The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Oscar Mauricio Hernandez Rodriguez - One of the best experts on this subject based on the ideXlab platform.
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Influence of Liquid Viscosity and Geometry on Vertical Gas/Liquid Two-Phase Annular-Duct Flow
SPE Journal, 2020Co-Authors: Alex Roger Almeida Colmanetti, Marcelo Souza De Castro, Marcel Cavallini Barbosa, Oscar Mauricio Hernandez RodriguezAbstract:Summary Two-phase flow in large Annular Ducts is very common in industrial applications. Nevertheless, many two-phase-flow phenomena in such a geometry have not been fully understood. This article addresses an investigation on the effects of Duct geometry and liquid viscosity on vertical upward Annular-Duct two-phase flows. Compressed air and tap water or mineral oil were the working fluids. Three oil viscosity ranges were tested, from 30 to 400 mPa·s. A 10.0-m-long inclinable experimental setup was designed and built for the experiments conDucted in two equivalent geometries. The first is made of a 95-mm-inner-diameter circular pipe (called the reference pipe), and the second is a concentric Annular Duct with 95-mm hydraulic diameter, the same diameter as the reference pipe. The radial geometries were similar to those observed in oil wells. Flow patterns, total pressure drop, and in-situ volumetric fractions were obtained. According to the results, the standard hydraulic diameter might not be appropriate for the modeling of vertical gas/liquid flow in large Annular Ducts.
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Analysis of Turbulence Characteristics in Two Large Concentric Annular Ducts Through Particle Image Velocimetry
Journal of Fluids Engineering, 2018Co-Authors: Marlon M. Hernández-cely, Victor E C Baptistella, Oscar Mauricio Hernandez RodriguezAbstract:An experimental study is presented on water single-phase flow in two 10.5 m-long Annular Ducts, with external pipe's internal diameter (De) of 155 mm and two concentric internal pipes of external diameters (Di) of 60 mm and 125 mm, i.e., radius ratio (α = Ri/e) of 0.39 and 0.80, respectively, with the aim of improving the understanding of flows in Annular Ducts. Particle image velocimetry (PIV) was applied to obtain instantaneous and averaged velocity measurements of the flow field. A charge-coupled device camera (2448 pixel × 2050 pixel, 5 Mpixel, 12-bit ) recorded pairs of images of the seeding particles and a double-pulsed PIV laser (Nd:YAG, frequency doubled to 532 nm), with a measured pulse intensity of 70 to 75 mJ/pulse, provided the illumination. Laminar flows were analyzed for validation purposes, experimental data on turbulent flows were compared with the classical law of the wall of the turbulent boundary-layer model, and the shear stresses derived from PIV data were compared with those calculated from the measured pressure drop. The effects of the Reynolds number and geometry on turbulent velocity profiles and Reynolds stresses are presented. The results suggest that the law of the wall for Annular-Duct flow is a function of radius ratio. The new experimental results are of great value for the development of computational fluid dynamics models and more refined pressure-drop prediction tools in Annular-Duct flow.
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phase inversion phenomena in vertical three phase flow experimental study on the influence of fluids viscosity Duct geometry and gas flow rate
Chemical Engineering Science, 2018Co-Authors: Alex Roger Almeida Colmanetti, Marcelo Souza De Castro, Marcel Cavallini Barbosa, Oscar Mauricio Hernandez RodriguezAbstract:Abstract Improper sizing of pipelines and proDuction tubings in the petroleum industry is often caused by lack of understanding of three-phase flow, which is characterized by gas flowing together with an immiscible water-oil mixture. There is the presence of a continuous liquid phase and a liquid dispersed phase, as observed in liquid-liquid flows, i.e., one can observe either a dispersion or emulsion of oil in water (o/w) or dispersion or emulsion of water in oil (w/o). The transition from o/w to w/o, or the other way around, is defined as phase-inversion. This phenomenon is characterized by a sharp increase in the pressure gradient, leading to significant pressure loss in the oil proDuction system. The goal is to investigate the effects of oil viscosity, channel geometry (circular pipe or Annular Duct) and superficial gas velocity on the phenomenon of phase inversion in vertical liquid-liquid-gas flows, using oil with three different viscosities (ranging from 70 mPa s to 280 mPa s), tap water and compressed air as working fluids. The experiments were carried out in three different geometries: (i) glass pipe of 50 mm i.d., (ii) glass pipe of 95 mm i.d. and (iii) concentric Annular Duct with 95 mm of hydraulic diameter (glass outer pipe and PVC internal pipe). The experiments were performed under equivalent Reynolds number to evaluate the geometry effect. New data as total pressure gradient and volumetric fractions were obtained for all geometries. The results suggest that the extrapolation of results obtained in circular pipes with low viscosity oil and low gas flow rates to Annular-Duct flow with viscous oils and high gas flow rates can lead to significant errors.
