Homogeneous bubble nucleation limit of mercury under the normal working conditions of the planned European Spallation Source

In spallation neutron sources, liquid mercury is the subject of big thermal and pressure shocks, upon adsorbing the proton beam. These changes can cause unstable bubbles in the liquid, which can damage the structural material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (i.e. for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low even after adsorbing several proton pulses, therefore the probability of temperature induced homogeneous bubble nucleation is negligible.Comment: 22 Pages, 11 figures, one of them is colour, we plan to publish it in Eur. Phys. J.

Direct Numerical Simulation of Turbulent Heat Transfer Modulation in Micro-Dispersed Channel Flow

The object of this paper is to study the influence of dispersed micrometer size particles on turbulent heat transfer mechanisms in wall-bounded flows. The strategic target of the current research is to set up a methodology to size and design new-concept heat transfer fluids with properties given by those of the base fluid modulated by the presence of dynamically-interacting, suitably-chosen, discrete micro- and nano- particles. We run Direct Numerical Simulation (DNS) for hydrodynamically fully-developed, thermally-developing turbulent channel flow at shear Reynolds number Re=150 and Prandtl number Pr=3, and we tracked two large swarms of particles, characterized by different inertia and thermal inertia. Preliminary results on velocity and temperature statistics for both phases show that, with respect to single-phase flow, heat transfer fluxes at the walls increase by roughly 2% when the flow is laden with the smaller particles, which exhibit a rather persistent stability against non-homogeneous distribution and near-wall concentration. An opposite trend (slight heat transfer flux decrease) is observed when the larger particles are dispersed into the flow. These results are consistent with previous experimental findings and are discussed in the frame of the current research activities in the field. Future developments are also outlined.Comment: Pages: 305-32

The pseudocritical regions for supercritical water

Vapour pressure curves and stability lines can be extended beyond the critical points into the supercritical domain by so-called Widom lines, along which some thermodynamic property undergoes a rapid change and liquid-like behaviour turns to vapour-like one. Knowledge about such lines is therefore important for thermohydraulic calculations and design. There are several properties that can be chosen as defining property of a Widom line. In this short note we calculate and compare several kinds of Widom lines for water. (C) 2012 Elsevier B.V. All rights reserved

Building a unique test section for local critical heat flux studies in light water reactor like accident conditions

Critical heat flux (CHF) has been studied for almost a century and yet there is no indisputable consensus reached on governing physical phenomena behind, not to mention, on modelling agreement of different correlations. When we are compelled to run our system at the safe distance from the CHF, and we can use all the accumulated knowledge so far, we will quite possibly cling to look-up tables delivered with that particular system. If this is not the case, than we will certainly stick to the system-specific correlation, which cannot be applied with confidence elsewhere. In the last two decades there were significant advancements applied both in numerical simulation capabilities and in unintrusive measuring techniques, which shed light on anticipated advancements in modelling the phenomenon. However, there are few reliable experimental measurements of instantaneous velocity and temperature fields in the wall boundary layer, and they are nil where local heat transfer coefficients are acquired. Therefore, at Reactor Engineering Division of Jožef Stefan Institute, a unique test section for local critical heat flux studies is under construction. The selected geometry and the test conditions will resemble light water reactor – like accident conditions. Moreover, to understand the phenomenon better, the design of the test section enables local measurements of heat transfer coefficients, and allows for control over the diabatic wall temperature. Measurements of single-phase convective heat transfer, conjugate heat transfer, flow boiling, convective condensation, and condensation-induced liquid hammer were all part of the test section’s design basis. In this context, the design and construction of the device is herein presented in considerable detail.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers