Boiling, flow local

Liquid distribution. Liquid needs to be uniformly distributed to the shell, particularly when boiling a multicomponent mixture. Uneven distribution may locally deplete the lighter component and result in localized pinching and loss of heat transfer. Uneven distribution can also promote uneven heating, resulting in further loss of heat transfer. In extreme cases, maldistribution can lead to stratified flow, local mist flow, excessive thermal stresses, and accelerated corrosion (430). 

The code determines the onset of nucleate boiling and critical heat flux corresponding to flow instability in each individual channel and burnout of fuel plate. Heat flux for the onset of nucleate boiling and critical heat flux are calculated using actual power distribution, coolant velocity, local pressure and saturation temperature at each individtial code. Margins to nucleate boiling, flow instability and burnout are also calculated. 

In physical objects involving thermal and fluid flow systems, the dual problem of how the heater is represented, and boiling as a local-instantaneous effect, should be considered. The temperature variations on the chip surface are a key characteristic of... 

On the other hand Bao et al. (2000) reported that the measured heat transfer coefficients for the air-water system are always higher than would be expected for the corresponding single-phase liquid flow, so that the addition of air can be considered to have an enhancing effect. This paper reports an experimental study of non-boiling air-water flows in a narrow horizontal tube (diameter 1.95 mm). Results are presented for pressure drop characteristics and for local heat transfer coefficients over a wide range of gas superficial velocity (0.1-50m/s), liquid superficial velocity (0.08-0.5 m/s) and wall heat flux (3-58 kW/m ). 

In this table the parameters are defined as follows Bo is the boiling number, d i is the hydraulic diameter, / is the friction factor, h is the local heat transfer coefficient, k is the thermal conductivity, Nu is the Nusselt number, Pr is the Prandtl number, q is the heat flux, v is the specific volume, X is the Martinelli parameter, Xvt is the Martinelli parameter for laminar liquid-turbulent vapor flow, Xw is the Martinelli parameter for laminar liquid-laminar vapor flow, Xq is thermodynamic equilibrium quality, z is the streamwise coordinate, fi is the viscosity, p is the density, <7 is the surface tension the subscripts are L for saturated fluid, LG for property difference between saturated vapor and saturated liquid, G for saturated vapor, sp for singlephase, and tp for two-phase. 

Kuo, CJ, Peles, Y (2007) Local measurements of flow boiling in structured surface micro-channels. Int. J. Heat Mass Transfer 50 4513-4526. 

To predict the detached voidage in region II, the experimental work of Thom et al. (1966), as modified by Tong (1967a), is recommended. Thom et al. measured the local void fraction in subcooled boiling water flows in a 1.52-m (5-ft)-long tube... 

A general expression for average void fraction in an adiabatic two-phase flow or in saturated boiling was suggested by Zuber and Findlay (1965). Defining the local superficial velocities, VSG(= Qc/A) and VSL (= QJA), as volumetric flux densities, they assumed the velocity and void distributions to be... 

Liquid core temperature and velocity distribution analysis. BankofT (1961) analyzed the convective heat transfer capability of a subcooled liquid core in local boiling by using the turbulent liquid flow equations. He found that boiling crisis occurs when the core is unable to remove the heat as fast as it can be transmitted by the wall. The temperature and velocity distributions were analyzed in the singlephase turbulent core of a boiling annular flow in a circular pipe of radius r. For fully developed steady flow, the momentum equation is given as... 

The flow friction of a channel having subcooled local boiling behaves in a manner similar to the flow friction of a channel having a rough surface. From air-water data obtained at low void fractions (Malnes, 1966), the two-phase flow friction factor without bubble departure was found (Tong, 1968b Thorgerson, 1969)to be... 

In order to evaluate Eq. (5-23) in the neighborhood of the boiling crisis for given local bulk conditions (i.e., fluid pressure, bulk mass flow rate, quality, and equivalent diameter), the following simplifications are made. 

Local flow velocity fluctuations at the exit portion have been observed in a long boiling channel as a result of the large fluid compressibility inside the channel (Proskuryakov, 1965). Exit velocity fluctuation frequency is usually the same as the natural frequency of the channel. 

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