A small fraction of oil circulates in refrigeration and air-conditioning systems. Oil return from the compressor discharge to the compressor suction is an important requirement for ensuring the reliability of the compressor. Oil return characteristics are affected by refrigerant/lubricant mixture properties, gas velocities, and piping geometry. In refrigeration systems, the oil film in vertical upward flow is hard to push upward since the refrigerant has to overcome the gravitational force and viscous force. The worst conditions hindering reliable oil return are the immiscible pairs of the refrigerant and oil when flowing from the evaporator to the compressor suction through the vertical upward tube because of their relatively high viscosity. This paper presents experimental results of the oil accumulation characteristics of R-134a and three oil mixtures in a vertical upward suction line by measuring the oil accumulation, and by visualizing the flow pattern. The test facility, which allows one to vary refrigerant/oil flow rates in a wide range and observe flow patterns, has been built. The amount of oil accumulated in the suction line was measured by the oil injection and extraction method. The flow visualization shows that oil films flow either by chum flow or annular flow, depending on the refrigerant flow rate. Mineral oil and other high viscosity oils result in larger oil accumulation and higher mean oil film thickness ratio in the suction line than in conventional oils. This paper reports the current status of a longer-range effort to develop a verified model for oil retention in vapor compression systems.

Stenosis is a constriction or narrowing of a duct or passage. This word is often used to refer to constrictions that occur in blood vessels, especially arteries. A stenotic vessel generally causes blood flow stagnation and hence formation of thrombus, which may be rather harmful to the human physiology. In this work we perform flow visualization experiments and finite-volume computation to study the flow of an yield-stress (or highly shear-thinning) liquid through a tube in the neighborhood of an axisymmetric constriction. The rheology of the liquid was chosen to be close to the one of blood with 55% hematocrit Reynolds and Yield number values characteristic to different arteries have been studied, as well as different levels of constriction. Results for a Newtonian liquid were also obtained, for comparison purposes. Numerical solutions of the mass and momentum balance equations were obtained. In these solutions it was assumed that the materials behave like a Generalized Newtonian Liquid with a biviscosity function, which mimics closely the Herschel-Bulkley equation but allows deformation below the yield stress limit. Among other findings, it has been observed that the non-Newtonian rheology causes a significant change in the flow pattern in the neighborhood of the stenosis. Because the conditions for thrombus formation are directly related to the flow pattern, studies of the flow through stenotic vessels that assume a Newtonian rheology may lead to erroneous conclusions.

Liquid crystal is one of homogeneous ER (Electrorheological) fluids in some range of temperature. Transient responses of pressure drop are examined when liquid crystal flows between two parallel-plate electrodes for constant flow rates. When voltages are applied on the liquid crystal and removed, the pressure responses of the inlet of electrodes are measured with the pressure transducer. At the same time, liquid crystal between the transparent electrodes made of glass is visualized with the high-speed video camera to investigate the time history of the director of the liquid crystal. Outlet of the flow channel with two parallel-plate electrodes is atmosphere. Relation between the flow visualization results and changes of pressure drop is investigated especially for transient period. In the present experiment the flow rates change from 0.001cc/sec (velocity is 1mm/sec) to 0.003cc/sec and the electric field intensity is from 0.2kV/mm to 1kV/mm. The gap of the electrodes is 0.2mm. The isotropic-nematic transition is 35.5°C and smectic-nematic transition is 23.1°C. The open-loop test facility with the liquid crystal is set in a pyrostat to keep the temperature constant.

An unsteady pressure measurement system was developed to measure the unsteady pressure field of an axial fan. The fan unsteady pressure fields of an inlet and outlet were obtained at three axial positions for seven-radial directions. The results showed that there is a relatively long response time for pressure drop both in inlet and outlet sections during the fan start-up. The measurements also showed that, due to the vortex shedding from the trailing edge of each fan blade, the fan outlet unsteady pressure distributions have a primary frequency related to the fan operating frequency. The time-dependent pressure measurements showed that pressure distributions of inlet and outlet during the fan start-up were severely unsteady and the main variation frequency of the pressure is much smaller than the fan rotational frequency. The pressure measurement on the fan blades showed that the pressure oscillations were mainly dominated by the vortex shedding from the fen blades. A flow visualization study was also performed to validate the flow characteristics near the blade surface. A complete set of time-dependent pressure measurements along the blade fan surfaces, fan inlet and outlet are suitable for an axial fan database for an industrial use as well as CFD code validation.

