Critical Heat Flux In Pool Research Articles

Coalescence‐Induced Jumping Bubbles during Pool Boiling

AbstractSurfaces with micro/nano‐structures significantly enhance the critical heat flux of nucleate boiling by increasing the wickability of the liquid phase to delay film boiling. An alternative strategy, of removing surface bubbles at smaller sizes, is largely ignored. Here, they fabricate a rationally designed micro‐structured surface that enables the coalescence‐induced departure of microscopic vapor bubbles during the pool boiling of water at diameters an order of magnitude smaller than single‐bubble buoyant departure. Arrays of micro‐cavities or micro‐grooves serve as tailored nucleation sites to nucleate close‐packed vapor bubbles, which coalesce at unusually small sizes compared to a homogeneous surface. Two different modes of coalescence‐induced bubble departure are observed and modeled: capillary‐inertial jumping for smaller micro‐bubbles and buoyant‐inertial liftoff for larger micro‐bubbles. Capillarity‐enhanced pool boiling on micro‐structured surfaces has the potential to dramatically enhance the heat transfer coefficient and critical heat flux of pool boiling.

A mechanistic model of critical heat flux for pool boiling based on supply failure mechanisms depending on the contact angle

Experimental and theoretical analyses are conducted to develop a mechanistic model of critical heat flux (CHF) for application to pool boiling of fluids with contact angles between 0° and 180°. To accurately predict the CHF over the entire range of the contact angle, it is postulated that an unquenchable dry patch occurs when liquid fails to be supplied to the dry patch; A supply failure mechanism is classified into two different mechanisms depending on the wettability of the boiling surface. The first is an evaporation mechanism which is dominant for non-wetting surfaces. This type occurs when the liquid supply rate in the liquid film due to the capillary force becomes lower than the evaporation rate. The second mechanism is a levitation mechanism which is dominant for wetting surfaces. This type occurs when the surface temperature under the dry patch exceeds the Leidenfrost point. To check the validity of the postulate, experiments involving the pool boiling of water and FC-72 are conducted on a titanium surface, which is a non-wetting surface for water but a wetting surface for FC-72. In the experiments, the temperature and heat flux distributions on the boiling surface are measured using infrared thermography for a quantitative characterization of boiling at the CHF. Experimental analyses confirm the postulate by showing that the CHF occurs due to the evaporation mechanism for the pool boiling of water, whereas the levitation mechanism for the pool boiling of FC-72. It is also observed that a large area of liquid film exists for the pool boiling of water, whereas this liquid film hardly exists for the pool boiling of FC-72. This observation indicates that the change in the dominant mechanism is due to the dependence of the liquid film area on the wettability of the boiling surface. After validating the postulate, a CHF correlation is developed that reflects the change of the supply failure mechanism depending on the contact angle. The developed correlation shows a good match to the extensive sets of CHF data previously obtained by various investigators to within 20%.

Investigation of critical heat flux enhancement on nanoengineered surfaces in pressurized subcooled flow boiling using infrared thermometry

Enhancing the flow boiling critical heat flux (CHF) is beneficial to the economics and safety margins of many industrial applications cooled by boiling heat transfer. While many studies have shown that surfaces with hydrophilic nanoscale and micro-scale features can enhance CHF in pool boiling, it is still not clear how these engineered surfaces affect the CHF in subcooled flow boiling at ambient pressure, let alone high-pressure conditions. Here, two nano-engineered surfaces, i.e., a surface coated with a porous layer of hydrophilic silica nanoparticles and a surface coated with zinc oxide nanowires, were tested. Flow boiling tests with a 10 K subcooling and a mass flux of 1000 kg/(m2·s) were conducted at 1 bar and 4 bars using infrared thermometry diagnostics. At 1 bar, the CHF enhancement is around 15% for both coatings. At 4 bars, the CHF enhancement is around 17% for the nanowire surface, and around 25% for the nano-porous surface. Infrared thermometry measurements reveal that the CHF enhancement comes from an increase of both two-phase heat transfer and single-phase heat transfer mechanisms, which is due to a change of bubble dynamics on the nanoengineered surfaces. It is also shown that the boiling crisis can be predicted using a percolation model based on Monte Carlo (MC) simulations.

