A new thermodynamic model for turbocharged diesel engines is developed for Miller cycle analysis and optimization. The effects of turbocharger efficiency, Miller degree, combustion mode, and air fuel ratio on engine efficiency, power, and NOx emissions are analyzed by the model. Engine performance is optimized by the thermodynamic model under the constraints of maximum cylinder pressure and NOx emissions. Although the Miller cycle is beneficial for NOx emission control, it has adverse effects on engine thermal efficiency and power without parameter redesign. Turbocharger efficiency is a key factor in highly boosted Miller cycle diesel engines. The trade-off relationships among thermal efficiency, break mean effective pressure, and NOx emissions can be improved remarkably by highly efficient turbochargers. Measures with high geometric compression ratio and boost pressure and intensified Miller degree must be applied to reduce NOx emissions and improve thermal efficiency simultaneously. The results provide guidance in designing a Miller cycle system for turbocharged diesel engines.
The flamelet model based on the concept of local flame speed S*is newly developed to describe a premixed flame with a finite thickness when defining G as the non-dimensional temperature. In the previous work the new flamelet model was validated by 1-D steady premixed flame successfully. In this paper the model of 3-D counter flow premixed methane-air is investigated. The local flame speed can be proposed as S*=Su+2Su(G-G0) where the scalar G=G0 represents the flame surface in the flamelet model. Considering the stretch effect the expression for the modified burning velocity is Su=Su0-Su0Lκ where Su is smaller than the burning velocity of the unstretched flame Su0. The new flamelet model results are compared with the original G-model results in the velocity and the temperature profiles. The results are also validated by the detailed chemical reaction solution of GRI-Mech3.0 by CHEMKIN.
In this paper, we present a simple combustion simulation technique based on a look-up table approach. In the proposed technique, a flow solver extracts the solutions of the ordinary differential equations (ODEs) of the chemical equations from the look-up table using the mixture fraction, mass fraction of products, and time scale of the reaction. The look-up table is constructed during combustion simulation. Thus, prior calculation is not needed in the proposed technique. The solutions of the ODEs are saved in the look-up table at points where the mixture fraction, mass fraction, and time scale are similar to those in the look-up table. Once the data are recorded, a direct integration to solve the chemical equations becomes unnecessary, and the time required to compute the reaction rates is shortened. The proposed technique is applied to an eddy dissipation concept (EDC) model and is validated through a simulation of a H2 turbulent non-premixed flame and a CH4 partially premixed flame. The results obtained through the proposed technique are then compared with experimental data and computational data obtained using the EDC model with direct integration. We found a good agreement between our method and the EDC model. Moreover, although the proposed technique is simple, the computation time for our technique is faster than the in situ tabulation method (ISAT) and is approximately 99% lower than that of the EDC model with direct integration.
In the present study, air-side entropy generation rate for an evaporative air-cooled heat exchanger has been investigated using thermodynamic second-law analysis. For this purpose, entropy generations due to heat transfer, friction loss and evaporation were taken into consideration. From the results of this study, it was observed that the total air-side entropy generation was increased by increasing the air-side Reynolds number. Actually, increasing of air mass flow rate, increases irreversibilities due to both evaporation and friction. Stepping up of ambient temperature leads to increasing of irreversibility due to water evaporation. The results showed when the ambient temperature approaches to inlet process fluid temperature, the contribution of the entropy generation rate due to water evaporation becomes the dominant term in the total entropy generation rate. Moreover, it was seen that the effect of deluge water mass flow rate changes on entropy generation rate is slight.
Energy sources independent of fossil fuels or atomic power are of great importance to achieve a sustainable society from the viewpoint of environmental conservation and benefits. Hydrogen is considered as one of the main candidates for new sources. The steam reforming method is a major contemporary process for manufacturing hydrogen, during which carbon dioxide is simultaneously generated. Therefore, an additional process that can separate hydrogen from carbon dioxide is necessary. For this reason, we proposed the use of a new method based on the Soret effect for the separation of hydrogen and carbon dioxide. The Soret effect is capable of producing a concentration difference only by imposing a temperature difference. However, in previous studies, our group found that a single-step separation process could increase the concentration by only a few percent. Thus, a multi-step separation process is necessary for obtaining high gas concentration. In this study, we adopted micro-electromechanical system (MEMS) fabrication technology to develop a separation device and performed single-step hydrogen-separation experiments. The MEMS technology applied in this study has shown potential for the miniaturization of the device and enhancement of separation cycle number for future experiments.