Micro-heat Pipe Research Articles

Electro-osmotic flow and heat transfer in Jeffery fluid: A multi-membrane microchannel model

Microscale heat transfer is vital for the performance of smart thermal devices like heat sinks, thermosyphons, and microheat pipes. This study introduces a biothermal pumping flow model based on a multi-membrane pumping mechanism that leverages microscale heat transfer. The model describes rhythmic contraction and relaxation of membranes, combined with electro-osmosis in Jeffery fluid flow within a vertical microchannel of finite length. Two membranes on the microchannel walls, with varying amplitudes, diameters, and phase lags, generate pressure that moves fluid in both directions through contraction and expansion cycles. The model is based on the conservation of mass and momentum, using a low Reynolds number approximation to capture microscale transport phenomena at biomedical scales. Dimensionless conservation equations are analytically solved under no-slip boundary conditions, with results computed in MATLAB for clarity. Axial velocity results are simulated and verified using the optimal homotopy analysis method. The model explores the influence of key parameters (UHS, me, λ, Gr, β) on pressure gradient, velocity distribution, volumetric flow rates, skin friction, Nusselt number, and stream function. The findings demonstrate that pressure from membrane motion is significantly affected by thermal effects and buoyancy forces, and flow and pumping characteristics are largely determined by the fluid's rheological qualities and the geometrical features of the membrane. This study provides novel ideas for enhancing the functionality and design of smart thermal devices while also advancing microscale heat transfer technology.

Comparative predictive analysis using ANN and RCA for experimental investigation on branched and conventional micro heat pipe

The demand for compact, high-performance electronic devices rises, making thermal management in limited space crucial. This study presents a novel branched micro heat pipe (MHP) to solve this pressing problem. This novel method improves thermal performance for small-scale electronic cooling applications, surpassing conventional cooling methods. Innovative design has branched condenser of 12.7 mm evaporator, 24.6 mm adiabatic section and 12.7 mm condenser, each, compared to conventional MHP of identical size. Investigation encompasses a study of various parameters including working fluids, wettability characteristics, fill ratio, heat input, and cross-sectional geometries. The significant improvement in thermal performance by applying a hydrophobic surface treatment to the condenser section, increasing capillary force and condensate transport. Ammonia and de-ionized water excel thermal performance at lower and higher applied heat input (Qh), respectively. At higher Qh (>2 W), ammonia and toluene in the conventional MHP experience faster drying compared to the branched MHP, especially for lower fill ratio (0.6 FR). The branched MHP outperforms the conventional MHP, showing improved fluid circulation and performance, for fill ratios ranging from 0.65 to 0.4 and effectively transporting maximum heat flux in range of 10–14.4 W/cm2 with square mono-grooved MHP exhibits inferior performance compared to the trapezoidal and triangular mono-grooved. To provide a framework for future studies, we establish an artificial neural network (ANN) and statistical prediction model of thermal performance using the non-dimensional Kutateladze number (Ku), showcasing a robust correlation with experimental results and demonstrating only a minor deviation of ± 3 %.

Study on Heat and Mass Transfer Performance of Ultra-Thin Micro-Heat Pipes

Study on Heat and Mass Transfer Performance of Ultra-Thin Micro-Heat Pipes

Performance analysis of micro heat pipe PV/T within and outside the greenhouse in northwest China

The study investigates the performance of a micro heat pipe (MHP) photovoltaic/thermal (PV/T) system operating with R141b as the working fluid under the climatic conditions of Lanzhou, situated in the northwest region of China—a locale noted for its high solar insolation. This study operates the PV/T system in the greenhouse, improving system performance while addressing the issue of wind and sand adhering to the surface of PV/T panels in the northwest region, which affects system performance. Empirical analysis was conducted in summer and winter to ascertain the system's performance. Results indicated that the PCE in winter inside and outside the greenhouse were found to be 7.1% and 13.1%, respectively, while the summer season values were 6.5% and 12.3%, respectively. Notably, the average thermal power generated within the greenhouse during winter was measured at 464.8 W/m2, surpassing the outdoor average thermal power by 61.9 W/m2; similarly, in summer, the internal thermal power was 313.9 W/m2, exceeding the external by 51.7 W/m2. The total efficiency was the highest inside the greenhouse in summer, at 61.5%, while the lowest total efficiency was observed outside the greenhouse in winter, at 34.3%.

