NUMERICAL ANALYSIS OF PASSIVE TWO PHASE FLUID FLOW IN A CLOSED LOOP PULSATING HEAT PIPE

Numerical analysis of passive two phase fluid flow in a 3D Closed Loop Pulsating Heat Pipe (CLPHP) with turns in evaporator and condenser section is carried out. Water is used as working fluid. The volume of fluid model (VOF) is used to simulate passive two-phase fluid flow in a Closed Loop Pulsating Heat Pipe. Filling ratio (FR) of is kept in the range of 60 to 70%. The evaporator temperature is set in the range of 353 K. The condenser temperature is set in the range of 298 K. The contours of volume fraction water, wall temperature and contours of mass flow rate is studied. Analysis of fluid flow is done with various geometrical parameter for 3D Closed Loop Pulsating Heat Pipe. Alternative liquid and vapor slug formation are observed and evaporation and condensation process are visualized in the analysis.


INTRODUCTION
The heat pipe is capable of transferring large quantities of heat with minimum temperature drop. With the size of electronics devices shrinking day by day there need to implement effective cooling technique which will cool the electronics devices without compromising on the performance, to achieve this goal heat pipe technology looks promising. In heat pipe liquid and vapor slug/bubble transport is caused by the thermally induced pressure pulsations inside the device and no external mechanical power is required. The type of working fluid and the operating pressure inside the pulsating heat pipe depends on the operating temperature of the heat pipe. The region between evaporator and condenser is adiabatic. The heat is transfer from evaporator to condenser by the means of pulsating action of vapor and liquid slug.
The development of CFD model provide deeper understanding of the principles of thermodynamics, fluid dynamics and heat transfer within a pipe, allowing the performance of heat pipe to be enhanced for many different applications. Furthermore, modelling the complicated twophase flow of boiling and condensation can be used for validating the experimental results. The boiling and condensation processes especially the boiling process does not depend on the actual shape of surface. The CFD model can reduce the experimental work necessary to predict the performance of system, which can then be optimised. The performance of final optimised model can be verified with the experimental results, which substantially reduces the cost.
Analysis is done to test performance of closed loop pulsating heat pipe with different working fluid and geometries to cool the temperature sensitive electronic equipment like CPU of laptop, graphics card, cooling of these devices is important to improve the performance of system, these devices generate heat during the high-end task. One of the objectives is to validate the experimental results with CFD tools and to understand the behaviour of fluid flow in narrow channels. * Corresponding author. Email: roshan.bhagat25@gmail.com Khandekar and Groll (2003) conducted experiments on a PHP made of copper capillary tube of 2 mm inner diameter for three different working fluids viz. water, ethanol and R-123. The PHP was tested in vertical (bottom heat mode) as well as horizontal orientation and indicated that, a 100% filled PHP (not working in the pulsating mode but instead as a single-phase buoyancy-induced thermosyphon) is thermally better performing than a partially filled pulsating mode device under certain operating conditions. Khandekar and Groll (2004) studied that, complete stop-over is within the loop happens a lot of times for filling magnitude relation < 50% as well as low heat input power. Stop-over development has additionally been determined for higher filling ratios. The 'self-sustained' periodic character is then lost; such a behaviour has never been according for multi-turn PHPs due to alternating periods during which bubble plugs are moving rapidly (activity phase) and 'stopping' (static phase).  indicated that the thermal resistance decreases with the rise of the heating power at an equivalent filling magnitude relation. For the pure operating fluid PHPs, the thermal resistance is decreases within the sequence of water, ethanol, methanol and acetone. Narasimha et al. (2012) studied that, at atmospheric condition; the saturation temperature is higher compared to evacuated situation. Thus, more liquid phase exists in the tube with a consequent increase in the heat transfer. Charoensawan et al. (2003) indicated that in vertical orientation for the 2.0 mm devices, water filled devices showed higher performance as compared to R-123 and ethanol. Whereas in 1.0 mm devices, R-123 and ethanol filled devices showed comparable performance but water showing very poor results.  observed that the Acetone as working fluid has higher value of bond number and lesser thermal resistance whereas Methanol has higher thermal resistance and lower value of bond number. So, the working fluid with higher value of bond number gives higher thermal performance than the working fluid with lower value of bond number.
With the development of semiconductor technology, various electronic equipment has entered people's lives extensively, and the trend of miniaturization and integration greatly increases the power consumption and heat generation per unit area of these electronic equipment. So, it is urgent to strengthen the research and applications of heat transfer devices of high thermal conductivity. Fairley and Patel studied that in order to meet the work and production requirements of higher heat load, smaller design space and lower cost, the development of passive cooling system with higher performance has become a focus of research. Qu and Zhao found that as a new heat transfer device, the PHP has a series of advantages such as high heat transfer efficiency, good adaptability, simple structure and low cost. So, since it was invented in the 1990s. Kearney state that the PHP has been paid close attention by researchers due to its unique working principle and excellent heat transfer efficiency. It has been applied in a variety of fields, such as heat dissipation of electronic equipment, heat collection of solar energy, recovery of waste heat, thermal management of power units.
During the start-up period the working fluid oscillate with large amplitude, after this period continuous circulation can occurs in the working fluid occurs. The direction of circulation for working fluid is consistent once circulation is obtained but the direction of circulation can be different for same experimental run.
Working fluid is vital issue that considerably influence the thermal performance of CLPHP, since operating fluid acts as heat transferring medium between source and sink. Thermal performance of the heat pipe considerably depends on thermo dynamical properties of operating fluid. The thermo dynamical properties involving two-part flow heat transfer encompass heat energy, heat capability, viscosity, physical phenomenon etc. the choice of the operating fluid employed in PHPs is depends on many variables. The approximate temperature ranges the system are going to be exposed to be most crucial in determining the right operating fluid. Exploitation to associated degree approximate temperature variation of −50 0 150 0 , suggests several potential operating fluids square measure doable choice for pulsating heat pipe.
The objective of research work is to do the numerical analysis of 3D Closed loop pulsating heat pipe. Numerical analysis is done on Various geometrical parameter having one or more turns, flat evaporator and condenser section, evaporator section with coil arrangement to visualize the complex phenomenon of evaporation and condensation.

