•In this paper, the onset of nucleate boiling (ONB) of a hypervapotron channel is studied.•The authors evaluate how the system parameters affect the ONB heat flux and analyze the causes ...thermal-hydraulically.•This study evaluates prediction performance of existing ONB correlations under the one-side high heat load condition.•A new ONB correlation of one-side heated hypervapotron channel is developed using Python code.
In present paper, onset of nucleate boiling (ONB) of one-side heated hypervapotron (HV) channel was studied. Through sub-cooled flow boiling experiments under the conditions of a pressure of 1–10 bar, a mass flow rate of 0.071–0.284 kg/s, an inelt fluid temperature of 40–140 °C, and a heat flux of up to 10765 kW/m2, ONB heat flux was experimentally explored. As a result, the HV channel recorded an average 186.91% improvement in ONB heat flux compared to the flat channel due to the vortex secondary flow induced by the fin structure arrangement. In addition, the effect of the system parameters on the ONB heat flux of the HV channel was analyzed. In addition, it was evaluated how well the existing ONB correlations predict the ONB of the HV channel under one-side high heat load conditions. Unfortunately, they tend to under-predict the enhanced ONB of the HV channel because they are correlations developed for the circular smooth channel. Therefore, the authors of this study developed a new correlation using a Python code grafted with machine learning technology.
Nucleate pool boiling in microgravity is influenced by stagnant vapor bubble dynamics, leading to early dryout of the heated surface compared to terrestrial conditions. The Asymmetric Sawtooth and ...Cavity-Enhanced Nucleation-driven Transport (ASCENT) investigation studied an engineered microstructure in a nucleate pool boiling setup aboard the International Space Station (ISS) from November 2022-January 2023. The experiment was housed in the Microgravity Science Glovebox (MSG) and used degassed FC-72 as the test liquid. The engineered microstructure consisted of repeating mm-sized 60°-30° sawtooth structures located within a hermetically sealed square ampoule as the test chamber. Experimental investigations were conducted to explore a range of input heat fluxes, ranging from 0.5 to 2.3 W/cm². The resulting vapor dynamics and heat transfer were compared against a flat baseline surface. Both surfaces contained 250 µm square cavities spaced 1-mm apart. The constrained dimensions of the square chamber severely influenced vapor bubble dynamics across the heated surface. The presence of a liquid layer beneath vapor slugs on the microstructure was observed, in contrast to the baseline surface, where no liquid layer was visually detected. The transient heat transfer coefficient from the microstructure was higher than the baseline surface as the vapor bubbles grew larger at constant heat flux due to liquid film access provided by the asymmetric ridges on the microstructure. A significant increase in the heat transfer coefficient was observed at 1.8 W/cm2, potentially due to vigorous nucleation in the observed liquid pockets along the microstructure, increasing the heat transfer coefficient from 2900 to 6200 W/m2K. Nucleation was observed at heat fluxes as low as 0.5 W/cm2 from the engineered cavities, and a quasi-steady heat transfer coefficient of 2200 W/m2K was obtained at 2.3 W/cm2 from the microstructure surface. The quasi-steady analysis also indicated that both surfaces performed similarly in microgravity and terrestrial gravity in the same experimental setup. The results demonstrate that the liquid film dynamics underneath vapor slugs influence heat transfer to a large extent in microgravity conditions.
The importance of flow boiling and condensing mini/micro-channel devices to a large number of modern applications has spurred an unusually large number of research efforts that culminated in many ...types of predictive tools. These efforts have inadvertently contributed enormous confusion when selecting a suitable predictive method. This study reviews methods for predicting heat transfer in condensing and boiling mini/micro-channel flows. Systematic assessment of predictive accuracy of individual methods requires the development of consolidated mini/micro-channel databases for condensation heat transfer, dryout incipience quality, and saturated boiling heat transfer. These databases cover numerous working fluids and broad ranges of geometrical and flow parameters. It is shown that, despite the success of previous predictive methods for specific fluids and narrow databases, these methods are incapable of providing accurate predictions against entire consolidated databases. The consolidated databases are used to develop 'universal' correlations with very broad application range. These include two separate correlations for condensation heat transfer, one for annular flow and the other for slug and bubbly flows. Also developed are correlations for dryout incipience quality and saturated boiling heat transfer; the later is shown to accurately tackle both nucleate boiling dominated and convective boiling dominated regimes up to the location of incipient dryout.
