Modelling airflow,heat transfer and moisture transport around a standing human body by computational fluid dynamics

In this study a combined computational model of a room with virtual thermal manikin with real dimensions and physiological shape was used to determine heat and mass transfer between human body and environment. Three dimensional fluid flow, temperature and moisture distribution, heat transfer (sensible and latent) between human body and ambient, radiation and convection heat transfer rates on human body surfaces, local and average convection coefficients and skin temperatures were calculated. The radiative heat transfer coefficient predicted for the whole-body was 4.6 W m^− 2 K^− 1, closely matching the generally accepted whole-body value of 4.7 W m^− 2 K^− 1. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.8 W m^− 2 K^− 1. Results of calculations were in agreement with available experimental and theoretical data in literature.     

3-D numerical and experimental models for flat and embedded heat pipes applied in high-end VGA card cooling system

The present study utilizes the three-dimension numerical and experimental methods to investigate the optimum thermal performance of a flat heat pipe-thermal module application in high-end VGA card cooling system, and compares that with a traditional copper metal based plate embedded three 6 mm diameter heat pipe-thermal module under three dissimilar inclination angles of 0°, 90° and 180°. The optimization for the thermal modules researches into various fin material, thickness and gap. Results show that the flat heat pipe-thermal module has the best thermal performance at high power GPU of 180 W and inclination angle of 180°. Simulation results show in good agreement with experimental results within 5%. Therefore, the thermal performance of a flat heat pipe-thermal module can be accurately simulated and analyzed by employed the manner introduced in this paper and is able to cope with the higher heat flux GPU over 62.5 W/cm² in the future.     

Thermoelectric air-cooling module for electronic devices

This article investigates the thermoelectric air-cooling module for electronic devices. The effects of heat load of heater and input current to thermoelectric cooler are experimentally determined. A theoretical model of thermal analogy network is developed to predict the thermal performance of the thermoelectric air-cooling module. The result shows that the prediction by the model agrees with the experimental data. At a specific heat load, the thermoelectric air-cooling module reaches the best cooling performance at an optimum input current. In this study, the optimum input currents are from 6 A to 7 A at the heat loads from 20 W to 100 W. The result also demonstrates that the thermoelectric air-cooling module performs better performance at a lower heat load. The lowest total temperature difference-heat load ratio is experimentally estimated as ?0.54 W K^?1 at the low heat load of 20 W, while it is 0.664 W K^?1 at the high heat load of 100 W. In some conditions, the thermoelectric air-cooling module performs worse than the air-cooling heat sink only. This article shows the effective operating range in which the cooling performance of the thermoelectric air-cooling module excels that of the air-cooling heat sink only.     

Improving power density and efficiency of miniature radioisotopic thermoelectric generators

We have built and tested a prototype miniaturized thermoelectric power source that generates 450 μW of electrical power in a system volume of 4.3 cm³. The measured power density of 104 μW cm⁻³ exceeds that of any previously reported thermoelectric power system of equivalent size. This improvement was achieved by implementing a novel thermopile design in which wagon wheel-shaped thermoelectric elements contact the entire circumference of the heat source whereas traditional approaches utilize only one heat source surface. The thermopile consists of 22 wagon wheel-shaped elements (11 P–N thermocouples) fabricated from 215-μm thick bismuth–telluride wafers having ZT = 0.97 at 30 °C. The power source operates on a 150 mW thermal input provided by an electrical resistance heater that simulates a capsule containing 0.4 g of ²³⁸PuO₂ located at the center of the device. Our primary research objective was to develop and demonstrate a prototype thermopile and radioisotopic thermoelectric generator (RTG) architecture with improved power density at small scales. Output power from this device, while optimized for efficiency, was not optimized for output voltage, and the maximum power was delivered at 41 mV. We also discuss modifications to our prototype design that result in significantly improved voltage and power. Numerical predictions show that a power output of 1.4 mW, power density of 329 μW cm⁻³, and voltage of 362 mV, is possible in the same package size.     