Extensive development of biomedical and chemical analytic microdevices involves microscale fluid flows. Merging of fluid streams is expected to be a key feature in such devices. An integrated microsystem consisting of merging microchannels and distributed pressure microsensors has been designed and characterized to study this phenomenon on a microscale. The two narrow, uniform and identical channels merged smoothly into a wide, straight and uniform channel downstream of a splitter plate. All of the devices were fabricated using standard micromachining techniques. Mass flow rates and pressure distributions were measured for single-phase gas flow in order to characterize the device. The experimental results indicated that the flow developed when both inlets were connected together to the gas source could be modeled as gas flow through a straight and uniform microchannel. The flow through a single branch while the other was blocked, however, could be modeled as gas flow through a pair of microchannels in series. Flow visualizations of two-phase flows have been conducted when driving liquid and gas through the inlet channels. Several instability modes of the gas/liquid interface have been observed as a function of the pressure difference between the two streams at the merging location.

The purpose of this study was to compare experimental electrokinetic plug flow velocities to computational flow models of microfabricated capillaries. Electroosmotic flow studies of dichlorofluorescein and electrophoretic separation of dopamine and catechol in a microfabricated capillary electrophoresis (CE) system were performed both experimentally and computationally. A “balanced cross design” consisting of a bent 2 cm long injection channel and a straight 2 cm long separation channel was used. The geometry of the capillary was 65 μm wide and 20 μm deep. For the fluorescein study, separation voltages ranging between 0.25 kV and 1 kV were applied, while voltages ranging from 100 V to 550 V were used in the separation studies. Laser Induced Fluorescent (LIF) images were obtained for flow visualization and qualitative analysis in the electroosmotic flow studies, while electrochemical potentials were acquired using “on-chip” electrodes interfaced to a custom-designed power supply and electrochemical detection (ECD) circuit. Finite element models of the experimental device were generated and flows were simulated using commercially available software. For the electroosmotic flow studies, the computational results were found to be within ± 11% of the experimentally obtained values. Similarly, the results of the computational separations of catechol and dopamine predicted plug velocities that were within ± 7.6% of the experimentally determined values.

A novel micro actuator for fluidic transport, mixing and particle filtering is presented. The technology uses a torsionally supported plate driven with an electrostatic signal at first-mode resonance to produce long-term reliable fluidic interactions at the chip level. Resonant frequencies from 10–20kHz have been tested with maximum fan deflections of approximately 30 microns. The resonant fans are fabricated using surface micro-machining techniques and flip-chip assembly. A simple flow visualization technique is also presented using laser illuminated smoke particles contained within the device packaging. The flow visualization setup allows for both overhead and side views as well as infinite ceiling and channel flow simulation in order to fully characterize the three dimensional flow produced by the resonant fans. Flow speeds of over 10 mm/s have been observed to exit single micro fans with large fan arrays producing over 2mL/min of volumetric flow. Fan testing in 600-micron deep channels has demonstrated the potential for suspended particle collection and filtering applications.

Numerical and experimental investigations have been conducted to study the flow and heat transfer characteristics for the buoyancy-induced flow inside an inclined arc-shape enclosure. Mathematical model in form of a stream function-vorticity formulation representing the laws of conservation in mass, momentum, and energy is expressed in a curvilinear coordinate frame and solved by a finite-volume discretization method. Heat transfer and flow pattern are predicted at various Grashof numbers and inclination angles. Meanwhile, an experimental system is developed and a flow-visualization technique using smoke is employed to observe the flow pattern. Results show that when the Grashof number is higher than 10 5 , the increase in natural convection heat transfer becomes appreciable. The vortex strength and pattern are found to be greatly dependent on the inclination angle. The range of the Grashof number considered in this study is up to 10 7 and the inclination angle is varied from 0 to π.

This paper presents experiments on the cooling effectiveness obtained for two different injection locations on the suction side of a turbine blade at transonic flow conditions. Previous results of a computational analysis and flow visualization indicated that a separation bubble is present on the suction side at a location x/L = 0.43 and the location x/L = 0.575 corresponds to a shock-boundary interaction zone [9]. The scientific interest is primarily focused on the realization of high film cooling efficiencies and its relevant parameters under these flow conditions. Streamwise aligned as well as inclined angled film coolant hole configurations have been investigated for each location. Due to the high number of interacting parameters the experimental simulation of turbine blade film cooling is extremely complex, which can only be solved by a simultaneous modeling using the experimentally measured results. Test rig, instrumentation and data analysis are described in detail. The goal of the investigations is to determine the optimum location of the film coolant injection.

This paper presents a review of the flow characteristics in microchannels and our own flow visualization work using a photochromic dye activation technique. The review reveals differences in flow behaviour between micro- and macro-channels, and the need to measure instantaneous velocity profiles to explain these differences. In the experiment, de-ionized water containing a photochromic dye was pumped through a square capillary tube with inner dimensions of 96 μm × 96 μm to yield a Reynolds number of 0.1. A pulsed ultraviolet laser beam was used to create dye traces in the flowing liquid, and images of the traces were recorded with a video camera. Friction factor in laminar flow was also determined from pressure drop measurements at several Reynolds numbers using tap water. The experimental results showed the flow in intermediate-sized microchannels at low Reynolds numbers to still conform to conventional fluid mechanics theory, possibly with slight deviation. The success in obtaining quantitative results with this approach holds promise for further studies of flows in microchannels with smaller diameters.

