Heat transfer enhancement in a small pipe by spinodal decomposition of a low viscosity,liquid–liquid,strongly non-regular mixture

We report experimental evidence of a 20–40 % enhancement of the effective heat transfer coefficient for laminar flow of a partially miscible binary liquid–liquid mixture in a small diameter horizontal tube that obtains when phase separation occurs in the tube. A mixture of acetone–hexadecane is quenched into the two-phase region so as to induce spinodal decomposition. The heat transfer rate is enhanced by self-induced convective effects sustained by the free energy liberated during phase separation. The experimental heat transfer coefficients obtained when separation occurs are compared to the corresponding values predicted for flow of a hypothetic mixture with identical properties but undergoing separation. For such comparison, the energy balance equation must carefully take into account both the sensible heat and the excess enthalpy difference between the inlet and the outlet streams because our liquid–liquid binary mixture is a very asymmetric system with large excess enthalpies. The non-ideal mixture thermodynamic properties needed for the energy balance are obtained by an empirical procedure from the experimental data available in the literature for our mixture. The experimental setup and calculation procedure is tested by experiments performed using single-phase water flow and single-phase mixture flow (above the critical point). The convective heat transfer augmentation that results in the presence of liquid–liquid phase separation may be exploited in the cooling or heating of small scale systems where turbulent convection cannot be achieved.     

“Investments and public finance in a green,low carbon,economy”

The paper evaluates the impacts on investments and public finance of a transition to a green, low carbon, economy induced by carbon taxation. Four global tax scenarios are examined using the integrated assessment model WITCH. Taxes are levied on all greenhouse gases (GHGs) and lead to global GHG concentrations equal to 680, 560, 500 and 460 ppm CO₂-eq in 2100. Investments in the power sector increase with respect to the Reference scenario only with the two highest taxes. Investments in energy-related R&D increase in all tax scenarios, but they are a small fraction of GDP. Investments in oil upstream decline in all scenarios. As a result, total investments decline with respect to the Reference scenario. Carbon tax revenues are high in absolute terms and as share of GDP. With high carbon taxes, tax revenues follow a “carbon Laffer” curve. The model assumes that tax revenues are flawlessly recycled lump-sum into the economy. In all scenarios, the power sector becomes a net recipient of subsidies to support the absorption of GHGs. In some regions, with high carbon taxes, subsidies to GHG removal are higher than tax revenues at the end of the century.     

Material performance of age-hardened beryllium–copper alloy,CDA-C17200, in a high-pressure,gaseous hydrogen environment

In order to develop safer and more energy-efficient, hydrogen pre-cooling systems for use in hydrogen refueling stations, it is necessary to identify a high-strength metallic material with greater thermal conductivity and lower susceptibility to hydrogen embrittlement, as compared with ordinary, stable austenitic stainless steels. To accomplish this task, the hydrogen compatibility of a precipitation-hardened, high-strength, copper-based alloy was investigated by slow-strain-rate tensile (SSRT), fatigue-life, fatigue-crack-growth (FCG) and fracture toughness tests in 115-MPa hydrogen gas at room temperature. The hydrogen solubility and diffusivity of the alloy were also determined. The hydrogen solubility of the alloy was two or three orders of magnitude lower than that of austenitic stainless steels. The alloy also demonstrated absolutely no hydrogen-induced degradation of its strength properties, a factor which could contribute to the reduction of costs related to the construction and maintenance of hydrogen refueling stations, owing to the downsizing and improved cooling performance of the pre-cooling systems.     

