ENHANCEMENT OF BOILING ON A SMALL DIAMETER TUBE DUE TO BUBBLES FROM BELOW

Transcription

1 Proceedings o Fith International Conerence on Enhanced, Compact and Ultra-Compact Heat Exchangers: Science, Engineering and Technology, Eds. R.K. Shah, M. Ishizuka, T.M. Rudy, and V.V. Wadekar, Engineering Conerences International, Hoboken, NJ, USA, September 5. CHE5 45 ENHANCEMENT OF BOILING ON A SMALL DIAMETER TUBE DUE TO BUBBLES FROM BELOW Ebenezer Adom 1, Peter A. Kew and Keith Cornwell 3 1 School o Engineering and Physical Sciences, Heriot Watt University, Edinburgh, U.K; ea11@hw.ac.uk School o Engineering and Physical Sciences, Heriot Watt University, Edinburgh, U.K ; p.a.kew@hw.ac.uk 3 School o Engineering and Physical Sciences, Heriot Watt University, Edinburgh, U.K; k.cornwell@hw.ac.uk ABSTRACT An experimental investigation was carried out using distilled water at nominal atmospheric pressure boiling outside horizontal tubes o diameter 3.mm arranged one above the other. This diameter is o industrial importance in compact systems and has the interesting characteristics o being similar to the bubble size. The heat lux on the lower tube is kept constant while that o the upper tube is varied. It is shown that at low to moderate heat luxes there tends to be an enhancement due to the inluence o bubbles rom the lower tube on the upper tube, but at high heat lux this eect is negated as the density o the bubbles increases. A superposition model has been proposed and is successul in explaining the enhancement and, at high heat lux the restriction to the heat transer caused by bubbles rom the lower tube. INTRODUCTION Boiling heat transer inds a wide application in process industries in areas such as reboilers, evaporators, condensers and tubular heat exchangers due to its ability to transer a large amount o energy at low temperature dierences. There is a move towards more compact equipment using smaller diameter tubes but the existing research and consequent correlations apply to large tubes o more than 6 mm diameter. This study extends the research to small tubes and attempts to evaluate the inluence o bubbles on tubes when the bubble is o similar size or larger than the tube. There has been considerable research in the area o boiling heat transer on the outside o tube bundles and various correlations have been developed to describe the phenomenon that occurs in these geometries. Correlations such as those developed by Mostinski s (1963) and Cooper (1984) are all independent o tube diameter. An attempt to model the pool boiling heat transer on tubes, which included the inluence o the tube diameter, was made by Cornwell et al (198) using a large data base. They concluded that the diameter has some small eect on the heat transer coeicient or larger diameter tubes. Recently a study by Kew et al () concluded that pool boiling correlations used or larger tubes can be used with reasonable accuracy to predict the boiling heat transer coeicient on tubes o the order 1-6mm. Boiling on tube bundles has been studied extensively by investigators such as Palen et al (197), Cornwell et al (1986; 199; 199), Fujita et al (1987; 1997), Jensen (1988) and Chan et al (1987), it was ound that there is an increase in the heat transer coeicient in the upper tubes due to rising bubbles rom tubes below sometime termed as the bundle eect. Fujita (199) developed a model based on micro layer evaporation to explain this bundle eect. Sliding bubbles have long been investigated as the mechanism by which this enhancement takes place. Cornwell and Schuller (198) showed that boiling on tube columns and also on a single tube was characterized by sliding bubbles that slid around the periphery. Studies by Kenning et al () have shown that these sliding bubbles do have a strong eect on the heat transer. Cornwell et al (199) were able to show experimentally the contribution using bubble boiling in a column o tubes. Studies on twin tube arrangement have been carried out by Wall et al (1978), Chan (1987), Cornwell (1994), Hahne (1983; 1991) and more recently by Kumar (), but these investigations all considered using tubes with diameter greater than 6 mm. All these investigations conirmed the earlier assertion that there is an increase o heat transer coeicient between upper and lower tube. Gorenlo et al () notes that sliding bubbles generated on the lower surace o a tube inluence both the convective and evaporative heat transer, the eect being most pronounced at an intermediate level o bubble generation. 343