This paper gives results of an experimental study to determine the effect that a surface reaction has on a boundary layer flow. An innovative technique has been developed to visualize the effect a surface reaction has on a flat plate boundary layer flow. A mixture of mineral oil and hexanoic acid flowed at low Reynolds number over a metal plate embedded with a small piece of sodium. The reaction between the hexanoic acid and the sodium metal produced hydrogen bubbles, which rose to the surface. Upstream of the reaction, dye was injected into the laminar flow, and the interaction between the bubbles and the dye was recorded. Results show that bubble entrainment stretch and bend the dye filaments, convect fluid away from the reaction, and create vortices downstream and to the sides of the reaction. These results show that a surface reaction can introduce velocity fluctuations into an otherwise laminar flow. Farther downstream the flow relaminarizes.

Flow visualization experiments on oscillating flow in a channel obstructed by an array of circular cylinders were performed. First of all, steady flow experiments without oscillation were carried out. The Karman vortices began to shed in a range of the Reynolds number: 53< Re <68. The Strouhal number showed a tendency to decrease with an increase of the Reynolds number. Oscillating flow experiments were carried out secondly. When the oscillation amplitude was small, Karman vortices shed periodically and its Strouhal number agreed with that in the steady flow. But, when the oscillation amplitude was large, shedding of Karman vortex was well controlled by the oscillating flow. The vortex shedding frequency became large with an increase of the cross-sectional mean velocity. On the other hand, the amplitude of velocity fluctuation in the wake was small in the accelerating phase and large in the decelerating phase.

A series of flow visualization experiments of pulsating flow obstructed by an array of square rods was carried out to investigate its characteristics. When the pulsation is absent, Karman vortices shed periodically from each rod. When the pulsation period is relatively long compared with the shedding period and its amplitude is large, the flow is stabilized during the accelerating phase and, during the decelarating phase, the flow is destabilized and Karman vortices break down. When the pulsation period is shorter than shedding period and its amplitude is large, the flow pulsation controls the generation and breakdown of the Karman vortices. A numerical simulation code was developed and compared with the experimental results. When the pressure gradient parameter of the code is changed sinusoidally with time, computed results become the pulsating flow. Time variations of the streamwise pressure gradient and cross-sectional averaged velocity show similarity between the experimental and computed results.

Previous study of carrier gas flow in rapid chemical vapor deposition (RCVD) reactors has been limited mostly to numerical simulations and flow visualizations. In the present work flow regimes were observed and temperatures were measured in a vertical axisymmetric pedestal RCVD reactor using flow visualization and Rayleigh light scattering (RLS) for noninvasive temperature measurement. Flow visualizations revealed that the flow undergoes complex transitions between stable flow regimes as heating occurs. The two dominant stable flow regimes were buoyant stable (BS) and momentum stable (MS). RLS was used to determine the instantaneous carrier gas temperature at discrete points in the test section. The flow regimes and their transition points were easily recognized and agreed with flow visualization data. The flow visualizations and RLS tests showed identifiable trends in transition points between flow regimes and in the types of regimes encountered. These trends were dependent on Grashof number and Reynolds number.

In order to explore the mechanics of EHD induced flow and heat transfer augmentation, an experimental study of the tube-side boiling heat transfer of refrigerant HFC-134a has been conducted in a single-pass, counter-flow heat exchanger. The electrode position was arranged concentric to the tube. Experiments are conducted for inlet qualities of 0% to 60%, mass fluxes from 100 kg/m 2 s to 500 kg/m 2 s, heat fluxes of 10 kW/m 2 and 20 kW/m 2 , and applied voltage from 0 kV to 8 kV DC. The threshold of EHD effects were analysed by a proposed dimensional analysis which predicts that the EHD forces become significant when the Dielectric Electric Rayleigh number is of the same order of magnitude as the square of the liquid Reynolds number, E ℓε ≈ Re ℓ 2 . This criterion is supported by experimental evidence. Flow visualization experiments have shown that, when the proposed dimensionless criterion is satisfied, EHD body forces may have a strong influence on the liquid-phase and consequently the flow pattern within the channel. The various flow configurations clearly affect heat transfer and pressure loss and need to be considered in any attempt to identify the effects of an applied electric field.

