Effect Of Mass Flux Research Articles

Void fractions in a rod bundle geometry at high pressure–Part Ⅱ: Drift-flux model assessment and development

The present paper is Part Ⅱ of an investigation into void fraction measurement and modeling in a triangular-array rod bundle geometry under high-pressure conditions (P = 5–9 MPa and G = 100–350 kg m−2 s−1). In Part I of the study, the experimental facility and measurement techniques were described, and the measured local, chordal, and area-averaged void fractions were presented and analyzed. In this paper, six existing drift-flux models for rod bundles are assessed using the present data. Results show that most of the models are inconsistent with the measured void fraction under low void fraction conditions. This is because the effects of pressure and mass flux on drift-flux parameters are not considered. Hence, a new drift-flux model is developed with flow parameters and flow regime transition being considered. Additionally, the general applicability of this new model is verified by both the present data and existing data in the literatures. Results show that the new drift-flux model agrees well with the experimental data, with the relative deviations of void fraction less than 20%, indicating that the new model is applicable under a wide range of mass-flux conditions.

Effects of wall superheat and mass flux on flow film boiling in cryogenic chilldown process

Chilldown process is usually the initial operation for storing and transporting cryogenic fluids. During this process, film boiling is of great importance in terms of the large time span. In this research, a numerical model was built to study the flow regime and heat transfer characteristics of the film boiling with cryogenic fluids. An algebraic interface area density framework was coupled in the model to calculate the drag force at the liquid-gas interface. Accordingly, the effects of wall superheat and mass flow rate on heat flux and film thickness were investigated. Furthermore, the variance analysis method was employed to evaluate the weight of the effects of superheat and inlet mass flow and their coupling effect on the heat transfer coefficient. The results reveal that the wall superheat effect prevails over the mass flux effect on the heat transfer coefficient. Moreover, the two factors have a relatively weak coupling impact on the heat transfer coefficient. Finally, the features of pressure drop in the tube were further analyzed.

A numerical investigation for dating 210Pbex and 137Cs vertical profiles in a coastal area: The Eastern Ligurian Sea, Italy

210Pb and 137Cs vertical profiles in coastal sediments are studied using a sedimentation-mixing model. The cores were sampled in a complex coastal area characterized by the presence of different impact sources and environmental uses (riverine inputs, commercial and military harbour, marine protected area) in the Eastern Ligurian Sea, Italy. The analysis of accumulation and dispersion processes is performed using a numerical advection and diffusion model, in terms of the independent variables - time and mass depth. The flux of 210Pb is considered in steady state while the time dependent input flux of 137Cs is estimated from the concentration of this radionuclide in seawater, starting from observed data from 1960 to date. Differently from the atmospheric fallout distribution, this input function contains, at least partly, the contribution that still continues to reach the sediment in the last 25 years as a result of coastal and riverine input. The analysis highlights some features obscured in experimental data, and allows comparison of the effects of different scenarios. The specific effect of a pulsed input is discussed by analysing the effects of the Chernobyl event. The effects of mass flux in non–steady state are also considered: we observe that since the pulsed inputs in 137Cs are now too old, a strong superficial mixing and a time-variable flux produce similar profiles for both radionuclides. Hence, the general environmental knowledge of the area remains the main instrument to fully define the active processes in some cores.

Condensation two-phase flow patterns for zeotropic mixtures of tetrafluoromethane/ethane in a horizontal smooth tube

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are essential components of mixed-refrigerants. In this paper, the adiabatic flow pattern map for R14/R170 mixtures was firstly developed based on previous flow pattern data. Then an experimental investigation on condensation two-phase flow patterns for R14/R170 mixtures (0.193/0.0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube with inner diameter of 4 mm was presented. Experiments were carried out at mass fluxes from 100 to 350 kg m−2s−1, saturation pressures from 1.5 to 2.5 MPa and heat fluxes from 8.4 to 42.2 kW m−2 over the entire range of vapor quality. The effects of mass flux, saturation pressure, heat flux and concentration on flow pattern evolutions were discussed and presented. Analysis results indicated that mass transfer resistance and heat flux account for the nonequilibrium effects of zeotropic mixture condensation, which leads to the different evolutionary trends between condensation and adiabatic two-phase flow. Finally, a new condensation flow pattern map was deduced by containing nonequilibrium effects into adiabatic flow pattern map.

