A study for the effect of enclosure insulation material on heat transfer and energy consumption at high temperatures

In this study, modelling of an enclosure, that can be considered as a field of fibrous insulation material application, is realized, then the heat transfer mechanisms in those materials and material microstructure parameters are investigated. Moreover, a device called Guarded Hot Plate (GHP), which is used to measure the total (effective) heat transfer coefficient of a material, is designed. First, numerical modelling of an enclosure is performed. In literature, the effect of heat transfer and flow mechanism were studied extensively. Therefore, firstly theoretical background of natural convection, turbulence and radiation heat transfer mechanisms are explained. Experimental setup and experiments are conducted to validate the numerical study. In experiments, instruments in ITU Mechanical Engineering Heat and Mass Transfer Laboratory, which are voltage regulator, data logger and panel including energy regulator and thermocouple sockets, are utilized. During the experiments, the temperature measurements of enclosure surfaces are recorded with data logger, and then the temperature profiles are obtained. Those temperature profiles from heaters are used as boundary conditions for numerical study. A commercial CFD program ANSYS Fluent® is used to model the enclosure. Issues that has been considered during the preparation of the geometry for the finite volume calculations and boundary conditions of the problem are indicated. For example, the crucial effect of enclosure wall thickness on numerical solution has been observed. That is why, they must be modelled. However, since their thicknesses are very low compared to the length of enclosure walls, these walls are modelled via shell conduction. Approximately half a million elements are created for this numerical study. A higher number of elements has been created near the enclosure walls prepared to achieve velocity and temperature profile more accurately. Different approaches used to model turbulence (Standard k-ε, Realizable k-ε models) and radiation (Surface-to-Surface and Discrete Ordinates models) are compared with each other. It has been observed that different turbulence and radiation models have no impact on solution. However, especially in high temperature applications, radiation must be modelled to obtain more reliable results. Solution methods, that can be considered to be suitable for modelling of natural turbulent convection at high temperatures, have been used. Since there are high temperature differences between the elements in domain, Ideal Gas Law is used to calculate the fluid density rather than Boussinesq Approximation, which can be used for low temperature differences. Methods, that are available in commercial program such as thermal effects option and full buoyancy effects option, are used. To discretize the pressure term, body force weighted discretization method, which can be considered to be suitable for natural convection problems, is used. Time dependent transport equations are solved using the implicit method. Therefore, it is not necessary to ensure Courant-Friedrichs-Lewy (CFL) condition. However, regarding the solution time, the determination of the time step is still critical. Time step of the problem is determined by using the average velocity observed in the enclosure and a characteristic length, which is the length of the enclosure walls. There are two main situations in analysis and experiments: One of them is using only fiberglass material on the backside of the enclosure and the other one is using fiberglass-ceramic wool, which is called as hybrid material. Fiberglass-ceramic wool hybrid material is prepared by using a method called needle punching. In this method, special needles are used to bond different insulation material to each other. In first situation, experimental and numerical temperature profiles on different surfaces of enclosure have been obtained and compared to each other in order to validate the numerical analysis. During the validation process, fiberglass insulation material is used. Maximum error between numerical solution and experimental results does not exceed 8% and temperature profiles from different walls are quite similar, thus it is convenient to use numerical model. In second situation, fiberglass-ceramic wool hybrid material is tested both experimentally and numerically. Next, energy consumptions and temperature profiles of two main situations are compared. As a result, using hybrid material as an insulation material of enclosure is not able to show the enhanced insulation effect that has been expected. Apart from this result of the first part of the thesis, the focus of the second part is micro structural properties of fibrous materials. Heat transfer mechanisms in fibrous materials namely - conduction, convection and radiation - have been discussed separately. Since high temperature applications are the main ones for fibrous insulation materials, radiation heat transfer become important and it is specifically studied. It is possible to categorize insulation materials in terms of their microstructure (fibrous, porous) or fiber base material (organic, inorganic). Fibrous