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study and characterization of gas liquid slug flow in an Annular Duct using high speed video camera wire mesh sensor and piv
Experimental Thermal and Fluid Science, 2018Co-Authors: Marlon Mauricio Hernandez Cely, Victor E C Baptistella, Oscar Mauricio Hernandez RodriguezAbstract:Abstract An experimental study is presented on air-water two-phase flow in a 10.5-m-long Annular Duct with an external diameter of 155 mm and an inner diameter of 60 mm. Particle image velocimetry (PIV) is applied to obtain instantaneous velocity measurements of the flow field. The Annular Duct inclination is of 5 Â ° from the horizontal. A CCD camera ( 2448 pixel × 2050 pixel, 5 Mpixel,12-bit ) was positioned in the test section to record the seeding particles. The illumination was provided by a double pulsed PIV laser (Nd:YAG, frequency doubled to 532 nm) with a measured pulse intensity of 70 mJ/pulse. It was used at 15 Hz (resulting in the independence of the velocity samples). Based on the instantaneous local velocities, Probability Density Functions (PDF) and mean velocities are calculated. Two-phase flow arranged in the slug-flow pattern is observed, at superficial velocities of jw = 0.154 m/s and ja = 0.044 m/s. 3000 samples per case are processed using cross-correlation procedure, for the PIV analysis. A home-made Annular Wire-Mesh Sensor (AWMS) was applied to obtain time-signal-measured void-fraction data as a function of the electrical permittivity. The average bubble velocity is estimated by two techniques, (i) High-Speed Video Recording and (ii) Particle Image Velocimetry (PIV) together with the AWMS. A comparison of the two techniques is presented. A new technique based on AWMS for the measurement of bubble-passage frequency, bubble length and slug length is proposed. It was observed: (i) deceleration of the water phase beneath the bubble as it passes, shown by velocity profiles at different bubble locations, (ii) an increase in bubble velocity as air superficial velocity is increased and (iii) the complexity of the flow pattern, shown in details by AWMS cross-sectional images. The new experimental results are of great value for comparison with CFD models and for the development of more refined pressure-drop prediction tools in two-phase Annular-Duct flows.
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piv measurements of the instantaneous velocities of flow in an Annular Duct
Instrumentation and Measurement Technology Conference, 2017Co-Authors: Marlon Mauricio Hernandez Cely, Andreza B F De Oliveira, Victor E C Baptistella, Oscar Mauricio Hernandez RodriguezAbstract:Particle image Velocimetry (PIV) is a well-established technique in the field of fluid flow measurement and provides instantaneous velocity fields over global domains. It has been applied to external and internal flows, in single and two-phase flows. Regarding internal flow, works about the application of PIV in Annular Ducts are scanty. An experimental work is presented, where flow of water is studied in an Annular Duct of inner diameter of 60 mm and outer diameter of 155 mm and 10.5-m length, with the goal of obtaining detailed velocity measurements. Experiments were also conDucted in a 21.1-mm inner-diameter horizontal transparent pipe of 3m length for comparison purposes. Depending on the flow rates of water, it can be laminar, transitional or turbulent. In this study, the water flow rate was kept at two different values for the horizontal pipe, for either laminar or turbulent flow, and three different values for the Annular Duct, allowing the analysis of one laminar and two turbulent flows. Velocity fields and statistic quantities of the turbulent flow were calculated.
