Coaxial Counterflow Research Articles

The role of coupled resistance–compliance in upper tracheobronchial airways under high frequency oscillatory ventilation

A large eddy simulation (LES) based computational fluid dynamics (CFD) study was conducted to investigate lung lobar ventilation and gas exchange under high frequency oscillatory ventilation conditions. Time-dependent pressure coupled with the airways resistance and compliance (R&C) were imposed as boundary conditions (BCs) in the upper tracheobronchial tree of patient-specific lung geometry. The flow distribution in the left and right lungs demonstrated significant variations compared to the case in which traditional BCs based on mass flow rate fractions was used and is in agreement with the in vivo data available in the literature. The gas transport due to the pendelluft mechanism was captured in the different lung lobes and units. The computed pendelluft elapsed time was consistent with available physiological data. In contrast to in vivo studies, our simulations were able to predict the volume associated with the pendelluft elapsed time at different frequencies. Significant differences in coaxial counter flow and flow structures were observed between different BCs. The consistency of the results with the physiological in vivo data indicates that computations with coupled R&C BCs provide a suitable alternative tool for understanding the gas transport, diagnosing lung pathway disease severity, and optimizing ventilation management techniques.

Numerical Study of High-Frequency Oscillatory Air Flow and Convective Mixing in a CT-Based Human Airway Model

High-frequency oscillatory ventilation (HFOV) is considered an efficient and safe respiratory technique to ventilate neonates and patients with acute respiratory distress syndrome. HFOV has very different characteristics from normal breathing physiology, with a much smaller tidal volume and a higher breathing frequency. In this study, the high-frequency oscillatory flow is studied using a computational fluid dynamics analysis in three different geometrical models with increasing complexity: a straight tube, a single-bifurcation tube model, and a computed tomography (CT)-based human airway model of up to seven generations. We aim to understand the counter-flow phenomenon at flow reversal and its role in convective mixing in these models using sinusoidal waveforms of different frequencies and Reynolds (Re) numbers. Mixing is quantified by the stretch rate analysis. In the straight-tube model, coaxial counter flow with opposing fluid streams is formed around flow reversal, agreeing with an analytical Womersley solution. However, counter flow yields no net convective mixing at end cycle. In the single-bifurcation model, counter flow at high Re is intervened with secondary vortices in the parent (child) branch at end expiration (inspiration), resulting in an irreversible mixing process. For the CT-based airway model three cases are considered, consisting of the normal breathing case, the high-frequency-normal-Re (HFNR) case, and the HFOV case. The counter-flow structure is more evident in the HFNR case than the HFOV case. The instantaneous and time-averaged stretch rates at the end of two breathing cycles and in the vicinity of flow reversal are computed. It is found that counter flow contributes about 20% to mixing in HFOV.

Fluid entrainment by isolated vortex rings

Of particular importance to the development of models for isolated vortex ring dynamics in a real fluid is knowledge of ambient fluid entrainment by the ring. This time-dependent process dictates changes in the volume of fluid that must share impulse delivered by the vortex ring generator. Therefore fluid entrainment is also of immediate significance to the unsteady forces that arise due to the presence of vortex rings in starting flows. Applications ranging from industrial and transportation, to animal locomotion and cardiac flows, are currently being investigated to understand the dynamical role of the observed vortex ring structures. Despite this growing interest, fully empirical measurements of fluid entrainment by isolated vortex rings have remained elusive. The primary difficulties arise in defining the unsteady boundary of the ring, as well as an inability to maintain the vortex ring in the test section sufficiently long to facilitate measurements. We present a new technique for entrainment measurement that utilizes a coaxial counter-flow to retard translation of vortex rings generated from a piston–cylinder apparatus, so that their growth due to fluid entrainment can be observed. Instantaneous streamlines of the flow are used to determine the unsteady vortex ring boundary and compute ambient fluid entrainment. Measurements indicate that the entrainment process does not promote self-similar vortex ring growth, but instead consists of a rapid convection-based entrainment phase during ring formation, followed by a slower diffusive mechanism that entrains ambient fluid into the isolated vortex ring. Entrained fluid typically constitutes 30% to 40% of the total volume of fluid carried with the vortex ring. Various counter-flow protocols were used to substantially manipulate the diffusive entrainment process, producing rings with entrained fluid fractions up to 65%. Measurements of vortex ring growth rate and vorticity distribution during diffusive entrainment are used to explain those observed effects, and a model is developed to relate the governing parameters of isolated vortex ring evolution. Measurement results are compared with previous studies of the process, and implications for the dynamics of starting flows are suggested.

