Gases Brayton cycle

The thermodynamic power cycles most commonly used today are the vapor Rankine cycle and the gas Brayton cycle (see Chapter 4). Both are characterized by two isobaric and two isentropic processes. The vapor... 

Rankine cycle operates mainly in the saturated region of its working fluid whereas the gas Brayton cycle processes are located entirely in the superheat or gas region. 

The gas Brayton cycle adds heat in a isobaric process over a large temperature range. The temperature level is independent of the pressure level. No blade erosion occurs in the gas turbine. However, the compression process of the gas Brayton cycle requires large work input. The back-work ratio is small. 

C, but with advanced materials, reactor outlet temperatures of 800°C may be possible. An indirect gas Brayton cycle is used to produce electrical power. 

The SSTAR reactor is coupled to a supercritical carbon dioxide (S-CO2) Brayton cycle power converter. It provides higher cycle efficiency than a helium ideal gas Brayton cycle or a Rankine saturated steam cycle operating at the same core outlet temperature. A key contributor to the high efficiency is the low amount of work (PdV work) to compress S-CO2 immediately above its critical temperature - due to the high S-CO2 density. Table XXII-6 compares the densities of S-CO2 at cycle conditions versus those for helium in the Eskom Pebble Bed Modular Reactor (PBMR) as well as typical liquid coolants the S-CO2 density is more like that of an ordinary liquid. Thus, the S-CO2 temperature and pressure at the low end of the cycle are designed close to but slightly greater than the critical temperature (30.98°C) and pressure (7.373 MPa) to exploit the small PdV work of compression. 

Since the Courant limit is controlled by both the length of the volume and fluid velocity, higher velocities, such as those seen in gas Brayton cycles, exacerbate the increase in problem run time. The parallel Brayton system model used in this study includes four coimter flow heat exchangers. In order to approach the analytically determined gas state points, it was necessary to model each heat exchanger with 80 control volumes. This resulted in a Courant hmit of approximately 0.5 milliseconds and problems that tan roughly 25 times slower than real time on the super computers used by the Naval Nuclear Program. Since most of the initial scoping jobs require 1000 seconds of problem run time, these jobs would take almost 7 hours to run. 

Figure 12-122 is a depiction of the RELAP5-3D modeJ creaited to simulate the transient behavior of the SNPP direct gas Brayton cycle reactor plant as seen in the SNAP user interface. The screen shot in Figure 12-122 has the gas cooled reactor portion of the model highlighted to indicate the demarcation point between the reactor fluid system and the Brayton cycle fluid system, since there is -no phyeieal ooporotion botwoon them-----------------------------------------------------... 

Gas-Cycle Systems. In principle, any permanent gas can be used for the closed gas-cycle refrigeration system however, the prevailing gas that is used is air. In the gas-cycle system operating on the Brayton cycle, all of the heat-transfer operations involve only sensible heat of the gas. Efficiencies are low because of the large volume of gas that must be handled for a relatively small refrigera tion effect. The advantage of air is that it is safe and inexpensive. 

Because of the very small bearing clearances in gas bearings, dust particles, moisture, and wear debris (from starting and stopping) should be kept to a minimum. Gas bearings have been used in precision spindles, gyroscopes, motor and turbine-driven circulators, compressors, fans, Brayton cycle turbomachinery, environmental simulation tables, and memory dmms. 

In apphcation to electric utihty power generation, MHD is combined with steam (qv) power generation, as shown in Figure 2. The MHD generator is used as a topping unit to the steam bottoming plant. From a thermodynamic point of view, the system is a combined cycle. The MHD generator operates in a Brayton cycle, similar to a gas turbine the steam plant operates in a conventional Rankine cycle (11). 

The surprise was finally clarified by remembering that this was an air operated plant built in a thermodynamic cycle, (the Brayton or gas turbine cycle) with a 18,000 HP air compressor. This generated 5 MW of salable... 

The modified Brayton cycle is used for both gas turbines and jet engines. The turbine is designed to produce a usable torque at the output shaft, while the jet engine allows most of the hot gases to expand into the atmosphere, producing usable thrust. Emissions from both turbines and jets are similar, as are their control methods. The emissions are primarily unbumed hydrocarbons, unbumed carbon which results in the visible exhaust, and oxides of nitrogen. Control of the unbumed hydrocarbons and the unburned... 

The Brayton cycle in its ideal form consists of two isobaric processes and two isentropic processes. The two isobaric processes consist of the combustor system of the gas turbine and the gas side of the HRSG. The two isentropic processes represent the compression (Compressor) and the expansion (Turbine Expander) processes in the gas turbine. Figure 2-1 shows the Ideal Brayton Cycle. 

