Sodium-Potassium Alloy Heat Pipe under Geyser Boiling Experimental Study: Heat Transfer Analysis

In the geyser boiling mode, the working fluid state is divided into a boiling process and a quiet process, and the sodium-potassium (Na-K) alloy heat pipe can discontinuously transfer heat at each boiling. The overheating of the liquid working fluid at the bottom causes short-term boiling and forms slug bubble, the strong condensing ability quickly conducts heat from the evaporator section. And geyser boiling can occur before the working fluid forms continuous flow, so it transfers more heat at lower temperatures than natural convection cooling. In this study, the heat transfer process of a Na-K alloy heat pipe with forced convection cooling under different heating power was experimental studied. The geyser boiling mode can make the Na-K alloy heat pipe work below 650 °C and reduce the start-up time. In the process of geyser boiling, the heat transfer quantity was increased by the boiling frequency and the amount of vapor produced in a single boiling. The boiling temperature had no obvious change with the increased of heating power, and the condenser section temperature increased with the heating power.

Heat dissipation performance of grooved- and copper foam -type vapor chambers

Both grooved-type and copper foam-type vapor chambers are explored and investigated. The overall performance of vapor chamber depends on both axial and spread thermal resistance mutually. The copper foam-type vapor chambers achieved the lowest axial thermal resistance less than 0.2 K/W. The grooved-type sample presented the lowest spread thermal resistance, indicating better temperature uniformity. The visual experiment demonstrated that the evaporating surface of the copper-foam vapor chamber was dominated by the thin film evaporation mechanism at low charging ratio while dominated by the pulsed pool boiling mechanism at large charging ratio. For the grooved-type vapor chamber, the pool boiling mode was observed at any charging ratio.

Characteristics of Pressure Buildup From Local Fuel-Coolant Interactions in a Simulated Molten Fuel Pool

Studies on local fuel-coolant interactions (FCI) in a molten pool are important for severe accident analyses of sodium-cooled fast reactors (SFRs). To clarify the mechanisms underlying this interaction, in this study a series of experiments was conducted by delivering a given quantity of water into a simulated molten fuel pool (formed with a low-melting-point alloy). Based on the experimental data obtained from a variety of conditions, including difference in water volume, melt temperature and water subcooling, the characteristics of pressure-buildup during local FCIs was investigated. It is found that under our experimental conditions the water volume and melt temperature have remarkable impact on the interaction, while the role of water subcooling seems to be less prominent. The performed analyses also suggest that the pressurization from local FCIs should be intrinsically limited, due to a suppressing role caused by the increasing of coolant volume entrapped within the pool as well as the transition of boiling mode. Current work, which gives a palette of favorable data for a better understanding and an improved estimation of severe accidents in SFRs, is expected to benefit future analyses and verifications of computer models developed in advanced fast reactor safety analysis codes.

Recent Work on Boiling and Condensation in Microchannels

Recent work on boiling of water and condensation of steam in single and parallel microchannels is reviewed in this paper. It is found that the amplitude and frequency of fluctuations of temperature and pressure during the unstable flow-boiling mode depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel or the installation of a throttling valve upstream of the test section, reversed flow of vapor bubbles can be suppressed resulting in a stable flow-boiling mode. Boiling heat transfer coefficient and pressure drop in microchannels under stable flow-boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow-boiling in macrochannels. Microbubble emission boiling phenomena, which can defer the arrival of critical heat flux, exist in a partially heated Pyrex glass microchannel at sufficiently high heat flux and high inlet subcooling conditions. For condensation in a microchannel, transition from annular flow to slug/bubbly flow is investigated. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate, and microchannel diameter tend to enhance the instability of the condensate film on the wall, resulting in the occurrence of injection flow further downstream at increasingly high frequency. The pressure drop in the condensing flow increases with the increase in mass flux and quality or with decreasing microchannel diameter. The existing correlations for pressure drop and heat transfer of condensing flow in macrochannels overestimate the experimental data in microchannels.

Boiling and Condensation in Microchannels

The characteristics of boiling of water and condensation of steam in a microchannel under heating or cooling conditions are discussed in this paper. Stable and unstable boiling modes occur in a microchannel, depending on the nucleated bubble size in the microchannel. Stable boiling mode with constant temperature variations exists when the size of the nucleated bubble is less than that of the microchannel diameter, while unstable boiling mode exists when the size of the bubble is greater than the microchannel diameter. The latter is owing to the fact that when a bubble grows to the size of the microchannel, it will expand in both upstream and downstream directions. Subsequently, the reversed flow of vapor bubble is swept downstream by the incoming subcooled liquid, leading to large cyclic fluctuations of temperature and pressure. The amplitude and frequency of these fluctuations depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel, the reversed flow of vapor can be suppressed, resulting in a stable flow boiling mode. Boiling heat transfer coefficient and pressure drop in a microchannel under stable flow boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow boiling in macrochannels. For condensation in a microchannel, mist flow, annular flow, injection flow, slug/bubbly flow exist depending on mass flux, condensation heat flux, and the location in the microchannel. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface because the surface tension effect is predominant in microchannels. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate and the microchannel diameter tend to enhance instability of the condensate film on the wall, resulting in occurrence of the injection flow further toward the outlet with an increase in occurrence frequency. At low mass fluxes, the pressure drop obtained for condensation in microchannels is substantially different from the correlation equations for macrochannels because of different flow patterns.

Pool Boiling of FC-72 and HFE-7100

This work reported the boiling characteristics of FC-72 and HFE-7100 at atmospheric pressure and at a liquid subcooling of 0–20 K. The FC-72 exhibits a more efficient nucleate boiling mode and a higher critical heat flux (CHF) than the HFE-7100. For film boiling mode, HFE-7100 becomes more efficient.

Relative Stability Between Nucleate and Film Boiling on a Nonuniformly Heated Plate Surface

Flow boiling of methanol over a nonuniform, indirect conduction heating surface is investigated experimentally. An axial (discrete) heat flux distribution corresponding to a neutral stability region where nucleate and film boiling can coexist steadily is identified. Below such a heat flux distribution, nucleate boiling mode is more stable. Above this distribution, film boiling mode becomes more stable. We had employed an equal-area criterion for interpreting the heat flux and wall superheat relationship. Analogy between the wire boiling system is proposed. The differences between average and real transition boiling curves are discussed as well.

Studies of Power Station Feed Pump Loss of Suction Pressure Incidents

Open cycle feed systems featuring a deaerator followed by the boiler feed pumps have been subject to unexpected loss in net positive suction head during transient operation. This has been traced to separation at the deaerator storage tank drain and the formation of a steam/water interface in the downcomer to the pumps. The outflow at separation when the storage tank water is subcooled is generally much greater than the maximum suction flow corresponding to the unit rating, but when the tank water boils, as during transients, it may be very much less. Thus with a high suction flow in the boiling mode, the new interface may descend in the downcomer to result in feed pump cavitation, pressure imbalance, vapor locking, severe vibration, damage to the pump and an inability to maintain forward flow. Model tests have been conducted to establish the criterion for the separation in terms of the critical outflow for specific operating conditions as a function of geometric ratios involving tank-water depth and tank/downcomer dimensions. The data obtained although scattered are in general agreement with the data from actual plant and may be used to assess safe operating parameters.