Condensation heat transfer characteristics and pressure drops of R410A, R22, R32, and R290 in a multiport rectangular channel

Experiments on the two-phase flow condensation heat transfer of R410A, R22, R32, and R290 (propane) inside a multiport minichannel tube are investigated in this study. The experiments are conducted with heat fluxes in the range of 3 to 15 kW/m2, working fluids mass fluxes in the range of 50 to 500 kg/m2s, and vapor quality up to 1.0, at a fixed saturation temperature of 48 °C. The test section is an aluminum multiport minichannel tube with a hydraulic diameter equal to 0.83 mm and 18 rectangular channels. The effects of mass flux, heat flux, and thermal properties of refrigerants on the heat transfer performance and pressure drop are illustrated and analyzed. The experimental heat transfer coefficients are compared with well-known correlations. Finally, from the experimental data, a new condensation heat transfer coefficient correlation for all refrigerants is proposed.

Shell-Side Condensation Characteristics of R410A on Horizontal Enhanced Tubes

Abstract An experimental investigation into heat transfer characteristics during condensation on two horizontal enhanced tubes (EHTs) was conducted. All the tested EHTs s have similar geometries with an outer diameter of 12.7 mm, and a plain tube was also tested for comparison. Investigated enhanced surfaces consist of dimples, protrusions, and grooves, which may produce more flow turbulence and enhanced the liquid drainage effect. The effects of mass fluxes and vapor quality were compared and analyzed. Test conditions were as follows: saturation temperature fixed at 45 °C, mass flux varying from 100 to 200 kg m−2 s−1, and vapor quality ranging from 0.3 to 0.8. The heat transfer coefficient was presented, and the results show that the proposed enhanced surfaces seem to have worse performance than the conventional tubes when the mass flux is less than 150 kg m−2 s−1, while one of the enhanced tubes (2EHT-1) produce an enhanced ratio of 1.03–1.14 when G = 200 kg m−2 s−1. Besides, it was found that the heat transfer coefficient increases with increasing vapor quality, which can be attributed to the increasing diffusion resistance. Mass flux seems to have little effect on the heat transfer performance of smooth tubes, while that of 1EHT increases obviously with increasing mass flux, especially for high vapor qualities.

Experimental investigation on the condensation regime and pressure oscillation characteristics of vertical upward steam jet condensation with low mass flux

An experimental study was conducted to investigate the condensation regime and pressure oscillation characteristics of vertical upward steam jet condensation with low mass flux. Steam mass flux and water temperature were 8.34–16.71 kg/(m2·s) and 40–85 °C, respectively. Steam bubble behavior, pressure oscillation amplitude and frequency were recorded and analyzed. Four typical regimes were found, namely, chugging, detached oscillatory, detached cracked and secondary bubble impinged regimes. A bubble condensation region map was developed by considering the effects of steam mass flux and water temperature. The average amplitude of pressure oscillation initially increased, and then decreased as water temperature increased. It reached its maximum value when water temperature was approximately 60–65 °C, which was the transition temperature range from the chugging regime to the detached oscillatory regime. Meanwhile, the frequency of pressure oscillation was within the range of 12–28 Hz. These values were higher than those for a vertical downward steam jet with the same steam mass flux and water temperature. A dimensionless correlation was obtained to predict the Strouhal number of pressure oscillation frequency. The predicted values corresponded well with the experimental data. The deviation was within the range of –9.12% to +8.30%.

Void fractions in a rod bundle geometry at high pressure–part Ⅰ: Experimental study

An investigation of void fraction measurement and prediction in a triangular-array rod bundle geometry was carried out under high pressure conditions (P = 5–9 MPa and G = 100–350 kg·m−2·s−1). The aims of this work were to expand the existing database and assess and modify the predictive models. The present paper is the Part I of this study, where the experimental work was presented to measure local, chordal and area-average void fractions respectively by a comprehensive measuring system composed by optical probes and gamma-ray densitometry. Non-uniform distribution of void fraction was investigated by local-to-local and local-to-average comparisons of void fractions. The results indicated that non-uniform distribution was not only related to the geometry, but also affected by the transition of flow regimes in rod bundle. The effects of pressure and mass flux on void fractions were also analyzed in detail in consideration of variations of fluid properties and drift-flux parameters. Additionally, both void fraction and interface frequency were compared with the flow regime map in TRACE code. The variation of interface frequency indicated that the droplet entrainment would occur in annular flow at high mass flux. In the Part Ⅱ of this study, an assessment of existing models for void fraction prediction in rod bundles was carried out according to the present data and a newly-developed drift-flux model was proposed.

