Channel Height Decreases Research Articles

Effects of channel height on the combustion efficiency and thermal performances of a micro Swiss-roll combustor

Micro combustors can be used as the heat sources for micro power generation systems. Recently, we developed a micro Swiss-roll combustor for non-premixed fuel and air. In the current work, the impacts of channel height (H2 = 7.5, 7.0, 6.5, and 6.0 mm) on the combustion efficiency of CH4, and the surface radiation efficiency of this combustor were numerically investigated under constant flow rates of the fuel and air. The results showed that both combustion efficiency and radiation efficiency varied non-monotonically with the decrease of channel height. The optimal channel height was found to be 6.5 mm under which a highest combustion efficiency of 99.05 % and a highest radiation efficiency of 27.03 % were obtained. These variation rules were theoretically analyzed in terms of the heat recirculation effect on fresh reactants, the buoyancy effect on flame structure and chemical reaction, and the non-dimensional residence time of the reactants (DaL). It was unraveled that the combustor with H2 = 6.5 mm achieved a best heat recirculation effect. In addition, the buoyancy effect was weakened as the channel height was decreased, resulting in a more symmetric flame structure in the vertical direction and a more intense chemical reaction. Furthermore, when the channel height descended from H2 = 7.5 mm to 6.5 mm, the region of DaL > 1 was broadened due to the intensified reaction, but narrowed as the channel height was further reduced to 6.0 mm due to increased flow velocity. The combined actions of these factors led to non-monotonic variations of combustion efficiency and radiation efficiency. The discoveries of the present study provided a guideline for optimal design of such kind of micro Swiss-roll combustors.

Effects of manifold design parameters on flow uniformity in parallel mini-channels

Flow maldistribution in parallel mini-channels is a critical obstacle to the efficient design of mini-channel heat sinks. The flow maldistribution is known to cause dryout incipience in some channels, which triggers critical heat flux, eventually leading to physical burnout and catastrophic device failure. Hence, enhancing flow uniformity by reducing flow maldistribution is critical. While several attempts have been made to reduce flow maldistribution, most have focused on providing strategic frameworks for enhancing flow uniformity in channels. The present study develops a new predictive tool for quantitatively assessing flow maldistribution in parallel channels based on the parametric effect. Five parameters are considered: channel height, channel width, header length, connection tube diameter, and total volumetric flow rate. Thirty-six different heat sinks are numerically investigated to study the influence of each parameter on flow uniformity in channels. A decrease in channel height, channel width, and total volumetric flow rate and an increase in header length and connection tube diameter reduces the maldistribution factor enhancing channel flow uniformity. By analyzing the effects of these parameters on the flow uniformity in channels, a new correlation for the maldistribution factor constructed with dimensionless groups, including the design parameters, is proposed, which fits the numerical predictions for the maldistribution factor with excellent accuracy, evidenced by a root-mean-squared-error of 1.0%. The parametric assessment of the effects of the different parameters on the flow maldistribution using the new correlation shows that the magnitude of the relative influence of each variable on flow maldistribution can be expressed in the following order: header length > channel height > connection tube diameter > total volumetric flow rate > channel width.

Theory for the Casimir effect and the partitioning of active matter.

Active Brownian particles (ABPs) distribute non-homogeneously near surfaces, and understanding how this depends on system properties-size, shape, activity level, etc.-is essential for predicting and exploiting the behavior of active matter systems. Active particles accumulate at no-flux surfaces owing to their persistent swimming, which depends on their intrinsic swim speed and reorientation time, and are subject to confinement effects when their run or persistence length is comparable to the characteristic size of the confining geometry. It has been observed in simulations that two parallel plates experience a "Casimir effect" and attract each other when placed in a dilute bath of ABPs. In this work, we provide a theoretical model based on the Smoluchowski equation and a macroscopic mechanical momentum balance to analytically predict this attractive force. We extend this method to describe the concentration partitioning of active particles between a confining channel and a reservoir, showing that the ratio of the concentration in the channel to that in the bulk increases as either run length increases or channel height decreases. The theoretical results agree well with Brownian dynamics simulations and finite element calculations.

