This paper presents a review of the literature describing the packing structure and effective thermal conductivity of randomly packed beds consisting of mono-sized particles. In this study particular ...attention was given to the packing structure (porosity, coordination number, and contact angles) and heat transfer by solid conduction, gas conduction, contact area, surface roughness, as well as thermal radiation. New methods to analyse the models were developed giving new insights into the shortcomings of the correlations to predict and define the packing structure, as well as to simulate the effective thermal conductivity in the near-wall region. This information is of particular importance in the design and operation of high temperature packed bed nuclear reactors.
Thermal-fluid simulations are used extensively to predict the maximum fuel temperatures, flows, pressure drops and thermal capacitance of pebble bed gas cooled reactors in support of the reactor ...safety case. The PBMR company developed the HTTU non-nuclear test facility in cooperation with M-Tech Industrial (Pty) Ltd. and the North-West University in South Africa to conduct comprehensive separate effects tests as well as integrated effects tests to study the different thermal-fluid phenomena. This paper describes the separate effects tests that were conducted to determine the effective thermal conductivity through the pebble bed under near-vacuum conditions and temperatures up to 1200°C. It also presents the measured temperature distributions and the methodology applied in the data analysis to derive the resultant values of effective thermal conductivity and its associated uncertainty.
► A new model to predict the effective thermal conductivity in a packed pebble bed. ► Specifically accounts for the porous structure. ► Enables improved prediction in the near-wall and wall regions. ...► Conduction component compares well with experimental data. ► Radiation component compares well for temperatures up to 1200°C.
This paper presents a new approach to the calculation of the effective thermal conductivity in packed pebble bed reactors, namely the Multi-sphere Unit Cell model. The model specifically accounts for the porous structure, which is characterised using the radial variation in porosity, coordination number and contact angles between adjacent spheres. It also accounts for solid and gas thermal conduction, contact area, surface roughness as well as the thermal radiation for pebble temperatures up to 1200°C. This more rigorous approach to characterising the porous structure enables improved prediction of the effective thermal conductivity in the near-wall and wall regions, resulting in better prediction of the temperatures at the reflector interface. Results obtained with the Multi-sphere Unit Cell model are compared with existing correlations and experimental data including those from the SANA-I experimental test facility.
Strategy is about making choices. Mining companies choose to do certain things and not to do other things. Mining is a long-term business, and the choices made typically have large investments ...attached to them, long payback periods, and significant socio-economic consequences. In today's uncertain world, it is important to make the right choices. The mining industry in South Africa finds itself in a difficult situation. Operating conditions are tough, the socio-political environment is complex, and financial performance is under pressure. The choices made by all the stakeholders in this industry in the short term will shape the future of the industry. This paper characterizes some of the big, difficult decisions faced by the mining industry in the South African context, and discusses how these decisions could be approached in a fact-based and robust way.
Surface mass loss from the Greenland ice sheet (GrIS) has accelerated over the past decades, mainly due to enhanced surface melting and liquid water runoff in response to atmospheric warming. A large ...portion of runoff from the GrIS originates from exposure of the darker bare ice in the ablation zone when the overlying snow melts, where surface albedo plays a critical role in modulating the energy available for melting. In this regard, it is imperative to understand the processes governing albedo variability to accurately project future mass loss from the GrIS. Bare-ice albedo is spatially and temporally variable and contingent on non-linear feedbacks and the presence of light-absorbing constituents. An assessment of models aiming at simulating albedo variability and associated impacts on meltwater production is crucial for improving our understanding of the processes governing these feedbacks and, in turn, surface mass loss from Greenland. Here, we report the results of a comparison of the bare-ice extent and albedo simulated by the regional climate model Modèle Atmosphérique Régional (MAR) with satellite imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) for the GrIS below 70∘ N. Our findings suggest that MAR overestimates bare-ice albedo by 22.8 % on average in this area during the 2000–2021 period with respect to the estimates obtained from MODIS. Using an energy balance model to parameterize meltwater production, we find this bare-ice albedo bias can lead to an underestimation of total meltwater production from the bare-ice zone below 70∘ N of 42.8 % during the summers of 2000–2021.
The environmental footprint of agricultural production can vary significantly both between countries and within a country based on regional conditions and agricultural practices. A life cycle ...assessment approach was used to model primary fossil fuel energy inputs and greenhouse gas emissions associated with the production of sugarcane in South Africa. Results were calculated for sugarcane produced in two distinct regions, the irrigated North and the non-irrigated North Coast. Regional differences also include terrain, soil, and use of mechanization. Models were adapted to estimate the impacts of burning prior to harvest, leaving a biomass mulch blanket with green cane harvesting, and increasing the level of mechanization for harvest and other field operations. Irrigation contributes to a higher energy input in the irrigated North but differences are mitigated by the lower fertilizer, agro-chemical, and diesel use per ton of sugarcane produced there. Despite higher energy inputs in the irrigated North, greenhouse gas emissions are similar for sugarcane produced in each region. Green cane harvesting reduces energy inputs and greenhouse gas emissions by 4% and 16%, respectively, in both regions. Impacts of mechanization on soil compaction and stool damage result in lower yields and proportionally higher energy inputs and greenhouse gas emissions. Results demonstrate the potential for variability in LCA results based on regional differences in production practices within a country.
•An LCA approach was used to model energy and GHGs for sugarcane in South Africa.•Effects of irrigation, cane burning, mechanized harvest, and regions were modeled.•Irrigated cane requires more energy but has similar emissions to non-irrigated cane.•Green cane harvesting reduces energy inputs (4%) and GHG emissions (16%).•Mechanization reduces yields and thus increases energy inputs and GHG emissions.
The term ‘regenerative agriculture’ describes a rehabilitation and conservation approach to farming, which aims to enhance the sustainability of production. The approach consists of an array of ...practices that focus on the maintenance and rehabilitation of soil health. In the South African sugar industry, major soil related problems include: (a) soil erosion; (b) compaction; (c) acidification; and (d) soil salinity/sodicity. While regenerative practices to guide growers have been developed at the South African Sugarcane Research Institute (SASRI), the remediation of poor soil health in the industry has generally been hampered by low levels of adoption of these practices. The importance of the small-scale grower (SSG) sector to economic development has been recognised as a key element of the South African Sugarcane Value Chain Master Plan to 2030, which aims to ensure the long-term sustainability of the industry. To understand the barriers to adoption, SASRI conducted a survey of a small-scale grower community. This revealed that conventional knowledge exchange methods were ineffective with SSGs. Consequently, SASRI is developing and implementing knowledge exchange approaches which are founded on: (a) networks of demonstration plots where regenerative practices can be showcased and (b) participative research methodologies which aim to empower SSGs in discovering their own practical solutions to production challenges. Complementing these is an initiative to upskill extension specialists and agricultural advisors in soil health regenerative practices supported by regular radio broadcasts and newsletters. In the short term, the impact of these knowledge exchange tactics is being monitored through analysis of SSG production and economic data by SASRI, in collaboration with stakeholders from the local grower associations and mills. To date, data from the small-scale grower community study indicated that the implementation of the demonstration plot methodology focused on matching variety to soil type has had a positive impact on SSG livelihoods.
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