2 More recently the group at Heriot-Watt University Cornwell et al () investigated the boiling mechanisms outside a column o horizontal wires and concluded that there is a small increase in the heat transer coeicient in bubbly low above that o nucleate boiling or the upper tubes, but vapour blanketing mitigated this eect at higher heat luxes. To the authors knowledge there is no published data on the eect o the lower tube on the upper tube or small tubes and wires or diameters below 3 mm. This work is pertinent to the growing interest in the area o process intensiication by reducing conventional equipment. EXPERIMENTAL APPARATUS AND PROCEDURE The apparatus used in this experiment, shown in Fig. 1, consists o an aluminum block o dimensions 18 high by 1 wide mm connected to a condenser. In the ront section there is a glass window or visualization studies. The tubes are made o stainless steel o outside diameter 3. mm and inside diameter.4 mm with a heating length o 85 mm which is connected to a terminal block whiles the centre distance between the tubes is 7mm. A 5W DC power supply is connected independently to each o the two test tubes with a voltage tapping connected to a digital voltmeter whiles the other two tubes are dummy tubes used to keep the instrumented tubes in horizontal position. A single Type K thermocouple in each tube was used to measure the internal temperature o the tubes. There are also two type K thermocouples to measure the saturation temperature o the liquid. Electrical heaters are embedded at the back o the test section to keep the liquid at the saturation temperature. The experiment was carried out by illing the test section with the liquid to a level o about 1 mm rom the base o the test section. The heaters at the back o the test section were switched on to heat the liquid to its saturation temperature and the liquid was allowed to boil continuously or about minutes to allow condensable gases to escape beore experimental readings are taken. The power supply to the tubes was switched on and the current and voltage across the tubes was measured ater a steady state had been achieved (-3minutes). The experiment was carried out by increasing the heat lux in predetermined steps and then at decreasing the heat lux through the same steps. Fig.1: Schematic diagram o the experimental set-up The heat lux on the upper tube was varied rom 7-19 kw/m while the heat lux on the lower tube was held at a series o dierent values between -19 kw/m. Readings rom the thermocouples are estimated as accurate to within.1 o C o the measured temperature and that o the digital voltmeter and the power supply are within ±3 % accurate. The relative error or the heat transer coeicient was ound to be 1%. Random error would be expected to less than this; systematic errors inluence the absolute values o heat transer coeicient calculated, but would be expected to have little eect on the enhancement. The heat lux q was ound rom VI q = (1) πd l o 344

3 and the heat transer coeicient is evaluated rom q α = () ( T wall T sat ) where T wall is the surace temperature o the tube and T sat is the saturation temperature o the luid. The surace temperature is evaluated by considering the heat conduction through a cylinder at the appropriate heat lux. The results obtained with distilled water as a working luid are shown in Figs. and 3. Heat lux (kw/m ) ( upper tube) q l ( kw/m ) qo= q1=19 q=37 q3=61 q4=91 q5=17 q6= Wall superheat (K) ( upper tube) Fig : Variation heat lux with wall superheat with distilled water as the working luid 5 Heat transer coeicient (kw/ m K) ( upper tube) q l ( kw/m ) qo= q1=19 q=37 q3=61 q4=91 q5=97 q6= Heat lux (kw/m ) (upper tube) Fig 3: Variation o heat transer coeicient with heat lux with distilled water as working luid. SUPERIMPOSITION MODEL FOR BOILING OF THE UPPER TUBE. Experimental results or the two tubes indicate that the heat transer coeicient o the upper tube is enhanced by the translating bubbles up to a particular value equating the point where the heat lux (q) yields a total enveloping o the tube. It is suggested that this is due to the eect o bubbles rom the lower tube passing over the upper tube. The model, shown schematically in Fig. 4 assumes that translating bubbles envelop a proportion, p, o the length o the tube and heat is transerred by nucleate boiling rom a raction (1-p) o the tube. The actor p may be applied 345

4 instantaneously to a length o the tube, or represent the proportion o the time during which a single point is traversed by translating bubbles. The resulting heat transer coeicient may thus be determined rom Eq. (3), α = pα + 1 ( p) α nb 1 t (3) where α 1 is the mean total heat transer coeicient or the upper tube, α nb is the nucleate boiling heat transer coeicient at the appropriate heat lux, and α t is the heat transer coeicient through the thin ilm laid down by a translating bubble originating rom the lower tube. Under boiling conditions without the lower tube the total heat transer coeicient is given by the nucleate boiling alone; The experimental analysis allows the estimation o p by the ollowing reasoning. Fig. 5 shows the enhancement to be zero at a heat lux o 15kW/m at all values o lower tube heat lux. Since observation indicates that p is not zero, then at this point α(t) =α(nb) and rom reerence to Fig 3 or lower tube q= yields approximately α(t) =14 kw/m K. α = α nb (4) The value o α t may be more than α nb (as immediately ater a passage o the translating bubble while a layer o the liquid is evaporating under the bubble) or less than α nb (as later in the passage when the liquid layer has evaporated away or when the intensity o passing bubbles precludes suicient liquid reaching the tubes, typically at high heat lux). The enhancement, α, o boiling on the upper tube over that or pure nucleate boiling is given by ( α α ) α = p (5) t nb Fig. 4: Diagram showing the superimposition model or boiling o the upper tube Enhancement (kw/m K) q l ( kw/m ) Heat lux o upper tube (kw/m ) q1=19 q=37 q3=61 q4=91 q5=17 q6=168 Fig 5: Variation o enhancement against heat lux o upper tube or distilled water This value may also be estimated by theoretical consideration o the thin ilm under the bubbles or this part o the tube. It is assumed that the heat transer due to translating bubbles on the same tube is essentially through a thin ilm rather like that under sliding bubbles on a large tube. Appendix 1 shows the analysis o conduction heat low through such a thin ilm thickness δ is given as k α t = (6) δ 346

