Two-phase Flow Region Research Articles

Preliminary Results for Flow Boiling Heat Transfer of R1233zd in Microchannels

The paper reports preliminary measurement results for flow boiling heat transfer of refrigerant R1233zd in parallel horizontal microchannels. The aluminum test section consists of two blocks and has 10 parallel channels with 1.0 mm in width, 1.5 mm in height and 440 mm in length. Five small thermocouple holes with diameter 0.6 mm and depth 20 mm were drilled at 10 locations along the channel on the two aluminum blocks, respectively. Local heat flux and channel surface temperature along the channel were obtained using the inverse heat conduction method [1] based on temperatures accurately measured by 100 thermocouples. Local saturation temperatures were obtained by assuming linear pressure distribution over the two-phase flow region along the channel. Experiments were conducted under different conditions. The refrigerant mass flow rate varied from 100 kg/m²s to 500 kg/ m²s. The heating water mass flow rate varied from 200 ml/min to 1300 ml/min. The heating water inlet temperature varied from 40℃ to 80℃. Preliminary results are presented and discussed.

Enhanced flow boiling in silicon nanowire-coated manifold microchannels

An experimental study was carried out to investigate the heat transfer, pressure drop and flow instability characteristics associated with the flow pattern of deionized water during two-phase boiling in a silicon-based manifold microchannel heat sink coated with silicon nanowires (SiNWs) compared to a plain-wall device. The manifold microchannel device featured parallel transverse microchannels etched on a silicon substrate and longitudinal microchannels etched on a glass cover plate. Silicon nanowires were generated on the bottom and the sidewalls of the silicon microchannels. A closed-loop experimental system was constructed to demonstrate thermal and hydraulic performance. Experimental results were presented with mass fluxes ranging from 250 to 1250 kg/m2 s and subcooled inlet temperatures from 15 K to 65 K. Results for the SiNWs device showed an approximate 20% improvement in heat flux rejection compared to the plain-wall device under the same wall superheat conditions. A subcooled inlet temperature of 65 K associated with a mass flux of 1250 kg/m2 s is shown to be capable of dissipating an effective heat flux of 431.3 W/cm2 with a wall superheat of about 85 K. Overall, the SiNW coatings proved positive effects on enhancing the flow boiling heat transfer with slower pressure drop increase, meanwhile the three-dimensional manifold microchannel design is revealed to effectively mitigate flow instability during the entire single and two-phase flow regions. This indicates great potential in utilizing three-dimensional flows by integrating SiNWs surface structures in high heat flux cooling applications.

Sizing charts of helical capillary tubes used in refrigeration and air conditioning

This paper presents new selection and correction factor charts for the sizing of adiabatic helical capillary tubes operating with alternative refrigerants. The mathematical model is based on the conservation of mass, energy, and momentum of fluids in capillary tubes. In the two-phase flow region, the developed model is based on the homogenous flow assumption. It is verified by the accuracy of the results of comparison between simulation results and experimental data in open literature. After the developed model is validated, selection and correction charts that include the important parameters are proposed for sizing adiabatic helical capillary tube. The present charts are proposed for R-134a, R-404A, R-407B, R-407C, and R-410A, and a wide range of operation. These newly developed selection and correction charts can be used to select helical capillary tube size from flow conditions and the mass flow rate or to predict the mass flow rate from capillary tube size and flow condition.

Effect of two-phase flow on transmission torque of oil film at high rotational speeds

PurposeThe purpose of this paper is to study the working characteristics of hydro-viscous clutch at high rotational speeds and obtain the trend of flow field variation of oil film.Design/methodology/approachThe FLUENT simulation model of the oil film between the friction disks is built. The effect of variation of working parameters such as input rotational speed, oil flow rate and film thickness on two-phase flow regime and transmission torque is studied by using the volume of fluid model.FindingsThe results show that the higher the rotational speed, the severer the cavitation is. In addition, the two-phase flow region makes the coverage of oil film over the friction pairs’ surface reduce, which results in a decrease in transmission torque for the hydro-viscous clutch.Originality/valueThese simulation results are of interest for the study of hydro-viscous drive and its applications. This study can also provide a theoretical basis for power transmission mechanism of oil film by considering the existence of a two-phase flow regime consisting of oil and air.

