Maximum Cycle Temperature Research Articles

Influence of Extremum Temperatures on TMF of a Ni-Base Superalloy

Significantly reducing the minimum temperature while maintaining maximum temperature of thermomechanical fatigue (TMF) cycles can reduce the life even when mechanical strain ranges are similar. This applies to in-phase (IP) and out-of-phase (OP) TMF cycles. This reduction in life has generally been attributed to a combination of changes in microstructure arising from aging and increases in the cyclic inelastic strain promoted by increases in the elastic modulus as the minimum cycle temperature is reduced. TMF cycles under both IP and OP conditions were conducted with maximum cycle temperatures within the 750-950C range and with minimum cycle temperatures of either 100 or 500C. A reduction in minimum temperature was observed to promote a decrease in TMF life by as much as a factor of ten for all TMF experiments. The reduction in TMF life is primarily controlled by increases in the inelastic strain range associated with increases in the elastic modulus that arise when the minimum temperature is reduced.

Gas turbine with heating during the expansion in the stator blades

Reheat is used in the gas turbine to achieve higher power output. However, the reheat process is constrained by the heat quantity given to it and the choice of reheat point. Consequently, this paper introduces a new gas turbine cycle to overcome the reheat drawbacks and having superior features. In this cycle, the reheat process is replaced by processes of heating the expanded gases while passing through different turbine stator blades. Small amount of combusted gases is utilized to flow inside such blades for heating and mixing with the expanded gases. Nevertheless, this is performed with precautions of turbine overheating by reducing significantly the maximum temperature of the present cycle. The simulated results demonstrate that the cycle performance is increased by raising the quantity of heating during the expansion. Additionally, this cycle achieves greater efficient output than the traditional reheat Brayton cycle operating with higher maximum cycle temperature. To boost the present cycle efficiency, regeneration is used making the possibility of such cycle to be competitive to the combined cycle.

Optimum parametric performance characterization of an irreversible gas turbine Brayton cycle

Abstract 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., η I ≥ 38 ≥ η II ≥ 60%, ECOP ≥ 1.65, x loss ≤ 0.150 MJ/kg, BWR ≤ 0.525, w net ≥ 0.300 MJ/kg, and q add ≤ 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.

Second law assessment of a syngas-fuelled triple thermodynamic cycle for sustainable power generation

This paper investigates the effects of change in cycle pressure ratio and maximum cycle temperature as well as the change in refrigerant in organic Rankine cycle (ORC) on energy and exergy performance of biomass-driven triple-power cycle. The results show that both first and second law efficiencies of the proposed triple-power cycle decreases with the increase in pressure ratio and increases with the increase in the maximum cycle temperature. The effect of change in the refrigerant in ORC is found to be marginal. Second law assessment of the cycle reveals that combustor, heat recovery steam generator (HRSG), and gasifier are the three main sources of irreversibility, where it is shown that out of the 100% fuel exergy input; around 25% of exergy is destructed in the combustor, 19% in HRSG, and 7.6% in the gasifier. Investigations show that the triple-power cycle has better thermodynamic performance than the combined power cycle driven by biomass.

Effect of Parameters Variation on the Performance of Adsorption Based Cooling Systems

In this study, an investigation of the influence of the adsorption refrigeration system parameters, within their limiting range, on the system performance and operation is introduced. A thermodynamic differential model is developed to analyze the system components and processes of the thermodynamic operating cycle. The analysis is based on a clear fundamental approach which is based on the energy conservation principle and dynamic equilibrium of the adsorption process. Activated carbon‎‎‎–‎methanol as working pair is used in this work. The effect of condensation, evaporator, ambient, and the maximum cycle temperatures on the adsorption cooling system behavior are explained and clarified. Moreover, the influences of the heat capacity of the metallic casing of the adsorption reactor as well as the total bed porosity variations are studied as well.

