Chemical reactions under highly intensive bubble boiling

A fundamentally new method is developed for enhancement of heat transfer under subcooled liquid boiling. The method is based on varying the stability of the “heater-liquid” system depending on the ways of controlling the delivery of heat load. This method is realized most simply for the case where boiling occurs on wire elements heated by electric current. The maximal effect in increasing the stability of bubble boiling is attained by using the stabilizer of integral mean resistance of sensor-heater as the source of electric power. It is experimentally found that very high heat loads (for example, up to 5000 W/cm² in the case of water) and the mechanism of their realization activate the liquid and lead to a large number of chemical reactions.     

Basic study on heat rejection system using high heat flux micro channel evaporator

Thermal management system which can reject very high amount of heat by small thermal devices will be required for future space systems. Our purpose is to develop miniaturized heat rejection system that can reject more than 100 W/cm². In the evaporator, thin liquid film vaporization which can dissipate very high heat flux, was utilized. The liquid film is stabilized in micro-channels by capillary forces. The microchannels are fabricated by chemical etching on silicon and copper plate. Also miniaturized condenser which utilized droplet condensation was tested. Droplets were produced on a cooled plate covered by non-wetting coating. After we built a heat rejection system constructed by above mentioned evaporator and condenser, influence of heat flux, coolant flow rate, and inlet temperature on the temperature of the heater element were investigated. Water is used as working fluid. Heat flux of 100 W/cm² could be achieved for water inlet temperature in flow rate of 3.0 mL/min. The temperature of the heater element is kept constant at about 120°C.     

Thermal Characteristics of High-Efficiency Photovoltaic Converters of High-Power Laser Light

The influence exerted by the heat removal conditions on the extent of overheating of photovoltaic converters of high-power (>10³ W/cm²) laser light has been studied. The temperature of the p–n junction of photovoltaic cells was measured by recording the instantaneous values of the open-circuit voltage generated by laser light. The effect of cooling in high-efficiency photovoltaic cells (efficiency = 55%) via removal of a substantial part of absorbed optical power by the photocurrent into the external load was demonstrated. It was shown that, at laser radiation power of 2.5 W, the overheating of a photocell with an area of 1.7 × 10^–3 cm² relative to the copper heatsink temperature is 48°C in the no-load conditions and 30°C in operation with the optimal load.     

A novel measurement theory for inventory of working fluid and vacuum pressure of heat pipe

Abstract

Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm² to 20 KW/cm² at extremely fast speeds. Therefore, heat pipes are widely used in 1U servers, notebooks, PCs, etc. A heat pipe is a heat removal device comprising a vacuum pipe that charges a certain amount of working fluid and seals the tube. Hence, the heat pipe performance depends not only on the geometric parameters such as wall thickness, tube material, and wick material but also on the thermal properties of the working fluid such as latent heat, vapor pressure, viscosity, and vacuum pressure.

Traditionally, the fluid inventory of heat pipes was measured by the lost weight method, that is measuring the weight of the heat pipe first, then, breaking the heat pipe, after drying in the oven, then weighting again, and the lost weight would be the weight of the working fluid. This paper presents a new methodological concept to measure the inventory by a basic energy mass balance equation. The Measurement theory not only calculates the fluid inventory but also the vacuum pressure data. The experimental results show that when the weight percentage of working fluid was larger than 10% of total pipe weight, the relative errors were within 4% when compared with the known inventory.     

Critical heat flux in a locally heated liquid film driven by gas flow in a minichannel

The rupture of a liquid film driven by friction with a gas flow in a horizontal minichannel and the heat-exchange crisis in this film locally heated by a 1 × 1 cm source in the channel wall has been experimentally studied. A heat flux of 250 W/cm² is achieved, which is greater by an order of magnitude than the limiting heat flux for a vertically falling liquid film with the same Reynolds number (Re _l = 21). These experiments confirmed good prospects for using gas-flow-driven liquid films in cooling systems of devices with intense local heat evolution.     

A DNA‐Threaded ZIF‐8 Membrane with High Proton Conductivity and Low Methanol Permeability

Natural biomolecules have potential as proton‐conducting materials, in which the hydrogen‐bond networks can facilitate proton transportation. Herein, a biomolecule/metal–organic framework (MOF) approach to develop hybrid proton‐conductive membranes is reported. Single‐strand DNA molecules are introduced into DNA@ZIF‐8 membranes through a solid‐confined conversion process. The DNA‐threaded ZIF‐8 membrane exhibits high proton conductivity of 3.40 × 10^?4 S cm^?1 at 25 °C and the highest one ever reported of 0.17 S cm^?1 at 75 °C, under 97% relatively humidity, attributed to the formed hydrogen‐bond networks between the DNA molecules and the water molecules inside the cavities of the ZIF‐8, but very low methanol permeability of 1.25 × 10^?8 cm² s^?1 due to the small pore entrance of the DNA@ZIF‐8 membranes. The selectivity of the DNA@ZIF‐8 membrane is thus significantly higher than that of developed proton‐exchange membranes for fuel cells. After assembling the DNA@ZIF‐8 hybrid membrane into direct methanol fuel cells, it exhibits a power density of 9.87 mW cm^?2 . This is the first MOF‐based proton‐conductivity membrane used for direct methanol fuel cells, providing bright promise for such hybrid membranes in this application.     

Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer

Materials coating has been proved to be an effective mean to increase the number of active nucleating sites, and therefore generate more vapor bubbles and lead to better pool boiling heat transfer performance. In this work, graphene oxide (GO) is coated on a boiling surface by self-assembly method, to enhance critical heat flux (CHF). The pool boiling is carried out on a smooth copper surface to study the effect of GO coating using distilled water as the working fluid along with bubble dynamic visualization. GO coating facilitates bubble nucleation by providing numerous microscale cavities. The visualization investigation of bubble dynamic behavior shows that the CO-coated surface exhibits a higher bubble departure frequency, a smaller bubble departure diameter and smaller bubble diameters in the pool, indicating greatly enhanced heat transfer effects. Meanwhile, the GO-coated surface exhibited a smaller contact angle than the copper surface, revealing that surface becomes more hydrophilic after GO coating. Consequently, GO-coated surface with a coating time of 4 h provides a CHF of 224.3 W/cm² and a heat transfer coefficient (HTC) of 8.79 W/(cm²·K), representing an improvement of 94.0% in CHF and 83.5% in HTC compared to smooth copper surface.     

Pulse tube cooler for flight hyperspectral imaging

A new version of TRW's miniature pulse tube cooler system maintains the short wave infrared–focal plane array (SWIR–FPA) (with wavelength spectrum of 0.9–2.5 μm in the hyperspectral imaging spectrometer for the Hyperion Instrument) interface at a temperature of 110 K. The cooler provides the nominally required cooling load of 0.84W to the FPA via a cold thermal strap, at 72% stroke consuming 14.7 W of electrical power, when the heat reject temperature is at 300 K. This cooler can operate up to 90% stroke, having 1.5 W cooling load, thus having 79% performance margin for the Hyperion mission. Before the installation and operation of the cooler onto the instrument, both the mechanical and the electronics assemblies underwent the environmental tests of launch vibration, thermal vacuum cycling, and burn-in. The cooler performance in terms of mechanical efficiency, electronics efficiency, load lines, temperature stability, self-induced vibrational force reduction, ripple current reduction, and magnetic radiated emission was measured and are reported here.     

Boiling flow through diverging microchannel

An experimental study of flow boiling through diverging microchannel has been carried out in this work, with the aim of understanding boiling in non-uniform cross-section microchannel. Diverging microchannel of 4° of divergence angle and 146 μm hydraulic diameter (calculated at mid-length) has been employed for the present study with deionised water as working fluid. Effect of mass flux (118–1182 kg/m²-s) and heat flux (1.6–19.2 W/cm²) on single and two-phase pressure drop and average heat transfer coefficient has been studied. Concurrently, flow visualization is carried out to document the various flow regimes and to correlate the pressure drop and average heat transfer coefficient to the underlying flow regime. Four flow regimes have been identified from the measurements: bubbly, slug, slug–annular and periodic dry-out/ rewetting. Variation of pressure drop with heat flux shows one maxima which corresponds to transition from bubbly to slug flow. It is shown that significantly large heat transfer coefficient (up to 107 kW/m²-K) can be attained for such systems, for small pressure drop penalty and with good flow stability.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Phase-field simulation of directional solidification of a binary alloy under different boundary heat flux conditions

Complicated morphologies of directional solidification structures attract a lot of theoretical studies and commercial uses. As known, the boundary heat flux has an important significance to the microstructures of directional solidification. In this article, the interface evolution of directional solidification with different boundary heat fluxes is discussed. In this study, only one interface has heat flow, and Neumann boundary conditions are imposed at the other three interfaces. From the calculated results, it is found that different heat fluxes cause different microstructures in the directional solidification. When the heat flux equal to 18 W/cm², the growth of lengthways side branches is accelerated and the growth of transverse side branches is restrained. At the same time, there is dendritic remelting in the calculating domain. When the heat flux equal to 36 W/cm², the growth of the transverse side branches and the growth of the lengthways side branches compete with each other. When the heat flux equal to 90 or 180 W/cm², the growth of transverse side branches absolutely dominates. The temperature field of dendritic growth is also analyzed and the relation between heat flux and temperature field is found.     

