Applied Thermal Engineering 236 (2024) 121864
Available online 28 October 2023
1359-4311/© 2023 Elsevier Ltd. All rights reserved.
Research Paper
Analysis of influence of inlet vapor quality on heat transfer and flow pattern in mini-channels during flow condensation process
Huiqing Shang, Ziheng Yan, Guodong Xia
*The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, China & Beijing Key Laboratory of Heat Transfer and Energy Utilization, Beijing University of Technology, Beijing 100124, China
A R T I C L E I N F O Keywords:
Mini-channel Flow pattern Condensation Vapor quality Heat transfer
A B S T R A C T
Micro/mini-channel heat sinks are extensively utilized in heat dissipation systems that involve high heat flux components. These heat sinks are preferred due to their compact size and excellent heat transfer efficiency. The selection of a refrigerant condensation system for the heat dissipation system under vapor conditions is crucial. In order to investigate the heat transfer characteristics of flow condensation in a mini-channel, a visual experiment system was used with a hydraulic diameter of 2 mm and a volume flowrate of 30 mL/min, 60 mL/min, 90 mL/
min, 120 mL/min respectively. The study focused on the effect of inlet vapor quality on the flow pattern and heat transfer characteristics of flow condensation in the mini-channel. The results indicate that the flow pattern in flow condensation process can be classified into annular flow, transition flow, slug flow and bubble flow in sequence along the flow direction. As the inlet vapor quality (xin) increases, the dominance of annular flow gradually increases, making it difficult to capture the bubble flow with high volume flowrate and high inlet vapor quality. This phenomenon is observed under experimental conditions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The increase in inlet vapor quality is beneficial to the improvement of condensation heat transfer coefficient. In this experiment, it was observed that the local heat transfer coefficient was decreased by up to 83.7 % when comparing the lowest vapor quality to the highest at the same volume flowrate, emphasizing the significance of inlet vapor quality in the flow condensation during the heat transfer process.
1. Introduction
At the present stage, the development of industrial technologies needs to follow the green and low-carbon goals and adapt to the new development philosophy, so that we can build a system that is green, low-carbon and sustainable. With the widespread response to the con- cepts of “carbon neutrality” and “carbon peaking”, the clean energy utilization has become one of the important measures to control the carbon emission in various industries [1]. In the utilization of clean energy sources, the photovoltaic technology has emerged as one of the cutting-edge technologies with some advantages such as wide applica- tion, high potential, and controllable cost. However, the heat dissipation has become an important bottleneck limiting the development of photovoltaic technology. It is showed that the conversion efficiency of solar panels in the concentrated photovoltaic system is limited to less than 40 %, with the remaining energy dissipated in the form of heat.
Furthermore, every 1 ℃ temperature rise in the solar cell would result in 0.2 % − 0.5 % reduction in the conversion efficiency. In this way, the rise of temperature and decrease of conversion efficiency form a vicious cycle that may seriously damage the power efficiency and lifespan of the battery [2–4]. Therefore, the control of temperature becomes an important part in this field.
Heat dissipation technologies based on liquid–vapor phase transition are extensively utilized in various industries due to their high heat dissipation capability and energy-saving benefits. Among these tech- nologies, micro/mini-channel heat sinks have garnered significant attention as direct cooling devices. Since the proposal of the concept of
“micro-channel heat sink”, these micro-channel heat sinks, known for their excellent heat dissipation performance and compact structure, have achieved significant advantages in the field of heat dissipation technology [5–6]. The research on the heat dissipation performance of micro/mini-channel heat sinks primarily focuses on the cross-sectional shape, the type and distribution of the channel, the wetting
* Corresponding author.
E-mail address: [email protected] (G. Xia).
