Flat Rectangular Cross-section Research Articles

On the contribution of the top and bottom walls in micro-pillar array columns and related high-aspect ratio chromatography systems

We report on a steady-state based, and hence highly accurate numerical modelling study of the effect of the top and bottom wall in the current generation of micro-pillar array columns. These have a mesoporous retention layer that not only covers the pillar walls but also the bottom wall. Our results show that the performance of these columns can in general not be improved by also covering the top wall with the same layer, despite the increased column symmetry this approach would offer. The reason for this is that the local species retardation caused by a retentive layer is much stronger than the pure flow arresting effect of an uncovered wall. At least, this has a crucial impact in high aspect-ratio systems such as micro-pillar array columns because these require a small inter-pillar distance to promote mass transfer together with a large channel depth to enable a sufficiently high flow rate. On the other hand, a notable improvement could be made if micro-pillar array would be produced without having a retentive layer at the bottom. At Péclet number Pe = 50 and aspect ratio AR = 5 for flow-channels, this gain amounts up to about 4.5 h-units at a zone retention factor k’’ = 2 and 1.75 h-units at k’’ = 16 (gain scales almost linearly with Pe).To verify these results, we also considered another high aspect-ratio system with a simplified geometry: the open-tubular channel with a flat-rectangular cross-section. This led to very similar observations, thus confirming the findings for the micro-pillar array.The results produced in the present study also allow us to conclude that the classic modelling paradigm adopted in chromatography, which is based on the independency and hence additivity of the hCm- and hCs-contributions, can lead to large modelling errors in chromatographic systems with a high aspect-ratio, even when their geometry is so simple as that of a straight open-tubular channel with constant cross-section. Indeed, when both zones are treated independently, the analysis misses how the vertical diffusion through the retentive layer helps suppressing the vertical gradients in the mobile zone. The diffusion through this layer occurs in a ratio of k’’Ds/Dm (Dm being the diffusion coefficient in mobile phase zone and Ds being the diffusion coefficient in stationary phase zone), such that at high retention factors this diffusion contribution even becomes the dominant one.

Effect of Aspect Ratio and Installation Direction on Channel Flow Heat Transfer Performance under a Wide Range of Reynolds Number

In recent years, in the cooling technology for high-power electronic devices such as power transistors used for drive motor control of electric vehicles and hybrid vehicles, a method of flowing a cooling fluid to a cooling substrate having a fin structure has become the main technology. The structure of the cooling fluid flow path is a channel flow through multiple narrow plate gaps to secure a heat transfer area. In this study, the heat transfer characteristics when the aspect ratio of the channel having a flat rectangular cross-section was changed were investigated in detail by experiments. Moreover, the difference in the heat transfer characteristic at the time of making a rectangular flow path into vertical installation and horizontal installation was also investigated.

Performance evaluation of flat rectangular earth-to-air heat exchangers in harmonically fluctuating thermal environments

An earth-to-air heat exchanger (EAHE) is an excellent passive technology that uses shallow geothermal energy directly to generate cooled or heated air for thermal regulation of buildings. Previous studies have focused on circular EAHEs, paying insufficient attention to the effect of cross-sectional shape. In this paper, we propose an EAHE having a flat rectangular cross section, develop a mathematical model to calculate its thermal performance in a harmonic thermal environment, and validate the theoretical results using a three-dimensional numerical model. Our results indicate that compared with a circular EAHE shape of the same cross-sectional area, the outlet air temperature fluctuation of the flat rectangular EAHE is 24.7% lesser and its phase shift is 22.67 days longer, and the soil's thermal disturbance range is shorter, resulting in a temperature wave in the soil around the buried pipe that decays faster and creates a more stable pipe wall temperature. The cooling/heating capacity of the flat rectangular EAHE is greater than that of the circular EAHE. The pressure drop and fan power consumption of the flat rectangular EAHE is greater than that of circular EAHE, and thus the coefficient of performance (COP) value of the flat rectangular EAHE is less than that of the circular EAHE.

