Effect of Cross-flow Velocity on the Critical Flux of Ceramic Membrane Filtration as a Pre-treatment for Seawater Desalination

Pre-treatment, which supplies a stable, high-quality feed for reverse osmosis （RO） membranes, is a criti- cal step for successful operation in a seawater reverse osmosis plant. In this study, ceramic membrane systems were employed as pre-treatment for seawater desalination. A laboratory experiment was performed to investigate the effect of the cross-flow velocity on the critical flux and consequently to optimize the permeate flux. Then a pilot test was performed to investigate the long-term performance. The result shows that there is no significant effect of the cross-flow velocity on the critical flux when the cross-flow velocity varies in laminar flow region only or in turbulent flow region only, but the effect is distinct when the cross-flow velocity varies in the transition region. The membrane fouling is slight at the permeate flux of 150 L·m^-2·h^-1 and the system is stable, producing a high-quality feed （the turbidity and silt density index are less than 0.1 NTU and 3.0, respectively） for RO to run for 2922.4 h without chemical cleaning. Thus the ceramic membranes are suitable to filtrate seawater as the pre-treatment for RO.

Critical Heat Flux with Subcooled Flowing Water in Tubes for Pressures from Atmosphere to Near-Critical Point

During last 45 years, two groups of the experimental data on critical heat flux were obtained in bare tubes, covering the pressures from atmosphere to near-critical point. One group of the data were obtained in the inner diameter of 2.32, 5.16, 8.05, 10.0 and 16.0 mm, respectively, with the ranges of pressure of 0.1-1.92 MPa, velocity of 1.47-23.3 m/s, local subcooling of 3.7-108.7 ℃ and heat flux of up to 38.3 MW/m2. Another group of the data were obtained in the inner diameter of 4.62, 7.98 and 10.89 mm, respectively, with the ranges of pressure of 1.7-20.6 MPa, mass flux of 454-4,055 kg/（m2.s） and inlet subcooling of 53-361 ℃. The results showed complicated effects of the pressure, mass flux, subcooling and diameter on the critical heat flux. They were formulated by two empirical correlations. A mechanistic model on the limit of heat transfer capability from the bubbly layer to the subcooled core was also proposed for all the results.

Improvement of Critical Heat Flux Performance by Wire Spacer

In order to investigate the coolability of a tight lattice core in BWR （boiling water reactors）, an experiment of CHF （critical heat flux） was conducted using a single heater pin flow channel with and without a wire spacer with the mass flux ranged from 200 kg/（m2.s） to 600 kg/（m2·s）, the inlet temperature from 50 ℃ to 90 ℃ at the pressure of 0.1 MPa. The results of CHF values were compared between two different types of heater pin with and without the wire spacer. The heat removability of flow channel was improved by the wire spacer in comparison with the heater pin flow channel without the wire spacer. The CHF value was higher in the heater pin channel with the wire spacer than in that without the wire spacer.

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(cp)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.

Critical Heat Flux in Forced Convective Boiling with a Wall Jet

The critical heat flux (CHF) in the forced convective boiling with a wall jet has been investigated.The experiments of CHF with a wall jet have been performed over a wide range of ρ l/ρ g=6.6-1 603 and ΔT sub =0-60 K. The mechanism on CHF is discussed and a CHF model based on heat balance in sublayer can provide a good clue for analyzing and deriving CHF.Finally,a generalized correlation is presented, which can predict CHF for saturated and subcooled conditions.

The Corrected Simulation Method of Critical Heat Flux Prediction for Water-Cooled Divertor Based on Euler Homogeneous Model

An accurate critical heat flux（CHF） prediction method is the key factor for realizing the steady-state operation of a water-cooled divertor that works under one-sided high heating flux conditions.An improved CHF prediction method based on Euler＇s homogeneous model for flow boiling combined with realizable k-ε model for single-phase flow is adopted in this paper in which time relaxation coefficients are corrected by the Hertz-Knudsen formula in order to improve the calculation accuracy of vapor-liquid conversion efficiency under high heating flux conditions.Moreover,local large differences of liquid physical properties due to the extreme nonuniform heating flux on cooling wall along the circumference direction are revised by formula IAPWSIF97.Therefore,this method can improve the calculation accuracy of heat and mass transfer between liquid phase and vapor phase in a CHF prediction simulation of water-cooled divertors under the one-sided high heating condition.An experimental example is simulated based on the improved and the uncorrected methods.The simulation results,such as temperature,void fraction and heat transfer coefficient,are analyzed to achieve the CHF prediction.The results show that the maximum error of CHF based on the improved method is 23.7%,while that of CHF based on uncorrected method is up to 188%,as compared with the experiment results of Ref.[12].Finally,this method is verified by comparison with the experimental data obtained by International Thermonuclear Experimental Reactor（ITER）,with a maximum error of 6% only.This method provides an efficient tool for the CHF prediction of water-cooled divertors.