Condensation heat transfer coefficients for R22 and R407C in gravity driven flow regime within a smooth horizontal tube

The quasi local heat transfer coefficients of R22 and R407C in the coaxial counterflow condenser (20 mm ID) of a refrigerating vapour compression plant have been experimentally measured. The experimental conditions under which the heat transfer coefficients were determined reflect a typical working situation for small scale refrigeration systems. The plant runs with low mass fluxes of refrigerant within the range of 45.5–120 kg/m2/s. During the experimental tests the pressure at the inlet of the test condenser varies within a fixed range between 15.2 and 14.3 bar. The results illustrate that the R22 heat transfer coefficient is always greater than that of R407C. Furthermore, comparisons between the experimental data and the values predicted by means of the most credited literature relationships for gravity-driven condensation are reported.

Experimental evaluation of R22 and R407C evaporative heat transfer coefficients in a vapour compression plant

The mean heat transfer coefficients of R22 and R407C in the coaxial counterflow evaporator (20 mm ID) of a refrigerating vapour compression plant have been experimentally measured. The experimental conditions under which heat transfer coefficients were determined reflect a typical working situation for small-scale refrigeration systems. The heat flux ranged from 1.9 to 9.1 kW/m 2 and the mass flux was varied from 30 to 140 kg/m 2 s. The results illustrate that the R22 heat transfer coefficient is always greater than that of R407C. Furthermore, a comparison carried out between the experimental data and those predicted by means of the most credited literature relationships showed a strongly overprediction for R407C coefficients.

Design and cost analysis for an ammonia-based solar thermochemical cavity absorber

A design and cost analysis is introduced for a solar thermochemical cavity absorber operated at the focus of a tracking paraboloidal concentrator and based on the ammonia dissociation reaction. The absorber design consists simply of a catalyst-filled nickel alloy tube wound into a spiral forming the inner cavity wall and shaped suitably to match the incident power density profile to the reactor heat requirements. The reactor tube is welded to a coaxial counterflow heat exchanger. The relationships among the power density profile, the reaction thermodynamics and kinetics and the heat transfer characteristics are examined in detail and it is shown that there is considerable potential that an installed cost goal of typically 10 U.S. dollars per square metre of solar collector area can be achieved through use of an appropriate high activity ammonia dissociation catalyst. There is a need for further tests of ammonia dissociation catalysts and also further nitridation tests of high-strength nickel alloys. The optimum absorber size for a given paraboloidal dish area is calculated for a system pressure of 20 MPa and it is shown that a cost effective absorber suitable for 100,000-hr operation would operate at 90 per cent efficiency at 750°C wall temperature.

Blood recirculation during hemodialysis with a coaxial counterflow single needle blood access catheter.

1. The average and the range of blood recirculation during hemodialysis employing a coaxial counterflow single needle blood access catheter are markedly reduced compared to those observed in the same patients using a standard "Y" type needle and external flow direction control device. 2. In a large group of patients, the average measured recirculation of blood of 3.8% at a blood flow rate of 200 ml/min is sufficiently small as to have little or no effect on the efficiency of dialysis. 3. Recirculation increases with increasing extracorporeal blood flow rate but remains sufficiently low to not significantly affect increased dialysis efficiency obtained at higher blood flow rates. 4. The coaxial counterflow single needle catheter permits single fistula puncture, eliminates the need for a flow direction control device, and is associated with negligible blood recirculation. It offers a most attractive alternative to both double needle and present single needle blood access techniques for hemodialysis.