Yousef M. Abdel-Rahim

Exergy analysis of radial inflow expansion turbines for power recovery

The paper presents an exergy analysis of the performance of a single-stage, radial-inflow expansion turbine. The analysis considers the effect of the design controlling parameters on the power output, and the first law and second law efficiencies of the turbine. The design parameters include: inlet temperature, wheel tip speed, exhaust Mach number and expansion pressure ratio. Limitations imposed by the second law of thermodynamics are presented and discussed, and the admissible ranges of variation of the controlling parameters are outlined and discussed. The analysis enables the process designer to quickly and realistically estimate the amount of power available from the exhaust gases of a process stream, the maximum effective pressure ratio and the temperature drop across the turbine and the exit Mach number.

Optimum parametric performance characterization of an irreversible gas turbine Brayton cycle

A general mathematical model is developed to specify the performance of an irreversible gas turbine Brayton cycle incorporating two-stage compressor, two-stage gas turbine, intercooler, reheater, and regenerator with irreversibilities due to finite heat transfer rates and pressure drops. Ranges of operating parameters resulting in optimum performance (i.e., ηI ≥ 38 ≥ ηII ≥ 60%, ECOP ≥ 1.65, xloss ≤ 0.150 MJ/kg, BWR ≤ 0.525, wnet ≥ 0.300 MJ/kg, and qadd ≤ 0.470 MJ/kg) are determined and discussed using the Monte Carlo method. These operating ranges are minimum cycle temperature ranges between 302 and 315 K, maximum cycle temperature ranges between 1,320 and 1,360 K, maximum cycle pressure ranges between 1.449 and 2.830 MPa, and conductance of the heat exchanger ranges between 20.7 and 29.6 kW/K. Exclusive effect of each of the operating parameters on each of the performance parameters is mathematically given in a general formulation that is applicable regardless of the values of the rest of the operating parameters and under any condition of operation of the cycle.

Modeling of Laminar Viscous Flow in a Semi-Porous Channel Under Uniform Magnetic Field

Laminar flow in porous channels or tubes has received a great attention in recent years due to its various applications. These applications include blood filtration in artificial kidney, blood flow in capillaries and oxygenations, transpiration, cooling of turbine blades, lubrication of ceramic machine parts, food processing, electronics cooling, gaseous diffusion, the extraction of geothermal energy, and nuclear reactor cooling systems. The present work, proposes the Galerkin method to solve the two dimensional electrically conducting flow in a semiporous channel under uniform magnetic field as governed by dimensionless: Hartman number (Ha), and Reynolds number (Re). The numerical results predict the flow field characteristics over wide ranges of Ha, and Re. These results are compared with other available analytical and numerical results at different values of Ha, and Re. Detailed results are presented and discussed.Copyright

Rapid thermodynamic simulation model for optimum performance of a four-stroke, direct-injection, and variable-compression-ratio diesel engine

A thermodynamic simulation model for the performance of a four-stroke, direct-injection diesel engine is developed. The simulation model includes detailed sub-models for fuel burning rate, combustion products, thermodynamic properties of working fluid, heat transfer, fluid flow, and both soot and oxides of nitrogen (NOx) formation mechanisms. To validate the model, comparisons between experimental and predicted results for different engines, operating under different conditions, were conducted. The comparisons show that there is a good concurrence between measured and predicted values. An optimization analysis is conducted for seeking an optimum variation of compression ratio to achieve pre-set objective targets such as constant minimum brake-specific fuel consumption and constant maximum torque. The optimization analysis is performed under the constraint that the maximum pressure and temperature inside the cylinder do not exceed the maximum allowable pressure and temperature of the conventional engine (constant compression ratio).The varying compression ratio is optimized with each of the previous conditions separately. Results indicated that varying the compression ratio to achieve previous targets leads to saving fuel consumption, higher brake efficiency and power, and reduction in soot emission from the engine. Also, an increase in NOx is noticed at low speed. This drawback is considerable and can be overcome by reducing the operation speed range.

