Optimum Fill Ratio Research Articles

Investigation of operational limit of a pulsating heat pipe by estimating local heat transfer

How pulsating heat pipes (PHPs) reach their operational limit has not yet been fully understood. This study aims to provide a complete picture of the termination mechanism of the self-oscillation of vapor and liquid. Experimental studies on a 10-turn PHP with HFC-134a were conducted and the filling ratio (FR) was from 20% to 80%. The thermo-fluid behavior in the PHP was investigated by temperature measurements with a high-resolution and high-speed infrared camera and estimation of fluid-to-wall heat flux distributions by solving inverse heat conduction problems. The results suggested that the PHP, increasing heat load, reached the operational limit due to different mechanisms depending on the filling ratio: at a high FR (80%), the liquid volume ratio increased with the increase of the operating temperature, resulting in the compressed liquid phase. At a low FR (20%), when a large amount of heat was applied, the fluid in the evaporator dried out and became a superheated vapor. The PHP with an optimum FR (50%) transferred the maximum heat under the same evaporator temperature, as the fluid in the PHP was able to keep the saturated two-phase state until the evaporator temperature exceeded the critical temperature.

Comparative analysis of an environmentally friendly refrigerant with DI water when used in a thermosiphon heat pipe

The focus on building better and efficient cooling techniques for electronic gadgets are gaining importance day-by-day. Among the various techniques being studied passive cooling techniques using heat pipes are gaining momentum; the challenge being finding a suitable working fluid which would enhance the performance of these devices. With the present scenario demanding eco-friendly alternatives, the researchers have tried using R600a, a hydrocarbon refrigerant with zero Ozone Depletion Potential. The optimum fill ratio of 50% was determined experimentally at an orientation of 90° from horizontal. Experiments were carried out to find the performance parameters of the thermosiphon heat pipe at the said fill ratio by varying the input heat from 20 W to 140 W with an increment of 20 W each. Next, the working fluid was replaced by de-ionized water and experiments were repeated. The results suggest that the resistance of the heat pipe thermosiphon reduced by 27% when R600a was used. Also, heat transfer coefficients at the evaporator and condenser section increased by a significant margin of 19% and 30% respectively when R600a was used in comparison to DI water.

Application of environment-friendly refrigerants in anodized grooved thermosyphon at high heat loads

Efficient heat transfer devices with eco-friendly refrigerants are essential to reduce the global warming. This work studies the heat transfer performance of both non-anodized and anodized aluminium grooved thermosyphon charged with R134a and R600a as working fluid. The grooved thermosyphon was tested under various parameters such as the fill ratio of 20–80%, the inclination angle of 00 –900, heat input in the range of 50 – 350 W. To estimate the anodizing effect the pore size and coating thickness were determined using SEM photographs. This study discusses the effect of inclination angle and fill ratio of working fluid for the different heat input concerning on the heat transfer performance. It was found that due to the anodization the reduction in thermal resistance of 25.62% and 26.7% as well as enhancement in heat transfer coefficient of 6.2% and 8.6% for R134a and R600a respectively at optimum fill ratio of 30% and inclination angle of 60°.

Feasibility of using multiport minichannel as thermosyphon for cooling of miniaturized electronic devices

AbstractThe feasibility of using multiport minichannel (MPMC) as thermosyphon for cooling miniaturized electronic products is experimentally investigated with acetone as the working fluid. A detailed analysis on thermal performance and entropy generation due to heat transfer and pressure drop with the effects of heat load (10‐50 W), filling ratio (FR; 40%, 50%, and 60%), and inclination angle (45°, 60°, and 90°) has been carried out. The results showed a reduction of 22.2% and 9.31% in thermal resistance and evaporator wall temperature at optimum filling ratio (OFR) of 50%. Reduction in entropy generation due to heat transfer and pressure drop of 16.6% and 12.3%, respectively, was observed at OFR. Internal fins in MPMC increase the surface area and evaporation rate by enhancing heat transfer leading to a decrease in the rate of entropy generation. Multiport increases surface tension of condensate at right angles to the flow direction along with the effects of gravity and enhancing rate of condensation. A new correlation is developed to predict evaporator wall temperature as a function of heat load and FR. The proposed correlation agrees well with a deviation of ±20% with present experimental results and also with the published literature. Thus, the obtained results will be useful in cooling miniaturized electronic devices.

An Experimental Study of a Two-Phase Closed Thermosyphon Rankine Cycle Performance

Decreasing the heat rejection of any power cycle is a promising agent in improving the overall performance and decreasing global warming. This can be done by different methods such as heat exchangers or thermosyphons. Water is used as a working fluid. The present experimental work used water as a working fluid and investigated the effects of filling ratio (27.2%≥Fr≥9%), total nozzle exit area to the turbine inlet area (4.3%≥Ar≥2.5%), and condenser cooling water flow rate (16LPM≥(V ) ≥8LPM). The results showed that the optimum-filling ratio is approximately 13.6% and the output power increases with increasing condenser cooling water flow rate. Also, the maximum output power produced at minimum area ratio but the optimum turbine rotational speed at area ratio of 3.1%. In addition, the maximum output power without load from the present turbine is 5.28 W at Fr= 13.6 %, Ar=2.5% and V =16 LPM

Cooling of high heat flux electronic devices using ultra-thin multiport minichannel thermosyphon

Studying multiport minichannels as thermosyphons are considered to be of great interest compared to cylindrical and flat shaped ones with single ports. In this study, a novel multiport minichannel (MPMC) thermosyphon (hydraulic diameter of 1.18 mm, length of 200 mm with 10 internal ports with thickness 0.1 mm each) with acetone as working fluid is experimentally investigated for cooling high heat flux electronic devices. Effect of heat load (10–50 W), filling ratio (40%, 50%, and 60%) and tilt angles (45°, 60° and 90°) on thermal resistance, convective heat transfer coefficients of evaporator and condenser, vapour velocity, flooding, sonic, boiling limits and efficiency are experimentally studied. Results showed 9.31% and 22.2% reduction in evaporator wall temperature and thermal resistance at optimum filling ratio (OFR) of 50%. Enhancement in evaporator heat transfer coefficient and thermal efficiency of 30.7% and 89.8% are respectively observed at OFR. Multiports present in minichannel acts as internal fins and increases the surface area and evaporation rate. Further multiports increases surface tension of condensate at right angles to the flow direction along with the effects of gravity enhancing the rate of condensation. Thus, the obtained experimental results will be useful in cooling of miniaturized high heat flux electronic devices..

Short communication: Thermal performance of a cryogenic helium pulsating heat pipe with three evaporator sections

This short communication reports surprising results from on-going measurements of a helium pulsating heat pipe (PHP) in which the adiabatic length has been increased from 0.3 m to 1 m. In addition, the PHP has been constructed with a total of three evaporator sections. The PHP contains 42 parallel stainless steel tubes with an outer and inner diameter of 0.8 mm and 0.5 mm, respectively. PHPs with this configuration can serve as thermal spreaders for cryocoolers and can be used to cool multiple hot spots. Results and configuration details of the PHP with a 300 mm long adiabatic section have been previously published (Fonseca et al., 2017, 2018). The same PHP, now modified with a 1-meter long adiabatic section has been tested at the same heat load and fill ratio values as the 0.3 m PHP. The PHP operates between 3 K and 5.2 K using a Sumitomo cryocooler model RDK-408A2 with a cooling capacity of 1 W at 4.2 K. Interestingly, the results show that the temperature difference (T) between the evaporator and condenser sections of the PHP with the 1-m adiabatic section length is almost identical to that measured with the 0.3 m adiabatic section length PHP when the two different configurations are each operated at their respective optimum fill ratio. Temperature differences below 1 K are achieved when the heat load is less than 0.26 W per evaporator section (0.78 W total). Very similar optimal thermal conductance (Q̇/ΔT) values are obtained for both configurations, where a maximum thermal conductance of 0.383 W/K at a fill ratio of 58% and 0.387 W/K at a fill ratio of 70% are achieved for the 1 m and 0.3 m PHP, respectively. In addition, the optimal thermal performance can be expressed as an effective thermal conductivity that is proportional to the adiabatic length, resulting in a higher value for the 1-m PHP of 150 kW/m-K; while the 0.3 m PHP achieved a maximum thermal conductivity of almost 53 kW/m-K. This paper will present results based on equal heat load distribution throughout the evaporator sections.

Modelling of two-phase closed thermosyphon based on SINDA/FLUINT

Two-phase closed thermosyphons (TPCTs) are simple and efficient heat exchangers. They have been explored for use in the renewable energy resource utilization marker and low grade thermal energy heat recovery systems. A transient simulation model for a TPCT was established by SINDA/FLUINT with low global warming potential Freon R245fa as working fluid. The TPCT was manufactured from a 40 mm inner diameter (Di), 3 m long smooth copper tube with a wall thickness of 2 mm. It consists of the evaporator, adiabatic and condenser sections with 1 m long for each section. The evaporator section was immersed in a water bath and the condenser section was cooled by recycled water. The effects of water bath temperature Tb and inlet temperature of cooling water Tcw,i were investigated by experimental and simulation methods. The results show the heat transfer rate Q and overall heat transfer coefficient U increase with the increase of Tb, and the decrease of Tcw,i. Good agreement between experimental and simulation results confirms the model is accurate and reliable. The influence of filling ratio (FR) and Di on the performance of TPCT was also investigated. The optimum FR for Di of 30 mm, 40 mm and 50 mm are 15%, 15% and 25% respectively. Moreover performances of 60–150 m long TPCTs were investigated based on the developed model.

Performance Study of Solar Heat Pipe with Different Working Fluids and Fill Ratios

This paper elaborates on the testing of solar heat pipes using different working fluids, fill ratios and tilt angles. Methanol, Acetone and water are used as working fluids, with fill ratios 25%, 50%, 75% and 100%. Experiments were carried out at 600 and 350 inclinations. Heat pipe condenser section is placed inside a water basin containing 200ml of water. The evaporator section is exposed to sunlight where the working fluid gets heated and it becomes vapour and moves towards the condenser section. In the condenser section the heat is given to the water in the basin and the vapour becomes liquid and comes back to the evaporator section due to gravitational force. Two modes of experiments are carried out: 1) using a parabolic collector and 2) using heat pipe with evacuated tubes. On comparative study, optimum fill ratio is been found to be 25% in every case and acetone exhibited slightly more efficiency than methanol and water. As far as the heat pipe orientation is concerned, 600 inclination of the heat pipe showed better performance than 350

Maximum Heat Transfer and Operating Temperature of Oscillating Heat Pipe

It is reported that the operating temperature of an oscillating heat pipe (OHP) at an operating limit is not dependent on the ambient temperature but that the maximum heat transfer is dependent on this. In this study, using different ambient temperature conditions, a 15-turn OHP filled with HFC-134a as a working fluid was operated until it dries out. The maximum heat transfer was found to vary with changes in the ambient temperature, but the operating temperature at an operating limit, which depends on the filling ratio (FR) of the working fluid, was found to be constant. At the operating limit, the operating temperature decreased with an increase in the FR when the ratio was greater than 50 wt.%. Visualization experiments and calculations were used to confirm that there is an increase in the liquid volume in the OHP in accordance with an increase in the heat input and that ultimately the OHP fills with the liquid, resulting in the failure of OHP operation. In contrast, at the operating limit, when the FR was less than 50%, the operating temperature increased in line with an increase in the FR. In this case, it is assumed that the volume of liquid slugs decreases as the heat input increases, thus causing the OHP to dry out. This theory is explained using a P–V diagram of the working fluid in the OHP. The OHP thermodynamic cycle reaches a saturated liquid or vapor line before it reaches a critical point if a specified volume is shifted from the specified volume at the critical point. The optimum FR for maximum heat transfer is therefore decided by the void ratio at the critical point of the working fluid.

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.

Effect of Carbon Nanotube Additive on the Thermal Performance of a Horizontal V-Grooved Heat Pipe

An experimental facility to examine the effect of carbon nanotubes (CNT) on the performance of a horizontal V-grooved heat pipe was build. The circular heat pipe was made of copper with an inner diameter of 10 mm and equipped with 10 axial V-grooves with 1 mm depth and 1 mm mouth. The experiment was performed using water as working fluid with mass percentage of CNT of 0%, 1%, 3% and 5%. The optimum fill ratio of the heat pipe was first determined by examining the fill ratio effect on the total resistance. This optimum fill ratio was then used throughout the whole experiment. The evaporator of the heat pipe was heated using a copper heater block equipped with 6 cartridge heaters. The power input to the evaporator was varied from 60 W to 240 W in steps of 60 W with maintaining steady-state operation at each power level. The condenser was cooled by a water jacket fed by a chiller. The surface temperature of the heat pipe was measured by 7 K-type self-adhesive thermocouples connected to data acquisition system. The experimental results showed that the performance of the heat pipe improved with the use of CNT-water mixture in terms of a reduction in the thermal resistance. The results also showed that this enhancement is a function of the amount of CNT additive.

The characteristics of extracellular polymeric substances and soluble microbial products in moving bed biofilm reactor-membrane bioreactor

The characteristics of extracellular polymeric substances (EPS) and soluble microbial products (SMP) in conventional membrane bioreactor (MBR) and in moving bed biofilm reactor-membrane bioreactors (MBBR-MBR) were investigated in long-term (170 days) experiments. The results showed that all reactors had high removal efficiency of ammonium and COD, despite very different fouling conditions. The MBBR-MBR with media fill ratio of 26.7% had much lower total membrane resistance and no obvious fouling were detected during the whole operation. In contrast, MBR and MBBR-MBR with lower and higher media fill experienced more significant fouling. Low fouling at optimum fill ratio may be due to the higher percentage of small molecular size (<1 kDa) and lower percentage of large molecular size (>100 kDa) of EPS and SMP in the reactor. The composition of EPS and SMP affected fouling due to different O-H bonds in hydroxyl functional groups, and less polysaccharides and lipids.

Experimental Investigations for Start up and Maximum Heat Load of Closed Loop Pulsating Heat Pipe

Startup heat load, maximum heat load and optimum fill ratio of Pulsating Heat Pipe (PHP) of 16 turn, 1 mm ID, 2 mm OD and 9.6 m total length are found out experimentally for water and ethanol as working fluids. PHP is operated in vertical bottom heat mode. Evaporator and condenser temperatures are maintained at 100 °C and 28 °C respectively. Temperature fluctuations of adiabatic section at startup and maximum heat loads are reported. Experimental results indicate that, startup heat load is independent of fill ratio, but maximum heat load depends on fill ratio. Optimum fill ratio for maximum heat load depends on working fluid for a given PHP and operating temperatures.