A simulated, non-pressurized hydrothermal system consisting of a fluid-superposed porous layer is fabricated and used for visualization and measurement of the temperature field using liquid crystal thermography. The system is used for various boundary conditions with pure glycerine as the working fluid and the porous layer is made of 3mm diameter glass beads. Experimental data is recorded using a color CCD camera and flow visualization is obtained through a long exposure video photography. A calibration is performed to relate the temperature with scattered colors at an orthogonal angle to the incoming white light sheet. Quantitative temperature data is obtained through this calibration and compared with the numerical predictions. For numerical studies the system is modeled as a composite layer of fluid and porous charge using the Darcy-Brinkman-Forchheimer flow model. A two-dimensional curvilinear algorithm using finite volume technique with a non-staggered grid is used to simulate the temperature field and transport phenomena for various Rayleigh–Darcy number combinations of varying aspect ratio. The results, for the first time, make an attempt towards understanding the transport process in hydrothermal system through both numerical simulation and experimental validation.

This paper describes the design, construction and testing of a thermochromic liquid crystal based, non-intrusive, surface flow visualization system. The system uses a pin-finned heat sink in contact with a thin sheet of liquid crystals and is easily mounted in a wind tunnel. When heated and placed in a fluid flow, the pin-finned heat sink produces teardrop-shaped thermal wakes that indicate the direction of the flow. We performed an optimization experiment and determined that the best flow visualization images are achieved for heat sink base temperatures that are 10–20 °C above the liquid crystal activation temperature. Under these conditions, the full range of the liquid crystals is utilized. We present liquid crystal flow visualization results for flow about streamlined and blunt objects and demonstrate the performance of the liquid crystal surface. We also compare liquid crystal images for flow about a blunt body to PIV data and show good qualitative agreement. The technique is repeatable, non-intrusive and instructive for learning about surface flow characteristics.

Radiation heat transfer affects natural convection of air inside an open ended cavity with a heated horizontal upper plate and an unheated lower parallel plate. The influence is mainly due to radiative heating of the lower plate, and plane fluid layer secondary motion could arise. In this paper an experimental study is carried out to describe and to detect the influence of radiation on air flow and on heat transfer coefficient by means of wall temperature profiles, smoke visualization, and air temperature measurements. The analysis is obtained for an emissivity of the horizontal plates equal to 0.8, for distances between the plates of 20.0, 32.3, and 40.0 mm. By means of flow visualization and local air temperature measurements in the cavity as a function of time, remarkable secondary motion in the cavity is observed when q Ω is equal to 120 W/m 2 . Measurement of the air temperature in the cavity also shows that radiation causes and damps secondary motion at the same time. Profiles of the mean value of the air temperature as a function of the x and y coordinates confirm both the main flow path inside the cavity and radiation effects on convective heat transfer. Finally, correlations related to average Nusselt number are proposed for natural convection as well as for heat transfer as a whole, that is convection along with radiation.

The performance of a typical numerical simulation for complex flows of viscoplastic materials was examined. The inertialess flow of viscoplastic materials through an axisymmetric channel formed by an abrupt expansion followed by a contraction was employed with this purpose. Flow visualization experiments were performed with a well characterized Carbopol aqueous solution. Numerical solutions of the mass and momentum balance equations were obtained, using the Generalized Newtonian Liquid model with a biviscosity function. The flow visualization results showed that the flow pattern is essentially Newtonian for large expansion lengths. For smaller expansion lengths, however, flow is observed only in an inner axisymmetric region whose diameter is approximately the same as the one of the inlet and outlet tubes. Outside this region the flow is stagnant, and a slip interface between these two regions seems to occur. The corresponding numerical solution was not capable of predicting the observed flow pattern.

Recently, considerable interest has developed in surfactant additives for use in district heating and cooling systems to lower the pumping energy requirement. Many studies in the case of surfactant solutions have been done for the flow behavior in a circular pipe. However, few studies have been conducted on flow near a rotating disk in surfactant solutions. In this paper, the flow characteristics near an enclosed rotating disk in surfactant solutions were studied by applying flow visualization techniques and analyzed by applying the momentum integral equations which are related to the three boundary layer problem. The test surfactant solution was Ethoquad 0/12 with sodium salicylate at a concentration of 200ppm and a temperature of 18°C. The flow patterns were obtained at Re = 2.5×10 5 and 3.5×10 5 so that the Reynolds number range corresponds with the transition region to turbulent flow in the boundary layer on the rotating disk for Newtonian fluids. Consequently, it has been clarified that the amplitude of the circular vortex on the rotating disk was reduced and the flow direction near the disk was turned outward to the circumferential direction comparing with that of tap water. In additional, the limiting maximum drag reduction asymptote for a moment coefficient of a rotating disk was obtained by applying the momentum integral equation for drag-reducing solutions based on previous papers on circular pipe flow.