Low Aspect Ratio Channel Research Articles

Performance analysis of thermoelectric generator system in different aspect ratio collector channels

Rectangular collector channels are widely used in thermoelectric generator (TEG) systems. The aspect ratio greatly influences the heat transfer of the channel and thus affects the TEG output performance. However, the effect of aspect ratio on TEG output performance has not been intensively analyzed. Thus this paper studies the effect of aspect ratio on TEG output performance under the same heated area for the first time and clarifies the reasons for the high output power of low aspect ratio channels. A numerical model of the TEG system is developed, and the model's accuracy is verified. Then, the output performance of the collector channel TEG system with different aspect ratios, such as output power, net power, and heat conversion efficiency, is investigated to obtain the optimal channel aspect ratio. Finally, the effects of the fin number and the load resistance on the output performance of the TEG system are further analyzed. The results show that the TEG system with low aspect ratio channels exhibits better output performance; the TEG system with an aspect ratio of 0.1 has an 18.3% higher output power than the TEG system with an aspect ratio of 1. When the Re number is 2400, the TEG with an aspect ratio of 0.1 has the highest net power of 2.77 W. The lower aspect ratio reduces the collector channel's hydraulic diameter and improves the TEG's heat transfer performance. When the fin number is increased from 16 to 34, the output power increases by 10%, but the lower number of fins exhibits a higher heat conversion efficiency. This study provides some new ideas for improving the output performance of TEG systems.

Conjugate heat transfer effects on flow boiling in microchannels

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D h = 229 µm, a uniform base heat flux of q b = 100 kW / m 2 , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ = 0.25 –4 and W f = D h / 8 − D h , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 0.25 , for same fin thickness W f = D h / 8 . In the two-phase flow regime, Nusselt numbers in the range Nu = 12 − 36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W f = D h / 8 and W f = D h / 4 , an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . For thicker fins, W f = D h / 2 and W f = D h , the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ = 0.5 –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.

Cost-effective microfabrication of sub-micron-depth channels by femto-laser anti-stiction texturing

Micro Electro Mechanical Systems (MEMS) and microfluidic devices have found numerous applications in the industrial sector. However, they require a fast, cost-effective and reliable manufacturing process in order to compete with conventional methods. Particularly, at the sub-micron scale, the manufacturing of devices are limited by the dimensional complexity. A proper bonding and stiction prevention of these sub-micron channels are two of the main challenges faced during the fabrication process of low aspect ratio channels. Especially, in the case of using flexible materials such as polydimethylsiloxane (PDMS). This study presents a direct laser microfabrication method of sub-micron channels using an infrared (IR) ultrashort pulse (femtosecond), capable of manufacturing extremely low aspect ratio channels. These microchannels are manufactured and tested varying their depth from 0.5 μm to 2 μm and width of 15, 20, 25, and 30 μm. The roughness of each pattern was measured by an interferometric microscope. Additionally, the static contact angle of each depth was studied to evaluate the influence of femtosecond laser fabrication method on the wettability of the glass substrate. PDMS, which is a biocompatible polymer, was used to provide a watertight property to the sub-micron channels and also to assist the assembly of external microfluidic hose connections. A 750 nm depth watertight channel was built using this methodology and successfully used as a blood plasma separator (BPS). The device was able to achieve 100% pure plasma without stiction of the PDMS layer to the sub-micron channel within an adequate time. This method provides a novel manufacturing approach useful for various applications such as point-of-care devices.

Constraining the sedimentology and stratigraphy of submarine intraslope lobe deposits using exhumed examples from the Karoo Basin, South Africa

Intraslope lobe deposits provide a process record of the infill of accommodation on submarine slopes and their recognition enables the accurate reconstruction of the stratigraphic evolution of submarine slope systems. Extensive exposures of discrete sand-prone packages in Units D/E and E, Fort Brown Formation, Karoo Basin, South Africa, permit analysis of the sedimentology and stacking patterns of three intraslope lobe complexes and their palaeogeographic reconstruction via bed-scale analysis and physical correlation of key stratal surfaces. The sand-prone packages comprise tabular, aggradationally to slightly compensationally stacked lobe deposits with constituent facies associations that can be attributed to lobe axis, lobe off-axis, lobe-fringe and distal lobe-fringe environments. Locally, intraslope lobe deposits are incised by low aspect ratio channels that mark basinward progradation of the deepwater system. The origin of accommodation on the slope for lobe deposition is interpreted to be due to differential compaction or healing of scars from mass wasting processes. The stacking patterns and sedimentary facies arrangement identified in this study are distinct from those of more commonly recognized basin-floor lobe deposits, thereby enabling the establishment of recognition criteria for intraslope lobe deposits in other less well exposed and studied fine-grained systems. Compared to basin floor lobes, intraslope lobes are smaller in volume, influenced by higher degrees of confinement, and tend to show aggradational stacking patterns.

