Small Diameter Channels Research Articles

Experimental Investigation and Flow Process Computer Simulation of the Single Mini Channel Condenser for Vapor Compression Refrigeration System

This study is a computer simulation of the temperature profiles and experimental investigation of three 100 mm x 50 mm x 18 mm single mini channel condensers with hydraulic diameters of 3 mm, 2 mm, and 1mm. The mini channels which were made of copper were designed, fabricated and tested. Each unit was connected in a vapor compression cycle with R-134a as the refrigerant. The average refrigerant mass flow rates were varied from 1.296 - 69.471 g/s, and the average inlet and outlet condenser pressure variations were 102.5 - 121.8 kPa and 101.74 -121.23 kPa, respectively. Each condenser was placed inside a mini wind tunnel system where forced draft air was introduced to initiate convective heat transfer. Each condenser was tested and data were gathered every five minute interval for one hour using a Lab View Software. Computer simulations on the flow process were conducted using Solid Works software. The experimental results presented the inlet and outlet condenser pressures, and pressure drops. The experimental heat transfer coefficients were calculated at different mass fluxes during condensation. The values ranged from 3900 to 5200 W/m2-°K for the 3 mm, 2600 to 9000 W/m2-°K for the 2 mm, and 13 to 98 W/m2-°K for the 1 mm. The heat transfer coefficients calculated from experiments were then compared with the computed values using the correlations developed by Dittus-Boelter and Lee-Son. The results showed increasing deviation as the diameter decreased. The discrepancies could be attributed to the appropriateness of the Dittus-Boelter and Lee-Son correlations in small diameter channels, complexities in the flow process which involved two phase flow heat transfer in very small tubes, and the difficulties in attaining very accurate measurements in small channels.

Comprehensive experimental investigation of two-phase heat transfer and pressure drop with propane in a minichannel

The use of hydrocarbons as natural refrigerants inside small diameter channels allows charge minimization and therefore it may be an interesting option in the refrigerating and heat pump technology. The aim of the present experimental work is to fully characterize the thermal performance of propane (R290) in minichannels by measuring frictional pressure drop, condensation and flow boiling heat transfer coefficients inside a circular cross section horizontal minichannel with an internal diameter of 0.96 mm and a rough inner surface. Measurements of frictional pressure drop during adiabatic two-phase flow have been performed at mass velocity ranging between 200 and 800 kg m−2 s−1. Local heat transfer coefficients have been measured during condensation and during flow boiling in the mass velocity range from 100 to 1000 kg m−2 s−1. The present database, including frictional pressure gradient, condensation and vaporization heat transfer coefficients, is compared against predicting correlations available in the open literature.

ONCE-THROUGH CONTACTLESS FLOW BOILING IN A MICRO EVAPORATOR

Equipment miniaturization offers several opportunities such as an increased surface-to-volume ratio combined with high heat transfer coefficients. However, moving toward small-diameter channels demands extra attention to fouling, reliability, and stable operation of the system. The present investigation explores the possibilities of hot gas flow through the porous wall of a pipe as a means to evaporate a liquid fully flowing in the pipe. A reduction of the pressure fluctuations of about 30% to 40% is obtained with this so-called contactless boiling. Because of the injected gas, wall temperatures measured are higher. The reduction in heat transfer coefficient corresponds to an insulating gas layer at the wall with an estimated thickness of 20 m.

Prototyping of poly(dimethylsiloxane) interfaces for flow gating, reagent mixing, and tubing connection in capillary electrophoresis

