Evaluation of convective heat transfer in a tube based on local exergy destruction rate

In this study, exergy efficiency is defined to evaluate convective heat transfer in a tube based on the local exergy destruction rate from the equilibrium equation of available potential. By calculating this destruction rate, the local irreversibility of convective heat transfer can be evaluated quantitatively. The exergy efficiency and distribution of local exergy destruction rate for a smooth tube, an enhanced tube into which short-width twisted tape has been inserted, and an optimized tube with exergy destruction minimization are analyzed by solving the governing equations through a finite volume method(FVM). For the smooth tube, the exergy efficiency increases with increasing Reynolds number(Re) and decreases as the heat flux increases, whereas the Nusselt number(Nu) remains constant. For the enhanced tube, the exergy efficiency increases with increasing Reynolds number and increases as the short-width rate(w) increases. An analysis of the distribution of the local exergy destruction rate for a smooth tube shows that exergy destruction in the annular region between the core flow and tube wall is the highest. Furthermore, the exergy destruction for the enhanced and optimized tubes is reduced compared with that of the smooth tube. When the Reynolds number varies from 500 to 1750, the exergy efficiencies for the smooth, enhanced, and optimized tubes are in the ranges 0.367–0.485, 0.705–0.857, and 0.885–0.906, respectively. The results show that exergy efficiency is an effective evaluation criterion for convective heat transfer and the distribution of the local exergy destruction rate reveals the distribution of local irreversible loss. Disturbance in the core flow can reduce exergy destruction, and improve the exergy efficiency as well as heat transfer rate. Besides, optimization with exergy destruction minimization can provide effective guidance to improve the technology of heat transfer enhancement.

Acoustic emission characteristics and stress release rate of coal samples in different dynamic destruction time

Considering the importance of the prediction of rock burst disasters, and in order to grasp the law of acoustic emission(AE) of coal samples in different dynamic destruction time, the SH-II AE monitoring system was adopted to monitor the failure process of coal samples. The study of the change rule of the AE numbers, energy, ‘b' value and spectrum in the micro crack propagation process of the coal samples shows that as dynamic damage time went by, AE presented high-energy counts and the accumulated counts increased during the compression phase. The AE energy and cumulative counts increased during the elastic stage. The AE blank area increased gradually and the blank lines were more and more obvious in the molding stage. The AE counts and energy showed a trend of decrease in the residual damage phase.AE ‘b' values gradually became sparse, and the large scale cracks percentage compared with micro cracks decreased and the degree of damage decreased. The AE frequency spectrum peak went from the residual damage phase to the molding phase, and finally it was nearly stable, besides the bandwidth of the main frequency is gradually narrowed. Also, the frequency peak changed from single peak frequency to bi-peak frequency and to the single peak frequency. Uniaxial compressive strength is more sensitive than the elastic modulus to dynamic damage time.

Energy-and exergy-based performance evaluation of solar powered combined cycle(recompression supercritical carbon dioxide cycle/organic Rankine cycle)

Nowadays,the recompression supercritical carbon dioxide(R-SCO_(2))cycle has emerged as a promising option for power conversion systems because of its boundless potential to tackle energy and environmental issues.In this study,we examined the performance of the solar parabolic trough collector(SPTC)integrated combined cogeneration system for the purpose of power generation as well as recovery of waste exhaust heat from the R-SCO_(2) cycle with the help of the organic Rankine cycle(ORC).An exergy and energy analysis was performed for a combined recompression cycle(R-SCO_(2)-ORC)by varying the input variables such as intensity of solar irradiation(Gb),pressure at the inlet of SCO_(2) turbine(P_(5)),mass flow rate of SCO_(2)()&mSCO_(2) inlet temperature of SCO_(2) turbine(T5),inlet temperature of main compressor(T_(9))and effectiveness of the high-and low-temperature recuperator( HTR and LTR).Eight organic working fluids were considered for the ORC:R123,R290,isobutane,R1234yf,R1234ze,toluene,isopentane and cyclohexane.The study revealed that R123-based R-SCO_(2)-ORC demonstrates the highest thermal and exergy efficiency:~73.4 and 40.89%at G_(b)=0.5 kW/m^(2);78.8 and 43.9%at P_(5)=14 MPa;63.86 and 35.57%at T5=650 K;74.84 and 41.69%at&mSCO 7kg s;2=/85.83 and 47.82%at T_(9)=300 K;84.57 and 47.11%at HTR 65;=0.85.06 and 47.38%at LTR 65,=0.respectively.Alternatively,R290 showed the minimum value of exergy and thermal efficiency.As can be seen,the maximum amount of exergy destruction or exergy loss occurs in a solar collector field,~58.25%of the total exergy destruction rate(i.e.6703 kW)and 18.99%of the solar inlet exergy(i.e.20562 kJ).Moreover,R123 has the highest net work output,~4594 kJ at T5=650 K and 6176 kJ at T_(9)=300 K.