Experimental Studies on Critical Heat Flux in a Uniformly Heated Vertical Tube at Low Pressure and Flowrates

Investigations into critical beat flux at low flow and pressure conditions are of particular interest when predicting the nuclear reactor core behavior during Loss of Coolant accident （LOCA）. Therefore, critical heat flux （CHF） has been investigated in a uniformly heated vertical round tube at two low system pressures and six low water flowrates. The results have been compared with two correlations which have different approaches and CHF look-up table. Good agreements have been obtained for the three comparisons at the lower sets of mass fluxes. The Bowring correlation was found to be the best to correlate the experimental results with Root Mean Square Error RMSE of 0.54% and 0.56% for the 5 bar and 15 bar system pressure respectively. A comparisons with the Shim and Lee correlation yielded RMSE of 0.23% and 5.74% for the two system pressure respectively. When the look-up table of Groeneveld et al. was used, RMES of 0.55% and 25.2% was obtained for the two system pressure respectively.

Enhancement of Critical Heat Flux in Tight Rod Bundle with Wire Spacer

The experiment of CHF （critical heat flux） was conducted for water boiling two-phase flow in three-pin tight rod bundle. The effects of with and without wire spacers and the pitch to diameter ratio p/d on CHF were investigated under the conditions of mass flux range 250-430 kg/（m2·s）, inlet temperature from 70 ℃ to 92 ℃ and the pressure of 0.1 MPa. The CHF was enhanced by wire spacers in comparison with the results of CHF without wire spacers. The CHF was enhanced by reducing thep/dfrom 1.18 to 1.10 under the same flow rate condition, although it did not change appreciably with the change ofp/d under the same mass flux condition.

Research on DTC System with Variable Flux for Switched Reluctance Motor

When switched reluctance motor(SRM)is in the status of the traditional direct torque control(DTC)system,due to the high saturation nonlinearity of the electromagnetic relationships of switched reluctance motors,the torque ripple and the stator phase current are larger.In order to resolve the above problems,through the analysis and deduction for SRM flux model and the influence of characteristics of flux and speed on torque ripple,this paper presents a variable-flux control strategy with the three closed-loop structure based on the critical flux supersaturated speed.And a DTC system of SRM with variable flux and three closed-loop is built up in Matlab/simulink.Moreover,the DSP hardware experiment platform based on the TMS320F2812 is established to validate the performance of the improved algorithm.The simulation and experimental results show that the new scheme has an obvious effect on torque ripple reduction,and the three-phase stator current is obviously reduced,which greatly reduces the stator winding copper consumption during the operation of SRM.Moreover,the improved system has good system stability.

The Hydrodynamic Crisis of Nucleate Boiling in a Horizontal Thin Layer of Dielectric Liquid HFE-7100

The results of an experimental study on critical heat fluxes(CHF)during the nucleate boiling of the HFE-7100 dielectric liquid in horizontal layers of different heights at atmospheric pressure are presented.The existence of a critical layer height has been established.In layers above the critical layer height,a hydrodynamic boiling crisis occurs;in thinner layers,a surface drying crisis occurs.At a layer height equal to the critical value,a dry spot first appears,followed by transition boiling,which gradually spreads to the entire heating surface.In these experiments,the critical layer height was equal to 6 mm.In a layer of liquid with a critical layer height of 6 mm,a two-dimensional Taylor instability was observed in the transition boiling mode when the ratio of the diameter of the"vapor jets"to the distance between them,as well as the void fractions in the layer(-π/16),corresponded to the main provisions of the Zuber theory.The calculation of CHF using the relations of Zuber’s theory,when approaching the crisis from the transition boiling side and taking into account the real geometric dimensions,aligns well with the experimental results.

