Boiling Heat Transfer Behavior Research Articles

Experimental study on axial self-propagation of heat transfer deterioration of near-critical carbon dioxide at low mass fluxes

Near-critical CO2 exhibits unique boiling heat transfer behavior due to its high saturated vapor density, low latent heat, and extremely low surface tension. In this study, an experimental investigation was conducted to study the heat transfer deterioration of near-critical CO2 inside a vertical circular channel at low mass fluxes. The experimental results show a rapid rise in wall temperature at the outlet side with minimal power increase (∆ power = 33 W), and the wall temperature rise continuously self-propagates toward the inlet side. The heat transfer deterioration mechanism responsible for the wall temperature rise was considered to be a departure from nucleate boiling type boiling crisis based on the low vapor quality of the bulk fluid. In addition, the evolution of the axial heat transfer deterioration was analyzed in detail. And a phenomenological model was developed to explain the axial self-propagation of heat transfer deterioration based on the low mass fluxes and unique bubble behavior resulting from the low surface tension, high vapor density and low latent heat of the near-critical CO2. The model was validated using different dry/wet interface movement speeds.

Investigation on the two-phase flow and heat transfer behaviors in a new central uniform dispersion-type heat exchanger

In order to improve the temperature uniformity of multiple chips, a new central uniform distributed heat exchanger (CUD) is designed. Firstly, adopting ethanol solution as coolant, the cooling ability of the heat exchanger is numerically investigated by using VOF gas-liquid two-phase flow model, and the two-phase flow experimental system is established to verify the correctness of the numerical results. The results show that the Tavg (average temperature of heat sources) error range between the experimental and numerical values is 1 K ~ 3 K, and the relative error of SDT (standard deviation of temperature) is 6% ~ 10%. This verifies the accuracy of the numerical simulation method. Secondly, the two-phase flow boiling heat transfer behavior are compared with that of single-phase flow with water as coolant. The results show that with the increase of inlet flow, the thermal resistance and average temperature of two-phase flow change are less than that of single-phase flow. This indicating that two-phase flow has better advantages in improving the temperature uniformity of multiple heat sources. Thirdly, the heat transfer characteristics of radiators with different Wi are studied. Compared with Case1, the boiling heat transfer coefficient of Case3 and Case4 can be increased by 12.3% and 18.3%, respectively. However, Case3 is a better choice by weighing the pressure drop. Finally, the prediction functions of the boiling heat transfer coefficient and pressure drop is established, and the multi-objective genetic algorithm is used to obtain the best parameters of this heat exchanger. The average heat transfer coefficient of the optimized heat exchanger can be increased by 7.56%, and the pressure drop can be reduced by 3.54% compared to the exchanger before optimization. Furthermore, the heat dissipation ability of the optimized heat exchanger design is studied numerically. The simulated results are in good agreement with the optimization results.

Nanofluid application for heat transfer, safety, and natural circulation enhancement in the NuScale nuclear reactor as a small modular reactor using computational fluid dynamic (CFD) modeling via neutronic and thermal-hydraulics coupling

In recent years, “Nanofluid” and “Energy-Efficient Cooling” have been achieved growing attention for use in new nuclear reactor technology. Also, the assurance of reactor safety has become of top priority in nuclear energy development because of the significant role of nuclear power in providing worldwide electricity. The use of nanofluid coolant is an effective way to enhance safety margins and heat transfer performance. Studies also prove that nanofluids are promising cooling fluids for applying as a future coolant in nuclear reactors. The current paper investigates the nanofluid effects as a coolant on the velocity of bulk, pressure drop, heat transfer coefficient, power peaking factors, and minimum departure from nucleate boiling ratio in a NuScale reactor, a natural circulation pressurized water reactor (PWR). It also analyzes the thermal characteristics of Al2O3 nanofluid with specific attention to their boiling heat transfer behavior. The study first presents the NuScale designed core with the use of water and different concentrations of Al2O3 nanofluid (0.001–10% volume fractions and 10–90 nm particle sizes). Then it shows the simulated equivalent cell with surrounding coolant utilizing computational fluid dynamic code (CFD). The study also describes the outcomes of replacing water with nanofluid coolant on natural circulation parameters, heat transfer coefficient, and minimum departure from nucleate boiling ratio with changes in nanoparticle concentrations and sizes. The results demonstrate the potential of this innovative coolant to enhance the minimum departure from nucleate boiling ratio and heat transfer coefficient. Overall, the presence of Alumina nanoparticles in water improves thermal performance and safety. The investigation also shows that the nanoparticles do not change the power peaking factor and neutronic performance significantly.