Experimental investigations on the boiling heat transfer of horizontal flow in the near-critical region

The critical point is the end point of a phase equilibrium curve; liquid and its vapor can coexist under designated points. Close to the critical point, thermophysical properties present clear variations, especially in the region of 0.85Pcr∼Pcr. Latent heat and liquid density in this region decrease more quickly than in lower-pressure areas, resulting in unique boiling heat transfer behavior. This region is also called the near-critical region. However, only a few scholars have discussed the heat transfer phenomenon; thus, it is difficult to ascertain the near-critical region’s properties and characteristics from extant literature. In the present study, we conduct experimental investigations to explore the specificities of the heat transfer characteristics of carbon dioxide in horizontal flow within the near-critical region in a circular channel with a diameter of 4 mm. The operating pressure ranges from 6.26 MPa to 7.3 MPa with a mass flow rate between 200 and 400 kg/m2 s, heat flux between 5 and 140 kW/m2, and test section inlet temperature of −5 °C. Then, we examine the inner-wall temperature and heat transfer coefficient profiles at different pressures within the near-critical region. The results show that at high heat flux, departure from nucleate boiling (DNB) phenomenon presents with a sudden decrease in the heat transfer coefficient in the subcooled region. The higher the heat flux, the more seriously deteriorating the heat transfer is. Interestingly, the temperature reaches its peak in the post-DNB region rather than at the critical vapor quality point. With an increase in pressure, DNB occurs early with lower vapor quality, and the temperature peak decreases at the given heat flux and mass flux. On the contrary, DNB is delayed with an increase in mass flux. A series of boiling heat transfer correlations in a subcooled region, two-phase flow region, and superheated region are proposed in addition to a new predictive correlation for critical heat flux in the near-critical region at a given mass flux.

Poroperm characteristics of high-rank coals from Southern Qinshui Basin by mercury intrusion, SEM-EDS, nuclear magnetic resonance and relative permeability analysis

Poroperm characteristics (porosity and permeability) of a coal seam play crucial roles in exploration and recovery of coalbed methane (CBM). In order to comprehensively understand the poroperm characteristics of high-rank coals, a series of laboratory tests including water-saturated porosity measurement, high-pressure mercury intrusion porosimetry (MIP), low-field nuclear magnetic resonance (NMR), scanning electron microscope and energy dispersive spectrometry (SEM-EDS) analysis and relative permeability tests were performed on the selected high-rank coal samples from underground coal mines in Southern Qinshui Basin, China. The cleat size distribution index (λ) and cleat tortuosity (η) were derived from the relative permeability experimental data and the correlations between these two parameters and coal rank parameters were analyzed. The results shows that: adsorption pores account for a dominant percentage in the total pore volume, while seepage-pores and fractures are poorly developed; clay minerals fill most of the micro-fractures and have a strongly negative impact on the coal permeability; the relative permeability curves for the coal samples is characterized by a higher residual water saturation, a higher water saturation at cross point, a narrow span of two-phase flow region, and a lower gas relative permeability. The cleat size distribution index (λ) is positively related to the water and gas relative permeability, while the cleat tortuosity (η) has a negative effect on fluid flow in coals. A mathematical model was proposed to relate η and λ to coal rank parameters respectively, which can be used to evaluate the relative permeability of high-rank coals. The producible porosity is very low, ranging from 0.17% to 0.37% for the high-rank coal samples, but it plays a dominant role in determining the permeability of coal. A modified producible porosity (PP) model was proposed to evaluate the absolute permeability and the effective gas permeability under the residual water saturation of high-rank coals.

