Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

Experimental Thermal and Fluid Science 33 (2008) 173–177 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal ho...

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Experimental Thermal and Fluid Science 33 (2008) 173–177

Contents lists available at ScienceDirect

Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes Przemysław R. Dominiczak, Janusz T. Cies´lin´ski * ´ sk University of Technology, Faculty of Mechanical Engineering, Narutowicza 11/12, 80-952 Gdan ´ sk, Poland Gdan

a r t i c l e

i n f o

Article history: Received 24 May 2007 Received in revised form 16 July 2008 Accepted 17 July 2008

Keywords: Pool boiling Horizontal tube Circumferential temperature distribution Porous layer

a b s t r a c t In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Pool boiling on horizontal tubes is one of the most extensively studied problems in the recent past because of its direct application. During nucleate boiling on an electrically heated horizontal tube the circumferential temperature distribution (CTD) is observed. Fedders [1] has reported CTD on the inside surface of a horizontal tube during pool boiling of water. The CTD reaches maximum and minimum values within lower and upper generatrix of the tube, respectively. Thus, assuming constant heat flux density along whole circumference, the heat transfer coefficient distribution has been estimated. Gorenflo et al. [2] have carried out experiments with hexane. The CTD has been recorded, as well. The difference between the maximum and the minimum temperature value in circumference was about 4 K. Bier et al. [3] have conducted research on refrigerant R11. The maximum temperature difference was about 0.07 K. Gorenflo and co-workers [4–8] carried out a systematic study regarding bubble formation on tubes with defined roughness or on surfaces modified by macrostructures for enhancement of heat transfer. Experiments with different organic fluids were performed in order to obtain results over wide range of reduced pressure. They established that the superheat of the tube wall varied significantly with an azimuthal angle. Most lately, Kang [9] determined heat transfer coefficient distribution on the outside surface of a stainless steel tube for water boiling at atmospheric pressure. Minimum local coefficients were observed at azimuthal angles of 45° and 180°, from the tube bottom, respec* Corresponding author. Tel.: +48 58 347 16 22; fax: +48 58 347 13 83. E-mail address: [email protected] (J.T. Cies´lin´ski). 0894-1777/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2008.07.007

tively. The maximum temperature difference between the temperatures at 45° and 180° was 1.8 K, for heat flux density equal to 50 kW/m2. According to Kang the temperature distribution results first of all from liquid agitation and bubble coalescence. As results from the literature, the circumferential temperature distribution depends on: heat flux density, pressure, tube diameter and thermo-physical properties of the fluid and the wall material. In the most general case, the conjugated heat transfer problem, where the thermal boundary condition on fluid side is unknown, should be solved [10]. On the other hand, Louahlia-Gualous et al. [11] have developed an iterative procedure to solve the two dimensional inverse heat conduction problem. CTD as well as local heat flux density have been determined for different heat flux densities taken as a boundary condition at the inner surface of a cylinder. The literature review shows that – except works by Gorenflo and co-workers, there is no CTD systematic investigation during pool boiling on horizontal tubes, with different tube diameters, surface finishing and thermo-physical fluid properties. There is also no complete explanation of the CTD nature. Therefore, in order to add some new data a study [12] was performed. In the present work, it is postulated, that CTD outside horizontal tubes is caused by the occurrence of different heat transfer mechanisms during nucleate pool boiling at low heat flux condition. So, if CTD really results from different heat transfer mechanisms existing outside horizontal tubes, thus in order to avoid temperature difference heat transfer should be locally enhanced. The techniques applied to heat transfer enhancement could be divided into passive and active ones [13]. Usually heat transfer enhancement techniques are applied to the whole heating surface and the entire heat flux density range, but the application of

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Nomenclature A D I t q Ra U

heat transfer area (pDoL) [m2] diameter [mm] electric current [A] temperature [°C] heat flux density [W/m2] roughness arithmetic average [lm] voltage [V]

Subscripts ax axial av average cr critical f fluid in inside m modified o outside s smooth

