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Rheological behavior, pressure drop, and skin friction coefficient of multiwalled carbon nanotubes (MWCNT)-oil nanofluid were investigated experimentally. Plain and microfin tubes were used as the test tubes under constant heat flux boundary conditions. Pure heat transfer oil and nanofluuids with particle weight concentrations of 0.05%, 0.1%, and 0.2% were utilized as the working fluuids. Shear stress, shear rate, and viscosity of base fluuid of all nanofluuids at different temperatures were measured and the effect of nanoparticle concentration on fluuid properties was investigated. Furthermore, effects of nanoparticles concentration, heat flux, natural convection, and inclination on pressure drop and skin friction coefficient were studied for Grashof number ranging from 10 3 to 10 4 , Reynolds number ranging from 10 to 150, and particle weight concentrations up to 0.2%. Results showed that pressure drop and skin friction increased as inclination was increased from 0 to 90 degrees. Results also showed that the effect of addition of nanoparticles on pressure drop in a plain tube is more compared to the microfin one. At the end, a correlation was developed to calculate the skin friction coefficient in different inclinations and nanoparticle weight fractions.

A B S T R A C TBackground: Although some investigations have shown higher rates of successful first attempt and fewer attempts by using ultrasound-guided Internal Jugular Vein (IJV) catheterization, arterial puncture is still common. Objectives: The present study aimed to investigate US-guided catheterization of the right IJV via medial-oblique technique and also compare this technique to short-axis technique in open-heart surgery patients. Patients and Methods: In this randomized clinical trial, 80 patients referred to cardiac operating room of Namazi hospital, Shiraz, Iran from March to July 2014 were selected using census method. Block randomization with website was also done. Then, the patients were divided into two groups of 40, Short Axis Group (SAG) and Medial-Oblique Group (M-OG). For short-axis technique, patient's head was positioned at zero degree angulation with his trunk. For medial-oblique technique, on the other hand, patient's head was tilted to left to 45 degrees between the head and trunk. Sex, age, Body Mass Index (BMI), access time, guidewire time, cannulation time, total attempts for catheterization, first, second, and third attempt success, arterial puncture, hematoma, bleeding, and catheter malposition were recorded. The overlap between the carotid artery and IJV in zero-and 45-degree angulation was estimated through ultrasound print. After all, Kolmogorov-Smirnov test was used to assess normal distribution of the data. Then, the data were analyzed through Student's t-test, Mann-Whitney U test, and chi-square test. P < 0.05 was considered as statistically significant. Results:The results showed no significant differences between the two groups regarding the duration of different catheterization steps (P = 0.376). In all the cases in both groups, accessing the vein was successful with three attempts or less. There were no clinical complications of catheterization in the two groups. The mean of overlap was 23.60 ± 33.47 in zero-degree angulation between the head and trunk and 32.72 ± 36.38 in 45-degree angulation and this difference was statistically significant (P = 0.001). Conclusions:The results of the present study showed that both US-guided techniques under investigation had the same duration in different catheterization steps, total success rate, and primary mechanical complications, and could be used in clinics.

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