Effect of the Geometrical Parameter of OpenMicrochannel on Pool Boiling Enhancement

High heat dissipation is required for miniaturization and increasing the power of electronic systems.Pool boiling is a promising option for achieving efficient heat dissipation at low wall superheat without the need for moving parts.Many studies have focused on improving heat transfer efficiency during boiling by modifying the surface of the heating element.This paper presents an experimental investigation on improving pool boiling heat transfer using an open microchannel.The primary goal of this work is to investigate the impact of the channel geometry characteristics on boiling heat transfer.Initially,rectangular microchannels were prepared on a circular copper test piece with a diameter of 20 mm.Then,the boiling characteristics of these microchannels were compared with those of a smooth surface under saturated conditions using deionized water.In this investigation,a wire-cutting electrical discharge machine(EDM)machine was used to produce parallel microchannels with channel widths of 0.2,0.4,and 0.8 mm.The fin thicknesses were 0.2,0.4,and 0.6 mm,while the channel depth remained constant at 0.4 mm.The results manifested that the surface featuring narrower fins and broader channels achieved superior performance.The heat transfer coefficient(HTC)was enhanced by a maximum of 248%,and the critical heat flux(CHF)was enhanced by a maximum of 101%compared to a plain surface.Eventually,the obtained results were compared with previous research and elucidated a good agreement.

HIGH EFFICIENCY COOLING TECHNOLOGY BASED ON GREEN MANUFACTURING

Green manufacturing （GM） and high efficiency machining technology are inevitable trends in the field of advanced manufacturing of the 21st century. To ensure green and high-efficiency machining, a new high efficiency cooling technology-cryogenic pneumatic mist jet impinging cooling （CPMJI） technology is presented. For obtaining the best cooling effect, a little quantity of coolant is carried by high speed cryogenic air （-20 C ） and reaches the machining zone in the form of mist jet to enhance heat transfer. Experimental results indicate that under the conditions of 40 m/s in the jet impinging speed and 10 mm in the jet impinging distance, the critical heat flux（CHF） nearly reaches 6× 10^7 W/m^2, more than six times of the CHF of the grinding burn with a value of （8～10）×10^6 W/m^2.

Liquid film dryout model for predicting critical heat flux in annular two-phase flow

Gas-liquid two-phase flow and heat transfer can be encountered in numerous fields, such as chemical engineering, refrigeration, nuclear power reactor, metallurgical industry, spaceflight. Its critical heat flux (CHF) is one of the most important factors for the system security of engineering applications. Since annular flow is the most common flow pattern in gas-liquid two-phase flow, predicting CHF of annular two-phase flow is more significant. Many studies have shown that the liquid film dryout model is successful for that prediction, and determining the following parameters will exert predominant effects on the accuracy of this model: onset of annular flow, inception criterion for droplets entrainment, entrainment fraction, droplets deposition and entrainment rates. The main theoretical results achieved on the above five parameters are reviewed; also, limitations in the existing studies and problems for further research are discussed.

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux(CHF) of departure from nucleate boiling(DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of Φ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m^(-2)·s^(-1), inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m^(-2). Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.

Critical Heat Flux(CHF)Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor,for which an accurate critical heat flux(CHF)correlation is necessary.Up to now,there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures.However,the CHF correlations’accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor.The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for some database.In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure,the present paper developed a new CHF correlation.Compared with other existing CHF correlations,the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices,such as those in the fusion reactor divertor.The validation results show that the new correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new CHF correlation is 7.4%less than that of the best existing one(W-2 correlation),further confirming its superior prediction accuracy and reliability.Besides,the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.

Critical Heat Flux of Blast Furnace Hearth in China

The critical heat flux surveys of thirteen Chinese blast furnaces were carried out. The mathematical model of hearth bottom was established and the temperature field was simulated by utilizing the method of inverse problem based on the collected parameters and temperature data. The critical heat flux and dangerous critical heat flux of hearth were defined and analyzed as well as the initial and investigative critical heat flux of hearth, and the influences of thermal conductivity and residual thickness of carbon bricks on critical heat flux were discussed. The relationships between critical heat flux of stave and hearth bricks were also compared. It is found that the dangerous critical heat flux of these blast furnaces ranged from 9.38 to 57 kW/mz. Therefore, there was no uniform critical heat flux of hearth due to the structure design, refractory materials selection, construction quality of hearth and other factors. The heat flux should be lower than the critical heat flux with corresponding thickness of carbon bricks to control the erosion of hearth. The critical heat flux of stave would be much lower than that of hearth bricks with the air gap. However, the critical heat flux of stave should be higher than that of hearth bricks when gas existed between furnace shell and staves.

