Gholamreza Keshavarzi

Two‐Dimensional Computational Analysis of Microbubbles in Hemodialysis

On average, an end-stage renal disease patient will undergo hemodialysis (HD) three or four times a week for 4-5 h per session. Any minor imperfection in the extracorporeal system may become significant in the treatment of these patients due to the cumulative exposure time. Recently, air traps (a safety feature of dialysis systems) have been reported to be inadequate in detecting microbubbles and may even create them. Microbubbles have been linked to lung injuries and damage to the brain in chronic HD patients; therefore the significance of microbubbles has been revisited. Bubbles may originate at the vascular access sites, sites of local turbulent blood flow, the air trap, or in the bloodlines after priming with saline prior to use. In this paper, computational fluid dynamics is used to model blood flow in the air trap to determine the likely mechanisms of microbubble dynamics. The results indicate that almost all bubbles with diameters less than 50 μm and most of the bubbles of 50-200 μm pass through the air trap. Consequently, the common air traps are not effective in removing bubbles less than 200 μm in diameter.

Numerical and experimental study of capillary-driven flow of PCR solution in hybrid hydrophobic microfluidic networks.

Capillary-driven microfluidics is essential for development of point-of-care diagnostic micro-devices. Polymerase chain reaction (PCR)-based micro-devices are widely developed and used in such point-of-care settings. It is imperative to characterize the fluid parameters of PCR solution for designing efficient capillary-driven microfluidic networks. Generally, for numeric modelling, the fluid parameters of PCR solution are approximated to that of water. This procedure leads to inaccurate results, which are discrepant to experimental data. This paper describes mathematical modeling and experimental validation of capillary-driven flow inside Poly-(dimethyl) siloxane (PDMS)-glass hybrid micro-channels. Using experimentally measured PCR fluid parameters, the capillary meniscus displacement in PDMS-glass microfluidic ladder network is simulated using computational fluid dynamic (CFD), and experimentally verified to match with the simulated data.

Comparison of the VOF and CLSVOF Methods in Interface Capturing of a Rising Bubble

The VOF (Volume of Fluids) and CLSVOF (Coupled Level Set and VOF) methods are both methodologies in capturing the interface of two phase fluids. Despite some comparison on the rising velocity and interface capturing for certain simple cases of bubbles at certain times of the simulation, no full comparison of the interface capturing for cases of breakups and coalescence along with the stage by stage interface comparison has been completed. In this paper, the shape and deformation of buoyancy driven spherical bubble rising in a 2D channel is numerically investigated. The VOF and CLSVOF models have been used in the numerical simulation and the results were compared both qualitatively and numerically for both models. This specific bubble size and case was chosen since it involves breaking up, coalescence and deformation which is essential in examining the ability of accurate interface capturing of different methodologies. The results show the effectiveness and faster mesh convergence of the CLSVOF method; how...

Pulsatility Produced by the Hemodialysis Roller Pump as Measured by Doppler Ultrasound

Microbubbles have previously been detected in the hemodialysis extracorporeal circuit and can enter the blood vessel leading to potential complications. A potential source of these microbubbles is highly pulsatile flow resulting in cavitation. This study quantified the pulsatility produced by the roller pump throughout the extracorporeal circuit. A Sonosite S-series ultrasound probe (FUJIFILM Sonosite Inc., Tokyo, Japan) was used on a single patient during normal hemodialysis treatment. The Doppler waveform showed highly pulsatile flow throughout the circuit with the greatest pulse occurring after the pump itself. The velocity pulse after the pump ranged from 57.6 ± 1.74 cm/s to -72 ± 4.13 cm/s. Flow reversal occurred when contact between the forward roller and tubing ended. The amplitude of the pulse was reduced from 129.6 cm/s to 16.25 cm/s and 6.87 cm/s following the dialyzer and venous air trap. This resulted in almost nonpulsatile, continuous flow returning to the patient through the venous needle. These results indicate that the roller pump may be a source of microbubble formation from cavitation due to the highly pulsatile blood flow. The venous air trap was identified as the most effective mechanism in reducing the pulsatility. The inclusion of multiple rollers is also recommended to offer an effective solution in dampening the pulse produced by the pump.

Investigation into the Existence of Cavitation within Haemodialysis Needles

The source of micro bubbles within the haemodialysis extracorporeal circuit is currently unknown. If micro bubbles enter the body they may result in several health issues. Micro bubbles may be formed near the needles if the local pressure drops below the vapour pressure of blood, causing cavitation. Cavitation has been hypothesised to occur at the arterial needle due to the lower local pressures from the suction forces created by the peristaltic blood pump. A CFD model was developed to study the possible inception of cavitation from various options. Variations in needle flow rate and needle orientation were studied using a transient waveform, to determine the clinical conditions in which cavitation might occur. Cavitation may occur within the needle bore when the flow rates are temporarily elevated in the waveform. Needle orientation was also found to have no effect on cavitation potential.

