Proceedings of the Institution of Mechanical Engineers,Part A: Journal of Power and Energy

Centrifugal compressors used for turbochargers are required to achieve high performance within wide flow ranges. However, their operability is limited at low mass flow rates by the flow instabilities leading to rotating stall and surge, which could be delayed by a suitable casing treatment. This article presents a numerical investigation aimed at studying the effects of a type of casing treatment on the flow structure through a radial compressor and the subsequent improvement of aerodynamic stability. The considered casing treatment consists of an oblique slot recess inserted in the shroud near the impeller throat and a large annular cavity connecting this slot to the bell-mouth. The flow computations were carried out by means of the code ANSYS-CFX, considering the shear stress transport (k–ω) turbulence model, in order to investigate the effects of the cavity dimensions on improving the stability limit of the compressor. The analysis of flow structure near the stall limit has revealed that the large vortex appearing near the blade tip is being controlled by such a type of casing treatment; hence the stall onset is delayed. As consequence, the operating range and the stall margin are improved by reducing the aerodynamic blockage.

This article reports the investigation of non-linear start-up processes in a looped-tube thermoacoustic engine. The engine utilises air as working gas, and has the fundamental frequency of around 111 Hz. It is observed that the mean pressure and the input heat power are the two key parameters controlling the start-up behaviour of the engine. When these are varied, a range of non-linear transient phenomena are observed, which include the ‘on–off’ effect, ‘fishbone-like’ oscillations (i.e. the quasi-periodic pressure amplitude bursts followed by a quasi steady state), and the normal smooth start-up process. The experiments show that the ‘fishbone-like’ bursts are a new mode of pressure amplitude growth. A series of experiments have been conducted to investigate in detail the influence of mean pressure, input heat power and regenerator type on the occurrence of such quasi-periodic amplitude bursts. It is observed that the duration of the pressure amplitude bursts depends on the combination of the heating power and the mean pressure. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator. It is thought that these can lead to the reported quasi-periodic unsteady behaviour of the engine.

The environmental wind speed, wind direction, and surroundings of the air-cooled condenser are parameters that may affect the cooling system performance. This article concentrates on the hot air recirculation phenomenon and its dependence on ambient winds are numerically simulated using the computational fluid dynamics code, FLUENT. In this study, two different wind directions (cases A and B) and two different wind speeds are considered. Results show that the hot air recirculation increases with the increment of velocity speed. Case A has a critical wind direction angle. Wind causes an air temperature increase at the fan inlet due to hot air recirculation, resulting in the deterioration of the heat transfer performance. In case A, fan inlet temperature is about 2.2 °C higher than fan inlet temperature in case B. Heat rejection in case B is 10.5% higher than case A. Increment of the wind speed from 3 to 9 m/s leads to 5.9 °C rise in fan inlet temperature and heat rejection reduction about 27%. The effect of the hot air recirculation phenomenon occurs at 9 m/s in case A.

In recent decades, analyzing and optimizing thermal systems have become of great interest to researchers. Recently, the engineers concentrated on variant concepts of artificial intelligence such as machine learning, simulation, fuzzy logic, game theory, and evolutionary computing to deal with complicated barriers and obstacles. Artificial intelligence and expert system techniques play an important role for surveying and controlling mechanical systems such as power plants and reservoirs. This is because of their interdisciplinary applications and versatile servicing potential in mathematical modeling of industrial systems. In this article, a new method called synchronous parallel shuffling self-organized Pareto strategy algorithm is presented which synthesizes different artificial techniques, nominally evolutionary computing, swarm intelligence techniques, and time adaptive self-organizing map that apply simultaneously incorporating with a stochastic data sharing behavior. Thereafter, it is applied to verify the optimum operating parameter of Damavand power plant as the biggest constructed power plant in Middle East with the potential of producing about 2300 MW electricity sited in Tehran, capital of Iran, as a multi-objective, multi-modal complex problem. It is also proved that implementing the governing equations of power plant leads to a multi-objective problem where some of these objectives are non-linear, non-convex, and multi-modal with different type of real-life engineering constraints. The results confirm the acceptable performance of proposed technique in optimizing the operating parameters of Damavand power plant.

This article presents the results of a two-dimensional computational study into the effects of rotor blade thickness and camber on the performance of a 5 kW scale vertical axis wind turbine. Validation is provided by reference to experimental data for a pitching aerofoil with dynamic stall phenomenon. The performance of the turbine is mapped out for a variety of different tip speed ratios and detailed investigations are presented to determine how and, most importantly, why the turbine performance varies with thickness and camber as it does. The rotor blades chosen were the NACA0012, NACA0022, NACA5522 and LS0421. The turbine rotor has a diameter of d = 3.1 m with a blade chord length of c = 0.18 m. Over the range of tip speed ratios examined, the NACA0012 profile performed with the highest overall performance of 50% at λ = 3.5. Slightly cambered aerofoils (such as the LS0421) can improve the overall performance of the vertical axis wind turbine, whereas a camber of 5% (NACA5522) results in unfavourable performance. Of the cambered blades tested, the LS0421 performs the best with maximum CP of 0.40 at λ = 3.5. A camber along the blade path causes the blades to generate higher values of torque in both the upwind and the downwind regions. It was also determined that inverted cambered profiles produce power mostly in the upwind region.