Biosynthesis of β-caryophyllene,a novel terpene-based high-density biofuel precursor,using engineered Escherichia coli

β-caryophyllene is a common sesquiterpene compound currently being studied as a promising precursor for the production of high-density fuels. Acute demand for high-density fuels has provided the impetus to pursue biosynthetic methods to produce β-caryophyllene from reproducible sources. In this study, we produced β-caryophyllene by assembling a biosynthetic pathway in an engineered Escherichia coli strain of which phosphoglucose isomerase gene has been deleted. The 1- deoxy-d-xylulose 5-phosphate (DXP) or heterologous mevalonate (MVA) pathways were employed. Meanwhile, geranyl diphosphate synthase, glucose-6-phosphate dehydrogenase and β-caryophyllene synthase genes were co-overexpressed in the above strain. The final genetically modified strain, YJM59, produced 220 ± 6 mg/L of β-caryophyllene in flask culture. We also evaluated the use of fed-batch fermentation for the production of β-caryophyllene. After induction for 60 h, the YJM59 strain produced β-caryophyllene at a concentration of 1520 mg/L. The volumetric production fermented in the aerobic fed-batch was 0.34 mg/(L·h·OD₆₀₀) and the conversion efficiency of glucose to β-caryophyllene (gram to gram) was 1.69%. Our results are the first successful attempt to produce β-caryophyllene using E. coli BL21(DE3), and provide a new strategy that is green and sustainable for the production of β-caryophyllene.     

Discrete ejector control solution design,characterization, and verification in a 5 kW PEMFC system

An ejector primary gas flow control solution based on three solenoid valves is designed, implemented and tested in a 5 kW proton exchange membrane fuel cell (PEMFC) system with ejector-based anode gas recirculation. The robust and cost effective combination of the tested flow control method and a single ejector is shown to achieve adequate anode gas recirculation rate on a wide PEMFC load range.In addition, the effect of anode gas inert content on ejector performance in the 5 kW PEMFC system is studied at varying load and anode pressure levels. Results show that increasing the inert content increases recirculated anode gas mass flow rate but decreases both the molar flow rate and the anode inlet humidity.Finally, the PEMFC power ramp-rate limitations are studied using two fuel supply strategies: 1) advancing fuel supply and venting out extra fuel and 2) not advancing fuel supply but instead using a large anode volume. Results indicate that the power of the present PEMFC system can be ramped from 1 kW to 4.2 kW within few hundred milliseconds using either of these strategies.     

Shock—Boundary Layer Interaction Control,Predictions Using a Viscous—Inviscid Interaction Procedure and a Navier—Stokes Solver

INTRODUCTIONTwomethodsarepresentedhereforthecalculationoftransonicfiowsarotmdairfoilsincludingshock-controldevices.Theuseofblowingandsuction,tIiroughaperforatedplatebelowtheshock-boundarylayerioteractionzone3ispresentinordertominidrizethewavedrag,duetothepresenceoftheshock,topreventtheshock-inducedseparationandinordertoalleviatetheoccurrenceoftheannoyingbthetphesnomenon.Theaimofthispaperistoinvestigatewaysofpredictingthecomplicatedphenomenapreseotintheshock-boundarylayerillteractionregion…     

Analysis of Natural Convection in an Elliptical Cavity,Using a Dini Expansion

INTRODUCTIONNaturalconvectioninanellipticalcavityheatedfrombelowwastreatednumericallybyM..him.ull],usingaFourierspectralfinitedifferencemethod,anditseffectivenesswasshownasinRef.[2].AlsopossibilityoftheextensiontovarioustypesofboundaryconditionsforthespectralfinitedeferenceschemewasproposedinReL[2-3].HerethespectralfinitedifferenceschemeisbeingextendedtoadoptDimexpansions(akindofBesselexpansions).NUMERICALANAlySISBasicAssumptionsbleatedistransiellttwo--dimensionalnaturallaminarco…     

The Initial, Boundary Value Problems for a Class of Generalized Diffusion Equations

The behavior for a class of initial, boundary value problems of generalized diffusion equations was studied utilizing the similarity transformation and shooting technique. Numerical solutions are presented for k(s)=s ^M, exponent M=1.0 to 5.0, and power law parameter N (N=0.3 to 3.0). The results shown that for each fixed M, the temperature distribution θ decreases with increasing in power law parameter N, and for each fixed N, the temperature distribution θ increases with the decreasing of M.     

Effects of dust,soiling, aging,and weather conditions on photovoltaic system performances in a Saharan environment—Case study in Algeria

Solar photovoltaic (PV) power represents one of the most promising future sources of energy in the world. Considered the cleanest form of energy, extensive research is being undertaken to widen its use. Notably, mega projects are being considered for installation in the Middle East and North Africa (MENA) region because of its high solar potential, with hopes of eventually feeding Europe from the PV electricity generated in this region and transported through high voltage direct current (DC) lines. However, current implementation of PV systems has shown that their reliability and efficiency depend upon surrounding environmental factors, such as the ambient temperature, wind, and rainfall, as well as soiling, pollution, and aging. The aim of this study was to investigate, through experimental tests, the effects of such factors on the power output of a grid connected PV station. The results showed that the output power and efficiency are deeply affected by various environmental factors, which are weather dependent. These findings may help us develop appropriate solutions to overcome these drawbacks.     

Performance of a triple-pressure-level absorption cycle with R125-N,N′-dimethylethylurea

In the developed triple-pressure-level (TPL) single stage absorption cycle, a specially designed jet ejector was introduced at the absorber inlet. The device served two major functions: it facilitated pressure recovery and improved the mixing between the weak solution and the refrigerant vapour coming from the evaporator. These effects enhanced the absorption of the refrigerant vapour into the solution drops. To facilitate the design of the jet ejector for such absorption machines, a numerical model of simultaneous heat-and-mass transfers between the liquid and the gas phases in the ejector was developed. The refrigerant pentafluoroethane (R125) and the absorbent N,N′-dimethylethylurea (DMEU) were used as the working fluid. A computerized simulation program was used to perform a parametric study of the TPL absorption cycle. The influence of the jet ejector on the performance of the TPL absorption cycle was evaluated, and the performance of the TPL absorption cycle was compared with that of a double-pressure level (DPL) cycle. Four cases were studied that represent the improvements in the TPL absorption cycle performances as a result of the incorporation of the jet ejector. The four cases are: the ability to reduce the circulation ratio f, the ability to lower the evaporator temperature, the ability to lower the generator temperature and the ability to use higher-temperature cooling water.     

A New Computation Method for Solidification Process in a Finite,Initially Overheated Slab

An approximate theory is presented for solidification in a finite,initially overheated slab in which one wall isinsulated and the other is subject to an instantaneous temperature drop below the freezing point.The simpleexpressions for the position of the phase change front as a function of time are derived.The results are com-pared with experimental data,numerical and approximate solutions as well as exact solution presented in otherliterature,and good agreement is attained.It is shown that the approximate method proposed herein is alsovalid for phase change problems with large Stefan numbers and different boundary conditions.     

Multi-criteria optimization of a district cogeneration plant integrating a solid oxide fuel cell–gas turbine combined cycle,heat pumps and chillers

A simultaneous optimization of the design and operation of a district heating, cooling and power generation plant supplying a small stock of residential buildings has been undertaken with regards to cost and CO₂ emissions. The simulation of the plant considers a superstructure including a solid oxide fuel cell–gas turbine combined cycle, a compression heat pump, a compression chiller and/or an absorption chiller and an additional gas boiler. The Pareto-frontier obtained as the global solution of the optimization problem delivers the minimal CO₂ emission rates, achievable with the technology considered for a given accepted investment, or respectively the minimal cost associated with a given emission abatement commitment.     

Non-linear,unsteady free convection in a vertical cylinder submitted to a horizontal thermal gradient: measurements in water between 6 and 21°C and a theoretical model of convection

The non-linear, unsteady behaviour of water contained in a vertical cylinder of yellow brass when submitted to a horizontal initial thermal gradient is investigated by following the temperature decay in the centre of a cylinder. Experimental results are interpreted by means of a theoretical model which allows the deduction of equations for temperature, velocity, pressure and density in the nucleus. The new equations are compared with those of conduction to provide an evaluation of the convective contribution to heat transfer. Our data indicate that when a characteristic dimensionless group which has the form of a Rayleigh number reaches a critical value of 1600 ± 50, the heat transfer may be described by a pure conduction equation.     