Experimental investigation of boiling regimes in a capillary-fed two-layer evaporator wick

Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 µm), array of liquid-feeding posts, and cap wick layer (800 µm) using 90–106 µm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 × 5 and 10 × 10 arrays of liquid feeding posts are characterized, along with a 200 µm-thick single-layer evaporator wick. The 10 × 10 array provides a >400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (∼0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here.

Experimental investigations on the boiling heat transfer of horizontal flow in the near-critical region

The critical point is the end point of a phase equilibrium curve; liquid and its vapor can coexist under designated points. Close to the critical point, thermophysical properties present clear variations, especially in the region of 0.85Pcr∼Pcr. Latent heat and liquid density in this region decrease more quickly than in lower-pressure areas, resulting in unique boiling heat transfer behavior. This region is also called the near-critical region. However, only a few scholars have discussed the heat transfer phenomenon; thus, it is difficult to ascertain the near-critical region’s properties and characteristics from extant literature. In the present study, we conduct experimental investigations to explore the specificities of the heat transfer characteristics of carbon dioxide in horizontal flow within the near-critical region in a circular channel with a diameter of 4 mm. The operating pressure ranges from 6.26 MPa to 7.3 MPa with a mass flow rate between 200 and 400 kg/m2 s, heat flux between 5 and 140 kW/m2, and test section inlet temperature of −5 °C. Then, we examine the inner-wall temperature and heat transfer coefficient profiles at different pressures within the near-critical region. The results show that at high heat flux, departure from nucleate boiling (DNB) phenomenon presents with a sudden decrease in the heat transfer coefficient in the subcooled region. The higher the heat flux, the more seriously deteriorating the heat transfer is. Interestingly, the temperature reaches its peak in the post-DNB region rather than at the critical vapor quality point. With an increase in pressure, DNB occurs early with lower vapor quality, and the temperature peak decreases at the given heat flux and mass flux. On the contrary, DNB is delayed with an increase in mass flux. A series of boiling heat transfer correlations in a subcooled region, two-phase flow region, and superheated region are proposed in addition to a new predictive correlation for critical heat flux in the near-critical region at a given mass flux.

Experimental investigation of boiling heat transfer in helically coiled tubes at high pressure

Helically coiled once-through steam generators have been used widely during the past several decades for small nuclear power reactors. The boiling heat transfer characteristics of helically coiled tubes are important to optimal design of helically coiled steam generators. In the present work, various experiments with helically coiled tubes are performed to investigate the boiling heat transfer characteristics at high pressure with water as the working medium. Six test sections are made of thin walled stainless steel tubes having inner diameters of 12.5 mm and 14.5 mm. The system pressure is varying from 2 to 8 MP. The effects of heat flux, mass flux and system pressure on boiling heat transfer behavior are discussed in detail. Increase in heat flux increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient. However, heat flux has no influence on saturated convective boiling heat transfer coefficient. Mass flux increases the saturated convective boiling heat transfer coefficient. Increase in system pressure increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient but decreases the saturated convective boiling heat transfer coefficient. For the investigated experimental conditions, the results show that the Baburajan's correlation gives the best prediction in subcooled boiling region and the Gungor and Winterton's correlation predicts well with the experimental data in saturated boiling region. For the investigated experimental conditions, the results also indicate that the helically coiled tubes have similar performance of heat transfer in boiling regions compared with straight tubes.

