Impact of coefficients of solar radiation attenuation of translucent cover and radiation absorption of heat-exchange panel on optimal thickness of closed air layer of “Translucent cover—radiation absorbing heat-exchange panel” system of flat solar collectors

Analytic expressions are presented for calculating the optimal (from the heat engineering standpoint) thickness of the closed air layer of the “translucent cover—radiation absorbing heat-exchange panel” system of flat solar collectors, under which the convective heat losses through the examined layer are minimal.     

Calculation of the glass cover temperature and the top heat loss coefficient for 60° vee corrugated solar collectors with single glazing

In this paper, the top heat losses from a 60° vee corrugated solar collector with single glazing have been investigated. An approximate method for computation of glass cover temperature and top heat loss coefficient has been followed. A modified equation from Akhtar and Mullick’s relation was proposed. The predicted values of the glass cover temperature and the top heat loss coefficient were compared with the results obtained by iterative solution of the energy balance equations over a wide range of operating conditions. A good accuracy is provided by the proposed equation which is recommended to be used in the energy analysis of the present configuration.     

Effects of heat absorption and thermal radiation on heat transfer in a fluid–particle flow past a surface in the presence of a gravity field

A continuum two-phase fluid–particle model accounting for fluid-phase heat generation or absorption and thermal radiation is developed and applied to the problem of heat transfer in a particulate suspension flow over a horizontal heated surface in the presence of a gravity field. Analytical solutions for the temperature distributions and the wall heat fluxes for both phases are obtained. Two cases of wall thermal conditions corresponding to stationary and periodic temperature distributions are considered. Numerical evaluations of the analytical solutions are performed and the results are reported graphically to elucidate special features of the solutions. The effects of heat absorption and thermal radiation are illustrated through representative results for the temperature distributions and heat fluxes of both phases for various fluid–particle suspensions. It is found that heat absorption increases the total heat transfer rate for various particulate volume fraction levels while thermal radiation decreases it.     

Influence of the structural and surface characteristics of activated carbon on the catalytic decomposition of hydrogen iodide in the sulfur–iodine cycle for hydrogen production

Coal-based activated carbon (AC-COAL) catalysts subjected to acid treatment were tested to evaluate their performance on hydrogen-iodide (HI) decomposition for hydrogen production in sulfur-iodine (SI or IS) cycle. The effects of acid treatment on catalysts and the relations between sample properties and catalytic activities were discussed. The AC-COAL obtained by non-oxidative acid treatments had the best catalytic activity. However, the catalytic activity of AC-COAL decreased after the treatment of nitric acid. Higher surface area, higher carbon contents, lower ash contents and fewer surface oxidation groups contributed to the catalytic activity of ACs. HI decomposition on the AC surface itself may be due to high densities of unpaired electrons associated with structural defects and edge plane sites with similar structural ordering. Moreover, the oxygen-containing groups reduced the electron transfer capability associated with the basal plane sites.     

Experimental investigation on the thermal efficiency and performance characteristics of a flat plate solar collector using SiO2/EG–water nanofluids

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO₂/ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO₂ nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles.     

Measurements of critical heat flux and liquid–vapor structure near the heating surface in pool boiling of 2-propanol/water mixtures

Saturated pool boiling of 2-propanol/water mixtures on a 12 mm diameter horizontal disk under atmospheric pressure was investigated. The CHF of the mixtures increased up to 1.7 times the CHF of water at 3.0–4.7 mol% concentrations of 2-propanol. To examine the mechanism of the CHF enhancement in the mixtures, liquid–vapor structures close to the heating surface were measured using a conductance probe. It was found that in the boiling of the mixtures, liquid–vapor structures show strong non-uniformity in the radial direction of the heating surface. The void fractions at 0.1–1 mm above the heating surface are small at the central region and large near the periphery of the heating surface. The liquid layer between the vapor mass and the heating surface is considerably thicker than that of water at the central region and becomes thinner near the periphery of the heating surface. This thicker liquid layer is likely to be the cause of the CHF enhancement in the 2-propanol/water mixtures.     

Influence of rib height on the local heat transfer distribution and pressure drop in a square channel with 90° continuous and 60° V-broken ribs

Internal channel cooling is employed in advanced gas turbines blade to allow high inlet temperatures so as to achieve high thrust/weight ratios and low specific fuel consumption. The objective of the present work is to study the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 90° continuous attached and 60° V-broken ribs. The effect of detachment of the rib in case of broken ribs on the heat transfer characteristics is also presented. Reynolds number based on duct hydraulic diameter is ranging from 10,000 to 30,000. A thin stainless steel foil of 0.05 mm thickness is used as heater and infrared thermography technique is used to obtain the local temperature distribution on the surface. The images are captured in the periodically fully developed region of the channel. It is observed that the heat transfer augmentations in the channel with 90° continuous attached ribs increase with increase in the rib height to hydraulic diameter ratio (e/D) but only at the cost of the pressure drop across the test section. The enhancements caused by 60° V-broken ribs are higher than those of 90° continuous attached ribs and also result in lower pressure drops. But, with an increase in the rib height, the enhancements are found to decrease in channel with broken ribs. The effect of detachment incase of broken ribs is not distinctly observed. The heat transfer characteristics degraded with increase in the rib height in both attached and detached broken ribbed cases.     

Production of hydrogen from methanol steam reforming using CuPd/ZrO2 catalysts – Influence of the catalytic surface on methanol conversion and CO selectivity

Electricity generation for mobile applications by proton exchange membrane fuel cells (PEMFCs) is typically hindered by the low volumetric energy density of hydrogen. Nevertheless, nearly pure hydrogen can be generated in-situ from methanol steam reforming (MSR), with Cu-based catalysts being the most common MSR catalysts. Cu-based catalysts display high catalytic performance, even at low temperatures (ca. 250 °C), but are easily deactivated. On the other hand, Pd-based catalysts are very stable but show poor MSR selectivity, producing high concentrations of CO as by-product. This work studies bimetallic catalysts where Cu was added as a promoter to increase MSR selectivity of Pd. Specifically, the surface composition was tuned by different sequences of Cu and Pd impregnation on a monoclinic ZrO₂ support. Both methanol conversion and MSR selectivity were higher for the catalyst with a CuPd-rich surface compared to the catalyst with a Pd-rich surface. Characterization analysis indicate that the higher MSR selectivity results from a strong interaction between the two metals when Pd is impregnated first (likely an alloy). This sequence also resulted in better metallic dispersion on the support, leading to higher methanol conversion. A H₂ production rate of 86.3 mmol h^?1 g^?1 was achieved at low temperature (220 °C) for the best performing catalyst.     

Influence of iridium content on the performance and stability of Pd–Ir/C catalysts for the decomposition of hydrogen iodide in the iodine–sulfur cycle

In this paper, a series of bimetallic palladium–iridium catalysts supported on active carbon (Pd–Ir/C) were prepared by the impregnation-reduction method. Effects of the composition on the performance and stability of bimetallic catalysts in the decomposition of HI were investigated. X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface area were employed to characterize their structure, morphology and surface area, respectively. The results of activity tests indicated that all the bimetallic Pd–Ir catalysts were more active than the monometallic 5% Pd/C and 5% Ir/C catalysts at both 400 °C and 500 °C. Among Pd, Ir and binary Pd–Ir catalysts, the 4% Pd–1% Ir/C was the most active for HI catalytic decomposition at 400 °C. Due to the low cost, high activity and good stability, the carbon supported bimetallic Pd–Ir catalysts are more potential candidates to replace Pt/C for HI decomposition in the IS thermochemical cycle.     

