Experimental and Computational Investigation of Heat Transfer and Fluid Dynamics in Shell-and-Tube Heat Exchangers with Helically Dimpled Tubes

1.1 Background overview

Heat exchangers are extremely important components that are heavily used across various industrial sectors [1-4]. These sectors include, but not limited to, the following: power generation, chemical processing, HVAC (heating, ventilating, and air conditioning), and refrigeration. Essentially, heat exchangers facilitate the transfer of heat between two or more fluids which makes them crucial for the general optimization of energy use and the enhancement of a selected system performance [5-7]. Figure 1 elaborates on the design and construction classification of heat exchangers.

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Figure 1. Typical heat exchangers design/construction classification

The functionality of heat exchangers hinges on the principles of thermodynamics and fluid mechanics [8]. By allowing heat to pass from a hotter to a cooler fluid, heat exchanges efficiently manage energy within systems through the minimizing of energy losses and overall improved efficiency procedures. The design and operational effectiveness of heat exchangers directly impact their performance that in turn influences some factors such as energy consumption, operational costs, and environmental impact. There are several types of heat exchangers, namely shell-and-tube, plate, and finned tube, each serve a distinct application and chosen based on factors like temperature ranges, pressure levels, and fluid properties [9]. The choice of a particular type of heat exchanger and its design considerations are critical for achieving desired heat transfer rates while maintaining pressure drop and material compatibility within acceptable limits. The importance of heat exchangers is being recognized more and more as industries press for improvement in energy efficiency and process optimization. Heat exchanger design innovations cater to demands for enhanced energy efficiency to meet the sustainability goal of reducing carbon emissions and enhancing the green credentials of industrial operations.

1.2 Fundamentals in heat exchanger design and heat transfer enhancement

Heat exchangers are classified by their flow arrangements—counterflow, parallel flow, and crossflow—each affecting thermal efficiency and industrial applicability. Counterflow maximizes the temperature gradient for higher efficiency, while parallel flow leads to quicker equilibrium and lower efficiency, and crossflow offers a middle ground, ideal for space-constrained applications. Common types include shell-and-tube, plate, and double-piped exchangers, with shell-and-tube being the most widely used due to its adaptability and potential for enhancements like dimples. Improving heat transfer efficiency involves surface modifications such as ribbing, corrugation, and dimpling to create turbulence, along with material advancements using high-conductivity composites like graphene or copper and phase change materials (PCMs) for thermal stabilization. Optimized flow arrangements, particularly counterflow, enhance efficiency by maintaining strong temperature gradients, while nanofluids improve thermal conductivity with minimal impact on fluid dynamics. This thesis focuses on helically dimpled tubes in shell-and-tube exchangers, examining their potential to enhance thermal performance through increased surface area and turbulence. Figure 2 summarizes these techniques in enhancement to bring out the diversity in usage and benefits.

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Figure 2. Heat transfer enhancement methodologies in heat exchangers

1.3 Significance and introduced novelties

In enhancing heat transfer in such industrial systems, the energy consumption will be reduced, operational costs will be minimized, and environmental degradation will be reduced. The emerging potentials of heat exchange technologies address global concerns in terms of sustainability and increased call towards stringent environmental regulations. A practical solution to the problem of effective heat transfer mechanisms reduces energy consumption since it ensures there is effective thermal energy exchange with minimal loss; this particular factor is valuable in systems where heat recovery reduces energy requirements, such as in power plants. Whereas innovations like dimpled tubes in shell-and-tube exchangers-this paper’s focus-disturb laminar flow to introduce turbulence, therefore enhancing heat transfer by breaking up thermal boundary layers [10]. Computational modeling and experimental methods have rapidly gained pace for engineers to iteratively refine these designs, setting new benchmarks in thermal efficiency. This study aims at contributing to solutions for the current energy and environmental challenges by exploring potential solutions that could result from the use of helically dimpled tubes when optimizing heat exchanger performance. The primary aim of this study is to enhance heat transfer and fluid flow properties in a shell-and-tube heat exchanger using a helically dimpled tube. The study’s novelty lies in the following objectives:

1. Investigating the effects of mass flow rate variations on heat transfer efficiency and pressure drop by comparing dimpled and non-dimpled tubes.
2. Conducting computational simulations in ANSYS Fluent to model heat transfer and fluid flow for smooth vs. dimpled tubes under varied boundary conditions.
3. Validating simulation results through experimental tests on a fabricated setup, ensuring alignment with real-world conditions.