A predictive model for confined and unconfined nucleate boiling heat transfer coefficient

The aim of this study is to investigate the influences of different parameters on the boiling phenomenon. In this paper, a new correlation to predict the pool boiling heat transfer coefficient is presented. The developed correlation was based on a database of experimental results, taking into account the effect of surface/liquid combination by considering the influence of contact the angle on pool boiling performance, and also the effect of the nucleate boiling in narrow spaces. The proposed correlation predicts accurately the boiling heat transfer behavior of fluids with significantly different thermophysical properties under confined and unconfined conditions. In order to validate the developed correlation, statistical analyses on the ratios of the experimental and correlated Nusselt numbers were performed. Also, the heat transfer coefficients calculated with the proposed correlation were compared with experimental results for different pool boiling conditions obtained by others authors.

B133 強制流動沸騰の熱伝達及び気泡挙動に及ぼす溶存気体の影響(OS-01:沸騰・凝縮伝熱および混相流の最近の進展(1))

B133 強制流動沸騰の熱伝達及び気泡挙動に及ぼす溶存気体の影響(OS-01:沸騰・凝縮伝熱および混相流の最近の進展(1))

Effects of surface roughness on flow boiling in silicon microgap heat sinks

Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500μm, 300μm and 200μm. The original silicon surface of surface roughness, Ra=0.6μm is modified to Ra=1.0μm and 1.6μm to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91°C at two different mass fluxes, G=390kg/m2s and 650kg/m2s and imposed effective heat flux, qeff″ ranging from 0W/cm2 to 85W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink.

Heat transfer characteristics during subcooled flow boiling of a kerosene kind hydrocarbon fuel in a 1 mm diameter channel

The subcooled flow boiling heat transfer characteristics of a kerosene kind hydrocarbon fuel were investigated in an electrically heated horizontal tube with an inner diameter of 1.0mm, in the range of heat flux: 20–1500kW/m2, fluid temperature: 25–400°C, mass flux: 1260–2160kg/m2s, and pressure: 0.25–2.5MPa. It was proposed that nucleate boiling heat transfer mechanism is dominant, as the heat transfer performance is dependent on heat flux imposed on the channel, rather than the fuel flow rate. It was found that the wall temperatures along the test section kept constant during the fully developed subcooled boiling (FDSB) of the non-azeotropic hydrocarbon fuel. After the onset of nucleate boiling, the temperature differences between inner wall and bulk fluid begin to decrease with the increase of heat flux. Experimental results show that the complicated boiling heat transfer behavior of hydrocarbon fuel is profoundly affected by the pressure and heat flux, especially by fuel subcooling. A correlation of heat transfer coefficients varying with heat fluxes and fuel subcooling was curve fitted. Excellent agreement is obtained between the predicted values and the experimental data.

Pool boiling characteristics of multiwalled carbon nanotube (CNT) based nanofluids over a flat plate heater

This paper is mainly concerned about the pool boiling heat transfer behavior of multi-walled carbon nanotubes (CNTs) suspension in pure water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS). Three different concentrations of 0.25%, 0.5% and 1.0% by volume of CNT dispersed with water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS) were prepared and boiling experiments were conducted over a stainless steel flat plate heater of size 30mm2 and 0.44mm thickness. The test results exhibit that the addition of carbon nanotubes increases boiling heat transfer coefficients of the base fluids. At a given heat flux of 500kW/m2, the enhancement of heat transfer coefficient was found to be 1.5, 2.6 and 3.0 times of water corresponding to 0.25%, 0.5% and 1.0% concentration of CNT by volume in water, respectively. In water–CNT–surfactant nanofluid, it was found that 0.5% of CNT concentration gives the highest enhancement of 1.7 compared with water. In both water and water–surfactant base fluids, it was observed that the enhancement factor for 0.25% of CNT first increases up to the heat flux of 66kW/m2 and then decreases for higher heat fluxes. Further, the overall heat transfer coefficient enhancement in the water–CNT nanofluids is approximately two times higher than that in the water–CNT–surfactant nanofluids. With increasing heat flux, however, the enhancement was concealed due to vigorous bubble generation for both water–CNT and water–CNT–surfactant nanofluids. Foaming was also observed over the liquid-free surface in water–CNT–surfactant nanofluids during the investigation. No fouling over the test-section surface was observed after experimentation.