Specific features of operation of a membrane-electrode assembly of an air-hydrogen fuel cell

Specific features of the operation of the membrane-electrode assembly with high catalytic activity that are a part of the simplified design of a low-temperature air-hydrogen fuel cell under conditions of forced and natural convection of air on the cathode are studied. The governing effect of water balance on the specific power of the fuel cell in the stationary mode (～1 h) is shown, and the range of the operating conditions of the cell with self-control is determined. The power of the fuel cell at an efficiency of ～50% and the surface density of platinum on a cathode of ≈0.2 mg/cm² is 200–250 and 100 mW/cm² in the forced and natural air-convection modes, respectively, which is comparable with the advanced results.     

Preparation and characterization of segmented stacking for thermoelectric power generation

Based on the Seebeck effect, thermoelectric generators can convert thermal energy directly into electrical power, which can be applied in waste heat recovery and clean energy generation. In this work, segmented thermoelectric legs were prepared with high-performance thermoelectric materials for the fabrication of multistage thermoelectric generators, which can be utilized in medium temperature energy harvesting. The P-type leg material was Pb_0.94Sr_0.04Na_0.02Te/Bi_0.5Sb_1.5Te₃, and the N-type leg material was Pb_0.94Ag_0.01La_0.05Te/Bi₂Te₃. The length ratio of the two segments was optimized based on the energy conversion efficiency under different working conditions. The segmented legs were measured with the four-probe method at different temperatures to evaluate their output performance. At a temperature difference of 420 K, the maximum output power density was 0.40 W/cm² for the P-type leg and 0.32 W/cm² for the N-type leg.     

3-Methyltrimethylammonium poly(2,6-dimethyl-1,4-phenylene oxide) based anion exchange membrane for alkaline polymer electrolyte fuel cells

Hydroxyl ion (OH^?) conducting anion exchange membranes based on modified poly (phenylene oxide) are fabricated for their application in alkaline polymer electrolyte fuel cells (APEFCs). In the present study, chloromethylation of poly(phenylene oxide) (PPO) is performed by aryl substitution rather than benzyl substitution and homogeneously quaternized to form an anion exchange membrane (AEM). ¹H NMR and FT-IR studies reveal successful incorporation of the above groups in the polymer backbone. The membrane is characterized for its ion exchange capacity and water uptake. The membrane formed by these processes show good ionic conductivity and when used in fuel cell exhibited an enhanced performance in comparison with the state-of-the-art commercial AHA membrane. A peak power density of 111 mW/cm² at a load current density of 250 mA/cm² is obtained for PPO based membrane in APEFCs at 30 °C.     

Effects of High Pressure on the Physical Properties of MgB2

The synthesis of MgB₂-based materials under high pressure gave the possibility to suppress the evaporation of magnesium and to obtain near theoretically dense nanograined structures with high superconducting, thermal conducting, and mechanical characteristics: critical current densities of 1.8?C1.0×10⁶ A/cm² in the self-field and 10³ A/cm² in a magnetic field of 8 T at 20 K, 5?C3×10⁵ A/cm² in self-field at 30 K, the corresponding critical fields being H _c2=15 T at 22 K and irreversible fields H _irr=13 T at 20 K, and H _irr=3.5 T at 30 K, thermal conduction of 53±2 W/(m?K), the Vickers hardness H _V =10.12±0.2 GPa under a load of 148.8 N and the fracture toughness K _1C =7.6±2.0 MPa?m^0.5 under the same load, the Young modulus E=213 GPa. Estimation of quenching current and AC losses allowed the conclusion that high-pressure-prepared materials are promising for application in transformer-type fault current limiters working at 20?C30 K.     

Fatigue performance and residual stresses in laser treated 50CrV4 steel

The effects of continuous CO₂ laser surface quenching of one side of a rectangular section of 50CrV4 steel samples are studied. The treatment is carried out in one or several passes, working with a square beam with a uniform power density of 2550 W cm^–2 at scan rates between 700 and 1400 mm min^–1, with either a single or overlapped sweep of the beam. The magnitude and distribution of internal longitudinal stresses created by the uniform treatment of the surface, relating them to the harness gradients created in depth, conditioned by structural variations, are considered. The uniform treatment of the surface leads to considerable improvement in fatigue performance. In the presence of the thermally affected zones produced by successive sweeps of the beam, however, the noted improvement is largely lost.     