The effect of the Ni/Cu ratio on H-induced ductility loss and its mechanism in Cu–Ni binary alloy system

The effect of the Ni/Cu ratio on hydrogen embrittlement (HE) behavior is studied in the context of the Cu–Ni binary alloy system. When classifying HE sensitivity as a function of the Ni fraction, two regimes are obtained: Regime 1 (wherein Ni fraction is less than 80 wt%) with moderate HE and Regime 2 (where Ni fraction exceeds 80 wt%) with more severe HE, although hydrogen- (H-)induced intergranular (IG) cracking is noted to be the common primary cause of H-induced degradation. Necessities of H-transportation towards grain boundaries (GBs) via H–dislocation interaction and/or dynamic H-diffusion are discussed via the slow strain rate tensile tests at −196 °C. Specifically, in Regime 1, H-transportation is necessary for the triggering of IG cracking. Conversely, in Regime 2, such H-transportation plays a minor role and the initially concentrated hydrogen along GBs can sorely cause IG cracking.     

Decision support system based on a hybrid genetic algorithm–Kohonen map for combined mode conduction–radiation heat transfer in a porous medium: A comparative assessment of three variations of the Kohonen map

A hybrid genetic algorithm (GA)–Kohonen map, with its three variants, is explored for the first time for the decision-making system in a porous ceramic matrix (PCM)-based burner through determination of the regime of operation. Four different attributes of PCMs such as convective coupling (P₂), extinction coefficient (β), downstream porosity (ϕ₂), and scattering albedo (ω) are selected for determining the regime of operation of a PCM-based burner. Changes in any of these attributes of a PCM lead to significant changes in the temperature profiles of the gas and solid phases. Temperature profiles of the gas and solid phases are computed by developing a numerical model. Various samples corresponding to different regimes are generated and used in a hybrid GA–Kohonen map. The best architectural details such as the neuron number and training epochs are obtained from GA as output. The best Kohonen map is trained with the input data, and regimes of operation for new temperature profiles are predicted. A supervised Kohonen map is able to provide the highest average class prediction of more than 40%. All the variants are assessed under two different types of neuron grids: hexagonal and rectangular. Comparative assessments of the three different variants of Kohonen maps, in terms of CPU time and average class prediction, are carried out.     

Study of CO2 stability and electrochemical oxygen activation of mixed conductors with low thermal expansion coefficient based on the TbBaCo3ZnO7+δ system

The influence of different application-oriented factors on the electrochemical activity and stability of TbBaCo₃ZnO_7+δ when used as a solid oxide fuel cell cathode has been studied. Calcination at temperatures above 900 °C (e.g. 1000 °C) leads to a significant increase in the electrode polarization resistance. The effect of the sintering temperature of the TbBaCo₃ZnO_7+δ cathode seems to be more important than the effect produced by the Tb substitution as observed when compared with 900 °C-sintered YBaCo₃ZnO_7+δ; and ErBaCo₃ZnO_7+δ electrode performances. The presence of CO₂ in the air flow leads to an increase of roughly 10% in the polarization resistance for the whole studied temperature range (500-850 °C) while this effect is reversible. Analysis of the impedance spectroscopy measurements shows that the exchange rate constant (k_G from Gerischer element) is significantly affected by CO₂ at temperatures below 700 °C, while the diffusion coefficient related parameter is slightly influenced at low temperatures. Electrode degrades with a low constant rate of 1 mΩ cm² h⁻¹ after 60 h. This cathode material exhibits high CO₂ tolerance, as shown by temperature programmed treatment under a continuous gas flow of air with 5% CO₂, and a relatively low thermal expansion coefficient.     

Effect of zirconium addition on the structure and properties of CuO/CeO2 catalysts for high-temperature water–gas shift in an IGCC system

A series of Ce_xZr_1−xO₂-based Cu catalysts was synthesized by the co-precipitation method. The influences of copper content, zirconium addition, and ratio of ceria to zirconia on the catalytic activity were investigated. BET, N₂O decomposition, XRD, TEM, SEM, EDS, Raman spectroscopy, H₂-TPR, TG/DTA, and XPS were used to characterize the catalysts. The catalytic activity was tested in terms of CO conversion and H₂ selectivity in H₂-rich coal-derived synthesis gas, simulating the actual gas composition of an integrated gasification combined cycle (IGCC) system. The long-term catalyst stability was also examined at 450 °C for 196 h. The addition of zirconium was found to be very important in enhancing catalytic performance. The surface area, copper dispersion, oxygen storage and mobility capacity, reducibility, as well as resistance to sintering all improved after zirconium addition.     