ANALYSIS OF HEAT AND MASS TRANSFER IN THE ADSORBENT BED OF A THERMAL WAVE ADSORPTION COOLING CYCLE

A coupled heat and mass transfer analysis of the adsorbent bed of a thermal wave adsorption cooling cycle is performed. The adsorbent bed is modeled two dimensionally. Governing equations for energy, mass and momentum transfers are solved by Comsol Multiphysics simultaneously. Variations of temperature, pressure, adsorption capacity, equilibrium adsorption capacity and mass transfer coefficient for a finless tube adsorbent bed are presented with multicolored plots. Desorption process is simulated in the model. Results show that the adsorbent bed reaches the maximum cycle temperature uniformly after 5000s. Uniform pressure assumption can be used in the bed due to small pressure gradient during the process. Adsorption capacity decreases from 26.1% to 8.89% in 5000s. Temperature front progresses along the bed creating a thermal wave. The thermal wave length is needed to be short to enhance the bed effectiveness regenerating more heat between the beds of the thermal wave cycle. It can be concluded that a parametric study is recommended for a future work in order to investigate the effects of design and operational parameters on the dependent variables and thermal wave length.

Kinetics of partial dehydroxylation in dioctahedral 2:1 layer clay minerals

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of partially dehydroxylated illite, aluminoceladonite, and dioctahedral smectite samples. The method consists of consecutive heating cycles separated by intervals of cooling to room temperature, with the maximum cycle temperatures (MCTs) set incrementally higher in each consecutive cycle. In the studied samples, dehydroxylation of each portion of the initial OH groups follows the kinetics of a homogeneous zero-order reaction in each heating cycle. The activation energies ( E a ) were calculated in terms of this model for separate heating cycles of each sample with regression coefficients R 2 ≥ 0.999. A zero-order reaction determined at each heating cycle indicates that at each stage of partial dehydroxylation, there is no formation of an intermediate phase and the reaction is probably the direct transformation of the original layers into completely dehydroxylated layers. The Wyoming montmorillonite and illite consisting of cis -vacant ( cv ) layers had the highest E a values (53–55 kcal/mol). In the samples consisting of trans -vacant ( tv ) layers and having almost the same octahedral cation composition the maximum E a values varied from 45 to 30 kcal/mol and the E a of each sample in this group are similar over a wide range of the D T . For the samples consisting of a mixture of cv and tv illite structures, a bimodal distribution of the E a values exists with increasing MCT and D T . The maximum E a values for dehydroxylation of the tv and cv illite structures are different. The activation energies from the tv aluminoceladonite and Otay tv montmorillonite samples have similar maximum E a values (39.4 to 41.8 kcal/mol), but the variation in E a with D T has a skewed bell-like distribution that is probably related to a heterogeneous octahedral cation composition of the 2:1 layers. The E a values calculated for the mineral structures in this study are compared with those published and the main factors controlling the E a variation at different stages of the partial dehydroxylation are discussed.

Mechanism‐based life model for out‐of‐phase thermomechanical fatigue in single crystal Ni‐base superalloys

ABSTRACTThis paper describes an enhanced physics‐inspired model to predict the life of the second‐generation single crystal superalloy PWA 1484 experiencing out‐of‐phase (OP) thermomechanical fatigue (TMF). Degradation due to either pure fatigue or a coupling between fatigue and environmental attack are the primary concerns under this loading. The life model incorporates the effects of material anisotropy by utilizing the inelastic shear strain on the slip system having the highest Schmid factor while accounting for the effects of temperature‐dependent slip spacing and stress‐assisted γ′ depletion. Both conventional TMF and special bithermal fatigue (BiF) experiments were conducted to isolate and therefore better understand the interactions between these degradation mechanisms. The influences of crystallographic orientation, applied mechanical strain range, cycle maximum temperature and high temperature hold times were assessed. The resulting physics‐inspired life estimation model for OP TMF and OP BiF predicts the number of cycles to crack initiation as a function of crystal orientation, applied strain amplitude and stresses, temperature, cycle time (including dwells), and surface roughness within a factor of 2.

A criterion to maximize the net-work output in a Miller cycle

The optimal performance parameters of a Miller heat-engine have been discussed in detail by using finite-time thermodynamics. The normalized net-work output and normalized thermal efficiency are obtained by introducing the compression ratio, the expansion–compression ratio and the specific heat ratio. The results show that, throughout the working range of the cycle, there is a maximum net-work output during the ranges of compression ratio and specific heat ratio, while the net-work output increases with increasing the expansion–compression ratio and the maximum cycle temperature ratio. The results also show that, throughout the working range of the cycle, the thermal efficiency increases with increasing the compression ratio, the expansion–compression ratio and the specific heat ratio, while the thermal efficiency increases with increasing the maximum cycle temperature ratio. The results obtained in this paper may provide guidance for the design of practical Miller engines.

