Heat transfer from in-line tube bundles to downward aqueous foam flow

Transcription

1 Energy and Sustainability II 389 Heat transfer from in-line tube bundles to downward aqueous foam flow J. Gylys 1, S. Sinkunas 2, T. Zdankus 1, R. Jonynas 2 & R. Maladauskas 2 1 Energy Technology Institute, Kaunas University of Technology, Lithuania 2 Department of Thermal and Nuclear Energy, Kaunas University of Technology, Lithuania Abstract An experimental investigation of heat transfer from in-line tube bundle to foam flow was performed. One type of aqueous foam statically stable foam was used as a coolant. The experimental setup consisted of the foam generator, experimental channel, in-line tube bundle, measurement instrumentation and auxiliary equipment. During the experiments aqueous foam initially moved vertically upward, then made a 180 degree turning and moved vertically downward crossing the in-line tube bundle. Three tube bundles with different geometry were used for investigation. Spacing among the centres of the tubes across the first in-line tube bundle was 0.03 m and spacing along the bundle was 0.03 m. In the second case spacing among the centres of the tubes across the bundle was 0.03 m; spacing along the bundle was 0.06 m. In the last case spacing was accordingly 0.06 m and 0.06 m. Initially, the first tube bundle was installed at the experimental channel, next the second tube bundle was installed instead of the previous bundle and so on. The laminar downward foam flow crossed the tube bundle and the mean velocity of foam flow was changed from 0.14 m/s to 0.30 m/s. The volumetric void fraction of foam was changed from to During an experimental investigation it was determined dependence of heat transfer intensity on flow parameters: flow velocity, volumetric void fraction of foam and liquid drainage from foam. Apart from this, influence of tube position of the bundle on heat transfer intensity was investigated. Keywords: aqueous foam flow, flow turn, in-line tube bundle, void fraction of foam. doi: /esu090341

2 390 Energy and Sustainability II 1 Introduction Application of aqueous foam flow for heated surfaces cooling is an area of our researches. Aqueous foam as coolant with small density has additional possibility to change the intensity of heat transfer by changing volumetric void fraction of foam. The foam type coolant enables to create compact, light and economic heat exchanger with simple and safe operation using two-phase foam flow. Heat exchangers with different type and geometry of tube bundles are usually used in industry. However, only heat transfer between different types of tube bundles and single-phase coolants (water, oil, air) has been investigated particularly [1, 2]. Investigation of heat transfer from tube bundles to foam flow are yet in initial stage. Therefore in our previous works heat transfer from alone cylindrical surface tube and then from tube line to upward foam flow was investigated [3]. Next an experimental series with staggered tube bundle in upward and downward foam flow followed [4, 5]. Research of heat transfer between in-line tube bundles of different geometry and upward foam flow [6, 7] was performed later. Presently heat transfer of in-line tube bundles to downward after 180 degree turning foam flow was investigated. Heat transfer from the tubes to upward and downward after 180 degree turning foam flow was investigated in order to obtain data necessary for the designing of real heat exchanger. Objectives of this research are to determine and estimate influence of in-line tube bundles geometry on the intensity of tube bundles heat transfer to foam flow, since bundles (behaving as an obstacles in a flow) induce substantial changes of foam structure: bubbles are transformed by collapsing, dividing, appearing new bubbles. This, together with gravity and capillary forces, facilitates the liquid drainage from the foam [8, 9] and affects the thickness of liquid film, composing on the heated surfaces and channel walls. Such effects can significantly increase or decrease heat transfer rates. Therefore experimental investigation of heat transfer process from three in-line tube bundles with different geometries to the vertical downward after 180 degree turning foam flow was performed. Dependence of heat transfer intensity on flow parameters: velocity, direction, volumetric void fraction and liquid drainage from foam, was determined. Apart of this, influence of tube position in the bundle was investigated. The further researches will be committed to estimate an optimal geometry of in-line tube bundle, which maximally increases the intensity of heat transfer to foam flow. 2 Experimental set-up An experimental investigation was performed on the experimental laboratory setup consisting of foam generator, experimental channel, tube bundle, measurement instrumentation and auxiliary equipment [5, 7]. One type of aqueous foam statically stable foam [3, 9] was used as coolant (heat carrier) for our experiments. Statically stable foam flow was generated from the detergents water solution. Concentration of the detergents was kept

3 Energy and Sustainability II 391 constant at 0.5% in all experiments. Foam-able liquid was supplied from the reservoir onto the special perforated plate; gas (air) was delivered through the plate from the bottom gas chamber. Foam flow was generated during gas and liquid contact. Foam flow parameters control was fulfilled using gas and liquid valves. Perforated plate for foam generation was installed at the bottom of the experimental channel and was made from stainless steel plate with a thickness of 2 mm; orifices were located in a staggered order; their diameter equal 1 mm; spacing among the centres of the holes equal 5 mm. The experimental channel was composed of three main parts: the vertical channel for upward foam flow; the channel for flow turn and the vertical channel for downward foam flow. The radius R of the channel turn was equal to 0.17 m. Cross-section dimensions of the channel were 0.14 x 0.14 m 2 ; height was equal to 1.8 m; walls were made from the transparent material in order to observe foam flow visually. The in-line tube bundle was installed in the vertical channel for downward foam flow. Three in-line tube bundles with different spacing between tubes centres were used during experimental investigation. A schematic view of the experimental channel with tube bundles is shown in the Fig. 1. Bundle No. 1 Bundle No. 2 s 1 s 1 Bundle No. 3 s 1 A1 B1 C1 s2 D1 E1 F1 s2 G1 s2 A2 B2 C2 A3 B3 C3 D2 E2 F2 G2 A4 B4 C4 d A5 B5 C5 D3 E3 F3 d G3 d A6 B6 C6 Foam Foam Foam Figure 1: In-line tube bundles: No. 1, No. 2 and No. 3. The in-line tube bundle No. 1 consisted of five columns with six tubes in each (Fig. 1). Spacing between centres of the tubes was s 1 =s 2 =0.03 m. The in-line tube bundle No. 2 consisted of five columns with three tubes in each. Spacing between centres of the tubes across the experimental channel was s 1 =0.03 m and spacing along the channel was s 2 =0.06 m. The in-line tube bundle No. 3 consisted of three columns with three tubes in each. Spacing between centres of the tubes was s 1 =s 2 =0.06 m. External diameter (d) of all the tubes was equal to

4 392 Energy and Sustainability II 0.02 m. Electrically heated tube calorimeter had an external diameter equal to 0.02 m also. Endings of the calorimeter were sealed and insulated. During the experiments calorimeter was placed instead of one tube of the bundle. An electric current value of heated tube was measured by an ammeter and voltage by a voltmeter. Temperature of the calorimeter surface was measured by eight calibrated thermocouples: six of them were placed around the central part of the tube and two of them were placed in both sides of the tube at a distance of 50 mm from the central part. Temperature of the foam flow was measured by two calibrated thermocouples: one in front of the bundle and one behind it. Measurement accuracies for flows, temperatures and heat fluxes range were correspondingly 1.5%, % and %. During the experimental investigation a relationship was obtained between an average heat transfer coefficient h from one side and foam flow volumetric void fraction β and gas flow Reynolds number Re g from the other side: Nu f = f ( β, Re g ). (1) Nusselt number was computed by formula hd Nu f =. (2) λ f Here λ f is the thermal conductivity of the statically stable foam flow, W/(m K), λ f was computed by the equation λ f = βλ g + ( 1 β ) λ l. (3) An average heat transfer coefficient was calculated as qw h =. (4) T Gas Reynolds number of foam flow was computed by formula Ggd Reg =. (5) Aν g Foam flow volumetric void fraction was expressed by the equation Gg β =. (6) Gg + Gl Experiments were performed within limits of Reynolds number diapason for gas (Re g ): (laminar flow regime) and foam volumetric void fraction (β): Gas velocity for foam flow was changed from 0.14 to 0.30 m/s. 3 Results In the case of the single-phase flow, heat transfer intensity of the fourth and further tubes of the in-line tube bundles are the same as that of the third tubes [1]. Distribution of local flow velocity across the experimental channel is the main flow parameter which influences on heat transfer intensity of different

5 Energy and Sustainability II 393 tubes of the bundle in that case. In our previous works [3, 4] there was noticed that it is different for the aqueous foam flow. Heat transfer from the tube bundle No. 1 to the vertical downward after 180 degree turning foam flow was investigated initially. Then the experiments with the tube bundle No. 2 were performed. Heat transfer intensities from the third tubes of the bundle No. 1 and No. 2 to downward wettest (β=0.996) foam flow were compared and analysed (see Fig. 2). Nu f A3 B3 C D3 E3 F Re g Figure 2: Heat transfer intensity of the tubes A3, B3, C3 (bundle No. 1) and D3, E3, F3 (bundle No. 2) to downward foam flow, β= Three main parameters [3] of the foam flow have an influence on the heat transfer intensity of the different tubes: changes of foam structure along the experimental channel, distribution of local flow velocity and distribution of local void fraction of foam across and along the experimental channel. While foam flow makes a 180 degree turn the gravity forces act across and along the foam flow. Liquid drains down from the upper channel wall and local void fraction increases (foam becomes drier) here as well. After the turn, local void fraction of foam is less (foam is wetter) on the inner left side of the cross-section (tubes A and D). Heat transfer intensity of the third tubes of the in-line bundle No. 2 is higher than that of the bundle No. 1. This phenomenon can be explained by the fact, that the effect of shadow is slight and heat transfer is higher for the tubes of the in-line tube bundle with more spacing between the tube centres along the bundle (tube bundle No. 2). Increasing foam flow gas Reynolds number (Re g ) from 190 to 410, heat transfer intensity (Nu f ) of the tube A3 increases by 1.8 times (from 677 to 1187), by 2.1 times (from 454 to 965) of the tube B3, and by

