Evaporator Section Research Articles

Mathematical Modeling of Closed Loop Pulsating Heat Pipe by Using Artificial Neural Networks

Pulsating heat pipe is one of the prominent technology for thermal management of electronic devices. It consists of three sections namely evaporator, adiabatic and condenser section. PHP is a two phase passive device having efficient and quick ability of transferring heat from evaporator section to condenser section. At first an 8 turn pulsating heat pipe of closed loop ends (CLPHP) with copper tube capillary dimensions is investigated experimentally for different fill ratios and for different inclinations by varying range of heat inputs. Different working fluids viz Water, Acetone, Ethanol and Methanol are considered for the experimentation. One of the recent analytical technology for modelling of CLPHPs is Artificial Neural Network (ANN) approach. The analytical models are having limited scope of applicability and they are simple in nature. The present paper describes Validation of experimental data by training prediction model ANN with available data. Three input nodes such as input heat, fill ratio and angle of inclination and one output node corresponding to PHP that is thermal resistance are considered. The feed forward neural network (FFNN) architecture is adopted for predictions. By using the physical phenomena of the system modelling are clearly known for obtaining feasible results which is main function of ANN. The predicted data validates experimental data in a satisfactory range and the results are found to be in good agreement with in the range of ± 10 percent.

Characterization and performance of a 3D-printed two-phase closed thermosyphon

We report our experimental study of the performance of a 3D-printed closed two-phase thermosyphon. Using laser powder-bed fusion (L-PBF) technique, the thermosyphon has been additively manufactured with stainless steel (316L), which has excellent resistance and compatibility with chemicals and coolants. The specific feature of the thermosyphon and the novelty of the study is that the device is printed as a single unit, including an extruded helical heat exchanger into the condenser body and integrated pillar arrays in the evaporation section. The evaporation, adiabatic, and condensation section lengths are 20 mm, 30 mm, and 50 mm, respectively. The micropillars size is 2.5 mm. We considered the effects of the filling ratio of acetone as a working fluid and heat flux on the thermal performance of the thermosyphon. The filling ratios used are 0%, 1.7%, 4.9%, 6.3%, 10.4%, 17.1%, 27.9%, 40.5%, 50.4%, 64.2%, 76.6%, and 84.1%. We found that the minimum total thermal resistance is 0.48 °C⋅W−1, which is achieved at the minimal value of the filling ratio of 1.7% and thermal power of 13.7 W. The filling ratio of 4.9% shows the best stable thermal performance of the thermosyphon.

Heat transfer characteristics of closed-end thermosyphon (CE-TPCT)

This study investigated the heat transfer characteristics of a Closed-End Thermosyphon (CE-TPCT) made of copper tubes with 5.5 mm diameter. The experiments were conducted with working substance filling rates of 0%, 30%, 50%, and 70% of the volumetric evaporator section, the evaporator temperatures of 50, 70, and 90 °C and inclination angle of 90°. The lengths of the evaporator section, adiabatic section and condenser section used in the experiments were 60 mm, the distance between the compartments was 5Di. Water was the working substance used in the experiments with the mass flow rate of the water inlet at the condenser section of 0.25 L/min. The water inlet temperature at the condenser was 20 °C. Obtained results were compared with convectional Thermosyphon (TPCT) which was tested under the same conditions. The results showed that CE-TPCT at the filling rate of 70% at the evaporator section volume had an average heat transfer rate and average heat flux of 140.72 ± 4.92 W and 33.41 ± 1.16 kW/m2, respectively, throughout the experiment. These results were higher than the other tested parameters. The highest heat transfer rate of 220.32 ± 7.71 W and average heat flux of 52.31 ± 1.83 kW/m2 were recorded at the evaporator temperatures of 90 °C. The experiment also showed a lower average total thermal resistance throughout when it was compared with other test parameters which was equal to 0.547 °C/W. This resulted in high heat transfer coefficients and relative thermal efficiency of 625.11 W/m2·°C and 1 respectively, the highest value of all studied experimental variables. When the maximum heat flux results between CE-TPCT and TPCT under the same condition parameters were compared, CE-TPCT demonstrated a higher heat flux value of 18.24%. The heat transfer characteristics of the CE-TPCT obtained from this study provides an alternative application choice for various types of heat exchanger that can reduce energy consumption in the future.

