Thermal Capacitance of Building MaterialsbyDan SchwarzSchool of EngineeringGrand Valley State UniversityEGR 468 – Heat TransferSection 02Instructor: Dr. M. SozenMarch 13, 20081OutlineI. Introduction/Purposea. Describe the importance of considering thermal properties when selectingbuilding materials.b. Purpose Statement: Compare the thermal capacitance of cement and celluloseinsulation.c. Describe the experimental system.II. Theorya. Describe the governing equation for the experimental system.III. Procedurea. Provide details about setting up the experimentb. Describe measurement schemec. Figure 1: Illustration of systemIV. Lab-equipmentV. Resultsa. Figure 1: Experimental temperature curves for concrete and cellulose samples.b. Interpret the data to compare concrete with cellulose.VI. Discussion/Conclusiona. Explain the results in the context of building energy efficiency.VII. Appendix Aa. Answer to Question 1b. Answer to Question 2c. Answer to Question 3d. Answer to Question 4e. Answer to Question 5f. Answer to Question 62Introduction/PurposeThe energy efficiency of a climate controlled building is governed by the thermalproperties of its building materials. Therefore, to conserve energy, an engineer must considerthermal properties when selecting materials for a climate controlled building. Since buildings are subjected to transient heat loading, the most important buildingmaterial property is thermal diffusivity. Thermal diffusivity determines the rate at which thermalenergy is dissipated through a material. Materials with low thermal diffusivity values tend toresist the dissipation of energy. In this experiment, the thermal diffusivity properties of concrete and cellulose werecompared. A block of each sample material was exposed to a constant heat flux at the topsurface. Temperature measurements were taken at several depths in each material to compare therate of energy dissipation through the material. The temperature measurements where graphedas a function of time and compared to determine the relative thermal diffusivity properties ofconcrete and cellulose.TheoryThe experimental system, shown in Figure 1, exhibits 3-Dimensional transient heattransfer without internal heat generation. The governing equation for such a system is given byEquation 1. TzTyTxT 1222222(1)On the left side of the equation there is a term for the rate of change in the temperaturegradient for each dimension of the object (height, width, and depth). Since the system has beensubjected to a transient heat load, the right side of the equation is balanced by the time rate oftemperature change multiplied by the thermal diffusivity property of the material. The right sideof Equation 1 clearly shows that an object with low thermal diffusivity will require more time todiffuse heat. ProcedureSamples of concrete and cellulose were made into equal sized blocks. Four blocks ofeach sample material were stacked with thermocouples between each layer. The thermocoupleswere connected to a data acquisition station to take temperature measurements as a function oftime. One halogen lamp was directed at the top surface of each stack to serve as a heat source.The complete experimental system is shown in Figure 1. The data acquisition was initialized and the lamps were turned on for approximately 2hours and 45 minutes. The resulting temperature measurements were graphed for comparison asshown in Figure 2.3Figure 1: Two materials were heated by halogen lamps. Each numeric label corresponds to a thermocouple.Lab Equipment- One National Instruments DAQ (CA-1000)- One National Instruments Voltage Meter (NI-4350)- One Computer with Labview Data Acquisition Software- Nine K-type Thermocouples- One Stack of Four Concrete Slabs (Each Slab ~1 in Thick)- One Stack of Four Cellulose Containers (Each Container ~1 in Thick)- Two Halogen LampsResultsThe temperature measurements taken during the experiment were graphed in Figure 2 asa function of time. Figure 2: Temperature curves were generated using thermocouples in the building materials. (See Figure 1)4Notice that the temperature difference between the first and second cellulose thermocouples waslarger (at every point in time) than the temperature difference between the first and secondconcrete thermocouples. This indicates that less energy was diffused through the top layer ofcellulose than the top layer of concrete. Therefore, cellulose has a relatively low thermaldiffusivity value when compared to concrete.Discussion/ConclusionThe lower thermal diffusivity exhibited by cellulose means that energy is diffusedthrough cellulose at a slower rate than energy is diffused through concrete. From thisobservation it follows that using cellulose to insulate buildings will conserve energy.In practical terms, cellulose insulation will reduce the rate of heat loss when the externaltemperature of a wall is lower than the desired internal temperature. Likewise, celluloseinsulation will reduce the rate of heat gain when the external temperature of the wall is higherthan the desired internal temperature.Although cellulose is a good insulator, it does not have very desirable mechanicalproperties. So, it should be used to insulate a wall but it should not be used to build a wall.Concrete, on the other hand, has very desirable mechanical properties and is often used to buildwalls that require high rigidity. The thermal properties of concrete walls can be improved byadding a layer of material with a lower thermal diffusivity value such as polystyrene. Appendix A: Answers to Lab QuestionsQuestion 1: How would you model the transient problem? (Which equation would you beginwith?)This system should be modeled as a 3-Dimensional transient heattransfer system with no internal heat generation. TzTyTxT 1222222Question 2: Do you expect 1-Dimensional behavior? Why or why not?For 1-Dimensional heat transfer to occur there must be an insulatedboundary around the heat source. This boundary would ideallyprevent heat transfer in any direction except through the samplematerial. However, the system shown in Figure 1 has no insulatedboundaries around the heat