Critical Heat Flux Point Research Articles

Fill-In and Boiling Transition Characteristics during the Liquid Oxygen Chill-Down Process in a Vertical Exit-Contracted Pipe

Liquid oxygen chill-down in a vertical exit-contracted pipe was investigated experimentally. The wall temperatures were recorded in detail to describe the filling and chill-down process of the experimental section. Two quenching fronts, the exit one and the inlet one, were detected, and their propagations were found. Results show that the chill-down process is controlled mainly by the formation and propagation of quenching front, which are determined by the pressure level. With the increase of pressure, the roles of both propagation of quenching front and inlet quenching front undergo decreasing. On the vertical section, the effect of circumferential position was discussed in detail and the dominant point was identified, which determines the boiling transition time of the dominated points on the current cross-section. Based on the experimental data, two correlations were suggested for dominant point and dominated points, respectively, to predict heat flux on Leidenfrost, heat transfer coefficient on Leidenfrost, and critical heat flux. One equation was approved to predict heat transfer coefficient on critical heat flux point for both sorts of points. All of these correlations could produce reliable predictions.

Development of a dimensionless rod-bundle CHF correlation based on stepwise regression method, Part II: determination of correction factors accounting for the effect of unheated guide tube and non-uniform axial heat flux profile

Correlation for rod-bundle critical heat flux (CHF) is of vital importance for design and assessment of a Pressurized Water Reactor (PWR). In our previous study, the basic form of a dimensionless rod-bundle CHF correlation has been developed based on stepwise regression with CHF data obtained in test bundles of uniform axial heat flux. It proves that stepwise regression is an effective method to develop simple, but reliable CHF correlation consisting of only dimensionless parameters with rigorous physical meanings. As a continuation of the previous investigation, revision of the basic form was conducted. Dimensionless correction factors accounting for the cold-wall effect due to unheated guide tube and the effect of non-uniform axial heat flux were developed. For the data set of 486 rod-bundle CHF points obtained in 7 test bundles, the average ratio of the measured to predicted CHF is 1.002 with a standard deviation from the mean of 5.51%.

Overview on critical heat flux experiment for the reactor fuel assemblies

The critical heat flux (CHF) experiment for the reactor fuel assemblies (FAs) is the most difficult and important one in the reactor thermo-hydraulic study. The FA CHF experiments are performed on the large scale thermo-hydraulic test facilities with large DC power suppliers, complicated test sections and rigid test process. The results are analyzed with subchannel codes to obtain the parameters at the CHF point. Based on the experimental data, the CHF correlation is derived as the tool for the reactor safety analysis and design. After many years’ work, the common understanding of the FA CHF was established and the method of the FA CHF experiment becomes similar. This paper reviewed the progress of the FA CHF experiments, analyzed the modeling of the test sections, introduced requirement of the test facilities, and discussed the hypothesis in the modelling. The benchmark qualification test adopted to verify the test method and test section used in the experiment are discussed in the paper, too.

Lattice Boltzmann Study on Influence of Gravitational Acceleration on Pool Nucleate Boiling Heat Transfer

Under both normal gravity and microgravity conditions, pool boiling is an efficient mode of heat transfer which has been widely applied in practice. Studying the influence of gravitational acceleration on boiling heat transfer is not only of academic significance, but also helpful for the design of space equipment related to boiling. With the development of computer technology, numerical method has been a new reliable way to investigate the boiling heat transfer under different gravities. Pseudopotential lattice Boltzmann (i.e., LB) model is one of the most popular multiphase LB models, in which the phase interface could be formed, disappeared and migrated naturally. In this paper, the Multi-Relaxation-Time (i.e., MRT) pseudopotential LB model coupled with phase-change model was applied to simulate the pool boiling heat transfer under different gravitational accelerations and wall superheats. Pool boiling curves under different gravities were obtained. It’s found that: 1) the pool boiling heat transfer coefficient at a given wall superheat decreases with a decrease in gravity; 2) the wall superheat, as well as heat flux, at the CHF (i.e., critical heat flux) point and ONB (i.e., onset of the nucleate boiling) decrease gradually with a decrease in gravity. In addition, based on the numerical results, a new gravity scaling model was proposed to predict the influence of gravitational acceleration on the nucleate boiling heat transfer under different wall superheats. Finally, the new gravity scaling model was proved to be capable of predicting the heat flux during the nucleate boiling under different wall superheats and gravities.

Experimental study on pool boiling regime transitions of vertical rod quenched in aqueous surfactant solutions using inverse heat transfer analysis

In this study, the effect of surfactants and liquid temperature on boiling regime transitions of high-temperature rod during quenching was examined using inverse heat transfer analysis. Liquid pools of the aqueous sodium dodecyl sulfate (SDS) and Triton X-100 solutions were used with pure water. In the present experimental range, the critical heat flux (CHF) and minimum film boiling (MFB) point for all test fluids increased as the liquid temperature decreased. On the other hand, the SDS and Triton X-100 surfactants suppressed the CHF and MFB point, which might be due to the enhancement of vapor film stability caused by reduced surface tension. Some previous experimental studies have reported that the surfactants enhance the boiling regime transitions, but their results seem to be contradictory to the present ones. Considering the present study with the previous works, the surfactants seem to have different effect on boiling phenomena depending on the boiling regime. In other words, in the aqueous surfactant solution, the boiling regime transition points seem to be markedly influenced by a path of boiling regime. In addition, boiling mode diagrams were presented for pure water and aqueous surfactant solutions.

Boiling flow of graphene nanoplatelets nano-suspension on a small copper disk

In the present work, an attempt was made to experimentally quantify the boiling heat transfer coefficient (BHTC) of graphene oxide-water nano-suspension (NS) inflow boiling heat transfer regime. The NS was prepared at weight fractions of 0.025, 0.05, and 0.1% using the two-step method and further stabilized for 17 days (at wt% = 0.1). Results showed that the presence of graphene oxide nanoplatelets (GNPs) imposed an extreme fouling thermal resistance (FTR) to the surface, which caused a reduction in the BHTC over 1000 min of continuous operation after the CHF point. This was mainly due to the presence of the graphene oxide on the surface, which created a surficial fouling layer and heat accumulation on the surface. Instead, the sedimentation layer promoted the critical heat flux (CHF) point such that the point for water was 1370 kW/m2 reaching 1640 kW/m2 for NS at wt% = 0.1. Likewise, the highest BHTC of 17.4 kW/(m2K) at Re = 10,950 was obtained. Also, with increasing the heat flux and flow rate, the BHTC increased. The same trend was also identified with a mass fraction of GNPs up to CHF point. The increase in the BHTC was attributed to the intensification of the Brownian motion and thermophoresis effect in the boiling micro-layer close to the surface.

Effect of saturation temperature and vapor quality on the boiling heat transfer and critical heat flux in a microchannel

In this paper, heat transfer coefficients and critical heat fluxes of the R134a fluid in two-phase flow was studied in a 27-channel, 421 μm hydraulic diameter and 40 mm long microchannel experimental set-up. Data were collected at constant mass flux of 1000 kg m−2 s−1, saturation inlet temperature range of 20–28 °C and at inlet vapor quality range of 0.01–0.20. The heat flux was initiated at 250 kW m−2 and gradually increased until reaching the critical heat flux (CHF). The effect of inlet vapor quality, saturation inlet temperature and heat flux on heat transfer coefficient and critical heat flux were investigated in detail and presented graphically. It can be observed that the CHF decreases with the increasing of saturation inlet temperature and inlet vapor quality. However, the heat transfer coefficient at the CHF point increased with the increasing of saturation temperature and the heat transfer coefficient at the CHF point was decreased with the increasing of vapor quality. The test results from the correlations, suggested in well-known articles, were compared with the experimental data, where the Kosaz et al. proposed correlation was showing well fit with the experimental data having absolute error of 9.86%. In this investigation, the new correlations for the heat transfer coefficient for the heat transfer coefficient and the critical heat flux have been proposed by taking into account the effect of the vapor quality.

Investigating the increased heat performance of direct steam generation of Fresnel power plant using nanoparticles

AbstractUtilization of direct steam generation is a significant point in the solar power plants particularly in linear Fresnel collectors. In this research, the increased heat transfer coefficient has been scrutinized in the used nanoparticles with volumetric concentrations of 0.01 to 2% in comparison with pure water in the linear Fresnel collectors for two mass fluxes embrace 100 and 500 kg/m2 s. In this process, determination of critical heat flux (CHF) is substantially important that in truth, it has been obtained with a numerical thermal analysis. Using the look up table method, the location of CHF has been determined for pure water and all nanofluids. According to obtained results for two types of mass fluxes and in along increasing of volumetric concentration from 0 up to 2% for gold nanoparticles, tube temperature in its internal surface has been decreased that it has been equaled 1.33 and 0.34% for mass fluxes 100 and 500 kg/m2 s, respectively. Also, length of CHF has been increased equal to 91.01 and 114.1% for mentioned mass fluxes. On the other hand, an increasing has been acquired about convection heat flux in CHF point that they are 1.49 and 0.28%.

Experimental investigation of transient pool boiling characteristics of [formula omitted] ferrofluid in comparison with deionized water

Pool boiling heat transfer and critical heat flux (CHF) in ferrofluid were experimentally studied under transient power conditions. The heat flux increment was begun from the free convection regime to the film boiling regime. Transient boiling heat transfer coefficient (TBHTC), transient CHF, scanning electronic microscopy (SEM) images of surface wire, and high-speed camera images were obtained. The following results were obtained for ferrofluid: as the time period increased, (1) the heat transfer coefficient of the nucleate boiling increased because of bubble production and its movement, then decreased in the transition from the nucleate to the film boiling because of an unstable layer of vapor around the wire heater and finally increased again in the film boiling because of the high temperature of the wire surface and radiation heat transfer; (2) the CHF and its wire superheat temperatures at CHF point decreased by up to a period of 100 s; (3) the incipient boiling point decreased. In the second part of the film boiling, the heat transfer coefficient was decreased because of the high heat flux and the more vapor film thickness around the wire. A comparison between two working fluids show that by increasing the time period, the nucleate boiling range of ferrofluid boiling was increased more than the deionized (DI) water boiling. Furthermore, the wire superheat temperature of the ferrofluid in all its regimes was decreased and the CHF of ferrofluid boiling were increased more than the DI water boiling. The ferrofluid CHF at the time period of 1 s decreased by 11.3% relative to DI water, and at the time period 5000 s increased by 160%.

Experimental and Numerical Investigations of Subcooled Boiling Heat Transfer in a Small-Diameter Tube

Abstract The boiling heat transfer for subcooled water flowing in a small-diameter tube was investigated experimentally and numerically. In the experiment, a platinum tube was used as an experimental tube (d = 1.0–2.0 mm) to conduct joule heating by direct current. The heat generation rate of the tube was controlled with an exponential function. The numerical simulation of boiling heat transfer for subcooled water flowing in the small-diameter tube was conducted using the commercial computational fluid dynamics (CFD) code, phoenics ver. 2013. The small-diameter tube was modeled in the simulation. As the boundary condition, the measured heat flux was given at the inner wall. The inlet temperature ranged from 302 to 312 K. The flow velocities of d = 1.0 mm and d = 2.0 mm were 9.29 m/s and 2.34 m/s, respectively. The three-dimensional analysis was carried out from non-boiling to the critical heat flux (CHF). Governing equations were discretized using the finite volume method in the phoenics. The semi-implicit method for pressure linked equation (SIMPLE) method was applied in the numerical simulation. For modeling boiling phenomena in the tube, the Eulerian–Eulerian two-fluid model was adopted using the interphase slip algorithm of phoenics. The surface temperature difference increased as the heat flux increased in the experiment. The numerical simulation predicted the experimental data well. When the heat flux of the experiment reached the CHF point, the predicted value of the heat transfer coefficient was approximately 3.5% lower than that of the experiment.

Experimental study on liquid oxygen chilldown in the horizontal pipe with an injector on the exit

Components downstream of the pump in the cryogenic liquid rocket engine were simplified to a horizontal pipe upstream of an injector. Cryogenic chilldown was investigated experimentally in this pipe by constant flow rate of LO2. The chilldown processes were discussed and the boiling curves were drawn to identify the boiling transition points, Leidenfrost point (LFP) and critical heat flux point (CHF). By fitting the present sets of data, correlation formats of the previous expressions were selected and revised linearly to give the suggested correlation expressions on TLeid, qLeid, TCHF and qCHF. Because these parameters could also be well correlated to the corresponding pressure, it could be supposed that the effects of pressure could be well represented by the physical properties of the fluid. The recommended correlations on TLeid, qLeid and TCHF include the physical properties of fluid only. The recommended correlation on qCHF include both the sensor position (Lse) and the physical properties of fluid. Furthermore, a couple of previous correlations based on cryogenic chilldown were validated, at least partially.

Measurement of the microlayer characteristics in the whole range of nucleate boiling for water by laser interferometry

During nucleate boiling, a thin liquid film (microlayer) is formed beneath a boiling bubble when the bubble undergoes rapid growth/expansion. The linear distribution of the microlayer has been previously confirmed, and its crest shape has been observed in the isolated bubble region of nucleate boiling. However, the microlayer behavior in larger heat flux regions up to critical heat flux has not yet been elucidated. In this study, to further understand the microlayer structure in the whole heat flux range of nucleate boiling, microlayer configuration was measured using laser interferometry. Water was adopted as the test fluid, and it is confirmed that the microlayer can be observed over a whole range of nucleate boiling containing the critical heat flux point. It is also confirmed that the deformation of the microlayer from axisymmetric shape was caused by complicated, irregular bubble motions such as bubble coalescence, which was observed for relatively higher heat flux. The crest shape of the microlayer, which appears near the periphery of its maximum diameter under relatively smaller heat flux, was not observed at a relatively higher heat flux. Finally, it is confirmed that heat flux does not obviously influence the thickness distribution of the initial microlayer.

Critical heat flux and Leidenfrost temperature on hemispherical stainless steel surface

In this experimental work, an investigation on the droplet evaporation lifetime using hemispherical stainless steel was carried out. The main objective of this experimental work is to obtain the critical heat flux (CHF) and Leidenfrost temperature, TL. The novelty of this study is the use of a hemispherical surface to evaporate the droplets. This hemispherical surface was constructed using Electrical Discharge Machining (EDM) which provided a nearly perfect and smooth hemispherical stainless steel surface. Ethanol was selected as the test liquid and the droplet size was approximately 3.628 mm in diameter. The droplet temperature was approximately 29.9 °C corresponding to liquid subcooling ΔTsub = 48.1 K. The droplet impact height was set to be around 65.0 mm corresponding to impact velocity of 1.129 m/s. The temperature range from a low temperature of Tw = 70 °C to a high temperature of Tw = 230 °C. As a result, the CHF point and Leidenfrost temperature was obtained at the surface temperatures of Tw = 120.1 °C and 193.6 °C respectively. The boiling curve obtained also showed a smooth exponential curve which was found to be similar to other literature.

Numerical study on saturated pool boiling heat transfer in presence of a uniform electric field using lattice Boltzmann method

For the industrial equipment with boiling, critical heat flux (CHF) is an important parameter that determines its upper limit of heat load for safe operation. Therefore, increasing CHF is of great importance in engineering field. Previous studies proved that it’s an effective way to enhance boiling heat transfer and CHF by electrohydrodynamics (EHD). However, it is difficult to investigate the influence mechanism of the electric field on the bubble dynamics and boiling heat transfer because of the limit of experimental technical methods. Thus, in this paper, a two-dimensional lattice Boltzmann model was established by coupling the pseudopotential model with phase-change model and electric field model to meet the lack of studies on simulating the pool boiling in presence of an electric field. After validating the rationality of the two-dimensional lattice Boltzmann model established at present study, the heat transfer during nucleate boiling and film boiling under a uniform electric field were studied in detail. In addition, the influence of electric field intensity on boiling curves and CHF was investigated quantitatively. In the partial nucleate boiling regime, a uniform electric field had limited influence on bubble dynamics and boiling heat transfer. In the fully developed nucleate boiling regime, increasing electric field intensity could enhance boiling heat transfer obviously. In the film boiling regime, increasing electric field intensity could slightly enhance averaged heat flux at first. After the electric field intensity reached a certain value, averaged heat flux increased rapidly. Increasing electric field intensity could expand the nucleate boiling regime, and enhance CHF and the wall temperature at CHF point.