Counterflow quantum turbulence in a square channel under the normal fluid with a Poiseuille flow

We perform a numerical analysis of superfluid turbulence produced by thermal counterflow in He II by using the vortex filament model. Counterflow in a low aspect ratio channel is known to show the transition from laminar flow to the two turbulent states TI and TII. The present understanding is that the TI has the turbulent superfluid and the laminar normal fluid but both fluids are turbulent in the TII state. This work studies the vortex tangle in the TI state. Solid boundary condition is applied to walls of a square channel, and the velocity field of the normal fluid is prescribed to be a laminar Poiseuille profile. An inhomogeneous vortex tangle, which concentrates near the solid boundaries, is obtained as the statistically steady state. It is sustained by its characteristic space-time oscillation. The inhomogeneity of the vortex tangle shows the characteristic dependence on temperature, which is caused by two effects, namely the profile of the counterflow velocity and the mutual friction.

Geometric design of microfluidic chambers: platelet adhesion versus accumulation

Arterial, platelet-rich thrombosis depends on shear rates and integrin binding to either a collagen surface or to the growing thrombus, which are mechanistically different. In general, small microfluidic test sections may favor platelet-surface adhesion without testing for the primary mode of intra-arterial thrombosis, i.e. platelet-platelet bonding and accumulation. In the present report, the ratio of platelet-platelet to platelet-surface interactions, R, and the percentage of platelet-platelet interactions, P, are estimated using an analytical approach for circular and rectangular test sections. Results show that the test section geometry strongly affects both R and P, with test section height in low-aspect ratio channels or diameter greater than 90 μm dominated by platelet-platelet interactions (R >10). Increasing rectangular test section aspect ratio decreases the required height. R increases linearly while P approaches 100 % asymptotically with increasing channel dimension. Analysis of platelet shape shows that the assumption of spherical platelets has a small effect on R compared to discoid platelets adhering flat against test section wall. However, an increase in average platelet volume resulted in a large decrease in R. Nonetheless, Monte Carlo simulations of a typical distribution of human platelet sizes show intrasubject variation in platelet size has only a 10 % net effect on R. Finally, experiments of thrombus formation show that platelet-surface lag times and platelet-platelet accumulation are similar for rectangular microfluidic test sections and round test sections when R >10. The findings show that the size of a microfluidic test section should be carefully considered in studies of cell-cell accumulation versus cell-surface adhesion.

Internal Cooling Using Porous Turbulators: Heat Transfer and Pressure Drop Measurements

Heat transfer and pressure drop characteristics were studied in a low aspect ratio channel with high temperature carbon foams. The convoluted fluid flow path that significantly increases the surface area is a primary reason for the high heat transfer augmentation. The experiment provided module-averaged heat transfer coefficients on the heated wall, from which the Nusselt number was calculated. Increasing the channel blockage increased the heat transfer coefficient due to the amplification in the surface area. On the contrary, the pressure drop was also high for a high channel blockage. By using a corrugated foam arrangement, heat transfer remained relatively high with a pressure drop reduction by a factor of four. The staggered arrangement of the porous foam continuously perturbs the thermal boundary layer, which helps to sustain a high thermal gradient between the hot wall and the fluid. Increased Reynolds numbers resulted in a corresponding rise in the heat transfer rate while the roughened to smooth wall heat transfer ratio decreased.

Driving a channel through a levee when the levee is high: An outcrop example of submarine down-dip entrenchment

Studies of modern submarine slope to basin floor systems suggest a smooth concave upward long profile is normal for channel systems where down-dip development is unhindered by bathymetric irregularities. Deviation from this ideal may occur where channel systems encounter positive or negative relief that drives down-dip incision and creation of a knick point. The Unit D system, exposed in the Karoo Basin, South Africa, provides a rare opportunity to examine the long profile of a submarine channel where the effects of bathymetry on geometry are evident over a down-dip distance of over 30 km. The Geelbek area, the focus of this paper, is located at the distal end of the study area. Four phases of stratigraphic development are recognised; D1, a distributary lobe complex incised by a younger genetically related high aspect ratio channel; D2, an accumulation of external levee deposits sourced from a channel unrelated to D3, which is a late-stage entrenched low aspect ratio channel, which incised from near the top of the D2 levee, through older Unit D deposits and into the underlying mudstones, before being partially removed by a late-stage slide (D4). Both D1 and D3 channel systems are oriented at a high angle to the regional palaeoslope of the Laingsburg depocentre. The accumulation of earliest Unit D deposits is thought to have influenced the orientation of the D1 channel and lobe, whereas the direction of the D3 channel is consistent with the maximum depositional gradient of the D2 levee. The depth of incision of the D3 channel at Geelbek suggests that connection to a bathymetrically deeper part of the basin was established late in D time, which locally steepened the channel gradient generating a knick point.

