Recent advances in micro/nanostructured heat pipes

This volume provides information on recent progress in spacecraft thermal control and the supporting disciplines of conduction, thermal radiation, and heat pipe theory and application. Four problem areas are considered: conduction heat transfer, radiation heat transfer, thermal control, and heat pipes. The topics covered include finite-element methodology for transient conduction/forced-convection thermal analysis; effects of surface finish on thermal contact resistance between different materials; mathematical models for wide-band nongray gas radiation in spherical and cylindrical geometries; thermal design, analysis and testing of the Shuttle remote manipulator arm; porous heat pipe; and transient behavior of liquid trap heat-pipe thermal diodes. Also discussed is the thermal design concept for a high-resolution UV spectrometer.

The rapid development of transportation electrification brings unprecedented opportunities and challenges as well to the technological improvement of high-power-density motors. As the major restriction of motor performance, thermal issues attract more attention recently and the heat pipe (HP), due to its high thermal conductivity and lightweight characteristics, is identified as a promising solution. Though HPs have been attempted in cooling systems of electrical machines, their thermal benefits still need to be fully understood. This article provides a comprehensive review and performs a series of experimental investigations to fill the technology gaps for the HP application in electrical machines. First, the HP technology is briefly introduced, followed by a detailed review on its applications in different locations of electrical machines with corresponding thermal improvements reported. Then, experimental tests are conducted to investigate the correlation between the HPs’ parameters and their cooling effectiveness, including cross-sectional shape, bending angle, fins arrangement, and inclination angle. The obtained results and conclusions could provide a guideline for the promotion of HPs in the next-generation electrical machines for future transportation.

The present paper describes the usage of heat pipes for phase transformation of steam that is steam condensation. The application of heat pipes were reviewed and it is found that heat pipes were never explored for steam condensation since its inception. Hence, a heat pipe had been designed for condensation purpose and the features of this designed heat pipe like, material of construction, length, and diameter were presented. The thermodynamic limits of the designed heat pipe were calculated theoretically. Practical results of experiments were also presented.

Advancements in energy-efficient building technologies: An enhanced lee model for precision simulation and optimization of heat pipe applications

The Compatibility of Mild Carbon Steel and Water in a Heat Pipe Application

This paper addresses the challenges the policymakers face concerning the EU decarbonization and total electrification roadmaps towards the Paris Agreement set forth to solve the global warming problem within the framework of a 100% renewable heating and cooling target. A new holistic model was developed based on the Rational Exergy Management Model (REMM). This model optimally solves the energy and exergy conflicts between the benefits of using widely available, low-temperature, low-exergy waste and renewable energy sources, like solar energy, and the inability of existing heating equipment, which requires higher exergy to cope with such low temperatures. In recognition of the challenges of retrofitting existing buildings in the EU stock, most of which are more than fifty years old, this study has developed a multi-pronged solution set. The first prong is the development of heating and cooling equipment with heat pipes that may be customized for supply temperatures as low as 35 °C in heating and as high as 17 °C in cooling, by which equipment oversizing is kept minimal, compared to standard equipment like conventional radiators or fan coils. It is shown that circulating pump capacity requirements are also minimized, leading to an overall reduction of CO2 emissions responsibility in terms of both direct, avoidable, and embodied terms. In this respect, a new heat pipe radiator prototype is presented, performance analyses are given, and the results are compared with a standard radiator. Comparative results show that such a new heat pipe radiator may be less than half of the weight of the conventional radiator, which needs to be oversized three times more to operate at 35 °C below the rated capacity. The application of heat pipes in renewable energy systems with the highest energy efficiency and exergy rationality establishes the second prong of the paper. A next-generation solar photo-voltaic-thermal (PVT) panel design is aimed to maximize the solar exergy utilization and minimize the exergy destruction taking place between the heating equipment. This solar panel design has an optimum power to heat ratio at low temperatures, perfectly fitting the heat pipe radiator demand. This design eliminates the onboard circulation pump, includes a phase-changing material (PCM) layer and thermoelectric generator (TEG) units for additional power generation, all sandwiched in a single panel. As a third prong, the paper introduces an optimum district sizing algorithm for minimum CO2 emissions responsibility for low-temperature heating systems by minimizing the exergy destructions. A solar prosumer house example is given addressing the three prongs with a heat pipe radiator system, next-generation solar PVT panels on the roof, and heat piped on-site thermal energy storage (TES). Results showed that total CO2 emissions responsibility is reduced by 96.8%. The results are discussed, aiming at recommendations, especially directed to policymakers, to satisfy the Paris Agreement.

Nanofluid is a new type of heat transfer medium formed by adding metal or non-metal in liquid medium in a certain proportion and manner, which has many advantages over the traditional working fluid. Combining heat pipes with nanofluids and using nanofluids as the working material of heat pipes can reduce thermal resistance and effectively improve the heat transfer performance of heat pipes. This paper provides a relevant overview of heat pipes and nanofluids, and introduces the relevant classifications of heat pipes, the working principle and the relevant research on nanofluid-enhanced boiling heat transfer. It conducts a literature review on the application of nanofluids in heat pipes, and finally proposes possible future research directions.

