Visual Study on Flow and Operational Characteristics of Flat Plate Closed Loop Pulsating Heat Pipes

This paper presents an experimental study including visualization on a flat plate closed loop pulsating heat pipes.It consists of a total of 40 channels with square cross section(2 mm×2 mm,165 mm long) machined directly on an aluminum plate(180 mm×120 mm×3 mm) covered by a transparent plate.The working fluid employed is ethanol.As a result,various flow patterns and their transitions are observed and found to be related to the fluid fill ratio,input heat load and the device orientation.Also the operational characteristics and working mechanism are discussed.

Parametric Influence on Thermal Performance of Flat Plate Closed Loop Pulsating Heat Pipes

This paper presents an experimental study on a flat plate closed loop pulsating heat pipes. It consisted of total 40 channels with square cross section （2 × 2 mm^2, 165 mm long） machined directly on an aluminum plate（180×120×3 nm^2）, which was covered by a transparent plate. The working fluid employed was ethanol. As the results, the influence parameters of thermal performance were investigated, such as filling ratio, heat load and operational orientations etc. Filling ratio was found to be a critical parameter, and its effect was rather complicated. According to its values the PHP plate could have four distinct working zones with different operational characteristics and heat transfer performance. The effect of heat load on thermal performance was found to be positive, and in general, iucrcasing the heat load would improve heat transfer performance. In order to analyze the effect of gravity on thermal performance, three different heat modes and total seven tilt angles were tested and compared. Successful operation at all orientations with respect to gravity was also achieved.

Two-Phase Flow Modeling in a Single Closed Loop Pulsating Heat Pipes

Mathematical modeling of pulsating heat pipes through ‘first’ principles is a contemporary problem which remains quite elusive. Simplifications and assumptions made in all the modeling approaches developed so far render them unsuitable for engineering design. In this paper, a more realistic modeling scheme is presented which provides considerable try for thought toward the next progressive step. At high enough heat flux level, closed loop pulsating heat pipes experience a bulk internal unidirectional fluid circulation. Under such a condition, conventional two-phase flow modeling in capillary tubes may be applied. This has been attempted for single-loop PHPs. A homogeneous model and a separated two-fluid flow model based on simultaneous conservation of mass, momentum and energy, have been developed for an equivalent ‘open flow’ system. The model allows prediction of two-phase flow parameters in each sub-section of the device thereby providing important insights into its operation. The concept of ‘void fraction constraint’ in pulsating heat pipe operation is introduced and its relevance to future modeling attempts is outlined.

Performances of Flat Plate Pulsating Heat Pipes

Flat plate pulsating heat pipe is useful for hot spot heat spreader. Two kinds of flat plate spreader of pulsating heat pipe are designed, fabricated and experimented. For the embedded circular capillary type, the transferred heat flux could reach 32 W/cm2, the smallest thermal resistance for acetone, methanol and FC-72 were respectively 0.50, 0.57 and 0.40℃/W. While for the square capillary type, the transferred heat flux could reach 26 W/cm2, the equivalent thermal conductivity could reach 3211 W/（m. ℃）. There are ranges of optimal transferred power and filling ratio for different working liquid. If the transferred power is constant, changing the heating area and the place has little effects on the performance.

Experimental Study of Flow Patterns and Improved Configurations for Pulsating Heat Pipes

A series of experiments were performed on three types of closed loop pulsating heat pipes （PHPs）, intending to investigate various kinds of flow patterns, and to develop some improved configurations for the PHPs. Optical visualization results indicated that there might exist three flow patterns, i.e. bubble-liquid slug flow, semi-annular flow and annular flow, corresponding to different working conditions. For a given geometry and an adequate fill ratio, the PHPs had the self-adjusting characteristic for the flow patterns （from slug flow to semi-annular and then to annular flow） to meet the demands of the increasing heat input. Two special configurations, one with alternately varying channel diameter, the other equipped with one section of thicker tube, were found to be advantageous in establishing and maintaining reliable circulation of the working fluid. The thermal performance of the PHPs was examined over a range of working conditions. Comparing with the normal PHP with uniform diameter, either of the improved PHPs exhibited higher thermal performance.

