Amir Houshmand, Ahmad Sedaghat, Kia Golmohamadi and Mohamadreza Salimpour

Transcription

1 J. Energy Power Sources Vol. 1, No. 4, 2014, pp Received: July 19, 2014, Published: October 30, 2014 Journal of Energy and Power Sources Experimental Study on Thermal and Hydrodynamics of TiO 2 /Water Nanofluid Turbulent Flow in a Constant Wall Temperature Circular Tube with Twisted Tape Inserts Amir Houshmand, Ahmad Sedaghat, Kia Golmohamadi and Mohamadreza Salimpour Department of Mechanical Engineering, Isfahan University of Technology, Isfahan , Iran Corresponding author: Ahmad Sedaghat (Sedaghat@cc.iut.ac.ir) Abstract: This study presents the performance of water/tio 2 nanofluid in a horizontal circular tube with twisted tape inserts for enhancing convective heat transfer. Experiments were conducted in the tube under constant wall temperature condition. Factors considered include the mass flow rate, the twist ratio of the tape and the volumetric portion of nano-particles to the water. The nanoparticles volume concentration varies within the range of 0 ϕ 5 %, and the twisted tape inserts utilize twist ratio of 0 H/D 15. The Reynolds number variation is within the turbulent flow regime of 3000 < Re < 2200 The measured results revealed that the addition of nanoparticles as well as twisted tape inserts had enhanced the convective heat transfer in all studied test cases. However, the effects of nanoparticles are more pronounced in high Reynolds number flows. Combining the positive effects of nanofluid and twisted tape, the convective heat transfer was significantly increased up to % in the test case with volumetric concentration of 5 % nanofluid and twist ratio of H/D = 5 at Reynolds number of 2000 Favorably, the addition of nanoparticles to the base fluid and twisted tape insert has marginally increased the pressure drop coefficient. Appropriate correlations are introduced for the thermal and hydrodynamics characteristics of the combined TiO 2 /water nanofluid and twisted tape inserts in circular tubes. Key words: Experimental heat transfer, TiO 2 /water nanofluid, twisted tape insert, convection, constant wall temperature. Nomenclature: Latin symbols C Specific heat, Jkg -1 K -1 D Tube diameter, m f Friction factor H Pitch of the twisted tape h Convective heat transfer coefficient, Wm -2 K -1 k Thermal conductivity, Wm -1 K -1 l Length of the tube, m ṁ Mass flow rate, kgs -1 Nu Nusselt number Pr Prandtle number Re Reynolds number T Temperature, K Greek symbols T Temperature difference, K P Pressure difference, Pa η Performance factor of the twisted tape ϕ Volumetric concentration of nanoparticles, % μ Dynamic viscosity, kgm -1 s -1 ρ Density, kgm -3 Subscripts w Water nf Nanofluid np Nanoparticles PT Plain tube TT Twisted tape 1. Introduction Fluids containing particles smaller than 100 nanometers are known as the nanofluid. In 1995 Choi [1] in Argone Institute, was the first that discovered nanofluid. As he said the term nanofluid refers to a suspension of nanoparticles in a base fluid. Nanoparticles used in these fluids often contain metal particles, metal oxides, carbides and carbon nanotubes