5 where Tk δ = δ i ( ) t (7) ρ h g From substitution o reasonable observed values rom experimental and also rom previous work on sliding bubbles, it is evident that or small diameter tubes δ δ i. That is the thin ilm suers very little thickness change due to evaporation during the short passage time (approximately a microsecond) o the enveloping bubble. Hydrodynamic analysis o the initial thickness o layer under a bubble on a surace in water by Addlesee and Kew () and thermal analysis by Kenning () have established this to be about 5µm under boiling conditions at 1 atmosphere. Substitutions in Eq. (6) yields very approximate a similar value o 14 kw/m K or α(t) under these conditions. This corresponds reasonably well with the value estimated experimentally and gives comort in using this experimental constant value o α (t) or the estimation o p. The values o p may now be ound or each experimental point by substituting measured values o α 1, α(nb) and α(t) into Eq. (5). A plot o p against heat lux o upper tube is shown in Fig. 6. As q upper approaches 15kW/m, the level at which α(nb) = α(t), p becomes indeterminate. At higher heat luxes vigorous nucleate boiling tends to displace translating bubbles and the model, as presented here, is not valid. Further work is necessary to determine the inluence o induced turbulence and bulk luid movement on the heat transer rom the upper tube. A tentative correlation has been attempted with the limited data obtained to date. Values o p or the range 1<q upper <1kW/m have been correlated against q lower. The orm o the correlation, given in Eq. (8), and shown on Fig. 7, was chosen to meet the requirement that q = p = and q = p = 1. 6 (4*1 q) p = 1 e (8) 1.5 p q ( kw/m ) q1=19 q=37 q3=61 q4=91 q5=17 q6=168 Heat lux o upper tube (kw/m ) Fig. 6: A plot o p vs. q (upper tube) using experimental values 347

6 p q(kw/m ) q1=19 q=37 q3=61 q4=91 q5=17 q6=168 lti Heat lux o lower tube (kw/m ) Fig. 7: A plot o p against heat lux o lower tube The predicted enhancement due to translating bubbles was then calculated using Eq. (4) and Eq. (7) with α(nb)taken as that or an isolated tube and a value o 14kW/m K used or α(t).. A plot showing the enhancement predicted by the model against heat lux o the upper tube is shown in Fig. 8. This shows reasonable agreement as expected, given the limited data employed in developing and testing the correlation. However, it demonstrates that the concepts underlying the model are valid and that the inluence o lower tubes on heat transer rom tubes above them may be explained in terms o translating bubbles. Further investigation is required to determine the apparently low enhancement when the upper tube heat lux is below 37kW/m. Enhancement ( kw/m K) 5 4 ql (kw/m ) 3 q1=19 q=37 q3=61 q4=91 q5=17 q6= Heat lux o upper tube (kw/m ) Fig. 8: Correlated plot o enhancement against heat lux or upper tube CONCLUSIONS An experimental investigation has been carried out during which the boiling heat transer coeicient rom a 3mm tube was measured at various heat luxes while the heat input to a similar tube situated below the test tube was varied. The study has led to the ollowing conclusions: 1. I the heat lux rom the upper tube is held constant at a low to moderate value, the presence o a heated tube below leads to an increase in heat transer coeicient or the upper tube. 348

