Experimental Investigation on the Temperature Distribution Characteristics of the Evaporation Section in a Pulsating Heat Pipe

With the increasing demand for heat dissipation in the electronics industry, pulsating heat pipe(PHP) has attracted wide attention due to its simple structure and excellent heat transfer ability. However, due to the unique operational mechanism of PHP, the temperature distribution in the evaporation section is obviously not even during the operational process of PHP. When the PHP is used as a heat dissipater, the evaporation section of the PHP directly contacts with the chips and has great influence on the performance of the chips, so it is very important to investigate the temperature distribution characteristics in the evaporation section. In this paper, both the effects of the filling ratio and heat flux on these characteristics were investigated. The experimental results indicated that the temperatures of the middle "U" turn were the highest. When the heat flux and the filling ratio were 364 W/cm~2 and 36.3%, respectively, the maximum temperature difference between the middle "U" turn and the other "U" turns could be as high as 18.92 K. Furthermore, the temperature differences between the middle "U" turn and the other "U" turns firstly increased and then decreased with the increase of heat flux and filling ratio.     

Numerical Investigation of the Performance of a U-Shaped Pulsating Heat Pipe

In this research the performance of a U-shaped pulsating heat pipe (PHP) was investigated using numerical methods. This heat pipe consists of two sections: The evaporator is set at the two ends of the pipe, and the middle part of the pipe comprises the condenser section. This heat pipe is a type of open looped pulsating heat pipe. The governing equations are derived analytically from the continuity, momentum, and energy equations and are solved implicitly. In this model, considering the liquid mesh, the rate of convection and boiling heat transfer in the U-shaped PHP, which has not been investigated as of yet, are examined. The effect of the evaporator temperature on the pulse amplitude and frequency, rate of convection, and boiling heat transfer is also investigated. The results show that by increasing the evaporator temperature, due to the increase in pulse amplitude and frequency, the rate of heat transfer due to convection and boiling in the pipe will increase too. Furthermore, it is derived that by increasing the evaporator temperature, the share of boiling heat transfer will increase. In order to validate the results, the calculated heat transfer is compared to experimental and analytical results, and it is seen that the suggested model correctly predicts the rate of heat transfer within a precise range.     

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.     

Flow and heat transfer of liquid plug and neighboring vapor slugs in a pulsating heat pipe

A model of fluid flow and heat transfer on liquid slug and neighboring vapor plugs in a pulsating heat pipe (PHP) is proposed. A new energy equation for the liquid slug is built by aid of Lagrange method. The shear stress term related with the fluid flow state is included in the motion equation of the liquid slug. A sensitive heat term is replaced by a phase change term in the energy equation of the vapor plug. Based on our analysis on the displacement variation of the liquid slug with time, it is known that the harmonic force acting on the liquid slug in PHPs is the pressure difference between the vapor plugs. The flow oscillation can be considered as a forced damping vibration of one degree of freedom system. The phase difference of the oscillating flow between with and without the gravity effect can reach 45°. The amplitude and angular frequency of flow oscillation is irrespective with the initial displacement of liquid slug. If the flow pattern remains strictly slug flow in the entire system, the contribution of the sensible heat exchange to the total heat transfer of the PHP is about 80%.     

Heat transfer in a pulsating heat pipe with open end

Heat transfer in the evaporator and condenser sections of a pulsating heat pipe (PHP) with open end is modeled by analyzing thin film evaporation and condensation. The heat transfer solutions are applied to the thermal model of the pulsating heat pipe and a parametric study was performed. The results show that the heat transfer in a PHP is mainly due to the exchange of sensible heat. The frequency and amplitude of the oscillation is almost unaffected by surface tension after steady oscillation has been established. The amplitude of oscillation decreases with decreasing diameter. The amplitude of oscillation also decreases when the wall temperature of the heating section is decreased, but the frequency of oscillation is almost unchanged.     

