Journal of Aerospace Mechanics
Year:2016 | Volume:12 | Issue:3 (45) (HEAT TRANSFER AND PROPULSION)|Papers Count:10

Formation of cavitation bubbles and their collapse regarding its advantages and disadvantages are some important engineering issues. We can point out to some cases like the presence of these bubbles in the satellite cooling system and the flow over hydrofoils, where cavitation bubbles are formed in fluid flows. Generally, when the fluid pressure decreases so close to the vapor pressure at a specific temperature, the bubbles are created together with the fluid vapor. One time, these bubbles enter the high pressure region, they collapse and their radius take the minimum size, simultaneously. In this case pressure and temperature increases in the bubbles such that the chemical reaction which lead to molecular dissociation becomes active and as a result of very high temperature, bubble emits light. Along with this numerical study, bubble changes and its characteristics in different conditions were investigated and the effects of various factors on bubble growth and collapse were studied.

Wetness loss is a thermodynamic loss, which is due to irreversible heat transfer between the liquid and vapour phase during condensation. Formation and subsequent behavior of the liquid decreases the performance of turbine last stages. This phenomenon has a major effect on the efficiency of steam turbines. In this study, the steady compressible condensation flow field with heat transfer in a two dimensional of convergent divergent nozzle has been studied. The ANSYS FLUENT V14.5 software is used to calculate flow properties. The CFD coded 2D- approximate Eulerian-Eulerian multi- phase method for non- equilibrium condensation in transonic steam flow is used to capture nucleation phenomenon in the wet steam flow. Density based finite- volume discritization is employed and flow is considered inviscid. To examine heat transfer effects on the condensation flow in the convergent section of nozzle, a thermal source is used. The perfect gas equation of state is used to calculate the pressure and the properties of steam condensation. The pressure distribution over the nozzle and the average radius of droplets in adiabatic case are compared with both experimental and analytical data and show a good agreement. After verifying the model, the condensation flow with volumetric heat transfer is studied. The results show that by adding volumetric heating in convergent section of the nozzle the condensation shock strength decreases and even is omitted which cause to reduce droplets produced during the condensation flow.

The most efficient equipment in which heat rejection processes may be realized is cooling towers. The mathematical models of cooling towers are developed and validated against the available experimental data. These devices basically consist of three zones; namely, spray zone, fill packing and rain zone. The spray and rain zones are often neglected even in large cooling towers, while a significant portion of the total heat that is rejected may occur in these zones. Therefore, in this paper the heat and mass transfer contribution of the spray and rain zones in cooling towers is discussed as well. Numerical results fall within the range of experimental measurements and shows a higher accuracy compared with the results of previous researchers. The rate of water evaporation under a wide range of operating conditions is also presented. For Mass flow rates bigger than one, evaporation controls heat transfer process. Increase of inlet water flow rate decreases evaporated water rate.

The aim of this paper is to decrease the energy consumption in a residential building and provide thermal comfort by using hybrid system instead of the conventional method (convection heating method). In this system, the temperature of the room is kept around 10oC by convection system and then is raised to the thermal comfort by radiant heater. Thermal comfort is affected by temperature, relative humidity and velocity of the air flow. These parameters can be controlled by changing surface and temperature of the heater, position and velocity of the inlet and outlet, and type of the materials. In this simulation a 3D model of a virtual standing thermal manikin with real dimensions is considered. The continuity, momentum, energy, species equations for turbulent flow and physiological equation for thermal comfort are numerically solved for heat, moisture and flow field. The obtained numerical results have a good agreement with the experimental results reported in the literature. The velocity, temperature and the relative humidity contribution around the manikin is more uniform in the hybrid system. The velocity that is needed to provide the thermal comfort is also equal to 0.2 m/s in this system. This reduction of the velocity causes the heat transfer losses and energy consumption to around 25% and 19%, respectively. Also, the energy consumption decreases equal to %8 due to thermal resistance variation in the walls.