Thermal performance and optimization of an integrated collector–storage solar air heater based on lap joint-type flat micro-heat pipe arrays

This study seeks to optimize the performance of an integrated collector–storage solar air heater (ICSSAH) based on lap joint-type (LJT) flat micro-heat pipe arrays (FMHPAs) and latent thermal storage through numerical simulation. A novel dynamic heat transfer model is developed by incorporating the heat transfer correlation describing the thermal characteristics of phase change material (PCM) in narrow and elongated confined spaces within plate fins and validated using experimental data to address the issue of deviation caused by the fixed thermal resistance of the PCM in conventional numerical simulation methods. The validated model is used to simulate and predict the dynamic performance of LJT-FMHPA-ICSSAH under different heat transfer structures. Several optimization parameters, including the air gap thickness, the optical properties of the collector and glass cover plates, and fins structure, are considered. Simulation results indicate that the heat lost to the external environment through the glass cover plate accounted for over 70 % of the total heat dissipation. The optimal combination included a 50 mm gas layer; copper-based blue titanium coating; ultra-white glass; and fin spacing, thickness, height, and length of 5, 0.2, 60, and 195 mm, respectively. Under autumn/winter operating conditions, the optimized thermal performance is 1.13–1.35 times higher than the performance prior to optimization.

Investigation on heat transfer performance of flat plate micro-heat pipe phase change heat storage/release device

In the domains of low/zero carbon energy, the ship energy storage system, coupled with phase-change storage and release technology, holds significant importance. The utilization of a flat micro-heat pipe as the primary heat transfer element prompts the development of a test platform and a transient heat transfer model for the heat storage device. This is in response to existing challenges such as a single structure, poor temperature uniformity, and low thermal efficiency in current heat storage technology. The study delves into the impact of varying temperatures and flow rates of heat transfer fluids on the overall performance of heat storage devices and heat transfer efficiency. The findings highlight that the staggered double plate micro-heat pipe structure can enhance total heat storage by 11%, average heat storage power by 19%, and heat storage efficiency by 21% compared to the heat storage device with a single flat micro-heat pipe structure. It becomes evident that the heat storage device with the staggered double heat pipe structure outperforms its counterpart. Additionally, Temperature and flow velocity play pivotal roles in determining the heat transfer performance of phase change materials. As the temperature difference between the heat transfer fluid and the phase change material increases, both the phase transition rate and the equivalent Nusselt number also rise, providing a crucial foundation for examining the heat transfer characteristics of phase change materials during different stages of phase transition.

Experimental study and performance enhancement of micro heat pipe PV/T system

The integration of photovoltaic/thermal (PV/T) systems, which enable the simultaneous conversion of solar energy into both electricity and thermal energy, holds great promise in the solar-rich northwest region of China. This study aims to assess the performance of a micro heat pipe (MHP) PV/T system through comprehensive experiments conducted over the four seasons in Lanzhou. The experimental setup involved the measurement of various parameters including environmental temperature, surface temperature of the PV/T panel, back temperature of the PV/T panel, and water temperature, as well as the determination of the power collection efficiency (PCE) and thermal conversion efficiency (TCE). The PV/T system was installed at a tilt angle of 45°, resulting in an average PCE of 12.42 % and TCE of 34.7 %. To further understand the system performance, a two-dimensional mathematical model was developed and validated using the experimental data, demonstrating good agreement between the simulated and actual results. The simulation provided valuable insights into the temperature distribution across different components of the PV/T module, such as the glass cover, solar cell, and single shell of the MHP. The findings revealed that increasing the number of MHPs from 12 to 20 led to a modest improvement of 0.21 % and 2.72 % in the PCE and TCE, respectively. Similarly, raising the flow rate from 0.108 L/s to 0.128 L/s resulted in a corresponding increase of 0.25 % and 3.01 % in the PCE and TCE, respectively. These experimental investigations and numerical simulations established a solid scientific foundation and offered practical guidance for the implementation of MHP-PV/T systems, thereby facilitating the efficient utilization of solar energy in future applications.