Two-phase closed thermosyphon vapor-chamber system for electronic cooling

This article experimentally investigates a two-phase closed thermosyphon vapor-chamber system for electronic cooling. A thermal resistance net work is developed in order to study the effects of heating power, fill ratio of working fluid, and evaporator surface structure on the thermal performance of the system. The results indicate that either a growing heating power or a decreasing fill ratio decreases the total thermal resistance, and the surface structure also influences the evaporator function prominently. A reasonable agreement with Rohesnow's empirical correlation is found for the evaporator. An optimum overall performance exists at 140 W heating power and 20% fill ratio with sintered surface, and the corresponding total thermal resistance is 0.495 °C W⁻¹.     

Effects of working fluids on the performance of a bi-directional thermodiode for solar energy utilization in buildings

A series of experiments were conducted to investigate the effects of different working fluids on the behavior and performance of a bi-directional thermodiode. The thermodiode was made up of two rectangular loops mounted between a collector plate and a radiator plate. The loops were filled with a working fluid for effective heat transfer when the thermodiode was forward biased. Five different working fluids were tested with thermal conductivity values ranging from 0.1 to 0.607 W/m-K, thermal expansion coefficient values ranging from 2.54 × 10⁻⁴ to 1.43 × 10⁻³ 1/K, and kinematic viscosity values ranging from 6.5 × 10⁻⁷ to 1 × 10⁻⁴ m²/s. The thermodiode was heated by a radiant heater consisting of 88 halogen lamps that generated a heat flux of about 10³ W/m² on the collector surface. Experimental results indicated that the onset time for natural convection to be induced throughout the diode system did not differ considerably when different working fluids were used. On the other hand the required fluid temperature differences in the loops for the onset of throughflow were quite different and depended strongly upon the viscosity and other properties such as thermal expansion coefficient and specific heat of the working fluid. Of the five fluids tested, water and low-viscosity silicon oil had the highest heat transfer rate. An analytical model was developed to predict and analyze the steady operation of the diode system when different working fluids are used.     

Thermal performance of a commercial alkaline water electrolyzer: Experimental study and mathematical modeling

In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1 N m³ H₂/h at an overall power consumption of 4.90 kW h/N m³ H₂ is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50–67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174 kJ °C⁻¹. The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3 W m⁻² °C⁻¹. The model has been implemented using ANSYS^® V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages.     

Determination of the thermal transport properties of ammonia borane and its thermolysis product (polyiminoborane) using the transient plane source technique

Reliable thermal property data are necessary to improve the fidelity of chemical hydride thermal decomposition models. The thermal diffusivity and conductivity of ammonia borane (NH₃BH₃) and its partial thermolysis product (polyiminoborane) were measured at various packing densities using a transient plane source technique under ambient conditions. The particle size of the ammonia borane powder was between 200 and 600 μm, while the particle size of the polyiminoborane powder was between 10 and 30 μm. The thermal diffusivity and conductivity of the ammonia borane increased from 0.17 to 0.24 mm²/s and 0.19 to 0.44 W/m K (±10%), respectively, when its packing density was increased from 0.37 to 0.58 g/cm³. The increase in thermal conductivity is due to the increase in contact area between particles and the increase in the thermal diffusivity is related to an increase in density and volumetric heat capacity caused by compaction. The thermal conductivity of the polyiminoborane powder was approximately three times lower, likely due to its higher porosity. The thermal diffusivity and conductivity of this product changed from 0.21 to 0.12 mm²/s and 0.068 to 0.23 W/m K (±10%), respectively, when its packing density was increased from 0.13 to 0.96 g/cm³.     