2 218 in conventional base fluids such as water, oil and ethylene glycol. From the theory of heat transfer for a constant Nusselt number, heat transferr coefficient is directly related to thermal conductivity of the fluid. Therefore, many researchers have attempted to accurately measure the thermal conductivity and the other thermo-physical properties of nanofluid. Masuda et al. [2] studiedd the properties of Al 2 O 3 and TiO 2 nanoparticles dispersed in water. Significant increase in the thermal conductivity of the nanofluid compared with water was reported. For example, the thermal conductivity of Al2O 3 and TiO 2 nanofluid with 3.4 % concentration has respectively showed an increase of 32 % and 11%. Pak and Cho [3] investigated the forced convective heat transfer of Al 2 O 3 and TiO 2 nanofluid in a horizontal tube with constant flux boundary condition on the wall of the tube. They also studied the properties of these fluids. They reported that the relative viscosity of water/tio and water/al2o 3 10 % nanofluid were increased by 3 and 200 percent, respectively; which is much larger than the values predicted by Batchelor equation [4]. In 2009 Duangthongsu and Wongwises [5] evaluated the thermal conductivity and viscosity of water/tio 2, and concluded that the thermal conductivity increased considerably with increasing temperature. They also reported that a direct relationship for the concentration of nanoparticles-viscosity exist which decreases by temperature. In addition, the viscosity of nanofluid was directly increased by ncreasing amount of the nano-particles and decreased with increasing temperature. One method of enhancing heat transfer in heat exchanger tubes is use of twisted tapes (Fig. 1). This increases heat transfer by means of (1) increasing heat transfer areaa at a constant volume; and (2) with increasing mixing augmentation and turbulence of the flow. Heat transfer and pressure drop characteristics of the systems with twisted tape are quite different from regular O 2 Fig. 1 Schematic of twisted tape for tube insertion. systems of heat exchanger tubes. Researchess in this area have been divided into two groups: (1) numerical and analytical studies; (2) experimental works. As noted above, several factors are involved in determining the nanofluid properties, so researchers have reported their models with examining each or more factors. Despite all these works, there still no inclusive and universal relationship has yet derived for nanofluid thermo-physical properties. The study of heat transfer and friction factor of systems with the twisted tapes can be pointed categorized the passive and to Bergles [6] who active methods of enhancing convective heat transfer in heat exchangers. Manglic and Bergles [7-8] found that the following factors are the most important factors in improving the heat transfer using twisted tapes: (1) increasing the flow rate divided by pipe insertion; (2) hydraulic diameter is reduced, which increases the heat transfer coefficient; (3) increase the path length due to the spiral shape of the twisted tape; (4) improve the heat transfer coefficient due to the secondary flow generated by the twisted tape; (5) the fin effects, if the tape connection with the tube wall is complete. Greatest factor in this improvement can be related to the horizontal mixing, due to the centrifugal force. For the experimental measurements of the heat transfer of nanofluid flow in a tube with twisted tape insert, few studies have been reported so far. Patipalka and Sivashanmugamm [9] numerically investigated the convectional heat transfer of Alumina nanofluid in a tube with constant heat flux boundary condition and with a twisted tape with twist ratio of They reported % enhancement in heat transfer r in Re = In another work Sharma et al. [10], experimentally determined heat transfer coefficient and friction factor in

3 219 transition flow regime for low concentration Alumina nanofluid with constant heat flux boundary condition. They conducted their experiments for a plain tube and with twisted tape insert. Finally Sundar et al. [11-12] studied the effect of twisted tape on heat transfer and friction factor of water/al 2 O 3 and water/magnetic Fe 3 O nanofluid flow in a plain tube under constant heat flux boundary condition. They presented correlations for determining Nusselt number and friction factor for their experiments. In this paper, a detail of the present experimental apparatus is explained in section 2. The thermophysica al properties of the test fluid are given in section 3. Section 4 summarizes calculation of friction factor. Heat transfer measurements are given in section 5 and conclusion is drawn in section Experimental Apparatus Fig. 2 shows a schematic view of the present experimental apparatus. This setup is designed and prepared for studying heat transfer and friction factor characteristics of flow in a plain tube and with twisted tape inserts in a constant wall temperature of a circular tube. Flow circuit is made of a circulation pump, reservoir, heat exchanger, test section, and a flow meter. A bypass valve was used with a line to reservoir to accurately control the desirable flow rate. The test section consists of a cubic water tank with five elements of 2000 W power at its bottom to heat up water and generate required steam. A copper tube in the test section was used with 935 m length and inner and outer diameter of 8 mm and 10 mm, respectively. The dimensions of the three twisted tape inserts in this tube are listed in Table 1. The fluid passing through the tube is heated up with the amount of surrounded steam. The flow is circulated with a pump of 5 hp. The flow rate is monitored by a flowmeter. Two thermocouples were located to sense the bulk temperatures of inlet and outlet of flow and five thermocouples were used to measure the surface O 4 Fig. 2 Schematic view of the experimental setup. Table 1 Geometrical characteristic of the twisted tapes. H (mm) D (mm) H/D ( mm) temperature of the copper tube. All of these are the K-type thermocouples and were connected to a temperature indicator. With two crotches before and after the test section and connecting those to a manometer the pressure drop in the section were measured. The flow enters to a Cross flow heat exchanger after the test section. Then, the fluid flows to a reservoir tank (a tank with 5 L capacity) and has been sucked from the bottom of it. To full vacation of circuit after each use, an air ventilation system was used. The connecting pipes of the system are mainly from 4-layered hoses and in certain parts are from galvanized steel pipes. Principally nanofluid is prepared in two ways, including one-step and two-step preparation methods. In the first one the particles are directly dispersed in the base fluid, but in the second one, initially the nanoparticles are produced and then added to the base fluid using an ultrasonic vibrator. We used the two-step method in this work to prepare the nanofluid. At first we dispersed particles in the distilled water. To complete preparation process the nanofluid were sonicated for 2 hours in an ultrasonic vibrator to stabilize the suspension of water and nanoparticles. 3. Evaluation of the Thermophysical Properties of the Test Fluid