Thermal performance of two evacuated tube solar collectors with flat heat pipes

To promote the transition to clean heating methods in rural areas and improve solar energy utilization efficiency, two new types of solar water collectors with evacuated tubes, namely finned and finless collectors, are constructed with multi-channel flat tubes and flat micro-heat pipe arrays, and comparison experiments are conducted. By integrating with multi-channel flat tubes, the micro-heat pipe arrays, which function as the central heat transfer component, are capable of attaining effective thermal transmission. The multi-channel flat tubes have a substantial surface area available for heat transfer with the thermal fluid, while the evacuated tubes exhibit superior thermal insulation performance. The structural discrepancy between the two collectors lies in the presence or absence of louvered fins attached to the flat heat pipes inside the evacuated tubes. The comparative analysis of the two collectors includes time constant, thermal resistance analysis, distribution of temperature, normalized temperature difference, pressure drop, and actual operation effect under different conditions. The average thermal efficiency of the finless collector (59.4 %) is lower than that of the finned collector (78.2 %) on a typical experimental day. Then, further investigations are conducted to explore the effects of heat transfer fluid volume flow rate and inlet temperature on the thermal performance of finned solar water collector. With the heat transfer fluid volume flow rate of 120 L/h and the inlet temperature at 15 °C, the finned collector can achieve its maximum thermal efficiency of 86.4 %.

Novel for the resistance welding of laminated wire mesh microgroove composite wicks with enhanced capillary performance

The efficient operation of micro heat pipes (MHPs) requires evaporation and rapid condensation of the working liquid reflux to the heat source, and the internal wick structures play a crucial role. With the advantages of high porosity, good flexibility, and low thickness, mesh is ideal for ultrathin thermal management systems, especially acting as wicks in MHPs. This study proposes a novel method of fabricating resistance-welded laminated mesh microgroove composite wick structures; this is different from the earlier composite wick structure that first processed microgrooves on a flat substrate and then laid the mesh for vacuum sintering. The new composite wick structures were fabricated by the resistance welding of laminated meshes, soaking them in resin, and laser processing them with microgrooves after solidification, which exhibited superhydrophilicity. The capillary rise experiment was used to test the capillary performance of the composite wick structure, and the ΔPcap·K value was used as an index to evaluate the capillary performance of samples. The results showed that the effective pore size gradually decreased, and the capillary performance gradually improved with an increase in the number of mesh welding layers. Compared with the single-layer mesh, the capillary height of the four-layer welded laminated mesh was increased by approximately 66.94 %. By modifying the laminated screen with resin soaking and processing microgrooves, forming porous microstructures on the sidewalls and bottom of the microgrooves, the capillary performance significantly improved again, with a maximum capillary height of 83.23 mm that was increased by approximately 394.83 % and 196.4 % than that of the single-layer screen and the four-layer laminated screen before modification, respectively, and the ΔPcap·K values were increased by approximately 157.38 % and 49.69 %, respectively. This study provides a new ideal for the fabrication of composite wicks for enhancing the thermal performance of MHPs.

The effects of transferred heat and wall material on thermal behavior of a nano-grooved micro-heat pipe, molecular dynamics simulation