Performance Tests on a Novel Un-Finned Thermosyphon Heat Exchanger Requiring a Single Charge

A novel design of an unfinned thermosyphon HPHX having a continuous closed tube loop which requires only a single charge is proposed for industrial waste heat recovery. The HPHX consists of 9×17 straight copper tubes in a staggered arrangement connected by 144 U bends. Without fins, not only are the pressure drops of the cooling air flow limited, but the cost, weight and maintenance effort can be greatly reduced. The thermal performance of this novel thermosyphon HPHX was tested with water at a filling ratio of 40%. The evaporator section is immersed in hot silicone oil, while the condenser section is cooled by air flow. The heat transfer rate (Q) reaches 6.65 kW at a heating pool temperature of 150 °C and a cooling air flow rate (F) of 1600 CMH, when the HPHX attains maximum effective thermal conductivity of 12,798 W/m-K. An ε-NTU theoretical model for single-tube thermosyphons was formulated with the boiling and film condensation modelled by empirical correlations. This model predicts the total resistance Rtot of the HPHX, which decreases with Q and F, with a total error of less than ±10%.

Experimental verification for the phases separation technique to improve the thermal performance of vertical and inclined wickless heat pipe

A flow separation technique is tested experimentally for the thermal performance enhancement of the Thermosyphon Heat Pipe (THP). Flow separation for the vapor rising upward and the condensate flowing downward is used to eliminate the complex interactions between the two phases. Such interactions are found and discussed by previous numerical work published by the authors. Tubes Packing (TP) made of two parallel copper tubes welded contrary to each other with a neighboring openings at central disk are used to grantee the smooth flow through the THP. The THP is made with a suitable dimension for the evaporator, adiabatic and condensation sections. Different operating conditions of filling ratios, supplied heat and inclination angles are tested to monitor the THP thermal performance during transient and steady state operation. The experimental tests results showed that the insertion of TP into the THP enhanced the thermal performance of THP by reducing the transient operation time about 37% and the maximum averaged temperature in the evaporator section about 50%. Also, reduces the variation effects of the filling ratio and the inclination angle on the thermal performance. In general, the results showed that the insertion of the TP reduces the THP thermal resistance about 17% to 62% based on the operating conditions.

Utilization of U-shaped finned heat pipe heat exchanger in energy-efficient HVAC systems

Energy-efficient Heating Ventilating and Air Conditioning (HVAC) system utilization is necessary to reduce energy consumption in buildings amid the supply difficulties of energy sources and the environmental impacts of high energy use. This study aims to investigate the utilization of a finned U-shaped heat pipe heat exchanger (HPHE) in a vertical configuration to reduce cooling and reheating energy in an HVAC system. The effectiveness of the U-shaped HPHE, coefficient of performance (COP) of the HVAC system coupled with U-shaped HPHE, and dehumidification capability are presented in this paper. The U-shaped HPHE was installed at the inlet fresh airstream ducts wrapped around the cooling coil, and tests were carried out with one-and two-row configurations arranged in eight heat pipes per row. The fresh air temperature and velocity across the evaporator section were varied at the ranges from 30 to 45 °C and 1.5 to 2.5 m/s, respectively. The results show that the two-row U-shaped HPHE significantly affects the precooling and reheating processes in the HVAC system compared with the one-row configuration. The two-row U-shaped HPHE can enhance the coefficient of performance (COP) by 39.9% over the HVAC without HPHE. The highest energy saving for precooling and reheating was 288.1 and 340.2 W, respectively, at 0.080 m3/s air volume and can reduce relative humidity by 21.6%. The investigation implies that the U-shaped heat pipe succeeded in becoming a solution to an HVAC system that requires cooling to reheat the HVAC system.

Heat transfer rate characteristics of two-phase closed thermosyphon heat exchanger

Two-phase closed thermosyphon heat exchangers (TPCTs) have considerable potential for use in ground source heat pump systems. This paper proposes a new metal-polyethylene TPCT that uses water as the working fluid and is composed of a galvanized steel pipe and a polyethylene pipe. The heat transfer rate characteristics of the metal-polyethylene TPCT were studied using a constant-temperature water bath test bench. The experimental results show that the metal-polyethylene TPCT has the best heat transfer rate when the vacuum degree and filling ratio are 1.00 kPa and 12.5%, respectively. The average heat transfer rate increased as the temperature of the heat source and the flow rate of the cooling water increased. Finally, a sandbox test bench was built, which provided seepage and non-seepage experimental conditions for the metal-polyethylene TPCT. In the sandbox experiment, the heat transfer rate capacity of the metal-polyethylene TPCT was initially studied and compared with that of a single U-tube heat exchanger, and the heat transfer rates were similar. When the evaporation section length was 0.5 m, the heat transfer rates were 34.89 W/m and 111.75 W/m under non-seepage and seepage conditions, respectively. The experimental results show that the metal-polyethylene TPCT is feasible as a new type of ground heat exchanger.

Code development and analysis on the operation of liquid metal high temperature heat pipes under full condition

Heat pipe cooled reactor (HPR) is an innovative solid-core reactor which utilizes heat pipes to extract heat from the core to the energy conversion system. In this paper, a mathematical model for high-temperature heat pipes from a frozen state to a steady state is developed based on experimental data. The model is simplified and solved numerically. The two-dimensional transient heat conduction model of the wall and wick and the one-dimensional quasi-steady compressible vapor flow model are coupled to simulate the operation of a heat pipe when vaporization and condensation occur at the liquid–vapor interface. And the equivalent heat capacity model and the rarefied vapor model are introduced to simulate the process of heat pipe startup from a frozen state. Numerical results indicate that the model of heat pipe can predict the operation of high-temperature heat pipe with an error of less than 1.04% at steady state and 23.4% during the startup. Based on the simulation results, the velocity of vapor decreases with the heat transfer power in the steady state and the velocity of potassium vapor is 58.4% of sodium vapor velocity under the same operation condition. During the startup, when the rarefied-continuum interface located in the end of the evaporator section, the maximum vapor velocity occurs which is 1.3 times of steady velocity. This work makes it possible to predict the operation of a high-temperature heat pipe and provides a reference for the design and application of HPR.

Evaporation characteristics and efficient working area of multi-stage high pressure and temperature reducing valve

A well understanding on the evaporation characteristics and efficient working area of multi-stage high pressure and temperature reducing valve (MSHPTRV) is important for improving the performance and safety of MSHPTRV. The water spraying and evaporation model are integrated into the flow model of MSHPTRV. Compared with the experimental data, the model can show the thermal process well. The flow characteristics and interaction between steam and droplets are presented. On this basis, the increase rate of entropy, Sdis, is adopted to analyze the thermodynamic loss and innovatively determine the efficient working area of MSHPTRV. The results show that the pressure reducing effect of the second orifice plate is prominent, which accounts for 41.6% of the total pressure drop. The "steam-water layer" is formed at the boundary of steam and water. At the inlet of second orifice plate, the maximum Sdis is 0.782, and the downstream of second orifice plate is the efficient working area of MSHPTRV. The length of evaporation section increases with the droplets diameter significantly.

Effect of Internal Diameter and Working Fluids on Thermal Performance of a Vertical Closed-Loop Oscillating Heat Pipe with Double Heat Sources

This research investigates the effects of internal diameter and working fluids on the thermal performance of a vertical closed-loop oscillating heat pipe with double heat sources (VCLOHP w/DHS). The closed-loop oscillating heat pipe (CLOHP) tested was made of a copper capillary tube with various inner diameters and working fluids. Two evaporator sections in the outer end of CLOHP were heated by a Ni-Cr alloy resistance wire heater. The heat was removed from the condenser section in the middle of CLOHP by forced convection heat transfer of ambient air blowing the section. The results showed that, for the inner diameters of 1.5 and 2.0 mm, the thermal resistance decreased when the inner diameter and the latent heat of evaporation increased.

A novel dual-diameter closed-loop pulsating heat pipe for a flat plate solar collector

The main idea of this investigation is to increase the thermal efficiency of a flat plate closed-loop pulsating heat pipe solar collector by introducing a novel closed-loop pulsating heat pipe (CLPHP) with a dual-diameter structure. The CLPHP with dual-diameter configuration is made of copper with two diameters; larger in the condensation section (inner diameter of 3.5 mm) and smaller in the evaporation and adiabatic sections (inner diameter of 2 mm). The thermal performance of this type of CLPHP was investigated at different filling ratios and inclination angles. Results showed that the thermal performance of the dual-diameter configuration is considerably better than the single-diameter in the vertical and non-vertical orientations. Moreover, in this study, the performance of a flat plate solar collector coupled with the proposed dual-diameter CLPHP (DD-CLPHPSC) is investigated experimentally in outdoor and indoor modes during July and August of 2020, under local weather conditions of Mashhad, Iran. The effects of the filling ratio, inclination angle, and solar intensity were assessed. Under sunny weather conditions, when the direct normal irradiance was 1030 W/m2, the thermal efficiency of the DD-CLPHPSC with a filling ratio of 60% reached 72.4%.

Correlation for Predicting Heat Transfer Characteristics of A Helical Oscillating Heat Pipe (HOHP) at Normal Operating Conditions

The helical oscillating heat pipe (HOHP) is a high heat transfer heat exchanger with high flexibility in its installation and can therefore be used in a wide variety of applications. In this study, the effect of various parameters on the heat transfer rates of HOHP were used to establish a correlation equation for use in the heat flux prediction, the dimensionless parameters studied were rv/rl, Bo, Nu, We, Ja, Pr, Fr, Co, Ga, Bi, Wo, Oh, and Ku. Experiments were conducted to find out their effects on the heat transfer rates of copper HOHP with internal diameters were 2.03, 3.5, and 4.5 mm. The lengths of evaporator and condenser sections were equal at 1500, 2000, and 2500 mm. The pitch coils were 10, 15, and 20 mm. The working fluids used were R134a, ethanol, and water with the filling ratios were 30%, 50%, and 80% of the total internal volume. The temperature of evaporator section were varied between 60, 70, and 80°C within normal operating conditions in a vertical position. The results of the experiment showed that the internal diameter, lengths of evaporator/condenser sections, pitch coil, type of working fluid, filling ratio and temperature of evaporator section affected the heat transfer rates of the HOHP. The results of dimensionless parameters can establish the correlation equation to predict the heat flux for the HOHP as shown in this paper. In addition, the results of this research can be applied in the designing and construction of HOHP heat exchangers.

Experimental investigation of Al2O3-water nanofluid as a secondary fluid in a refrigeration system

In the present study, a new technique of utilizing nanofluid as a secondary fluid in a secondary loop of refrigeration system is examined. This secondary loop serves for providing cooling in specific applications such as walk-in coolers and freezers. The secondary loop is obtained across the evaporator section wherein the refrigerant R134a extracted heat from the nanofluid. The evaporator here is classified as shell and coil heat exchanger where nanofluid flowed in the shell side and refrigerant flowed in the coil. A compressor, expansion valve and water cooled condenser are the major integral parts of the system. Experiments were conducted for various volume concentrations of Al2O3 nanofluid (0-15%), mass flow rates (40-80 g/s) and nanofluid inlet temperatures (30-40 °C). The modified system exhibited superior performance when Al2O3 nanofluid was employed in the secondary loop as compared to base fluid (distilled water) while operating at the same mass flow rate and inlet temperature. A maximum COP of 6.5 was achieved for nanofluid inlet temperature 40 C at mass flow rate 80 g/s and volume concentration 15%. The enhancement in refrigeration effect and coefficient of performance is attributed to micro conduction effect due to the rise in thermal conductivity of nanofluid as compared to distilled water.

Experimental Study on the Enhanced Thermal Performance of Two-Phase Closed Thermosyphon Using Mechanical and Chemical Treated MWCNTs Nanofluids

Nowadays the two-phase closed thermosyphon (TPCT) has been widely used as an important heat transfer medium for passive cooling systems. Since the enhancement of the performance of electronic device releases a considerable amount of heat, the development of working fluids which can improve thermal performance of the TPCT is necessary. In this experiment, two different type of MWCNT nanofluids were successfully made by using ball-mill process and alkaline treatment. The morphological and structural analysis were conducted by using transmission electron microscope (TEM), fourier-transform infrared spectroscopy (FT-IR). Also thermal conductivity of modified MWCNT nanofluids were investigated by introducing transient hot wire method. The results of TEM, FT-IR and thermal conductivity showed that the MWCNTs were successfully functionalized as well as had higher thermal conductivity. The TPCT was fabricated with copper tube and total length of the TPCT is 700mm, in which the evaporator section, the adiabatic section and the condenser section are 150mm, 400mm, and 150mm, respectively. The experiment of the thermal performance of the TPCT was evaluated by introducing RMS value of the temperature of the evaporation section, thermal resistance and heat transfer coefficient at five different filling ratios, 10%, 30%, 50% and 70% by ratcheting up heat loads in five levels, 5W, 8W, 10W, 13W and 15W. The overall results showed that the both of modified MWCNT nanofluids can decrease thermal resistance and increase the heat transfer coefficient, especially at 10% filling ratio with 8W. M-CNT nanofluid can even enhance the thermal performance of TPCT at 30% filling ratio with all heat loads.

Study of diffusion bonded flat plate closed loop pulsating heat pipes with alternating porous media

This work aims to discuss the application of diffusion bonding process in the fabrication of flat plate pulsating heat pipes and to analyze the thermal performance effect of porous media structures applied to alternating channels in the evaporator section and compare it with a smooth conventional flat plate pulsating heat pipe. For that, two different copper flat plate pulsating heat pipes with five turns and 1.5 mm of inner diameter channels were fabricated, using diffusion bonding. Samples were made with two different diffusion process to evaluate the ability of the resulting device to keep the vacuum, avoiding the use of filler material during the welding process, which could result in contamination and, therefore, undesirable corrosion on both internal and external heat pipe surfaces. The results allowed to assess the quality of the bonded interface of two different configurations of the diffusion process. Microscopic results obtained on controllable pressure furnace and non-pressure control were analyzed, where the diffusion process with controlled pressure presented best results of continuity, alignment, and tightness. A comparison of the heat transfer characteristics between the two devices was performed under the same experimental conditions; for both devices, the thermal resistance in horizontal position showed to be like heat transfer of pure conductive copper plate. However, pulsating heat pipe containing smooth channels showed the lower thermal resistance in vertical position and filling ratio of 50%.

Performance of a small diameter two-phase closed thermosyphon in geyser boiling condition

In the present article, the geyser boiling phenomenon is experimentally investigated in a small diameter two-phase closed thermosyphon. The studied thermosyphon has an inner diameter of 4.8 mm and total length of 918 mm. Three specific filling ratios and four temperatures for the thermal bath used to provide cooling water to the condenser were tested. Power was inputted in steps of 20 W, up to 100 W. Variation of the temperatures on the walls of the evaporator, adiabatic and condenser sections as well as variations on pressure, obtained from a pressure transducer installed on the thermosyphon, indicated the presence of Geyser boiling. The results show that the studied variables affect directly the intensity and periodicity of the geyser boiling phenomenon.

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.

Operational Limitations of Heat Pipes with MgO-Water Nanofluid: An Experimental Investigation

This paper presents the study of functional limits of heat pipes with MgO deionized (DI) water nano fluid as working fluid. The nano fluid is made with the help of two step method and it is surfactant free. The output has been analyzed with the increase in concentrations of MgO / DI water nano fluid (0.005%, 0.025% and 0.05 %) and at power input of (10 W, 30 W & 60 W), for covering operational range temperature of the electronic instruments. (<600C). On the basis of the thermo physical properties of the MgO/DI water nano fluid, the functional limits of heat pipe are determined. The changed of working fluids shows an improvement in boiling, pick up(entrainment), viscous and sonic limit except the capillary limit of heat pipe. The surfactant free nano fluids give good safety margin to the boiling limit of the heat pipe, with elimination of chances of locking due to vapor. The capillary limit reduces by the application of nano fluids due to increase in the vapor and liquid flow path resistance rather than capillary pressure in the evaporator section of heatpipe.

The influence of integrated Micro-ZnO and Nano-CuO particles/paraffin wax as a thermal booster on the performance of heat pipe evacuated solar tube collector

The research aims to investigate the possibility of enhancing the functionality of a heat pipe evacuated tube collector (HPETC) with a new method of, one or both an evacuated tube (ET) and two separate storage tanks, incorporating nano or micro-enhanced phase change materials as a thermal energy storage system (TESS). In the proposed systems, the (ET) was filled with enhanced medical paraffin wax, while the two separated tanks were filled with enhanced paraffin wax (grade-A). These two types of paraffin wax were enhanced with 5wt% CuO nanoparticle (NP-CuO) or ZnO microparticles (MP-ZnO). In this study, Six tests were carried out to do the investigation. Three tests were done to investigate the effect of the integration of enhanced-PCM in the only (ET) of HPETC. While the other three tests performed to investigate the effect of enhanced-PCM integrated into both (ET) of HPETC and two additional storage tanks. The main advantage of this new system is that it improved solar water heater efficiency by minimizing hot water temperature fluctuations. Thereby, the efficiently thermal isolation of the (ET) and storage tanks and the efficient energy accumulation in PCM which enables the system to supply hot water even when there is no solar radiation at night or in overcast weather. The results showed that the integration of enhanced-paraffin wax with NP-CuO and MP-ZnO in both the (ET) and the separated tanks gives an efficiency improvement range of (36.8-50%) and (25.43-41.4%) respectively, While the integration of enhanced-paraffin wax with NP-CuO and MP-ZnO in the only (ET) led to an increase in the efficiency with a range of (33.8-45.7%) and (23.8-26.7%) respectively, compared with a typical reference collector without PCM. Also, the NP-CuO enhanced paraffin wax integrated with the ET gave a better improvement in the evaporation & condensation coefficients in the evaporator and condenser sections where their values were (401 and 2366.4 W/m2·°C) respectively at a flow rate of 1L/h.

Filling Ratio Optimization for High-Performance Nanoengineered Copper-Water Heat Pipes

Abstract This experimental test investigates the effect of filling ratio (FR) and inclination angle on the thermal performance of a nanoengineered copper-water heat pipe. A hydrophilic copper oxide coating (CuO) is synthesized and integrated on the inner wall of the evaporation section of the heat pipe. The heat pipe is fabricated from an inner grooved copper pipe with dimensions of 12.7 mm outer diameter, 11 mm inner diameter, and 440 mm length. Ultra-filtered de-ionized (DI) water is used as a working fluid. Four different FRs of DI water 3%, 5%, 10%, and 15% are investigated to determine the optimum configuration. All samples are tested at various inclination angles and working loads. Experimental results show that the optimum filling ratio is the 5% FR, which was indicated by the lowest thermal resistance of 0.019 K/W.