Experimental Investigation of an Innovative Coaxial with Shell-and-Tube Heat Exchanger: Thermo-Hydraulic Performance Evaluation and Comparative Analysis

Heat exchangers are categorized into various types according to their structural configurations, including helical coil and plate-frame heat exchangers. However, the widely favored choice in industrial applications remains the shell-and-tube heat exchanger (STHE). Renowned for its large surface area, compact design, and efficient heat transfer, the STHE finds extensive use in sectors reliant on heat transfer to facilitate chemical processes. Such processes include distillation, synthesis, and combustion, commonly observed in petrochemical, food, and pharmaceutical industries. Additionally, STHE plays a crucial role in converting atomic and fossil energy into electricity, as seen in power stations. Their applications also extend to air-conditioning, heating, and heat pump systems [1], where they contribute to temperature control and thermal regulation in enclosed environments such as cold storage facilities.

The necessity to enhance heat exchanger thermo-hydraulic performances arises from the depletion of energy resources, environmental challenges linked to pollution-induced climate change, and the global pursuit of economic prosperity by nations. A substantial body of literature is dedicated to heat exchanger studies, encompassing both numerical and experimental perspectives. The predominant goal across these works is the enhancement of heat transfer [2] and the reduction of pressure drop within these devices. Several representative studies and enhancement approaches related to heat exchanger thermo-hydraulic performance are briefly reviewed below. 

The thermo-hydraulic performance of heat exchangers is strongly influenced by the characteristics of the heat transfer tubes. Several studies have delved into the impact of tube shapes, including those that are finned, twisted or coiled. Tan et al. [3] employed simulation to investigate heat exchangers featuring twisted oval tubes and finned tubes. They found that these non-circular shapes enhanced turbulence, particularly in comparison to smooth circular tubes. The utilization of finned tubes, in particular, was observed to improve the transfer surface area, consequently boosting the heat exchange rate. In the case of twisted oval tube heat exchangers, the length of the torsion pitch was identified as a significant factor affecting transfer performance. Zhang et al. [4] investigated the thermal-hydraulic characteristics of twisted tubes in turbulent flow and reported that a reduction in twist pitch yields higher heat transfer rates, though this is accompanied by a proportional increase in the friction factor. Dizaji et al. [5] conducted experiments on a vertical heat exchanger employing coil tubes and injecting air bubbles. Their findings demonstrated that this process elevated efficiency and the Number of Transfer Units (NTU) of the examined exchanger. Additionally, Genić et al. [6] experimentally investigated the thermo-hydraulic performance of a spiral-coil heat exchanger and demonstrated that heat transfer enhancement could be achieved by increasing the coil diameter, pitch length, and flow rate.

Other research has examined the characteristics of baffles and how they affect the thermos-hydraulic performances of heat exchangers. An experimental investigation comparing segmented and overlapping helical baffles in STHEs was carried out by Zhang et al. [7] by employing oil as the fluid flow. According to their research, the latter kind of exchanger outperformed the former in terms of heat transfer per unit pressure drop at the same flow rate and showed a smaller pressure drop throughout the shell. When Xiao et al. [8] simulated an STHE using helical baffles at different inclination degrees and Prandtl values, they discovered that water had superior heat transfer at a 40° baffle angle, whereas fluids with large Prandtl values showed improved heat transfer at lower inclination angles. The study highlighted that helical baffle exchangers incurred lower pressure drop compared to those featuring segmented baffles. Li et al. [9] used a tube bundle of 19 tubes in a single pass and creative baffle designs to do computational and experimental studies on an STHE. A 2.32 million cell mesh grid was used with a realizable k-ε model in their investigation. Ben Slimene et al. [10] employed simulation to explore turbulent flows in an STHE with different passes, introducing baffles to enhance thermos-hydraulic performances. They used the Shear Stress Transport (SST) k-ω turbulence model and extended it to an STHE configuration with a rectangular shell. Ali et al. [11] provided a concise summary of significant types of heat exchangers and passive techniques for improving heat transfer by focusing on the use of baffles [12].

Additional research has explored various types of baffles, including rod baffles. The STHEs with rod baffles, three-flow baffles, and pore plate baffles were the subjects of comparative research by Chen et al. [13]. In conclusion, the segmented baffle heat exchanger was found to have inferior thermo-hydraulic performance to all of these configurations. Wang et al. [14] examined an STHE with two shell passes and rod baffles in experimental research. And according to the results, it performed better than a single shell pass exchanger with the same kind of baffles. Liu et al. [15] performed a comparative simulation of two rod-baffle STHEs, evaluating the thermo-hydraulic performances of plain tubes against spirally corrugated tubes. The results demonstrated that the corrugated tube configuration achieved superior thermo-hydraulic performances.

Fascinating and innovative research endeavors have yielded novel designs and structures. In a study by Ji et al. [16], a simulation was conducted on an STHE featuring elastic tubes. The findings revealed that an increasing heat transfer was achieved by adding vibrations to the shell side, particularly for low Reynolds number flows. Bougriou and Baadache [17] introduced a novel heat exchanger termed a dual concentric tube and shell heat exchanger. This design not only amplifies the exchange surface but also augments heat transfer within a more compact volume. Van Trang et al. [18] conducted an experiment to evaluate the thermo-hydraulic performances of a four-pass micro-scooter radiator that uses ambient air to cool hot water. The study unveiled that, under certain water flow rates, the cooling efficiency improved with the use of a fan, surpassing that of conventional radiators with larger dimensions. Additionally, it was observed that while fuel consumption decreased, engine power increased, leading to a reduction in the mini radiator operating cost compared to larger conventional counterparts.

Considerable attention has been devoted to plate heat exchangers and the application of nanofluids for heat transfer enhancement. Lozano et al. [19] conducted numerical and experimental investigations on a grooved plate heat exchanger using oil and water as working fluids in automotive applications. The study revealed non-uniform fluid flows and indicated a tendency for the two fluids to move along the plates' lateral extremities. Elias et al. [20], Leong et al. [21], and Elias et al. [22] investigated the effects of nanofluids on STHE. The first group investigated baffles with different inclination angles and reported that nanofluids enhanced thermo-hydraulic performance. The second group compared nanofluids with ethylene glycol and water, determining that nanofluids resulted in superior overall heat transfer and convection coefficients. The third group concluded that the shape, volume concentration, and particle size of the constituents in nanofluids significantly influenced heat exchanger thermo-hydraulic performance. Philip et al. [23] carried out experiments on the properties of magnetic nanoparticles and observed an increase in thermal conductivity when controlled by an external magnetic field. Yu et al. [24] further explored graphene nanosheets dispersed in ethylene glycol and reported an increase in thermal conductivity owing to their favourable thermophysical properties.

An alternative method for studying heat exchangers is to use optimization methods of non-linear calculation algorithms. Touatit and Bougriou [25] conducted an optimization study on a triple concentric tube heat exchanger in which nitrogen, oxygen, and hydrogen were circulated through three separate channels. They used a Fortran-based computation code and a techno-economic method to calculate heat transfer coefficients, total friction power, and temperature profiles. Ultimately, they identified optimum diameters that minimized energy consumption and manufacturing costs. Xie et al. [26], Fettaka et al. [27], and Guo et al. [28] optimized the STHE thermo-hydraulic performance using non-linear prediction systems. The first group employed the artificial neural method, the second utilized the multi-objective optimization method, and the third employed the genetic algorithm. Optimization research on an STHE was carried out by Şahin et al. [29] utilizing the artificial bee colony approach. Upon comparing their results with existing literature, they concluded that this method effectively minimizes manufacturing and operating costs. In another study, Guo et al. [30] optimized an STHE by maximizing the objective function describing the synergy field number. This number signifies the synergy between heat transfer and the velocity field, leading to the conclusion that its use yields superior thermo-hydraulic performances and design cost compared to the traditional method for reducing the objective function that characterizes the overall cost. To minimize total expenditures, Fesanghary et al. [31] explored STHE optimization using the harmony search method combined with global sensitivity analysis. Their findings indicate that this integrated approach yields significantly more precise results compared to traditional genetic algorithms.

This study presents an experimental investigation of a coaxial with shell-and-tube heat exchanger (CWST), comparing its thermo-hydraulic performances to a conventional STHE design. The results demonstrate that the CWST achieves higher heat flux and lower pressure drop. While heat transfer rates are comparable between the two models, the CWST exhibits a superior heat transfer rate to pressure drop ratio. Furthermore, these experimental findings are consistent with the numerical simulations reported by Medjdoub et al. [32] regarding both thermal and hydraulic performances.

The heat exchangers experimented on the test bench circulate water in counter-current and have inlet temperatures and material properties shown in Table 3. A theoretical calculation on the STHE was carried out to obtain outlet temperatures that were compared with those of the two tested heat exchangers.