Numerical analysis of photovoltaic-thermal collector using nanofluid as a coolant

The PV cells and thermal collector are integrated into a system to form the photovoltaic-thermal (PV/T) collector, and nanofluid is utilized as a coolant to decrease temperature of photovoltaic cells (PV). This work is aimed at the numerical analysis of a PV/T collector using nanofluid. For acquiring more results about the effects of operation parameters on performances of PV/T collector, the mathematical models of PV/T collector are proposed. In this work, the influences of nanofluid type and volume concentration on PV conversion efficiency, PV cell temperature, thermal and electrical power are investigated. The effects of PV collector parameters on the performances are also analyzed and discussed. The results indicate that the performances of PV/T collector with Al2O3/water nanofluid are better than that of the PV/T collector using TiO2/water nanofluid. When mass flow rate of nanofluid is 0.03 kg/s, the electrical power of the PV/T collector is much higher than that of the PV/T collector when mass flow rate of nanofluid is 0.0005 kg/s, 0.001 kg/s and 0.01 kg/s. As mass flow rate of nanofluid is 0.03 kg/s, the thermal power of the PV/T collector is 12.11% higher than that of the PV/T collector as mass flow rate of nanofluid is 0.0005 kg/s. When the channel height decreases, the removed heat of the PV/T collector by the nanofluid increases, the largest thermal power difference of the PV/T collector between 0.005 m and 0.015 m tube diameter is 24.00 W.

Experimental study and numerical analysis of the scaling effect on the flame stabilization of propane/air mixture in the micro-scale porous combustor

In order to investigate the effect of heat recirculation through the porous medium on the flame stabilization limits for different channel heights, micro-combustors with three different channel heights 3 mm, 2.5 mm and 2 mm are fabricated and tested. A three-dimensional numerical model is established to explain the inherent mechanisms for the experimental results. It is shown that the stable flame can be established inside the micro-scale porous combustor with the channel height 2 mm which is smaller than the quenching distance. The porous medium combustion limit remains nearly unchanged as the channel height decreases due to the enhanced heat recirculation through the porous medium and the solid wall. The extinction limit increases obviously with the decrease of channel height due to the excess heat loss. The simulation results show that the flame thickness becomes larger as the channel height decreases. With the decreasing channel height, the wall temperature increases due to the increase of inner-wall convection heat transfer coefficient. The thermal balance analysis shows that the decrease of channel height results in lower thermal efficiency. The 2 mm (ε = 4) height combustor is more suitable for the micro thermoelectric generator. Extra heat exchange channel is need to utilize the output energy sufficiently.

Investigation on three dimensional squeezing flow of mixture base fluid (ethylene glycol-water) suspended by hybrid nanoparticle (Fe3O4-Ag) dependent on shape factor

In this paper, three dimensional squeezing flows and heat transfer of (Fe3O4)-Ag/ethylene glycol-water hybrid nanoparticles inside a rotating channel with a fixed and impermeable bottom walls in the presence of thermal radiation has been investigated and analyzed. It should be noted that the hybrid base fluid consists of 50% water and 50% ethylene glycol. PDEs governing the problem have been solved after converting to ODEs using numerical method. The effect of different parameters variations on velocity and temperature profiles and also on skin friction coefficient and local Nusselt number are shown in graphs for two modes of increasing and decreasing channel height for three shape factor of nanoparticles such as, Bricks, cylinders and platelets. Increasing velocity profile due to the decrease in channel height, reducing the temperature profile and increasing heat transfer rate due to the increase in shape factors, the greater effect of hybrid nanofluid in comparison with nanofluid on reducing temperature due to the large difference in the thermal conductivity of two nanoparticles and the greater local Nusselt number in the top wall of the channel for hybrid nanoparticles compared to the nanoparticle is the most important results.

