Flat Plate Pulsating Heat Pipe Research Articles

Space Structures With Embedded Flat Plate Pulsating Heat Pipe Built by Additive Manufacturing Technology: Development, Test and Performance Analysis

In this paper is described the development and the experimental study of a flat plate pulsating heat pipe (FPPHP) built by means of metal additive manufacturing (AM). In the recent years, small/medium aerospace companies have gained interest in the development of small satellites. The small dimensions, coupled with the need for high-power devices for science and communications, increase the interest in thermally functional structures. The space business is characterized by a very small production lot, and custom designs from project to project. The AM exactly fits these needs and, in the past years, the use of this technology in aerospace projects has grown significantly. This paper, after a brief review of the pulsating heat pipe (PHP), focuses on the development and testing of a panel with an embedded closed-loop FPPHP built by means of metal AM technique. The article presents a trade-off analysis between the metal AM technologies available on the market; by means of the trade-off analysis, a design strategy is proposed by the authors. A comparison between available FPPHP results in literature and the three-dimensional (3D) printed structure will show the differences between the common subtractive technology and the innovative AM technique.

Thermo-Hydraulic Analysis of Semi-Transparent Flat Plate Pulsating Heat Pipes Tested in 1 g and Microgravity Conditions

Four Closed-Loop Flat Plate Pulsating Heat Pipes (FPPHP) with four different channel internal diameters have been tested under terrestrial (1 g), hyper (1.8-2 g) and micro-gravity conditions, during the ESA 64th Parabolic Flight campaign, at the same operating conditions (heat power range, cooling fluid temperature, vertical BHM orientation). During terrestrial gravity periods, the fluid stratifies with the liquid at the bottom of the system, and, in the lower part of the channels, where the heat is applied (Bottom Heated Mode), the thermo-hydraulic heat and mass transfer mode is purely linked to pool boiling, regardless the diameter. Instead, during microgravity periods, the fluid circulates naturally into a slug and plug flow pattern regime. Dry-out phenomena occur almost immediately after the change in gravity level followed by a fast motion of the liquid plugs, promoting a mass transfer through all PHP channels. A comparative analysis of the evaporator temperatures and the menisci velocities obtained through video post-processing, shows the influence of the channel diameter on the heat and mass transfers occurring inside the different PHPs. A quantitative comparison of the FPPHP thermal performance shows that, during microgravity transient phases, there is a limit of the channel diameter beyond which the thermal performances does not increase anymore, despite the decrease in viscous pressure losses.

Thermal Characteristic and Analysis of Microchannel Structure Flat Plate Pulsating Heat Pipe With Silver Nanofluid

This paper investigates the effect of silver nanofluid on a flat-plate pulsating heat pipe (FP-PHP). The simulation was performed using FLUENT 15.0 software, for which a three-dimensional model having a microchannel structure in the condensation section was developed. The developed model adopted the volume of fluid (VOF) method to track the internal vapor-liquid interface of the FP-PHP and to observe the state of the two-phase flow. The result revealed the evident presence of various types of bubble flows, including dispersed bubble flows, slug flows, annular flows, and column flows in the evaporation section. Trends in thermal resistance variation during simulation were studied by changing the volume fraction, liquid filling rate, and heating power. The thermal resistance of the FP-PHP containing silver nanofluid was lower and the FP-PHP exhibited stable state when the heating power reached 120 W. The optimal volume fraction of silver nanofluid was 1%, and the FP-PHP containing sliver nanofluid exhibited increased heat transfer efficiency.

Comparative thermal performance evaluation between ultrathin flat plate pulsating heat pipe and graphite sheet for mobile electronic devices at various operating conditions

The objective of this study is to evaluate the thermal performance improvement of an ultrathin, flat plate pulsating heat pipe (PHP) against a graphite sheet for use in mobile electronic devices. Initially, the thermal performances of five PHP models with various channel designs are quantified, and an optimum PHP is selected to achieve the best performance. In the optimum PHP, the thermal performance is analyzed as a function of the heat input and condenser temperature. Finally, the finished product of the flat plate PHP is fabricated using a diffusion bonding technique. The thermal performance of the finished flat plate PHP is then compared with that of the graphite sheet based on conducted tests at the inclination angles of 90°, 0°, and −90°. The thermal resistances of the finished flat plate PHP at the inclination angles of 90° and 0° are 63% and 56%, respectively, lower than those of the graphite sheet, whereas the resistance at the inclination angle of −90° is almost the same as that of the graphite sheet.

