Real Time Upright Freezer Evaporator Performance Under Frosted Conditions

Frost formation on an upright freezer evaporator is investigated experimentally on real refrigerator geometry. It’s effects on overall heat transfer value (UA), airside pressure drop and airflow rate is observed with respect to time. That is, the real time evaporator performance under frosted conditions is evaluated. Airflow rate is obtained with respect to time by using fan characteristic curve. INTRODUCTION When moist air comes across a surface, which has a surface temperature below dew point of air, the moisture in the air condenses on the surface. If the surface’s temperature is below the freezing point of the water, the condensed moisture on the surface changes its phase from liquid to solid, namely from water to frost. This phenomenon so-called frost formation is mostly encountered in the field of refrigeration and air-conditioning and has a significant adverse effect upon the heat transfer and airside pressure drop. During the cooling cycle, frost accumulates on the evaporator of the system. The frost does not only insulate the heat transfer surface of the evaporator, but also becomes thick enough to restrict and blocks the airflow. Moreover, the compressor decreases the evaporation temperature to increase the heat transfer during frosting. However, this precaution increases the energy consumption of the system. Furthermore, frost layer on the evaporator has to be melted away periodically to prevent the above effects. However, during defrosting, energy has to be spent for heating the evaporator. Due to these adverse effects, frost formation phenomenon has been investigated by several researchers. Aluminum heat exchanger fins under frosting conditions were investigated experimentally [1] by using a laser scanning system. It was found that frost thickness close to the leading edge and fin base was higher than the frost that accumulated on the other parts of the fin. In another study, frost growth around a cylinder was investigated numerically [2]. It was stated that frost thickness near the leading edge was thicker than the trailing edge. In addition to these studies, the whole no-frost evaporator geometry was investigated [3,4,5]. It was determined that the overall heat transfer value (UA) and evaporator airside pressure drop increased during frosting [3,4]. These conclusions matched with mathematical model result [4], which was based on constant airflow rate and homogenous frost accumulation on the evaporator. The other model [5] that was based on non-homogenous frost distribution, real time airflow rate and pass-by-pass evaporator frosting evaluation gave different trends. It was found that, UA value increased in the first hours and a decreasing trend followed this augment. Airside pressure drop trend gave reasonable results with the other studies. Almost all of the experimental studies were performed in wind tunnels, however there are some differences between wind tunnels and real refrigerator geometries. For instance in refrigerators, airflow through the evaporator is not same in wind tunnels due to the refrigerator geometry and airflow rate follows a decreasing trend due to the frosting, however in wind tunnels constant airflow rate can be maintained through the evaporator. In addition to airside, refrigerant side in refrigerators cannot be adjusted easily as it can be in wind tunnels. Because of these facts, a real refrigerator is utilized in this study to obtain more reliable and realistic results. EXPERIMENTAL SETUP AND PROCEDURE Experimental Setup Descriptions Refrigerator cabinet In this study, an upright freezer charged with R134a is utilized in the experiments, however a few modifications shown in figure 1 have been made. For instance, the separation wall so-called mullion between the freezer and the fresh food cabinet is cut out, so that instead of the two cabinets, a relatively big one is obtained. Inner volume of this modified refrigerator is about 464 L. Evaporator An aluminum finned-tube evaporator is used in this refrigerator. The outside diameter of the refrigerant tube is 8 mm and the heat transfer surface area is 0.81 m. Despite modification of the refrigerator, the location of the evaporator in the refrigerator is not changed.