A review of recent experimental investigations and theoretical analyses for pulsating heat pipes

Pulsating heat pipe （PHP）, or oscillating heat pipe （OHP）, a novel type of highly efficient heat transfer component, has been widely applied in many fields, such as in space-borne two-phase thermal control systems, in the cooling of electronic devices and in energy-saving technology, etc. In the present paper, the characteristics and working principles of the PHPs are introduced and the current researches in the field are described from the viewpoint of experimental tests, theoretical analyses as well as practical applications. Besides, it is found that the state-of-the-art experimental investigations on the PHPs are mainly focused on the flow visualization and the applications of nanofluids and other functional fluids, aiming at enhancing the heat transfer performance of the PHPs. In addition, it is also pointed out that the present theoretical analyses of the PHP are restricted by further development of two-phase flow theories, and are concentrated in the non-linear analyses. Numerical simulations are expected to be another research focus, in particular of the combination of the nanofluids and functional fluids.

Heat Transfer Properties and Chaotic Analysis of Parallel Type Pulsating Heat Pipe

The refrigerant mixture of ethanol aqueous was applied to the parallel type pulsating heat pipe (PHP). The operation characteristics of the PHP were analyzed by means of experiment and nonlinear chaotic theory. Moreover, the relationship between the running state and attractor was described. The results indicate that starting power, stable running power and dry burning transition power are about 64.08 W, 148.68 W and 234.0 W respectively. The cycle and amplitude of PHP initially decrease and then increase with the increasing power. However, the data are welldistributed in a certain range. The running state is in agreement with the attractors, and the changing process for attractors is as follows: the attractors first disperse in the whole phase space, then present mass status, and finally show band distribution.

Performance Evaluation of Small-channel Pulsating Heat Pipe Based on Dimensional Analysis and ANN Model

The pulsating heat pipe is a very promising heat dissipation device to address the challenge of higher heat-flux electronic chips,as it is characterised by excellent heat transfer ability and flexibility for miniaturisation.To boost the application of PHP,reliable heat transfer performance evaluationmodels are especially important.In this paper,a heat transfer correlation was firstly proposed for closed PHP with various working fluids(water,ethanol,methanol,R123,acetone)based on collected experimental data.Dimensional analysis was used to group the parameters.It was shown that the average absolute deviation(AAD)and correlation coefficient(r)of the correlation were 40.67%and 0.7556,respectively.For 95%of the data,the prediction of thermal resistance and the temperature difference between evaporation and condensation section fell within 1.13K/Wand 40.76K,respectively.Meanwhile,an artificial neural networkmodelwas also proposed.The ANN model showed a better prediction accuracy with a mean square error(MSE)and correlation coefficient(r)of 7.88e-7 and 0.9821,respectively.

Experimental Investigation of the Operating Characteristics of a Pulsating Heat Pipe with Ultra-Pure Water and Micro Encapsulated Phase Change Material Suspension

The specific heat capacity of working fluid is an important influence factor on heat transfer characteristic of the pulsating heat pipe(PHP).Due to the relatively large specific heat capacity of micro encapsulated phase change material(MEPCM) suspension,a heat transfer performance experimental facility of the PHP was established.The heat transfer characteristic with MEPCM suspension of different mass concentrations(0.5% and 1.0%) and ultra-pure water were compared experimentally.It was found that when the PHP uses MEPCM suspension as its working fluid,operating stability is impoverished under lower heating power and the operating stability is better under higher heating power.At the inclination angle of 90°,the temperature at heating side decreases compared to ultra-pure water and the temperature at heating side decreases with the raising of MEPCM suspension mass concentration.The heat transfer characteristic of the PHP is positively correlated with the inclination angle and the 90° is optimum.The unfavorable effect of the inclination angle decreases with heating power increasing.When the inclination angle is 90°,the PHP with MEPCM suspension at 1.0% of mass concentration has the lowest thermal transfer resistance and followed by ultra-pure water and MEPCM suspension at 0.5% of mass concentration has the highest thermal transfer resistance.When the inclination angles are 60° and30°,the effect of gravity on the flow direction is reduced to 86.6% and 50% of that on the inclination angle of 90°,respectively,and the promoting effect of gravity on the working fluid is further weakened as the inclination angle further decreases.Due to the high viscosity of MEPCM suspension,the PHP with ultra-pure water has the lowest heat transfer resistance.When the inclination angles is 60°,the thermal resistance with MEPCM suspension at0.5% of the mass concentration is lower than that at 1.0% at the heating power below 230 W.The thermal resistance of MEPCM suspension tends to be similar for heating power of 230-250 W.At the heating power above 270 W,the thermal resistance with MEPCM suspension at 1.0% of the mass concentration is lower than that at 0.5%.

