the stability of the flow. The flow evolution between each transition is illustrated by means of temporal sequences of the meridional streamlines and transition curves. The present results indicate that all the flow transitions in a gap between two rotating spheres are produced symmetrically with respect to the equator.

Turbine blade heat transfer prediction in flow transition using k-omega two-equation model

The proper design of a turbine blade has an important effect on engine performance and efficiency. The thermal design of a turbine blade is also greatly influenced by the flow transition phenomena of the boundary layer. Transition in turbine flowfields is a complex process and is influenced by the Reynolds number, pressure gradient, geometry, and freestream turbulence, etc. In this study, a k-o> turbulence model is used to investigate the flow transition as well as problems concerned with heat transfer. The model predicts well the transition point and the associated transitional heat transfer coefficients for flat plate flows. With a suitable surface curvature correction to closure coefficients of the model, reasonable heat transfer predictions in flow transition are simulated for turbine blade flows.

Performance analysis of single-stage air source heat pump utilizing indirect vapor injection design:

This study investigated the performance of a single-stage scroll compressed air source heat pump coupled with a flash tank indirect vapor injection. In the refrigerant circulating piping of the heat pump, an indirect vapor injection piping connecting a flash tank at the refrigerant outlet of a condenser and the suction of a scroll compressor was designed. By the indirect refrigerant vapor injection from the flash tank, the saturation pressure and temperature of the refrigerant (R134a) in the evaporator can be raised and the sub-cooling of the refrigerant at the inlet of expansion valve can be enlarged as well. Thus, energy consumption of the compressor can be reduced and cooling capacity of the evaporator can be boosted. It was found that the suitable amount of indirect vapor refrigerant strongly depends on the operational environmental temperatures. By adjusting the suitable indirect vapor injection volume into the compressor, the performance of the heat pump was enhanced. In this study, a suitable indirect vapor injection volume was found and 5~15% performance increments can be obtained while the heat pump operated under ambient temperatures in a range from 5°C to 35°C.

Development and analysis of a new multi-function heat recovery split air conditioner with parallel refrigerant pipe

This study developed a heat recovery air conditioner to supply on-demand air conditioning and reclaim waste heat during operation. In addition to the conventional refrigerant pipe, a plate heat exchanger was designed and connected in parallel to the condenser and evaporator. While refrigerant with high pressure and high temperature flows through the plate heat exchanger, the refrigerant flow is cooled down and condensed by cooling water from a water tank, and condensation heat in the exchanger is absorbed by the cooling water and stored in the water tank. In this study, solenoid valves were installed in the pipe to regulate the refrigerant volume in parallel connected pipes based on the tank water temperature by duty control logic. This multi-function heat recovery air conditioner has various operation modes. In summer, the air conditioner not only provides on-demand cooled air to air-conditioned rooms but also reclaims waste condensation heat through the plate heat exchanger. In winter, the air conditioner supplies on-demand heated air to indoor spaces and heats the water tank by simultaneously reclaiming waste condensation heat. According to an experimental operation test, the value of average water heating efficiency (COPh) of this air conditioner can reach 3.77 in the summer operating mode and 1.91 in the winter operating mode.