Studying thermal stresses of a solar absorber in single and two-phase regimes and effects of various coatings on the absorber

In the solar parabolic trough collectors, functioning based on direct steam generation system, heat transfer system and appropriate thermal stress are indicated as two most important thermal parameters. High amount of the critical heat flux creates an extensive application range for the system so heat transfer systems can function without being subjected to high thermal stresses. In this regard, by using a coat on the external surface of the absorber thermal stresses can be reduces in single-phase, sub-cooled, boiling flow regimes and even at the critical heat flux point. In this study, by considering a numerical model of heat transfer, the input heat flux of the absorber which is considered to be a boundary condition for the absorber’s external surface is calculated. Energy conservation is assessed for three different zones of the fluid flow, single-phase, sub-cooled and boiling. Obtaining the temperature profile of the absorber’s surface from the beginning of the fluid flow to the critical heat flux point, leads to calculation of thermal stresses, which has been accomplished by finite element modeling of Misses and Tresca Model, and these two criteria have been compared. Mentioned analyses are performed and compared for coated and uncoated tubes. Three different coats (zinc, chromium and tungsten) are considered. It is shown that zinc is much more preferred than the two other coats, due to 39% and 83% reduction of thermal stress at the onset of critical thermal flux in both Mises and Tresca, respectively.

Experimental study of subcooled flow boiling heat transfer on micro-pin-finned surfaces in short-term microgravity

The flow boiling heat transfer of subcooled air-dissolved FC-72 on micro-pin-finned surfaces was studied in microgravity by utilizing the drop tower facility in Beijing. The micro-pin-fins with the dimension of 30 × 30 × 60 μm3 (width × thickness × height), named PF30-60, were fabricated on a silicon chip by using the dry etching technique. For comparison, experiments of flow boiling heat transfer in terrestrial gravity were also conducted. The effects of inlet velocity on both flow boiling heat transfer and bubble behavior were explored. It was found that gravity has nearly no effect on flow boiling heat transfer for the departure of the inertial-force-dominant bubbles in the low and moderate heat fluxes regions. In contrast, in the high-heat-flux region, the flow boiling heat transfer deteriorates and the critical heat flux (CHF) decreases due to the bubble accumulation in the channel. For PF30-60 at V = 0.5 m/s, the CHF point can be inferred to be between 20.8 and 24.5 W/cm2, which is 63.0–74.2% of that in normal gravity. Regarding PF30-60 at V = 1.0 m/s, the CHF point can be inferred to be between 25.4 and 31.6 W/cm2, which is 67.6–84.0% of that in normal gravity. The impact of gravity on CHF is closely linked to the channel geometry parameter and surface modification. The dimensionless numbers, Ch (Channel number) and Sf (Surface number), were proposed to describe the effect of the channel geometry and surface modification on the ratio of CHF in microgravity to that in normal gravity (CHFμg/CHF1g). An empirical correlation based on We (Weber number), Ch and Sf was proposed to predict the value of CHFμg/CHF1g ratio in good agreement with the experimental data. This study provides a new perspective to determine the threshold inlet velocity of inertial-force-dominant flow boiling under different experimental conditions at different gravity levels.

Pool boiling heat transfer to zinc oxide-ethylene glycol nano-suspension near the critical heat flux

Experimental study is conducted on the thermal performance of the ZnO nanoparticles dispersed in water-ethylene glycol (water/EG) as the base fluid under the pool boiling condition. Experimental facility provides condition to apply heat fluxes up to the critical heat flux point. Influence of different operating parameters such as heat flux and mass concentration on the boiling heat transfer coefficient and thermal fouling resistance is investigated. Results showed that with an increase in heat flux and mass concentration, the heat transfer coefficient increases; however, at concentration of 0.5 % by weight for ZnO/water/EG nano-fluid. Deposition of particles on the heating surface was a significant disadvantage of the nano-fluid. However, with an increase in mass concentration of nanoparticles, the critical heat flux point is enhanced, which is due to the enhancement in deposition layer of the nanoparticles, resulting in capillary wick and keeping more liquid inside the deposition layer.