Performance Analysis of a Diesel Cycle under the Restriction of Maximum Cycle Temperature with Considerations of Heat Loss, Friction, and Variable Specific Heats

of the cycle decrease with increasing friction loss. It is noteworthy that the eects of heat loss characterized by a percentage of fuel’s energy, friction and variable specific heats of working fluid on the performance of a Diesel-cycle engine are significant and should be considered in practice cycle analysis. The results obtained in the present study are of importance to provide a good guidance for the performance evaluation and improvement of practical Diesel engines.

Optimization of Gas Turbine Cogeneration Systemfor Various Heat Exchanger Configurations

The present paper investigates and compares the performance of three configurations of Gas Turbine systems allowing cogeneration of heat and electricity, on the basis of an irreversible regenerative Brayton-Joule cycle. The proposed model is developed for two different cycle constraints, namely, an imposed heat transfer rate released by the fuel combustion, or an imposed maximum cycle temperature. The model also includes the irreversibility due to the friction in the compressor and turbine, and due to the heat losses in the combustion chamber and heat exchangers. Energy efficiency for the system without and with cogeneration, and the exergetic efficiency are used in order to emphasize the cogeneration advantages, but also to help the designer to choose the best configuration of the Gas Turbine system that suits to his needs. Experimental data from a real operating microturbine were used to validate the model. The power output and the energy and exergetic efficiencies are optimized with respect to a set of operating parameters. The optimum values of the Gas Turbine engine parameters corresponding to maximum power output and respectively to maximum thermodynamic efficiency are discussed. The results show same optimal values of the compression ratio corresponding to almost all maximum performances for an imposed heat transfer rate released by the fuel combustion, excepting the maximum exergetic efficiency that requires higher optimal values of the compression ratio than the maximum exergy rate one. A performance comparison of the three configurations is done and future perspectives of the work are proposed.

Kinetics of thermal transformation of partially dehydroxylated pyrophyllite

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of two partially dehydroxylated pyrophyllite samples. In the original state, the S076 sample contains only trans-vacant (tv) layers, whereas in sample S037 tv and cis-vacant (cv) layers are randomly interstratified. The method consists of consecutive heating cycles (rate 2 °C/min) separated by intervals of cooling to room temperature, with the maximum cycle temperature set (MCT) incrementally higher than in each previous cycle.

A Simple Parametric Model for the Analysis of Cooled Gas Turbines

A natural gas fired gas turbine combined cycle power plant is the most efficient option for fossil fuel based electric power generation that is commercially available. Trade publications report that currently available technology is rated near 60% thermal efficiency. Research and development efforts are in place targeting even higher efficiencies in the next two decades. In the face of diminishing natural resources and increasing carbon dioxide emissions, leading to greenhouse gas effect and global warming, these efforts are even more critical today than in the last century. The main performance driver in a combined cycle power plant is the gas turbine. The basic thermodynamics of the gas turbine, described by the well-known Brayton cycle, dictates that the key design parameters that determine the gas turbine performance are the cycle pressure ratio and maximum cycle temperature at the turbine inlet. While performance calculations for an ideal gas turbine are straightforward with compact mathematical formulations, detailed engineering analysis of real machines with turbine hot gas path cooling requires complex models. Such models, requisite for detailed engineering design work, involve highly empirical heat transfer formulations embedded in a complex system of equations that are amenable only to numerical solutions. A cooled turbine modeling system incorporating all pertinent physical phenomena into compact formulations is developed and presented in this paper. The model is fully physics-based and amenable to simple spreadsheet calculations while illustrating the basic principles with sufficient accuracy and extreme qualitative rigor. This model is valuable not only as a teaching and training tool, it is also suitable to preliminary gas turbine combined cycle design calculations in narrowing down the field of feasible design options.