6 394 Energy and Sustainability II 1.6 times (from 219 to 345) of the tube C3 for foam volumetric void fraction β= Heat transfer intensity of the tubes D3 for the same Re g increases by 1.7 times (from 743 to 1287), by 2.1 times (from 468 to 975) of the tube E3, and by 2.3 (from 252 to 591) of the tube F3 for β= Heat transfer intensity of the tube D3 is higher than that of the tube A3 on average by 9%, the heat transfer of the tube E3 is higher than that of the tube B3 on average by 1.5%, and the heat transfer of the tube F3 is higher than that of the tube C3 on average by 56% for β=0.996 and Re g = Comparison of heat transfer intensity of the tubes A3, B3, C3 and D3, E3, F3 to the downward foam flow at the volumetric void fraction β=0.998 is shown in Fig. 3. Heat transfer intensity of the tube D3 is higher than that of the tube A3 on average by 19%, the heat transfer of the tube E3 is higher than that of the tube B3 on average by 18%, and the heat transfer of the tube F3 is higher than that of the tube C3 on average by 27% for β=0.998 and Re g = Nu f A3 B3 C3 D3 E3 F Re g Figure 3: Heat transfer intensity of the tubes A3, B3, C3 (bundle No. 1) and D3, E3, F3 (bundle No. 2) to downward foam flow, β= An average heat transfer rate of middle-column tubes was calculated in order to analyse and compare the experimental results of different in-line tube bundles. An average heat transfer intensity of the middle-column tubes of the bundle No. 1 and No. 2 to downward after 180 degree turning foam flow is shown in Fig. 4. An average heat transfer intensity of middle-column tubes of the bundle No. 2 is higher than that of the bundle No. 1 on average by 7% for β=0.997 and by 20% for β = When foam volumetric void fraction (β) is equal to heat transfer intensity of middle-column tubes of the bundle No. 1 is higher than that of the bundle No. 2 till Re g =270.

7 Energy and Sustainability II 395 Nu f (No. 1) (No. 1) (No. 1) (No. 2) (No. 2) (No. 2) Re g Figure 4: An average heat transfer intensity of the middle-column tubes of the bundles No. 1 and No. 2 to downward foam flow, β=0.996, and Next, the experiments with tube bundle No. 3 followed. Comparison of heat transfer intensity of the tubes E3 and G3 to the downward foam flow at the volumetric void fraction β=0.996, and is shown in Fig. 5. Heat transfer intensity of the third tubes of the in-line bundle No. 2 is higher than that of the bundle No. 3. Increasing foam flow gas Reynolds number (Re g ) from 190 to 410, heat transfer intensity (Nu f ) of the tube G3 increases by 1.5 times (from 369 to 549) for β=0.996, by 1.5 times (from 297 to 433) for β=0.997, and by 1.4 times (from 204 to 292) for β= Heat transfer intensity of the tube E3 (bundle No. 2) is higher than that of the tube G3 (bundle No. 3) on average by 56% for β=0.996, by 55% for β=0.997 and by 86% for β=0.998 to downward foam flow for Re g = An average heat transfer intensity of the middle-column tubes of the bundle No. 2 and No. 3 to downward after turning foam flow is shown in Fig. 6. The local foam flow velocity increases while foam passes the rows of tubes. The value of local velocity is higher in the cases of tube rows with more tubes in each (tube bundle No. 1 and No. 2). Therefore the heat transfer intensity from the tubes of the bundle No. 3 to foam flow is less in comparison with tube bundles No. 1 and No. 2. Changing Re g from 190 to 410, an average heat transfer intensity of the middle-column tubes of the in-line bundle No. 2 to downward foam flow increases by 2.4 times for β=0.996; by 2.1 times for β=0.997, and by 1.8 times for β=0.998; and that for the tubes of the in-line bundle No. 3 is by 1.5 times for β=0.996; by 1.4 times for β=0.997, and by 1.3 times for β=0.998.

