Flat Plate Heat Pipe Research Articles

Effects of wall material, working fluid, and barriers on performance of a nano flat-plate heat pipe: Molecular dynamics simulation

The use of microscale heat generating or heat transfer equipment with higher capacity and smaller dimensions requires more accurate management and better disposal of their produced heat. This makes the necessity of designing and manufacturing superconductors completely clear. In the meantime, the nano-grooved flat plate heat pipes (FPHP) have gained the industry’s attention. Due to the provision of the conditions for manufacturing devices on a micro scale, it is possible to integrate this type of heat pipe with other microscale devices. In the meantime, understanding their behavior on small scales requires more studies. In this paper, the effect of using barriers to improve the thermal performance of a nano FPHP (1050×220×95Å) is evaluated. Simulations are performed on a molecular scale through molecular dynamics (MD) simulations using LAMMPS® software. Platinum (Pt), copper (Cu), and aluminum (Al) are used for HP’s body. In addition, argon (Ar), water (H2O), and ethanol (EtOH) are used as working fluids. The results show that increasing the number of barriers leads to improved thermal performance. Different cases include different teeth called barriers augmented inside HP are simulated. The combination of Cu-EtOH showed the best thermal performance (about 18% better than other cases). Cubical barriers have more heat flux improvement than conical ones (from 3.5% up to about 10.7%). Among the three fluids used, EtOH leads to a better heat flux (about 6% for Pt and up to 15% for Cu). Using 24 barriers, a very favorable result of 1992 W/cm2 heat flux is achieved. In this case, the minimum heat flux is obtained by using Pt and Ar and is 1609 W/cm2. Using Ar and the cub shape barriers, the highest mass transfer rates for Pt, Cu, and Al are about 35.2%, 38.9%, and 38%, respectively.

Study on the permeability and capillary pressure of spiral woven mesh for flat-plate heat pipes

Due to its excellent capillary characteristics, spiral woven mesh is widely used as the wick structure in ultra-thin flat-plate heat pipes. The performance of these heat pipes surpasses that of conventional ones using sintered powder or multi-layer mesh structures. However, precise quantitative studies on the permeability and capillary pressure characteristics of this unique porous structure are lacking. In this work, we systematically investigated its permeability and capillary pressure, as well as their relationship with the structure. Based on experimental results, we demonstrated that numerical simulation of periodic microelements and subsequent area weighting can accurately predict the permeability of actual structures. Additionally, a parametric study was conducted, yielding a unified empirical relationship between permeability and the structural parameters. In the part of capillary pressure, U-shape tube gravity method was utilized and the relationship between capillary pressure and structure was also disclosed. This study will provide reliable data and theoretical guidance for calculating the capillary limit in heat pipes using spiral woven mesh as the wick.

Three-dimensional simulation and artificial neural network optimization of a flat plate heat pipe for the cooling of telecommunication equipment

To address the significantly increased heat dissipation requirements of 5G base stations, a flat plate heat pipe (FPHP) of 500 × 200 × 3 mm3 comprising of 19 independent channels was developed. To solve the problem of simulation distortion due to the high superheat of the initial boiling of the working fluid in the ultra-long channel of 490 × 5 × 1.5 mm3, this paper adopts the Lee model considering the superheat, and simulates based on the Volume of fluid method, and the simulation results are compared with the experiments, and the average relative error is less than 5 %. The FPHP was explored at 100–500 W heat power and filling ratio of 30 %-70 %, depending on the application scenarios of the base station. By partitioning the 19 channels and calculating the thermal resistance of the FPHP based, an Artificial neural network (ANN) model with 10 hidden layer nodes is established based on 675 data sets. The results showed that the mean square error and correlation coefficient were 1.30 × 10-5 and 0.9990. The ANN prediction results are validated, the mean relative error is 5.4 %, demonstrating that the ANN model can be an effective tool to optimize the FPHP for the telecommunication equipment.

Design, Heat Leakage Analysis and Stirling Cryocooler Option of Stirling-type Lyophilizer

Background: Based on referring to the relevant patents, it was found that the traditional lyophilizer refrigeration system has the disadvantages of large volume and high power consumption. Meanwhile, considering the context of advocating low carbon emission and environmental protection, the use of energy-saving refrigeration technology and the use of non-fluorine and harmless refrigerants are the trends of future development. Objective: In order to solve the traditional lyophilizer problems and enhance the lyophilizer reliability, the paper designs a Stirling-type lyophilizer with a working temperature of -70°C. Methods: Stirling cooling has the advantages of ultra-low refrigeration temperature and high cooling speed. Based on the above advantages, combining the characteristics of the lyophilization box, a Stirling- type lyophilizer with a capacity of 0.120 m3 and a shelf area of 0.4 m2 is designed. The Stirlingtype lyophilizer refrigeration system structure is analyzed. The flat plate heat pipe to transfer the cold is used. The rigid polyurethane as the insulation material of the lyophilization box is studied. The heat leakage calculation method of the lyophilization box is given. Results: Stirling cryocooler is selected based on the lyophilization box heat leakage. The experimental test of the Stirling cryocooler is conducted. The feasibility of the design is verified. The Stirling cryocooler, with a charging pressure of 2.9 MPa and an operating frequency of 68 Hz, could achieve -87.057°C within 24 min. The lowest refrigeration temperature could be maintained at - 100.286°C after 97 min. The cooling capacity obtained is 40.0 W at an input power of 99.2 W, and the COP is 0.403. Conclusion: Stirling refrigeration technology is suitable for use in lyophilizer and Stirling refrigeration technology improves the robustness of the lyophilizer refrigeration system.

Performance analysis of flat plate heat pipe and heat pump heating device based on solar energy

To solve the problem of poor heating performance of air source heat pumps when the air temperature is relatively low, this paper proposes a flat heat pipe and heat pump heating device based on solar energy. The device can store solar energy and continuously replenish the heat pump. The performance of the device is studied through experiments and simulation calculations. The results show that the difference in temperature between the evaporating section and the condensing section of the flat heat pipe is about 1°C, which shows that the heat pipe has good thermal conductivity. The device can produce a temperature rise of 7~61°C for the air entering the heat pump, which can realize the purpose of heat replenishment for the heat pump. Under the premise of no heat supply, the device can continuously heat -5°C fresh air to 5°C within 3.6 hours through the storage of solar energy. It shows that the device has good heat storage capacity. The above research shows that the device can realize the function of continuous replenishment of air source heat pump by solar energy, and has certain practical application value.

Thermal performance of flat plate heat pipe array with surface wettability modification: An experimental and simulation study

Surface wettability has a significant effect on the heat transfer characteristics of the flat plate heat pipe array (FPHPA). In this study, an aluminum based FPHPA measuring 250 × 100 × 3 mm3, comprising of 9 independent channels, was developed to investigate the impact on wettability segmented modification through experiment and simulation. After chemical etching, the micro-nano structure of the surface can produce a hydrophilic effect, while hydrophobic effect can be achieved after spraying polydimethylsiloxane mixture, and the two modification methods passed the heat resistance and durability tests. The FPHPA, equipped with a hydrophilic evaporator, a hydrophobic adiabatic, and a condenser, exhibited enhanced heat transfer characteristics, lower wall temperature, a higher heat transfer limit, and reduced sensitivity to inclination angle. Simulation results revealed that the hydrophilic surface promoted bubbles to break away from the wall surface and improved the critical heat flux of the evaporator, while the hydrophobic surface had a faster droplet return rate and a thinner liquid film thickness, which improved the convective heat transfer coefficient of the condenser. Compared to conventional flat heat pipes, hybrid wetting modified FPHPA shows a higher overall effective thermal conductivity at thermal powers above 90 W. The findings from this study contribute to a better understanding of the heat transfer mechanism following surface modification and offer a practical method for modifying FPHPA.