T.f. Lin - One of the best experts on this subject based on the ideXlab platform.
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time periodic evaporation heat transfer of r 134a in a narrow Annular Duct due to mass flow rate oscillation
International Journal of Heat and Mass Transfer, 2018Co-Authors: C.a. Chen, T.f. Lin, Wei-mon Yan, Mohammad AmaniAbstract:Abstract An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal Annular Duct was experimentally conDucted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a Duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations.
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Bubble characteristics in time periodic saturated flow boiling of R-134a in a narrow Annular pipe due to heat flux oscillation
International Journal of Heat and Mass Transfer, 2016Co-Authors: C.a. Chen, T.f. Lin, Wei-mon YanAbstract:Abstract In this work, experiments have been conDucted to investigate how the imposed time periodic heat flux oscillation affects the bubble characteristics of saturated flow boiling with refrigerant R-134a in a horizontal narrow Annular pipe. The test section for the horizontal Annular Duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow Annular Duct. In particular, attention is focused on the time periodic saturated flow boiling characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The results show that the bubble departure diameter, bubble frequency and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the boiling the resulting oscillation amplitudes of the bubble parameters, such as the bubble departure diameter, bubble frequency and active nucleation site density, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux.
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time periodic flow boiling heat transfer of r 134a and associated bubble characteristics in a narrow Annular Duct due to flow rate oscillation
International Journal of Heat and Mass Transfer, 2010Co-Authors: C.a. Chen, W R Chang, T.f. LinAbstract:An experiment is conDucted here to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on refrigeriant R-134a flow boiling heat transfer and associated bubble characteristics in a horizontal narrow Annular Duct with the Duct gap fixed at 2.0 mm. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface does not exist in an entire periodic cycle. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation only slightly affects the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting heated wall temperature oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in the bubble characteristics are noted. The effects of the mass flux oscillation on the size of the departing bubble and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing mass flux in the flow boiling, opposing to that in the single-phase flow. But they are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the wall temperature oscillation is also noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the R-134a flow boiling in the Annular Duct.
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subcooled flow boiling heat transfer of r 407c and associated bubble characteristics in a narrow Annular Duct
International Journal of Heat and Mass Transfer, 2008Co-Authors: C.a. Chen, Y M Lie, W R Chang, T.f. LinAbstract:An experiment is conDucted here to investigate how the channel size affects the subcooled flow boiling heat transfer and the associated bubble characteristics of refrigerant R-407C in a horizontal narrow Annular Duct with the gap of the Duct fixed at 1.0 and 2.0 mm. The measured boiling curves indicate that the temperature overshoot at ONB is relatively significant for the subcooled flow boiling of R-407C in the Duct. Besides, the subcooled flow boiling heat transfer coefficient increases with a reDuction in the Duct gap, but decreases with an increase in the inlet liquid subcooling. Moreover, raising the heat flux imposed on the Duct can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are slighter. Visualization of the subcooled flow boiling processes in the Duct reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Raising the imposed heat flux, however, produces positive effects on the bubble population, coalescence and departure frequency. Meanwhile, the present heat transfer data for R-407C are compared with the R-134a data measured in the same Duct and with some existing correlations. We also propose empirical correlations for the present data for the R-407C subcooled flow boiling heat transfer and some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density.
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saturated flow boiling heat transfer and associated bubble characteristics of r 134a in a narrow Annular Duct
International Journal of Heat and Mass Transfer, 2005Co-Authors: Y M Lie, T.f. LinAbstract:Experiments are conDucted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow Annular Duct. The gap of the Duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the Duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reDuction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow Duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the Duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow Annular Duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.
Seungbok Choi - One of the best experts on this subject based on the ideXlab platform.
