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DOI: http://dx.doi.org/10.20534/ESR-17-3.4-102-104
Sukhrob Telyaev Kudratillayevich, Junior Researcher of laboratory "Ion plasma technologies and Thermo multiphase systems", department of Engineering Physics,
Tashkent State Technical University named after Islam Karimov, Tashkent, Republic of Uzbekistan.
Iskandarov Asilbek Akrom ugli, Bachelor, department of Thermal Engineering, Tashkent State Technical University named after Islam Karimov, Tashkent, Republic of Uzbekistan.
E-mail: asilbek.iskandarov17@gmail.com
Measurement of coefficient of convective heat transfer based on silicon oxide nanofluid in the cylindrical channel
Abstract: Coefficient ofheat transfer is investigated experimentally in nanofluidits flow in the cylindrical channel. The nanofluid has been prepared on the basis of distilled water and nanoparticle SiO2 Concentration of nanoparticles was varied ranging from 0.5 to 5% by volume. Significant intensification of heat transfer is established. At particle concentrations above 0.5%, the nanofluid was Non-Newtonian. Consequently, estimates of the rheological parameters of the nanofluid and coefficient ofthermal conductivity. Keywords: Thermal conductivity, nanofluid, viscosity, laminar-turbulent transition, coefficient of heat transfer.
Attempts to use liquid using microparticles for objectives of heat exchange intensification are known since the mid 70 's. On the way to achieve significant effects failed because large particles of sediment quickly enough. The liquid in which the component is dispersed nanoparticles (nanofluid), deprived of this shortcoming. Early experiments have shown that even small additives nanopar-ticles to fluids can lead to significant growth in its thermal conductivity and heat transfer and critical heat flow can be increased many times (see, for example, [1-4]).
Despite the huge amount of work, in which thermal conductivity of nanofluid and their heat transfer is studied, the results are often contradictory. In most works of increasing heat transfer using nanoparticles. There are, however, and publications, which demonstrates its decrease when you add nanoparticles [5]. There is a need to further study of all specified processes. The key in this issue is not thermal conductivity of nanofluid, that's its heat transfer coefficient. In this work for water based nanofluid and nanoparticle, SiO2 is experimentally studied.
Through pump fluid from the tank served in a heated area, after which enters the thermostat. Flow in the circuit is governed by Voltage transformer system. Plot is a heated copper cylindrical pipe with a wall thickness of 1 mm, diameter of 15 mm and a length of 1 m as the heater uses a coiled up on a nichrome channel thread thickness 0.1 mm with an overall resistance 320 n. Channel with the heater is insulated. Heating power regulated power source Lauda Alpha A 24. For measurement of the local temperature channel on its walls at equal distances from each other docked 6 Chromel-Copel thermocouples. As a heat carrier, a nanofluid was used based on distilled water and SiO2 nanoparticles with volume concentration y, equal to 0.5, 0.75, 1.5, 2, 3 and 5%. For preparation of a nano-
fluid applied standard two-step process and used SiO2 powder production of "Evonik Industries AG ", (63403 Hanau, Germany). Spherical nanoparticles, the bulk density of the powder equal to 2.2 g/cm 3. The average size of the nanoparticles was 12 nm. After add the required amount of water tank of nanopowders to destroy conglomerates nanoparticles fit in an ultrasonic mixer UZDN-4. Liquid consumption path changed from 50 to 550 g/min these ranges correspond to a laminar flow for all carriers, except water and nanofluid with concentration of 0.5%. In the last case, laminarturbulent transition occurs since the flow of about 400 g/min. Thus, three liquids laminar flow regime had taken place, and the fourth is laminar and transition. Conducted measurements showed that adding nanoparticles significantly increase local and convective heat transfer coefficients of the fluid medium. The degree of this extend rises with increasing concentration of nanoparticles. At low flow quantity (up to about 300 g/min), when with certainty we can talk about laminar flow mode as for the nanofluid, and water, the extent of this increase are growing almost proportional to the bulk concentration of nanoparticles. With further increase in consumption observed the rise in the average heat transfer coefficient of water. Starting with 350 g/min flow quantity average heat transfer coefficient of water compared with the corresponding coefficient for nanofluid with small concentrations of nanoparticles, and then begins to exceed them. At the expense of the order of400 g/min average heat transfer coefficient ofwater becomes higher than 2 per cent of a nanofluid. Such behaviour is associated with the turbulence of the water flow. This also indirectly indicates, that turbulence of nano-fluids at given flow quantity does still not occur. Laminar-turbulent transition is determined by the number of Reynolds. Given the expense of variation values for water and nanofluid can be associated
Measurement of coefficient of convective heat transfer based on silicon oxide nanofluid in the cylindrical channel
only with changes in density and coefficient of nanofluid viscosity. But when the volumetric concentrations of nanoparticles change the density of the liquid only leads to slight (no more than a few per cent) Reynolds number change and may not significantly shift the boundary of laminar-turbulent transition. Therefore, it is important examine the viscosity of a nanofluid. Viscosity of nanofluid experimentally studied in the last decade systematically (see [6-9]
quoted there literature). Reliably determined that adding nanoparticles, firstly, significantly increases the viscosity of the liquid carrier, and secondly, can lead to changes in fluid rheology. Viscosity of nanofluid experiments submitted was measured by means of rotation viscometer Rheostress 600 at a temperature of 20oC. Accuracy was about 1%. First of all, it was found that the rate of investigated nanofluid viscosity significantly depends on the speed of the shift.