Thermodynamic analyses and optimization of a recompression N2O Brayton power cycle

Thermodynamic analyses and simultaneous optimizations of cycle pressure ratio and flow split fraction to get maximum efficiency of N2O recompression Brayton cycle have been performed to study the effects of various operating conditions and component performances. The energetic as well as exergetic performance comparison with its counterpart recompression CO2 cycle is presented as well. Optimization shows that the optimum minimum cycle pressure is close to pseudo-critical pressure for supercritical cycle, whereas saturation pressure corresponding to minimum cycle temperature for condensation cycle. Results show that the maximum thermal efficiency increases with decrease in minimum cycle temperature and increase in both maximum cycle pressure and temperature. Influence of turbine performance on cycle efficiency is more compared to that of compressors, HTR (high temperature recuperator) and LTR (low temperature recuperator). Comparison shows that N2O gives better thermal efficiency (maximum deviation of 1.2%) as well as second law efficiency compared to CO2 for studied operating conditions. Component wise irreversibility distribution shows the similar trends for both working fluids. Present study reveals that N2O is a potential option for the recompression power cycle.

Study of the Thermal Properties of a Ni3Ta Shape Memory Alloy

Dilatation characteristics, thermal diffusivity, and thermal conductivity of a Ni3Ta shape memory alloy were studied over the temperature range from room temperature up to 950 °C. The Ni3Ta alloy was investigated in both polycrystalline and single crystal forms. The shape memory effect was positive for the polycrystalline samples and negative for the single crystal. While the transformation temperature of the M (martensite) → A (austenite) phase transformation was the same for both types of alloys and all measurements, the transformation temperature for the reverse phase transformation A → M was dependent on the maximum cycle temperature. Higher maximum temperatures of the thermal cycle yielded lower transformation temperatures for the A → M transformation. The thermal diffusivity and thermal conductivity of the austenite were higher than those of the martensite. No latent heat was found for the phase transformations.

Exergy analysis of a gas turbine trigeneration system using the Brayton refrigeration cycle for inlet air cooling

Trigeneration with gas turbines as prime mover can improve energy utilization efficiency significantly because of their potential of high economic and energy saving characteristic. In this article, a novel method for compressor inlet air cooling in gas turbine trigeneration is achieved by the Brayton refrigeration cycle driven by the gas turbine and air is used as a working fluid. Introducing the air refrigeration cycle for inlet air cooling provides the advantage of quite low temperatures close to 0 °C and even lower, and enhances the performance significantly. The performance was evaluated by both energy and exergy efficiencies, with the latter providing good guidance for system improvement. The influences of key parameters, which include extraction pressure ratio, extraction mass rate, turbine inlet temperature (TIT), process heat pressure, and ambient relative humidity on the cycle performance parameters, have been investigated. It is found that the inlet air-cooled gas turbine trigeneration cycle has a good thermal performance, with first and second law efficiencies of 91 per cent and 55 per cent, respectively, for the base case studied (having a maximum cycle temperature of 1327 °C). For a minimum process heat pressure of 4 bar, the energy and exergy efficiencies were obtained as 89 per cent and 51 per cent, respectively. Gas turbine trigeneration plants with and without inlet air cooled by the Brayton refrigeration cycle are compared with each other, and the effect of inlet air cooling on energy and exergy efficiencies is clarified. Inlet air cooling increases the energy and exergy efficiencies and decreases the electrical to thermal energy ratio significantly. Exergy analysis indicates that maximum exergy is destroyed during combustion and steam generation, which represents around 65 per cent of the total exergy destruction in the overall system. The exergy destruction in major components of trigeneration varies significantly with TIT and process heat pressure. The present analysis provides further information on the role of inlet air cooling through extracted mass rate influence on the performance of a gas turbine-based trigeneration system.

Effects of combustion efficiency on a Dual cycle

This paper studies a Dual cycle model containing irreversibilities coming exclusively from expansion and compression processes. It is assumed that any fraction of the fuel’s chemical energy can not fully released inside the engine because of the incomplete combustion. Utilizing the combustion efficiency is found to be more useful to realize the cycle feasibility. Amount of the released energy from fuel into the cylinder restricts the compression ratio. It is presented how the upper limit of compression ratio is evaluated by means of using some constraints for realizing a Dual cycle. Valid ranges of the constraints given in literature seriously affect the feasibility of cycle. Developed mathematical model leads to a qualitative understanding of how engine loss can be reduced. Thermal efficiency–work curves cannot have a closed loop shape because there is a close relationship between the fuel energy, air–fuel mass ratio, combustion efficiency, maximum cycle temperature and the heat losses into the cylinder wall. If these are all omitted, while heat losses are determined independently without establishing any relationship between the released fuel energy, the thermal efficiency versus work curves will just be able to have a closed loop shape. This is the original perspective and contribution of paper.