Development of a bendable pyramidal kagome structure and its structural characteristics

A sandwich construction is an effective lightweight structure. Conventionally, sandwich structures have been fabricated as plane panels with limitation of application to curved components. In this work, a bendable structure that can be applied to the curved surface was developed based on the pyramidal kagome (PK) structure. The bending zone adaptable to the curved surface was designed between the neighboring PK structures. The PK sandwich structure was strengthened by developing the cross-sectional design of the strut. The mechanical characteristics were investigated by three-point bending test and compression test. The semi-circular cross-section (SCC)-based PK sandwich structure showed improved bending stiffness and maximum bending load over those of the conventional flat rectangular cross-section (FRC)-based PK sandwich structure. The curved PK sandwich structure was applied to the curved component.

Optimum kinetic performance of open‐tubular separations in microfluidic devices

A survey is made of the different factors contributing to the kinetic performance of open-tubular separation channels. Being representative for most of the channels used in microfluidic devices, the main focus is on channels with a rectangular format. Kinetic plots of t(0)/N(2 )versus N are established to allow for a visual selection of the ideal channel format and dimensions. These plots for example show that in the pressure-driven mode a channel with a flat-rectangular crosssection (top and bottom wall covered by a retentive layer) can always yield slightly faster (some 15%) separations than a cylindrical capillary, provided the channel depth is optimized. If the channel depth is fixed, the optimal w/d-ratio depends on the required plate number. In electrically driven flows, the situation is reversed and rectangular channels with a small width are to be preferred, and the cylindrical capillary format becomes the best format.

Noise reduction by vortex suppression in air flow systems

Acoustic vibrations generated by von Karman vortex streets are reduced by shaping members interposed in moving air flowstreams, such as fan guards and grilles used in forced flow air conditioning systems, to have either a cylindrical cross section or a non-cylindrical cross-section with non-linear or interrupted leading or trailing edges presented to the air flowstream. Relatively flat rectangular cross-section members with interrupted or non-linear leading or trailing edges formed by somewhat sawtooth or sinusoidal wave forms or connected to spaced apart support members, or cylindrical members formed in the shape of a sawtooth or sinusoidal wave form, or presented with spaced apart rings or grooves interrupting the cylindrical cross-section of the member are typical configurations which exhibit reduced or substantially eliminated acoustic vibrations caused by von Karman vortex shedding.

Pushing the miniaturisation of LC with shear-driven flows

Recently, a novel driving principle for the mobile phase flow in open-tubular chromatographic systems has been proposed to evade the pressure drop limitation of pressure-driven chromatographic systems. The concept, further referred to as shear-driven chromatography (SDC), gives hitherto unachievably large mobile phase velocities through channels with a sub-micron thickness. Applying the SDC-concept in channels with a flat rectangular cross-section (e.g., 100 micrometer wide and only a few hundred nanometer thick), we hope to combine the high separation speed and resolution offered by the small channel thickness with a sufficient mass loadability arising from the large channel width. An attractive feature of the concept is furthermore that all the mobile and stationary phases of conventional LC can be used without any restriction or modification. The concept hence also provides a powerful alternative to CEC.

On the possibility of shear-driven chromatography:: a theoretical performance analysis

The use of shear forces for the generation of the mobile phase flow in chromatographic separations is proposed. This novel chromatographic operating principle, referred to as shear-driven chromatography (SDC), completely circumvents the pressure-drop limitation of conventional pressure-driven GC and LC without affecting the operational flexibility (choice of mobile and stationary phases, possibility of solvent and/or temperature programming, etc.). In the present paper, the expression for the height equivalent to a theoretical plate in SDC in a channel with a flat rectangular cross-section is established and is used to demonstrate the large gain in analysis speed under LC, GC and supercritical fluid chromatography conditions.

On the application of Reynolds' analogy and the heat-exchange factor to the design of heat exchangers I

Part I. Reynolds' analogy between heat transfer and friction leads to the wellknown equationh=1/2fcpϱv. According to improved theories, for turbulent flow of gases in long straight ducts, the factor 1/2 is to be replaced byc=0.55 It is shown that for laminar flow in long ductsc is equal to 0.3 for a circular, and equal to 0.4 for a flat rectangular cross-section. Further in very short ducts an approximate theory is shown to give values that are of the same order of magnitude. Though the equationh=cfcpϱv has an approximative character, it has the advantage of being simple and generally valid for gases flowing in ducts. Applications and experimental data will be given in a subsequent article.