A comparative study of flow regimes and thermal performance between flat plate pulsating heat pipe and capillary tube pulsating heat pipe

This paper reports the results of an experimental study to investigate the performance comparison between two Pulsating Heat Pipes namely, a Flat Plate Pulsating Heat Pipe (FPPHP) and a Capillary Tube Pulsating Heat Pipe (CTPHP). The comparison is made based on the flow regimes and the corresponding thermal performances at heat inputs varying from 20 W to 180 W with filling ratios of 40%, 60%, and 80%. Experiments are performed in the vertical bottom heating mode with ethanol as the working fluid. The pressure inside the PHPs and temperatures at the evaporator and condenser region are measured along with a recording of the internal flow regimes using a high-speed camera. Slug-plug flow is observed to be the dominant flow regime in both the PHPs. However, the amplitude of oscillations is found to be higher in CTPHP as compared to FPPHP. The reduction in thermal resistance of FPPHP and CTPHP due to the presence of working fluid is about 83% and 35% of the corresponding thermal resistances without any working fluid respectively. CTPHP shows better thermal performance than FPPHP due to the presence of lateral conduction arising in the latter which has a detrimental effect on the slug-plug oscillations.

Numerical and experimental investigation of flat-plate pulsating heat pipes with extra branches in the evaporator section

In addition to some approaches such as changing the working fluid or number of turns in a flat-plate pulsating heat pipe (FP-PHP), geometrical changes are also appealing for enhancing the thermal performance of this type of heat pipes. The main idea of this investigation is to increase heat transfer rate by increasing flow circulation of working fluid. By placing additional branches in the evaporator section, secondary bubble pumps were created which improved the circulation of fluid inside the FP-PHP. In order to investigate the impact of these additional branches, two similar four-turn aluminum FP-PHPs were fabricated. One of them was the conventional FP-PHP and the other had four additional branches and is named additional branch FP-PHP (AB-FP-PHP). Thermal performances of these two types of heat pipes were investigated at different filling ratios (40, 50, 60, and 70 percent) and heat inputs (from 40 to 200 W). Results showed that the thermal resistance of the AB-FP-PHP was 11–20% lower on average compared to the thermal resistance of the conventional FP-PHP at different examined filling ratios. Additionally, for heat inputs around 80 W and above, thermal performances of both devices were better at 50% filling ratio. Furthermore, flow visualization indicated that additional branches affect the flow regime and enhance flow circulation in PHPs. Also, a numerical procedure was conducted for the two-phase system before the experimental investigation to show the role of additional branches in achieving a better circulation of the working fluid.

Experimental analysis of a Flat Plate Pulsating Heat Pipe with Self-ReWetting Fluids during a parabolic flight campaign

A Flat Plate Pulsating Heat Pipe (FPPHP) filled with an ordinary liquid (water) and a self-rewetting mixture (dilutes aqueous solutions of long-chain alcohols with unusual surface tension behavior) is investigated under variable gravity conditions on board a ‘Zero-g’ plane during the 65th Parabolic Flight Campaign of the European Space Agency. The FPPHP thermal performance in terms of evaporator and condenser temperatures, start-up levels and flow regimes is characterized for the two working fluids and a power input ranging from 0 to 200 W (up to 17 W/cm2 at the heater/evaporator wall interface). The experimental set-up also includes a transparent plate enabling the visualization of the oscillating flow patterns during the experiments. For a low power input (4 W/cm2), the pulsating heat pipe filled with pure water is not able to work under low-g conditions, because the evaporator immediately exhibits dry-out conditions and the fluid oscillations stops, preventing heat transfer between the hot and cold side and resulting in a global increase of the temperatures. On the other hand, the FPPHP filled with the self-rewetting fluid runs also during the microgravity phase. The liquid rewets several times the evaporator zone triggering the oscillatory regime. The self-rewetting fluid helps both the start-up and the thermal performance of the FPPHP in microgravity conditions.