Combined effects of free convection and chemical reaction with heat–mass flux conditions: A semianalytical technique

This paper discusses the effect of heat and mass flux on the natural convective laminar flow of a viscous incompressible fluid under the influence of radiation, magnetic field and Joule heating. The partial differential equations related to the problem have been changed as a set of ordinary differential equations employing non-dimensional quantities. Semianalytical approach such as the Adomian decomposition method (ADM) is employed to solve the system of ordinary differential equations. The behaviour of characterising parameters on the velocity, heat and mass transfer profiles, and the engineering quantities of interest, i.e. skin friction, heat and mass transfer rates and other indices are presented through graphs.

Enhanced flow boiling heat transfer characteristics of R134a on graphene-Cu nanocomposite coating on copper substrate

This study presents the flow boiling heat transfer characteristics of graphene-Cu nanocomposite coated copper surfaces. First, recently reported studies regarding efforts to enhance flow boiling heat transfer are discussed, then related studies on the impacts of using nanocomposite coated surfaces on flow boiling heat transfer characteristics are reviewed. Experimental investigations explore the effects of graphene concentration and feed mass flux of a refrigerant (R134a) on the heat flux and the heat transfer coefficient. The test chamber is a rectangular channel of 60 mm width, 3 mm height, and 460 mm length, and the dimensions of the graphene-Cu composite test plates are 10 mm × 10 mm. The feed mass fluxes used in the experiments are 240 and 480 kg/m2 s, while the wall heat flux are varied from 0.1 to 20 W/cm2. Based on the experimental analysis, the heat flux slightly increased before the onset of nucleating bubbles. Subsequently, the values drastically increased as the wall superheat continued to increase. Additionally, in experiments with higher graphene concentration, higher heat flux and heat transfer coefficient were demonstrated, and the increase in feed mass flux promoted flow boiling heat transfer.

Effect of a mass flux at bubble-liquid interface on propagation properties of weakly nonlinear waves

Toward the establishment and realization of medical applications accompanied with a phase change such as a drug delivery system, a formulation of the effect of mass transfer (i.e., mass flux) at the bubble-liquid interface on a ultrasound propagation process has been desired. Since a large amplitude ultrasound is utilized in such an application, the consideration of both the mass flux and the wave nonlinearity is essential. Although Fuster and Montel [J. Fluid Mech.(2015)] investigated linear wave propagation, nonlinear analysis has not been performed. In this paper, we theoretically clarify the effect of mass flux at the bubble-liquid interface on weakly nonlinear (i.e., finite but small amplitude) propagation of pressure waves in bubbly liquids. The effect of total evaporation or condensation as a mass flux across the bubble interface is taken into account as a simple classical model. Although various types of dissipation are installed, the effect of viscosity of gas inside the bubble is neglected. The set of basic equations for bubbly flows is composed of the conservation laws of mass, momentum, and energy, equation of motion for radial oscillations of bubble wall, equations of state, and so on. From the method of multiple scales, some nonlinear wave equations (e.g., KdV-Burgers equation) are derived. We then discuss the effects of mass flux on the propagation processes of nonlinear waves.

Thermodynamic performance analysis of supercritical pressure CO2 in tubes

Thermodynamic characteristics of supercritical pressure CO2 were numerically investigated in tubes under cooling and heating conditions. The effects of mass and heat fluxes, pressure, cross section shape and buoyancy were studied, and the determinant of the peak heat transfer coefficient was discussed. The results indicated that smaller ratio of heat flux to mass flux q/G and pressure are beneficial to the heat transfer and could lead to smaller fluid friction and entropy generation. Among three tubes with different cross sections, the circular one has the best bulk heat transfer performance in terms of both first and second laws of thermodynamics. Under heating conditions, the heat transfer deterioration may occur at the entrance when the ratio of q/G is relatively large. Replacing the circular tube with a semicircular one could alleviate local deteriorations efficiently. The local heat transfer coefficient has a lot to do with the near-wall effective thermal conductivity instead of the static thermal conductivity. The heat transfer augmentation caused by buoyancy and the deterioration mitigation in the semicircular tube could be attributed to improved near-wall effective thermal conductivity as well.

