Natural convection liquids

Convection is the transfer of heat from one point to another within a fluid, gas, or liquid by the mixing of one portion of the fluid with another. In natural convection, the motion of the flmd is entirely the result of differences in density resiilting from temperature differences in forced convection, the motion is produced by mechanical means. When the forced velocity is relatively low, it should be reahzed that Tree-convection factors, such as density and temperature difference, may have an important influence. 

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

The lower Emit of applicability of the nucleate-boiling equations is from 0.1 to 0.2 of the maximum limit and depends upon the magnitude of natural-convection heat transfer for the liquid. The best method of determining the lower limit is to plot two curves one of h versus At for natural convection, the other ofh versus At for nucleate boiling. The intersection of these two cui ves may be considered the lower limit of apphcability of the equations. 

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

For organic liquids, evaluate the natural convection film coefficient from Figure 10-103. Equation 10-29 may be used for the inside horizontal tube by multiplying the right side of the equation by 2.25 (1 + 0.010 Gr,i/")/logRe. 

Convection. Heat transfer by convection arises from the mixing of elements of fluid. If this mixing occurs as a result of density differences as, for example, when a pool of liquid is heated from below, the process is known as natural convection. If the mixing results from eddy movement in the fluid, for example when a fluid flows through a pipe heated on the outside, it is called forced convection. It is important to note that convection requires mixing of fluid elements, and is not governed by temperature difference alone as is the case in conduction and radiation. 

Heat transfer by convection occurs as a result of the movement of fluid on a macroscopic scale in the form of eddies or circulating currents. If the currents arise from the heat transfer process itself, natural convection occurs, such as in the heating of a vessel containing liquid by means of a heat source situated beneath it. The liquid at the bottom of the vessel becomes heated and expands and rises because its density has become less than that of the remaining liquid. Cold liquid of higher density takes its place and a circulating current is thus set up. 

In interface evaporation, the bubbles of vapour formed on the heated surface move to the vapour-liquid interface by natural convection and exert very little agitation on the liquid. The results are given by ... 

Figure 2.42 shows boiling curves obtained in an annular channel with length 24 mm and different gap size (Bond numbers). The heat flux q is plotted versus the wall excess temperature AT = 7w — 7s (the natural convection data are not shown). The horizontal arrows indicate the critical heat flux. In these experiments we did not observe any signs of hysteresis. The wall excess temperature was reduced as the Bond number (gap size) decreased. One can see that the bubbles grew in the narrow channel, and the liquid layer between the wall and the base of the bubble was enlarged. It facilitates evaporation and increases latent heat transfer. 

This discrepancy arises primarily from the fact that spontaneous liquid flows will always develop in any hquid even without artificial stirring (e.g., under the action of density gradients caused by local temperature or concentration fluctuations). This phenomenon has been termed natural convection. Electrochemical reactions reinforce natural convection, since the concentrations of substances involved in the reaction will change near the electrode surfaces, and also since heat is evolved. Gas evolution attending the reactions has a particularly strong effect on naturaf convection. 

Natural convection depends strongly on cell geometry. No convection can arise in capillaries or in the thin liquid layers found in narrow gaps between electrodes. The rates of natural convective flows and the associated diffusion-layer thicknesses depend on numerous factors and cannot be calculated in a general form. Very rough estimates show that the diffusion-layer thickness under a variety of conditions may be between 100 and 500 pm. 

Natural convection can be eliminated entirely when electrolytes held in a matrix or porous support are used instead of free liquids. Natural convection will not develop in a pore space when the individual pores are sufficiently narrow. When such electrolytes are used, the diffusion layer propagates across the entire matrix (i.e., across the full electrode gap). 

In the pores of the electrodes, practically no natural convection of the liquid takes place. Reactants dissolved in the liquid can be supplied in two ways from the external surface to the internal reaction zones (and reaction products transported away in the opposite direction) (1) by diffusion in the motionless liquid diffusion electrode),... 