Solar-driven steam-based gasification of sugarcane bagasse in a combined drop-tube and fixed-bed reactor – Thermodynamic,kinetic, and experimental analyses

Syngas production via steam-based thermochemical gasification of Brazilian sugarcane bagasse, using concentrated solar energy for process heat, was thermodynamically and experimentally investigated. Energy and exergy analyses revealed the potential benefits of solar-driven over conventional autothermal gasification that included superior quality of syngas composition and higher yield per unit of feedstock. Reaction rates for the gasification of fast pyrolyzed bagasse char were measured by thermogravimetric analysis and a rate law based on the oxygen exchange mechanism was formulated. In order to provide residence times long enough for adequate char conversion, a laboratory-scale entrained flow reactor that combines drop-tube and fixed-bed concepts was developed. Testing was performed in an electric furnace with the final aim to supply heat by concentrated solar radiation. Experimental runs at reactor temperatures of 1073–1573 K and a biomass feed rate of 0.48 g/min yielded high-quality syngas of molar ratios H₂/CO = 1.6 and CO₂/CO = 0.31, and with heating values of 15.3–16.9 MJ/kg, resulting in an upgrade factor (ratio of heating value of syngas produced over that of the feedstock) of 112%. Theoretical upgrade factors of up to 126%, along with the treatment of wet feedstock and elimination of the air separation unit, support the potential benefits of solar-driven over autothermal gasification.     

Heat Transfer to a Particle Exposed to a Rarefied Plasma with a Great Temperature Gradient

A kinetic-theory analysis is presented concerning the heat transfer from a rarefied plasma to a spherical particle for the extreme case of free-molecule regime and thin plasma sheath. A great temperature gradient is assumed to exist in the plasma, and thus a non-Maxwellian velocity distribution function is employed for each of the gas species. Analytical results show that the existence of a temperature gradient in the plasma causes a nonuniform distribution of the local heat flux density on the sphere surface, while the total heat flux to the whole particle is independent of the temperature gradient. The nonuniformity of the local heat flux distributioln is small even for the case with a temperature gradient as great as 10~6 K/m, but it may significantly enhance the thermophoretic force on an evaporating particle. Heat transfer is mainly caused by atoms at low gas temperatures with negligible ionization degree, while it can be attributed to ions and electrons at high plasma temperatures.     

Thermodynamic analysis of a combined PV/T–fuel cell system for power,heat, fresh water and hydrogen production

A hybrid renewable energy system is proposed and analyzed for electricity, heated air, purified water and hydrogen production. Energy, exergy and economic analyses are performed to analyze and determine the performance of the system under different operating conditions. The photovoltaic/thermal (PV/T) system produces heat and electricity for residential applications. Excess power is used to operate electrolyser which produces hydrogen to be fed directly to a fuel cell. Fuel cell is operated during high power demand, and it produces electricity, heat and water for residential applications. The water produced as a by-product by the fuel cell is used for drinking water supply. The parametric studies are conducted to determine the efficiencies of the system with and without fuel cell network for hot air, power and purified water. When fuel cell heat is used, the overall system efficiency increases to 5.65% for energy and 19.8% for exergy. Up to 80 L of drinkable water can be collected from the fuel cell when operated for extended periods. The present study confirms a significant economic gain when fuel cell heat and water are utilized as useful outputs.     

Thermoeconomic assessment of a multi-engine,multi-heat-pump CCHP (combined cooling,heating and power generation) system – A case study

Design and operation of complex systems for combined cooling, heating and power generation (CCHP) are always a matter of matching performance and demand characteristics of a thermal system set to supply electrical, cooling and heating loads, according to specific usage demands. Equipment selection and operation require the characterization of power, heating and cooling load demands, and their time variation during years, seasons, months and even hours or minutes. The paper aims at utilizing a general model for complex CCHP systems. The proposed model is based on the general theory of exergy cost and structural coefficients of internal links. A general model is presented, and a simple hypothetical cogeneration case is studied. The system operates with two heat engines, with waste heat recovery driving a chiller, in order to meet electrical power and refrigeration loads.