| Pulsating heat pipe(PHP)was invented in 1990 s as a non-conventional heat pipe which is passive yet efficient heat managing device that have potential applications in solar cell,fuel cell,space and electronic cooling and hybrid vehicles.Divided in three parts viz.,evaporator,adiabatic region and condenser,PHP has no wick structure to assist flow of working fluid from one region to another,rather the capillary action assists in this regard.Many studied have revealed that thermo-fluid dynamics and phase change behaviour are mostly dependent on the type of working fluid,the Filling Ratio(FR),the heat input,evacuation pressure and many other factors.Yet,clear-cut explanation of working mechanism and effect of each parameter that defines the thermal performance of PHP is still unclear.On the other hand,researchers are focused on finding new working fluids which would enhance the heat transfer rate by the device in efficient manner.Many experimental researches have been conducted in the past,and PHP has been showing promising results in tackling the need of efficient heat dissipation from compact spaces.Yet further researches are needed to fully understand the complex chaotic flow dynamics and heat transfer phenomenon of PHP to achieve optimal operation parameters.Many studies have indicated that,unlike conventional heat pipes,PHPs have potential to handle much larger thermal loads while meeting demanding weight and size constraints.Over the time,higher degree of thermal performance of PHP is achieved with the introduction of non-conventional fluids like nano-fluids and surfactant solution as the working fluids.Till date,it is clear that parameters like geometry and inclination,fill ratio,evacuation pressure,heat input range,mode of heating and cooling,very much affect the performance.But the degree and dominance of one parameter onto other is not yet fully realized.In this research non-visualization and visualization experiment are carried out in the PHP with different working fluids at varying FR to observe the thermal response of the device at varying heat input.For non-visualization experiment,PHP considered is closed type and had 8 numbers of turns with copper tube of 2 mm inside diameter and 0.5 mm thickness.The entire experimental setup was composed of a PHP,heating and cooling system,charging and evacuation system,temperature sensors and data acquisition system.The PHP was first evacuated with the help of vacuum pump and the absolute pressure of 90 k Pa was maintained.Then,the working fluid was charged through the charging tube into the PHP with the help of syringe.De-ionized(DI)water,Methanol and Ethanol are conventional working fluids considered in this research.Likewise,and Cetyltrimethyl Ammonium chloride C19H42 Cl N(CTAC),a surfactant,is non-conventional working fluids whose surface tension less than DI water,depending on the concentrations.The experiment is initially performed with 0% FR,without evacuation,which serve as a reference measurement,where heat transfer is purely by conduction via copper tube and air inside it.The thermal resistance of the PHP system is obtained as 3 K/W at 15 W of heat supply.Thereafter,100% FR of DI water is used,which yield thermal resistance as 2.8 K/W.Here,the heat transfer is only due to the circulation prompted by buoyancy on single-phase fluid.In every case,the PHP is kept in vertical position.The test is stopped when the temperature shot up which represents non-working of the PHP as pulsation flow is ceased.Time series of temperature on the evaporator and condenser region are recorded to observe the heat transfer dissipation rate and to calculate the thermal resistance at different heat load.Higher fluctuations in the difference of temperature between evaporator and condenser region is attained with the case of Methanol,which is mainly due to its greater value of ratio of saturation pressure gradient and temperature(d P/d Tsat).The lowest thermal resistance of the PHP with DI water is 0.34 K/W achieved with 50% FR at 75 W.Dry-out condition is attained with 50 ppm,100 ppm,and 1000 ppm CTAC at 35% FR before 80 W of heat supply.2000 ppm avoid this condition of dry-out and achieve the lowest thermal resistance value of 0.30 K/W at 35% FR and 50% FR.Lower surface tension values of surfactants,which decreases from 51.30 m N/m to 38.46 m N/m when the concentration increases from 50 ppm to 2000 ppm,tend to promote the easy rising and falling of fluids from evaporator to condenser and vice-versa.On the other hand,viscosity increases(from 1.51×10-3 Pa·s to 2.09×10-3 Pa·s)as the concentration increase(from 50 ppm to 2000 ppm)which hinders the flow within the capillary.So the flow movement is the trade-off between the surface tension and viscosity.At 50% FR,2000 ppm CTAC has the lowest thermal resistance at all values of heat supply and 50 ppm CTAC has the highest of all.And at 65% FR,2000 ppm CTAC yields higher thermal resistance at lower heat supply which can be blamed