Heat Transfer Model And Experimental Study On Concrete Ceiling Radiant Cooling Panel

The radiant cooling system is a cooling way for the indoor environment and personnel based on the cold radiation surfaces through lowering one or multiple surfaces in a room in the modes of radiative heat transfer and convective heat transfer. The radiative heat transfer occurs among the surfaces of the cold radiation, human bodies, heat resources, indoor equipment, and inner space enclosing structures, while the convective heat transfer happens between the cold radiation surfaces and the indoor air. The radiative heat transfer plays the main role of the heat transfer of the radiant cooling system, and the convective heat transfer plays the supplementary role.The concrete ceiling radiant cooling system is formed with the plastic water pipes embedded in the cast-in-place concrete floors of the construction. It takes the construction floor as the terminal of cooling directly. The concrete ceiling radiant cooling system has scores of advantages, such as energy saving, good thermal comfort, transferring peak power consumption, save indoor space and so on.The research of the concrete ceiling radiant cooling system mainly focuses on two aspects: heat transfer of the cold radiant panel and indoor thermal environment. This article concentrates on the former the study of the heat transfer of cold radiant panel that is the premise for the indoor thermal environment to meet the design and comfort requirements in an energy-efficient way.The predecessors mostly adopt the simplified analysis method in the research of heat transfer of cold radiant ceilings, and adopt the equivalent or simplified method for the approximate solution of the inner heat transfer of the cold radiation ceiling. In this research, in order to study the heat transfer of the concrete cold radiation ceiling more accurately, we adopt the finite difference method to build the model, program to solve the internal temperature field of the concrete cold radiant panel, and then acquire the actual measured internal temperature field through experiments to verify the correctness of the model.The concrete itself is a kind of porous medium material, whose process of heat transfer process and moisture transmission are correlative to each other. In this paper, the heat and moisture transfer of the concrete is analyzed based on the porous medium theory. However, the solution process of the coupled moisture and heat transfer is quite complicated, together with lots of uncertainties, plus that the predecessors’ researches on the features of heat and moisture transfer of the concrete discovered that both the rate and strength of moisture transmission inside the concrete are far more less than those of the heat transfer process at the relatively small time scale. Consequently, at the relatively small time scale, the influence of the concrete moisture transmission on the heat transfer can be ignored, and just the heat conduction process of the inner concrete is studiedThe heat transfer process between the surface of the concrete cold radiation ceiling and the indoor environment is analyzed and the computational formula of the heat transfer between the surface of the concrete cold radiation ceiling and the indoor environment under the condition of ceiling cooling is derived. Based on the features of heat transfer of the concrete cold radiation ceiling, we make reasonable assumptions on its heat transfer process, establish the finite difference model that addresses the 3-D steady heat transfer and 2-D steady heat transfer of the concrete cold radiation ceiling, deduce and establish the computational formula that solves the internal nodes and boundary nodes of the region, determine the calculation method of Gauss- Seidel iterative method and the convergence criterion, and analyze the factors of model error, space truncation error and time truncation error that affect the calculation accuracy of the numerical model.The raised finite difference model is adopted to calculate the heat transfer of steady state and unsteady state of the concrete cold radiation ceiling. And the experiment table of the concrete ceiling radiant cooling system is established to verify the finite difference model. After comparing the calculated results with the actually measured results, we find that the calculated results of heat transfer of the steady state and unsteady state match well with the actually measured results, thus authenticating the high accuracy of finite difference model.The verified finite difference model is adopted to analyze the influences of the different operating parameters(water supply temperature, water flow rate) of the concrete cold radiation ceiling on its temperature distribution and heat flux density distribution; and the effect of the different structures of the cold radiation panels(distance and depths of embedded pipes, thermal insulation layer on the floor) on its temperature distribution and heat flux density distribution.At the relatively long time scale, after the concrete moisture changes, the influence of the moisture transmission on the heat transfer is computed and analyzed. And the effect of the moisture transmission on the heat conductivity coefficient and the internal heat transfer of the concrete cold radiant panel is also studied.The experiment table of the concrete radiant cooling ceiling is set up, actual operating features of the concrete ceiling radiant cooling system are analyzed, and the actual operation features at its steady state operation, varying-duty operation, start-up stage and shutdown stage are studiedThe main innovative work of this paper mainly includes the following points:(1) Under the premise that the concrete is a porous medium material, based on its features of heat transfer and moisture transmission, and combining the complexity of the coupled heat and moisture transfer of porous medium, different simplified methods are determined.For the heat transfer of the concrete cold radiation ceiling at the relatively small time scale, the influence of the moisture transmission on the heat transfer is ignored, and only the heat conduction of the concrete cold radiant panel is studied, namely, the temperature field and heat flow density distribution of the concrete cold radiation ceiling is investigated.At the relatively long time scale, the concrete moisture change cannot be ignored. The impact of the moisture transmission on the heat transfer needs to be analyzed. This research raises the method of indirect simplified research to analyze the effect of the moisture transmission on the heat conductivity coefficient of concrete itself, and then study the influence of the change of the heat conductivity coefficient on the internal heat transfer of the concrete cold radiant panel.(2) This paper put forward finite difference numerical model used to solve steady and unsteady heat transfer of the concrete ceiling cold radiation, establishing the boundary radiation heat transfer and convective heat transfer calculation method based on the ceiling radiant cooling conditions and researching concrete ceiling radiant cooling system heat transfer conditions in different operation parameters and radiant panel structure parameters.(3) In this research, the experiment table of radiant cooling of concrete ceiling is established, and through simulating the actual operating conditions of cooling including the operating conditions of the steady state and the transient state, collecting and analyzing the related experimental data, the actual operating features of the concrete ceiling radiant cooling system, which lays foundation for the adjustment and control of the actual operation of the concrete.