聚乙烯在固体燃料冲压发动机中燃烧特性的数值研究

为准确模拟聚乙烯在固体燃料冲压发动机中的燃烧状态,估算了乙烯-空气两步反应中,相关组分的粘性、导热系数随温度的变化,并进行多项式拟合；同时使用UDF进行质量、动量、能量添加。数值计算的结果表明:燃面退移速率的计算误差不超过11.3%,其变化规律与已有实验相同；补燃室温度的误差不大于5.2%；补燃室温度随来流空气质量流率的增大而减小；高来流空气总温或低来流空气质量流率的工况下,燃料充分燃烧所需的补燃室更长；几何相似,其它条件相同时,小尺寸发动机内流场传向燃料表面的热流密度更大,燃面退移速率更高。      

横向射流起爆爆震波数值研究

采用CFD软件对横向喷射的射流在预先充满丙烷/空气恰当比混合物的爆震室中起爆爆震波的过程和机理进行了数值模拟,并讨论了射流的压力、速度以及温度对爆震波起爆特性的影响。结果表明,对于计算的物理模型,爆震射流不能在爆震室中直接起爆爆震波；射流与壁面的碰撞形成激波反射,激波反射产生的横波、热点和局部爆震引发起爆；能够成功起爆爆震波的横向射流存在最小的射流压力,该射流压力为0.9MPa；较小的射流压力、较小的射流速度或较低的射流温度都不利于横向射流在爆震室中起爆爆震波。      

吸气式脉冲爆震发动机反压传播规律研究

以汽油为燃料,空气为氧化剂,在内径为60mm的吸气式脉冲爆震发动机上进行了反压传播规律实验研究。测量了10~30Hz频率范围内进气道内的反传压力。实验结果表明,进气道内的反压峰值随着工作频率的增加而增加,两者基本呈线性关系。随着工作频率的增加,压力波动的时间占每个工作循环时间的比例增加。压力脉动比在20Hz时达到最大。建立了数值模型,采用小能量点火及温度梯度自适应方法,计算得到了反压的形成及传播特性。计算结果印证了反压是由于回传爆震引起的。将计算结果与实验结果进行了比较,结果表明两者符合地较好。      

Numerical investigation of the effects of the clearance gap between the inducer and impeller of an axial blood pump

A series of numerical models are generated to investigate the flow characteristics and performance of an axial blood pump. The pump model includes a straightener, an inducer-impeller, and diffuser. Numerical studies of the effects of angular alignment of the inducer and impeller blades and the axial clearance gap between the inducer and impeller are presented in this article. The pump characteristics derived from numerical simulation are validated with experimental data. Numerically simulated results showed a sinusoidal variation in the pressure generated across the pump with changes in angular alignment between the inducer and impeller. This is attributed to additional losses when flow is forced or diverted from the trailing edge of the inducer to either the pressure or suction side of the impeller blade when the alignment between the two sets of blades is not optimal. The pressure generated is a maximum when the impeller blades are at 0 or 30 degrees with respect to the inducer. The effect of rotating the impeller with respect to the inducer causes the sinusoidal pressure variation. In addition, it was observed that when the clearance gap between the inducer and impeller is reduced to 1 mm, the pressure generated is a minimum when compared to the other models. This is attributed to the interference between the inducer and impeller when the gap separating them is too small. The location of the maximum pressure on the pressure side of the impeller blade shifts upstream while its magnitude decreases for small clearance gap between the inducer and the impeller. There was no flow separation in the inducer while small regions of backflow are observed at the impeller trailing edge. Recommendations for future modifications and improvements to the pump design and model simulation are also given.     

