Boiling heat transfer during rewetting

An analytical one-dimensional conduction model is developed for the rewetting of a hot surface. The solution scheme is not limited to a specific heat flux profile. Experimental results falling within the range of validity of the one-dimensional solution are correlated using a generalized pool boiling curve. The effect of various heat transfer parameters such as inlet velocity and local subcooling is discussed. A unique relationship is shown to exist between the dimensionless quench front velocity, which is defined here as an eigenvalue of the governing equation, the ratio of the integral of the dimensionless heat flux, and the integral of the surface axial temperature gradient.     

Calculations on the rewetting of hot surfaces

Previous theoretical and experimental work has shown that the rate of advance of a quench front over a hot surface is a function of the initial temperature, the thickness and the physical properties of the hot surface, and the cooling water flow conditions. The heat conduction problem, which determines the quench rate, is solved accurately by an analytic method which requires a computer. The results, based on a constant heat transfer coefficient in the wetted region, can be applied to most conditions of practical interest, and are applied to determining the quenching heat transfer coefficient and the sputtering temperature from previous experimental results at high pressures, with cooling water near its saturation temperature.     

An analysis of the wet-side heat-transfer coefficient during rewetting of a hot dry patch

A physical model for the rewetting of a hot dry patch is proposed. The mechanisms governing the rate of resorption of the dry area are the conduction of heat from the dry to the wet area and the removal of this heat by a large local heat-transfer coefficient. The coefficient is assumed to be directly proportional to the cube of the difference between the surface and saturation temperature (h = RT^ß). The wall temperature at the interface between the wet and dry side is assumed equal to the sputtering value, and the cubic relationship between the heat-transfer coefficient and surface temperature difference is assumed valid up to this temperature. Experimental data from Harwell on rewetting rates for a stainless steel tube in a steam environment at various pressures are analysed to determine R. The energy equations are solved numerically by a quasi-linearization technique combined with the finite-difference method.     

Experimental studies on rewetting of hot vertical annular channel

Studies on the rewetting behaviour of hot vertical annular channels are of interest in the context of emergency core cooling in nuclear reactors following LOCA. Experimental studies were carried out to study the rewetting behaviour of a hot vertical annular channel, with hot inner tube, for bottom flooding and top flow rewetting conditions. The length of the inner tube of the test section was 3030 mm for bottom flooding rewetting experiments and 2630 mm for top flow rewetting experiments. The tube was made of stainless steel. Experiments were conducted for water flow rates in the annulus upto 7 lpm (11.7×10⁻⁵ m³ s⁻¹). The initial surface temperature of the inner tube was varied from 200 to 500°C. The experimental studies show that for a given initial surface temperature of the tube, the rewetting velocity increases with an increase in flow rate of water and it decreases with an increase in the initial surface temperature for a given water flow rate. For a given water flow rate and initial surface temperature, the rewetting velocity is higher in the case of rewetting under bottom flooding conditions as compared to that in the case of rewetting under top flow conditions. These conclusions agree with the conclusions reported in the earlier literature. Using the experimental data of the present work, correlations for bottom flooding and top flow rewetting velocities are developed.     

Heat transfer characteristics of horizontal steam generators under natural circulation conditions

This paper deals with the heat transfer characteristics of horizontal steam generators, particularly under natural circulation (decay heat removal) conditions on the primary side. Special emphasis is on the inherent features of horizontal steam generator behaviour. A mathematical model of the horizontal steam generator primary side is developed and qualitative results are obtained analytically. A computer code, called HSG, is developed to solve the model numerically, and its predictions are compared with experimental data. The code is employed to obtain for VVER 440 steam generators quantitative results concerning the dependence of primary-to-secondary heat transfer efficiency on the primary side flow rate, temperature and secondary level. It turns out that the depletion of the secondary inventory leads to an inherent limitation of the decay energy removal in VVER steam generators. The limitation arises as a consequence of the steam generator tube bundle geometry. As an example, it is shown that the grace period associated with pressurizer safety valve opening during a station black-out is hours instead of the 5–6 hours reported in several earlier studies. (However, the change in core heat-up timing is much less—about 1 h at most.) The heat transfer limitation explains the fact that, in the Greifswald VVER 440 station black-out accident in 1975, the steam generators never boiled dry. In addition, the stability of single-phase natural circulation is discussed and insights on the modelling of horizontal steam generators with general-purpose thermal-hydraulic system codes are also presented.     

Heat transfer analysis of the bent cooling channels in SSRF light-blocked components

The front end light-blocked components of the third generation of synchrotron radiation facility,which are subjected to high heat load,are cooled with flowing water through the cooling channels.The convective heat transfer coefficient and the flow resistance(or pressure drop) are two important parameters for evaluating the heat transfer performance of the cooling channels and should be strictly quantified.In this research,two typical bent cooling channels in Shanghai Synchrotron Radiation Facility(SSRF) were modeled and their in-pipe turbulent flows were simulated.The two criteria obtained under different channel water velocities met the SSRF technical requirements.To reduce the total pressure drop,arc transitions were proposed to replace the right angle transitions in the cooling channels.At the same time,an experiment was performed to measure the convective heat transfer coefficient of a typical bent channel unit.The experimental results were in good agreement with the simulation ones.     

