In the context of computational fluid dynamic simulations of boiling flows using time-averaged Eulerian multi-phase approaches, the many sub-models required to describe such a complex phenomena are ...of particular importance. Of interest here, wall boiling requires calculation of the contribution of evaporation to global heat transfer, which in turn relies on determination of the active nucleation site density, bubble departure diameter and frequency of bubble departure. In this paper, an improved mechanistic model for the bubble departure diameter during flow boiling is developed. The model is based on the balance of forces acting on a bubble at a single nucleation site, with a new equation governing bubble growth proposed. The formulation accounts for evaporation of the micro-layer under the bubble, heat transfer from superheated liquid around the bubble surface, and condensation on the bubble cap due to the presence of sub-cooled liquid. Validation of the growth equation is provided through comparison against experiments in both pool boiling and flow boiling conditions. Introduction of condensation on the bubble cap allows reproduction of the growth of the bubble for different sub-cooling temperatures of the surrounding liquid. In addition, a sensitivity study guarantees dependency of the bubble departure diameter on relevant physical quantities such as mass flow rate, heat flux, liquid sub-cooling and pressure, with any inclination of the channel walls correctly accounted for. Predictions of bubble departure diameter and bubble lift-off are validated against three different databases on sub-cooled flow boiling with water and an additional database on saturated boiling with refrigerant R113. The whole data set guarantees validation is performed over a range of parameters and operating conditions as broad as possible. Satisfactory predictive accuracy is obtained in all conditions. The present formulation provides an appropriate starting point for prediction of the behaviour of vapour bubbles under more general conditions which include lift-off after sliding, the frequency of bubble departure, bubble merging and bubble shrinking and collapse due to condensation.
•Pool-boiling mechanism and HTCs for the specific C-shape PRHR HX are investigated.•A revised heat transfer correlation with q″ and H/D for vertical bundle is proposed.•Typical pool-boiling ...correlation is valid to predict the HTCs for lower horizontal bundle.•Forced convective flow boiling correlations are assessed for upper horizontal bundle.
In advanced third generation reactor AP1000, the C-shape Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) is the key equipment in the passive safety system. The mutual influences of the C-shape bundle and the irregular IRWST lead to the complicated heat transfer characteristics. It is still unclear which heat transfer correlations are appropriate for the vertical and horizontal bundle sections, respectively. In the present work, an overall scaled-down IRWST and PRHR HX experimental bench has been built up to simulate the pool boiling performance of the C-shape PRHR HX bundle. The key thermal parameters are measured to evaluate the heat transfer mechanisms and heat transfer coefficients (HTCs) for different bundle sections. The traditional and the revised correlations are further compared with the experimental data of the C-shape PRHR HX bundle. The validated correlations provide applicable and effective references for the reliable operation and system analysis of the passive safety system.
A forced convection boiling system with liquid dielectric coolant is proposed as a new cooling system for SR motors (switched reluctance motors) for electric vehicles. In a previous study, the ...superiority of cooling by liquid dielectric coolant was confirmed compared to conventional cooling by epoxy resin. We have improved the cooling method using this liquid dielectric coolant and devised a method to cool the coils by forced convection boiling of the liquid dielectric coolant inside the motor. The liquid dielectric coolant is divided into a "heating room" that is directly heated by coils and a "cooling room" that is cooled by external cooling water, and the liquid dielectric coolant is circulated in the two rooms. This system is named the "Boiling Immersion Cooling System". However, if boiling cannot be controlled, the coils are exposed from the liquid surface and the cooling capacity is lost. In this experiment, this cooling system was verified by experiment. In addition, to reduce the cost of performance estimation and model improvement, a thermo-fluid analysis method was established to reproduce the boiling phenomenon of liquid dielectric coolant in the simulation. Thermo-fluid analysis and experiments confirmed that thermal runaway due to coil exposure does not occur even when the heat generated by the coil is assumed to be 19 kW at the motor's steady-state output.
Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, ...and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 /spl mu/m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.
Pool and forced-convection boiling of FC-72 was conducted in earth gravity and microgravity. A platinum wire heater was used to generate bubbles and provide simultaneous measurement of the heater ...surface mean temperature. An electrical circuit was built to control the temperature of heater surface. Boiling curves of FC-72 for different flow rates were obtained in the experiment. By using photographic visualization, it was observed that the microgravity environment decreased the critical heat flux (CHF) significantly and force the boiling to switch to transition or film regime. With an increase of the flow rate, the CHF increased and the boiling curves moved upward in both terrestrial gravity and microgravity. In this study, it was also found that the forced convection tends to offset the microgravity effect on CHF when the flow rate was sufficiently high.
In-pile flow-boiling experiments were performed to investigate the possible enhancement of heat transfer by the radiation induced surface activation (RISA) effect. The test section was a 2-mm ...diameter 100-mm long bore in a SUS-316L stainless steel block heated electrically. The test section, housed in an irradiation capsule, was inserted into one of the irradiation holes in the Japan Materials Testing Reactor (JMTR) of the Japan Atomic Energy Agency (JAEA). Quasi-steady state experiments were conducted before irradiation (out-of-pile and in-pile before reactor operation), during irradiation and after irradiation (in-pile), for the same boundary conditions using the same test section block. This approach allowed direct evaluation of the RISA effect through comparison of experimental data. Boiling curves were obtained up to the onset of dryout in an annular dispersed flow, for mass fluxes ranging from 180 to 630kg/(m
2
s) under a fixed pressure of 420 kPa. The critical heat flux obtained during and after irradiation indicated an about 17% increase, on average, from that before irradiation. Meanwhile, the wall superheat at subcritical heat fluxes generally became greater than that before irradiation.
This paper tackles a subject of present interest. It contributes to the promotion of small capacity refrigerating systems working with ammonia – an ecologically and highly efficient refrigerant – as ...an alternative to the traditional commercial refrigeration, heat pumps and air-conditioning systems working with CFC refrigerants.
Since a literature survey showed that available theoretical methods have not been sufficiently experimentally validated, the authors focused on constructing a test facility, namely a test loop, assisted by a data acquisition system. Experiments have been run in order to develop a data bank for the heat transfer of ammonia inside an electrically heated tube. The local heat transfer coefficient for boiling ammonia was measured in 10 different locations along the tube and the flowing regime was visualized in three locations, using segments of glass tube. Sequences of computerized collected data (from pressure transducers, thermocouples, platinum RTD temperature sensors, and an ultrasound flowmeter) were treated with a digital filtration. A comparison between theoretical and experimental results indicated that the Steiner–Taborek and Gungor–Winterton correlations make the best predictions for the local heat transfer coefficient of boiling ammonia.
In addition, the authors experimentally determined the fluid coefficient to be used in Kandlikar's correlation in order to extend its applicability to ammonia boiling.
It is shown the influence that the main parameters have on the boiling process, over a range of the input parameters. Both the developing of the two-phase flow mechanisms and the heat transfer coefficients characterizing ammonia boiling inside tubes, are strongly influenced by those parameters.
The entropy balance for boiling flow Collado, Francisco-Javier
Fusion engineering and design,
10/2001, Volume:
56
Journal Article, Conference Proceeding
Peer reviewed
Subcooled forced convection boiling of water is recognized as one of the best means of accommodating the very high heat fluxes that plasma facing components of fusion reactors have to withstand. The ...boiling curve, giving the wall temperature in function of the applied flux and flow conditions, is essential for the design of such cooling configurations. In this paper, a new entropy balance for subcooled boiling flow, which allows the wall temperature to be obtained, is presented and successfully compared with experimental data from the Joint US–Euratom R&D Program. The derivation of this entropy balance is based on a new strict application of the Reynolds theorem to multiphase flows recently proposed by the author.
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