Shape of Taylor bubbles in vertical tubes

The shape of a Taylor bubble in a vertical downward slug flow in a pipe is studied. The phase distribution in the Taylor bubble region is investigated by still pictures and video film. The shape of the Taylor bubble is reported for different condition. These experimental results consists of the position of the interface x(r) obtained using image processing and represents the new view to the shape of the Taylor Bubble and gives some light on the basic phenomena for the hydrodynamics of the two-phase slug flow.     

Numerical investigation of refrigerant flow through non-adiabatic capillary tubes

A mathematical model is developed to study flow characteristics in non-adiabatic capillary tubes. The theoretical model is based on conservation of mass, energy and momentum of fluids in the capillary tube and suction line. The mathematical model is categorized into three different cases, depending on the position of the heat exchange process. The first case is considered when the heat exchange process starts in the single-phase flow region, the second case is determined when the heat exchange process starts at the end of the single-phase flow region, and the last case is considered when the heat exchange process takes place in the two-phase flow region. A set of differential equations is solved by the explicit method of finite-difference scheme. The model is validated by comparing with the experimental data obtained from previous works. The results obtained from the present model show reasonable agreement with the experimental data. The present non-adiabatic capillary tube model can be used to integrate with system models working with alternative refrigerants for design and optimization.     

Numerical modeling of two-phase refrigerant flow through adiabatic capillary tubes

Literature shows that the homogeneous flow assumption has been commonly used in most of the adiabatic capillary tube modeling studies due to its simplicity. The slip effect between the two phases was often not considered in this small diameter capillary tube. This paper attempts to exploit the possibility of applying the equilibrium two-phase drift flux model to simulate the flow of refrigerant in the capillary tube expansion devices. Attempts have been made to compare predictions with experimental results. The details flow characteristics of R134a in a capillary tube, such as distribution of pressure, void fraction, dryness fraction, phase’s velocities and their drift velocity relative to the center of the mass of the mixture are presented.     

Numerical simulation and experimental validation of coiled adiabatic capillary tubes

The objective of this study is to extend and validate the model developed and presented in previous works [O. García-Valladares, C.D. Pérez-Segarra, A. Oliva, Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part I: mathematical formulation and numerical model, Applied Thermal Engineering 22 (2) (2002) 173–182; O. García-Valladares, C.D. Pérez-Segarra, A. Oliva, Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part II: experimental validation and parametric studies, Applied Thermal Engineering 22 (4) (2002) 379–391] to coiled adiabatic capillary tube expansion devices working with pure and mixed refrigerants. The discretized governing equations are coupled using an implicit step by step method. A special treatment has been implemented in order to consider transitions (subcooled liquid region, metastable liquid region, metastable two-phase region and equilibrium two-phase region). All the flow variables (enthalpies, temperatures, pressures, vapor qualities, velocities, heat fluxes, etc.) together with the thermophysical properties are evaluated at each point of the grid in which the domain is discretized. The numerical model allows analysis of aspects such as geometry, type of fluid (pure substances and mixtures), critical or non-critical flow conditions, metastable regions, and transient aspects. Comparison of the numerical simulation with a wide range of experimental data presented in the technical literature will be shown in the present article in order to validate the model developed.     

Numerical simulation of gas-contaminated refrigerant two-phase flow through adiabatic capillary tubes

An unfavorable effect of gas impurities on the throttling process inside a small-diameter tube, i.e. a capillary tube, has been studied in detail. A special testing capillary tube equipped with precise temperature and pressure sensors has been used for an experimental investigation of the capillary flow of a saturated fluorocarbon refrigerant, R218, contaminated by dissolved nitrogen. The gas impurities significantly affected the throttling process, since the two-phase flow started notably earlier than in the case of pure refrigerant flow. Moreover, the gas contamination resulted in a decreased mass flow rate of refrigerant delivered through the capillary tube. A comprehensive numerical model has been developed to simulate the capillary flow of gas-contaminated refrigerant. The model takes into account two coincident thermodynamic events: the throttling process of the refrigerant (solvent) and the gradual release of the dissolved gas impurities (solute) from the refrigerant liquid phase. The gas release is in principle described by using the temperature correlation of the Henry’s law constant. The model considers adiabatic, thermodynamically equilibrated capillary flow with homogeneous two-phase flow. The numerical simulation is in good agreement with our experimental data measured for R218 contaminated by nitrogen.     

