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Heat exchangers have its major application in automobile, air condition, refrigerator, power plants, and many others. Heat transfer characteristics and performance of Copper spiral heat exchanger are investigated and compared with pure water. Nanofluid can enhance thermos-physical properties. Experiment is carried out for water based SiO
_{2}
Nanofluid with 15
nm average sized nanoparticle at varying air velocity and mass flow rate of fluid to investigate its effect on heat transfer coefficient. From the experimental data, a closed form solution for Nusselt number has been calculated using ∈
-NTU method. A new correlation has been proposed as a function of Reynolds number and Prandtl number. The heat transfer rate, effectiveness
,
has been significantly higher compare
d
to pure water and with increasing volume fraction of nanoparticles.

Radiators are used for transferring heat or thermal energy for cooling in case of automobile engine and heating in case of refrigerators and air conditions. In past few years, several configurations have been developed for heat exchangers or radiators to maximize the heat transfer rates and utilize the available space effectively [

Ramesh et al. [

Nanoparticle, because of its smaller size, always has issues related to stability and sedimentation, use of sonification and surfactant is necessary. Yu et al. [_{2} Nanofluid with different concentration of nanoparticle in car radiator. They concluded that the use of 1% concentration by volume of TiO_{2} could enhance the heat transfer efficiency up to 45% compared to pure water. Sundar et al. [

This paper is inspired by the need of co-relation for water-based Nanofluid as coolant; therefore, an experimental setup is designed using Spiral Heat Exchanger for achieving relevant data. An appropriate data treatment and analysis are conducted to calculate Nusselt No., Effectiveness, Reynold’s No. and over all heat transfer coefficients. An empirical correlation is developed between non-dimensional quantities for the setup.

Experimental setup _{2} Nanofluid passes at higher temperature with the help of pump. The spiral radiator is fixed with the help of clamps. A fan at different velocity is used to cool the Nanofluid flowing inside Spiral heat exchanger. The velocity of air is measured with the use of anemometer. Several thermocouples are used to measure the temperature of inlet and outlet Nanofluid, ambient temperature and

air temperature. A constant temperature electric heater is used to heat Nanofluid at before entering into spiral heat exchanger to a constant temperature. Rotameter measures fluid flow rate and flow control valve limits the flow rate. Nanoparticles mixed with pure water by the process of sonication to achieve homogenous mixture. Since, the nanoparticle used is amorphous hydrophilic SiO_{2}, surfactant is not required for mixture.

Constant temperature electric heater heats the Nanofluid to a constant temperature of 65˚C. Flow control valve controls the flow rate of Nanofluid entering to the inlet of the spiral tube. Once the system is in steady state, ambient temperature and fluid inlet temperature is noted, fan blows air to the spiral radiator at constant speed. Observe Outlet temperature of Nanofluid using another thermocouple and allow fluid to flow into sump again where electric heater heats fluid to 65˚C again. Repeat the experiment for various concentration of Nanofluid, different mass flow rate and air velocity. From the observed experimental data, heat transfer rate, other heat transfer coefficients is calculated and co-relation between the non-dimensional terms is developed using the Î-NTU approach and Excel regression curve by power tool. Experimental testing is carried out on the spiral heat exchanger for different volume fraction of Nanofluid and the result is compared with the pure water flow. Average inlet temperature was kept constant and outlet fluid temperature was compared at different flow rate of fluid from 15 cc/s to 100 cc/s and for different air flow velocity viz. 5, 5.5 and 6.5 m/s (

After achieving experimental data, calculation has been done using below mentioned equations and

Water | Air | |
---|---|---|

ρ | 990 | 1.225 |

c | 4183 | 1006.43 |

K | 0.64 | 0.0242 |

μ | 0.00048213 | 0.00001789 |

∅ = 0.05 | ∅ = 0.1 | ∅ = 0.2 | ∅ = 0.3 | ∅ = 0.4 | |
---|---|---|---|---|---|

ρ | 990.705 | 991.41 | 992.82 | 994.23 | 995.64 |

c | 4180.90 | 4178.81 | 4174.6 | 4170.45 | 4166.27 |

K | 0.6404 | 0.6408 | 0.6417 | 0.6425 | 0.6434 |

μ | 0.0008504 | 0.00085085 | 0.0008517 | 0.0008525 | 0.0008534 |

Volume fraction of Nanofluid

∅ = [ m p ρ p m p ρ p + m w ρ w ] × 100 (1)

Density of Nanofluid

ρ f = ( 1 − ∅ ) ρ w + ∅ × ρ p (2)

Specific heat capacity of Nanofluid [

c f = ( 1 − ∅ ) × c w + ∅ × c p (3)

Thermal Conductivity of Nanofluid [

K f = K p + K w + 2 ∅ ( K p − K w ) K p + 2 K w − ∅ ( K p − K w ) × K w (4)

Viscosity of Nanofluid [

μ f = μ w ( 1 + 2.5 ) × ∅ (5)

Reynolds number of Nanofluid passing through spiral heat exchanger

R e = ρ f d v f μ f (6)

Prandtl number of Nanofluid passing through spiral heat exchanger

P r = μ f c f K f (7)

Heat capacity of both air and fluid

C a = m a c a (8)

