فهرست مطالب محمود فرزانه گرد

In this paper, a sub-coled CAES unit has been investigated to produce electricity and cooling demand during peak times, and energy efficiency and payback period have been calculated in different operational conditions. For this purpose, a computer code has been developed in MATLAB software by solving the relevant equations in repetition process with appropriate time step. The results showed that by increasing the compression and expansion stages, the energy efficiency improves. Also, increasing in compression’s stages, the system payback period has been decreased, while increasing in expansion’s stages, the system’s payback period has been increased. The system has the best its performance when the temperature of tank is equal to that of ambient. In the study of a real sample that provides 900 kW of output power over a period of 4 hours, the payback period for the average price of electricity consumed was 15 cents per kWh, excluding the 8-year inflation rate, but based on the current net value method After 11 years, the proposed system becomes profitable.

In recent years, with the continuous growth of natural gas consumption in Iran, the number of pressure drop stations has increased significantly. In throttling valves of these stations, the temperature drop due to the Joule-Thomson effect causes the gas to hydrate, freeze the valves, and block the transmission path. Hence, about 14,000 indirect-fired water-bath heaters have a duty for preheating high-pressure gas before entering them. Unfortunately, the 30% average efficiency of indirectly fired water-bath heaters wastes nearly one billion cubic meters of processed natural gas every year, equivalent to a 400 MW power plant capacity. In this article, intending to optimize, indirect-fired water-bath heaters were modeled thermodynamically and thermo-economically, and three objective functions including thermal efficiency, entropy generation number, and wasted cost number are defined and the mathematical model was proposed in two scenarios. Then the model was solved based on the multi-objective genetic algorithm, using the entropy generation minimization method, and the Pareto optimal fronts of the scenarios were determined. The model implementation results with a deviation of less than ±10% compared to the results of a real sample indicate its acceptable performance. Based on the techno-economic justified results, it is possible to improve the efficiency of indirectly fired water-bath heaters between 48 and 55% depending on the gas volume flow rate. The relations, curves, and dimensionless groups obtained, can be used as a reference for the optimal design of indirect-fired water-bath heaters.

Organic Rankine Cycles are appropriate technology for the conversion of low-quality thermal energy to electrical power. In this research, the organic Rankine cycle driven by an evacuated tube collector was studied from energy, exergy, and exergo-economic points of view. Then, various cases such as the temperature and pressure input to the turbine, the temperature difference of the evaporator of the organic cycle, and the solar flux were evaluated. Simulation results show that power energy and exergy efficiency are 45.2 and 9.7 and the amount of work output and total irreversibility are 51.5 and 1291.8, respectively. The solar collector and the storage tank are the most important components from the exergo-economic point of view, due to the high initial cost rate and Exergy destruction. The parametric analysis result shows that the temperature increase in the evaporator has a positive effect on cycle operation and, the increase in pinch point temperature difference of evaporator has a negative effect on cycle operation. From an economic point of view, the increase in evaporator and pinch point temperature difference of evaporator causes the decrease in capital coast rate. Also, the solar radiation change finds to have a positive effect on system operation, based on an exergo-economic point of view, which causes the increase in energy and exergy efficiency.

The fast filling process is the main process that occurs in the CNG refueling station. The main objective of the present study is to apply a thermodynamic analysis for a complete modeling of a CNG refueling station with several consecutive refueling . The results of the thermodynamic analysis have been validated against the experimental data and previous studies. The AGA8 Equation of State has been used to calculate the thermodynamic properties of CNG. There are many researches concentrated on the thermodynamic modeling of CNG refueling station, but didn't address the optimal algorithm of priority panel. Filling time minimizing, filling mass maximizing and average specific energy consumption minimizing have been employed as the objective function for the optimization problem. Finally, the optimal algorithm of priority panel has been determined as the main goal of the present study. The conclusive results of the optimization problem are presented. Achieving more accurate modeling and modeling the process of several consecutive refueling is one of the most important results of the present study. The optimal algorithm of priority panel are obtained for PPM225 algorithm . An economic analysis is performed to see the results more clearly. The results of the economic analysis show that 65,700,000,000 Rials (43,800,000 kWh) can be saved in one year.

