• Energy Research

Cooling of the electrical vehicles’ battery is of crucial importance, as it affects the performance of the electrical power system, discharging duration, and subsequently their market acceptance. Present numerical work aims at analysing the improvement of the thermal management system by topological changes, which can be easily and affordably performed. The influence of AgO-water nanofluid with a volume fraction of 3% is also evaluated. A pack involving 10 cylindrical batteries with a constant heat flux is studied, which is merged by the nanofluid flow. Effects of changing the location of the inlet/outlet ports and inserting one or two plates to guide the fluid flow are assessed. The analysis was performed for a range of Reynolds number (based on the inlet pack diameter) from 1000 to 2000. It is shown that the topological modifications can improve the Nusselt number by more than 25%, while the Reynolds number rising from 1000 to 2000 makes a maximum increase of 30%. However, it follows a maximum 50% increment in the pressure drop. The proposed geometries indicate a more uniform temperature distribution compared to the simple cooling system without guiding plates by about 50%.

Abstract Present work examines numerically the asymmetric behavior of hydrogen/air flame in a micro-channel subjected to a non-uniform wall temperature distribution. A high resolution (with cell size of 25 μm × 25 μm) of two-dimensional transient Navier–Stokes simulation is conducted in the low-Mach number formulation using detailed chemistry evolving 9 chemical species and 21 elementary reactions. Firstly, effects of hydrodynamic and diffusive-thermal instabilities are studied by performing the computations for different Lewis numbers. Then, the effects of preferential diffusion of heat and mass transfer on the asymmetric behavior of the hydrogen flame are analyzed for different inlet velocities and equivalence ratios. Results show that for the flames in micro-channels, interactions between thermal diffusion and molecular diffusion play major role in evolution of a symmetric flame into an asymmetric one. Furthermore, the role of Darrieus–Landau instability found to be minor. It is also found that in symmetric flames, the Lewis number decreases behind the flame front. This is related to the curvature of flame which leads to the inclination of thermal and mass fluxes. The mass diffusion vectors point toward the walls and the thermal diffusion vectors point toward the centerline. Asymmetric flame is observed when the length of flame front is about 1.1–1.15 times of the channel width.

Abstract This paper studies experimentally the forced convection heat transfer of turbulent flow in a cylindrical pebble bed channel with internal heat generation. Exergy and entropy generation analyses are performed to optimize energy conversion in the system identify the destruction of exergy in the pebble bed channel. Stainless steel spheres are used in stacked pebble bed channel. Internal heating is generated uniformly by electromagnetic induction heating method in metallic spheres. Dry air is used as the working fluid in the process of cooling of the heated spheres. The experiment is performed for turbulent flow regimes with Reynolds (Red) number (based on the diameter of the spheres) in the range of 920–2570, which is equal to Reynolds (Re) number, based on channel diameter, in the range of 4500–10,000. The effects of different parameters, including spheres diameter (d = 5.5, 6.5 and 7.5 mm), inlet volumetric flow rate ( V ) and internal heat generation (Q) on the forced convection heat transfer, exergy transfer and entropy generation are studied. For second law and exergy analyses, mean exergy transfer Nusselt number (Nue) and entropy generation number (Ns) are investigated. Results show that for a fixed d and Q, the mean exergy transfer Nusselt number (Nue) decreases with the increase of Red number until it becomes zero for a critical Red number. This critical Red number found to be about 1450, 1800 and 2300 for d = 5.5, 6.5 and 7.5 mm, respectively. Further increase in the Red number, decreases Nue to negative values. It is found for spheres with diameter of d = 5.5 mm and for a fixed Q, as Red increases, the entropy generation number Ns increases monotonically. While, for d > 5.5 mm and fixed Q, the entropy generation number (Ns) decreases with the increase of Red number up to a critical Red value that makes Ns to be minimum. Further increase in Red number, increases Ns. It is also found that for Red > 1800, among the sphere diameters studied in this work, balls with highest diameters yield the minimum entropy generation in the system.

AbstractThe present work investigates analytically the problem of forced convection heat transfer of a pulsating flow, in a channel filled with a porous medium under local thermal non-equilibrium condition. Internal heat generation is considered in the porous medium, and the channel walls are subjected to constant heat flux boundary condition. Exact solutions are obtained for velocity, Nusselt number and temperature distributions of the fluid and solid phases in the porous medium. The influence of pertinent parameters, including Biot number, Darcy number, fluid-to-solid effective thermal conductivity ratio and Prandtl number are discussed. The applied pressure gradient is considered in a sinusoidal waveform. The effect of dimensionless frequency and coefficient of the pressure amplitude on the system’s velocity and temperature fields are discussed. The general shape of the unsteady velocity for different times is found to be very similar to the steady data. Results show that the amplitudes of the unsteady temperatures for the fluid and solid phases decrease with the increase in Biot number or thermal conductivity ratio. For large Biot numbers, dimensionless temperatures of the solid and fluid phases are similar and are close to their steady counterparts. Results for the Nusselt number indicate that increasing Biot number or thermal conductivity ratio decreases the amplitude of Nusselt number. Increase in the internal heat generation in the solid phase does not have a significant influence on the ratio of amplitude-to-mean value of the Nusselt number, while internal heat generation in the fluid phase enhances this ratio.

Abstract Present work examines the effect of a low-level oxygen concentration enhancement in the oxidizer stream, on the generation of direct combustion noise in a non-premixed turbulent free flame. A hybrid approach utilizing a large eddy simulation, partially stirred reactor combustion model, and Lighthill analogy was employed to predict the sound pressure level in the farfield of the flame. Contributions of different noise sources, including heat release rate and, mole consumption and production rate fluctuations are calculated to predict the sound pressure level in the farfield. Results show that, in comparison to the flame burning with air, the higher temperature in oxygen enhanced flame leads to an increase in the heat release rate and an increase in the rate of consumption and production of the species. The total sound pressure level found to increase by adding oxygen to the oxidizer stream, especially in low frequency ranges. Enhancement of molar oxygen by 10%, increases the noise contribution of the heat release rate fluctuations about 20 dB in low frequency range, and about 10 dB in high frequencies. Additionally, the noise contribution from the mole consumption and production rate decreases about 5 dB in low frequency range and 10 dB in high frequencies.

The present work studies the effect of entropy dispersion on the level of combustion noise at the turbine outlet of the Rolls-Royce ANTLE aero-engine. A new model for the decay of entropy waves, based on modelling dispersion effects, is developed and utilised in a low-order network model of the combustor (i.e. LOTAN code that solves the unsteady Euler equations). The proposed model for the dispersion of entropy waves only requires the mean velocity field as an input, obtained by RANS computations of the demonstrator combustor. LOTAN is then coupled with a low order model code (LINEARB) based on the semi-actuator disk model that studies propagation of combustion noise through turbine blades. Thus, by combining LOTAN and LINERAB we predict the combustion noise and its counterparts, direct and indirect noise, generated at the turbine exit. In comparison with experimental data it is found that without the inclusion of entropy dispersion, the level of combustion noise at the turbine exit is over-predicted by almost two orders of magnitude. The introduction of entropy dispersion in LOTAN results in much better agreement with the experimental data, highlighting the importance of entropy wave dispersion for the prediction of combustion noise in real engines. In more detail, the agreement with the experiment for high and low frequencies was very good. At intermediate frequencies the experimental measurements are still over-predicted, however the predicted noise is much smaller compared to the case without entropy dispersion. This discrepancy is attributed to (i) the role of turbulent mixing in the overall decay of the entropy fluctuations inside the combustor, not considered in the model developed for the decay of entropy waves, and (ii) the absence of a proper model in LINEARB for the decay of entropy waves as they pass through the turbine blade rows. These are areas that still need further development to improve the prediction of low-order network codes.