Investigation of thermo-hydrodynamic characteristics in micro-oscillating heat pipe by alternatively-arranged ratchet microchannels

In this study, three flat-plate micro-oscillating heat pipes (FPMOHPs) with different alternatively-arranged ratchet microchannels were designed and fabricated. Then, a combined visualization and thermal experiment was conducted to examine the regulation of thermo-hydrodynamic characteristics in the FPMOHPs by the alternatively-arranged ratchet microchannels, via the comparison with a traditional FPMOHP with completely uniform microchannel. The detailed fluid flow motions and corresponding temperature oscillations as well as thermal performance of the FPMOHPs were illustrated and analyzed to clarify the advantages of this microchannel design. It is indicated that the oscillating frequency and velocity of the fluid are diminished by the ratchet microchannels because of the higher flowing resistance. However, the ratchet microchannels can trigger the stronger flow disturbance, leading to the ascending “wave-like” annular flow broken into the dispersive slugs or bubbles with irregular shape. Among the three different ratchet microchannels, the asymmetric ratchet structure with the larger contraction-expansion ratio near the inlet of the fluid can trigger more intense boiling in the evaporator, which also energizes the downward flow from the condenser. Furthermore, the additional actions on the bubbles/slugs caused by the Laplace pressure jump in three different ratchet microchannels play distinct roles in upward and downward bubble/slug flow, i.e., driving force or resistance force or little role, which determine the most favorable ratchet structure for the fluid flow and heat transfer in the FPMOHPs. In general, the positive effects of the enhanced boiling and the stronger fluid flow disturbance caused by the ratchet microchannel dominates the negative effect of growing flow resistance, leading to the enhanced convective heat transfer. Therefore, the equivalent thermal conductivity can be improved up to 33.8 % on average as compared to the traditional FPMOHP with completely uniform rectangular channel.

PREPARATION METHOD AND THERMAL PERFORMANCE OF A NEW ULTRA-THIN FLEXIBLE FLAT PLATE HEAT PIPE

Ultra-thin flat plate heat pipes must provide a degree of flexibility to meet foldable electronics heat dissipation requirements. In this paper, a new flexible ultra-thin flat plate heat pipe with a thickness of 0.75 mm has been designed and fabricated. Compared with the traditional flexible ultra-thin flat heat pipes, the innovation lies in the flexible insulation section formed by epoxy resin pouring of the shell. The design of the shell ensures that the flexible ultra-thin plate heat pipe can respond quickly to the external temperature change, and also has good flexibility, which provides a new choice for the material and structure design of the flexible ultra-thin plate heat pipe shell. The gas-liquid coplanar type mesh is used as the capillary wick to reduce the flow resistance of steam inside the heat pipe, and the wick is hydrophilically modified to improve its capillary pumping performance; a sandwich support structure is used to prevent the steam chamber from collapsing. The thermal performance of the three liquid filling ratios of 0.3, 0.4, and 0.5 was tested at different tilt angles and bending angles. The results show that in the cases of filling ratios of 0.3, 0.4, and 0.5, the ultra-thin flexible flat plate heat pipe with the liquid filling ratio of 0.3 has the best heat transfer performance under different working conditions; the tilt angle has different effects on the heat transfer performance and starting speed of the ultra-thin flexible flat plate heat pipe with different filling ratios, and the bending angle changes the steam condensation position inside the ultra-thin flexible flat plate heat pipe and increases the thermal resistance.

Comparison of air-cooled and spray-cooled heat transfer performance with phase change material coupled with microgroove flat plate heat pipe

Comparison of air-cooled and spray-cooled heat transfer performance with phase change material coupled with microgroove flat plate heat pipe

Thermal and hydrodynamic characteristics of an ultra-thin flattened centered-wick heat pipe: Experiments and numerical analyses

An ultra-thin flattened heat pipe with a centered-wick structure is a promising option for the thermal management of ultra-slim electronic devices. However, while experimental studies are being actively conducted, the thermal and hydrodynamic characteristics of the ultra-thin flattened centered-wick heat pipe have not been deeply analyzed. In the present study, therefore, experiments and three-dimensional numerical analyses were conducted to clarify the characteristics specific to this type of heat pipe. Because a semi-transparent flat-plate centered-wick heat pipe with inner height of 0.5 mm was fabricated in the authors’ previous study, the present study was performed based on this semi-transparent heat pipe. The experiments were conducted to obtain experimental results for comparison with numerical results. In the numerical analyses, the heat pipe was represented by a model comprising vapor, liquid–wick, upper wall, and lower wall regions; the numerical results were obtained for the velocity, pressure, and temperature distributions within the heat pipe. The experimental and numerical results confirmed that their good agreement was obtained within ± 6 % when a condensation end position was set 20 mm from the inlet of the condenser section. In contrast to conventional heat pipes, the present centered-wick heat pipe contains vapor-wick-wall contact lines, which causes the concentration of heat flux. The numerical results clarified that the concentration of heat flux occurred in the wick structure within the range of 0.2 mm from the bottom. This range needs to be considered to improve the wick structure of the centered-wick heat pipe. Simple one-dimensional equations were derived to discuss the vapor pressure difference within the heat pipe. Agreement was obtained within ± 10 % between the vapor pressure differences from one- and three-dimensional calculations.

Performance tests on triple composite mesh-groove-powder wicks in a visualizable flat-plate heat pipe

Two types of novel triple composite mesh-groove-powder wicks are proposed and tested in a visualizable flat-plate heat pipe. The powders are only added in the evaporator in the triple composite wicks to increase the capillarity therein. Outside the evaporator is the mesh-groove wick, with a layer of 200-mesh copper screen sintered over parallel grooves, which has been shown to exhibit high permeability and anti-gravity ability. The first type (Triple A) is to add powders in the mesh-groove wick at the evaporator section, while the second type (Triple B) contains an evaporator with only copper powders. The parallel grooves have a semi-elliptic cross-section with a width of 0.18 mm and a depth of 0.075 mm. According to visualization, the powders in the mesh-groove evaporator appear to increase the flow resistance and prompt partial dry-out in some grooves at high heat loads. The experiment results for the Triple B wick with a powder-only evaporator demonstrate a maximum heat load (Qmax) 20 % higher than that for the double mesh-groove wick at the horizontal orientation, with evaporator resistances also slightly higher. However, the Triple A wick, with powders added in the mesh-groove evaporator, displays a slightly lower Qmax than the original mesh-groove evaporator due to the excessive flow resistance associated with powder-filled meshed grooves.

Experimental investigation on the optimization of different filling ratios for large-size flat plate heat pipe array

To meet the increased heat dissipation demands of 5G base stations, a flat plate heat pipe array (FPHPA) of 500 × 200 × 3 mm3 comprising of 19 independent channels is developed. To investigate the thermal performance of the FPHPA under different filling ratios (FR) and heat powers (30–300 W), a multi-channel independent filling system is designed to conduct two experimental cases: uniform or asymmetric filling of all channels. Results indicate that 30 %–70 % FR exhibits better thermal performance and can replace the aluminum plate over a broader heat power range. Furthermore, separate tests are conducted to evaluate the heat transfer and start-up performance of the FPHPA. The optimal filling ratio interval of single channel at different heat fluxes is derived through asymmetric filling. Subsequently, a mixture model is proposed and compared with the uniform filling ratio case, resulting in optimized heat transfer performance. The optimized FPHPA provides better heat transfer performance, effectively reducing the area averaged wall temperature and ensuring the normal operation of the 5G base station.

