Three-dimensional numerical study on fin-and-oval-tube heat exchanger with longitudinal vortex generators

Three-dimensional numerical study was performed for heat transfer characteristics and fluid flow structure of fin-and-oval-tube heat exchangers with longitudinal vortex generators (LVGs). For Re (based on the hydraulic diameter) ranges from 500 to 2500, it was found that the average Nu for the three-row fin-and-oval-tube heat exchanger with longitudinal vortex generators increased by 13.6–32.9% over the baseline case and the corresponding pressure loss increased by 29.2–40.6%. The results were analyzed on the basis of the field synergy principle to provide fundamental understanding of the relation between local flow structure and heat transfer augmentation. It was confirmed that the reduction of the intersection angle θ between the velocity field and the temperature field was one of the essential factors influencing heat transfer enhancement. Three geometrical parameters – placement of LVGs (upstream and downstream), angles of attack (α = 15°, 30°, 45° and 60°) and tube-row number (n = 2, 3, 4 and 5) – were also investigated for parameter optimization. The LVGs with placement of downstream, angles of attack α = 30° and minimum tube-row number provide the best heat transfer performance. The effects of the three geometrical parameters on heat transfer enhancement were also analyzed from the view point of the field synergy principle and it was found that the results can be well explained by the field synergy principle.     

Biogas reforming for hydrogen-rich syngas production over a Ni–K/Al2O3 catalyst using a temperature-controlled plasma reactor

Plasma-enhanced catalytic biogas reforming for hydrogen-rich syngas production over a Ni–K/Al₂O₃ catalyst was investigated using a tabular dielectric barrier discharge non-thermal plasma reactor. To better understand the plasma catalysis synergy at elevated temperatures, we compared different reaction modes: plasma catalysis, plasma alone, and catalysis alone in a reaction temperature range of 160–400 °C. The combination of Ni–K/Al₂O₃ and plasma produced synergistic effects. Notably, the plasma-catalytic synergy was temperature-dependent and varied at different reaction temperatures. Using plasma catalysis, the maximum conversion of CH₄ and CO₂ (31.6% and 22.8%, respectively) was attained over Ni–K/Al₂O₃ at 160 °C, while increasing the reaction temperature to 340 °C noticeably enhanced the H₂/CO ratio to 2.71. Moreover, compared to plasma-catalytic biogas reforming at 160 °C, increasing the reaction temperature to 400 °C suppressed biogas conversion with dramatically reduced coke formation on the Ni–K/Al₂O₃ surface from 6.81 wt% to 3.37 wt%.     

Modeling study on a two-stage hydrogen purification process of pressure swing adsorption and carbon monoxide selective methanation for proton exchange membrane fuel cells

A two-stage hydrogen purification process based on pressure swing adsorption (PSA) and CO selective methanation (CO-SMET) is proposed to meet the stringent requirements of H₂-rich fuel for kW-scale skid-mounted or distributed proton exchange membrane fuel cell systems. The reforming gas is purified using dynamic adsorption model of PSA with activated carbon for initial purification and then kinetic model of CO-SMET with 50 wt% Ni/Al₂O₃ for CO deep removal. Sensitive analyses of the gas hourly space velocity, adsorption time and adsorption pressure etc. are studied. The results show that excellent H₂ purity and CO concentration below 1000 ppm for the initial target using the three-bed and four-bed PSA system at shorter adsorption time and higher pressure, and then CO concentration below 10 ppm with H₂ purity over 99.94% on CO-SMET. This work provides a small-scale and hydrogen-saving process for hydrogen purification can be achieved by the two-stage process.     

Investigation on laminar convection heat transfer in fin-and-tube heat exchanger in aligned arrangement with longitudinal vortex generator from the viewpoint of field synergy principle

3-D numerical simulation results are presented for laminar flow heat transfer of the fin-and-tube surface with vortex generators. The effects of Reynolds number (from 800 to 2000), the attack angle (30° and 45°) of delta winglet vortex generator are examined. The numerical results are analyzed from the viewpoint of field synergy principle. It is found that the inherent mechanism of heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, the second flow generated by the vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. In addition, the heat transfer enhancement of delta winglet with the attack angle of 45° is larger than that of 30°, while the delta winglet with the attack angle of 45° results in an increase of the pressure drop, however, the delta winglet with the attack angle of 30° results in a slight decrease.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Phase-separated Ce–Co–O catalysts for CO oxidation

As a major gas pollutant, the control of CO gas emission from chemical industry and vehicle exhausts has aroused global attention in the past years and it is extremely essential to develop efficient, low-cost and environmental friendly catalysts for CO conversion and removal. In this work, we report the facile synthesis of Ce–Co–O catalysts through a simple ultrasonic spray pyrolysis process and investigate their application for low-temperature CO oxidation. The Ce–Co–O catalysts comprising of separated CeO₂ and Co₃O₄ phases show superior CO oxidation capability below 473 K (200 °C) without the assistance of any other co-catalyst or noble metal. With the increase of Co₃O₄ concentration, the CO oxidation temperature of Ce–Co–O catalysts decreases quickly and reaches a complete conversion temperature of 410 K (137 °C) in the case of the optimized Co content. Microstructure analysis using high-resolution transmission electron microscope reveals that the tiny CeO₂ and Co₃O₄ phases assembled into porous particles are well crystallized and show high chemical purity. The porous feature of Ce–Co–O catalysts synthesized from feasible ultrasonic spray pyrolysis makes them more competitive and promising towards gaseous environmental pollution processing including CO oxidation, TWCs, SCR, etc.     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]     

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition.     

Effects of plate temperature on heat transfer and emissions of impinging flames

Experiments were conducted to investigate the heat transfer and CO/NO_X emissions of a premixed LPG/air circular flame jet impinging upwards normally to a flat rectangular plate. Temperatures of the impingement plate were controlled by cooling water at 38 °C, 58 °C and 78 °C which was circulating at its back in order to create different plate temperatures. Under each plate temperature, the effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the heat transfer and CO/NO_X emissions were examined. The Re was selected to be 500, 1000 and 1500 to ensure laminar flame jets. The values of Ф were chosen to cover fuel-lean, stoichiometric and fuel-rich conditions. The H varied from 3d to 7d with an interval of 1d.The flame-side temperature of the impingement plate is enhanced when the cooling water temperature increases, but the temperature difference across the impingement plate is reduced. Heat transfer from the flame to the plate is suppressed at higher cooling water temperature. The heat transfer rate is the highest when the cooling water temperature is at 38 °C and the lowest heat flux is obtained at 78 °C. At the highest cooling water temperature of 78 °C, the CO emission is reduced whereas the NO_X emission is enhanced. However, this trend is reversed at the lowest cooling water temperature of 38 °C.     

Performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) using CO2 as heat transfer fluid

In this paper, a performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) based on parabolic troughs using CO₂ as heat transfer fluid is reported on. The use of CO₂ instead of the more conventional thermal oil as heat transfer fluid allows an increase in the temperature of the heat transfer fluid and thus in solar energy conversion efficiency. In particular, the ISCCS plant considered here was developed on the basis of a triple-pressure, reheated combined cycle power plant rated about 250 MW. Two different solutions for the solar steam generator are considered and compared.The results of the performance assessment show that the solar energy conversion efficiency ranges from 23% to 25% for a CO₂ maximum temperature of 550 °C. For a CO₂ temperature of 450 °C, solar efficiency decreases by about 1.5–2.0% points. The use of a solar steam generator including only the evaporation section instead of the preheating, evaporation and superheating sections allows the achievement of slightly better conversion efficiencies. However, the adoption of this solution leads to a maximum value of the solar share of around 10% on the ISCCS power output. The solar conversion efficiencies of the ISCCS systems considered here are slightly greater than those of the more conventional Concentrating Solar Power (CSP) systems based on steam cycles (20–23%) and are very similar to the predicted conversion efficiencies of the more advanced direct steam generation solar plants (22–27%).The results of a preliminary cost analysis show that due to the installation of the solar field, the electrical energy production cost for ISCCS power plants increases in comparison to the natural gas combined cycle (NGCC). In particular, the specific cost of electrical energy produced from solar energy is much greater (about two-fold) than that of electrical energy produced from natural gas.     

