Transition in Microchannels

Moiini GL (2004) Laminar-to-turbulent transition in microchannels. Microscale Thermophys Eng... 

Morini, G. L., Laminar-to-turbulentflow transition in microchannels. Microscale Thermophysical Engineering, Vol. 8, pp. 15-30, (2004). 

Coleman, J. W., and Garimella, S. (2000) Two-Phase Flow Regime Transitions in MicroChannel Tubes The Effect of Hydraulic Diameter, American Society of Mechanical Engineers, Heat Transfer Division, Orlando, EL, Vol. HTD-366, American Society of Mechanical Engineers, pp. 71-83. 

Brackbill TP, Kandlikar SG (2007) Effect of sawtooth roughness on pressure drop and turbulent transition in microchannels. Heat Trans Eng 28(8-9) 662-669... 

Friction factor in microchannels Pressure drop in microchannels Transition in microchannels... 

The recent development of microscopic particle image velocimetry (microPIV) [7] has provided researchers with the capability of measuring instantaneous velocity fields in microchaimels and hence provided an additional tool for investigating transition in microchannels. Because microPIV can be used to directly observe features of the velocity field in the microchannel, data collected using this technique provide a more direct indication of transition than pressure drop measurements where the transitional behavior must be inferred. In the first study of transition using microPIV, Sharp and Adrian [8] measured velocities in round... 

Hetsroni et al. [6] also reexamined previous studies of friction factor in microchannels and drew the same conclusions that they did for transition in microchannels. They found that the anomalous results reported in some studies could be explained by the same factors that contributed to the observation of anomalous transitional behavior. Indeed, in the only study performed to date combining both microPIV and extensive pressure drop measurements. Sharp and Adrian [8] found that transition as measured by microPIV agreed well transition as inferred from friction factor data and also found that their measured friction factors agreed well with macroscale results. As with transition to turbulence, the experimental evidence on friction factors in turbulent microchannel flow shows that microscale flow exhibits the same behavior as macroscale flows. 

In Fig. 6 the experimental results obtained by Rands et al. [5] are compared with Eq. 27 it is evident that the comparison between Eq. 27 and the experimental results allows one to individuate the laminar-to-turbulent transition in microchannels without the need to use pressure gauges. 

Hassan, 1., Vaillancourt, M., and Pehlivan, K. (2005) Two-phase flow regime transitions in microchannels a comparative experimental study. Microscale Thermophys. Eng., 9 (2), 165-182. 

In an attempt to clarify the seemingly contradictory results on transition in microchannels presented by various researchers, Hetsroni et al. [6] reexamined the results of previous researchers, with a particular emphasis on determining possible sources of the anamoulous results found in some studies. They determined that the anamoulous results reported by some researchers were due to either improperly defined experiments, heating of the fluid due to viscous dissipation resulting in a change in fluid viscosity, or due to measurement error. After examining the extensive body of experimental studies on transition in microchannels, they concluded that the vast preponderance of evidence reveals that the transitional behavior of flow in microchannels shows no differences with macroscale flow. 

The friction factor can thus be determined without measuring the pressure drop along the microchannel but by means of temperature and flow rate measurements. This kind of measurement is unsuitable when macrochannels are tested. For this reason Eq. (25) can be considered as an example of the role of scaling effects and to suggest new measurement procedures at the microscale. This relation has been used by Celata et al. [4] in order to determine the friction factor in microchannels. In addition, since Eq. (25) is valid only in the laminar regime, one can use it to individuate the laminar-to-turbulent transition in microchannels. This has been experimentally demonstrated by Celata et al. [4] and Rands et al. [5]. 

Rands C, Webb BW, Maynes D (2006) Characterization of transition to turbulence in microchannels. Int J Heat Mass Transfer 49 2924-2930 Ren L, Qu W, Li D (2001) Interfacial electrokinetic effects on liquid flow in micro-channels. Int J Heat Mass Transfer 44 3125-3134... 

Galbiati L, Andreini P (1992) Elow patterns transition for vertical downward two-phase flow in capUlary tubes. Inlet mixing effects. Int Comm Heat Mass Transfer 19 791-799 Garimella S, Sobhan C (2003) Transport in microchannels - a critical review. Ann Rev Heat Transfer 13 1-50... 

Taitel Y, Bamea D, Dukler AE (1980) Modeling flow pattern transitions for steady upward gas-liquid flow in vertical tubes. AlChE J 26 345-354 Triplett KA, Ghiaasiaan SM, Adbel-Khalik SI, Sadowski DL (1999a) Gas-liquid two-phase flow in microchannels. Part 1 two-phase flow patterns. Int J Multiphase Flow 25 377-394 Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, LeMouel A, McCord BN (1999b) Gas-liquid two-phase flow in microchannels. Part 11 void fraction and pressure drop. Int J Multiphase Flow 25 395 10... 

Flow is typically laminar in microchannel devices, although not always rigorously so. Correlations for fully developed laminar flow in perfectly rectangular microchannels have been validated in the literature [33-35]. Transition and turbulent flows in a microchannel have no such consistent treatise, and are highly dependent upon channel shape, aspect ratio, and surface characteristics [36, 37]. 

In the laminar and transition regimes in microchannels, the behavior of convective heat transfer coefficient is very different compared with the conventionally-sized situation. In the laminar regime, Nu decreases with increasing Re, which has not been explained. 

The Nu in the laminar and transition regimes in microchannels is correlated with Br, in addition to Re, Pr, and a geometric parameter of the microcharmels. The role of Br in the laminar regime is supported by an analysis of the experimental data. 

Experiments were conducted to measure flow and heat transfer characteristics of gaseous flows in microchannels in [12]. Their experimental result of the Poiseuille number is 118 for laminar flow, which is higher than the expected value. They also reported that the flow transition from laminar to turbulent occurs at Reynolds numbers around 400 to 900, which is lower than the conventional value of... 

Convective heat transfer analysis for a gaseous flow in microchannels was performed in [24]. A Knudsen range of 0.06-1.1 was considered. In this range, flow is called transition flow. Since the eontinuum assumption is not valid, DSMC technique was applied. Reference [24] considered the uniform heat flux boundary condition for two-dimensional flow, where the channel height varied between 0.03125 and 1 micrometer. It was concluded that the slip flow approximation is valid for Knudsen numbers less than 0.1. The results showed a reduction in Nusselt number with increasing rarefaetion in both slip and transition regimes. 

Development of additional models for pressure drop in noncircular channels, and for heat transfer coefficients and transition criteria based on nondimensional parameters is underway. This integrated approach using flow visualization, pressure drop and heat transfer measurements, and analytical modeling, is yielding a comprehensive understanding of condensation in microchannels. 

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