I
J. P. STONE, C. T. EWING, C. H. BLACHLY, B. E. WALKER, and R.
U. S.
Naval Research Laboratory, Washington,
R. MILLER
D. C.
Heat Transfer Studies on a Forced Convection loop with Biphenyl and Biphenyl Polymers
A of liquid organic materials have been used as heat transfer media at NUMBER
temperatures to about GOO0 F. for a number of years. More recently certain stable aromatic and alkyl-aromatic compounds, such as biphenyl, terphenyl mixtures, and monoisopropylbiphenyl, have been considered for possible primary coolants a t temperatures to about 800’ F. These materials have poorer
heat transfer properties and stability than water; however, their relative inertness to container materials and high boiling temperatures make them attractive for many applications. Equipment The closed loop is a,welded system of low-carbon steel with a maximum working pressure of 500 p.s.i.g. at bulk
The pumped loop was designed to study heat transfer characteristics of organic fluids and the effect of pyrolytic decomposition on heat transfer surfaces. Many studies were based on direct determination of film and/or over-all heat transfer coefficients. Reliable fllm coefficients are presented for biphenyl and two polymer-biphenyl mixtures (25 and 40% polymer) and are effectively correlated with an equation of the Dittus-Boelter type. Physical properties for a number of phenyl compounds and mixtures were determined. Density and viscosity measurements were made for biphenyl, metaterphenyl, paraterphenyl, monoisopropylbiphenyl, and two polymerbiphenyl mixtures, and equations developed for extrapolation of the property results to higher temperatures. A main objective was to study the influence of pyrolytic product formation on heat transfer surfaces at high fluid velocity and high heat flux. Conclusions concerning the lack of fouling were made after short-term experiments with biphenyl and the low-polymer mixture. Inorganic fouling was found with a lead-contaminated biphenyl-polymer mixture, Although experiments with lead are relatively unimportant to the organic program-commercial biphenyl does not contain leadgeneral conclusions regarding fouling have been drawn. Solubility effects during loop operation with p-quaterphenyl and possibly higher ppolyphenyls are discussed and solubilities reported for a number of phenyl compounds in biphenyl and other solvents,
fluid temperatures of GOO0 F. and was designed to deliver coolant at cbntrolled flow and temperature through the test section. This section was a Type 347 stainless steel tube, heated by passing an electric current through it. In the fabrication of the system and its units, welds were examined by x-ray, the components were pickled before and after welding where possible, and the system was hydrostatically tested to 2000 p.s.i.g. at room temperature. All the units and final assembly were helium leak-tested. Flow through the test section is controlled by the throttling action of a motor valve located on the discharge side of a bypassed, turbine-type centrifugal pump. Positioning of this valve is controlled by a wide-band proportioning instrument with automatic reset action and is based on the volume rate of flow measured by a variable-area meter with a float that is insensitive to viscosity changes over the range encountered. Density corrections to flowmeter readings varied from 0 to 6% for biphenyl and the polymer mixtures. Bulk temperature of the coolant in the test section is controlled by an annular-type, single-pass heat exchanger heated electrically with external band heaters. The heaters are in two banks, one manually controlled and the other controlled by a time-proportioning, “on-off” instrument which gives a constant desired temperature a t the test section entrance. Normally, heat lost in the piping of the system and at the cooling boxes of the pump packing glands is sufficient to remove power input at the test section; however, an exchanger in which a controlled amount of water is flashed to steam at atmosVOL. 50, NO. 6
JUNE 1958
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INDUSTRIAL AND ENGINEERING CHEMISTRY
BIPHENYL HEAT TRANSFER pheric pressure may be used. A cover pressure of argon is applied over the surge and make-up tanks as necessary to prevent boiling at the heat transfer surface. Constant voltage, 60-cycle power is supplied to the preheater and the test section by 10- and 20-kv.-amp. voltage-regulating transformers, respectively. All piping and units are lagged and traced as required with armored flexible heating cables. Test Section
The teit section assembly is composed of two multitubular mixing chambers at entrance and exit, two stainless steel flanges, one of which is electrically insulated, and the test section tube. The assembly is housed in Sil-0-Cel insulation, and installed in the loop in such a manner that it is free to expand and contract thermally without tube deformation. Bulk temperatures of the fluid are measured by thermocouples located to a depth of 9 inches in the mixing pots. The flanges permit removal and replacement of test sections. Silver bus bars (18 inches apart) and stainless steel voltage taps (12 inches apart) are silver-soldered uniformly with respect to the center section of the tube. Current is passed through the 18-inch length and power is measured over the center 12 inches by a Weston Model 310,' Form 2, wattmeter in conjunction with a Weston Model 321, Type 1, current transformer. A continuous range of 60-cycle power from 5 to 18 kw. is delivered to the tube by matched Sola constant-voltage transformers and a low-voltage transformer. Nine thermocouples formed from 11-mil wire are spot-welded, as shown, along the length of the tube, each couple being located 90' around the circumference of the tube from the adjacent couple. Five couples are located approximately 2 inches apart on the center 12-inch section of the tube, where heat transfer coefficients are measured. The nine tube couples are intercalibrated in place. against the two previously stabilized and calibrated mixing pot couples. These 11 platinum-platinum-10% rhodium couples are connected to pure copper, shielded lead wire at the cold junction and through a bank of lowresistance, knife-type switches located in an isothermal nest of aluminum boxes to a Rubicon Type B potentiometer.