This paper is organized into six sections: The introduction provides an overview, aim, and objectives. The literature review examines recent advancements in heat exchangers with a focus on dimpled tubes for heat transfer enhancement. The Theoretical Approach outlines ANSYS simulations of heat transfer and flow dynamics. Section 4 describes the experimental work of the setup and procedures for validating simulations. The results and discussion are in section 5 where it presents and analyzes findings, while conclusions and recommendations summarize key insights and suggest future research directions in section 6.

3.1 Introduction

Performances of the heat exchangers mean much in many industrial applications and hence continuous development in the technology of transferring heat. Computational simulations will be executed in this part with the aid of ANSYS Fluent for thermal and hydraulic performance evaluation of helically dimpled tube shell-and-tube heat exchanger on account of its potential improvement over conventional geometries concerning both heat transport enhancement and flow resistance. The real value of these simulations lies in determining the quantitative fluid flow and heat transfer behaviors for various configurations and operational conditions that so far have been understood only qualitatively. They also optimize the designs of dimpled tubes for practical applications. The expected outcome is to establish theoretical gains in practice, with a view toward refining design parameters to support the most efficient solution to heat transfer problems in industry.

3.2 Overview of simulation methodologies

In the pursuit of optimizing heat exchanger performance, the choice of simulation methodology plays a critical role. For this study, FEA serves as the cornerstone of our computational approach [43, 44]. This method that is particularly when implemented via ANSYS fluent software offers a robust platform for simulating complex physical phenomena, which includes the thermal and fluid flow dynamics within heat exchangers equipped with helically dimpled tubes. ANSYS fluent facilitates detailed modeling of physical systems by solving the governing equations of fluid dynamics and heat transfer and this is why it is renowned for its comprehensive simulation capabilities. The software's ability to handle complex geometries and advanced boundary condition settings makes it an ideal tool for this research [45-48]. It supports a variety of solver techniques that are crucial for achieving accurate and reliable results in multiphysics simulations that include coupled thermal-fluid problems. The simulation process in ANSYS fluent involves several key steps that start from the creation of a detailed geometric model of the heat exchanger [15, 49]. The geometry of the helically dimpled tubes requires precise representation in the simulation environment to ensure that all relevant physical phenomena are captured. This is considered to be more complex than standard tube designs. Once the geometry is defined, appropriate material properties must be assigned to different components of the model. These properties include thermal conductivity, density, specific heat capacity, and viscosity, which influence the heat transfer and fluid flow behaviors within the system.

Meshing is the next critical step in preparing for simulation. The mesh must be refined enough to capture the intricate details of the helical dimples while also being computationally manageable. ANSYS fluent provides advanced meshing tools that allow for the creation of finely tuned meshes that conform to the complex surfaces of dimpled tubes [50]. The quality of the mesh directly affects the accuracy and convergence of the simulation results which generally makes this a focal point of the currently presented methodology. After setting up the model with the proper boundary conditions—such as inlet temperature, flow rate, and pressure—simulation parameters are consequently defined. These parameters include the contours of heat distribution after flow occurs, and other operational conditions under which the heat exchanger is expected to perform. The choice of solver settings depends on whether the simulation is steady-state or transient, which is a steady-state in the current thesis. These settings are collectively crucial for the capturing of the dynamic interactions between the heat transfer and fluid flow within the heat exchanger. By integrating these elements—geometry modeling, material property assignment, meshing strategy, and solver configuration—ANSYS fluent provides a comprehensive environment for the prediction of the performance of helically dimpled tube heat exchangers. This methodology supports the fundamental understanding of heat transfer enhancement mechanisms in addition to the fact that it aids in the design and optimization of more efficient heat exchanger systems.

3.3 Modeling and configurations

3.3.1 Physical models of heat exchangers

The geometric modeling of heat exchangers forms the foundation for any computational simulation. For this study, two distinct configurations were modeled: a standard tube HE and a helically dimpled tube within an HE. Both of these configurations are designed with precise dimensions to facilitate comparative analysis of their thermal and fluid dynamics performance. The basic geometry of a traditional tube heat exchanger is characterized by simpler, smooth surfaces without any modifications. Table 3 details these dimensions of the traditional HE. The dimensions for this model include a total length (L) of 600 mm, coil diameter (Dcoil) of 75 mm, shell diameter (Dshell) of 150 mm, and pipe diameter (Dpipe) of 12.5 mm. The pitch (P) is set at four times the diameter of the pipe, equating to 50 mm, and the number of coils (N) is 12, leading to a curvature ratio (Dpipe/Dcoil) of 0.166. This configuration is collectively depicted in Figure 6.

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Figure 6. Traditional shell-and-tube of helical coils: (a) helical copper tube, (b) shell

Table 3. Heat exchanger dimensional properties

This configuration introduces spherical dimples staggered along the tube surface to enhance the heat transfer capabilities by disrupting the flow pattern and increasing surface area. The helical dimple properties are in Table 4 and are meticulously set with a diameter of 3.5 mm, depth of 1.75 mm, dimple spacing of 10 mm, and an angle of 90 degrees between each dimple. This modified design aims to intensify the turbulence within the flow, thereby potentially guarantees an increment in the heat transfer rate. The geometric representation of this configuration is shown in Figure 7, in addition to the dimples. Figure 8 illustrates the dimples in a zoomed in configuration.

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Figure 7. Helically-dimpled shell-and-tube HE of helical coils: (a) helical copper tube, (b) shell

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Figure 8. Dimples’ configuration

Table 4. Dimple properties

3.3.2 Effect of dimples geometry

The geometric configuration of dimples on the inner tube of the heat exchanger significantly influences the heat transfer performance. The Table 5 below summarizes the effects of different dimple parameters, such as diameter, depth, spacing, and distribution angle, on the overall performance of heat transfer.

Table 5. Dimple properties for the optimal choice

3.4 The assumptions and governing equations

Fluid Flow Fluent was used for the ANSYS simulation with 1000 interactions for convergence. The governing equations are a set of partial differential equations for continuity, momentum, and energy equations, as shown below:

Continuity equation:

$\frac{\partial(\rho u)}{\partial x}+\frac{\partial(\rho v)}{\partial y}+\frac{\partial(\rho w)}{\partial z}=0$     (1)

Momentum equation:

$u$-momentum (x-direction):

$\frac{\partial\left(\rho u^2\right)}{\partial x}+\frac{\partial(\rho u v)}{\partial y}+\frac{\partial(\rho u w)}{\partial z}=-\frac{\partial p}{\partial x}+\mu\left[\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}+\frac{\partial^2 u}{\partial z^2}\right]$     (2)

$v$-momentum (y-direction):

$\begin{aligned} \frac{\partial(\rho u v)}{\partial x}+\frac{0 \partial\left(\rho v^2\right)}{\partial y}+ & \frac{\partial(\rho v w)}{\partial z} =-\frac{\partial p}{\partial y}+\mu\left[\frac{\partial^2 v}{\partial x^2}+\frac{\partial^2 v}{\partial y^2}+\frac{\partial^2 v}{\partial z^2}\right]+\rho g\end{aligned}$     (3)

$w$-momentum (z-direction):

$\begin{gathered}\frac{\partial(\rho u w)}{\partial x}+\frac{\partial(\rho v w)}{\partial y}+\frac{\partial\left(\rho w^2\right)}{\partial z}=-\frac{\partial p}{\partial z}+\mu\left[\frac{\partial^2 w}{\partial x^2}+\right. \left.\frac{\partial^2 w}{\partial y^2}+\frac{\partial^2 w}{\partial z^2}\right]\end{gathered}$     (4)

Energy equation:

$\frac{\partial(\rho u T)}{\partial x}+\frac{\partial(\rho v T)}{\partial y}+\frac{\partial(\rho w T)}{\partial z}=\Gamma_{e f f}\left[\frac{\partial^2 T}{\partial x^2}+\frac{\partial^2 T}{\partial y^2}+\frac{\partial^2 T}{\partial z^2}\right]$     (5)

$\Gamma_{e f f}=\frac{\mu_{e f f}}{P r_{e f f}}$     (6)

where, $\Gamma_{e f f}$ is the effective diffusion coefficient, $\mu_{e f f}$ is the effective viscosity coefficient, and $P r_{e f f}$: is the effective Prandtl number.

