Evaporator Section Research Articles

Experimental study on characteristics of gravity heat pipe with threaded evaporator

This work is for the first time an experimental study of thermal characteristics of an copper gravity heat pipe (thermosyrhon) with an outer diameter of 12 mm and a length of 230 mm with a threaded evaporator and freon R141b as working fluid under natural convection. The study demonstrates how a change in power input in the range from 5 W to 70 W (with a 5 W step) at natural convection affects such thermal characteristics of the gravity heat pipe (GPH) as temperature in evaporator section, temperature distribution along the length of the GPH, total thermal resistance, thermal resistance in evaporation and condensation sections, and heat transfer coefficients in evaporation and condensation sections. Also studied is the influence of tilt angle of the GPH changing from 0 to 90° (with a 15° step) on its thermal characteristics. The coefficient of convective heat exchange between the heat sink and air at different values of heat flux and tilt angle is determined experimentally. The obtained research results indicate that the GPH with a threaded evaporator can be effectively used in natural air cooling systems for electronic equipment with high heat flux values, for example, LED devices, power amplifiers of T/R modules, etc.

Heat Transfer Enhancement in Gravity Heat Pipes Using AAO Nanostructure Generated on Condenser Section Inner Surface

This study mainly focuses on the influence of anodic aluminum oxide (AAO) nanostructure generated on condenser section inner surface on the heat transfer performance of gravity heat pipes. AAO nanotubes were first grown by anodizing the inner wall surface of the condenser section of aluminum alloy gravity heat pipes through different anodizing voltages and treatment times. The nanostructure effect on the temperature distribution and overall thermal resistance was then investigated by using a thermal performance test system under different input heat powers. The experimental results showed that the generation of AAO nanostructure on the inner surface significantly enhances heat transfer performance; that is, the temperature difference between the evaporator and condenser sections and overall thermal resistance are reduced. Such an effect can be more significant in the case of a lower heat source. The percentage decreases in temperature difference and overall thermal resistance can be reduced by up to 58.83% and 58.79%, respectively, compared to the unprocessed heat pipe.

Experimental parametric passive solar desalination prototype analysis

Passive solar desalination is a simple system that is convenient for use in arid areas. In this paper, an experimental investigation with a theoretical parametric analysis was performed for a passive solar desalination system composed of an evacuated tube collector integrated with a steel condenser. The impacts of the porosity of the stainless-steel wool in the evaporation section and the radiation shading for the condenser were examined. It was found that the less porosity of the stainless-steel wool achieves an overall efficiency enhancement of 5% compared to the maximum porosity. The condenser's shading achieved an overall efficiency enhancement of 12.4%, compared to the unshaded condenser's case. The theoretical analysis showed that the condensate increases with the increase of both ambient temperature and global solar irradiance, while decreases with the increase of wind speed. The theoretical model was validated with the experimental shading case with maximum error of 8.6% in the overall efficiency. The cost of the produced desalinated water reached 0.01287 $/kg/m2 in the proposed system.

An Experimental Study on Micro-Enhanced TESM Incorporated Inside Evacuated Tube Solar Collector Equipped with Heat Pipe

The incorporation of thermal energy storage materials (TESMs) into solar energy systems is a factor that boosts the performance of these systems. In this paper, an experimental study was addressed for enhancing the heat pipe’s thermal performance that works with an Evacuated Solar Tube Collector with Heat Pipe (ETCHP) as a solar water heater system. This is done by adding micro-zinc oxide (ZnO-MP) to the paraffin wax integrated as TESM into the evacuated tube (ET) of the system, where the evaporator section of the heat pipe is completely submerged within the micro-enhanced paraffin wax. Three experimental prototype rigs with one evacuated tube were designed, built, and tested to do the investigation. The most important parameters that have been studied in this study are the thermal resistance and the temperature distribution pattern along the heat pipe. The results show a clear indication of the decrease in the thermal resistance of the heat pipe of the proposed system compared to the system in which pure paraffin wax was incorporated. Also, it was noticed that there is a significant improvement in the temperature distribution along the heat pipe due to the improvement in the conductivity of the micro-enhanced wax compared to the pure wax.