低碳钢表面氧化效应对加热面朝下CHF的影响

先进压水堆压力容器下封头采用材料为低碳钢SA508,其表面容易氧化,可能对堆内熔融物滞留(IVR,in-vessel retention)策略临界热流密度(CHF,critical heat flux)产生显著影响,从而影响该策略安全余量。针对这一问题,本文首先研究了低碳钢SA508不同程度表面氧化对加热面朝下CHF的影响,其次研究了在低碳钢表面氧化情况下纳米流体强化CHF效果。结果发现,低碳钢SA508表面氧化对加热面朝下池式沸腾CHF具有显著影响,随着氧化程度的加深,CHF快速增长并最终达到稳定,表面完全氧化情况下CHF稳定值是表面轻微氧化情况下的2倍左右;将完全氧化低碳钢表面放入纳米流体中进行沸腾实验,发现纳米流体可以继续提高完全氧化表面CHF但效果大幅减弱,CHF增强原因在于表面纳米颗粒沉积层具有很强的吸水性。

Critical Heat Flux of Boiling Heat Transfer in aModerate Narrow Space

Boiling heat transfer process is analyzed in a moderate narrow space consisted of two horizontal plates.The main difference between this process and the conventional unconfined pool boilillg is the liquidsupply mechanism which is absolutely prevented by the growth of coalescence bubble along with theheated surface in the narrow space. As a result, the macrolayer becomes thinner due to the evaportion of the individual bubbles within the macrolayer during the period of bubble coalescence, with orwithout dryout that depends on both the gap size of narrow space and the size of heated surface. Fora specified size of the heated surface, the initial thickness of the liquid layer has a critical value whichapproaches a constant while the space height is larger than a critical value. The individual bubblebehaviors and tlie heat transfer can be considered as the same as that in the unconfined pool boiling, ifthe space gap is large. However, the individual bubbles do not generate in the last period of the bubblecoalescence and a lower maximum heat fiux will be resulted if the space gap is reduced. In such a case,the macrolayer is dryout.

High-Quality Critical Heat Flux in Horizontally Coiled Tubes

An investigation on the high-quality dryout in two electrically heated coiled tubes with horizontally helix axes is reported. The temperature profiles both along the tube and around the circumference are measured, and it is found that the temperature profiles around the circumference are not idelltical for the cross-sections at different parts of the coil. The 'local condition hypothesis' seems applicable under present conditions, and the critical heat flux qcr decreases with increasing critical quality xcr The CHF increases as mass velocity and ratio of tube diameter to coil diameter (d/D) increases, and it seems not to be effected by the system pressure. The CHF is larger with coils than that with straight tubes, and the difference increases with increasing mass velocity and d/D.

A Numerical Study of the Irradiated Ignition of Obliquely Oriented Thermally-Deformable Polymer

This study numerically investigates the influence of molten matter dynamics on the gasification and subsequent ignitability of an inclined thermoplastic specimen subjected to localized irradiation heat flux normal to the surface. A thermoplastic material is modeled as a phase change material with predefined solidification and melting temperatures, respectively, and the gasification process is modeled by the Arrhenius law of molten matter. Gas phase kinetics is not considered for simplicity purposes;instead, the onset of ignition of polymer is estimated on the basis of the critical mass flux concept. According to the numerical results, as the inclination angle becomes steeper (toward the vertical angle), the estimated ignition delay becomes shorter, showing ignition is promoted, whereas it is turned to be difficult to occur when inclination angles are above the vertical angle (>90?) having a longer delay time for the onset of gasification. With careful observation, the thermal interaction between the hot molten matter and unmelted (cold) solid is found to play an important role in gasification. The formation of a bulge due to resolidification to suppress the dripping downstream could be the source to promote ignition. By contrast, the hot molten matter is enforced to detach from the unmelted solid and “freely fall-off” to prohibit ignition for inclination angles beyond 90?. This supports the notion that high-enthalpy caused by the external heating is simply lost because of dripping, and there is less chance of catching fire there.