Study of Nanofluid Natural Convection Phenomena in Rectangular Enclosures

Buoyancy induced flows in rectangular enclosures using nanofluids were investigated. The effects of mass fraction concentration of nanoparticles, enclosure aspect ratio and inclination were observed. The nanofluid under investigation was a water-based alumina nanofluid. Since water exhibits an anomalous density extremum near 4°C the additional effect of buoyancy force reversal will also be observed. The opacity of nanofluid does not permit the use of particle image velocimetry, laser induced fluorescence or any other means of flow visualization or visual temperature measurement of the local fluid temperature. Therefore to investigate the temperature field a non-invasive method, namely ultrasound thermometry, will be used to observe the temperature field. The experimental enclosure was validated using water as the initial fluid; measured values of the local fluid temperature were compared with numerical simulations utilizing COMSOL Multiphysics. Nanofluid mass fractions of 10% and 25% were used for comparative purposes of the effects of concentration on the temperature field. Buoyancy force reversal effects were witnessed in both 10% and 25% concentrations. The nanofluid also prolonged the multicellular effects that occur in buoyancy inversion flows. A Rayleigh number inversion was observed for the 25% mass fraction nanofluid. The multicellular regime transitions to boundary layer regime at about Ra=1E+07 when the aspect ratio is 2.625 and at about Ra=2E+08 when the aspect ratio is 1.000, for different concentrations of nanofluid. For these concentrations of nanofluid and aspect ratio equal to 2.625, instability in the core region occurred at about Ra=1.2E+07.

Process Model for the Production of Syngas via High Temperature Co-Electrolysis

A process model has been developed to evaluate the potential performance of a large-scale high-temperature co-electrolysis plant for the production of syngas from steam and carbon dioxide. The co-electrolysis process allows for direct electrochemical reduction of the steam-carbon dioxide gas mixture, yielding hydrogen and carbon monoxide, or syngas. The process model has been developed using the Honeywell UniSim systems analysis code. Using this code, a detailed process flow sheet has been defined that includes all the components that would be present in an actual plant such as pumps, compressors, heat exchangers, turbines, and the electrolyzer. Since the electrolyzer is not a standard UniSim component, a custom one-dimensional co-electrolysis model was developed for incorporation into the overall UniSim process flow sheet. The one dimensional co-electrolysis model assumes local chemical equilibrium among the four process-gas species via the gas shift reaction. The electrolyzer model allows for the determination of co-electrolysis outlet temperature, composition (anode and cathode sides); mean Nernst potential, operating voltage and electrolyzer power based on specified inlet gas flow rates, heat loss or gain, current density, and cell area-specific resistance. The one-dimensional electrolyzer model was validated by comparison with results obtained from a fully three dimensional computational fluid dynamics model developed using FLUENT, and by comparison to experimental data. This paper provides representative results obtained from the UniSim flow sheet model for a 300 MW co-electrolysis plant, coupled to a high-temperature gas-cooled nuclear reactor. The co-electrolysis process, coupled to a nuclear reactor, provides a means of recycling carbon dioxide back into a useful liquid fuel. If the carbon dioxide source is based on biomass, the entire process would be climate neutral.

An Air-Brayton Nuclear-Hydrogen Combined-Cycle Peak- and Base-Load Electric Plant

A combined-cycle power plant is proposed that uses heat from a high-temperature nuclear reactor and hydrogen produced by the high-temperature reactor to meet base-load and peak-load electrical demands. For base-load electricity production, air is compressed; flows through a heat exchanger, where it is heated to between 700 and 900°C; and exits through a high-temperature gas turbine to produce electricity. The heat, via an intermediate heat-transport loop, is provided by a high-temperature reactor. The hot exhaust from the Brayton-cycle turbine is then fed to a heat recovery steam generator that provides steam to a steam turbine for added electrical power production. To meet peak electricity demand, after nuclear heating of the compressed air, hydrogen is injected into the combustion chamber, combusts, and heats the air to 1300°C—the operating conditions for a standard natural-gas-fired combined-cycle plant. This process increases the plant efficiency and power output. Hydrogen is produced at night by electrolysis or other methods using energy from the nuclear reactor and is stored until needed. Therefore, the electricity output to the electric grid can vary from zero (i.e., when hydrogen is being produced) to the maximum peak power while the nuclear reactor operates at constant load. Because nuclear heat raises air temperatures above the auto-ignition temperatures of the hydrogen and powers the air compressor, the power output can be varied rapidly (compared with the capabilities of fossil-fired turbines) to meet spinning reserve requirements and stabilize the grid.

An Experimental Investigation of the Auto-Ignition Properties of Two C5 Esters: Methyl Butanoate and Butyl Methanoate

Ignition studies of two small esters were performed using a rapid compression facility (RCF). The esters (methyl butanoate and butyl methanoate) were chosen to have matching molecular weights, and C:H:O ratios, while varying the lengths of the constituent alkyl chains. The effect of functional group size on ignition delay time was investigated using pressure time-histories and high speed digital imaging. The mixtures studied covered a range of conditions relevant to oxygenated fuels and fuel additives, including bio-derived fuels. Low temperature and moderate pressure conditions were selected for study due to their relevance to advanced low temperature combustion strategies, and internal combustion engine conditions. The results are discussed in terms of the reaction pathways affecting the ignition properties.