Integrated microfluidic systems coupled with electrophoretic separations have broad application in biologic and chemical analysis. Interfaces for the connection of various functional parts play a major role in the performance of a system. Here, we developed a rapid prototyping method to fabricate monolithic poly(dimethylsiloxane) (PDMS) interfaces for flow-gated injection, online reagent mixing, and tube-to-tube connection in an integrated capillary electrophoresis (CE) system. The basic idea was based on the properties of PDMS: elasticity, transparency, and suitability for prototyping. The molds for these interfaces were prepared by using commercially available stainless steel wires and nylon lines or silica capillaries. A steel wire was inserted through the diameter of a nylon line and a cross format was obtained as the mold for PDMS casting of flow gates and 4-way mixers. These interfaces accommodated tubing connection through PDMS elasticity and provided easy visual trouble shooting. The flow gate used smaller channel diameters, thus reducing flow rate by 25-fold for effective gating compared with mechanically machined counterparts. Both PDMS mixers and the tube-to-tube connectors could minimize the sample dead volume by using an appropriate capillary configuration. As a whole, the prototyped PDMS interfaces are reusable, inexpensive, convenient for connection, and robust when integrated with the CE detection system. Therefore, these interfaces could see potential applications in CE and CE-coupled systems.

Numerical Simulation of Evaporation Process of Two-Phase Flow in Small-Diameter Channels

In this work, the task of resolving the evolution of bubble growth in a flow field during boiling is investigated. A distinction between phases is been made using a diffuse interface method. Surface tension forces are introduced as a volumetric force smeared in the interface. Finally, the evaporation rate is incorporated into the model as a parameter also smeared in the interface region, taking advantage of the diffuse interface approach. The method used involves the coupling of conservation of energy, mass, and momentum using the least squares finite-element method. The method presents a good balance between accuracy and computational cost.

Evaporative Annular Flow in Micro/Minichannels: A Simple Heat Transfer Model

The present study collected and analyzed flow boiling data points which fall in the annular flow regime with an increasing heat transfer coefficient h - vapor quality x trend (h increases with increasing x) in small diameter channels (0.1 < dh < 3.1 mm) for halogenated refrigerants, CO2 and water. In this annular flow regime, heat transfer coefficient also depends on both heat flux and mass flux. It is proposed that the heat flux dependence comes mainly through its effect on interfacial waves and the fact that bubble growth and coalescence in isolated bubble flow and elongated bubble flow propagate oscillations downwards into the annular flow. In other words, heat flux affects the heat transfer coefficient in the annular flow regime by upstream effects or historical effects. A semi-empirical model for annular flow was developed by starting with pure thin film evaporation and then corrections were applied based on the Boiling number and the liquid Reynolds number. The resulting simple model can predict about 89.1% of the entire database within a ± 30% error band. Almost all data points can be predicted within a ± 50% error band. It is shown that the parametric trends are well captured by the new model. Besides, no noticeable macro-to-micro/miniscale transition was observed for the entire database of annular flow. Therefore, the new model can be applied to model annular flow covering from microchannels to relatively large channels.

Investigation of liquid phase axial dispersion in Taylor bubble flow by radiotracer residence time distribution analysis

A gas-liquid Taylor bubble flow occurs in small diameter channels in which gas bubbles are separated by slugs of pure liquid. This type of flow regime is well suited for solid catalyzed gas-liquid reactors in which the reaction efficiency is a strong function of axial dispersion in the regions of pure liquid. This paper presents an experimental study of liquid phase axial dispersion in a Taylor bubble flow developed in a horizontal tube using high speed photography and radiotracer residence time distribution (RTD) analysis. A parametric dependence of axial dispersion on average volume fraction of gas phase was also investigated by varying the relative volumetric flow rates of the two phases. 137m Ba produced from a 137 Cs/ 137m Ba radionuclide generator was used as radiotracer and measurements were made using the NaI(Tl) scintillation detectors. Validation of 137m Ba in the form of barium chloride as aqueous phase radiotracer was also carried out. Axial Dispersion Model (ADM) was used to simulate the hydrodynamics of the system and the results of the experiment are presented. It was observed that the system is characterized by very high values of Peclet Number (Pe∼10 2 ) which reveals an approaching plug type flow. The experimental and model estimated values of mean residence times were observed in agreement with each other.