Experimental Study on the Starting-Up and Heat Transfer Characteristics of a Pulsating Heat Pipe under Local Low-Frequency Vibrations

Vibrations have attracted much attention as an effective method for enhancing heat transfer in pulsating heat pipes (PHPs). This study mainly investigates and explores the effects of local low-frequency vibrations on the starting-up and heat transfer characteristics of a PHP. The starting-up temperature and average temperatures along the evaporation section of the pulsating heat pipe were experimentally scrutinized, along with thermal performance, under local vibrations on evaporation, condensation and adiabatic sections, respectively. The following important conclusions can be derived by the experimental study: (1) The effect of vibrations at the evaporation section and at the adiabatic section during the starting-up time of the PHP were more significant than that at the condensation section; (2) vibrations at different positions could reduce the starting-up temperature of the PHP—the effect of the vibrations at the evaporation section was the best when heat power was lower, while the effect of vibrations on the adiabatic section was the best when heat power was higher; (3) vibrations at the evaporation and adiabatic sections could reduce the thermal resistance of the PHP, but vibrations at the condensation section had little effect on the thermal resistance of the PHP; (4) vibrations at the evaporation and adiabatic sections could effectively reduce the temperature at the evaporation section of the PHP, but the vibrations at the condensation section had no effect on the temperature at the evaporation section of the PHP. This paper shows that local low-frequency vibrations have positive effects on the heat transfer performances of PHPs.

The effect of boiling in a thermosyphon on surface tension and contact angle of silica and graphene oxide nanofluids

Thermosyphons are heat transfer devices characterised by high efficiency due to simultaneous phase changes occurring in the evaporation-condensation cycle of working fluid. One of the most promising solutions to enhance their heat transfer capacity of the device is the use of nanofluids – suspensions of particles with at least one dimension below 100 nm. It was determined that nanofluid does not influence the work of thermosyphons condenser section and the focus should be put on the boiling process in the evaporator section. During boiling, nanoparticles tend to deposit on the heater’s surface, what alters characteristics of this surface and near surface hydrodynamics. This changes the appearance of nanofluid, but the precise effect on how the deposition of particles affects the properties of nanofluid is unknown. Changes in surface tension and wettability affect boiling regimes (e.g. reduced surface tension reduces the size of departing bubbles and inhibits geysering), and efficiency of heat transfer through the device. Understanding of those parameters is crucial for the development of appropriate models describing heat transfer in thermosyphon working with nanofluids. The main goal of this study is to determine surface tension and contact angle of nanofluids based on silica nanoparticles and nano-sized graphene oxide flakes before and after the experimental boiling cycle in the thermosyphon. Results show that, in comparison with water, silica nanofluid (2 vol%) is characterised by lower surface tension and contact angles on both analysed surfaces. After-use silica nanofluid exhibited noticeably higher averaged surface tension and smaller contact angles in comparison to the fresh working medium. The change was most likely due to the decreased concentration caused by the deposition of nanoparticles during the thermosyphon operation. Still, the differences between before-use and after-use samples were smaller than the measurement uncertainties. Before-use graphene oxide nanofluid already showed surface tension and contact angle similar to water due to low concentration of graphene flakes (0.1 g/L). Consequently, the properties of after-use graphene oxide fluid were also not much different from water. Additional measurements of surface tension for graphene oxide nanofluid with and without addition of sodium dodecyl sulphate surfactant allowed to differentiate the effects caused by graphene flakes and surfactant. The surfactant reduced the surface tension of the nanofluid, but the change was smaller than in case of surfactant addition to pure water.

Thermal performance and flow characteristics in additive manufactured polycarbonate pulsating heat pipes with Novec 7000

• A 3D-printed polycarbonate-based transparent PHP was successfully fabricated. • An aluminum plate bonded to the 3D-printed channel allows for better heat conduction. • The 3D-printed transparent PHP enables flexible channel design and flow visualization. • The maximum effective thermal conductivity of 7.0 × 10 3 W/(m⋅K) was obtained. To improve the performance of electronic devices, incorporating an effective cooling system in their thermal design is important. A pulsating heat pipe (PHP) is a passive cooling device that efficiently transports heat as sensible heat and latent heat from an evaporator section to condenser section via an oscillatory two-phase flow. In recent years, with the progress of additive manufacturing technology the design of PHPs with increased flexibility and wider industrial applicability has been expected. To optimize a plastic PHP flow channel structure for practical uses, the effects of channel geometry and flow regime on appropriate operating conditions need to be clarified at higher input power. The present study focuses on the flow visualization of a plastic-based PHP having a practical channel size and configuration. Using additive manufacturing, a serpentine flow channel of the PHP was fabricated with transparent polycarbonate. The PHP was filled with Novec 7000 as a working fluid, and the effects of the flow channel size and fill ratio of the working fluid on PHP’s heat transport performance were investigated. Based on the flow visualization images, we evaluated the flow pattern and two-phase flow parameters such as vapor-slug lengths, velocity, and passing frequency, and clarified the relationship between the oscillating flow and thermal performance during the PHP operation.

The Thermal—Flow Processes and Flow Pattern in a Pulsating Heat Pipe—Numerical Modelling and Experimental Validation

The aim of the article is to numerically model a two-dimensional multiphase flow based on the volume of fluid method (VOF) in a pulsating heat pipe (PHP). The current state of knowledge regarding the modeling of these devices was studied and summarised. The proposed model is developed within open source software, OpenFOAM, based on the predefined solver called interPhaseChangeFoam. The analyses were carried out in terms of the influence of four different mass transfer models between the phases, proposed by Tanasawa, Lee, Kafeel and Turan, and Xu et al. on the shape and dynamics of the internal flow structures. The numerical models were validated against data obtained from a specially designed experimental setup, consisting of three bends of pulsating heat pipes. The numerical calculations were carried out with ethanol being treated as a working medium and the initial and boundary conditions taken directly from the measurement procedures. The variable input parameter for the model was the heat flux implemented in the evaporation section and a fixed temperature applied to the condensation section. The flow structures obtained from the numerical analyses were compared and discussed with the flow structures gained from experimental studies by employing a high speed camera. In addition, to verify the quantitative results obtained from the numerical analyses with the experimental data, a technique called particle image velocimetry (PIV) was used for the velocity vector field. For the analysed velocity ranges, the relative error obtained was reached at the level of 10%.

Investigation of the flow and heat transfer performance for a water-cooling grate in a biomass boiler

ABSTRACT Biomass direct combustion power generation technology can effectively take the advantage of the biomass resources. Water-cooled reciprocating grate has become the research hotpot of biomass boilers for its advantages such as excellent performance and controllable cost. This paper mainly studies the flow and heat transfer performance of the water-cooled reciprocating grate, according to the different combustion state, the boiler can be divided into four sections: the moisture evaporation section, the volatile release section, the char combustion section and the cooling section. A porous model is used to simulate the biomass combustion process in different sections. Results indicate that the porous model can show more details about the temperature distribution of the grate, which is closer to the real situation. Compared to the traditional model, the relative deviations of main parameters against the heat transfer performance are less than 5%. With the increase of mass flow rate, the descend amplitude of the temperature decreases, but the ascend amplitude increases sharply, which would increase the operation cost. The least mass flow rate needed to keep the temperature rise between the inlet and outlet less than 5°C for the moisture evaporation section, the volatile release section, the char combustion section and cooling section are 0.46 kg·s−1, 0.55 kg·s−1, 4.60 kg·s−1, and 0.23 kg·s−1, respectively. For the moisture evaporation section, the volatile release section and the cooling section, the temperature of the grate and cooling water is low on the middle but high on the tail, while it is opposite for the char combustion section, due to the variation of combustion state and biomass thickness in different sections. Besides, it is found that the longitudinal vortices could pose great influence on the uniform distribution of cooling water. To optimize the cooling effect, it is important to reorganize the water channel and reduce the longitudinal vortices.

Exergy Analysis of a Plate Type Freshwater Generator Used in Marine Vessels

The freshwater generation from seawater in marine vessels is outlined. Freshwater is a requirement for human consumption as well as for the operation of onboard machinery. As per the World Health Organization (WHO) guidelines, dissolved solids in the drinking water should be less than 500 mg/litre. Technical water (used for the operation of onboard machinery) will contain less than 2.5 mg/litre of boiler feed water. The submerged plate-type evaporator is mainly used on merchant's vessels in order to produce freshwater. An exergy analysis for utilizing waste heat as a source for a single effect desalination process to meet the freshwater needs is proposed with due consideration to practical aspects. A plate-type freshwater generator installed at the Training Ship of Tolani Maritime Institute, Pune, India, is analyzed. Thermal properties are calculated using a "C" program. The practical approach to exergy analysis of the freshwater generating process is to calculate the total exergy loss, which is then compared with the partial exergy losses in the system in order to establish the impact of the specific process on the overall system efficiency. This exergy study calculates the proportion and the distribution of unavailable work in the evaporator, condenser, and brine section of a plate-type freshwater generator. The exergy efficiency in freshwater generators with plate-type freshwater generators is 64%. Since the exergy losses in the freshwater generator are disclosed, the task of effectively directing efforts in performance and energy conservations is simplified.

Mathematical model of bent-flattened sintered-grooved heat pipe with concept of receding-and-excessing fluid

A mathematical model that uses the concept of evaporation and condensation to explain the process of heat and mass transfer within the bent, flattened heat pipe has been established to predict its wall temperature and thermal resistance. When the heat pipe was bent and flattened in the application, the wick was damaged, resulting in less liquid being returned. It caused the wick to become unsaturated at the evaporator section. It can be determined that the liquid-vapor interface will gradually recede into the wick. This is defined as the “Receding” model (R). In the condenser section, the remaining condensate was collected as excess fluid. This is referred to as the “Excess Fluid” model (EF). It was observed that the low thermal conductivity of the unsaturated wick decreased the effective thermal conductivity at the evaporator section. The effective thermal conductivity at the condenser section also decreased due to the low thermal conductivity of the liquid film. The low effective thermal conductivity resulted in higher evaporator temperatures and lower condenser temperatures, resulting in higher thermal resistance. The established “REF” model has been validated in the same conditions by the experimental results and provides accurate predictions for the heat transfer performance of heat pipes.

Heat Transfer Enhancement for On-Chip Cooling Application Using Novel Composite Heat Sink—Comparative Numerical Study

This study investigates the enhancement of thermal performance of a composite heat sink (CHS). The novelty is that instead of embedding a conventional heat pipe at the bottom of base plate of the heat sink, a series of microheat pipes (MHPs) are embedded above, in such a manner that it replaces the solid fin arrangement. With this novel idea, the comparison of thermal performance of novel CHS with embedded MHP and micro-oscillating heat pipe (MOHP) is carried computationally using ANSYS-FLUENT-2019R1. The parametric study is carried out by varying the heat input in evaporator section and the convective condition in condenser section in accordance with the single fin of heat sink, and additionally different fill ratios in both MHP and MOHP are also considered. The thermal resistance modeling is used to calculate the effective total thermal resistance of the novel CHS using the volume averaged properties. The average temperature variation in single MOHP is found to be more which results in better performance than single MHP. The lowest thermal resistance of single MOHP is 0.827 K/W and single MHP is 0.969 K/W for 3.2-W distributed heat input and 35-W/ m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·K convective condition. The total thermal resistance of novel CHS is more efficient than the conventional heat sink with solid fins by an average of 64.7% and 72.3% for MHP and MOHP, respectively, at considered maximum convective condition. Finally, the thermal resistance of novel CHS with MHP or MOHP is compared with the previously investigated CHS which seems to have enhanced heat transfer.

Heat Transfer Performance in a Vertical Grooved Thermosyphon

The performance of a grooved copper-water thermosyphon with a modest bend between a vertical evaporator section and inclined condenser section was characterized for a moderate fluid loading. The time-averaged temperature varied significantly along the evaporator for heat fluxes up to 10 W/cm 2 before becoming more uniform at higher heat fluxes. Transients of the measured temperatures show evidence of different flow regimes that were compared to existing flow pattern maps. The evaporator performance for both the pool region, which had the highest temperature, and the film region were not well predicted for heat fluxes up to 10 W/cm 2 . The local evaporator performance changed with the change in flow regime, suggesting the need for a flow regime-based performance model. The performance of the condenser appeared to depend on the reduced pressure and was significantly overpredicted by the standard condensation models. A modified model for the condenser was proposed.

Effect of particle diameter and heating position of evaporation section on thermal performance of a vapor–liquid–solid three-phase closed thermosyphon

A vapor-liquid-solid three-phase closed thermosyphon (THPCT) with variable heating position at evaporation section is built by combining the fluidized bed heat transfer technology with the two-phase closed thermosyphon (TPCT). The effect of particle diameter and heating position on the thermal performance is investigated by varying the heating power (100–300 W) and solid holdup (5–15%). Results show that the addition of the glass beads with different diameters can enhance the heat transfer under certain conditions. The maximum reduction rates of the overall thermal resistance are 25.4% and 27.7% for Le = 200 mm and Le = 200 mm − up40, respectively. The upward movement of the heating position is beneficial to reducing the overall thermal resistances of the TPCT and the THPCT with small particles, but is not conducive to the heat transfer of the THPCT with large particles. A medium particle diameter and a low solid holdup are suggested for the THPCT with different heating positions.

Study on adaptive heat transfer performance of high temperature heat pipe

High temperature heat pipe (HTHP) employs alkali metal or its alloy as working fluid. Due to the large latent heat of its working fluid during phase change heat transfer, HTHP has the advantages of high temperature uniformity, high working temperature and high intensity heat transfer capacity, which has broad application prospects in the field of HTHP cooled nuclear reactors and the thermal protection of hypersonic vehicles. In order to simplify the structure of heat transfer system in nuclear reactors, so that it can safely and reliably realize passive heat transfer without additional pump power under various operating conditions, HTHP must have adaptive heat transfer capacity. In this paper, several HTHPs with excellent heat transfer performance are designed and prepared. The test-bed for frozen startup and heat transfer performance of HTHP is set up, and the electromagnetic induction high frequency heater is used as high temperature heat source. The thermal performance of HTHP with different heating power, heating position and the length of evaporation section (adaptive heat transfer characteristics) is experimentally studied, and the theoretical model is used to validate the experimental result. The experimental and theoretical results show that high vacuum degree is the prerequisite for excellent heat transfer performance of HTHP; high capillary driving force and suction force, reasonable filling ratio of working fluid are the key factors for HTHP to have adaptive heat transfer capacity, and the diameter of HTHP also has important influence on adaptive heat transfer capacity.

An investigation of the performance of groundwater-based heat pipes in heating lawn systems

Owing to the depletion of global fossil energy and the increasing environmental awareness of environmental protection, the use of shallow geothermal energy has been receiving widespread attention. In this study, two sets of groundwater-based heat pipe (GBHP) systems were fabricated to explore the application of heat pipes in shallow geothermal energy and tested through lawn heating experiments during the winter. Results of the experiment showed that the vertical diameter of the heat pipe did not have any significant effect on its heat transfer performance, and the maximum difference in temperature between the horizontal section of the east and west heat pipes during the observation period was 1.6 °C. Furthermore, the heat pipe exhibited good isothermicity with a constant operation, with the average temperature difference between the condensation and evaporation sections were 2.1 °C. The heat pipe also exhibited good thermal conductivity, the thermal resistance accounting for only 1% of the total thermal resistance in the heat transfer process. The maximum heat transfer power of a single heat pipe was found to be 202 W. This study presents a new approach for the application of shallow geothermal energy.

Numerical Investigation on the Thermal Performance of Three Advanced Micro Heat Pipes

Three micro heat pipes belonging to different types are selected as the research objects due to their lowest thermal resistance and the steadiest operation according to published literatures. They are the pulsating heat pipe with corrugated configuration (CPHP), the flat-plate heat pipe with eight microchannels (FPMHP) and the radial pulsating heat pipe (RPHP). A mathematical model common to the three micro heat pipes is built, and the maximum deviation between simulation and actual results is 16.1%. It is found that CPHP has the best heat transfer performance with the minimum thermal resistance of 1.14 K·W−1. Its average wall temperature and center-line velocity regularly oscillate after the start-up point. There is a pause phenomenon of motion for CPHP, and the pause period of motion decreases from 8.93 s to 7.25 s with the increasing input heat flux. RPHP possesses the most excellent temperature uniformity with the maximum axial temperature difference of 5.24 K. Furthermore, FPMHP is suitable for cooling temperature-resistant devices because its average wall temperature of evaporation section is the highest.

Experimental Studies on Geometrically Changed Thermosiphons Using Al2O3 Nanofluid

Objectives: To understand the two-phase closed thermosiphons (TPCT) which is being used nowadays to reduce heat by using thermosiphon.Methods: The experiment is carried out with different inclination angles and by calculating the heat transfer parameters (mention them) 0°,45°,90° with the heat input of 50-250w and compare with previous results.Findings: The TPCT mainly has many advantages, having the capacity of transferring a high amount of heat in a small size. Nanofluid play an important role in the aluminum oxide (Al2O3) Nanofluid is used because of high thermal conductivity and low thermal resistance, Due to the presence of this Nanofluid, the heat transfer rate in the evaporator section gets decreased and at the time of applying the highest heat flux the thermal resistance tents to be low.Novelty: Aluminum oxide (Al2O3) Nanofluid was tested in the various setting angles 0°, 45°,90°. Among these 45° have shown better performance in all the properties such as low temperature and high heat transfer rate and high thermal conductivity. So the 45° have shown better performance compared to the other two setting angles.

Evaluation of internal structure modifications effect of two-phase closed thermosyphon on performance: An experimental study

In this paper, an experimental study has been carried to enhance the effectiveness of a two-phase closed thermosyphon (TPCT) type heat pipe. For this purpose, three different designs have been developed by modifying the internal structure so as not to interfere with the physical operation of TPCT. Three heat pipes with different internal structures were manufactured with additional pipes placed longitudinally inside the evaporator and condenser sections of the normal (conventional) TPCT. Ionized pure water is used as a working fluid at the same rate in Normal, Type-1, Type-2, and Type-3 heat pipes. TPCTs are positioned at three different inclination angles, 26°, 41°, and 56°, to see the performance effect not only among themselves but also on the changes made by the gravitational effect. For each tilt angle, four TPCTs were simultaneously powered by solar energy on three different days. As a result of the experiments carried out in July, it has been observed that the optimum inclination angle for solar water heated TPCT is 26°. In addition, it was found that with the increase of the angle of tilt, the thermal resistance (R) of the heat pipes also increased and the heat transferability of the TPCT decreased. Comparing the new designs and normal TPCT, Type-2 acted better at tilt angles of both 26° and 41oAs a result, the heat transfer and efficiency of normal TPCT have been improved by almost 14% with Type-2 at the tilt angle of 26°.

Experimental investigation of heat pipe performance under translational acceleration

Heat pipes provide highly efficient heat transfer and are powerful tools in thermal management systems. In contrast to most established systems, heat pipes operating within moving systems such as electric vehicles or machine tools are exposed to unsteady acceleration forces. In order to guarantee proper heat pipe function in scenarios involving accelerated movement, it is necessary to examine the effect of acceleration forces on the heat pipe behavior. This paper presents experimental investigations on the thermal performance of translationally moving heat pipes. The designed experimental set-up consists of a measurement platform mounted to a linear direct drive. By obtaining the temperature gradient between evaporator and condenser section, the thermal resistance of heat pipes is determined at standstill and under linear motion. The experiments were conducted using heat pipes with sintered, mesh and grooved wicks. The results demonstrate the motion influence on the thermal resistances depending on the heat pipe orientation and heat input.