Micro channel walls

Evaporative two-phase flow in a heated micro-channel resembles a two-phase slug flow with distinct domains of liquid and vapor. These domains are divided by the infinitely thin evaporating front, which propagates relatively to the fluid with a velocity u f equal (numerically) to the linear rate of liquid evaporation. In the frame of reference associated with micro-channel walls, the velocity of the evaporation front is... 

The development of the two-phase flow in a heated capillary at different Peclet number is illustrated in Fig. 9.13. It shows that different mechanisms of two-phase flow formation may occur depending on the value of Peu. At small Pcl the fine bubble formation (on the micro-channel wall) plays a dominant role. Growth of these bubbles leads to a blockage of the micro-channel, to a sharp change of the hydraulic... 

Velocity of the evaporating front in the system of coordinates associated with the micro-channel walls... 

Fig. 10.11 Stable liquid velocity and meniscus location vs. heat flux on the micro-channel wall (L = 2 X10 m,d= m, g = 9.8 m/s ). Reprinted from Yarin et al. (2002) with permission...

After activation by heating, the catalyst was dusted over the surface of a thin polydimethylsiloxane (PDMS) layer, being coated on the PDMS top plate of the micro reactor [19]. Such a modified plate was baked for 1 h at 100 °C. A high surface area and firm immobilization of the catalyst resulted. Then, the micro reactor was assembled from the top and another bottom plate, having at one micro-channel wall the catalyst layer. Stable operation with the PDMS micro reactor up to 175 °C could be confirmed. 

Peptide analysis is an important field of application for which several approaches exist, all based on the same functional principle of enzymatic cleavage as initial step followed by classical fragment analysis. Lab-on-a-chip approaches aim at real-time high-throughput analysis of peptides with minimal possible amounts of sample substance. The peptides are handled in aqueous solution as they are funneled to the enzyme immobilized onto the micro-channel wall. The fission products are then detected by classical methods [25-28] such as GC, LC-MS, MS/MS, MALDI-TOF, ESI-TOF, CE, UV-Vis, and fluorescence spectrometry. 

Chapter 4 is devoted to single-phase heat transfer. Data on heat transfer in circular micro-tubes and in rectangular, trapezoidal and triangular ducts are presented. Attention is drawn to the effect of energy dissipation, axial conduction and wall roughness on the thermal characteristics of flow. Specific problems connected with electro-osmotic heat transfer in micro-channels, three-dimensional heat transfer in micro-channel heat sinks and optimization of micro-heat exchangers are also discussed. 

For flow at a given rate, the only way to significantly increase the heat transfer coefficient is to reduce the channel size, whose optimum can be calculated assuming a practical limit on the available pressure. Recourse to multiple channels, instead of continuous coolant flow over the entire back substrate surface, enables one to multiply the substrate area by a factor (jp, representing the total surface area of the channel walls which are in contact with fluid. Single-row micro-channels etched dir-... 

The IR technique also yielded temperature distributions (Fig. 2.17) in the symmetry plane at Re = 30 and g = 19 x lO W/m. The wall temperature decreases by axial conduction through the solid walls in the last part of the micro-channel (x/L > 0.75) since this part is not heated. Neither the wall nor the fluid bulk temperature distribution can be approximated as linear. 

However, for flow in micro-channels, the wall thickness can be of the same order of channel diameter and will affect the heat transfer significantly. For example, Choi et al. (1991) reported that the average Nusselt numbers in micro-channels were much lower than for standard channels and increased with the Reynolds number. 

After venting of the elongated bubble, the region of liquid droplets begins. The vapor phase occupies most of the channel core. The distinctive feature of this region is the periodic dryout and wetting phenomenon. The duration of the two-phase period, i.e., the presence of a vapor phase and micro-droplet clusters on the heated wall, affects the wall temperature and heat transfer in micro-channels. As the heat flux increases, while other experimental conditions remain unchanged, the duration of the two-phase period decreases, and CHF is closer. 

One drawback of a micro-channel heat sink is a relatively high temperature rise along the micro-channel compared to that for the traditional heat sink designs. In the direction of the flow, the wall temperature rises in a single-phase flow even when the wall heat flux is uniform. In a micro-channel heat sink, the large amount... 

A method that creates patterned micro-structures distributed on the bottom wall of the micro-channel was proposed by Yang et al. (2006). A roughened bottom wall was created using the crystal orientation characteristics of the wafers. 

Fig. 2.76 SEM micrograph of roughened micro-channel bottom wall with distributed hexagonal micro-structures. Reprinted from Yang et al. (2006) with permission...

Yang H, Lee F, Chein R (2006) Micro-channel heat sink fabrication with roughened bottom walls. Microsyst Technol 12 760-765... 

Several investigators obtained friction factors in micro-channels with rough walls that were greater than those in smooth wall channels. These observations should be considered taking into account the entrance effects, losses from change in channel size, etc. 

The paper by Davies et al. (2006) reports results of a numerical investigation of the laminar, periodically repeating flow in a parallel-plate micro-channel with superhydrophobic walls. In particular, the influence of the Reynolds number and the vapor cavity size on the overall flow dynamics was explored. A schematic of the near-wall and cavity regions is shown in Fig. 3.18. 

Celata GP, Cumo M, McPhail S, Zummo G (2006) Characterization of fluid dynamics behavior and channel wall effects in micro-tube. Int J Heat Fluid Flow 27 135-143... 

Cross- Micro-channel size Working Walls Re... 

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