Boundary Conditions
Working fluids is tested with different combination of pulsating heat pipe. More attention is given on developing 3D model with the help of CFD tools that includes the complex physical phenomenon of the heat transfer, the process of evaporation and condensation. The fluid flow behavior is observed with different sets of parameters. The use of CFD model can reduce the experimental work necessary to predict the performance of system, which can then be optimized. The performance of final optimized model can be verified with the experimental results, which substantially reduces the cost of fabrication.

Physical Characteristics and Assumption
The pulsating heat pipe used in the analysis here has inner diameter of 2mm. Various geometries with single turn, flat, multiple turns and evaporator coils. The filling ratio is taken in the range of 60 to 70%. Three phases are utilized liquid, vapor and air. Volume of fluid model is most suitable as it tracks the interphase of phases. The three Eulerian phases are air, water vapor and water liquid, even though there is a vacuum inside the pipe, which is defined later, air is still defined as one of the phases. The reason for including a phase with air is that if only water liquid and vapor are defined, the calculation starts as if there is already water vapor inside the pipe. The k-epsilon model is selected, with enhance wall treatment, thermal effect and curvature correction.

Governing Equations
The flow inside the pulsating heat pipe is having liquid and vapor slug transformation. As the phase change takes place at the saturation temperature, the conservation of mas equation needs to be taken into consideration.
The momentum equation is solved throughout the domain Eq.
(2) which is reliant on volume fractions of all the phases.
The energy equation shared among the phases is shown in Eq. (3). Here, ℎ is energy source caused by phase change.

Cell Zone Condition
In this section the vacuum inside the pipe is introduced. Clicking on "Cell Zone Conditions" and then on "Operating Conditions" set the "Operating Pressure" as 4000 (this is the saturation pressure of the water at a saturation temperature of 29 °C. Boundary conditions are applied in the evaporator section and temperature specified as 353 K. The condenser temperature is kept at as 298 K. As capillary action has to takes place due to smaller diameter of tube contact angle of working fluid need to be specified at all wall geometry. The contact angle is specified as 20 0 .

Initializing Simulation and Patch
After configuring the other two boundary conditions i.e., evaporator and condenser the last step before starting the simulation is initializing it and attribute initial positions for the liquid and air for the beginning of the simulation. After the initialization the zones patching is done where the geometry is patch with water as working fluid. Steam portion here is considered at 0 at initial stage.

Simulation Run
The issue with simulation of multiphase flows is that the time step needs to be sufficiently small to capture the movement of the particles, and at the same time it needs to be big enough to reduce the computational time.

Single Turn Flat Closed Loop Pulsating Heat Pipe
The 3D geometry of single turn flat closed loop pulsating heat pipe was generated with the help of Ansys Design Modeler. Default Meshing is generated with the meshing tool used in Ansys.

Single Turn Closed Loop Pulsating Heat Pipe
The 3D geometry of single turn CLPHP is generated with the help of Ansys Design Modeler. Default Mesh is generated with the help of meshing tool available in Ansys. The reason for default mesh is to reduce the computational time during the process of iteration.