A Novel Integrated Photovoltaic System with a Three-Dimensional Pulsating Heat Pipe

Solar energy is a valuable renewable energy source,and photovoltaic(PV)systems are a practical approach to harnessing this energy.Nevertheless,low energy efficiency is considered a major setback of the system.Moreover,high cell temperature and reflection of solar irradiance from the panel are considered chief culprits in this regard.Employing pulsating heat pipes(PHPs)is an innovative and useful approach to improving solar panel performance.This study presents the results of the power performance of a PV panel attached to a newly designed spiral pulsating heat pipe,while graphene oxide nanofluid with three different concentrations was used as a working fluid to maximize the efficacy of the solar panel.The study proved that the cooling method delivered high efficiency by reducing the temperature,especially in the middle of the day.Using nanofluid graphene oxide at concentrations of 0.2,0.4,and 0.8 gr/lit as the working fluid can reduce the thermal resistance of PHPs by over 30%,24%,and 15%,respectively.This,in turn,enhances the system’s electrical power output by approximately 9%,7%,and 6%,respectively.

Experimental Analyses of Flow Pattern and Heat Transfer in a Horizontally Oriented Polymer Pulsating Heat Pipe withMerged Liquid Slugs

Extended experiments were conducted on the oscillation characteristics of merged liquid slugs in a horizontally oriented polymer pulsating heat pipe(PHP).The PHP’s serpentine channel comprised 14 parallel channels with a width of 1.3 mm and a height of 1.1 mm.The evaporator and condenser sections were 25 and 50 mm long,respectively,and the adiabatic section in between was 75mmlong.Using a plastic 3D printer and semi-transparent filament made from acrylonitrile butadiene styrene,the serpentine channel was printed directly onto a thin polycarbonate sheet to form the PHP.The PHP was charged with hydrofluoroether-7100.In the experiments,the evaporator section was heated,and the condenser section was cooled using high-temperature and low-temperature thermostatic baths,respectively.Flow patterns of the working fluid were obtained with temperature distributions of the PHP.A mathematical model was developed to analyze the flow patterns.Themerged liquid slugs were observed in every two channels,and their oscillation characteristics were found to be approximately the same in time and space.It was also found that the oscillations of the merged liquid slugs became slower,but the heat transfer rate of the PHP increased with a decrease in the filling ratio of the working fluid.This is because vapor condensation was enhanced in vapor plugs as the filling ratio decreased.However,the filling ratio had a lower limit,and the heat transfer rate was maximum when the filling ratio was 40.6%in the present experimental range.

Evaporating Temperature Uniformity of the Pulsating Heat Pipe with Surfactant Solutions at Different Concentrations

The evaporating section of the pulsating heat pipe(PHP)is in direct contact with the electronics when it is used for heat dissipation,and thus the evaporating temperature uniformity has an important effect on the safe and reliable operation of electronic equipment.On the basis of these conditions,an experimental study on the evaporating temperature uniformity of the PHP with surfactant solutions at different concentrations was conducted at the heat fluxes of(1911–19427)W/m^(2).Sodium stearate was utilized for the solute;the surfactant solutions were prepared with the concentrations of 0.001 wt%,0.002 wt%,and 0.004 wt%,respectively,and the filling ratios of the PHP were 0.31,0.44 and 0.57,respectively.The experimental results revealed that under all tested working conditions,the highest temperature always appeared in the intermediate zone of the evaporating section.As the heat flux increased,the temperature differences among different zones rose initially and then reduced due to the change of the flow motion and the flow pattern.The evaporating temperature uniformity of the sodium stearate solutions-PHP was better than that of the deionized water-PHP,which suggested that the evaporating temperature uniformity might be improved through decreasing the surface tension.Furthermore,combined with the effect of surface tension and viscosity,for different filling ratios,the required concentration was different when the best evaporating temperature uniformity was achieved.To be specific,when the filling ratio were 0.31 and 0.44,the best evaporating temperature uniformity was achieved at the concentration of 0.004 wt%,while at the filling ratio of 0.57,the best evaporating temperature uniformity was attained at the concentration of 0.002 wt%.

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.

An Experimental Investigation on Heat Transfer Performance of Nanofluid Pulsating Heat Pipe

The effect of SiO2 particles on heat transfer performance of a pulsating heat pipe （PHP） was investigated experi- mentally. DI water was used as the base fluid and contrast medium for the PHP. In order to study and measure the character, there are SiO2/H20 nanofluids with different concentration and applying with various heating powers during the experiment investigation. According to the experimental result, the high fraction of SiO2/H20 will de- teriorate the performance of PHP compared with DI water, i.e. the thermal resistance and the temperature of evaporation section increases. It is in contrary in the case of low fraction of SiO2/H20. Finally, the comparison of the thermal performances between the normal operation system and the static settlement system is given. It is found that both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after standing for a period, and it is the same trend for the temperature fluctuation at the identical heating power for PHP.