A parametric study of refrigerant flow in non-adiabatic capillary tubes

This paper presents a parametric analysis of refrigerant flow through capillary tube–suction line heat exchangers, used in domestic refrigeration systems. The analysis is based on a homogeneous model developed by the authors. The model is based on the numerical solution of fundamental equations of conservation of mass, momentum and energy of refrigerant flow. The refrigerant flow characteristics are investigated by varying thermodynamic (e.g. condensing temperature, evaporating temperature, inlet sub-cooling, suction line superheat) and geometric parameters (e.g. inlet adiabatic length, heat exchanger length and internal diameter of the capillary tube) of the capillary flow. The source of divergence in the numerical solution process is found to be the discontinuity in non-adiabatic capillary tube flow characteristics caused by re-condensation of the refrigerant within the capillary heat exchanger.     

Experimental evaluation on the oil influence in capillary tubes

This work presents the results of an experimental study on pure refrigerant R-134a and refrigerant–oil mixtures flowing through capillary tubes in order to analyse the oil influence in component performance. Tests were carried out for capillary tubes internal diameters of 0.69 mm and 0.82 mm, condensing temperatures ranging from 40 °C to 50 °C, and subcooling degrees between 3 °C and 12 °C. Pure refrigerant flow measurements were compared to those for refrigerant–oil mixtures with oil concentrations of 1.0% and 3.0%.     

Numerical analysis of refrigerant flow along non-adiabatic capillary tubes using a two-fluid model

This work presents a numerical model to simulate steady state refrigerant flow along capillary tube-suction line heat exchangers, commonly used in small refrigeration systems. The flow along the straight and horizontal capillary tube is divided into two regions: a single-phase and a two-phase flow region. The flow is taken as one-dimensional and the metastable flow phenomenon is neglected. The two-fluid model is employed for the two-phase flow region, considering the hydrodynamic and the thermodynamic non-equilibrium between the liquid and vapor phases. Comparisons are made with experimental measurements of the mass flow rate and temperature distribution along capillary tube-suction line heat exchangers working with refrigerant R134a in different operating conditions. The results indicate that the present model provides a good estimation of the refrigerant mass flow rate. Moreover, comparisons with a homogeneous model are also made. Some computational results referring to the quality, void fraction and velocities of each phase are also presented and discussed.     

Numerical modelling of alternative refrigerants to HCFC‐22 through capillary tubes

In this paper, a numerical model is presented for predicting capillary tube performance using new alternative refrigerants to HCFC‐22. The model has been established after the fluid flow conservation equations written for a homogeneous refrigerant fluid flow under saturated, sub‐cooled and two‐phase conditions. Numerical results showed that the proposed model in question fairly simulated our experimental data and fairly predicted the capillary tube behaviour under different conditions. The results also indicated that a system using R‐407C would experience smaller pressure drop compared to R‐410A and R‐410B. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical study of the interactions and merge of multiple bubbles during convective boiling in micro channels

Multi bubbles interaction and merger in a micro-channel flow boiling has been numerically studied. Effects of mass flux (56, 112, 200, and 335 kg/m² 1 s), wall heat flux (5, 10, and 15 kW/m²) and saturated temperature (300.15 and 303.15 K) are investigated. The coupled level set and volume of fluid (CLSVOF) method and non-equilibrium phase model are implemented to capture the two-phase interface, and the lateral merger process. It is found that the whole transition process can be divided to three sub-stages: sliding, merger, and post-merger. The evaporation rate is much higher in the first two stages due to the boundary layer effects in. Both the mass flux and heat flux affect bubble growth. Specifically, the bubble growth rate increase with the increase of heat flux, or the decrease of mass flux.     

