1. Introduction
The air heat exchanger is the core component of an air-conditioning system because heat exchange occurs in it. An air heat exchanger comprises copper tubes with a spiral-grooved inner surface and hydrophilic aluminium foil fins. The grooved inner surface of the tubes, collar design, and uneven structures on both sides of the fin increase the heat transfer efficiency by 20–30% and reduce energy consumption by 15% compared to a smooth inner structure [
1,
2,
3]. This heat exchanger design is widely applied in power generation, air conditioning, and many other industries. Energy-saving air conditioners are suitable for applications requiring reductions in energy consumption and emissions [
4].
Figure 1 shows an air heat exchanger.
A heat exchanger can be designed with small-diameter heat exchange tubes and fins, which help maintain the heat transfer performance and reduce the weight, package volume, airflow resistance, and manufacturing complexity [
5]. Although reducing the tube diameter decreases the heat exchange area, it improves the heat transfer performance. For example, using a
Φ5 mm tube instead of a
Φ9.52 mm tube increases the heat transfer coefficient by 10%. In addition, the reduced amount of copper reduces the cost by more than 50% and the filling volume by about 25% [
6]. Therefore, research on the joints of small-diameter heat exchange tubes and fins is of great significance.
To improve the joints of tubes and fins, tube hydroforming (THF) is an alternative to mechanical roll expansion that uses hydraulic fluid as a uniform forming medium to transform metal tubes into complex hollow parts [
7]. THF can be applied to heat exchange tubes and fins to solve problems such as the wrinkles and bending caused by mechanical roll expansion. THF does not damage the inner grooved structure. Typically, THF results in a 1–3% wall reduction due to the uniform hydraulic pressure; in contrast, mechanical roll expansion results in a reduction of 3–12% because of the uneven axial compression [
8]. Consequently, THF can potentially be applied to the joints of heat exchangers, which undergo an expansion process, as shown in
Figure 2. However, research on the hydraulic expansion of tubes has been limited.
Tube hammering/pulsating hydroforming was first proposed by Rikimaru and Ito [
9] in 2001 and was applied to the hydraulic expansion of a tube to improve performance. They found that applying pulsating hydroforming could delay the rupture time of tubes [
9]. In 2004, Di Lorenzo et al. [
10] used artificial intelligence techniques, numerical simulations, and experiments to realize a tube hydroforming design. Other research on tube hammering/pulsating hydroforming has shown that hydraulic expansion increases the height of the expanded tube and produces a more uniform wall thickness in the bulging zone of the tube [
11,
12]. Yang et al. [
13] and Mohamed et al. [
14] simulated the pulsating hydroforming process while considering the expansion pressure and time increment. They proposed new indicators for evaluating the uniformity of the tube wall thickness, reliability, moulds fillability. They found that the expansion of the tube increased as the expansion pressure and time increment decreased and the pulsating hydraulic amplitude increased. Thus, tube hammering/pulsating hydroforming is considered a promising method for fabricating the joints of heat exchange tubes and fins and improving the reliability of heat exchangers.
An important mechanical property [
15] for the performance of the heat exchanger is the expansion strength [
16] of the joint between a tube and fin, which is denoted by the pull-out force. The pull-out force after hydraulic expansion is mostly evaluated through tensile tests. Merah et al. [
17,
18] performed finite element analysis and found that, for low clearances, the calculated residual contact pressure of the analytical results compares well with those inferred from the experimentally measured pull-out force. Wang et al. [
19] used the finite element method to analyse a shell-and-tube multiple fixed-tube plate heat exchanger for three cases and calculated the pull-out force for a heat exchanger and tube-sheets at different temperature distributions. Their results showed that the effect of the pull-out force on the different locations of a heat exchanger tube is greater for multiple exchangers. However, most of the above studies were based on mechanical roll expansion. Few studies have evaluated the pull-out force of tubes and fins in air heat exchangers fabricated by hydraulic expansion.
In this study, a device was designed for the hydraulic expansion of heat exchange tubes and fins; the device is reliable and adaptable to tubes and fins of different numbers and lengths. A non-pulsating hydraulic expansion experiment was performed on test samples comprising copper tubes and fins. Based on the experimental results, the appropriate hydraulic pressure range was determined. The same test results were used to evaluate the pull-out force of different bulging zones of the same test sample expanded at the proper hydraulic pressure. The influence of different bulging zones on the pull-out force of heat transfer tubes was evaluated. A hammering/pulsating hydraulic expansion test was applied to the joints of tubes and fins with a varying pulsating amplitudes and frequencies. After the expansion, a tensile test was performed on the samples, and the maximum pull-out force with different pulsation parameters was determined. The results were used to characterise the influence of the pulsation parameters on the pull-out force.