upon the higher viscosity value that resists the heated solution at evaporator region to rise up into cooler section at the adiabatic and condenser regions.The value of surface tension and viscosity changes with the increase in temperature.And,this phenomenon affected the thermal performance of the PHP.At higher heat loads,the 2000 ppm solution reaches dry-out condition.The thermal resistance of 1000 ppm is lowest,compared to 100 ppm and 50 ppm solution.The lowest thermal resistance attained by 2000 ppm CTAC at 35% and 50% FRs ascertains than the CTAC solution can perform better than DI water in the PHP.The thermo-physical properties of surfactant solution have positive impact on the thermal performance of the PHP.Influence of specific heat capacity,latent heat of vaporization,surface tension and viscosity on the thermal performance of the PHP varies with the FR and heat load conditions.It is concluded that better thermal performance can be achieved with surfactant solutions,compared to conventional working fluids,at different FRs and heat loads.From all the experimental investigation it is observed that the solutions with different thermo-physical properties response differently in the PHP.Their possessions are reflected in the temperature variation and thermal resistance values.But the effects on phase change phenomenon and bubble formation and bubble's movement cannot be seen in this set of experiment,a visualization system is needed.The most common technique to visualize the operation of PHP is to use the transparent tube.Such test system would enable to get the insight of flow regime inside an operational PHP.The start-up characteristics can be understood by exactly monitoring the flow regime while the temperature fluctuation occurs once the heat is supplied in the evaporator region.Likewise,the flow pattern with two-phase interaction,bubble formation,growth and its dissipation,flow direction and chaotic flow can be captured by high speed camera.Heat transfer mechanism inside a PHP is governed mainly by the flow regimes of the working fluids,which can be further disintegrated as bubble generation,bubble growth and annular flow.Bubbles in the evaporator are produced as enough heat is supplied at this region.These bubbles then moves into the liquid column at condenser which is at a lower pressure and temperature.And the heat transfer phenomenon inside the PHP with the continuous cycle of generation,growth and dissipation of bubbles in a pulsating flow path is a very complex to understand and establishing global correlation for this process in another challenge.Understanding the working of PHP is greatly assisted by the visualization technique.Abundant research works could be found where researchers have been working to find ways of enhancing the thermal performance and to fully understand the working mechanism of Pulsating Heat Pipe(PHP).Experimental investigations bring out the difference in thermal performance characteristics of PHP with different parameters like: geometric designs,working fluids and operating conditions.There are not much of explanations to varying response of PHP to these parameters which can be extracted from such experiments.Visualization experiment is a must to understand the operating characteristics of PHP under different working conditions.It can reveal the underlying working mechanism that could delineate the features of operational PHP.In visualization experiment set,a closed PHP of glass tube(thermal conductivity: 1.2 W/m·K)with 4 numbers of turns is used.The inside and outside diameter of the PHP is 2mm and 3mm.From the visualization experiment of PHP with conventional and non-conventional working fluids,the difference in operational mechanism is quite clearly observed.To complete the experimental set up,beside PHP,filling and evacuating system,heating and cooling system,temperature sensors,data acquisition system and image capturing system are integrated.The sizes of the bubbles and their flow characteristics are certain to alter the thermal performance of PHP.A high speed camera was arranged to capture the flow behaviour within the operational PHP.It can capture the gas-liquid two-phase change from the start of functioning of the PHP to the stable operation stage.It was connected to a computer to record and store the images for further investigation.The time interval considered for this purpose is 25 ms.The image with 1280x1024 resolutions was taken into consideration.The shooting speed is 400 frames per second.The movement and formation of bubbles,evaporation and condensation are recorded and analyzed.High power illuminating light bulb was placed behind the PHP to assist the good capture of images by the camera.Prior to heating,when the check valve at the filling connection is opened,working fluids rushed into the PHP from the syringe due to the lower pressure inside the PHP.Because of the dominance of surface tension over the gravitational force,working fluids randomly