Contents lists available at ScienceDirect
Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng
https://doi.org/10.1016/j.applthermaleng.2023.121864
Received 28 August 2023; Received in revised form 19 October 2023; Accepted 25 October 2023
characteristics of the inner surface of the channel, and the type and physical properties of refrigerants [7–10]. In the study of two-phase flow heat transfer, researchers focus not only on the heat dissipation of liquid vaporization, but also on the condensation of the refrigerant vapour [11–17]. In order to study the heat transfer characteristics of refrigerant vapor condensation, researchers have conducted comprehensive studies on the various influencing factors mentioned earlier. It has been observed that the hydraulic diameter plays a significant role in deter- mining the final performance of the condensation heat transfer in channels [18]. Jing et al.[19] investigated the performance of rectan- gular, elliptical and isosceles triangle section shapes with a fixed section area and circumference. They found that the heat transfer coefficient decreases as the hydraulic diameter increases. Moreover, rectangular and triangular channel cross-sectional shapes are relatively more favorable for condensation heat transfer [20]. In the study on the condensation heat transfer of refrigerant vapor in micro/mini-channels, Keniar et al.[21] investigated the condensation process of refrigerants R134a, R245fa and R1234ze(E) in both circular and square micro- channels. Their experiments revealed that, under similar conditions, refrigerant R245fa exhibited a significantly higher heat transfer coeffi- cient compared to the other two refrigerants when the mass flow rate exceeded 150 kg/(m2⋅s). Jige et al.[22] conducted an experiment using horizontal multiple circular mini-channels with varying inner diameters as carriers to investigate condensation heat transfer coefficients. The results showed that the heat transfer coefficient of R32 was higher than that of R1234yf and increased as the channel size decreased. For instance, in a channel with an inner diameter of 0.49 mm, the heat transfer coefficient of R32 was found to be approximately 40 % higher compared to a channel with an inner diameter of 0.81 mm. However, the heat flux exhibited an opposite trend, with smaller inner diameters having a greater impact on the channel. These researches have revealed that the shape of the channel significantly affects the process of condensation heat transfer. Important parameters such as the heat transfer coefficient and the flow pattern of condensation show signifi- cant variations under different aspect ratios of the regular channel cross- section. In their study, Yan et al.[23] focused on the aspect ratio of the channel and conducted a comparative analysis of the steam condensa- tion heat transfer characteristics in rectangular tubes with aspect ratios of 2:1, 3:1, and 5:1. The researchers found that under experimental conditions with large aspect ratios, the influence of aspect ratio on the
condensation heat transfer process was amplified. The aspect ratio had an important influence on the condensation heat transfer process by affecting the two-phase flow pattern in the channel. Any changes in the flow pattern resulted in significant differences in the condensation heat transfer coefficient. Furthermore, the difference in condensation heat transfer coefficient is more significant when the vapor quality is below 0.5. Singh et al.[24–25] investigated the impact of mini-channel aspect ratios on condensation heat transfer and identified different trends. The study focused on the influence of R134a and R410A vapor, with the channel depth kept constant (1 mm) while the channel width was altered (0.5 mm, 0.7 mm or 1 mm), and the researchers analyzed the condensation heat transfer in mini-channels with different aspect ratios.
Notably, they found that the experimental scenario with a reduced channel width (corresponding to a small aspect ratio) exhibited a distinct condensation heat transfer effect compared to the situation with a large aspect ratio. Moreover, the vapor quality also affects this process, and the growth rate of condensation heat transfer coefficient is greater when it is within the range of 0.5 to 0.83. The two different trends mentioned here are related to the differences in channel size and aspect ratio setting methods. The former trend involves using a larger channel size with a fixed channel width of 13.5 mm, and adjusting the channel height to achieve different aspect ratios. On the other hand, the latter trend involves using a fixed channel height of 1 mm and adjusting the channel width to achieve different aspect ratios. In addition, the per- formance of the micro/mini-channel heat sink is more significantly affected by the type of channel arrangement. The type of channel used in a heat sink has a direct impact on the internal condensation flow pattern layout. The study has shown that printed circuit heat exchangers with the wavy micro-channel have fewer flow patterns compared to straight channels, but they exhibit better heat transfer capabilities [26]. Vaisi et al.[27] also conducted a study on wavy micro-channels and developed a three-fluid plate-fin compact heat exchanger on the vapor condensa- tion heat transfer. The upper and lower layers of the exchanger consist of strip channels through which hot and cold fluids flow, while the middle layer with wavy channels is dedicated to vapor flow. The research findings indicate that the heat exchanger exhibits optimal heat transfer performance when the heat exchange area ratio of the two sections is approximately 1. There are other several factors that affect this process, including micro/nano-structures on the channel surface or wettability of the inner surface in channels [28–31]. The micro/nano-structure and Nomenclature
Q Heat exchange capacity (W) m Mass flowrate (kg/(m2⋅s))
cp Specific heat at constant pressure (J/(kg⋅℃)) T Temperature (℃)
hfg Latent heat of vaporization of liquid refrigerant R141b (J/
q kg) Heat flux (kW/m2) d Distance (mm)
W Width (mm)
K Thermal conductivity of copper (W/(m⋅K)) htp Local heat transfer coefficient (W/(m2‧K)) R Uncertainty
x Vapor quality Subscripts
1 First row thermocouples’ position 2 Second row thermocouples’ position
max Maximum value for specific vapor quality conditions min Minimum value for specific vapor quality conditions in, h Hot water inlet of the plate heat exchanger
out, h Hot water outlet of the plate heat exchanger in Refrigerant inlet of experimental section pre Evaporation section
pre, in Refrigerant inlet of evaporation section pre, out Refrigerant outlet of evaporation section r Refrigerant R141b
h Hot water
sat Saturation state ch Mini-channel
th1 First row of thermocouples th2 Second row of thermocouples w Wall surface in mini-channels
Cu Copper
Superscript
– Reduction
Greek symbols
η Fin efficiency γ Fin efficiency factor
δ Reduction ratio of the heat transfer coefficient φ Reduction ratio of the wall temperature
coating on the inner surface of the channel have a significant impact on the performance of condensation heat transfer and the type of flow pattern [32], such as the appearance of droplet-streak flow, similar to the factors mentioned above. Additionally, the hydrophobic treatment on the channel inner surface promotes the flow condensation heat transfer [33]. Chen et al.[34] conducted an experiment where they sprayed a 0.2 mm thick coating on the inner surface of a smooth channel, altering its structure. Compared to an equivalent smooth tube, this modification resulted in a remarkable increase of up to 44.78 % in the condensation heat transfer coefficient within the vapor quality range of 0.2 to 0.9. These findings demonstrate substantial advantages in the condensation heat exchange process. In addition, the refrigerant cate- gory can also determine the performance of condensation heat transfer by its physical characteristics, it is necessary to choose the suitable refrigerant [35–36].
