Experimental investigation of convective heat transfer performance using sporadic flow divider type

Abstract

Keywords: Flow Divider Type; Heat Transfer Enhancement; Inserts; Reynolds Number

Introduction

Convective heat transfer holds notable importance in diverse engineering applications, influencing the efficacy of heat exchangers, cooling systems, and thermal management devices. The present research focuses on the exploration and enhancement of convective heat transfer by strategically incorporating flow divider inserts. Flow dividers, integral components in fluidic systems, have the potential to bring about substantial changes in flow patterns and heat transfer characteristics. The thoughtful integration of these inserts enables the manipulation of fluid dynamics, fostering improved convective heat transfer. This paper presents a comprehensive investigation into the effects, mechanisms, and optimization strategies associated with integrating flow divider inserts to enhance convective heat transfer. Through extensive experimentation, this study aims to provide valuable insights into the thermal performance enhancements achievable through the strategic use of flow dividers. The research considers various parameters, including geometric configurations, flow rates, and operational conditions, to unravel the intricate interplay between flow dividers and convective heat transfer. The objective of the study is to investigate experimentally, forced convection heat transfer and pressure drop characteristics with sporadic introduction of bisectional passive inserts. The findings of this study not only deepen the understanding of convective heat transfer enhancement but also offer practical implications for the design and optimization of heat exchange systems in diverse engineering sectors. This comprehensive exploration highlights the potential advantages of integrating flow divider inserts for improved convective heat transfer, paving the way for more efficient and sustainable thermal management solutions. The persistent pursuit of enhancing convective heat transfer has been a longstanding focus in thermal engineering. Researchers have explored various techniques to improve the efficiency of heat exchange systems. Among the innovative approaches gaining attention is the strategic implementation of flow divider inserts, introducing alterations in fluid dynamics to optimize convective heat transfer. Literature available in the field examines key studies contributing to the understanding of convective heat transfer enhancement, laying the groundwork for the current comprehensive investigation. Kumar and Jhinge [1] conducted experiments on a Shell-and-Tube Heat Exchanger (STHE) employing segmental baffles at various locations and with varying Reynolds numbers. Bichkar et al. [2] performed numerical simulations with different types of baffles, focusing on the impact of pressure drop in the STHE. They demonstrated that the use of helical baffles led to an improvement in thermal efficiency. Ali et al. [3] investigated the influence of tube length and

shell diameter on pressure drop (ΔP) and Heat Transfer Coefficient (HTC) for the shell side, considering triangular and square pitch arrangements. Their findings indicated an increase in HTC with reduced cutting space and baffle spacing. Joemer et al. [4] utilized Computational Fluid Dynamics (CFD) software to optimize the baffle cut and angle in the STHE. Son and Shin [5] explored heat exchangers utilizing spiral baffle plates, emphasizing the rational fluid contact with the inner shell. Petrik and Szepesi [6] statistically studied shell-and-tube exchangers with horizontal baffles, comparing their results with the commercial program SC – Tetra V11. Edward and Volker [7] investigated the influence of pressure drop, particularly on the shell side, in heat exchangers with segmental baffles. They used correlations for pressure drop calculations in an ideal tube bank and applied correction factors to account for leakage and bypass streams. Their study identified ranges of geometric and operational parameters where the experimental and theoretical results differed by no more than ± 35%. Gu et al. [8] performed experimental and numerical analyses on a trapezoidal baffled heat exchanger to evaluate the heat transfer coefficient, flow resistance, and overall thermal performance on the shell side. They found that compared to a shutter baffle heat exchanger, a twisty flow heat exchanger can enhance the heat transfer coefficient by 7.3–10.2%, decrease the pressure drop by 18.5–21%, and improve the thermal performance factor (TEF) by 14.9–19.2%. Akpabio et al. [9] studied the effect of baffle spacing on the overall heat transfer coefficient in a heat exchanger with a fixed baffle cut. Their research indicated that the optimal overall heat transfer coefficients in shell-and-tube heat exchangers are generally achieved with a baffle cut of 20 to 25 percent of the diameter. The maximum baffle spacing is determined by the tube support requirements. Singh et al. [10] explored a specific type of shell-and-tube heat exchanger (STHE), where heated water flows through one tube and cold water flows around it. They conducted numerical simulations to model fluid flow and heat transfer, comparing the simulated results with experimental data for validation. Their analysis showed discrepancies between the experimentally measured temperatures and those predicted using ANSYS 13.0. Irshad et al. [11] compared various STHEs featuring segmental baffles using ANSYS software, aiming to develop and explore the flow and temperatures within the shell and tubes for different baffle assemblies and orientations. Menni et al. [12] employed computational simulations to enhance the thermal behaviour of shell-and-tube heat exchangers by incorporating W-shaped Baffle-type Vortex Generators. Biçer et al. [13] utilized the Taguchi technique after determining various design parameters for a specific baffle type to identify optimal design configurations for optimal performance. Feng et al. [14] investigated a STHE for organic fluid vaporization, highlighting the potential improvement in overall performance by