A study on the effect of the CO2/steam mixtures and the addition of natural minerals on the reactivity of Adaro coal gasification

The synergetic effect in reactivity and gas yield on the various ratio of CO₂/steam mixtures was investigated. The isothermal gasification was conducted at three different temperatures. The synergy effect was evaluated on the ratio of CO₂/steam mixtures and reaction temperatures. In order to analyze the synergy quantitively, two reaction indexes were calculated from carbon conversion. The effect of natural minerals like Dolomite and Kaolin was investigated as well. The influence of synergy was varied upon the ratio of CO₂/steam mixtures and the optimal synergy was observed when the ratio of CO₂/steam mixtures was 1:2. The best synergy in reactivity and gas yield was shown at 800 °C and at 900 °C, respectively. By adding Dolomite, the synergetic effect in both reactivity and H₂ yield was promoted at 800 °C. Conclusively, the ratio of CO₂/steam mixtures and Dolomite played an important role to facilitate the synergy in the coal gasification.     

Influence of the supports ZrO2 on selective methanation of CO over the nickel supported catalysts

It is attempted to optimize preparation of ZrO₂ as support of the nickel catalysts for selective methanation of CO in H₂-rich gas (CO-SMET). Therefore, the supports ZrO₂ were prepared at first by thermal decomposition method from zirconium oxynitrate and zirconium oxychloride at the calcination temperature of 400 °C and 800 °C, respectively. It is illustrated that the salt kind and calcination temperature affected phase state (tetragonal, monoclinic), crystallite size and specific surface area (SSA) of the supports. The difference in property of the supports influenced catalytic performance of the catalysts Ni/ZrO₂ for CO-SMET reaction. Especially, the chlorine ion residues in the support ZrO₂ prepared from zirconium oxychloride was beneficial for CO removal selectively. Furthermore, a precipitation method was adopted to prepare ZrO₂ for comparison with the thermal decomposition method with use of the zirconium oxychloride as starting material. It is found that the supports ZrO₂ prepared by the precipitation method induced a better dispersion of metallic Ni on its surface. The catalyst Ni/ZrO₂ with use of the support ZrO₂ prepared by the precipitation method and calcination at 400 °C exhibited a good performance at the reaction temperature of 220 °C in the 100 h durability test, where CO outlet concentration was kept below 10 ppm and the selectivity remained constant at 100%. Relation of Ni crystallite size and chlorine ion residues with the catalytic performance was discussed.     

Steam reforming of hot gas from gasified wood types and miscanthus biomass

The reforming of hot gas generated from biomass gasification and high temperature gas filtration was studied in order to reach the goal of the CHRISGAS project: a 60% of synthesis gas (as x(H₂)+ x(CO) on a N₂ and dry basis) in the exit gas, which can be converted either into H₂ or fuels. A Ni-MgAl₂O₄ commercial-like catalyst was tested downstream the gasification of clean wood made of saw dust, waste wood and miscanthus as herbaceous biomass. The effect of the temperature and contact time on the hydrocarbon conversion as well as the characterization of the used catalysts was studied. Low (<600 °C), medium (750°C–900 °C) and high temperature (900°C–1050 °C) tests were carried out in order to study, respectively, the tar cracking, the lowest operating reformer temperature for clean biomass, the methane conversion achievable as function of the temperature and the catalyst deactivation. The results demonstrate the possibility to produce an enriched syngas by the upgrading of the gasification stream of woody biomass with low sulphur content. However, for miscanthusthe development of catalysts with an enhanced resistance to sulphur poison will be the key point in the process development.     

Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier

The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H₂/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H₂/CO, CO₂ yield and carbon conversion efficiency gradually increased, while the yield of H₂, CO and CH₄ and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H₂, CH₄ and CO₂, H₂/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 °C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H₂, H₂/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/C of 1.5, 800 °C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H₂ content of 35.64%, syngas content of 56.40%, H₂/CO ratio of 1.72 and LHV of 12.25 MJ/Nm³.     

Biomass-derived CO2 rich syngas conversion to higher hydrocarbon via Fischer-Tropsch process over Fe–Co bimetallic catalyst