Visualization of Flow Patterns in Closed Loop Flat Plate Pulsating Heat Pipe Acting as Hybrid Thermosyphons under Various Gravity Levels

ABSTRACTA particular flat plate pulsating heat pipe (FPPHP), filled with FC72, is tested during the 62th and 64th ESA parabolic flight campaigns under vertical orientation. The FPPHP is made of a thin copper plate, in which a curved channel disposed with 11 U-turns is milled and closed on the top face by a transparent borosilicate plate. The particular characteristics is that the equivalent hydraulic diameter of the square channel (2.5 × 2.5 mm²) is above the working fluid capillary diameter on ground, inducing a stratification of the liquid/vapor phases under ground and hyper-gravity conditions, whatever the orientation. The energy transfer mode in such conditions is represented either by pure pool boiling inside the channels almost completely filled by the liquid phase or by an annular flow pattern inside the channels mostly filled by the refrigerant vapor. Instead, during the microgravity phases, the fluid regime naturally turns into a slug-plug flow pattern. During the transition from 1.8 g to 0 g a rapid dry-out may occur in some of the channels, followed by a similarly fast reaction of liquid plugs moving towards the evaporator from the condenser zone. Such stop-and-start motion events continue during the whole microgravity period, leading to strong temperature oscillations, but also to a still acceptable thermal performance of the device.

Flow patterns and heat transfer characteristics of flat plate pulsating heat pipes with various asymmetric and aspect ratios of the channels

The thermal performance of flat plate pulsating heat pipes (PHPs) in compact electronic devices can be improved by adopting asymmetric channels with increased pressure differences and an unbalanced driving force. The objective of this study is to investigate the heat transfer characteristics of flat plate PHPs with various asymmetric ratios and aspect ratios in the channels. The thermal performance and flow pattern of the flat plate PHPs were measured by varying the asymmetric ratio from 1.0 to 4.0, aspect ratio from 2.5 to 5.0, and heat input from 2 to 28W. The effects of the asymmetric ratio and aspect ratio on the thermal resistance were analyzed with the measured evaporator temperature and flow patterns at various heat inputs. With heat inputs of 6W and 12W, the optimum asymmetric ratio and aspect ratio for the flat plate PHPs were determined to be 4.0 and 2.5, respectively. With the heat input of 18W, the optimum asymmetric ratio and aspect ratio were determined to be 1.5 and 2.5, respectively.

Experimental study on rack cooling system based on a pulsating heat pipe

A rack cooling system based on a large scale flat plate pulsating heat pipe is proposed. The heat generated from IT equipment in a closed rack is transferred by the rear door pulsating heat pipe to the chilled air passage and is avoided to release into the room. The influence of the start-up performance of the heat pipe, the load of the rack and the load dissipation to the temperature and the velocity distribution in the rack are discussed. It is found that the temperature would be lower and the temperature distribution would be more uniform in the rack when the pulsating heat pipe is in operation. Also, the effect of rack electricity load on temperature distribution is analyzed. It is indicated that higher velocity of chilled air will improve heat transfer of the rack.

Experimental study of a closed loop flat plate pulsating heat pipe under a varying gravity force

This paper reports on an experimental study of a closed loop Flat Plate Pulsating Heat Pipe (FPPHP) tested on ground and on board of an aircraft during the 60th ESA parabolic flight campaign, during which hyper- and microgravity conditions were reproduced. The tested FPPHP consists of two brazed copper plates, into one of which a continuous rectangular channel (1.6 × 1.7 mm2) with 12 bends in the evaporator is machined. The channel is filled with FC-72 as working fluid with a volumetric filling ratio of 50%. Tests have been conducted with the FPPHP positioned both horizontally and vertically (bottom-heated). The FPPHP presents an innovative design, involving the milling of grooves between the channels. Experimental results on the ground show that the thermal device can transfer more than 180 W in both inclinations, and that the horizontal operation is characterized by repeated stop-and-start phases and lower thermal performance. The FPPHP can operate under microgravity conditions and with a transient gravity force, with global thermal resistance reaching 50% and 25% of that of the empty plate (or around 66% and 35% of a full copper spreader of same overall dimensions), in horizontal and vertical orientation respectively. The temperature homogeneity remains within 10 K in the evaporator section and 3 K in the condenser section with thermal power transfer up to 180 W. Minimum thermal resistance of 0.12 K W−1 was recorded, with its value rising as heating grew more powerful. A parabolic flight test demonstrated that the FPPHP in vertical inclination is rapidly influenced by variation of gravity field, even if, due to the novel geometry, it continues to operate under microgravity. In horizontal inclination, on the other hand, there was no observable parameter change during gravity field variations.