Compact cooler for electronics on the basis of a pulsating heat pipe

The paper presents the results of developing and investigating a compact cooler for electronics made on the basis of a closed loop pulsating heat pipe (CLPHP). The cooler is made of a copper tube 5.6 m long with OD of 2 mm and ID of 1.2 mm in the form a 3D spiral containing 17 turns. The device is equipped with a light copper radiator with a finning area of 1670 cm², which was blown by an axial fan located inside the spiral. The thermal interface of the cooler situated in the heating zone is made of a copper plate with a thermocontact surface measuring 40 × 35 mm, which was in thermal contact with all the turns of the device. The cooler overall dimensions are 105 × 100 × 60 mm, its mass is 350 g.The operation of the cooler has been investigated with water, methanol and R141b as working fluids at a uniform and concentrated supply of a heat load in different heating modes. A reliable operation of the device has been demonstrated in the range of heat loads from 5 to 250 W. A minimum thermal resistance “heat source–ambient air” equal to 0.32 °C/W was attained with water and methanol as working fluids at a uniform heat load of 250 W. With a heat load concentrated on a section of the thermal interface limited by an area of 1 cm², a minimum value of thermal resistance equal to 0.62 °C/W was attained at a heat load of 125 W when methanol was used as a working fluid.     

Thermal performance of a meso-scale liquid-fuel combustor

Combustion in small scale devices poses significant challenges due to the quenching of reactions from wall heat losses as well as the significantly reduced time available for mixing and combustion. In the case of liquid fuels there are additional challenges related to atomization, vaporization and mixing with the oxidant in the very short time-scale liquid-fuel combustor. The liquid fuel employed here is methanol with air as the oxidizer. The combustor was designed based on the heat recirculating concept wherein the incoming reactants are preheated by the combustion products through heat exchange occurring via combustor walls. The combustor was fabricated from Zirconium phosphate, a ceramic with very low thermal conductivity (0.8 W m⁻¹ K⁻¹). The combustor had rectangular shaped double spiral geometry with combustion chamber in the center of the spiral formed by inlet and exhaust channels. Methanol and air were introduced immediately upstream at inlet of the combustor. The preheated walls of the inlet channel also act as a pre-vaporizer for liquid fuel which vaporizes the liquid fuel and then mixes with air prior to the fuel–air mixture reaching the combustion chamber. Rapid pre-vaporization of the liquid fuel by the hot narrow channel walls eliminated the necessity for a fuel atomizer. Self-sustained combustion of methanol–air was achieved in a chamber volume as small as 32.6 mm³. The results showed stable combustion under fuel-rich conditions. High reactant preheat temperatures (675 K–825 K) were obtained; however, the product temperatures measured at the exhaust were on the lower side (475 K–615 K). The estimated combustor heat load was in the range 50 W–280 W and maximum power density of about 8.5 GW/m³. This is very high when compared to macro-scale combustors. Overall energy efficiency of the combustor was estimated to be in the range of 12–20%. This suggests further scope of improvements in fuel–air mixing and mixture preparation.     

Preparation and thermal properties of polyethylene glycol/expanded graphite blends for energy storage

Expanded graphite is a promising heat transfer promoter due to its high conductivity, which improves the thermal conductivity of organic phase change materials. Moreover, it can also serve as supporting materials to keep the shape of the blends stable during the phase transition. After various investigation, the results showed that the maximum weight percentage of polyethylene glycol was as high as 90% in this paper without any leakage during the melting period, with the latent heat of 161.2 J g⁻¹ and the melting point of 61.46 °C. It was found that the value of the latent heat was related to the polyethylene glycol portion, increased with the increase in polyethylene glycol content. Moreover, the measured enthalpy of the composite phase change materials was proportional to the mass ratio of the polyethylene glycol component. The melting temperatures were almost the same with different ratios of composites. The conductivity of blends was improved significantly with the high value of 1.324 W m⁻¹ K⁻¹ compared to the pure polyethylene glycol conductivity of 0.2985 W m⁻¹ K⁻¹.     