Forced convection heat transfer of a cam-tube and a cylinder in external laminar flow and at Reynolds number in the interval 7´104 to 1.2´105 areexperimentally implemented. Nusselt number distribution and its varying in terms of Reynolds number for prototypes (Cam-tube and cylinder) are treated. The principle result indicates applying the Cam-tube instead of cylinder make it possible increase the heat transfer between 5 to 22 percent. The uncertainty of the gathered data was at minimum 12 percent and at maximum 22 percent. Furthermore, the results show that the Hilpert’s equation is authentic for the treated cam-tube.

In this research, cut cells level set method despite of classic level set method is presented to evaluate more accurate grain burn-back analysis. This method is included of a Cartesian grid and by determination of the minimum distance to the grain boundary, boundary cells are specified. In order to investigate grain boundary condition with respect to Cartesian grids, two algorithms of perfect cut cells and selected cut cells are considered. In the perfect algorithm, all possible states (thirty states) for cutting cells by boundary lines, are considered. However, in the selected algorithm, more application states are considered. Comparison of results between classic, perfect and selected cut cells level set method prove, that CPU time increase, from classic to cut cells methods. However, accuracy of results increases also. The results of the selected method have a similar accuracy with a significant reduction in CPU time with respect to perfect method.

A finite volume method is used to numerically study the mixed convection of aluminum oxide-water Nano fluid inside a square cavity containing hot quadrilaterals obstacles on its bottom wall. The effects of Rayleigh number, volume fraction of Nano particles and the height of hot obstacles are investigated. The results are shown through streamlines and isotherm curves as well as Nusselt number. The average size of Nano particles and the temperature of Nano fluid are 40 nm and 300 K respectively. The applied rang of Rayleigh number is between 104 and 105, for volume fraction of Nano particles between 0 and 0.06, and for height of obstacles between 0.1 and 0.2 of the height of cavity. The results show that the increase of Rayleigh number as well as volume fraction of Nano particles will increase the heat transfer inside the cavity. Furthermore, the increase of the height of obstacles reduces the average Nusselt number. A comparison with experiments was made to validate the results.

To investigate the effect of oxygen enrichment in internal combustion engines, we used GT-POWER code to simulate the XU7 engine. The result show an increase of %8 in power and torque, and a decrease of %7 in brake fuel consumption for %23 volumetric excess oxygen. However, the increase of NOX is the only undesired side effect. To reduce exhaust NOX a convenient time variable is selected which can modify this effect, while prevents too much reduction in excess power and torque. Spark timing is chosen as tha mentioned time variable. To optimize oxygen amount and spark timing, we link GT-POWER and SIMULINK-MATLAB as well as neural network toolbox of MATLAB. The target of optimization is %50 reduction in NOX with respect to %23 volumetric oxygen enrichment case and %5 increment in power with respect to natural aspiration case. The designed algorithm is capable of optimizing excess oxygen and spark timing.

In this research, an experimental investigation has been carried out to study the heat transfer characteristics during evaporation of R-134a inside a corrugated tube. The corrugated tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, a. The Devices used in this study works based on a vapor compression refrigeration cycle. This test system includes an evaporator, which is composed of a standard corrugated tube. Heat required for evaporation of refrigerant in the evaporator is supplied by the heating element that is wrapped around it. The experiments were performed for seven different tube inclination angles, a, in a range of -90o to +90o and four mass velocities of 46, 81,110 and 136 kgm-2s-1 for each tube inclination angle during evaporation of R-134a. The results demonstrate that the tube inclination angle, a, affects the boiling heat transfer coefficient in a significant manner. For all refrigerant mass velocities, the best performing tube is that having inclination angle of a=+90o. The effect of tube inclination angle, a, on heat-transfer coefficient, h, is more prominent at low vapor quality and low mass velocity. In the low vapor quality region the heat transfer coefficient, h, for +90o inclined tube is about 62% more than that of the -90o inclined tube.