تجزیه و تحلیل ناپایدار عملکرد حرارتی یک اتاق با سیستم گرمایشی سریال و موازی کف تابشی داکت با استفاده از جریان هوا داغ

In this study, the unsteady thermal performance of a test room heated by circulating hot airflow under the floor was analyzed with a developed mathematical model based on heat transfer equilibrium among the air flow, the floor and the indoor air. The time variations in the indoor air temperature for the serial duct floor heating system were investigated theoretically and experimentally. The time variations in the floor surface and the indoor air temperatures were predicted theoretically for the parallel duct floor heating system. Experiments on the time variations of the dimensionless numbers such as Nu for the airflow in duct and the indoor air, Gr for the indoor air and the heat ratios of convection and radiation to total heat for the serial duct floor heating system were performed. The theoretical and experimental results showed a good agreement.

Floor heating systems have been used since ancient times due to their advantages compared to other heating systems. However, the higher temperature of the floor surface may not pleasant for everybody. The floor heating system for large volumes and high spaces such as hangars, gymnasiums, churches, mosques, etc. seems a preferable alternative for human physiology as the temperature gradient on vertical direction in a room heating from floor is negative. The cost of this heating system is also very reasonable for large volumes and high spaces. In floor heating, a more comfortable room can be obtained due to the fact that the velocities of air flow resulted from heat transfer with natural convection are smaller than 0.1 m/s as the temperature distribution in indoor air is more homogenous than other heating systems. Floor heating systems are affected less in cold days when sudden temperature drops occurred because heat is accumulated in the floor. On-off valve control and control with two parameters for radiant floor heating systems with hot water experimentally investigated [1]. The temperatures of floor and indoor air selected as two parameters and controlled by a valve. They concluded that the control system with two parameters provided a better temperature control for indoor air. The study on the optimal control of a floor heating with hot water [2] showed that the room temperature during a day can approximately be kept constant by controlling inlet energy to boiler and mass flow rate of hot water. In another study, a building heated by using a simulator program the permanent heating was more stable and efficient than temporary heating [3]. The floor heating of a classroom by hot water obtained with a solar energy system [4] and the floor heating of a room, whose floor consists of electrically heated panels and whose window is integrated with a passive solar energy system were reported [5]. The floor heating of a room, which has a rock bed under its floor to accumulate the heat of circulating air heated with solar energy was also investigated [6]. In the literature, studies are mainly on the radiant floor heating systems using hot water but to the authors’ knowledge there is not enough study about radiant serial and parallel duct floor heating by circulating the hot airflow. The primary objective of this study is to obtain unsteady thermal performance of rooms having serial and parallel duct floor heating systems by applying a mathematical heat equilibrium model for unsteady heat transfer among the hot air flow, floor material and indoor air. The second objective is to define the availability of these heat sources in radiant serial and parallel duct floor heating systems if there are waste heat sources such as available hot airflow, hot exhaust or chimney gases.

This study defines the theoretical and experimental unsteady heat transfer among the hot air flow, the floor and indoor air. A mathematical model has been developed which is based on heat equilibrium equation widely used for solids in literature. A good agreement between the theoretical and experimental indoor air temperatures was obtained. Although hot air radiant floor heating system has low heating efficiency, it can be used as an alternative heating system to the conventional hot water radiant floor heating system if waste heat sources such as hot exhaust or chimney gases are available. The performance of radiant floor heating system with hot air can be satisfactory for well-insulated houses in mild climate conditions and much more insulated houses in more severe climates, because the radiant floor heating system provides higher quality in heating and more comfort than the direct air heating system, and heat distribution in a radiant floor heating system is generally even and consistent throughout the room: there are no drafts, no hot spots nor cold spots. It can be emphasized that the effective heat transfer area for radiant floor heating with hot airflow is greater than that of the radiant floor heating systems with hot water because of the use of ducts surrounding hot airflow in spade of pipe network under the floor. The floor materials such as marble and ceramic for the test cell of a single room can be preferred to mosaic due to the fact that they have the higher conductive heat transfer coefficients. In this case, the same floor surface and/or the indoor air temperatures may be reached in lower heating time.