A Mathematical Model of an Oscillating Heat Pipe

A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the system multidegree oscillation of vapor bubbles and liquid plugs, including the effects of filling ratio, operating temperature, gravitational force, and temperature difference between the evaporator and condenser. The model shows that the average velocity of liquid slugs is determined by the temperature difference between the evaporator and condenser. As the turn number increases, the temperature difference for the system to start the oscillating motion decreases. Increasing the bubble number will make the system more unstable and the system can be easily started up. The existence of gravity at the bottom heating mode will make the system easily produce the oscillating motion and decrease the temperature difference as well. Results presented here will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscillating heat pipe.     

Experimental evidences of distinct heat transfer regimes in pulsating heat pipes (PHP)

Various experiments were conducted on two full-size pulsating heat pipes (PHP) which differed from their diameter, number of turns, and working fluid. The analysis of the experimental results showed two kind of operating curves (overall thermal resistance vs. heat rate): for low heat fluxes, the curve is irregular and the PHP performance is sensitive to the orientation. For high heat fluxes, the operating curve is smooth and independent from the orientation. To contribute to the analysis of these results, experiments were conducted at the scale of a single branch of a PHP. An oscillating motion was imposed to a single liquid plug surrounded by two vapour slugs in a capillary tube and high speed visualizations were performed. The test section was either adiabatic or heated. The adiabatic experiments brought to the fore the importance of dynamic contact angles in the flow and the dissymmetry between the advancing and receding contact angle. The non-adiabatic experiments showed that at low flux, the flow is disturbed by bubble nucleation, while at high heat flux, the main heat transfer mechanism is thin film evaporation, with a completely different thermal and hydrodynamic behaviour.     

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.     

Flower-type pulsating heat pipe for a solar collector

Inspired by the sunflower, we report a new structure of a solar collector that integrates a pulsating heat pipe (PHP) into a flat-plate collector. The proposed flower-type PHP solar collector is designed after a sunflower with petals that absorb sunlight and transfer nutrients to the stem after photosynthesis. The evaporator section adopts the shape of a flower to absorb sunlight fully, and the condenser section is rolled into a cylinder and placed in the lower part of the structure. A systematic experimental study is conducted upon start-up, and the performance characteristics, with acetone as the working fluid, are evaluated. We also did a heat loss analysis, which has a deviation of 8%. The effects of the mass flow rate of cooling water, filling ratio, length of the condenser section, and solar intensity are assessed. As the temperature of the heat absorber plate increases, the thermal resistance of the PHP can decrease to a minimum of 0.14°C/W. Under sunny weather conditions, the instantaneous thermal efficiency of the system with a filling ratio of 50% reaches 50%. Besides, we discussed the unstable operation conditions and possible dryout phenomenon that happened inside the PHP.     

Experimental investigation of pulsating heat pipe performance with regard to fuel cell cooling application

A pulsating heat pipe (PHP) is a closed loop, passive heat transfer device. Its operation depends on the phase change of a working fluid within the loop. Design and performance testing of a pulsating heat pipe was conducted under conditions to simulate heat dissipation requirements of a proton exchange membrane (PEM) fuel cell stack. Integration of pulsating heat pipes within bipolar plates of the stack would eliminate the need for ancillary cooling equipment, thus also reducing parasitic losses and increasing energy output. The PHP under investigation, having dimensions of 46.80 cm long and 14.70 cm wide, was constructed from 0.3175 cm copper tube. Heat pipes effectiveness was found to be dependent upon several factors such as energy input, types of working fluid and its filling ratio. Power inputs to the evaporator side of the pulsating heat pipe varied from 80 to 180 W. Working fluids tested included acetone, methanol, and deionized water. Filling ratios between 30 and 70 percent of the total working volume were also examined. Methanol outperformed other fluids tested; with a 45 percent fluid fill ratio and a 120 W power input, the apparatus took the shortest time to reach steady state and had one of the smallest steady state temperature differences. The various conditions studied were chosen to assess the heat pipe's potential as cooling media for PEM fuel cells.