Direct numerical simulations of pool boiling curves including heater's thermal responses and the effect of vapor phase's thermal conductivity

Effects of heater's thermal properties and vapor phase's thermal conductivity on saturated pool boiling above a large horizontal heater are simulated numerically based on an improved pseudo-potential liquid-vapor phase change lattice Boltzmann model. A transient conjugate heat transfer problem is under consideration, where the conjugate thermal boundary condition is imposed and heater's thermal responses during boiling processes are investigated. Saturated pool boiling curves from onset of nucleate boiling to critical heat flux (CHF), to transition boiling regime to stable film boiling regime are obtained numerically. It is found that the simulated critical heat flux (CHF) agrees reasonably well with existing analytical models. Also, the simulated boiling heat fluxes in stable film boiling regime are shown to be in good agreement with the existing analytical solution. Thus, this improved pseudo-potential liquid-vapor phase change lattice Boltzmann model is quantitatively validated. Simulation results demonstrate that there is significant maldistribution in temperature distribution near the top of heater surface in nucleate boiling regime, CHF point and transition boiling regime. As a result, two-dimensional heat conduction can not be ignored when evaluating heat flux closely beneath the heater's top surface. It is also shown that both heater's thermal conductivity and thermal mass (the product of density and specific heat at constant pressure) have no effect on CHF value as well as the boiling curve in nucleate boiling regime and film boiling regime for a thick heater. However, the transition boiling regime of the boiling curve moves to the left with the increasing heater thermal conductivity and heater thermal mass for a thick heater. Increasing the vapor theraml conductivity has no effect on CHF but would increase boiling heat flux in film boiling regime, and hence shortening the transition boiling regime.

Effect of geometrical parameters of open microchannel surfaces on pool boiling heat transfer

This study focuses on the effect of channel depth on the heat transfer coefficient during nucleate pool boiling. Experimental studies were performed for saturated deionized water, and Novec-649 as working fluids at atmospheric pressure. Copper surfaces were modified to form microchannels with different geometrical properties. The microchannels were from 0.2 mm to 0.4 mm deep, 0.3 mm wide and spaced every 0.1 mm. The experiment was conducted for increasing heat flux up to the critical heat flux point. The surface modification provided an appreciably higher heat transfer coefficient compared to the smooth surface for all boiling liquids. The maximum heat transfer coefficient obtained exceeded 60 kW/m2 K.

Influence of boiling initiation surface superheat on subcooled water flow boiling critical heat flux in a SUS304 circular tube at high liquid Reynolds number

The subcooled boiling heat transfer and the steady-state critical heat flux (CHF) in a vertical circular tube for the liquid Reynolds numbers (Red=3.65×104–3.08×105) and the flow velocities (u=3.95–30.80m/s) were systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tube of inner diameter (d=6mm) and heated length (L=59.5mm) was used in this work. The boiling initiation noise of outer surface of the test tube in the open air was simultaneously measured up to CHF point by the sound level meter (SLM) and the microphone of a video camera (MP). The outer surface temperatures of the SUS304 test tube with heating were also observed by an infrared thermal imaging camera (ITIC) and the color temperatures of outer surface of the test tube in the open air were observed by a video camera (VC). The subcooled boiling heat transfer and CHF for SUS304 circular tube were compared with the values calculated by authors’ and other researchers’ correlations for the subcooled flow boiling heat transfer. The influences of flow velocity on the boiling initiation surface heat flux, the boiling initiation surface superheat, the subcooled boiling heat transfer and the CHF were investigated into details based on the experimental data. At the flow velocities higher than 13.3m/s, boiling initiation surface heat fluxes were close to the CHFs and surface superheats at the CHF were over to the homogeneous spontaneous nucleation temperature as well as the lower limit of the heterogeneous spontaneous nucleation temperature. The dominant mechanism of the subcooled water flow boiling CHF on the SUS304 circular tube was discussed at high liquid Reynolds number.