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Venkat Eswarlu Tangirala – One of the best experts on this subject based on the ideXlab platform.

  • Turbine Efficiency for Unsteady, Periodic Flows
    Journal of Turbomachinery, 2011
    Co-Authors: Ambady Suresh, Douglas Carl Hofer, Venkat Eswarlu Tangirala

    Abstract:

    The definition of turbine efficiency for a machine subjected to unsteady periodic inflows is studied. Since mass and energy are conserved quantities over a period, there is no ambiguity in calculating the Actual Work Output of the turbine over a period. The main difficulty lies in calculating the isentropic Work Output of an ideal turbine operating under the “same” conditions. Two definitions of ideal Work Output are presented. In the first, the ideal turbine is assumed to operate under the same time traces of inlet and exit total pressures as the Actual turbine. The expression for the efficiency that results involves no averages of total pressure. In the second approach, the ideal turbine is assumed to operate under the same average conditions as the Actual turbine. Total pressure averages that preserve the isentropic Work Output are derived and used to calculate an efficiency of the turbine. The two expressions are calculated explicitly for the case of a turbine blade row downstream of a pulse detonation tube. It is found that the definition of efficiency using averages is approximately 10 points lower than the first definition.

C J Barclay – One of the best experts on this subject based on the ideXlab platform.

  • Comparison of the efficiency of rat papillary muscles during afterloaded isotonic contractions and contractions with sinusoidal length changes.
    The Journal of experimental biology, 2001
    Co-Authors: L J Mellors, C L Gibbs, C J Barclay

    Abstract:

    The results of previous studies suggest that the maximum mechanical efficiency of rat papillary muscles is lower during a contraction protocol involving sinusoidal length changes than during one involving afterloaded isotonic contractions. The aim of this study was to compare directly the efficiency of isolated rat papillary muscle preparations in isotonic and sinusoidal contraction protocols. Experiments were performed in vitro (27 degrees C) using left ventricular papillary muscles from adult rats. Each preparation performed three contraction protocols: (i) low-frequency afterloaded isotonic contractions (10 twitches at 0.2 Hz), (ii) sinusoidal length change contractions with phasic stimulation (40 twitches at 2 Hz) and (iii) high-frequency afterloaded isotonic contractions (40 twitches at 2 Hz). The first two protocols resembled those used in previous studies and the third combined the characteristics of the first two. The parameters for each protocol were adjusted to those that gave maximum efficiency. For the afterloaded isotonic protocols, the afterload was set to 0.3 of the maximum developed force. The sinusoidal length change protocol incorporated a cycle amplitude of +/-5% resting length and a stimulus phase of -10 degrees. Measurements of force Output, muscle length change and muscle temperature change were used to calculate the Work and heat produced during and after each protocol. Net mechanical efficiency was defined as the proportion of the energy (enthalpy) liberated by the muscle that appeared as Work. The efficiency in the low-frequency, isotonic contraction protocol was 21.1+/-1.4% (mean +/- s.e.m., N=6) and that in the sinusoidal protocol was 13.2+/-0.7%, consistent with previous results. This difference was not due to the higher frequency or greater number of twitches because efficiency in the high-frequency, isotonic protocol was 21.5+/-1.0%. Although these results apparently confirm that efficiency is protocol-dependent, additional experiments designed to measure Work Output unambiguously indicated that the method used to calculate Work Output in isotonic contractions overestimated Actual Work Output. When net Work Output, which excludes Work done by parallel elastic elements, rather than total Work Output was used to determine efficiency in afterloaded isotonic contractions, efficiency was similar to that for sinusoidal contractions. The maximum net mechanical efficiency of rat papillary muscles performing afterloaded isotonic or sinusoidal length change contractions was between 10 and 15%.

Ambady Suresh – One of the best experts on this subject based on the ideXlab platform.

  • Turbine Efficiency for Unsteady, Periodic Flows
    Journal of Turbomachinery, 2011
    Co-Authors: Ambady Suresh, Douglas Carl Hofer, Venkat Eswarlu Tangirala

    Abstract:

    The definition of turbine efficiency for a machine subjected to unsteady periodic inflows is studied. Since mass and energy are conserved quantities over a period, there is no ambiguity in calculating the Actual Work Output of the turbine over a period. The main difficulty lies in calculating the isentropic Work Output of an ideal turbine operating under the “same” conditions. Two definitions of ideal Work Output are presented. In the first, the ideal turbine is assumed to operate under the same time traces of inlet and exit total pressures as the Actual turbine. The expression for the efficiency that results involves no averages of total pressure. In the second approach, the ideal turbine is assumed to operate under the same average conditions as the Actual turbine. Total pressure averages that preserve the isentropic Work Output are derived and used to calculate an efficiency of the turbine. The two expressions are calculated explicitly for the case of a turbine blade row downstream of a pulse detonation tube. It is found that the definition of efficiency using averages is approximately 10 points lower than the first definition.