A REVIEW OF ENHANCEMENT OF BOILING HEAT TRANSFER THROUGH NANOFLUIDS AND NANOPARTICLE COATINGS

This review traces the development of nanofluid pool boiling from its beginning (1984) to the present through a sampling of studies that have interested the authors and which have led to the latest findings at the University of Texas at Arlington (UTA). The studies of thermophysical properties of nanofluids are briefly covered. Several works in the last 7 years are highlighted to illustrate the modes of nanofluid pool boiling testing, the variability of nanofluid boiling heat transfer (BHT), and the postulations of causes of this behavior. Starting in 2006, the wettability increase in the nanoparticle coating, generated during the nanofluid pool boiling, is recognized as the source of critical heat flux (CHF) enhancement through its effect on the dynamics of hot spots and departing bubbles. The reasons for the observed contradictory BHT behavior are not yet fully clear, but recently at UTA, nanofluid boiling heat transfer has shown to be transient due to the dynamic nature of the formation of the nanoparticle coating. Also at UTA, the mechanism of nanoparticle deposition on the heated surface has been further confirmed. Thus, nanofluid boiling has led back to heat transfer enhancement through surface modification in nanoscale. These developments from 2006 are covered in more detail.

Pool Boiling Characteristics of Carbon Nanotube Based Nanofluids Over a Horizontal Tube

This paper is mainly concerned about the pool boiling heat transfer behavior of multiwalled carbon nanotube (CNT) suspensions in water and water containing 9.0% by weight of sodium lauryl sulfate anionic surfactant (SDS) has been carried out. Three different concentrations of 0.25%, 0.5%, and 1.0% by volume of CNT in base fluids, i.e., water and water containing 9.0% by weight of sodium lauryl SDS, were prepared and boiling experiments were conducted over a 115 mm long stainless steel horizontal tube heater of 9.0 mm outer diameter and 0.5 mm thickness. The test results show that the addition of carbon nanotubes increases the boiling heat transfer coefficient of base fluids. The enhancement of heat transfer coefficient was found at 1.75, 1.2, and 1.2 folds corresponding to 0.25%, 0.5%, and 1.0% concentration of CNT by volume in water at the critical heat flux of 961 kW/m2, 611 kW/m2, and 508 kW/m2, respectively. It was also observed that the enhancement factor was higher for lower heat fluxes, which decrease for higher heat fluxes. And also, the heat transfer coefficient enhancement in the water-CNT-surfactant nanofluids of 0.25%, 0.5%, and 1.0% of CNT concentrations are found to be 1.13, 1.44, and 1.5 times that of water at the critical heat fluxes of 295 kW/m2, 231 kW/m2, and 213 kW/m2, respectively. Foaming was found over the free surface of all water-CNT-surfactant nanofluids during the boiling experiments. Furthermore, it was observed that there was no fouling over the test-section. For further confirmation, however, a long term study needs to be carried out.

Flow boiling heat transfer study of R-134a/R-290/R-600a mixture in 9.52 and 12.7 mm smooth horizontal tubes: Experimental investigation

A detailed experimental investigation is carried out to study the flow boiling heat transfer behavior of R-134a/R-290/R-600a (91%/4.068%/4.932% by mass) refrigerant mixture in smooth horizontal tubes of diameter 9.52 and 12.7 mm. The heat transfer coefficients of the mixture are experimentally measured under varied heat flux conditions for stratified flow patterns using a coaxial counter-current heat exchanger test section. The tests are conducted for refrigerant inlet temperatures between −9 and 5 °C and mass flow rates ranging from 3 to 5 g s −1. Kattan–Thome–Favrat maps are used to confirm the flow patterns for the tested conditions. The magnitude of the heat transfer coefficient with respect to flow patterns and different mechanisms of boiling are discussed. The heat transfer coefficient of the refrigerant mixture is also compared with that of R-134a for selected working conditions. The significance of nucleate boiling in the overall heat transfer process under these testing conditions is highlighted.