selecting appropriate working fluid, heated water mass flow rate, and total tube number. The analysis demonstrates that the complex function sacrifices some heat transfer performance while significantly enhancing fluid flow performance, leading to a noticeable reduction in the overall complex function. Among the seven working fluids considered, R152a exhibits the lowest complex function, whereas R236fa shows the highest complex function. Arani and Moradi [15] investigated the enhancement of fluid flow and heat transfer in a segmental baffle shelland-tube heat exchanger (SBSTHE) by integrating a baffle design with longitudinally ribbed tubes. Their study revealed that a disk baffle shell-and-tube heat exchanger with longitudinal triangular ribbed tubes (DB-TR) outperforms conventional baffle designs, achieving a 39% improvement. Additionally, the combined segmental-disk baffle shell-and-tube heat exchanger with longitudinal triangular ribbed tubes (CSDB-TR) showed a 37% increase in performance, while the configuration with longitudinal circular ribbed tubes (CSDB-CR) resulted in a 13% enhancement. Marzouk et al. [16] explored the use of bubbles injected with inserts within tubes to enhance thermal performance in shell-and-tube heat exchangers. Their study involved pumping air into tube sides containing wiring nails-circular rod insertion (WNCR) in a straightforward manner, with a constant water tube side flow rate. Marzouk and colleagues [16] have recently drawn considerable interest to the application of bubbles injected with inserts within tubes as a method to improve the thermal efficiency of Shell-and-Tube Heat Exchangers (STHEs). In their specific investigation, air was introduced into the tube sides featuring circular rod insertions composed of wiring nails (WNCR) in a direct manner. The flow rate on the water tube side (TSFR) was adjusted within the range of 14 to 18 Liters per minute (LPM), while the water flow rate on the shell side remained consistently maintained at a suitable level of

18. LPM. In a separate investigation, Safarian et al. [17]

employed a shell-and-tube exchanger with a 25% baffle reduction. The study focused on exploring the impact of tube location on heat transfer. The findings indicated that tubes positioned around the shell perimeter had a more significant influence on heat transmission compared to tubes located at the centre of the shell. These selected studies collectively form the foundation for the present research, guiding the exploration of convective heat transfer enhancement through the comprehensive study of flow divider inserts. By building upon and synthesizing insights from these works, the current investigation aims to contribute novel perspectives and practical implications for the advancement of thermal management technologies. The different researcher [1722] has tried to find some other techniques of enhancing convective heat transfer like using nano fluid. Kale and Gawade [22] has mentioned various methods of