Effect of heat-saving measures on the CO2 savings attributable to micro-combined heat and power (μCHP) systems in UK dwellings

This paper considers the relationship between heat-saving and micro-combined heat and power (μCHP) technological interventions for reducing the carbon footprint of existing domestic dwellings within the UK housing stock. The relationship between the annual heat requirement of individual dwellings and the CO₂ savings attributable to different μCHP systems is investigated (by means of predictive modelling based on heat and power demand datasets recorded on a 1-min time base for nine dwellings). An assessment is made of the effects of various heat-saving measures upon the annual CO₂ savings predictions for candidate μCHP system implementations, when applied to ‘domestic building variants’ (as defined within the Carbon Vision TARBASE research programme). The increasing application of heat-saving interventions serves to reduce the CO₂ savings solely attributable to a μCHP system. The magnitude of this effect is a function of the μCHP system's electrical efficiency and electrical power output. For example, a 1 kW prime mover of 10% electrical efficiency is predicted to reduce annual CO₂ emissions by 72 kg CO₂ for a dwelling with an annual heat requirement of 11.9 MWh, but if the identified set of heat-saving measures is implemented first the demand falls to 5.0 MWh and the μCHP system will actually result in an emissions increase of 100 kg CO₂ p.a. By comparison, relative savings of 467 and 294 kg CO₂ p.a. are predicted if this dwelling is fitted with a 1 kW prime mover of 30% electrical efficiency. Still greater savings are predicted for higher power output systems of high efficiency, but a relatively large proportion of the generated electricity (44–75% depending on the heat and electrical demand of the dwelling) must then be exported.     

Reversible hydrogen storage in the Li–Mg–N–H system – The effects of Ru doped single walled carbon nanotubes on NH3 emission and kinetics

In this study, the LiNH₂–MgH₂ (2:1.1) complex hydride system (Li–Mg–N–H is investigated in terms of hydrogen ab/desorption kinetics and the concomitant NH₃ emission levels. By selecting more intense ball milling parameters, the hydrogen ab/desorption kinetics were improved and the NH₃ emission reduced. However, it is shown that NH₃ emission cannot be completely eliminated during ball milling. Single walled carbon nanotubes (SWCNTs) and 20 wt.% Ru doped SWCNTs are utilized as catalysts to study their effects on NH₃ emission and kinetics characteristics of the Li–Mg–N–H system. The SWCNT doped sample did not show any kinetics improvement, whereas the SWCNT-20Ru doped sample showed similar kinetics performance as that of the base sample. More importantly, the presence of SWCNT increased the NH₃ emission as compared to the base sample. On the other hand, SWCNT-20Ru doping reduced the NH₃ emission compared to the SWCNT doping, but did not eliminate it completely. As revealed from the mass spectrometry signals, the SWCNT-20Ru catalyst starts to decompose NH₃ at a temperature as low as 200 °C.     

Comment on “Effects of chemical reaction on free convective flow of a polar fluid through a porous medium in the presence of internal heat generation” by Patil and Kulkarni,Int. J. Thermal Sci. 47 (2008) 1043–1054

In this Letter we show that the paper by Patil and Kulkarni [P.M. Patil, P.S. Kulkarni, Effects of chemical reaction on free convective flow of a polar fluid through a porous medium in the presence of internal heat generation, Int. J. Thermal Sci. 47 (2008) 1043–1054] contains two errors which render their conclusions invalid. The first of these is a simple sign error. But the second involves the omission of part of the viscous dissipation term. We also show that the inclusion of this term yields an equation for the temperature field which cannot satisfy both the imposed boundary conditions.     

Comment on the paper “Heat transfer enhancement in hydromagnetic dissipative flow past a moving wedge suspended by H2O-aluminum alloy nanoparticles in the presence of thermal radiation,Umar Khan,Adnan, Naveed Ahmed,Syed Tauseef Mohyud-Din” [Int J Hydrogen Energy 42 (2017) 24634–24644]

The present comment concerns some doubtful results included in the above paper.