Vapor Volume Fraction Research Articles

Numerical simulation of cryogenic cavitating flow in LRE oxygen turbopump inducer

Turbopump is a critical component of modern liquid rocket engine (LRE), which employs cryogen as propellant. Its working performance is closely relevant to the reliability of LRE. The performance improvement of turbopump is limited by cryogenic cavitation condition. While the cryogenic cavitating flow characteristic is not well understood till now. A novel numerical method for cryogenic cavitation simulation was developed based on a transport-based cavitation model in the present investigation. The real thermodynamic properties of fluids and an additional energy source item which takes the heat transfer during phase change into account were imbedded into the code, and the empirical constants of the cavitation model are unnecessary to alter. It is found that a moderate reduction of released heat by vapor condensation in the energy source item provides better prediction accuracy for temperature distributions, and an optimal reduction proportion has been suggested. The cavitating flow features inside a three-bladed inducer using liquid oxygen as working fluid have been investigated in detail based on the proposed method, thermal effect reduces both the cavitation regions and the vapor volume fraction inside cavities remarkably, producing less blocking effect on the blade flow channels, which delays the head breakdown. Higher temperature shows stronger thermal effect and better cavitation performance. A classical semiempirical theoretical model is introduced to quantitatively predict the influence of thermal effect on cavitation performance. The predicted results are in consistent with the calculated results. The developed numerical model shows favorable universality to act as a reliable tool for cryogenic turbopump design.

Thermal analysis of drilling in titanium under flood and cryogenic cooling using coupled CFD and FEM

Measuring drill temperature in drilling in the presence of cutting fluids is always a challenge. Therefore, numerical techniques have been adopted to study heat generation during drilling. In this work, the distribution of temperature in drilling of titanium workpiece under various cooling strategies was investigated by developing a coupled computational fluid dynamics (CFD) and finite element model (FEM). The CFD model simulates some of the realistic aspects of drilling, such as cutting fluid boiling in the vicinity of drill cutting edges and rotation of the drill inside the fluidic domain. The CFD model gives temperature distribution and flow properties of cutting fluids around drill cutting edges. The temperatures, stresses, and cutting forces encountered during drilling were evaluated using 3D FEM. Chip formation was modelled by combining material model with Johnson Cook plasticity, and failure models. It was evident that coolant vapor volume fraction is the highest around drill cutting edges, and increases with cutting speed. Liquid nitrogen with low boiling point, undergoes a rapid phase change during drilling. A spiral streamlines as well as reversing and circular streamlines of coolants were observed. In the flood and cryogenic cooling conditions, the coolant velocities range from 0.71–1.05 m/s and 0.79–1.15 m/s, respectively. The cryogenic and flood conditions reduce the drill maximum temperature by 33–50 % and 25–38 %, respectively, as compared to dry condition. The numerically predicted temperature is ~4–12 % off the average experimental temperature. The numerical thrust and torque lie within ~2–18 % of the experimental values.

Predicting reverse flow characteristics in equal-height-difference passive heat removal system for ocean nuclear power

The equal-height-difference passive heat removal system (EHDPHRS) via natural circulation driving and utilizes ocean as infinite heat sink for heat dissipation, showing broad application prospect in ocean nuclear power field; but the heat removal pipe normally encounters the reverse flow of cool seawater into the pipe, leading to operation instability and even condensation-induced water hammer. In this study, the integration of volume of fluid model and condensation model based on surface renewal theory is used to simulate the reverse flow characteristics in the heat removal pipe under single-and two-phase natural circulation conditions. The interfacial heat and mass transfer are calculated by user-defined functions (UDF). The numerical results reveal that the buoyancy force (Fb) and condensation driving force (FΔP) both accelerate the reverse flow, while the inertial force (Fi) restrains the reverse flow; the reverse flow length presents monotonous decrease with increasing the flow velocity under single-phase conditions and parabolic-like trend for two-phase flow. A prediction correlation for the reverse flow length is developed using dimensionless Reynolds number, Grashof number and vapor volume fraction, covering single- and two-phase conditions. The predicted reverse flow length by the correlation agrees with 94% of the results within deviations of ±15%.

Coolant flow in drilling titanium considering two phase boiling

In drilling titanium alloys, significant heat build-up on drill cutting edges due to insufficient heat dissipation raises tool temperature, leading to its rapid wear. Researchers have experimented with a variety of cooling methods to reduce temperature as well as tool damage. The in-situ monitoring of temperature of cutting edges presence of fluids in the drilling is difficult. Therefore, computational fluid dynamics (CFD) based model is proposed to investigate the temperature distribution along the drill cutting edges, and the flow characteristics of the cutting fluids such as water, mist and liquid nitrogen around the cutting edges. The boiling of cutting fluids near the cutting edges of the drill was considered by using Volume of Fluid (VOF) multiphase model in the analysis. The mist cooling was handled using a discrete phase model, and the Euler–Lagrangian technique was employed to deal with interactions between air and droplets. The highest vapour volume fraction of coolants was detected along the chisel edge of the drill cutting edge in flood and cryogenic cooling conditions. Whereas, for mist cooling, maximum vapour volume fraction of coolants was detected end of cutting edges. As the liquid nitrogen has an unusually low boiling point, it undergoes a quick phase change during the initial stages of drilling. Coolant velocities around the cutting edges in flood, cryogenic, and thick-thin mist cooling were 0.03–0.12 m/s, 0.7–0.9 m/s, 4–9 m/s, and 6–13 m/s, respectively. The maximum temperature of the drill drops by 45–48%, 32–38%, 29–36%, and 24–32% in cryogenic, thick mist, thin mist and flood conditions, respectively, over the dry condition. The numerically predicted temperature lies within an error of ∼4–18% of that of the average experimental temperature.

Parameter study of the grid spacer position on subcooled boiling in an upward vertical tube at low pressures

One of the principal issues in the phase change heat transfer at the vertical channel that changes the vapour fraction in the two-phase system is the presence of a grid spacer which prompts the temperature and velocity jump. In this study, the subcooled fluid stream rising inside the annular tube with grid spacer has been concentrated to investigate the effect of its location on changes in the gas volume fraction ratio, grid spacer temperature, and fluid flow jumping. The system of non-equilibrium two-phase flow of liquid-vapour exposed to an external heat transfer is modelled by mass, Euler-Euler model momentum, and energy equation of each phase. Validation of developed code is done by experimental results. Results show that extending the gas volume part as a result of a grid spacer presence decreases the Nusselt number, in conclusion, a temperature jump will occur. In the same manner, the presence of the grid spacer changes fluid streamlines. In this assessment, by contemplating the grid spacer in the vertical annular tube, the rising of fluid velocity in the vertical direction is demonstrated. As well the results show that the presence of a grid spacer moves to the liquid stage is less redirected conversely with vapour stage stream because of having more power; along these lines, around the completion of the grid spacer in the channel, the proportion of vapour volume fraction near the divider rises strongly. It has moreover been observed that if there are grid spacer inside the vertical annular tube, there will be no gigantic change in the liquid stream and gas volume division in various parts of the vertical annular tube when stood out from the customary vertical annular tube. The results demonstrate that the grid spacer before the heating zone will not change the vapour volume fraction inside the tube. The worst case happens for the grid spacer near the middle of the heated zone. The local peaks happen after the grid spacer when it is located after the heated zone.

A parametric study of subcooled flow boiling of Al2O3/water nanofluid using numerical simulation and artificial neural networks

ABSTRACT Utilizing an Euler-mixture three-dimensional numerical simulation for Al2O3/water nanofluid subcooled flow boiling in a mini channel, we study the effects of pressure, heat flux, nanoparticle concentration, surface roughness, and subcooled temperature on heat transfer quantities (average and local heat transfer coefficient, average and local vapor volume fraction, and average and local wall temperature) and bubble dynamics quantities (bubble departure diameter, bubble detachment frequency, bubble detachment waiting time, and nucleation site density). The numerical results demonstrate that the nanoparticles particularly impact the bubble dynamics significantly by increasing wettability and decreasing contact angle. In order to reduce the computational burden of such an expensive multiphase flow simulation, we also present a machine learning approach based on artificial neural networks (ANN). The numerical experiments show that using the ANN model, we can achieve highly accurate results with much less computational time and resources.

Numerical Study of the Effect of the Rolling Motion on the Subcooled Flow Boiling in the Subchannel

The marine environment may change the force on the fluid and inevitably influence bubble behavior and the two-phase flow in the reactor core, which are vital to the safety margin of a nuclear reactor. To explore the effect of the marine motion on the flow and heat transfer features of subcooled flow boiling in the reactor core, the volume of fluid (VOF) method is employed to reveal the interaction between the interface structure and two-phase flow in the subchannel under rolling motion. The variations of several physical parameters are obtained, including the transverse flow, the vapor volume fraction, the vapor adhesion ratio, and the phase distribution of boiling two-phase flow with time. Sensitivity analyses of the amplitude and the period of the rolling motion were performed to demonstrate the mechanisms of the influence of the rolling motion. We found that the transverse flow in the subchannel was mainly affected by the Euler force under the rolling motion. In contrast to the two-phase flow in the static state, the vapor volume fraction and vapor adhesion ratio show different characteristics under rolling motion. Additionally, the onset of significant void (OSV) point changes periodically under rolling motion.

Research on flow resistance characteristics of reactor coolant pump under large break LOCA at inlet

In the event of a large break loss-of-coolant accident (LBLOCA) at the inlet of the nuclear Reactor Coolant Pump (RCP), the RCP is likely to enter an extreme working condition of the reverse pump mode. In this mode, the pump actually acts as a stopper to the coolant out of the circulation system. When the flow rate remains constant, the head reflects the flow resistance capacity. In previous studies, we have found that the head of the RCP increases and then decreases with increasing cavitation under the reverse pump mode. Objective to research into this situation, numerical simulation of the RCP under the reverse pump mode has been used to analyze the flow resistance characteristics inside the impeller. Results show that the high-velocity flow generated by the cavitation region is the crucial factor causing the variation in pump head in comparison with the vapor volume fractions of the impeller blade surface. To investigate the effect of high-velocity flow on the pump head under different cavitation conditions and to even better reveal the flow resistance characteristics, entropy production analysis and vorticity analysis are carried out for the internal flow field of the impeller.

Numerical and experimental study of the transient conjugate heat transfer and fluid flow of passive residual heat removal heat exchanger

The transient heat transfer performance of passive residual heat removal heat exchanger (PRHR HX) with C-tube bundle directly influences nuclear reactor safety under accident condition. This paper presents an investigation of transient conjugate heat transfer and fluid flow phenomena and characteristics of scaled-down single C-tube submerged in water tank under atmospheric pressure at the initial period before saturation pool boiling occurs. Empirical calculation model, experimental setup and numerical model were established separately, and both numerical and experimental studies were carried out. The distributions of tube outlet temperature and water temperatures at horizontal and vertical monitoring points in tank were obtained. Detailed comparisons of the results show good agreements. Also, the change tendencies of heat flux, heat transfer coefficients and heat load were analyzed. The fluctuation of heat flux along normalized length from tube inlet firstly increases but finally tends to keep stable with the change of temperature difference and transformation of heat transfer mechanism. The heat transfer coefficient inside tube shows a slight downtrend along normalized length from tube inlet, but the heat transfer coefficient outside tube shows obvious change in the transient process. The total heat load continuously decreases with time. Furthermore, the volume fraction of vapor is relatively low during the subcooled boiling period. The two-phase flow enhances the agitation in water tank and the fluid velocity near tube outside wall is much larger.

Influence of Cutter Head on Cavitation of Non-Jammed Submerged Grinder Pump

For the investigation of the cavitation of non-jammed submersible grinder pumps, a GSP-22 model pump was numerically simulated based on CFX. ICEM-CFD was applied to a structured mesh for the flow components. Pump performance and the influence of the cutter head on cavitation with different cutter head numbers and shapes were investigated. The results were as follows: with increases in the number of cutter heads, the effects of the cutter heads on the water increased, and the flow rate near the cutter head increased correspondingly—which eventually led to aggravated cavitation near the cutter head of the non-jammed submersible grinder pump. The head of the submersible grinder pump with a streamlined cutter changed little compared to the pump with a non-streamlined cutter; the overall power declined by 13.2% and the highest efficiency increased by 6%. For all pumps with different numbers of cutter heads, the vapor volume fraction of the streamlined cutter head was lower than that of the non-streamlined cutter head, and the vapor distribution area size of the streamlined cutter head was smaller than that of the non-streamlined cutter head. This means that changing the cutter head shape to streamlined can effectively control the cavitation intensity near the cutter head.

Mass transfer rate effects on the cavitating vortex shedding flow around a circular cylinder at a low Reynolds number

Cavitating vortex shedding is a flow phenomenon commonly encountered behind bluff bodies in hydraulic machinery and systems. The generated vapor bubbles significantly change the dynamic behaviors of the vortices. In addition, the collapses of the cavities become major sources of unwanted effects such as vibration, noise and material erosion. In this study, the numerical results of the cavitating vortex shedding behind a circular cylinder at a low Reynolds number have been compared and analyzed using different mass transfer rate values between the vapor and the liquid water phases. For that, the transport equation for the vapor volume fraction coupled with the source term based on the simplification of the Rayleigh-Plesset equation has been solved. Commonly, this source term that accounts for the mass transfer rate is a function of the pressure difference, the vapor volume fraction, and the empirical factors. To understand the effects of this rate, its has been gradually increased from its default value up to the infinite when the equilibrium assumption is satisfied. The current results show that as the rate increases, higher gradients of the vapor volume fraction and of the pressure near the interface between the vapor and the liquid phases are predicted. As a consequence, the frequency of the vortex shedding is also affected by the variation of the mass transfer rate.

Numerical investigation of Cavitation in Francis Turbine

Cavitation is one of the major challenges in the Himalayan rivers of Nepal that cause damage and degradation of the performance of hydraulic turbine components. Numerical analysis has been performed to investigate the cavitation effect in the Francis turbine. For detection of cavitation, the SST turbulence model using Rayleigh plesset equation has been used in ANSYS-CFX. The turbine is investigated for three different operating conditions i.e. part load, BEP and full load under cavitation and without cavitation. The results are shown based on efficiency, pressure fluctuation, vortex rope and vapor volume fraction. It has been observed that changes in efficiency and pressure distribution are found to be minimal between cavitation and without cavitation conditions at rated discharge and head. Under part load condition, turbine efficiency loss and vapor bubbles formation at the leading edge of the runner blade is found to be maximum.

Influence of Rotor Geometry on Cavitation Characteristics of Rotational Hydrodynamic Cavitation Generator

The RNG k-ε turbulence model and mass transport cavitation model are applied to simulate the cavitating flows in a rotational hydrodynamic cavitation generator with a rotor and a stator. The cavitation generator is designed to have surface dimples on the rotor. The cavitating flows analyses in cavitation generator are focused mainly in the rotor. The accuracy and reliability of the calculation model and method are demonstrated by the good agreement between with experimental and numerical results. The pressure distribution, streamline distribution and vapor volume fraction distribution in cavitation generator with five different structures (5 different diameter of rotor dimples d=11mm, 13mm, 15mm, 17mm, 19mm) under the same operation condition are analysed. The calculation results show that the characteristics of streamline and cavitaion area and pressure area are quite different based on the magnitude of the dimple diameter of the rotor. It is observed that with the rotor dimple diameter increase, the cavity bubbles firstly appear in the low pressures area of dimple bottom and develops rapidly towards the wall and top of dimple; the mass transfer between water and vapor is even more intense; the low pressures area increases in dimple and the vortex area expands and the number of vortices increases, which induces the strong turbulence on the surface of the rotor and inside the rotor. The increasing the diameter of the rotor dimple is an effective means to enhance the cavitation effect of the rotational hydrodynamic cavitation generator.