7 . At higher values o upper tube heat lux, the increase in heat transer coeicient with lower tube heat lux is less marked, and may be zero or negative above a certain threshold. 3. A new superposition model combining nucleate boiling with heat transer to translating bubbles has been developed which is consistent with the experimental observations at moderate heat luxes. 4. Future work will include examination o photographic and video recordings and comparison o observed values o p with those inerred rom equation 3. Work is continuing to urther develop the model by studying heat transer rom tubes in the range 1-3mm diameter and with dierent luids. The model will also be tested on tube bundles. APPENDIX 1 Tk i t h δ = δ (A6) ρ g where δ i is the initial thickness o ilm. NOMENCLATURE d o outside diameter o tube (m) I electrical current (A) l length o heated tube (m) q heat lux (kw/m ) T temperature (K) V voltage (V) p proportion o tube covered with vapour Greek Symbols α heat transer coeicient(kw/m K) change in heat transer coeicient (kw/m K) Subscripts Fig A: Heat conduction through a thin ilm For heat low through the ilm, Q dm dδ dt l = h g = ρ A h (A1) g dt Also Q = q A (A) t Equating Eq. (A1) and Eq. (A) we have qt dδ = dt (A3) ρ h g The heat transer through the ilm is by linear conduction such that T qt = k (A4) δ Substituting equation Eq. (A3) into Eq. (A4) we have δ t k T δdδ = dt ρ h δi g this gives (A5) sat saturation condition wall wall temperature o tube nb nucleate boiling t thin ilm upper upper tube lower lower tube REFERENCES Addlesee, A. J. and Kew, P. A.,, Development o the liquid ilm above a sliding bubble.trans IChemE 8 Part A,pp Chan, A. M. C. and Shoukri, M., 1987, Boiling characteristics o small multitube bundles.asme.j.heat Transer Vol. 19 pp Cooper, M. G., 1984, Heat transer rate in saturated nucleate boiling.advance Heat Transer Vol. 16 pp Cornwell, K.,198, The inluence o diameter on nucleate boiling. 7th International Heat Transer Conerence.Munich and published as "Heat Transer 198" pp Cornwell, K., 199, The Inluence o bubbly low on boiling rom a tube bundle.int. J. Heat Mass Transer Vol.33(1) pp Cornwell, K.,199, The Role o Sliding Bubbles on tube bundles. 9th Int. Heat Transer Con., Hemisphere.Jerusalem pp Cornwell, K., Einarsson, J. G. and Andrews, P. R.,1986, Studies on boiling in tube bundles. 8th International Heat transer Conerence,. Hemisphere.San Francisco, pp

8 Cornwell, K. and Houston, S. D., 1994, Nucleate Pool Boiling on horizontal tubes: a convection based correlation. Int. J. Heat Mass Transer Vol. 37(1) pp Cornwell, K., Houston, S. D. and Adlesee, A. J.,199, Sliding Bubbles heat transer on a tube under heating and cooling conditions. Proceedings o Engineering Foundation on Pool and Flow Boiling.Santa Barbara,Caliornia Cornwell, K. and Kew, P. A.,, Boiling on Column o Horizontal Wires. 8th UK National Heat Transer Conerence.Oxord Fujita, Y., 199, Heat Transer in Nucleate Boiling Outside Tube bundles-prediction or Tube Bundle Eect.Heat Transer Japanese Research Vol.1() pp Fujita, Y.,1997, Heat transer and low pattern in horizontal tube bundles under pool and low boiling conditions. Convective Flow and Pool boiling Conerence, Engineering Foundation.Germany Fujita, Y., Ohta, H., Yoshida, K. and Nishikawa, K.,1987, Boiling heat transer on horizontal tube bundles,. Proc. Heat Transer Science and Technology.Beijing, China. pp Gorenlo, D., Danger, E., Luke, A., Stephan, K., Chandra, U. and Ranganayakulu, C.,, Bubble ormation with pool boiling on tubes with or without basic surace modiications or enhancement. 5th International Conerence on Boiling Heat Transer,.Jamaica pp Hahne, E. and Mueller, J., 1983, Boiling on a inned tube and inned tube bundle.int. J. Heat Mass Transer Vol. 6(6) pp Hahne, E., Qiu-Rong, C. and Windisch, R., 1991, Pool Boiling experiments on inned tubes -an expereimental study.int. J. Heat Mass Transer Vol. 34(8) pp Jensen, M. K. and Hsu, J. T., 1988, A parametric study o Boiling Heat Transer in a Horizontal Tube Bundle.ASME.J.Heat Transer Vol.11 pp Kenning, D. B. R., Butress, O. E. and Yan, Y.,, Heat Transer to a sliding Bubble. Proc. UEF Conerence, Boiling :Phenomena and Emerging Applications,, Begell House.Alaska pp Kew, P. A. and Houston, S. D.,, The eect o diameter on boiling heat transer rom the outside o small horizontal tubes.trans IChemE Vol.8(A) pp Kumar, S., Mohanty, B. and Gupta, S. C.,, Boiling heat transer rom a vertical row o horizontal tubes.int.j.heat Mass Transer Vol. 45 pp Mostinski, I. L., 1963, Calculation o heat transer and critical heat lux in boiling liquids based on the law o corresponding states.teploenergetica Vol. 1(4,) pp Palen, J. W., Yarden, A. and Taborek, J., 197, Characteristics o boiling outside large scale multi tube bundles.aiche Symposium pp Wall, K. W. and Park Jnr, E. L., 1978, Nucleate boiling o n-pentane, n-hexane and several mixtures o the two rom various tube arrays.int. J. Heat Mass Transer Vol. 1 pp