Numerical study of radial temperature gradient effect on separation efficiency in capillary electrophoresis

Joule heating effect is one of the factors that can decrease the separation efficiency in capillary electrophoresis. A mathematical model, including the Poisson–Boltzmann equation, the modified Navier–Stokes equation and the energy equation, is used to calculate temperature and velocity distributions in the capillary under different capillary inner radii, heat transfer coefficients and solution concentrations. By substituting the velocity distributions under these conditions into two different expressions for the thermal plate height, various effects on the thermal plate height and separation efficiency in capillary electrophoresis are analyzed. It is found that at the same electrical field strength, larger capillary radius, lower heat transfer coefficient and higher solution concentration can lead to a larger thermal plate height and therefore lowering the separation efficiency. The results of the separation efficiency based on the two different expressions agree well with each other.     

Numerical Study of the Effects of the Secondary Diaphragm and Contact Surface in Expansion Tubes

The interaction of the shock reflected at a secondary diaphragm with the primary contact in six cases, i.e. the strengths and shapes of the contact surface are different, were explored by numerical study. The influences of the strength and shape of the contact on the developing wave pattern and the quality of the test gas are illustrated by many kinds of figures (e.g. the time-distance diagrams of the acoustic impedance contours on the axis, the acoustic impedance contours, and the time histories of pilot and static pressures).     

Numerical analysis of the dynamics of moving vapor bubbles

Bubbles have been observed rapidly sweeping along very fine heated wires during subcooled nucleate boiling with jet flows emanating from the tops of the vapor bubbles. This paper analyzes the physical mechanisms driving the bubble and the jet flows from the tops of these moving bubbles. The flows are analyzed by numerically solving the governing equations for the velocity and temperature distributions around the bubble and the heated wire as the bubble moves along the wire. The bubble motion is due to the non-uniform temperature distribution in the liquid and in the wire caused by the bubble as it moves along the wire. The flow is driven by the horizontal Marangoni flow induced by the temperature difference across the bubble which thrusts the bubble forward. Comparisons with experimental observations suggest that the condensation heat transfer at the bubble interface is restricted by non-condensable gases that increases the surface temperature gradient and the resulting Marangoni flow.     

Numerical study of single bubbles with dynamic contact angle during nucleate pool boiling

Nucleate pool boiling is typically characterized by cyclic growth and departure of vapor bubbles from a heated wall. It has been experimentally observed that the contact angle at the bubble base varies during the ebullition cycle. In the present numerical study, a static contact angle model and dynamic contact angle models based on the contact line velocity and the sign of the contact line velocity have been used at the base of a vapor bubble growing on a heated wall. The complete Navier–Stokes equations are solved and the liquid–vapor interface is captured using the level-set technique. The effect of dynamic contact angle on bubble dynamics and vapor volume growth rate is compared with results obtained with the static contact angle model.     

Numerical study of laminar mixed convection of a nanofluid in horizontal curved tubes

Fully developed laminar mixed convection of a nanofluid consisting of water and Al₂O₃ in a horizontal curved tube have been studied numerically. Three-dimensional elliptic governing equations have been used. Simultaneous effects of the buoyancy force, centrifugal force and nanoparticles concentration has been presented and discussed. The nanoparticles volume fraction does not have a direct effect on the secondary flow, axial velocity and the skin friction coefficient. However, its effect on the entire fluid temperature could affect the hydrodynamic parameters when the order of magnitude of the buoyancy force becomes significant compared to the centrifugal force. For a given Reynold number, buoyancy force has a negative effect on the Nusselt number while the nanoparticles concentration has a positive effect on the heat transfer enhancement and also on the skin friction reduction.     

Numerical study of flow and heat transfer characteristics in outward convex corrugated tubes

The flow and heat transfer characteristics in convex corrugated tubes have been investigated through numerical simulations in this paper. Two kinds of tube types named as symmetric corrugated tube (SCT) and asymmetric corrugated tube (ACT) are modeled and studied numerically based on the k-ε model. The heat transfer working fluid at shell and tube sides are nitrogen and helium gases respectively. 2D axisymmetric model is adopted to simplify 3D model in order to reduce the computation cost greatly. Numerical simulation results for flow and heat transfer performances in SCT and ACT with various geometrical parameters, including corrugation pitch, corrugation height and corrugation trough radii are systematically analyzed. The mechanisms behind the improvement of overall performances of the simulated outward convex corrugated tube are discussed through investigating the details of turbulent velocity fields at both tube and shell sides. Compared to SCT, ACT exhibits 8–18% higher overall heat transfer performance.     