4 220 In this study TiO 2 nanoparticles with 99 % pureness are utilized. The shape of these particles is spherical and average diameter of 15 nm. With studying heat transfer and pressure drop behavior of any fluid require knowledge of the thermophysical properties of that fluid. In the present work, some of the widely used correlations were adopted to calculate the thermophysical properties of the used nanofluid. The density and specificc heat of the suspension are calculated by Pak and Cho [3] laws, as written below: 1 (1 1) C 1 C Also Maxwell s [13] relation was used to estimate the thermal conductivity of the nanofluid: knf knp 2kw 2 ( knp kw ) k k 2 k ( k k ) viscosity of the nanofluid: nf w nf np w C (2 np Here, the factor 2.5 corresponds to a solid sphere model of the nanoparticles. w w np w (3) Enstine s equation [14] is used to determine the 2) (4) Fig. 3 Variation of friction factor versus Reynolds number for water flow in circular tube. 4. Pressuree Drop Measurements Pressure drop measurements were accomplished in a hydrodynamically fully developed region. Then, the experiments hanged on with the twisted tape insertions. The equation for friction factor is defined as: P f In this relation, the Reynolds number i l D 2 v 2 (5) We used Blasius equation [15] to determine the friction factor: f 3164 Re 25 (6) is 4ṁ. Also, Q πd i μ is volumetric flow rate, ρ is density, D i is internal diameter of the copper pipe, and μ is the dynamic viscosity of the fluid. With above relations, we have compared the Blasius expressions and present experimental results in Fig. 3. Clearly, there is a good agreement between these two sets of results. Fig. 4 Variation of friction factor for different nanofluids versus flow Reynolds number in the circular tube. Fig. 1 shows a schematic view of a twisted tape with twist length of H, and width of D. We used the width of the tapes 1 mm less than the inner diameter of the pipe. In Fig. 4, the experimental results are shown for the friction factor of different nanofluid flows in the plain tube. With increasing of the Reynolds number, there is no a considerable difference between various nanofluid friction factors as we see in this figure. We may relate this behavior to the mixing of the flow by its turbulence than the mixing by nanoparticles. Also with ncreasing in nanoparticle concentration, the friction factor will be increased. This is expected, because nanoparticle acrostic

5 221 Fig. 5 Friction factor ratios of different geometries of twisted tapes to plain tube for 2 % vol. nanofluid flow. twisted tapes at the same Reynolds number, so we could relate twisted tape friction factors for a specific Reynolds number to the plain tube. The friction factor of the flow in a plain tube and with twisted tape inserts is a function of the Reynolds number, the volumetric concentration of the nanofluid, and the twist ratio of the tape inserted. So we have a functional form as below: Using linear regression of the experimental data, some correlations are obtained for the friction factor as follow: Re Re f Re Re The introduced f Re,, H D experimental data in the error band of ( %, %) as shown in Fig ,H D 0,H D 15,H D 10,H D 5 (7) (8) correlation (8) predicts the 5. Convective Heat Transferr Measurements Fig. 6 Comparison of the experimental data with correlation (8) for predicting the friction factor. motions in the flow are intensive to provoke this undesirable increase in the friction factor. The primary reason may also be related to the increase in dynamic viscosity. From Fig. 4, the maximumm increase in friction factor of the 5 % vol. nanofluid flow with the tapes with twist ratios of 15 and 5 in Reynolds numbers of and 8806 are observed as % and 19.4 %, respectively. In Fig. 5 we showed friction factor ratio of the twisted tapes per plain tube for 2 % vol. nanofluid flow. It is necessary to obtain this ratio for different The main goal of these experiments is to gain the average Nusselt number of fluid flow in a plain tube and with various twisted tape inserts. We may determine the convective heat transfer coefficient by Newton's law of refrigeration as follows: mc P ( Tout Tin ) h (9) D.. l TLM MTD Where TLMTD is the logarithmic mean temperature Tout Tin difference of flow ( TLMTD lnts Tin Ts Tout ). To determine Nusselt number, we can use the general expression as follow: hd Nu (10) k The obtained experimental results have been compared to the well known model Eqs. (11)-(12) to evaluate the validity of the experimental results Nu 027 Re Pr, s (11) 7 Pr 16, 700 Re 10000, LD10