required. The physical measurement techniques, which were imposed by the required precision, were superior to those normally encountered in an engineering application. The test section design was carefully analyzed in an effort to reduce error in the coefficient resulting from entrance effects, and losses, control variables, and radial heat losses. The coefficient measurements were derived from a 12-inch central portion of the 18-inch heated section with a calming length of like diameter ( L I D = 27) preceding the measurement section. Although entrance effects are not too well understood, calculations with entrance region equations, such as those recommended by McAdams (7), would indicate negligible influence on the coefficient at the maximum velocity of 50 feet per second. For the type 347 stainless steel test section, conduction of heat axially and change in heat generation with electrical resistance along the tube length were considered negligible. Radial heat losses between the bulk temperature measuring points and the 12-inch measurement section were very low, because of the effeGtive Sil-0-Cel insulation. For example, in the central measurement section radial heat losses at minimum heat flux and maximum wall temperature represented less than 0.770 of the power input to the section. At maximum heat flux and maximum wall temperature this figure drops to 0.2%. Both ends of the test section assembly were positioned in bearing surfaces to prevent thermal buckling or deformation of the thin test section tubing. One end of the assembly was free to move in its bearing surface and was attached by pulleys to a heavy weight, which was just large enough to maintain axial tension on the test section tubing, permitting it to expand and contract freely with temperature. All required side attachments to the test section, such as the silver bus bars, were supported under spring tension. The thermocouple measurement system was patterned after those designed for precision thermal conductivity studies at this laboratory ( 4 , 5 ) . The platinumplatinum-lO% rhodium thermocouples
which were to be spot-welded on the tube wall and those to be installed in the mixing pots were stabilized before use at 1150' C. The mixing pot couples were also intercalibrated to +0.5 pv. ( d ~ 0 . 0 5C.) ~ at 100' C. intervals over the measurement range, using primary standard couples from the National Bureau of Standards to provide the absolute-temperature reference. Because of the required spot-welding of the wall couples, these couples were Calibrated in place against the mixing-pot reference couples, by operating the loop at high-fluid velocity and at various bulk-temperature conditions with no application of heat to the section. AS the mixing pot and test section assembly were well insulated, this procedure provided an effective calibration. To reduce thermocouple pickup. complete shielding of the thermocouple lead system was required. A low-resistance, isothermal switch box of double aluminum construction permitted direct reading of each couple on the Rubicon Type B potentiometer. Film coefficients were measured for biphenyl and for two biphenyl-polymer mixtures. The biphenyl results (Table I) cover a wide range of test conditions. Velocity in the test section was varied from 5 to 50 feet per second, bulk temperature from 400' to 615' F., heat flux from 100,000 to 340,000 B.t.u./hr. sq. ft., and heat transfer surface temperature from 440' to 850' F. Coefficients for the polymer mixtures (Table 11) were determined during fouling studies and are more limited in range. For the coefficient determinations three test sections were employed. The room temperature dimensions of these tubes were: Test Section
i 2 3
I.D., Ft. 0.02623 0.02587 0.02575
O.D.. Ft. 0.03167 0.03164 0.03162
The inside dimension of each tube was obtained with a mercury calibration and, to ensure uniformity, a linear check with a tube gage. The absolute roughness or mean projection for test
Film Coefficients of Heat Transfer A number of studies made with the loop were based on direct determination of film and/or over-all heat transfer coefficients. T o obtain reliable fouling results in a reasonable time, high reliability in the coefficient measurement was
Test section assembly includes two mixing chambers and stainless steel heat transfer section which is heated by an electric current (one half of assembly shown) VOL. 50, NO. 6
JUNE 1958
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'
Table I.
Expt. No.
Av. Temp., F. Bulk Surface O
Heat Flux B.t.u./Hr. Sq. Ft.