Heat transfer augmentation in rectangular micro channel covered with vertically aligned carbon nanotubes

An experimental heat transfer investigation was carried out to examine the influence of carbon nanotubes (CNTs) layer deposits on the convective heat transfer performance inside rectangular microchannels. Successful synthesis of vertically aligned CNTs was achieved using a catalytic vapor deposition (CVD) process on a silicon sample substrate. By varying the synthesis time, on average 6μm and 20μm thick layers of CNTs were made with surface roughness of (Sa=1.062μm, Sq=1.333μm) and (Sa=0.717μm, Sq=0.954μm) respectively. The external surface area of the samples increased 7 times compared to the bare silicon chip. The heat transfer performance of each sample was measured inside two rectangular microchannels with cross-section of 125μm×9mm and 200μm×9mm. For the 125μm channel height, the 6μm and 20μm thick layer of CNTs resulted in 12% and 26% increase in pressure drop respectively. The pressure drop obtained from the 200μm channel height show a similar trend with an increase of 6% and 16.4% for 6μm and 20μm CNTs layer thickness respectively. An average heat transfer enhancement of 19% and 74% is obtained inside the 125μm height microchannel with 6μm and 20μm CNTs layer thickness respectively. Whereas, the average heat transfer enhancement of 22% and 62% are obtained inside the 200μm channel with respective CNTs layer thicknesses of 6μm and 20μm. Enhancements are attributed to an increase in surface area and effective thermal conductivity inside the thermal boundary layer. However, the frictional heating (viscous dissipation) of a particular nanostructured sample increases with a decrease in channel height. This difference in channel size results in stronger competition between heat transfer enhancement potential that can be achieved by the deposited surface and the decrease in Nusselt number due to viscous dissipation.

Dynamical role of slip heterogeneities in confined flows.

We demonstrate that flows in confined systems are controlled by slip heterogeneities below a certain size. To show this we image the motion of soft glassy suspensions in microchannels whose inner walls impose different slip velocities. As the channel height decreases, the flow ceases to have the symmetric shape expected for yield-stress fluids. A theoretical model accounts for the role of slip heterogeneities and captures the velocity profiles. We generalize these results by introducing a length scale, valid for all fluids, below which slip heterogeneities dominate the flow in confined systems. General implications of this notion, concerning the interplay between slip and confinement, are presented.

A three-dimensional modeling of transport phenomena of proton exchange membrane fuel cells with various flow fields

In this work, a two-phase three-dimensional numerical transport model based on the two-fluid method for the proton exchange membrane fuel cells (PEMFCs) with parallel flow field, interdigitated flow field, and serpentine flow field has been presented to study the cell performance and transport phenomena in the PEMFCs. The effects of width, height and aspect ratio of the flow channel on the cell performance and water removal with different flow fields are under investigation. Results show the liquid water removal increases as the channel height decreases, however, the cell performance decreases as well. It is also found that the cell performance decreases as the channel width increases in parallel flow field due to lower gas velocity with less water removal. The effect of channel aspect ratio approximate to 1 is particularly studied in this paper. It reveals that the cell performance is better as the channel cross-section area is smaller because of higher gas velocities.

Numerical characterization of laminar bulk flow over textured surfaces

Textured surfaces have been fabricated in the past using several methods and were characterized for relatively simple texture geometries and especially with static drops on them. Numerical tools may provide an effective alternative to understand the effect of complex texture geometries and help understand the effect of texturing on flowing liquids. This work focuses on the numerical simulation of laminar bulk flow of water through microchannels with textured surfaces and provides a comparison with their smooth counterparts. The interpillar spacing on the textured surfaces is found to influence the pressure drop. This is because the trapped air is able to exit the surface cavities at a relatively high rate for larger interpillar spacings. A decrease in channel height increases the percentage pressure drop reduction because of an increase in the ratio of slip length to channel height. The probability of water penetrating the surface cavities increases with flow velocity. Pressure drop measurements on microchannels with textured surfaces, fabricated using two-step photolithography, show that the theoretical pressure drop for smooth microchannels is marginally closer to the experimental value than those observed with textured microchannels. The study therefore helps understand the effect of individual parameters on the overall flow field and engineering parameters such as wall shear stress.

Power generation from concentration gradient by reverse electrodialysis in ion-selective nanochannels

In an aqueous solution, the surface of inorganic nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions. In the presence of a concentration gradient, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of one type of ions. The Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using inorganic ion-selective nanochannels. Silica nanochannels with heights of 4, 26, and 80 nm were used in this study. We experimentally investigated the power generation from these nanochannels placed between two potassium chloride solutions with various combinations of concentrations. The power generation per unit channel volume increases when the concentration gradient increases, and also increases as channel height decreases. The highest power density measured was 7.7 W/m2. Our data also indicate that the energy conversion efficiency and the ion selectivity increase with a decrease of concentrations and channel height. The best efficiency obtained was 31%. Power generation from concentration gradients in inorganic ion-selective nanochannels could be used in a variety of applications, including micro batteries and micro power generators.

