Natural convection gases, 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. 

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. 

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.]. 

Conductive and Convective Heat Transfer, Thermo Explosion by. There are three fundamental types of heat transfer conduction, convection radiation. All three types may occur at the same time, but it is advisable to consider the heat thransfer by each type in any particular case. Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of either body. 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 fluid is entirely the result of differences in density resulting from temp differences in forced convection, the motion is produced by mechanical means. Radiation is the transfer of heat from one body to another, not in contact with it, by means of wave motion thru space (Ref 5)... 

For ideal solutions, the partial pressure of a component is directly proportional to the mole fraction of that component in solution and depends on the temperature and the vapor pressure of the pure component. The situation with group III-V systems is somewhat more complicated because of polymerization reactions in the gas phase (e.g., the formation of P2 or P4). Maximum evaporation rates can become comparable with deposition rates (0.01-0.1 xm/min) when the partial pressure is in the order of 0.01-1.0 Pa, a situation sometimes encountered in LPE. This problem is analogous to the problem of solute loss during bakeout, and the concentration variation in the melt is given by equation 1, with l replaced by the distance below the gas-liquid interface and z taken from equation 19. The concentration variation will penetrate the liquid solution from the top surface to a depth that is nearly independent of zlDx and comparable with the penetration depth produced by film growth. As result of solute loss at each boundary, the variation in solute concentration will show a maximum located in the melt. The density will show an extremum, and the system could be unstable with respect to natural convection. 

Rate of Heat Transfer. Whatever the source of the heat, the principal method for its dissipation is undoubtedly conduction through the bed to the container walls, and thence, to the boiling liquid bath. Radiation, being proportional to the fourth power of the absolute temperature, is negligible. Some natural convection in the gas above the sample will occur and help to cool the top surface of the bed as well as the sides. The outside of the bed will cool quickly to bath temperature, but the center of the bed will cool much more slowly. This is the well known (9) cooling rate problem, for which mathematical solutions have been developed giving the temperature at various points in the bed. These solutions always involve some sort of exponential approach to thermal equilibrium and the physical constants of the system appear in the following expression ... 

The use of fins is most effective in applications involving a low convection heat transfer coefficient. Thus, the use of fins is more easily justified when the medium is a gas instead of a liquid and the heat transfer is by natural convection instead of by forced convection. Therefore, it is no coincidence that in liquid-to-gas heat exchangers such as the car radiator, fins are placed on the gas side. 

The value of /i in a fluid film, i.e., that o h, is originally proportional to the value of A of the fluid itself As the value of the temperature gradient in a fluid film increases on account of the decrease in thickness of the film, however, the value of h increases even in the identical fluid. In other words, the value of hi of a liquid in which the forced convective flow is made is necessarily larger than that of the liquid in which the natural convective flow is allowed, because the thickness of the fluid film decreases with the increase in rate of the forced convective flow. Besides, it is also possible to point out a fact that the value of A of an ordinary liquid in general are much larger than that of an ordinary gas. 

At 35 C, the gas-solid mass transfer coefficient increases dramatically near the critical point, has its maximum value near 100 atm, and then decreases gradually as pressure increases. The mass transfer rate under supercritical conditions is much higher than at standard conditions (1 atm and 25 C) for liquid-solid and gas-solid systems, due to strong natural convection effects. Both natural and forced convection are important for supercritical mass transfer. 

In Eqs. (12.24) and (12.25), n is the viscosity at the arithmetic mean temperature of the fluid, T + T(,)/2, and is the viscosity at the wall temperature T ,. For liquids fi < n and

1.0 when the liquid is being heated, and > n and < 1.0 when the liquid is being cooled. The viscosity of a gas increases with temperature, so these inequalities are reversed for a gas. The change in viscosity of a gas, however, is relatively small, and the term is usually omitted when dealing with gases. The change in gas density with temperature is of more importance, and this will be discussed when dealing with natural convection. 

Natural convection currents will develop if there exists a significant variation in density within a liquid or gas phase. The density variations may be due to temperature differences or to relatively large concentration differences. Consider natural convection involving mass transfer from a vertical plane wall to an adjacent fluid. Use the Buckingham method to determine the dimensionless groups formed from the variables significant to this problem. 

The mobile nature of the gas-liquid interface induces radial convection in the liquid phase. The velocity of the liquid is, in general, different from that of the interface, except at r = R t). 

From a heat balance at the vapor-liquid interface, it is found that the net mass-transfer energy must equal the difference between the heat transferred from the gas to the interface and the heat transferred from the interface through the liquid. This difference is represented by the previously developed natural convection heat-transfer relation for the gas-phase portion of the balance (7) and by a moving boundary steady-state conduction heat-transfer relation for the liquid-phase portion of the balance. This leads to the expression... 