Biphenyl Heat Transfer Results Heat Transfer Bulk Coeff. Vel., B.t.u./Hr. Ft./Sec. Sq. Ft. F x 10-4
70, Dev. DlttusBoelter (Calcd. -0bsd.)
Test Section 1 1 2 3
396 42 1 417
50 1 465 45 1
100,800 102,800 102,800
9.4 28.4 36.9
946 2333 2940
5.62 18.26 23.49
5.77 5.34 5.43
327.5 808.1 1019
-0.4 f3.7 +2.7
4 5 6
416 599 585
445 685 622
103,100 100,600 103,200
46.2 9.9 29.1
3640 1151 2792
29.27 10.35 29.60
5.43 3.34 3.46
1261 402.8 976.3
-0.2 -2.6 -0.7
7 8 9
594 589 585
665 643 650
202,800 202,800 101,700
29.5 39.2 14.6
2815 3690 1552
30.62 40.16 14.85
3.39 3.42 3.46
985.0 1291 542.9
-3.7
10 11 12 13
599 615 608 609
732 74 7 775 828
202,700 202,400 255,100 342,300
14.7 14.7 14.7 14.7
1514 1511 1503 1544
15.49 16.04 15.77 15.79
3.34 3.23 3.27 3.27
530.5 529.8 527.0 541.6
t3.8 "4.9 +4.8 +2.0
58 59 60
607 606 601
734 78 1 798
203,000 129,100 145,200
15.5 5.4 5.7
1581 730 728
16.38 5.74 5.93
3.28 3.29 3.35
546.6 252.5 251.9
+4.4 -6.2 -2.3
61 62 63
605 606 603
835 83 1 807
169,300 160,200 142,700
5.6 5.6 5.6
724 70 1 702
5.85 5.87 5.92
3.29 3.29 3.31
250.6 242.6 242.9
-4.0 -0.4 +0.5
71 72 73
597 602 595
659 665 789
101,600 101,600 147,700
15.9 15.9 5.7
1631 1589 755
16.26 16.49 5.82
3.37 3.33 3.38
556.8 542.7 261.2
$3.6 +6.7 -6.7
74 75 76
600 399 396
824 571 566
169,900 110,700 107,700
5.6 5.4 5.4
750 64 1 627
5.77 3.17 3.15
3.34 5.71 5.76
247.9 218.0 212.2
-3.1 -8.0 -5.6
77
500 586
697 739
138,600 117,000
5.5 5.6
693 755
4.38 5.63
4.30 3.45
237.2 259.6
-6.4 -7.5
0.0 0.0
Test Section 2
Test Section 3
78
Table II.
Expt. NO.
14 15 16
17
Av. Temp., F. Bulk Surface
597 591 593 606
666 73 1 803 842
Biphenyl-Polymer Heat Transfer Results Heat Transfer Heat Flux, Bulk Coeff., [EpQ] B.t.u./Hr., Vel., B.t.u./Hr., Sq. Ft. Ft./Sec. Sq. Ft. F. x 10-4 Test Section 1, Low Polymer Mixture (75% Biphenyl, 20% Terphenyls, 4-5% Higher Polymers) O
100,100 201,300 304,900 343,300
14.4 14.4 14.5 14.6
1439 1427 1425 1436
12.81 12.60 12.74 13.19
% ,Dev. DlttusBoelter
[?I
(Calcd.
- Obsd.)