Numerical study on mixture formation characteristics in a direct-injection hydrogen engine

Numerical modeling of direct hydrogen injection and in-cylinder mixture formation is performed in this paper. Numerical studies on direct-injection hydrogen engines are very limited due mainly to the complexity in modeling the physical phenomena associated with the high-velocity gas jet. The high injection pressure will result in a choked flow and develop an underexpanded jet at the nozzle exit, which consists of oblique and normal shock waves. A robust numerical model and a very fine computational mesh are required to model these phenomena. However, a very fine mesh may not be feasible in the practical engine application. Therefore, in this study a gas jet injection model is implemented into a multidimensional engine simulation code to simulate the hydrogen injection process, starting from the downstream of the nozzle. The fuel jet is modeled on a coarse mesh using an adaptive mesh refinement algorithm in order to accurately capture the gas jet structure. The model is validated using experimental and theoretical results on the penetrations of single and multiple jets. The model is able to successfully predict the gas jet penetration and structure using a coarse mesh with reasonable computer time. The model is further applied to simulate a direct-injection hydrogen engine to study the effects of injection parameters on the in-cylinder mixture characteristics. The effects of the start of fuel injection, orientation of the jets, and the injector location on the mixture quality are determined. Results show that the hydrogen jets impinge on the walls soon after injection due to the high velocity of the gas jet. The mixing of hydrogen and air takes place mainly after wall impingement. The optimal injection parameters are selected based on the homogeneity of the in-cylinder mixture. It is found that early injection can result in more homogeneous mixture at the time of ignition. Results also indicate that it is more favorable to position the injector near the intake valve to take advantage of the interaction of hydrogen jets and the intake flow to create a more homogeneous mixture.     

Algebraic solution of capillary tube flows: Part I: Adiabatic capillary tubes

Capillary tube flows have been solved through both numerical and analytical approaches. The former requires a reasonable understanding of the governing equations of heat and fluid flow, thermodynamic relations, numerical methods, and computer programming, and therefore are not the suitable approach for most refrigeration and air-conditioning practitioners. Some simpler procedures based on different strategies for analytically solving the capillary tube flow have been proposed in the literature, although iterative loops for calculating the mass flow rate are still required. The aim of this work is to advance a semi-empirical algebraic model to solve adiabatic capillary tube flows using a relatively simple set of thermodynamic relations and being explicit for the mass flow rate calculation. Comparisons with a comprehensive experimental data set, taken with the refrigerants HFC-134a and HC-600a, has shown that the model predicts more than 90% and nearly 100% of all data within ±10% and ±15% error bands, respectively.     

Numerical study of the hydrodynamics and heat transfer characteristics of liquid–liquid Taylor flow in microchannel

A numerical study of an oil–water Taylor flow is presented in this paper to explore its flow and heat transfer characteristics. Due to the large surface area to volume ratio in narrow channels, using slug flows, high heat and mass transfer rates could be achieved. Sound knowledge of the underlying physics of slug flow is required for the practical design of microfluidic devices. In this study, hydrodynamics and heat transfer characteristics of dispersed oil droplets flowing inside a vertically upward circular microchannel (D = 0.1 mm) with water being the carrier phase have been explored numerically. ANSYS Fluent was employed to capture the liquid–liquid interface using volume of fluid method. Two different boundary conditions were considered in the present study. First, an isothermal wall of 373 K and later a constant wall heat flux (420 kW/m²) were, respectively, prescribed over the wall of the microchannel. The numerical code was validated against the results available in the literature, and the significant results in the form of pressure drop and heat transfer rates have been discussed. A considerable increase in Nusselt number, up to 180% and 210%, was observed with the oil–water slug flow in contrast to the liquid‐only single‐phase flow inside the microchannel for isothermal and constant wall heat flux conditions, respectively.