C f = m f c f (9)

Heat lost by Nanofluid from one end of spiral tube to other end

Q = C f × ( T f i − T f o ) (10)

Convective heat transfer coefficient of Nanofluid

h f = Q A × ( T m − T a ) (11)

Nusselt number of Nanofluid

N u = h f d K f (12)

Make use of Excel regression curve by power tool to find correlation between R e and P r , Prandtl number for given volume fraction of Nanofluid remains constant so its power can be considered constant as in Dittus-Boelter’s Equation for parallel and counter flow

N u = 0.023 R e 0.8 P r 0.3 (13)

Overall heat transfer coefficient of the system, since there are three steps involved for heat transfer which includes convection because of hot Nanofluid inside tube, conduction because of Copper tube wall, and convection from tube wall to air.

1 U = 1 h a A 2 + ln r 2 r 1 2 π L K t + 1 h f A 1 (14)

The value of C max and C min is the maximum and minimum value from C a and C f

C max = M A X [ C a : C f ] (15)

C min = M I N [ C a : C f ] (16)

Maximum Heat transfer rate is given by

Q max = C min × ( T f i − T a ) (17)

Effectiveness of spiral radiator is

∈ = Q Q max (18)

In

for constant volume flow rate of fluid up to 6.5˚C for the reason being, as the concentration increases, thermal conductivity of fluid also increases and hence the heat transfer rate.

It can be observed in

There is an increase in Nusselt number with the increasing value of Reynolds number in

Since the fluid is different, the predefined equations to calculate Nusselt No. cannot be used so a correlation is found between Nusselt No., Reynolds No. and Prandtl No.

Since the Prandtl No. is the function of viscosity, specific heat and conductivity of the fluid, and these values are predetermined, there is no variation in Prandtl number, it remains constant for the given volume concentration of Nanofluid. So the major variation is found in the coefficient and the power coefficient of Reynolds number. The Nusselt Number equation is found in the form of:

N u = c 1 R e c 2 P r c 3

Excel power correlation is used to find out the value of coefficients c 1 , c 2 and c 3 (

Effectiveness is the ratio of the actual heat transfer to the maximum heat transfer. From Figures 6-8, it can be observed that with increase in Reynolds

Volume concentration(Ø) | Air velocity, m/s | C1 | C2 | C3 |
---|---|---|---|---|

0.2% | 5 | 0.2358 | 0.3611 | 0.3 |

5.5 | 0.4469 | 0.3122 | 0.3 | |

6.5 | 0.1945 | 0.4327 | 0.3 | |

0.3% | 5 | 0.2536 | 0.3645 | 0.3 |

5.5 | 0.1667 | 0.4460 | 0.3 | |

6.5 | 0.1326 | 0.4982 | 0.3 | |

0.4% | 5 | 0.1915 | 0.4158 | 0.3 |

5.5 | 0.0764 | 0.5452 | 0.3 | |

6.5 | 0.1081 | 0.5289 | 0.3 |

no., effectiveness decreases sharply and after certain value around 10,000, effectiveness almost remains constant. Effectiveness decreases because the flow changes from laminar flow to turbulent flow and there is transition in the flow. Further, the value of effectiveness increases with the increase in volume concentration of Nanofluid, which is because higher concentration of nanoparticle in fluid will increase its conductivity, hence heat transfer rate and finally effectiveness. For a constant Reynolds No., and volume concentration effectiveness increases with increase in flow rate of air, as more air molecules takes away heat and actual heat transfer increases. The increase in effectiveness compare to 0.05% volume concentration of Nanofluid by using 0.4% volume concentration is from 50% to 170%. Higher increase difference in effectiveness is at higher Reynolds number.

From the experimental observation conclusion that can be drawn are: maximum heat transfer occurs at 0.4% volume concentration and 100 cc/s volume flow rate of Nanofluid at 6.5 m/s air velocity. Outlet temperature of fluid is minimum for 0.4% volume fraction of Nanofluid and 15 cc/s volume flow rate of Nanofluid at 6.5 m/s air velocity. There is a substantial increase in the heat transfer coefficient by using Nanofluid compared to water. Effectiveness tends to decrease with the increase in Reynolds number, which may be attributed for change in the efficiency of spiral heat exchanger. Since, the heat transfer rate is significantly affected by both internal fluid flow rate and external air velocity, there should be some tread off between these two while choosing the operating parameters. The increment in heat transfer rate was found from 160% to 400% then by the use of normal water. Correlation among Nusselt number, Reynolds number and Prandtl number is proposed for different cases.

The author is very grateful to Dr. Naga Srinivasulu G and Dr. K. Kiran Kumar, Associate Professor of Department of Mechanical Engineering at National Institute of Technology-Warangal, India for providing necessary equipment at Heat and Mass Transfer Lab during experiment.

The authors declare no conflicts of interest regarding the publication of this paper.

Shah, S. and Kumar, K.K. (2018) Experimental Study & Heat Transfer Analysis on Copper Spiral Heat Exchanger Using Water Based SiO_{2} Nanofluid as Coolant. World Journal of Nano Science and Engineering, 8, 57-68. https://doi.org/10.4236/wjnse.2018.84004