Coal spontaneous combustion is one of the major disasters in coal storage and coal transportation. Consequently, coal fire prevention and control technology is one of great significance to the safe operation of coal mines. Heat pipe is a passive heat transfer device by phase change method. Thermal energy can be transported from the underground to the surface continuously by the heat pipe effect, so that it is expected to diffuse concentrated heat and reduce the temperature of the coal pile, which is effective for preventing coal spontaneous combustion. The experiment removing thermal energy from the underground coal is carried out in order to identify the feasibility of the heat pipe heat extraction heat from the coal dump. The results out of the present experimental conditions were shown that the application of heat pipes are contributing to extinguishing coal fire as well as for energy recovery utilization purpose.

A comprehensive review of heat pipe: Its types, incorporation techniques, methods of analysis and applications

A heat pipe is an excellent heat transfer device, widely used in electronics cooling. Nanofluids have superior thermophysical properties. Researchers noticed that nanofluids are very effective working fluids as they enhanced the heat transfer characteristics of heat pipes (HPs). Mono nanofluids have their limitations regarding the applications in heat pipes. Therefore, the research community extends its work toward the application of hybrid nanofluids. In the present investigation, the authors used TiO2/H2O and (TiO2 + MWCNT)/H2O as working fluid in the HP for a wide range of power input (50–100 watts). The authors noticed the significant enhancement in the performance of HP using hybrid nanofluids. The mechanisms/causes of enhancement in the TPHP have been identified.

Muti-channel flat heat pipe is an innovative technology recently used at the rear of photovoltaic cells to absorb and reuse the wasted heat. To better understand the fundamentals of two-phase heat transfer (boiling and condensation) taking place inside multi-channel heat pipes, a unique three-leg heat pipe has been built. This one-of-a-kind heat pipe was used to develop both computational fluid dynamic (CFD) and theoretical models of a multi-channel heat pipe. To simulate the heat pipe operation with ANSYS Fluent, the Volume of Fluid (VOF) approach and Lee model were investigated. Different types of Lee models using user defined function (UDF) were compared and the influence of the condenser's boundary condition, saturation temperature, and mass transfer coefficient on the simulations was studied. For the first time, major limits of the Lee model for the simulation of heat pipes are identified. It is concluded that the available Lee model cannot predict the heat pipe temperature as it shows low physical meaning and can easily be manipulated to adjust the simulation's results. Based on the three-leg heat pipe experimental data, a new multi-channel theoretical model was developed that uses the thermal-electrical resistance analogy to predict the three-leg heat pipe thermal resistance. By selecting the optimum correlations for pool boiling and filmwise condensation, the developed iterative theoretical model was able to predict the three-leg heat pipe thermal resistance with an error of 8.2%.

The contribution is listed possible application of heat pipes in systems for obtaining heat from flue gas of small heat sources. It is also stated in the contribution design an experimental device on which to study the impact of fill (the quantity, type of load) at various temperature parameters (temperature heating and cooling) thermal power transferred to the heat pipe. Is listed measurement methodology using heat pipes designed experimental facility, measurement results and analysis of the results obtained.

In an earlier investigation of evaporative heat transfer in low-temperature heat pipes, it was concluded, that nucleate boiling was the dominating mechanism for heat removal from the wall. Experimental data for vaporization of water and freon in screen wicks was shown to agree well with a boiling correlation. Data used in the correlation were obtained from Abhat and Seban for a water/screen wick vaporization experiment and from Ponnappan and Mahefkey for a copper/water double-wall artery heat pipe. During verification testing of this heat pipe, heat input to the evaporator was provided by electrical resistance heating. In such cases the evaporator wall heat flux is the independent parameter and the temperature drop across the evaporator is measured as a function of applied heat flux. The temperature drop is given by T{sub w} {minus} T{sub s}, where T{sub w} is the wall temperature and the T{sub s} is saturated vapor temperature. The criterion used to signify that the maximum heat flux had been reached was an observed evaporator temperature drop larger than 10C. In low-temperature heat pipes, before the boiling limit is reached, dryout will usually occur due to the inability of the wick to pump sufficient liquid to the evaporator (capillarymore » limit). The capillary limit Q{sub max} is a limitation on total heat input rate Q for a given heat pipe. Temperatures in the heat pipe evaporator increase rapidly if the maximum heat input rate is maintained, and the rise in internal pressure may cause failure of the container material. In this paper, a unique apparatus is used to control the wall temperature of the heat pipe evaporator independently. With this method, an increase in T{sub w} above that corresponding to Q{sub max} can only cause a decrease in Q. Although it is more difficult to control the temperature than the heat flux, this method allows a more accurate determination of Q{sub max} and precludes the possibility of runaway temperature.« less

Heat transfer characteristics of nanofluids in heat pipes: A review

Heat transfer and fluid flow characteristics in a micro heat pipe with curved triangular grooves are investigated using numerical and experimental methods. In the numerical part, a one-dimensional mathematical model for micro heat pipe with curved triangular grooves is developed and solved to obtain the maximum heat transport rate, the capillary radius distribution, the liquid and the vapor pressure distributions along the axial direction of the micro heat pipe under the steady-state condition. In particular, the modified Shah method is applied to calculate the pressure drop induced by the liquid-vapor interfacial shear stress. Experiments are conducted to validate the numerical model. In the experiments, the micro heat pipe with 0.56 mm in hydraulic diameter and 50 mm in length is tested. The experimental results for the maximum heat transport rate agree well with those of the numerical investigations. Finally, thermal optimization of the micro heat pipe with curved triangular grooves is performed using the numerical model.