فهرست مطالب محمدرضا سلیم پور

Having been developed in accordance with the Montreal Protocol, R134a has been proposed as a potential replacement for R12. In accordance with the Kyoto Protocol, the use of R134a, which has a significant global warming potential, must be restricted. It has been stated that there is no one refrigerant or combination available that can address both the ozone depletion potential (ODP) and the global warming potential (GWP) concerns at the same time. According to the research, the goal of this effort was to produce an environmentally friendly refrigerant combination with minimal ODP and GWP values that perform virtually identically to R12 in terms of performance. With extremely low ODP and GWP values, R406a has the potential to be a useful refrigerant. Experimental investigations conducted on R406a refrigerant have shown that it may be a viable replacement for R12. In this research, the pressure drop created by the torsion in the steam condensation of R-406a inside the horizontal tubes and also changes in the heat transfer coefficient are investigated experimentally. In each experiment, parameters such as mass flow, refrigerant temperature and pressure, and water at the inlet and outlet of the condensers were measured. Examination of the results for annular and torsional fins pipes, it was determined that heat transfer and pressure drop increase with the reduction of the pitch. The use of spirals increases the average heat transfer coefficient and pressure drop by about 47% and 220%, respectively, compared to the pipe without fins. Finally, using the results from the twisted tubes and based on the relationships that best match the experimental results related to the pressure drop, a new relationship for the pressure drop in the twisted tubes was obtained. The values calculated by it are in the range of approximately 15% of the experimental values.

Pool boiling has many applications in industrial processes. Use of nanoparticle, heater surface expansion and magnetic field applying are important and effective elements on boiling heat transfer. In this article, pool boiling of dionized water and γ-Fe2O3/water magnetic nanofluid have been analyzed on smooth and copper grooved surface at one atmosphere pressure in the presence and absence of magnetic field, experimentally. For comfirmation of results accuracy, the test of dionized water was done in three different days that had good agreement with reference relations. Nanofluid has been built in one stage and has high stability. The result showed that the boiling heat transfer coefficient of dionized water has increased in circular and rectangular grooved surfaces and has decreased in triangular grooved suface toward the smooth surface. The boiling heat transfer coefficient of nanofluid has increased 24% in circular grooved surface and has decreased 8% and 37% in rectangular and triangular grooved sufaces, respectively, toward the smooth surface. The corners existence and wettability reduction in vertical wall of rectangular and triangular grooves and the bubbles slipping cause thermal resistanse increasing toward circle groove. Two flat constant magnets have been used in two sides of boiling reservoir for magnetic field creation. By applying magnetic field with negative gradient, the boiling heat transfer cofficient of nanofluid has enhanced in circular and rectangular grooved surfaces and has reduced in triangular grooved surface. The upward magnetic force causes the formed bubbles diameter decreasing, but groove type is effective on result, too.

Frostbite is severest form of cold injury that can have serious consequences on limbs such as digits, ears, cheek and nose which are far from the heart. Therefore, in this article, it has been tried to simulate the temperature distribution and find the starting time of frostbite in a human finger as a member that is sensitive to cold stress with a realistic approach. In this research, anatomical data has been extracted from MR images which approximate bone, soft tissues and vessels volume independently. The approach that is presented for converting the MR images to a 3-D finite element model is one of the novelties of this research. A coupled thermo-fluid model is applied to simulate a finger exposed to cold weather. The effect of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between tissues and large vessels are considered in energy balance equations. Environmental situations such as ambient temperature and wind speed and also glove thermal resistance have been investigated and compared to each other.

Investigating of boiling process is one of the attractive fields for researchers, because of many applications in industry such as heat exchangers and air condition systems. One of the important and effective factors in pool boiling heat transfer is the heating surface geometry. In present article, pool boiling of dionized water and Fe3O4/water nanofluid at atmospheric pressure have been analyzed on smooth and grooved copper surfaces, experimentally. The effect of rectangular, circular and triangular grooves with the same pitch on boiling heat transfer is the main aim of present article. The results have showed that the boiling heat transfer coefficient of dionized water in circular and rectangular grooved surfaces has enhanced 92% and 48.9%, respectively, and has reduced 33.1% in triangular grooved surface toward the smooth surface. Also, the boiling heat transfer coefficient of Fe3O4/water nanofluid in circular grooved surfaces has increased 40.7% and has decreased 21.8% and 88.7% in rectangular and triangular grooved surfaces, respectively, toward the smooth surface. The corners existence in rectangular and triangular geometries causes thermal resistance increasing and heat transfer coefficient decreasing toward circular geometry. Also, the groove area, the mechanism of bubbles creation and nanoparticles deposition content on different surfaces are effective on the boiling heat transfer. For investigation of depth effect, the grooves depth was increased in different geometries. By adding depth, the boiling heat transfer coefficient of water and nanofluid has increased up to 43.5% and 40.6%, respectively, because of heat transfer surface and nucleation sites density augmentation.

In the present study, three different fluids with different volumes of nano materials(Al2O3) in the Reynolds range of 3 to 48 thousand were investigated for the purpose of evaluating the effect of using nanofluid in heat exchangers. The converter in the submarine engine come to use of as a motor oil cooler. The results show that the addition of nanoparticles to the base fluid in the lower reynolds results in a higher performance factor in the exchanger. By increasing the volumetric percentage of nanoparticles, the converter's performance factor decreases. Optimum mode and maximum heat exchanger performance, in all volumetric percentages, occurs in Reynolds 20,000 for water and 12,000 for ethylene glycol. In other words, the choice of base fluid with a higher performance factor depends on the Reynolds number range. Also, this study examined sea water as a cooling fluid available to the marine diesel engine.

