• Energy Research

Abstract Hydrogen production through supercritical water gasification (SWG) of biomass has been widely studied. This study reviews the main factors from exergy aspect, and these include feedstock characteristics, biomass concentration, gasification temperature, residence time, reaction catalyst, and reactor pressure. The results show that the exergy efficiencies of hydrogen production are mainly in the range of 0.04–42.05%. Biomass feedstock may affect hydrogen production by changing the H2 yield and the heating value of biomass. Increases in biomass concentrations decrease the exergy efficiencies, increases in gasification temperatures generally increase the exergy efficiencies, and increases in residence times may initially increase and finally decrease the exergy efficiencies. Reaction catalysts also have positive effects on the exergy efficiencies, and the reviewed results show that the effects are followed KOH > K2CO3 > NaOH > Na2CO3. Reactor pressure may have positive, negative or negligible effects on the exergy efficiencies.

Abstract A multi-objective optimization of corrugated tube with loose-fit twisted tape (CLT) is conducted to obtain the optimal performance, using Response Surface Methodology (RSM) and Non-dominated Sorting Genetic Algorithm (NSGA-II). A series of simulation runs are performed to optimize the design variables on the objective functions. The corrugation parameters are fixed at constant values. The design variables include the Reynolds number (Re), twisted ratio (Y), and clearance ratio (CR). The objective functions involving the Nusselt number ratio (Nu/Nus), friction factor ratio (f/fs), and overall heat transfer performance (η) are calculated from numerical simulations. Regression models for Nu/Nus, f/fs, and η are tested, the statistical importance of each is assessed by means of the analysis of variance (ANOVA), and the models are expressed in quadratic polynomial forms. The multi-objective NSGA II is adopted to obtain the Pareto-optimal fronts. The results show that the most significant factor is the linear term of Y for Nuc/Nus and fc/fs, while that for η is the linear term of Re. Pareto optimal solution can be obtained with the optimum Nuc/Nus = 2.484 and fc/fs = 3.324. The corresponding values of the design variables are Re = 4000.08, Y = 2.79, and CR = 0.20. The optimal design variables against Re are also obtained, which possesses practical significances for designing heat exchanger.

The present study numerically investigates the second law analysis of turbulent flow in novel outward helical corrugated tubes by using a Reynolds Stress Model (RSM). A case of a transverse corrugated tube and five cases of helical corrugated tubes with different height-to-diameter ratios and pitch-to-diameter ratios are examined with Reynolds number ranging from 3800 to 43,800 at a constant wall temperature condition. The results indicate that the secondary flow significantly increases the thermal and viscous dissipation irreversibilities. However, the spiral flow inhibits the secondary flow and the production of local entropy. The heat transfer effective zone (Belocal > 0.5) is mainly located at the sub-layer and buffer-layer, and the ineffective zone (Belocal < 0.2) is located at the turbulent severe pulsation zone. A comprehensive consideration of the total entropy generation number (Ns-tot) and average Bejan number (Beave) indicates that the optimal Re is less than 31,000 for all the cases although Re does not exceed 18,800 in the case of Hl/D = 0.15, pl/D = 1.0 and Hl/D = 0.10, pl/D = 0.5. Additionally, the helical corrugated tube exhibits an overall advantage when compared with a transverse corrugated tube with the same geometrical parameters. (Less)

Rice husk and straw are by-products of rice cultivation and processing industry and can be used as an energy source. Proper understanding of the physical properties of rice residues is necessary for utilizing them in thermochemical conversion processes such as gasification and combustion. The physical properties (moisture content, particle size, bulk density and porosity) of rice husks and straws obtained from three countries (Egypt, Cuba and China) were evaluated in this study. The moisture contents of rice husks and straws were in the ranges of 4.60-6.07% and 6.58-6.92%, respectively. For all rice varieties tested, the moisture content of the straws was higher than these of the husks. The particle sizes of rice husks and straws were in the ranges of 0.212-0.850 mm and 0-0.710 mm, respectively. All the rice husk varieties had a normal distribution of particle size around the main value of 0.6 mm while the particle size distribution for the rice straws showed a decreasing trend, the larger the particle size the higher was the weight percentage. The bulk density of rice husks and straws were in the ranges of 331.59-380.54 kg m-3 and 162.03-194.48 kg m-3, respectively. The bulk density values of rice straws were lower than those of rice husks. A negative linear relationship between the bulk density and the average particle size was observed for rice husks and straws. The porosity of rice husks and straws were in the ranges of 63.64-68.94% and 71.21-85.28%, respectively. A positive linear relationship between the porosity and the average particle size was observed for rice husks and straws. Also, a negative linear relationship between the porosity and the bulk density was observed. The results obtained from this study showed significant differences in the physical properties of the rice husks and straws collected from different countries (located in three different continents). These differences may be due to variations in climatic conditions, soil type, methods of cultivation and type of fertilizer used. The results also indicated that different parts of rice plant (straw and husk) had different physical properties. Also, significant differences were observed among rice varieties even though they were grown under the same climatic conditions using same soil type and cultivation method as in the case of the long and short grain rice variety of Egypt.