Theoretical and experimental study on the heat transport in metallic nanofilms heated by ultra-short pulsed laser

In this paper the mechanism of heat transport in metallic nanofilms under ultra-short pulsed laser heating is examined theoretically and experimentally. In order to easily understand the non-equilibrium heat transport in metallic nanofilms the study of heat transport behavior is first carried out in dielectrics. The analyses indicate that there may be two kinds of wave phenomena in dielectrics subjected to a periodic surface temperature. One is the thermal wave governed by the C-V model based hyperbolic equation and the other is the diffusive wave governed by the Fourier model based parabolic equation. According to the hyperbolic two step model for non-equilibrium heat transport, such two kinds of wave phenomena can also occur simultaneously in the metallic nanofilms under pulsed laser heating, where the diffusive wave is induced by the electron temperature oscillation at the surface due to the non-equilibrium between electrons and lattices. Unlike the propagation speed of the thermal wave, the propagation speed of the diffusive wave depends not only on the medium properties but also the period of the temperature oscillation at the boundary. Hence, the propagation speed of the diffusive wave in the electron gas may be of as high as 10⁶ m s⁻¹, when the laser pulse duration is less than 1 ps. A transient thermoreflectance (TTR) system has been built to measure the transient electron temperature responses caused by the femtosecond laser heating and a pump-probe technique is used to ensure the femtosecond temporal resolution in the experiments. Different from the commonly used front heating-front detecting (FF) method for measuring the material properties, a rear heating-front detecting (RF) method is applied, so that measuring the propagation speed of heat becomes available. The non-equilibrium heat diffusion model is used to fit the measured normalized electron temperature profiles of 27.2 nm, 39.9 nm and 55.5 nm Au films. The best-fitted coupling factor G basically agrees with the theoretical value 2.3 × 10¹⁶ W m⁻³ K⁻¹. The propagation speed of the diffusive wave in the electron gas can be obtained by comparing the measured delay time of peak electron temperatures of Au films with different thicknesses. The average propagation speed of the temperature oscillation or diffusive wave in Au films for the range of thickness from 27.2 nm to 55.5 nm is equal to 8.1 × 10⁵ m s⁻¹, which is close to the value predicted by the non-equilibrium heat diffusion model.     

Galactitol hexa stearate and galactitol hexa palmitate as novel solid-liquid phase change materials for thermal energy storage

Galactitol has a melting point of 187.41 °C and a fusion enthalpy of 401.76 J g⁻¹. Its melting temperature is not suitable for many thermal energy storage applications although it has good latent heat storage capacity compared to the several traditional phase change materials (PCMs). The galactitol also has high supercooling degree as about 72 °C. These unfavorable properties limit the usage potential of galactitol in thermal energy storage applications. However, the phase change temperature and supercooling degree of galactitol can be reduced to a reasonable value and therefore its feasibility for energy storage systems can be increased. For this aim, in this study, galactitol hexa stearate (GHS) and galactitol hexa palmitate (GHP) were prepared as novel solid-liquid PCM by means of esterification reaction of the galactitol with palmitic acid and stearic acid. The GHP and GHS esters were characterized chemically using FT-IR and ¹H NMR techniques. By using DSC analysis method, the melting temperature and latent heat value of the PCMs were determined as 31.78 °C and 201.66 J g⁻¹ for GHP ester and 47.79 °C and 251.05 J g⁻¹ for GHS ester. Thermal cycling test showed that the prepared PCMs had good thermal reliability after thermal 1000 melting-freezing cycles. Thermogravimetric analysis (TGA) results revealed that the PCMs have good thermal stability over their working temperatures. In addition, thermal conductivity of the prepared PCMs was increased as about 26.3% for GHP and 53.3% for GHS by addition of 5 wt.% expanded graphite. Based on all results it can be concluded that the prepared GHP and GHS esters can be considered as promising solid-liquid PCMs for many energy storage applications such as solar energy storage, indoor temperature controlling in buildings, production of smart textile and insulation clothing due to their good energy storage properties.     

