Experimental Study on Heat Transfer Enhancement of Modified CuO Nanofluid in Helical Grooved Tube

This study proposes a novel technology to enhance heat transfer by combining nanofluid and helical grooved tubes. The researchers used a two-step method to obtain a stable modified CuO nanofluid. They then constructed an experimental setup to investigate the single-phase heat transfer of the nanofluid in a helical grooved tube, and compared its performance to traditional methods under varying volume fractions and Reynolds numbers. The experimental results indicate that the use of nanofluid in the helical grooved tube significantly improves the convective heat transfer coefficient. Furthermore, the heat transfer coefficient increases gradually with the addition of nanoparticle volume fraction and Reynolds number. The helical grooved tube’s internal structure causes local turbulence at low Reynolds numbers, intensifying the nano-scale eddies and turbulence inside the nanofluid, enhancing fluid mixing and thermal diffusion rates, and thus improving the heat transfer efficiency.


Introduction
The rise in energy consumption and material costs is a significant driving force for the development of energy-efficient and cost-effective heat exchange equipment.With the global economy's growth and population increase, the demand for energy has also risen.Consequently, enhancing energy utilization efficiency and promoting energy conservation and emission reduction has become a crucial global issue.The development of energy-saving heat exchange equipment can significantly reduce energy consumption and carbon emissions, leading to sustainable development [1].Additionally, the high cost of materials used in traditional heat exchange equipment is a significant driving factor for the development of efficient and energy-saving equipment.Such equipment often employs expensive materials, resulting in high production and maintenance costs [2].Therefore, there is a need to develop more economical and efficient heat exchange equipment that can reduce costs and enhance efficiency.
Currently, research on nanofluids centers around the following areas of interest: Nanofluid preparation and characterization [3]: Researchers investigate the effect of modifying nanofluid material, particle size, concentration, and other factors on heat transfer performance.Meanwhile, various characterization methods such as TEM and DLS were used to observe nanoparticle dispersion and fluid structure.
Researchers employ experimental and modelling approaches to analyze the heat transfer performance of nanofluids and investigate heat transfer enhancement mechanisms such as improved thermal conductivity and surface enhancement impacts of nanoparticles on the fluid surface [4] .
Research on the application of nanofluids in heat exchangers [5]: Using experiments and numerical simulations, researchers investigate the application impacts and engineering possibilities of nanofluids in various types of heat exchangers.
Heat transfer of multi-scale nanofluids research [6]: Researchers combine the heat transfer performance of nanofluids with micro, meso, and macro scales to examine the heat transfer performance and mechanism of multi-scale nanofluids and explore ways to further increase their heat transfer efficiency.
When fluid flows out of a helical grooved tube, it induces spiral flow near the pipe wall due to its convex and concave outer walls.The thinning of the laminar layer and the fluctuation of the wall surface increase fluid turbulence, speed up heat transfer from the wall to the main body of the fluid, and improve heat transmission.Currently, a large amount of research work on helical grooved tubes has been conducted worldwide, and the internal and external heat transfer characteristics, as well as the resistance characteristics of various specifications of helical grooved tubes, have been systematically tested.The present study on helical grooved tubes, as an essential heat transfer technique, is primarily focused on the following aspects: Optimization of helical grooved tube structure and characteristics [7]: Researchers want to improve heat transmission by modifying helical grooved tube factors such as groove form, groove depth, groove spacing, and spiral angle.
Exploring the heat transmission process [8]: In order to offer theoretical basis for future optimization, researchers did extensive study on the heat transfer mechanism of helical grooved tubes using numerical simulations, tests, and other methods.
Developing new materials [9]: Researchers are constantly creating new materials to improve the heat transfer enhancement effect and corrosion resistance of helical grooved tubes.
Practical application research [10]: Researchers have used helical grooved tube heat exchangers to numerous industrial domains, such as chemical, power, and pharmaceutical, to investigate their application impacts in practical engineering, with the goal of boosting their use.
Previous research on nanofluids has demonstrated that Gnielinski and other traditional correlations cannot be used to calculate the heat transfer coefficient of nanofluids [11], and that increasing the convective heat transfer coefficient of nanofluids is significantly greater than increasing the overall thermal conductivity.Because of the intricacy, standard correlations cannot be employed to solve the heat transport problem of nanofluids in tubes.Despite substantial study on nanofluids and spiral grooved tubes, there is currently very little research on nanofluids in spiral grooved tubes around the world.In this research, modified CuO nanoparticles were dispersed in water in two steps to generate homogenous and stable nanofluids, and their convective heat transfer properties were investigated and compared at various percentages and Reynolds numbers.

Preparation of nanofluids
Choose surface modified CuO nanoparticles (the characteristics are presented in Table 1) and create stable CuO/H2O nanofluids in two steps.Weigh a specific number of CuO nanoparticles, then add a specific volume of deionized water, then set them in an ultrasonic vibrator for ultrasonic vibration.After the same amount of time of ultrasonic vibration, place the beaker holding the nanoparticles in a constant temperature water bath and add oleic acid as a surface modification with a pipette.Simultaneously, turn on an electric stirrer for mechanical stirring to ensure adequate reaction.3 CuO nanofluids with volume fractions of 0.3 percent, 0.6 percent, 0.9 percent, and 1.2 percent were generated in this experiment following the procedures described above.