Influence of Rotor Dimple Geometry on Cavitation Characteristics of Rotational Cavitation Generator

The current investigation is aimed to simulate the internal flows in a rotational cavitation generator with a rotor and a stator by using the RNG k-ε turbulence model and Zwart-Gerber-Belamri cavitation model. The design of the cavitation generator is that the rotor is provided with surface dimples. A detailed analysis of the pressure distribution, vapor volume fraction distribution and velocity distribution in cavitation generator with five different structures (5 different depth of rotor dimples h=25mm, 35mm, 45mm, 55mm, 65mm) under the same operation condition, is presented. The simulation results demonstrate that the characteristics of velocity and pressure area and cavitaion area are quite different based on the magnitude of the dimple depth of the rotor. It is showed that with the rotor dimple depth increase, cavitation area appears in the low pressures area at the bottom of the dimple and expands rapidly inside dimple; near the top of the dimple, the pressure gradient becomes larger and larger and velocity gradient changes dramatically; which induces strong flow disturbance at the top of the dimple and on the surface of the rotor, the water vapor exchange effect is enhancing, and the cavitation effect is getting stronger and stronger. The cavitation effect of the rotational hydrodynamic cavitation generator can be enhanced by increasing the depth of the rotor dimple.

A Novel Integrated Design for Heat and Water Recovery from Exhaust Flue Gas of Bandar Abbas Power Plant

This study concerns a theoretical design of a condensing heat exchanger for a 320 MW unit of Bandar Abbas thermal power plant in the south of Iran. A film theory in conjunction with heat and mass transfer analogy is used as the theoretical basis of the design. The condensing unit is used for heat and mass recovery from the natural gas-fired boiler flue gases. The assumed condensing unit includes 4 equal capacity condensing heat exchangers, each of which is supposed to reduce the flue gas temperature from 160 ℃ to 53℃. Decreasing the flue gas temperature to below the dew point temperature of its water vapor causes condensation (latent) and sensible heat transfer. The analysis was done for 13%, 15%, and 17% of the water vapor volume fraction in the flue gases, and based on the 17% water vapor fraction, 52.8 tons/hr of water was recovered. This recovered water could be used as the cooling tower makeup, and accordingly, almost 14% of water consumption is saved. The recovered heat by the condensing unit is also being used as the heat source of an ORC cycle, and up to 2.8 MW power is estimated to be generated depending on the evaporation temperature.

Numerical Study on Pool Boiling of Hybrid Nanofluids Using RPI Model

The performance of deionized (DI) water and hybrid nanofluids for pool boiling from a horizontal copper heater under atmospheric pressure conditions is numerically examined in the current study. The Eulerian–Eulerian scheme is adopted with a Rensselaer Polytechnic Institute (RPI) sub-boiling model to simulate the boiling phenomena and predict the heat and mass transfer in the interior of the pool boiling vessel. This paper attempts to correct the coefficient of the bubble waiting time (BWTC) in the quenching heat flux partition as a proportion of the total heat flux and then correlate this coefficient to the superheat temperature. The pool boiling curve and pool boiling heat transfer coefficient (PBHTC) obtained for the present model are verified against experimental data from the literature and show good agreement. In addition, this work comprehensively discusses the transient analysis of the vapor volume fraction contours, the vapor velocity vectors, and the streamlines of water velocity at different superheat temperatures. Finally, for BWTC, new proposed correlations with high coefficients of determination of 0.999, 0.932, and 0.923 are introduced for DI water and 0.05 vol.% and 0.1 vol.% hybrid nanofluids, respectively.

Analysis of Orifice in Biodiesel Reactor with Hydrodynamic Cavitation System using Computational Fluid Dynamics

Reactor technology for the transesterification process of vegetable oils or animal fats with methanol has been widely developed to obtain biodiesel products that comply with standards but at the lowest cost. The hydrodynamic cavitation reactor with orifice type is a choice for this purpose. This research aims to determine the optimal orifice design from several orifice designs tested through CFD simulation. Computer simulations performed on orifices A, B, C and D using the Schnerr and Sauer models show that orifice C is optimal for cavitation at an absolute inlet pressure of 3x105 N/m2 with the use of methanol as fluid. The parameters studied in the computer simulation are velocity, pressure, turbulent kinetic energy and vapor volume fraction. At the absolute inlet pressure of 3x105 N/m2, the maximum speed is 28.69 m/s, the minimum pressure is 12266 N/m2, the maximum vapor volume fraction is 0.98, and the maximum turbulent kinetic energy is 12.75 m2/s2. The results of simulation were compared with experiments conducted on a hydrodynamic cavitation reactor using orifices C and D. Measurements of the velocity and pressure parameters showed that there were no significant deviations between the results of the computer simulation and the experiment.