Experimental research on heat transfer of pulsating heat pipe

Experimental research was conducted to understand heat transfer characteristic of pulsating heat pipe in this paper, and the PHP is made of high quality glass capillary tube. Under different fill ratio, heat transfer rate and many other influence factors, the flow patterns were observed in the start-up, transition and stable stage. The effects of heating position on heat transfer were discussed. The experimental results indicate that no annular flow appears in top heating condition. Under different fall ratios and heat transfer rate, the flow pattern in PHP is transferred from bulk flow to semi-annular flow and annular flow, and the performance of heat transfer is improved for down heating case. The experimental results indicate that the total heat resistant of PHP is increased with fill ratio, and heat transfer rate achieves optimum at filling rate 50%. But for pulsating heat pipe with changing diameters the thermal resistance is higher than that with uniform diameters.

Experimental Study on Heat Transfer Performance of Pulsating Heat Pipe with Refrigerants

The effects of different refrigerants on heat transfer performance of pulsating heat pipe(PHP) are investigated experimentally.The working temperature of pulsating heat pipe is kept in the range of 20℃-50℃.The startup time of the pulsating heat pipe with refrigerants can be shorter than 4 min,when heating power is in the range of 10W-100W.The startup time decreases with heating power.Thermal resistances of PHP with filling ratio 20.55% were obviously larger than those with other filling ratios.Thermal resistance of the PHP with R134a is much smaller than that with R404A and R600a.It indicates that the heat transfer ability of R134a is better.In addition,a correlation to predict thermal resistance of PHP with refrigerants was suggested.

Visualization of Thermo-Hydrodynamic Behavior in Flat-Plate Pulsating Heat Pipe with HFE-347

In this paper,the visualization of the thermo-hydrodynamic behavior in flat-plate pulsating heat pipe(FP-PHP)with HFE-347 is experimentally investigated.The FP-PHP is vertically placed with filling rate of 20%to 70%and heating power of 20 W to 140 W.A high-speed camera is used to record the two-phase flow in the FP-PHP.Four flow pattern types and four flow directions are observed.The flow directions of the two-phase flow inside the FP-PHP with medium filling rate(40%–60%)are the most complex,and the FP-PHP with high filling rate(70%)is most likely to form a directional circulating flow.At high heating power(100 W to 140 W),the flow patterns in FP-PHP with medium(40%–60%)and high filling rate(70%)are dominated by mixed flow.The wall temperature fluctuates greatly at moderate heating power(60 W to 80 W)owing to the uncertainty of the flow direction.The temperature distribution of the FP-PHP is highly affected by the heat transfer intensity of the working fluid under different flow states,so that the state of fluid flow and the thermal performance of FP-PHP can be evaluated through the infrared thermal image of the FP-PHP.

Full visualization and startup performance of an ammonia pulsating heat pipe

In this paper,a novel study on full visualization and startup performances of pulsating heat pipe using ammonia as working fluid are experimented.The tested pulsating heat pipe,consisting of 6 turns,is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter.The filling ratio is 70%.Wall temperature fluctuations of several key positions are recorded under a series charge of heat inputs.The visualization investigation is conducted to observe the oscillations and circulation flows with the advantage of high quality digital video camera,by which the unique thermodynamic behaviors are able to recognize and analyze more easily.The experimental results show that the startup power required by the ammonia Pulsating Heat Pipe(PHP)is very small,owing to particular identities of ammonia.It is observed that there are also some unevenly distributions in slug-train during initial and operating state.Phenomena such as circulation flows and local oscillations coupling breaking up of bubbles and formation of slugs are observed.

Experimental study on effect of inclination angles to ammonia pulsating heat pipe

In this paper, a novel study on performance of closed loop pulsating heat pipe（CLPHP）using ammonia as working fluid is experimented. The tested CLPHP, consisting of six turns, is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter. The filling ratio is50%. The visualization investigation is conducted to observe the oscillation and circulation flow in the CLPHP. In order to investigate the effects of inclination angles to thermal performance in the ammonia CLPHP, four case tests are studied. The trends of temperature fluctuation and thermal resistance as the input power increases at different inclination angles are highlighted. The results show that it is very easy to start up and circulate for the ammonia CLPHP at an inclining angle.The thermal resistance is low to 0.02 K/W, presenting that heat fluxes can be transferred from heating section to cooling section very quickly. It is found that the thermal resistance decreases as the inclination angle increases. At the horizontal operation, the ammonia CLPHP can be easy to start up at low input power, but hard to circulate. In this case, once the input power is high,the capillary tube in heating section will be burnt out, leading to worse thermal performance with high thermal resistance.

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.