The effect of insulation material on convection heat transfer in airframe

Natural convection and forced convection are among the topics studied in engineering applications such as thermal energy storage systems, underground electrical transmission cables, solar energy collectors, nuclear reactors, airframe insulation materials. During the flight, moisture accumulation occurs in the porous insulation material of the aircraft, and due to the temperature difference of the cabin and outer airframe, the water vapor in the insulation material can become liquid or solid by changing the state. The moisture accumulation in the insulation material increases the weight of the aircraft and may cause the material to become corroded and lose heat and sound insulation properties. In this study, natural convection in concentric cylinders, natural convection in annulus with partially filled porous medium and forced convection in the cylinder, which has a certain thickness porous media located in the inner part of the outer wall, have been investigated and numerical solutions for these three problems have been presented. By means of these three problems which are solved numerically, another aim of the thesis study is to create theoretical framework to investigate and examine the air flow between the cabin and the fuselage, the moisture accumulation and water vapor change occurring in porous insulation material. The governing equations for fluid and porous media separated by finite volume method (FVM) were solved by means of the SIMPLE algorithm used for pressure and velocity correction. In this study, collocated grid which all flow variables are stored at the same locations was used. By using alternating direction implicit method and tridiagonal matrix algorithm, the systems of linear algebraic equations were solved. Firstly, natural convection in between concentric cylinders with constant inner and outer wall temperature, $T_i>T_d$, was analysed. The temperature and stream function profiles of air flow caused by buoyancy force due to temperature difference in annulus were presented and temperature and velocity distributions at certain angles, average Nusselt numbers were obtained in different Rayleigh numbers. While conduction heat transfer is dominated for low Ra number, with increasing Ra number increases, convection heat transfer becomes dominant. The thermal boundary layer is closer to the walls and the temperature gradients increase in laminar flow for high Ra nummber. Temperature change in low Ra numbers shows linear properties. As the Ra number increases, the thermal boundary layer gets thinner and the temperature change in the middle regions is very low. In high Ra numbers, the fluid in the middle regions becomes almost stagnation and the temperature gradient in these regions is close to zero. In low Ra numbers, a single vortex formation is observed in approximately 90 degrees angle zone. As the number of Ra increases, the single vortex moves further towards the upper left wall area by taking an elliptical structure. As a result of incresing Ra number, the velocity boundary layer is getting thinner and the velocity gradients increase . The maximum velocity is increased, and also it is closer to the wall with rising Ra number. The second problem examined was the natural convection in annulus with porous insulation material which located in the inner part of the outer wall. Heat transfer and air flow calculations between concentric cylinders with constant wall temperatures were obtained in different Rayleigh numbers, Darcy number and porous insulation material thicknesses and the effects of these three parameters were investigated. The second problem was solved numerically by using thermal non-equilibrium model. In the second problem, it shows similar characteristics with the first problem in high Ra numbers. As Darcy number decreases, velocities and temperature in the annulus will increase but the velocities decrease in the porous insulation material. The average Nusselt number increases with higher fine porous media thickness and higher Ra and Da numbers. Finally, the problem of forced convection in a cylinder with porous insulation material between the outer wall and the air duct was discussed. The air entering the channel with a certain temperature and velocity by the bottom of the cylinder on the vertical symmetry axis passes through the porous media and channel located side by side, then enters the inner of the cylinder from the top of the symmetry axis. Finally, the air is evacuated from the zone located just above the channel entrance. The effects of different Reynolds numbers, Darcy numbers and porous media thickness on the forced Convection phenomenon were discussed. In this problem, fluid and solid temperature profile, stream function, vector velocities and mean Nusselt number were calculated. As Re number increases, the absolute maximum stream function value increases. the temperature rise is observed in the cylinder. As Darcy number representing permeability in the per unit area in porous medium reduces, the flow ability to fluid is restricted within the porous insulation material. The absolute value of maximum current function increases and the temperature values in the cylinder increase as Darcy number is reduced. For high Darcy numbers, the vectoral velocities in porous insulation material have higher values than ones in low Da number. As the the porous media thickness increases in the cylinder, the stream function curves change into circular shapes from elliptic shape and the absolute maximum stream function value increases. The dimensionless temperature values tends to rise in the cylinder for thicker porous material. As the thickness decreases and Re number increases, the average number of Nusselt increases. In the second step of the third problem, where the channel wall is adiabatic and horizontal impermeable walls, the Re number increases, the solid skeleton temperature values of the fluid, the vector velocity values, the average Nusselt number increase. As Darcy number decreases, the temperature values in the upper and lower parts of the cylinder, the velocity values increase, the porous structure vector velocity values are very low, the maximum stream function absolute value increases and the flow becomes stronger. At the upper side of the cylinder representing the cabin part, the temperature values are always higher than the area under the impermeable horizontal wall. The lowest temperature values in the cylinder, except for the porous insulating material region, are located just below the horizontal impermeable wall where the flow is the weakest. For the most recent problem, the critical insulation thickness and the temperature distribution in two different Darcy numbers were obtained under certain conditions and the Darcy number which is suitable for human health is determined by comparing the temperature values at the top of the cylinder which represents the aircraft cabin section. Also the results were obtained and evaluated for different air channels and exit sections. The natural convection and forced convection investigated in this study were expressed by means of numerical results obtained for three problems. These results will be useful in establishing a basic perspective and framework for the heat and mass transfer phenomena between the airframe and cabin that will be examined after the thesis study.