heat transfer augmentation for various range of Reynolds number. Various heat transfer augmentation method can also be used to improve efficiency of solar panels [23]. Singh et al. [24] explored the use of aluminium helically corrugated twisted tape inserts to enhance heat transfer in turbulent flow. Their study evaluated the thermal performance, abrasion characteristics, and heat transfer enhancement of a helically fluted duct. They analysed nine spirally corrugated ducts with varying rib-to-width ratios (0.26, 0.17, and 0.22) and rib-height to width ratios (0.06, 0.02, and 0.04). Yadav et al. [25] examined the performance improvement of a solar air heater with rib roughening. The findings revealed that the rib with a shorter pitch achieved the highest Nusselt number at the highest Reynolds number. Additionally, it was observed that this rib with a shorter pitch resulted in the highest friction factor at the lowest Reynolds number. Liaw et al. [26] investigated convective heat transfer in helical twisted multilobe tubes. Hussen et al. [27] explored the enhancement of heat transfer by employing twisted tape in both horizontal and inclined tubes. They compared the thermal performance of plain tubes and those with twisted tape inserts, finding a strong correlation between experimental and numerical results, with a maximum deviation of 10%. Chandrashekaraiah et al. [28] have worked on the heat transfer enhancement in solar systems. Currently the literature review is not available on the sporadic bisectional inserts, where as the non continious/intermittent twisted tape research is available. sporadic bisectional inserts shows better results compare to the non – continios twisted tape, that is the overall optimal performance was noted, with ψ increasing by 23% and η by 55% compared to the plain tube [28-34]. A perforated conical baffle plate with rectangular air deflectors of different inclination angles was used by Md. Atiqur Rahman et al. [35], experimented at Reynolds number ranged between 93,500 and 160,500. Their findings demonstrate that both the inclination angle and the pitch ratio significantly influence the Nusselt number, with the pitch ratio having a more pronounced effect on the friction factor. In a numerical study conducted by Attou et al. [36], the impact of V-shaped grooves on the outer walls of a rotary kiln was analyzed in terms of turbulent flow and heat transfer enhancement. The results showed that increasing the groove depth reduces the outer wall temperature of the rotary kiln compared to smooth walls and results in the highest Nusselt number, particularly for groove depths with h/D ratios of 0.3 and 0.4. Additionally, Raval et al. [37] and Raja et al. [38] have reviewed various techniques for enhancing heat transfer in solar collectors. Various studies reviewed indicate that inserts enhance heat transfer by disrupting the fluid’s boundary layer. However, the presence of inserts also causes area blockage, which increases the friction factor and consequently requires more pumping power. Presently, there exists considerable research focusing on inserts, particularly

exploring different configurations of twisted tape. Some researchers have also investigated flow divider-type inserts, although literature specifically addressing non-continuous types, such as sporadic flow divider inserts, remains scarce. This study aims to bridge this gap by exploring non-continuous flow divider inserts. The hypothesis predicts that the sporadic inserts reduce the blocking of the section and hence the pumping power required will be less. Due to sporadic inserts the pumping power required will be reduced and hence it will help for the increasing Overall performance criteria (η) In the current research work Overall Performance Criteria (η) were calculated at various space ratios. The results showed a moderate improvement, with the highest η occurring at a space ratio of 0.59, which corresponds to a 0.015m space between the inserts. For this space ratio, η values for different twist angles were as follows: 1.12 to 1.80 for 90°, 1.12 to 1.35 for 60°, 1.15 to 2.22 for 45°, and 1.05 to

Theory

Figure 1 presents schematic diagram outlining the methodology and experimental configuration employed in this study. The experimental setup centers around a mild steel pipe with an internal diameter of 26 mm, facilitating smooth airflow. A robust 2 hp blower propels air through the pipe, featuring strategically positioned flow control valves at the blower outlet for precise airflow regulation.

