Direct steam generation (DSG) for concentrating solar power (CSP) is an emerging technology that can unlock new avenues for efficient and affordable energy utilisation and expand the current capabilities of CSP. However, the direct evaporation of steam in parabolic trough solar collectors presents control and operational challenges due to the inherently complex two-phase nature and inherent unsteadiness of the boiling flow inside the tubes. Thus, a fundamental understanding of the hydrodynamic and heat transfer characteristics of this two-phase flow is required. A bespoke flow boiling facility has been constructed for the investigation of boiling flows using laser diagnostic techniques to gain accurate spatio-temporally resolved information on the flow characteristics. The facility has been validated and a proof of concept of the application of particle image velocimetry to boiling flows performed.

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full field nonintrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m(2) and mass fluxes from 300 to 460 kg/m(2)s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapor bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapor bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. This research demonstrates the applicability of laser-based diagnostic techniques to boiling slug flows, to which such experimental methods have not been widely applied in macroscale geometries. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

This paper outlines a recently designed programme of training for graduate teaching assistants (GTAs), focusing on peer elements embedded in the programme. In particular, we describe our approach to co-facilitation of training, with sessions facilitated by the GTA programme lead and an experienced Peer GTA. Peer GTAs have at least one year of teaching experience and are able to provide practical, contextualised and discipline-specific input, which helps to address the challenge of balancing generic and discipline-specific training in GTA programmes. We describe a small case study of co-facilitation and reflect on the benefits of this approach for new GTAs and for the Peer GTAs themselves. The paper is co-authored by the GTA programme lead and a Peer GTA, who provides first-hand reflections on her experiences.