Flat-plate Oscillating Heat Pipe Research Articles

Performance characteristics of ultra-thin flat plate oscillating heat pipes with alternate channels and auxiliary micro-grooved structures

Two flat plate oscillating heat pipes (FPOHPs) characterized by the same thickness of 1 mm and alternating channel widths were fabricated using the wet etching technique. HFE-7100 was used as a working fluid at volumetric filling ratios ranging from 30%–60%. The startup and heat transfer characteristics of FPOHPs with and without embedded parallel microgrooves on the bottom wall of wider channels were experimentally compared and evaluated. The dual-diameter FPOHPs could operate at both horizontal and vertical orientations. The introduction of microgrooves reduced the duration of the startup time, lowered the startup temperature, and avoided the temperature overshoot. Furthermore, the microgrooves significantly reduced the temperature of the evaporator. At the combined condition of 30% filling ratio and 24 W power input, the effective thermal conductivities (ETCs) of the FPOHP with microgrooves were 1241 and 1226 W/(m·°C), respectively, at horizontal and vertical orientations, approximately 25.6% and 18.8% higher than those without microgrooves, or approximately 3.1 times higher than pure copper. At the horizontal orientation, the heat transfer enhancement was more pronounced for the micro-grooved FPOHP compared to that without micro-grooves. The micro-grooved structures facilitated the unidirectional fluid circulation and reduced the gravity dependence, showing the application prospects at both terrestrial and microgravity conditions.

Heat Transfer Performance of a 3D-Printed Aluminum Flat-Plate Oscillating Heat Pipe Finned Radiator.

As electronic components progressively downsize and their power intensifies, thermal management has emerged as a paramount challenge. This study presents a novel, high-efficiency finned heat exchanger, termed Flat-Plate Oscillating Heat Pipe Finned Radiator (FOHPFR), which employs arrayed flat-plate oscillating heat pipes (OHP) as heat dissipation fins. Three-dimensional (3D)-printed techniques allow the internal microchannels of the FOHPFR to become rougher, providing excellent surface wettability and capillary forces, which in turn significantly improves the device's ability to dissipate heat. In this study, the 3D-printed FOHPFR is compared with traditional solid finned radiators made of identical materials and designs. The impacts of filling ratio, inclination angle, and cold-end conditions on the heat transfer performance of the 3D-printed FOHPFR are investigated. It is demonstrated by the results that compared to solid finned radiators, the FOHPFR exhibits superior transient heat absorption and steady-state heat transfer capabilities. When the heating power is set at 140 W, a decrease in thermal resistance from 0.32 °C/W in the solid type to 0.11 °C/W is observed in the FOHPFR, marking a reduction of 65.6%. Similarly, a drop in the average temperature of the heat source from 160 °C in the solid version to 125 °C, a decrease of 21.8%, is noted. An optimal filling ratio of 50% was identified for the vertical 3D-printed FOHPFR, with the minimal thermal resistance achieving 0.11 °C/W. Moreover, the thermal resistance of the 3D-printed FOHPFR is effectively reduced compared to that of the solid finned radiator at all inclination angles. This indicates that the FOHPFR possessed notable adaptability to various working angles.

Heat Transfer Performance of 3D-Printed Aluminium Flat-Plate Oscillating Heat Pipes for the Thermal Management of LEDs