•This review explores effects of additives and nanofluids on nucleate boiling heat transfer coefficient and critical heat flux (CHF).•Surfactants are shown to enhance nucleate boiling, but ...enhancement potential of polymers is polymer specific.•Nanofluids enhance CHF significantly, but may have negative impact on nucleate boiling heat transfer coefficient.•Serious practical concerns in deploying nanofluids in cooling applications are discussed.
Enhancement of nucleate pool boiling by modifying fluid properties has drawn considerable attention in recent years. This paper provides a comprehensive review of published literature concerning enhancement methodologies of surfactant and polymer additives, and nanofluids. Each method is discussed in detail in terms of measured impact on the nucleate boiling heat transfer coefficient and critical heat flux (CHF), mechanisms proposed for any heat transfer enhancement, and predictive models. It is shown that adding surfactant to base liquid shifts the nucleate boiling region of the boiling curve towards lower surface superheats, thereby promoting earlier boiling incipience and increasing the nucleate boiling heat transfer coefficient, but the heat transfer merits of polymer addition are polymer specific. Despite significant enhancement in CHF with most nanofluids, there are many contradictory findings concerning influence of nanofluids on nucleate boiling heat transfer coefficient. These contradictions are the result of many complex influences of base liquid, nanoparticles, and initial surface roughness. Despite the potential heat transfer benefits of nanofluids, there are several serious practical concerns that must be considered carefully before deploying nanofluids in practical cooling applications.
This study introduces a methodology to compute spray cooling in the nucleate boiling regime. It examines the cooling of a horizontal circular tube, although the approach is not restricted to this ...geometry. While numerous investigations have considered nucleate boiling in the past, these are usually restricted to spray impingement onto a flat plate and not onto more complex surfaces. Furthermore, most prior studies are highly empirical in nature. The present study circumvents both of these restrictions. On the one hand, with more complex surfaces additional factors must be considered, in particular the influence of the local impingement angle on the number flux of impinging drops. On the other hand, the heat transfer for single droplets is first computed using existing theoretical solutions and then the principle of superposition is used to compute the global heat transfer from the surface over which the spray impinges. Accompanying this computation, a drop size distribution is specified in order to also investigate the influence of not only one representative drop size, but also the variance of drop size distribution on the heat transfer, an influence seldom reported on. Furthermore, the probability of drop-drop interaction on the surface has been estimated and the heat transfer correspondingly adjusted. This approach of computing total net heat transfer on a surface is first validated using available measurements in which the heat transfer in the nucleate boiling regime from a flat plate were used for comparison.
The heat transfer from a circular tube was then computed as a function of mass flow rate, drop size distribution (expectation and variance) and wall temperature. Conditions for maximum heat transfer, corresponding to the critical heat flux, were identified. In all cases the net heat transfer was limited by the onset of drop-drop interaction on the substrate, i.e. the net coverage of liquid on the surface. The results of these parametric studies indicate clearly how the mass flow rate can be adjusted to maximize heat transfer while avoiding excess fluid consumption. Furthermore, the Sauter mean diameter is shown to be an appropriate drop size to use, if no further information is available concerning the size distribution.
•A non-empirical methodology to computing heat transfer in the nucleate boiling regime.•Spray cooling computation taking into account explicitly the size distribution of drops.•Insight into spray conditions, which maximizes heat transfer while minimizing liquid consumption.•Consideration of highly varying number density of drops when cooling a surface of complex geometry.