4.31 4.37 4.37 4.24
541.6 538.5 539.0 544.0
+1.0 f1.8 +2.1
5.64 5.78 5.71
571.3 562.7 560.8
t0.4 +1.5 $1.8
$1.0
Test Section 1, High Polymer Mixture (60% Biphenyl, 15% Terphenyls, 25% Higher Polymers) 18 19 20
599 590 593
672 740 846
100,600 204,700 343,800
section 1, as obtained from photomicrographs of tube sections, was 0.000027 foot. The differential pyrovane controller, activating a portion of the heaters on the preheater, provided effective bulktemperature control of the fluid entering the test section. The degree of temperature and flow control was evidenced by the small mean deviation for coefficients measured consecutively over 12 to 48 hours. I n a normal series of experiments at equilibrium conditions, the mean deviation of the measured coefficient was around =k0.4'%. In most cases each coefficient in Tables I and I1 represents an average value for several
898
14.4 14.5 14.3
1360 1345 1337
11.18 10.94 11.02
experiments, for which measurements were made a t 2-hour intervals. The temperature profile for each test section was linear with length for any position on the circumference of the tube, but the profile around the circumference of the tube for any tube section showed reproducible variation, with a temperature difference of 2' to 4 ' F. existing between top and bottom; the bottom was hotter than the top. This radial temperature difference, being a function of velocity, heat flux, and (tS - tb), can be attributed to either natural convection superimposed on forced convection or some other charac-
INDUSTRIAL AND ENGINEERING CHEMISTRY
teristic of a horizontal tube. The coefficients for biphenyl and the other fluids are based on the average linear profile from four positions on the tube circumference. With possible influence from the strong electrical fields in the vicinity of the tube and couples, a series of coefficient determinations was made in which couple size and type of installation were varied. No significant change in coefficient or type of profile has been observed, In work now in progress with monoisopropylbiphenyl, nine thermocouples (8, 12, and 16 mils in diameter) are installed on the 12-inch
B I P H E N Y L HEAT TRANSFER measurement section, six on the top of the tube and three on the bottom. Couples are again attached by spotwelding, but the leads are positioned for 2 inches parallel to the section and then extended straight out from either the top or bottom of the test section assembly. For these experiments, the profiles for top and bottom are linear with length, but show circumference variation similar to that encountered in the biphenyl work. The auxiliary loop equipment provided for the calibration of the flowmeter was not used, as an effective check on the flowmeter was permitted by comparing the flow reading with that calculated from the observed power input and bulk temperature rise in the heated section. For the coefficients in Tables I and 11, the mean deviation of the flow rate calculated from that observed was 4~5.6%; but a major part of this error resulted from experiments with bulk temperature changes less than 8' F., in which the temperature error was magnified. I n more recent work with monoisopropylbiphenyl, a flow calibration by the heat input method as outlined for biphenyl was checked by an additional calibration in which the actual volume of metered fluid was measured. The two methods agreed to within zt2.901,.
62
60
58
56
cLL
.
2 52 >
k
2W
Heat transfer coefficients were calculated by Equation 2. The power input, 412, represents the observed power into the 12-inch measurement section as corrected for radial losses.
Velocity Check with Bulk Temperature Rise in Test Section. As a check on flow meter measurements, the fluid velocity in the test section was calculated by Equation 3. The net heat input for the full section, q 1 8 , was obtained from the net q 1 2 and the measured length ratios, with suitable corrections for conduction and heat generation in the silver bus bars. The specific heat and density
5c
0
4e
4E
44
100
I
I
200
300
I
I
400 500 TEMP. " E
I
I
600
700
I
800
Figure 1 . Density of biphenyl and two polymer mixtures decreases linearly with temperature I.
Calculation
Heat Transfer Film Coefficient. The average bulk temperature, tb, was taken as the arithmetical mean of the two mixing pot temperatures. The average wall temperature, t,, represents an average of the linear temperature profiles. The average inside wall temperature, t,, was calculated by Equation 1. This equation for uniform heat generation in the tube wall was derived using the integration procedure outlined by Bernard0 and Eian (2).
54
3 0
II. 111.
Biphenyl
75% biphenyl, 20% terphenyls, 4 to 5% polymers 60% biphenyl, 15% terphenyls, 25% polymers
values were taken at the average bulk temperature. q18
= WCdAkise)
(3 1
Correlation of Film Heat Transfer Coefficients
The availability of reliable coefficients for the organics covering a wide range of velocity, temperature, and heat flux makes it possible to include in this article ' a correlation study. Properties for the three fluids, upon which the correlation work is based, are presented in Figures 1 to 3. The density and viscosity values were determined at this laboratory and extended to higher temperatures by the equations presented under physical properties. The specific heat values for the correlation, which are not included, were obtained from unpublished measurements at this laboratory. The thermal conductivity values chosen were those reported by McEwen (8). In the heat transfer results for biphenyl (Table I), the film coefficient at constant bulk temperature does not appear to be a function of the surface temperature. Thus, a general equation of the type proposed by Sieder and Tate (9) does not provide an adequate corre-
lation. On the other hand, an equation of the Dittus-Boelter type ( 3 ) , which is uninfluenced by the surface temperature, is effective. Equation 4 was used to correlate all heat transfer results. An equation of similar type using a like exponent for Reynolds number was used in the recent water study by Kaufman and Isely ( 6 ) .
A 'graphical correlation of the heat transfer results for biphenyl and the two polymer mixtures is presented in Figure 4. The degree of fit obtained with data in a graphical plot cannot be well shown; thus, the per cent deviation of the calculated Nusselt number from that observed, for each experiment, is included as the last column of Tables I and 11. The same equation is equally effective for biphenyl and the polymer mixtures. The mean deviation for all the biphenyl experiments is =t3.5% and for the polymer work, -L 1.4%. Fouling of Heat Transfer Surfaces
One of the main objectives with the loop was to study the influence of pyrolytic product formation on heat transfer VOL. 50, NO. 6
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