With the rapid development of electronic technologies towards high integration, high power and miniaturization, thermal management has become an increasingly important issue to guarantee the reliability and service life of electronic devices. The oscillating heat pipe (OHP), which was governed by thermally excited oscillating motion, was considered as a promising technology to dissipate high-density heat and had excellent application prospects in many important industrial processes. A flat-plate OHP, however, was fabricated by traditional welding methods, which were difficult and inefficient, resulting in increasing the cost and wasting the production time. In this work, we adopted a new metal 3D printing technology to develop an aluminum flat-plate OHP, which made it facile to build complex inner channels with high-precision molding at one time. AlSi10Mg powders, as raw materials, were selectively melted and solidified to form the container of the flat-plate OHP. The sintered inner surface presented excellent wettability to the working fluid, which facilitated the evaporation of the working fluid. Acetone was chosen as the working fluid, and the filling ratios with a range of 40-70% were loaded into the flat-plate oscillating heat pipe to analyze its effect on heat transfer performance. It was found that the 3D-printed flat-plate OHP with a 60% filling ratio had a better heat transfer performance and a lower thermal resistance, and it was able to work properly in both vertical and horizontal operation modes. The 3D-printed flat-plate OHP had been successfully applied for the thermal management of high-power LEDs, and the results showed that the temperature of LEDs was maintained within 60 °C, and its service life was prolonged.

Heat transfer investigation of a flat-plate oscillating heat pipe with tandem dual channels under nonuniform heating

To provide cooling of electronics under nonuniform heating from multiple heat sources, a novel tandem dual-channel flat-plate oscillating heat pipe (OHP) is tailored and experimentally studied with a “center-multiple-heat-source nonuniform heating and two-end air cooling” layout under different inclination angles, heat loads and nonuniform heating levels. Flow pattern diagrams together with thermal resistance contour maps are organized for two tandem mirror-symmetric halves of the flat-plate OHP, and the thermal performance differences under nonuniform and uniform heating are compared. It is indicated that, compared with uniform heating, nonuniform heating produces better thermohydrodynamic performance at a low heat load with quasi-steady “stop-oscillation” fluid motion in the flat-plate OHP, while it results in poorer thermohydrodynamic performance when the heat load is large enough to maintain sustainable fluid flow in the flat-plate OHP. Under nonuniform heating conditions, concentrating more heat to overcome the negative effects rather than to amplify the positive effects on the fluid flow and heat transfer in OHPs is more beneficial to the overall thermal performance. For the 540 tested conditions, the flat-plate OHP has a much smaller overall thermal resistance than a pure 6063 aluminum alloy plate with the same configuration (approximately 15.3% on average) along a heat transfer distance of 101.8 mm. In addition, it possesses good adaptability to gravity, with coefficients of variation of 27.6% in the overall thermal resistance for inclination angles from 0° to 90°. Accordingly, this flat-plate OHP has good potential for high-efficiency and good-adaptability cooling of electronics under nonuniform heating by multiple heat sources.

Three-dimensional heat transfer analysis of flat-plate oscillating heat pipes

• Three-dimensional heat transfer model for flat-plate OHP is proposed. • Model includes both thermal hydrodynamic phenomena and thermal diffusion in casing. • The model is validated with experimental results of Aluminum flat-plate OHP. • Relation of temperature distribution in casing to liquid-vapor behavior is studied. • Novel outcome is that moving hot spots on flat-plate OHP surface are evaluated. This paper presents a three-dimensional heat transfer analysis of a flat-plate oscillating heat pipe (OHP). To enhance implementation of OHPs in various applications, it is important to understand the comprehensive phenomena that include thermal diffusion in the whole structure and thermal hydraulics in the channel. We developed an OHP model that includes the effect of thermal diffusion and thermo-fluid behavior. The model was validated with experimental results of a flat-plate aluminum OHP which has a size of 200 mm × 90 mm × 3.8 mm, a channel diameter of 1.0 mm, and a turn number of 42. The effect of surface roughness in the channel and liquid film thickness on operating temperature is investigated. The relation between thermo-fluid behavior in the channel and temperature distribution of the OHP is analyzed. For aluminum OHP, the temperature difference across the thickness direction is 0.1 °C which is relatively small, whereas, the moving hot spot: local high-temperature region, is found in the planer surface. The maximum hot spot superheat temperature reached 5.4 °C for 200 W. The novelties of this paper are to develop the three-dimensional comprehensive OHP model and to reveal the combined effect of thermo-hydraulic phenomena and thermal diffusion in OHP structure.

Thermal performance of a novel dual-serpentine-channel flat-plate oscillating heat pipe used for multiple heat sources and sinks

A novel dual-serpentine-channel flat-plate oscillating heat pipe (FPOHP) is designed and fabricated for electronic cooling with multiple heat sources and sinks. The FPOHP has two mirror symmetric tandem branches of capillary serpentine square channel with a hydraulic diameter of 2 mm (i.e., Bond number Bo = 1.137~1.265 here), which is made of 6063 aluminum alloy and 55% partially filled with acetone. A corresponding experimental study is conducted to investigate the start-up characteristics and quasi-steady thermal performance of the FPOHP with arrangement of “uniform heating by multiple heat sources at center and air cooling at both ends” under different inclination angles. The combined flow pattern diagrams as well as thermal resistance and equivalent thermal conductivity contour maps are provided for two mirror symmetric tandem sections of the FPOHP, based on which the thermo-hydrodynamic performance deviations between these two sections are compared and analyzed. The results show that the quasi-steady fluid motions in the FPOHP can be characterized by intermittent or independent occurrence of three elements, namely, stop, oscillation, and circulation, depending on the heat load and inclination angle. Significantly, the thermo-hydrodynamic behaviors in two tandem sections of the FPOHP both cooperate and compete with each other, as a result of the heat, mass and momentum exchanges via the tandem structure. The FPOHP can start up successfully under all inclination angles from 0° to 90° with no obvious difference, and its start-up temperature increases smoothly and approximately reaches a plateau of 41~46 °C as the heat load goes up. For the 175 tested conditions, the coefficients of variation for the thermal resistance of two tandem sections are smaller than 21.5% with the average value of 6.8%, showing a fine heat transfer uniformity of the FPOHP over a wide range of operating conditions. Significantly, in general, the FPOHP has much larger overall equivalent thermal conductivity (about 5.8 times on average) and possesses only a weight 83.6% of a pure 6063 aluminum alloy plate with the same geometry. In addition, the FPOHP exhibits a well gravity adaptability with the relative deviation ranging from 7.1% to 25.2% in the total thermal resistances for all inclination angles. Accordingly, the FPOHP shows a good potential for the high-heat-flux electronic cooling applications with multiple heat sources and sinks.

Experimental Investigation of a Flat-Plate Oscillating Heat Pipe With Groove-Enhanced Minichannels

Abstract The thermal and capillary performance of a groove-enhanced, or “microchannel-embedded,” flat-plate oscillating heat pipe (MC FP-OHP) was experimentally investigated while varying heating width, orientation, working fluid and operating temperature. The copper MC FP-OHP possessed two layers of 1.02 × 1.02 mm2 square channels, with the center 14 channels possessing two embedded microchannels (0.25 × 0.13 mm2) aligned coaxially with the primary minichannels. A FP-OHP without embedded microchannels, but with deeper minichannels (DC FP-OHP), was also tested for comparison. The FP-OHPs were filled with Novec 7200 or water (both at 80% ± 2% by volume), and the heating widths were varied between full-width and localized configurations: 38.71 cm2 and 14.52 cm2, respectively. Experimental results demonstrate that the MC FP-OHP is significantly less sensitive to operating orientation and can perform with less detriment as heat flux increases. The MC FP-OHP has a lower startup heating requirement and provides more fluid wetting along the FP-OHP structure—which is advantageous for pumping liquid from the evaporator to the condenser. The MC FP-OHP has enhanced convective heat transfer during operation, as it was observed to have similar or lower thermal resistances to that of the DC FP-OHP for a wide range of operating conditions. The groove-enhanced minichannel within the MC FP-OHP also provides for enhanced heat transfer because there being more thin-film evaporation sites and vapor–liquid mixing between the minichannel and microchannels.

