Effective Thermal Conductivity of a Glass Fiber Blanket Standard Reference Material
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5 A 11 ID EFFECTIVE THERMAL CONDUCTIVITY OF A GLASS FIBERBLANKET STANDARD REFERENCE MATERIAL David R. Smith Jerome G. Hust Lambert J. Van Poolen Thermophysical Properties Division National Engineering Laboratory National Bureau of Standards Boulder, Colorado 80303 HQC February 1981 100 .1156 SI-JAW 1981 national buhjcad Or 8TANBAJU)* UllAIT 81-1640 NBSIR 1 3 1»' r <r * MAY EFFECTIVE THERMAL CONDUCTIVITY OF A GLASS FIBERBLANKET STANDARD REFERENCE MATERIAL David R. Smith Jerome G. Hust Lambert J. Van Poolen Thermophysical Properties Division National Engineering Laboratory National Bureau of Standards Boulder, Colorado 80303 February 1981 U.S. DEPARTMENT OF COMMERCE, Malcolm Baldridge, Secretary NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director Ttt > * UJ • 5K S .£ Y«f CONTENTS Page Introduction 1 Apparatus and Specimen Characterization 1 Results .............................. 3 References (Annotated Bibliography) ................ 12 iii . Effective Thermal Conductivity of a Glass Fiberblanket Standard Reference Material David R. Smith, Jerome G. Hust, and Lambert J. Van Poolen This paper* describes the results of thermal conductivity measurements at temperatures from 84 K to 360 K on a glass fiber- blanket insulation which is intended to be a Standard Reference Material (SRM) . The measurements were performed in an atmosphere of dry nitrogen at a pressure near 84 kPa (630 mm Hg) . The results are analyzed and compared to literature data for similar material. An annotated bibliography of literature sources is included Key words: Glass fiberblanket; insulation; low temperature; standard reference material; thermal conductivity. Introduction As part of the National Bureau of Standards - Department of Energy Program to establish insulation Standard Reference Materials (SRM’s), work is continu- ing on various candidate materials. The results on SRM 1450 (glass fiberboard) were reported earlier [l]. Results of measurements are reported here for a \ specimen of a glass fiberblanket insulation for specimen temperatures covering the range from 84 K to 360 K, in an atmosphere of dry nitrogen gas at 84 kPa (630 mm Hg) , local atmospheric pressure. Apparatus and Specimen Characterization A commercial guarded-hot-plate thermal conductivity instrument was exten- sively modified [2] to permit efficient use of cryogenic or refrigerated cool- ants, to obtain greater stability of controlled temperatures, and to achieve *This work was done for the Department of Energy, Oak Ridge Operation, Oak Ridge, TN 37830, under Interagency Agreement DoE No. DE-AI05-780R05965 1 . lower imprecision and uncertainty of measurements. This apparatus was designed to measure heat transfer through a pair of specimens surrounding the horizontal hot plate in the typical double-sided guarded hot plate configuration (ASTM STM C-177) The pair of specimens studied were disks of nominal 10 cm (4 in) diameter and 2.6 cm (1.0 in) thickness. They were cut from larger slabs measuring approximately 58 cm square as received from the National Bureau of Standards, Office of Standard Reference Materials stock. The pair was chosen on the basis of near equality of area densities (0.0381 g/cm^ and 0.0383 g/cm^). The thickness, Ax, of each specimen was determined during the conductivity measurement by three stainless steel spacer tubes positioned at three equidistant points around the perimeter of the guard material. The guard material was similar to that of the specimens with an inside diameter of 10 cm (4 in) and an outside diameter of 20 cm (8 in). The measured thickness was 2.59 cm and the resulting volume density was 14.75 kg/m^. This thickness was corrected for thermal expansion of the spacers (less than 0.3 percent). All hot plate/cold plate surfaces facing the specimens were black anodized aluminum. The cold surfaces were electrically heated, temperature-controlled plates, cooled by resistive thermal contacts with fluid-cooled copper plates; thus the cold surfaces of the specimens could be brought to a temperature near that of the coolant, or raised in temperature, as desired. The main heater (metered section) was a disk 10.16 cm in diameter separated by a 1.6 mm gap from the annular guard of 5.1 cm width. A concentric, cylindrical outer guard surrounded the stack of auxiliary heaters, specimens, and main heater. It established a temperature gradient approximating that of the stack about 2 cm away from the stack edge surfaces. 