Results 1 - 10 of 3647
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[en] A cryogenic regenerator for a Stirling cycle is discussed by minimizing the entropy gain as the criterion of performance. Only the gas losses are treated here. The authors argue that the optimum design corresponds to uniform channel flow with minimum turbulence. The optimization depends upon minimizing the sum of three sources of entropy generation, those due to transverse and parallel heat conduction and that due to friction with the wall. This leads to criteria for the width, length, and velocity of the gas, which for helium become W= 1.6x10-4 TO/(σPO) cm, L= 6.7x10-5 TO/(σ2PO) cm, and v/Cs = σ/2 respectively where σ is the ratio (entropy gain)/(entropy transferred), Cs is sound speed, PO is the pressure in atmospheres, and TO is the ratio of temperature to room temperature. The thermal properties of the channel wall must then accommodate the heat flow of the gas without substantially increasing the loss fraction. That problem is reserved to another paper
[en] This work deals with the study of an irreversible refrigeration cycle controlled by three heat sources. Following the basic idea to treat the entire system as a combined cycle of an internally irreversible two heat source engine driving an internally irreversible two heat source refrigerator, we select a classical configuration to derive a fundamental optimum formula. Each one of these irreversible engine are characterized by the irreversibility factors Iw and Ir, respectively. Therefore, the predicted optimal plant performance coefficient and the relationship of that with the cooling effect reveal a unified description of various refrigerators of three heat sources previously reported in finite time thermodynamics. The numerical results show clearly the physical influence of these irreversibility factors together with the thermal resistances on the global performance of the refrigeration plant
[en] In order to predict the maximum performance of scramjet engine at flight conditions with high freestream Mach numbers, a thermodynamic model of Brayton cycle was utilized to analyze the effects of inlet pressure ratio, fuel equivalence ratio and the upper limit of gas temperature to the specific thrust and the fuel impulse of the scramjet considering the characteristics of non-isentropic compression in the inlet. The results show that both the inlet efficiency and the temperature limit in the combustor have remarkable effects on the overall engine performances. Different with the ideal Brayton cycles assuming isentropic compression without upper limit of gas temperature, both the maximum specific thrust and the maximum fuel impulse of a scramjet present non-monotonic trends against the fuel equivalence ratio in this study. Considering the empirical design efficiencies of inlet, there is a wide range of fuel equivalence ratios in which the fuel impulses remain at high values. Moreover, the maximum specific thrust can also be achieved with a fuel equivalence ratio near this range. Therefore, it is possible to achieve an overall high performance in a scramjet at high Mach numbers. - Highlights: • Thermodynamic analysis with Brayton cycle on overall performances of scramjet. • The compression loss in the inlet was considered in predicting scram-mode operation. • Non-monotonic trends of engine performances against fuel equivalence ratio.
[en] Highlights: • HPCCE contains precooling method, dual-mode feature and multi-cycle coupling mechanism. • Large-scale frequent energy exchange enables HPCCE to utilize energy in high speed intake air. • Core compressor connects cycles which should efficiently work in a wide pressure ratio range (4.0–10.0). • Engine design reflects compromises between hypersonic and subsonic work conditions. • Over-high coolant flow rate shadows benefits. Innovation of flow path and its detail discussion are needed. - Abstract: Propulsion system is a crucial issue of hypersonic civil aviation. Precooled engine shows attractive advantages in high flight Mach numbers, and the hypersonic precooled combined cycle engine (HPCCE) is one of the most potential plans. Multi-cycle coupling is the most remarkable feature of HPCCE, reflected by the large-scale frequent energy exchange among subsystems, i.e. opened cycle, closed cycle and propellant system, and the interaction between the designs for different working conditions. The mechanism is studied through engine cycle analysis with enthalpy-entropy relations. Quantitative discussion on major design parameters based on a component level simulation model shows the trends of inter-impact among subsystems and parameters, and sketches the multi-cycle mechanism and energy transfer relations in detail. Vital issues that should be emphasized through the design process are pointed out.
[en] In an expander for use in a split Stirling cycle refrigeration system of the type wherein a displacer moves with reciprocating motion inside an expander housing, and wherein a plunger force and a regenerator force are formed on the displacer, the plunger force cyclically varying and having a time of minimum and maximum plunger force amplitude, and the regenerator force cyclically varying and having a time of minimum and maximum regenerator force amplitude, the improvement is described comprising: (a) means for maintaining displacer forces, such that the maximum plunger force amplitude is substantially equal to the maximum regenerator force amplitude; and (b) means for adjusting a time difference, the time difference being the time between the time of maximum plunger force and the time of maximum regenerator force such that a measure of the cooling power of the refrigeration system is maximized
[en] In this paper, the theory of finite time thermodynamics is used in the performance analysis of an irreversible closed intercooled regenerated Brayton cycle coupled to variable temperature heat reservoirs. The analytical formulae for dimensionless power and efficiency, as functions of the total pressure ratio, the intercooling pressure ratio, the component (regenerator, intercooler, hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficients, the heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio, are derived. The intercooling pressure ratio is optimized for optimal power and optimal efficiency, respectively. The effects of component (regenerator, intercooler and hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies, the pressure recovery coefficients, the heat reservoir inlet temperature ratio and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio on optimal power and its corresponding intercooling pressure ratio, as well as optimal efficiency and its corresponding intercooling pressure ratio are analyzed by detailed numerical examples. When the heat transfers between the working fluid and the heat reservoirs are executed ideally, the pressure drop losses are small enough to be neglected and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper replicate those obtained in recent literature
[en] The work reported herein represents a significant step in the preliminary design of heat exchanger options (material options, thermal design, selection and evaluation methodology with existing challenges). The primary purpose of this study is to aid in the development and selection of the required heat exchanger for power production using either a subcritical or supercritical Rankine cycle.