Liquid Metals are a specific class of coolants. Their basic advantage is a high molecular thermal conductivity which, for identical flow parameters, enhances heat transfer coefficients. Another distinguishing feature of liquid metals is the low pressure of their vapors, which allows their use in power engineering equipment at high temperatures and low pressure, thus alleviating solution of mechanical strength problems. The most widespread liquid metals used in engineering are alkali metals.
Among them sodium is first and foremost, used as a coolant of fast reactors and a working fluid of high-temperature heat pipes.
Potassium is a promising working medium for space power plants. The high thermal conductivity and, hence, low Prandtl numbers of liquid metals imply that heat transfer by molecular thermal conduction liquid metal heat exchanger significant not only in the near-wall layer, but also in the flow core even in a liquid metal heat exchanger developed turbulent flow.
The thickness of the thermal boundary layer proves to be substantially larger than the thickness of the hydrodynamic boundary layer see Single-phase Forced Convection Heat Transfer. The hydrodynamic characteristics of liquid metal flows friction factor and the coefficient of local resistance are calculated by conventional formulas.
Fully developed heat transfer to liquid metals in tubes may be calculated using the following generalized relations:. At high values of Pe number Eqs. An approximate calculation of mean heat transfer in the entrance region in the case of turbulent flow can be performed with Eqs. The thermal properties of liquid metal depend only slightly on temperature.
Taking into account the small transverse temperature difference in liquid metal flow due to the high thermal conductivity, the effect of nonisothermal conditions in heat transfer is not significant and, as a rule, is not considered.
Bundles of fuel rods in triangular or square arrays are often used in reactors with a liquid metal coolant. In this case, due to the nonuniform flow past the central, peripheral and angular fuel rods, the heat transfer to them is different. There are also variations in heat transfer around the perimeter of the fuel rod. Normally, heat transfer is determined above all by the Pe number.
However, the pitch to diameter ratio of the rods in the bundle, the arrangement of the rods and the presence of plugs mounted for equalizing coolant flow rate over the bundle section have proved to be important. A uniform temperature distribution around the fuel rod perimeter, except for the above factors, depends on the ratio of the coefficient of thermal conductivity of the liquid metal to that of the rod enclosure.
These factors assume particular importance in tightly-packed bundles. Heat transfer for the cases indicated above is liquid metal heat exchanger using unwieldy empirical relations that are valid, as a rule, within a narrow range of parameters. Details of these are presented in handbooks. It has been established that heat transfer in liquid metals depends, to a high degree, on fouling resistances at the wall-liquid interface.
These resistances appear due to chemical or electrochemical interaction between the wall material and a coolant to produce a surface layer of intermetallics, carbides and other compounds or solid solutions with reduced thermal conductivity. Mass transfer and deposition of corrosion products are also possible on the heat exchange surface. In all cases, much importance is attached not only to the chemical compatibility of liquid metal heat exchanger liquid metal coolant and the wall material, but also to the degree of metal purity.
This is given a liquid metal heat exchanger attention with special in-line systems for metal purification used in many systems. However, the design of such apparatus — primarily shell-and-tube heat exchangers — has a specific feature. Due to the relatively low specific heat of liquid metals, heating of the coolant is comparable to, and in some cases appreciably exceeds, the value of the governing temperature difference.
Thus, a correct allowance for the bypass leakages of the coolant along the shell and the contribution of zones of deteriorated flow along the liquid metal heat exchanger exchange surface are of crucial significance. In this case a "zone-by-zone" approach in designing heat exchangers has proven to be efficient. One of the principal specific features of boiling of most of liquid metals — alkali metals above liquid metal heat exchanger — is that the superheat for the incipience of boiling may amount to tens, and in some cases, hundreds of degree.
Thus, measures should be taken, whenever necessary, for reducing the incipient boiling superheat and ensuring the reproducibility of its value. Note that mercury, which poorly wets most technical surfaces, has extremely low incipient superheat. Pool boiling of alkali metals at moderate heat loads is characterized by unstable boiling, i.
Pool boiling of liquid metals: The remainder of the heat is transferred by convection to the liquid bulk, or is removed with the superheated liquid surrounding the bubbles rising to the free surface. These effects are linked with the high thermal conductivity of liquid metals. Critical heat fluxes which bring about liquid metal heat exchanger transition from nucleate to film boiling, are described for alkali metals by the empirical formula.
Flow regimes of two-phase liquid metal heat exchanger metal flows are the same as those of ordinary liquids see Forced Convection Boiling.
However, owing to the high incipient liquid metal heat exchanger superheat and a high ratio of specific volumes of vapor and liquid phase low operating pressuresthe regions of bubble and slug flow regimes liquid metal heat exchanger correspond to an extremely narrow range of vapor quality or be missing altogether. The annular-dispersed flow regime see Annular Flow is predominant.
This means that classic boiling liquid metal heat exchanger vapor bubbles growing on the wall is usually absent. Phase transition occurs by evaporation from the interface, to which heat is supplied by thermal conduction through liquid metal heat exchanger liquid film. Burnout for a forced, two-phase liquid metal flow is commonly linked with the dryout of the near-wall liquid film.
The boundary quality which gives rise to the dryout of the film depends on pressure and mass liquid metal heat exchanger rate. It is fairly high and varies from 0. A further increase in can be attained liquid metal heat exchanger porous coatings, coiled tubes or other means facilitating retention on the wall of moisture deposited from the flow core. Condensation of liquid metals readily wetting the surface obeys the same laws as condensation of conventional liquids does see Condensation.
As a rule, this is a film-type condensation. The high thermal conductivity of liquid metals leads to a drastic reduction of the contribution of liquid film thermal resistance to overall heat transfer resistance during condensation for nonmetallic liquids it is the basic contribution. Simultaneously, the contribution of resistance at the vapor-film interface and, in particular, of diffusion resistance grows if noncondensable gases are present or chemical reactions proceed in vapor phase, which require that efforts be made to expel impurities from a vapor.
Fully developed heat transfer to liquid metals in tubes may be calculated using the following generalized relations:
To be completely fair the user goes on to explain he did not put two- factor authentication so it' s not really on the website as much as it is on. Introducing Gujjar The bot responsible for Bitcoin price fluctuation in Coinbase. But not in Sweden, where robots are just another way to make companies more efficient.