6 222 Nu 5 015Re These equations are valid in the ranges shown. Eqs. (11)-(12) are adopted from the works of Sider and Tate [16] and Notter and Slicher [17], respectively. As shown in Fig. 7, the maximum differences between Eqs. (11)-(12) and the present experimental data are 5.37 % and 5.53 %, respectively. It is observed that with increase in the Reynolds number resulted in the increase in mass flow rate and with growth in flow turbulence, the heat transfer coefficient and the Nusselt number of the flow are increased. enhancement of this factor is observed with using of tapes with less twist ratios. Secondary flow generation and increasing in augmentation of the flow may be the main reasons of this enhancement. In Fig. 9, we showed Nusselt number variation for different concentration of nanofluids in the tube with H/D = 5. By adding nanoparticles to the base fluid, the nature of the flow is transformed from single phase into two-phase flow. On the other hand, the thermal conductivity of the fluid is increased. In addition, dispersal effects of the nanoparticles, random motions, and distributional effects of them are the intensifier factors to enhancement in heat transfer. It seems that with reduce in the twist ratio, the centrifugal forces are increased and a good mixing in bulk of the flow occurred. For a clear comparison between different twisted tapes, we show the ratio of Nusselt number of the twisted tapes to plain tube in Fig. 10 for 2 % vol. nanofluid flow. As we see from this figure, the maximum increase is observed for a tape with twist ratio of 5 and the maximumm value of this ratio is at Reynolds number of The Nusselt number of the fluid flow in a circular tube and with twisted tapes insert is a function of the Reynolds number, Prandtle number, volumetric concentrationn of the nanofluid, and the twist ratio of the Pr Pr 10, 10 Re 10, (12) In Fig. 8, we see the heat transfer coefficient variation of water flow in a tube with different geometries of twisted tapes insert. Considerable Fig. 7 Nusselt number validations of the present experimental data for water flow in circular tube. Fig. 8 Heat transfer coefficient variation with the Reynolds number for water flow in circular tube with various twisted tape inserts. tapes. The functional relation is: Nu Re,Pr,, H D A regression fit equation for the Nusselt number of water and nanofluid flow, with or without the twisted tapes are derived by the least square method as follow: Re Pr 1,H D Re Pr 1,H D 15 Nu (14) Re Pr 1,H D Re Pr 1,H D 5 As shown in Fig. 11, the obtained correlation equations (13)

7 223 Fig. 9 Nusselt number variation for different concentration of nanofluids in the tube with twisted tape insert with twist ratio of H/D = 5. Fig. 111 Compariso on of the experimental data with correlation Eq. (14) for predicting Nusselt number of the flow. Fig. 10 Nusselt number of different twisted tapes insert to the plain tube for the flow of 2 % vol. nanofluid. predict the experimental data with an error confine of (-7.28 %, %). As mentioned in the previous sections, using of both the twisted tapes and nanofluid may cause with more pressure drop. On the other hand, this procedure is resulted for the heat transfer. We must evaluate the quantitative proportion of prominence of heat transfer to pressure drop. To show the quantitative value of profit of using the twisted tapes, we defined the twisted tape performance factor as below: NuTT Nu PT ftt fpt 1 3 (15) Fig. 12 Performance factor of twisted tape inserts. where Nu TT, NuPT, f TT and f PT are Nusselt number with using of the twisted tape, without it, friction factor with using of the twisted tape, and without it, respectively. The results are presented in Fig. 12. The maximum performance factor is for 5 % vol. Nanofluid and with the twisted tape of H/D = 5 at the Reynolds number of Conclusions We used water/tio 2 nanofluid and twisted tape inserts to study the thermal and hydrodynamical behavior of the flow. As the results obtained in this

