OPTIMIZATION OF REALISTIC REFRIGERATION PLANT UNDER FIXED TOTAL THERMAL CONDUCTANCE CONSTRAINT

2007 ◽  
Vol 31 (2) ◽  
pp. 243-253
Author(s):  
Tong-Bou Chang
Keyword(s):  
Thermal Conductance ◽  
Internal Heat ◽  
Heat Losses ◽  
Numerical Results ◽  
Coefficient Of Performance ◽  
Design Constraint ◽  
Optimal Coefficient ◽  
Conductance Ratio ◽  
Internal Irreversibility

This study analyzes the internal irreversibility of a realistic refrigeration plant under the design constraint of a fixed total thermal conductance. The internal heat losses are determined using a heat by-pass model. The optimal thermal conductance allocation and optimal coefficient of performance are derived from a series of detailed analyses and formulations. The numerical results indicate that the optimal thermal conductance ratio of the hot end of a realistic refrigeration plant is slightly higher than 0.5.

Efficiency Optimizations of an Irreversible Brayton Heat Engine

1998 ◽  
Vol 120 (2) ◽  
pp. 143-148 ◽  
Author(s):  
C.-Y. Cheng ◽  
C.-K. Chen
Keyword(s):  
Heat Transfer ◽  
Thermal Efficiency ◽  
Heat Exchangers ◽  
Heat Engine ◽  
Thermal Conductance ◽  
Working Fluid ◽  
Reservoir Temperature ◽  
Temperature Ratio ◽  
Conductance Ratio ◽  
Internal Irreversibility

A steady-flow approach for finite-time thermodynamics is used to calculate the maximum thermal efficiency, its corresponding power output, adiabatic temperature ratio, and thermal-conductance ratio of heat transfer equipment of a closed Brayton heat engine. The physical model considers three types of irreversibilities: finite thermal conductance between the working fluid and the reservoirs, heat leaks between the reservoirs, and internal irreversibility inside the closed Brayton heat engine. The effects of heat leaks, hot-cold reservoir temperature ratios, turbine and compressor isentropic efficiencies, and total conductances of heat exchangers on the maximum thermal efficiency and its corresponding parameters are studied. The optimum conductance ratio could be found to effectively use the heat transfer equipment, and this ratio is increased as the component efficiencies and total conductances of heat exchangers are increased, and always less than or equal to 0.5.

scholarly journals Finite Physical Dimensions Thermodynamics Analysis and Design of Closed Irreversible Cycles

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3416
Author(s):  
Gheorghe Dumitrașcu ◽  
Michel Feidt ◽  
Ştefan Grigorean
Keyword(s):  
Thermal Conductance ◽  
Heat Output ◽  
Operational Parameters ◽  
Analysis And Design ◽  
The Mean ◽  
Internal Irreversibility ◽  
Heat Transfers ◽  
Operational Constraints ◽  
Log Temperatures ◽  
Physical Dimensions

This paper develops simplifying entropic models of irreversible closed cycles. The entropic models involve the irreversible connections between external and internal main operational parameters with finite physical dimensions. The external parameters are the mean temperatures of external heat reservoirs, the heat transfers thermal conductance, and the heat transfer mean log temperatures differences. The internal involved parameters are the reference entropy of the cycle and the internal irreversibility number. The cycle’s design might use four possible operational constraints in order to find out the reference entropy. The internal irreversibility number allows the evaluation of the reversible heat output function of the reversible heat input. Thus the cycle entropy balance equation to design the trigeneration cycles only through external operational parameters might be involved. In designing trigeneration systems, they must know the requirements of all consumers of the useful energies delivered by the trigeneration system. The conclusions emphasize the complexity in designing and/or optimizing the irreversible trigeneration systems.

scholarly journals Analysis of the internal heat losses in a thermoelectric generator

2014 ◽  
Vol 85 ◽  
pp. 12-20 ◽  
Author(s):  
R. Bjørk ◽  
D.V. Christensen ◽  
D. Eriksen ◽  
N. Pryds
Keyword(s):  
Thermoelectric Generator ◽  
Internal Heat ◽  
Heat Losses

Decreased thermal conductance during the luteal phase of the menstrual cycle in women

1990 ◽  
Vol 69 (6) ◽  
pp. 2029-2033 ◽  
Author(s):  
P. Frascarolo ◽  
Y. Schutz ◽  
E. Jequier
Keyword(s):  
Menstrual Cycle ◽  
Thermal Equilibrium ◽  
Luteal Phase ◽  
Thermal Conductance ◽  
Heat Losses ◽  
Tympanic Temperature ◽  
Whole Body ◽  
Direct Calorimetry ◽  
Plasma Progesterone ◽  
Metabolic Heat

To study the influence of the menstrual cycle on whole body thermal balance and on thermoregulatory mechanisms, metabolic heat production (M) was measured by indirect calorimetry and total heat losses (H) were measured by direct calorimetry in nine women during the follicular (F) and the luteal (L) phases of the menstrual cycle. The subjects were studied while exposed for 90 min to neutral environmental conditions (ambient temperature 28 degrees C, relative humidity 40%) in a direct calorimeter. The values of M and H were not modified by the phase of the menstrual cycle. Furthermore, in both phases the subjects were in thermal equilibrium because M was similar to H (69.7 +/- 1.8 and 72.1 +/- 1.8 W in F and 70.4 +/- 1.9 and 71.4 +/- 1.7 W in L phases, respectively). Tympanic temperature (Tty) was 0.24 +/- 0.07 degrees C higher in the L than in the F phase (P less than 0.05), whereas mean skin temperature (Tsk) was unchanged. Calculated skin thermal conductance (Ksk) was lower in the L (17.9 +/- 0.6 W.m-2.degrees C-1) than in the F phase (20.1 +/- 1.1 W.m-2.degrees C-1; P less than 0.05). Calculated skin blood flow (Fsk) was also lower in the L (0.101 +/- 0.008 l.min-1.m-2) than in the F phase (0.131 +/- 0.015 l.min-1.m-2; P less than 0.05). Differences in Tty, Ksk, and Fsk were not correlated with changes in plasma progesterone concentration. It is concluded that, during the L phase, a decreased thermal conductance in women exposed to a neutral environment allows the maintenance of a higher internal temperature.

Second Law Analysis of a Stirling Cryocooler With Optimal Design of the Regenerator and Losses

2001 ◽  
Author(s):  
E. D. Rogdakis ◽  
N. A. Bormpilas
Keyword(s):  
Gas Flow ◽  
Optimization Technique ◽  
Internal Heat ◽  
Thermodynamic Characteristics ◽  
Coefficient Of Performance ◽  
Solid Matrix ◽  
Second Law ◽  
External Temperature ◽  
Technical Requirements ◽  
Second Law Analysis

Abstract The aim of the research in this paper is a second law analysis of a Stirling cryocooler. A one-dimensional model is proposed for the simulation of the gas flow in the expansion space, the regenerator, the warm-end, the compression space and the compressor. Helium gas is selected as the working medium. An algorithm has been developed considering parametrically the most from the main operational tasks of the thermodynamic cycle. Performance indices such as heat input, efficiency, external dimensions of the engine and technical requirements are taken into account as constraints. Engine operating parameters i.e. speed, external temperature, mean pressure are fixed. The regenerator loss has a critical influence on the cryocooler efficiency and the reduction of this kind of internal irreversibilities is extremely difficult due to the generator is subject to rapidly cycling flows accompanied by steep temperature gradients and large pressure variations. The second flow analysis of the regenerator identifies two principal losses, the irreversible internal heat transfer into the solid matrix and the hydraulic resistance. An optimization technique leads to entropy generation charts, extremely useful for a good design of the regenerator. Finally the main thermodynamic characteristics (net refrigeration, power input and the coefficient of performance) of the cryocooler are given both cases with and without external and internal irreversibilities.

Potential of an air curtain system orientated to create non-uniform indoor thermal environment and save energy

2016 ◽  
Vol 26 (2) ◽  
pp. 152-165 ◽  
Author(s):  
Chong Shen ◽  
Xiaoliang Shao ◽  
Xianting Li
Keyword(s):  
Thermal Environment ◽  
Internal Heat ◽  
Internal Heat Source ◽  
Coefficient Of Performance ◽  
Maximum Temperature ◽  
Building Energy Efficiency ◽  
Maximum Temperature Difference ◽  
Air Curtain ◽  
Discharge Velocity ◽  
Save Energy

Non-uniform indoor environment has shown its potential for building energy efficiency and improving indoor air quality compared with traditional uniform environment created by mixing ventilation. An air curtain was employed to create non-uniform thermal environment in this study. The performance of an optimal ventilation strategy between an air curtain and the background ventilation in a typical air-conditioned room was investigated numerically. The air curtain's effectiveness and coefficient of performance (COP) are proposed as evaluation indices for assessing the efficacy of air curtain for this usage. The numerical model was validated by experiments. The energy saving potential of an air curtain system for maintaining a thermal environment of a compartment was studied under different internal heat-source characteristics. The discharge velocity of the air curtain was optimized. The results show that the efficacy of an air curtain system to create a non-uniform thermal environment is high, where the maximum temperature difference between two sides of an air curtain could be as high as 7.4℃. Both the flow rate of the air curtain and background ventilation can be reduced to save energy. An air curtain is more efficient when the heat is concentrated mainly in the unoccupied room partition and the unoccupied zone contains external walls.