Inertial microfluidics in parallel channels for high-throughput applications

Passive particle focusing based on inertial microfluidics was recently introduced as a high-throughput alternative to active focusing methods that require an external force-field to manipulate particles. In this study, we introduce inertial microfluidics in flows through straight, multiple parallel channels. The scalable, single inlet and two outlet, parallel channel system is enabled by a novel, high-density 3D PDMS microchannel manufacturing technology, mediated via a targeted inhibition of PDMS polymerization. Using single channels, we first demonstrate how randomly distributed particles can be focused into the centre position of the channel in flows through low aspect ratio channels and can be effectively fractionated. As a proof of principle, continuous focusing and filtration of 10 μm particles from a suspension mixture using 4- and 16-parallel-channel devices with a single inlet and two outlets are demonstrated. A filtration efficiency of 95-97% was achieved at throughputs several orders of magnitude higher than previously shown for flows through straight channels. The scalable and low-footprint focusing device requiring neither external force fields nor mechanical parts to operate is readily applicable for high-throughput focusing and filtration applications as a stand-alone device or integrated with lab-on-a-chip systems.

The oscillatory channel flow with arbitrary wall injection

In this article, we consider the laminar oscillatory flow in a low aspect ratio channel with porous walls. For small-amplitude pressure oscillations, we derive asymptotic formulations for the flow parameters using three different perturbation approaches. The undisturbed state is represented by an arbitrary mean-flow solution satisfying the Berman equation. For uniform wall injection, symmetric solutions are obtained for the temporal field from both the linearized vorticity and momentum transport equations. Asymptotic solutions that have dissimilar expressions are compared and shown to agree favourably with one another and with numerical experiments. In fact, numerical simulations of both linearly perturbed and nonlinear Navier-Stokes equations are used for validation purposes. As we insist on verifications, the absolute error associated with the total time-dependent velocities is analysed. The order of the cumulative error is established and the formulation based on the two-variable multiple-scale approach is found to be the most general and accurate. The explicit formulations help unveil interesting technical features and vortical structures associated with the oscillatory wave character. A similarity parameter is shown to exist in all formulations regardless of the mean-flow selection.

Convective Heat Transfer Distribution on the Surface of an Electronic Package

Measurements of the distribution of convective heat transfer over the five exposed faces of a low profile electronic package are described. The package, of square planform and length-to-height ratio, L/a = 6, is part of a regular array of such elements attached to one wall of a low aspect ratio channel. The coolant is air, and experiments are described for the Reynolds number range, 3000&lt;Re&lt;7000. The average heat transfer coefficient for the top face is found to be nearly equal to the overall average heat transfer coefficient for the element. The average heat transfer coefficient for the upstream face and two side faces are higher than the overall average by approximately 30–40 percent and 20–30 percent, respectively while that for the downstream face is 20–30 percent less than the overall average. Furthermore, the distribution in local heat transfer coefficient over the five surfaces of the element is approximately independent of variations in Reynolds number.

Forced Convection Air Cooling of Simulated Low Profile Electronic Components: Part 1—Base Case

Forced convection cooling of a simulated array of card-mounted electronic components has been investigated. An important feature of the simulated components is their relatively low profile (height/length = 0.058). Laboratory measurements of heat transfer rates resulting from convective air flow through a low aspect ratio channel are reported. The effect of variations in array position, channel spacing and flow rate is discussed. In the flow range considered laminar, transitional and turbulent heat transfer behavior have been observed. The behavior due to variations in flow rate and channel spacing is well correlated using a Reynolds number based on component length.

A CONCEPTUAL STUDY FOR A NEW WINGED SURFACE EFFECT VEHICLE SYSTEM

ABSTRACTThis article describes the results of a feasibility study on a class of winged surface effect vehicles (WSEV's) intended for naval use. The objective of the study was to define a system which would: 1) possess speed and operational capabilities substantially greater than existing surface effect vessels, 2) circumvent size limitations inherent in conventional aircraft designs, and 3) offer an economically and operationally viable alternative to existing water based and/or airborne naval systems for a variety of missions. The proposed WSEV's are a family of low aspect ratio channel shaped vessels operating on the principle of an aerodynamic lifting wing in ground effect during cruise and initially intended for cargo transportation over short‐to‐intermediate ranges (up to 3000 nautical miles). The proposed configuration is quite flexible, however, and could be suitable for a variety of missions. It is concluded that the proposed WSEV configuration shows substantial promise for satisfying the design objectives specified. However, several aspects of the system require additional research.