Plastic Mesoscale Combustors/Heat Exchangers

Recent experimental and theoretical studies of heat-recirculating combustors have demonstrated the importance of thermal conduction through the structure of the combustor on its performance. In particular, this solid-phase heat conduction inevitably degrades performance via transfer of heat out of the reaction zone to the surrounding structure, which is then lost to ambient. This in turn leads to a reduction of reaction temperature and thus sustainable reaction rates. By use of platinum-based catalysts in spiral counterflow “Swiss roll” heat-recirculating combustors, we have been able to sustain nearly complete combustion of propane-air mixtures at temperatures less than 150 °C using combustors built with titanium (thermal conductivity (k) of 7 W/m°C). Such low temperatures suggest that high-temperature polymers (e.g. polyimides, k ∼ 0.3 W/m°C) may be employed as a combustor material. With this motivation, a polyimide Swiss roll combustor was built using CNC milling and tested over a range of Reynolds numbers with propane fuel and Pt catalyst. The combustor survived prolonged testing at temperatures up to 450 °C. Reynolds numbers as low as 2 supported combustion, with thermal power as low as 3 watts and temperatures as low as 72 °C. These initial results suggest that polymer combustors may prove more practical for meso- or microscale thermochemical devices due to their lower thermal conductivity and ease of manufacturing. Applications to electric power generation via single-chamber solid oxide fuel cells are discussed.

An Alternative Field Test for Spot Air Conditioning Units

U.S. governmental standards require that newly produced air conditioners have a SEER (seasonal energy efficiency ratio) rating of over 13, Federal Register (2001), [1]. This rating is closely tied to the COP (coefficient of performance). In fact, the SEER is 3.792 times the COP. Since COP varies with temperature loads, a standard testing method requires the unit to be tested at standard conditions of temperature and humidity. This requires the use of expensive climate control chambers, where the system can be loaded to the specified temperatures. The scope of this paper proposes a simpler, less expensive method to test spot AC (air conditioning) units, as an alternative field test to ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Standard 128 (2001), [2]. By taking temperature measurements of the appropriate control volume, the COP can be calculated. To obtain steady state, the control volume will be treated as very large (or infinite), placing the unit to be tested outdoors or in a room big enough so that delta T will remain constant. Themocouples in conjunction with data logging software are used to take the temperature measurements, and the mass flow rate is measured by assuming uniform flow and placing a flow meter in the center of the air exhaust, on the evaporator side. The entire system can be assembled into a portable unit composed of a computer, thermocouples, flow meter and a digital multimeter, alternatively, a handheld relative humidity and temperature sensor can be used, ASHRAE (2003), [3]. This would allow not only testing of units before they go into production, but having technicians in the field test the efficiency of units already in operation. The need may be there since there could be a significant drop in the SEER between factory conditions and installed unit, due to variations in duct sizes, losses due to non-ideal installations. Owing to the fact that the COP varies with loading, and our testing method requires no artificial control over loading temperatures, the current study is being conducted to find if the AC unit can perform up to its rating. The second law COP at environmental loading conditions is also evaluated for each of the five AC units tested. The calculated COPII (based on exergy) of the AC units tested do not vary as much (percentage-wise) as the rated COP. Their relative detrimental effects to the environment are probably not that much different from each other.

Three-Dimensional Fluid Flow and Coupled Heat Transfer in Simplified Bipolar Plates

Numerical simulations were performed for three-dimensional fluid flow and coupled heat transfer in simplified bipolar plates. The Reynolds number of inlet flow is varied from 100 to 900 on the anode side while the Reynolds number is maintained as a constant of 100 on the cathode side. The solid wall surfaces of the bipolar plates are assumed to be adiabatically insulated, except that the active areas of the channels are supplied with uniform heat flux. Results of velocity and temperature distributions for different Reynolds numbers are presented and discussed. It is shown that effects of flow pattern on temperature distributions in channels becomes negligible when the Reynolds number is as high as 900.

A Scenario for a Secure Transportation System Based on Fuel From Biomass

This article presents a scenario to meet the future fuel needs of the US ground transportation system that does not require hydrogen, can use existing technology and eventually transition to ethanol from biomass. This scenario is based on a combination of reduction of liquid fuel use by means of plug-in hybrid electric vehicles and generation of ethanol from biomass. The article also demonstrates the reduction in CO2 generation with this technology and the urgency of initiating a strategy for reducing gasoline consumption as soon as possible.

Transient Simulations for Economical Analysis of Energy Systems in Industry and Buildings

In this paper, criteria which indicate the usage of transient models and dynamic simulation environments for such energy systems are presented. A complex energy system for heating and cooling of industrial facilities and industrial processes is presented as a reference model. A model of a hot water storage tank is presented, which is optimized for the simulation in whole years, in which a very accurate transient response at much quicker simulation times compared to conventional geometric models can be delivered. The model was validated with measurement data from a large cogeneration plant. In addition, the economical impact of system simulation is emphasized on by an optimization study carried out on a large industrial system. Furthermore, the impact of a transient system model is compared to that of a steady state approach of the same system.

Thermodynamic Analysis of the CO2 Gas Removal Process

The present paper describes the thermodynamic analysis of the carbon dioxide (CO2) gas removal process in two separated columns with absorption/stripping sections respectively. This process is characterized as mass transfer enhanced by chemical reaction, in which the presence of an alkanolamine enhances the solubility of an acid gas in the aqueous phase at a constant value of the equilibrium partial pressure. A very useful procedure for analyzing a process is by means of the Second Law of Thermodynamics. Thermodynamic analyses based on the concepts of irreversible entropy increase have frequently been suggested as pointers to sources of inefficiency in chemical processes. Furthermore, they point out where the irreversibilities of the process are located, and provide a generalized discussion from the successful application of the technique.