•Microlayer depletion model for simulation of bubble growth during nucleate boiling.•Effect of temperature on mass accommodation coefficient considered for simulation.•Model validated against pool ...boiling measurements in water and ethanol.•Potential to accurately capture the motion of the vapor–liquid interface shown.
In this study, a new microlayer depletion model was developed to simulate bubble growth during the nucleate boiling of water and ethanol. By improving the coupled volume-of-fluid and level set model, the macro-two-phase flow was accurately tracked, eliminating velocity jumps at the interface. The proposed model was validated against pool boiling measurements in water and ethanol at atmospheric pressure. The results demonstrated that the microlayer contributed approximately 15 and 33.17 % to the growth of water and ethanol bubbles, respectively. Under the influence of the interfacial temperature, the mass accommodation coefficient of water could vary from 0.004 to 0.027. Considering the impact of the interfacial temperature on the mass accommodation coefficient could reduce the prediction error of the evaporative thermal resistance from 17.2 to 4.67 %. Furthermore, the study revealed that the evaporative thermal resistance of the water is comparable to the microlayer conductive thermal resistance, whereas the maximum evaporative thermal resistance of ethanol is less than 5 % of the microlayer conductive thermal resistance and can be neglected.
Effects of wettability on saturated pool boiling heat transfer from a smooth superheated substrate, with a finite thickness at constant wall temperatures at the bottom, are investigated based on a ...recently developed liquid–vapor phase-change lattice Boltzmann method. For a hydrophilic surface, it is shown that bubble departure diameter and bubble departure frequency increase with the increase of contact angle and superheat, and the whole bubble departs from the surface with a waiting period in the ebullition cycle. A microlayer of liquid exists between the bubble and the heated hydrophilic surface and this region exhibits a high local heat flux during bubble growth period. On the other hand, a residual bubble will be left on the surface when bubble departs from a hydrophobic surface and hence no waiting period exists. No microlayer exists on the heated hydrophobic surface, and the three-phase contact line region has the highest local heat flux and lowest local temperature. Time histories of the wall temperature on the top of a hydrophilic substrate and a hydrophobic substrate as well as the associated heat flux are studied in details during the nucleate boiling process. Effects of wettability, superheat and heater size on number of nucleation sites in saturated pool boiling are investigated. Saturated boiling curves (from onset of nucleate boiling to critical heat flux, to transition boiling to stable film boiling) for hydrophilic and hydrophobic heating surfaces are obtained by numerical simulation for the first time.
•An adaptive second order SMC of a PWR with Xenon oscillation suppression.•Adaptive twisting and twisting algorithm comparison.•We showed a dynamic gain of the controller with unbounded ...disturbances.•We showed that the knowledge of upper bound of disturbances and nature of uncertainty are not required.•Comparison of controllers using IAE, ITAE, ISE, ITSE indices.
This study proposes an adaptive second-order sliding mode control based on a twisting algorithm to control nuclear reactor core power during load-following power maneuvers. The control system was designed based on the concept of an extended equivalent control. The control technique does not require knowledge of the uncertainty or upper bound of the disturbance in the system. Additionally, the gain of the control system is a dynamic gain that increases or decreases according to the system requirements within a specified period. Consequently, chattering was attenuated. Chattering excites high-frequency dynamics and causes wear out of the control mechanism, making chattering a severe challenge in sliding-mode control. A nuclear reactor is a time-varying, complex, nonlinear, and constrained system. The characteristics of a nuclear reactor are a function of its operating power level, fuel burnup, and aging. In addition, the load-following mode of operation causes Xenon oscillation, which can further cause instability in the core. Therefore, the non-base load operation of the system, including load following, further aggravates uncertainty and disturbances in the system. To this end, an adaptive second-order sliding mode control that does not require knowledge of the uncertainty or the upper bound of the disturbance in the system was designed. The reactor core was modelled using an experimentally verified and validated multi-point kinetic model with four nodes. Simulation experiments were conducted using a multi-point kinetics model and an adaptive second-order sliding mode control. The results of the simulation experiments indicated that reactor core integrity is guaranteed, and the core is protected against peak power densities, such as linear heat generation rates (LHGRs) and lower departure from nucleate boiling ratios (DNBR). Moreover, the control system achieved the load-following objective and suppressed Xenon oscillation. The performance of the proposed control system was further compared with that of a twisting control system and classical proportional integral derivative control system to validate the effectiveness and reliability of the adaptive twisting second-order sliding mode control system.