Depending on the temperature and pressure of the heat source, single-effect absorption chillers are categorized in two types of hot water and steam single-effect chillers. Due to the ability to use the waste steam in oil, gas and petrochemical industries for air conditioning and process cooling purposes, the steam type chiller is more widely used. In this study, the artificial neural network is exploited in the prediction of the steam single-effect absorption chiller performance since it is faster and has lower computational cost compared to thermodynamic modeling methods. The perceptron multilayer neural network with the error backpropagation algorithm, the hyperbolic tangent excitation function and the Levenberg-Marquardt learning method with 15285 data points and also the mean squared error estimation index are used. Inputs of the artificial neural network are the inlet cooling tower water temperature, inlet chilled water temperature, inlet steam temperature, outlet chilled water temperature and the solution heat exchanger efficiency respectively. Also, outputs of the neural network are the coefficient of performance and thermal energy consumption of the chiller. Results of this study show that the artificial neural network is capable to predict the coefficient of performance and the thermal energy consumed by the single-effect absorption chiller while the values of mean squared error are 3.183×10^(-7) and 7.466×10^(-8) respectively which verify the accuracy of the method proposed here in absorption chiller performance prediction.

Prior to entering to the throttling valve of the City Gate Stations (CGS), high-pressure natural gas flow in pipelines is transmitted through Water Bath Indirect Heaters (WBIH), which is increasing its temperature to compensate for the temperature drop caused by the Joule-Thomson effect and preventing the occurrence of the hydration phenomenon, gas freezing, and subsequent blockage of the gas flow path. Because of feeding of processed gas of the network on a large scale, optimizing the WBIHs has a lot of significance. In the present study, each WBIH is simulated by a type of thermodynamic machine, consisting of two distinct thermal systems. According to the problem geometry and governing equations, the thermodynamic analysis of these two systems results in the formulation of a relationship between their thermal efficiencies together and the definition of a parameter was defined as the Thermodynamic Similarity Coefficient (TSC). Then, the results showed that always, a constant logarithmic relationship exists between of the Number of Heat Transfer Units (NTU) values difference of the fire tube and heat coil of the WBIHs with their TSC as well as a constant power relationship between their NTU values ratio with this coefficient too. Finally, by solving the equation system obtained from these two relations, it was possible to determine the optimal values of NTU for the fire tube and heat coil as functions of TSC of the WBIH and to achieve the relationship between their optimum geometric dimensions together in the most ideal heat transfer state with a maximum relative error of about 13%.

Due to the cold air in the cold season, natural gas consumption will be greater than the standard mode and much higher than the normal. In this situation for networks comes a critical state, consumption increased as well as pressure reduced for natural gas, this condition has always been one of the main problems to deliver natural gas to various locations. In this study, a natural gas pipeline network in critical condition has been investigated. The proposed method for the analysis of pipeline network compared with previous studies, the validation results show the accuracy of 0.37% for the proposed method. The effect of important parameters on the pipeline network in critical state such as: friction factor and natural gas composition have been studied. The results show, the friction factor has a great impact on the natural gas pipeline network, also natural gas with higher molecular weights have less pressure drop in the all of natural gas pipeline network’s nodes.

In this study, a two-stage natural gas reciprocating compressor is simulated based on ideal and real models. To this end, a zero-dimensional numerical method based on the crank angle is developed. For this simulation, control volumes including compressor cylinders, suction, and discharge chambers with equivalent mass and energy equations along with piston movement, valve dynamic, and mass flow rate through valve and orifice equations and heat transfer equation for heat-exchanger are investigated. For real gas model, the AGA8 equation of state has been used for computing thermodynamic properties. Simulated results compared and validated with the previous experimental results for air reciprocating compressor. Then, the developed model is used to predict compressor behavior and performance parameters. It suction and discharge pressure was considered 4.122 and 9.795 MPa respectively. Predicted results show that intermediate pressure for simulation based on real gas (4.015 MPa) is lower than ideal gas (4.093 MPa). Furthermore, the mass flow rate based on the real model (730.67 kg/h) is higher than the ideal model (710.3 kg/h). In addition, discharge gas temperature prediction with the ideal model is lower than the real model.