The vapor quality significantly influences the distribution of flow patterns in mini-channels, subsequently impacting the performance of flow condensation, but the relevant data are inadequate. In the context of panel heat dissipation in concentrator photovoltaic power generation systems, this study aims to investigate the impact of vapor quality on condensation heat transfer in mini-channels and explore the inherent relationship between vapor quality and the type of flow pattern in these mini-channels. We conducted a comprehensive experimental study on the flow condensation heat transfer of the refrigerant vapor R141b. Our study aimed to characterize the thermal performance by measuring the wall temperature distributions and evaluating the overall and local heat transfer coefficients of mini-channels. We also distinguished the flow pattern in the mini-channel and assessed the influence of refrigerant vapor quality. The present study can provide data support and reveal the laws governing the flow condensation heat transfer characteristics and the change of flow patterns in mini-channels using refrigerant R141b as
the working fluid.
2. Experimental system and equipment 2.1. Experimental system
Fig. 1 presents a schematic diagram of the mini-channel flow condensation heat transfer system, which comprises three subsystems and one auxiliary system. The first subsystem involves the circulation of refrigerant R141b, while the second subsystem focuses on the circula- tion in the condensation section. The third subsystem deals with the circulation in the evaporation section, and the auxiliary system en- compasses the data acquisition system for video and temperature. In detail, the experimental system consists of constant temperature water tank, water pump, heat exchanger, mass flowmeter, peristaltic pump, liquid storage tank and data acquisition system, etc. The refrigerant R141b undergoes a phase transition from liquid to gas in the evapora- tion section circulation subsystem due to heating. The resulting satu- rated R141b vapor then enters the parallel mini-channel condensation section. The driving force for condensation is provided by the counter- current flow in the condensation section circulation subsystem. Then, the system continues to operate and completes a cycle. The parameters of the data acquisition system for the experimental setup can be found in Table 1. Using this system, we collected visualization data on the flow condensation heat transfer of mini-channels and the temperature fluc- tuation range. We also analyzed the influence of inlet vapor quality on the heat transfer and flow pattern in this experiment.
2.2. Design of the parallel mini-channel experimental piece
The experimental work on flow condensation heat transfer in mini-
Fig. 1. Parallel mini-channel flow condensation experiment system diagram.
channels was conducted using a copper-based parallel mini-channel.
This mini-channel heat sink is machined with a machining accuracy of
± 5 μm. Fig. 2 illustrates the explosion and physical diagram of the experimental piece. The mini-channel heat sink structure has a total of 8 channels, and the width of the entire channel area is 30 mm. This test section includes four main components, including the aluminium alloy cover plate, the quartz glass, the mini-channel test section, and the condensation base. The components of the experimental piece were connected using screws and sealed with a polytetrafluoroethylene sealing gasket to ensure that there was no refrigerant leakage during system operation. Transparent quartz glass was used as the material for the visualization window in this experiment. Table 2 provides the overall size parameters of the mini-channel experimental piece.
A working fluid buffer chamber is incorporated into the copper- based experimental piece both at the inlet and outlet of the mini- channel, with a length, width and depth of 30 mm ×10 mm ×8 mm.
The temperature and pressure measuring holes, with a diameter of 6 mm, are designed on both sides of the working fluid buffer chamber.
Additionally, one side of the copper-based mini-channel features two rows of wall temperature measuring holes, each with a diameter of 1.5 mm, arranged at a distance of 50 mm. The schematic diagram of the copper-based parallel mini-channel structure is illustrated in Fig. 3. The mini-channel with a length of 300 mm, a width of 2 mm (Wch), a height of 2 mm (Hch), and a rib width of 2 mm (Wfin). The other measurement specifications and dimensions are shown in Table 3.
The water-cooling head made of aluminum alloy is connected to the bottom surface of the copper-based parallel mini-channel, that is, the aluminum alloy base. The water-cooling head is designed with a cooling
water channel that has a length, width, and depth of 300 mm ×30 mm
×10 mm inside. The inlet and outlet are respectively equipped with a measuring hole 6 mm in diameter, at the front and back ends of this channel. The working principle about it is illustrated in the accompa- nying picture in Fig. 2.
3. Experimental data and uncertainty analysis 3.1. Experimental conditions and data processing
In this experiment, the independent variables are the inlet vapor quality and mass flow rate, and the working fluid used was the low boiling point refrigerant R141b. The experimental conditions of inde- pendent variable are presented in Table 4, and the physical property parameters of the working fluid are shown in Table 5. In addition to the relevant parameters of the independent variable in Table 4, detailed experimental conditions for other parts are provided. The set tempera- ture range of the constant temperature water tank in the evaporation section of the system is 30 ℃ to 55 ℃, and the high-temperature water flow rate is 7.5 ×10-3 kg/s. The set temperature of the constant tem- perature water tank in the condensation section of the system is 10 ℃, and the condensate flow rate is 21.67 ×10-3 kg/s.