EXPERIMENTAL STUDY ON NATURAL CONVECTION COOLING OF LED USING A FLAT-PLATE PULSATING HEAT PIPE

Heat dissipation structure is important for high-power LED chips because of a high heat flux. The experimental study of cooling high-power LED (up to 100 W) based on the combination of flat-plate pulsating heat pipe (PHP) and natural convection fins was conducted. The effects of heating power, working fluids, filling ratio (rate of liquid-filled volume to the total volume of channels), configuration parameters, and fin area on the start-up time and the thermal characteristics of the plate PHP were investigated experimentally. The experimental results indicated that cooling by a flat-plate PHP with natural convection fins was effective in lowering the temperature of high-power LED. However, the thermal characteristics of a flat-plate PHP with natural convection cooling possessed large disparity compared to that of forced convection cooling. The optimal filling ratio range was 30% to 50% in this research and the thermal performance of a flat-plate PHP with acetone as a working fluid was better than that with ethanol. Moreover, a flat-plate PHP with a trianglular cross-section channel led to a lower thermal resistance than that with a square cross-section channel. The study of thermal resistance based on channel cross-sectional area was suggested.

Thermo-hydrodynamics analysis of vapor–liquid two-phase flow in the flat-plate pulsating heat pipe

A three-dimensional unsteady model of vapor–liquid two-phase flow and heat transfer in a flat-plate pulsating heat pipe (FP-PHP) is developed and numerically analyzed to study the thermal-hydrodynamic characteristics in two different configurations of FP-PHPs. The thermo-hydrodynamics characteristics under steady unidirectional circulation condition of the studied FP-PHPs are numerically investigated and discussed. The results indicate that the bubbly flow, slug flow and semi-annular/annular flow occur in the FP-PHP under the condition of steady unidirectional circulation, when the adjacent tubes of the FP-PHP become ‘upheaders’ and ‘downcomers’ of working fluid. The periodical oscillations of fluid temperature and vapor volume fraction are observed to be synchronous, while the temperature oscillation amplitude at adiabatic section is larger than that at condenser section but less than that at evaporator section. The increases in the heat load lead to the high temperature level and small integral equivalent thermal resistance of the FP-PHP. Additionally, compared with the traditional FP-PHP with uniform channels, the FP-PHP with micro grooves incorporated in the evaporator section is effective for the heat transfer enhancement and possesses a smaller thermal resistance at high heat loads.

Experimental study on the start up performance of flat plate pulsating heat pipe

An experimental system of flat plate pulsating heat pipe was established and experimental research was carried out in this system to know the mechanism of heat transfer, start-up and operating characteristics. The factors, such as filling rate, heating power, heating method etc, which have great influence on the thermal performance of the plate pulsating heat pipe were discussed. The results indicate that heating power and filling rate are the important factors for the start-up of the plate pulsating heat pipe. The different start-up power is needed with different filling rate, and the start-up of the heat pipe in case of bottom heated is much easier than that of top heated. Increasing the heating power and enlarging the heating area can make the start-up easier. Heating power can also affect the start-up time of heat pipe under the condition of bottom heated, while it does not have some influence to the heat pipe of top heated. The thermal resistance of plate pulsating heat pipe is related with the heating power, and the higher the heating power is, the smaller the thermal resistance is. But the best filling rate which the heat pipe needs is different with different heating methods, and the performance of the heat pipe in the case of bottom heated is better than the others.

Experimental Investigation of Oscillating Motions in a Flat Plate Pulsating Heat Pipe

An experimental study was conducted to evaluate the motion of vapor bubbles and liquid plugs within a flat plate pulsating heat pipe to determine the effects of working fluids (water, ethanol, Flutec PP2, and Flourinert), power input, filling ratio, and angle of orientation on the pulsating fluid flow. Experimental investigations quantify the position and velocity of the liquid-vapor interface for various conditions. Through the use of a Photron high speed camera, precise locations of the liquid-vapor interface were tracked and analyzed. Experimental data show that both the macromovement and oscillating motion of vapor bubbles and liquid plugs exist for a functional pulsating heat pipe. The amplitude of these oscillations was increased as more power was inputted into the pulsating heat pipe. The pulsating heat pipe investigated here would not function when charged with 50% high performance liquid chromatography grade water and positioned horizontally. On the other hand, the experimental results show that when the heat pipe was charged with ethanol, it created the largest amplitudes and velocities. The oscillating motion of vapor bubbles and liquid plugs including the macromovement is very sensitive to the tilted angle, and large increases in amplitude were observed when the angle was increased (bottom heating mode) from horizontal.