Ahmet Durmayaz

An application of the degree-hours method to estimate the residential heating energy requirement and fuel consumption in Istanbul

Cold season heating energy requirements in buildings can be estimated with the degree-hours method based on human comfort levels and available meteorological temperature records for a given area. Such estimations are especially significant for cities where fossil fuel consumption must be eliminated in favor of clean energy alternatives to reduce air pollution. This paper considers the city of Istanbul in Turkey and presents a detailed account for practical energy requirements and fuel consumption calculations.

Ecological coefficient of performance (ECOP) optimization for an irreversible Brayton heat engine with variable-temperature thermal reservoirs

An ecological performance analysis for an irreversible Brayton heat engine with variabletemperature thermal reservoirs based on the ecological criterion called Ecological Coefficient of Performance (ECOP) is presented. The model considered includes irreversibilities due to finiterate heat transfer and internal dissipations. The effects of design parameters such as isentropic temperature ratio, heat exchanger effectiveness, thermal reservoir inlet temperature ratio and the ratio of hot-to-cold thermal capacity rates of thermal reservoirs, on the general and optimal ecological performances have been investigated in detail. Comparisons of the results with those of an alternative ecological objective function defined in the literature, the maximum power output conditions, and thermal efficiency are also provided.

Exergy analysis of a pressurized-water reactor nuclear-power plant

An exergy analysis based on the second law of thermodynamics is performed to evaluate the plant and subsystem irreversibility of a nuclear power plant (NPP) with a pressurized-water reactor (PWR). The construction of such a system having a maximum reactor core thermal power of 4250 MW is proposed in Turkey and China. This study concentrates on the questions of where and how much of the available work is lost in such a plant. The evaluated exergy destruction of this plant indicates that the reactor pressure vessel including PWR is the most inefficient equipment in the whole NPP, while the turbines take the second place.

Heating energy requirements and fuel consumptions in the biggest city centers of Turkey

Seasonal energy requirements and fuel consumption for heating purposes in the buildings in a city can be estimated from the predetermined architectural design and material characteristics of the buildings, the regular meteorological temperature measurements and the population of the city. This paper considers the city centers of Istanbul, Ankara, Bursa, Adana and Konya in Turkey and presents a detailed account of the practical seasonal heating energy requirements in an apartment building and fuel consumption estimations in each city center by using the degree-hours method. Since 50.8% of the total city center population in Turkey is considered, the total amounts of these estimations may be interpreted as a good indicator to give an idea about the energy requirements and fuel consumption for heating purposes in the buildings in all the city centers of Turkey.

Ecological performance optimisation of a solar driven heat engine

An optimal performance analysis of a parabolic trough direct steam generation solar driven Rankine cycle power plant at maximum ecological objective function conditions is performed numerically to investigate the optimum design parameters and the optimal ecological performances. Comparisons for thermal efficiencies of the heat engines at maximum power and maximum power density conditions are provided. The effect of different heat transfer mechanisms between thermal reservoirs and working fluid is also investigated and the results are discussed in detail. The main finding of the present paper is that the thermal efficiency of the solar driven power plant at maximum ecological function conditions considered in the present study is always higher than those of the heat engines at maximum power and maximum power density conditions for all values of the allocation parameters concerned and the realistic range of thermal reservoir temperature ratio.

Approximate functions for the fast computation of the thermodynamic properties of heavy water

A set of some approximate functions derived for the fast computation of the thermodynamic properties of heavy water at saturation, in subcooled liquid and superheated vapor states is presented. To derive these functions, the data given in the steam tables by Hill et al. AECL 7531 (1981) were accurately and successfully fitted with curves by using the least-squares method. Specific volume (or density), specific enthalpy, specific entropy, constant-pressure specific heat and temperature at saturation were approximated by a number of piecewise continuous functions of pressure whereas pressure at saturation was approximated by a piecewise continuous function of temperature for heavy water. Density in subcooled liquid state, specific volume in superheated vapor state, specific enthalpy, specific entropy and constant-pressure specific heat in both of these states were also approximated as piecewise continuous functions of pressure and temperature for heavy water. The correlations presented in this study can be used in the two-phase thermalhydraulic system analysis of CANDU-PHW reactor with confidence.

Effect of heat leakage on the performance of a twin-spool turbofan engine

The effect of heat leakage from a twin-spool turbofan engine to the ambient air on the performance of the engine for a commercial aircraft is investigated. Effects of heat leakage on the variation of the performance indicators of coefficient of ecological performance, overall efficiency, exergy destruction factor, thrust specific fuel consumption and entropy generation rate with respect to the design parameters of compressor and fan pressure ratios, by-pass ratio and turbine inlet temperature are investigated numerically. The main findings are as follows: (i) Effect of heat leakage from the combustion chamber to the by-pass air on the coefficient of ecological performance, exergy destruction factor and entropy generation rate is considerably large when compared to that on the other performance indicators especially for small values of design parameters except for turbine inlet temperature. (ii) Effect of heat leakage on the performance indicators is significant for small values of turbine inlet temperature and large values of the other design parameters.

Exergy–based ecological optimisation of a turbofan engine

A novel ecological optimisation is performed by introducing a new coefficient of ecological performance (CEP) defined by propulsive power per unit exergy destruction rate as the objective function for a single spool turbofan engine with an unmixed exhaust considering finite–rate heat transfer from a thermal reservoir at constant temperature. The optimum values for the selected design parameters of compressor pressure ratio, fan pressure ratio, bypass ratio and turbine inlet temperature are determined numerically to maximise this objective function. Furthermore, the overall efficiency, specific thrust, exergy destruction factor, and exergetic sustainability index are considered as the additional objective functions and optimised with respect to the same parameters for comparison. The results show that: i) the compressor pressure ratio range of 15–25.8 provides a good compromise between energy efficiency and ecological performance; ii) a relatively high pressure ratio with a limited flow rate of air through the fan increases ecological performance of the turbofan engine for commercial aircrafts.

Assessment of Selected Two-Phase Friction Multiplier Correlations for Steam-Water Flows in Vertical Heated and Unheated Tubes

In this study, around 10 000 experimental two-phase (2ϕ) flow pressure drop (ΔP) data for steam-water in uniformly heated and adiabatic vertical tubes have been compiled from the literature.The compiled data have been used to assess the prediction accuracy of two-phase friction multiplier (Display Formulaϕlo2) correlations selected from the literature and the homogeneous-flow model for steam-water flow in vertical tubes. The predictions of the Display Formulaϕlo2 correlations have also been compared to the predictions of the homogeneous-flow model. The assessments are based on comparisons of the average and the RMS errors.Additionally, ΔP components (due to gravitation, acceleration and friction) have been examined for a variety of flow conditions in vertical heated and unheated tubes.The results of the assessments show that the predictions of Muller-Steinhagen and Heck (1986) correlation yield the minimum RMS error for flow in heated tubes and the predictions of the Chisholm (1973) correlation yield the minimum RMS error for flow in adiabatic tubes when these predictions are compared with both the data base and the predictions of the homogeneous-flow model.It is concluded from the research and a review of the literature that the current state of the art is such that none of the investigated prediction methods is very good because of their large RMS errors. More work is needed to expand the ΔP data base especially for adiabatic two-phase flow. Also, a systematic experimental study should be performed on the effect of heating on the 2ϕ-flow ΔP.Copyright