Thermal Resistance Of Pulsating Heat Pipe Research Articles

Experimental study on heat transfer performance enhancement of pulsating heat pipes induced by surfactants

Pulsating heat pipe (PHP) is considered as a promising technique to address the thermal dissipation issues in micro-electronic devices, but its heat transfer performance needs to be further enhanced urgently. In this study, the PHP comprehensive performance experimental system was established. Then, the influence of surfactant sodium dodecyl sulfate (SDS) aqueous solution on the heat transfer performance of PHP was experimentally investigated and the visualization of PHP with surfactant was also accomplished. Additionally, based on Marangoni effect and surface tension, a comprehensive analysis was conducted on the impact mechanism of surfactants on PHP. The experimental results indicated that the oscillatory disturbance of the liquid slug inside the PHP is enhanced by the addition of SDS, and the resulting pressure imbalance and temperature difference enhance the directional flow of the working fluid. Meanwhile, under the operating conditions of 30 W working thermal power and the solution concentration of 0.001 wt%, the thermal resistance of PHP was reduced by 30.7 % compared with that of deionized water (DI water). This indicated that the working fluid with low concentration SDS could increase the area of liquid film and reduce the thermal resistance, which is induced by the surface tension and Marangoni effect due to the uneven distribution of surfactants. However, the thermal resistance of PHP with the SDS concentration of 0.2 wt% is higher than that of DI. This means that high concentration of SDS would deteriorate the startup characteristics and the heat transfer performance of PHP. Therefore, high concentrations of surfactants should be avoided from practical application of PHP.

Fluid flow and thermal performance of the pulsating heat pipes facilitated with solar collectors: Experiments, theories and GABPNN machine learning

Solar energy could be effectively exploited and transferred by the solar collectors, where thermal transport efficiency was inhibited by actual operations and applications. In present research, solar collector combined with pulsating heat pipes (PHP) was proposed to enhance solar energy conversion and management. Fluid temperature oscillation, flow patterns and motions, heat transfer mechanisms inside present facility were comprehensively investigated, which aimed to suitably determine the flow stability of working fluid for PHPs, such that the whole system performance could be fully optimized. Firstly, an experimental investigation was comprehensively conducted to obtain the flow motions, temperature oscillation, and thermal resistance of PHPs, under multi-parameter conditions. Our results indicated that heat transfer performance was promoted with an increase in thermally driven force, accompanied by which low frequency with “small amplitude” transformed to high frequency with “bulk amplitude”. In addition, filling ratio of 25% even a minimum input power generally results in dry-out phenomena, superior liquid-vapor patterns and heat transfer performance were observed as filling ratio was 60%. Following that, theoretical modeling results confirmed that, within the critical pipe diameter, larger saturated vapor pressure and lower surface tension were significantly beneficial for the application of PHPs-solar collector. Considering dimensionless parameters and experimental data, Genetic Algorithm-Back Propagation Neural Network (GA-BPNN) was further built to predict the heat transfer performance of PHPs, which was in great agreement with the experimental data. Optimal filling ratios were further confirmed depending on the best Ku performance, within average deviation being no more than 10%. Present theoretical and experimental researches could facilitate the optimization and application of PHP- solar collectors, greatly exploiting solar energy for buildings.

Investigation on start-up and thermal performance of the single-loop pulsating heat pipe with variable diameter

The start-up performance and heat transfer performance of three-dimensional single-loop pulsating heat pipes (PHP) with three diameter ratios are numerically probed in present work. The various heating power (20W-50W) and inclination angle (0°-90°) are employed in present numerical simulation. The results show that the start-up and heat transfer performance of the uniform PHP are significantly affected by inclination angle. When PHP is placed vertically, two start-up processes are observed for the PHP with three diameter ratios and the heating power of 20W. The start-up of PHP with variable diameter has a sharp decline in temperature, while the temperature variation of uniform PHP in start-up is continuous and stable. By comparing three PHPs with different diameters, it can be found that the variable diameter structure can increase additional pressure difference to reduce the sensitivity of PHP to gravity, and even the start-up phenomenon can be observed with horizontal arrangement. In addition, under the condition of the heating power of 20W, the PHP with diameter ratio of 4:2 has the greatest thermal resistance, while the PHP with diameter ratio of 4:3 has the least thermal resistance, among which the thermal resistance of PHP with diameter ratio of 4:3 reduces by 3.37% under the inclination angle of 10°. Besides, the larger the diameter ratio is, the easier the PHP is to dry-out due to less liquid mass and greater flow resistance with the same filling ratio.