Electrowetting modulated enhancement of critical heat flux in pool boiling

Critical heat flux (CHF) represents the upper limit for nucleate boiling, beyond which heat transfer deteriorates due to the boiling regime transition to film boiling. This paper reports a new strategy for CHF enhancement by employing electrowetting (EW) to tailor the far-field and near-field effects of CHF. With electrowetting, the instability of liquid-vapor interface is amplified due to the reduction in the hydrodynamic wavelength, apparent contact angle is reduced and the bubble's lateral expansion over the heating surface is refrained by a resistive electric force, all contributing to CHF improvement. A comprehensive theoretical model is developed to understand the effects of the electric field on various aspects of CHF. Experiments are conducted to demonstrate the feasibility of this CHF enhancement strategy and to validate the theoretical model.

Effects of super-hydrophilicity and orientation of heater surface on bubble behavior and the critical heat flux in pool boiling

This study investigated the boiling heat transfer characteristics of a super-hydrophilic surface with de-ionized water at different orientations. Pool boiling experiments were conducted at inclination angles of 45°, 90°, and 135° by rotating the experimental apparatus. The experimental results were analyzed to investigate the influence of inclination angle and hydrophilicity of the surface on the boiling heat transfer characteristics. Parameters such as the maximum and minimum vapor thicknesses, bubble wavelength, bubble velocity, BHTC (Boiling Heat Transfer Coefficient) and CHF (Critical Heat Flux) were measured during the experiment. An enhancement in the values of the parameters of the bubble dynamics was observed during the boiling experiment using the super-hydrophilic PCB (Printed Circuit Board) heater, indicating improved heat transfer during the experiment. The CHF of the super-hydrophilic PCB heater was enhanced by a maximum of 28% at an inclination angle of 45°; the enhancement reduced as the inclination angle increased.

Enhanced critical heat flux in pool boiling applying the method of different-mode-interacting boiling for water

Boiling heat transfer was explored with different-mode-interacting boiling (DMIB) to improve critical heat flux (CHF) in a water pool. Previous research show that nonuniform heating in a geometric copper plate arrangement with different thermal conductance materials achieves remarkably high heat transfer in narrow gaps. As DMIB does not have any structure on the heating surface, it is also characterized by its resistance to surface deterioration. In this study, DMIB is adopted to improve CHF in pool boiling for water. The effects of the pitch P and ratio RL between different materials, and pattern of material arrangements, cross and parallel, inside the heating plate are examined. An enhancement in heat transfer performance is shown for nonuniform heating plates compared with a conventional uniform plate. The CHFs present increases with decreasing P and RL for both material arrangements. Furthermore, the heat transfer performances for the nonuniform plates with a cross material arrangement are higher than for a parallel material arrangement. The maximum CHF and ratio of CHF enhancements reach 1.64 MW/m2 and 56%, respectively.

Enhanced critical heat flux of pool boiling by vapor management

The critical heat flux (CHF) is an important parameter in pool boiling, establishing the limit in heat flux and temperature in most applications. A higher CHF offers the system a wider operational range or an additional safety factor under normal conditions. In this work, a new vapor management approach is adopted by using a chimney-like vapor shroud during the pool boiling process. The vapor shroud footprint is equal to the area of the heated surface, and it is placed directly above the heated surface with a specified gap between its lower edge and the surface. The CHF increases for most vapor shroud arrangements (shroud length and gap), with a maximum increase of 83% compared to the baseline. The CHF shows a monotonic increase with increasing shroud length and passes through a maximum (unimodal) as the gap is changed. The maximum CHF occurs for gaps ranging from 1.8 mm to 3.1 mm, corresponding to a Bond number of approximately one, based on gap size. The increase in CHF is believed to be caused by improved liquid feeding and vapor release dynamics caused by the shroud. Analysis shows that the hydrostatic pressure and the interfacial stability are important to the nonlinearity in CHF behavior. A simple model based on scale analysis has been provided to predict the shroud effects on the CHF.

Accurate prediction of the critical heat flux for pool boiling on the heater substrate

While the influence of liquid qualities, surface morphology, and operating circumstances on critical heat flux (CHF) in pool boiling has been extensively studied, the effect of the heater substrate has not. Based on the force balance analysis, a theoretical model has been developed to accurately predict the CHF in pool boiling on a heater substrate. An analytical expression for the CHF of a heater substrate is obtained in terms of the surface thermophysical property. It is indicated that the ratio of thermal conductivity (k) to the product of density (ρ) and specific heat (c p) is an essential substrate property that influences the CHF. By modifying the well-known force-balance-based CHF model (Kandlikar model), the thermal characteristics of the substrate are taken into consideration. The bias of predicted CHF values are within 5% compared with the experimental results.