Suppression of oscillations of the vapor–liquid phase boundary in boiling of superfluid helium inside a porous body

Introduction. The recent decade has seen an active development of the technology of space applications of superfluid helium. Among the achievements one should note the launching of the two space observatories-telescopes Herschel and Planck, which took place in 2009. For the Herschel observatory, 2300 liters of liquid helium at a temperature of 1.65 K, placed in the cryostat before the launch, determine the lifetime of the entire structure (expected life is about 4 years) and ensure the efficiency and stability of both the guidance systems and the scientific instrumental base [1]. At the same time, creation of the Laboratory of Low-Temperature Investigations within the framework of the Russian Segment of the International Space Station assumes the extension of the base of basic research. Therefore, for a number of years, we have seen the development of the project of experiment investigations of boiling of superfluid helium under zero gravity. To ensure the preservation of helium in the cryostat volume, it is proposed that a porous shell coaxial with a cylindrical heater be used [2]. For corresponding experiments under laboratory conditions, it is planned to use a bench of the Department of Low Temperatures of the National Research University "Moscow Power Institute;" which bench has been created to investigate processes in superfluid helium in heat supply, including the case of constrained conditions. The corresponding mathematical model of an experimental cell is presented below. Furthermore, it is of separate interest to investigate the dynamics of a vapor film and flow of a quantum liquid in small-diameter channels under a thermal load of 10 4 W/m 2 . Formulation of the Problem and Mathematical Description. The heater in the form of a thin wire of prescribed radius R w is arranged inside a coaxial shell with internal radius R 0 and thickness L; the wire is manufactured from a material with known structural characteristics (porosity m and characteristic dimension d). The experimental cell is closed, on the ends, by optically transparent plugs and is fully submerged in the liquid volume to a depth h. Helium-II fills the internal space and the porous-shell channels. Above the free surface of the liquid, a constant pressure P b is maintained, to which there corresponds the liquid temperature T b as far as the saturation line is concerned. Once the thermal load q w has been applied, a vapor film of radius R 1 is formed on the heater surface, which expels the liquid through the channels in the porous body (Fig. 1). The objective of the work is to calculate the process of heat and mass transfer in the experimental cell with the aim of determining a combination of parameters at which a steady state can be reached. The system of equations is formulated under the assumption that the heat- and mass-transfer processes in helium-II are quasisteady. The liquid is assumed incompressible; the dependence of the processes on the local temperature is not considered.

Dynamics of ionization wave splitting and merging of atmospheric-pressure plasmas in branched dielectric tubes and channels

Atmospheric-pressure fast ionization waves (FIWs) generated by nanosecond, high voltage pulses are able to propagate long distances through small diameter dielectric tubes or channels, and so deliver UV fluxes, electric fields, charged and excited species to remote locations. In this paper, the dynamics of FIW splitting and merging in a branched dielectric channel are numerically investigated using a two-dimensional plasma hydrodynamics model with radiation transport, and the results are compared with experiments. The channel consists of a straight inlet section branching 90° into a circular loop which terminates to form a second straight outlet section aligned with the inlet section. The plasma is sustained in neon gas with a trace amount of xenon at atmospheric pressure. The FIW generated at the inlet approaches the first branch point with speeds of ≈108 cm s−1, and produces a streamer at the inlet–loop junction. The induced streamer then splits into two FIW fronts, each propagating in opposite directions through half of the loop channel. The FIWs slow as they traverse the circular sections due to a shorting of the electric field by the other FIW. Approaching the loop–outlet junction, the two FIW fronts nearly come to a halt, induce another streamer which goes through further splitting and finally develops into a new FIW front. The new FIW increases in speed and plasma density propagating in the straight outlet channel. The electrical structure of the FIWs and the induced streamers during the splitting and merging processes are discussed with an emphasis on their mutual influence and their interaction with the channel wall. The FIW propagation pattern is in good agreement with experimental observations. Based on numerical and experimental investigations, a model for the splitting and merging FIWs in the branched loop channel is proposed.

