• Energy Research

Abstract Flat-plate solar collectors are one of the cleanest and most efficient heating systems available. This experimental study explores the effect of replacing water with surfactant-free rutile TiO2–water nanofluids as the working fluid in a symmetric flat-plate solar collector. The efficiency of the collector was investigated in Aghajari, a city in the south of Iran. Analysis was performed according to the ASHRAE standard, taking into account the flow rate of the heat transfer fluid (HTF), the solar irradiance, and the temperature difference between the inlet and outlet. Results show that the use of TiO2–water nanofluids can improve thermal efficiency relative to water. As the solar irradiance or the HTF flow rate increases, the effect of nanoparticle addition on the collector efficiency gains becomes more pronounced. The maximum efficiency of the collector, when filled with a 1 wt% TiO2-water nanofluid, is found to be approximately 78%; this represents maximum and average efficiency gains of 9.80% and 6.64%, respectively, relative to the water baseline. In addition, the maximum efficiency gains are 17.41%, 27.09%, and 33.54% for 1 wt%, 3 wt%, and 5 wt% nanoparticle concentration, respectively.

© 2019 The Combustion Institute In many combustion devices, strong self-excited flow oscillations can arise from feedback between unsteady heat release and acoustics, resulting in increased vibration and pollutant emissions. Open-loop acoustic forcing has been shown to be effective in weakening such thermoacoustic oscillations, but current implementations of this control strategy require the forcing to be continuously applied. In this proof-of-concept study, we experimentally demonstrate an alternative method of weakening thermoacoustic oscillations in a self-excited combustion system – a laminar premixed flame in a double open-ended tube. Unlike existing methods, the proposed method combines the use of transient forcing with hysteresis and mode switching, thus avoiding the need to continuously supply energy to the control system. Control is achieved by exploiting the fact that most combustors have a multitude of natural thermoacoustic modes, some of which are linearly unstable but some are nonlinearly unstable. By applying open-loop acoustic forcing at an off-resonance frequency and at an amplitude higher than that required for synchronization, we find that the combustor can switch to one of the nonlinearly unstable natural modes (f2) and remain there, even after the forcing is removed. Dynamic mode decomposition of high-speed chemiluminescence videos shows that this mode switching occurs because the flame structure at f2 is more robust than that at the original linearly unstable natural mode. The final unforced state has a thermoacoustic amplitude of just half that of the initial unforced state, even though the Rayleigh index of the former is higher than that of the latter. Although this 50% reduction in thermoacoustic amplitude is not as large as the 95% reduction achieved with asynchronous quenching, it is achieved without the use of continuous forcing. This is a distinct advantage over existing control strategies as it allows the complexity and power requirements of the control system to be reduced. With further development and testing, particularly on turbulent swirling combustors, the proposed control strategy could pave the way for a new class of open-loop control techniques based on transient forcing rather than continuous forcing.

We explore the transition to chaos in a prototypical hydrodynamic oscillator, namely a globally unstable low-density jet subjected to external time-periodic forcing. As the forcing strengthens at an off-resonant frequency, we find that the jet exhibits a sequence of nonlinear states: period-1 limit cycle $\rightarrow $ quasiperiodicity $\rightarrow$ intermittency $\rightarrow$ low-dimensional chaos. We show that the intermittency obeys type-II Pomeau–Manneville dynamics by analysing the first return map and the scaling properties of the quasiperiodic lifetimes between successive chaotic epochs. By providing experimental evidence of the type-II intermittency route to chaos in a globally unstable jet, this study reinforces the idea that strange attractors emerge via universal mechanisms in open self-excited flows, facilitating the development of instability control strategies based on chaos theory.

In this study, heterogeneous combustion of dust clouds containing polydisperse porous iron particles was numerically investigated. The main aim was to develop a discrete three-dimensional model to quantify the effects of particle size, porosity, cloud concentration, and polydispersity on flame propagation speed. The developed numerical model was validated against experimental data to show its promising accuracy. The modeling results show that increasing the cloud concentration increases flame propagation speed significantly, regardless of the particle size distribution, by about 3 times. Increasing the particle porosity can increase flame propagation remarkably, i.e., for particle sizes in the range of 1−3, 1−10, and 1−30 um, flame propagation speed was elevated by up to 24.2%, 36.7%, and 22.6%, respectively, when particle porosity increases from 0 to 0.1. However, increasing the particle size itself was found to decrease flame propagation speed as larger particles tend to be more difficult to ignite. For example, when the particle size distribution changes from 1−3 to 1−30 um, flame propagation speed decreases by a factor of 3.6. These findings serve to improve our understanding of heterogeneous combustion of dust clouds containing polydisperse porous iron particles.

Abstract Open-loop forcing is known to be an effective strategy for controlling self-excited thermoacoustic oscillations, but the details of this synchronization process have yet to be comprehensively explored. In this study, we experimentally examine the synchronization dynamics of a laminar conical premixed flame in a tube combustor subjected to periodic acoustic forcing. We compare the response of this forced self-excited system with that of a forced Duffing–van der Pol oscillator, and find many similarities but also some differences. The similarities include (i) a torus-birth bifurcation from periodicity to quasiperiodicity at low forcing amplitudes, producing a stable ergodic T 2 torus attractor in phase space; (ii) a transition from T 2 quasiperiodicity to lock-in above a critical forcing amplitude, which increases linearly as the forcing frequency ff deviates from the natural frequency f1; (iii) two distinct routes to lock-in, one via a torus-death bifurcation if ff is far from f1 and one via a saddle-node bifurcation if ff is close to f1; and (iv) asynchronous quenching (AQ), which coincides with a torus-death bifurcation to lock-in and reduces the oscillation amplitude – by up to 90% in the combustor. There are, however, quantitative differences between the two systems, which pertain mainly to (i) the magnitude of the amplitude reduction achieved by AQ and (ii) the size of the AQ region in the ff/f1–forcing-amplitude plane. This study has three main contributions. First, it shows that studying open-loop control from a synchronization perspective can provide valuable insight into the optimal forcing conditions. Second, it shows that the optimal forcing condition for weakening thermoacoustic oscillations is that which causes the onset of lock-in via a torus-death bifurcation, as this is where AQ occurs. Third, it shows that the synchronization dynamics of a real combustor can be qualitatively modelled with a low-order universal oscillator. This suggests that it may be possible to develop and test new control strategies by analyzing the solutions to such an oscillator.