Experimental Investigation on the Thermal Performance of Pulsating Heat Pipe Heat Exchangers

In this study, the vertically-oriented pulsating heat pipe (PHP) heat exchangers charged with either water or HFE-7000 in a filling ratio of 35% or 50% were fabricated to exchange the thermal energy between two air streams in a parallel-flow arrangement. Both the effectiveness of the heat exchangers and the thermal resistance of PHP with a size of 132 × 44 × 200 mm, at a specific evaporator temperature ranging from 55 to 100 °C and a specific airflow velocity ranging from 0.5 to 2.0 m/s were estimated. The results show that the heat pipe charged with HFE-7000 in either filling ratio is likely to function as an interconnected array of thermosiphon under all tested conditions because of the unfavorable tube inner diameter, whereas the water-charged PHP possibly creates the pulsating movement of the liquid and vapor slugs once the evaporator temperature is high enough, especially in a filling ratio of 50%. The degradation in the thermal performance of the HFE-7000-charged PHP heat exchanger resulted from the non-condensable gas in the tube became diminished as the evaporator temperature was increased. By examining the effectiveness of the present heat exchangers, it is suggested that water is a suitable working fluid while employing the PHP heat exchanger at an evaporator temperature higher than 70 °C. On the other hand, HFE-7000 is applicable to the PHP used at an evaporator temperature lower than 70 °C.

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°C-50°C. 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.

A study of the heat transfer performance of a pulsating heat pipe with ethanol-based mixtures

The heat transfer performances of a pulsating heat pipe (PHP) with ethanol–water, ethanol–methanol and ethanol–acetone are investigated experimentally. The mixing ratios (MRs) of the ethanol-based mixed working fluids are 2:1 and 4:1, the volume filling ratios (FRs) range from 45% to 90% and the heat input ranges from 10W to 100W. The experimental results are as follows: When the mixing ratio is 2:1, the heat transfer performance of PHP with ethanol–water is better than other working fluids at a filling ratio of 45% because of the phase-change inhibition in ethanol–water; at a filling ratio of 55%, PHP with ethanol–acetone shows better performance among those with mixed working fluids. Acetone with a relatively high value of (dp/dT)sat (saturation pressure gradient versus temperature) and relatively lower dynamic viscosity can lead to relatively high velocity in the PHP, which can decrease the temperature difference between the evaporation section and condensation section. When the mixing ratio is 4:1, the thermal resistance of PHP with ethanol–water that is close to being dried out under a filling ratio of 45% rises faster than that at a mixing ratio of 2:1 due to the presence of less deionized water (DI water); the heat transfer performance of PHP with ethanol–acetone is excellent at a filling ratio of 55% among the ethanol-based mixed working fluids because the thermal resistance is relatively small and the maximum heat input that PHP can endure is highest. When the volume filling ratio reaches 62%, 70% and 90%, the heat transfer performance of PHP with pure working fluids is better than that with ethanol-based mixed working fluids. This may be partially attributed to the offset of flow driving force caused by the mass transfer due to the concentration difference between the liquid and the vapour phase of mixtures. The filling ratio of 62% shows a marginal leading in terms of lower thermal resistance.

Simulation and optimization of a pulsating heat pipe using artificial neural network and genetic algorithm

In this paper, a novel approach has been presented to simulate and optimize the pulsating heat pipes (PHPs). The used pulsating heat pipe setup was designed and constructed for this study. Due to the lack of a general mathematical model for exact analysis of the PHPs, a method has been applied for simulation and optimization using the natural algorithms. In this way, the simulator consists of a kind of multilayer perceptron neural network, which is trained by experimental results obtained from our PHP setup. The results show that the complex behavior of PHPs can be successfully described by the non-linear structure of this simulator. The input variables of the neural network are input heat flux to evaporator (q″), filling ratio (FR) and inclined angle (IA) and its output is thermal resistance of PHP. Finally, based upon the simulation results and considering the heat pipe’s operating constraints, the optimum operating point of the system is obtained by using genetic algorithm (GA). The experimental results show that the optimum FR (38.25 %), input heat flux to evaporator (39.93 W) and IA (55°) that obtained from GA are acceptable.