Optimization of recompression S-CO 2 power cycle with reheating

In the present study, optimization of compressor pressure ratio and intermediate pressure between HP and LP turbines leading to maximum thermal efficiency is implemented for a supercritical CO 2 recompression cycle with reheating applicable to next generation nuclear reactors. Effects of various operating conditions and component performances on the optimal values and cycle efficiency are studied as well. Finally, a comparison between supercritical CO 2 recompression cycles with and without reheating is presented. The optimization of recompression mass fraction shows that the present cycle configuration yields maximum thermal efficiency for nearly equal heat capacity values of both fluids in LTR. The effect of maximum cycle temperature on the optimum pressure ratio is negligible for lower values of minimum cycle temperature and maximum pressure, and the effect becomes significant with an increase in these parameters. Results show that the thermal efficiency improves with a decrease in minimum cycle temperature and with increase in maximum cycle temperature and pressure. The optimum intermediate pressure is exhibited to be higher than the geometric mean value between minimum and maximum pressure and a simple empirical correlation useful for optimum design is presented. The maximum efficiency improvement using reheating is calculated as 3.5% at optimum conditions.

Delamination toughness of electron beam physical vapor deposition (EB-PVD) Y2O3–ZrO2 thermal barrier coatings by the pushout method: Effect of thermal cycling temperature

A pushout test method was used to quantify effect of thermal cycling temperatures on the delamination toughness of an electron beam physical vapor deposited thermal barrier coating (EB-PVD TBC). The delamination toughness, Γi, was related to the maximum thermal cycling temperature, Th, equal to 1000, 1025, 1050, and 1100 °C. The measured delamination toughness varied from 9 to 95 J/m2. At Th = 1000 °C, Γi attained a maximum value, larger than that of the as-deposited sample and decreasing with increased Th. During the thermal cycling tests, the thermally grown oxide (TGO) was formed between the TBC and the bond coat deposited onto the superalloy substrate. Inside the TGO layer, mixture of Al2O3 and ZrO2 oxides was observed close to the TBC side with nearly pure Al2O3 phases close to the bond-coat side. During the pushout test, delamination occurred at the interface of the mixture and pure Al2O3 layer with an exception for Th = 1100 °C specimens where delamination also occurred at the interface between the TGO and bond-coat layers. The effect of thermal cycling temperatures on the delamination toughness is discussed in terms of the microstructural change and delamination behavior.

Unified Approach to Exergy Efficiency, Environmental Impact and Sustainable Development for Standard Thermodynamic Cycles

The exergy efficiency of three standard thermodynamic cycles, i.e., Brayton, Rankine and Otto cycles, are developed and the corresponding analytical equations are derived accordingly. The resultant expressions are applied to typical operating conditions and numerical results are obtained, when the heat of each engine is supplied by burning natural gas as a fuel with 100 percent theoretical air. A common result is the significant effect of the maximum cycle temperature, which causes an increase of exergy efficiency. It is shown that the compression ratio of the Brayton and Otto cycles, as well as the turbine inlet pressure in a steam power plant, raise the exergy efficiency. Moreover, increasing the ambient temperature has a negative influence on the exergy efficiency in the Brayton and Otto cycles, which occurs due to ambient air fed to these systems, thereby decreasing the deviation of the system from ambient conditions and reducing the exergy efficiency. Further findings include an optimal performance point of the Brayton and Rankine cycle, with a high sustainability and exergy efficiency. For instance, at the optimal operating point of the Brayton cycle with a compression ratio of 8 (or 12 for a second case), the exergy efficiency is 73 (60) percent, CO2 emissions is 530 (590) g/kWh and the sustainability index is 3.8 (2.8). The optimal operating point for an example of a Rankine cycle is found to be 50 percent for the exergy efficiency, with 440g/kWh of emitted CO2 and a sustainability index of two.