8 224 investigation, we have: 1. Validation of the experimental results with relations of illustrative thermophysical properties is validated for constant wall temperature boundary conditions in the present study. 2. The concluded regression correlations on the friction factor of various nanofluid flow in a circular tube with different twisted tape inserts is given by: Re 1,H D Re 1,H D 15 f Re 1,H D Re 1,H D 5 3. The resulted regression correlation for the Nusselt number of various nanofluid flow in a circular tube with different twisted tape inserts is obtained as: Re Pr 1,H D Re Pr 1,H D 15 Nu Re Pr 1,H D Re Pr 1,H D 5 4. The performance factor in the range of these experiments is usually greater than unity. This conclude that using of both twisted tape and water/tio 2 nanofluid can be useful to enhance thermal behavior of a heat exchanger with constant wall temperature boundary condition. References [1] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, in: D.A. Siginer, H.P. Wang (Eds.), Developments and Applications of Non-Newtonian Flows, ASME, New York, NY, USA, 1995, pp [2] H. Masuda, A. Ebata, K. Teramae, N. Hishinuma, Alteration of thermal conductivity and viscosity of liquid by dispersing ultra-fine particles (dispersions of γ-al 2 O 3, SiO 2, and TiO 2 ultra-fine particles), Netsu Bussei 7 (4) (1993) (in Japanese) [3] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Experimental Heat Transfer 11 (2) (1998) [4] G.K. Batchelor, The effect of Brownian motion on the bulk stress in a suspension of spherical particle, J. of Fluid Mech. 83 (01) (1997) [5] W. Duangthongsu, S. Wongwises, Measurement of temperature-dependent thermal conductivity and viscosity of TiO 2 -water nanofluids, Exp. Thermal and Fluid Sci. 33 (2009) [6] A.E. Bergles, Techniques to augment heat transfer, in: W.M. Rohsnow et al. (Eds.), Hand Book of Heat Transfer Applications, 2nd ed., [7] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes: Part I Laminar flows, J. Heat Trans. 115 (1993) [8] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes: Part II Transition and turbulent flows, J. Heat Trans. 115 (1993) [9] G. Pathipakka, P. Sivashanmugam, Heat transfer behaviour of nanofluid in a uniformly heated circular tube fitted with helical inserts in laminar flow, Superlattices and Microstructures, 47 (2) (2010) [10] K.V. Sharma, L.S. Sundar, P.K. Sarma, Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al 2 O 3 nanofluid flowing in a circular tube and with twisted tape insert, Int. Comm. in Heat and Mass Trans. 36 (5) (2009) [11] L.S. Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al 2 O 3 nanofluid in circular tube with twisted tape inserts, Int. J. of Heat and Mass Trans. 53 (7) (2010) [12] L.S. Sundar, N.R. Kumar, M.T. Naik, K.V. Sharma, Effect of full length twisted tape inserts on heat transfer and friction factor enhancement with Fe 3 O 4 magnetic nanofluid inside a plain tube: An experimental study, Int. J. of Heat and Mass Trans. 55 (11-12) (2012) [13] J.C. Maxwell, A Treatise on Electricity and Magnetism, Vol. 1, Clarendon Press, [14] A. Einstein, Berichtigung zu meiner Arbeit: Eine neue Bestimmung der Moleküldimensionen, Annalen der Physik 339 (3) (1911) [15] F.P. Incropera, A.S. Lavine, D.P. DeWitt, Fundamentals of Heat and Mass Transfer, John Wiley & Sons Incorporated, [16] E.N. Sieder, G.E. Tate, Heat transfer and pressure drop of liquids in tubes, Industrial & Engineering Chemistry 28 (12) (1936) [17] R.H. Notter, C.A. Sleicher, A solution to the turbulent Graetz problem III fully developed and entry region heat transfer rates, Chemical Engineering Science 27 (11) (1972)