Reduction of thermally induced stress in a solid disk heated with radially periodic expanding and contracting ring heat flux

The purpose of this study is to reduce the thermally induced stress in a solid disk heated by moving ring heat flux (radially periodic expanding and contracting) and cooled by means of coolant following this heat flux (the subsequent cooling process). It was assumed that the ring heat flux per unit area at the each ring surface was uniform. The applied heat transfer rate, Q, regularly increases from 3.14 to 311 W and then decreases to 3.14 W in one period depending on the area of heated ring. The FLUENT 6.1 program was chosen as computer code to calculate these numerical solutions. Furthermore, a computer program, applying the SIMPSON integration method to the obtained temperature distributions from the heat transfer calculations, has been developed to calculate numerically the governing thermal stress distributions. The calculations were performed individually for a wide range of coolant (liquid) heat transfer coefficient, from 1000 to 10,000 W/m(2) K stepped by 1000 W/m(2) K and for the various ring heat flux expansion and contraction speeds, from 0.0005 to 0.001 m/s stepped by 0.0001 m/s, under transient conditions. The thermal stress differences in the axial direction are quite high around the heated ring and the coolant rings with respect to the other rings due to the non-uniform heating at the surface. However, the levels of the thermal stress in the disk are reduced (from 6 to 31% depending on coolant heat transfer coefficient), by means of the subsequent cooling process. (c) 2006 Elsevier B.V. All rights reserved

Numerical study of effect of oxygen fraction on local entropy generation in a methane-air burner

This study considers numerical simulation of the combustion of methane with air, including oxygen and nitrogen, in a burner and the numerical solution of local entropy generation rate due to high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (phi) and oxygen percentage (gamma) on combustion and entropy generation rates are investigated for different 0 (from 0.5 to 1.0) and gamma values (from 10 to 30%). Combustion is simulated for the fuel mass flow rate resulting in the same heat transfer rate (Q)over dot to the combustion chamber in each case. Numerical calculation of combustion is performed individually for All cases with the use of the Fluent CFD code. Furthermore, a computer program has been developed to calculate the volumetric entropy generation rate and the other thermodynamic parameters numerically by using the results of the calculations performed with the FLUENT code. The predictions. show that the increase of phi (or the decrease of lambda) significantly reduces the reaction rate levels. Average temperature in the combustion chamber increases by about 70 and 35% with increase of gamma (from 10 to 30%) and phi (from 0.5 to 1.0) respectively. With increase of gamma from 10 to 30%, volumetric local entropy generation rate decreases by about 9 and 4% for phi = 0.5 and 1.0 respectively, while total entropy generation rate decreases exponentially and the merit numbers increase. The ratio of the rates useful energy transfer to irreversibility therefore improves as the oxygen percentage increases

Study on transient local entropy generation in pulsating fully developed laminar flow through an externally heated pipe

This study presents the investigation of transient local entropy generation rate in pulsating fully developed laminar flow through an externally heated pipe. The flow inlet to the pipe is considered as pulsating at a constant period and amplitude (only the velocity oscillates). The simulations are extended to include different pulsating flow cases (sinusoidal flow, step flow, and saw-down flow). To determine the effects of the mean velocity, the period and the amplitude of the pulsating flow on the entropy generation rate, the pulsating flow is examined for various cases of these parameters. Two-dimensional flow and temperature fields are computed numerically with the help of the fluent computational fluid dynamics (CFD) code. In addition to this CFD code, a computer program has been developed to calculate numerically the entropy generation and other thermodynamic parameters by using the results of the calculations performed for the flow and temperature fields. In all investigated cases, the irreversibility due to the heat transfer dominates. The step flow constitutes the highest temperature (about 919 K) and generates the highest total entropy rate (about 0.033 W/K) within the pipe. The results of this study indicate that in the considered situations, the inverse of square of temperature (1/T-2) is more dominant on the entropy generation than the temperature gradients, and that the increase of the mean velocity of the pulsating flow has an adverse effect on the ratio of the useful energy transfer rate to irreversibility rate

Effect of oxygen fraction on local entropy generation in a hydrogen-air burner

This study considers numerical simulation of the combustion of hydrogen with air, including oxygen and nitrogen, in a burner and the numerical solution of local entropy generation rate due to the high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (phi) and oxygen percentage ( c) on the combustion and entropy generation rate are investigated for different phi s (from 0.5 to 1.0) and gamma s (from 10 to 30%). The combustion is simulated for the fuel mass flow rate providing the same heat transfer rate (Q) the combustion chamber in the each case. The numerical calculation of combustion is performed individually for all cases with the help of the Fluent CFD code. Furthermore, a computer program has been developed to calculate numerically the volumetric entropy generation rate distributions and the other thermodynamic parameters by using the results of the calculations performed with the FLUENT code. The calculations bring out that the increase of phi (or the decrease of lambda) reduces significantly the reaction rate levels. The average temperatures in the combustion chamber increase about 70 and 23% with the increases of gamma (from 10 to 30%) and phi (from 0.5 to 1.0), respectively. With the increase of gamma from 10 to 30%, the volumetric local entropy generation rates decrease about 9 and 4% in the cases of phi =0.5 and 1.0, respectively, and while the total entropy generation rates decrease exponentially, the merit numbers increase. The useful energy transfer rate to irreversibility rate therefore improves as the oxygen percentage increases