Figure 2. dependence of viscosity of nanofluid from shear
Table 1. - Dependence of rheological parameters of a nanofluid
9,% n K
5 0.255 0.03333
3 0.387 0.02511
2 0.502 0.00709
1.5 0.697 0.06433
0.75 0.203 0.01036
0.5 0.173 0.01039
perature the walls of the tube, according to the average of six received thermocouples, Cp - the heat carrier Cp = p - 1 [(1 - 9)pf Cpf + 9 ppC], where p^ - the density of the fluid transport pp - the density of the material of nanoparticle, Cpf and Cp - heat carrier liquid and material particles, respectively.
According to Mikheyev's formula, we can dedicate that, Nu = 0.021-Re a8-Pr 043
Table 2. - The dependence of the effective thermal conductivity coefficientfrom concentration
In Figure 2 provided dependence of effective viscosity coefficient y of nanofluid from speed shift j. Viscosity coefficient of a nanofluid with minimal concentration of nanoparticles (0.25%) does not change with increasing speed shift, i. e. the nanofluid is Newtonian. All the rest nanofluid are non-Newtonian. Found that their rheology is well described by the model power liquid:
y = K Jn-1. Included in this formula parameters are presented in Table 1. With increasing concentration of nanoparticles nanofluid index n decreases, and parameter K on the contrary, is increasing. Because of researched liquids viscosity significantly different currents of fluid carrier and nanofluid when the specified expense will match different Reynolds numbers, which means and different regimes of flow. For this reason, it is useful to examine the dependence of coefficient of heat transfer from the Reynolds number. Appropriate dependence is shown in Fig. 3, a. Because experimental nanofluid has non-Newtonian properties, the Reynolds number in this case was determined by the standard for power fluid way [7]:
pU 2-ndn
0.5 0.75 1.5 2 3 5
X 1.131 1.18 1.33 1.4 1.48 1.41
Re = -
K • 8"
3n +1 4n
where U - the average speed of the current, which is measured by expenditure, d - the diameter of the tube. The Figure 3 shows the data for average ratio a = GCp (t. - t^S"1 (t'w - f )-1. Here G - flow, S - area of the lateral surface of the tube, t, t. - temperature of the liquid at the outlet and inlet channel, ts - the average temperature of the liquid in the tube, t = (t. + to)/2, t'w - the arithmetic mean tem-
Intensification of heat transfer when using nanofluid with a fixed number of Reynolds is very significant. So, the heat transfer coefficient for 1% nanofluid more than 40% higher values for water practically at all Re. Obtained for this nanofluid dependence of Reynolds number heat transfer from an extremely cool goes up (fig. 3). Approximate dependence of the coefficient of heat transfer from the Reynolds number for the used nanofluid, you can say with certainty that the excess of heat transfer coefficient turns out twice or more. Naturally, with the decrease concentration of nanoparticles, this effect is monotonically decreasing. When the value of the Reynolds number above 2000 for water takes place laminar-turbulent move that intensifies its heat.
If we assume that Nusselt number Nu = (a d)/X from Reynolds number is universal, according to submitted by experimentation you can evaluate effective values coefficient of thermal conductivity X reviewed by nanofluid. For this, we should pick up X so that the dependence of Nusselt number from Reynolds number for these liquids coincided with similar dependent for water. Such a comparison conducted on fig. 3. As you can see, all the good data are consistent if the corresponding coefficients of thermal conductivity values are set, listed in the table 2 (here X = X/X , X and X - coefficients of thermal
v r n w n w
conductivity water and nanofluid accordingly). Received thermal conductivity coefficient on the basis of a nanofluid water and particles of SiO2 are consistent and direct its dimensions [8].
Figure. 3. Dependence of Nusselt number from the Reynolds number with various concentration
Figure 4. Dependence of coefficient of heat transfer As you can see on fig. 4, with increase concentration of nanopar-ticles SiO2, the thermal conductivity grows. However, with 3% concentration, in our case, it is the highest and at the same time the heat transfer coefficient decreases. It's also registered that with 5% concentration heat transfer coefficient is the lowest [9]. The values obtained for the thermal conductivity exceed the values predicted by Maxwell's theory, and good approximate ratio A = 1 + 28.2 y - 400 y 2
Thermal conductivity of nanofluid depends on the size and, according to data [10], increases with increasing particle size. Therefore, heat transfer coefficient can be increased, if the use of a nanofluid with larger particles. Energy is also more profitable because larger particles
from thermal conductivity with various concentration.
have lower viscosity [7; 8; 10]. Specifies the ratio for the nanofluid received here at a fixed temperature. However, in [7; 9] it was shown that at not too high concentrations of nanoparticles the dependence of the viscosity of the nanofluid on temperature is determined by the corresponding dependence of the carrier fluid. Therefore, the defining relation obtained here is also sufficient universally.
This work was partially supported by the Grant № FA-A4-F063 "Development of technology for new building materials based on vermiculite with high thermal and sound insulating properties" and "RENAFISA". We would like to thank Professor Sirojiddin Mirzaev for providing valuable feedback on this article.
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