Two Turn Closed Loop Pulsating Heat Pipe
The 3D geometry of two turn closed loop pulsating heat pipe is generated with the help of Ansys Design Modeler. Default Meshing is done to reduce the computational time during the process of iteration. This heat pipe has two small turn in evaporator section and one large and one small turn in condenser section. The excess area of tube in condenser is to avoid dry out condition. The geometry has inner diameter of 2mm, smaller diameter is selected to have capillary action inside the pipe.

Three Turn Closed Loop Pulsating Heat Pipe
The 3D geometry of three turn closed loop pulsating heat pipe is generated with the help of Ansys Design Modeler. Default Mesh is generated with the meshing tool used in Ansys. The evaporator section has three small turn and the condenser section has two small and one large turn. The geometry has inner diameter of 2mm, smaller diameter is selected to have capillary action inside the pipe.

Six Turn Closed Loop Pulsating Heat Pipe
The 3D geometry is created with inner diameter of 2mm.  The geometry of heat pipe has six small turns in evaporator section and five small and one large turn in condenser section. Surface area of copper tube in condenser section is ensure to have higher value than the evaporator section to avoid the dry out of working fluid.

Evaporator Coil Closed Loop Pulsating Heat Pipe
The 3D geometry with inner diameter of 2mm generated in Ansys Design Modeler. Sweep operation is used to prepare the turns in evaporator section. Default meshing is done to reduce computational time required. The geometry is sliced in the evaporator and condenser section, geometry slice is useful to define the heater and condenser section. Specification of heater and condenser section is required to apply the boundary condition in setup.

Single Turn Flat Closed Loop Pulsating Heat Pipe
Thermal analysis of single turn flat CLPHP is carried out using ANSYS fluent the contours of water volume fraction, wall temperature, contours of mass transfer rate and its behavior is studied at various flow time  shows the fluid flow has been developed and due to condensation, some vapor slug converted to liquid while they reached towards the condenser section. Latent heat as well as sensible heat transfer takes place. Fig. 9 Contours of water volume fraction at 0.30 Seconds As the pulsating action takes place difference of pressure and density drives the flow of fluid from evaporator to condenser section. Heat transfer takes place as combination of sensible and latent heat of working fluid.

Contours of Mass Transfer rate
The contours of mass transfer rate help to understand the flow and behavior of fluid during the process of evaporation and condensation.    Fig.13 shows the contours of wall temperature to understand the process of evaporation and condensation the temperature at evaporator and condenser will be observed during the flow of fluid. At initial flow time of 0 seconds higher temperature are observed at evaporator.

Single Turn Closed Loop Pulsating Heat Pipe
The analysis of single turn closed loop pulsating heat pipe is carried out as shown in the fig. 16 to 19. Contours of water volume fraction. Wall temperature are visualized at various flow time.

Fig. 16 Contours of water volume fraction at 0 seconds
The alternate liquid and vapor slug formation can be understood with single turn closed loop pulsating heat pipe. Evaporator section has one turn and the condenser section has one turn, combine sensible and latent heat will be responsible for heat transfer.

Contours of Wall Temperature
The contours of wall temperature are studied at different flow time. At initial flow time of 0 seconds higher temperatures are observed in the evaporator section of closed loop pulsating heat pipe. The temperature reduction is observed when these liquid and vapor slugs enter the condenser section.   The contours of wall temperature are studied at various flow time starting from 0 seconds to 0.72 seconds. The temperature distribution can be more specific while observing the simulation results. Maximum temperature is observed at the evaporator section of heat pipe and minimum temperature is observed at condenser section. As heat transfer occurs heat absorption and rejection take place at evaporator and condenser section of Single Turn Pulsating Heat Pipe.

Two Turn Closed Loop Pulsating Heat Pipe
The fluid flow in multiple turn CLPHP is observed in fig. 24 to 29.

Fig. 25 Contours of water volume fraction at 0 seconds
The contours of water volume fraction at flow time of 0 seconds shows that the steam formation not started during the initial stage of heating. At start of heating the small bubble get condensed at evaporator section before reaching to the other end.

Fig. 26
Contours of water volume fraction at 0.14 seconds The liquid and vapor slug formation started from the flow time of 0.14 seconds. It is found to be more developed when reached to the flow time of 0.49 seconds. Contours of water volume fraction can be visualized at various flow time that is helpful to understand the behavior of water as working fluid when two turns are present in evaporator section.