Performance Parameters Following are the performance parameters which determines the performance of the system. Various equations of Fluid Mechanics and Heat Transfer is used for performance evaluation. Equation no 1, is taken form book of White and Xue [33], the equation 2, 3, 4 are taken form the book of Incropera et al. [34], the equation no 5 and 6 is taken from the research paper Nalavade et al. [32]. The Reynolds number for the air flow in the test-section is defined as

(1) where u is the mean velocity of air in the tube. ρ is density of air in kg/m3, D is diameter of test section pipe in m, µ is dynamic viscosity in Pa-s. The total heat transferred to air is estimated by (2) where Ti and T0 is inlet and exit temperature of air, across the test-section. is mass flow rate of air in kg/sec, Cp is specific heat of air in kJ/kgK.

The heat transfer coefficient is given by (3) where As​denotes the surface area of the tube test-section, h represents the average convective heat transfer coefficient, Tw​is the average temperature of the tube wall surface, and Tmb​ is the mean bulk fluid temperature obtained through the averaging of air entrance and exit temperatures. Space ratio (S/D) is defined as the ratio of Space between two blades of insert (S) to the Diameter of the Pipe (D) The influence of heat transfer and fluid flow is respectively demonstrated through the Nusselt number (Nu) and the friction factor (f), elucidating the relationship between them. (4) Where Nu is Nusselt number, h is heat transfer coefficient in W/m2.k, D is the diameter of the pipe in m, k is the thermal conductivity of working fluid in W/m.k (5) where ΔP is the pressure drop across the test section length in Pa, k is thermal conductivity in W/mk , Thermal Enhancement factor (ψ) is defined as; Where h = Corvective heat transer coefficient for plain tube fitted with flow divider type insert (W/m2K) and, h0 = Convective heat transfer coefficient for plain tube (W/ m2K). Plain tube is tube without any insert or empty tube. Performance Evaluation Criteria (η): The evaluation of performance criteria (η) is defined as the ratio of the Nusselt number ratio

As the twist angle decreases, the size of the fluid pockets also decreases, which in turn intensifies turbulence. This reduction in twist angle correlates with an increase in the Nusselt number. The highest Nusselt number is observed at a 45° twist angle, where the average Nusselt number is 2.21 times higher than that of a plain tube. Experimental data were collected using different twist angles—specifically 90°, 60°, 45°, and 30°—across Reynolds numbers ranging from 7000 to 21000. Predicted correlations are valid for the Re ranges from 7000 to 12000. To evaluate the effect of spacing between the inserts (sporadic insert), results were obtained for different space ratios (0.19, 0.39, 0.59, 0.78) combined with various twist angles. The data was analyzed by plotting various graphs, with result parameters such as Nusselt number, friction factor, thermal enhancement factor, and performance evaluation criteria plotted against Reynolds number to draw conclusions and observe trends. The graph is constructed for four twist angles: 90°, 60°, 45°, and 30°. Correspondingly, there are four distinct lines on the graph. Analysis of the graph reveals that the Nusselt number is dependent on the Reynolds number. The 45° twist angle yields the highest Nusselt number. The trend line equation for the 45° twist angle is determined to beNu = 6.2129Re + 61.918

The equations vary for different angles: for a 60° twist angle, the equation is Nu = 6.5662Re + 41.026

for a 30° twist angle, the equation is Nu = 5.217Re + 42.535

Results And Discussion

The experimental results are analyzed using various graphical representations, with different observational parameters recorded at varying Reynolds numbers. The study investigates the effect of changing the twist angle of a flow divider-type insert on convective heat transfer. The findings indicate that increasing the Reynolds number leads to a higher convective heat transfer rate, due to the increased flow rate that enhances turbulence. Figure 7 demonstrates the effect of altering the twist angle on the Nusselt number.

