Free convection condensation

FIGURE 14.8 Influence of vapor drag during laminar free convection condensation on a vertical plate [31]. (Reprinted with permission from JSME International Journal, Tokyo, Japan.)... 

Forced convection in air Forced convection in water Forced convection in liquid metals Free convection in air Free convection in water Boiling in water Condensing in steam... 

In this paper, we now report measurements of heat transfer coefficients for three systems at a variety of compositions near their lower consolute points. The first two, n-pentane--CO2 and n-decane--C02 are supercritical. The third is a liquid--liquid mixture, triethylamine (TEA)--H20, at atmospheric pressure. It seems to be quite analogous and exhibits similar behavior. All measurements were made using an electrically heated, horizontal copper cylinder in free convection. An attempt to interpret the results is given based on a scale analysis. This leads us to the conclusion that no attempt at modeling the observed condensation behavior will be possible without taking into account the possibility of interfacial tension-driven flows. However, other factors, which have so far eluded definition, appear to be involved. 

In addition to this the heat transfer coefficient aL is dependent on the temperature i9j. As free convection frequently occurs in the vapour, the term aG (i9G — i9j) can often not be neglected, but can be of the same magnitude as the other expressions in (4.67), in particular when the temperature at the phase interface i91 is close to the wall temperature. This means that the driving temperature drop d — d(l will be small in the condensate, whilst in contrast, that in the vapour t G — dj will be large. The practical calculation of the temperature at the condensate surface is time consuming and, even in the case discussed here for a binary mixture, cannot be easily carried out without a computer. 

Laminar Free Convection. When a stagnant vapor condenses on a vertical plate, the motion of the condensate will be governed by body forces, and it will be laminar over the upper part of the plate where the condensate film is very thin. In this region, the heat transfer coefficient can be readily derived following the classical approximate method of Nusselt [12], Consider the situation depicted in Fig. 14.4 where the vapor is at a saturation temperature Ts and the plate surface temperature is T . Neglecting momentum effects in the condensate film, a force balance in the z-direction on a differential element in the film yields... 

Laminar Free Convection. Sparrow and Gregg [33] were the first to use the boundary layer method to study laminar, gravity-driven film condensation on a vertical plate. They improved upon the approximate analysis of Nusselt by including fluid acceleration and energy convection terms in the momentum and energy equations, respectively. Their numerical results can be expressed as ... 

Laminar Free Convection. Laminar film condensation of a quiescent vapor on an isothermal, smooth horizontal tube, as depicted in Fig. 14.11, may be treated approximately by a Nusselt-type analysis, yielding the following average heat transfer coefficient ... 

Calculate the temperature at which the steam condenses on the tube wall. Neglect the thermal resistance of the inner copper tube wall. Do not neglect the effects of free convection. 

The TRIO-VF has been validated in many number of configurations free-convection, forced-convection, thermal coupling between fluid and surfaces, flows with water-condensation,. 

Indices have been developed to express the instability of the environment in quantitative terms. An example is the lifted index, the computation of which is illustrated in Fig. 5 for an environment typical of that in which tomadic thunderstorms form. Surface air is lifted dry adiabatically (dashed curve) until it becomes saturated (at the lifted condensation level). Then it is lifted moist adiabatically to the 500-millibar (mbar) pressure level, passing, in this example, the level of free convection, above which the... 

Ernst Schmidt (1892-1975) a German scientist in the field of heat and mass transfer who measured the radiation properties of solids and developed the use of aluminum foils as radiation shields. He was the first to measure velocity and temperature fields in free convection boundary layers and discovered the large heat transfer coefficients occurring in condensation. A paper on the analogy between heat and mass transfer caused the dimensionless quantity involved to be called the Sc number. 

The analysis which follows assumes that where the wall exchanges heat with its environment it does so either throughaheattransfer coefficient (71, or which is constant and/or with constant heat flux q/A)y, into the wall. Such an assumption produces an approximate result but permits its analytical treatment. Actually little is known of the nature of the coefficients of heat transfer on the inside of the vessel during discharge. It is a transient process which most likely also involves condensation from the gas on the wall in the region of the liquid-gas interface. One result of this present study is to indicate approximate values of these coefficients from a fit with the experimental data. Numerical values of the heat transfer coefficients used to reduce the theoretical results were taken from established free-convection correlations [3] and previous experimental measurements [1]. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

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