The main process that occurs in the CNG refueling station is filling fast process. The main objective of the present study is to apply a more accurate thermodynamic analysis of fast filling process than previous works. By using the thermodynamic analysis, a CNG refueling station with the cascade system, has been simulated. Comparing real and ideal gas models, it can be realized that the temperature profiles are highly different and temperature rise is much more for ideal gas model. the lack of Joule-Thompson cooling effect is the most important factor for this different. The results indicated that there is a temperature rise in order 50 K for real gas and 80 K for the case of ideal gas. The results of the present work have been validated against the experimental data and previous studies. The AGA8 Equation of State has been used to calculate the thermodynamic properties of CNG. The results show that the energy consumed by the compressor for a cycle is equal to 55.1 kWh and the average energy consumption is equal to 1.6 kWh for a vehicle.

Accurate natural gas measurement is an important issue especially in domestic usage levelCapillary tube thermal mass flow meter is one of the most common types of mass flow meters which are mostly used for low mass flow rates. In this work, a capillary tube flow meter was simulated numerically. A three-dimensional steady state heat transfer in its sensor tube was numerically analyzed and the sensitivity of this type of flow meter was investigated. In order to validate the simulation approach and conditions taken in this study, the simulation was also accomplished for Nitrogen, for which experimental data was available in the literature, leading to satisfactory results. Considering all the possible effective parameters, the uncertainty of the flow meter was also calculated. Numerical results show that the maximum amount of uncertainty in measurement for this type of flow meter is less than 1.24% that is acceptable for residential customers.

In this research, the amount of natural gas leakage from a small hole located on the lateral surface of the above-ground or buried distribution gas pipe lines is estimated. The geometry was designed and meshed by GAMBIT software and then numerical calculation is carried out by FLUENT software. The working fluid was considered as compressible ideal gas. The fluid flow was assumed turbulent and soil considered as a porous zone. The results indicate that for small hole diameters, discharge speed reaches to the sound speed and at the so-called, choking occurs at exit position. Also, the volumetric flow rate of leaked gas has a linear relation with pressure of initial point and second order relation with hole diameter. In the case of buried pipes, permeation depth of gas in soil at the small diameter holes, is more than large holes but volumetric rate of leaked gas is lower. Finally, with the goal of making the results applicable for the natural gas industry, two correlations were developed for estimating the amount of leaked gas in aboveground and buried pipelines.

In this study, the cooling, heating and electrical energy demands of a sample residential apartment in Mashhad region (Iran) has been estimated hourly during a year. By employing a CCHP system for this apartment compared to the separate production system, hourly and monthly amounts of optimal consumption of NG as prime fuel and electrical energy imported from/sold back to grid during a year and cost saving ratio and payback ratio has been investigated. For this purpose an optimization algorithm was proposed to find the best optimized operation of the cogeneration system based on operation of the power generation unit (PGU) according to nonlinear relationship of electrical efficiency of the engine (Prime Mover) so that the cost of the energy in CCHP system, as an objective function, be least amount in each time step during a year. The results of proposed algorithm, specifying the best operation, show that PGU with 12 kW capacities has the lowest payback ratio of 3.3 years and that PGU with 5 kW capacities has the lowest energy cost saving ratio of 57%.

Replacement of expansion valves which are used in natural gas pressure reduction stations, with expansion engine, to recycle wasted energy to generate electricity is the main objective of this study. In these engines, ports are used for inlet and outlet the gas. Control valve geometry set on these ports has a great impact on performance of this engine. In this research, simulation and optimization of the optimum opening and closing time of these valves according to two types of valve, piston and spool valves, to maximize the Exergy Efficiency has been done for the first time which Genetic Algorithm is used for this optimization. Simulation has been conducted with regard to the composition of gas stations which to calculate the thermodynamic properties of natural gas, AGA8 standard is used. For optimization constrain of having no back flow has been applied. Results showed that exergy destruction due to outlet processing in piston valve and due to inlet processing in cylinder valve is more than other destruction sources. Overall engine with cylinder valve has better performance than engine with piston valve. Variation of supply line pressure showed that engine with cylinder valve could not be used in inlet pressure of 30 bar, because engine doesn’t produce power.