The design and construction of suitable superconductors for dissipating the heat produced by microprocessors and concentrator photovoltaic (CPV) is one of the bottlenecks in their design. Accordingly, any progress in these industries depends onthethermal management of produced heat in such devices. Among all available options, heat pipes (HPs) are desirable options due to their independence and proper performance. In this article, the performance of a flat plate micro-HP with embedded nano groovesasthecondensate returning mechanism is investigated. A part of an HP with a length of 1 μm, and 100 nm and 200 nm in two other dimensions is selected and simulated using molecular dynamics (MD). Platinum (Pt), copper (Cu), and aluminum (Al) are used for body and argon (Ar), ethanol (Eth), and water (H2O) are also used asworking fluids. The effects of using each of these materials on efficiency, effectiveness, density, temperature,velocity, heat flux,and several parameters related to mass transferinside the HP are investigated. The results show that among the three fluids used, the highest efficiency is related to Eth (23.7 %). Evaluation of the effectiveness of the HP indicates that under all examined conditions, using HP as a superconductor is justified. Among the three metals used, Cu shows the best heat flux and provides better results for evaporation and condensation rates. The highest heat flux is obtained by using Cu-Eth and is 1828 W/cm2.

Experimental and heat transfer studies of MHP-PV/T by Reynolds number and heat transfer coefficient enhancement

The photovoltaic/thermal (PV/T) system, as an energy conversion system to generate electricity and heat, has great application potential in northwest zone of ample solar energy resource in China. The working media inside the micro heat pipe (MHP) of previous studies was acetone. Compared to acetone, R141b has better stability and lower solubility. For working fluid as R141b in the MHP, higher Reynolds Number (Re) theoretically means better heat transfer. During the typical winter season, when the inclination of the PV panel was 45°, the average power conversion efficiency (PCE) and thermal conversion efficiency (TCE) can reach 12.8 and 26.4 %. Furthermore, in order to reduce the simulation time and facilitate the research, the study establishes the fitting equation of MHP-PV/T surface temperature based on solar radiation intensity and environmental temperature with an average error of 7.6 %. Furthermore, a three-dimensional mathematical model of MHP-PV/T system was developed and validated with experimental results, investigating the Re of R141b in the MHPs and calculating the related heat transfer coefficient (h) based on Re. The simulation showed that the Re and h at the condensation section of the MHP were bigger than those at the evaporation section. The Re and h increased with the water temperature decrease of airfoil heat exchanger and solar radiation intensity rise. Lastly, when water temperature of airfoil heat exchanger was unchanged, the impact of solar radiation intensity on h was greater than Re. When the solar radiation intensity remained unchanged and the water temperature decreased, Re was the main reason for affecting the change of h. The research results will give a scientific foundation and technical application for the MHP-PV/T, as well as more efficient solar energy applications in the future.

Thermal resistance analysis of micro heat pipes incorporated with carbon nanotubes nanocapillaries

The remarkable performance improvement and thermal optimization of micro heat pipes (MHPs) incorporated with carbon nanotubes (CNTs) of different density are analysed using a thermal resistance model. The ultrafast water transport characteristic of CNT-nanocapillaries induces large-surface-area water film where the evaporation, condensation, and condensate circulation processes, which govern the MHP's overall performance, are simultaneously enhanced. By employing a thermal resistance model, the pertinent parameters inside an MHP, which are not measurable due to the miniature size of the microchannels, are estimated. By benchmarking with the uncoated MHP, the condenser and evaporator heat transfer coefficients of the optimal 5 mg/CNTs MHPs are enhanced remarkably by the up to 249% and 264%, respectively. Furthermore, the circulation rate attains a maximum increase of 212%. Consequently, an enhancement up to 250% in the overall performance of MHP gauged by the effective thermal conductivity and a significant temperature drop of 20.8 °C on the heat source as compared to the uncoated MHP case are achieved. By minimizing the total thermal resistance of MHP, the optimized length ratio of the evaporator and condenser, which is useful in the optimal design of the MHP, can be obtained. The thermal resistance model serves as a useful analytical tool to provide insightful thermal analysis and optimized design of an MHP.