Fabrication of a Hybrid Microfluidic System Incorporating both Lithographically Patterned Microchannels and a 3D Fiber‐Formed Microfluidic Network

A device containing a 3D microchannel network (fabricated using sacrificial melt-spun microfibers) sandwiched between lithographically patterned microfluidic channels offers improved delivery of soluble compounds to a large volume compared to a simple stack of two microfluidic channel layers. With this improved delivery ability comes an increased fluidic resistance due to the tortuous network of small-diameter channels.

Thermal and flow characteristics of the condensation of R404A refrigerant in pipe minichannels

This article presents the results of experimental investigations of heat exchange and pressure drop during the condensation of R404A refrigerant in stainless steel pipe minichannels with internal diameters of 1.4–3.30mm. A review is provided of the present state of knowledge concerning the condensation of this refrigerant in conventional channels and in small-diameter channels. It is emphasized that there are few prior publications concerning this issue. The test setup is described as well as the results of the experimental tests. We discuss the dependence of the heat transfer coefficient and the pressure drop of the R404A refrigerant on both minichannel diameter and process parameters. The pressure drop during the condensation of this refrigerant is satisfactorily described by the Friedel and Garimella correlations. Based on the experimental tests, we propose a new correlation for the calculation of the local heat transfer coefficient. The values calculated from this correlation were in agreement with the experimental results to within ±20%.

Template synthesis of ultra-thin and short carbon nanotubes with two open ends

Anodic aluminium oxide (AAO) films containing small diameter channels with both ends open were prepared for use as a growth template for carbon nanotubes (CNTs). The smallest channel diameter was 7 nm and the smallest film thickness was 2 μm. The CNTs produced from the AAO template by chemical vapor deposition of acetylene had controllable outer and inner diameters and length, and open ends. The pore texture, structure and crystallinity of the CNTs were characterized by electron microscopy, nitrogen gas adsorption–desorption, and thermogravimetric analysis. The inner diameters of the CNTs were adjustable by selecting AAO templates with specific channel diameters and/or tuning the duration of chemical vapor deposition. CNTs with an outer diameter, inner diameter and length of 7 nm, 4.5 nm and 2 μm, respectively, were obtained, the narrowest and shortest ever reported for CNTs with two open ends. These thin, short, and open CNTs would render easy access, filling and transport of foreign matter into their one-dimensional cavity with prominent confinement effects.

Entropic unfolding of flexible polymers in nanotubes: a Langevin dynamics study

Langevin dynamics simulations of the bead–spring model are employed to study the unfolding dynamics of a flexible hairpin polymer which has been inserted in a cylindrical nanochannel. Because the hairpin polymer is not in an equilibrium configuration the molecule starts unfolding until reaching a stretched configuration inside the tube. We study the effect of the channel diameter D on the unfolding relaxation time. The effect of the number of monomers N in the folded arm on the relaxation time is also investigated. We show the unfolding time τR ∼ DN2 for small channel diameters. These results can be explained and understood using scaling arguments of the polymer segregation theory. Our simulations are in agreement with experimental observations of DNA unfolding in solid-state nanochannels.

Plasma parameters of square superlattice pattern in a dielectric barrier discharge

Dielectric barrier discharge is an important method of producing nonequilibrium low-temperature plasma. Measurement of the plasma parameters is highly valuable for its industrial application. Plasma parameters of square superlattice pattern are investigated by optical emission spectroscopy in a dielectric barrier discharge by using a two-liquid-electrode dielectric barrier discharge device in argon at atmospheric pressure. It is found that the light intensity of the large diameter microdischarge channel(big dot) is different from that of the small diameter channel (small dot). Vibrational temperature is investigated by using the N_{2} second positive spectrum. Electronic excitation temperature is measured by means of spectral line intensity ratio. Electron density is obtained by using the stark broadening of Ar atom 696.54 nm spectral line. The results show that the electron density and the vibrational temperature of the small dot are larger than those of the big dot but the electronic excitation temperature is lower than that of the big dot. It is suggested that the plasma state of the big dot is different from that of the small dot in the stable square superlattice pattern.