In this study, the performance of some hydrocarbon refrigerants (R290, R600a and R1270) as alternatives to R22 in vapour compression refrigeration system was investigated theoretically employing a sub-cooling heat exchanger. The effects of sub-cooling on the performance of the investigated refrigerants were quantified in terms of relative capacity index. The results obtained showed that the saturated vapour pressure and specific volume of R290 and R1270 are very close to those of R22; therefore, they could be used as substitutes for R22. The condenser duty using R290, R600a and R1270 are better than that of R22 with the highest value obtained using R1270, which shows that they could perform very well as refrigerants in heat pump systems. The coefficient of performances of all the investigated refrigerants were found to be in the range 3.10–3.51. The degree of sub-cooling greatly increased the coefficient of performance and it had positive effects on the refrigerating capacity. The average relative capacity indices obtained for R290, R600a and R1270 were 35.4%, 29.5% and 27.3% higher than that of R22, respectively. Generally, the thermophysical properties of R290 and R1270 matched those of R22 and the two refrigerants exhibited better performance than R22 in sub-cooling heat exchanger refrigeration system. R600a has slightly better performance in terms of lower power per ton of refrigeration and higher coefficient of performance, but its saturation pressure and specific volume deviate significantly from that of R22, which will require major system redesign before it can be employed as R22 substitute.

A stereoscopic particle image velocimetry system was used to obtain the velocity field of a can-type combustor in the non-reacting condition. In order for these measurements to be taken, an optically accessible can-type forward flow combustor was manufactured. The combustor has a 10-vane swirler in the dome as well as a primary zone with six 9.5 mm holes, a secondary zone with eight 5 mm holes and a dilution zone with ten 11.8 mm holes. The two cooling rings have 30 and 50 × 1.2 mm holes and are placed between the three zones. The main flow features were captured such as the recirculation zones and jets. The more subtle features such as flow entering the swirler, entering the dilution holes from the annulus, and converging into smaller annuli around the secondary and dilution zones were also evident in some sections of the data. An unexpected flow recirculation was observed in the dilution zone. The departure of the flow from the recirculation regions to join the bulk flow in the dilution zone was also shown.

A three-dimensional, single-phase, isothermal, explicit electrochemistry polymer electrolyte membrane fuel cell model has been developed and the developed computational model has been used to compare various effective diffusivity models of the gas diffusion layer. The Bruggeman model has traditionally been used to represent the diffusion of species in the porous gas diffusion layer. In this study, the Bruggeman model has been compared against models based on particle porous media, multi-length scale particles and the percolation-type correlation. The effects of isotropic and anisotropic permeability on flow dynamics and fuel cell performance have also been investigated. This study shows that the modelling of the effective diffusivity has significant effects on the fuel cell performance prediction. The percolation-based anisotropic model provides better accuracy for the fuel cell performance prediction. The effects of permeability have been found to be negligible and the specification of any realistic value for permeability has been found to be sufficient for polymer electrolyte membrane fuel cell modelling.

This article is aimed at carrying out a performance analysis of a combined cycle gas turbine power plant with various inlet air-cooling systems. A comparison is performed using different cooling systems, including evaporative cooling, fogging, mechanical (electric) chiller cooling, and absorption cooling. A performance analysis is completed for each system. The performance characteristics are determined for a set of actual operational parameters including ambient conditions (temperature and relative humidity), turbine inlet temperature, pressure ratio, etc. To evaluate the net power output and efficiency for each system, a combined cycle unit system consisting of inlet air cooling system, gas turbine, dual pressure heat recovery steam generators, and steam turbine has been analyzed. The results show that power and efficiency improvements depend on ambient air temperature. In addition, by decreasing the ambient temperature and increasing the humidity of the air, the output power can be increased. The carbon dioxide emissions for each cooling system are also discussed. The results and the sensitivity analysis show that the absorption cooling system is most effective.

A combined finite element analysis and computational fluid dynamics was developed to investigate the discharge characteristics and flow field of pocket damper seals at high pressure (up to 7.2 MPa) and high rotational speed (up to 50,200 r/min) flow condition. At first, the leakage flow rates of the published experimental pocket damper seals with the eight-bladed and eight-pocket were conducted at three different pressure drops and three different rotational speeds, and a comparison between finite element analysis and computational fluid dynamics results, experimental data and computational fluid dynamics results was presented. It showed that the utilized finite element analysis/computational fluid dynamics numerical method has sufficient precision (the maximum relative deviation is less than 1.3%) to predict the leakage flow rate through the pocket damper seals. The importance of taking rotor growth with rotational speed into consideration was made apparent by the rotor growth and leakage results. Then, six pressure ratios and seven rotational speeds were utilized to study the effects of these two factors on the leakage flow characteristics of the pocket damper seals. Numerical simulation results show that the leakage through a pocket damper seals increases with decreasing pressure ratio and asymptotically approach a maximum value corresponding to the choked flow condition when the pressure ratio is small enough (less than 0.26). The leakage flow rate with and without rotor growth included both decrease with the increasing rotational speed. In addition, it is quite necessary to take rotor growth with rotational speed into consideration when the rotational speed is very high (larger than 20,200 r/min) according to the numerical results and the engineering accuracy requirement. The influence of the pressure ratio on the structure of the leakage flow field in seal cavity is negligible. A significant influence of the rotational speed on the flow field in the active cavity has been observed. The circumferential partition wall can significantly decrease the circumferential flow in active cavity of the pocket damper seals.