Thermal model of the MUPUS penetrator

The insertion device for the MUPUS penetrator on Phylae, the lander to be set by the Rosetta mission on the nucleus of Tchurumov-Gerasimenko comet, will have to work in extreme environmental condition. The electronic unit inside the device should be kept at the temperature greater than −55 °C (220 K) in order to work properly. The expected temperature of the nucleus is of about 130 K, while the cosmic background temperature is as low as 3 K. Therefore, the electronic box must be heated continuously to balance the heat loss by radiation. The experiments performed in a vacuum-thermal chamber on the flight model of MUPUS has shown that the temperature of the electronics increases, when heated with a power of 1.33 W, from 140 K by about 120-130 K after more than 1 h of heating. We present a numerical thermal (FEM) model that gives a very good fit to the measured temperature dependence on time. In the process of fitting, several important thermal parameters of the insertion device have been determined, for instance the emissivity of electronic boards and their specific heat. The numerical models agree qualitatively with the results of simple thermal analysis. The main conclusion concerning performance of the instrument on the comet is that it should safely operate above the lover limit of acceptable temperatures against expected variations of the external temperature in the day-night cycle of the comet.     

Evaluation of phase change materials for thermal regulation enhancement of building integrated photovoltaics

Regulating the temperature of building integrated photovoltaics (BIPV) using phase change materials (PCMs) reduces the loss of temperature dependent photovoltaic (PV) efficiency. Five PCMs were selected for evaluation all with melting temperatures ∼25 ± 4 °C and heat of fusion between 140 and 213 kJ/kg. Experiments were conducted at three insolation intensities to evaluate the performance of each PCM in four different PV/PCM systems. The effect on thermal regulation of PV was determined by changing the (i) mass of PCM and (ii) thermal conductivities of the PCM and PV/PCM system. A maximum temperature reduction of 18 °C was achieved for 30 min while 10 °C temperature reduction was maintained for 5 h at −1000 W/m² insolation.     

Improved performance of air-cathode microbial fuel cell through additional Tween 80

The ability of electron transfer from microbe cell to anode electrode plays a key role in microbial fuel cell (MFC). This study explores a new approach to improve the MFC performance and electron transfer rate through addition of Tween 80. Results demonstrate that, for an air-cathode MFC operating on 1 g L⁻¹ glucose, when the addition of Tween 80 increases from 0 to 80 mg L⁻¹, the maximum power density increases from 21.5 to 187 W m⁻³ (0.6-5.2 W m⁻²), the corresponding current density increases from 1.8 to 17 A m⁻², and the resistance of MFC decreases from 27.0 to 5.7 Ω. Electrochemical impedance spectroscopy (EIS) analysis suggests that the improvement of overall performance of the MFC can be attributed to the addition of Tween 80. The high power density achieved here may be due to the increase of permeability of cell membranes by addition of Tween 80, which reduces the electron transfer resistance through the cell membrane and increases the electron transfer rate and number, consequently enhances the current and power output. A promising way of utilizing surfactant to improve energy generation of MFC is demonstrated.     

Chemical compatibility, thermal expansion matches and electrochemical performance of SrCo0.8Fe0.2O3−δ-La0.45Ce0.55O2−δ composite cathodes for intermediate-temperature solid oxide fuel cells

The chemical compatibility, thermal expansion and electrochemical property measurements of the SrCo_0.8Fe_0.2O_3−δ (SCF)-La_0.45Ce_0.55O_2−δ (LDC) composite cathodes for solid oxide fuel cells (SOFCs) were investigated by X-ray diffraction (XRD), thermal expansion coefficients (TECs) and cathodic polarization measurements together with electrochemical impedance spectroscopy (EIS). The results indicated that LDC had good chemical compatibility with SCF and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM), and the addition of LDC to SCF markedly reduced the polarization resistance. When the content of LDC reached 50 wt%, the SCF50 cathode showed the best electrochemical performance, with a cathodic overpotential of 0.1 V at the current density of 1102.0 mA cm⁻², together with a polarization resistance of 0.149 Ω cm² at 800 °C. The improved electrochemical performance was attributed to the expansion of the electrochemical reaction region into the electrode, and offering an easier path for the oxygen ion transport. Furthermore, the SCF-LDC composite cathodes match better with the LSGM electrolyte.     