In the experiment, the inlet vapor quality (xin) can be obtained by thermal balance calculation in the experimental system, and the calcu- lation formula is
xin=Qpre− mrcp,r
(Tpre,out− Tpre,in
)
mrhfg (1)
where Qpre is the absorbed heat by refrigerant R141b in the evaporation section. mr is the mass flow rate of liquid refrigerant R141b. cp,r is the specific heat at constant pressure of liquid refrigerant R141b. Tpre,in and Tpre,out are the temperature of liquid refrigerant R141b entering and exiting the evaporation section, respectively. hfg is the latent heat of vaporization of liquid refrigerant R141b at saturation temperature.
The amount of absorbed heat in the evaporation section can be ob- tained from the heat release of hot water in the evaporation section. This is done by considering the inlet water temperature of the plate heat exchanger (Tin,h), the mass flowrate of hot water in the evaporation circuit (mh), and the outlet water temperature of the plate heat exchanger (Tout,h); the calculation formulas are
Qh=mhcp,h
(Tin,h− Tout,h
) (2)
Qpre=mrcp,r
(Tpre,out− Tpre,in
)+xinmrhfg (3)
Table 1
Data collection system parameters.
Instrument names Model Characteristic parameters Differential pressure
transmitter EJA110EJL 12 temperature measuring points in total: 2 temperature measuring points at the inlet and outlet of the channel and 10 temperature measuring points along the channel. Measurement accuracy: 0.2 % T-type
thermocouple WRT-187 Temperature range: − 100 ~ 350 ℃. Probe length: 10 mm. Measurement accuracy: ± 0.2 ℃ (Standardization)
PT100-type thermal
resistance WZP-187 Temperature range: − 50 ~ 450 ℃. Probe length: 10 mm. Measurement accuracy: ± 0.2 ℃ (Standardization)
Data acquisition
instrument Agilent
34970A Scanning rate of 250 per second.
Fig. 2. Explosion and physical diagram of the experimental piece.
according to the heat conservation, the calculation formula is
Qpre=Qh (4)
where Qh is the heat release of hot water in the evaporation section. cp,h
is the specific heat at constant pressure of hot water in the evaporation circuit.
The heat transfer coefficient and heat flux along the parallel mini- channel can be obtained by the following formulas. The local heat transfer coefficient of the parallel small channel can be calculated as htp= q(
Wch+Wfin
)
(Tsat− Tw)(2ηHch+Wch) (5)
where q is the local heat flux on the wall of parallel mini-channel. Tsat is the saturation temperature of refrigerant R141b. Tw is the local inner wall temperature. η is the fin efficiency. Since the heat transfer in the experiment is mainly concentrated in the vertical direction, the heat transfer process along the vertical direction can be simplified as one- dimensional steady heat conduction. Therefore, the wall heat flux q of the parallel mini-channel can be calculated by equation (6):
q=KCu(Tth1− Tth2)
d2 (6)
where Tth1 is the temperature measured by the first row of thermocou- ples and Tth2 is the second row. d2 is the distance between two rows of thermocouples. KCu is the thermal conductivity of copper. The inner wall temperature of the parallel mini-channel can be calculated by equation (7):
Table 2
The total size parameters of the experimental piece.
Names Total length Total width Total thickness Cover plate thickness Glass thickness Channel layer thickness Base thickness
Sizes 360 mm 60 mm 50 mm 10 mm 10 mm 15 mm 15 mm
Fig. 3. Schematic diagram of the copper-based parallel mini-channel.
Table 3
Measurement specifications and dimensions of the copper-based parallel mini- channel.
Names d1 d2 d3
Sizes 3 mm 5 mm 5 mm
Table 4
Experimental conditions of independent variable.
Names Characteristics
Refrigerant R141b
Mass flow rate (kg/(m2⋅s)) 19.0625–––76.25
Inlet vapor quality 0.15–––0.85
Table 5
Physical property parameters of refrigerant R141b.
Names Parameters
Molecular formula C2H3FCl2
Boiling point (℃) 32.05
Liquid phase density (kg/m3) 1227
Gas phase density (kg/m3) 4.1
Latent heat of vaporization (kJ/kg) 223.0
Specific heat at constant pressure (kJ/(kg‧℃)) 1.16
Liquid phase surface tension (m‧N/m) 18.7
Tw=Tth1+qd1
KCu (7)
where d1 is the distance between the first row of thermocouples and the bottom surface of the parallel mini-channel.
The fin efficiency (η) and the fin parameter (γ) can be calculated by the following formulas [37–38]:
η=tanh(γ⋅Hch)
γ⋅Hch (8)
γ=
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
2htp
KCu⋅Wfin
√
(9)
3.2. Experimental uncertainty analysis
The analysis of experimental uncertainty is crucial during the experiment, and the calculation of uncertainty R is determined by Equation (10) [39]. The main error parameters and uncertainty in the experiment are described below.
R= [(∂R
∂x1
Δx1
)2 +
(∂R
∂x2
Δx2
)2 +⋯+
(∂R
∂xn
Δxn
)2]
(10)
where ×is the uncertainty of the variable, and different variables are represented by 1, 2, ……, n.
In this study, the machining accuracy of the mini-channel can be controlled within 5 μm. Additionally, the temperature measurement error range can be controlled within 0.2 ℃ after experimental calibra- tion. The measurement accuracy of the differential pressure transmitter can also be controlled around 0.2 %. Consequently, during the whole experimental process, the maximum relative error of the mass flowrate is 4.2 %, the maximum uncertainty of the heat flux is 6.7 %, and the maximum uncertainty of the heat transfer coefficient is 8.0 %.
4. Results and discussion
4.1. Flow pattern analysis in parallel mini-channel flow condensation In the experimental research of gas–liquid two-phase flow, the flow pattern is an important parameter to consider in mini-channels. It not only plays a crucial role in explaining heat transfer and dynamic char- acteristics in the channel, but also helps predict and monitor the flow stability of channels in practical engineering applications. To gain a better understanding of certain mechanisms, it is important to delve into topics such as the approximate theory of control in condensate film form and the intrinsic correlation between transfer and flow patterns in the
channel [40–41], among others. The condensation flow characteristics in mini-channels differ from those in conventional channels due to the presence of constraints. Additionally, the flow condensation force in mini-channels is influenced by surface tension, which cannot be ignored.
For this experiment, a copper-based mini-parallel channel experimental piece with high wettability and surface energy was used. Fig. 4 presents the flow pattern diagram of flow condensation in parallel mini-channels.
The diagram corresponds to the conditions of an inlet vapor quality (xin) of 0.35 and a volume flowrate of 60 mL/min (equivalent to a mass flowrate of 38.125 kg/(m2⋅s)). The flow pattern is characterized by the following observations: when there is no liquid blockage in any channel of the experimental piece, the flow pattern is annular flow. However, if a
“liquid bridge” is present in any channel, the vapor flow is no longer continuous, forming an intermittent vapor flow, and the flow pattern transitions to transition flow. At this stage, the definition we introduced as “liquid bridge” can be more clearly expressed, which refers to the connection of liquid between two intermittent vapor columns [42], as shown in Fig. 4. While relatively uniform and slender bubbles appear in the mini-channel, it indicates the onset of slug flow. Finally, if circular bubbles are observed in the channel while the remaining area is filled with liquid, the flow pattern transitions from slug flow to bubble flow.
According to Fig. 4, it can be observed that all flow pattern categories in the flow condensation process can be encompassed within this working condition, including annular flow, transition flow, slug flow and bubble flow along the channel in sequence. This is consistent with the flow pattern evolution of flow condensation in micro/mini-channels proposed in previous studies [43–44]. As depicted in Fig. 4, the flow pattern in the inlet segment of the channel is characterized by annular flow. During this stage, a thin liquid film of condensate is created when the refrigerant vapor comes into contact with the cold wall upon entering the channel. This film adheres to the inner wall of the channel, effectively wrapping the refrigerant vapor in the middle. It is worth noting that the thickness of the thin liquid film increases as the flow distance increases, due to the influence of surface tension. Eventually, this film forms a curved interface with a specific curvature. As the flow distance increases, the gas–liquid interface of the refrigerant is driven by shear force, causing the condensate to flow towards the back end of the channel. In this scenario, the main thermal resistance for flow conden- sation heat transfer is generated by the condensate film. As a result, the annular flow pattern exhibits a higher heat transfer coefficient compared to other flow patterns in the subsequent stage. This can be attributed to the relatively uniform distribution and thin thickness of the liquid film. As the refrigerant flows through the middle and upper segment of the channel, the vapor quality decreases further due to the continuous condensation of the refrigerant vapor in the channel. During this stage, the liquid film is no longer stably attached to the wall due to the combined effect of shear force and surface tension. Instead, it is
x
Fig. 4. Schematic diagram of flow patterns for flow condensation in parallel mini-channel.
blocked by formation of a “liquid bridge”. This transition leads to a change in the flow pattern from annular flow to transition flow. The condensate further accumulates to form a slug flow in the middle and inferior segment of the channel. Within the slug flow, there are slender cylindrical bubbles, which further reduce the vapor quality of the
refrigerant. Upon entering the outlet segment of the channel, the cy- lindrical bubble transforms into a spherical bubble due to the effect of surface tension, resulting in a transition to bubble flow. During this stage, the liquid film thickness in the channel reaches its maximum, while the condensation efficiency is at its lowest. Moreover,
x
x
x
x
x
x
x
x
x
x
x
x
Fig. 5.The effect of inlet vapor quality (xin) on condensation flow patterns at various volume flowrates: (a) the volume flowrate is 30 mL/min; (b) the volume flowrate is 60 mL/min; (c) the volume flowrate is 90 mL/min; (d) the volume flowrate is 120 mL/min. The flow pattern inside of the dark red dotted circle is annular flow, the red dotted circle is transition flow, the yellow dotted circle is slug flow and the blue dotted circle is bubble flow. Inside the dark blue dotted circle is the refrigerant liquid.
experimental studies of Kim et al. [43–44] have shown a limited value that the bubble flow becomes more apparent when the mass flow rate is below 68 kg/(m2⋅s), while the visualization view of bubble flow in Fig. 5 also confirms this viewpoint.
Fig. 5 illustrates the impact of vapor quality on the flow pattern during the flow condensation process at various volume flowrates. The visual diagram in Fig. 5(a) illustrates the influence of different inlet vapor quality on flow patterns at a volume flowrate of 30 mL/min. When xin =0.25, both the vapor quality and flow rate are in a low state, resulting in the observation of annular flow only within a short distance from the inlet segment. As condensation continues in the channel, the flow pattern transitions from annular flow to transition flow, slug flow successively, and then to bubble flow. Moreover, the second half of the channel is predominantly filled with liquid refrigerant due to the lower vapor quality and flowrate. It has been observed that the quality of the inlet vapor is proportional to the length of the annular flow. Addition- ally, it has been found that with higher volume flowrate and inlet vapor quality, it becomes progressively more challenging to capture the bubble flow. For instance, as shown in Fig. 5(b) − 5(d), the bubble flow cannot be accurately detected in cases with inlet vapor quality (xin) and volume flowrate being (0.75, 60 mL/min), (0.55/0.70, 90 mL/min) and (0.25/
0.35/0.45, 120 mL/min). While in the group (0.70, 90 mL/min) and group (0.45, 120 mL/min), the mini-channel is fully occupied by annular flow, resulting in the heat transfer coefficient of condensation in the channel reaching its maximum value. Compared to previous research work, there are some differences in this experiment. Wu et al.
[45] observed a greater variety of flow patterns in their experiment.
Specifically, they observed an injection flow which was not observed in our experiment. This discrepancy may be attributed to the hydraulic diameter of the channel, which has a significant influence. In Al-Zaidi’s research work [46], the types of flow patterns observed were generally consistent with our experiment. Their work included annular flow, slug flow, bubble flow, and a neck. In our experiment, we considered the existence of transition flow to be crucial in order to facilitate the un- derstanding of flow pattern evolution.
Fig. 6(a) shows the distribution diagram of flow patterns at the channel outlet for various inlet vapor qualities and volume flowrates.
This figure can be referenced in practical applications to determine the appropriate volume flowrate and inlet vapor quality for ensuring com- plete or near-complete vapor condensation when it leaves the conden- sation heat sink. To ensure optimal performance, the inlet vapor quality should be selected within the dashed line based on the chosen volume Fig. 5. (continued).
Fig. 6. Flow pattern distribution: (a) the distribution diagram of flow patterns at the channel outlet at the condensate flowrate is 21.67 ×10-3 kg/s and the inlet temperature of cooling water is 12.2 ±0.4 ℃; (b) the transient variation in dimensionless length of a vapor slug at 30 mL/min and xin =0.45.
flowrate. For a volume flowrate of around 90 mL/min, the inlet vapor quality should be 0.40 or below. Similarly, for a volume flowrate of around 60 mL/min, the inlet vapor quality should be 0.55 or below, etc.
Fig. 6(b) tracks a specific vapor slug and plots its dimensionless length varying with time, at 30 mL/min and xin =0.45. This process starts from the transition flow with a slender vapor segment that eventually condensed into a small bubble. This experimental condition was chosen because it encompasses all the flow patterns observed in our tests. Fig. 6 (b) indicated that the variation in dimensionless length can also be used to distinguish the flow patterns in the mini-channel.
4.2. Heat transfer analysis in parallel mini-channel flow condensation The condensation heat transfer of refrigerant vapor in mini-channels is affected by several factors. The condensation flow pattern illustrated in Fig. 5 not only experiences a transition due to the influence of volume flowrate and vapor quality, but also the heat transfer within the channel plays a crucial role in the transition of the flow pattern. Fig. 7 shows the heat flux of condensation at various volume flowrates. As mentioned above, the set temperature of the constant temperature water tank in the condensation section of the system is 10 ℃, and the condensate flowrate is 21.67 ×10-3 kg/s. In the cooling circuit, the inlet temperature of cooling water is 12.2 ±0.4 ℃, and the outlet temperature is slightly different under various experimental conditions.
Fig. 7.The heat flux of condensation at various volume flowrates: (a) the volume flowrate is 30 mL/min; (b) the volume flowrate is 60 mL/min; (c) the volume flowrate is 90 mL/min; (d) the volume flowrate is 120 mL/min; (e) comparison of heat flux at the volume flowrate of 30 mL/min and 120 mL/min.
Fig. 7 shows a changing trend in the heat flux, indicating that the heat flux gradually decreases along the steam flow direction at the condition of low volume and low vapor quality. This change in heat flux is accompanied by flow pattern transition in the channel, including annular flow, transition flow, slug flow and bubble flow in sequence.
These flow patterns result in an increasing proportion of liquid in the mini-channel, with the annular flow representing a very small portion of the total channel length. This is depicted by xin =0.25 in Fig. 7(a). As the vapor quality improves, the heat flux initially increases and then de- creases, appearing an approximate peak. This corresponds to a transition in the flow pattern in the channel, where the proportion of annular flow begins to increase. In the vicinity after the peak point, the flow pattern starts to change, as depicted by xin = 0.85 in Fig. 7(a). Under the experimental conditions of high-volume flowrate, the heat flux under each vapor quality shows an overall increasing trend as the volume flowrate continues to increase. For instance, the trend at xin =0.70 in Fig. 7(c) and xin =0.45 in Fig. 7(d). During this time, the channel is fully occupied by the annular flow, and there is no conversion in the flow pattern. By comparing the changing trends of heat flux at the highest and lowest vapor quality for each volume flowrate, the above conclusion can be further supported, as shown in Fig. 7(e).
The flow pattern in mini-channels is influenced by the inlet vapor quality, which in turn affects the flow condensation heat transfer char- acteristics in parallel mini-channels. Consequently, focusing research line on the inlet vapor quality, the condensation heat transfer in the mini-channel will vary gradually due to fluctuations in the inlet vapor quality. Fig. 8 illustrates the impact of the inlet vapor quality (xin) of refrigerant R141b on the comprehensive condensation heat transfer coefficient at various mass flowrates. The figure demonstrates that the comprehensive condensation heat transfer coefficient rises as both the inlet vapor quality and mass flowrate increase. When the inlet vapor quality is constant, the condensate film moves downstream rapidly as the mass flow rate increases. This phenomenon can be attributed to the effect of shear force. Furthermore, when the mass flowrate remains the same, the flow speed of refrigerant vapor will increase with the increase of inlet vapor quality (xin). This can be explained by the significant difference in density between the gas and liquid phases. Both of the above conditions can result in a decrease in liquid film thickness and an
increase in the condensation heat transfer coefficient, thereby improving the heat transfer performance in the mini-channel. In this study, the comprehensive condensation heat transfer coefficient of 2800 W/(m2‧K) was used as an example, as shown in Fig. 8. When the mass flow rate is 76 kg/(m2‧s), the inlet vapor quality (xin) is less than 30 %.
However, when the mass flowrate is 19 kg/(m2‧s), the inlet vapor quality (xin) is about 85 %. This means that increasing the mass flowrate by 3 times leads to a reduction in the inlet vapor quality (xin) by approxi- mately 65 %. Eventually, these findings demonstrate the significance of inlet vapor quality (xin) in the flow condensation heat transfer process in mini-channels.
In the aforementioned experimental study on parallel mini-channel flow condensation heat transfer, the thermocouples were positioned at distances of 50 mm, 100 mm, 150 mm, 200 mm and 250 mm from the inlet of the channel. This allowed for the calculation of the local condensation heat transfer coefficient and the local wall temperature, enabling real-time monitoring of the heat transfer characteristics of the condensation system. The experimental data are compared with the experimental correlation of micro/mini-channel flow condensation heat transfer proposed in previous studies [24]. Fig. 9 demonstrates that the experimental data in this paper agree greatly with this correlation, with an average error of within 20 %.
In order to ensure the universality of the experimental results, the only independent variable that was changed was the inlet vapor quality (xin) and the volume flowrate of refrigerant was kept constant at 30 mL/
min, 60 mL/min, 90 mL/min and 120 mL/min, respectively. The main objective of these experiments was to observe the effect of different inlet vapor quality on condensation heat transfer performance in mini- channels. Fig. 10 illustrates the influence of inlet vapor quality of refrigerant R141b on the heat transfer coefficient of condensation along the mini-channel at various volume flowrates. Similarly, Fig. 11 dem- onstrates the effect of inlet vapor quality on the wall temperature at different volume flowrates. It is observed that as the flow distance in- creases, the influence of inlet vapor quality on the local heat transfer coefficient and on the local wall temperature along the mini-channel weakens. Additionally, this influence is found to be inversely propor- tional to the inlet vapor quality. δ- in figures represents the reduction ratio of the heat transfer coefficient when the inlet vapor quality (xin) decreases from its maximum value to its minimum value at a specific flowrate, while φ- represents the reduction ratio of the wall temperature, and calculation formulas are
Fig. 8.The effect of inlet vapor quality (xin) on comprehensive condensation heat transfer coefficient at various mass flow rates. The figure shows a random heat transfer coefficient line of the comprehensive condensation, represented by the red dotted line. The four vertical dotted lines correspond to auxiliary lines with the same color as the mass flowrate labels. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.) Fig. 9.Comparison between experimental results of heat transfer coefficient
and correlation calculation results.
δ− =htp,max− htp,min
htp,max
×100% (11)
and
φ− =Tw,max− Tw,min
Tw,max
×100% (12)
Fig. 10(a) and Fig. 11(a) respectively illustrate the influence of each inlet vapor quality (xin) on both the local heat transfer coefficient and the local wall temperature when the volume flowrate is set at 30 mL/
min. For example, at X =50 mm, the local wall temperature with inlet vapor quality (xin) of 0.25 is approximately 11℃ lower than that with xin =0.85. When the inlet vapor quality (xin) is reduced from 0.85 to 0.25, it leads to a significant decrease of about 83.7 % in the local heat transfer coefficient. Furthermore, the local heat transfer coefficient and local wall temperature exhibit a significant decrease along the flow di- rection of the refrigerant particularly in the first four temperature measurement points. This phenomenon can be attributed to the
alteration in flow pattern within the mini-channel. The thickness of the condensate film gradually increases as the flow pattern in the channel changes. This increase in thickness results in a higher liquid thermal resistance, which in turn reduces the heat transfer performance. The proportion of annular flow in the mini-channel increases as the inlet vapor quality (xin) increases. When the annular flow occurs with a higher proportion, the resulting liquid film is thinner and has a smaller thermal resistance. This phenomenon explains why the local heat transfer coefficient and local wall temperature have a smaller drop at higher volume flowrate and higher inlet vapor quality (xin). As depicted in Fig. 5, the predominant flow pattern observed in mini-channels is annular flow. This flow pattern is observed under experimental condi- tions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The coverage rate of liquid film increases with the continuous flow of the refrigerant vapor in the mini-channel. As the Fig. 10. The effect of the inlet vapor quality (xin) on the heat transfer coefficient of condensation at various volume flowrates: (a) the volume flowrate is 30 mL/min;
(b) the volume flowrate is 60 mL/min; (c) the volume flowrate is 90 mL/min; (d) the volume flowrate is 120 mL/min.
flow condensation enters into the back section of the channel, the liquid state of the refrigerant increases, leading to a rapid decrease in the fluid flowrate in the second half of the channel. This countercurrent heat transfer also has a severe hindrance effect. At high volume flowrates, analysis of the experimental phenomena and effects reveals that a higher inlet vapor quality results in a higher heat transfer coefficient. Mean- while, flow condensation experiments preferably provide valuable data for selecting appropriate flowrates and inlet vapor quality for conden- sation operations.
By comparing the data in Fig. 10 and Fig. 11, it is observed that there is a direct relationship between the local wall temperature of the channel and the local heat transfer coefficient. This relationship has been analyzed in previous experiments [47]. Under the same experi- mental conditions, when the wall temperature is low, it promotes condensation and heat transfer of the refrigerant working vapor. How- ever, excessive condensation under these conditions leads to a decrease
in the volume flowrate of the gas phase, resulting in a decrease in flow velocity. As a result, the thickness of the condensate film continues to increase, leading to higher thermal resistance. Consequently, this has an opposite effect on the vapor condensate, causing the heat transfer co- efficient to decrease as the wall temperature decreases. In this experi- ment, it was observed that the inlet vapor quality (xin) had a significant influence on the local heat transfer coefficient and wall temperature when the volume flowrate was low. However, this effect was relatively reduced towards the end of the mini-channel. Conversely, under high volume flow rate conditions, the opposite trend was observed. This research presents a trend diagram, as shown in Fig. 12, illustrating the variation tendency of the local heat transfer coefficient and wall tem- perature under the influence of the refrigerant inlet vapor quality at the last temperature measurement point (X =250 mm). The diagram pro- vides valuable insights into the variations of these parameters at the specified location. Fig. 12 clearly shows that the temperature at X = Fig. 11. The effect of the inlet vapor quality (xin) on the wall temperature at various volume flowrates: (a) the volume flowrate is 30 mL/min; (b) the volume flowrate is 60 mL/min; (c) the volume flowrate is 90 mL/min; (d) the volume flowrate is 120 mL/min.
250 mm is significantly influenced by the inlet vapor quality (xin). When the volume flowrate is lower, this point is less affected by the inlet vapor quality. For instance, at a volume flowrate of 30 mL/min, the temper- ature changes within a controlled range of 1 ◦C. As the volume flowrate increases, the temperature fluctuation range also increases, but it is still observed that the change remains within a controlled range of 10 ◦C.
Under the same test conditions, it was observed that the condensate film at the channel end under a lower volume flowrate is significantly thicker compared to that under a higher flowrate; the channel may even be entirely filled with condensed liquid at low volume flowrates. This leads to a higher thermal resistance and reduced heat transfer to the wall, which can be clearly observed in Fig. 5, specifically in the case of xin = 0.25 at 30 mL/min. Consequently, this results in a relatively lower temperature at this location.
5. Conclusion
This study presents the findings of visual experiments conducted to investigate the flow condensation heat transfer and flow pattern change of refrigerant R141b in copper-based parallel mini-channels. The mini- channel used in the experiments had a rectangular shape with a hy- draulic diameter of 2 mm. The study focuses on understanding the type and transformation mechanism of flow patterns during the flow condensation process in the mini-channel. Additionally, the influence of inlet vapor quality (xin) on the performance of flow condensation heat transfer is analyzed. The results provide valuable insights for practical engineers working with vapor condensation of refrigerants in mini- channel heat dissipation systems. Based on the findings, the following conclusions can be drawn.
(1) By analyzing the flow condensation patterns of refrigerant R141b in parallel mini-channels, four categories of the flow condensa- tion patterns can be classified, that is, annular flow, transition flow, slug flow and bubble flow. This experimental result can be referenced in practical applications to determine the appropriate volume flowrate and inlet vapor quality for ensuring complete or near-complete vapor condensation when it leaves the condensa- tion heat sink.
(2) Affected by the inlet vapor quality, the condensed flow patterns of bubble flow and even slug flow cannot be accurately captured as the mass flowrate increases.
(3) The local heat transfer coefficient and wall temperature decrease along the mini-channel in the direction of the flow. However, both the heat transfer coefficient and wall temperature increase with an increase in the inlet vapor quality.
(4) As the inlet vapor quality decreases, the proportion of annular flow gradually decreases, leading to a maximum reduction of the local heat transfer coefficient of approximately 83.7 %. On the other hand, with an increase in the inlet vapor quality, the vapor flowrate of refrigerant in the channel increases, resulting in an increased shear force at the gas–liquid interface. Consequently, the condensate film thickness decreases, reducing the thermal resistance of heat transfer in the channel and increasing the heat transfer coefficient of condensation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51976002 & No. 52206191).
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