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A new approach for an analytical solution of unsteady laminar flow in dispensing processes
Proceedings of the Institution of Mechanical Engineers Part C: Journal of Mechanical Engineering Science, 2010Co-Authors: Quoc Hung Nguyen, Seungbok ChoiAbstract:AbstractIn this study, a new method to analytically model unsteady laminar flows of both Newtonian fluid and power-laws fluid is developed. The proposed approach is based on the assumption that the pressure drop of the unsteady flow results from the pressure drop due to inertia and the pressure drop due to fluid viscosity, separately. After an overview of dispensing technology and historical modelling methods of the unsteady flows, a new modelling approach of the unsteady laminar flow is developed for the flow in a circular pipe and the axial Couette flow in an Annular Duct. Using the proposed model, simulation results of the unsteady flow in an arbitrary circular pipe and the Annular Duct are obtained and compared with other approach solutions. In addition, a comparative work between the proposed analytical dynamic model and the finite-element model is undertaken in order to demonstrate the effectiveness of the proposed modelling methodology.
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dynamic modeling of an electrorheological damper considering the unsteady behavior of electrorheological fluid flow
Smart Materials and Structures, 2009Co-Authors: Quoc Hung Nguyen, Seungbok ChoiAbstract:In this study, dynamic modeling of an electrorheological (ER) damper is performed considering the unsteady behaviors of ER fluid flow through the Annular Duct of the damper. After describing the configuration of the ER damper, quasi-static modeling of the damper is conDucted on the basis of the Bingham model of ER fluid. The pressure drop of the unsteady ER fluid flow through the Annular Duct between the electrodes of the damper with a known variation of flow rate is then obtained by solving the momentum equation of the ER fluid flow using the Laplace transform technique. Based on the proposed unsteady flow solution, the simulated results are obtained and compared with measured ones in order to evaluate the effectiveness of the proposed model. In addition, in order to reduce the computation load, a simplified solution of the dynamic damping force is proposed with validation.
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modeling of unsteady laminar flow based on steady solution in jetting dispensing process
IEEE Transactions on Electronics Packaging Manufacturing, 2008Co-Authors: Quoc Hung Nguyen, Seungbok ChoiAbstract:This paper proposes a new approach to model unsteady laminar flows of dispenser systems in the semiconDuctor packaging industry. The approach is based on the exact steady solution. After an overview of dispensing technology and historical modeling of the unsteady flows, a new modeling approach of the unsteady laminar flow in a circular pipe and Annular Duct is developed. From the proposed model, the modeling results of unsteady flow in an arbitrary circular pipe and Annular Duct are obtained and compared with the spectral solution. In addition, a comparative work between the proposed analytical dynamic model and the finite-element model is undertaken in order to demonstrate the effectiveness of the proposed modeling methodology.
C. Nouar - One of the best experts on this subject based on the ideXlab platform.
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Study of cooling with solidification of a laminar thermodependent Herschel-Bulkley fluid flow in a convectively cooled Annular Duct
Acta Mechanica, 2001Co-Authors: Z. Youbi, B. Benaouda-zouaoui, C. NouarAbstract:This article presents a numerical analysis of cooling with solidification for laminar Herschel-Bulkley fluid flow in an Annular Duct. The outer cylinder is subject to uniform cooling with the surrounding sink temperature below the freezing temperature of the fluid. The inner cylinder is considered adiabatic. It is assumed that all the physical properties of the fluid except the consistency K are constant. The K-T relation used is K=a exp (− bT ). Under this cooling condition, the thermal entrance region consists of two parts. The first one is a liquid-solidification free zone. The second one corresponds to the region where the solidification grows inward along the Annular Duct. The problem is governed by nine independent dimensionless numbers. Here, we focus on the effect of the rheological parameters and the thermo-dependency of K on the dynamic and thermal fields. The effects of the rheological parameters are analyzed through the flow behavior index n , and the relative dimension of the plug core flow ap _ e . Concerning the K-T variation effects, they can be described by the Pearson number Pn . Numerical results are obtained for the liquid-solidification free length z _ f , liquid-solid interface profile, pressure drop, axial velocity evolution, plug core dimension and local Nusselt number. They indicate how the effect of Pn depends on ap _ e and n .
