جستجوی مقالات مرتبط با کلیدواژه « ضریب انتقال حرارت جابجایی » در نشریات گروه « مکانیک »

The cooling system is one of the most important components that affects the performance of an engine. This system increases the heat transfer and fuel economy, leading to improved the engine performance. Most internal combustion engines have a coolant fluid that can be air or a liquid coolant that flows through a heat exchanger (radiator) and is cooled by air. The rheological and thermophysical properties of the nanofluid are of great importance for calculating the performance of pumping in this system. The heat transfer inside the radiator can be increased by increasing the heat transfer surface area and increasing the heat transfer coefficient. The heat transfer coefficient can be increased by appropriate heat transfer methods or by improving the thermophysical properties of the heat transfer medium such as coolant fluids. In this study, two samples of the thermophysical properties of the base fluid, which is a solution of distilled water and ethylene glycol with equal presence percentages, were investigated after adding titanium dioxide and silicon dioxide nanoparticles, at the temperatures of 30°C and 60°C. The viscosity, shear stress, and thermal conductivity coefficient of the prepared nanofluid were measured at two volume concentrations of 1% and 2% and then, validated using various theoretical models. The results indicated an improvement in heat transfer by 8% and 17% for the temperatures of 30°C and 60°C, respectively. The rheological behavior of the prepared nanofluid at two volume concentrations of 1% and 2% was also examined and validated using different theoretical models. Additionally, the results showed a decrease in the shear stress with increasing the temperature and an increase in the shear stress with increasing the volume concentration of nanoparticles.

The present paper aimed to conduct an experimental study on the hydro-thermodynamic performance of the double-tube heat exchanger using the water-magnesium oxide nanofluid and double twisted tape with three different steps (3.5,5and7 cm).The experiments were performed in the turbulent flow for the Reynolds numbers from 4400 to 16000 and the nanofluid volume fractions of 0.7%and 1.0%.The results were obtained at different flow rates with or without the use of twisted tape and nanofluid designed and manufactured in the experimental setup to calculate the convection heat transfer coefficient.Also the effect of using double twisted tapes at three steps along with the magnesium oxide nanofluid was greater in the higher Reynolds numbers.In the Reynolds numbers 4400,the convection heat transfer coefficient increased about17%for the volume fraction of0.7%and about 29% for the volume fraction of 1%.Also in the Reynolds number of 16000,it increased about48%for the volume fraction of 0.7% and about53% for the volume fraction of 1%.The simultaneous use of magnesium oxide nanofluid and double twisted tapes with the step of 3.5 cm in the Reynolds number 6400 was more effective than deionized water compared to other results.The performance evaluation criteria were examined in all cases and in case of using nanofluid and twisted tape with the step size of 3.5 cm, the highest value of the performance evaluation criterion is obtained with80% increase compared to the base state.

This paper reports a review and comparison of the methods used for calculating convective heat transfer coefficient in combustion chambers and in diverging-converging nozzles. Therefore, a history of applying different methods for calculating the convective heat transfer coefficient is explained first. Then, the nozzle flow is numerically solved, using the explicit McCormack method. In a Bates nozzle, The methods of Bartz, Stanton, Preiskorn, and Adami were selected among the proposed methods and were compared with CFD. Convective heat transfer coefficient of a solid fuel engine was calculated by taking into account the flow parameters in the engine chamber. Consequently, it was found that as wet move to the nozzle, heat transfer coefficient increases with velocity of the flow. This results revealed that in analytical methods, the maximum convective heat transfer coefficient occurs in the nozzle throat, while CFD results show that the maximum occurs upstream of the nozzle throat. These methods require less computational time than CFD, however CFD has to be considered more accurately. As a result, during a preliminary design procedure, the much faster and slightly less precise method can be used, in particular at the throat where the relative difference between the methods is quite low. Finally, it was shown that the innovative approach of combining Adami and Bartz methods has the lowest possible error, compared to the CFD.

The present study aimed to investigate the convective heat transfer coefficient of nanofluids containing magnesium oxide nanoparticles in water-based fluid in the exchanger plates have been carried out. Tests on the volume fraction of (0.005, 0.01, 0.015and 0.02) were used in this study have Ast.nanvzrh which has a diameter of 20 nm. Also the results show that with increasing fraction nanofluid volume, heat transfer coefficient and thus increases the heat transfer. Also heat transfer nanofluids% increase in volume fraction of 0.02 to 0.005 respectively 52% and 4.3% is on.

Removal of fluid in solar energy for maintenance and environmental problems is an important issue in recent years studied and tested operation. This technology is known as direct steam generation technology (DSG). The boiling liquid absorbing power plants is problematic in this article to help nanofluid trying to be that the problem is less absorbent and improve performance. In this study, based on numerical calculations of heat transfer and thermodynamics that govern the major parameters affecting the efficiency of the power plant is built in two pure fluid and nanofluid is investigated. Using equations related to it (single-phase or two-phase fluid), temperature, quality of steam and heat transfer coefficient of fluid throughout the pipes have been investigated and the positive impact of added nanoparticles was concluded. By adding 0.05% nanoparticles of copper, copper oxide, silver oxide, titanium was found to boil water faster and heat transfer coefficient of around 12.5 percent.

In this study, numerical simulation of the effect of hyperthermia using FePt magnetic nanoparticles with alternating magnetic field for skin cancer treatment is carried out. The numerical solution is presented to analyze bioheat transfer and magnetic induction equations, in cylindrical skin tumor situated in healthy tissue with considering evaporation and convection heat transfer. In order to show the validity of the study, the obtained results are compared with those of the studies already exist in the literature. Bioheat equation is used to predict the temperature rise in terms of characteristics of the magnetic nanoparticles, applied magnetic field and the tissue. The results reveal that nanoparticles diameter has a major effect on the temperature rise. The results also show that the temperature field in axial direction (from surface to depth) of tissue and the effects of hyperthermia decreases by increasing the convection heat transfer coefficient. In other words hyperthermia is more effective in the presence of the environmental natural convection. Moreover, the position of maximum temperature inside the tumor, varies by changing the heat transfer coefficient. Also, the amount of evaporation has a negligible effect on hyperthermia treatment.

The purpose of this study is to simulate fluid flow in a weir-type cascade solar still, assuming steady-state and air being the ideal gas and incompressible. The commercial software FLUENT is used for numerical simulation of the process. SIMPLE algorithm is used to calculate the relationship between pressure and velocity. In addition, first order upwind method is applied for separation of displacement and interpolation of pressure. Numerical simulations are performed for Solar Still with four to ten and also with twelve steps. Results of investigations on a Solar Still with different number of steps, in which the height of the steps is constant value of 2 cm, show that increasing the number of steps causes to increase water production up to eight steps. Furthermore, the highest rate of water production was 232 ml/m3hr for solar still with eight steps.

The purpose The purpose of this study is to simulate fluid flow in a weir-type cascade solar still, assuming steady-state and air being the ideal gas and incompressible. The commercial software FLUENT is used for numerical simulation of the process. SIMPLE algorithm is used to calculate the relationship between pressure and velocity. In addition, first order upwind method is applied for separation of displacement and interpolation of pressure. The effective parameters on unit productivity are the distance between glass surface and water, height of solar still, the number of vortexes and convective heat transfer coefficient. Results of investigations on a Solar Still with different height of steps, indicated that the productivity increases with increasing the height of stairs. Although, it should be noted that in each model with specific number of stairs there is an upward-downward trend; in other words, there is an optimum point in each model with specific number of stairs. Also, the still in the specific model of six-stairs with the height of 3.5 cm of stairs had maximum productivity.