Numerical Analysis of Pouch Filling and Emptying After Laparoscopic Gastric Banding Surgery

Background Previous studies have indicated that pouch volume and stoma size are two important factors related to weight loss after laparoscopic gastric banding in morbid obese patients. We hypothesized that there was association among the wall stress, pouch volume, and stoma size in a model for the filling and emptying phases of the pouch. Methods A numerical pouch model with variable pouch volume and stoma size was generated. Uniaxial tensile testing was performed on fundus strips from fresh pig stomach and the mass flow of filling and emptying of the pouch was simulated numerically. Results There was an overall qualitative agreement on the volume change between the simulated results and the clinical recording. Increasing the pouch volume size from 22 to 105 ml caused a decrease of the maximum circumferential stress from 14.14 to 11.80 kPa and the maximum longitudinal stress from 9.87 to 6.70 kPa in the pouch wall at the same degree of filling. Decreasing the stoma diameter from 27 to 10 mm caused an increase of the maximum circumferential stress from 11.46 to 12.78 kPa and a decrease of the maximum longitudinal stress from 10.34 to 8.69 kPa. Conclusion Both the pouch volume and stoma size are important determinants of mechanical wall stress, wall strain, and pouch emptying and hence may affect satiety and weight loss. This information may be important in understanding the mechanical behavior of pouches and for the development of more advanced numerical models in the clinical management of the surgery.     

Numerical investigation of the effect of blade geometry on blood trauma in a centrifugal blood pump

Fluid dynamic forces in centrifugal blood pump impellers are of key importance in destruction of red blood cells (RBCs) because high rotational speed leads to strong interaction between the impeller and the RBCs. In this paper, three-dimensional models of five different blade geometries are investigated numerically using the commercial software CFX-TASCflow, and the streaklines of RBCs are obtained using the Lagrangian particle tracking method. In reality, RBCs pass through the pump along complicated paths resulting in a highly irregular loading condition for each RBC. In order to enable the prediction of blood damage under the action of these complex-loading conditions, a cumulative damage model for RBCs was adopted in this paper. The numerically simulated percent hemoglobin (%HB) released as RBCs traversed the impeller and volute was examined. It was observed that the residence time of particles in the blade passage is a critical factor in determining hemolytic effects. This, in turn, is a function of the blade geometry. In addition, it was observed that the volute profile is an important influence on the computed HB% released.     

The Application of Bileaflet Mechanical Heart Valves in the Polish Ventricular Assist Device: Physical and Numerical Study and First Clinical Usage

The Polish ventricular assist device (Polvad) has been used successfully in clinical contexts for many years. The device contains two single‐disc valves, one at the inlet and one at the outlet connector of the pneumatic pump. Unfortunately, in recent years, a problem has occurred with the availability of single‐disc valves. This article presents the possibility of using bileaflet mechanical heart valve prostheses in the Polvad to avoid a discontinuity in clinical use. The study is based on experimental and numerical simulations and comparison of the distribution of flow, pressure, and stress (wall, shear, and turbulent) inside the Polvad chamber and the inlet/outlet connectors fitted with Sorin Monodisc and Sorin Bicarbon Fitline valves. The type and orientation of the inlet valve affects valve performance and flow distribution inside the chamber. Near‐wall flow is observed for single‐disc valves. In the case of bileaflet valves, the main jet is directed more centrally, with lower shear stress but higher turbulent stress in comparison with single‐disc valves. For clinical usage, a 45° orientation of the bileaflet inlet valve was chosen, as this achieves good washing of the inlet area near the membrane paste surface. The Polvad with bileaflet valves has now been used successfully in our clinic for over a year and will continue to be used until new assist devices for heart support are developed.     

Numerical and In Vitro Investigation of a Novel Mechanical Circulatory Support Device Installed in the Descending Aorta

Traditional implantation techniques of assist devices from the apex of left ventricle to the ascending or descending aorta are highly invasive and carry substantial complications for end‐stage heart failure patients. This study has shown that the descending aorta can be a promising location to install an implantable mechanical circulatory support with minimally invasive surgery. Herein, the hemodynamic effect of an in‐house prototyped pump implanted in the descending aorta was investigated numerically as well as experimentally. The objective of the experimental study is met by using the in‐house simulator of the cardiovascular loop replicating congestive heart failure conditions. The objective of the numerical study was met by using the modified version of the concentrated lumped parameter model developed by the same team. The results show that the pump placement in the descending aorta can lead to an improvement in pulsatility. The pressure drop, generated at the upstream of the pump, facilitates the cardiac output as a result of after‐load reduction, but at the same time, it induces a slight drop in the carotid as well as the coronary perfusion. The pressure rise, generated at the downstream of the pump, improves the blood perfusion in the renal circulation.     