materials have a wide application area in the industry. Some significant features of fibrous materials such as solid volume fraction, porosity, anisotropy factor are defined. Scanning Electron Microscope (SEM) analysis results are given. Modelling of radiation heat transfer in fibrous and porous materials are being studied widely. Basically, two main approaches are present: numerical and experimental modelling of material microstructure and optical properties. In the study of numerical modelling of radiation heat transfer, effects of parameters such as bulk density of insulation material and fiber orientation are discussed and compared with similar studies in the literature. An important parameter called size parameter determines the behavior of scattering of radiation-namely Mie or Rayleigh scattering. In this study it is accepted that fibers have larger diameters compared to wavelength of a ray; therefore, geometric reflection assumption can be made. It is concluded that, material temperature decreases rapidly with increasing material solid volume fraction. Moreover, increased material solid volume fraction suppresses radiation heat transfer transmission throughout the material. It is also concluded that different fiber diameters with a varying solid volume fraction have negligible impact on average fiber temperature. When it comes to the theoretical and experimental studies, the main focus is the practical methods, which make it possible to take radiation part of the heat transfer into consideration. Two-flux approximation is one of the methods that can be used to model the radiation heat transfer. Two-flux approximation equations are solved numerically through MATLAB® assumption of non-absorbing media. Experimental study is conducted by using Fourier Transform Infrared Spectroscopy (FTIR) with optically thick approximation to determine the material spectrum and then, to obtain radiation heat transfer coefficient. Radiation heat transfer in optically thick medium can be considered to be a diffusion process. Radiation heat transfer equation is solved by using FLUENT® with present parameters, namely scattering, absorption coefficients and refractive index. Radiation heat transfer flux is obtained from different hot side temperatures and compared with experimental and numerical data on literature. Effect of absorbing coefficient of material is also discussed. It is observed that increasing absorbing coefficient of material decreases heat flux across the material. Moreover, homogenous radiant temperature profile is obtained if the material has low absorbing coefficient, which means that media do not participate in the radiation heat transfer. Another discussion topic is the conduction and convection heat transfer mechanisms in fibrous materials. Air in fibrous materials is generally stationary. Therefore, convection heat transfer in those materials can be neglected compared to the other heat transfer mechanisms. Conduction heat transfer in fibrous materials takes place in gas (air) and solid (fiber) phases. There are various approaches to identify the interaction between those two phases. Empirical expressions found in literature are used to model the conduction heat transfer. It is showed that solid phase conduction heat transfer is very low compared to gas phase, especially if the material consists of low conduction heat transfer coefficient fibers. Using heat transfer coefficients from different mechanisms, total (effective) heat transfer coefficient of material is obtained and compared with experimental study that is conducted via Guarded Hot Plate (GHP). The last part of the thesis explains the design steps of GHP in detail. GHP is widely used to determine the total heat transfer coefficient of materials. Working principle of GHP is based on one-dimensional Fourier Law. A comprehensive explanation for the design of hot and cold plate of GHP as well as for the control mechanism of GHP is provided. Finally, it is concluded that mesh construction and solution methods have a significant effect on enclosure modelling. Especially in high temperature applications, modeling of radiation also becomes very crucial to obtain rational results. Enhancement of an insulation material basically depends on material microstructure such as solid volume fraction, fiber diameter, orientation and optical properties. Through the optimization of those parameters, an effective thermal insulation material can be designed. It is crucial to use correct radiation heat transfer approximations to obtain reasonable results. For example, two-flux approximation is suitable for one dimensional heat transfer problems and diffusion approximation is only valid for optically thick media. Therefore, one must be careful during the use of those approximations. FTIR method is widely used in textile and chemistry engineering applications to determine the material microstructure. In order to use this method in radiation heat transfer area, it is necessary to consider the device capacity. One of the most critical issues in the design of a GHP is how to get homogeneous temperature distribution from both hot and cold side plates. Moreover, structural construction of GHP must also be taken into consideration.