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Laminar mixed convection in a horizontal Annular Duct. Case of thermodependent non-Newtonian fluid
European Journal of Mechanics - B Fluids, 2000Co-Authors: C. Nouar, B. Benaouda-zouaoui, Christophe DesaubryAbstract:Abstract The experimental and numerical analysis of combined forced and free convection heat transfer of a non-Newtonian fluid in a horizontal Annular Duct is presented. The flow is laminar and Prandtl and Boussinesq hypotheses are adopted. The outer and inner cylinders are heated uniformly with a constant heat flux density. At the inlet of the Annular Duct, the flow is fully developed and the temperature profile is uniform. The governing equations are solved numerically using finite differences. The variation of the rheological properties with temperature is taken into account. Near the entrance, forced convection is the dominant mechanism. The core flow is decelerated because of the decrease of the consistency K as the temperature T increases near the heated walls. Simultaneously, the downstream flow between the two cylinders caused by the displacement of the secondary boundary layer induces an acceleration of the flow in the lower part of the Annular Duct and a deceleration in the upper part. Further downstream, the fluid warms up and the buoyancy force effect becomes strong enough to overcome the forced flow. The critical Cameron number X + c , above which the convection mechanism becomes dominated by natural convection is determined using scaling analysis. The results are found to agree well with the numerical solution. For X + X + c and for a large Peclet number, an asymptotic solution is obtained by perturbing the forced convection solution. However, far from the entrance region, the thermal stratification induced by buoyancy force combined with the variation of K with T lead to another flow reorganization. There is an acceleration of the flow in the upper half part of the Annular Duct and a deceleration in the lower half one.
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numerical and experimental investigation of thermal convection for a thermodependent herschel bulkley fluid in an Annular Duct with rotating inner cylinder
European Journal of Mechanics B-fluids, 1998Co-Authors: C. Nouar, C Desaubry, H ZenaidiAbstract:Abstract Thermal convection for an incompressible Herschel-Bulkley fluid along an Annular Duct, whose inner cylinder is rotating and outer is at rest, is analyzed numerically and experimentally. The outer cylinder is heated at constant heat flux density and the inner one is assumed adiabatic. The first part of this study deals with the effect of the rheological behavior of the fluid and that of the rotation of the inner cylinder on the flow field and heat transfer coefficient. All the physical properties are assumed constant and the flow is assumed fully developed. The critical Rossby number Ro c = (R 1 Ω/U d ) c , for which the dimension of the plug flow is reduced to zero is determined with respect to the flow behavior index, the radius ratio and the Herschel-Bulkley number for axial flow. The rotation of the inner cylinder induces a decrease of the axial velocity gradient at the outer cylinder thereby reducing the heat transfer between the heated wall and the fluid. The second part of this study introduces the variation of the consistency K with temperature and analyzes the evolution of the flow pattern and heat transfer coefficient along the heating zone. Two cases are distinguished depending on the Rossby number: (i) Ro
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Thermal convection for a thermodependent herschel-bulkley fluid in an Annular Duct
Heat and Mass Transfer, 1996Co-Authors: C. Nouar, M. LebouchéAbstract:This article presents a numerical and experimental investigation of the thermal convection for a thermodependent Herschel-Bulkley fluid in an Annular Duct under the conditions of a uniform heat flux density on the outer wall and an insulated inner wall. In the numerical analysis, it is assumed that: (i) the rheological behavior of the fluid can be expressed through the Herschel-Bulkley law:\(\tau = \tau _s + K\dot \gamma ^n \); (ii) the flow is fully developed at the inlet; (iii) all fluid properties except consistency indexK are constant. TheK−T relation used isK=K0exp(−bT). The results obtained enable us to characterize completely the dynamical and thermal fields. The numerical solution is in good agreement with the experimental data, showing the reasonableness of the computed results.
C.a. Chen - One of the best experts on this subject based on the ideXlab platform.