Influence of the channel profile on the thermal resistance of closed-loop flat-plate oscillating heat pipe

The closed-loop flat-plate oscillating heat pipe (CL-FPOHP) is a new two-phase heat transfer device for an effective thermal management in different systems. A number of theoretical and experimental investigations have been carried out on the CL-FPOHP in the past decades after its invention. However, due to the operational mechanism of the CL-FPOHP, the effects of channel profile on the thermal resistance have not been completely revealed so far. This paper aims at discussing the thermal resistance and thermal conductivity by changing the shape and size of the CL-FPOHPs channel. The thermal resistances of the CL-FPOHPs were investigated by varying the channel shape from square to circular, channel sizes from 2 × 2 mm2 to 5 × 5 mm2, and heat load from 10 to 120 W. The pure copper was used to develop the CL-FPOHP and charged with the acetone with a charge ratio of 70%. The most suitable channel shape for the CL-FPOHP was found to be a square channel, and the most suitable channel size was observed to be 2 × 2 mm2. The highest thermal conductivity of the CL-FPOHP reached to 2137 W/m °C.

Experimental investigation of the thermal performance of closed loop flat plate oscillating heat pipe

ABSTRACT A number of theoretical studies and experimental investigations have been carried out on the closed loop flat plate oscillating heat pipe (CLFP-OHP) in the past decades. However, due to the complex working mechanism of CLFP-OHP, the effects of operational and geometrical parameters on the thermal performance CLFP-OHP have not been completely revealed so far. In the present study, the CLFP-OHP was investigated with different operational parameters (charge ratio, orientation, and heat load) and geometrical parameters (channel size and shape). In order to investigate the effect of these parameters, parallel mini channels were machined onto the copper plate and acetone was used as working fluid to form CLFP-OHP. An attempt has been made to resolve the critical charge ratio of acetone for CLFP-OHP. The measured thermal resistance found strongly dependent on operational and geometrical parameters. The best performance was observed by acetone with a charge ratio of 60% in the vertical orientation for square (2 x 2 mm2) channels. The lowest thermal resistance of 0.39°C/W achieved for acetone at 100 W. The thermal conductivity of CLFP-OHP reached to 3205 W/m °C.

Experimental Investigation of Flat Plate Cryogenic Oscillating Heat Pipe

Recent developments in applications such as high-temperature superconducting magnets, infrared detectors, and electronics components have led to an alarming increase in heat dissipation rate, which now far exceeds the capability of conventional heat pipe cooling systems. This trend is responsible for a recent transition to flat plate oscillating heat pipes. A new flat plate cryogenic oscillating heat pipe (FPC-OHP) has been developed and validated through experimentations. The performance evaluation of FPC-OHP was investigated with temperature measurements. FPC-OHP consisted of evaporator, condenser, and adiabatic section with the dimensions of 93 × 70 × 8 mm3. The FPC-OHP was made of copper alloy and fabricated by a vertical milling machine, having square channels with a hydraulic radius of 0.66 mm. Liquid nitrogen was used as a working fluid with a charge ratio of 60%. Experimental results revealed the maximum heat transport capability up to 300 W. Moreover, the thermal resistance decreased from 0.25 to 0.11 K/W corresponding to an increase in the heat load from 25 to 300 W. The average temperature difference between evaporator section and condenser section reached up to 34 K for 300 W. The measured effective thermal conductivities were found to be 7353 W/m K.

Effect of Operational Parameters on the Thermal Performance of Flat Plate Oscillating Heat Pipe

Abstract Flat plate oscillating heat pipe (FP-OHP) is a unique heat transfer device and considered as a promising candidate for effective heat transfer device in electronics industries. A number of theoretical studies and experimental investigations have been carried out on FP-OHP in the past decades after its invention. However, due to the operational characteristics of FP-OHP, the effect of various parameters on the thermal performance of FP-OHP has not been completely revealed so far. This paper attempts to discuss the effect of operational parameters on the thermal performance of FP-OHP. In this study, the FP-OHP was investigated with different charge ratios, orientations, working fluids, and heat loads from 10 W to 150 W. In order to investigate the effect, 18 parallel square channels of 2 × 2 mm2 are machined onto pure copper plate (93 × 70 × 8 mm3) to form FP-OHP. DI water, ethanol, methanol, acetone, and FC-72 are investigated. The measured thermal resistance was strongly dependent on operational parameters. The optimum performance was observed with acetone with a charge ratio of 70% in the vertical orientation. The lowest thermal resistance of 0.39 °C/W is achieved using acetone as a working fluid at 100 W. A Kutateladze number (Ku) was used to compare the experimental data and found to be suitable for prediction of the thermal performance of FP-OHP with standard deviation of 15%.

Influence of Working Fluids on Thermal Performance of Flat Plate Oscillating Heat Pipe

Influence of Working Fluids on Thermal Performance of Flat Plate Oscillating Heat Pipe

Performance characteristics of flat-plate oscillating heat pipe with porous metal-foam wicks

In this study, a flat-plate oscillating heat pipe (FPOHP) with its channel bottom covered by sintered copper-foam wicks was fabricated. The overall size of FPOHP is 150 mm × 100 mm × 7.5 mm, having rectangular cross-section channels with a hydraulic diameter of about 1.64 mm. The start-up and heat transfer performance of the porous-based FPOHP were experimentally investigated and compared with a conventional smooth-channel FPOHP at the vertical orientation. Ethanol was used as the working fluid at a volumetric filling ratio of about 50%. Experimental results showed that the copper-foam wicks enabled a profound improvement in the FPOHP performance and reduced the start-up temperature and time duration significantly. Compared with the conventional FPOHP, temperature fluctuations with smaller amplitudes and better temperature uniformity were observed for the porous-based FPOHP. However, local dry-out appeared earlier in the porous-based FPOHP when the power input was greater than about 207 W because the robust nucleation boiling induced vapor layer covered the wicks, preventing the backflow of liquid to the evaporator and causing the heat transport limitation. To extend the applicable possibility of porous-based FPOHPs in thermal management of high energy utilization systems, the re-design and re-fabrication of copper-foam wicking structures are required.

Variation in oscillating heat pipe performance

Oscillating heat pipes have been shown to be an effective means for thermal management. However, the constant, phase-change-powered oscillation can also produce variations in performance. The goal of this research is to quantify performance variation of oscillating heat pipes through repeated tests with closed-loop flat plate oscillating heat pipes. The heat pipes were filled with two different working fluids, butane and R134a, and tested in both double-sided and single-sided cooling configurations, two or three different orientations, and four different power levels, and repeated to quantify the variation in performance, including average thermal resistance, intratest variation (repeatability), intertest variation (temperature fluctuations), and reliability (likelihood of temperature overshoot). Orientation, configuration, working fluid, and power level all influenced the level of variation in performance. In a double-sided configuration, OHPs had lower thermal resistance when oriented vertically. In a single-sided configuration, OHPs had lowest thermal resistance when the evaporator was below the condenser. OHPs were significantly more reliable and experienced smaller temperature fluctuations when in the double-sided than single-sided configuration.

Analysis of Heat Transfer of a Flat Plate Oscillating Heat Pipe, for Different Surfaces

Analysis of Heat Transfer of a Flat Plate Oscillating Heat Pipe, for Different Surfaces

Design and study of a Flat Plate Oscillating Heat Pipe. Flow Pattern analysis and Heat Transfer Performance

Design and study of a Flat Plate Oscillating Heat Pipe. Flow Pattern analysis and Heat Transfer Performance

Effect of Hydrophilic Nanostructured Cupric Oxide Surfaces on the Heat Transport Capability of a Flat-Plate Oscillating Heat Pipe

With a surface treatment of hydrophilic cupric oxide (CuO) nanostructures on the channels inside a flat-plate oscillating heat pipe (FP-OHP), the wetting effect on the thermal performance of an FP-OHP was experimentally investigated. Three FP-OHP configurations were tested: (1) evaporator treated, (2) condenser treated, and (3) untreated. Both evaporator- and condenser-treated FP-OHPs show significantly enhanced performance. The greatest improvement was seen in the condenser-treated FP-OHP, a 60% increase in thermal performance. Neutron imaging provided insight into the fluid dynamics inside the FP-OHPs. These findings show that hydrophilic nanostructures and their placement play a key role in an OHP's performance.

