Reynolds number boiling

Pressure drop due to hydrostatic head can be calculated from hquid holdup B.]. For nonfoaming dilute aqueous solutions, R] can be estimated from f i = 1/[1 + 2.5(V/E)(pi/pJ ]. Liquid holdup, which represents the ratio of liqmd-only velocity to actual hquid velocity, also appears to be the principal determinant of the convective coefficient in the boiling zone (Dengler, Sc.D. thesis, MIT, 1952). In other words, the convective coefficient is that calciilated from Eq. (5-50) by using the liquid-only velocity divided by in the Reynolds number. Nucleate boiling augments conveclive heat transfer, primarily when AT s are high and the convective coefficient is low [Chen, Ind Eng. Chem. Process Des. Dev., 5, 322 (1966)]. 

Figure 5.47 shows a plot of the ratio of the experimental heat transfer coefficient obtained by Bao et al. (2000) divided by the predicted values of Chen (1966) and Gungor and Winterton (1986) for heat transfer to saturated flow boiling in tubes versus liquid Reynolds number. It can be seen that both methods provide reasonable predictions for Rcls > 500, but that both overpredict the heat transfer coefficient at lower values of Rols- For comparison it was assumed that the boiling term of these correlations is zero. 

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. 

The boiling Reynolds number or bubble Reynolds number (Re,) is defined as the ratio of the bubble inertial force to the liquid viscous force, which indicates the intensity of liquid agitation induced by the bubble motion ... 

The Rohsenow approach is to use Eq. (1) modified for boiling. The correct Reynolds number is taken to be the Reynolds number of a bubble just after it breaks loose from the hot solid. Previous workers (J2) found experimentally that the velocity of a released bubble is constant for a short time. In addition it was shown that the diameters of the released bubbles of water or carbon tetrachloride are inversely proportional to the frequency of emission of bubbles that size, or... 

The magnitude of these coefficients is determined by physical properties of the fluid and by fluid dynamics, the degree of turbulence known as the Reynolds number or its equivalent. Heat transfer within a fluid, due to its motion, occurs by convection fluid at the bulk temperature comes in contact with fluid adjacent to the wall. Thus, turbulence and mixing are important factors to be considered, even when a change in phase occurs as in condensing steam or a boiling liquid. 

For the great majority of reaction schemes, piston flow is optimal. Thus the reactor designer normally wants to build a tubular reactor and to operate it at high Reynolds numbers so that piston flow is closely approximated. This may not be possible. There are many situations where a tubular reactor is infeasible and where CSTRs are used instead. Typical examples are reactions involving suspended solids and autorefrigerated reactors where the reaction mass is held at its boiling point. 

The other studies, which included the effect of vibration on steam condensation, nucleate boiling heat transfer, and scale deposition, investigated the relation of frequency and amplitude of vibration of the heat transfer surface to increases in these heat transfer mechanisms. The study of the effect of acoustic vibrations in water on forced convection heat transfer investigated the influence of frequency and amplitude of the standing waves on increasing heat transfer rates and the flow Reynolds numbers at which increases could be obtained. 

Figure 8 from [8] presents the total minichannel pressure loss as a function of the inlet Reynolds number. The total pressure loss includes liquid, two-phase, and vapor depending on the boiling stage in the minichannel. The curve behavior is a classical N-shape observed for all the heat fluxes studied. The total minichannel pressure loss is the sum of fluid pressure loss for each zone and arises when the friction term in the... 

Flow Boiling Instability, Fig. 8 Average pressure loss versus inlet Reynolds number when the buffer is not connected to the loop for flve heat fluxes... 

Flow Boiling Instability, Fig. 9 Pressure loss scaling law for all heat flux densities provided non-dimensioned pressure loss function of the ratio between the phase change number and the Reynolds number for only exit vapm qualities strictly between 0 and 1... 

The following equation has been widely used to predict nucleate boiling heat transfer. The equation includes a term to treat variation of coefficient with pressure. A special Reynolds number is also defined. 

The vapor quality at the unit outlet was calculated for the various test runs from the known unit inlet conditions and the heat input. Quality and Reynolds number undoubtedly influence the boiling heat transfer, but the predominant factor is pressure level. The curves show the heat transfer rate increases with an increase in pressure. 

The coefficient f depends on the Reynolds number for flow within the tube. In laminar flow, the Hagen-Poiseuille law can be applied. In turbulent flow the Blasius equation is used. The main difficulty is the evaluation of water pressure drop during transition boiling. The pressure drop consists of three components friction (APf), acceleration (APJ and static pressure (APg). In once-through horizontal tubes boiler APg=0. The Lockard-Martinelli formulation is used to estimate the friction term. 

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