Effect of Micro Honeycomb Porous Plate on the Critical Heat Flux in Saturated Pool Boiling

Enhancement of the critical heat flux in pool boiling by the attachment of a micro HPP on a heated surface is investigated experimentally using water under saturated boiling conditions. HPP enhance the CHF from the capillary supply of liquid onto the heated surface through the pores and the release of vapor generated through the channels. When the height of the micro HPP on the heated surface is 1.4 mm, the CHF increases to 2.5 MW/m2. While the CHF began to decrease when the thickness was more than1.4 mm. In regions of thin HPP thickness, the CHF can be explained by the Haramura-Katto model. Therefore, in this presentation we examined the CHF mechanism in the thicker HPP region using the capillary limit model.

Relationship between heat flux at minimum evaporation time of droplet and critical heat flux for pool boiling (Extension to Quenching temperature of high temperature surface with droplets)

Relationship between heat flux at minimum evaporation time of droplet and critical heat flux for pool boiling (Extension to Quenching temperature of high temperature surface with droplets)

Effect of Nanofluids on the Enhancement of Boiling Heat Transfer: A Review

Boiling heat transfer can play a vital role in the two-phase flow applications. The analysis of the boiling hat transfer enhancement is of importance in such applications and the enhancement can be mostly conducted by using various active and passive techniques. One type of passive techniques is the enhancement of heat transfer by nanofluids. This article presents an extensive review on the effect of different nanofluids on the enhancement of heat transfer coefficient (HTC) and critical heat flux (CHF) for both pool as well as flow boiling. Nanoparticles addition to a working fluid is done arbitrarily to improve the thermophysical properties which in turn improves heat transfer rate. Numerous works have been done in the studies on nanofluid boiling. Among various nanoparticles, the most frequently used nanoparticles are Al2O3 and TiO2. In the case of binary nanoparticles, the most commonly used combination is Al2O3 and TiO2. After reviewing the relevant literatures, it is found that for pool boiling, the maximum HTC is increased to 138% for TiO2 nanoparticles and the maximum CHF is increased to 274.2% for MWCNTs. Conversely, in flow boiling the maximum HTC is increased to 126% for ZnO nanoparticles and the maximum CHF increased to as 100% for GO nanoparticles. In addition, when two or more nanoparticles in succession or binary nanofluids are used the CHF in pool boiling increased up to 100% for Al2O3 and TiO2 as well as the CHF in flow boiling increased up to 100% for Al2O3, ZnO, and Diamond. Though the information of the coefficient of heat transfer and the critical heat flux varied for different nanofluids and vary from experiment to experiment for each of the nanofluids. This variation happens because the coefficient of heat transfer and the critical heat flux in boiling is dependent upon several factors.

Relationship between heat flux at minimum evaporation time of droplet and critical heat flux for pool boiling

Relationship between heat flux at minimum evaporation time of droplet and critical heat flux for pool boiling

Understanding triggering mechanisms for critical heat flux in pool boiling based on direct numerical simulations

Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulations of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs.

Enhancement of Boiling Heat Transfer Performance Using Nano Coating - A Review

In various engineering as well as non-engineering i.e. in our daily lives boiling phenomenon is using. Boiling heat transfer is a systematic and essential mechanism for transfer of thermal energy form one surface to the relatively lower temperature surface. The phenomenon of boiling heat transfer is being applied in many industrial fields to require for maximization of the critical heat flux. Objective of this present paper is to present the effect of surface coating on heat transfer coefficient and critical heat flux in pool and flow boiling phenomenon. Present paper includes summary of existing work on heat transfer enhancement techniques using nano coating. Effect of surface wettability and nanoparticle deposition has been discussed.

Influence of surface wettability increase induced by Gamma-ray irradiation on critical heat flux in downward-facing flow boiling

To increase the surface wettability is a feasible way to enhance critical heat flux (CHF) according to some CHF theories and many previous researches. It is found that surface wettability of metal can be increased after the surface being irradiated by Gamma-ray irradiation and CHF in pool boiling can thus be enhanced. In this study, whether CHF in downward-facing flow boiling can be enhanced after the surface being irradiated by Gamma-ray is experimentally investigated. Copper surface and metal porous honeycomb plate are irradiated by Gamma-ray and then used in the downward-facing flow boiling experiments. However, the experiment results show that downward-facing flow boiling CHF is not enhanced even though the surface wettability of bare surface and metal porous honeycomb plate increase a lot. The reason is analyzed considering the characteristic of downward-facing flow boiling.