Experimental investigation of gravity and channel size effects on flow boiling heat transfer under hypergravity

Abstract Understanding of flow boiling heat transfer under hypergravity is needed due to its applications to modern flight vehicles. Few investigations on this issue have been reported. The present paper presents the experimental results of R134a flow boiling heat transfer in 1.002 and 2.168 mm ID tubes under hypergravity up to 3.16 g. The results reveal effects of gravity, channel size, heat flux, mass flux, quality, and pressure on and their interrelations in flow boiling heat transfer. Gravity has strong effects on flow boiling heat transfer. The heat transfer coefficients (HTCs) under hypergravity levels up to 3.16 g are normally greater than those under Earth's gravity. The heat transfer characteristics in the 1.002 and 2.168 mm tubes are very different, indicating that gravity effects on flow boiling heat transfer strongly interrelate with channel size. Also, gravity effects on flow boiling heat transfer are influenced by heat flux, mass flux, and pressure, and somewhat related to quality.

Flow boiling in horizontal annuli outside horizontal smooth, herringbone and three-dimensional enhanced tubes

Heat transfer and pressure drop measurements of R410A saturated flow boiling in horizontal annuli were performed at a saturation temperature of 6 °C over the mass flux range of 8–107 kg/(m2 s). The inner tubes with the same inside diameter of 12.70 mm contain a smooth tube, a herringbone micro-fin tube and a 1EHT tube (a dimpled tube enhanced by petal-shaped background patterns). In this study, the test section is a typical double-tube type heat exchanger and the emphasis is focused on the effects of mass flux, vapor quality, annular width and surface structures on the annulus-side heat transfer. Wilson plot method was adopted to ensure the correction factor of water-side coefficient of 1EHT tube. For flow boiling tests in annuli with an annular gap of 6.35 mm, both the evaporating coefficient and pressure drop increase as the mass velocity increases. Better thermal performance was seen at a outlet vapor quality of 0.6. In this case, the heat transfer rate ratio of enhanced annulus to smooth annulus is 1.7–2.5 for the herringbone micro-fin tube and 1.4–1.7 for the 1EHT tube, respectively. When the annular spacing decreases, heat transfer deterioration will worsen accompanied with a sudden drop in boiling heat-transfer coefficient. Besides, a comparison of flow boiling characteristics of annulus-side and tube-side heat transfer was made. The annulus-side heat transfer coefficient is much larger than that of tube-side case.

Analytical model for the condensation heat and mass transfer characteristics of binary zeotropic mixtures

An analytical model was developed to study the condensation heat and mass transfer characteristics of binary zeotropic mixtures for annular flow inside a horizontal tube. The model was based on non-equilibrium film theory including consideration of the surface tension variation in the liquid phase as the concentration changes. The model was validated by comparing the calculated heat transfer coefficient with experimental data from the literature for methane/ethane mixtures with various compositions and mass fluxes. The model was then used to study the effects of composition, mass flux, inlet pressure and tube diameter on the heat and mass transfer resistances in the vapor phase and the heat transfer coefficient during condensation of methane/ethane mixtures. The results showed that the mass transfer resistance in the vapor phase was important in the high vapor quality region with the heat transfer resistance in the liquid phase being more significant in some cases.

Two-phase flow condensation heat transfer characteristic of R-600a inside the horizontal smooth and dimpled helical coiled tube in shell type heat exchanger

This paper presents an experimental study on condensation heat transfer of R-600a inside smooth and dimpled helical coiled tubes. The experiments were executed at saturation temperature of 35°, and 45 °C with mass fluxes of 75, 115, 156 and 191 kg m−2s−1. The effect of vapor quality, mass flux and saturation temperature on the heat transfer coefficient is examined. The experiments have been carried out for smooth and dimpled helical tube. The dimpled helical coiled tube showed higher heat transfer coefficient compared to the smooth helically coiled tube. The heat transfer data of R-600a are compared with R-134a from a previous study with the same operating condition for both tubes. We developed correlations to predict the condensation heat transfer coefficients in smooth and dimpled tubes that predict data within ±20%.

Effect of SiO2 nanoparticles on the heat transfer characteristics of refrigerant and tribological behaviour of lubricant