Hydrodynamics of CO2-ethanol flow in a microchannel under elevated pressure

The hydrodynamics of gaseous and supercritical CO2-ethanol flows in microchannels under elevated pressure (45–90 bar) at 40 °C have been visualized experimentally. The effect of pressure and CO2-ethanol mixture composition on the flow regimes developed has been investigated and related to the thermodynamic phase equilibrium diagram. The characteristics of gas-liquid Taylor flow formed at elevated pressure have been analyzed and compared with Taylor flows at ambient conditions. The results of this study show that depending on the pressure and mixture composition, different flow regimes are formed, namely two-phase Taylor flow, dissolving Taylor flow, single-phase jetting-dissolving flow and single-phase supercritical jetting flow. The formation of these flows is explained with respect to the phase equilibrium diagram of CO2-ethanol. In the two-phase flow region, Taylor bubble size has been shown to depend on both CO2-ethanol mixture composition and operating pressure due to changes in CO2 mass flow rate and density, respectively. The bubble size formed has shown to increase linearly with UG/UL closely following the well-known bubble formation models for ambient conditions. Due to the non-negligible solubility of CO2 in ethanol, the Taylor bubble size decreases in the channel until equilibrium is reached. Mass transfer is not easily quantified however due to the mutually soluble nature of CO2 and ethanol and the dependency of this solubility on pressure.

Self-oscillatory regimes of the penetration of vertical free turbulent jets through a liquid surface

The results of an experimental investigation of the penetration of vertical plane and round free turbulent jets through the surface of a liquid contained in a relatively narrow channel are presented. It is established that there exist the ranges of jet thicknesses, their velocities, and free region lengths, on which regular self-oscillatory regimes of the displacement of submerged jet regions and two-phase flow regions are observable. The mechanism of the generation of these regimes and the special features of the observable flows are discussed. The dependences of the self-oscillation periods on the main control parameters of the problem are established.

Study on gas-liquid relative permeability experiments of fractured-porous reservoirs

Abstract For carbonate reservoirs, gas-water and oil-gas relative permeability curve are of importance for parameters calculation in oil-field development, dynamic analysis and numerical reservoir simulation. The oil-gas/gas-water relative permeability curves were measured with unstable method under normal temperature and low pressure, using fifteen artificial fractured cores. The shape and change characteristics of gas/water and oil/gas relative permeability curve were mainly analyzed. The results showed that for the relative permeability curve of water driving gas. The saturation of equal permeability point, the saturation range of two-phase flow region and the displacement efficiency has a good power-function relation with core permeability. In addition, the saturation of equal permeability point, the saturation range of two-phase flow region and the displacement efficiency will decrease with the increasing of core permeability. However, for oil-gas relative permeability curves, the saturation of equal permeability point, the saturation range of two-phase flow region and the displacement efficiency will increase with the increasing of core permeability. It should be noted that the gas relative permeability presents a straight line in the normalized relative permeability curve, which indicates that the main flow is dominated by fracture with low displacement efficiency. The research results provide theory basis for fracture-vuggy carbonate reservoir development and new research idea for the further study of this kind of reservoir.

Experimental study on flow condensation of mixture in a hydrophobic microchannel

The condensation flow patterns of ethanol–water mixture in a wide range of ethanol mass concentration inside a hydrophobic microchannel is experimentally studied via a high speed imaging system. The effects of channel surface wettability and ethanol concentration on flow condensation are compared and discussed. The experimental results indicate that the surface hydrophobic modification and ethanol concentration play a significant role in the flow condensation of mixture in a microchannel. The droplet condensation appears almost the whole two-phase flow region when the water steam is the main component, in which the droplet flow, droplet-streak flow, droplet-annular flow, droplet-injection flow and droplet-slug/bubble flow occur sequentially in a hydrophobic microchannel. With increasing ethanol concentration, the droplet-slug/bubble flow after droplet-injection flow disappears and is replaced by the pure slug/bubble flow. When the ethanol vapor is the main component, the droplet condensation almost disappears, and the annular-streak flow, annular flow, injection flow and slug/bubble flow appear sequentially along the flow direction. Both an increase in vapor Reynolds number and a decrease in ethanol concentration cause the injection location move toward the channel outlet. In addition, the surface hydrophobic modification introduces the droplet condensation, which is beneficial for the flow condensation heat transfer.

Real-time dynamic analysis for complete loop of direct steam generation solar trough collector

Direct steam generation is a potential approach to further reduce the levelized electricity cost of solar trough. Dynamic modeling of the collector loop is essential for operation and control of direct steam generation solar trough. However, the dynamic behavior of fluid based on direct steam generation is complex because of the two-phase flow in the pipeline. In this work, a nonlinear distribution parameter model has been developed to model the dynamic behaviors of direct steam generation parabolic trough collector loops under either full or partial solar irradiance disturbance. Compared with available dynamic model, the proposed model possesses two advantages: (1) real-time local values of heat transfer coefficient and friction resistance coefficient, and (2) considering of the complete loop of collectors, including subcooled water region, two-phase flow region and superheated steam region. The proposed model has shown superior performance, particularly in case of sensitivity study of fluid parameters when the pipe is partially shaded. The proposed model has been validated using experimental data from Solar Thermal Energy Laboratory of University of New South Wales, with an outlet fluid temperature relative error of only 1.91%. The validation results show that: (1) The proposed model successfully outperforms two reference models in predicting the behavior of direct steam generation solar trough. (2) The model theoretically predicts that, during solar irradiance disturbance, the discontinuities of fluid physical property parameters and the moving back and forth of two-phase flow ending location are the reasons that result in the high-frequency chattering of outlet fluid flow. (3) The model validates that the solar irradiance disturbance at subcooled water region would generates larger fluctuation of fluid parameters than two-phase flow region or superheated steam region.

Heat transfer coefficient for flow boiling in an annular mini gap

The aim of this paper was to present the concept of mathematical models of heat transfer in flow boiling in an annular mini gap between the metal pipe with enhanced exterior surface and the external glass pipe. The one- and two-dimensional mathematical models were proposed to describe stationary heat transfer in the gap. A set of experimental data governed both the form of energy equations in cylindrical coordinates and the boundary conditions. The models were formulated to minimize the number of experimentally determined constants. Known temperature distributions in the enhanced surface and in the fluid helped to determine, from the Robin condition, the local heat transfer coefficients at the enhanced surface – fluid contact. The Trefftz method was used to find two-dimensional temperature distributions for the thermal conductive filler layer, enhanced surface and flowing fluid. The method of temperature calculation depended on whether the area of single-phase convection ended with boiling incipience in the gap or the two-phase flow region prevailed, with either fully developed bubbly flow or bubbly-slug flow. In the two–phase flow, the fluid temperature was calculated by Trefftz method. Trefftz functions for the Laplace equation and for the energy equation were used in the calculations.

Approach for Modeling Rocket Plume Impingement and Dust Dispersal on the Moon

When a lander approaches the lunar surface, the plume from the descent engine impinges on the ground and entrains loose regolith into a high-velocity spray. This problem is simulated with a hybrid continuum-kinetic solver, where the internal nozzle flow and near-field plume are simulated with a continuum solver (data parallel line relaxation) and the regions of two-phase flow are simulated via the direct simulation Monte Carlo method. Fully coupled two-phase dust and gas motions and an inelastic grain–grain collision model have been implemented for a polydisperse distribution of particles sizes. This work examines the sensitivity of the solutions to these models for the case of steady axisymmetric plume impingement at different hovering altitudes. Of the physical phenomena modeled in this work, the dust sprays were found to be most sensitive to grain–grain collisions. The coefficient of restitution for colliding grains was parametrically studied and has a significant impact on how grain–grain collisions scatter the dust sprays.

SURFACE TENSION EFFECTS ON A CONJUGATE LAMINAR FILM-CONDENSATION PROCESS FOR A VERTICAL FIN PLACED IN A POROUS MEDIUM

In this work we treat theoretically the conjugate film-condensation process on a vertical fin embedded in a homogeneous porous medium filled with a saturated vapor. The presence of the solid matrix results in the ocurrence of a two-phase flow region governed by gravity and capillarity. In order to predict the influence of surface tension on the condensed thickness, an overall energy balance in the liquid, the two-phase region and through the fin was conducted. Therefore, the conservation equations of mass, momentum and energy for the condensed film, together with the energy equation in the fin are reduced to a nonlinear system of two differential equations containing five dimensionless numbers: the Bond number, Bo; the Jakob number, Ja; the Rayleigh number Ra; a conjugate heat transfer parameter, α, which represents the competition between the heat conducted by the fin in the longitudinal direction and the heat conducted through the condensate film, and the aspect ratio of the fin, e. Using the limit of Ja ≪ 1 with Ra ≫ 1, and finite values of Bo, together with the boundary layer approximation for the film condensation process, the nondimensional heat transfer or Nusselt number and the overall mass flow rates of the condensed fluid have been obtained as functions of the involved dimensionless parameters.

