Transition to Turbulence in Microchannels

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... 

Rands C, Webb BW, Maynes D (2006) Characterization of transition to turbulence in microchannels. Int J Heat Mass Transf 49 2924-2930... 

Although a number of researchers in the 1990s have suggested that the transition to turbulence occurs at lower Reynolds numbers, some of the recent experiments conducted under careful conditions [5-7] have shown that for smooth microchannels, the transition Reynolds number remains the same as for the macroscale channels. 

In turbulent flows. Re is important because it can be used to determine when the flow transitions from laminar to turbulent flow and also when the flow achieves a fully turbulent state. For macroscale pipe and channel flows, transition to turbulence can occur in the range 1,800 < Re < 2,300, depending of flow conditions, and the flow becomes fully turbulent at a Reynolds number of approximately 4,000. As will be shown later in this chapter, whether flows in microchannels agree with this macroscale behavior has long been a topic of debate, as some researchers have reported transitional Reynolds numbers in microchannel flows far lower than the 1,800-2,300 reported for macroscale flows. However, recent studies using newly developed experimental techniques and also careful reexamination of the results of some of the earlier studies strongly suggest that microchannel flows... 

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. 

The transition from laminar to turbulent flow in macroscale channels occurs at the critical Reynolds number of about Re=2300. For flow in microchannels, an earlier transition to turbulent flow was indicated in earUer publications. However, recent studies show that in fluid flow at the microscale the critical Reynolds number is in the... 

Mixing in microchannels in another major application where the transition to turbulent flows can be utilized for enhanced mixing possible in turbulent flows. The roughness elements then act to destabilize the flow and induce local instabilities. The localized transition to turbulence will enhance the mixing, and relaminarization process downstream will provide the laminar flow with its lower friction factor and associated lower pressure drop penalties. 

In early studies of flow in microchannels, the onset of transition to turbulence was inferred from measurements of bulk properties of the flow, such as pressure drop or heat transfer rate. The behavior of the friction factor or Nusselt number undergoes a drastic change when the flow transitions to turbulence, and thus an observation of such a behavioral change can be indicative of the onset of turbulent flow. 

Many of these early studies of flow in microchannels suggested transition to turbulence occurring at lower Reynolds numbers than those observed at the macroscale. For example, Wu and Little [2] measured friction factors and Nus-... 

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

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... 

The laminar regime in a channel holds until the Reynolds number reaches a critical value above which the laminar motion becomes unstable and a transition to a turbulent flow will generally occur. It has been demonstrated experimentally that for a microchannel the critical value of the Reynolds number depends on the entrance conditions, on the cross-sectional geometry, and on the wall roughness. The effect of the roughness on the laminar-to-turbulent transition is very important, and it can be evidenced by observing Fig. 8 in which the experimental values of the Poiseuille number (f Re) are shown as a function of the Reynolds number for two microtubes made in stainless steel and in fused silica. The stainless steel microtube has an internal... 

Transition from laminar to turbulent flow is identified by a departure from the laminar flow velocity profile and the presence of time-varying velocity component in the flow, especially near the wall. The wall shear stress becomes higher following the departure from the laminar flow. The transition occurs over a range of Reynolds number and is dependent on the local wall and flow conditions. Microchannels are defined as channels with the minimum channel dimension in the range from 1 to 200 pm [1]. 

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 tnicrochannels. This has been experimentally demonstrated by Celata et al. [4] and Rands et al. [5]. 

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. 

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