Influence of Artificial Roughness on Convective and Boiling Heat Transfer in the Rotating Flow

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1 Influence of Atificial Roughness on Convective and Boiling Heat Tansfe in the Rotating Flo T. MAGRAKVELIDZE, N. BANTSADZE, N. LEKVEISHVILI, A. MIKASHAVIDZE, J.RUSISHVILI, KH. LOMIDZE V. Gomelaui Depatment of Poe Engineeing Poblems, A. Eliashvili Institute of Contol Systems Geogian Technical Univesity 10, Mindeli st., 0186, Tbilisi GEORGIA Abstact: - The esults of an expeimental study of the pocess of convective and boiling heat tansfe in the vessel ith stie fo smooth and ough ing-shaped pipes ae pesented. Expeimentally as found thee egimes of the influence of atificial oughness on the convective heat tansfe: 1. egime, hee oughness has no influence on heat tansfe; 2. egime of patial influence of oughness on heat tansfe; 3. fully developed oughness effect egime. It as established that ceation of to-dimensional atificial oughness on the heated suface causes the essential (~90%) intensification of convective heat tansfe in case of fully developed oughness effect egime. The similitude equation fo calculating convective heat tansfe coefficient, hich genealizes ell expeimental data both fo the smooth and the ough sufaces is poposed. In case of boiling the influence of oughness appeas on the initial stage of boiling and in case of fully developed nucleate boiling thee as no intensification of heat tansfe. Key-Wods: - Heat, Tansfe, Boiling, Roughness, Mixing. 1 Intoduction In the heat exchanges geneally, and paticulaly in stied tanks, hich ae idely used in chemical and food industy, heat tansfe pocesses fequently accompany the chemical eactions. As a esult, it is obvious that heat exchange intensification in such kind of appaatus has geat pactical value. Among of numeous methods of intensification of stiing and heat tansfe cuently the mostly effective and ell studied is the method of using eflective spaces [1]. Togethe ith this, the effectiveness of the method of atificial oughness appeaed quite high in case of tubulent heat exchange in channels [2-6] has not been investigated in case of otating flo. Fist oks dedicated to this issue, as fa as e kno, ee published by authos [7-9]. Boiling pocess in such conditions as fa as e kno as not studied as ell. 2 Test Unit The test unit (Fig.1) - stainless steel cylindical vessel (1), ith inne diamete D=200mm and height 300 mm as manufactued in ode to investigate the influence of atificial oughness on heat tansfe intensity in the otating flo. The heating lo voltage AC electic poe as supplied to expeimental ing shaped stainless steel pipe (5) via coppe conductos (7) installed though the bottom of the vessel. The oute diamete of pipe as 10 mm, aveage diamete of pipe ing -140 mm. The heated pipe ing as located at 50 mm fom the bottom of the vessel coaxially ith mixe s shaft. The admitted heat as disposed ith liquid (distilled ate) in the vessel. The all of the vessel as cooled fom outside by ate (16) using cooling jacket (13). In case of satuated boiling vessel as insulated and vapou as condensed on the ate (15) cooled coil (6). On the cove of the vessel as installed electic moto (4) connected to the shaft of the mixe (2). The paddles (3) ith vaious dimensions ee mounted on the mixe s shaft on the specific level fom the bottom of the vessel. Themocouple (10) ith case (9) as immesed in the vessel ith liquid fo measuing liquid tempeatue. Expeiments ee caied out in case hen the paddles of the mixe ee located in the diffeent levels fom the heated expeimental pipe. Impelle mixes ith vetical paddles ee used. Mixes ith vaious diametes (d), ith diffeent quantity (Z) and idth (b) of the paddles, ee used: d=35 mm, 50 mm, 65 mm, 100 mm, 120 mm, 180 mm; Z=2, 4, ISBN:

2 6; b=10 mm, 20 mm, 30 mm. The angles beteen paddles ee equal. h=0.25 mm, 0.5 mm, 1.15 mm, 1.4 mm; s/h=3.5, 7.1, 7.5, 8, 10, 20, 40. The inne suface tempeatue of the expeimental pipe as measued by thee chomelalumel themocouples, placed in Teflon chambe (12). The tempeatue of the oute suface of expeimental pipe as calculated using ell-knon fomula. The tempeatue of the distilled ate in the vessel as measued also using chomel-alumel themocouple, hich as placed in the case (10) filled ith oil. The voltage on the ends of themocouples as measued by digital multimete. Heat tansfe coefficient as detemined by fomula: q α = (1) t t f The esults of the expeiments ae pefomed using modified Re numbe in the ange: 10 4 Re ; P numbe in the ange: 2 P Convective Heat Tansfe Expeiments ee caied out both fo smooth and ough sufaces (pipes). Pat of expeimental data is shon in Fig.2-4. Fig.1 Test unit 1-Vessel; 2-Shaft; 3- Paddle;4-El. moto; 5-Ringshaped pipe; 6-Cooling coil; 7-Conductos; 8- Auxiliay heate; 9-Case; 10,11 Themocouples; 12 Chambe; 13 Cooling jacket; 14 oughness elements; 15,16-Cooling ate. Roughness elements- ie ings o ashes ee attached on the heated expeimental pipe. The height (h) of the element of the oughness and the aveage pitch (s) beteen the elements vaied: Fig.2. Relation of heat tansfe intensity fom Reynolds numbe, d=65 mm, b=10 mm; H=0. 1. Smooth suface; Rough sufaces: 2. h=0.25mm, s/h=10; 3. h=0.5mm, s/h=10; 4. h=1.15 mm, s/h=10; 5. h=1.4mm, s/h=7.1. I Accoding to fomula (3) (Smooth); II Accoding to fomula (3) (Rough). ISBN:

3 In the Fig. 2 expeimental esults fo smooth and ough sufaces ae epesented as a elation A=f (Re), hee Nu A = m k 0.25 (2) P ( D / d ) ( D / H ) Fo smooth sufaces m=0.33; k=. Fo ough sufaces m=0.38; k=0.2. These esults ee obtained fo H=0. In the Fig. 2 thee diffeent egimes can be seen: 1. Roughness has no influence on heat tansfe. 2. Regime of patial appeaance of oughness effect. 3. Regime, hee oughness effect is fully developed. In the egime 2, inceasing oughness element s height heat tansfe intensity inceases accodingly and in the egime 3 heat tansfe intensity is not elated ith oughness element s height. In the Fig. 3 expeimental data ae pesented fo smooth and ough (h=0.25mm, s/h=10) sufaces hen H=30mm. Hee up mentioned thee egimes ae seen moe clealy. Fig.4 Relation of heat tansfe intensity fom geometical paamete s/h. Re= , P=3, d=120 mm, b=10mm. 1. Expeimental data, H=0; 2.Expeimental data, H=30mm. I Accoding to fomula (3), H=0; II Accoding to fomula (3), Η=30 mm. As it is clea fom the Fig.4, the chaacte of elation of Nu /Nu s =f(s/h) is simila despite of change of Η. It as found that the optimal ange of atificial oughness geometical paamete pitch-to height atio is 7 s/h 10. In the ange 7 s/h 10 the meaning of Nu /Nu s is highe fo 20% in case of Η=30mm, than in case of H=0. The atio Nu /Nu s is the highest (1.9) in case of Η=30mm and 7 s/h 10. Based on expeimental study the folloing similitude equation as obtained: Nu = 0.82 Re l Hl. 1+ b P Z D d ( µ / µ ) 0.14 ε, 0.25 D 25b (3) Fig.3 Relation of heat tansfe intensity fom Reynolds numbe, b=10 mm; H=30 mm. 1. Smooth suface; Rough sufaces: 2. h=0.25mm, s/h=10; I Accoding to fomula (3) (Smooth); II Accoding to fomula (3) (Rough). Accoding to Fig. 2 and Fig. 3 the effect of oughness as highe hen H=30mm. Expeimental data, shon in Fig. 4 epesent elation of heat tansfe intensification on the geometical paamete (s/h). Whee in case of smooth suface ε =1, and fo ough sufaces D l Hl ε = P 1+ d b ( ( s / h)exp( 0.1( s / h))). 