Table 3. Properties of the material

The calculation steps that provide us with the output temperatures are as follows: 

1. Calculation of the Nusselt $N u_t$ inside the tubes: to do this, we use the Nitsche correlation [33] for lamimar flows ($\operatorname{Re}<2300$), after calculating the Reynolds number $R e_t$ and the Prandtl number $\operatorname{Pr}$ as follows:

$R e_t=\frac{G_t \cdot d i}{\mu}$        (1)

$P r=\frac{\mu \cdot C p}{\lambda}$         (2)

$N u_t=1.86 \cdot\left[\frac{R e_t \cdot P r \cdot d i}{L}\right]^{0.33}$         (3)

2. Calculation of the Nusselt $N u_s$ inside the shell: for this purpose, we use the Prończuk-Krzanowska correlation [34], and we calculate the equivalent diameter $d_{e q}$ for the triangular pattern, then the Reynolds number $R e_s$ as follows:

$d_{e q}=4 \cdot \frac{\left(\frac{P t^2 \cdot \sqrt{3}}{4}-\frac{\pi \cdot d o^2}{8}\right)}{\frac{\pi \cdot d o}{2}}$         (4)

$R e_s=\frac{G_s \cdot d_{e q}}{\mu}$         (5)

$N u_s=0.0813 \cdot\left[R e_s\right]^{0.834} \cdot[P r]^{0.33}$            (6)

3. Overall heat transfer coefficient $K$ calculation: for this purpose, we calculate the convection heat transfer coefficients inside and outside the tubes ($h_t$ and $h_s$) and the average exchange surface $S_m$ as follows:

$h_t=\frac{N u_t \cdot \lambda}{d i}$        (7)

$h_s=\frac{N u_s \cdot \lambda}{d_{e q}}$           (8)

$S_m=\frac{\pi \cdot[d i+(d i+2 \cdot e)] \cdot L \cdot N t}{2}$            (9)

$K \cdot S_m=$ ${\frac{1}{\frac{1}{h_s \cdot \pi \cdot(d i+2 \cdot e) \cdot L \cdot N t}+\frac{1}{h_t \cdot \pi \cdot d i \cdot L \cdot N t}+\frac{\ln \left(\frac{d i+2 \cdot e}{d i}\right)}{2 \cdot \pi \cdot \lambda_t \cdot L \cdot N t}}}$           (10)

$K=\frac{K \cdot S_m}{S_m}$           (11)

4. Calculation of the heat transfer rate $\phi$: to do this, we calculate the efficiency $\varepsilon$, the $N T U$ and the maximum heat transfer rate $\phi_{\max }$ as follows [35]:

$N T U=\frac{K \cdot S_m}{(q m \cdot C p)_{\min }}$          (12)

$C r=\frac{(q m \cdot C p)_{\min }}{(q m \cdot C p)_{\max }}$            (13)

$\begin{aligned} & \varepsilon= 2 \cdot\left[1+C r+\left(1+C r^2\right)^{\frac{1}{2}} \cdot \frac{1+\exp \left[-N T U \cdot\left(1+C r^2\right)^{\frac{1}{2}}\right]}{1-\exp \left[-N T U \cdot\left(1+C r^2\right)^{\frac{1}{2}}\right]}\right]^{-1}\end{aligned}$           (14)

$\phi_{\max }=(q m \cdot C p)_{\min } \cdot\left(T t_i-T s_i\right)$            (15)

$\phi=\phi_{\max } \cdot \varepsilon$         (16)

5. Calculation of outlet temperatures $T t_o$ and $T s_o$: by using the heat transfer rate $\phi$ as follows:

$T t_o=T t_i-\frac{\phi}{q m_t \cdot C p}$           (17)

$T s_o=\frac{\phi}{q m_s \cdot C p}+T s_i$              (18)

In this article, an experimental investigation was conducted on the CWST heat exchanger in comparison with the STHE, and a theoretical calculation on the STHE was carried out to compare its results with the experimental ones. This study led to the following conclusions:

• The CWST exchanger achieves a greater heat flux than the STHE for hot fluid volume flow rates inside the tubes varying from 6.5 l/min to 9.5 l/min and with a cold fluid volume flow rate of 7.7 l/min.
• The CWST heat exchanger delivers heat transfer rates close to those of the STHE for the low volume flow rates studied in this scientific paper.
• The CWST exchanger results in a lower pressure drop than the STHE, and therefore favors an increase of its heat transfer rate to pressure drop ratio.
• The experimental results obtained in this article are in line with the simulation results obtained in the article of Medjdoub et al. [32].
• The results obtained by theoretical calculation for the STHE are close to those obtained experimentally.

Future research will focus on broadening the operating range of the CWST heat exchanger, notably by investigating fluids with different viscosities. The development of generalized correlations based on dimensionless numbers will be essential to optimize the geometric parameters and ensure the reliability of the system when scaled up to industrial dimensions.