The influence of changing the cyclic angle of twist of the flow divider type insert on the friction factor is depicted in Figure 8. Remarkably, the pressure drops for the tube equipped with the flow divider type insert is noted to rise as the cyclic angle of twist decreases, indicating a consistent trend in the friction factor. Figures 8 and 9 collectively depict the impacts of varying the cyclic angle of twist of the flow divider type insert on the friction factor and friction factor ratio, respectively. Specifically, the friction factor is found to vary from

Conclusion

In this research, an experimental investigation was conducted to study the fluid flow and heat transfer characteristics of a circular tube fitted with a sporadic flow divider type turbulator under turbulent flow conditions. The study focused on the performance of heat exchanger tubes equipped with flow dividers with space to diameter ratios (S/D) of 0.19, 0.39, 0.59, and 0.78. The results indicate that the flow divider turbulator effectively disrupts the boundary layer, thereby enhancing thermal performance. However, the solid flow divider also partially obstructs the test section, leading to increased friction and pumping power. Future studies could explore the use of flow dividers with specific porosities to optimize heat transfer efficiency and reduce friction. The key findings from this experimental study are as follows: 1) Flow divider inserts with twist angles of 90°, 60°, 45°, and 30° were tested. As the twist angle decreases, fluid flow blockage increases, which enhances fluid mixing from the core to the tube wall and improves convective heat transfer. The corresponding Nusselt numbers range from 40.12 to 74.45 for a 90° twist, 46.95 to 90.56 for a 60° twist, 70.12 to 110.12 for a 45° twist, and 47.97 to 82.9 for a 30° twist. 2) The thermal enhancement factor (ψ) shows significant improvement across different twist angles. For a 90° twist, ψ increases by 1.28 to 1.43 times compared to a plain tube; for a 60° twist, ψ increases by 1.38 to 1.65 times; for a 45° twist, ψ rises by 1.48 to 2.13 times; and for a 30° twist, ψ increases by 1.39 to 1.72 times. 3) The highest friction factor is observed at a 30° twist angle, ranging from 0.30 to 0.37, due to the significant flow area blockage. 4) The overall performance criteria (η) were calculated at different space ratios, with the highest η observed at a space ratio of 0.59, corresponding to a 0.015 m space between inserts. For this space ratio, η values are 1.12 to

Nomenclature

Surface area of the experimental test section, measured in square meters (m²). Specific heat of air at constant pressure, expressed in joules per kelvin (J/k). Inner diameter of the plain tube, measured in meters (m). Friction factor, a dimensionless number. Average convective heat transfer coefficient (W/m2.k). Thermal Conductivity (W/m·K) - Ability of a material to conduct heat, in watts per meter-kelvin. Length of the test section, expressed in meters (m). Mass Flow Rate(kg/s) - Amount of mass flowing per unit time, in kilograms per second) Nusselt number, a dimensionless quantity. Insert pitch, measured in meters (m). Prandtl number, a dimensionless quantity. Heat transfer rate, measured in watts (W). Reynolds number, a dimensionless quantity. Space between the two blades of the insert, in meters (m). Initial temperature of the fluid, in degrees Celsius (°C). Iinal temperature of the fluid, in degrees Celsius (°C). Temperature of the wall surface, in degrees Celsius (°C). Average temperature of the bulk fluid, in degrees Celsius (°C). Volume Flow Rate (m³/s) - Volume of fluid passing through per unit time, in cubic meters per second

Greek symbols ρ Density (kg/m³) - Mass per unit volume, measured in kilograms per cubic meter µ Dynamic Viscosity (Ns/m²) - Measure of fluid’s internal resistance to flow, expressed in Newton-seconds per square meter. ψ Thermal Enhancement Factor, TEF - Dimensionless factor representing thermal enhancement. η Performance Evaluation Criteria - Criteria used for performance evaluation. Subscripts i Inlet o Outlet bm Bulk mean 0 Plain tube s Surface

Acknowledgement

The authors sincerely acknowledge the invaluable support and research facilities provided by Nagesh Karajagi Orchid College of Engineering and Technology, Solapur. Their generous assistance and resources played a crucial role in facilitating the successful completion of this research work.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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