Greek letters b angle [°] k heat conductivity [W/mK]

turbulent inserts, with a shape memory as a local heat transfer enhancement, shows expediency of such solution when the insert is activated above ‘‘programmed” temperature. As pool boiling investigations show, heat transfer coefficient can be many times higher than for a smooth surface when porous metallic coating is used [14]. Therefore, in the present study, porous metallic coating as an enhancement technique was applied. The present paper is focussed on a study of CTD during pool boiling on electrically heated horizontal tubes, and particularly is devoted to development of local heat transfer enhancement technique in order to avoid CTD. 2. Experimental set up The test section was mounted in a double-walled vessel made of stainless steel. A heating jacket was used for indirect heating of the liquid and keeping the vessel warm. Insulation was provided on the top, the bottom and the sides of the tank. Four inspection windows, at the top, side and bottom walls of the vessel were furnished for visual observation. The fluid temperature tf was measured with six iron-constantan thermocouples and controlled by monitoring the temperature of the thermostatic fluid in a heating jacket. The test specimens were heated by using the tubes themselves as resistance heaters. Experiments of nucleate pool boiling have been conducted using stainless steel tubes with outside diameters: 8.15 mm, 13.52 mm, 18.04 mm and 23.60 mm with the active length of 250 mm. The heat flux was calculated from potential difference and the current flowing through the test section. The temperature of the inside surface of the test tube was measured in two manners: using single thermocouple which displays an average temperature of the inside surface tax, or applying a specially designed probe which allows measurement of the circumferential temperature distribution. The hot junction of the single thermocouple was placed on the centre line of the tube at the midpoint of the test section. The probe consists of 6 or 12 thermocouples, which were evenly spaced and cemented to the teflon rod and there was no contact between the thermocouple hot junction and the tube surface. The outside surface temperature, to, used for wall superheat determination, was obtained as an analytical solution of the one-dimensional, steady-state heat conduction equation with uniform heat generation in the tube wall

t o ¼ t ax þ

" # UIDo 2 lnðDo =Din Þ  1 4kA ðD2o =D2in Þ  1

of producing current in excess of 1300 A and maximum power of 10 kW. The current was controlled by a transformer choke. The collection of data began by supplying minimum power to the tube. At each level of heat input, equilibrium was established before taking data. During each run successive sets of thermocouple outputs were taken and processed until the readings at each power level differed only by small amounts so that averages were duplicated. This was taken to be an indication that boiling process was at steady state. It generally took up to 15 min to achieve steady conditions after the power level was changed. A precision mercury barometer was used to measure the atmospheric pressure. The liquid level in the tank was maintained at about 120 mm above the heating surface. The details of the experimental set up and experimental procedure are given in [12]. Experiments of nucleate pool boiling were conducted within low heat flux density (q < 0.1qcr) with two fluids: distilled water and HFC-141 b. The investigation was divided into two stages. The first one devoted to tubes with heating surface condition Ra 6 0.25 lm which are called smooth. The smooth tubes, when the first stage were completed, have been partially coated by porous layer made of aluminium. The porous layer area fraction has been determined according to measured CTD’s and direct observations during experiments with smooth tubes. Thus, the porous layer area, indicated by angle b = 120° – Fig. 1, has been assumed. A metalographic analysis was applied to determine porous layer parameters for all investigated tubes. The mean thickness of the porous layer (0.27 mm, 0.30 mm, 0.45 mm, and 0.40 mm), average porosity (64.24%, 27.96%, 25.69%, and 55.40%), and mean pore radius (2.29 lm, 2.57 lm, 3.74 lm, and 2.32 lm) were estimated by microscope Reichert MF2, for the tube diameter 8.15 mm, 13.52 mm, 18.04 mm and 23.60 mm, respectively. The uncertainties of the measured and calculated parameters are estimated by the mean-square method. The maximum overall experimental limits of error for heat flux density extended from ±1.3% for maximum heat flux up to ±5.7% for minimum heat flux density, the maximum error for average heat transfer coefficient was estimated to ±7% and in wall superheat to ±10%.

ð1Þ

The day before starting the main experiment, each of the heating surfaces was boiled for an hour in saturated pool in order to ‘‘age” the heating surface and remove gases from the liquid. Power was supplied as low voltage direct current from an assembly of four welding transformers. The circuit was capable

Fig. 1. Porous layer location: 1– tube, 2 – porous layer.

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3. Results and discussion CTD within the whole investigated heat flux range, all tube diameters and for both liquids has been observed. Selected CTD’s measured on internal wall surface during pool boiling experiments for water and R 141b boiling on smooth tube are presented in Figs. 2 and 3, respectively. Trigonometric polynomials were used for approximation of the temperature distribution

TðhÞ ¼

n a0 X ðaj cos jh þ bj sin jhÞ þ 2 j¼1

ð1Þ

where polynomial coefficients were calculated (using MATLAB environment) from the formulas

aj ¼

X 1 2L1 pij f ðxi Þ cos L i¼0 L

ð2Þ

bj ¼

X 1 2L1 pij f ðxi Þ sin L i¼0 L

ð3Þ

for i = 0, 1, 2 . . . 2L  1, j = 1, 2 . . . n and n < L, where L denotes the number of thermocouples – 6 or 12. Temperature tax measured by axially placed single thermocouple is represented by a dashed line and solid line corresponds to the average temperature tav,s calculated as arithmetic mean from 6 or 12 thermocouple readings. The maximum and the minimum temperature appears near the upper and the lower generatrix, respectively. In case of low heat flux density in pool boiling on horizontal, smooth tubes two stagnation points can be differentiated, one near the lower generatrix, and the second one near the

Fig. 2. Circumferential temperature distribution on an internal wall surface for water at q = 14.1 kW/m2 and a smooth tube of OD = 13.52 mm.

