Parallel micro

Efforts have been made, however, to extend the range or extent of samples that can be analysed by using a two-dimensional separation when used in heart-cut mode. This has been reported to include the use of numerous parallel micro-traps to essentially store the primary column eluent fractions ready for second-column separation, and the use of parallel second-dimension columns. 

The concepts of boiling in micro-channels and comparison to conventional size channels are considered in Chap. 6. The mechanism of the onset of nucleate boiling is treated. Specific problems such as explosive boiling in parallel micro-channels, drag reduction and heat transfer in surfactant solutions are also considered. 

The behavior of the flow pattern in a parallel micro-channel is different from that in a single micro-channel. It was shown by Hetsroni et al. (2003b) that at the same value of heat flux, different flow regimes may be observed in different micro-channels, depending on the time interval. Moreover, at the same time interval different flow regimes may exist in each of the component micro-channels. In Fig. 2.33 two-phase steam-water flow in the central part of such a parallel system db = 100 pm is shown as the top view observed through a transparent cover. The... 

Fig. 2.36 Flow regime map for parallel micro-channels. Region A is the low-heat flux region, and region B the high-heat flux region. Reprinted from Hetsroni et al. (2003b) with permission...

Figure 2.40 shows the unsteady flow upstream of the ONE in one of the parallel micro-channels of d = 130 pm at = 228kW/m, m = 0.044 g/s (Hetsroni et al. 2001b). In this part of the micro-channel single-phase water flow was mainly observed. Clusters of water appeared as a jet, penetrating the bulk of the water (Fig. 2.40a). The vapor jet moved in the upstream direction, and the space that it occupied increased (Fig. 2.40b). In Fig. 2.40a,b the flow moved from bottom to top. These pictures were obtained at the same part of the micro-channel but not simultaneously. The time interval between events shown in Fig. 2.40a and Fig. 2.40b is 0.055 s. As a result, the vapor accumulated in the inlet plenum and led to increased inlet temperature and to increased temperature and pressure fluctuations.

Several studies (Klein et al. 2005 Mishan et al. 2007) showed that manifold design plays an important role in the liquid distribution among parallel micro-channels, which can lead to spanwise temperature gradients on the device surface, increase the thermal stresses and reduce reliability. To study the effect of entrance conditions... 

A variety of studies can be found in the literature for the solution of the convection heat transfer problem in micro-channels. Some of the analytical methods are very powerful, computationally very fast, and provide highly accurate results. Usually, their application is shown only for those channels and thermal boundary conditions for which solutions already exist, such as circular tube and parallel plates for constant heat flux or constant temperature thermal boundary conditions. The majority of experimental investigations are carried out under other thermal boundary conditions (e.g., experiments in rectangular and trapezoidal channels were conducted with heating only the bottom and/or the top of the channel). These experiments should be compared to solutions obtained for a given channel geometry at the same thermal boundary conditions. Results obtained in devices that are built up from a number of parallel micro-channels should account for heat flux and temperature distribution not only due to heat conduction in the streamwise direction but also conduction across the experimental set-up, and new computational models should be elaborated to compare the measurements with theory. 

Depending on the particular design of inlet and outlet manifolds, the difference between the flow rates into some parallel micro-channels may be up to 20%. Idealizing the flow rate as uniform can result in significant error in prediction of the temperature distribution of a heated electronic device. 

Hetsroni G, Mosyak A, Segal Z (2001) Nonuniform temperature distribution in electronic devices cooled by flow in parallel micro-channels. IEEE Trans Comp Packag Technol 24(1) 16-23 Ho CM, Tai Y-C (1998) Micro-electronic mechanic systems (MEMS) and fluid flows. Ann Rev Fluid Mech 30 5-33... 

Hetsroni et al. (2003a) investigated two-phase air-water flow in 21 triangular parallel micro-channels of dh = 130 pm. The experimental test facility and flow loop, for liquid flowing through micro-channels is shown in Fig. 5.13. 

Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in different microchannels. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. 

Fig. 6.3 Dependence of wall superheat on heat flux. Experiments performed by Qu and Mudawar (2002) in rectangular parallel micro-channels 231 pm wide and 713 pm deep...

Onset of Nucleate Boiling in Parallel Micro-Channels... 

Onset of Nucleate Boiling in Parallel Micro-Channels 6.2.1 Physical Model of the Explosive Boiling... 

The bubble dynamics in a confined space, in particular in micro-channels, is quite different from that in infinity still fluid. In micro-channels the bubble evolution depends on a number of different factors such as existence of solid walls restricting bubble expansion in the transversal direction, a large gradient of the velocity and temperature field, etc. Some of these problems were discussed by Kandlikar (2002), Dhir (1998), and Peng et al. (1997). A detailed experimental study of bubble dynamics in a single and two parallel micro-channels was performed by Lee et al. (2004) and Li et al. (2004). 

The bubble dynamics under conditions corresponding to flow in two parallel trapezoidal micro-channels with hydraulic diameter 47.7 pm was studied by Li et al. (2004). The bubbles in two parallel micro-channels generally grow similarly to that in a single micro-channel. The authors reported on the presence of two-phase flow... 

Explosive Boiling of Water in Parallel Micro-Channels 309... 

In the study by Hetsroni et al. (2006b) the test module was made from a squareshaped silicon substrate 15 x 15 mm, 530 pm thick, and utilized a Pyrex cover, 500 pm thick, which served as both an insulator and a transparent cover through which flow in the micro-channels could be observed. The Pyrex cover was anod-ically bonded to the silicon chip, in order to seal the channels. In the silicon substrate parallel micro-channels were etched, the cross-section of each channel was an isosceles triangle. The main parameters that affect the explosive boiling oscillations (EBO) in an individual channel of the heat sink such as hydraulic diameter, mass flux, and heat flux were studied. During EBO the pressure drop oscillations were always accompanied by wall temperature oscillations. The period of these oscillations was very short and the oscillation amplitude increased with an increase in heat input. This type of oscillation was found to occur at low vapor quality. 

The experimental investigations of boiling instability in parallel micro-channels have been carried out by simultaneous measurements of temporal variations of pressure drop, fluid and heater temperatures. The channel-to-channel interactions may affect pressure drop between the inlet and the outlet manifold as well as associated temperature of the fluid in the outlet manifold and heater temperature. Figure 6.37 illustrates this phenomenon for pressure drop in the heat sink that contains 13 micro-channels of d = 220 pm at mass flux G = 93.3kg/m s and heat flux q = 200kW/m. The temporal behavior of the pressure drop in the whole boiling system is shown in Fig. 6.37a. The considerable oscillations were caused by the flow pattern alternation, that is, by the liquid/two-phase alternating flow in the micro-channels. The pressure drop FFT is presented in Fig. 6.37b. Under... 

The pressure drop fluctuation provides insight into the temperature behavior of the fluid in the outlet manifold. The pressure drop fluctuation frequency is representative of the oscillations in the system. Figure 6.38a,b shows time variation and FFT of the fluctuation component of the fluid temperature. From Fig. 6.38a one can see that the average fluid temperature at the outlet manifold is less than the saturation temperature. This results in the fact that only single liquid comes to the outlet manifold through some of the parallel micro-channels. 

When D < 1 (Tin C Ts) incipient boiling heat flux increases with increasing mass velocity. When D incipient boiling heat flux weakly depends on mass velocity. For micro-channels boiling incipient heat flux may weakly depend on inlet temperature. This case corresponds to flow boiling in parallel micro-channels, in which vapor penetrates the inlet manifold. 

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