distribute as liquid slugs and vapor plugs inside the tube of PHP.Start-up characteristics was measured based on the temperature fluctuation in the outside wall of the PHP.And,dominating flow pattern for each case was considered to be the one which persisted for longer time once the pseudo-stable condition is attained.Nucleate boiling at the evaporation region marks the preparation for the start-up phenomenon of PHP while in operation.Vapour bubbles start to form when the thin film of liquid absorbed sufficient amount of heat from the heating wall.When large amount of vapor bubbles are formed and the increase in vapor pressure is adequate to push the upper liquid regime,the start-up phenomenon is triggered.At the same working condition of 50% FR and 50 W heat input,four different working fluids viz.DI water,Methanol,Ethanol and 300 ppm CTAC yielded different start-up characteristics.Since the outside wall of the glass could not give the exact temperature recording with the considered set up,the start-up is perceived as the two phase flow start to rapidly oscillate inside the PHP.Heating was turned off when the outside wall reached the temperature beyond 120 °C.The start-up time was comparable for the DI Water and CTAC,and it was also similar for Methanol and Ethanol.Start-up time was in the range of 60 sec to 70 sec for DI water and CTAC,and it was lesser by 10 sec to 20 sec for alcohols.For this,it can be settled that the boiling point is the dominating properties.300 ppm CTAC with base fluid water has preserved this property to certain extent.However,there was major difference in the bubble formation process near the evaporator region and nature of bubble movement.In case of DI water,the flow is very much dominated by the annular flow and it supports the efficient transfer of heat from the evaporator region.Methanol and Ethanol,due to their low surface tension values than that of DI water gave rise to smaller sizes of bubbles.Also due to high(d P/d Tsat)of Methanol and Ethanol,bubbles in PHP with these fluids are moving rapidly.With CTAC,quite unique flow characteristics can be observed.The swirling motion of bubbles is most likely the reason behind the rapid transfer of heat from the evaporator region.Moreover,movement of cluster of micro-bubbles in the condenser region is the special feature of PHP with CTAC solution.For numerical analysis of the PHP,transient analysis is opted in commercial software for the fluid flow simulation.Mass transfer with phase change prompted by temperature and pressure gradient needs to be addressed upon the mesh generated.For this purpose,Volume of Fluid(VOF)model that can be applied on a fixed Eulerian mesh is considered to capture the interface between liquid and vapor phases in this analysis.This inbuilt model incorporated in commercial software for computational fluid flow is mainly controlled by 3 governing equations for mass,momentum and energy.This numerical analysis is mainly focused on observing and understanding the effects of surface tension and viscosity of working fluids on thermal performance of PHP at different heat loads and fills ratios.Study on the influence of change in surface tension and viscosity on thermal performance of PHP by computational method revealed the probable reason for better performance of PHP with Surfactant at 50% FR and 65% FR.It is observed that the PHP with Surfactant has lower thermal resistance values at higher heat loads as compared to PHP with Water-0.07.It suggests that the lowering of surface tension increased the thermal performance.Because of lower surface tension,Surfactant changed the phase easily and more vapors are formed at the same heat input level when compared to Water-0.07.Though the increase in viscosity limited the maximum velocity attained by the fluid within the capillary tube of PHP,its effect is dominated by the effects of surface tension.35% FR is not the favourable condition for PHP with both Water-0.07 and Surfactant.Thermal performance of PHP with Surfactant is worst at 35% FR,and its higher thermal resistance values than that of Water-0.07 suggests that lowering of surface tension affects the performance of PHP adversely by promoting rapid phase change,and thereby stimulating early dry-out condition.Thus,the PHP with surfactant solution performs better at higher fill ratios,and the performance is enhanced at higher heat loads.Further exploration of operation PHP that can realize the velocity of the flow field is certain to reveal more explanations to the different characteristics of flow field.And,laser penetration technique to insert the thermocouples into the glass PHP will give out more accurate thermal response of the PHP.Likewise,inputs from the improvised visualization technique as the initial boundary conditions in the numerical modeling will surely improve the results of simulation thereby giving more accurate flow field and extended results. |
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