In this paper by using Fluent software, is paid to numerical simulation of natural gas leakage from a small hole located on the lateral surface of the urban distribution gas pipe lines. The geometry is designed and meshed by Gambit software and methane with Ideal-gas assumption considered as natural gas and flow is turbulent. The results indicate that for small hole diameters, discharge speed reaches the sound speed and at the so-called, choking occurs in the flow. The volumetric flow rate of leaked gas has a second order relation with hole diameter and linear relation with pressure of initial point. Finally a simple useful correlation is developed to calculate the volumetric flow rate of leaked gas. Also data analysis shows that in most of the investigated cases, the percentage of relative error is below 10% which implies high accuracy of the presented correlation.

This paper, paid to calculate and develop an equation to determine the amount of the natural gas leakage, from a small hole located on the surface of the buried gas pipe lines. The studied case designed and meshed by Gambit software and then numerical solution is done by using Fluent software. The pipe is located in the town border distribution pipelines, flow is turbulent and soil considered as a porous zone. The results indicate that at the small hole diameters, discharge speed reaches the sound speed and at the so-called, choking occurs in the flow. Also after permeation of natural gas into the soil, and hitting the soil particles and the air moving through soil, a pair of vortex is created inside the soil. Volumetric flow rate of leaked gas has a second order and linear relation with hole diameter and pressure of initial point. Finally, its paid to develop an equation to calculate the amount of leaked gas.

Annular vortex tube is a vortex tube which allows the hot flow pass again over the hot tube. It is introduced for first time in this work. Hot Flow is not allowed to exit after passing conic valve in annular vortex tube, but it is redirected over hot tube. This back flow absorbs heat from outer wall of hot tube. To study temperature separation which occurs in an annular vortex tube; the performance of this type of vortex tube has been experimentally tested and compared with the performance of a typical vortex tube. Inlet test pressure is 4 bars and natural gas is being used as working fluid. For both type of vortex tubes, ratio of length to diameter of tube is 10. Cold orifice diameter of vortex generator is set to 6.4 mm. It was observed that redirecting hot flow over the hot tube in annular vortex tube improves cooling efficiency up to 24% respect to a typical vortex tube at the maximum temperature difference. The results show that cold mass fraction in which the coldest temperature occurs is lower for annular vortex tube comparing with a typical vortex tube.

At a Compressed Natural Gas (CNG) filling station، natural gas is stored in the high pressure reservoirs. The pressures within these reservoirs have huge effects on fast filling process of a natural gas vehicle’s (NGV) cylinder and the difficulties associated with the filling process. The accurate modeling of the fast-fill process is a complex procedure which should be thoroughly studied to optimize the filling process. Here، a theoretical analysis has been developed to study the effects of various parameters on the CNG filling process and the conditions. The analysis is based on the first and second laws of thermodynamics، conservation of mass and the AGA8 equation of state. The required properties of natural gas mixtures have been calculated making use of the AGA8 equation of state (EOS) and thermodynamics relationships. It is found that، the composition of natural gas is very effective on the CNG filling process and final in-cylinder values. For various Iranian natural gas compositions، the optimized filling stations'' reservoirs pressure has been found.

Pre-swirl is often used in the internal cooling-air systems of gas turbines to reduce the temperature of the cooling air relative to the rotating turbine blades. In a "direct-transfer" system, the air passes axially across the wheel-space from stationary pre-swirl nozzles to receiver holes in the rotating turbine disc. This paper investigates the effects of inlet flow conditions and rotational speed on the flow and the heat transfer in such system, using a simplified computational model for three-dimensional steady, incompressible turbulent flow based on an in-house solver developed by the author. The computed results are compared with measurements of tangential velocity and Nusselt number and show that there are significant mixing losses near the inlet nozzles, resulting in a reduced “effective” pre-swirl ratio at inlet. The computed tangential velocity distributions suggest free-vortex-type behaviour for the flow between the pre-swirl nozzle radius and that of the receiver holes. There is mainly good agreements between the computed and the measured velocities, although the measured values are generally lower than the computed ones and follow free-vortex behaviour less closely. There is less agreements between the computed and the measured values of Nusselt number, although the correct trends are obtained for the effects of the main flow parameters.