Thermal
Conductivity
Improveoriginal merits design in the of the
of Liquids
apparatus previously reported have resulted in an apparatus of great flexibility and reliability in determining by a single run the true, average, and temperature coefficients of thermal conductivity of liquids over a temperature range. All factors which had a tendency to cause undesirable effects in the former apparatus have either been eliminated or ’ very greatly reduced. The evidence is quite conclusive that the coefficient of therCOMPLETE and No. 30 copper wire carefully mal conductivity of pure glycerol bibliography and c a li b r a t e d against a precision is independent of temperacriticisms of the thermometer calibrated by the United ture over the range methods of determining the thermal States Bureau of Standards. The rate of of this investiconductivity of liquids employed by preheat flow per unit area was determined by gat3on. vious investigators were given in a previous the calorimeters, B,shown in Figure 1.
@A
paper (1) by the author. The apparatus used in the present work was essentially the same as the one previously reported in detail. Several important changes in details and construction were made, however, which materially improved the operation of’ the apparatus. These changes will be discussed here. Figure 1 shows that the general method, as previously explained, consists in employing the “tall column or thick disk” method with suitable thermal guarding. The thickness of the liquid layer under test may be varied in different tests from a few millimeters (0.125 inch) to approximately 100 mm. (4 inches), The thickness of the layer of liquid employed in this and previous work was fixed a t approximately 50 mm. (2 inches). The liquid was electrically heated a t the top and water-cooled a t the bottom, and the temperature gradient through the liquid was obtained by the use of thermocouples. The thermocouples were made with No. 26 constantan wire
r
TIGURE1. DIAGRAMOF A P PARAT us
Changes in Construction BAKELITE WALL. Bakelite and linen tubing, 38.1 cm. (15 inches) in diameter, 0.95 cm. (0.375 inch) thick, and 20.0 cm. (approximately 8 inches) high was fastened and sealed to the annular guard-ring calorimeter (Figure 1). This shell retained the liquid between the two chromium-plated surfaces and held the small brass tubes containing the temperature gradient thermocouples. By employing a Bakelite wall in place of the brass previously described, the heat flow from the heater through the wall to the guard-ring calorimeter was materially reduced. Before, it was necessary to place a secondary guard-ring heater around the brass wall in order that the heater surfaces and the retaining wall might have the same temperature a t the same height. This caused a large flow of heat down the wall, with a resulting increase in temDerature nea; the edges of th’e guardring calorimeter. This was fully explained in the previous paper. T h i s u n e v e n temperature d i s t r i b u t i o n , increasing rapidly near the edge of the guard-ring calorimeter, w a s t o t a l l y eliminated, and as far as could be experimentally d e t e r mined, a uniform temperature existed a t a l l p o i n t s across t h e c a l o r i m e t e r faces. A f u r t h e r check on this point was obtained by comparing the rate of heat flow determined by the test and guard-ring calorimeters. The heat flow per unit area of t h e t e s t c a l o r i m e t e r c h e c k e d identically w i t h that of the guard-ring calor i m e t e r . T h i s showed t h a t t h e e f f e c t of c o n -
’7
d u c t i o n down t h e w a l l was CHROMIUM-PLATED TESTSURFACES. In the previously renegligible. DIAMETEROF TESTCALORIMEported work, the test surfaces were copper, polished before each TER. Whereas the diameter of the guard-ring calorimeter was run. These surfaces t a r n i s h e d unaltered, the diameter of the during runs. In order to elimiDate any surface effect which central test calorimeter, B (Figmight result from such a surface, ures 1 and 2) was increased from all surfaces exposed to the test 7.6 cm. (3 inches) to 12.7 cm. (5 inches). There still remained OSCAR KEXNETH BATES liquid were chromium-plated in The St. Lawrence University, Canton. N. Y . the new apparatus. The resultample thermal guarding by the h e surfaces remained unchanged guard-ring c a l o r i m e t e r . This d&ng all the test runs. greatly increased the test area; IMPROVED SPIUNOARRANQEthe result was that with all other mm. Figure 1 (Dand E ) and Figure 4 show the improved previously employed conditions remainifg unchanged, the spring arrangement for the temperature gradient thermamount of heat measured per unit of tlme was Increased couples. Barometer glass tubing, fitting snugly in the brass approximately threefold. The water rates through the test tubing fixed in the Bakelite wall, reached from the liquid to flow calorimeter under test conditions were approximately the outside top of the apparatus. This glass tubing was sealed 170 cc. per minute, and the temperature increase Was in the brass tubing, and the fine thermocouple wires filled the from 0.5" to l.Oo C. With multiple differential thermocouples central capillary. As a result the thermocouple wires were (composed of fix thermocouples) and suitable electrical kept taut under all conditions and were extremely flexible in measuring instruments, temperature differences of 0.001"C. adjustment. The liquid under test was also confined wholly were readdy detected. A complete description of method to the central portion. and procedure was reported in the previous paper by the author (1). Procedure As mentioned ahove, a uniform temperature existed a t all points across the calorimeter faces, from the center to the The experimental procedure was explained in detail in wall, in the improved apparatus. the previous paper by the author (1) and will not be repeated BEVELEDEDGES.In order to minimize any conductive here. The values of the several coefficients were calculated effect which might possibly exist between the test and guardfrom the temperature gradient curves drawn for each series ring calorimeters, beveled edges were made on the calorimeters ( B on Figure 1, and Figure 2). This meant that the two calorimeters broke metallic contact from each other a t the test surface by approximately 0.4 mm. (0.016 inch), but receded rapidly from the surface, leaving shortly some 6.4 mm. (0.25 inch) between them. This space was 6lled with litharge and glycerol in order to seal the opening between the c a b rimeters and to fix them in place. Since great care was exercked to have the two calorimeters the same temperature, the beveled edges may have been only a desirable precaution. Figures 2 and 3 show the new and old test calorimeters. The concentric countefiow channels in the calorimeters and the beveled edge on the new test calorimeter are clearly shown. CALORIMETER INLETS. In order to insure that the water inlets to the two calorimeters were the same temperature, FIGURE 3. OLD TESTCAMRIMMETER the inlets were placed as near as possible to each other (C, Figure 1). The water flow through the test calorimeter was very small in comparison to that of the guard-ring calorimeter. of runs. Figure 5 shows a representative temperature gradient Consequently the guard-ring calorimeter inlet was run directly curve for distilled water, and one for a 95.3 per cent glycerolunder t,he test calorimeter (Figure 1), and the water for the 4.7 per cent water solution. A series of tests was made for test calorimeter wae bled from it in the most direct path liquid mixtures of the following compositions (in per cent by possible. weight) :
Binary Mixtures of
water and Glycerol
Y
Distilled water 9.7 glycerol-90.3 water c. 20.1 glycerol-79.9 water d. 39.5 glycerol40.5 water e. 59.2 glyrero140.8 water f. 76.6 glycerol-23.4 water g. 95.3 glycerol- 4.7 water a.
6.
From the calorimetric determinations and the temperature gradient CUNCS, the true coefficients of thermal conductivity were determined for binary liquid mixtures of compositions (I to g over the range of temperature covered by the particular runs. It was noted in every case that, within the accuracy of the tests, the true coefficient of thermal Conductivity was a linear function of the temperature-that is, a straight-line relation. From these curves, composition us. true coefficient of thermal conductivity curves were drawn for every IOo interval from 20" to 80" C. 495
INDUSTRIAL A 5 D ENGINEERINC CHEMISTRY
496 ....
........
TABLE I. \later
Glyeeiol
.......... 10" C .
20" C.
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n.o01:
