Study on the Dynamic Generation of Subcooled Water Using a Compact Heat Exchanger
Abstract
:1. Introduction
2. Experimental Setup and Experimental Conditions
2.1. Selection of Compact Heat Exchanger
2.2. Experimental Setup
2.3. Experimental Method
- (1)
- Before the experiment starts, the temperature setting of the thermostatic device is established according to the experimental conditions. It takes approximately 25~30 min for the low-temperature thermostatic device to cool the NFL from room temperature so as to the set temperature. The high- and constant-temperature device takes approximately 25 min to cool the tap water from room temperature to 3.0 °C. Once the tap water has cooled to the set temperature, it is necessary to turn on the variable frequency pump. This step enables the dynamic generation of subcooled water in the loop to begin its cycle (one adjusts the high flow rate to quickly establish the loop’s thermal equilibrium) and continue until the loop temperature is stable. Then, the experiment for the subsequent dynamic generation of subcooled water is carried out.
- (2)
- Then, one turns on the low-temperature NFL loop’s variable-frequency water pump and adjusts its flow rate to 1.900 L/min. When the inlet NFL temperature of the compact heat exchanger reaches a stable level, this is followed by further fine-tuning of the flow rate of the other loop in order to dynamically generate subcooled water at the desired subcooling degree. The process of adjusting and achieving the stable generation of subcooled water lasts for approximately 3 min, after which the water enters a stable generation phase until ice blockage occurs. The experiment is recorded.
- (3)
- For the next experiment, it is necessary to wait for the inlet temperature of the non-freezing fluid to rise to around 2.0 °C in order to ensure that the ice formed in the compact heat exchanger is completely melted. Furthermore, as the expansion of the ice slurry in the heat exchanger channels may damage the heat exchanger through its dynamic cold capacity, it is necessary to be vigilant of ice blockage occurrences. Therefore, to minimize and prevent ice blockage in the compact exchanger, one must turn off the pump at the side of the low-temperature non-freezing liquid loop at the right time.
3. Experimental Results and Analysis
3.1. Experimental Results
3.1.1. Subcooled Water with Different Subcooling Degrees
3.1.2. Maximum Degree of Subcooling and Maximum Duration Time
3.1.3. Influence of Specific Flux i
3.2. Analysis of Ice Blockage Factors
3.3. Discussion
4. Conclusions
- (1)
- The dynamic generation of subcooled water with a high subcooling degree from tap water is feasible. The generation of ice or ice slurry through this processes shows the system’s potential for applications in food engineering, indoor snowmaking, ice storage, and other fields. The experiments also proved that the compact heat exchanger is suitable for the dynamic generation of subcooled water. The concept of using the high-efficiency heat transfer performance of the compact heat exchanger for instantaneous heat transfer and shortening the channel length and small hydraulic diameter is also correct. Through the experiments, we achieved the generation of subcooled water at a minimum of −3.8 °C, and the −2.8 °C average-temperature subcooled water generation lasted for 108 min.
- (2)
- The deployment of compact heat exchangers can achieve higher inlet water temperatures, greater subcooling degrees, and longer dynamic generation durations than the use of other types of heat exchangers. With the increase in the subcooling degree, the range of specific flux i suitable for the dynamic generation of subcooled water is narrowed, and a specific flux i of around 1.5 is more appropriate. The increase in the subcooling degree is more effective in releasing the subcooled water to form ice or ice slurry. The dynamic generation durations of the subcooled water determine the amount of ice produced.
- (3)
- Ice blockage also occurs when generating subcooled water with tap water using a compact heat exchanger. The reasons for ice blockage are as follows: (1) the impure particles in the tap water are more likely to promote the formation of ice nuclei at high subcooling degrees; (2) the micro-icing on the surface of the compact heat exchanger may provide ice nuclei for the subcooled water at the outlet; and (3) the two turns at the outlet of the compact heat exchanger may also cause minor energy disturbance affecting the subcooled water flowing out of the heat exchanger for release through subcooling, especially at high subcooling degrees at which ice blockage is more likely to occur.
- (4)
- The analysis of the results clarified that compact heat exchangers for the dynamic generation of subcooled water require further design guidelines to shorten the channels, reduce the hydraulic diameter, optimize the shape of the channels, and increase the number of channels. In addition, the heat exchanger’s surface structure prevents icing and achieves greater subcooling degrees, laying a foundation for later explorations of the dynamic generation of deep subcooled water.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Nomenclature
NFL | non-freezing liquid |
t | duration time of subcooled water (min) |
T | temperature (°C) |
ΔT | heat exchange temperature difference (K) |
T1,in | NFL inlet temperature (°C) |
T1,out | NFL outlet temperature (°C) |
ΔT1 | NFL ΔT (K) |
T2,in | tap water inlet temperature (°C) |
T2,out | tap water outlet temperature (°C) |
ΔT2 | tap water ΔT (K) |
q | volume flow (L/min) |
q1 | NFL volume flow (L/min) |
q2 | tap water volume flow (L/min) |
i | specific flux (q1/q2) |
Tf | freezing temperature of the water |
Tw1 | inner surface temperature of the subcooled tube |
Tb1 | temperature of the water at the subcooled water outlet |
ls | distance from the position where the average temperature of the test fluid reaches the freezing temperature to the cooling tube exit |
di | hydraulic diameter |
Pr | Prandtl number |
Re | Reynolds number |
θf | dimensionless subcooling degree |
Gz* | Graetz number |
Cf | wall friction |
Nu | Nusselt number |
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Item | Unit | Content |
---|---|---|
Dimensions | mm | 40 L × 40 W × 12.6 H |
Hydraulic diameter | mm | 0.32 |
Heat exchange channel length | mm | 21.6 |
Working fluid | - | High temperature: tap water; low temperature: NFL |
Rated flow | L/min | 3.00 L/min, normal temperature |
Material | - | SUS316L, mirror surface |
Channel processing | - | Photoetching |
Structural processing | - | Atomic diffusion bonding |
Performance testing | - | Leakage: <10−10 Pa·m3/s; pressure > 6.9 MPa |
Item | Condition | Remark | |
---|---|---|---|
Tap water | T2,in (°C) | 3.0 | |
q2 (L/min) | 1.000~1.300 | Depending on cooling conditions | |
NFL | T1,in (°C) | −6.0, −6.5, −7.0, −8.0 | |
q1 (L/min) | 1.900 | Determined by the inlet temperature T1,in |
Subcooling Degree/K | 2.4~2.8 | 2.9~3.3 | 3.4~3.8 | Remarks | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Number of samples | 46 | 31 | 11 | |||||||
Time range/min | >10 | >16 | >25 | >10 | >16 | >25 | >10 | >16 | >25 | |
Quantity | 21 | 10 | 4 (108) | 18 | 8 | 5 (46) | 3 | 1 | 1 (28) | (Maximum duration/min) |
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Cheng, P.; Wang, K.; Sinkolongo, S. Study on the Dynamic Generation of Subcooled Water Using a Compact Heat Exchanger. Appl. Sci. 2023, 13, 4369. https://doi.org/10.3390/app13074369
Cheng P, Wang K, Sinkolongo S. Study on the Dynamic Generation of Subcooled Water Using a Compact Heat Exchanger. Applied Sciences. 2023; 13(7):4369. https://doi.org/10.3390/app13074369
Chicago/Turabian StyleCheng, Pengwei, Kaijian Wang, and Solomon Sinkolongo. 2023. "Study on the Dynamic Generation of Subcooled Water Using a Compact Heat Exchanger" Applied Sciences 13, no. 7: 4369. https://doi.org/10.3390/app13074369