8 396 Energy and Sustainability II Nu f E3 (0.996) E3 (0.997) E3 (0.998) G3 (0.996) G3 (0.997) G3 (0.998) Re g Figure 5: Heat transfer intensity of the tubes E3 (bundle No. 2) and G3 (bundle No. 3), β=0.996, and Nu f (No. 2) (No. 2) (No. 2) (No. 3) (No. 3) (No. 3) Re g Figure 6: An average heat transfer intensity of the middle-column tubes of the bundles No. 2 and No. 3 to downward foam flow: β=0.996, and An average heat transfer intensity of the middle-column tubes of the in-line bundle No. 2 is higher than that of the tubes of the in-line bundle No. 3 on average by 43% for β=0.996, by 48% for β=0.997 and by 67% for β=0.998 to downward foam flow for Re g =

9 Energy and Sustainability II 397 Experimental results of investigation of heat transfer of the in-line tube bundles to downward after 180 degree turning statically stable foam flow were generalized by criterion equation using dependence between Nusselt number Nu f and gas Reynolds Re g number. This dependence within the interval 190<Re g <410 for the in-line tube bundle in downward foam flow with the volumetric void fraction β=0.996, 0.997, and can be expressed as follows: n m Nu f = cβ Reg. (7) On average, for whole middle-column (B) of the in-line tube bundle No. 1 (s 1 =s 2 =0.03 m) to the downward foam flow: c=16.1, n=518, m=140.7(1.003 β). On average, for whole middle-column (E) of the in-line tube bundle No. 2 (s 1 =0.03 and s 2 =0.06 m) to the downward foam flow: c=29.6, n=952, m=202.5(1.002 β). On average, for whole middle-column (G) of the in-line tube bundle No. 3 (s 1 =s 2 =0.06 m) to the downward foam flow: c=29.4, n=53.3, m=59(1.005 β). 4 Conclusions Heat transfer from three in-line tube bundles with different geometries to downward laminar statically stable foam flow was investigated experimentally. Three main parameters of foam flow influence on heat transfer intensity of different tubes of the bundles: changes of foam structure along the experimental channel, distribution of local flow velocity and distribution of local foam void fraction across and along the experimental channel. The effect of shadow is slight and heat transfer intensity is higher for the tubes of the in-line tube bundle No. 2 (with more spacing between the tube centres along the bundle) in comparison with tube bundle No. 1. Heat transfer is higher for the tubes of the in-line tube bundle No. 2 (with less spacing between the tube centres across the bundle) in comparison with tube bundle No. 3. Results of investigation were generalized by criterion equation, which can be used for the calculation and design of the statically stable foam heat exchangers with in-line tube bundles. Nomenclature A cross section area of experimental channel, m 2 ; c, m, n coefficients; d outside diameter of tube, m; G volumetric flow rate, m 3 /s; h average coefficient of heat transfer, W/(m 2 K); Nu Nusselt number; q heat flux density, W/m 2 ; Re Reynolds number; T average temperature, K; β volumetric void fraction; λ thermal conductivity, W/(m K); ν kinematic viscosity, m 2 /s. Indexes f foam; g gas;

10 398 Energy and Sustainability II l liquid; w wall of heated tube. References [1] Zukauskas, A., Convectional Heat Transfer in Heat Exchangers, Moscow: Nauka, pp , [2] Kuppan, T., Heat Exchanger Design Handbook, New York: Marcel Dekker, pp , [3] Gylys, J., Hydrodynamics and Heat Transfer Under the Cellular Foam Systems, Kaunas: Technologija, pp , [4] Gylys, J., Zdankus, T., Miliauskas, G.; Sinkunas, S., Influence of vertical foam flow liquid drainage on tube bundle heat transfer intensity. Experimental Heat Transfer, 20(2), pp , [5] Gylys, J., Sinkunas, S., Zdankus, T., Analysis of staggered tube bundle heat transfer to vertical foam flow. International Journal of Heat and Mass Transfer, 51(1 2), pp , [6] Gylys, J., Zdankus, T., Sinkunas, S., Giedraitis, V., Study of In-line Tube Bundle Cooling in Vertical Foam Flow, WSEAS Transactions on Heat and Mass Transfer, 6(1), pp , [7] Gylys, J., Sinkunas, S., Zdankus, T., Giedraitis, V., Different type tube bundle heat transfer to vertical foam flow. Proc. of the 5th International Conference on Nanochannels, Microchannels and Minichannels, ICNMM 2007, Pueble, Mexico, pp , [8] Nguyen A. V., Liquid Drainage in Single Plateau Borders of Foam. Journal of Colloid and Interface Science, 249(1), pp , [9] Tichomirov V., Foams. Theory and Practice of Foam Generation and Destruction, Chimija: Moscow, pp , 1983.