An experimental study was conducted in top-heated finned horizontal tubes to study the effect of enhancement devices on flow boiling heat transfer in coolant channels. The objectives are to examine the variations in both the mean and local (axial and circumferential) heat transfer coefficients for circular coolant channels with spiral finned walls and/or spiral fins with a twisted tape, and improve the data reduction technique of a previous investigator. The working fluid is freon-11 with an inlet temperature of 22.2 C (approximately 21 C subcooling). The coolant channel's exit pressure and mass velocity are 0.19 M Pa (absolute) and 0.21 Mg/sq. ms, respectively. Two tube configurations were examined; i.e., tubes had either 6.52 (small pitch) or 4.0 (large pitch) fins/cm of the circumferential length (26 and 16 fins, respectively). The large pitch fins were also examined with a twisted tape insert. The inside nominal diameter of the copper channels at the root of the fins was 1.0 cm. The results show that by adding enhancement devices, boiling occurs almost simultaneously at all axial locations. The case of spiral fins with large pitch resulted in larger mean (circumferentially averaged) heat transfer coefficients, h sub m, at all axial locations. Finally, when twisted tape is added to the tube with large-pitched fins, the power required for the onset of boiling is reduced at all axial and circumferential locations

Boyd, Ronald D., Sr.

Smith, Alvin

English

NASA Technical Reports Server

N91-28 116 :'
SUBCOOLED FREON-I1 FLOW BOILING IN TOP-HEATED FINNED
COOLANT CHANNELS WITH AND WITHOUT A TWISTED TAPE
\
Alvin Smith and Ronald D. Boyd, Sr.
Department Of Mechanical Engineering
P.O. Box 397
Prairie View A&M University
Prairie View, Texas
ABSTRACT
An experimental study has been conducted in top-heated finned horizontal tubes to study the
effect of enhancement devices on flow boiling heat transfer in coolant channels.The objectives of the
present work are to;_ (1) examine the variations in both the mean and local (axial and circumferential)
heat transfer coefficients for circular coolant channels with spiral finned walls and/or spiral fins with a
twisted tape, and (2) improve the data reduction technique of a previous investigator. The working fluid
is freon-11 with an inlet temperature of 22.2_C (approximately 21°-C subcooling). The coolant channel's
exit pressure and mass velocity are 0.19 M Pa (absolute) and 0.21 Mg/m2s, respectively. Two tube
configurations were examined; i.e., tubes had either 6.52 (small pitch) or 4.0 (large pitch) fins/cm of the
circumferential length (26 and 16 fins, respectively). The large pitch fins were also examined with a
twisted tape insert. The inside nominal diameter of the copper channels at the root of the fins was 1.0
cm.
The results show that by adding enhancement devices, boiling occurs almost simultaneously at all
axial locations. The case of spiral fins with large pitch resulted in larger mean (circumferentially
averaged) heat transfer coefficients, hm, at all axial locations. Finally, when a twisted tape is added to
the tube with the large-pitched fins, the power required for the onset of boiling is reduced at all axial
and circumferential locations.
39O
https://ntrs.nasa.gov/search.jsp?R=19910018802 2020-03-19T16:42:43+00:00Z
SUBCOOLED FREON-11 FLOW BOILING IN TOP-HEATED FINNED
COOLANT CHANNELS WITH AND WITHOUT A TWISTED TAPE
Nomenclature
ac
As
CHF
D
Dh
G_
h
hm
H
K
L
2Cross sectional area, m
Surface area, m 2
Critical heat flux, W/m 2
Tube diameter, cm
Heated hydraulic diameter, m
Mass velocity, kg/rnZs
Local heat transfer coefficient, W/m2°C
Mean heat transfer coefficient, W/m2°C
fin height
Thermal conductivity, W/rn K
Heated length of test section, m
y
N Number of fins per test section
p Static exit pressure, M Pa
q Heat flux W/m 2
r Radial coordiante for data reduction model, m
R Tube radius, cm
Tf Bulk temperature of the flowing fluid, °C
Tm Local measured outside wall temp. of the tube, °C
Ts Local inside wall temp. of the tube, °C
Tsar Saturation temperature, °C
T,, Ambient temperature, °C
Z Axial coordinate for heated portin of test section, cm
Greek Letters