Additive manufacturing of heat exchangers: A case study on a multi-layered Ti–6Al–4V oscillating heat pipe

Additive manufacturing (AM) allows for layer-by-layer fabrication of complex metallic parts with features typically unobtainable via conventional manufacturing. For heat exchangers, such complex features are desirable for enhancing their heat transfer capability and conformability to specific applications. In this case study, Selective Laser Melting (SLM), a laser-based additive manufacturing process, was utilized to fabricate a compact (5.08cm×3.81cm×1.58cm) flat-plate oscillating heat pipe (FP-OHP) with innovative design features, including a Ti–6Al–4V casing and a closed-loop, circular mini-channel (1.53mm in diameter) consisting of four interconnected layers. Venting holes were integrated to intersect each layer to allow for a unique layer-by-layer, plug-and-pressurize de-powdering procedure. The device channel surface was inspected via Scanning Electron Microscopy (SEM) – and it was found that the channel wall consisted of partially un-melted particles, as well as amorphous melt regions; surface characteristics influential on surface/fluid capillarity and heat transfer. This study also highlights important design and manufacturing concerns encountered during SLM of channel-embedded parts, such as channel surface quality and de-powdering. The Ti–6Al–4V FP-OHP was found to operate successfully with an effective thermal conductivity of approximately 110W/mK at a power input of 50W; demonstrating a 400–500% increase relative to solid Ti–6Al–4V.

An Experimental Investigation of an Oscillating Heat Pipe Heat Spreader

With ever increasing technological advances in electronics, modern computer components continue to produce higher power densities that present a challenge to thermal management. A radial flat-plate oscillating heat pipe (RFP-OHP) heat spreader is investigated to study the effect of central heating on the heat transport capability in an OHP. The investigated OHP has dimensions of 100 mm × 100 mm × 2.5 mm with central heating using a 30 mm × 30 mm heater. Experimental results show that when heat is added to the center section of one side of the radial flat-plate OHP, and when heat is removed from the whole surface of another side of the heat pipe, the startup power for the oscillating motion increases. In addition, the spacer effect on the heat transport capability including the startup is investigated experimentally. The spacer added between the cooling block and OHP could lower the startup power for oscillatory motion. When compared to a copper slab of the same dimensions in the same test configuration, the temperature difference for the OHP with and without the additional copper spacer was reduced by a maximum of 46% and 25%, respectively, at a power input of 525 W and a heat flux of 58 W/cm2.

Time–frequency analysis of flat-plate oscillating heat pipes

Time-varying surface temperatures of two flat-plate oscillating heat pipes (OHPs), one with Tesla valves and one without, were analyzed in the time–frequency domain using the short-time Fourier and Hilbert–Huang transforms. Tesla valves were installed along the channel structure of the OHP for the purpose of rectifying the oscillatory internal flow field, thereby enhancing heat transfer. Spatial-averaged surface temperatures in the evaporator region of both OHPs were investigated in the time–frequency domain at three separate heat inputs in order to detect salient effects of the Tesla valves. In all cases, the temperature signals from both OHPs were found to contain intermittent, aperiodic oscillations with most energy concentrated at frequencies in the 0–200 mHz range. The energies of oscillations in both OHPs were found to decrease with increases in the heat input, suggesting more consistent inter-channel flow circulation at these heat inputs heat inputs. The non-valved OHP temperature signals contained oscillations of larger amplitude and over a broader frequency range than the valved OHP temperature signals, indicating that the Tesla valves reduced the occurrence of intermittent high-energy oscillations in the OHP evaporator surface temperature.