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time periodic evaporation heat transfer of r 134a in a narrow Annular Duct due to mass flow rate oscillation
International Journal of Heat and Mass Transfer, 2018Co-Authors: C.a. Chen, T.f. Lin, Wei-mon Yan, Mohammad AmaniAbstract:Abstract An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal Annular Duct was experimentally conDucted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a Duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations.
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Bubble characteristics in time periodic saturated flow boiling of R-134a in a narrow Annular pipe due to heat flux oscillation
International Journal of Heat and Mass Transfer, 2016Co-Authors: C.a. Chen, T.f. Lin, Wei-mon YanAbstract:Abstract In this work, experiments have been conDucted to investigate how the imposed time periodic heat flux oscillation affects the bubble characteristics of saturated flow boiling with refrigerant R-134a in a horizontal narrow Annular pipe. The test section for the horizontal Annular Duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow Annular Duct. In particular, attention is focused on the time periodic saturated flow boiling characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The results show that the bubble departure diameter, bubble frequency and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the boiling the resulting oscillation amplitudes of the bubble parameters, such as the bubble departure diameter, bubble frequency and active nucleation site density, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux.
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oscillatory subcooled flow boiling heat transfer of r 134a and associated bubble characteristics in a narrow Annular Duct due to flow rate oscillation
International Journal of Heat and Mass Transfer, 2013Co-Authors: Sunghao Wang, C.a. Chen, Tingyu LinAbstract:An experiment is conDucted here to investigate how an imposed time periodic flow rate oscillation in the form of a triangular wave affects the long time subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow Annular Duct. In the experiment the mean R134a mass flux G varies from 200 to 500 kg/m 2 s, imposed heat flux ranges from 0 to 45 kW/m 2 , and the amplitude of the mass flux oscillation changes from 0 to 30% of G with the period of the mass flux oscillation varied from 20 to 120 s for the inlet liquid subcooling ranging from 0 to 6 C. The Duct gap is fixed at 2.0 mm. The results indicate that the inlet liquid subcooling significantly affects the oscillatory flow boiling heat transfer characteristics. Besides, when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears. The intermittent boiling prevails in a very different range of the Boiling number for a change in the inlet subcooling. Moreover, in the subcooled boiling the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time. Furthermore, in the persistent boiling at high imposed heat flux the resulting Tw oscillation is stronger for a higher inlet liquid subcooling and for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in dp, f and nac are noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the oscillatory R-134a subcooled flow boiling in the Annular Duct.
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time periodic flow boiling heat transfer of r 134a and associated bubble characteristics in a narrow Annular Duct due to flow rate oscillation
International Journal of Heat and Mass Transfer, 2010Co-Authors: C.a. Chen, W R Chang, T.f. LinAbstract:An experiment is conDucted here to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on refrigeriant R-134a flow boiling heat transfer and associated bubble characteristics in a horizontal narrow Annular Duct with the Duct gap fixed at 2.0 mm. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface does not exist in an entire periodic cycle. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation only slightly affects the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting heated wall temperature oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in the bubble characteristics are noted. The effects of the mass flux oscillation on the size of the departing bubble and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing mass flux in the flow boiling, opposing to that in the single-phase flow. But they are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the wall temperature oscillation is also noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the R-134a flow boiling in the Annular Duct.
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Experimental study of R-134a evaporation heat transfer in a narrow Annular Duct
International Journal of Heat and Mass Transfer, 2010Co-Authors: C.a. Chen, Chia-han Lee, Tsing-fa LinAbstract:Abstract An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow Annular Duct. The gap of the Duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the Duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient hr are examined in detail. For the Duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m2 s, imposed heat flux q from 5 to 15 kW/m2, vapor quality xm from 0.05 to 0.95, and refrigerant saturation temperature Tsat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m2 s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in Tsat, but the effects are less pronounced in the narrower Duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the Duct gap causes a significant increase in hr. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the Duct side show the change of the dominant two-phase flow pattern in the Duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow Annular Ducts is proposed.