Influence of the Compliance of the Pump Housing and Cannulas of a Paracorporeal Pneumatic Ventricular Assist Device on Transient Pressure Characteristics

Abstract: The dependence of transient pressure characteristics of a ventricular assist device (VAD) on the compliance of its housing and cannulas was investigated in a mock circulation. The peak rate of change of pressure ( dP/dt _max) values was greater in the cannulas than other compartments and was associated with valve closure-induced pressure oscillations. When cannula compliance was increased from 0.0057 to 0.0129 cm³/mm Hg, these values decreased by ˜20%, and outflow cannula pressure oscillation frequency decreased from 17.5 Hz by 35%. This trend was also apparent in the inflow. A VAD housing compliance increase from 0.0162 to 0.0483 cm³/mm Hg caused a dP/dt _max decrease of 30% in both the blood chamber and the outflow cannula. The effect of this change on the inflow was weaker implying that housing absorbs the energy associated with outflow deceleration more effectively than the inflow. These findings suggest that increasing VAD housing and cannulas compliance can improve hydrodynamic performance.     

Effect of Variable Body Mass on Plantar Foot Pressure and Off-Loading Device Efficacy

An increasing body of evidence has implicated obesity as having a negative effect on the development, treatment, and outcome of lower extremity pathologic entities, including diabetic foot disease. The objective of the present study was to increase the body of knowledge with respect to the effects of obesity on foot function. Specifically, we attempted to (1) describe the relationship between an increasing body mass index (BMI) on plantar foot pressures during gait, and (2) evaluate the efficacy of commonly prescribed off-loading devices with an increasing BMI. A repeated measures design was used to compare the peak plantar foot pressures under multiple test conditions, with the volunteers acting as their own controls. The primary outcome measure was the mean peak plantar pressure in the heel, midfoot, forefoot, and first metatarsal, and the 2 variables were modification of patient weight (from “normal” BMI to “overweight,” “obese,” and “morbidly obese”) and footwear (from an athletic sneaker to a surgical shoe, controlled ankle motion walker, and total contact cast). Statistically significant increases in the peak plantar pressures were observed with increasing volunteer BMI weight class, regardless of the off-loading device used. The present investigation has provided unique and specific data with respect to the changes that occur in the peak plantar pressures with variable BMIs across different anatomic levels and with commonly used off-loading devices. From our results, we have concluded that although the plantar pressures increase with increasing weight, it appears that at least some reduction in pressure can be achieved with an off-loading device, most effectively with the total contact cast, regardless of the patient's BMI.     

Numerical simulation of cardiovascular dynamics with left heart failure and in-series pulsatile ventricular assist device

This article presents a numerical model for investigations of the human cardiovascular circulation system response, where the function of the impaired left ventricle is augmented by the pumping action of a pulsatile ventricular assist device (VAD) connected in series to the native heart. The numerical model includes a module for detailed heart valve dynamics, which helps to improve the accuracy of simulation in studying the pulsatile type VAD designs. Simulation results show that, for the case with left ventricular (LV) failure, the VAD support successfully compensates the impaired cardiovascular response, and greatly reduces the after-load of the diseased ventricle, thus assisting possible recovery of the ventricle from the diseased condition. The effects of these conditions on pulmonary circulation are also shown. To investigate the effect of different pumping-activation functions (VAD motion profiles) on the cardiovascular response, three different VAD motion profiles are investigated. The numerical results suggest that Hermitian type motion profiles (smooth curves skewed toward early systole) have the advantage of requiring minimum power to the VAD, and producing the minimum after-load to the left ventricle, minimum ventricular wall stress, and minimum ventricular work to the diseased ventricle; while sawtooth type motions need slightly more power input, and induce slightly increased aortic pressure in diastole, thus improving coronary perfusion.