Channel Inlet Conditions

2 Channel Inlet Conditions A known gas stream inlet temperature is assigned as 

It can be seen from this plot that the heat generated within the tri-layer fuel cell dissipates by conduction through the tri-layers of electrodes and 

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The influence of inlet conditions on stability of flow boiling in micro-channels was analyzed by Brutin and Tadrist (2004). The set-up with rectangular micro-channel 500 X 4,000 jm was used to study flow boiling at two kinds of upstream conditions, which corresponded to constant liquid velocity at channel entrance (confinement condition) and constant velocity at the syringe outlet. The flow characteristics corresponding to steady and unsteady regimes were studied and the Reynolds number that subdivided these states was found. 

Brutin B, Tadrist L (2004) Pressure drop and heat transfer analysis of flow boUing in micro-channel influence of the inlet condition on two-phase flow stability. Int J Heat Mass Transfer 47 2367-2377... 

The boundary conditions to be satisfied are that the lateral pressure difference between subchannels should be zero at the channel inlet and exit. Having passed once along the channel, this implies that iteration over the channel length may be necessary by using improved guesses of flow division between subchannels at the inlet. In practice, only one pass may be necessary, particularly for hydraulic model, in which lateral momentum transfer is neglected or only notionally included. Rowe (1969) has shown that for a single-pass solution to be stable and acceptable,... 

Transfer coefficients in catalytic monolith for automotive applications typically exhibit a maximum at the channel inlet and then decrease relatively fast (within the length of several millimeters) to the limit values for fully developed concentration and temperature profiles in laminar flow. Proper heat and mass transfer coefficients are important for correct prediction of cold-start behavior and catalyst light-off. The basic issue is to obtain accurate asymptotic Nu and Sh numbers for particular shape of the channel and washcoat layer (Hayes et al., 2004 Ramanathan et al., 2003). Even if different correlations provide different kc and profiles at the inlet region of the monolith, these differences usually have minor influence on the computed outlet values of concentrations and temperature under typical operating conditions. 

A single channel description is no longer applicable and the entire DPF needs to be simulated in case of no radially uniform inlet conditions to the trap. 

The boundary-layer equations represent a coupled, nonlinear system of parabolic partial-differential equations. Boundary conditions are required at the channel inlet and at the extremeties of the y domain. (The inlet boundary conditions mathematically play the role of initial conditions, since in these parabolic equations x plays the role of the time-like independent variable.) At the inlet, profiles of the dependent variables (w(y), T(y), and Tt(y)) must be specified. The v(y) profile must also be specified, but as discussed in Section 7.6.1, v(y) cannot be specified independently. When heterogeneous chemistry occurs on a wall the initial species profile Yk (y) must be specified such that the gas-phase composition at the wall is consistent with the surface composition and temperature and the heterogeneous reaction mechanism. The inlet pressure must also be specified. 

At the channel inlet, the radial-velocity v profile cannot be specified independently of the axial-velocity and temperature profiles. Explain why this is so. Explain the implications concerning consistent initial conditions from the differential-algebraic-equation perspective. Develop an algorithm to determine the consistent initial v profile, given the u profile. 

Fig. 17.19 Comparison of the CH4 and CO2 mass-fraction contour maps for the Navier-Stokes (lower panels) and Boundary-Layer (upper panels) models. The upper channels show Re,/ = 200 cases and the lower channels show a Re,/ = 2000 case. The Reynolds numbers (Re,/ = Vmd/v) are evaluated using the fluid properties at the inlet conditions. For visual clarity, the radial coordinates have been multiplied by a factor of 20. In fact, the channel is much narrower than it appears.

The mole fractions and the total molar flow rate at the anode channel inlet, %i in and Ti , serve as initial conditions ... 

To calculate the required initial conditions for Eq. (20) and the molar flow into the anode channel, namely j, and f, some mixing rules are required. They result from the total and the component balances at the mixing point at the channel inlet ... 

A high flow rate in the gas channels and a low fuel utilization are assumed. In this case, the temperature and the composition in the gas channels are approximately constant and identical to the inlet conditions. Therefore, the mass and energy balances (1, 2, 3, 4) are no longer needed, and the reaction kinetics (7, 8) can be simplified to zero-order kinetics. 

In this expression, XMet is the inlet vapor quality, which can be negative when considering a subcooled inlet condition, i.e. the inlet subcooling enthalpy relative to the latent heat. As CHF normally occurs at the outlet, the heated length is taken as the channel length. The parameter Yshah is ... 

It is necessary to remark that a large number of comparisons between experimental and theoretical results for microchannels have revealed a deep mismatch in the thermal boundary and inlet conditions that can preclude the use of the conventional correlations. In addition, in experiments on flow and heat transfer in microchannels, some parameters, like the channel dimensions, the average roughness, the local convective heat transfer, the local value of the static pressure along a microchannel, and so on, are difficult to measure accurately. For this reason, a large number of inconsistencies in published data are very likely to be due to experimental inaccuracies. 

A uniform velocity profile (equal to the mean velocity W) at the entrance with negligible transversal components (v = W v = Vy =0). In this case, the vorticity defined as w = (dvjdy — dvyldz dvjdz — dvjdx dvyidx — dvjdy) is not zero at the entrance. This situation can be obtained experimentally only by using a screen at the channel inlet. This is the entrance boundary condition that is most frequently used for hydrodynamic entrance problems. 

When using CFX-10, the solid and fluid are treated as a unitary computational domain. The interface between the solid and fluid is automatically connected by equal temperature magnitudes and heat flux values thus, only the boundaries for the unit cell are needed. Specifically, for the hydraulic boundary conditions, a uniform velocity is applied at the channel inlet ... 

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