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Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
Microchannel cooling technology exhibits several advantages that provide highly efficient cooling in both earth- and space-based systems, such as higher heat transfer coefficient at the lower wall superheat temperature, eliminating contact resistance associated heat exchangers through directly fabricated-onchip substrates, and potential application in space confined area (three-dimensional stacks of chips). However, the fluid flow and heat transfer in microchannel heat exchangers is not well understood, especially in two-phase flow regimes where flow maldistributions may be present. For very small channels, on the order of a micron or less, the heat transfer rates may not be predicted using the correlations developed for macrochannels. The design of microchannel cooling devices is still state-of-the-art, strongly depending on fabrication, channel geometry, channel surface, materials, coolant, pump, packaging, and practical system integration.
CMOS Technology for Wireless Applications
Published in Krzysztof Iniewski, Wireless Technologies, 2017
Since the designer can control gate geometry through layout, the key concern for maintaining low 1/f performance across generation is preventing trap density from increasing as the result of process-related factors such as strain and changes in oxide composition (e.g., increase in nitrogen content). To explore the trend in trap density across four technology generations of NFET from 0.25 μm to 90 nm, we plot SVg versus frequency in Figure 18.17. Gate geometry is held constant at 0.25 × 10 μm2 while COX scaling is normalized by measuring each node at a different gate voltage so as to maintain a constant channel surface field and thus a fixed channel charge. Plotted in this manner, differences in SVg should reflect trap density directly. We observe that each generation approximately overlays the next, suggesting that gate oxide quality is currently being held constant through the 90 nm generation despite changes in oxide formation processes. However, the spectrum plotted for the minimum gate length at the 90 nm node shows noise power scaling inversely with gate area.
Estimation of Surface Runoff from Storm Water
Published in G.L. Sivakumar Babu, Prithvi S. Kandhal, Nivedya Mandankara Kottayi, Rajib Basu Mallick, Amirthalingam Veeraragavan, Pavement Drainage, 2019
G.L. Sivakumar Babu, Prithvi S. Kandhal, Nivedya Mandankara Kottayi, Rajib Basu Mallick, Amirthalingam Veeraragavan
On the hydrograph, channel runoff appears at the start of the storm, remains throughout the storm and varies with rainfall intensity. The surface runoff appears on hydrograph once the demands of the interception, infiltration and surface storages are met. It varies during the storm and ends during or soon after the storm. The subsurface flow contributes to the hydrograph during or soon after the storm. The channel, surface and subsurface flow are combined into direct runoff, which is estimated in the NRCS method by the Curve Number (CN) method. The CN is based on soil permeability, surface cover, hydrologic condition and antecedent moisture. The dimensionless unit hydrograph is based on the drainage area time of concentration.
Local Measurements in Heat Exchangers: A Systematic Review and Regression Analysis
Published in Heat Transfer Engineering, 2022
Bengt Sunden, Josua Meyer, Jaco Dirker, Bibin John, Yagnavalkya Mukkamala
Given the extremely small geometry (dh < 250 µm), phase-change measurements in microchannels are particularly challenging. Channel wall and fluid bulk temperature measurements, estimating the local vapor quality and heat flux rate require precision microsensors and well-defined channel surface for computation. Boudouh et al. [8] deployed fourteen K-type microthermocouples (75 µm wires) for measuring the channel wall temperatures. The axially located microthermocouples were calibrated against a precision sensor probe to within ±0.03 °C accuracy. A finite-difference-based numerical algorithm was used to solve the inverse heat conduction problem to estimate the local heat flux and wall temperatures from the measured axial wall temperatures, while the fluid bulk temperature was assumed to be saturation temperature for flow boiling. Del Col et al. [29, 30] measured the surface roughness of the inner channel surface with a digital surface roughness machine. The mean surface roughness that averaged between 0.98–1.61 µm had a significant impact on the local flow boiling HTC. The variable heat flux along the channel wall was computed from the secondary heating fluid temperature profile. The channel inner diameter was used for estimating the heat transfer area, while a polynomial function was used to approximate the secondary heating fluid temperature gradient. The correction in the calculated local boiling HTC due to axial channel wall heat conduction was estimated to be 5%.
On friction factor of fluid channels fabricated using selective laser melting
Published in Virtual and Physical Prototyping, 2020
Yi Zhu, Lei Zhou, Shuai Wang, Chao Zhang, Cong Zhao, Lei Zhang, Huayong Yang
Schmelzle et al. (2015) proposed non-circular channels (e.g. diamond, teardrop shapes) in an SLM fabricated hydraulic manifold, which significantly reduces the overhang regions. However, compared to a circular channel, non-circular channels inevitably have stress concentration. As a result, wall thickness needs to be increased to compensate, which adds extra weights. On the other hand, fluid flow in a channel also depends on the channel surface roughness. Friction loss is the pressure loss due to the effect of the fluid’s viscosity when viscous fluid flows through a pipe. It is related to the flow rate, viscosity, properties of the fluid and the pipe, etc. Similar to the coefficient of friction, friction factor describes the value of the friction loss under a given condition. Based on the classical theory (Darcy 1857; Fanning 1877; Moody 1944), the friction factor is proportional to the Reynolds number in the laminar flow. While in the turbulent flow, the relation between the friction factor and Reynolds number becomes complex. The pipe roughness starts to affect when a large Reynolds number appears (further detailed in the discussion part). A horizontal fluid channel (without supports) has poor dimensional accuracy and extremely high surface roughness (particularly on the top). How do those characteristics affect friction loss remains unknown, which is crucial in designing complex fluid channels.