Experimental study on heat transfer performance using a hybrid cooling method combined of a flat-plate heat pipe and spray cooling

A hybrid cooling system consists of spray cooling as condensing end and flat-plate heat pipe (FPHP) as heat-transfer medium was previously studied on the effects of the volume flow rate and the inlet temperature. As an extension, this paper using multi-nozzle arrays to promote the forced convection heat transfer and changing the surface roughness of the FPHP to improve the phase change heat transfer. It was found that the impinging energy and drainage channel formed by multi-nozzle arrays were the main reasons for the enhancement in convection region. However, in the phase-change section, a uniform thin liquid film and surface roughness apparently improved the evaporation and nucleate boiling. Relative to our previous study, a maximum heat flux removal was increased from 70 W/cm2 to 90 W/cm2 at surface roughness (Ra = 1.62 μm) with a surface superheat only 4.17℃, the heat transfer coefficient of 21.6 W·cm-2·K-1 and an enhancement of 28.6 % were obtained. Furthermore, the transition temperature and the temperature change rate were studied to explore the vapor flow modes and thermal response characteristics of the FPHP. As the transition temperature Ttr = 251 K, and the Knusden number＜0.01, the FPHP consistently maintained a continuum flow regime during startup, stabilization, and dry-out processes. Impressively, the temperature change rate of the FPHP remained below 0.015℃/s, resulting in a temperature change of only 0.9℃ within one minute. This exceptional temperature stability ensured reliable operation for electronic components.

Experimental investigation on the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe

This study experimentally investigates the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe (PHP). Rectangular grooves are engraved on the sidewalls of the channel in the evaporator section in a direction transverse to the oscillating flow, and these grooves extend from the top surface to the bottom surface of the sidewalls. Two PHPs are fabricated for experimental evaluation using MEMS techniques: one with a grooved channel and another with a smooth channel. Both PHPs have overall dimensions of 29 × 29 × 1.5 mm3. A rectangular meandering channel is etched on a silicon wafer, and this channel possesses dual hydraulic diameters with alternating widths of 1 mm and 0.4 mm and a height of 0.5 mm. In the PHP with the grooved channel, the dimensions of the grooves are analytically determined to maximize the film evaporation heat transfer rate: the depth, the width, and the spacing are determined to be 50 μm, 20 μm, and 20 μm, respectively. R-236fa is used as the working fluid, and the filling ratio is approximately 55 % by volume. Thermal performances of the PHP with the grooved channel and the PHP with the smooth channel are evaluated and compared through thermometry. Experimental results reveal that engraving transverse grooves on sidewalls of the channel effectively enhances the thermal performance of the PHP: the thermal resistance is decreased by an average of 34.6 % across the entire range of input heat fluxes, from the heat flux required for startup to the maximum allowable heat flux. High-speed photography is used, and it is observed that the liquid film in the grooved channel exhibits an 69.2 % thinner average film thickness than that in the smooth channel. This decrease in the average film thickness contributes to the increased evaporation heat transfer rate.

Experimental study on heat transfer and storage of a heating system coupled with solar flat heat pipe and phase change material unit

Using renewable energy, especially solar energy, is essential to achieve a low-carbon society. PCMs suffer from low thermal conductivity, which hinders the efficiency of phase change thermal storage systems. Heat pipes exhibit vastly superior thermal conductivity, making them a promising candidate for enhancing PCM-based systems. To solve the contradiction between building insulation and importing solar energy into buildings in winter, this study proposes a new building heating ceiling system consisting of a flat-plate heat pipe (FHP) coordinated with a phase change material (PCM), and establishes a platform for a heat stockpiling device integrating the FHP and PCM. This research investigates the impact of heating power and bending angle on the heat transfer efficiency of the heat pipe and phase change heat storage system. The experimental findings demonstrate a direct relationship between increasing heating power and the enhancement of the FHP's heat transfer efficiency. When the FHP is bent at a 5° angle and heating power is increased from 20 W to 80 W, to store heat energy into the PCM cavities, the effective thermal conductivity rises significantly, escalating from 7.26 × 104 W/(m·°C) to 15.1 × 104 W/(m·°C), marking a remarkable 108 % rise. Moreover, although increasing the bending angle leads to heightened effective thermal conductivity of the FHP under constant heat input, and the heat transfer in PCM cavities will be weakened within a bending angle of 15°, experimental results demonstrate that the melting time of PCM still decreases with the increase of bending angle; the heat transfer inertia of phase change materials is dominant.