Optimum performance comparative study of combined gas turbine partial oxidation cycle and reheat cycle

The present paper studies the general characteristics and evaluate the optimum performance of 2-stage POGT cycle augmented by a recuperation process. A comparative study of the performance of POGT cycle with that of a reheat cycle (RHGT) is discussed. The ranges of values of the controlling parameters of both cycles that give optimum performance (e.g. first law and second law efficiencies and net work output /spl eta//sub I/, /spl eta//sub II/, and w/sub net/ respectively) are determined and fitted into functional correlation equations. The effects of irreversibilities of the different composing units of the two cycles are accounted for in the study. The present findings can form a very important basis for a complete phenomenological design of POGT and RHGT cycles to achieve optimum performance. The study shows that for POGT cycle, at constant /spl theta/, the effect of x on q is very small compared to its effect on /spl eta//sub I/ and /spl eta//sub II/, reaching their maximum values at about x=2.8. Mass ratio in has a constant decreasing effect on all cycle performance characteristics. Higher values of /spl theta/ result in higher values of all cycle characteristics. For the RHGT cycle, x greatly effects all performance characteristics and has some limiting values at which w/sub net/=/spl eta//sub I/=0. Variation trends of cycle characteristics are similar in regards to the effect of /spl theta//spl mu/ with their optimum values tends to lie between values of x of 1.6 and 2.6. Higher values of x decrease cycle performance. The comparative study shows that /spl eta//sub I/ for the RHGT cycle always exceeds that of the POGT cycle only at low values of x. Beyond these values of x, the POGT is better than the RHGT. The correlation equations relate optimum values of both cycle characteristics to the controlling parameters of each cycle. Irreversibilities of different units of each cycle have been accounted for in these equations.

Optimum Compression Ratio Variation of 4-Stroke, Direct Injection Diesel Engine for Optimum Performance

A thermodynamic model for simulation the performance of a four-stroke direct-injection (DI) diesel engine is developed. The simulation model includes detailed sub-models for fuel burning rate, combustion products, thermodynamic properties of working fluid, heat transfer, fluid flow, and both soot and NOx formation mechanisms. To validate the model, comparisons between experimental and predicted results for different engines, operating under different conditions were conducted. Comparisons show that there is a good concurrence between measured and predicted values. An optimization analysis is conducted for seeking an optimum variation of compression ratio to achieve pre-set objective target of constant minimum brake specific fuel consumption (bsfc). The optimization analysis is performed under the constrain that the maximum pressure and temperature inside the cylinder not exceed the maximum allowable pressure and temperature of the conventional engine (constant rc ). Varying compression ratio is optimized with the previous condition. Results indicated that, at the values of rc ranged between 16.4 and 17.8, the optimum bsfc is attained with an increase in brake power by about 3.8%, while the bsfc and soot emission are reduced by about 4.4% and 21%, respectively. In addition to an increase in NOx , maximum pressure (pmax ), and maximum temperature (Tmax ) by about 75%, 6% and 4.3%, respectively.Copyright

Monte Carlo Optimization of Two-Stage Cascade R134A Refrigeration System With Flash Chamber

Present paper studies the optimal characteristics of the two-stage cascade R134A refrigeration system with flash and mixing chambers over its operating ranges of all cycle controlling parameters. The COP, total heat rate in Qin , total work rate in Win and second law efficiency ηII are used as cycle performance parameters. Compared to the practically-limited other rate-based optimization methods and to other experimentally-optimized specific cases of cycle parameters, the application of Monte Carlo method has proved to be very effective for optimizing the cycle performance in its global sense over all cycle controlling parameters. Correlations relating performance and cycle controlling parameters are presented and discussed. Study shows that COP of the cycle can reach a value of 8 at intermediate pressure P2 of about 200 kPa, and a maximum value of 9.92 at about 370 kPa and 720 kPa, beyond which COP goes as low as 4.2. P2 alone has no significant effect on Qin , Win and ηII unless values of other controlling parameters are specified. Values of Qin , Win and ηII can reach as high as 94 kW, 23 kW and 0.85 and as low as 6.8 kW, 1.1 kW and 0.57 respectively depending on other cycle parameters. Neither pressure ratio nor volume ratio of the HP compressor has any effect on Qin , Win or ηII . However, the ratio of inlet to exit temperatures of the condenser has the greatest effect on both ηII and the volumetric specific work of the HP compressor, which is about double the value of the volumetric specific work of the LP compressor. Study shows an almost linear relationship between the two mass flow rates in the upper and lower loops of the cycle, where its value in the lower LP loop is about 75% that in the upper HP loop. Findings of the present work as well as the elaborate application of Monte Carlo method to real cycles can greatly open the way for reducing the trade-off design methods currently used in developing such systems as well as direct the useful experimentations and assessment of such designed systems.Copyright