Thermophysical properties of CuO nanofluids
The thermophysical properties of nanofluids include density, specific heat, viscosity, and thermal conductivity.The density calculation can be done using the following formula: ρ 1 ∅ ρ ∅ρ in which ρ ，ρ and ρ is the density of the nanofluid, base liquid, and nanoparticles, respectively; ∅ is the volume concentration.
The formula for calculating specific heat is: in which C ，C and  is the specific heat of the nanofluid, base liquid, and nanoparticles, respectively.
According to the basic theory of convective heat transfer, the convective heat transfer coefficient of a fluid is related to its own physical parameters.For the thermal conductivity of nanofluids, Bruggeman revised the formula based on Maxwell, taking into account the mutual influence between particles, and obtained a more accurate formula for calculating the effective thermal conductivity: In which, k is the thermal conductivity of the solid-liquid suspension,  is the thermal conductivity of the base liquid,  is the thermal conductivity of the nanoparticles.

Experimental setup
Electric heating wire wall heating is employed in the experiment to guarantee that heat transmission occurs under continuous heat flux.An insulating layer is put around the test tube to prevent heat dissipation into the environment.Figure 1 depicts the intended experimental system diagram.In the experiment, two helical grooved tubes of the same length and diameter were compared to a straight tube with L=1.00m and D=0.075m.Before exiting the condenser, the nanofluid is heated within the tube and completely mixed in the output mixing box.The cooling tower supplies the condenser with cooling water.A constant temperature auxiliary electric heater heats the lowtemperature fluid from the condenser to a predetermined temperature to provide a constant temperature at the tube's input portion.The circulating fluid is kept in a water storage tank and is powered by a circulating pump.The flow rate of the fluid is regulated by altering the power of the circulating pump.In the experimental part, two tubes, one helical grooved and one straight tube, were placed.To make it easier to compare heat exchange conditions across different tube types, tube types were chosen by manipulating the opening of valves V1, V2, V3, and V4.
Keep all valves closed during the experiment, then inject heat transfer oil into the liquid storage tank, open all valves in turn, turn on the circulating pump, turn on the power switch of the electric heating wire, the auxiliary electric heater, and the cooling tower, adjust the power of the circulating pump to change the Reynolds number, and record the data after the experimental data is stable.After the test is finished, turn off the electric heater, the cooling tower, valve 7, and finally the circulating pump.Pour the heat transfer oil out of the storage tank through the bottom outlet, and then inject the pre-prepared nanofluid through the higher injection port.Repeat the preceding procedures until all nanofluids have been removed.

Data processing
Ignoring the heat dissipation of the electric heating wire to the environment, the heat obtained from the pipe wall when the fluid passes through the pipeline is calculated by the following formula Q hA t t ̅ The average temperature of the pipe wall is obtained by averaging the readings obtained from thermocouples distributed on the pipe wall, and the temperature of the fluid inlet and outlet is directly measured by the thermocouples arranged at both ends of the pipe.Heat is calculated through temperature rise during fluid flow simultaneously.
Q cm t t According to the conservation of energy, Q 1 =Q 2 , the convective heat transfer coefficient inside the tube can be obtained.ℎ cm t t A t t ̅ For helical grooved tubes, their effective heat transfer area can be calculated using the following formula:

Experimental Results and Discussion
The heat transfer characteristics of pure heat conducting oil and CuO nanofluids with volume fractions of 0.3 percent, 0.6 percent, 0.9 percent, and 1.2 percent in smooth tube and helical grooved tube were investigated, as well as the laws of Nusselt number and convective heat transfer coefficient changing with Reynolds number.The graphic shows that when the Reynolds number (flow rate of fluid in the pipe) increases, so does the heat transfer performance in both plain and helical grooved tubes.The 0.3 percent concentration of nanofluid in the helical grooved tube may achieve an average heat transfer increase of 91 percent when compared to the base liquid thermal oil in the straight tube.As the volume fraction of the nanofluid grows, so does its strengthening, which ultimately flattens out.
Further investigation reveals that even at low Reynolds numbers, there is still a significant increase in heat transfer capacity, owing to the interior structure of helical grooved tubes, which creates local turbulence at low Reynolds numbers.This effect aggravates the nanoscale eddies and turbulence inside the nanofluid, increasing the degree of fluid mixing and the rate of thermal diffusion, hence enhancing heat transfer efficiency.

Conclusion
The influence of modified CuO nanofluids on heat transmission in helical grooved tubes is investigated in this paper.The following results are reached from setting up a test bench in the laboratory and measuring the Nusselt number and heat transfer coefficient of nanofluids with varying volume fractions as a function of Reynolds number: (1) Using modified CuO nanofluids in helical grooved tubes can considerably increase the fluid's heat transfer enhancing effect.
(2) As the volume percentage of nanoparticles increases, so does the Nusselt number and heat transmission coefficient.However, as the volume concentration rises, the upward trend flattens out.
(3) The Reynolds number has a significant impact on nanofluid heat transfer enhancement.Nusselt number and heat transfer coefficient begin to stabilize when Reynolds number reaches a specific threshold.
The results reveal that using modified CuO nanofluids in spirally grooved tubes may significantly increase heat transfer enhancement, and Reynolds number is an essential element influencing heat transfer improvement.These findings are extremely important for furthering the development of nanofluids in engineering applications.