Thermal effect on cavitation characteristics of a hydraulic torque converter

The performance of torque converters, which use hydraulic oil as the working medium to transmit power, are severely affected by the operating temperature because various oil properties, such as viscosity, density and saturated pressure, vary drastically with temperature change. To study the thermal effect on the cavitation characteristics of viscous oil, we developed a full flow passage geometry and computational fluid dynamics (CFD) model with cavitation based on the finite volume method (FVM) to predict and analyze cavitation behavior and thermal effect in a torque converter. The results show that the critical cavitation number and critical cavitation temperature of the tested torque converter are 9.2 and 30 °C, respectively, and increasing the temperature reduces the cavitation number and induces the generation and intensification of cavitation. Besides, although higher temperature improves the transmission efficiency and capacity constant of the torque converter, it greatly magnifies the influence of cavitation on the hydrodynamic performance. In severe cases, it reduces the hydraulic performance by 9.3% and the flow velocity by 11.8% at 100 °C. Meanwhile, the cavitation flow field and the configuration of cavitation are also greatly affected by temperature. About 11.9% of the surface of the stator blade was covered by cavitation bubbles with the vapor volume fraction of 88.1% at 100 °C. Moreover, 30–60 °C is the optimum operating temperature range in terms of cavitations effect with no pronounced cavitation occurrence or that the cavitation degree is too weak to affect the performance in the mentioned temperature range.

Numerical Simulation of Unsteady Cavitation Flow in a Low-Specific-Speed Centrifugal Pump with an Inducer

Cavitation is an undesirable phenomenon in the pumps. In this paper, unsteady cavitation flow in a low-specific-speed centrifugal pump with an inducer is investigated based on the full cavitation model and standard k-ε turbulence model using the commercial software PumpLinx. The numerical results of external performance curve and cavitation performance curve of design condition agree well with that of the experiment. The bubbles in the inducer mainly appear on the outer blade leading edge. The regions of larger vapor volume fraction in the inducer and impeller increase with the decreasing of NPSHa. The regions of larger vapor volume fraction in the inducer expand from the outer edge of the blade inlet to the hub on suction surface. Under very low NPSHa, the impeller may be filled with bubbles in the passage and the pump head drops drastically, and there exist distinct back flows near the suction surface of the blade and impeller outlet. Affected by pre-swirl of inducer outlet and the circle flow of impeller blade inlet, the amplitude of pressure fluctuation near the impeller inlet is obviously larger than that at the inlet in the inducer. The dominant frequency of pressure fluctuation for four monitoring points is shaft frequency. Compared with the non-cavitation flows, the maximum amplitudes of pressure pulsation increase for cavitating flow.

Influence of Solid–Liquid Two-Phase Flow on Cavitation of Tubular Turbine Blades Under Combined Conditions

The influence of sediment media on the blade pressure and cavitation of a tubular turbine was investigated in this study. The Zwart–Geber–Belamri cavitation model and the shear stress transport k–w turbulence model were applied to numerically simulate and experimentally validate the full flow path of the tubular turbine under combined conditions for sediment particle sizes of 0.01 and 0.05 mm and concentrations of 1%, 1.5%, and 2%. The results show that the pressure of the blade increases with the sediment concentration. Cavitation mainly occurs between the blade shroud and the leading edge. The higher the sediment concentration, the lower the vapor volume fraction at the same sediment particle size. The presence of sediments inhibits further cavitation development, and the inhibition effect is significant. At the same concentration, the larger the particle size of the sediment, the lower the vapor volume fraction. Thus, the increase in the particle size inhibits cavitation, but the inhibition effect is not significant.