Thermal stress and thermo-electrochemical analysis of a planar anode-supported solid oxide fuel cell: Effects of anode porosity

A 3D integrated numerical model is constructed to evaluate the thermal-fluid behavior and thermal stress characteristics of a planar anode-supported solid oxide fuel cell (SOFC). Effects of anode porosity on performance, temperature gradient and thermal stress are investigated. Using commercial Star-CD software with the es-sofc module, simulations are performed to obtain the current-voltage (I-V) characteristics of a fuel cell as a function of the anode porosity and the temperature distribution within the fuel cell under various operating conditions. The temperature field is then imported into the MARC finite element analysis (FEA) program to analyze thermal stresses induced within the cell. The numerical results are found to be in good agreement with the experimental data. It is shown that the maximum principal stress within the positive electrode-electrolyte-negative electrode (PEN) increases at a higher current and a higher temperature gradient. It is recommended that the temperature gradient should be limited to less than 10.6 °C mm⁻¹ to maintain the structural integrity of the PEN.     

Thermal stability of disordered carbon negative-electrode materials prepared from peanut shells

The thermal stability of electrochemically lithiated disordered carbon with a poly(vinylidene difluoride) binder and 1 mol dm⁻³ LiPF₆ dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was investigated by differential scanning calorimetry (DSC) using a hermetically sealed pan. The disordered carbon used was prepared by pyrolyzing peanut shells with porogen at temperatures above 500 °C. The disordered carbon gave much larger charge and discharge capacities than graphite when a weight ratio of porogen to peanut shells was set at 5. In DSC curves, several exothermic peaks were observed at temperatures ranging from 120 to 310 °C. This behavior was similar to that for electrochemically lithiated graphite, except for an exothermic peak at around 250 °C. However, the lithiated disordered carbon had a higher heat value, which was evaluated by integrating a DSC curve, compared to lithiated graphite. The heat values increased with an increase in accumulated irreversible capacities. These results suggest that heat generation at elevated temperatures should increase as an amount of irreversibly trapped lithium-ion increases. On the other hand, heat values per reversible capacities for disordered carbon, which showed larger capacities than graphite, were almost comparable to that for graphite. These results indicate that several types of disordered carbon showed larger capacity than graphite, while their thermal stability was lowered accordingly.     

The effects of different entrance sections lengths and heating on free and forced convective heat transfer inside a horizontal circular tube

An experimental study was done for hydrodynamically fully developed and thermally developing laminar air flows in a horizontal circular tube has a 30 mm inside diameter and 900 mm heated length (L/D = 30) under a constant wall heat flux boundary condition, with different aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths of 600 mm (L/D = 20), 1200 mm (L/D = 40), 1800 mm (L/D = 60), and 2400 mm (L/D = 80). The Reynolds number ranged from 400 to 1600 and the heat flux is varied from 60 W m^− 2 to 400 W m^− 2. This paper examines the effects of the entrance sections lengths and heating on the free and forced convection heat transfer process. The surface temperature data were measured and heat transfer rates at different heat flux levels as well as different Reynolds numbers were calculated and correlated in the form of relevant parameters. The buoyancy force has a significant effect on the heat transfer and the combined convection factor was approximately varied form 0.13 ≤ Gr/Re² ≤ 7.125. It was found that the surface temperature increases as the entrance section length increases. It was inferred that the heat transfer decreases as the entrance section length increases due to the flow resistance and the mass flow rate. The proposed correlation was compared with available literature and with laminar forced convection and showed satisfactory agreement.