Mixtures boiling heat transfer coefficient

J. R. Thome, Prediction of Binary Mixture Boiling Heat Transfer Coefficients Using Only Phase Equilibrium Data, Int. J. Heat Mass Transfer (26) 965,1983. 

Estimate the nucleate-pool-boiling heat-transfer coefficient for a water-26% glycerin mixture at 1 atm in contact with a copper surface and AT, = 15°C. 

In the macroscopic heat-transfer term of equation 9, the first group in brackets represents the usual Dittus-Boelter equation for heat-transfer coefficients. The second bracket is the ratio of frictional pressure drop per unit length for two-phase flow to that for Hquid phase alone. The Prandd-number function is an empirical correction term. The final bracket is the ratio of the binary macroscopic heat-transfer coefficient to the heat-transfer coefficient that would be calculated for a pure fluid with properties identical to those of the fluid mixture. This term is built on the postulate that mass transfer does not affect the boiling mechanism itself but does affect the driving force. 

Likewise, the microscopic heat-transfer term takes accepted empirical correlations for pure-component pool boiling and adds corrections for mass-transfer and convection effects on the driving forces present in pool boiling. In addition to dependence on the usual physical properties, the extent of superheat, the saturation pressure change related to the superheat, and a suppression factor relating mixture behavior to equivalent pure-component heat-transfer coefficients are correlating functions. 

The equations for estimating nucleate boiling coefficients given in Section 12.11.1 can be used for close boiling mixtures, say less than 5°C, but will overestimate the coefficient if used for mixtures with a wide boiling range. Palen and Small (1964) give an empirical correction factor for mixtures which can be used to estimate the heat-transfer coefficient in the absence of experimental data ... 

The overall heat-transfer coefficient between the reacting gaseous mixture ar.d the boiling cocxlant is assumed to be 10 Btu/ h °F. This coefficient is toward ... 

Liquid temperatures in the tubes of an LTV evaporator are far from uniform and are difficult to predict. At the lower end, the liquid is usually not boiling, and the liquor picks up heat as sensible heat. Since entering liquid velocities are usually very low, true heat-transfer coefficients are low in this nonboiling zone. At some point up the tube, the liquid starts to boil, and from that point on the liquid temperature decreases because of the reduction in static, friction, and acceleration heads until the vapor-liquid mixture reaches the top of the tubes at substantially vapor-head temperature. Thus the true temperature difference in the boiling zone is always less than the total temperature difference as measured from steam and vapor-head temperatures. 

Similarly to Nusselt s film condensation theory, in the condensation of vapour mixtures, the heat flux transferred increases with the driving temperature difference — 1 0. According to Nusselt s film condensation theory, the heat transfer coefficient decreases with the driving temperature difference according to a ( oo- o) 1/4 (4-12). The heat flux increases in accordance with q co — q)3/4. Fig. 4.21a shows clearly that a minimum for the transferred heat flux exists at a certain temperature oa. This is because the temperature difference dj — r)(j between the condensate surface and the wall, which is decisive for heat transfer, also assumes a minimum this can be explained by the boiling diagram, Fig. 4.21. 

Effect of Multicomponent Mixtures in Nucleate Pool Boiling. For a binary liquid mixture of composition xu we may define a heat transfer coefficient h as... 

Commonly, mixtures of three or more components are boiled Fig. 15.55 shows data reported by Schlunder [125] for the boiling of acetone-methanol-water mixtures. The ratio of measured to ideal heat transfer coefficient (the latter being calculated by extending Eq. 15.112 to include the third component) varies in a complex way with composition several azeotropic compositions are noted at which the ratio of measured to ideal coefficient approaches unity. 

FIGURE 15.53 Comparison of ideal actual heat transfer coefficient for pool boiling of CF2CI2ISF6 mixtures (from Schlunder [125]). 

FIGURE 15.111 Variation of heat transfer coefficient with composition in the forced convective boiling of R134a/R123 mixtures (from Fujita and Tsutsui [279], with permission from Taylor Francis, Washington, DC. All rights reserved). 

FIGURE 15.113 Average heat transfer coefficient for boiling of R22/R114 mixtures in a 9-mm bore tube (from Jung et al. [284], with permission from Elsevier Science). 

Y. Fujita, Predictive Methods of Heat Transfer Coefficient and Critical Heat Flux in Mixture Boiling, in Proc. 4th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Brussels, Belgium, vol. 2, pp. 831-842,1997. 

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