Experimental research on the dynamic response characteristics of proton exchange membrane fuel cell thermal management using micro-heat pipe array

Experimental research on the dynamic response characteristics of proton exchange membrane fuel cell thermal management using micro-heat pipe array

Study of low-level pool boiling in vapor chamber of wickless micro-heat pipe based on Lattice Boltzmann pseudopotential method—Effects of superheat and wall wettability

The lattice Boltzmann method, a mesoscopic numerical method with a solid kinetic basis, is used to establish a low-level pool boiling model to exhibit the transport phenomena inside an evaporation chamber of a wickless micro heat pipe using DI water as the working fluid. The effects of wall superheat and wall wettability on bubbles’ nucleation, growth, and detachment in the evaporation chamber were compared and analysed. Moreover, the influence of the wall superheat on the thermal performance of the micro heat pipe was specifically examined. It is found that the low-level pool boiling occurs with a wall superheat that is higher than 0.357T c, indicating a high heat flux for heat removal. Interestingly, when the relatively hydrophobic chamber wall is superheated by 0.46T c, there are more nucleation sites, and the complete bubbles cycle containing nucleation, growth, and detachment is longer. In contrast, on a hydrophilic surface under the same and sufficient superheat condition, the number of bubble nucleation sites is less than that on the hydrophobic surface, and the complete cycle of bubbles is shorter. It is worth mentioning that the model has values for predictions of homogeneous nucleation of bubbles and thermal transport on hydrophobic or hydrophilic surfaces.

Metal flow hysteresis behavior and shape control strategy during porthole die extrusion of micro heat pipe

The shape control strategy of micro grooves is still unclear and challenging during the porthole die extrusion of grooved micro heat pipe (MHP). Through the simulation and experiment of porthole die extrusion of a MHP profile, the metal flow hysteresis behavior within micro features and the effect of ram speed and extrusion temperature on it and the resulting forming integrity was elucidated. Innovatively, Taguchi design and variance analysis (ANOVA) were introduced to determine their influence magnitude on the metal flow uniformity calculated by simulation results. The main findings are given below. The metal flow hysteresis derives from part feature size effect. The negligible friction-affected area during conventional extrusion severely slows down the metal flow within micro features during the MHP profile extrusion, which is due to the surge in the area ratio of the friction-affected area to the region in which it is located. Neither ram speed nor extrusion temperature can change the distribution of the friction-affected area. However, increasing ram speed multiplies the metal flow hysteresis and severely reduces the forming integrity, whereas extrusion temperature has little effect. Following this strategy, batch extrusion of the profile with micro-grooved width of 0.27 ± 0.02 mm was achieved in industrialized conditions.

Heat transfer performance of flat micro-heat pipe with sintered multi-size copper powder wick

The heat flux rises dramatically as electronic circuits become more integrated. Flat micro-heat pipe (FMHP) is a popular device for high heat flux management. A novel sintered multi-size copper powder wick (SMW) is designed in this paper. The evaporator and condenser section of the SMW is a double-layer structure with two copper powder diameters. The adiabatic section is a single-layer structure. Copper powder sintering is a compelling method to achieve graded pore size design. SMW and six sintered single-size copper powder wicks (SSWs) are prepared by vacuum sintering of copper powder. The capillary pressure of wicks and heat transfer characteristics of FMHPs are experimentally investigated. The results show that the liquid transfer time of the SMW is shortened 16.59% compared to that of the SSW. The dynamic response time constant of the SMW FMHP is decreased by 0.9 s, and the overall thermal resistance is reduced by 14.22%. The heat pipe made in this study is more agreeable to heat dissipation and is of importance for thermal management in power devices.

Remarkable Thermal Performance Enhancement of Micro Heat Pipes with Graphene-Nanoplatelet Nano-Wicks

The ultrafast water permeation property of graphene nanoplatelets (GNPs) synergically enhances the evaporation and water circulation processes in a micro heat pipe (MHP). An MHP is a promising phase-change heat-transfer device capable of transferring large amounts of heat energy efficiently. The hydrophobic, atomically smooth carbon walls of GNPs nanostructures provide a network of nanocapillaries that allows water molecules to intercalate frictionlessly among the graphene layers. Together with the attraction force of the oxygenated functional groups, a series of hydrophobic and hydrophilic surfaces are formed that significantly improve the water circulation rate. The intercalation of water molecules encourages the formation of water-thin film for film-wise evaporation. The effect of nano-wick thickness on the thermal performance of the MHP is investigated. A thinner GNP nano-wick is more favorable to film-wise evaporation while a thicker nano-wick promotes a higher water circulation rate from the condenser to the evaporator, leading to the existence of an optimal thickness. By benchmarking with the uncoated MHP, the thermal conductance of an MHP with a 46.9-µm GNP nano-wick manifests a maximum enhancement of 128%. This study provides insights on the feasible implementation of GNP nano-wicks into a highly efficient micro-scale electronics cooling device for environmental sustainability.

