Internal flow natural convection

Oosthuizen, P.H. and Paul, J.T., Natural Convective Flow in a Cavity with Conducting Top and Bottom Walls , Proc. 9th International Heat Trans. Conf., Vol. 2, Hemisphere Publisher, New York, pp. 263-268, 1990. 

Convection is classified as natural (or free) and forced convection, depend ing on how the fluid motion is initiated. In forced convection, the fluid is forced to flow over a surface or in a pipe by external means such as a pump or a fan. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, which manifests itself as the rise of warmer fluid and the fall of the cooler fluid. Convection is also classified a.s external and internal, depending on whether the fluid is forced to flow over a surface or in a pipe. 

Because of his relatively larger differences in drop diameter, Garwin s results cannot be properly considered here. They conform with the concept that internal natural convection currents enhance the mobility of the interface which nevertheless fail in the light of Johnson s as well as Woodward s experimental results. The question is obviously open, and the need for controlled experiments under identical temperature and flow conditions is indicated. The theoretical study of Young et al. (Yl) on the motion of bubbles... 

A dip tube was added to the reactor vessel so that the F-T catalyst slurry could be recycled internally via a natural convection loop. The unreacted syngas, F-T products, and slurry exited into a side port near the top of the reactor vessel and entered a riser tube. The driving force for the recirculation flow was essentially the difference in density between the fluid column in the riser (slurry and gas) and that of the dip-tube (slurry only). The dip tube provided a downward flow path for the slurry without interfering with the upward flow of the turbulent syngas slurry mixture. Thus, to some degree, back mixing of the slurry phase and wall effects in the narrow reactor [3] tube were minimized. 

In the case of airlift reactors, the flow pattern may be similar to that in bubble columns or closer to that two-phase flow in pipes (when the internal circulation is good), in which case the use of suitable correlations developed for pipes may be justified [55]. Blakebrough et al. studied the heat transfer characteristics of systems with microorganisms in an external loop airlift reactor and reported an increase in the rate of heat transfer [56], In an analytical study, Kawase and Kumagai [57] invoked the similarity between gas sparged pneumatic bioreactors and turbulent natural convection to develop a semi-theoretical framework for the prediction of Nusselt number in bubble columns and airlift reactors the predictions were in fair agreement with the limited experimental results [7,58] for polymer solutions and particulate slurries. 

Used widi steam or coofing. Liquid flow velocities are low and the flow is poorly distributed. Natural convection equations are suitable and cooling coefficients have low values. Conventional jackets are best applied to small vessels or high pressure applications, where flie vessel internal pressure is twice the jacket pressure as a minimum. The conventional jacket is the most common... 

Men et al. (2014) have conducted experiments on the natural convection heat transfer for a PRHRS HEX in an in-containment refueling water storage tank. Several empirical correlations for the forced convection flow internal to the HEX tube and the natural convection heat transfer outside of the tube in the tank, for the vertical and horizontal portion of the tube, were compared with experimental data. The Dittus-Boelter forced convection correlation and the McAdams correlations for natural convection proved to give the better model of the data. Wenbin et al. (2014) have conducted experiments for the secondary loop of the Chinese Advance Pressurized Water Reactor for validation of the MIS AP20 models and code. These and other papers are in a special issue of the Science and Technology of Nuclear Installations journal published in 2014 as indicated by the cited references. 

Chatoorgoon, V., 2001. StabiUty of supercritical fluid flow in a single channel natural-convection loop. International Journal of Heat and Mass Transfer 44, 1963—1972. 

Mat, M., Aldas, K., 2005. Application of a two-phase flow model for natural convection in an electrochemical cell. International Journal of Hydrogen Energy 30 (4), 411—420. 

A reactor with vertically-upward flowing core coolant, unconstrained by internal flow channels within the core, has relatively stable hydraulics. Employing a vertical core ensures a potential to initiate and continue buoyancy-driven natural circulation. An upward normal core coolant flow direction avoids any concern of momentary core flow stagnation during transition from forced-convection to natural-circulation core cooling. A safety margin penalty is incurred with upward... 

In addition to the linearization assumption, another feature which is common to all solutions is the neglect of the gravitational term [i.e., the last term in Eq. (113)]. This may at first seem rather odd, since in the thermally driven countercurrent, the internal circulation induced by unequal plate temperatures is a type of natural convection. In thermal natural convection problems in ordinary flow situations, the acceleration of gravity is a crucial feature of the fluid behavior. In the thermally driven centrifuge, however, the expansion-compression work term on the left of Eq. (127) replaces the gravitational term as the mechanism by which small temperature inequalities are transformed into fluid motion (see Section 1TI,E,1). 

Equipment designs based on indirect conduction usually transfer the heat from the primary heat transfer fluid to the intermediate wall within some kind of internal duct or channel. Transfer coefficients for these cases depend on the nature of the flow (laminar or turbulent) and the geometry of the duct or channel (short or long). Expressions for evaluating the transfer coefficients for these cases are available in standard texts. An expression for the convective thermal resistance can be generated similar to that derived for the conductive resistance ... 

The high sensitivity of the Allendoerfer cell makes it of great value in the detection of unstable radicals but, for the study of the kinetics and mechanism of radical decay, the use of a hydrodynamic flow is required. The use of a controlled, defined, and laminar flow of solution past the electrode allows the criteria of mechanism to be established from the solution of the appropriate convective diffusion equation. The uncertain hydrodynamics of earlier in-situ cells employing flow, e.g. Dohrmann [42-45] and Kastening [40, 41], makes such a computational process uncertain and difficult. Similarly, the complex flow between helical electrode surface and internal wall of the quartz cell in the Allendoerfer cell [54, 55] means that the nature of the flow cannot be predicted and so the convective diffusion equation cannot be readily written down, let alone solved Such problems are not experienced by the channel electrode [59], which has well-defined hydrodynamic properties. Compton and Coles [60] adopted the channel electrode as an in-situ ESR cell. 

Many formulations based upon these assumptions can be derived. One formulation can be converted into another using the definitions of density, internal energy and the ideal gas law. Though equivalent analytically, these formulations differ in their numerical properties. Each formulation can be expressed in terms of mass and enthalpy flow. These rates represent the exchange of mass and enthalpy between zones due to physical phenomena such as plumes, natural and forced ventilation, convective and radiative heat transfer, and so on. For example, a vent exchanges mass and enthalpy between zones in connected rooms, a fire plume typically adds heal to the upper layer and transfers entrained mass and enthalpy from the lower to the upper layer, and convection transfers enthalpy from the gas layers to the surrounding walls. 

The above theoretical approaches apply estimating MTCs for the forced convection of fluid flowing parallel to a surface. Typically, the fluid forcing process is external to the fluid body. In the case of natural or free convection fluid motion occurs because of gravitational forces (i.e., g = 9.81 m/s ) acting upon fluid density differences within (i.e., internal to) regions of the fluid. Temperature differences across fluid boundary layers are a major factor enhancing chemical mass transport in these locales. Concentration differences may be present as well, and these produce density differences that also drive internal fluid motion (i.e., free convection). 

---