Fig. 3. Circumferential temperature distribution on an internal wall surface for R 141b at q = 14.2 kW/m2 and a smooth tube of OD = 13.52 mm.

Fig. 4. Influence of a smooth tube diameter on an circumferential temperature distribution on an internal wall surface for water at q = 11.2 kW/m2; (a) OD = 8.15 mm, (b) OD = 18.04 mm.

upper generatrix of the tube. In the case of free convection heat transfer outside horizontal tube the most preheated liquid layer is near the lower generatrix of the tube. So, these are favourable nucleation conditions, what results in the minimum temperature occurrence. The CTD is almost symmetric according to the vertical axis of the tube and it varies due to changes of heat flux density. The difference between maximum and minimum temperature readings, in presented CTD’s (Fig. 2 and Fig. 3), was about Dt = 3.5 K for both tested liquids. Fig. 4 illustrates the effect of the tube diameter on CFD observed during nucleate pool boiling of water outside smooth tubes for almost the same heat flux density 11.2 kW/m2. It is seen in Fig. 4 that for the greater tube diameter bigger maximum temperature difference was recorded – 2.5 K and 3.5 K for tube with 8.15 mm and 18.04 mm in diameter, respectively. Furthermore, for the tube with the smaller diameter (8.15 mm) in the region of upper generatrix wall the temperature is almost constant, contrary to the tube with the bigger diameter (18.04 mm), where temperature changes monotonically along the tube periphery. In the case of water boiling on modified tubes four parts of the boiling curve have been distinguished – Fig. 5. Within line A–B nucleate boiling occurred only on the porous layer and the remaining part of the heating surface was occupied by single phase convection. The first vapour bubbles along lower generatrix appear for Dt = 4 K – point B in Fig. 5. The temperature drop B–C is related to heat transfer mechanism replacement from single phase convection to nucleate boiling within the area of lower generatrix. The part C–D of the boiling curve is associated with an unstable boiling process within lower generatrix. The stable boiling process on the smooth part of a heating surface starts over Dt = 5.5 K – point D, and causes another temperature drop D–E. The line E–F–G corresponds to stable boiling on the whole heating surface area (porous layer and smooth part). In the case of boiling R 141b on modified tubes three parts of the boiling curve have been distinguished – Fig. 6. Within line A–B the single phase convection on the whole heating surface has been observed. The first vapour bubbles appear only on the porous layer for Dt = 12 K – point B, and single phase convection

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Fig. 7. Circumferential temperature distribution on an internal wall surface for water at q  14 kW/m2 and a tube of OD = 13.52 mm: d – smooth, N – modified.

Fig. 5. Boiling curve for water on a modified tube with OD = 13.52 mm.

q[W/m2]

F G

D

1E+4

C B Fig. 8. Circumferential temperature distribution on an internal wall surface for R 141b at q  14 kW/m2 and a tube of OD = 13.52 mm: d – smooth, N – modified.

A-B

1E+3

A

Application of a porous layer on a part of a tube during R 141b boiling does not result in CTD smoothing and alleviating – Fig. 8, nevertheless the lowering of average temperature tav,m by ca. 3.4 K has been achieved.

C-D

E-F E

3.1. Practical usefulness

1E+2 1

10

t o - t f [k]

Fig. 6. Boiling curve for R141b on a modified tube with OD = 13.52 mm.

on smooth part of the heating surface has been observed. The temperature drop B–C is related to heat transfer mechanism replacement from single phase convection to nucleate boiling on the porous layer. The part C–D of the boiling curve is associated with stable boiling process within upper generatrix on the porous layer. The first vapour bubbles along lower generatrix appear for Dt = 15 K – point D, which causes another temperature drop D– G. The line G–F–E corresponds to stable boiling on the whole heating surface area. Selected CTDs measured on the modified tube internal wall surface during pool boiling experiments for water and R 141b are presented in Fig. 7 and Fig. 8, respectively. The porous layer provides favourable nucleation conditions so the lowest temperature in circumference was recorded within the upper generatrix. The maximum temperature was observed near the lower generatrix. In the case of the boiling water porous layer application results in two effects. The first one is smoothing and alleviating of CTD and the second one is a distinct lowering of average temperature tav,m.

