INFLUENCE OF VARIOUS FACTORS ON THE HEAT TRANSFER CHARACTERISTICS OF MINIATURE TWO-PHASE THERMOSYPHONS WITH NANOFLUIDS
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-12-20 |
| Journal | Energy Technologies & Resource Saving |
| Authors | V. Yu. Đravets, V.N. Moraru, D.I. Gurov |
| Institutions | National Academy of Sciences of Ukraine, National Technical University of Ukraine âIgor Sikorsky Kyiv Polytechnic Instituteâ |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the performance enhancement of miniature two-phase closed thermosyphons (5 mm inner diameter) using various aqueous nanofluids (NFs) compared to pure water. The study demonstrates significant improvements in heat transfer characteristics critical for high-density electronics cooling.
- Performance Enhancement: Nanofluids based on Carbon Nanotubes (CNT) and Synthetic Diamond (SD) achieved a greater than 1.5-fold, and up to a twofold, increase in maximum transferable heat flux (Qmax) compared to distilled water.
- Thermal Resistance Reduction: The use of NFs resulted in a sharp decrease in total thermal resistance (R), indicating highly efficient heat removal.
- Optimal Operating Parameters: Maximum performance (Qmax) was consistently observed at low fill factors (Kf) ranging from 0.4 to 0.5, and an optimal inclination angle of 45° relative to the horizontal.
- CNT Superiority: The 0.1% CNT nanofluid (NF 4-1) demonstrated the best results, achieving Qmax up to 180 W, significantly surpassing the 90 W baseline for water.
- Mechanism of Improvement: The enhanced performance is attributed not only to the higher intrinsic thermal conductivity of the nanoparticles but also to the formation of a porous surface structure on the heating wall, which suppresses the formation of a continuous vapor film (boiling crisis).
- Equivalent Conductivity: Equivalent thermal conductivity (λeq) reached up to 120,000 W/mK at high heat fluxes when using CNT nanofluids, confirming the high efficiency of the two-phase heat transfer cycle.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Thermosyphon Inner Diameter (din) | 5 | mm | All samples tested |
| Thermosyphon Total Length (LÎŁ) | 700 | mm | All samples tested |
| Condensation Zone Length (Lc) | 210 | mm | Fixed length |
| Heating Zone Length (Lhz) Range | 45 to 200 | mm | Varied during testing |
| Optimal Fill Factor (Kf) Range | 0.4 to 0.5 | - | For maximum Qmax |
| Optimal Inclination Angle (Ί) | 45 | ° | For minimum thermal resistance |
| Max Heat Flux (Qmax) - Water | ~90 | W | Baseline coolant |
| Max Heat Flux (Qmax) - CNT NF (0.1%) | ~180 | W | Best observed performance |
| Max Heat Flux (Qmax) - SD NF (0.3%) | ~140 | W | Synthetic Diamond NF |
| Max Equivalent Thermal Conductivity (λeq) | 120,000 | W/mK | Achieved with CNT NF at high Q |
| CNT Nanoparticle Concentration (4-1) | 0.1 | mass % | Water + Carbon Nanotubes |
| Synthetic Diamond Nanoparticle Conc. (5-1) | 0.3 | mass % | Water + Synthetic Diamond |
| Carbon Black Nanoparticle Conc. (6-1) | 0.31 | mass % | Water + Carbon Black (DG-100) |
| CNT Nanoparticle Thermal Conductivity | 2000 to 2300 | W/mK | Intrinsic property |
| Water Thermal Conductivity | 0.613 | W/mK | Base fluid property |
| Cooling Water Flow Rate (G) | 4.9 x 10-3 | kg/s | Constant flow rate |
| Temperature Measurement Accuracy | 0.05 | °C | Maximum error in heat flux measurement |
Key Methodologies
Section titled âKey Methodologiesâ- Nanofluid Preparation: Nanofluids (NFs) were synthesized by ultrasonic dispersion of solid nanoparticles (Carbon Nanotubes (CNT), Synthetic Diamond (SD), Carbon Black (CB)) in deaerated distilled water using an UZDN-2T disperser (22 kHz, 500 W).