Design and Thermal Performance of Ultra-Thin Flat Plate Heat Pipes with Composite Wick for Electronic Cooling

The rapidly increasing heat load and high-density packaging of electronic devices demand heat dissipation devices that possess miniaturization and high thermal performance. A series of ultra-thin flat plate heat pipes (UPHPs) with thicknesses of 1.2 mm was prepared to satisfy this demand. Strengthening rib optimization and capillary optimization were processed to restrict the entrainment limit and capillary limit of the UPHPs. Furthermore, surface modification technology with composite nanostructure coated was introduced to improve the surface wettability of capillary wick. Finally, the heat transfer capacity of the UPHPs was experimentally tested. The influence of capillary structure and liquid filling ratio on the heat transfer characteristic was investigated in detail. This work provides insight into the design and application of UPHPs for the new generation of electronic devices.

Effect of evaporation and condensation section length ratio on thermal performance of aluminum flat plate heat pipe with different micro grooved wicks

The length of the evaporation or condensation section has a significant impact on conventional heat pipe or copper flat heat pipe heat transfer performance. Based on this demand, a novel concept of the length ratio of evaporation and condensation section (LREC) of the flat plate micro grooved heat pipe (FPMHP) was proposed and the effect of the LRECs on the thermal performance of aluminum-based FPMHP with different micro grooved wicks was experimentally investigated for the first time. The experiment explored the effects of nine different LRECs on the thermal performance of FPMHP with the same section size, filling mass, rectangular and triangular micro grooved wicks installed at 90°, a constantly increasing input heat flux, and forced air cooling. The findings demonstrate that rectangular micro grooved wick flat heat pipe (FPMHP-r) and triangular grooved wick flat heat pipe (FPMHP-t) have the shortest startup times when LREC is 1.0, with FPMHP-r starting 70 s sooner than FPMHP-t. When 0.8 ≤ LREC ≤ 1.25, the heat transfer performance of FPMHP-t and FPMHP-r is excellent, better than performance when LREC < 0.8 or LREC > 1.25, and the LREC is recommended to be 0.8 ∼ 1.25 in practical applications. The optimal LREC value of aluminum-based FPMHP is 1.0, which can maximizes the heat transfer performance. Under the same conditions, the heat transfer performance of FPMHP-r is better than FPMHP-t, flat heat pipe wicks with high permeability, low porosity and effective pore radius should be designed in practice. The results can provide a theoretical basis for the design and optimization of aluminum-based FPMHP application.

Experimental investigation of pulsating heat pipes with different evaporator roughness values

Three transparent experimental setups of flat-plate pulsating heat pipes were investigated, each with a different channel surface roughness in the evaporator section. Roughness was varied using milling and sand blasting techniques. Experiments were conducted in different orientations with the refrigerant R1233zd(E) with filling ratios of 30%, 50% and 70%. Heat flux values between 5 and 45 W and condenser temperatures of 0 °C, 20 °C and 40 °C were applied. Low filling ratios, high condenser temperatures and high heat flux values were found to achieve the lowest thermal resistances. Increased surface roughness in the evaporator section also reduced the thermal resistance. The effect was especially pronounced for low heat flux values. The surface roughness in the evaporator section was analyzed with a confocal and a novel evaluation method with particular regard to the influence of the surface structure on the bubble nucleation behavior was introduced. The evaluation method yielded estimated bubble radius distribution profiles of the cavities found in the surface structure. The data was then used to calculate a mean bubble nucleation radius for the surface structure of each test setup. Combined with the pool boiling nucleation equation this resulted in a theoretical relative bubble nucleation threshold calculated for each measurement point. It was found that the theoretical bubble nucleation threshold could be directly correlated with the thermal resistance and overall performance of the pulsating heat pipe. In the experiments, bubble nucleation rates were determined by evaluating high-speed camera recordings using a bubble tracking algorithm. For a 70% filling ratio the bubble nucleation rate was found to increase up to a maximum with increasing heat flux. When approaching high heat flux values the bubble nucleation rate dropped again. The quantification of the connection between bubble nucleation threshold and thermal performance is a novelty in this work, as well as the quantification of the bubble nucleation rate in an experimental setup. The work offers meaningful insights into the thermo-hydraulic behavior of a PHP.

