Research On The Characteristics Of Non-steady State Heat Transfer In The Room With Concrete Ceiling Radiant Cooling Panels

Much attention has been devoted to the ceiling radiant cooling panels(CRCP) systems in recent years because of the advantages in energy conservation, health, thermal comfort and so on. Concrete ceiling radiant cooling panels(C-CRCP) with embedded pipes is one type of the terminal devices in CRCP systems. The operating principle of the terminal devices is to make the ceiling a kind of cold radiating surface with lowering temperature, and achieve the purpose of indoor cooling by the radiation heat transfer between the ceiling and other indoor surfaces such as the internal surfaces of building envelope and the surfaces of furniture and human bodies. The natural convection heat transfer between these surfaces and indoor air is enhanced due to the temperature decrease of the indoor surfaces. As a result, the temperature of the indoor air is reducing at the same time, which would further strengthen the cooling effect. The dominant heat transfer mode between the concrete ceiling radiant cooling panels and the indoor environment is the combined heat transfer of thermal radiation and natural convection, which is different from the forced convection heat transfer mode of traditional air-conditioning systems.The construction method of C-CRCP is binding the plastic or steel pipes on the rebar and pouring into a concrete slab, or fixing the pipes on the ceiling then covering the surface course. C-CRCP can be regarded as a building part with large thermal storage, that is to say, the slab serves as a part of the radiant cooling system to participate directly into the cooling process. Currently, the studies of C-CRCP are mostly focus on the heat transfer inside the concrete slab or the heat transfer of the whole cooling system. Few studies related to the non-steady state radiation heat transfer between the indoor surface and the C-CRCP.There are a lot of reasons causing non-steady state radiation heat transfer. For instance, the heat gain of building envelope keeps changing with the variations of exterior disturbances such as sunlight and outdoor ambient air temperature, thus the inner surface temperatures of the walls are always changing. The thermal environment may also change because of the start-stop of CRCP system and indoor heat sources or personnel entering and leaving. The heat transfer mechanism and the thermal response characteristics in the indoor thermal radiation environment of non-steady state are deficient in theory and experimental data support at present. Moreover, general methodology fails to solve the situation that the temperatures of some inner surfaces are lacking but the heat flows are known. This dissertation combines theoretical research, experimental study and numerical simulation, aiming at solving the problems mentioned above.A calculation methodology called the Gebhart method is improved for modeling the non-steady state radiation heat transfer in the room equipped with CRCP. The non-steady state radiation heat transfer is solved by transforming the given parameter variations into discrete column vectors of time steps and calculated with matrix equations. Multiple calculation of equations can be avoided, which provides great convenience for computer programming. The solution of the radiation heat transfer in unclosed space with windows is also provided.The complex heat transfer problem in the room equipped with C-CRCP is simplified to an non-steady coupling heat transfer problem(involved in thermal conduction, thermal radiation and natural convection heat transfer) in a three-dimensional enclosed cavity. The numerical method of this problem and the theoretical derivation process are also provided. The additional source term method is adopted to solve the boundary conditions of the energy conservation equations and the radiation heat transfer on the solid surface. The corresponding additional source term and the mathematical computational formulas of unknown surface temperature are derived.A thermal experimental platform was established to simulate the dynamic thermal response process in enclosed space with the C-CRCP system operating in summer. Several non-steady state conditions was designed to conduct the experiments, which were the start and the stop of the C-CRCP system, the sudden increasing and decreasing of the supply water temperature, the sudden increasing and decreasing of the indoor load, and the cyclical fluctuation of the supply water temperature. The non-steady state radiation heat transfer model in enclosed space and the non-steady coupling heat transfer numerical model are verified in this dissertation.Experimental results are analyzed in detail. The temperature variation curves during the start and the stop process of the C-CRCP system are fitting with the first-order exponential function. The C-CRCP system should be started at least 8 hours before working time to guarantee the indoor thermal environment relatively steady. The temperature increasing rates per hour during the first 8 hours after the C-CRCP system stop are listed in the dissertation, to estimate the temperature variations rapidly after the C-CRCP system stop. Furthermore, the amplitudes and the rates of temperature variations in the test space is very small when the supply water temperature or the indoor load suddenly change. The large fluctuating margin of the supply water temperature may slightly effect the temperature of the C-CRCP surface while hardly effect the temperatures of other surfaces and the air in the test space.The temperature distribution, the airflow field and the radiation heat flux field in the stabilized numerical model are also analyzed. It is observed that the temperature distribution in the test space is approximately uniform and the natural convection is not significant. The radiation heat flux on the micro-unit surface is affected by the temperatures of all the other micro-unit surfaces and the relative position with other micro-unit surfaces. The natural convection in the test space at time zero is very weak, but becoming stronger as the change of the indoor thermal environment. The eddy around the heat source is formed gradually.The innovative works in this dissertation mainly are:(1) The model of the non-steady radiation heat transfer in the room with concrete ceiling radiant cooling panels and a heat source in it is established.(2) The numerical model of the non-steady coupling heat transfer involved in thermal conduction, thermal radiation and natural convection heat transfer in the room with concrete ceiling radiant cooling panels is established and solved.(3) The experimental results of the dynamic thermal response process in enclosed space with the C-CRCP system operating in summer are fitting with function, and the empirical equations are provided.