•Experimental investigation of the effect of surface wettability and microporous coating on nucleate boiling performance.•Pool boiling with water and surface wettability ranging between ...superhydrophilic to hydrophobic.•Higher CHF but lower h due to enhanced wettability with oxidized Cu-HTCMC surface over a pristine microporous surface.•Improved boiling heat transfer and delayed dryout by expanding the vapor film over an entire extended hydrophobic surface.
Microporous coated surfaces significantly enhance nucleate boiling heat transfer performance over plain surfaces. Pool boiling experiments are performed on a copper high-temperature, thermally-conductive, microporous coating (Cu-HTCMC) with altered surface wettability. The combined effect of surface wettability and microporous coating on nucleate boiling performance is experimentally investigated on copper surfaces. Specifically, the surface condition is pristine, oxidized, and Teflon-coated, yielding a surface wettability ranging between superhydrophilic (∼0° apparent contact angle) to hydrophobic (131° apparent contact angle). The experimental results show that the hydrophobic microporous surface promotes early bubble incipience, yielding a high heat transfer coefficient at low heat flux. However, with enhanced wettability, the hydrophilic microporous surfaces result in high critical heat flux. The enhanced wettability of the oxidized Cu-HTCMC surface offers a 19% increase in CHF (2350 kW/m2) over the pristine microporous surface, but the corresponding heat transfer coefficient for the pristine Cu-HTCMC surface (466 kW/m2K) is 37% higher than the oxidized. The trends are found to be consistent with plain copper surfaces, where the enhanced wettability (oxidized plain surface) offers a 57% increase in CHF over the pristine plain surface but h for the pristine is 9% higher at CHF.
Direct numerical simulation of nucleate boiling in micro-layer regime Urbano, A.; Tanguy, S.; Huber, G. ...
International Journal of Heat and Mass Transfer/International journal of heat and mass transfer,
August 2018, 2018-08-00, 20180801, 2018-08, Volume:
123
Journal Article
Peer reviewed
Open access
•First DNS of nucleate pool boiling in micro-layer regime (fully resolved).•Parametric study on the transition between micro-layer and contact line regimes.•A micro-layer is formed if the growth of ...the bubble exceeds a limit velocity.•The depletion is mainly induced by capillary actions.•Correlation on the limit between micro-layer and contact line regimes.
The physical mechanisms associated with the evolution of a micro-layer beneath a bubble and the transition between contact line and micro-layer regimes are investigated with fully resolved numerical simulations, in the framework of nucleate pool boiling. Capturing the transition between these two regimes has been possible for the first time using very refined grids and parallel computations. Indeed, grids with a cell size under 1 μm must be used in order to capture thermal and dynamical effects in the micro-layer. Such multiscale computations require advanced code capabilities. The present simulations are used to analyse the physical processes involved in the formation and depletion of a micro-layer. A parametric study is carried out to investigate the impact of the main parameters affecting the presence of the micro-layer. From these results, the limit conditions between nucleate boiling in micro-layer and contact line regimes are deduced. Neglecting the micro-layer would lead to erroneous results because it has a strong influence on the overall bubble growth. Therefore the present results could be of major interest for designing models of nucleate pool boiling in larger scales computations, when the micro-layer cannot be resolved.
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