Experimental study of the thermal characteristics of a heat storage wall with micro-heat pipe array (MHPA) and PCM in solar greenhouse

Solar greenhouses are agricultural facilities that use solar energy for growing vegetables. The thermal characteristics of a solar greenhouse wall have an important influence on the creation of the microclimate in the greenhouse and improving the heat storage capacity of the wall materials can prevent freezing damage of greenhouse crops. To increase the temperature of the internal temperature stabilisation layer and the heat storage and release characteristics of the wall, a novel greenhouse wall with micro-heat pipe arrays (MHPAs) and phase-change materials (PCMs) was proposed, and an experimental greenhouse with the proposed wall and an ordinary greenhouse were built in this study. The thermal performance of the greenhouse wall and the improvement effect on the greenhouse microclimate for typical weather conditions were analysed using dynamic evaluation indices, such as the air temperature, the inner surface temperature of the experimental wall, and the heat storage and release rate of the greenhouse wall. The combined use of MHPA and PCM solves the problem of the heat transport bottleneck caused by the low thermal conductivity of PCM and the traditional wall temperature stabilisation layer, and effectively increases the amount of heat stored in the wall, thereby ensuring the growth of crops at night. For typical days, the average total heat storage of the experimental wall was 18.89 MJ/m3, while that of the ordinary wall was 9.67 MJ/m3 (an increase of 95.35%). The average total heat release was 17.58 MJ/m3, while that of the ordinary wall was 8.95 MJ/m3 (an increase of 96.42%), which led to an increase in the air temperature during the night and provided an environment suitable for the crop growth.

Experimental and numerical study on thermal management of air-cooled proton exchange membrane fuel cell stack with micro heat pipe arrays

In this study, micro-heat pipe arrays (MHPAs) are integrated into air-cooled proton exchange membrane fuel cells (PEMFCs) to enhance their load level and heat dissipation. Experiments are carried out to compare the relevant thermal performance of the stack consisting of 50 units at different operating conditions. A three-dimensional, non-isothermal, and steady-state numerical model of a single cell in the stack is then established to analyze the effect of a MHPA on the electrochemical parameters within the cell. The results of the experiments show that the load level of the stack with MHPAs is 40 % higher than that without MHPAs near the limit temperature. Meanwhile, the average temperature of the stack with MHPAs is 8.3 °C lower than that without MHPAs at 33 A, and MHPAs make the temperature more uniform. Numerical study shows that the decrease of cell temperature by MHPA enables the proton exchange membrane to maintain better hydration and higher current density. At an operating voltage of 0.628 V, the average temperature in the membrane of a single cell with MHPA is 8.8 °C lower than that of PEMFC, and the average water content and current density are 2.01 and 241.06 A/m2 higher than those without MHPA, respectively.

Modeling of Phase Change in Nanoconfinement Using Moment Methods

Abstract Accurate prediction of liquid–vapor phase change phenomena is critical in the design of thin vapor chambers and microheat pipes for the thermal management of miniaturized electronic systems. In view of this, we have considered the heat and mass transfer between two-liquid meniscuses separated by a thin gap of its own vapor. Assuming the heat and mass flow are to be steady and one-dimensional, analytic solutions are obtained to the linearized equations from the regularized 26-moment framework. Our analytic solutions provide excellent predictions for the effective heat conductivity of a dilute gas with those from the molecular dynamics (MD) and Boltzmann equation where Fourier's law fails. We also verified that the predicted heat and mass flow rates over the whole range of the Knudsen number are consistent with the kinetic theory of gases. Further, the model has been used to predict the effect of evaporation and accommodation coefficients on the heat and mass transfer between the liquid layers.