SRINIVASAN, G. et al., Thermo-mechanical behaviour of FBTR reactor vessel due to natural convection in cover gas space, Proc. 4 Inti Conf Liquid Metal Engineering and Technology (LIMET 88), organized by Societe Francaise d Energie Nucleaire 17-21 October 1988, Avignon, France (1988). 

The /th species mass flux, j, and the total heat flux, q, can be expressed in terms of transfer coefficients. This is useful in situations where the liquid or gas phase is not completely resolved, or when the flow conditions are not exactly known. Often, these transfer coefficients are determined experimentally for a particular flow situation. For instance, different expressions are used, depending on whether the transfer is due to pure conduction or whether it is dominated by ccaivection. Also, the type of convection plays a role, that is, if the convection is forced or non-forced. A forced convection has a non-zero relative velocity between droplet and environment, whereas for a non-forced convection, the relative drop-gas velocity is zero and only the Stefan flow dominates. Note that the natural convection due to gravity is taken to be zero since gravity is an external force, and external forces are neglected in this article. In addition, in forced convection, the nature of the flow, that is, whether the flow is laminar or turbulent, plays an important role. These issues will be discussed in more detail in the following subsections. 

There are different kinds of DAFC operation conditions depending of the way the fuel and the oxidant (oxygen/air) are fed into the cell. In complete active fuel cells the liquid fuel (neat alcohol or aqueous solution) is pumped and gas is compressed, using auxiliary pumps and blowers, in order to improve mass transport and reduce concentration polarization losses in the system. On the other hand, in complete passive DAFC the alcohol reaches the anode catalyst layer by natural convection and the cathode breathes oxygen directly from the air. A number of intermediate options have been also studied and tested. 

Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. Density differences in the ffuid arising from the heating process provide the buoyancy force required to move the ffuid. Free or natural convection is observed as a result of the motion of the fluid. An example of heat transfer by natural convection is a hot radiator used for heating a room. Cold air encountering the radiator is heated and rises in natural convection because of buoyancy forces. The theoretical derivation of equations for natural convection heat-transfer coefficients requires the solution of motion and energy equations. 

An important heat-transfer system occurring in process engineering is that in which heat is being transferred from a hot vertical plate to a gas or liquid adjacent to it by natural convection. The fluid is not moving by forced convection but only by natural or free convection. In Fig. 4.7-1 the vertical flat plate is heated and the free-convection boundary layer is formed. The velocity profile differs from that in a forced-convection system in that the velocity at the wall is zero and also is zero at the other edge of the boundary layer since the free-stream velocity is zero for natural convection. The boundary layer initially is laminar as shown, but at some distance from the leading edge it starts to become turbulent. The wall temperature is T K and the bulk temperature T. ... 

Cross-flow exchanger. When a gas such as air is being heated or cooled, a common device used is the cross-flow heat exchanger shown in Fig. 4.9-3a. One of the fluids, which is a liquid, flows inside through the tubes and the exterior gas flows across the tube bundle by forced or sometimes natural convection. The fluid inside the tubes is considered to be unmixed since it is confined and cannot mix with any other stream. The gas flow outside the tubes is mixed since it can move about freely between the tubes and there will be a tendency for the gas temperature to equalize in the direction normal to the flow. For the unmixed fluid inside the tubes there will be a temperature gradient both parallel and normal to the direction of flow. 

The analytical model is shown in F ig. 1 for heat flow interaction with an environment consisting of Case I, heat transfer with an ambient through a convective heat transfer coefficient and. Case II, heat transfer with an ambient consisting of an imposed, constant heat flux. The system consists of a container (or, pipe as far as this analysis is concerned) initially filled with liquid at temperature T, Initially, both the wall and the liquid are at temperature Tj. At zero time, a pressurizing gas having temperature is introduced into the top of the container at x = 0. At this same time liquid discharge or flow is commenced such that the gas-liquid interface immediately starts to move downward at a velocity V. In addition, heat flow interaction of the nature of Case I or Case II between the outside of the container and the ambient starts. As a consequence of this, a transient process is introduced in the temperatures of both the wall and the gas. It is the purpose of this paper to present a solution for the thermal response of the gas and the wall. 

Heat transfer coefficient Thermal parameter that encompasses all of the complex effects occurring in the convection heat transfer mode, including the properties of the fluid gas/liquid, the nature of the fluid motion, and the geometry of the structure. 

---