Author Contributions
Conceptualization, H.H. and L.Y.; Data curation, H.H. and J.M.; Formal analysis, H.H. and J.J.; Funding acquisition, L.Y.; Investigation, H.H., J.J. and J.M.; Methodology, H.H.; Project administration, L.Y.; Resources, H.H., L.Y. and J.J.; Supervision, L.Y.; Validation, H.H., J.J. and J.M.; Writing—original draft, H.H.; Writing—review & editing, H.H. and L.Y. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the National Natural Science Foundation of China (52065014), Natural Science Foundation of Guangxi Province (2017GXNSFAA198133), and the Innovation Project of Guangxi Graduate Education (JGY2019074).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Air heat exchanger, comprising inner grooved copper tubes and hydrophilic aluminium foil fins.
Figure 2.
Diagram of the hydraulic expansion process in the air heat exchanger.
Figure 3.
Test sample comprising details of cross-section of tubes and fins.
Figure 4.
The hydraulic expansion device and sealing part. (a) The design of the device; (b) The photo of device.
Figure 5.
Non-pulsating and pulsating hydraulic expansion experimental platform. (a,d) External pressure liquid supply device; (b) data acquisition system; (c) stamping machine; (e) self-designed hydraulic expansion device; (f) non-pulsating hydraulic pressure; (g) pulsating hydraulic pressure.
Figure 6.
(a) Bending failure and (b) bursting failure in the sample due to hydraulic expansion.
Figure 7.
Tensile test platform.
Figure 8.
Eight samples in the tensile test cut along the dashed lines.
Figure 9.
Sample jointed at a hydraulic pressure of (a) 14 MPa and (b) 20 MPa.
Figure 10.
Variation in the pull-out force with the loading time at different hydraulic expansion pressures: (a) 16, (b) 17, and (c) 18 MPa.
Figure 11.
Variation in Fmax at different bulging zones of tensile test samples and 16, 17, and 18 MPa.
Figure 12.
Variation in Fmax with ΔP at the same frequency.
Figure 13.
Variation in the pull-out force over time for the three amplitudes and three frequencies: (a) 0.67, (b) 1.29, and (c) 1.80 c/s.
Table 1.
Geometric and material parameters of the TP2 tube and fin made of 8006 aluminium.
Tube | Value | Fin | Value |
---|
Outer diameter Φ0 (mm) | 5 | Arrangement length l1 (mm) | 355 |
Wall thickness δ (mm) | 0.5 | Width d0 (mm) | 40 |
Length l0 (mm) | 375 | Height h0 (mm) | 11 |
Density (g·cm−3) | 8.916 | Density (g·cm−3) | 2.780 |
Young’s modulus (GPa) | 127 | Young’s modulus (GPa) | 68 |
Poisson’s ratio | 0.33 | Poisson’s ratio | 0.33 |
Tensile stress (MPa) | 205.807 | Tensile stress (MPa) | 136.889 |
Yield stress (MPa) | 66 | Yield stress (MPa) | 132 |
Table 2.
Settings for the stroke and speed of servo stamping machine and the pulsation parameters of the amplitude and frequency.
Stroke s (mm) | Speed n (r/min) | Amplitude ΔP (MPa) | Frequency f (c/s) |
---|
2 | 200 | 1.72 | 0.67 |
4 | 500 | 2.83 | 1.29 |
6 | 800 | 3.53 | 1.8 |
Table 3.
Pulsation parameters of the amplitude and frequency in the experiment.
Stroke s (mm) | Speed n (r/min) | Amplitude ΔP (MPa) | Frequency f (c/s) |
---|
2 | 200 | 1.72 | 0.67 |
2 | 500 | 1.72 | 1.29 |
2 | 800 | 1.72 | 1.80 |
4 | 200 | 2.83 | 0.67 |
4 | 500 | 2.83 | 1.29 |
4 | 800 | 2.83 | 1.80 |
6 | 200 | 3.53 | 0.67 |
6 | 500 | 3.53 | 1.29 |
6 | 800 | 3.53 | 1.80 |
Table 4.
Average value of Fmax of tensile test samples at Pmax of 16, 17, and 18 MPa.
Pmax (MPa) | Fmax (kN) |
---|
16 | 0.36 |
17 | 0.38 |
18 | 0.47 |
Table 5.
Fmax that each part of the test sample could withstand at Pmax = 16, 17, and 18 MPa.
Pmax (MPa) | Fmax (kN) |
---|
A | B | C | D |
---|
16 | 0.35 | 0.36 | 0.38 | 0.36 |
17 | 0.36 | 0.39 | 0.40 | 0.36 |
18 | 0.40 | 0.50 | 0.51 | 0.45 |
Table 6.
Fmax corresponding to each group of ΔP and f.
ΔP (MPa) | f (c/s) | Fmax (kN) |
---|
1.72 | 0.67 | 0.391 |
1.72 | 1.29 | 0.394 |
1.72 | 1.80 | 0.359 |
2.83 | 0.67 | 0.404 |
2.83 | 1.29 | 0.396 |
2.83 | 1.80 | 0.419 |
3.53 | 0.67 | 0.389 |
3.53 | 1.29 | 0.403 |
3.53 | 1.80 | 0.391 |
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