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EXPERIMENTAL RESEARCH OF CAVITATION PERFORMANCE OF A LEAD COOLANT AND MODELS OF MAIN CIRCULATION PUMP IN HEAVY LIQUID- METAL COOLANT REACTOR PLANTS
A.V. Beznosov, T.A. Bokova, A.V. Lvov, P.A. Bokov (NNSTU n.a. R.E. Alekseyev, Nizhny Novgorod, Russia)
The conventional concept of cavitation in vane-type pumps combines the characteristics of a pumped medium and the design features of a particular vane-type pump.
Different specialists give slightly different definitions to the term cavitation". Almost in all definitions, a sign of cavitation is recognized as a discontinuity, or a break in the flow of liquid drops with the formation of bubbles, voids, etc. moving with the flow of liquid and then collapsing. As applied to vane-type pumps in calculations related to their operating cavitation modes, a condition of cavitation occurrence is recognized when a critical pressure is reached in the intervane space. In this regard, a critical pressure at which cavitation occurs means a pressure of saturated liquid vapors at a given temperature.
Analysis of the references shows that during examining cavitation in pumps occurring implicitly or explicitly, all the authors in any cases take water or liquids close to it in physical properties (salt solutions, organic compounds, etc.) as pumped liquids. In rare cases of calculating the pump flow part with organic liquids different from water: oil, kerosene, gasoline, etc., the differences of their properties from water are taken into account using empiric coefficients. The densities of the pumped medium are taken for the actual pumped liquids. Cavitation and other characteristics are calculated using empiric formulas obtained in water tests, although the same authors admit that the cavitation processes depend on the thermodynamic properties of the liquid, the pressure of saturated vapors, the surface tension, etc.
As evidenced by the opinion of all experts, it is obvious that in order to create a new pump, it is necessary to know, first of all, the cavitation characteristic of a pumped medium, by which all the authors mean saturated vapor pressure of such medium. Other properties (thermodynamic, physical, etc.) of the pumped medium may be taken into account by means of introduced empiric coefficients or ignored.
One of the factors determining the cavitation performance of each particular vane-type pump includes physical properties of the pumped liquid. The incipient cavitation condition is accepted to be a condition where an underpressure equal to the pressure at which the continuity of the pumped medium is violated, and a new phase having a much smaller density than the pumped medium is formed in the local areas of the transported pump flow. This condition may be considered the most general formulation of cavitation, but it can be determined only as a result of appropriate cavitation tests. The most obvious and simple incipient cavitation condition, in explicit or implicit form, which is adopted by all the authors, includes a condition of reducing the flow pressure to boiling, namely, vapor bubbling. This value, saturated vapor pressure, is easily determined from relevant references and liquid state tables.
The cavitation performance of a lead coolant significantly differs from that of other coolants in nuclear reactors, such as water and sodium, and is probably close to such characteristics of a lead-bismuth coolant.
Specific properties of a lead coolant, which determine the cavitation process in vane- type pumps, include:
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