Thermographic Investigation on Fluid Oscillations and Transverse Interactions in a Fully Metallic Flat-Plate Pulsating Heat Pipe

The present investigation deals with the quantification of fluid oscillation frequencies in a metallic pulsating heat pipe tested at varying heat loads and orientations. The aim is to design a robust technique for the study of the inner fluid dynamics without adopting typical experimental solutions, such as direct fluid visualizations through transparent inserts. The studied device is made of copper, and it is partially filled with a water–ethanol mixture (20 wt.% of ethanol). Heat fluxes locally exchanged between the working fluid and the device walls are first assessed through the inverse heat conduction problem resolution approach by processing outer wall temperature distributions acquired by thermography. The estimated local heat transfer quantities are therefore processed to quantify the fluid oscillatory behavior in every device branch during the intermittent flow and full activation regimes, thus providing a deeper insight into the heat transfer modes. After dealing with a further validation of the inverse approach in terms of oscillation frequency restoration capability, the wall-to-fluid heat fluxes referred to each channel are processed by means of the wavelet method. Scalograms and power spectra of the considered signals are presented for a time-based analysis of the working fluid oscillations, as well as for the identification of dominant oscillation frequencies. Fluid motion is then quantified in terms of the continuity of fluid oscillations and activity of channels by applying a scalogram denoising technique named K-means clustering method. Moreover, a statistical reduction of the channel-wise dominant oscillation frequencies is performed to provide useful references for the interpretation of the overall oscillatory behavior. The link between oscillations and transverse interactions is finally investigated. The vertical bottom-heated mode exhibits stronger fluid oscillations with respect to the horizontal mode, with fluid oscillation frequencies ranging from 0.78 up to 1 Hz. Nonetheless, the fluid motion is more stable in terms of oscillation frequency between channels when the device operates in the horizontal orientation probably due to negligible buoyancy effects. Moreover, thermal interactions between adjacent channels are found to be stronger when the oscillatory behavior presents similar features from channel to channel in horizontal orientation. The proposed method for fluid oscillation analyses in fully metallic flat-plate pulsating heat pipes can be effectively adopted to other flat-plate layouts without any need for transparent windows, thus reducing the overall complexity of experimental set-ups and providing, at the same time, a good insight into the inner fluid dynamics.

Experimental operating range evaluation of flat-plate pulsating heat pipes for high-heat flux automotive power electronics cooling

Flat-plate pulsating heat pipes are a promising future technology to passively increase the power density of power electronics in mobile applications by means of a more efficient thermal management. However, in the design phase, it is not known how different thermal boundary conditions are affecting the operation range of a pulsating heat pipe. To experimentally assess this dependency, the present work aims to evaluate the performance of a flat-plate pulsating heat pipe for electric mobility and different operating conditions. To this purpose, a flexible test section was designed allowing to raise different heating and cooling conditions. A novel array of multiple hotspots was employed which realistically represents a power semiconductor layout. This way, a local heat flux of up to 200W/cm2 was introduced to the pulsating heat pipe. In the condenser section, the coolant temperatures were varied between 10°C and 60°C, representing real conditions of a driving cycle. The investigated aluminum pulsating heat pipe consists of 22 rectangular channels and two different newest generation refrigerants R1233zdE and R1234yf were used as working fluids. Based on the new experimental evidence, our previously published thermal resistance model was adapted. A posteriori comparison with previous calculations confirms the validity of the numerical assumptions and shows a good agreement with the obtained results. For the first time, our investigation shows that pulsating heat pipes can robustly achieve heat flux densities above 100W/cm2 at conditions representative for automotive electronics applications. The real implementation potential was demonstrated, as an considerable 5 to 11 times higher effective thermal conductivity than solid aluminum was obtained depending on the integration concept.