Investigation to Improve the Pool Boiling Heat Transfer Characteristics Using Laser-Textured Copper-Grooved Surfaces

Improving the performance of pool boiling with critical heat flux of pool boiling and enhancing the coefficient of heat transfer through surface modification technique have gained a lot of attention. These surface modifications can be done at different scales using various techniques. However, along with the performance improvement, the durability and stability of the surface modification are very crucial. Laser machining is an attractive option in this aspect and is gaining a lot of attention. In the present experimentation research work, pool boiling attributed performance of copper-grooved surfaces obtained through picosecond laser machining method is investigated. The performance of the modified surfaces was compared with the plain surface serving as reference. In this, three square grooved patterns with the same pitch (100 μm) and width (100 μm) but different depths (30, 70, and 100 μm) were investigated. Different depths were obtained by varying the scanning speed of the laser machine. In addition to the microchannel effect, the grain structuring during the laser machining process creates additional nucleation sites which has proven its effectiveness in improving the pool boiling performance. In all aspects, the pool boiling performance of the grooved laser-textured surface has showed increased surface characterisation as compared with the surface of copper.

Local measurements on vertical subcooled flow boiling of refrigerant Novec 649

Local measurements from optical fiber micro probes are presented on vertical subcooled flow boiling of refrigerant Novec 649. Experiments were conducted in a rectangular flow channel with a cross section of 40 mm by 40 mm at a mass flux density of 1000kgm−2s−1 and a subcooling of 13 K. Boiling regimes ranged from single-phase convection to fully developed film boiling. Detailed description of the measurement methods and in particular of the fiber optical micro probes is given. Average boiling curves are provided for three measurement positions along the direction of flow. Void profiles above the copper heater surface are presented for void fraction, void ligament velocity, void ligament size and detection frequency. Time-resolved data of the void phase at critical heat flux was obtained at a distance of 100 µm away from the heater surface. The dataset is discussed with respect to a previously developed conceptualization for the critical heat flux process in flow boiling and the recent consensus on dry spot formation and re-wettability being the main cause of critical heat flux in pool boiling. A phenomenological extension to flow boiling is presented.

Evaluation of critical heat flux of ATF candidate coating materials in pool boiling

The critical heat flux (CHF) of potential accident tolerant fuel (ATF) cladding materials and concepts, Cr and FeCrAl coatings on zirconium-alloy substrates, was investigated experimentally. A cold spray process, which involves the propulsion of high velocity powder particles on to the substrate surface to form a coating upon impact, was used to create the coatings. Surface parameters such as contact angle, surface roughness, and morphology that are associated with CHF in pool boiling were characterized. Non-hydrodynamic effects of newly developed ATF candidate coating materials on CHF were also evaluated. The effects of thermal properties and coating thickness and expected challenges for evaluation of CHF for new ATF cladding materials development are discussed.

Enhancement of nanofluid stability and critical heat flux in pool boiling with nanocellulose

A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing critical heat flux (CHF); however, its instability over a long period of time due to sedimentation and aggregation has impeded its successful application in industry. In this study, lightweight negatively charged TEMPO-oxidized cellulose nanofibers (CNFs) were utilized as a nano-sized substance in water and examined to enhance both the CHF performance and thermal stability of nanofluids. Owing to low density of the CNFs and long range repulsion between negatively charged CNFs, there were no aggregation and sedimentation of CNFs with multiple boiling/cooling cycles. In addition, with CNF concentrations of 0.01, 0.03, 0.05, and 0.10 wt%, CHF enhancement increases of 40.7%, 45.1%, 54.9%, and 69.4%, respectively, were achieved over that of pure water. The present results demonstrated the great potential of CNFs as eco-friendly and cost-effective nano-substances that can overcome the instability of nanofluids.

Boiling heat transfer of nanofluids: A review of recent studies

Adding solid particles of nanometer scale to fluids is one of the most important passive methods of enhancing heat transfer performance. However, this gives numerous chances to investigate new frontiers, but also raises remarkable difficulties. Nanofluids act as suspension that can be obtained by dispersing nanometer-sized nanoparticles (1-100nm) in host fluids with the aim of enhancing thermal properties. This paper is a review of recent studies on boiling heat transfer of nanofluids for pool and convective flow boiling of nanofluids. The research results, collected since 2012 to present of the recent survey are reviewed and briefly outlined. An emphasis is put on the enhancement and the deterioration of the boiling heat transfer coefficient and critical heat flux of pool and convective flow boiling of nanofluids. Other important parameters affecting the boiling of nanofluids are identified and discussed in this review. While preparing future studies is greatly encouraged in order make this phenomenon well understood.