A numerical investigation and parametric study of cooling an array of multiple protruding heat sources by a laminar slot air jet

The present article reports on the fluid flow and heat transfer characteristics associated with cooling an in-line array of discrete protruding heated blocks in a channel by using a single laminar slot air jet. Numerical experiments have been carried out for different values of jet Reynolds number, channel height, slot width, spacing between blocks, block height, and block thermal conductivity. The effects of variation of these parameters are detailed to illustrate important fundamental and practical results that are relevant to the thermal management of electronic packages. In general, the effective cooling of blocks has been observed to increase with the increase of Reynolds number and the decrease of channel height. Heat transfer rates are enhanced for shorter and widely spaced heated blocks. Circulation cells that may appear on the top surface of the downstream blocks have been shown to decrease the value of Nusselt number for these blocks. The values of surface averaged Nusselt number attain their maximum at the block just underneath the impinging air jet, decrease for the downstream blocks, and approximately reach a constant value after the third block. Useful design correlations have been obtained for the mean Nusselt number for the heated blocks underneath and downstream the impinging jet.

Opposed-flow flame spread in a circular duct of a solid fuel: Influence of channel height on spread rate

The influence of channel height on flame spread in a circular duct of the solid fuel in an opposed-flow configuration was examined. Polymethylmethacrylate cylinders with a circular duct (diameter of 1, 2, or 3 mm) were used as fuel specimens, and both flame-spreading and stabilized combustion were observed. In the case of stabilized combustion, the flame cannot spread into the duct because of the high oxygen velocity. The flame-traveling velocity is the velocity at which the flame widens the duct by fuel consumption. Therefore, the flame-traveling velocity in stabilized combustion is significantly low compared with flame-spreading combustion. In the case of flame-spreading combustion, the equivalence velocity, which contains channel height information, defines whether the regime is the thermal or the chemical regime. When the equivalent velocity is higher than a certain value, the flame-spread rate is controlled by chemical effects. On the whole, the flame-spread rate decreases with the decrease of channel height in the case of flame-spreading combustion because of the curvature effect. Owing to the curvature effect, the area ratio of the flame to that of the solid surface decreases with decreasing channel height, and this is conspicuous when the channel height is low. The curvature effect is negligible when the channel height is sufficiently large compared with the flame stand-off distance.

Analysis of Hydroelastic Instabilities of Rectangular Parallel-Plate Assemblies

Flow-induced vibrations and hydroelastic instabilities of rectangular parallel-plate assemblies are studied theoretically. The extended Galerkin method is employed to solve the plate equation, while the Fourier transform technique is used to solve for the perturbation pressure from the potential flow equations. The mixed fluid-plate interaction boundary condition leads to a singular integral equation, which is solved numerically. The complex frequencies versus flow velocity and critical velocities are obtained for various plate aspect ratios and channel-height-to-plate-width ratios. The frequencies and critical velocities are found to decrease as the plate length increases and as the channel height decreases. Two types of instability are found, namely, single-mode divergence, mostly in the first mode, and coupled-mode flutter involving adjacent modes. Damping has a significant effect on post-divergence stability and coupled-mode flutter of the system. [S0094-9930(00)00403-0]

Fluid Dynamic Characteristics of the Flow Over an Array of Large Roughness Elements

Flow visualization and measurements of velocity and turbulence intensity using laser Doppler velocimetry are used to investigate separation and reattachment processes in the flow over an array of protruding elements mounted on the bottom wall of a rectangular water channel. The concept of an array shear layer is introduced to demarcate the region of influence over which the resistance of the array retards the flow. Flow separation at the leading edge of the elements is documented. The confined or interacting nature of the flow in the cavities between elements is established as a function of element spacing. The reattachment length downstream of the element varies from 4 to 1.5 element heights, decreasing both with an increase in Reynolds number and a decrease in channel height.