Monte-Carlo Parametric Performance Analysis of A/C and H/P Transcritical R744 Compressor

Thermal engineers usually face two kinds of problems: design of new systems, where multiple alternatives for each composing unit of the system can fulfill the design requirements and/or assessment of exiting systems, where they usually work at off-the-design conditions due to deterioration of their efficiencies. The present paper addresses possible solutions to such problems via the Monte-Carlo method (MCM) applied to compressors as major units in airconditioning (A/C) and heat pump (H/P) systems having Carbon Dioxide (R744) as working fluid. Diversity of the effects of system design, process parameters and fluid properties within and across the saturation line hinders an overall-view of hidden capabilities and limitations of the compressors. Thus, it is hard to apply any appropriate optimization methods for their better design, besides the difficulty in assessing their performance along their working lifetime. In an answer to the above two points, the present paper utilizes the powerful MCM to investigate and expose the effects on compressor performance of all controlling parameters, randomly selected from their variation ranges within and across wet/saturation region to the transcritical dry region of R744. For each selected set of parameters, a complete analysis is calculated to determine the performance of the compressor. This process is repeated about 500 times to randomly cover the variation spaces of the parameters. The study reveals that MCM can fairly handle the parametric design and evaluation study of compressor performance. It shows that the initial temperature has a greatly decreasing effect on compressor specific displacement volume per unit work and to a less extent on size of compressor, while its effects on all other parameters are insignificant. Initial pressure has similar expected trends. Final to initial temperature ratio has increasing effects on both final pressure and work of compressor and decreasing effects on both second law efficiency and specific displacement volume per unit work. The application of MCM to the parametric compressor performance presented here could open many ways for further theoretical design optimization and experimental justification of the compressors and hence greatly limits the trade-off methods that are usually adopted in the selection of such units.Copyright

Generalized A/C and H/P Transcritical R744 Compressor Performance in Wet and Dry Regions

Performance of the compression process of Carbon Dioxide (R744) in (A/C) and (H/P) systems depends on many factors related to fluid properties, equipment design and process parameters. Diversity of these factors represents a difficulty to have an overall-view of the hidden capabilities and limitations of the compression process. Thus, it is hard to apply any appropriate design optimization methods for the compressors, specifically for two reasons: (1) when the compression starts from the wet or saturation region and commences through the transcritical dry region of the fluid and (2) when the compressor works at off-design operating point during its working life-time. In an answer to these two points, the present paper utilizes the first and second laws of thermodynamics augmented by Monte-Carlo method to investigate, expose and discuss the compression performance (e.g. final temperature and pressure, work and size of compressor and second law efficiency) starting randomly from the wet/saturation region to the transcritical dry region of R744. The study shows that initial temperature has a greatly decreasing effect on compressor specific displacement volume per unit work and to a less extent on size of compressor, while its effects on all other parameters are insignificant. Initial pressure has similar expected trends as those of the initial temperature due to their linear relationship in the wet region. Final to initial temperature ratio has increasing effects on both final pressure and work of compressor and decreasing effects on both second law efficiency and specific displacement volume per unit work. The method presented here could open many ways for further theoretical design optimization applications of the compressor and hence greatly limits the trade-off methods that are usually adopted in the selection of such units.Copyright