3.5 Mathematical model

The standard $\mathrm{k}-\varepsilon$ model uses the following transport equations for $k$ and $\varepsilon$:

Turbulence kinetic energy ($k$):

$\begin{gathered}\frac{\partial(\rho u k)}{\partial x}+\frac{\partial(\rho v k)}{\partial y}+\frac{\partial(\rho w k)}{\partial z}=\frac{\partial}{\partial x}\left(\frac{\mu_t}{P r_k} \frac{\partial k}{\partial x}\right)+\frac{\partial}{\partial y}\left(\frac{\mu_t}{P r_k} \frac{\partial k}{\partial y}\right)+\frac{\partial}{\partial z}\left(\frac{\mu_t}{P r_k} \frac{\partial k}{\partial z}\right)+P_k \\ +G_k-\rho \varepsilon\end{gathered}$     (7)

Dissipation rate ($\varepsilon$):

$\begin{aligned} \frac{\partial(\rho u \varepsilon)}{\partial x} & +\frac{\partial(\rho v \varepsilon)}{\partial y}+\frac{\partial(\rho w \varepsilon)}{\partial z} =\frac{\partial}{\partial x}\left(\frac{\mu_t}{P r_{\varepsilon}} \frac{\partial \varepsilon}{\partial x}\right)+\frac{\partial}{\partial y}\left(\frac{\mu_t}{P r_{\varepsilon}} \frac{\partial \varepsilon}{\partial y}\right)+\frac{\partial}{\partial z}\left(\frac{\mu_t}{P r_{\varepsilon}} \frac{\partial \varepsilon}{\partial z}\right)+C_{1 \varepsilon} \frac{\varepsilon}{k}\left(P_k+G_k\right)-C_{2 \varepsilon} \rho \frac{\varepsilon^2}{k}\end{aligned}$     (8)

where:

$P r_k$: is the turbulent Prandtl number for $k$ (empirical constant).

$P r_{\varepsilon}$: is the turbulent Prandtl number for $\varepsilon$ (empirical constant).

$G_k$: is the kinetic energy generation by buoyancy.

$\begin{aligned} P_k=\mu_t\left(2\left[\left(\frac{\partial u}{\partial x}\right)^2\right.\right. & \left.+\left(\frac{\partial v}{\partial y}\right)^2+\left(\frac{\partial w}{\partial z}\right)^2\right] \left.+\left(\frac{\partial u}{\partial x}+\frac{\partial u}{\partial y}+\frac{\partial u}{\partial z}\right)^2\right)\end{aligned}$     (9)

$G_k=\frac{\mu_t}{P r_t} g \beta \frac{\partial T}{\partial y}$     (10)

This section presents and discusses the results from experimental tests and numerical simulations on both normal and helically dimpled tube heat exchangers, focusing on evaluating the helical dimples' impact on heat transfer under varying mass flow rates. The analysis compares temperature change, pressure drop, and overall performance, highlighting key trends and validating computational predictions. The findings provide insights into how the helical dimples improve efficiency by disrupting boundary layers and promoting turbulence. Finally, the section compares experimental and simulation results, offering recommendations for optimizing design and operational parameters for enhanced heat transfer efficiency and reliability.

5.1 Experimental results

This section delineates the experimental outcomes obtained from the testing of both the traditional and dimple-added helical-tubed heat exchangers. The primary focus here is on the temperature and pressure measurements collected under varying operational conditions, which are crucial for assessing the thermal performance and the efficacy of the helical dimpling enhancement on heat transfer processes. The experimental results concerning temperature changes are critical for understanding the impact of helical dimpling on heat transfer efficiency, as depicted in Figure 12. Figure 12(a) and Figure 12(b) visually show the findings, with clear distinctions in temperature reductions across the range of tested flow rate. The data indicates that at every given mass flow rate, the dimple- From the data, we observe a clear trend: as the mass flow rate increases, so does the exit temperature of the water in both heat exchanger designs. This trend indicates that higher flow rates, while increasing the rate of heat transfer by mass, do not allow sufficient time for the water to fully absorb heat, thus exiting the system at higher temperatures. For example, in the traditional heat exchanger, the exit temperature increases from 36℃ at a flow rate of 0.03 kg/s to 47℃ at 0.15 kg/s. Similarly, in the dimple-added heat exchanger, temperatures increase from 32℃ to 44℃ under the same conditions. The lower temperatures observed in the dimple-added heat exchanger across all flow rates highlight the efficiency of dimples in enhancing heat transfer. The dimples increase the surface area and create turbulent flows that disrupt the boundary layer formation, facilitating higher rates of heat transfer even as flow rates increase. For instance, at a flow rate of 0.15 kg/s, the dimple-added model shows a 3℃ lower exit temperature compared to the traditional model, which is a significant improvement in thermal performance.