Boiling heat transfer behavior of lead–bismuth–steam–water direct contact two-phase flow

The boiling heat transfer behavior of lead–bismuth (Pb–Bi)–steam–water direct contact two-phase flow was experimentally investigated. Experimental study was performed using Pb–Bi–steam–water direct contact boiling two-phase flow loop. The heat transfer rate was estimated from data of one-dimensional flow direct contact boiling of water in Pb–Bi. It is assumed in the analysis that film boiling occurs at the surfaces of a small water droplet after water is injected into hot Pb–Bi flow, because of the large temperature difference between water and Pb–Bi (i.e. 493 K and 733 K for injection water and Pb–Bi temperature, respectively). The heat transfer occurs between Pb–Bi and steam without phase change after all water completely evaporates. The overall heat transfer coefficient decreased with the superheat at low injection flow rate and was nearly constant for high injection flow rate. The local heat transfer coefficient was higher than average one in the whole tube, which means that the direct contact boiling heat transfer coefficient was high and it decreased in the downstream direction. Almost all the water vaporized in the test tube at high pressure according to the local and total heat transfer rate.

Pool Boiling Heat Transfer of Aqueous TiO2-Based Nanofluids

Stable TiO2−H2O nanofluids are formulated by using the electrostatic stabilization method for nucleate pool boiling heat transfer experiments. The results show that the stable nanofluids significantly enhances boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼50% at a particle concentration of 0.7% by volume. Discussion of the results suggests that the reported controversy in the pool boiling heat transfer behavior be associated with the properties and behavior of both nanofluids and boiling surface, as well as their interaction.

Transient flow pattern based microscale boiling heat transfer mechanisms

In our previous paper (J L Xu et al 2005 J. Micromech. Microeng. 15 362–76), it is identified that the transient flow patterns for microscale boiling heat transfer are repeated on the timescale of milliseconds. A full cycle could be subdivided into three substages: liquid refilling stage, bubble nucleation, growth and coalescence stage and transient annular flow stage. Five heat transfer mechanisms could be deduced from the transient flow patterns. This paper extends the above work and mainly focuses on the boiling heat transfer behavior, which was performed for 102 runs with the following data ranges: inlet pressures of 1–2 bar, inlet liquid temperatures of 24–45 °C, pressure drops of 10–100 kPa, mass fluxes of 64–600 kg m−2 s−1, heat fluxes of 150–480 kW m−2, exit vapor qualities of 0.07–1.15 and the boiling numbers of 0.69 × 10−3–5.046 × 10−3. The silicon wafer test section consists of ten triangular microchannels with the hydraulic diameter of 155.4 µm. Acetone is selected as the working fluid. The heat transfer coefficients were analyzed with the effects of the heat fluxes, the mass fluxes and the vapor mass qualities. We provide a link between the transient flow patterns and the heat transfer process. The boiling numbers can be used to characterize the microscale boiling heat transfer, which can display three distinct regions by dividing the boiling numbers into three subranges. The transient flow pattern based heat transfer mechanisms are very consistent with the heat transfer coefficient measurements with the effects of the heat fluxes, mass fluxes and vapor mass qualities. The transition boundaries among the three heat transfer regions are given.

Water-Injection Effect on Boiling Heat Transfer in a Water-Saturated Porous Bed

A number of experimental studies of boiling heat transfer in liquid-saturated porous media have been motivated by such diverse technological problems as high-flux heat transfer porous surface, geothermal energy extraction, and heat transport characteristics in heat pipes. From the literature survey, it is apparent that very little is known about the boiling heat transfer behavior in a saturated porous bed with liquid injection to the heating surface, which appears to provide the fundamental data for the geothermal energy-extraction technology. The objective of this technical note is to report the results of an experimental study of the effect of saturated-water injection on boiling heat transfer in a water-saturated porous bed.