2 Temperature controllers were used to regulate conditions in the inner and outer guards and to establish steady temperatures in the auxiliary heaters at the cold sides of the specimens. The offset signal for the inner-guard con- troller was developed by a 20-pair thermopile of type E thermocouples, with alternate junctions on opposite sides of the gap separating the main and inner- guard heater plates. Type K thermocouples (32 gauge) for determining temperatures of the speci- men faces, positioned at the center and near the gap surrounding the main heat- er, were varnished to the surfaces of the hot and auxiliary plates over a length of 15 cm. These thermocouples were also thermally tempered by means of 15 cm of wire wrapped and varnished to copper lugs attached to the periphery of the hot and auxiliary plates. Type K thermocouple wire was calibrated using a calibrated platinum resis- tance thermometer and baths of liquid helium, liquid nitrogen, dry-ice-and- alcohol, ice-water, room-temperature water, and boiling water. The departures from the standard tabular values* of emf vs. temperature were represented by a 3-term polynomial and a corrected calibration curve was formulated for the particular spools of thermocouple wire used. The dc power, Q, for the main heater was obtained from a constant current power supply of better than 0.01 percent stability and accuracy. The power in the metered heater was found from the product of voltage drop, V, across the heater and the current, I, through the heater. Results Measurements of heat flow and of temperatures of the hot and cold surfaces were performed at mean temperatures from 95 K to 350 K with temperature thermocouple Reference Tables based on IPTS-68, NBS Monograph 125, March, 1974. 3 . , differences of about 25 K (gradient approximately 1 deg/mm). The metered area was taken to be that contained within the 10.32 cm diameter circle including the main heater and one-half of the surrounding gap. The area through which the metered heat energy flows through the two matched specimens is 2^. This area was corrected for thermal expansion (less than 0.7 percent). The mean thermal conductivity was computed according t0 ^ = = , ¥, ^ where T and T^ are the average hot and cold side temperatures ? ( t^tJ* respectively. The resulting data set of values k, Q, T]_, and T 2 were then functionally represented using a thermal conductivity integral (TCI) fitting technique* to yield the thermal conductivity as a continuous function of temperature over the range from the lowest value of T^ (84 K) to the highest value of T K) . This resulted in the values of the parameters Aj 2 (360 , 4 . in the equation k = £ A.T . This technique is valid only to the extent i=l 1 that the mechanisms of heat transfer are conductive in nature (as opposed to direct radiation or convection). Deviations from this condition can be detect- ed by searching for systematic trends in the deviations of the fitted data as a function of boundary conditions. This search will be conducted on more exten- sive data to be published later. The values of k vs. temperature obtained by means of this technique for the specimens are given in Table 1 and plotted in Fig. 1 (solid line). The values of ¥ are also plotted in Fig. 1 (circles) at the mean temperature, T. *Hust, J. G. and Lankford, A., Comments on the Measurement of Thermal Conduc- tivity and Presentation of the Thermal Conductivity Integral Method, to be published 4 The fact that the mean values of thermal conductivity, k, are consistent with the solid line for k at T is indicative of the absence of appreciable non- conductive heat transfer mechanisms, i.e., direct radiation and convection. Deviations between k and k at T can also be caused by non-linear behavior of k vs. T for just conductive heat transfer, but, in this case k vs. T is nearly linear. Table 2 gives the coefficients, A^, and the range over which the fitted curve is valid. The TCI deviations, (TCI0 b s - TCI ca ^ c ) x lOO/TCIo^g, between the data (obs) and the fitted (calc) curve are given in Fig. 2 as a function of mean temperature. The horizontal bars indicate the temperature range for each measurement and the range of the TCI. A three term polynomial fit showed large systematic deviations from the data but the four term polynomial exhibited mainly random deviations. The remaining small systematic deviations (approximately +1/2 percent) were essentially unchanged from a four terra to a seven term polynomial. Further analysis is planned on other data sets to ascertain if these systematic deviations reflect real behavior or experimental error. At about 320 K two triplets of points are shown in Fig. 2 which differ appreciably from the mean (+ 3 percent). These data are the result of running the apparatus in the single-sided mode, i.e., each triplet is a measure of only one or the other of the two specimens. Further analysis is required to determine if these deviations are caused by specimen variations or by differences in the apparatus configuration. Other investigators [3-14] have reported similar trends in the dependence of k on temperature for glass fiber insulations over a wide range of densities including our value. The characteristic differences of these insulations, 5 such as binder or lubricant content, fiber size or atmospheric pressure actually present at each laboratory, are not known in great enough detail to permit definitive statements about the agreement or disagreement between results.