Channel flow boundary-layer limitations

The flow in the diffuser is usually assumed to be of a steady nature to obtain the overall geometric configuration of the diffuser. In a channel-type diffuser the viscous shearing forces create a boundary layer with reduced kinetic energy. If the kinetic energy is reduced below a certain limit, the flow in this layer becomes stagnant and then reverses. This flow reversal causes... 

The flow conditions are chosen to represent a range of gas-turbine-combustor conditions, covering a range of physical parameters that include inlet velocities from 0.5 to 5 m/s and pressures from 1 to 10 bar. These conditions can be characterized in terms of a Reynolds number based on channel diameter and inlet flow conditions, which is varied over the range 20 < Rej = V nd/v < 2000. The upper limit of Rej = 2000 is chosen to ensure laminar flow, hence removing the need to model turbulence. It should be noted that the validity of the boundary-layer approximations improve as the Reynolds number increases. 

Two limiting types of flow can exist. If the channel is short and the Rayleigh number is high (see later), the flow will essentially consist of boundary layers on each wall with a uniform flow at temperature T between the boundary layers as shown in Fig. 8.18. Under these circumstances it is to be expected that a boundary-type relation for the heat transfer rate will apply. 

In the limiting boundary layer situation here being considered, the total flow through the channel will be small, i.e., effectively ... 

Ideally the mixing layer should start with zero thickness and grow linearly with increasing x. However, the presence of boundary layers on the splitter plate leads to a displacement of the virtual origin of the mixing layer by a distance x (see fig.1). Moreover, the initial disturbances take some time to die away so that undisturbed development of the mixing layer first starts at X =100 mm. Because of the limited cross-sectional area of the channel this development only proceeds up to about 300 mm, after which point the walls strongly disturb the flow. 

The approximations given by Equations 8.35 are the solution to Leveque s problem given in Equation 8.30 with a linear wall reaction. Since the formulation of the problem leads to a linearized velocity profile in a planar boundary layer, laminar flows (parabolic velocity profiles) in curved channels are more susceptible to present higher deviations from these results. For a fully developed flow in a round tube, the error associated with Equation 8.35b is 1.4 and 0.13% for aPe ,lz equal to 100 and 1000, respectively. Lopes et al. [40] observed that these differences are visible mainly for Da — 00 and calculated corrections to account for these effects. It was shown that in the mass transfer-controlled limit. 

Optical Imaging Lens resolution-can be sub-micron Limited to camera resolution, can be >1000 fps Lens field-of-view Yes—with a modified cell to allow optical access Accurate tool for flow for channel-level flow only. Catalyst layer images require alteration of the diffusion media and heat transfer boundary conditions. 

---