Investigation of ice sublimation

Sublimation is a mechanism that has been worked on for many years, expressing the phase change between the solid and the gas phase of the material. For sublimation; the substance must be below the triple point temperature and pressure values on the phase diagram. Despite the fact that studies on sublimation have been done for a long time, sublimation phenomena is still a hot topic. Sublimation phenomena is important especially for cryogenic cooling, medical studies, air-space studies, and atmospheric studies in cold regions. Also, the energy transferred in the phase change between the solid and the gas phases is higher than the solid-liquid or liquid-gas phases. This feature makes the sublimation-driven cryogenic cooling cycles be more important than the conventional vapor compression cycles. Since the aerospace and aeronautical studies have extreme ambient conditions (temperature and pressure), the issue of sublimation needs to be taken into consideration. Materials, sublimate in room conditions, such as naphthalene, solid carbon dioxide and camphor have usually been preferred in the literature studies. However, there are also a number of studies in which the ambient conditions are specially conditioned, conducted with different materials. One of the most common difficulties in sublimation studies is the measurement of the sublimation rate. In most of the existing studies mass measurements were done to determine the amount of sublimated mass. However, in studies where mass measurement is not possible, there is no general agreed method for measuring the rate of sublimation. Scientists working on issues such as meteorology and space have tried to develop their own methods to investigate the amount of sublimation that takes place on a geographical location on Earth or another planet. They performed sublimation measurements in laboratory environment using different equipments. Another difficulty in measuring rate of sublimation is that the sublimation phenomena is slower than other phase changes. In order to overcome this difficulty and make the sublimation observable, scientists attempted to provide sublimation-accelerating environmental conditions in their study or used very precise measurement techniques. The most obvious examples of this situation are studies about the snow and ice sublimation at the cold regions. A number of models have been developed to determine the total amount of sublimation occurring in large land or airborne snowflakes, but these models usually deal with the sublimation of a single snow crystal or a specific snow mass to perform experimental measurements. In this thesis, three different methods have been used, namely weighing, analytical calculations and visualization studies, to measure the rate of sublimation in time-dependent ambient conditions of ice particles in a closed volume freezer. The aim of this study is to investigate the ice sublimation and observe the effect of different parameters on the rate of sublimation. An analytical model and experimental setups were developed to study ice sublimation under time-dependent conditions. Measurement of the sublimation by means of the visualization method which is mentioned in the literature is determined as a secondary objective for this thesis. Weighing is the most used method to determine the sublimation rate. This method was used to determine the total sublimation rate by measuring the mass difference between the first and last states of ice. Hemispherical ice particles and ice cubes made with ice tray were used in this thesis. A mathematical model which is compatible with the ambient conditions and hemispherical geometry was created for analytical calculations. This model was transferred to MATLAB and an algorithm was developed for the mass transfer calculations. Experimental temperatures and humidity values were recorded to use in MATLAB algorithm calculations. It is seen that the model which was created for time related sublimation rate in hemispherical geometry is compatible with experimental results. Nusselt and Sherwood relations which used in analytical model was obtained from general equations in literature and experimental results. It is seen that this Nu and Sh relations are compatible with literature. Periodic photo recording method that is recommended to observe sublimation in the literature was used to determine sublimation rate in this study. Projection area photographs of ice particle was recorded hourly. Image processing algorithm which was created with MATLAB was used to analyze the photographs. The changes in the ice volume and mass were calculated from the changes in the projection area of the particle. It has been observed that the results of sublimation obtained by image processing for hemispherical ice particles are consistent with analytical study and weighing results. Temperature, air velocity, time, ice size and ice geometry are taken as the main parameters. Experimental studies were carried out at -18 ° C and -24 ° C set temperatures of the freezer. The amounts of sublimation at high air velocity (forced convection) and at low air velocity (mixed convection) are investigated. The amount of sublimation was compared in terms of geometric shape and size for hemispherical ice particles and ice cubes made with ice tray. Experiments were conducted at different durations to observe the effect of experiment time on the sublimation rate. In the freezer compartment, two separate experimental setups were constituted for two different transport situations. Cameras were placed to periodically take photos of the projection area of the ice particles in both experimental setups. These cameras were fixed to the freezer shelves to minimize the vibration effects and external influences. In order to facilitate the identification of ice particle by the MATLAB algorithm, the background area where the ice particle is placed is covered with matte black paint. Led lights were placed on the experimental setups to provide the required ambient light while taking photographs. The taken photographs were analyzed by MATLAB image processing method. K-type thermocouples were used to acquire temperature data from the ice surfaces and ambient in experimental setups. Relative humidity sensors are used to acquire relative humidity data from the freezer ambient. The data were recorded with the VTS (data acquisition) system. Ice particles which were placed in the freezer compartment subjected to air circulation for the mixed convection experiments. Velocity measurements were made with anemometer on the mixed convection setup where the ice particles placed. It was determined that the average velocity of the air influencing the ice is 0.07 m/s. The Richardson number for different parameters was calculated and it was determined that the experimental setup provided the conditions for mixed convection studies. An air tunnel was created for the forced convection studies and located vertically in the freezer compartment. The air tunnel consists of an axial fan, two flow straighteners and a grid which ice cubes put. PIV (particle image velocimetry) method was used to determine the air velocity which flows over the ice particle in the tunnel. Also flow homogeneity was checked with PIV method. Air velocities are measured for different volts of the axial fan. Three different air velocities (0,7; 1 and 1,2 m/s) are used in forced convection experiments based on the axial fan volts (6; 9 and 12 volts). Experimental studies showed that the rate of sublimation in mixed convection is less than in the forced convection. It has been determined that temperature and air velocity are directly proportional to the sublimation rate. However, it has been observed that there may be a limit value for the air velocity which affecting the sublimation rate at the constant temperature. The limit air velocity value is different for each temperature value. Also, it is thought that there is an inverse proportion between the temperature and the limit air velocity. The rate of sublimation does not change significantly depending on the duration of experiments. However, the rate of sublimation decreases markedly when the ice size falls below 20% from the initial state. This was observed with a decrease in the slope of the remaining mass curve. The amounts of sublimation from hemispherical ice particle are calculated with using analytical model in terms of different temperatures, relative humidities, air velocities. When relative humidity in the environment is 100%, sublimation does not occurs for different convection types regardless of temperature. The maximum rate of sublimation occurs in dry ambient conditions which relative humidity in the environment is 0%. It is observed that as the air velocity increases, the amount of sublimation increases in both types of convection, but this increase depends on the temperature. If the amount of sublimation at a given temperature is determined; it is seen that the rate of sublimation is directly proportional to the air velocity. However, it is also observed that the increase in the rate of sublimation is inversely proportional to the air velocity. This supports the presence of a limit air velocity for the rate of sublimation at a given temperature. In order to minimize the ice sublimation rate, it has been observed that the relative humidity values should be high and the temperature values should be low in the ice storage environment. This study which done with single ice particles is the basis for determining the interactions of the ice cube particles with each other. The generated mathematical model and image processing algorithm can also be used in sublimation studies for different materials. In addition, the developed image processing algorithm can be used in different applications that requiring visualization by editing the filter properties.