Scientists and researchers are enthusiastically utilizing novel technologies that can substantially reduce energy consumption in diverse thermal systems. The foremost aim of the present investigation is to exploit the potential of nanoparticles on the enhancement of heat transfer characteristics of refrigerant and tribological capabilities of lubricant. Experimental investigations on the forced convection heat transfer characteristics of R134a/SiO2 nanorefrigerant (R134a + SiO2-polyalkylene glycol nanolubricant) are carried out. The effect of particle concentration, heat flux, mass flux, and vapour quality are investigated. The friction reduction and anti-wear properties of the nanolubricant are evaluated using four-ball tribo-tester. The results reveal that the presence of nanoparticles in the refrigerant significantly enhances the heat transfer coefficient of refrigerant and the tribological behaviour of lubricant. The maximum increase in heat transfer coefficient is 163.2% corresponding to a particle concentration of 0.4%. The presence of nanoparticles in refrigerant results in a marginal increase in pressure drop. However, the penalty in pressure drop is insignificant compared to the enhancement in heat transfer. Excellent friction reduction and anti-wear performance are also manifested by the nanolubricant. The maximum reduction in the friction coefficient is 37.76% and that of wear scar diameter is 40.83%. Considering the heat transfer and tribological characteristics, the optimum value of the particle volume fraction is 0.4%. SiO2-PAG nanolubricant can be considered as a potential energy-saving alternative to refrigerant compressor oils.

Experimental investigation of flow boiling heat transfer performance in zigzag microchannel heat sink for electronic cooling devices

This study proposes an improved microchannel heat sink with offset zigzag cavities to enhance boiling performance. Flow and heat transfer characteristics are experimentally investigated via a microscale boiling heat transfer visualization system using acetone as the working fluid. At mass fluxes of 285, 402, and 684 kg/(m2·s) and heat fluxes of 1.31–80.26 W/cm2, the effects of heat and mass fluxes on flow pattern, heat transfer, wall temperature, and pressure drop of zigzag microchannel heat sink are analyzed and compared with conventional rectangle microchannel heat sink. Results show that the fluctuating amplitude first increases and then decreases with the increment of heat fluxes, thereby causing a quasi-steady and unstable boiling with small fluctuations. Instability becomes increasingly severe for the nucleate boiling area moves toward the inlet and flow reversal intensity increases with heat fluxes under low heat flux. With heat flux increased furtherly. The proportion of steam in the channel increases to reduce fluctuations. Compared with the rectangular microchannel, the zigzag microchannel evidently enhances heat transfer characteristic with lower wall temperature at the onset of nucleate boiling (ONB), higher heat transfer coefficient (HTC) and critical heat flux (CHF) and improves boiling stability by suppressing flow reversal and reducing two-phase pressure drop. The mechanism of zigzag microchannel enhancement boiling performance can be attributed to (1) the improved disturbance of the working fluid, which enhances forced convective boiling heat transfer and mitigates bubble coalescence and flow reversal strength; (2) the continuously developing liquid film and improved wettability maintain a stable liquid film evaporation and delay partial dry out. Increase in mass flux can improve CHF and reduce pressure drop albeit with high wall temperature of ONB. The local HTC is enhanced, except for the low heat fluxes, because the bubbles are condensed by liquid and cannot grow to absorb heat at low heat fluxes.

Boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel

This study experimentally investigated the flow boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel with a hydraulic diameter of 2.0 mm, and the effects of mass flux, vapor quality, heat flux, and refrigerant properties on the flow boiling characteristics were clarified. The heat transfer coefficient and pressure drop of R32 and R1234yf were measured in a mass flux range of 50–400 kgm−2s−1 at a saturation temperature of 15 °C. The flow pattern of the square minichannel outlet was observed and was classified as plug, wavy, churn, and annular flows. The heat transfer coefficients in the square minichannel were larger than those in the circular minichannel with a similar hydraulic diameter at low mass flux conditions. The heat transfer coefficients of R32 indicated higher values compared with those of R1234yf at same mass flux and qualities. An empirical heat transfer model taking into account the forced convection, nucleate boiling, and thin liquid film evaporation was developed for horizontal square and circular minichannels. The frictional pressure drop of R32 was 1.5–2 times higher than that of R1234yf at same mass flux and vapor quality condition, and the effect of channel shape on the frictional pressure drop was small unlike the boiling heat transfer.

CFD simulation of Departure from Nucleate Boiling in vertical tubes under high pressure and high flow conditions

In this study, Departure from Nucleate Boiling (DNB) is investigated in vertical tubes under high pressure and high mass flux conditions using two-fluid Eulerian approach coupled with improved wall boiling model. It is essential to determine the near wall void fraction accurately in order to predict DNB. After accessing the theoretical and experimental basis of the correlations used for bubble departure diameter and nucleation site density, existing correlations are recalibrated to capture the phenomena at high pressure conditions. Sensitivity analysis is performed to analyze the impact of boiling and momentum closures. The effect of mass flux, inlet temperature, and exit quality on DNB is studied. The proposed model predicted DNB within 15%. This result indicates that CFD is a promising tool for predicting DNB.