Friction reduction by inlet temperature variation in microchannel flow

2021 ◽  
Vol 33 (6) ◽  
pp. 062003
Author(s):  
Dmitry S. Gluzdov ◽  
Elizaveta Ya. Gatapova
Keyword(s):  
Temperature Variation ◽  
Friction Reduction ◽  
Inlet Temperature ◽  
Microchannel Flow

Stability Analysis of Cassie-Baxter State Under Pressure Driven Flow

2010 ◽  
Author(s):  
Tae Jin Kim ◽  
Carlos H. Hidrovo
Keyword(s):  
Liquid Layer ◽  
Friction Reduction ◽  
Wetting Transition ◽  
Textured Surface ◽  
Microchannel Flow ◽  
Flow Conditions ◽  
Cassie State ◽  
The Stability ◽  
Gas Layer ◽  
Micro Cavity

The Cassie-Baxter state is a phenomenon in which a liquid rests on top of a textured surface with a gas layer trapped underneath the liquid layer. This gas layer introduces an effective shear free boundary that induces slip at the liquid-gas interface, allowing for friction reduction in liquid channel flows. Multiple studies have shown that different surface configurations result in different friction reduction characteristics, and most work is aimed at controlling the roughness factor and its shape in order to achieve an increased slip flow. This paper investigates the effects that different texturing geometries have on the stability of the Cassie state under pressurized microchannel flow conditions. To test the stability effects associated with the pressurized microchannel flow conditions, microfluidic channels with microstructures on the side walls were designed and fabricated. The microstructures were designed to induce the Cassie state with a liquid-air interface forming between the texturing trenches. The air trapped within the microstructure is treated as an ideal gas, with the compressibility induced pressure rise acting as a restrictive force against the Wenzel wetting transition. The model was validated against experimental flow data obtained using microchannel samples with microtextured boundaries. The microchannels were fabricated in PDMS (poly-dimethylsiloxane) using soft lithography and were baked on a hot plate to ensure the hydrophobicity of the microtexture. Pressure versus flow rate data was obtained using a constant gravitational pressure head setup and a flow meter. The liquid-gas interface layer in the microchannel was visualized using bright field microscopy that allowed measurement of the liquid penetration depth into the microtexturing throughout the microhannel. The experimental results indicate that air trapped in the pockets created by micro-cavity structures prevented the liquid layer from completely filling the void. As expected, the pressure drop in the micro-cavity textured channel showed a considerable decrease compared to that in the flat surfaced channel. These results also suggest that micro-cavities can maintain the Cassie state of a liquid meniscus, resting on top of the surface, in larger pressure ranges than open spaced micro-pillars arrays.

Effect of Wetted Microtexturing on Friction in Microchannel Flow

2020 ◽  
Author(s):  
Nastaran Rabiei ◽  
Carlos Hidrovo
Keyword(s):  
Aspect Ratio ◽  
Hydraulic Resistance ◽  
Open Space ◽  
Friction Reduction ◽  
Solid Boundary ◽  
Textured Surface ◽  
Microchannel Flow ◽  
Small Diffusion ◽  
Pressure Drag ◽  
Aspect Ratios

Abstract Microchannel flows are widely used in applications where small diffusion length scales are important. However, their inherent dimensional constrain also translates into high pumping power requirements. Inspired by nature, one possible method to reduce the large viscous pressure losses is to introduce textures in a microchannel. Depending on the interaction between the textured surface and the liquid, the microstructures can either be wetted or nonwetted. Less adhesion between solid and liquid in nonwetted state has made it popular in most of the friction reduction studies. However, in the nonwetted state, preventing liquid from penetrating into the grooves under pressurized conditions and the gas-liquid interface acting like a solid boundary open space to consider the wetted state for friction reduction as well. When dealing with the wetted state we should be aware that penetration of the flow inside the grooves can induce the pressure drag alongside the skin drag. Therefore, the wetted state will lead to a trade-off between skin and pressure drag. The aim of this work is to understand how different microtextures affect the total drag in a laminar microchannel flow. Textured microchannels with width-to-depth aspect ratios of 1, 10 and 50 and different width of the land region have been tested. In order to perform correct comparisons, the textured and baseline microchannels are designed to have the same volume. The results show that increasing the aspect ratio of the trenches introduces an extermum point in the hydraulic resistance of the microchannels. The optimum aspect ratio for the tested microchannels is 10, in which the trenches are not wide enough for streamlines to bend inside the trenches and increase the skin drag and they are not highly dense along the microchannel to reveal the negative effect of the pressure drag. On the whole, the hydraulic resistance of the textured channels is higher than the equivalent baseline for all the tested geometries.

A Transient Heat Exchanger Evaluation Test for Arbitrary Fluid Inlet Temperature Variation and Longitudinal Core Conduction

1986 ◽  
Vol 108 (2) ◽  
pp. 370-376 ◽  
Author(s):  
R. S. Mullisen ◽  
R. I. Loehrke
Keyword(s):  
Heat Exchanger ◽  
Finite Difference ◽  
Temperature Variation ◽  
Inlet Temperature ◽  
Fluid Temperature ◽  
Difference Model ◽  
Reduction Procedure ◽  
Finite Difference Model ◽  
Evaluation Test ◽  
Computer Based

An improved version of the transient technique is described which utilizes a finite-difference model of the heat exchanger for the evaluation of an average hem transfer coefficient. The model, which can accommodate arbitrary inlet fluid temperature variation as well as longitudinal conduction in the heat exchanger core, is well suited for a computer-based data reduction procedure. The finite-difference model is validated by comparison with the predictions of exact solutions for a step change in inlet temperature. Actual tests on a core of known performance indicate that the overall accuracy of the technique can be within ± 2 percent.

Simulation of the internal temperature in the Passol Laboratory, University of Debrecen

2012 ◽  
Vol 3 (1) ◽  
pp. 63-73 ◽  
Author(s):  
I. Csáky ◽  
F. Kalmár
Keyword(s):  
Temperature Variation ◽  
Air Temperature ◽  
Indoor Air ◽  
Storage Capacity ◽  
Building Structures ◽  
Internal Temperature ◽  
The One ◽  
Heavy Structure ◽  
East West ◽  
Made In

Abstract Nowadays the facades of newly built buildings have significant glazed surfaces. The solar gains in these buildings can produce discomfort caused by direct solar radiation on the one hand and by the higher indoor air temperature on the other hand. The amplitude of the indoor air temperature variation depends on the glazed area, orientation of the facade and heat storage capacity of the building. This paper presents the results of a simulation, which were made in the Passol Laboratory of University of Debrecen in order to define the internal temperature variation. The simulation proved that the highest amplitudes of the internal temperature are obtained for East orientation of the facade. The upper acceptable limit of the internal air temperature is exceeded for each analyzed orientation: North, South, East, West. Comparing different building structures, according to the obtained results, in case of the heavy structure more cooling hours are obtained, but the energy consumption for cooling is lower.

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