Pinch point analysis and design considerations of CO2 gas cooler for heat pump water heaters

Due to strong nonlinear variation of supercritical CO₂ specific heat capacity with temperature, pinch point would occur in water-cooled CO₂ gas cooler, which has great impacts on the heat transfer characteristics of gas cooler and overall system performance. Pinch point analysis was conducted for CO₂ gas cooler in the present study. The effects of refrigerant pressure, mass flow ratio (m_w/m_c), inlet water temperature and heat transfer area on pinch point location, approach temperature difference and heat transfer rate were analyzed in detail. Based on the analysis of pinch point location in CO₂ gas cooler, the critical flow ratios were proposed to effectively control the approach temperature difference. Furthermore, the actual conductance of gas cooler was calculated and compared with that estimated by LMTD method. The results showed that CO₂ gas cooler may be undersized by as much as a factor of 30–60% for different pressures if LMTD method is used. However, the UA value evaluated by LMTD method also may be overestimated under high refrigerant pressures when the approach temperature difference tends to be zero. Results of the present study are helpful to practical designs of CO₂ gas cooler and heat pump water heaters.     

The properties of InGaAsP/InP heterolasers with step-divergent waveguides

InGaAsP/InP laser heterostructures with step-divergent waveguides and two stressed quantum wells were obtained by metalorganic VPE. The lasers emitting at 1.55 μm provide for an intrinsic quantum yield of η_i=85%. An optical power of 5.2 W in the continuous operation mode was achieved at a laser diode temperature of 10°C. The internal optical losses in the laser heterostructure studied amount to 3.6 cm⁻¹, which is comparable with the level of losses in similar structures with uniform divergent waveguides.     

Numerical Investigation of the Effects of Orientation and Gravity in a Closed Loop Pulsating Heat Pipe

The Closed Loop Pulsating Heat Pipe (CLPHP) is a very promising passive two-phase heat transfer device for relatively high heat fluxes (up to 30 W/cm²) patented by Akachi (1990, 1993). Although the CLPHP has a simple structure, its working principles are very complex compared to the standard heat pipe with a porous wick. One of the most debated issues deals on how the thermal performance is affected by the inclination and by the action of different gravity fields (terrestrial, lunar, martian and microgravity). Even if the internal tube diameter satisfies the conventional slug flow regime requirement on the Bond number, gravity force still plays an important role on the PHP behaviour. Heat input and the number of turns are two of the most important indirect parameters linked to the gravity issue. A complete numerical campaign has been performed by means of a FORTRAN code at different inclination angles and gravity levels on various PHP. The numerical model is able to estimate both the hydrodynamic and the thermal performance of a CLPHP with different working fluids. The analysis shows that the effect of local pressure losses due to bends is important and must be taken into account, in particular in the horizontal operation which is the reference point for space applications. Numerical results are matched with the experimental data quoted in literature and both good qualitative and quantitative agreement have been found.     

Influence of sputtering power on the physical properties of magnetron sputtered molybdenum oxide films

Thin films of molybdenum oxide were formed on glass and silicon substrates by sputtering of molybdenum target under various sputtering powers in the range 2.3–6.8 W/cm², at a constant oxygen partial pressure of 2 × 10⁻⁴ mbar and substrate temperature 523 K employing DC magnetron sputtering technique. The effect of sputtering power on the core level binding energies, chemical binding configurations, crystallographic structure, surface morphology and electrical and optical properties was systematically studied. X-ray photoelectron spectroscopic studies revealed that the films formed at sputtering powers less than 5.7 W/cm² were mixed oxidation states of Mo⁵⁺ and Mo⁶⁺. The films formed at 5.7 W/cm² contained the oxidation state Mo⁶⁺ of MoO₃. Fourier transform infrared spectra contained the characteristic optical vibrations. The presence of a sharp absorption band at 1,000 cm⁻¹ in the case of the films formed at 5.7 W/cm² was also conformed the existence of α-phase MoO₃. X-ray diffraction studies also confirmed that the films formed at sputtering powers less than 5.7 W/cm² showed the mixed phase of α-and β-phase of MoO₃ where as at sputtering power of 5.7 W/cm² showed single phase α-MoO₃. The electrical conductivity of the films increased from 8 × 10⁻⁶ to 1.2 × 10⁻⁴ Ω⁻¹ cm⁻¹, the optical band gap decreased from 3.28 to 3.12 eV and the refractive index decreased from 2.12 to 1.94 with the increase of sputtering power from 2.3 to 6.8 W/cm², respectively.