p Density, kg/m3; io Circumferential coordinate; :r Half of a full rotation or 180 °(bottom of test section)
Introduction
Subcooled flow boiling (locally boiling fluid whose mean temperature is below its saturation
temperature while flowing over a surface exposed to a high heat flux) has the greatest potential of
accommodating high heat fluxes [1]. In space, the active cooling of cold plates, turbine blades, fusion
reactor components, chemical reactor components, electronic components, refrigeration systems com-
ponents, and other systems necessary for the successful commercialization of space will require an
efficient heat transfer system. Therefore, the need to understand the behavior of two-phase flow boiling
in both the space (zero-g or reduced-g) and earth environments is essential to thermal designers,
especially where pumping power requirements must be optimized. Since thermal designers are forced
on occasion to use data from uniform heat flux applications for non-uniform heat flux applications, it
is important to assess the effect of the non-uniformity on the local and mean heat transfer.
Ideal heat transfer is achieved when the sum of the component thermal resistances is reduced to
a minimum. However, flow dynamics through plain tubes rarely provides the ideal conditions needed,
because the stationary fluid layer at the tube wall leads to the buildup of a slow moving laminar boundary
layer caused by frictional drag forces. This results in heat transfer being principally controlled by fluid
conductivity and the thickness of this layer.
Research related to enhanced surfaces for heat transfer has increased in recent years [2]. The
immediate benefit from recent research has led to increasing efficiency of heat exchanges and to the
development of the spirally fluted tube [3]. Fins (sometimes called extended surfaces) are often used
to increase heat transfer rates to fluids in cases where low heat transfer coefficients exist. If the wall
temperature is sufficiently high to cause film boiling and all other resistances are small, fins will cause
an increase in the heat transfer rates in a boiling situation [4]. Examples include cooling of nuclear fuel
391
rods, cooling of electrical equipment such as radio-power tubes, and some evaporators. Devices for the
establishment of fluid swirl are often used to increase heat transfer in tube flows. The increase in
pressure drop, however, is less than the increase in heat transfer in many cases. Twisted tape inserts
have been studied by Hong and Bergles [5], who found that in laminar flow, heat transfer could be
increased by a factor of as much as 9 whereas pressure drop increased by a factor of less than 4 over
empty tube values. In turbulent flow, twisted tape inserts can improve heat transfer by a factor of 2 over
empty tube values, but the pressure drop is increased by several orders of magnitude. Brown Fintube
Company reported in its literature that increases in heat transfer rate of up to 300 percent have been
achieved in viscous fluids with nominal increases in pressure drop with the addition of twisted tapes.
Experimental Investigation
The flow loop (Figure 1) is a closed system designed to control the working fluid (freon-11) and
reservoir temperatures, pressures, heating capabilities, and overall system stability. The reader is
referred to references 6-8 for detailed descriptions of the experimental flow loop, procedures, and data
acquisition.
The loop was designed to study both saturated and subcooled flow boiling, although only subcooled
flow boiling was considered here. By allowing both saturated and subcooled flow regimes, the heat
rejection can be either isothermal or at a slightly variable axial temperature. The latter case (subcooled
flow boiling) will result in enhanced heat rejection capability (above the saturated case) to the extent
that the given application will allow a variable axial temperature.
Referring to Figure 1, the reader should be made aware of the possible circulation patterns of the
working fluid that can be accomplished by opening and closing various valves within the loop. However,
for the tests performed within this investigation, only reservoir #1 was utilized.