The same principle is used for the preparative separation of mixtures of biological materials, the extraction of different individual components from these mixtures, and their purification. In this case one uses an electrophoretic method with continued introduction of individual portions of the mixture and withdrawal of portions of pure fractions. There have been reports that such processes were accomplished in spacecraft where, since gravitational forces are absent, the liquid solutions can be used without the danger of natural convection. 

Experimental gas-solid mass-transfer data have been obtained for naphthalene in CO2 to develop correlations for mass-transfer coefficients [Lim, Holder, and Shah, Am. Chem. Soc. Symp. Ser, 406, 379 (1989)]. The mass-transfer coefficient increases dramatically near the critical point, goes through a maximum, and then decreases gradually. The strong natural convection at SCF conditions leads to higher mass-transfer rates than in liquid solvents. A comprehensive mass-transfer model has been developed for SCF extraction from an aqueous phase to CO2 in countercurrent columns [Seibert and Moosberg, Sep. Sci. Techrwl, 23, 2049 (1988) Brunner, op. cit.]. 

The flow artifacts detected in the droplet size measurements are similar to those reported by Goux et al. [79] and Mohoric and Stepisnik [80]. In their work natural convection effects led to an increase in the decay of signal attenuation curves, causing over-prediction in the self-diffusion coefficient of pure liquids. In order to avoid flow effects in droplet size distributions, flow compensating pulse sequences such as the double PGSTE should be used. It has been demonstrated recently that this sequence facilitates droplet size measurements in pipe flows [81]. 

The mechanism of heat transfer from a submerged surface to a pool of liquid depends on the temperature difference between the heated surface and the liquid Figure 12.54. At low-temperature differences, when the liquid is below its boiling point, heat is transferred by natural convection. As the surface temperature is raised incipient boiling occurs, vapour... 

In forced convection, the velocity of the liquid must be characterized by a suitable characteristic value Vih e.g. the mean velocity of the liquid flow through a tube or the velocity of the edge of a disk rotating in the liquid, etc. For natural convection, this characteristic velocity can be set equal to zero. The dimension of the system in which liquid flow occurs has a certain characteristic value /, e.g. the length of a tube or the longitudinal dimension of the plate along which the liquid flows or the radius of a disk rotating in the liquid, etc. Solution of the differential equations (2.7.5), (2.7.7) and (2.7.8) should yield the value of the material flux at the phase boundary of the liquid with another phase, where the concentration equals c. ... 

Equation (2-113) means that any cylindrical cavity for any liquid-solid combination under a given pressure has a minimum heat flux below which boiling will not be stable, and a transition between natural convection and stable nucleate boiling (bumping) is always observed. 

Their results showed the following. Surface 1 gave direct transition from liquid-phase natural-convection heat transfer to film boiling with CHF values of 160,000 Btu/hr ft2 (503 kW/m2), independent of the pressure. Surface 2 gave stable nucleate boiling with CHF values much greater than those obtained with surface 1, and... 

Monde, M., H. Kusuda, and H. Uehara, 1982, Critical Heat Flux during Natural Convective Boiling in Vertical Rectangular Channels Submerged in Saturated Liquid, Trans. ASME, J. Heal Transfer 704 300-303. (5)... 

Example 6.2 Estimate the mass flux evaporated for methanol in dry air at 25 °C and 1 atm. Assume natural convection conditions apply at the liquid-vapor surface with h = 8 W/m2 K. 

Example 6.3 Consider Example 6.2 for a shallow pool of methanol with its bottom surface maintained at 25 °C. Assume that natural convection occurs in the liquid with an effective convective heat transfer coefficient in the liquid taken as 10 W/m2 K. Find the surface temperature, surface vapor mass fraction and the evaporation flux for this pool. 

A burning pool of liquid or a volatile solid fuel will establish a stagnant film height due to the natural convection that ensues. From analogies to heat transfer without mass transfer, a first approximation to the liquid pool burning rate may be written as... 

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