Fig. 27
Contours of water volume fraction at 0.26 seconds While the vapor slug moves towards the condenser section it was observed that it gets disappear at initial stage of condensation process, these vapor and liquid slug carries heat in the form of sensible and latent heat towards the condenser section.

Contours of Mass Transfer Rate
The contours of mass transfer rate are studied at various flow times of 0 second to 0.80 seconds. This analysis gives the mass transfer rate of liquid and vapor slug while they move over the entire length of tube.

Three Turn Closed Loop Pulsating Heat Pipe
Behavior of closed loop pulsating heat pipe with three small turn in evaporator section, two small and one large turn in condenser section is studied from the fig.35 To 46, at different flow time and time step.

Contours of Mass Transfer Rate
The mass flow rate is visualized at different flow time as shown below.

Contours of Wall Temperature
The contours of wall temperature are important to get the temperature at evaporator and condenser section. At given flow time of 0 seconds. Maximum temperature is observed at evaporator only.  Fig. 44. The evaporator section observed to at higher temperature than the condenser section. Heat is first transfer from water bath to the copper tube by convection which subsequently transfer to the working fluid present inside by the conduction heat transfer.  The contours of wall temperature at the evaporator section are found to be higher at flow time of 1.11 seconds, the temperature increased is also observed near to condenser section. The minimum value of temperature is observed exactly in the condenser it indicates the heat is rejected by working fluid.

Six Turn Closed Loop Pulsating Heat Pipe
The flow visualization can be observed from fig. 47 to 58 having six small turns in condenser section, one large turn in condenser section and five small turns in condenser section. The contours of water volume fraction, wall temperature and mass flow rates are studied.

Contours of Water Volume Fraction
The fig. 51 shows the contours of water volume fraction at flow time of 0 seconds. As per the visualization the heating has started and the flow is yet to get developed.

Contours of Mass Transfer Rate
The contours of mass transfer rate for water shows the mixture of water and steam the phase transformation during the process of heating. Better visualization is observed from the flow time of 0 seconds to the flow time of 0.44 seconds. Negligible mass transfer rate is observed at flow time of 0 seconds, however the same is developed as shown in Fig. 55 to 58.

Evaporator Coil Closed Loop Pulsating Heat Pipe
Closed loop pulsating heat pipe with coil turns in evaporator section is studied, with contours of water volume fraction, contours of wall temperature. The flow behavior is observed at various flow time.  The liquid and vapor slug flow can be observed at given flow time of 0.14 seconds. The liquid and vapor slug movement are observed from evaporator to condenser section. During the initial stage the vapor gets condensed as soon as it enters the condenser section of evaporator coil closed loop pulsating heat pipe.    The alternate liquid and vapor slug transport take the heat from evaporator to the condenser section. Contours of wall temperature helps in understanding the evaporation and condensation process in evaporator coil closed loop pulsating heat pipe. The difference of density is responsible for fluid flow from evaporator to the condenser section The increase in wall temperatures is observed at given flow time of 0.14 seconds. The variation in the color contours can be observed as the heating in done in the evaporator section. The gradual temperature variation is observed from evaporator to condenser section.

Fig. 66
Contours of wall temperature at 0.14 seconds The contours of wall temperature show the temperature distribution along the length of the pulsating heat pipe. There is increase in the value of temperature at condenser at flow time of 0.28 seconds. The sensible and latent heat transfer is primary reason for the temperature change at the condenser section. The contours of wall temperature show the heat carried by liquid and vapor slug while moving from evaporator to condenser section. The drop in temperature was observed in condenser section. The closed loop pulsating heat pipe with coil in evaporator section looks promising in higher heat transfer.

Fig. 68
Contours of wall temperature at 0.5 seconds

Conclusion
The Numerical analysis of closed loop pulsating heat pipe is carried out by using different 3D geometries. The flow visualization using CFD tools helped in predicting the flow of fluid, the behavior is studied at various flow time. The contours of liquid volume fraction, contours of wall temperature and contours of mass flow rate are visualized. For few geometries the flow is getting developed at lower flow time while for some other it has developed at higher flow time.
The phenomenon of evaporation and condensation best understood with CFD results thus, this will be helpful in selection of geometry for given closed loop pulsating heat pipe. Also, it is observed that as all the different combination of 3D geometries used has same inner diameter of 2mm the liquid and vapor slug formation take place, if the diameter is more than 2mm these pulsations does not take place. The aspect ratio i.e., length to diameter ratio is also important in order to have the pulsating action so as to transfer heat in the form of sensible and latent heat. Evaporator coils closed loop pulsating heat pipe seems promising for cooling of battery and other electronic components. Best visualization results are obtained with the evaporator coils closed loop pulsating heat pipe.