Two-Phase Microchannel Heat Sinks: Theory, Applications, and Limitations

Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase microchannel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase microchannel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macrochannels. Conversely, other fluids may exhibit considerable “confinement” even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nanofluids and carbon nanotube coatings, with proven merits to single-phase macrosystems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of microchannel flow with those of jet impingement.

Streamer Breakdown Development in Undercritical Electric Field

The different regimes of nanosecond pulsed-periodic discharge development in a point-to-plane geometry are investigated. The development of a discharge burst at a frequency of 1 kHz is investigated with nanosecond temporal resolution. Kinetic and gasdynamic effects that control the transition from streamer to spark discharge are demonstrated. It was shown that subsequent streamers have higher velocities and smaller channel diameters than the initial streamer. Thus, the self-focusing of the periodic discharge due to the inhomogeneous excitation and heating of the gas in the previous discharge channel has been demonstrated. Discharge transitions into a short-circuit mode cause an increased release of energy. This energy release leads to a sharp increase in the intensity of gasdynamic perturbations, effective mixing of the recombining channel's plasma with the surrounding air, loss of the symmetry of the initial conditions, and generation of multiple channels. This process leads to the self-restriction of the energy release during the high-current phase of the discharge because of an effective decrease of inhomogeneities in the heat and concentration.

Experimental and theoretical study of dryout in annular flow in small diameter channels

Experimental and theoretical study of dryout in annular flow in small diameter channels In the paper the experimental analysis of dryout in small diameter channels is presented. The investigations were carried out in vertical pipes of internal diameter equal to 1.15 mm and 2.3 mm. Low-boiling point fluids such as SES36 and R123 were examined. The modern experimental techniques were applied to record liquid film dryout on the wall, among the others the infrared camera. On the basis of experimental data an empirical correlation for predictions of critical heat flux was proposed. It shows a good agreement with experimental data within the error band of 30%. Additionally, a unique approach to liquid film dryout modeling in annular flow was presented. It led to the development of the three-equation model based on consideration of liquid mass balance in the film, a two-phase mixture in the core and gas. The results of experimental validation of the model exhibit improvement in comparison to other models from literature.

The Development of Anodic Aluminum Oxide Based Micro-channel Plate for Large-area Photo-detector

ABSTRACTAnodized Aluminum Oxide (AAO) based micro-channel plates (MCP) are fabricated in order to develop economical large-area photodetectors. Commercially available glass capillary array has a limitation to reach channel diameter below ~10 microns. However, smaller channel diameter is desired for better spatial and fast timing resolution. AAO based MCP is a good candidate to produce channel diameter less than 10 um by taking advantage of the nano-scale intrinsic pores during etching process. In this study, various channel diameters are fabricated with use of lithographic patterning techniques and wet etching; and characterized with optical, atomic force, and scanning electron microscopies. The channel diameter, channel length and related aspect ratio, as well as the open area are varied in order to maximize the MCP photon amplification.

Development of fluidic weft insertion sensors

Measuring the speed of weft insertion into the shed of warp threads on a jet loom is a difficult problem, sometimes described as nearly impossible to solve. A solution was found by the author in an inversion of the basic principle of shuttle-less air-jet loom, i.e. using the dragging action between the weft and air flow. Initially, the analogy went as far as using also an air jet – issuing from a nozzle through which the weft passes – as the flow influenced by the weft motion. Recent studies, described in this paper, have shown that better performance is obtained by the weft/air interaction in long, small-diameter channels.

A New Method for Determination of Flow Boiling Heat Transfer Coefficient in Conventional-Diameter Channels and Minichannels

Presented in this article are considerations regarding modeling of flow boiling in conventional, small-diameter channels and minichannels. A concise survey of available methods for prediction of heat transfer coefficient in saturated boiling regime is given and in that light a modified author's own model is presented. The presented model, contrary to other approaches, finds application in the cases of both conventional and small-diameter channels. The results of calculations are compared with some experimental data available from literature on conventional-size tubes and also minichannels. Obtained agreement is satisfactory.