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Figure 12. The experimental-based hot exit water temperature bars across the inner tube of the HE under all working condition of variant mass flow rates: (a) traditional, (b) dimpled

Next, we analyze the pressure drops recorded during the experimental testing of both the traditional and dimple-added helical-tubed heat exchangers. Pressure drop is an essential parameter for evaluating the hydraulic performance of heat exchangers, as it impacts the pump power requirement and overall system efficiency, as Figure 13 illustrates. The data presented in Figures 13(a) and 13(b) show an expected trend where the pressure drop across the heat exchanger increases with the mass flow rate. This is due to the increased resistance encountered by the flowing fluid, particularly as it navigates through the helical and dimpled tubes. From the traditional heat exchanger data, we see a pressure drop from 211 Pa at a flow rate of 0.03 kg/s to 5012.09 Pa at 0.15 kg/s, indicating a substantial increase as the flow rate rises. Similarly, in the dimple-added heat exchanger, the pressure drop starts at 247.1 Pa at the lowest flow rate and escalates to 6736 Pa at the highest flow rate. Notably, the dimple-added heat exchanger exhibits higher pressure drops at corresponding flow rates compared to the traditional model. For instance, at a flow rate of 0.15 kg/s, the pressure drop in the dimple-added model is approximately 34% higher than in the traditional model. This increase in pressure drop for the dimple-added heat exchanger can be attributed to the enhanced surface roughness and geometric complexity introduced by the dimples. While dimples improve thermal performance by increasing turbulence and surface area for heat transfer, they also add to the hydraulic resistance, thereby elevating the pressure drop.

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Figure 13. The experimental-based pressure drop bars across the inner tube of the HE under all working condition of variant mass flow rates: (a) traditional, (b) dimpled

5.2 Numerical simulation results

This section presents the numerical simulation results of the heat transfer performance for both traditional and dimple-added helical-tubed heat exchangers. The simulations were conducted to evaluate the impact of mass flow rate variations on the hot water exit temperatures under controlled conditions, aiming to complement and compare with the experimental results. The temperature analysis from the numerical simulations offers critical insights into how different configurations of heat exchangers react under various operational conditions. The results are presented in Figures 14, 15 and 16, to provide a comprehensive view of temperature dynamics within these systems. The simulation results demonstrate a clear trend of increasing hot water exit temperatures with increasing mass flow rates for both heat exchanger types. In the traditional model, temperatures gradually rise from 34.9℃ at a flow rate of 0.03 kg/s to 47.7℃ at 0.15 kg/s. This increment highlights the reduced heat transfer efficiency at higher flow rates due to decreased residence time of water within the heat exchanger. Conversely, the dimple-added model exhibits consistently lower exit temperatures across all mass flow rates compared to the traditional model, confirming the enhanced heat transfer capabilities of the dimples. For instance, at the highest flow rate of 0.15 kg/s, the dimple-added heat exchanger achieves a temperature of 44.4℃, which is significantly lower than the 47.7℃ observed in the traditional model.

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Figure 14. Traditional helical-tubed HE temperature profile across varying mass flow rates of: (a) 0.03 kg/s, (b) 0.06 kg/s, (c) 0.09 kg/s, (d) 0.12 kg/s, (e) 0.15 kg/s

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Figure 15. Dimpled helical-tubed HE temperature profile across varying mass flow rates of: (a) 0.03 kg/s, (b) 0.06 kg/s, (c) 0.09 kg/s, (d) 0.12 kg/s, (e) 0.15 kg/s

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