Figure 1. Freon- 11 flow loop for both subcooled and saturated flow boiling experiments.
392
ORIGINAL PAGE IS
OF POOR QUALITY
The measured outside wall temperatures of the test sections were used to calculate the unknown
heat transfer coefficient, h. This was achieved through a data reduction technique developed by
Turknett and Boyd [7], based on the heated hydraulic diameter, Dh. This approach will result in, at most,
a qualitative indication of the local (axial and circumferential) distribution of h. A piece-wise linear
profile method was used to evaluate the mean (circumferentially averaged) distribution of hm. A finite
difference technique (although not used for the present work) was developed to evaluate the local h
[81.
Results
The results are presented for the case of subcooled freon-ll boiling in top-heated horizontal
channels with internal fins and/or twisted tape inserts.
Figure 2 shows the circumferential variations in the heat transfer coefficient at the axial location
Z4 (center of the heated portion of the test section). It is evident that the variations in heat transfer
coefficients in the circumferential location are increased by adding more enhancement devices. The
sl_al fin with large pitch shows no variations in the single phase region located at the bottom
:rt
(_:- and _r) of the tube, and minor variations at the top (0 and _) of the tube in the film boiling region.
Adding a twisted tape (spiral fin large pitch tape) decreased variations in heat transfer coefficients in
the film boiling region (0 _r ___and ), but increased the variations in the single phase region ( and :r).
The spiral fin small pitch shows the largest variation of h in the single phase region (_- and :r) and only
minor variation in the film boiling region (0 and _).a
t4
The axial variation of h as a function of power for the three (3) cases is presented in Figure 3. The
axial locations Z1 and Z7 are excluded due to end losses and the effect of axial conduction. At
circumferential location 0 (top of the tube), the onset of boiling appears to occur more uniformly along
:r
the length of the channels than at circumferential location _-. At circumferential location _, the spiral
fin large pitch (3a.2) provides higher heat transfer coefficients than any case presented. By adding a
twisted tape to the large pitch tube, the power required for the onset of nucleate boiling was reduced
at all axial and circumferential locations, but the average heat transfer coefficients were reduced
_r (Figure 3c.2), provided a(Figure 3b.2). The spiral fin small pitch tube at circumferential location _-
more uniform onset of boiling in the axial direction than the tubes with large pitch or large pitch with
a twisted tape insert. However, at circumferential location 0, the tube with twisted tape had a more
uniform onset of nucleate boiling. The average axial heat transfer coefficients at circumferential
location 0 (top of the tube) were higher for the case of the tube with a small pitch than the other cases
(spiral fin large pitch and spiral fin large pitch with tape), but were found considerably lower at
•
circumferential location _- for the same cases.
Figure 4 is a plot of the mean (circumferentially averaged) heat transfer coefficient hm, as a
function of power. The averaged h (i.e., hm) appears to be consistently higher for the case of the spiral
fin large pitch (4a) than the cases with a tape (4b) and small pitch (4c). In some cases, the addition of
the tape to the spiral fin large pitch reduced the average heat transfer coefficients in the axial direction.
However, adding a twisted tape or reducing the pitch of the fins caused boiling to occur more uniformly
over the length of the test section. Heat transfer coefficients literally remained unchanged in the spiral
pitch tube (4c) in the axial direction when compared with Figure 3.
393
E.E
SPf'_L F_ _ RTCH (Hl01t)
SYMBOL ¢IRO.X_I[REhTW. LOGA'noN
• 0/4
'_ 3M/4-; -
200 _00 600 800
Power CW) Power (W)
zoooo
e_E
J::: 10000
5000.
q" = CONSTANT
_._ _ " 0_+/ _-'-- qloc a 1
I:I) = IT INSULATED
,:;, ;, ,',;', ,'; __ ............
Power L'W)
Figure 2. Heat transfer coefficient versus power at axial location Z = Z4 (center of