Effects of the Heat Transfer at the Side Walls on Natural Convection in Cavities

1990 ◽  
Vol 112 (2) ◽  
pp. 370-378 ◽  
Author(s):  
Y. Le Peutrec ◽  
G. Lauriat
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Thermal Conductance ◽  
Fluid Flows ◽  
Heat Losses ◽  
Transfer Rates ◽  
Side Walls ◽  
The Difference

Numerical solutions are obtained for fluid flows and heat transfer rates for three-dimensional natural convection in rectangular enclosures. The effects of heat losses at the conducting side walls are investigated. The problem is related to the design of cavities suitable for visualizing the flow field. The computations cover Rayleigh numbers from 103 to 107 and the thermal conductance of side walls ranging from adiabatic to commonly used glazed walls. The effect of the difference between the ambient temperature and the average temperature of the two isothermal walls is discussed for both air and water-filled enclosures. The results reported in the paper allow quantitative evaluations of the effects of heat losses to the surroundings, which are important considerations in the design of a test cell.

scholarly journals Performance Optimization of an Air-Standard Irreversible Dual-Atkinson Cycle Engine Based on the Ecological Coefficient of Performance Criterion

2014 ◽  
Vol 2014 ◽  
pp. 1-10 ◽  
Author(s):  
Guven Gonca ◽  
Bahri Sahin
Keyword(s):  
Performance Optimization ◽  
Pressure Ratio ◽  
Performance Criterion ◽  
Coefficient Of Performance ◽  
Cycle Performance ◽  
Temperature Ratio ◽  
Finite Rate ◽  
Ecological Performance ◽  
Atkinson Cycle ◽  
Internal Irreversibility

This paper presents an ecological performance analysis and optimization for an air-standard irreversible Dual-Atkinson cycle (DAC) based on the ecological coefficient of performance (ECOP) criterion which includes internal irreversibilities, heat leak, and finite-rate of heat transfer. A comprehensive numerical analysis has been realized so as to investigate the global and optimal performances of the cycle. The results obtained based on the ECOP criterion are compared with a different ecological function which is named as the ecologic objective-function and with the maximum power output conditions. The results have been attained introducing the compression ratio, cut-off ratio, pressure ratio, Atkinson cycle ratio, source temperature ratio, and internal irreversibility parameter. The change of cycle performance with respect to these parameters is investigated and graphically presented.

Theoretical Analysis and Experimental Researches on Internal Heat Exchanger in CO2 Trans-Critical Cycle

2012 ◽  
Vol 204-208 ◽  
pp. 4336-4342
Author(s):  
Hui Xia Lu ◽  
Jing Lv ◽  
Zhe Bin He ◽  
Jin Yu Wang ◽  
Jia Wei Zhou
Keyword(s):  
Heat Exchanger ◽  
Internal Heat ◽  
Operating Conditions ◽  
Coefficient Of Performance ◽  
Outlet Temperature ◽  
Internal Heat Exchanger ◽  
High Side ◽  
Side Pressure ◽  
Experimental Researches ◽  
Regenerative Cycle

The change of system performance caused by regenerative cycle in different operating conditions was analyzed in this paper, comparing the cycle with or without internal heat exchanger in a CO2 trans-critical cycle. We analyzed theoretically the performance of CO2 trans-critical cycle with the internal heat exchanger, and found that the coefficient increased with the decreasing of the high side pressure and the increasing of outlet temperature in gas cooler, in a certain range of the high side pressure and outlet temperature. The evaporation temperature could be raised when the system with internal heat exchanger and at the same time the coefficient of performance could be improved obviously. At lower high side pressure, the performance coefficient could be improved significantly by increasing the suction superheat. The higher the gas cooler outlet temperature was, the more obvious the increase was.

Heat Transfer Enhancement for Gas Turbine Internal Cooling by Application of Double Swirl Cooling Chambers

2013 ◽  
Author(s):  
Karsten Kusterer ◽  
Gang Lin ◽  
Dieter Bohn ◽  
Takao Sugimoto ◽  
Ryozo Tanaka ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Gas Turbine ◽  
Internal Heat ◽  
Numerical Results ◽  
Internal Cooling ◽  
Transfer Enhancement ◽  
Cooling Channels ◽  
Swirl Chamber ◽  
Internal Heat Transfer

Improvement of the gas turbine thermal efficiency can be achieved by reducing the cooling fluid amount in internal cooling channels with enhanced convective cooling. Nowadays the state of the art internal cooling technology for thermally high-loaded gas turbine blades consists of multiple serpentine-shaped cooling channels with angled ribs. Besides, huge effort is put on the development of more advanced internal cooling configurations with further internal heat transfer enhancements. Swirl chamber flow configurations, in which air is flowing through a pipe with a swirling motion generated by tangential jet inlet, have a potential for application as such advanced technology. This paper presents the validation of numerical results for a standard swirl chamber, which has been investigated experimentally in a reference publication. The numerical results obtained with application of the SST k-ω model show the best agreement with the experiment data in compare with other turbulence models. It has been found at the inlet region that the augmentation of the heat transfer is nearly seven times larger than the fully developed non-swirl flow. Within the further numerical study, another cooling configuration named Double Swirl Chambers (DSC) has been obtained and investigated. The numerical results are compared to the reference case. With the same boundary conditions and Reynolds number, the heat transfer coefficients are higher for the DSC configuration than for the reference configuration. In particular at the inlet region, the DSC configuration has even higher circumferentially averaged heat transfer enhancement in one section by approximately 41%. The globally-averaged heat transfer enhancement in DSC configuration is 34.5% higher than the value in the reference SC configuration. This paper presents the configuration of the DSC as an alternative internal cooling technology and explains its major physical phenomena, which are the reasons for the improvement of internal heat transfer.

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