Heat transfer performance tests using a half scale horizontal storage module

Spent nuclear fuel generated at nuclear power plants must be safely stored during interim storage periods. A horizontal storage module to safely store the spent nuclear fuel should be able to adequately emit the decay heat from the spent nuclear fuel. The horizontal storage module must ensure that the temperatures of the spent nuclear fuel assemblies are maintained within the allowable values for normal, off-normal, and accident conditions. Therefore, the horizontal storage module must be designed including heat removal capabilities with an appropriate reliability. However, the thermal conductivity of concrete is not good and the allowable temperature of concrete is lower than that of steel. Therefore, heat transfer performance tests were performed to evaluate the heat transfer performance in accordance with the ratio of the outlet to inlet of the air as well as the direction of the inlet and outlet of the air in the horizontal storage module. The influence that the direction of the inlet and outlet of the air reaches to the heat transfer performance was estimated to be minimal. The overall heat removal efficiencies were reduced as the mass flow rate at the outlet was reduced.     

Heat transfer during molten corium-concrete interactions

Severe accidents in light water-cooled nuclear power plants involved heat transfer from molten reactor core materials or “corium” penetrating the reactor pressure vessel and coming to rest upon the containment building concrete floor covered by water. This paper discusses the difficulties of getting good information about the properties of the components and the flow structure during molten corium-concrete-water interactions. Also, potential heat transfer mechanisms are described and available prototypical tests are utilized to show that the enhancement in heat transfer by rising gas bubbles is the most likely mechanism, particularly if heat transfer by iradiation across the gas bubbles is included.     

On the process of rewetting a hot surface by a falling liquid film

A two-dimensional transient heat conduction model for rewetting a hot surface by a falling liquid film predicts that for stainless steel, Inconel or Zircaloy only a wall thickness of some 0.020 in. takes part in the rewetting process in steam at 100–1000 psia. The rewetting rate is nearly independent of heat flux and thermal conductivity, but increases with pressure and decreases with volumetric heat capacity.     

An analytical solution to a two-dimensional model of the rewetting of a hot dry rod

The model considers a hot dry rod of infinite length cooled by a film of liquid moving along its surface. The heat transfer coefficient is assumed to be constant on the wet side and zero on the dry side of the rewetting front, and the liquid film is assumed to move at constant speed. We derive an analytical formula relating the temperature difference in the rod, the temperature at the rewetting front, the wet side heat transfer coefficient, and the rewetting speed. The formula is thought to apply to the rewetting of a fuel rod during emergency cooling by flooding.     

Supercritical flow stability in horizontal channels

An analytical study of supercritical flow instability in two horizontal parallel channels is reported herein—a topic that would be of immense value to new reactor designs that propose to use supercritical light water on the primary side. The finding is that supercritical flow instability in horizontal channels is driven primarily by the state property variation of density with enthalpy; in particular, the second derivative of density with enthalpy is a dominant term for the onset of flow oscillations in horizontal flow.     

抛物体核废料容器自埋过程中传热问题的研究

对装有核废料的抛物体容器在地球引力的作用下，依靠其放出的热量将其周围介质熔化而进行的自埋过程进行了研究，利用边界层理论，对熔化区的液体传热与运动进行了分析，求得容器的自埋速度与表面温度，并对影响它们的诸因素进行了讨论。     

Heat transfer crisis during boiling in simulators of rod assembly cells

Translated from Atomnaya Énergiya, Vol. 69, No. 5, pp. 282–285, November, 1990.     

Heat transfer during boiling of a liquid metal during emergency cooldown of a fast-neutron reactor

The physics of the processes, the characteristics, and the stability of different regimes, of boiling (nucleate, projectile, disperse-ring), which are observed in experiments investigating the boiling of liquid-metal coolant in a model of a fuel assembly for a fast-neutron reactor in the emergency cooldown regime with low circulation velocity, are analyzed. The experimental setup, the, methods for performing measurements, and the experimental data on the boiling of a liquid metal are described. A mathematical model of the process of boiling of a liquid-metal, coolant in a natural-circulation loop is described, and the results of test calculations for regimes with an increase in heating and with sharp pressure drop are prresented. 7 figures, 12 references. State Science Center of the Russian Federration–A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 87, No. 5, pp. 337–342, November, 1999.     

A correlation of rewetting data

Following a loss-of-coolant accident in a water reactor the fuel pins dry out and overheat and it becomes necessary to rewet them to restore normal temperatures. A thermal conduction analysis of rewetting is presented in which it is shown that the heat transfer coefficient associated with rewetting may be taken as an arbitrary function of surface temperature, rather than a constant, without changing the dependency of rewetting velocity on the other variables. An effective heat transfer coefficient then replaces the constant value used in previous expressions for the rewetting velocity. Experiments at atmospheric pressure show that the rewetting rate increases with inlet water subcooling. The available rewetting data at both atmospheric and elevated pressure have been analysed using an existing theoretical model. Taking the effective heat transfer coefficient as proportional to the product of mass flow rate and inlet subcooling a data fit has been achieved to within a factor of two. Expressions are given which predict rewetting rates for a wide range of pressures, wall temperatures, subcoolings, clad materials and geometries.     

Heat transfer in liquid metals

This article is a survey of basic work on heat transfer between solid surfaces and flowing liquefied metal that has been reported by both Soviet and foreign authors. The results of experiments on heat transfer to liquid metals are analyzed for cases where flow took place in both long and short tubes, flat channels, over bundles of rods and over a plate along their longitudinal directions, across cylinders, with free convection and condensation of liquid metal vapors. The effect of additives on the rate of heat transfer is examined. Questions that arise in connection with boiling of liquid metals are discussed, as are problems of wetting effects on fluid friction and on heat transfer rate. Equations for calculation of heat transfer rates are given.