A mathematical model for the prediction of Internal recirculation of complex impinging stream reactors has been presented. The model constitutes of a repetition of a series of ideal plug flow reactors and CSTR reactors with recirculation. The simplicity of the repeating motif allows for the derivation of an algebraic relation of the whole system using the Laplace transform. An impinging streams reactor system with one axial and two tangential inlet fluid streams was constructed and considered as a case study. The model predicts satisfactorily the complex and flow rate dependent experimental residence time distribution functions obtained employing a pulse tracer method for different total flow rates of the incoming feed. The variation of the controlling parameters with changing the total inlet flow rate are discussed. The presented model can predict complex Internal recirculation streams within the impinging streams reactor system.

Dear Editor: Recently, with great interest we read the review article by Zeraati et al. entitled "A review article: access recirculation among end stage renal disease patients undergoing maintenance hemodialysis" in your most valuable journal. The author summarized some of observations about causes and clinical significance of arteriovenous (AV) fistula recirculation, techniques for accurate assessment and main source of pitfall in calculation of access recirculation (AR). Hemodialysis AR occurs when the blood urea concentration in arterial line is lower than that of systemic circulation, indicating that dialyzed blood returning through the venous needle reenters the HD machine through the arterial needle.

Sound is one of the forms of mechanical energy and the two main characteristics of sound are intensity or power and frequency or wavelength of sound.their performance against the incoming sound wave, industrial silencers can be divided into two general groups of resonating and absorption silencers, the main difference between these silencers is the release of sound energy from the channeling system, which is one of the common examples of the use of resonating type silencers, their use in It is the Internal combustion engines that distinguish absorption silencers from the resonator type based on the fact that the main and visible part of the act of muting the sound is achieved by changing sound energy to heat energy.The goal of this article is to design a muffler based on the breaking of sound frequencies resulting from the movement of fluid in the exhaust output of vehicles, which leads to a reduction of at least 50 db of sound and gives the operator enough peace and concentration. In this article, after examining three types of mufflers, absorbent mufflers that use the properties of porous absorbent material to absorb passing sound and are the simplest form of mufflers, have been selected, analyzed and reviewed and are suitable for the OM457 engine of Idem Industrial Company. It designed for maximum inlet exhaust temperature is 520 and for the maximum kW power is 315 with the maximum discharge relative pressure of 185 mbar for homogenization with the standard atmosphere.

Treatment for end stage renal disease patients is based on dialysis; however, the presence of access recirculation (AR) decreases dialysis efficiency and adequacy. This study was conducted to determine the recirculation rate in dialysis patients undergoing hemodialysis through using a permanent catheter. 60 patients including 23 males and 37 females were enrolled. Mean age of the participants was 57. 66 (± 14. 08) years. Mean AR in the subjects was 9. 36%, and 16 (27%) of patients had mean AR above 10%. Moreover, there was a significant correlation between AR and catheter longevity (P <. 001). It is suggested to limit the use of permanent catheters to specific cases and not to use them in place of arteriovenous fistulas.

Hemodialysis access recirculation occurs when dialyzed blood returning through the venous needle reenters the extracorporeal circuit through the arterial needle rather than returning to the systemic circulation. It is important to meausre hemodialysis access recirculation for two reasons: The reentry of dialyzed blood into the extracorporeal circuite reduces solute concentration gradients across the dialysis membrane by mixing already dialyzed with undialyzed blood. As a result, the efficiency of dialysis is reduced. High degrees of recirculation can lead to a significant discrepancy between the amount of hemodialysis prescribed (prescribed Kt/V urea) and the amount of hemodialysis delivdred (delivered Kt/V urea). High degrees of access recirculation indicate the presence of access stenoses, the most common cause of access thrombosis. Prospective identification and treamtnet of access stenoses improves long-term access patency rate.

Background: Prior investigators have demonstrated that determining recirculation in aterovenous fistula is a reliable tool for evaluation of hemodialysis. Dialysis patients were reported to be 22000 cases in our country, for whom aterovenous fistula were prepared with different techniques. To our knowledge, there is not any study clarifying the quality of hemodialysis in the affected subjects. Materials and methods: A descriptive study was conducted on 97 patients in Hasheminejad Dialysis Center in Tehran, all of whom had native aterovenous fistula. recirculation was determined by low blood flow technique. Type and site of the fistula were compared according to the recirculation rate. Results: recirculation varied from zero to 16% with the mean of 2.5%. The highest mean of recirculation was found in end to side proximal fistulas and the lowest was reported in side to side proximal fistulas. Conclusion: recirculation was within the normal range. It does not show correlation with site and type of fistula at flow of 300 ml/min.

Continuous Variable Valve Timing System or CVVT has a duty of variation of continuous open timing of the valves. This system has been put on the intake camshaft in EF7 engine. In this article, thermodynamic analysis of variation for open selective angles of valve according to variation of engine speed and load has been investigated. Opening angles in two state of gas and gasoline fuel by selecting of best point on the basis of data results of engine test has been achieved and this article analyses of the cause of this selection from thermodynamic point of view.

The impeller of a centrifugal compressor is traditionally designed using some formula for only one design point which makes it less efficient in all other situations. This is especially important for compressors not experiencing a constant working condition. To improve the performance at low mass flow rates and retard the surge, an innovative concept is introduced for a centrifugal compressor. In this method pressurized air is injected at the compressor inlet to improve the flow field. With better incidence angle, related losses at off design conditions are minimized and the surge is delayed. This system is designed, modeled and adjusted for providing an optimal flow pattern at the inlet. Its implementation on a compressor has shown an increase of efficiency at low mass flow rates. It has also improved flow pattern in impeller passages and decreased the blade loading near surge condition. It is also shown that the swirl generator system can be fed up from the compressor volute or diffuser, and thus widening the compressor performance map by retarding the surge margin.

Introduction: The measurement of Arterio-Venous (A-V) Fistula recirculation has important diagnostic implications because the efficiency of haemodialysis (HD) may be limited, resulting in dialysis delivery being less than that prescribed. The purpose of the study is to determine its prevalence and causes in our HD patients.Methods and Materials: We randomly selected 100 end stage renal disease patients with AV fistula that they were on HD more than 3 months. The degree of recirculation was also measured with Urea based two needle technique method. For each patient distances between arterial and venous and distances of needles from fistula and its directions was recorded.Echocardiography and A-V fistula Color Doppler Ultrasound were also performed.Results: Blood flow rate and dialysate flow rate were 300 mL/min and 500 mL/min respectively.The prevalence of A-V fistula recirculation was 17% (17patients).Average degree of recirculation between these patients was 9.56±2.32 %. The most common cause was misplacement and or misdirection of needles (17 patients). The second cause was heart failure with Ejection Fraction>40% (8 patients). No difference was seen between diabetic versus non diabetic (P=0.28) and hypertensive versus normotensive (P=0.21%) HD patients.Conclusions: A-V fistula recirculation is common occurrence in HD patients and the most common cause of recirculation is misplacement and or misdirection of needles so we should have more emphasis on education and training of HD staffs.