Computational Simulations of Microbubbles

Accurate tracking of microbubbles plays a significant role in many engineering processes. Computational methods, including the volume of fluid (VOF), and coupled level set and volume of fluid (CLSVOF) are validated against a comprehensive experimental dataset, including detailed information describing the interface deformation, and transient development of the stage by stage shape data. Using the fully developed shape and subsequent deformation of rising microbubbles that have been captured experimentally and analyzed in detail using image processing, the corresponding VOF and CLSVOF results are accurately assessed for the small-scale differences between these interface capturing methods. Computational prediction on the removal of microbubbles is also examined in a haemodialysis airtrap. Such a model can provide useful information about the effectiveness and performance of an airtrap which is a commonly device used in a clinical setting for kidney failure patients.

Effectiveness of microbubble removal in an airtrap with a free surface interface

An end stage renal disease patient will undergo haemodialysis (HD) three or four times a week for four to five hours per session. Because of the chronic nature of the treatment, any minor imperfection in the extracorporeal system may become significant over time. Clinical studies have raised concerns relating to small microbubbles entering HD patients. These bubbles lead to further pathophysiological complications with the size of the bubble being a major factor. Microbubbles of different sizes can be generated throughout the extra-corporeal HD circuit. It is important to understand the possibility of these bubbles passing through the air trap or successfully being removed which indicates the performance of the air trap, the only mechanics of removing air bubbles. Chronic exposure to various sizes of microbubbles was analysed in detail for haemodialysis patients. However, smaller microbubbles are shown to be able to pass our modelled air trap. While studies have reported the presence of bubbles before and after the air trap, because these bubbles are only counted and not tracked, the performance of the air trap for removing different bubble sizes is not understood. Here, the performance of the air trap in filtering bubbles and the possibility of different bubble sizes passing through the air trap with the presence of the free surface interface have been evaluated. The modelled air trap is shown to be ineffective for filtering small micro bubbles.

Transient Analysis of Rising Bubble Using Image Analysis

Image analysis can play an important role in the validation and analysis of computational studies. Computational Fluid Dynamics (CFD) compared to experiments, has the advantage of detailed information analysis across the domain of interest. However, with detailed image analysis not only better detailed information can be extracted from experimental results, but also numerical methods and CFD can benefit from better detailed post processing information. In this paper, we show how image analysis may be applied to CFD data to better understand the results. More specifically, we simulate the transient behaviour of a 9 mm bubble rising in a 20 mm wide flow cell and extract the precise kinematics and characteristics of the bubble using image analysis. In order to better understand the kinematics of the bubble, we extract and plot the transient position and shape characteristics of the bubble. These parameters provide detailed comparison and analysis of the ability and differences of the interface capturing methods. This leads to better understanding of the applicability and accuracy of these models for various applications.

Investigation of the 3D Flow in Hemodialysis Venous Air Traps

Hemodialysis is an extracorporeal system which removes the waste product from kidneys for patients with kidney failure. Air bubbles within the system can cause several deficiencies to the system, and more importantly serious health issues to the patients. Therefore, different types of air traps (artery and venous side) are situated in the setup to prevent air bubbles passing through the system and being sent to the body. There have been evidence of the air trap deficiency. In order to understand these deficiencies the flow inside these air traps need to be understood. The investigation of the flow structures in air traps allow us to predict the efficiency of the air traps in capturing the air bubbles and preventing them from passing through. Computational fluid dynamic (CFD) has been used to compare the flow inside both these air traps. The results show interesting flow phenomena leading to explanations of the air bubble capturing effect.

Development of Two-Dimensional Bubble Movement and Development Benchmark Dataset for Numerical Validation

The motion and transport of bubbles in fluid flows have many engineering applications. The rise of a bubble has been a point of interest for both numerical and experimental studies. Various tracking methodologies have been developed, including markers, level sets and volume tracking. In order to validate numerical models of bubble flow, detailed experimental data describing the transient bubble shape is needed. This is best found from a 2D comparison rather than 3D experiment because computational resources for determining an accurate shape can be maximized. No real full time shape and subsequent deformation of this 2D bubble has yet been demonstrated. In this paper 2D bubble experiments have been conducted, in which a single bubble has been injected inside a close-walled tank and the rising of the bubble has been captured through a high speed camera. This data is now being used as a benchmark for numerical interface capturing and two phase flow methodology validations.© 2012 ASME