Channels single-phase flow

In Chap. 3 the problems of single-phase flow are considered. Detailed data on flows of incompressible fluid and gas in smooth and rough micro-channels are presented. The chapter focuses on the transition from laminar to turbulent flow, and the thermal effects that cause oscillatory regimes. 

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.

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... 

Calame JP, Myers RE, Binari SC, Wood FN, Garven M (2007) Experimental investigation of micro-channel coolers for the high heat flux thermal management of GaN-on-SiC semiconductor devices. Int J Heat Mass Transfer 50 4767-4779 Celata GP, Cumo M, Zummo G (2004) Thermal-hydraulic characteristics of single- phase flow in capillary pipes. Exp Thermal Fluid Sci 28 87-95 Celata GP (2004). Heat transfer and fluid flow in micro-channels. Begell House, N.Y. 

The problems of micro-hydrodynamics were considered in different contexts (1) drag in micro-channels with a hydraulic diameter from 10 m to 10 m at laminar, transient and turbulent single-phase flows, (2) heat transfer in liquid and gas flows in small channels, and (3) two-phase flow in adiabatic and heated microchannels. The smdies performed in these directions encompass a vast class of problems related to flow of incompressible and compressible fluids in regular and irregular micro-channels under adiabatic conditions, heat transfer, as well as phase change. 

We attempt here to reveal the acmal reasons of disparity between the theoretical predictions and measurements obtained for single-phase flow in micro-channels. For this purpose, we consider the effect of different factors (roughness, energy dissipation, etc.) on flow characteristics. Some of these factors were also discussed by Sharp et al. (2001), and Sharp and Adrian (2004). 

The experimental results of single-phase flow in smooth micro-channels are summarized in Table 3.3. 

In experiments related to flow and heat transfer in micro-channels, some parameters, such as the flow rate and channel dimensions are difficult to measure accurately because they are very small. For a single-phase flow in micro-channels the uncertainty of ARe is (Guo and Li 2002,2003)... 

Bo = q/Gh] Q, where t is the period between successive events, U is the mean velocity of single-phase flow in the micro-channel, Jh is the hydraulic diameter of the channel, q is heat flux, m is mass flux, /zlg is the latent heat of vaporization). The dependence t on Bo can be approximated, with a standard deviation of 16%, by... 

Figure 6.35 shows dependence of the dimensionless initial liquid thickness of water and ethanol 5, on the boiling number Bo, where 5 = 5U/v, f/ is the mean velocity of single-phase flow in the micro-channel, and v is the kinematic viscosity of the... 

The investigations of fluid flow in micro-channels may be divided in two groups (1) single-phase flow, and (2) evaporative two-phase flow. The first was intensively investigated beginning from the pioneer work by Tukermann and Pease (1981). Two-phase flow is much less understood. 

It should also be noted that in single-phase flow heat transfer, the effect of channel size is expressed by equivalent diameter. This concept, however, should be... 

Agostini B, Watel B, Bontemps A, Thonon B. Experimental study of single-phase flow friction factor and heat transfer coefficient in mini-channels. CHE Symposium, Grenoble Edizioni ETS, August 2002, 85-89. 

These definitions rely upon the molecular mean free path in a single-phase flow, surface tension effects and flow patterns in two-phase flow applications. In recent studies in minichannels the hydraulic diameter ranges from 100 /jm to 2-3 mm. The channel cross sections were either circular or rectangular and much of the research concerned boihng. Commonly, classical correlations have been used with or without modifications to predict flow boihng results in minichannels. However agreement was poor and the need for new correlations was evident. 

Figure 6 (a) Nusselt number and (b) pressure drop for single-phase flow in narrow channels. 

It was assumed here that at low gas content the rate of energy dissipation in two-phase flow closes to it value for single-phase flow. If the liquid flow rate will be less then critical value then the walls of the channel will effect on heat transfer rate. 

Current experimentation on micro-channel two-phase flows has provided some evidence of the heat transfer mechanisms that govern the micro-scale flow boiling process (i) at low vapor qualities, when bubbly flow is the dominant flow pattern, thermal transport is primarily associated to nucleate boiling, (ii) at intermediate vapor qualities, with the intermittent passage of elongated bubbles and slugs of liquid, heat is transferred by single phase... 

The boundary conditions are zero velocity at the walls and zero slope at any planes of symmetry. Analytical solutions for the velocity profile in square and rectangular ducts are available but cumbersome, and a numerical solution is usually preferred. This is the reason for the transient term in Equation 16.7. A flat velocity profile is usually assumed as the initial condition. As in Chapter 8, is assumed to vary slowly, if at all, in the axial direction. For single-phase flows, u can vary in the axial direction due to changes in mass density and possibly to changes in cross-sectional area. The continuity equation is just AcUp = constant because the cross-channel velocity components are ignored. 

Developed sub-model A 2D single-phase flow model - down the channel and across the EC sandwich. 

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