An experimental study of turbulent two-phase flow in hydraulic jumps and application of a triple decomposition technique

Intense turbulence develops in the two-phase flow region of hydraulic jump, with a broad range of tur- bulent length and time scales. Detailed air-water flow measurements using intrusive phase-detection probes enabled turbulence characterisation of the bubbly flow, although the phenomenon is not a truly random process because of the existence of low-frequency, pseudo-periodic fluctuating motion in the jump roller. This paper presents new measurements of turbulent properties in hydraulic jumps, including turbulence intensity, longitudinal and transverse integral length and time scales. The results characterised very high turbulent levels and reflected a combination of both fast and slow turbulent components. The respective contributions of the fast and slow motions were quantified using a triple decomposition technique. The decomposition of air-water detection signal revealed ''true'' turbulent characteristics linked with the fast, microscopic velocity turbulence of hydraulic jumps. The high-frequency turbulence intensities were between 0.5 and 1.5 close to the jump toe, and maximum integral turbulent length scales were found next to the bottom. Both decreased in the flow direction with longitudinal turbulence dissipation. The results highlighted the considerable influence of hydrodynamic instabilities of the flow on the turbulence characterisation. The successful application of triple decomposition technique provided the means for the true turbulence properties of hydraulic jumps.

A two-fluid model for refrigerant flow through adiabatic capillary tubes

This work presents a numerical model to simulate refrigerant flow through capillary tubes, commonly used as expansion devices in refrigeration systems. The flow is divided in a single-phase region, where the refrigerant is in the subcooled liquid state, and a region of two-phase flow. The capillary tube is considered straight and horizontal. The flow is taken as one-dimensional and adiabatic. Steady-state condition is also assumed and the metastable flow phenomena are neglected. The two-fluid model, considering the hydrodynamic and thermal non-equilibrium between the liquid and vapor phases, is applied to the two-phase flow region. Comparisons are made with experimental measurements of the mass flow rate and pressure distribution along two capillary tubes working with refrigerant R-134a in different operating conditions. The results indicate that the present model provides a better estimation than the commonly employed homogeneous model. Some computational results referring to the quality, void fraction, velocities, and temperatures of each phase are presented and discussed.

Boiling two-phase flow and efficiency of co- and counter-current microchannel heat exchangers with gas heating

To learn how to utilize the exhaust heat from a high-temperature gas product of a methanol reformer, the present study experimentally investigates the boiling two-phase flow in co- and counter-current microchannel heat exchangers (MCHE) with gas heating. Boiling two-phase flow patterns, two-phase flow instability, and efficiency are explored. The working fluid on the hot and cold sides are helium and liquid methanol, respectively. The silicon-based MCHE, which has dimensions of 20mm×20mm, is designed with 18 parallel microchannels on both sides and is prepared using microfabrication processes. Four types of two-phase flow patterns – bubbly-elongated slug flow, annular flow, annular flow with liquid film breakup, and dryout are identified in both types of MCHE that are studied. A flow pattern map is then constructed on the plane of the methanol mass flux versus heat flux for both types of MCHE. In the counter-current MCHE, the efficiency increases significantly with an increase in the mass flux in both the single- and two-phase flow regions, while the effect of mass flux is insignificant in the co-current MCHE. In the two-phase flow region, the efficiency of both types of MCHEs gradually increases with an increase in the hot-side thermal power until the CHF is approached. The highest efficiency obtained in the present study is about 0.85 and 0.90 for the co- and counter-current MCHEs, respectively.