Biomass-derived syngas (CO₂ + CO + H₂) has emerged as a potential non-fossil fuel source to yield transportation fuel via Fischer Tropsch Synthesis (FTS) reaction. Thus, the present study demonstrates the conversion of CO₂ containing syngas into fuel range hydrocarbon via Fischer Tropsch Synthesis over Fe–Co bimetallic catalyst. The experimental tests were carried out in a fixed bed continuous reactor to investigate the effect of CO₂ on CO/CO₂ conversion. Accordingly, obtained data were validated by FTS kinetic model for a plug flow reactor. It was found that the unique combination of Fe and Co bimetallic catalyst facilitates both FTS and water gas shift (WGS) reaction simultaneously that helps to convert CO₂ along with CO. It was also observed that the presence of iron in the catalyst helps in conversion of CO₂ into hydrocarbons, only when a particular concentration of CO₂ in syngas is reached, i.e., critical ratio R_C (CO₂/CO + CO₂) due to the occurrence of reverse water gas reaction (RWGS) which varies with the temperature and the feed gas composition (H₂/CO/CO₂ molar ratio). At 240 °C and hydrogen deficient condition, the critical ratio was measured to be 0.74 whereas for hydrogen balanced condition, it was measured 0.6. The kinetic model developed in the present study predicted trends for % CO conversion, % carbon conversion, and % CO₂ conversion which is applicable for a wide range of critical ratio R_C (CO₂/(CO + CO₂) = 0 to 1). The model also predicted that a positive conversion of CO₂ could be achieved at lower CO₂ concentration by increasing the reaction temperature. At 260 °C and 280 °C, the value of Rc were 0.31 and 0.18 were measured.     

Numerical study of heat and mass transfer in the plate methanol steam micro-reformer channels

Effects of geometric and thermo-fluid parameters on performance and heat and mass transfer phenomena in micro-reformer channels were investigated by mathematical modeling. The geometric parameters considered were the channel length, channel height, catalyst thickness and catalyst porosity, while the thermo-fluid parameters included wall temperature, inlet fuel temperature, fuel ratio and Reynolds number. The results of the modeling suggest that the methanol conversion could be improved by 49%-points by increasing the wall temperature from 200 °C to 260 °C. The results also show that the CO concentration would be reduced from 1.72% to 0.95% with the H₂O/CH₃OH molar ratio values ranging from 1.0 to 1.6. The values of parameters that enhance the performance of micro-reformer were identified, such as longer channel length, smaller channel height, thicker catalyst layer, larger catalyst porosity, lower Reynolds number and higher wall temperature.     

Emission characteristics and axial flame temperature distribution of producer gas fired premixed burner

This paper presents the emission characteristics and axial flame temperature distribution of producer gas fired premixed burner. The producer gas fired premixed burner of 150 kW capacity was tested on open core throat less down draft gasifier system in the present study. A stable and uniform flame was observed with this burner. An instrumented test set up was developed to evaluate the performance of the burner. The conventional bluff body having blockage ratio of 0.65 was used for flame stabilization. With respect to maximum flame temperature, minimum pressure drop and minimum emissions, a swirl angle of 60° seems to be optimal. The experimental results also showed that the NO_x emissions are inversely proportional to swirl angle and CO emissions are independent of swirl angle. The minimum emission levels of CO and NO_x are observed to be 0.167% and 384 ppm respectively at the swirl angle of 45–60°. The experimental results showed that the maximum axial flame temperature distribution was achieved at A/F ratio of 1.0. The adiabatic flame temperature of 1653 °C was calculated theoretically at A/F ratio of 1.0. Experimental results are in tune with theoretical results. It was also concluded that the CO and UHC emissions decreases with increasing A/F ratio while NO_x emissions decreases on either side of A/F ratio of 1.0.     

Design,simulation and experimental study of a directly-irradiated solar chemical reactor for hydrogen and syngas production from continuous solar-driven wood biomass gasification

The use of concentrated solar energy as the high-temperature heat source for the thermochemical gasification of biomass is a promising prospect for producing CO₂-neutral chemical fuels (syngas). The solar process saves biomass resource because partial combustion of the feedstock is avoided, it increases the energy conversion efficiency because the calorific value of the feedstock is upgraded by the solar power input, and it also reduces the need for downstream gas cleaning and separation because the gas products are not contaminated by combustion by-products. A new concept of solar spouted bed reactor with continuous biomass injection was designed in order to enhance heat transfer in the reactor, to improve the gasification rates and gas yields by providing constant stirring of the particles, and to enable continuous operation. Thermal simulations of the prototype were performed to calculate temperature distributions and validate the reactor design at 1.5 kW scale. The reliable operation of the solar reactor based on this new design was also experimentally demonstrated under real solar irradiation using a parabolic dish concentrator. Wood particles were continuously gasified at temperatures ranging from 1100 °C to 1300 °C using either CO₂ or steam as oxidizing agent. Carbon conversion rates over 94% and gas productions over 70 mmol/g_biomass were achieved. The energy contained in the biomass was upgraded thanks to the solar energy input by a factor of up to 1.21.