• Energy Research

Abstract Subcooled flow boiling is frequently encountered in hot assemblies in nuclear power plant. Considerable efforts have been made to improve the understanding of boiling process and prediction of critical heat flux (CHF). In this study, the effects of non-uniform heat input on heat transfer characteristics are further investigated, so that more realistic descriptions of practical systems can be achieved vis-a-vis usual heat input assumptions. A CFD model has been built to predict subcooled flow boiling process in a vertical round pipe. The capability of the model is validated against experimental results in the publication. A variety of non-uniform heat input profiles are tested using this validated CFD model. Results show that a regular triangle-shape heat input profile will cause stronger vapour generation in the middle of the heating region. Also, the risk of reaching CHF may be increased with the position of maximum heat flux moving downstream.

AbstractThis paper reports a new innovative re‐compression technology for solid oxide fuel cell (SOFC) hybrid systems necessary to increase pressure at compressor outlet level (as required by fuel cell systems for managing cathodic recurculation and to increase SOFC efficiency). This work was based on a collaboration between the University of Manchester (United Kingdom) and the University of Genoa (Italy). The re‐compression study will be performed with the hybrid system emulator rig by TPG. This device is composed of the following technology: a microturbine package able to produce up to 100 kWe which was modified for external connections, external pipes designed for several purposes (by‐pass, measurement or bleed), and a high temperature modular vessel necessary to emulate the dimension of an SOFC stack. For the purpose of re‐compression, this test rig is planned to be equipped with a turbocharger capable of increasing pressure using part of recuperator outlet flow. Theoretical activity was considered before carrying out the real experimental tests to avoid plant risky conditions. So, it was necessary to develop a transient model (Matlab®‐Simulink® environment) to simulate the hybrid system emulator including the re‐compression system. The results obtained with the model were carried out considering the start‐up/shutdown phases of the turbocharger device.

This paper is concerned with the dynamic system nonlinear behaviour encountered in classical thermo-acoustic instability. The Poincare map is adopted to analyse the stability of a simple non-autonomous system considering a harmonic oscillation behaviour for the combustion environment. The bifurcation diagram of a one-mode model is obtained where the analysis reveals a variety of chaotic behaviours for some select ranges of the bifurcation parameter. The bifurcation parameter and the corresponding period of a two-mode dynamic model are calculated using both analytical and numerical methods. The results computed by different methods are in good agreement. In addition, the dependence of the bifurcation parameter and the period on all the relevant coefficients in the model is investigated in depth.

Based on similarity theory, this research details a thermal-state model experiment, concerning the evolution of the air/water temperature profiles inside a Natural Draft Wet Cooling Tower (NDWCT) under windless and crosswind conditions. Prior studies have shown that the air/water temperature distribution is fairly uniform and stable under windless (stagnant) conditions, but the uniformity is destroyed in the presence of windy conditions, and the air/water temperature of different points displays a large variation subject to the same crosswind velocity. Generally speaking, the highest air/water temperature values inside the whole tower lie on the windward and leeward direction, but the highest air temperature at the tower outlet appears near the leeward side zone, rather than exactly on the leeward side. Based on this research, the air/water temperature profiles regarding measurement of values can be obtained accurately under windless and crosswind conditions, a fact that can help confirm the specific location of vortex on the windward and leeward side. All of above findings can provide an important theoretical foundation concerning further research, specifically for energy-saving aspects NDWCTs.

Unitized regenerative proton exchange membrane fuel cell (URPEMFC) performs the functions of an electrolyser and a fuel cell. Currently, power hysteresis effect (PHE) is a key technological challenge for the URPEMFC because it reduces the efficiency of the system as it switches from electrolyser mode to fuel cell mode and vice versa. Here, a modelling and simulation approach is used to investigate the PHE based on its thermodynamic and electrochemical attributes. URPEMFC model was validated against an experimental study and then used for parametric studies. The results indicate that the PHE occurs when the number of cells is 1, 5 and 10. Moreover, an increase in the lost internal current density and total resistance resulted in an increase in overpotentials of the system. Although the theoretical thermodynamic efficiency of a URPEMFC is about 68.86%, the current study predicted an efficiency of 44% for a stack of 10 cells at current density of 0.5 A cm−2. A case study of an integrated photovoltaic-URPEMFC system for power generation using actual meteorological data is also presented. If optimised, URPEMFC can be applied with renewable energy sources for power-to-gas technologies, power-to-power technologies, hydrogen filling stations or distributed hybrid energy systems.

As renewable energy technologies (RETs) replace fossil fuel-based energy systems, the need to address the risks and reliability of emerging RETs suitable for integration into energy infrastructures becomes urgent. The intermittency of renewable energy sources and individual characteristics of the components of RETs are potential causes of power curtailment, system failure, techno-economic costs, and the inertia to transit to renewable energy utilisation. Here, we applied the classical failure modes, effects, and criticality analyses to assess the effects of failure modes of the components of an integrated photovoltaic-thermal-fuel cell system. Risk items were identified with their possible failure modes, mechanisms, and effects to generate risk priority numbers and Criticality values. Results showed that the risk of no solar radiation, hydrogen leakage, failure of photovoltaic module, leakage of oxygen had risk priority number of 450, 270, 240, 240 whilst their corresponding Criticality values were 90, 54, 80, 48, respectively. Generating power with both battery and fuel cell may improve the overall reliability of the system. Eventually, some recommendations were made to improve the system for off-grid and grid-connected applications to supply electrical energy and hot water. Therefore, addressing the identified risk items could significantly improve the reliability and operational efficiency the system.

Abstract In this paper, of primary concern is a time-delayed thermoacoustic system, viz. a horizontal Rijke tube. A continuation approach is employed to capture the nonlinear behaviour inherent to the system. Unlike the conventional approach by the Galerkin method, a dynamic system is naturally built up by discretizing the acoustic momentum and energy equations incorporating appropriate boundary conditions using a finite difference method. In addition, the interaction of Rijke tube velocity with oscillatory heat release is modeled using a modified form of King's law. A comparison of the numerical results with experimental data and the calculations reported reveals that the current approach can yield very good predictions. Moreover, subcritical Hopf bifurcations and fold bifurcations are captured with the evolution of dimensionless heat release coefficient, generic damping coefficient and time delay. Linear stability boundary, nonlinear stability boundary, bistable region and limit cycles are thus determined to gain an understanding of the intrinsic nonlinear behaviours.

Abstract A vertical two phase closed thermosyphon is analyzed numerically using the two-fluid methodology within Eulerian multiphase domain. The steadily operating thermosyphon is first simulated using a full scale axi-symmetric model. The model is then used to predict the behaviour of the thermosyphon under different pulsed heat increment conditions. The effects of evaporation, condensation and interfacial heat and mass transfer are taken into account within the two phase domain. The cooling water jacket is also modelled along with the wall of thermosyphon to simulate the effect of conjugate heat transfer between the wall and fluid phase. Results obtained show in detail the overall thermal response of the thermosyphon along with the dynamics of fluid flow within its core. It is established that two-fluid methodology along with the applied techniques can be used effectively for the purpose of simulation of two phase system like a typical thermosyphon.

The two-phase closed thermosyphon (TPCT) or wickless heat pipe has been widely considered as an extremely effective and low-cost heat removal device for various applications. A computational fluid dynamics (CFD) investigation of the TPCT can provide detailed information regarding its design and development. In this study, the effect of non-uniform heat-input profiles on a vertical TPCT has been investigated. A CFD model has been built to simulate the evaporation and condensation processes within the TPCT investigated, using a solver based on OpenFOAM which has been modified and validated against experimental data reported in the open literature. Four non-uniform heating profiles of the TPCT have been investigated, and the effects these have on the internal flow field within the pipe are discussed. Simulation results show that the non-uniform heat flux profiles can impact thermal performance depending on the overall heat loading and the heat flux profile.