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A second law analysis is carried out on convective heat transfer from a fluid flowing in a duct with constant heat flux. The entropy generated is expressed as a function of the initial temperature difference and the frictional pressure drop. Since the loss in available energy is directly proportional to the entropy generated, an optimum value of the initial temperature difference is found where the entropy generated is the minimum. A similar optimum is found for the ratio of heat transfer to pumping power. An optimum fluid velocity which corresponds to the minimum loss of available power is recommended.

SUMMARYIn the present paper thermodynamic analyses, i.e. both energy and exergy analyses have been conducted for a coal based combined cycle power plant, which consists of pressurized circulating fluidized bed (PCFB) partial gasification unit and an atmospheric circulating fluidized bed (ACFB) char combustion unit. Dual pressure steam cycle is considered for the bottoming cycle to reduce irreversibilities during heat transfer from gas to water/steam. The effect of operating variables such as pressure ratio, gas turbine inlet temperature on the performance of combined cycle power plant has been investigated. The pressure ratio and maximum temperature (gas turbine inlet temperature) are identified as the dominant parameters having impact on the combined cycle plant performance. The work output of the topping cycle is found to increase with pressure ratio, while for the bottoming cycle it decreases. However, for the same gas turbine inlet temperature the overall work output of the combined cycle plant increases up to a certain pressure ratio, and thereafter not much increase is observed. The entropy generation, the irreversibilities in each component of the combined cycle and the exergy destruction/losses are also estimated.

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