Circumferential temperature distribution, particularly with such a big difference between maximum and minimum temperature value, as reported in [2], could be unsuitable during chemical processing, e.g. fractional distilling column. Owing to temperature distribution, a multi fraction mixture is extracted instead of a single component one. The conducted investigation exhibits that application of a porous layer on a part of a heating surface during R 141b boiling – an organic fluid, has not resulted in satisfactory CTD smoothing and alleviating. Probably, porous layers of different parameters, i.e. thickness, porosity and mean pore radius can give better results. For both tested fluids distinct lower average temperature of the heating surface has been observed, so application of partly covered surfaces may be useful in all cooling systems. 4. Conclusions Systematic experimental determination of CTD during pool boiling of water and R 141b outside horizontal tube has been reported. The CTD appears in the whole investigated heat flux density range for both boiling liquids. In the case of smooth tubes the maximum and minimum temperature appears near the upper generatrix and lower generatrix for both investigated liquids, respectively.

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The porous layer application on a part of the tube results in two effects. The first one is smoothing and alleviating of CTD for boiling water, and the second one is lowering of average wall temperature both for water and R 141b. Acknowledgements Authors wish to acknowledge the financial support from the State Committee for Scientific Research under Grant: 8 T10B 048 15. References [1] H. Fedders, Messung des Wärmeübergangs beim Blasensieden von Wasser an metallischen Rohren, KFA Jülich, Jül 740 RB 1971. [2] D. Gorenflo, H. Schömann, P. Sokol, S. Caplanis, Zum Einfluß der Oberflächenrauhigkeit und des Rohrdurchmessers beim Blasensieden an einzelnen Glatt- und Rippenrohren, Wärme- und Stoffübertragung 25 (1990) 265–272. [3] K. Bier, J. Goetz, D. Gorenflo, Zum Einfluß des Umfangwinkels auf den Wärmeübergang beim Blasensieden an horizontalen Rohren, Wärme- und Stoffübertragung 15 (1981) 159–169. [4] D. Gorenflo, W. Fust, A. Luke, E. Danger, U. Chandra, Pool boiling heat transfer from tubes with basic surface modifications for enhancement. Design of the test tubes and first measurements, in: Third European Thermal Sciences Conference, Heidelberg, 2000, pp. 743–747.

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[5] D. Gorenflo, U. Chandra, E. Danger, A. Luke, Pool boiling heat transfer from tubes with and without basic surface modification for enhancement, in: 12th International Heat Transfer Conference, Grenoble, 2002. [6] D. Gorenflo, E. Danger, A. Luke, S. Kotthof, U. Chandra, Ch. Ranganayakulu, Bubble formation with pool boiling on tubes with or without basic surface modifications for enhancement, in: 5th International Conference on Boiling Heat Transfer, Montego Bay, Jamaica, 2003. [7] D. Gorenflo, S. Kotthof, Heat transfer and bubble formation on horizontal copper tubes with different diameters and roughness structures, in: ECI Conference on Boiling Heat Transfer, Spoleto, 2006. [8] S. Kotthof, D. Gorenflo, E. Danger, A. Luke, Heat transfer and bubble formation in pool boiling: effect of basic surface modifications for heat transfer enhancement., International Journal of Thermal Sciences 45 (2006) 217–236. [9] M.G. Kang, Local pool boiling coefficients on the outside surface of a horizontal tube, ASME Journal of Heat Transfer 127 (2005) 949–953. [10] Y. Lee, Y. Shigechi, T. Zeng, Conjugated heat transfer of nucleate pool boiling on a horizontal tube, Journal of Multiphase Flow 16 (3) (1990) 421–428. [11] H. Louahlia-Gualous, P.K. Panday, E.A. Artioukhine, Inverse determination of the local heat transfer coefficients for nucleate boiling on a horizontal cylinder, ASME Journal of Heat Transfer 125 (2003) 1087–1095. [12] P. Dominiczak, Theoretical and Experimental Investigation of the Outside Temperature Distribution during Nucleate Pool Boiling on Horizontal Tubes for Small Heat Fluxes, PhD Thesis, TU Gdansk, Gdansk, 2002 (in Polish). [13] A.E. Bergles, ExHFT for fourth-generation heat transfer technology. Experimental heat transfer fluid dynamics and thermodynamics, in: 5th World Conference on Experimental Heat Transfer Fluid Dynamics and Thermodynamics, Thessaloniki, 2001. [14] J.T. Cies´lin´ski, Nucleate pool boiling on porous metallic coatings, Experimental Thermal and Fluid Science 25 (2002) 557–564.