Abstract A model considering evaporator wettability in terms of the contact angle is developed in this study. In addition, experiments are conducted using a copper thermosyphon with a length of 240 mm and internal and external diameters of 22.2 mm and 25.4 mm. The influences of the inclination angle and evaporator wettability on the heat performance of a thermosyphon charged with water are investigated. It is observed that simulated temperatures agree well with experimental data with a relative error of 0.12% for a modified thermosyphon. The results show that bubbles attaching to the wall of the evaporator decrease as the inclination angle increases from 15° to 90°, which decreases thermal resistance by 59.5%. As the heating power increases from 10 W to 14 W, thermal resistance reduces more significantly (44.1% decrease) for an evaporator with a hydrophilic surface than for an evaporator with a hydrophobic surface (20.6% decrease) because the bubble emission frequency of a hydrophilic surface rises more sharply (265% increase) than that of a hydrophobic surface (100% increase) at an inclination angle of 90°.

The present work numerically studied the enhanced heat transfer mechanism of the coupled fields of detached and spiral vortices in symmetrical and asymmetrical spirally corrugated tubes. The heat t...

Microwave-assisted pyrolysis of waste suffers from the problem that the waste generally has low microwave absorptivity thereby resulting in low heating rate and low pyrolysis temperature. In this case, fast microwave-assisted pyrolysis is proposed and developed to help the pyrolysis of waste. This study describes two methods that can be used to realize fast microwave-assisted pyrolysis of waste: (1) premixed method (wastes are mixed with microwave absorbent) and (2) non-premixed method (wastes are poured onto the heated microwave absorbent bed). Then, biofuels (bio-oil, bio-gas, and bio-char) produced from fast microwave-assisted pyrolysis of wastes are reviewed. The review results show that the yields of bio-oil, bio-gas, and bio-char obtained from fast microwave-assisted pyrolysis of wastes varied significantly in the ranges of 2-96 wt%, 2.4-86.8 wt%, and 0.3-83.2 wt%, respectively. Although the present research focused mainly on the premixed method, non-premixed/continuous fast microwave-assisted pyrolysis is still promising and challenging.

Abstract Szargut et al. [1] proposed an appropriative equation for calculating the exergy of woody biomass. Cumbersome work is involved in calculating the exergy of woody biomass as the complicated ash compositions are included for obtaining the ash exergy in the equation. Based on the exergy values of ash contents of sixty four woody biomass investigated in this study, the average value of wood ash is obtained and a positive linear relationship between ash exergy and ash content is observed. An equation based on the average exergy of wood ash with relative errors of −0.76% to 1.38% and another equation based on the relationship between ash exergy and ash content with relative errors of −1.16% to 0.88% are observed. These two easier equations can be well used for expeditious estimation of the exergy of woody biomass with high accuracies.

This study proposes a sectorial-finned pipe in the hydronic road heating system with a three-dimensional road model to enhance the road heating performance. Three primary fin parameters, including sectorial fin angle, height, and spacing, were analyzed. The range analysis and ANOVA (analysis of variance) of the final average road surface temperature (Tasf), the maximum temperature difference of road surface (Tds), and the overall utility (Ut) were conducted through the orthogonal test method. The results showed that compared with the bare pipe, Tasf and Tds of the finned pipe increased by 0.63–1.37 K and 0.27–0.66 K, respectively, and ts0 of the finned pipe decreased by 0.22–0.44 h when the fin parameters were changed in specific ranges at the average air temperature of 258.15 K. The fin angle of 210° had the most suitable performances for road heating in terms of the thermal and economic targets. The range analysis revealed that the ranks of influence degree for Tasf, Tds, and Ut were height > spacing > angle, height > spacing > angle, and angle > spacing > height, respectively. ANOVA indicated that the parameters of height, spacing, and angle contributed Tasf in proportions of 80.56%, 10.25%, and 9.19%, those contributed Tds in proportions of 98.58%, 1.29%, and 0.13%, and those contributed Ut in proportions of 60.06%, 36.20%, and 3.74%, respectively. The optimum parameter combination for Ut was 210°, 20 mm, and 45 mm for the fin angle, height, and spacing, respectively. The results can provide a good guide for the design of sectorial-finned pipes in the hydronic road heating system.