Inereasing the pressure ratio and the turbine firing temperature inereases the Brayton cycle efficiency. This relationship of overall cycle efficiency is based on certain simplification assumptions such as (1) liia > nif, (2) the gas is caloricaly and thermally perfect, which means that... 

The work required to drive the turbine eompressor is reduced by lowering the compressor inlet temperature thus increasing the output work of the turbine. Figure 2-35 is a schematic of the evaporative gas turbine and its effect on the Brayton cycle. The volumetric flow of most turbines is constant and therefore by increasing the mass flow, power increases in an inverse proportion to the temperature of the inlet air. The psychometric chart shown shows that the cooling is limited especially in high humid conditions. It is a very low cost option and can be installed very easily. This technique does not however increase the efficiency of the turbine. The turbine inlet temperature is lowered by about 18 °F (10 °C), if the outside temperature is around 90 °F (32 °C). The cost of an evaporative cooling system runs around 50/kw. 

In the gas turbine (Brayton cycle), the compression and expansion processes are adiabatic and isentropic processes. Thus, for an isentropic adiabatic process 7 = where Cp and c are the specific heats of the gas at constant pressure and volume respectively and can be written as ... 

For an irreversible gas turbine cycle (the irreversible Joule-Brayton (UB) cycle of Fig. 1.9), less than unity) and < 1 so that the thermal efficiency is... 

Figure 16-8. This Brayton cycle describes the basic operation of a gas turbine.

Figure 15.5 shows the ideal open cycle for the gas turbine that is based on the Brayton Cycle. By assuming that the chemical energy released on combustion is equivalent to a transfer of heat at constant pressure to a working fluid of constant specific heat, this simplified approach allows the actual process to be compared with the ideal, and is represented in Figure 15.5 by a broken line. The processes for compression 1-2 and expansion 3-4 are irreversible adiabatic and differ, as shown from the ideal isentropic processes between the same pressures P and P2 -... 

This cycle also uses continuous counterflow heat exchanger and is closely related to the Joule-Thomson and Claude cycles as shown in Fig. 5.15(a) [60], The cryocooling or reverse Brayton cycle derives from a reciprocating gas engine patented by G. B. Brayton in... 

Gas turbine systems operate on the thermodynamic cycle known as the Brayton cycle. In a Brayton cycle, atmospheric... 

On-site combined heat and power (CHP) which has existed for years, includes turbines, reciprocating engines and steam turbines. Gas turbines in the 500-kW to 250-MW produce electricity and heat using a thermodynamic cycle known as the Brayton cycle. They produce about 40,000-MW of the total CHP in the United States. The electric efficiency for units of less... 

The results of the performance calculations are summarized in Table 9-24. The efficiency of the overall power system, work output divided by the lower heating value (LHV) of the CH4 fuel, is increased from 57% for the fuel cell alone to 82% for the overall system with a 30 F difference in the recuperative exchanger and to 76% for an 80 F difference. This regenerative Brayton cycle heat rejection and heat-fuel recovery arrangement is perhaps the simplest approach to heat recovery. It makes minimal demands on fuel cell heat removal and gas turbine arrangements, has minimal number of system components, and makes the most of the inherent high efficiency of the fuel cell. 

Absorption Air-Conditioning Brayton Gas Refrigeration Cycle Stirling Refrigeration Cycle Ericsson Cycle Liquefaction of Gases Nonazeotropic Mixture Refrigeration Cycle Design Examples Summary... 

The ideal Brayton gas turbine cycle (sometimes called Joule cycle) is named after an American engineer, George Brayton, who proposed the cycle in the 1870s. The gas turbine cycle consists of four processes an isentropic compression process 1-2, a constant-pressure combustion process 2-3, an isentropic expansion process 3-4, and a constant-pressure cooling process 4-1. The p-v and T-s diagrams for an ideal Brayton cycle are illustrated in Fig. 4.1. 

The gas turbine cycle may be either closed or open. The more common cycle is the open one, in which atmospheric air is continuously drawn into the compressor, heat is added to the air by the combustion of fuel in the combustion chamber, and the working fluid expands through the turbine and exhausts to the atmosphere. A schematic diagram of an open Brayton cycle, which is assumed to operate steadily as an open system, is shown in Fig. 4.2. 

Increasing the average temperature during the heat-addition process with a reheater without increasing the compressor pressure ratio increases the net work of the Brayton cycle. A multistage turbine is used. Gas is reheated between stages. 

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