heated section of the test section) for top-heated finned tubes at 0.19 MPa and 0.210
Mg/m2s. Channel diameter = 1.27 cm, Heated length = 1.22 m.
394 ORIGINAL PAGE iS
OF POOR QUALITY
$YUEOL5 AXIAL LOCAT_'ONT(HYO 1n)
• _'" °_" _ ° --,o,,oo-/
• Z2 _ 2032 cm a Z$ - Sl28 c_
• Z3 = 4064cm 1/_
Q Z4 = 50.96 cm
= Z5 - 81 28 crn
n Z6 - 10160 crn _E
, ,,, ............ 1 0
........ ;_' ....... _=;....... _," ;,&, ,=o :
• 5PIPJ_L RN LARC_ PITCH '¢I/TAFI tPttl-Pl/4)
S'(IJROLS A.X_L LrXAI_(,NS01112_'O)
: z,- ooo_._
20 32 cm
F f Z3 4064 cmI _ SPrRAL F_N LAP_[ PIrCll (PHI.PI,/4) /_ 4 (_Og6
l _ $_UBO(.S AXIAl LOCA_S(HIOliS) _ Z_ 81 )R ¢m
o Z6 101.60 cm
• l_' m 4061 cm _
o Z6 - 101 60 cm
=lol __ 1_tg_ cro J:
...... :_ ........ :,_' ....... _/'" U.,"" ' ;d== ' .........' "_,; ..... ',&'....... =_,;' ' '_' ....... ;¢_
Po_*r (W) pommr (W)
(3b. i) (3b.2)
SPIneL TIN SMALL PITCH (pHI_ 0)
SYMBOLS AXIAL LOCATIONS{H 1/ I BA) !,tAIL _ItCIIrPHt-_I/4)
• ""°"' V i :"°:'°Z4 i 60,_6 Cm " _." . 4064 cm_t Z6 = 812B _:m =_ ZA - 6096 cm
'_ Z6 - 10i60 ¢m _.
v Z7 - 121.92 cm _., , _65 =. 10IG081:0 cmCm
= _ = Z7 m 121,92 cm
...................... .,-, , ..... _ ....... ._,. ....... _ ....... ,_,. ....... .;_
Power (W )
(3c. l) (3c.2)
Figure 3. Axial variations of h versus power at circumferential locations 0 and _- for
spiral fin large pitch (3a), spiral fin large pitch with tape (3b), and spiral fin small pitch
(3c).
ORIGINAL PAGE IS
OF POOR QUALF_Y
395
I1_O-
143o.
2 :
j :
200-
i
_1_ Mr._IH _l_JTlOtl$ (141011-A)
SYMSOl.3 /,,X_I. LOC_11(_5
• Zl " 0.00 ¢m
\_,. k • 7.3 - 40._
% & Z4 - 80,D8
\K \ ° . : _o,:m
(a)
o ;
000 8O0 I0o0
Po*er (W)
eoo-
400.
E
1_6,N OIST_'IgUTION_ (H I122-_
= SPIRAL ;IN LARGE Pi'I_.H W/T_,>E
, 3'r_g0L_ AXIAL LOCATICw_
f_ . Z1 - 0,00 am
I _9 , Z2 - 20.32 ©m
• 3 m 40 64 cm
_li \A \ , Z_,= e;.:_B cm
I_ |_ _ a ZO = 101._0 cm
(b)
, + ..... l_r i 111 ii iii l rl 1+ ill r] ill i, 111 4 1 11 _4_ _00 _ ,
Power (W)
Power (W)
Figure 4. Mean heat transfer coefficient versus power for spiral fin large pitch (4a),
spiral fin large pitch with tape (4b), and spiral fin small pitch (4c).
396 ORIGiHAL P_GE iS
OF POOR QUAUTY
Acknowledgments
Support for this study was provided by Sandia National Laboratories and NASA (JSC). The
authors would like to thank the personnel at these facilities for their support.
References
. Boyd, R.D., "Subcooled Flow Boiling Experiments Using Small Diameter Uniformly-
Heated Tubes," ASME, 86-HT-23, AIAA/ASME 3th Joint Thermophysics and Heat
Transfer Conference, Boston, Mass., June, 1986.
. Bergles, A.E., and Webb, R.L., "A Guide to the Literature on Convective Heat Transfer
Augmentation," Advances in Enhanced Heat Transfer - 1985, ASME Publication, HTD-
Vol. 43, 1985, pp. 81-89.
. Panchal, C.B., et al., "A Spirally Fluted Tube Heat Exchanger as a Condenser and
Evaporator," Proc. 4th Miami Intl. Symp. on Multi-Phase Transport and Particulate
Phenomena, Dec. 1986.
4. Burmeister, L.C., Convective Heat Transfer, John Wiley & Sons, New York, 1983,
pp. 615-616.
. Hong, S.W. and Bergles, A.E., "Augmentation of Laminar Flow Heat Transfer in Tubes by
Means of Twisted-Tape Inserts," Journal of Heat Transfer, Trans. ASME, Vol. 98, May,
1976, pp. 251-256.
. Boyd, R.D., "Flow Boiling With and Without Enhancement Devices for Horizontal, Top-
Heated Coolant Channels for Cold Plate Design Applications, Preliminary Report,"
Department of Mechanical Engineering, Prairie View A&M University, Prairie View, TX.,
Submitted to NASA(JSC), Contract No. 9-16899 (Task-5), December, 1986.
. Boyd, R.D. and Turknett, J.C., "Forced Convection and Flow Boiling With and Without
Enhancement Devices for Top-Side-Heated Horizontal Channels," Department of
Mechanical Engineering, Prairie View A&M University, Prairie View, TX., (presented
at these proceedings).
. Smith, A., "Subcooled Freon-11 Flow Boiling Heat Transfer With and Without
Enhancement Devices for Top-Heated Horizontal Tubes," MS Thesis, Department of
Mechanical Engineering, Prairie View A&M University, Prairie View, TX.,
(to be published).
397


Subcooled freon-11 flow boiling in top-heated finned coolant channels with and without a twisted tape

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