- Characterization: NF stability and properties were assessed by measuring zeta potential (ζ), specific electrical conductivity (Ï), pH, and surface tension (Ï20°C) using specialized instruments (ZetaSizer NANO-ZS, K6 KRĂSS tensiometer, COND 5022, pH-150 M).
- Experimental Setup: Miniature copper thermosyphons (700 mm length, 5 mm ID) were tested in a closed loop system. Heat was supplied via an ohmic heater in the heating zone (Lhz) and removed via a âpipe-in-pipeâ condenser in the condensation zone (Lc = 210 mm).
- Thermal Control and Measurement: Heat input (Q) was controlled by a laboratory autotransformer and measured by a wattmeter. Temperatures along the thermosyphon wall and cooling water were monitored using copper-constantan thermocouples (Type T), with data acquisition at 1 Hz frequency.
- Parameter Variation: Experiments systematically varied the heat flux (Q), the fill factor (Kf, ranging from 0.44 to 1.93), and the angle of inclination (Ί, ranging from 5° to 90°).
- Performance Metrics: Thermal resistance (R), heat transfer coefficients in the heating zone (αhz) and condensation zone (αc), and equivalent thermal conductivity (λeq) were calculated based on measured Q and steady-state temperature differences.
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated high heat transfer capacity and low thermal resistance of miniature thermosyphons utilizing nanofluids make them ideal for advanced thermal management in several high-power, compact systems:
- High-Performance Computing (HPC): Cooling high-density processors (CPUs, GPUs) and memory modules in servers and data centers, where heat fluxes exceed conventional limits.
- Power Electronics and Inverters: Thermal stabilization of high-power semiconductor devices (IGBTs, MOSFETs) used in electric vehicles, industrial drives, and renewable energy grid interfaces.
- Compact Medical Devices: Heat removal from portable diagnostic and imaging equipment where size and weight constraints are severe.
- Aerospace and Satellite Thermal Control: Passive cooling systems for avionics and sensitive instruments, leveraging the thermosyphonâs ability to operate efficiently across a range of orientations (optimal at 45°).
- High-Power LED and Laser Systems: Maintaining stable operating temperatures for industrial lasers and high-flux solid-state lighting arrays to ensure longevity and performance.
View Original Abstract
Currently, various types of nanofluids are of increasing interest as heat carriers for heat transfer in thermosiphons and other evaporative-condensation devices. This paper presents and analyzes experimental data on heat transfer characteristics (total thermal resistances, maximum transferable heat fluxes and equivalent thermal conductivity) of two-phase miniature thermosyphons with nanofluids. Geometric parameters of thermosiphons for all experimental samples were identical and were: total length 700 mm, inner diameter 5 mm. The length of the heating zone was changed stepwise from 45 mm to 200 mm. The length of the condensation zone was 200 mm for all investigated thermosyphons. The amount of coolant in the thermosiphons was the same, and its height in the heating zone before the start of the study was 88 mm. Distilled water and aqueous nanofluids with nanoparticles of carbon nanotubes, synthetic diamond, and carbon black were used as heat carriers. The main attention is paid to the study of the influence of the filling factor and the angle of inclination of the thermosyphon, the value of the transferred heat flux and the chemical nature of the coolant (nanofluid) on the heat transfer characteristics of thermosyphons. The strong influence of these factors on the efficiency of a miniature closed two-phase thermosyphon has been demonstrated. A more than twofold increase in the heat transfer characteristics of thermosyphons (the maximal transferred heat flux) was obtained with a sharp decrease in their thermal resistance. It is assumed that the significantly higher heat transfer capacity of such thermosiphons compared to those filled with water is explained not only by the higher thermal conductivity of the coolant, but also by the appearance of a peculiar porous structure that prevents the appearance of a vapor film and promotes the intensification of heat transfer processes during boiling. Bibl. 16, Fig. 10, Tab. 2.