0.1 Z In the Fig.5 is epesented compaison beteen fomula (3) and expeimental data as a elation A * =f(re), hee * Nu A = (4) f (P, G) ISBN:

4 f (P, G) = P l Hl. 1+ b D d Z 2 D ( µ / µ ) b ε (5) Fig.5 Relation of heat tansfe intensity fom Re numbe. I Accoding to fomula (3); Expeimental data.smooth suface: b=10 mm, 1. Z=2; 2. Z=4; 3.Z=6;4. b =20 mm; 5. b=30 mm; Rough sufaces: b=10 mm, Z=2: 6. s/h=5; 7. s/h=8; 8. s/h=10; 9. s/h=20; 10. s/h=40. 4 Boiling Reseaches done in [10] sho that in case of nucleate boiling of subcooled liquid (distilled ate) flo in annulus atificial oughness inceased heat tansfe intensity in the egime of initial boiling and did not affect in case of developed nucleate boiling. To find if the simila mechanism oks in case of otating liquid flo expeiments ee done in appaatus ith stie both fo smooth and ough pipes. Flat to paddle stie as used, d=65mm. Roughness as ceated attaching ie ings to ingshaped stainless steel pipe. Atificial oughness geometical paametes ee: element height - h=1mm, pitch-to-height atio- s/h=7. Distilled ate as used as coolant. In the Fig. 6 expeimental data is pesented both fo smooth and ough pipes. Data can be divided by thee zones: convective zone, tansitional nucleate boiling zone and developed nucleate boiling zone. In the convective and tansitional nucleate boiling zones influence of atificial oughness on the heat tansfe is significant. Inceasing heat flux mentioned influence becomes less and in the egime of developed nucleate boiling is equal to zeo. Accoding to the obtained esults heat tansfe intensification due to atificial oughness in appaatus ith mixe is less, than in ough canals. This diffeence can be explained based on ideas belo. Let us conside that in mixed liquid movement consists of thee components tangential, adial and axial. If e conside that one pat of pipe is ovefloed ith tangential component and anothe ith nomal (adial and axial) fo smooth suface can be itten: Nu s = c1nus т + c2nus n (6) hee c 1 and c 2 can be detemined expeimentally. Since in ou case oughness elements ae displaced tansvesal ith tangential component of the flo, obviously heat tansfe intensification can be caused only due to this component. Taking into consideation above mentioned, Nu = c1ε Nusт + c2nusn (7) Accoding to data, obtained in the case of canals hen s/h=10-14, ε =2.4 by V. Gomelaui [3]. It can be seen fom fomula (7) that in ou case the goth of heat tansfe is less than in canals. Fig. 6 Relation beteen heat tansfe coefficient and heat flux t sc = t st - t tf =7 0 C,d=65mm, b=10mm. 1- smooth suface, n=2.33 RPS; 2- smooth suface, n=10 RPS; 3-ough suface, h=1mm, s/h=7, n=2.33rps; 4-ough suface, h=1mm, s/h=7, n=10 RPS. I- developed nucleate boiling, n=0; II, IIa- acoding to fomula (3) fo smooth and ough sufaces coespondingly, n=2.33rps; III, IIIa- acoding to fomula (3) fo smooth and ough sufaces coeespondingly, n=10 RPS. ISBN:

5 Fig. 6 shos that in the convective zone coincidence of expeimental data to the fomula (3) hen n=10 RPS is good, and hen n=2.33rps is not. This should be caused because at high RPS and accodingly at high Re numbes oughness effect is fully developed. At lo RPS oughness effect develops patially. 5 Conclusions In case of convective heat tansfe ceating to-dimensional atificial oughness on the suface of ing shaped heated pipe immesed in the appaatus ith mixe significantly inceases heat tansfe intensity; oughness effect is moe significant hen mixe and heated pipe ae placed at diffeent levels. Optimal ange of to-dimensional atificial oughness geometical paamete as found 7.5< s/h <10, hee maximal intensification of heat tansfe intensity (90%) as eached. Similitude equation fo calculating heat tansfe coefficient genealizing ell expeimental data has been obtained. In case of nucleate boiling to-dimensional atificial oughness inceased heat tansfe intensity in the egime of initial boiling, and did not in case of developed nucleate boiling. 6 Nomenclatue A [m 2 /c] themal diffusivity α [/m 2 K] heat tansfe coefficient B c [m] [-] idth of the paddles constant D [m] diamete of the vessel d [m] diamete of the mixe H [m] level diffeence beteen mixe and heated pipe H [m] level of ate in the vessel h [m] height of oughness elements λ [W/mK] heat conductivity coefficient ν [m 2 /c] cinematic viscosity n [1/c] mixe s RPS Nu=αD/λ [-] Nusselt numbe P=ν/a [-] Pandtl numbe q [/m 2 ] heat flo density Re=nd 2 /ν s [-] [m] Modified Reynolds numbe pitch beteen oughness elements t [K] tempeatue z [-] quantity of the paddles Subscipts f Fluid s sc ough Smooth Sub cooled all The designated poject has been fulfilled by financial suppot of the Geogia National Science Foundation (Gant# GNSF/ST08/7-482). Any idea in this publication is possessed by the autho and may not epesent the opinion of the Geogia National Science Foundation itself. Refeences: [1] Baginski L.N., Begachev V.I., Baabash V.M. Mixing in liquid media. (In Russian), L.,Chem., 1984, 336p. [2] Nunne W. Wameubengang und duckabfall in ouhen ohen.vdi Foschungscheft, 1956, 455s. [3] Gomelaui V. Influence of Todimensional Atificial Roughness on Convective Heat Tansfe. Int. J. Heat and Mass Tansfe, v.7, N6, 1964, pp [4] Dippey D.F. and Sabesky R.H. Heat and momentum Tansfe in Smooth and Rough Tubes at Vaious P Numbes. Int. J. Heat and Mass Tansfe, v.6, N5, 1963, pp [5] Kalinin E.K., Deitse G.A., Yakho C.A. Intensification of heat tansfe in channels (In Russian). М., Mashinostoenie, 1972, 219p. [6] Webb R.L. Ecket E.R., Goldstein R.J. Heat tansfe and fiction in tubes ith epeated-ib oughness. Int. Jounal Heat and mass tansfe. Vol.14, 4, 1971, p [7] Magakvelidze T. Bantsadze N. Lekveishvili N. Influence of Atificial Roughness on Heat Tansfe to Tubulent Mixed Liquid in a Pool. Bulletin of the Geogian Academy of Sciences, N3, 1996, pp [8] Magakvelidze T. Bantsadze N. Lekveishvili N Similitude Equations fo Calculating Heat Tansfe Coefficient in Stied Tanks. Bulletin of the Geogian Academy of Sciences, N3, 1999, pp [9] Magakvelidze T., Bantsadze N., Lekveishvili N., Lomidze Kh. //Heat tansfe intensification in stied tanks using ISBN:

6 atificial oughness method. 7-th Intenational Confeence on Heat Tansfe, Fluid Mechanics and Themodynamics. Antalya, Tukey, 2010, pp [10] Gomelaui V.I., Magakvelidze T. Sh. Expeimental Study of Influence of Todimensional Roughness on Citical Heat Fluxes and Heat Tansfe in Case of Subcooled Wate Flo Boiling. Heat Engineeing, 1976, N6, pp ISBN: