Simulation Methods Research On Transient Heat Transfer Through Building Envelope

Transient heat transfer through the building envelope is one of the principalcomponents of space cooling/heating loads and energy requirements. Especially forthe hot-summer and cold-winter zone in China, the conduction heat transfer throughbuilding envelope is highly erratic because of the bad weather conditions. It is still nota trivial exercise to predict the thermal performance of the fabric envelope.As there are two type of building envelope (opaque or transperant), the researchof dynamic heat transfer in this paper is also divided into two parts: the first part is forconventional wall constructions and the second part is for ventilated double-skinfacade (DSF) as they are now the types of envelopes mostly applied in commercialbuildings in China.In the first part of this paper, a method based on the equivalence of dynamicmodels for a linear system and the frequency characteristics of building transient heattransfer models are introduced for verification of the CTF coefficients and responsefactors over the whole frequency range.Various causes may lead to incorrect dynamic thermal behavior data used inbuilding simulation and dynamic space cooling/heating load calculation. Variousdynamic models of a linear system should be equivalent. The agreement between thefrequency characteristics of the models based on dynamic thermal behavior data andthe theoretical frequency characteristics of the construction is used to evaluate theircorrectness and reliability. The Bode diagrams for the dynamic models and theoreticalfrequency characteristics are visual aids to judge whether or not the dynamic thermalbehavior data is correct. Two examples have demonstrated the verification of dynamicthermal behavior data using the Bode diagrams and the percentage error. As thesteady-state conduction tests only checks the response at a single frequency, the workdescribed in this paper can tests the response over a range of frequencies. Furthermore,the method is both comprehensive and reasonably easy to implement.In the second part of this paper, models for simultaneous thermal, optical, andfluid flow processes are analyzed in turn to establish an improved zonal model tosimulate the complex dynamic thermal process occupied in the ventilated DSF in hotsummertime. As building energy consumption of buildings with DSF strictly depends on thesolar heat gain which differs with seasons and latitude location, the solar radiation,the sun-heights angle and the beam incident angle for the vertical skin in Changshaand Beijing are calculated respectively. These results show the importance of dynamicoptical model for the transient heat transfer process through the ventilated DSFThis paper also considered the spectral properties of the slab-type blinds whichshould change with different sun-height angle. A cell model of blinds is applied tocalculate the slab’s spectral properties for beam radiation and diffuse radiationrespectively. Then treat the blinds system as a transparent material whose extinctioncoefficient is given. Based on the radiation balance on the surface, the optical modelfor the multilayer transparent system is easily established to get its dynamic spectralproperties. The reflectivity, absorption and transmittance for direct and diffuse solarenergy of the whole DSF system are obtained respectively to calculate heat sources inglass panes and on opaque surfaces of shading devices. These heat sources aresubsequently implemented in the DSF simulation model.When simulating the thermal process in DSF system with or without slab-typeblind device in summertime, an improved zone model which is simple and explicit isprovided instead of those commonly used software which are very complex. Thisthermal model combines the control-volume method and zonal approach. Each layerof the DSF system is divided into many two-dimensional sub-zones. The mass andenergy conservation equations for each sub-zone are then given and calculated byfinite difference method. The Power Law Model (PLM) based on the relationshipbetween pressure difference and mass flow is applied to calculate the vertical air flowrate in the channel and the cross air flow rate through the slabs of the blinds. Thetemperatures and airflow results for each sub-zone are obtained at last.An existent summer experiment on the ventilated DSF system with slab-type blinddevice in the air channel is used to verify the introduced simulation model. The resultsshow that the improved zone model is reliable and simple which is suitable foranalyzing the dynamic heat transfer through the double-skin facade under the weatherconditions in the hot-summer and cold-winter zone in China.