Boiling heat transfer is one of the most applicable heat transfer processes within the industry. In this paper, the pool boiling heat transfer of Fe3O4 /water nanofluid (ferrofluid) in atmospheric pressure has been analyzed, experimentally. The nanofluid in this study, has been synthesized in a single step and retains high stability. The replication and accuracy of the testing machine has been studied for deionized water for three times, indicating an appropriate concordance with the literature. Considering different volume concentrations of the nanofluid has revealed that boiling heat transfer in high concentrations decreases with an increase of concentration, while it rises with the increase of concentration in low concentrations. Hence, boiling heat transfer coefficient in 0.1% volume concentration nanofluid has been measured to be the optimum value which increases up to 43%. The roughness of boiling surface was varied with the deposition of nanoparticles in various conditions of nanofluid concentration, and heat flux. It is noteworthy that in the present research, the effects of surface roughness changes due to nanoparticles deposition and the impact of passing time on boiling process have been investigated, for the first time. Therefore, several experiments have been designed in order to study the change of nanoparticles deposition due to the change of nanofluid concentration and boiling surface heat flux. The results indicate that boiling heat transfer of deposited surfaces at low heat fluxes decreases; while it rises at high heat fluxes.

In the present work, constructal design of annular finned tube has been studied. Geometrical parameters include fin diameter, fin thickness, fin pitch, tube outer diameter, tube length while physical parameters involve pressure drop number, Stanton number, fin-to-air conductivity ratio, and in-tube fluid-to- air conductivity ratio. The aim of this study is to enhance heat transfer by letting the geometrical degrees of freedom to morph. It was observed that at certain flow conditions, there exist optimal geometry and fin number for the finned tube construct in which its thermal resistance is minimum. Fin efficiency and tube-side convective heat transfer coefficient are higher at low pressure drops and Stanton numbers. In these conditions, analytical relationships were proposed to predict optimal heat transfer, optimal fin number and optimal geometry. It was seen that the optimal fin thickness-to-fin pitch ratio is merely dependent on the fin volume fraction; and it rises with the increase in fin volume fraction. Moreover, the optimum fin number is directly proportional to fin spacing – to- fin pitch ratio and inversely proportional to Stanton number. Furthermore, it was seen that in the range of parameters considered in this study, the tube with 3400 fins and aspect ratio of 0.63 has the most heat transfer rate.

In this paper, the heat shocks in the heat flux profile are estimated with known temperature distribution using Kalman filtering. For this task, the temperature distribution is calculated and artificial noise is added to adequately simulate the real temperature measurements. Next by using Kalman filtering, estimation of online heat flux is assessed which appears as heat shocks in the discussed problem. Results indicate that the present method can accurately estimate jumps in the heat flux.

Energy costs have soared rapidly in the last decades. Thus there is a tremendous need for new kinds of working fluids to improve the heating systems performances and to reduce energy consumption. One of the most applicable heating systems are heat exchangers. Study of thermal hydraulic characteristics for laminar fully developed flow through conduits with different cross-sections is significant in the design of heat exchangers. In the present investigation, the thermo-hydraulic behavior of nanofluid trough conduits with circular, square and rectangular cross sectional shapes is studied, experimentally. This investigation aims to study the effect of Reynolds number and shape of cross-section on heat transfer and pressure drop of nanofluid flow. The experiments were conducted for TiO2/water nanofluid with three volume fractions 0, 0.2 and 0.5 under laminar flow regime with constant wall temperature. Analyzed data indicate that friction factors of square and rectangular cases are more than that of circular cross section. Also, it is seen that the addition of nano powder with low volume fraction (0-0.5%) to base fluids does not increase the friction factor remarkably at both circular and non-circular cases. The results show that the Nusselt number of flow through the conduit with circular cross-section is higher than that of non-circular cases. Moreover, it is observed that adding nano powder o base fluid improves heat transfer in sharp corners and therefore its effect is more pronounced in non-circular cross sections.

One of the challenging problems of tribology is cam and follower elastohydrodynamic lubrication due to the simultaneous effect of various lubrication mechanisms. These mechanisms are transient, squeeze film, elastic deformation of contacting surfaces and variation of lubricant properties with pressure. In this paper, besides studying the mentioned factors, the effect of using a non-Newtonian lubricant such as grease is numerically investigated. The lubrication governing equations and Oswald’s grease behavior equation have been discretized using finite difference technique. The system of equation has been solved via Multi-Grid method which is an advanced iterative method in solving system of partial differential equations. The results are shown for Newtonian oil compared to grease for different cam rotational speed. Also, different grease behaviors are investigated. The results are verified by comparison of the results obtained using the famous Newton-Raphson method. Findings show that the minimum lubricant thickness as well as the maximum pressure is lower when using grease compared to the case that a Newtonian lubricant is used. In the case of Newtonian lubricant, increasing the speed results in an increase in the lubricant film thickness but it is shown that, in the case of Non-Newtonian lubricant, speed does not affect the lubricant thickness.

In rocket systems، the re-entry speed to atmosphere is very high which leads to compression of air molecules and appearance of strong bow shock waves in the leading edge; consequently، this yields aerodynamic heating. Using ablating-dispensing materials on the leading edge surfaces، it is important to accurately determine heat flux on these moving boundaries. Measuring heat flux directly is very difficult or impossible in some situations. In the present study، the online Kalman filtering is used to determine heat flux accurately. Since the heat flux is estimated in online (non-iterative) fashion، the optimum location of temperature sensors can be effectively determined. In addition، the results of this study can be used to design heat flux sensors. In this paper، the optimum locations of three temperature sensors are calculated on the basis that the disturbances occur due to burning of sensors are reduced. More robust solutions are obtained for heat flux on the ablating surfaces.