Negative-Saturation Approach for Compositional Flow Simulations of Mixed CO2/Water Injection Into Geothermal Reservoirs Including Phase Appearance and Disappearance

Summary Cold mixed CO2/water injection into hot-water reservoirs can be used for simultaneous geothermal-energy (heat) production and subsurface CO2 storage. This paper studies this process in a 2D geothermal homogeneous reservoir, a layered reservoir, and a heterogeneous reservoir represented by a stochastic-random field. We give a set of simulations for a variety of CO2/water-injection ratios. In this process, often regions of two-phase flow are connected to regions of single-phase flow. Different systems of equations apply for single-phase and two-phase regions. We develop a solution approach, called the nonisothermal-negative-saturation (NegSat) solution approach, to solve efficiently nonisothermal compositional flow problems (e.g., CO2/water injection into geothermal reservoirs) that involve phase appearance, phase disappearance, and phase transitions. The advantage of this solution approach is that it circumvents using different equations for single-phase and two-phase regions and the ensuing unstable switching procedure. In the NegSat approach, a single-phase multicomponent fluid is replaced by an equivalent fictitious two-phase fluid with specific properties. The equivalent properties are such that the extended saturation of a fictitious gas is negative in the single-phase aqueous region. We discuss the salient features of the simulations in detail. When two phases are present at the injection side, heterogeneity and layering lead to more CO2 storage compared with the homogeneous case because of capillary trapping. In addition, layering avoids movement of the CO2 to the upper part of the reservoir and thus reduces the risk of leakage. Our results also show that heterogeneity and layering change the character of the solution in terms of useful-energy production and CO2 storage. The simulations can be used to construct a plot of the recovered useful energy vs. maximally stored CO2. Increasing the amount of CO2 in the injection mixture leads to bifurcation points at which the character of the solution in terms of energy production and CO2 storage changes. For overall injected-CO2 mole fractions less than 0.04, the result with gravity is the same as the result without gravity. For larger overall injected-CO2 mole fractions, however, the plot without gravity differs from the plot with gravity because of early breakthrough of a supercritical-CO2 tongue near the caprock. The plot of the useful energy (exergy) vs. the CO2-storage capacity in the presence of gravity shows a Z-shape. The top horizontal part represents a branch of high exergy recovery and a relatively lower storage capacity, whereas the bottom part represents a branch of lower exergy recovery and a higher storage capacity.

Integration of heat-energy recovery and carbon sequestration

Cold mixed CO2–water injection into geothermal reservoirs can be used to integrate geothermal-energy production and subsurface CO2 storage. This article studies this process in a 2D geothermal reservoir derived from the Delft Sandstone Member below the city Delft (The Netherlands). In this process, often regions of two-phase flow are connected to regions of single-phase flow. Different systems of equations apply for single-phase and for two-phase regions. For this purpose, we have applied a new and effective solution approach, the so-called “negative saturation” (NegSat) solution approach. The results show that permeability and porosity heterogeneities in a geothermal aquifer significantly influence both heat extraction and CO2 storage. Hence, reservoir characterization plays an important role in assessing the benefit of CO2 storage and energy extraction. The CO2-trapping mechanism is more efficient in the heterogeneous-permeability field because of the larger permeability heterogeneity contrasts and consequently larger capillary effects, compared to the homogeneous-permeability field. In particular, CO2 banks are mainly formed in the highly permeable zones that are surrounded by less permeable zones. However, the existence of non-isolated highly permeable zones for some injected CO2 concentrations leads to earlier breakthrough. Moreover, heterogeneity considerably weakens gravity effects. The frequent occurrence of evaporation and condensation, which is particularly effective close to the bubble point, substantially delays CO2 breakthrough and leads to a larger amount of heat-energy production and CO2 storage. Based on the simulations, it is possible to construct a plot of the recuperated heat energy versus the maximally stored CO2 for a variety of conditions.

Study of the influence of axial conduction in a boiling heated pipe

In this work a characterization of a horizontal heated pipe has been performed. This characterization consists in a steady state analysis of the thermohydraulic behavior of a boiling heated channel with subcooled liquid at the inlet. The temperature, velocities and pressure profiles along the heated section have been analyzed and the length of the single- and two-phase flow regions have been characterized. Without axial conduction and due to the big difference in the magnitude of the single- and two-phase heat transfer coefficient, steep temperature gradients were observed. By adding the effect of axial conduction, more heat is removed from the single phase region and added to the two-phase region through the wall. The net effect was a decrease in length of the two-phase flow region and consequent increase of the single-phase region. It was also observed that the axial conduction decreases the gradients in the wall temperature profile but it does not influence markedly its temperature differences with the fluid.